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1

Geochemical properties and nuclear chemical characteristics of Oklo natural fission reactors  

Energy Technology Data Exchange (ETDEWEB)

There are six uranium deposits in the Gabonese Republic in the cnetral Africa. `Fission reactor zone`, the fission chain reactions generated about 200 billion years ago, was existed in a part of them. CEA begun geochemical researches of Oklo deposits etc. in 1991. The geochemical and nuclear chemical properties of Oklo were reviewed from the results of researches. Oklo deposits is consisted of main five sedimentary faces such as sandstone (FA), Black Shale formation (FB), mudstone (FC), tuff (FD) and volcaniclastic sandstone (FE) from the bottom on the base rock of granite in the Precambrian era. Uranium is enriched in the upper part of FA layer and the under part of FB layer. {sup 235}U/{sup 238}U, U content, fission proportion, duration time, neutron fluence, temperature, restitution factor of {sup 235}U and epithermal index ({gamma}) were investigated and compared. The geochemical properties of Oklo are as followed: large enrich of uranium, the abundance ratio of {sup 235}U as same as that of enriched uranium, interaction of natural water and small rear earth elements. These factors made casually Oklo fission reactor. (S.Y.)

Hidaka, Hiroshi [Hiroshima Univ., Higashi-Hiroshima (Japan). Faculty of Science

1997-07-01

2

Role of organic matter in the Proterozoic Oklo natural fission reactors, Gabon, Africa  

International Nuclear Information System (INIS)

Of the sixteen known Oklo and the Bangombe natural fission reactors (hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals), reactors 1 to 6 at Oklo contain only traces of organic matter, but the others are rich in organic substances. Reactors 7 to 9 are the subjects of this study. These organic-rich reactors may serve as time-tested analogues for anthropogenic nuclear-waste containment strategies. Organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions. Liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality. The bitumen soon became a solid, consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite, which enclosed and immobilized uraninite and the fission-generated isotopes entrapped in uraninite. This mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1.2 b.y. 24 refs., 4 figs

3

Organic free radicals and micropores in solid graphitic carbonaceous matter at the Oklo natural fission reactors, Gabon  

International Nuclear Information System (INIS)

The presence, concentration, and distribution of organic free radicals as well as their association with specific surface areas and microporosities help characterize the evolution and behavior of the Oklo carbonaceous matter. Such information is necessary in order to evaluate uranium mineralization, liquid bitumen solidification, and radio nuclide containment at Oklo. In the Oklo ore deposits and natural fission reactors carbonaceous matter is often referred to as solid graphitic bitumen. The carbonaceous parts of the natural reactors may contain as much as 65.9% organic C by weight in heterogeneous distribution within the clay-rich matrix. The solid carbonaceous matter immobilized small uraninite crystals and some fission products enclosed in this uraninite and thereby facilitated radio nuclide containment in the reactors. Hence, the Oklo natural fission reactors are currently the subjects of detailed studies because they may be useful analogues to support performance assessment of radio nuclide containment at anthropogenic radioactive waste repository sites. Seven carbonaceous matter rich samples from the 1968 ± 50 Ma old natural fission reactors and the associated Oklo uranium ore deposit were studied by electron spin resonance (ESR) spectroscopy and by measurements of specific surface areas (BET method). Humic acid, fulvic acid, and fully crystalline graphite standards were also examined by ESR spectroscopy for comparison with the Oklo solid graphitic bitumens. Wwith the Oklo solid graphitic bitumens. With one exception, the ancient Oklo bitumens have higher organic free radical concentrations than the modem humic and fulvic acid samples. The presence of carbon free radicals in the graphite standard could not be determined due to the conductivity of this material. 72 refs., 7 figs., 1 tab

4

Uraninite recrystallization and Pb loss in the Oklo and Bangombé natural fission reactors, Gabon  

Science.gov (United States)

The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO 2 in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombé, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones. The analyzed uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2?). Thus, the first major Pb loss from uraninite occurred at both Oklo and Bangombé during regional extension and the intrusion of a dyke swarm in the Franceville basin, ˜860-890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the "chemical" ages of the uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that uraninite at both the Oklo and Bangombé natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the uraninite structure.

Evins, Lena Z.; Jensen, Keld A.; Ewing, Rodney C.

2005-03-01

5

Isotopic study of natural fission reactors at Oklo and Bangombe, Gabon  

International Nuclear Information System (INIS)

The Oklo and Bangombe uranium ores in the Republic of Gabon are fossils of natural fission reactors. Many elements in these natural fission reactors show isotopic anomalies derived from fission and neutron capture reactions. Isotopic analyses of uraninites and some other minerals provide useful information on the geochemical behavior of fission products and nuclear chemical characterization of the reactors. Integrated isotopic measurements by whole rock analysis with inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS) and by in-situ analysis with secondary ion mass spectrometry (SIMS) make it possible to clarify the migration processes of fissiogenic nuclides over a range of scales from micro meters to meters. (author)

6

Illite in the Oklo natural fission reactors in Gabon: Considerations for Cs containment  

International Nuclear Information System (INIS)

The ? 2 Ga old Oklo, Okelobondo and Bangombe natural reactors in the Republic of Gabon contain solid graphitic bitumens and clay minerals, both of which have effected the containment, or partial containment, of 235U and several fission products. In laboratory experiments, sorption of 134Cs by illite, and illite coated with petroleum was measured in aqueous NaCl solutions to simulate subsurface (connate) waters in sedimentary rocks. Elevated temperatures and increasing salinity of the NaCl solutions facilitated the removal of sorbed cesium from illite

7

Oklo, the world's first nuclear fission reactor  

International Nuclear Information System (INIS)

The world's oldest identified nuclear fission reactor was fashioned by nature approximately two billion years ago in western equatorial Africa. The remains of the six-reactor complex, or what is now known as the Oklo Phenomenon, was discovered by the French Commissariate Energie Atomique (CEA) during 1972, in a rich uranium mine located at Oklo in the Republic of Gabon. Dr. George A. Cowan of LASL, one of the members of the team that produced the first man-made nuclear reactor in Chicago, recognized the relevance of Oklo to the long-term management of high-level radioactive waste. The US effort on Oklo is concentrated on evaluating the relevance of Oklo and other naturally occurring nuclear reactors on the terminal placement of radioactiv waste in geologic formations. US efforts to locate additional natural reactors in Canada and Australia will be discussed. Recognition that God or nature created the oldest known nuclear fission reactor and essentially contained its associated waste long before the time of man is probably the most significant thing about Oklo

8

Heterogeneity and alteration of uraninite from the natural fission reactor 10 at Oklo, Gabon  

International Nuclear Information System (INIS)

A mineralogical study of uranium ore from reactor zone 10 revealed that uraninite in the natural reactors at Oklo, Gabon, has been altered through partial dissolution, Pb loss, and replacement by coffinite, USiO4.nH2O. The dissolution occurred during formation of the clay mantle surrounding the ore body and was probably caused by hydrothermal saline solutions under reducing conditions. The loss of lead (up to 11 wt%) from uraninite occurred approximately one billion years after the operation of the reactors. As a result, there are two generations of uraninite in the reactor zone that differ in chemical composition and unit cell parameters [a1 = 0.5495(2) and a2 = 0.5455(2) nm]. Minor coffinitization of uraninite has also occurred. Thus, the Oklo deposit has been altered since the event of nuclear criticality. This provides several distinct geochemical environments in which one may analyze the corrosion of uraninite and the subsequent retention or migration of fission products. (author). 20 refs., 3 figs., 1 tab

9

XPS and XRD studies of samples from the natural fission reactors in the Oklo uranium deposits  

International Nuclear Information System (INIS)

Mineral samples from the natural fission reactors 10 and 13 in the Oklo uranium deposits were studied using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to gain information about the long-term behaviour of UO2 fuel in a geological disposal vault. Two samples from reactor zone 10 (samples No. D81N-190292 and D73-88) and one sample from reactor zone 13 (sample No. SD37-S2/CD) were analysed. Low-resolution XPS spectra were recorded to determine the major elements present in the ore. High-resolution spectra were recorded to gain information about the chemical state of the elements present in the mineral samples. The samples show low values for the U6+/U4+ ratio. The oxidation state of uranium in these samples is even lower than that in U4O9.The binding energies of the Pb 4f bands indicate most of the Pb is in the +2 oxidation state in these samples. The C ls band indicates the presence of organic carbon. XRD analysis shows that the main uranium-bearing phase is uraninite and lead is present mainly as galena. The significance of the results for nuclear fuel waste management is discussed

10

The Oklo natural reactor, Gabon  

International Nuclear Information System (INIS)

This paper reports how the Oklo natural reactor, Gabon, remains the best natural analog for assessing the behavior of fission products, actinides, and actinide daughters in rocks. The rocks at Oklo are porous and permeable, as well as being fractured and containing abundant water, yet many of the fission products and actinides have remained in place or close to their formation sites. The actinides Th, U, Np, Pu, and Am are similar in their crystal chemical characteristics, and all were retained in the host pitchblende. Elements incompatible in the pitchblende structure were lost by diffusion into the rocks surrounding the high grade reactor ore. Alkali and Alkaline earth elements Rb, Sr, Cs, and Ba were fixed in and very close to the reactor ores in clay minerals and in some secondary carbonates and sulfates. Local oxidizing conditions in the reactor zones caused some loss of Tc, Mo, Cd, and Ru, but the Tc, Mo, and Ru were fixed in sulfides formed close by under chemically reducing conditions. Local migration for Ag and Sn has been documented for some samples

11

Oklo natural reactors: geological and geochemical conditions  

International Nuclear Information System (INIS)

Published as well as unpublished material on the Oklo natural reactors in Gabon was evaluated with regard to the long-term aspects of nuclear waste disposal. Even though the vast data base available at present can provide only a site specific description of the phenomenon, already this material gives relevant information on plutonium retention, metamictization, fission product release, hydrogeochemical stability and migration of fission products. Generalized conclusions applicable to other nuclear waste repository would require the quantitative reconstruction of t s coupled thermo-hydrologic-chemical processes. This could be achieved by studying the deviations in the 2H/1H and 18O/16O ratios of minerals at Oklo. A further generalization of the findings from Oklo could be realized by examining the newly-discovered reactor zone 10, which was active under very different thermal conditions than the other reactors. 205 refs

12

X-ray photoelectron spectroscopic study of a sample from a natural fission reactor at Oklo, Gabon  

International Nuclear Information System (INIS)

X-ray photoelectron spectroscopy (XPS) was used to study a sample from a natural fission reactor, reaction zone number 13, at Oklo (Gabon). Peaks due to uranium, oxygen, carbon, lead, silicon, calcium, aluminum, sulfur thorium and bismuth were seen in the XPS spectra. High-resolution spectra were recorded for the U, Pb, O, and C bands to determine the chemical state of these elements. The oxidation state of uranium in the sample is predominantly U4+, with a U6+/U4+ ratio of about 0.11. Lead was in the +2 oxidation state. The carbon signal indicated the presence of organic carbon, while the oxygen bands indicated the presence of water. The significance of the results for nuclear fuel waste management is discussed. (author). 27 refs., 7 figs., 2 tabs

13

The Oklo natural nuclear reactors: neutron parameters, age and duration of the reactions, uranium and fission products migrations  

International Nuclear Information System (INIS)

Mass spectrometry and isotopic dilution technique are used in order to carry out, on various samples from the fossil nuclear reactors at Oklo, Gabon, isotopic and chemical analyses of some particular elements involved in the nuclear reactions: uranium, lead, bismuth, thorium, rare gases (krypton, xenon), rare earths (neodymium, samarium, europium, gadolinium, dysprosium), ruthenium and palladium. Interpretations of these analyses lead to the determination of many neutron parameters such as the neutron fluence received by the samples, the spectrum index, the conversion coefficient, and also the percentages of fissions due to uranium-238 and plutonium-239 and the total number of fissions relative to uranium. All these results make it possible to determine the age of the nuclear reactions by measuring the amounts of fission rare earths formed, i.e. 1.97 billion years. This study brings some informations to the general problem of radioactive wastes storage in deep geological formations, the storage of uranium, plutonium and many fission products having been carried out naturally, and for about two billion years

14

The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16  

International Nuclear Information System (INIS)

In an underground nuclear waste repository, the chemical behavior of some stored fission products and actinides depends on the redox conditions during their long-term evolution. In this respect, radiolysis is an important phenomenon which can significantly modify the local redox conditions. The Oklo natural fission zones are good examples where the effect of radiolysis can be deduced from a mineralogical and geochemical study. Zones 10 and 16 were studied because they are located at depth of 270 m in an area devoid of any recent water circulation and not subject to the effect of the lateritic alteration occurring elsewhere in this area. In zone 10, there is a marked evolution of the U-Pb-Fe-S mineralogy from the center to the periphery of the reactor zone. In the center, uraninite shows silicification and coffinitisation with the formation of galena and native lead; the PbO content of uraninite can be as much as 20 wt%. In the periphery of the reactor zone, some radiogenic lead is present as minimum (Pb3O4) and in Pb-bearing calcite. In the surrounding sandstones, hematite is widespread. In zone 16, the mineral paragenesis is generally comparable with that of zone 10 but with some differences. Galena is the only Pb-bearing mineral associated with uraninite crystals. The PbO content of uraninite is always 2O-H2 ± CH4 inclusions were observed in healed microcracks in the detrital quartz grains. Based on microthermometric measurements, the salinity of the aqueous solution ranges from 0.2 to 18 wt% eq. NaCl. Raman analysis of the gas phase indicates that the hydrogen to oxygen ratio differs from an inclusion to the other. 41 refs., 15 figs., 3 tabs

15

3D modelling of thermal and fluid transfers around a natural fission reactor (Oklo, Gabon)  

International Nuclear Information System (INIS)

Numerical modelling is used to quantify heat and mass transfers around the Oklo site. A 3D model of a reactor, at decametric scale, built with the GOCAD software shows that the functioning of the reactor acts as a powerful but local thermal perturbation. This perturbation increases the temperature within a range of 50 to 250 deg C, according to assumed heat production, with a spatial extent less than 50 m. The steady state regime is reached very rapidly, in less that 100 years. The heat dissipation is essentially conductive, the reactor inducing only weak fluid movements. A forced convection model has been also investigated, where fluids come from basin scale circulations. It shows that, in the range of the studied filtration velocities, temperatures are not significantly affected by these circulations. Nevertheless, they induce an asymmetry between upstream and downstream parts of the flow. Assuming low permeability, the high temperature increase could have caused local fluid overpressures, which could lead to the development of a radial hydraulic fracturing near the reactor, as has been observed around the reactor 10. (authors)

16

Oklo natural fission reactor program. Progress report, April 1-August 31, 1980  

Energy Technology Data Exchange (ETDEWEB)

An interim report has been published on the redistribution of uranium, thorium, and lead in samples representing several million cubic meters of sandstone and metamorphosed sediments in the Athabasca Basin which is located in the northwest corner of the Canadian province of Saskatchewan. The region of study includes zones of uranium mineralization at Key Lake. Mineralization occurs at the unconformity between the Athabasca sandstone and the underlying metasediments and in fault zones within the metasediments. Lead isotopes record a radiometric age of 1300 +- 150 m.y. in samples from above and below the unconformity. This age probably reflects the time of deposition of the sandstones and an associated redistribution of uranium and/or lead in the underlying rocks. Many of the samples have been fractionated with respect to radiogenic lead and the actinide parent elements since that time. Sandstones and altered rocks from the region above the unconformity have been a transport path and are a repository for lead. In contrast, mineralized rocks are deficient in radiogenic lead and must be an important source of lead in the local geologic environment. Samples from Oklo reactor zone 9 and nearby host rocks have been prepared for isotopic analyses of ruthenium, molybdenum, uranium and lead.

Curtis, D.B. (comp.)

1980-12-01

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Fission product retention in newly discovered organic-rich natural fission reactors at Oklo and Bangombe, Gabon  

International Nuclear Information System (INIS)

The discovery of naturally occurring fission reactors in the rock strata of the Paleoproterozoic Francevillian Basin in the Republic of Gabon in equatorial West Africa led to several programs to define migration and/or retention of uranium and fissiogenic isotopes from/in the natural reactor zones. Although much understanding has been gained, new insight is needed regarding the chemical and physical parameters that control movement and retention of fission products over almost two billion years from/in the natural reactors. Seventeen known natural fission reactors sustained criticality for 0.1 to 1 million years in hydrothermally altered sedimentary rocks 1968 +/- 50 million years ago. These natural nuclear reactors attained criticality because of high concentrations of uranium in small pockets in uranium ores, the lack of neutron poisons, and because at the time they reached criticality, the abundance of 235U was five times greater than it is today. Water acted as a moderator, and temperature in the natural reactors was between 160 and 360 degrees C. Both the uranium-rich pockets and the uranium ore bodies in which these pockets are located were formed when aqueous solutions moving through highly fractured zones in the Francevillian sedimentary rocks met organic-rich sediments. This resulted in the reduction of U(VI) in the dissolved uranyl ions to U(IV), causing the precipitation of pitchblende and uraninite. It has been proposed that between 2.2 and 1.9 as been proposed that between 2.2 and 1.9 billion years ago, the earth's atmosphere experienced a remarkable temporary rise in O2 content; this event may account for the uranium-bearing, oxidizing aqueous solutions in the Francevillian rocks

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The effects of water radiolysis on local redox conditions in the Oklo, Gabon, natural fission reactors 10 and 16  

Science.gov (United States)

In an underground nuclear waste repository, the chemical behavior of some stored fission products and actinides depends on the redox conditions during their long-term evolution. In this respect, radiolysis is an important phenomenon which can significantly modify the local redox conditions. The Oklo natural fission zones are good examples where the effect of radiolysis can be deduced from a mineralogical and geochemical study. Zones 10 and 16 were studied because they are located at depth of 270 m in an area devoid of any recent water circulation and not subject to the effect of the lateritic alteration occurring elsewhere in this area. In zone 10, there is a marked evolution of the U?Pb?Fe?S mineralogy from the center to the periphery of the reactor zone. In the center, uraninite shows silicification and coffinitisation with the formation of galena and native lead; the PbO content of uraninite can be as much as 20 wt%. In the periphery of the reactor zone, some radiogenic lead is present as minium (Pb 30 4) and in Pb-bearing calcite. In the surrounding sandstones, hematite is widespread. In zone 16, the mineral paragenesis is generally comparable with that of zone 10 but with some differences. Galena is the only Pb-bearing mineral associated with uraninite crystals. The PbO content of uraninite is always hematite is sometimes replaced by pyrite. In an area where the fission zone 10 is in contact with sandstones devoid of organic matter, H 2O?H 2O 2 and H 20?H 2 ± CH 4 inclusions were observed in healed microcracks in the detrital quartz grains. Based on microthermometric measurements, the salinity of the aqueous solution ranges from 0.2 to 18 wt% eq. NaCl. Raman analysis of the gas phase indicates that the hydrogen to oxygen ratio differs from an inclusion to the other. The presence of H 2- and O 2-bearing fluid inclusions confirms the existence of water radiolysis in the reactor zone. In zones rich in organic matter, oxygen was preferentially consumed in reactions with organic matter and its radiolytic products. In the absence of organic matter and its radiolytic products, oxidizing conditions prevailed and hematite and minium have formed. Consequently, presence of highly oxidizing conditions has important consequences on uranium and fission products mobility. Water radiolysis is also evidenced during the subsequent 1.97 By history of the reactor. During the fission reactions, redox conditions deduced from Pb-bearing minerals indicate that the sulfur fugacity was very low in zone 10 as shown by the formation of minium and higher in zone 16 with the formation of barite and pyrite. In this paper we discuss the geochemical consequences of such contrasting redox zones and the resulting redox heterogeneity created by water radiolysis.

Savary, Véronique; Pagel, Maurice

1997-11-01

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Nuclear fission is no invention of humans: The natural reactor of Oklo two billion years ago  

International Nuclear Information System (INIS)

r by neutrons formed by secondary reactions of cosmic radiation. From changes in isotope ratios, however, with modern mass spectrometry one can learn a lot about the migration of fission products. Thus it is possible to obtain a 'glimpse' of a final repository after millions of years, a better picture than any computer simulation has been able to provide. And although no technical barriers and no optimized geological conditions hindered the migration of fission products and plutonium, this picture shows that uranium, neptunium, plutonium, niobium, yttrium, technetium, zirconium and the rare earth metals have remained in their original places. Elements that have migrated are alkali metals, e.g. rubidium and caesium, the alkaline earth metals barium and strontium, the noble gases, molybdenum, cadmium, lead and iodine. It must also be noted that, in this historical final repository, the heat production of approximately 50 W/m2 (through the intermittent chain reaction) was many times higher than in the final repositories planned today for highly radioactive waste

20

Chemical and isotopic studies of uranium, lead and several fission products in a sample of the ore of the Oklo natural reactor  

International Nuclear Information System (INIS)

Isotopic dilution technique and mass spectrometry are used in order to measure isotopic compositions and concentrations of some particular elements on the Oklo sample KN50-3548, which are interesting for describing the nuclear reaction. These analyses give coherent results leading to the determination of the following parameters: neutron fluence, spectrum index, total number of fissions relative to uranium, and percentage of 238U and 239Pu fissions. From the percentage of plutonium fissions, the duration of the nuclear reaction is estimated to be 800,000 years. The age of the nuclear reaction is measured from the lead analysis and from the number of uranium fissions; these two determinations give respectively d=1730+-10 m.y. and d=2000+-100 m.y. The second method, based on the assumption that uranium and rare earths have not migrated, could mean that the nuclear reaction might be older than the Francevillian. It is also shown that no contamination by natural elements has occurred for lead, rare earths, palladium and rare gases, after the reaction. (Auth.)

 
 
 
 
21

The analyses of various fissiogenic nuclides from Oklo natural reactor  

International Nuclear Information System (INIS)

The isotopic compositions of Rb, Sr, Zr, Mo, Ru, Pd, Ag, Te, Ba, rare earth elements (REE) and U in four Oklo uranium ore samples were determined by thermal ionization mass spectrometer. The analyses of these elements make possible the reconstruction of relative fission product yield curves covering mass number range from 85 to 176 of Oklo samples. Relative mobility or retentivity of fissiogenic nuclides in the Oklo ore can be recognized from the deviations between empirical fission yields for uranium fuel and measured ones for Oklo samples. The degree of retention for some fissiogenic nuclides would be discussed. The comparison of empirical fission yields with measured ones suggests that Ru, Pd, Te and most of REE have been retained well in the reactor. On the other hand, considerably large amounts of fissiogenic Rb, Sr and Ba have been removed from the reactor. Fissiogenic Zr, Mo and Aa have been partially removed. (author)

22

The deposit of Oklo and its natural nuclear reactors  

International Nuclear Information System (INIS)

In the uranium deposit of Oklo (Republic of Gabon), seven zones have been discovered since 1972, in which natural fission reactions took place. Since 1974, a thorough geological study of these zones has been undertaken. It includes field studies, observations of drilled samples and laboratory studies. These studies permit the authors to define the geological environment of the reactors and to point out the influence of nuclear reactions on the surrounding formations. All this work was completed by a geological and metallogenic study of the deposit of Oklo and of the uraniferous basin of Franceville. The deposit of Oklo is situated in a detrital, sandstone-like and pelitic series belonging to the Francevillian. The Francevillian and the mineralization are dated as Middle Precambrian (1800-2000 M.A.). The ore of Oklo is the result of two concentration stages. In the first, uranium seems to have been fixed by hydrocarbons that were concentrated in oil traps. After a tectonic event, circulations of oxidizing solutions generated reconcentrations that are associated with hematite and have contents of UO2 between 1 and 20%. The fission reactions developed in the high-graded ores which had formed during the last phase of UO2 concentration. A thorough tectonic analysis of the ore deposit shows that high-graded ores and fission reactors are controlled by fractures. The working of nuclear reactors results in a local increase of temperature which gave a rise increase of temperature which gave a rise to circulation of warm water. The results of this hydrothermal circulation and of the neutron bombardment are seen in a succession of facies surrounding the reactors. At the centre of the reactor all sedimentary structures have been destroyed; within the reaction zone the following clays mineral zones are founded: (1) 1 Md illite and ferrous chlorite corresponding to the common Francevillian sediment; (2) 2 Md illite, (3) magnesium chlorite and (4) 1 Md illite and chlorite-vermiculite in the very rich uraninite ore. These dydrothermal circulations also caused removal of silica toward the outer zones which reacted affecting the geometry of the reactors by fracturing

23

Recent outputs of the Oklo (Gabon) natural analogue study to nuclear waste disposal  

International Nuclear Information System (INIS)

In the past twenty five years, the natural nuclear reactors of Oklo have been the subject of numerous detailed studies. First investigated for the physical and neutron aspects of the nuclear reaction, they were then reconsidered because they provide a unique opportunity in the world to study the containment of actinides and fission products in a geological formation over a broad timescale (two billion years). Although the sites investigated do not represent a complete analogue of a repository system, many of the processes studied (mass transfer to the surface, transport, migration / retention), the spatial extent of these processes, and the timescales involved, are compatible with processes liable to occur during the lifespan of a repository for the deep geological disposal of spent nuclear fuel. A fresh program was therefore initiated as a European Commission project in 1990, entitled''Oklo as a natural analog for transfer processes in a radioactive waste repository'- phase 7, and then extended by a phase 2 entitled Oklo, Natural Analogue - Behavior of Nuclear Reaction Products in a Natural Environment''. Researches conducted in phase I served to determine the physical conditions of the operation of the natural reactor, reconstruct the geological history of the reactor environment, and decode the behavior of actinides as well as fission products in the surrounding geological formations. Phase N, which ended in June 1999, had three main objectives: i) to assess radionuclide migration and retention processes from the reactor zones to the geological environment, ii) to define the confinement properties and long-term behavior of geological materials; iii) to test models of processes related to radionuclide migration and retention, and eventually to provide suitable data and scenarios for performance assessment of nuclear waste disposal. This paper proposes a synthesis of the main outputs of the Oklo project to the performance assessment of nuclear waste disposal, the study of the long-term evolution of spent fuel and the long-term behavior of geological materials with respect to the containment of actinides and fission products. The Oklo natural analogue displays a number of specific features that make it unique in the world. The Oklo basin is characterized by the occurrence of meter scale uraninite lenses, that were affected by nuclear fission 2 billion years ago. These ''reactor zones'' exist in three sites: Oklo, Okelobondo and Bagombe. By analogy with a repository system, they are considered as representative of the 'Source' term. Numerous isotopic and geochemical tracers are thus available in order to restrict the migration or retention processes of actinides and fission products present in these zones. The near environment of the reactor zones, called ''Near field'' by analogy, is mainly composed of clayey materials (i.e. chlorite, illite, kaolinite). Reactor zones are found at present from the surface (Bagombe under oxidizing and acid conditions, with supergene weathering) to deep (Okelobondo under reducing conditions, with a low groundwater dynamics) conditions. Some reactor zones, e.g. R.Z. 13 in Oklo mine, have been subjected to strong hydrothermal disturbances (with temperatures above 350 deg C), linked to the geological history of the Franceville basin. On the other hand, the old age of the Oklo reactors (2 Ga) implies that pressure, temperature and chemical conditions have evolved during a long geological history, with associated basin scale movements of fluids. The Oklo-natural analogue Phase II project compiled useful information and tools for persons involved in Performance Assessment of waste disposal, wasteform conception or long term behavior [10] in four main areas corresponding to major investigation fields: 1/ ''Source'' term evolution, 2/ Long term containment properties of geological materials, 3/ Migration and retention of actinides and fission or end products, and 4/Geochemical and transport modeling. The main outputs of the European Oklo project to the nuclear waste repository issue are shown. The

24

Natural analogue studies in the Oklo uranium ore deposit, Republic of Gabon  

International Nuclear Information System (INIS)

The Oklo uranium ore in the Republic of Gabon is one of the adequate natural analogue for radioactive waste disposal in geological formation, because it has preserved abundant fissiogenic nuclides for 2 Ga. Isotopic studies of the Oklo fission reactor zones have been carried out to estimate the geochemical behavior of fissiogenic nuclides and to evaluate some nuclear parameters for the reactor characterization. Cooperative SEM studies, isotopic measurements by whole rock analysis with TIMS and ICP-MS and by the observation with SIMS make possible to clarify the retention and/or migration process of nuclides over wide range from ?m to m. In this paper we describe the latest results obtained in the CEC Oklo project on the migration process of fissiogenic nuclides, uranium and plutonium, and discuss the application of these results to the safety assessment of geological disposal. (author)

25

Inception and evolution of Oklo natural nuclear reactors; Genese et evolution des reacteurs nucleaires fossiles d'Oklo  

Energy Technology Data Exchange (ETDEWEB)

The occurrence of more than 15 natural nuclear Reactor Zones (RZ) in a geological environment remains a mystery even 40 years after their discovery. The present work gives for the first time an explanation of the chemical and physical processes that caused the start-up of the fission reactions with two opposite processes, uranium enrichments and progressive impoverishment in {sup 235}U. Based on Monte-Carlo neutronics simulations, a solution space was defined taking into account realistic combinations of relevant parameters acting on geological conditions and neutron transport physics. This study explains criticality occurrence, operation, expansion and end of life conditions of Oklo natural nuclear reactors, from the smallest to the biggest ones. (authors)

Bentridi, Salah-Eddine [UMR 7517, laboratoire d' hydrologie et de geochimie de Strasbourg, CNRS/universite de Strasbourg, 1, rue Blessig, 67084 Strasbourg (France); Laboratoire de l' energie et des systemes intelligents, CUKM, route de Theniet, El-Hed 44225 (Algeria); Gall, Benoit [UMR 7178, institut pluridisciplinaire Hubert-Curien, CNRS-IN2P3/universite de Strasbourg, 23, rue du Loess, 67037 Strasbourg (France); Gauthier-Lafaye, Francois [UMR 7517, laboratoire d' hydrologie et de geochimie de Strasbourg, CNRS/universite de Strasbourg, 1, rue Blessig, 67084 Strasbourg (France); Seghour, Abdeslam [Centre de recherches nucleaires d' Alger - CRNA, 2, boulevard Frantz-Fanon, 16000 Alger (Algeria); Medjadi, Djamel-Eddine [Ecole normale superieure, Vieux-Kouba, 16050 Alger (Algeria)

2011-11-15

26

Fine structure in the mass yield curve and the Oklo natural reactors (Preprint No. IT-14)  

International Nuclear Information System (INIS)

Fission yields can be determined by radiochemical or mass-spectrometric techniques. In the radiochemical method the number of fissions can be determined by direct counting of the radioactive fission products after exposing the fissionable materials to a neutron flux. However, the accuracy of radiochemically determined fission yields, particularly in the symmetric region where the probability of fission is low and uncertainties in isomeric ratios in the many of the nuclides of interest occur, is often poor. Fine structure effects can usually only be detected by the more accurate mass spectrometric technique. An important region in the study of fine structure is the symmetric valley where discontinuities in neutron yield distribution may occur. Although it must be acknowledged that an extrapolation of the results of the Oklo natural reactor to the storage of radioactive wastes in geological repositories is not a straightforward mater, nevertheless the isotopic studies carried out at Oklo have demonstrated that the retention of many fission products in well-preserved crystalline lattices has been achieved on a time scale far longer than is required for practical purposes. (author). 24 refs

27

Natural chain reaction in the Oklo mine in Gabon  

International Nuclear Information System (INIS)

An isotopic analysis of a sample of uranium ore from the Oklo mine in Gabon showed a significantly reduced content of 235U (down to 0.296% 235U). The isotopic composition of neodymium (shown), lead, rubidium, ruthenium, strontium, krypton, and xenon proved that a chain reaction took place in the ore deposit approximately 1,800 million years ago. The reaction proceeded for about 10 to 70 thousand years. About 200 kg of 235U has undergone fission and the burn-up amounted to 25,000 MWd/t. (B.S.)

28

Oklo, natural analogue of the radionuclides migration through the geological barrier  

International Nuclear Information System (INIS)

One of the main part of he CEC project 'Oklo-Natural Analogue' is devoted to present time migration studies. This part comprises hydrogeology, groundwater chemistry, isotope geochemistry and modelling. Two sites are being investigated: the less perturbed reactor zone of the Oklo mine (OK84 in the southern mine extension of Okelobondo) at around 400 meters depth and the Bangombe reactor zone, sited in a shallow environment 30 km south of Oklo. The present contribution aims to define regional hydrogeology and hydro-chemistry boundary conditions for the modelling exercise, to assess the present day water-rock interaction in the vicinity of reactor zones, to gather information on the geochemical conditions which allowed the preservation of reactor zones for two billions years, to estimate the uranium migration from the reactor zone in using a natural marker (the depleted 235U/238U ratio resulting from the fission) and to compare these data with predictive modelling. Based on the hydrogeological conceptual modelling, we have sampled waters in recharge areas, discharge areas above and below reactors, and in major local aquifers. We have been able to reconstruct the evolution of the groundwaters, in a way which is consistent with the hydrogeology, using major elements and environmental isotopes. (author)

29

Gamma-ray fluxes in Oklo natural reactors  

CERN Document Server

Uncertainty in the operating temperatures of Oklo reactor zones impacts the precision of bounds derived for time variation of the fine structure constant $\\alpha$. Improved $^{176}$Lu/$^{175}$Lu thermometry has been discussed but its usefulness may be complicated by photo excitation of the isomeric state $^{176m}$Lu by $^{176}$Lu($\\gamma,\\gamma^\\prime $) fluorescence. We calculate prompt, delayed and equilibrium $\\gamma$-ray fluxes due to fission of $^{235}$U in pulsed mode operation of Oklo zone RZ10. We use Monte Carlo modeling to calculate the prompt flux. We use improved data libraries to estimate delayed and equilibrium spectra and fluxes. We find $\\gamma$-ray fluxes as a function of energy and derive values for the coefficients $\\lambda_{\\gamma,\\gamma^\\prime}$ that describe burn-up of $^{176}$Lu through the isomeric $^{176m}$Lu state. The contribution of the ($\\gamma,\\gamma^\\prime $) channel to the $^{176}$Lu/$^{175}$Lu isotopic ratio is negligible in comparison to the neutron burn-up channels. Lutetium...

Gould, C R; Sonzogni, A A; 10.1103/PhysRevC.86.054602

2012-01-01

30

Oklo natural reactor. Study of uranium and rare earths migration on a core drilled through a reaction zone. Application to determination of the date of the nuclear reaction by measurement of fission products  

International Nuclear Information System (INIS)

Isotopic and chemical analysis of uranium and five rare earths: neodymium, samarium, europium, gadolinium and dysprosium were effected on fourteen samples taken in the same core drilled through a reaction zone of the Oklo uranium deposit. This study points out the general stability of uranium and fission rare earths; spatial distributions of these elements are quite analogous. Migrations have affected about 5% only of fission neodymium in the core of the reaction zone; corresponding values for samarium and gadolinium are slightly higher. These migration phenomena have carried rare earths to no more than 80 cm out of the core. By study of the europium it is shown that nuclear reactions have stayed in the ground since the time of reactions. On the other hand it is shown by analysis of the dysprosium that rare earths have not undergone an important movement. This study allow also the datation of nuclear reactions from the measurement of the quantity of fission neodymium produced. A value of 1.98x109 years is obtained slightly higher than the value obtained by geochronology

31

Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon  

Science.gov (United States)

Using selective laser extraction technique combined with sensitive ion-counting mass spectrometry, we have analyzed the isotopic structure of fission noble gases in U-free La-Ce-Sr-Ca aluminous hydroxy phosphate associated with the 2 billion yr old Oklo natural nuclear reactor. In addition to elevated abundances of fission-produced Zr, Ce, and Sr, we discovered high (up to 0.03 cm3 STP/g) concentrations of fission Xe and Kr, the largest ever observed in any natural material. The specific isotopic structure of xenon in this mineral defines a cycling operation for the reactor with 30-min active pulses separated by 2.5h dormant periods. Thus, nature not only created conditions for self-sustained nuclear chain reactions, but also provided clues on how to retain nuclear wastes, including fission Xe and Kr, and prevent uncontrolled runaway chain reaction.

Meshik, A. P.; Hohenberg, C. M.; Pravdivtseva, O. V.

2004-10-01

32

Record of cycling operation of the natural nuclear reactor in the Oklo/Okelobondo area in Gabon  

International Nuclear Information System (INIS)

Using selective laser extraction technique combined with sensitive ion-counting mass spectrometry, we have analyzed the isotopic structure of fission noble gases in U-free La-Ce-Sr-Ca aluminous hydroxy phosphate associated with the 2 billion yr old Oklo natural nuclear reactor. In addition to elevated abundances of fission-produced Zr, Ce, and Sr, we discovered high (up to 0.03 cm3 STP/g) concentrations of fission Xe and Kr, the largest ever observed in any natural material. The specific isotopic structure of xenon in this mineral defines a cycling operation for the reactor with 30-min active pulses separated by 2.5 h dormant periods. Thus, nature not only created conditions for self-sustained nuclear chain reactions, but also provided clues on how to retain nuclear wastes, including fission Xe and Kr, and prevent uncontrolled runaway chain reaction

33

What we learn from the nuclear data in Oklo natural reactor  

International Nuclear Information System (INIS)

We reexamined the constraint for the time variation of the coupling constant of the fundamental interaction by studying the isotropic abundance of Sm observed at Oklo natural reactor. Using the most modern and reliable data, together with the study of the isotropic abundance of Gd, we found that the original finding of Shlyakhter is essentially correct, that is, the Oklo data provides us the most stringent limit for the time variation compared with any other methods. (author)

34

Isotopic abundances measurements a key to understanding the Oklo phenomenon  

International Nuclear Information System (INIS)

The Oklo natural nuclear reactors have been studied mainly by isotopic and chemical analyses of uranium and fission products. Interpretation of these analyses allows parameters which characterize these reactions (flux, fluence,...), and also the age and duration of the nuclear reactions to be evaluated. The mechanisms and extent of fission-product migration can also be discussed. The behaviour of various elements formed in the Oklo uraninite and in oxyde fuels of pressurized water reactors is compared and found to be similar. Therefore, experience gained from the study of the Oklo phenomenon can provide valuable information on the long-term storage of radioactive wastes in geological media. (orig.)

35

Investigations of the Natural Fission Reactor Program. Progress report, October 1976--September 1977  

Energy Technology Data Exchange (ETDEWEB)

During Fiscal Year 1977, efforts of the Natural Fission Reactor Program have been directed toward (1) further measurements and predictions of reactor product migration at Oklo, (2) uraninite solubility studies, (3) radiogenic equilibria studies of natural Pu, Ra, and Th in U ores, (4) search for evidence of other natural fission reactors, and (5) supportive analytical measurements and procedures development.

Apt, K.E. (ed.)

1977-11-01

36

Natural fission reactors from Gabon. Contribution to the study of the conditions of stability of a natural radioactive wastes storage site (2 Ga)  

International Nuclear Information System (INIS)

The natural fission reactors of Oklo consists of a core of uraninite (60%) with fission products, embedded in a pure clay matrix. Thus, the aim of geological, mineral, and geochemical studies of the Oklo Reactors is to assess the behaviour of fission products in an artificial waste depository. Previous studies have shown that Reactor Zone 10, located in the Oklo mine, represents an example for an exceptional confinement of fission products since 2 Ga. In reactor Zone 9, located in Oklo open pit, migrations are more important. Reactor ZOne 13 was influenced by a thermal event due to a doleritic intrusion, located some twenty meters far away, one Ga years after fission reaction operations. In this study,we characterized temperature and redox conditions of fluids by using stable isotopes of uraninites and clays. Moreover mineralogical and chemical characteristics were defined. (author)

37

Oklo-Natural analogue. Hydrochemical constraints on the radionuclide transport modelling at Bagombe  

International Nuclear Information System (INIS)

One of the main part of the Oklo-Natural Analogue project has been devoted to estimate the uranium migration from the fossil natural reactor zones in using a natural tracer, the depleted 235/238U ratio resulting from the fission in dissolved uranium, and to compare these data with predictive modelling of U transport under present groundwater condition. At each stage of the model construction, hydro-chemistry has been used to verify the model consistency and to constraint its parameters and boundary conditions. Water chemistry and environmental isotopes are used to verify the consistency of the hydrogeological modelling and to define regional hydrogeology and hydro-chemistry boundary conditions to improve the modelling. The steady-state flow model coupled with transient solute transport is then calibrated by using environmental isotope distribution in groundwater. The geochemical system is modelled using redox conditions buffered by the iron. The modelling of buffer consumption by oxidising water predicts a displacement of the redox front of 1 m in 105 years. The uranium transport from the Bagombe nuclear reaction zone into present-day groundwater system is investigated by the 235U/238U isotopic ratio measurement in groundwater. The uranium transport from the reactor zone into the groundwater system is modelled and the distribution plume of depleted isotopic ratio of dissolved uranium is coherent with isotopic ratio measured is coherent with isotopic ratio measured on groundwater samples. (author)

38

Characterization and modeling of groundwater flow and transport around the nuclear natural reactors at Oklo, Gabon  

International Nuclear Information System (INIS)

The purpose of the study of the Oklo Natural Analogue - Transport in the Reactor for field to identify and model current groundwater flow which may mobilize elements produced in the fossil fission reaction zones. The first important step is to understand the circulation patter, of the water involved in transporting the elements and then to describe the geochemical system by which it is governed. Hydrogeologic studies were carried out at two sites: a deep site at Okelobondo which offers an opportunity to study potential radionuclide migration from the reaction zone, over a distance of 400 m. Through very heterogeneous layers and a superficial site at Bangombe with a more homogeneous layering but situated in a area of meteoric alteration. The rationale and scope of performed field studies are presented together with results of hydrogeological characterization, conceptualizations (at different scales) and results of modelling of groundwater flow and transport. After an overview of the geology and hydrogeology which inspired our treatment of the flow in the near far fields of the reaction zones, the different mathematical models used are described. The Okelobondo site is discussed first. For this site we used a hydrodynamic model together with a model of the migration of natural environmental tracers. The results lead us to propose a flow pattern associated with transfer times between the reaction zone and the surface discharge zone. Then a similar approach is used for Bane. Then a similar approach is used for Bangombe. For this site, we added a geochemistry model describing the migration of uranium in the vicinity of the reaction zone using the METIS, CHIMERE and STELE software, developed in the MIRAGE programme. The isotopic ratio 235U/238U was used as a natural, original, tracer of the reactor zone. (author)

39

The Oklo phenomenon as an analogue of radioactive waste disposal. A review  

International Nuclear Information System (INIS)

This work demonstrates the utility of the Oklo uranium ore deposit and natural fission reactors as a long time scale analogue for man-made radioactive waste repositories. Oklo has opened a new horizon representing an unrivalled opportunity to apply isotopic geochemistry to the study of migrations of fission products after an extremely long cooling and storage time and to define the processes involved in the transport of these elements through geological materials. This is the topic of the first section of this report. In the second section the information available on retention or migration at Oklo of the most interesting fission products is presented trying to illustrate how relevant the Oklo experience is in formulating predictions on the destiny of high activity waste disposed of in stable geological formations

40

Preliminary investigations of samples of uranium ore from the Oklo natural reactor in Gabon  

International Nuclear Information System (INIS)

The authors present the preliminary results of an investigation of samples from the Oklo deposits which were kindly supplied to the USSR by the French authorities. The theory of the existence of a natural nuclear reactor in the Oklo ore, put forward by French scientists and supported by analysis of the isotopic composition of the uranium and the large number of elements in the middle part of the periodic system, is assumed to be adequately proven. The present study aimed at finding additional confirmation for this theory. To this end the possibility of (n, ?), (n, 2n) and other reactions occurring in the uranium isotopes and certain of their long-lived decay products was analysed and isotopic and radiochemical investigations were performed in an effort to find traces of these reactions. The authors also carried out mineragraphic, X-ray-diffraction and electron-microscopic investigations on samples from the reactor zone and made an ontogenetic analysis of individual constituents and aggregates. Mineralogically the following were established together with uraninite: coffinite, pitchblende, galenite, chalcopyrite, pyrite, hematite, chlorite and hydromicas. The results of preliminary isotopic, radiochemical, mineragraphic and structural studies on Oklo samples do not contradict the theory of the existence long ago of a mineral reactor, but nevertheless it remains important to carry out more complete and systematic investigations of all aspects of the problemf all aspects of the problem

 
 
 
 
41

Oklo. A review and critical evaluation of literature  

Energy Technology Data Exchange (ETDEWEB)

The Oklo natural fossil fission reactors in Gabon, Equatorial Africa, have been studied as a natural analogue for spent nuclear fuel in a geological environment. For these studies, it is important to know what has happened to these reactors since they formed. This review is focussed on existing geological and geochronological information concerning the Oklo reactors and the surrounding ore. A sequence of geological and geochemical events in the Oklo area, as described in the literature, is given. The data and the studies behind this established geochronology are discussed and evaluated. Of the regional geology, special attention is given to the dating of the Francevillian sediments, and the intrusion of a dolerite dyke swarm. The processes that led to the mineralisation at Oklo, the subsequent formation of the nuclear reactors and later migration of fission products are described. Further discussion concerns the studies of the dolerite dyke swarm, since this appears to be one of the most important events related to fission product migration. A close look at the data related to this event shows that further study of the age of the dolerite dykes, and their effect on the uraninite in the Oklo reactors, is needed.

Zetterstroem, Lena [Swedish Museum of Natural History, Stockholm (Sweden). Lab. for Isotope Geology

2000-10-01

42

Oklo. A review and critical evaluation of literature  

International Nuclear Information System (INIS)

The Oklo natural fossil fission reactors in Gabon, Equatorial Africa, have been studied as a natural analogue for spent nuclear fuel in a geological environment. For these studies, it is important to know what has happened to these reactors since they formed. This review is focussed on existing geological and geochronological information concerning the Oklo reactors and the surrounding ore. A sequence of geological and geochemical events in the Oklo area, as described in the literature, is given. The data and the studies behind this established geochronology are discussed and evaluated. Of the regional geology, special attention is given to the dating of the Francevillian sediments, and the intrusion of a dolerite dyke swarm. The processes that led to the mineralisation at Oklo, the subsequent formation of the nuclear reactors and later migration of fission products are described. Further discussion concerns the studies of the dolerite dyke swarm, since this appears to be one of the most important events related to fission product migration. A close look at the data related to this event shows that further study of the age of the dolerite dykes, and their effect on the uraninite in the Oklo reactors, is needed

43

Neutron Moderation in the Oklo Natural Reactor and the Time Variation of alpha  

CERN Document Server

In the analysis of the Oklo (gabon) natural reactor to test for a possible time variation of the fine structure constant alpha, a Maxwell-Boltzmann low energy neutron spectrum was assumed. We present here an analysis where a more realistic spectrum is employed and show that the most recent isotopic analysis of samples implies a non-zero change in alpha, over the last two billion years since the reactor was operating, of \\Delta\\alpha/\\alpha\\geq 2.2\\times 10^{-7} (6\\sigma confidence). Issues regarding the interpretation of the shifts of the low energy neutron resonances are discussed.

Lamoreaux, S K

2003-01-01

44

Reactive transport modelling of uranium around a natural nuclear reactor at Bangombe (Oklo, Gabon)  

International Nuclear Information System (INIS)

On the Bangombe site (Oklo, Gabon), a natural nuclear reactor 1.95 Gyr old, and 12 m deep is submitted to weathering. The geochemical behaviour of uranium and trace elements around the reaction zone has been carried out using a reactive transport code HYTEC-2D. The buffer redox capacity of the organic matter associated with FeII/FeIII minerals around the reactor can explain the uraninite stability into the reaction zone and its weak migration during the geological time. (authors)

45

Isotope anomalies in natural nuclear reactor - Oklo uranium deposit (Gabon Republic, Africa)  

International Nuclear Information System (INIS)

Samples from two ore zones of the Oklo uranium deposit are studied. The survey chemical analysis is conducted using the mass-spectrometer with a spart source. Large and various isotopic shifts of many elements explained by the chain fission reaction proceeding under conditions of intensive neutron ore irradiation are established. Isotopic composition of xenon and krypton is studied in detail. Isotopic anomalies of xenon and krypton observed earlier in samples of other holes have been discovered in samples from SC-36 and SC-52 holes. Thus, it is confirmed that these anomalies are probably typical for the whole deposit. Excess of the neutrondeficit 130Xe isotope explained by secondary reactions on neutrons has been found out for the first time

46

The epsilon Phase in the UO2 of the Oklo Natural Reactors  

International Nuclear Information System (INIS)

In spent nuclear fuel (SNF), the metal epsilon phase consists of an alloy of Mo-Ru-Pd-Tc-Rh, occurring at a micron to sub-micron scale. 99Tc has a long half life (2.13 x 105 years) and can be an important contributor to dose in safety assessments of nuclear waste repositories. Under oxidizing conditions, TcO4- is the predominant species of Tc. In this form, Tc is highly soluble and weakly adsorbed onto mineral surfaces. Because the Oklo reactors are 2.0 billion years old, a majority of the 99Tc formed by natural fission reactions has decayed to 99Ru. Thus, this study is focused on Ru and the other constituents of the epsilon phase in order to investigate the occurrence and the fate of epsilon phase elements during the corrosion of this natural SNF. Samples from reactor zone (RZ)-10 (836, 819, 687); from RZ-13 (864, 910); from Okklobondo (943) were studied. High resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) with energy dispersive X-ray analysis (EDX) were completed on thin foil specimens of uraninite from each reactor zone. Among these samples, no Ru-bearing phase is observed in 910 and 943. A Bi-Pd particle (40-60 nm), froodite, PdBi2, occurs with trace amounts of As, Fe, and Te surrounded by an amorphous Pb-rich area (No.864). A Ru-As particle (?300nm) occurs surrounded by Pb-rich inclusion (400-500 nm) in uraninite (No.819). Based on EDX analysis the composition is: As, 59.9 Co, 2.5: Ni, 5.2; Ru, 18.6; Th, 8.4; Pd, 3.1; Sb, 2.4 in atomic%. The Ru-As phase is not a single particle, but an aggregate of 100-200 nm-sized ruthenarsenite, (Ru,Ni)As, particles. Another Ru-particle (600-700 nm) shows that Pb occurs at the core of the particle, and the rim portion consists of Ni, Co, and As without Ru (No.819). Ru-particles, ruthenarsenite, occur with Ni between the core and the rim. A Mo-particle (2As. All of the Ru-phases are associated with polycrystalline galena. There is a wide variation in the composition of the Ru-phase. The Ru-particles are, in most cases, ruthenarsenite, and do not contain detectable amounts of Mo, although the Mo-concentration for the epsilon phase in SNF is reported to be as high as 40 atomic%. Mo is only observed as a separate nano-scale phase in a nanocrystalline aggregate of galena

47

Oklo working group meeting  

International Nuclear Information System (INIS)

Natural analogue studies have been carried out for several years in the framework of the European Community's R and D programme on radioactive waste; and within its recent fourth five-year programme on 'Management and storage of radioactive waste (1990-94)' the Community is participating in the Oklo study, natural analogue for transfer processes in a geological repository. The Oklo project is coordinated by CEA-IPSN (F) and involves laboratories from several CEA directorates (IPSN, DTA and DCC) which collaborate with other institutions from France: CREGU, Nancy; CNRS, Strasbourg and ENSMD, Fontainebleau. Moreover, institutes from non-EC member States are also taking part in the Oklo study. The second joint CEC-CEA progress meeting of the Oklo Working Group was held in April 1992 in Brussels and gave the possibility of reviewing and discussing progress made since its first meeting in February 1991 at CEA in Fontenay-aux-Roses. About 40 participants from 15 laboratories and organizations coming from France, Canada, Gabon, Japan, Sweden and the USA underline the great interest in the ongoing research activities. The meeting focused on the different tasks within the CEC-CEA Oklo project concerning (i) field survey and sampling, (ii) characterization of the source term, (iii) studies of the petrographical and geochemical system, and (iv) studies of the hydrogeological system and hydrodynamic modelling. (author) 17 papers are presented

48

Formation and geochemical significance of micrometallic aggregates including fissiogenic platinum group elements in the Oklo natural reactor, Gabon  

Science.gov (United States)

Metallic aggregates with a size of a few tens ?m and consisting mainly of Ru, Rh, Pd, Te, Pb, As, Sb, S and Bi were found in the acid residue of SD37-S2/CD uraninite taken from Oklo natural reactor zone (RZ) 13. Quantitative analyses of major elements using an electron probe microanalyzer and in situ isotopic analyses of Zr, Mo, Ru, Pb and U using a sensitive high-resolution ion microprobe were performed on the metallic aggregates to determine the geochemical behaviors of fission products and actinides and to ascertain the processes of formation of the aggregates in the RZs. The chemical compositions of the aggregates investigated in this study are significantly different from those reported previously, showing lower Pb content and no correlation between the contents of Pb and S in the individual grains. The 235U/ 238U ratios in metallic aggregates vary significantly from 0.00478 to 0.01466, indicating chemical fractionation between U and Pu during the formation of the aggregates. The Pb isotopic data indicate that most of the Pb in the aggregates decayed from 2.05 Ga-old uraninite that existed in the RZ originally and that there was chemical fractionation between U and Pb in some aggregates. The Zr and Mo isotopic ratios, 90Zr/ 91Zr and 95Mo/ 97Mo, for most of the aggregates had small variations, which can be simply explained by constant separate mixing of fissiogenic and nonfissiogenic components. On the other hand, a large variation in the 99Ru/ 101Ru ratio (0.324-1.73) cannot be explained only by a two component mixing theory; thus, chemical fractionation between Tc and Ru during the reactor criticality is suggested. The large variations in the 235U/ 238U and 99Ru/ 101Ru isotopic ratios suggest that the aggregates formed under various redox conditions owing to the radiolysis of water.

Kikuchi, Makiko; Hidaka, Hiroshi; Gauthier-Lafaye, François

2010-08-01

49

Natural Nuclear Reactor Oklo and Variation of Fundamental Constants Part 1: Computation of Neutronic of Fresh Core  

CERN Document Server

Using a modern methods of reactor physics we have performed the full-scale calculations of the natural reactor Oklo. For reliability we have used the recent version of two Monte Carlo codes: the Russian code MCU REA and world wide known code MCNP (USA). Both codes produce close results. We constructed computer model of zone RZ2 of reactor Oklo which takes into account all details of design and composition. The calculations were performed for the three fresh cores with different uranium contents. Multiplication factors, reactivities and neutron fluxes were calculated. We estimated also the temperature and void effects for the fresh core. As would be expected, we have found for the fresh core a great difference between reactor spectra and Maxwell's one, which was used before for averaging cross sections in the Oklo reactor. The averaged cross section of Sm and its dependence on the shift of resonance position (due to variation of fundamental constants) are significantly different from previous results. Contrary...

Petrov, Yu V; Onegin, M S; Petrov, V Yu; Sakhnovskii, E G; Petrov, Yu.V.

2006-01-01

50

Oklo 2 Billion Years Before Fermi  

International Nuclear Information System (INIS)

The author aims to present the little-known story of the Oklo natural reactors. He recalls the historical aspects of the Oklo reactors discovery by the CEA in 1972, he explains the scientific phenomenon and the interest, notably as a ''natural analogue'' for the geological disposal of high level radioactive wastes. (A.L.B.)

51

THE FATE OF THE EPSILON PHASE IN UO2 OF THE OKLO NATURAL FISSON REACTORS  

Energy Technology Data Exchange (ETDEWEB)

In spent nuclear fuel (SNF), the micron- to nano-sized epsilon phase (Mo-Ru-Pd-Tc-Rh) is an important host of {sup 99}Tc which has a long half life (2.13 x 10{sup 5} years) and can be an important contributor to dose in safety assessments of nuclear waste repositories. In order to examine the occurrence and the fate of the epsilon phase during the corrosion of SNF over long time periods, samples of uraninite from the Oklo natural reactors ({approx}2.0 Ga) have been investigated using transmission electron microscopy (TEM). Because essentially all of the {sup 99}Tc has decayed to {sup 99}Ru, this study focuses on 4d-elements of the epsilon phase. Samples were obtained from the research collection at University of Michigan representing reactor zone (RZ) 10 (836, 819,687) and from RZ 13 (864,910). Several phases with 4d-metals have been identified within UO{sub 2} matrix at the scale of 50-700 nm; fioodite, PdBi{sub 2}, with trace amounts of As, Fe, and Te, and palladodymite or rhodarsenide, (Pd,Rh){sub 2}As. The most abundant 4d-metal phase is ruthenarsinite, (Ru,Ni)As, which has a representative composition: As, 59.9; Coy 2.5; Ni, 5.2; Ru, 18.6; Rh, 8.4; Pd, 3.1; Sb, 2.4 in atomic%. Ruthenarsenite nanoparticles are typically surrounded by Pb-rich domains, galena in most cases; whereas, some particles reveal a complexly zoned composition within the grain, such as a Pb-rich domain at the core and enrichment of Ni, Co, and As at the rim. Some ruthenarsenites and Rh-Bi-particles are embedded in surrounding alteration products, e.g., chlorite, adjacent to uraninite (no further than {approx}5 {micro}m). A few of those particles are still coated by a Pb-rich layer. Based on these results, the history that epsilon phases have experienced can be described as follows: (1) The original epsilon phase was changed to, in most cases, ruthenarsenite, by As-rich fluids with other trace metals. Dissolution and a simultaneous precipitation may be responsible for the phase change. (2) All Mo and most of the Tc were released from the epsilon phase. Galena precipitated surrounding the 4d-metal phases. (3) Once the uraninite matrix has dissolved, the epsilon nanoparticles were released and ''captured'' within alteration phases that are immediately adjacent to the uraninite.

S. Utsunomiya; R.C. Ewing

2005-09-08

52

Radioactive wastes in Oklo  

International Nuclear Information System (INIS)

The acceptance of the Nuclear Energy as electric power supply implies to give answer to the population on the two main challenges to conquer in the public opinion: the nuclear accidents and the radioactive wastes. Several of the questions that are made on the radioactive wastes, its are the mobility migration of them, the geologic stability of the place where its are deposited and the possible migration toward the aquifer mantels. Since the half lives of the radioactive waste of a Nuclear Reactor are of several hundred of thousands of years, the technical explanations to the previous questions little convince to the public in general. In this work summary the results of the radioactive waste generated in a natural reactor, denominated Oklo effect that took place in Gabon, Africa, it makes several thousands of millions of years, a lot before the man appeared in the Earth. The identification of at least 17 reactors in Oklo it was carried out thanks to the difference in the concentrations of Uranium 235 and 238 prospective, and to the analysis of the non-mobility of the radioactive waste in the site. It was able by this way to determine that the reactors with sizes of hardly some decimeter and powers of around 100 kilowatts were operating in intermittent and spontaneous form for space of 150,000 years, with operation cycles of around 30 minutes. Recent studies have contributed information valuable on the natural confinement of the radioactive waste of the Oklo reactors in matrixes of minerals of aluminum phosphate that caught and immobilized them for thousands of millions of years. This extracted information from the nature contributes guides and it allows 'to verify' the validity of the current proposals on the immobilization of radioactive wastes of a nuclear reactor. This work presents in clear and accessible form to the public in general on the secure 'design', operation, 'decommissioning' and 'storage' of the radioactive waste of the reactors that the nature put into operation in Oklo before the man appeared in the Earth. (Author)

53

Investigations of the natural fission reactor program. Progress report, October 1977--September 1978  

Energy Technology Data Exchange (ETDEWEB)

The U.S. study of the Oklo natural reactor began in 1973 with the principal objectives of understanding the processes that produced the reactor and that led to the retention of many of its products. Major facets of the program have been the chemical separation and mass spectrometric analysis of the reactor components and products, the petrological and mineralogical examination of samples taken from the reactor zones, and an interdisciplinary modeling of possible processes consistent with reactor physics, geophysics, and geochemistry. Most of the past work has been on samples taken within the reactor zones. Presently, these studies give greater emphasis to the measurement of mobile products in additional suites of samples collected peripherally and ''downstream'' from the reactor zones. This report summarizes the current status of research and the views of U.S. investigators, with particular reference to the extensive work of the French scientists, concerning the main features of the Oklo natural fission reactor. Also mentioned briefly is the U.S. search for natural fission reactors at other locations.

Cowan, G.A.; Norris, A.E. (eds.)

1978-10-01

54

Gas, benefits and question marks. The Oklo reactors: 100 % natural. The Kyoto protocol: use it or lose it?. Small hydro power: a great leap forward. The energy mix of South Korea  

Energy Technology Data Exchange (ETDEWEB)

This issue of Alternatives newsletter contains a main press-kit about natural gas economics worldwide and 4 articles dealing with the Oklo natural reactor, the Kyoto protocol, the small hydro-power in China, and the energy mix of South Korea: 1 - 'Gas benefits and question marks': The world's most widely distributed fossil fuel, natural gas is also the fastest-growing energy source of the past thirty years. Its position as the fuel of choice in the global energy mix is due in large part to its many domestic and industrial applications. 2 - 'The Oklo reactors: 100% natural': Another look at this extraordinary 2 billion year-old phenomenon in words and pictures: the nuclear fission reaction that created the natural reactors of Gabon. 3 - 'The Kyoto Protocol: use it or lose it?': Nearly eight years after its signature, the Kyoto Protocol is still hotly debated. Two experts give us their views: Spencer Abraham, former U.S. Secretary for Energy, and Jean-Charles Hourcade of CIRED, the international center for research on the environment and development. 4 - 'Small hydro power: a great leap forward': The Chinese government has responded to the need for rural electrification with an aid program for the country's poorest cantons. Enter the small hydro plant in northern Guangxi province. 5 - 'The energy mix of South Korea': Faced with continuing strong economic growth and energy demand, South Korea has multiplied its projects, from hydropower to tidal power to nuclear and even hydrogen in the longer term.

Anon

2005-07-01

55

The Oklo bound on the time variation of the fine-structure constant revisited  

CERN Document Server

It has been pointed out by Shlyakhter that data from the natural fission reactors which operated about two billion years ago at Oklo (Gabon) had the potential of providing an extremely tight bound on the variability of the fine-structure constant alpha. We revisit the derivation of such a bound by: (i) reanalyzing a large selection of published rare-earth data from Oklo, (ii) critically taking into account the very large uncertainty of the temperature at which the reactors operated, and (iii) connecting in a new way (using isotope shift measurements) the Oklo-derived constraint on a possible shift of thermal neutron-capture resonances with a bound on the time variation of alpha. Our final (95% C.L.) results are: -0.9 \\times 10^{-7} <(alpha^{Oklo} - alpha^{now})/alpha <1.2\\times 10^{-7} and -6.7 \\times 10^{-17} {yr}^{-1} < {\\dot alpha}^{averaged}/alpha <5.0\\times10^{-17} {yr}^{-1}.

Damour, Thibault Marie Alban Guillaume; Damour, Thibault; Dyson, Freeman

1996-01-01

56

Gamma-ray spectrometer measurement of 238U/235U in uranium ore from a natural reactor at Oklo, Gabon  

International Nuclear Information System (INIS)

A sample from the Oklo deposit containing about 0.51 atom percent 235U (by mass spectrometer) was analyzed by a gamma-ray spectrometer system, using a high-purity planar germanium detector. The 238U was determined from its daughter's (234Th) 63.3 keV photopeak; the 235U was determined from its 143.8 and 163.4 keV photopeaks. The ratios of these photopeaks were compared with that from a standard having normal uranium isotopic content; the resulting calculations give a 235U abundance of 0.54 atom percent in the Oklo sample. The gamma-ray spectrum also contains lines from five other isotopes in the uranium series, which indicate the Oklo sample to be at or near secular equilibrium, as the time elapsed since the nuclear reaction ended was sufficient to permit the daughters to achieve equilibrium

57

The Oklo phenomenon  

International Nuclear Information System (INIS)

Until recently, scientists believed that the chemical elements were synthesized only in stars. The discovery of the Oklo phenomenon in the Republic of Gabon in 1972 has revealed, however, that a nuclear ''fire'' had existed on the earth and large-scale transmutations of the elements were occurring on our planet 1.7x109 years ago. The formation of natural (or Pre-Fermi) reactors is closely related to the appearance of life on our planet earth. The Pre-Fermi reactors were probably never formed until about 2x109 years ago, when oxygen was injected into the earth's atmosphere by a new generation of living organisms carrying out photosynthesis. (orig.)

58

Behavior of radionuclides around Oklo reactor  

International Nuclear Information System (INIS)

Re-examination of a vast amount of data on the Oklo reactors which have been accumulated in the past reveals that radionuclides which have been retained and preserved at the site of nuclear reactions during the past 1.7 billion years have high melting and boiling points. Gaseous elements and elements with melting points lower than that of tellurium (452degC) appear to have mostly migrated out of the reactor. About one percent of fissiogenic xenon isotopes have been retained, however, and the isotopic compositions of small amounts of xenon released from the natural reactors were found to be abnormal in that the relative abundances of 131Xe and 132Xe are markedly enhanced. It seems that the origin of this anomalous xenon can be attributed to the nonlinear oscillatory mode of operation of the reactors at temperatures of about 400degC, periodically being turned on and off in a manner similar to the present-day geysers or intermittent hot springs. The time period during which the reactor was turned off was calculated to be about 3 hours from the observed ratios of 132Xe, 134Xe and 136Xe in the anomalous xenon. The knowledge gained from the study of the Oklo phenomenon has recently played a key role in solving some of the most difficult problems in the field of geochemistry and cosmochemistry: for example, the so-called 'puzzle of the xenon isotopes', which have remained unsolved for a period of a quarter-century since the 1960's. Results of latest calculations indicate that the carbonaceous chondrites and lunar fines contain amounts of 244Pu fission xenon which are much greater than hitherto believed and they appear to have started to retain their xenon 4800 to 4900 million years ago. (author)

59

Two billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon (Africa)  

International Nuclear Information System (INIS)

Two billion years ago, the increase of oxygen in atmosphere and the high 235U/238U uranium ratio (> 3%) made possible the occurrence of natural nuclear reactors on Earth. These reactors are considered to be a good natural analogue for nuclear waste disposal. Their preservation during such a long period of time is mainly due to the geological stability of the site, the occurrence of clays surrounding the reactors and acting as an impermeable shield, and the occurrence of organic matter that maintained the environment in reducing conditions, favourable for the stability of uraninite. Hydrogeochemical studies and modelling have shown the complexity of the geochemical system at Oklo and Bangombe (Gabon) and the lack of precise data about uranium and fission products retention and migration mechanisms in geological environments. (author)

60

OKLO: fossil reactors  

International Nuclear Information System (INIS)

Events leading up to the discovery during the summer of 1972 of the Oklo fossil reactor in Gabon and its subsequent exploration are reviewed. Results of studies are summarized; future investigations are outlined

 
 
 
 
61

Far field hydrogeochemistry in the Oklo reactor area (Gabon)  

International Nuclear Information System (INIS)

In the frame of a general study of the Oklo natural reactor, which takes into account the natural analogue aspect, a complete hydrogeological and hydrogeochemical study is undertaken. The partners of this study are the following: - Section de geochimie, CEA (France): P. Toulhoat, J.P. Gallien, P. L'Henoret, V. Moulin (groundwater chemistry and colloids). - Ecole des Mines de Paris (CIG, Fontainebleau) E. Ledoux, I. Gurban (hydrogeology and modelling) - SKB and Conterra AB (Sweden) J.A.T. Smellie, A. Winberg (hydrogeology, isotope geochemistry). The aim of this study is to try to understand and to characterize the possible mobilization of elements or isotopes when groundwaters come in contact with nuclear reaction zones. The first step of the study is presented here, which comprises a general geochemical and hydrodynamical characterization of the site. In this presentation, the site of Bagombe is also mentioned as it has been confirmed as sector in which nuclear fission reactions occurred as in Oklo. (author). 10 refs., 6 figs., 6 tabs

62

Oklo reactors and implications for nuclear science  

CERN Document Server

We summarize the nuclear physics interests in the Oklo natural nuclear reactors, focusing particularly on developments over the past two decades. Modeling of the reactors has become increasingly sophisticated, employing Monte Carlo simulations with realistic geometries and materials that can generate both the thermal and epithermal fractions. The water content and the temperatures of the reactors have been uncertain parameters. We discuss recent work pointing to lower temperatures than earlier assumed. Nuclear cross sections are input to all Oklo modeling and we discuss a parameter, the $^{175}$Lu ground state cross section for thermal neutron capture leading to the isomer $^{176\\mathrm{m}}$ Lu, that warrants further investigation. Studies of the time dependence of dimensionless fundamental constants have been a driver for much of the recent work on Oklo. We critically review neutron resonance energy shifts and their dependence on the fine structure constant $\\alpha$ and the ratio $X_q=m_q/\\Lambda$ (where $m_...

Davis, E D; Sharapov, E I

2014-01-01

63

Radioactive wastes in Oklo; Desechos radiactivos en Oklo  

Energy Technology Data Exchange (ETDEWEB)

The acceptance of the Nuclear Energy as electric power supply implies to give answer to the population on the two main challenges to conquer in the public opinion: the nuclear accidents and the radioactive wastes. Several of the questions that are made on the radioactive wastes, its are the mobility migration of them, the geologic stability of the place where its are deposited and the possible migration toward the aquifer mantels. Since the half lives of the radioactive waste of a Nuclear Reactor are of several hundred of thousands of years, the technical explanations to the previous questions little convince to the public in general. In this work summary the results of the radioactive waste generated in a natural reactor, denominated Oklo effect that took place in Gabon, Africa, it makes several thousands of millions of years, a lot before the man appeared in the Earth. The identification of at least 17 reactors in Oklo it was carried out thanks to the difference in the concentrations of Uranium 235 and 238 prospective, and to the analysis of the non-mobility of the radioactive waste in the site. It was able by this way to determine that the reactors with sizes of hardly some decimeter and powers of around 100 kilowatts were operating in intermittent and spontaneous form for space of 150,000 years, with operation cycles of around 30 minutes. Recent studies have contributed information valuable on the natural confinement of the radioactive waste of the Oklo reactors in matrixes of minerals of aluminum phosphate that caught and immobilized them for thousands of millions of years. This extracted information from the nature contributes guides and it allows 'to verify' the validity of the current proposals on the immobilization of radioactive wastes of a nuclear reactor. This work presents in clear and accessible form to the public in general on the secure 'design', operation, 'decommissioning' and 'storage' of the radioactive waste of the reactors that the nature put into operation in Oklo before the man appeared in the Earth. (Author)

Balcazar, M.; Flores R, J.H.; Pena, P.; Lopez, A. [ININ, 52045 Ocoyoacac, Estado de Mexico (Mexico)

2006-07-01

64

Mineralogical, chemical and oxygen isotopic data on uraninites from natural fission reactors (Gabon): effects of weathering conditions  

International Nuclear Information System (INIS)

Nuclear fission reactions occurred 2000 Ma ago in the uraninites of the Oklo-Okelobondo and Bangombe uranium deposits (Gabon). These uraninites contain actinides and fission products and therefore are considered as natural analogues for artificial nuclear spent fuel of PWR type nuclear plant. Mineralogical, chemical and oxygen isotopic analyses have been performed in order to determine the stability of the uraninites in weathering conditions. Results show that the alteration of uraninites by meteoric waters affects the cell parameters and the chemistry of the uraninites but not the oxygen isotopic composition. Radiolysis of water is invoked as a possible mechanism for the very low values of ?18O (-22.0 0/00 SMOW) measured in the uraninites. (authors)

65

Oklo 2 Billion Years Before Fermi; Les reacteurs naturels d'Oklo (Gabon): 2 milliards d'annees avant Fermi  

Energy Technology Data Exchange (ETDEWEB)

The author aims to present the little-known story of the Oklo natural reactors. He recalls the historical aspects of the Oklo reactors discovery by the CEA in 1972, he explains the scientific phenomenon and the interest, notably as a 'natural analogue' for the geological disposal of high level radioactive wastes. (A.L.B.)

Barre, B

2005-02-15

66

Oklo Constraint on the Time-Variability of the Fine-Structure Constant  

CERN Document Server

The Oklo phenomenon, natural fission reactors which had taken place in Gabon about 2 billion years ago, porvides one of the most stringent constraints on the possible time-variability of the fine-structure constant $\\alpha$. We first review briefly what it is and how reliable it is in constraining $\\alpha$. We then compare the result with a more recent result on the nonzero change of $\\alpha$ obtained from the observation of the QSO absoorption lines. We suggest a possible way to make these results consistent with each other in terms of the behavior of a scalar field which is expected to be responsible for the accelaration of the universe.

Fujii, Y

2003-01-01

67

Uranium redistribution under oxidizing conditions in Oklo natural reactor zone 2, Gabon  

International Nuclear Information System (INIS)

This mineralogical study was completed to elucidate the relationships between uranium distribution and alteration products of the host rock of natural reactor zone clays just below the reactor core. Uraninite is preserved without any alteration in the reactor core. Uranium minerals are found to be present in the fractures in the reactor zone clays associated with iron-mineral veins, galena and Ti-bearing minerals. Uranium, for which the phases could not be identified, occurs in iron-mineral veins and the iron-mineral rim of pyrite grains in the reactor zone clays. Uranium is not associated with granular iron minerals occurring in the illite matrix of the reactor zone clays. The degree of crystallinity and uranium content of the three iron-bearing alteration products suggest that they formed under different conditions; the granular iron minerals, under alteration conditions where uranium was not mobilized while the iron-mineral veins and the iron-mineral rim of pyrite, under conditions in which uranium is mobilized after the formation of the granular iron minerals

68

Eh-pH diagrams for elements from Z = 40 to Z = 52: application to the Oklo natural reactor, Gabon  

International Nuclear Information System (INIS)

Eh-pH diagrams for elements from Z = 40 (Zr) to Z = 52 (Te) have been constructed in order to comment on migration/retention of these elements at Oklo. Although data for fissiogenic amounts of some of these elements are lacking, where such data are available to agreement between predicted migration/retention based on the Eh-pH diagrams and actual measurement is excellent. Based on Eh-pH diagrams, migration (to what degree is uncertain) of Mo and Cd is predicted whereas retention of Y, Zr, Tc, Ru, Rh, Pd, Ag and Te is also predicted. An earlier report of Frejacques et al. of Tc migration is in disagreement with Eh-pH prediction, and recent (unpublished) data argue for Tc retention. In view of the agreement between prediction and observation, the possible migration of Sb and retention of In and Sn is proposed. These data again demonstrate the usefulness of Eh-pH diagrams for the Oklo fossil nuclear reactor but, more important, allow constraints to be placed on repositories for nuclear waste now under consideration. (Auth.)

69

Retention of transuranic and actinide elements and bismuth at the Oklo natural reactor, Gabon: application of Eh-pH diagrams  

International Nuclear Information System (INIS)

Several tons of 239Pu and lesser amounts of 240Pu and 241Pu were produced during the 0.5 Ma lifetime of the Oklo natural reactor, Gabon, some 1.8 Ga ago. Production of Pu isotopes is evidenced by slight excesses of 235U in otherwise 235U-depleted U-rich zones as well as by even slighter excesses of 232Th and 209Bi in the same zones. The low-grade ore, unaffected by nuclear reactions, contains very little Bi and Th, and U possesses a normal isotopic composition. The 241Pu-produced Bi-excess demands that 241Am, 237Np and other members of the 241Pu decay chain must not have migrated, since loss of any member from the 241Pu decay chain would prevent the Bi excesses from accumulating in the high-grade ore zones. Eh-pH diagrams for all species of importance indicate the following: Po is retained as the native metal in the Eh-pH range of interest, whereas Pu and Np are retained as PuO2 and NpO2, respectively. Am is probably retained as Am(OH)3 or as an (Eu,Am)2(CO3)3 species. The field of Bi2S3 is wide enough to prevent migration even under slightly oxidizing conditions. Collectively, these data argue for the retention of Pu, Am, Np, U, Po and Bi at Oklo and suggest that geologic sites elsewhere should be suitable for waste isolation. (Auth.) waste isolation. (Auth.)

70

Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon)--inferences from U, REE, Zr, Mo and Se isotopes  

International Nuclear Information System (INIS)

Mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the Oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides. To address these issues, in situ isotopic analyses by SHRIMP and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products (exchangeable and non exchangeable) and their association with mineral phases. The distribution patterns of U, REE, Zr and Mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances, owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality. The results emphasize the mobility of Zr at Oklo, this element being often considered as 'immobile' during water-rock interactions. The main output is the demonstration of the net effects of sorption and coprecipitation processes. Chlorite and to a lesser extent illite were found to have adsorbed significant amounts of U, REE, Zr (and probably Th) and less sorbing elements such as Mo. Coprecipitation of secondary UO2 and P-rich coffinite within the alteration halo is also an important means of retardation. The concentration of radionuclides released from the reactor were probably high and t from the reactor were probably high and they display solubility limited transport behaviour. No retention effect was found for Se in the immediate vicinity of the reactor and this element may have moved farther from its source of production. These results have interesting implications for the evaluation of long-term containment of radionuclides. They provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary U-minerals. This study also demonstrates the robustness of these retention processes over extremely long periods of time

71

Oklo reactors and implications for nuclear science  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We summarize the nuclear physics interests in the Oklo natural nuclear reactors, focusing particularly on developments over the past two decades. Modeling of the reactors has become increasingly sophisticated, employing Monte Carlo simulations with realistic geometries and materials that can generate both the thermal and epithermal fractions. The water content and the temperatures of the reactors have been uncertain parameters. We discuss recent work pointing to lower temper...

Davis, E. D.; Gould, C. R.; Sharapov, E. I.

2014-01-01

72

Natural fission reactors in the Franceville basin, Gabon: A review of the conditions and results of a open-quotes critical eventclose quotes in a geologic system  

International Nuclear Information System (INIS)

Natural nuclear fission reactors are only known in two uranium deposits in the world, the Oklo and Bangombe deposits of the Franceville basin: Gabon. Since 1982, five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative European program. New geological, mineralogical, and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2.0 Ga years. The Franceville basin and the uranium deposits remained geologically stable over a long period of time. Therefore, the sites of Oklo and Bangombe are well preserved. For the reactors, two main periods of actinide and radionuclides migration have been observed: during the criticality, under P-T conditions of 300 bars and 400-500 degrees C, respectively, and during a distention event which affected the Franceville basin 800 to 900 Ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors. New isotopic analyses on uranium dioxides, clays, and phosphates allow us to determine their respective importance for the retention of fission products. The UO2 matrix appears to be efficient at retaining most actinides and fission products such as REEs, Y, and Zr but not the volatile fission products (Cd, Cs, Xe, and Kr) nor Rb, Sr, and Ba. Some fissiogenic elements such as Mo, Tc, Ru, Rh, Pd, and Te could have formed metallic and oxide i Te could have formed metallic and oxide inclusion in the UO2 matrix which are similar to those observed in artificial spent fuel. Clays and phosphate minerals also appear to have played a role in the retention of fissiogenic REEs and also of Pu. 82 refs., 21 figs., 12 tabs

73

Reactive transport modelling of uranium at Bangombe (Oklo, Gabon)  

International Nuclear Information System (INIS)

The Oklo uranium deposit in the Francevillian sedimentary series, SE Gabon, has retained abundant fission fragments produced from 2.2 Ga ago. Sixteen natural fission reactors have been recognised at Bangombe uranium ores. The geochemical behaviour of uranium and trace elements around the reaction zone has been carried out using an approach based on the reactive transport modelling HYTEC-2D. The buffer redox capacity of the organic matter associated with FeII/FeIII minerals around the reactor can be explain the uraninite stability into the reaction zone and its weak migration. Nevertheless, the oxidising groundwater dissolves the redox buffer zone and this one cannot protect the reactor of oxidation. Thus, a plume of depleted uranium (235U/238U ratio) coming to the dissolution of the uraninite from the reactor can be simulated by the coupled reactive transport codes. The aqueous uranium concentration predicted is in good agreement with the groundwater analysis in boreholes. The redox buffer controlled by the organic matter could explain the preservation of reactor along the geological time. Thus, the organic matter (graphite type) seems a very interesting mineral for the stabilisation of hydro-geochemical parameters into the nuclear waste storage. (authors)

74

Etching of fission tracks in silicate glasses by means of deionized water  

International Nuclear Information System (INIS)

Fission tracks have been revealed in silicate glasses with deionized water. Their sharp conical shape implies a marked enhancement of the dissolution rate along their core and consequently a cone angle and an etching efficiency (close to 100%) much higher than previously reported for glasses. We show that etching of fission tracks in natural environments has generally very limited geochemical implications except in specific cases such as that found in the Oklo uranium ores

75

Search for other natural fission reactors  

International Nuclear Information System (INIS)

Precambrian uranium ores have been surveyed for evidence of other natural fission reactors. The requirements for formation of a natural reactor direct investigations to uranium deposits with large, high-grade ore zones. Massive zones with volumes approximately greater than 1 m3 and concentrations approximately greater than 20 percent uranium are likely places for a fossil reactor if they are approximately greater than 0.6 b.a. old and if they contained sufficient water but lacked neutron-absorbing impurities. While uranium deposits of northern Canada and northern Australia have received most attention, ore samples have been obtained from the following worldwide locations: the Shinkolobwe and Katanga regions of Zaire; Southwest Africa; Rio Grande do Norte, Brazil; the Jabiluka, Nabarlek, Koongarra, Ranger, and El Sharana ore bodies of the Northern Territory, Australia; the Beaverlodge, Maurice Bay, Key Lake, Cluff Lake, and Rabbit Lake ore bodies and the Great Bear Lake region, Canada. The ore samples were tested for isotopic variations in uranium, neodymium, samarium, and ruthenium which would indicate natural fission. Isotopic anomalies were not detected. Criticality was not achieved in these deposits because they did not have sufficient 235U content (a function of age and total uranium content) and/or because they had significant impurities and insufficient moderation. A uranium mill monitoring technique has been considered where the ''yellowcake'' output from appropriate mills would be monitored for isotopic alterations indicative of the exhumation and processing of a natural reactor

76

Search for other natural fission reactors  

Energy Technology Data Exchange (ETDEWEB)

Precambrian uranium ores have been surveyed for evidence of other natural fission reactors. The requirements for formation of a natural reactor direct investigations to uranium deposits with large, high-grade ore zones. Massive zones with volumes approximately greater than 1 m/sup 3/ and concentrations approximately greater than 20 percent uranium are likely places for a fossil reactor if they are approximately greater than 0.6 b.a. old and if they contained sufficient water but lacked neutron-absorbing impurities. While uranium deposits of northern Canada and northern Australia have received most attention, ore samples have been obtained from the following worldwide locations: the Shinkolobwe and Katanga regions of Zaire; Southwest Africa; Rio Grande do Norte, Brazil; the Jabiluka, Nabarlek, Koongarra, Ranger, and El Sharana ore bodies of the Northern Territory, Australia; the Beaverlodge, Maurice Bay, Key Lake, Cluff Lake, and Rabbit Lake ore bodies and the Great Bear Lake region, Canada. The ore samples were tested for isotopic variations in uranium, neodymium, samarium, and ruthenium which would indicate natural fission. Isotopic anomalies were not detected. Criticality was not achieved in these deposits because they did not have sufficient /sup 235/U content (a function of age and total uranium content) and/or because they had significant impurities and insufficient moderation. A uranium mill monitoring technique has been considered where the ''yellowcake'' output from appropriate mills would be monitored for isotopic alterations indicative of the exhumation and processing of a natural reactor.

Apt, K.E.; Balagna, J.P.; Bryant, E.A.; Cowan, G.A.; Daniels, W.R.; Vidale, R.J.

1977-01-01

77

Uranium mill monitoring for natural fission reactors  

Energy Technology Data Exchange (ETDEWEB)

Isotopic monitoring of the product stream from operating uranium mills is proposed for discovering other possible natural fission reactors; aspects of their occurrence and discovery are considered. Uranium mill operating characteristics are formulated in terms of the total uranium capacity, the uranium throughput, and the dilution half-time of the mill. The requirements for detection of milled reactor-zone uranium are expressed in terms of the dilution half-time and the sampling frequency. Detection of different amounts of reactor ore with varying degrees of /sup 235/U depletion is considered.

Apt, K.E.

1977-12-01

78

Tectonic analysis of the Oklo deposit  

International Nuclear Information System (INIS)

A large folded structure with a 400 incline and extending 500 m in the north-south direction has been uncovered at the Oklo mine. This structure has been analysed from the geometric and dynamic points of view in order to determine the possible role of tectonic activity in the creation of the uraniferous concentrations. Compression and extension zones which at certain points control the shape and arrangement of the lodes are associated with the structure. The natural reactors are situated in an extension zone where compartmentation and slippage, which explain the arrangement of the reactors, are observed

79

Characterization of near- to far-field ancient migrations around Oklo reaction zones (Gabon) using minerals as geochemical tracers  

International Nuclear Information System (INIS)

We developed a method allowing the identification of ancient fluid circulations through trace-element patterns in hydrothermal minerals. Application of this technique to Oklo nuclear reactors as natural analogues involves apatite and lanthanides for early circulations, and sulfides and chalcophile elements for late events. At least four different fluid generations have been found, and are tentatively assigned to episodes of Oklo site history. (1) Zr- and U-rich fluids predate criticality near reaction zone 10. (2) Isotopic anomalies of lanthanides provide evidence for mobility of fission products during criticality. Up to now, anomalies are restricted to the near field (desilicified zone around reaction zones). (3) A first pyrite stage is focused around reaction zones, up to a few tens of meters. (4) The latest event is most probably of regional extent. Besides direct evidence of fission-product migration, the present study is expected to provide the geological background and geochemical constraints for modelling ancient fluid circulation and consequent element migration near reaction zones. (orig.)

80

Geochemistry of neo-formed minerals at Oklo (Gabon), geologic history of the Oklo basin: a contribution for the studies of geologic disposals of radioactive wastes  

International Nuclear Information System (INIS)

Oklo uranium ore deposit (Francevillian basin, Gabon) is the unique place in the world where 2000 Ma old fossil nuclear reactors were described. The geological and thermal history of this basin, since 2000 Ma was retraced. Tholeiitic intrusion was 755 ± 83 Ma with Sm-Nd isochron on whole rock and plagioclase and 746 ± 16 Ma old with U-Pb dating on zircons. This event was linked to a pre-Pan-African rifting stage. A green schist facies metamorphism was detected on the granitic rocks of the substratum and seemed to affect the tholeiitic intrusion. Apatite fission tracks dating performed on granitic basement revealed a thermal event between Permian and middle-Jurassic time, linked to the Atlantic ocean opening. Fission track ages distribution suggest a brittle tectonics (T < 60 deg. C) occurred after middle Jurassic times. Geochemical and isotopic studies on apatites and zircons which crystallized on natural nuclear reactors let compare the confinement of these two crystalline structures. These minerals were affected by self-irradiation due to actinide incorporation. U, Pu and fission products (REE, Rb, Sr) were trapped by apatites. Zircons contain fissiogenic REE and radiogenic Ba. Apatites crystallized during the nuclear reactions, zircons at the end of this phenomenon. Isotopic study of tholeiitic intrusion minerals point out fissiogenic Nd and Sm incorporation in clinopyroxenes. This result implies a fissiogenic products remobilization during the tholeiitic intrusibilization during the tholeiitic intrusion event. (author)

 
 
 
 
81

Galena crystallization and the origin of sulfur in the Oklo and Bangombe natural reactors: the effects of ca. 900 Ma thermal event  

International Nuclear Information System (INIS)

Galena in the ca. 1950 Ma old natural fossil fission reactors in Gabon crystallized sometime between 980 Ma and 750 Ma during a period of regional extension and the intrusion of a dolerite dyke swarm. The S isotopic composition of galena, containing radiogenic Pb from uraninite, gives information about the origin of the S. Results from ion microprobe analyses of galena from the reactor zones indicate that S mainly originates from the surrounding sediment. Galena in a thin, altered dolerite dyke also contains non-magmatic S. The presented data gives no positive evidence for the involvement of magmatic S during the ca. 900 Ma galena crystallisation, however, the possibility cannot be ruled out. Copyright (2001) Material Research Society

82

Time-variability of alpha from realistic models of Oklo reactors  

CERN Document Server

We reanalyze Oklo $^{149}$Sm data using realistic models of the natural nuclear reactors. Disagreements among recent Oklo determinations of the time evolution of $\\alpha$, the electromagnetic fine structure constant, are shown to be due to different reactor models, which led to different neutron spectra used in the calculations. We use known Oklo reactor epithermal spectral indices as criteria for selecting realistic reactor models. Two Oklo reactors, RZ2 and RZ10, were modeled with MCNP. The resulting neutron spectra were used to calculate the change in the $^{149}$Sm effective neutron capture cross section as a function of a possible shift in the energy of the 97.3-meV resonance. We independently deduce ancient $^{149}$Sm effective cross sections, and use these values to set limits on the time-variation of $\\alpha$. Our study resolves a contradictory situation with previous Oklo $\\alpha$-results. Our suggested $2 \\sigma$ bound on a possible time variation of $\\alpha$ over two billion years is stringent: $ -...

Gould, C R; Lamoreaux, S K

2006-01-01

83

Criticality in a high level waste repository. A review of some important factors and an assessment of the lessons that can be learned from the Oklo reactors  

Energy Technology Data Exchange (ETDEWEB)

The conditions and scenarios that might allow sufficient {sup 239}Pu and/or {sup 235}U to accumulate together with enough water to allow for moderation of neutron energies and thereby achieving a state where neutron-induced fission reactions could be sustained at a rate significantly above the natural rate of spontaneous fission is discussed. The uranium deposit in Oklo, Gabon, which was the site of naturally-occurring neutron-induced fission reactions approximately 2000 My ago is described. The chemistry, mineralogy, and conditions of the nuclear reactor operations are reviewed. Results of modelling the conditions for criticality at Oklo are used to estimate the amounts of spent fuel uranium that must be assembled in a favorable geometry in order to produce a similar reactive situation in a geologic repository. The amounts of uranium that must be transported and redeposited to reach a critical configuration are extremely large in relation to those that could be transported under any reasonably achievable conditions. In addition, transport and redeposition scenarios often require opposite chemical characteristics. It is concluded that the likelihood of achieving a critical condition due to accumulation of a critical mass of uranium outside the canisters after disposal is negligible. Criticality inside the canister is rendered impossible by the use of low-solubility materials inside the canisters that fill space and prevent the entry of enough water to allow moderation of neutron energies. Criticality due to plutonium outside the canister can be ruled out because it requires a series of processes, each of which has a vanishingly small probability. 25 refs, 9 tabs, 8 figs.

Oversby, V.M. [VMO Konsult, Stockholm (Sweden)

1996-06-01

84

Criticality in a high level waste repository. A review of some important factors and an assessment of the lessons that can be learned from the Oklo reactors  

International Nuclear Information System (INIS)

The conditions and scenarios that might allow sufficient 239Pu and/or 235U to accumulate together with enough water to allow for moderation of neutron energies and thereby achieving a state where neutron-induced fission reactions could be sustained at a rate significantly above the natural rate of spontaneous fission is discussed. The uranium deposit in Oklo, Gabon, which was the site of naturally-occurring neutron-induced fission reactions approximately 2000 My ago is described. The chemistry, mineralogy, and conditions of the nuclear reactor operations are reviewed. Results of modelling the conditions for criticality at Oklo are used to estimate the amounts of spent fuel uranium that must be assembled in a favorable geometry in order to produce a similar reactive situation in a geologic repository. The amounts of uranium that must be transported and redeposited to reach a critical configuration are extremely large in relation to those that could be transported under any reasonably achievable conditions. In addition, transport and redeposition scenarios often require opposite chemical characteristics. It is concluded that the likelihood of achieving a critical condition due to accumulation of a critical mass of uranium outside the canisters after disposal is negligible. Criticality inside the canister is rendered impossible by the use of low-solubility materials inside the canisters that fill space and prevent the entry of enough water to allow moderation of neutron energies. Criticality due to plutonium outside the canister can be ruled out because it requires a series of processes, each of which has a vanishingly small probability. 25 refs, 9 tabs, 8 figs

85

The Oklo reactors: five years of exploration of the site  

International Nuclear Information System (INIS)

The main phases of the exploration of the Oklo site since the discovery of the ''reactor'' phenomenon are outlined briefly. Over 180 sampling holes were drilled during the interruption of the mining activities in the sector concerned. Several new zones have been found. Mining was resumed in the second half of 1975, providing an opportunity for highly fruitful geological follow-up work: more precise knowledge was gained of the morphology of the reactors, and very many additional samples were taken. Plant treatment of the ore and the systematic analysis of batches have made it possible to establish a balance of missing uranium-235. A small portion containing sites of intense reaction has been preserved by being anchored to the quarry wall. Mining in this sector has now finished, but new indications of fission have been found, especially in the Okelobondo sector. (author)

86

Relevance of nuclide migration at Oklo to the problem of geologic storage of radioactive waste  

International Nuclear Information System (INIS)

Analyses of Oklo samples published by French scientists and the data presented by United States scientists at this symposium indicate that escape of radioactive products from the Oklo reactor zones was quite limited. Apparently, the major radioactive products that might have been measured in the surrounding environment at the time of the reaction would have been due to 85Kr and, possibly, to 137Cs and 90Sr. Other fission-product elements which have been significantly displaced over the intervening epochs include Xe, Rb, Ba, Mo, and I. The heavy elements have remained relatively fixed. These conclusions must remain tentative until additional analyses are completed on samples taken near the interfaces between reactor and non-reactor zones. Relevance of these tentative conclusions to other geologic formations must await further study of comparative geochemistry and geology. (author)

87

Relevance of nuclide migration at Oklo to the problem of geologic storage of radioactive waste  

International Nuclear Information System (INIS)

Analyses of Oklo samples published by French scientists and the data presented by U. S. scientists at this symposium indicate that escape of radioactive products from the Oklo reactor zones was quite limited. Apparently, the major radioactive products that might have been measured in the surrounding environment at the time of the reaction would have been due to 85Kr and, possibly, 137Cs and 90Sr. Other fission product elements which have been significantly displaced over the intervening epochs include Xe, Rb, Ba, Mo, and I. The heavy elements have remained relatively fixed. These conclusions must remain tentative until additional analyses are completed on samples taken near the interfaces between reactor and non-reactor zones. Relevance of these tentative conclusions to other geologic formations must await further study of comparative geochemistry and geology. (U.S.)

88

Occurrence of Pu and fissiogenic REE in hydrothermal apatites om the fossil nuclear reactor 16 at Oklo (Gabon)  

International Nuclear Information System (INIS)

Neutron-fission induced nuclear reactions took place 2 Ga ago in the Oklo uranium deposit which is located in the Franceville sedimentary basin (Gabon). The heat released during the running time of the natural reactors led to extensive alteration and desilicification of the sandstones enclosing the reactors and subsequent new formation of clays and apatites, among other phases. The purpose of this study is to determine the capacity of hydrothermally formed apatite to retain actinides and fission products. Combined optical microscopy, scanning electron microscopy and electron microprobe observations revealed the occurrence of fluoroapatite in reaction zone 16. Isotopic and elemental analysis of U, Sm-Nd, Rb-Sr, Bi and Th were performed by ICP-MS and MS-thermoionization on separated apatite grains of various sizes devoid of uraninite inclusions. Most fractions show a slight depletion in 235U. However, one sample shows a significant 235U enrichment (235U/238U = 0.00804 instead of 0.007254 for the natural U) together with high contributions of fissiogenic Nd and Sm. This result shows that 239Pu which is produced by 238U epithermal neutron capture was trapped by the apatite structure during the nuclear reactions or soon after the end of the reactions. It is suggested that combined in situ epithermal neutron irradiation and chemical Pu/U fractionation during the high temperature leaching had contributed to the occurrence of the high 235U/238U ratio in the sample. (orig.)

89

Geochemistry of apatites, pyrites and galenas in near- and far-field veins and sandstones around the Oklo fossil reactors (Gabon): identification of ancient hydrothermal circulations  

International Nuclear Information System (INIS)

The identification and the modelling of fission-product transfer from reactor cores to near-and far-field surrounding rocks, are a prerequisite to the use of the Oklo fossil reactors as a natural analogue of radioactive waste disposal. It is thus necessary to characterize the different hydrothermal circulation stages which occurred in the deposit, for their ability to carry away radionuclides to be assessed. The problems to be solved are as follows:(1) to identify each stage, (2) to constrain the chemical composition of these fluids as precisely as possible, (3) to determine fluid pathways, and if possible, (4) to assign the source(s) of matter carried by each fluid, and the source of energy needed for fluid convection. This paper presents the geochemistry of apatites, pyrites and galenas. (author). 3 refs., 4 figs., 2 tabs

90

Hydrogeology of the Oklo-Okelobondo site  

International Nuclear Information System (INIS)

This study gives an account of all the hydrogeological data from the Oklo-Okelobondo site obtained so far. This hydrogeological overview has led to the proposal of a hydrodynamic flow pattern for the system and the choice of a study area as a basis for a preliminary modelling of groundwater flow and solute transport in the far field. (author). 5 refs., 4 figs

91

The nature of dynamics of the last stages of the fission process  

Energy Technology Data Exchange (ETDEWEB)

It is shown that the existing data for the different fissioning nuclei on the proton odd-even effect {delta}{sub p}, conditional charge variance ({sigma}{sub Z}{sup 2}(Z:A)) and the light-charged-particle accompanied ternary fission-to-binary fission ratio T/B, can be treated in a coherent way to understand the nature of dynamics that comes into play during the last stages of the fission process through the rupture of the neck joining the nascent fission fragments. (author)

Bouzid, B.; Djebara, M.; Medkour, M. [Institut de Physique, USTHB, BP El Alia, Bab Ezzouar (Algeria); Asghar, M. [Institut des Sciences Nucleaires, Universite Joseph Fourier, F-38026 Grenoble (France)

1998-05-01

92

Fluid phases contemporaneous with sandstone diagenesis, tectonic movements and functioning of the Oklo nuclear reactors (Gabon)  

International Nuclear Information System (INIS)

A joint study of the fluid inclusions and petrography of the sandstones in the FA at Oklo enables us to estimate the temperature and pressure conditions for the various diageneses. It also enables us to fix the time of the formation and functioning of the natural reactors, in relation to the geological events. The conditions for the siliceous diagenesis, just as for the formation of the Oklo tectonic structure at the beginning, are estimated at approximately 2400C and 1000 bar. The carbonate diagenesis, which comes after the functioning of the natural reactors, would appear to have occurred at about 1800C and 800 bar. The density of the fluids and pressure stresses (between 1000 and 800 bar) suggests that, during the nuclear reaction, the aqueous solutions may have reached temperatures between 450 and at least 6000C. (author)

93

Uranium deposits of Gabon and Oklo reactors. Metallogenic model for rich deposits of the lower proterozoic  

International Nuclear Information System (INIS)

The geology of the Franceville basin (Gabon) is examined: stratigraphy, tectonics and geodynamics. The mobile zone of the Ogooue is specially studied: lithology, metamorphism and tectonics, isotopic geochronologic data are given. The different uranium deposits are described. A whole chapter is devoted to the study of Oklo natural nuclear reactor. A metallogenic model is proposed evidencing conditions required for deposit genesis. Tectonics, microstructures sedimentology, organic matter, diagenesis and uraniferous mineralizations are examined

94

Oklo - A nuclear reactor 1800 million years ago  

International Nuclear Information System (INIS)

e uranium-235 concentration was slightly less than what one would expect from natural uranium, namely 0.7171% instead of the value just mentioned. If the scientists at Pierrelatte had not made a point of being so careful they might have overlooked this result. However, they carried out a series of checks and discovered that it was not a question of measurement error, but that they were dealing with uranium which had a composition differing from that of 'normal' uranium, with additional variations between different samples. It was soon established that the uranium had come from Gabon ? or more precisely from a mine, OKLO, in the vicinity of Franceville in south-east Gabon

95

Thermal history and redox conditions in the Oklo reactor zones (Gabon)  

International Nuclear Information System (INIS)

In the uranium ore deposit of Oklo-Okelobondo (Gabon), the mineralization contains U-enriched zones, that have fissioned spontaneously 1.97 Ga ago. In the Okelobondo, the salinity of the diagenetic fluid is below 3 wt % NaCl for a minimal temperature ranging from 120 deg to 200 deg C. At Lastoursville, presence of a H2O-NaCl-CH4 + CO2 immiscibility case indicate a temperature of 160-190 deg and a pressure of 1.0 + 0.2 kbar. In zone 10 core, sulphides (galena, pyrite,...), native Pb and organic matter (OM) indicate a reduced environment. At the border, minium and hematite indicate very oxidized conditions. H2O-H2-O2 fluid inclusions are related to water radiolysis. The conclusion is that, OM trap O2 produced by radiolysis leading to a very reduced environment. In the absence of OM, the environment becomes very oxidized. These specific conditions have lead to a stability of uraninite and fission products in the reactors but local heterogeneities are to be considered. Fluid inclusions reveal that temperatures have reached 400 deg C at the reactor border, but decreased sharply along a few meters. Fluid salinity increased drastically ( 23 %), whereas temperature was decreasing. Salt enrichment is attributed to interactions between fluids and decays. The dolerite impact in the Oklo carrier has also been investigated. (author)ted. (author)

96

Implications of the Oklo Phenomenon in a Chiral Approach to Nuclear Matter  

Science.gov (United States)

It has been customary to use data from the Oklo natural nuclear reactor to place bounds on the change that has occurred in the electromagnetic fine structure constant ? over the last 2 billion years. Alternatively, an analysis could be based on a recently proposed expression for shifts in resonance energies which relates them to changes in both ? and the average m q of the u and d current quark masses, and which makes explicit the dependence on mass number A and atomic number Z. (Recent model independent results on hadronic {?} -terms suggest sensitivity to the strange quark mass is negligible.) The most sophisticated analysis, to date, of the quark mass term invokes a calculation of the nuclear mean-field within the Walecka model of quantum hadrodynamics. We comment on this study and consider an alternative in which the link to low-energy quantum chromodynamics and its pattern of chiral symmetry-breaking is more readily discernible. Specifically, we investigate the sensitivity to changes in the pion mass {M_?} of a single nucleon potential determined by an in-medium chiral perturbation theory ({?} PT) calculation which includes virtual {{?}} -excitations. Subject to some reasonable assumptions about low-energy constants, we confirm that the m q -contribution to resonance shifts is enhanced by a factor of 10 or so relative to the {?} -term and deduce that the Oklo data for Sm imply that {|m_q(Oklo)- m_q(now)| ? 10^{-9}m_q(now)}.

Davis, Edward D.

2014-09-01

97

Oklo/obsidian/ancient glasses: applications to nuclear waste public information programs  

International Nuclear Information System (INIS)

Natural analogs such as the Oklo natural nuclear reactor and archaeological data from ancient, man-made glasses can be helpful in making a more interesting presentation on nuclear waste disposal. The extent to which natural analogs and archaeological data can also be used as benchmarks for our current understanding of processes over long periods of time is sometimes overlooked. The potential pitfalls of using natural analogs and archaeological evidence in a presentation in nuclear waste disposal, as well as the benefits, must be carefully considered

98

Description of the Oklo uranium deposit  

International Nuclear Information System (INIS)

The Oklo uranium deposit, situated 1500 m to the south of the Mounana deposit, is close to a basal rise which influenced sedimentation and conditioned the tectonics. The deposit is a sedimentary one in a sandstone layer 4-10 m thick and forms the extreme upper layer of the sandstone basal formation of the Francevillian series. It is overlaid by predominantly pelitic facies. The layer has an eastward monoclinal dip complicated locally by north-south tectonic features. The maximum size of the mineralized zone is 900 m by 600 m. The variations in the degree of mineralization are due to sedimentological phenomena on which are superposed later tectonic features (local enrichment factors)

99

The origin of the chemical elements and the Oklo phenomenon  

International Nuclear Information System (INIS)

Major developments in the field of nuclear geochemistry and cosmochemistry are reviewed in this monograph. Following a brief introduction, an historical account of the early ideas concerning the cosmic abundance of the elements and the searches made for the ''missing'' elements 43 (Tc) and 61 (Pm) in nature are given. The sequence of events which culminated in the discovery of the Oklo Phenomenon (Pre-Fermi reactor), and the topics related to the synthesis of the elements in stars are then discussed as are the ideas concerning the extinct radioactivities and the discoveries of the extinct nuclides 129I and 244Pu. In the final chapter on isotopic anomalies in the early solar system, the author presents an unbiased review of an area that - although dating back to the days of ancient Greek philosophers and regarded by many as the most fundamental in the entire compass of our modern science - is far from settled and is perhaps not quite ready for incorporation into textbooks. (orig.)

100

Thermal stability of fission tracks in natural and synthetic glasses: An assessment of compositional effects  

CERN Document Server

A series of laboratory annealing experiments are designed to evaluate the thermal annealing characteristics of fission tracks for a large number of glasses of varying chemical compositions. A comparative study of thermal stability has been made by comparing the calculated value of activation energy (Ea) for each glass. The present investigations suggest that fission tracks in glass samples with a high silica content are more resistant to annealing than those with a low silica content. In addition, the influence of chemical composition on etching characteristics for both synthetic and natural glasses has also been investigated.

Singh-Sandhu, A; Preece, S J

1999-01-01

 
 
 
 
101

Excitation function of 4He-ion-induced fission of Dy (natural)  

International Nuclear Information System (INIS)

As part of a long-range programme of work on the measurement of fission excitation functions of low Z (Z 4He-ions in the energy range 35-50 MeV were measured using the sensitive ''fission track'' technique using lexan polycarbonate plastic as detectors. Targets of (spec. pure grade) high purity Dy2O3, further purified by a series of anion exchange technique, deposited on high purity (99.9999%) silver foils were irradiated with 4He-ions of different energies from the Variable Energy Cyclotron at Calcutta. The heavy elements contents of both the dysprosium oxide and the silver foils were estimated to be not more than 1-3 ppb. The experimental 4He-ion-induced fission cross sections of natural Dysprosium (162.5Dy) are presented. 7 refs, 1 tab

102

Chemically fractionated fission-xenon in meteorites and on the earth  

Science.gov (United States)

This is a report on the nature of isotopically anomalous xenon, which has been detected in two Ca-Al-rich inclusions of the Allende carbonaceous chondrite. It is extremely enriched in 132Xe, 129Xe, and to a lesser extent in 131Xe. Similar large excesses of 132Xe as well as of 131Xe, 134Xe, and 129Xe have previously been found in material processed in a natural nuclear reactor (Oklo phenomenon). Excess of these isotopes had also been encountered in MORB-glasses, in an ancient Greenland anorthosite. Thus, this Xe-type, which had previously been termed "alien" ( JORDON et al., 1980a) does not seem to be unique. To determine the origin of "alien" Xe, we analysed Xe (a) in neutron irradiated pitchblende and in the irradiation capsule, (b) in non-irradiated extremely fine-grained pitchblende (so-called Colorado-type deposit), and (c) in sandstone taken from the epicentre of an atomic explosion. In addition, the isotopic composition of xenon released by stepwise degassing and after selective dissolving of rocks from the Oklo natural reactor was determined. The results of these dedicated experiments demonstrate that the formation of alien Xe is due to the migration of the radioactive precursors of the stable isotopes 134Xe, 132Xe, 131Xe, and 129Xe. Due to this reason we now call it CFF-Xe - Chemically Fractionated Fission Xenon. Prerequisites for its formation are the simultaneous prevalence of two conditions: (1) fission (of 238U, 235U, and/ or 244Pu) and (2) a physicochemical environment (temperature, pressure, fluidity) at which the precursors of xenon (mainly Te and I) are mobile. Taking into account the occurrence of xenon in meteorites and terrestrial rocks, not all excesses of 129Xe in mantle rocks and natural gases are necessarily connected with the decay of primordial 129I.

Shukolyukov, Yuri A.; Jessberger, Elmar K.; Meshik, Alexander P.; Vu Minh, Dang; Jordan, Jimmy L.

1994-07-01

103

Study on natural deposition of fission product aerosol in severe accidents  

International Nuclear Information System (INIS)

Aerosol natural deposition model of gravitational sedimentation, diffusionphoresis, inertial impaction and thermophoresis are established based on integrated safety analysis model for 600 MW pressurized water reactor. Typical severe accidents are chosen, and natural deposition phenomenon of fission product aerosol is analyzed. Additionally, gravitational sedimentation model of MELCOR is coupled into integrated safety analysis model, and fraction of gravitational sedimentation is compared. The results show that gravitational sedimentation is the most important deposition mechanism, and deposition effect of gravitational sedimentation model in this paper is stronger than MELCOR. (authors)

104

Migration of U-series radionuclides around the Bangombe natural fission reactor (Gabon)  

International Nuclear Information System (INIS)

The Bangombe natural fission reactors has undergone extensive weathering phenomena and continues to be affected by the penetration of meteoric waters. Hence this system provides a model for studying the stability of spent fuel uraninite and the influence of various rock matrices on the mobilization/retardation of various actinides and fission products. The Bangombe uranium deposit has been investigated by drilling on a grid. Radiochemical analysis by alpha- and gamma-spectroscopy of the obtained rocks show significant disequilibria of the 234U/238U, 230Th/234U, and 226Ra/230Th parent-daughter pairs. In this paper, a conceptual model for spatio/temporal evolution of the Bangombe system is proposed. (J.P.N.)

105

Radiolysis in nature: Evidence from the Oklo natural reactors  

International Nuclear Information System (INIS)

radiation and the inferred hydrogen production suggest an effective radiation yield of 0.06 molecules of hydrogen per 100 eV of energy imparted to the fluid phase. Considering radiation from both alpha and beta sources, the G value for hydrogen production is reduced to 0.01 to 0.002 molecules H2/100 eV. (author)

106

Nuclear fission as a global energy resource  

International Nuclear Information System (INIS)

This chapter describes the role and the extent of nuclear fission as a global energy resource. The chapter begins with a history of nuclear fission, starting with the Oklo Reactor of about 2 billion years ago and continues through man-made reactors and their current ramifications and problems. Energy resources are described under the topics of impact, fossil resources, fission-water reactors, and fusion-fission-coal. Nuclear applications for oil replacement, propulsion, electric utilities, cogeneration, desalting, and enhanced oil recovery are discussed. Other topics covered in this chapter are: energy requirements and energy availability of uranium enrichment; plant construction of breeder reactors along with antiproliferation and the energy available from breeder reactors in comparison with water reactors; and the energy available from enrichment tailings. The author concludes that nuclear fission is indeed the largest world energy resource available today and hopes the US and the world use it wisely

107

Investigation of the fundamental constants stability based on the reactor Oklo burn-up analysis  

CERN Document Server

The burn-up for SC56-1472 sample of the natural Oklo reactor zone 3 was calculated using the modern Monte Carlo codes. We reconstructed the neutron spectrum in the core by means of the isotope ratios: $^{147}$Sm/$^{148}$Sm and $^{176}$Lu/$^{175}$Lu. These ratios unambiguously determine the spectrum index and core temperature. The effective neutron absorption cross section of $^{149}$Sm calculated using this spectrum was compared with experimental one. The disagreement between these two values allows to limit a possible shift of the low laying resonance of $^{149}$Sm even more . Then, these limits were converted to the limits for the change of the fine structure constant $\\alpha$. We found that for the rate of $\\alpha$ change the inequality $|\\delta \\dot{\\alpha}/\\alpha| \\le 5\\cdot 10^{-18}$ is fulfilled, which is of the next higher order than our previous limit.

Onegin, M S

2010-01-01

108

Problems posed by the development of the Oklo phenomenon: tentative global interpretation  

International Nuclear Information System (INIS)

This paper discusses the basic problems posed by the development of the Oklo phenomenon: the conditions in which the reactions are triggered and propagated and how they have been controlled. The reactions were maintained by the destruction of neutron poisons in the ore and were controlled by temperature. Oklo is made up of a large number of contiguous reactors. Geological problems of the origin of the clays, desilification, and uranium concentration are discussed. Oklo is shown to be a very complex phenomenon which developed in space and time. Besides the thermal, neutron, and geochemical coupling, there is also a tectonic coupling

109

Anomalies of isotopic composition of Xe and Kr from Oklo deposit, Gabon, Africa  

International Nuclear Information System (INIS)

Isotopic content of Xe and Kr from mineral fractions of the sample N1348, taken from the SC20 Hole (ore zone N2) of Oklo deposit, was investigated using the method of thermal annealing. A method of sample separation into mineral components is described. The tables of Xe and Kr isotopic content, the diagrams of Xe isotopic correlations, and also Xe and Kr isotopic variations at stepped annealing of fraction mineral components are presented. It was found that Xe isotopic content in the fractions, rich with uraninite is close to Xe isotopic content, formed under artificial conditions at irradiation of 235U by thermal neutrons. The Xe isotopic content in the uranium free fractions is sharply anomalous. 134Xe/136Xe, 132Xe/136Xe, 131Xe/136Xe, 129Xe/136Xe ratios amount to 1.59; 3.46; 1.20, 0.358 respectively. Linear correlations between isotopic ratios are interpreted as a result of mixing up 2 main mineral phases: the one containing normal Xe from 235U fission and the second one containing anomalous Xe with isotopic ratio 136:134:132:131:129=1.00:1.65:3.50:1.48:0.41 Anomalous Xe is also present in uraninite. It was found that sharp maximum of the extraction rate from minerals at 800 deg C, recurring in all fractions, but uraninite, is characteristic of thXe extraction kinetics. The Xe carrier is a mineral with the density of 3-4 g/cm/3. The maximum 1 3-4 g/cm/3. The maximum 136Xesub(a) content equals 1.9x10-6 cm3/g, its quantity being in some fractions several times higher than that of the normal fission Xe

110

Parametric study of the criticality of natural reactors  

International Nuclear Information System (INIS)

Conditions for the criticality of natural reactors are investigated from a general point of view; a parametric study is presented, which expresses the possibility of chain reactions as functions of five parameters: the age of the deposit, the ore's uranium content, the volume of high-grade ore, the neutron capture of the vein of ore and the amount of water associated with the uranium. It is demonstrated that although criticality could theoretically be attained for ages that are not in excess of 1000 to 1200 MA, conditions would have to be exceptionally favorable for it since the deposits are clearly much younger than those at Oklo. The study offers a much better appreciation of the probability for discovery of other natural fissionable reactors

111

On the thermal stability of fossil and neutron-induced fission-tracks in natural titanite  

International Nuclear Information System (INIS)

Results are presented of 625 deg. C and 675 deg. C-annealing experiments on fossil and neutron-induced fission-tracks in 44 natural-titanite samples from the Fish Canyon Tuff. Both track density and track length decrease in a complex manner with annealing time. Possible causes are: (a) a phase transition during annealing, (b) parallel annealing of recoil-tracks or (c) differences in the defect structure along the tracks at the atomic scale. The relationship between the decrease of track density and the reduction of track length is similar to that in apatite, i.e. proportional up to ?35%, followed by a collapse of track density without a comparable reduction of track length. The latter is ascribed to the appearance of unetchable gaps. The initial decrease is not exactly 1:1, partly as a result of a decrease of track counting efficiency ?q with decreasing track length. The mean plateau-age of all samples before the break in slope at ?35% is 31.8±0.6 Ma, i.e. ?15% higher than the reference age of the Fish Canyon Tuff (27.8±0.2 Ma). It is thought that this results neither from inaccurate measurement of the thermal neutron fluence, nor from a different degree of metamictisation of the crystals with spontaneous and with induced tracks, but from unidentified systematic factors affecting the track-density measurements. The systematic differences between fossil and induced tracks in these experiments call into question the generality of the principle of equivalent timf the principle of equivalent time, that underlies all fission-track temperature-time path modelling

112

A fission-fusion hybrid reactor in steady-state L-mode tokamak configuration with natural uranium  

Energy Technology Data Exchange (ETDEWEB)

This work develops a conceptual design for a fusion-fission hybrid reactor operating in steady-state L-mode tokamak configuration with a subcritical natural or depleted uranium pebble bed blanket. A liquid lithium-lead alloy breeds enough tritium to replenish that consumed by the D-T fusion reaction. The fission blanket augments the fusion power such that the fusion core itself need not have a high power gain, thus allowing for fully non-inductive (steady-state) low confinement mode (L-mode) operation at relatively small physical dimensions. A neutron transport Monte Carlo code models the natural uranium fission blanket. Maximizing the fission power gain while breeding sufficient tritium allows for the selection of an optimal set of blanket parameters, which yields a maximum prudent fission power gain of approximately 7. A 0-D tokamak model suffices to analyze approximate tokamak operating conditions. This fission blanket would allow the fusion component of a hybrid reactor with the same dimensions as ITER to operate in steady-state L-mode very comfortably with a fusion power gain of 6.7 and a thermal fusion power of 2.1 GW. Taking this further can determine the approximate minimum scale for a steady-state L-mode tokamak hybrid reactor, which is a major radius of 5.2 m and an aspect ratio of 2.8. This minimum scale device operates barely within the steady-state L-mode realm with a thermal fusion power of 1.7 GW. Basic thermal hydraulic analysis demonstrates that pressurized helium could cool the pebble bed fission blanket with a flow rate below 10 m/s. The Brayton cycle thermal efficiency is 41%. This reactor, dubbed the Steady-state L-mode non-Enriched Uranium Tokamak Hybrid (SLEUTH), with its very fast neutron spectrum, could be superior to pure fission reactors in terms of breeding fissile fuel and transmuting deleterious fission products. It would likely function best as a prolific plutonium breeder, and the plutonium it produces could actually be more proliferation-resistant than that bred by conventional fast reactors. Furthermore, it can maintain constant total hybrid power output as burnup proceeds by varying the neutron source strength.

Reed, Mark; Parker, Ronald R.; Forget, Benoit [Department of Nuclear Science and Engineering, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139 (United States)

2012-06-19

113

A fission-fusion hybrid reactor in steady-state L-mode tokamak configuration with natural uranium  

Science.gov (United States)

This work develops a conceptual design for a fusion-fission hybrid reactor operating in steady-state L-mode tokamak configuration with a subcritical natural or depleted uranium pebble bed blanket. A liquid lithium-lead alloy breeds enough tritium to replenish that consumed by the D-T fusion reaction. The fission blanket augments the fusion power such that the fusion core itself need not have a high power gain, thus allowing for fully non-inductive (steady-state) low confinement mode (L-mode) operation at relatively small physical dimensions. A neutron transport Monte Carlo code models the natural uranium fission blanket. Maximizing the fission power gain while breeding sufficient tritium allows for the selection of an optimal set of blanket parameters, which yields a maximum prudent fission power gain of approximately 7. A 0-D tokamak model suffices to analyze approximate tokamak operating conditions. This fission blanket would allow the fusion component of a hybrid reactor with the same dimensions as ITER to operate in steady-state L-mode very comfortably with a fusion power gain of 6.7 and a thermal fusion power of 2.1 GW. Taking this further can determine the approximate minimum scale for a steady-state L-mode tokamak hybrid reactor, which is a major radius of 5.2 m and an aspect ratio of 2.8. This minimum scale device operates barely within the steady-state L-mode realm with a thermal fusion power of 1.7 GW. Basic thermal hydraulic analysis demonstrates that pressurized helium could cool the pebble bed fission blanket with a flow rate below 10 m/s. The Brayton cycle thermal efficiency is 41%. This reactor, dubbed the Steady-state L-mode non-Enriched Uranium Tokamak Hybrid (SLEUTH), with its very fast neutron spectrum, could be superior to pure fission reactors in terms of breeding fissile fuel and transmuting deleterious fission products. It would likely function best as a prolific plutonium breeder, and the plutonium it produces could actually be more proliferation-resistant than that bred by conventional fast reactors. Furthermore, it can maintain constant total hybrid power output as burnup proceeds by varying the neutron source strength.

Reed, Mark; Parker, Ronald R.; Forget, Benoit

2012-06-01

114

A fission-fusion hybrid reactor in steady-state L-mode tokamak configuration with natural uranium  

International Nuclear Information System (INIS)

This work develops a conceptual design for a fusion-fission hybrid reactor operating in steady-state L-mode tokamak configuration with a subcritical natural or depleted uranium pebble bed blanket. A liquid lithium-lead alloy breeds enough tritium to replenish that consumed by the D-T fusion reaction. The fission blanket augments the fusion power such that the fusion core itself need not have a high power gain, thus allowing for fully non-inductive (steady-state) low confinement mode (L-mode) operation at relatively small physical dimensions. A neutron transport Monte Carlo code models the natural uranium fission blanket. Maximizing the fission power gain while breeding sufficient tritium allows for the selection of an optimal set of blanket parameters, which yields a maximum prudent fission power gain of approximately 7. A 0-D tokamak model suffices to analyze approximate tokamak operating conditions. This fission blanket would allow the fusion component of a hybrid reactor with the same dimensions as ITER to operate in steady-state L-mode very comfortably with a fusion power gain of 6.7 and a thermal fusion power of 2.1 GW. Taking this further can determine the approximate minimum scale for a steady-state L-mode tokamak hybrid reactor, which is a major radius of 5.2 m and an aspect ratio of 2.8. This minimum scale device operates barely within the steady-state L-mode realm with a thermal fusion power of 1.7 GW. Basic thermal hydraulic analysis demonstrates that presshydraulic analysis demonstrates that pressurized helium could cool the pebble bed fission blanket with a flow rate below 10 m/s. The Brayton cycle thermal efficiency is 41%. This reactor, dubbed the Steady-state L-mode non-Enriched Uranium Tokamak Hybrid (SLEUTH), with its very fast neutron spectrum, could be superior to pure fission reactors in terms of breeding fissile fuel and transmuting deleterious fission products. It would likely function best as a prolific plutonium breeder, and the plutonium it produces could actually be more proliferation-resistant than that bred by conventional fast reactors. Furthermore, it can maintain constant total hybrid power output as burnup proceeds by varying the neutron source strength.

115

Naturally etched tracks in apatites and the correction of fission track dating  

CERN Document Server

Naturally etched tracks have been found in apatites from the rapid cooled, high-level Kunon pluton in the Zhangzhou Igneous Complex, SE China. This is manifested by the fact that the apatite fission track (FT) age derived from conventional counting of spontaneous and induced tracks yields a result of 140.6+-6.5 Ma, which is much older than the ages determined using other methods on different minerals from the same rock. When tracks are observed after etching the polished inner sections of the apatite grains, the naturally etched tracks characterized by having hazy boundaries can be distinguished from the normal tracks with sharp boundaries. The age obtained by omitting these fading-resistant hazy tracks, 76.5+-4.0 Ma, indicates the time of the Kunon pluton cooling down to approx 100 deg. C. The corrected peak age (73.8 Ma) is consistent with the other apatite FT peak ages (79.2 to 70.2 Ma) of the nearly contemporaneous plutons in the same igneous complex.

Tien, J L

1999-01-01

116

Measurement of absoulute fission rate of natural, depleted oranium and thorium and microscopic cross section ratios in the radial beam of the RA3  

International Nuclear Information System (INIS)

The capability to measure absolute fission rates per nuclei has been established to a precision level of around ±2 %. Thin deposits of ThO2 and U3O8 were prepared by molecular plating and the isotopic masses determined by two techniques. A miniature fission chamber and SSNTD fission fragment detection techniques were used in the radial neutron beam of the RA3. Fission fragments selfabsorption in the deposits are corrected. The results of this investigation are: 1. Optical efficiency of SSNTD detectors. 2. The microscopic integral fission cross section ratio of Th232 to U238 by two independent techniques. 3. Experimental ratios of natural, depleted uranium and thorium fission rate per ?g to Mn56 (mn thermal flux monitor) desintegration rate per ?g. The agreement with calculated results is excellent. (author)

117

Enhanced effect of quark mass variation in 229Th and limits from Oklo data  

CERN Document Server

The effects of the variation of the dimensionless strong interaction parameter Xq=mq/Lambda{QCD} (mq is the quark mass, Lambda{QCD} is the QCD scale) are enhanced about 1.5 x 10**5 times in the 7.6 eV "nuclear clock" transition between the ground and first excited states in the 229Th nucleus and about 1 x 10**8 times in the relative shift of the 0.1 eV compound resonance in 150Sm.The best terrestrial limit on the temporal variation of the fundamental constants, |delta(Xq)/Xq| < 4 x 10**-9 at 1.8 billion years ago (|d(Xq/Xq)/dt| < 2.2 x 10**-18 y**-1), is obtained from the shift of this Sm resonance derived from the Oklo natural nuclear reactor data. The results for 229Th and 150Sm are obtained by extrapolation from light nuclei where the many-body calculations can be performed more accurately. The errors produced by such extrapolation may be smaller than the errors of direct calculations in heavy nuclei. The extrapolation results are compared with the "direct" estimates obtained using the Walecka model....

Flambaum, V V

2008-01-01

118

Leaching of actinides and fission products from ILW embedded in cement and bitumen, and their mobility in natural salt rock  

International Nuclear Information System (INIS)

Real and simulated ILW embedded in cement and bitumen has been subjected to leaching through various binary brines. The resulting solutions containing americium, plutonium, cesium, antimony, ruthenium, cobalt, and strontium have been led through columns packed with the natural Na3? salt rock from the Gorleben salt dome, in order to determine the mobility characteristics of these elements in the near-field range of a projected waste repository in the Gorleben salt dome, specifically for the case of water intrusion. Leaching data and experimental results are explained and discussed, special attention being given to the impact of the pH-value of the systems studied, and to the formation of carrier (or 'pseudo') colloids during radionuclide release. The paper also gives data obtained on the mobility of transuranium elements and fission products, together with information on differences in behaviour of the actinides and the fission products (ruthenium in particular). (orig.)

119

Calculation and measurement the fission products and their distribution in natural ceramic UO2 nuclear fuel  

International Nuclear Information System (INIS)

In order to improve the fuel fabrication processes and develop them further, the determination of the fission products both qualitatively and quantitatively, as well as their distribution over the entire length of the fuel pin swelling especially at the regions where some gaseous fission products are accumulated. This study is concentrating at the post-irradiation evaluation of the ceramic UO2 nuclear fuel, by utilizing the existing facilities and equipment at the hot laboratories. Cs-137 was chosen as a precursor for the burn-up level of the irradiated fuel because of the case with which its gamma radiation is measured and due to its relatively long half-life. 19 figs.; 13 tabs.; 42 refs.; 9 apps

120

Natural uranium impurities in fission track detectors and associated geocronological parameters  

International Nuclear Information System (INIS)

A technique, based in counting neutron induced fission tracks, has been developed for the measurement of uranium impurities in mica. Uranium concentrations of 10-10 and 10-9 (U atom/mica atom) have been measured. As a part of the development of this technique, the mica geological age was also measured, by fossil and induced track detection. The agreement obtained by this method, T = (472+-52) x 106 years with that of (450+-15) x 106 years obtained by the Ar-K technique is satisfactory and is an indirect test of the fission track technique used. A careful analysis of the neutron field parameters and nuclear data used in the age determination was made. This analysis is useful for applications in geocronology. According to this analysis a value of lambdasub(f)=(7.1+-0.1) x 10-17 years-1 is recommended for the spontaneous fission of U238. However, in order to compare the results, the quoted age, T=(472+-52) x 106 years, was obtained with the generally accepted value of lambdasub(f)=(6.85-0.20) x 10-17 years-1 (Fleischer and Price 1964). (author)

 
 
 
 
121

Oklo: The fossil nuclear reactors. Physics study - Translation of chapters 6, 13 and conclusions  

International Nuclear Information System (INIS)

Three parts of the 1991 book 'Oklo: reacteurs nucleaires fossiles. Etude physique' have been translated in this report. The chapters bear the titles 'Study of criticality'(45 p.), 'Some problems with the overall functioning of the reactor zones'(45 p.) and 'Conclusions' (15 p.), respectively

122

Petrographic analysis of samples from the uranium deposit at Oklo, Republic of Gabon  

International Nuclear Information System (INIS)

This study is preliminary to detailed X-ray diffraction analysis and additional electron microprobe analyses. Twenty samples were examined; seven from reactor zone 9 (RZ-9), ten from RZ-10, two from RZ-13 and one from RZ-16. This suite of samples includes examples from reactor cores and their associated hydrothermal alteration haloes. The most striking characteristic of the Oklo samples is their heterogeneity. In particular, the samples vary with respect to texture, mineralogy, uranium mineral content, and evidence for deformation. Deformation features provide evidence for both shear and extensional stresses and include brecciation and a variety of mineralized fractures. Opaque phases include orgainc matter, uraninite, coffinite, and sulfides. In many cases, the opaque assemblage is concentrated along fractures. Considerable textural evidence, viz., embayed grain margins and fractures with nonparallel margins, suggests partial dissolution of uraninite in the Oklo ores. Uraninite with embayed margins is commonly accompanied by organic matter. Oklo uraninites are, in many cases, altered to produce coffinite (USiO4.nH2O) and are associated with varying quantities of galena at grain boundaries, within fractures, and within individual grains at intracrystalline locations. Textural evidence suggests multiple periods of uraninite formation. Electron microprobe data are presented for uraninites from RZ-13. Analytical data are discussed for Oklo reactor zical data are discussed for Oklo reactor zones 2, 9, 10, 13, and 16 and for the reactor at Bangombe. Uraninites are compositionally similar with respect to Pb for RZ-2, RZ-9, RZ-13, RZ-16 and for the Bangombe reactor (average content of PbO is 5.92 weight %). Lead contents for uraninites from RZ-10 are generally higher and more variable than for the other reactor zones (e.g., PbO content for RZ-10 uraninites ranges from approximately 12 to 18 weight %). (orig.)

123

Uraninite: A 2 Ga spent nuclear fuel from the natural fission reactor at Bangombe in Gabon, West Africa  

International Nuclear Information System (INIS)

Uraninites from the Bangombe natural fission reactor (RZB) and normal uranium-ore occur as fine veins in the sandstone host-rock as well as altered, broken, and slightly displaced grains in an illitic matrix, and in nodules and veins of solid bitumen. Inclusions of galena, (Y,Gd)-rich phosphates, a Pb-oxide and a Ti-oxide? were observed. Uraninites just below RZB were partially altered to a uranyl-sulfate. Three generations of uraninite were identified based on their PbO-contents of 8--11.06 wt%, 6 wt% (the largest population), and a younger generation with 3 wt%. Diffusional loss of Pb is indicated by the presence of a Pb-oxide at the interface to the uraninites. The behavior of the metallic fission products, incompatible with the uraninite structure, may mimic the behavior of Pb in these uraninites. The averaged impurity-content ranges from 4.29 to 6.89 wt%, and consists mainly of SiO2, TiO2, ZrO2, FeO, CaO, Al2O3 and P2O5. The averaged content of Y2O3 and the Ln''s is less than 0.78 wt% and there is a scattered positive correlation with P2O5. The content of Y + Ln''s is generally highest in the uraninites from RZB. Uraninite hydration and the formation of uranopelite/zippeite have caused complete loss of Y and the Ln''s. The analytical results indicate that Y and the Ln''s, which are high yield fission products, may be released from uraninite duroducts, may be released from uraninite during alteration in the presence of P

124

Imprints left by natural radioactivity in geological materials: uranium fission tracks and thermoluminescence applications in earth sciences  

International Nuclear Information System (INIS)

In a rock, all minerals which contain uranium are host to a number of spontaneous fission phenomena forming a single damaged area called a ''latent track'', observations of which may lead to dating, uranium mapping and finding paleo-geo-thermometers (thermal history, used in oil exploration). The radioactive elements during the decay process release energy which is trapped as electrons into the physical or chemical defects of the crystalline lattice; this energy can be later released by heating the mineral (thermic stimulated luminescence); the thermoluminescence is characterized by a glow which spectrum constitutes a typical feature of the mineral, its crystallization conditions and the subsequent evolution of the material. Natural and induced glow curve may be produced. 6 figs., 52 refs

125

Delayed fission  

Energy Technology Data Exchange (ETDEWEB)

Delayed fission is a nuclear decay process that couples {beta} decay and fission. In the delayed fission process, a parent nucleus undergoes {beta} decay and thereby populates excited states in the daughter. If these states are of energies comparable to or greater than the fission barrier of the daughter, then fission may compete with other decay modes of the excited states in the daughter. In this paper, mechanism and some experiments of the delayed fission will be discussed. (author)

Hatsukawa, Yuichi [Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment

1997-07-01

126

Indications of uranium transport around the reactor zone at Bagombe (Oklo)  

Energy Technology Data Exchange (ETDEWEB)

The aim of this study is to use the hydrogeological and hydrochemical data from Oklo Natural Analogue to compare the outcome of two independent modelling approaches (HYTEC-2D and M3) which can be used to model natural conditions surrounding the reactor. HYTEC-2D represents a 2D, deterministic, transport and multi-solutes reactive coupled code developed at Ecole des Mines de Paris. M3 (named Multivariate Mixing and Mass balance) is a mathematical-statistical concept code developed for SKB. The M3 results are visualised using the Voxel Analyst code and the outcome of the uranium transport predictions are made from a performance assessment point of view. This exercise was in the beginning intended to represent a validation for M3, by comparing this statistic approach with the standard hydrodynamic - geochemical coupled code HYTEC-2D. It was realized that the codes complete each other and a better understanding of the geochemical studied system is obtained. Thus, M3 can relatively easily be used to calculate mixing portions and to identify sinks or sources of element concentrations that may exist in a geochemical system. This can help to address the reactions in the coupled code such as HYTEC-2D, to identify the hydrodynamic and hydrochemical system and to reduce the computation time. M3 shows the existence of the buffer around the reactor. No transport of uranium was indicated downstream the reactor. HYTEC-2D gives the same result in the case when we consider the existence of the redox buffer in the model. M3 shows an increase of the alkalinity in the reactor zone. The increase of the alkalinity was indicated by the M3 modelling to be associated with microbial decomposition of organic material which added reducing capacity to the system. The modelling result was supported by new results from the last field campaign, which included in-situ Eh measurements and microbial sampling and identification. The effects from the same process was indicated also by the HYTEC-2D predictions which show an increase of the pH in the reactor zone, due to the existence of the buffer. The two modelling approaches can be used to complete each other and to better understand the processes that can take place in nature. Thus, we can build confident tools which can be used to support performance assessment 16 refs, 16 figs

Gurban, I.; Laaksoharju, M. [INTERA KB, Sollentuna (Sweden); Ledoux, E.; Made, B.; Salignac, A.L. [Ecole des Mines, Paris (France)

1998-08-01

127

On The Geometric Nature of ``Singlet Fission'' in Certain Crystalline Conjugated Polymers  

Science.gov (United States)

In recent years, the coherent fission of low-lying singlet electronic excitations in conjugated polymers has attracted interest as a possible way to exceed the Shockley-Queisser limit in organic photovoltaics. Femtosecond spectroscopic and fluorescence measurements of such singlets and the resulting triplets in crystalline anthracene, tetracene and naphthalene reveal curious phenomena associated with certain vibrational modes, such as ultrafast propagation on a timescale inconsistent with conventional intersystem crossing, long-lived electronic coherence, and triplet magnetic anisotropy whose structure is consistent across all three materials. This conflicts with NRG and quantum chemical simulations, which posit isotropic triplets. I explain this by a dynamical Rashba spin-orbit interaction that decays as R-6. This arises from a geometric SU(2) gauge potential generated by a nuclear-motion-induced parametric near-degeneracy of the molecular electronic states. The anisotropy is shown to follow from the work of Affleck and Oshikawa on spin one-half Heisenberg chains. Possible directions for future work are discussed, especially with regard to adiabatic pumping and topological insulators.

Rahman, Noah

2013-03-01

128

Fast fission phenomena  

International Nuclear Information System (INIS)

Experimental studies of fast fission phenomena are presented. The paper is divided into three parts. In the first part, problems associated with fast fission processes are examined in terms of interaction potentials and a dynamic model is presented in which highly elastic collisions, the formation of compound nuclei and fast fission appear naturally. In the second part, a description is given of the experimental methods employed, the observations made and the preliminary interpretation of measurements suggesting the occurence of fast fission processes. In the third part, our dynamic model is incorporated in a general theory of the dissipative processes studied. This theory enables fluctuations associated with collective variables to be calculated. It is applied to highly inelastic collisions, to fast fission and to the fission dynamics of compound nuclei (for which a schematic representation is given). It is with these calculations that the main results of the second part can be interpreted

129

Geochemical behavior of radionuclides in highly altered zircon above the Bangombé natural fission reactor, Gabon  

Science.gov (United States)

The isotopic compositions of rare earth elements (REE), Pb and U of highly altered zircons from the clay and black shale layers above the Bangombé natural reactor, Gabon, were determined by a sensitive high resolution ion microprobe (SHRIMP) to discuss the redistribution processes of elements into zircons under the supergene weathering. The clay layer trapped most of the fissiogenic Nd, Sm and Eu derived from the reactor and prevented them migrating into the black shale layer. On the other hand, only the Ce isotopic ratios of the clay and black shale layers have about 2 times larger variations than the other REE. This result suggests that a large chemical fractionation between Ce and other REE above the reactor occurred under the oxidizing condition. The U-Pb data of zircons suggest that the U-Pb system was largely disturbed by migration of chemically fractionated Pb and U from the 2.0 Ga-old uraninite in association with recent weathering.

Kikuchi, Makiko; Hidaka, Hiroshi; Horie, Kenji

130

Coulomb fission  

International Nuclear Information System (INIS)

A semiquantal theory of Coulomb fission is presented. It is shown that Coulomb fission is a two-step process, involving multiple Coulomb excitation of an actinide target nuclear by a heavy projectile :Zp >approx. 50) followed by radioactive decay. The historical background; dynamical formalism of the theory; phenomenological models of the theory; Coulomb fission experiments; comparison of theory and experiment; and prompt Coulomb fission theory; are all discussed. (U.K.)

131

Ternary fission  

International Nuclear Information System (INIS)

This is a report on experimental results obtained with the double torus ionization chamber 'DIOGENES', which has been developed for the investigation of particle associated fission. The role of ternary fission for the understandig of the fission will be discussed. (orig.)

132

Sorption of 239Np and 235U fission products by zeolite Y, Mexican natural erionite, and bentonite  

International Nuclear Information System (INIS)

Zeolite Y, erionite, and bentonite have been used in this work to remove 239Np and 235U fission products from aqueous solutions at various pH values. It was found that the sorption of fission products by aluminosilicates takes place by different mechanisms, mainly ion exchange, precipitation, and electrostatic surface interaction. The radionuclides content was determined by ?-spectrometry, and X-ray diffraction was used to learn whether the solids maintained their crystallinity at different pH values

133

Nature and properties of the fission modes of the neutron deficient 220,224,226Th nuclides  

International Nuclear Information System (INIS)

On the U-400 accelerator of FLNR and the Tandem (Catania) by means of a double arm DEMAS spectrometer were measured the mass-energy distributions of fission fragments, the emission of pre- and post-fission neutrons and ?-multiplicity of 220,224,226Th nuclei produced in the reactions 204,208Pb(16O,f) with energies E16O=75, 78, 85, 108 MeV and 208Pb(18O,f) with energies E18O=75, 78 MeV. The range of excitation energies at the saddle point for these nuclei was E*sp=16-40 MeV. The contribution of the asymmetric fission mode characterized by the total yields ratio Ya/Ys increases sharply for heavier isotopes of thorium. Such a behaviour of Ya/Ys is in qualitative agreement with theoretical calculations predicting predominantly symmetric fission for the lightest thorium isotopes and predominantly asymmetric - for heavy ones at low E*sp. The sorted out asymmetric fission mode Ya=Ytot - Ys for all studied isotopes has a complex structure characterized by two components Ya=Ya1 -Ya2 with average masses Ma1 132 and Ma2 139. The energy characteristics of fragments: kinetic energy Ek and its dispersion ?2E for the lowest E*sp also have certain irregularities which are characteristics of the multimodal fission of nuclei - increase of Ek and a peak in ?2E in the region where Ya manifests itself

134

Manifestations of a spatial variation of fundamental constants on atomic clocks, Oklo, meteorites, and cosmological phenomena  

CERN Document Server

The remarkable detection of a spatial variation in the fine-structure constant, alpha, from quasar absorption systems must be independently confirmed by complementary searches. In this letter, we discuss how terrestrial measurements of time-variation of the fundamental constants in the laboratory, meteorite data, and analysis of the Oklo nuclear reactor can be used to corroborate the spatial variation seen by astronomers. Furthermore, we show that spatial variation of the fundamental constants may be observable as spatial anisotropy in the cosmic microwave background, the accelerated expansion (dark energy), and large-scale structure of the Universe.

Berengut, J C

2010-01-01

135

Syngenetic model for some early Proterozoic uranium deposits. Evidence from Oklo  

International Nuclear Information System (INIS)

Massive, vein-like accumulations of uranium in Early Proterozoic metamorphic rocks have conventionally been explained by hydrothermal processes, metamorphism of original syngenetic or early epigenetic sedimentary deposits, mechanical accumulations, supergene processes, or some combination of these processes. Many of these deposits are located at unconformities and may be controlled by basement fault systems, but the unconformities are not necessarily the primary control for ore formation. Exploration of Proterozoic sedimentary basins is, for economic factors, usually restricted to the basin edges where distance to the unconformities is minimal. The 2.0+-0.1 billion year old Oklo uranium deposit, Republic of Gabon, is unique in that very high grade ore has apparently been derived from low grade ore in unmetamorphosed sedimentary rocks. The low grade ore is similar to Phanerozoic syngenetic or early epigenetic sandstone type deposits; the assemblage pitchblende-illite (1 Md polytype)-chlorite-organic carbon-dolomite is common. Uranium was mobilized from the low grade ore zones and concentrated as high grade ore in fractures. Pitchblende-illite (2M1 polytype)-chlorite-hematite-pyrite in the virtual absence of quartz and carbon is common in the high grade ore. Crystallization temperatures near 2000C at P=0.5-2 kb and Eh conditions favouring formation of either pyrite or hematite, in the pH range of 6-8 were likely. The reductant for the mobilized uraikely. The reductant for the mobilized uranium was apparently carbon which was oxidized as fractures opened, thus also accounting for the absence of quartz. The high grade ore, concentrated in shale-rich material down-filled into broken sandstone layers, was then severely folded and fractured. In cross sections, then, much of the Oklo ore resembles cross sections of high grade uranium ore in metamorphic rocks. It is proposed that Oklo may either be a hybrid type of deposit or a precursor to typical deposits in metamorphic rocks noted in Canada and elsewhere. It is considered significant that the Oklo deposit is located within the sedimentary sequence and not at the unconformity between this sequence and the basement granitic massif

136

Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombé, Gabon  

Science.gov (United States)

To study geochemical processes for migration and fixation of fissiogenic rare earth elements (REE) in association with uranium dissolution, in situ isotopic analyses using an ion microprobe were performed on U- and REE-bearing secondary minerals, such as coffinite, françoisite, uraniferous goethite, and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at Bangombé, Gabon. Phosphate minerals such as phosphatian coffinite and françoisite with depleted 235U ( 235U/ 238U = 0.00609 to 0.00638) contained large amount of fissiogenic light REE, while micro-sized uraninite grains in a solid bitumen aggregate have normal U isotopic values ( 235U/ 238U = 0.00725) and small amount of fissiogenic REE components. The proportions of fissiogenic and non-fissiogenic REE components in four samples from the core of BAX03 vary in depth ranging from 30 cm to 130 cm beneath the reactor, which suggests mixing between fissiogenic isotopes from the reactor and non-fissiogenic isotopes from original minerals in the sandstone. Significant chemical fractionation was observed between Ce and the other REE in the secondary minerals, which shows evidence of an oxidizing atmosphere during their formation. Pb-isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of U-Pb decay system due to recent geologic alteration. However, systematic Pb-isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes, Pb and U from the reactor in association with dolerite dyke intrusion ˜0.798 Ga ago and the formation of the secondary minerals by mixing event between 2.05 Ga-old original minerals and reactor materials due to recent alteration.

Hidaka, Hiroshi; Janeczek, Janusz; Skomurski, Frances N.; Ewing, Rodney C.; Gauthier-Lafaye, François

2005-02-01

137

Sorption of {sup 239}Np and {sup 235}U fission products by zeolite Y, Mexican natural erionite, and bentonite  

Energy Technology Data Exchange (ETDEWEB)

Zeolite Y, erionite, and bentonite have been used in this work to remove {sup 239}Np and {sup 235}U fission products from aqueous solutions at various pH values. It was found that the sorption of fission products by aluminosilicates takes place by different mechanisms, mainly ion exchange, precipitation, and electrostatic surface interaction. The radionuclides content was determined by {gamma}-spectrometry, and X-ray diffraction was used to learn whether the solids maintained their crystallinity at different pH values.

Olguin, M.T.; Solache, M.; Iturbe, J.L. [Instituto Nacional de Investigaciones Nucleares, C.P. (Mexico)]|[Universidad Autonoma Metropolitana, C.P. (Mexico)] [and others

1996-09-01

138

Entrance-channel dependence of fission transients  

CERN Document Server

Fission transients describe the fission rate as it evolves towards the quasistationary value given by Kramers' formula. The nature of fission transients is dependent on the assumed initial distribution of the compound nuclei along the fission coordinate. Although the standard initial assumption of a near-spherical object leads to a transient suppression of the fission rate (fission delay), a moderate initial fissionlike deformation can reduce the magnitude of this suppression. For still larger initial deformations, transient fission enhancements are possible. Examples of this behavior are illustrated via a one-dimensional Langevin simulation. It is argued that the initial conditions are determined by the fusion dynamics and thus are entrance-channel dependent. Transient fission may be considered intermediate between statistical fission and quasifission as the rapid time scale of transient fission may not lead to an equilibrium of the angular and mass-asymmetry coordinates. The relationship between the mean fi...

Charity, R J

2004-01-01

139

Spontaneous fission  

International Nuclear Information System (INIS)

Recent experimental results for spontaneous fission half-lives and fission fragment mass and kinetic-energy distributions and other properties of the fragments are reviewed and compared with recent theoretical models. The experimental data lend support to the existence of the predicted deformed shells near Z = 108 and N = 162. Prospects for extending detailed studies of spontaneous fission properties to elements beyond hahnium (element 105) are considered. (orig.)

140

On the use of thin natural uranium film dosimetry in mineral dating by the fission track method  

International Nuclear Information System (INIS)

Three obsidian samples were irradiated in a neutron facility and their age was measured by the fission track method; using a thin uranium film dosemeter. The results were compared to others made previously on the same type of rock using conventional neutron dosimetry. The use of thin uranium film for age determination is discussed. (F.E.). 20 refs, 4 tabs

 
 
 
 
141

Spontaneous Fission  

Energy Technology Data Exchange (ETDEWEB)

The first attempt to discover spontaneous fission in uranium was made by Libby, who, however, failed to detect it on account of the smallness of effect. In 1940, Petrzhak and Flerov, using more sensitive methods, discovered spontaneous fission in uranium and gave some rough estimates of the spontaneous fission decay constant of this substance. Subsequently, extensive experimental work on the subject has been performed by several investigators and will be quoted in the various sections. Bohr and Wheeler have given a theory of the effect based on the usual ideas of penetration of potential barriers. On this project spontaneous fission has been studied for the past several years in an effort to obtain a complete picture of the phenomenon. For this purpose the spontaneous fission decay constants {lambda} have been measured for separated isotopes of the heavy elements wherever possible. Moreover, the number {nu} of neutrons emitted per fission has been measured wherever feasible, and other characteristics of the spontaneous fission process have been studied. This report summarizes the spontaneous fission work done at Los Alamos up to January 1, 1945. A chronological record of the work is contained in the Los Alamos monthly reports.

Segre, Emilio

1950-11-22

142

The topography of the nuclear fission barrier  

International Nuclear Information System (INIS)

Fission theory first developed within the framework of the liquid drop model. Shell model concepts were introduced into fission theory much later than they were in nuclear structure theory but then with spectacular success in explaining striking experimental results then emerging in actinide fission. In the last two decades the complex topography of the fission barrier that is the result of shell model theory has been a major theme in the expanding knowledge of fission, most experimental data finding a natural explanation within this theme. The development of the concept of shell model structure in the fission barrier is outlined in this review. (author). 140 refs., 35 figs

143

Fission isomers  

International Nuclear Information System (INIS)

After a short historical account of superdeformed nuclei which are able to de-excite by spontaneous fission (hence their name of fission isomers) the principles of different experimental set-ups are described. These set-ups were used to study various properties: half-life, excitation energy, spin, gyromagnetic factor, moment of inertia and quadrupole moment. The most significant values are given. Finally, the question of the various types of excited states of fission isomers is tackled. (author) 20 refs.; 17 figs

144

The discovery of fission  

International Nuclear Information System (INIS)

In this article by the retired head of the Separation Processes Group of the Chemistry Division, Atomic Energy Research Establishment, Harwell, U.K., the author recalls what he terms 'an exciting drama, the unravelling of the nature of the atomic nucleus' in the years before the Second World War, including the discovery of fission. 12 references. (author)

145

Underground mining by horizontal top slicing under concrete in the uranium mine of Oklo Fond in Gabon  

International Nuclear Information System (INIS)

This article presents the mining method adopted in the uranium mine of Oklo Fond which belongs to Comut in Gabon, i.e. a method by horizontal top slicing under concreted backfilling. It discusses in particular: - the mechanical characteristics of the different types of concreted backfilling corresponding to their composition and their admixture of cement; - methods of placing this backfilling and their progressive improvement; - the bearing of this method on the problems of selectivity

146

Influence of experimental factors on dating natural and man-made glasses by the fission track method  

International Nuclear Information System (INIS)

In age determination work by counting fission tracks the precision, besides the statistical counting error of spontaneous and induced tracks, largely depends on exact determination of the neutron dose and close maintenance of etching conditions. Slight variations in temperature and concentrations of etch batches may lead to considerable deviations. By dating several 'Early Victorian' U-stained glasses the authors discovered the fact that the spontaneous fission decay constant ?sp. f. is still not known with sufficient accuracy. Since an age of ? 150 yr can be inferred from the style of these glasses, the most probable value of ?sp. f. should lie closely to 8 x 10-17 yr-1. In establishing the coincidence of two geological events the uncertainty of ?sp. f. may be less important. The authors were able to confirm the agreement in the age of moldavites (14.9 ± 1.6) x 106 yr and the Ries Crater glass (14.9 ± 1.5) x 106 yr. (author)

147

Fission in Spallation Reactions  

Science.gov (United States)

Some properties of fission in spallation reactions in the GeV range are examined. It is shown on theoretical grounds that the charge, mass and excitation energy are strongly fluctuating. The range of accessible excitation energies is determined. The ability of a particular intranuclear plus evaporation model, namely the INCL4+ABLA model to describe the existing data is demonstrated. In view of the numerous parameters used in the fission model, the sensitivity of the results to these parameters is investigated. It is shown that, due to the complexity of the fission modeling, it is hard to get reliable information on the level density parameters at high excitation energy. Finally the influence of the nature of the incident projectile is shortly discussed.

Cugnon, J.; Aoust, Th.; Boudard, A.

2008-04-01

148

Current topics in nuclear fission research  

International Nuclear Information System (INIS)

This year is the 50th anniversary of the discovery of nuclear fission by Otto Hahn and Fritz Strassmann. From the point of view of the fundamental physics research, nuclear fission was one of the oldest motivation of the nuclear physics. In this 50 years, many aspects of nuclear fission phenomena have been investigated. Owing to the nature that the nuclear fission involves a dynamical treatment of finite-many-body systems, there have been always some challenging new topics in nuclear fission research in these 50 years. By solving these problems one by one, we have come closer to the understanding of the nuclear fission dynamics. As examples of current topics in nuclear fission research, I will choose and discuss the following three: 1) Cold fission phenomena. 2) Heavy-particle radioactivity phenomena. 3) Transient effect phenomena. (author)

149

Dissolution studies of natural analogues spent fuel and U(VI)-Silicon phases of and oxidative alteration process  

International Nuclear Information System (INIS)

In order to understand the long-term behavior of the nuclear spent fuel in geological repository conditions, we have performed dissolution studies with natural analogues to UO2 as well as with solid phases representatives of the oxidative alteration pathway of uranium dioxide, as observed in both natural environment and laboratory studies. In all cases, we have studied the influence of the bicarbonate concentration in the dissolution process, as a first approximation to the groundwater composition of a granitic environment, where carbonate is one of the most important complexing agents. As a natural analogue to the nuclear spent fuel some uraninite samples from the Oklo are deposit in Gabon, where chain fission reactions took place 2000 millions years ago, as well as a pitchblende sample from the mine Fe ore deposit, in Salamanca (spain) have been studied. The studies have been performed at 25 and 60 deg C and 60 deg C, and they have focussed on the determination of both the thermodynamic and the kinetic properties of the different samples studied, using batch and continuous experimental methodologies, respectively. (Author)

150

Spin determination of fission resonances  

International Nuclear Information System (INIS)

Spin determination of neutron induced fission resonances is important to the basic understanding of the fission process and, in particular, to understanding both the role and the nature of the fission transition states. Of the several experimental techniques which have been used in the past for determining spins in fissionable nuclei, only the technique using a polarized neutron beam and a polarized target seems to be definitive. For the fissionable isotopes it is assumed that certain fission resonance properties, such as ?, the number of neutrons emitted in fission, and the kinetic energy and mass distribution of the fragments are strongly correlated with either the spin J or the spin projection K on the nuclear symmetry axis. However, the question of whether J or K plays the definitive role remains unanswered. Although it has also long been assumed that K = J for nuclear ground states, even this assumption has never been experimentally verified. Experimental techniques and results using polarized neutrons and polarized and aligned targets are reviewed and the program currently being pursued in this area is described. Also recommendations are made for future measurements which are seen as most important in contributing to an understanding of the fission process

151

Bimodal fission  

International Nuclear Information System (INIS)

In recent years, the author has measured the mass and kinetic-energy distributions from the spontaneous fission of 258Fm, 259Md, 260Md, 258No, 262No, and 260[104]. All are observed to fission with a symmetrical division of mass, whereas the total-kinetic-energy (TKE) distributions strongly deviated from the Gaussian shape characteristically found in the fission of all other actinides. When the TKE distributions are resolved into two Gaussians, the constituent peaks lie near 200 and near 233 MeV. He concludes two modes or bimodal fission is occurring in five of the six nuclides studied. Both modes are possible in the same nuclide, but one generally predominates. He also concludes the low-energy but mass-symmetrical model is likely to extend to far heavier nuclei; while the high-energy mode will be restricted to a smaller region, a region of nuclei defined by the proximity of the fragments to the strong neutron and proton shells in 132Sn

152

From transmutation to fission  

International Nuclear Information System (INIS)

The article explains the historical background of the discovery of nuclear fission, observed by O. Hahn and F. Strassmann. Becquerel's discovery of the natural radioactivity, in 1986, had made physicists waver in their belief in their fundamental concept which then was based on classical mechanics, Maxwell's electrodynamics, and Gibbs' theory of thermodynamics. The novel research activities then started have led to the discoveries and findings by E. Rutherford, Pierre and Marie Curie, F. Soddy, E. Fermi, and many other scientists. The article traces back the events which span the first observed transmutations as a result of studies on the nature of emanations, the first application of alpha particles for exploring the atomic structure, the development of particle accelerators, the discovery of artificial radioactivity, and the application of neutrons for inducing nuclear fission processes. (RB)

153

Budding and fission of vesicles.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report on budding and fission of protein-free vesicles swollen from a natural lipid mixture of bovine brain sphingomyelins. Budding was induced by increasing the area-to-volume ratio through heating. Morphological changes were monitored by phase contrast microscopy and correlated with the thermal behavior of the bilayer by differential scanning calorimetry. Freeze fracture electron microscopy revealed that budding and fission are not restricted to giant vesicles but also occur on length sc...

Do?bereiner, H. G.; Ka?s, J.; Noppl, D.; Sprenger, I.; Sackmann, E.

1993-01-01

154

Fission isomers  

International Nuclear Information System (INIS)

The single particle (shell) effects giving rise to spontaneously fissioning shape isomeric states are outlined. Empirical data on static properties such as excitation energies and barrier heights are reviewed and compared with potential energy surface calculations. Half-life and branching ratio information are related to barrier penetration. Spectroscopic information on moments of inertia, quadrupole moments, spins and g-factors are reviewed. Recent developments on shape isomerism in other regions of the Chart of the Nuclides are presented. 22 refs., 5 figs

155

An evaluation of the dissolution process of natural uranium ore as an analogue of nuclear fuel  

International Nuclear Information System (INIS)

The assumption of congruent dissolution of uraninite as a mechanism for the dissolution behaviour of spent fuel was critically examined with regard to the fate of toxic radionuclides. The fission and daughter products of uranium are typically present in spent unreprocessed fuel rods in trace abundances. The principles of trace element geochemistry were applied in assessing the behaviour of these radionuclides during fluid/solid interactions. It is shown that the behaviour of radionuclides in trace abundances that reside in the crystal structure can be better predicted from the ionic properties of these nuclides rather than from assuming that they are controlled by the dissolution of uraninite. Geochemical evidence from natural uranium ore deposits (Athabasca Basin, Northern Territories of Australia, Oklo) suggests that in most cases the toxic radionuclides are released from uraninite in amounts that are independent of the solution behaviour of uranium oxide. Only those elements that have ionic and thus chemical properties similar to U4+, such as plutonium, americium, cadmium, neptunium and thorium can be satisfactorily modelled by the solution properties of uranium dioxide and then only if the environment is reducing. (84 refs., 7 tabs.)

156

Calculations of fission rates for r-process nucleosynthesis  

Energy Technology Data Exchange (ETDEWEB)

Fission plays an important role in the r-process which is responsible not only for the yields of transuranium isotopes, but may have a strong influence on the formation of the majority of heavy nuclei due to fission recycling. We present calculations of beta-delayed and neutron-induced fission rates, taking into account different fission barriers predictions and mass formulae. It is shown that an increase of fission barriers results naturally in a reduction of fission rates, but that nevertheless fission leads to the termination of the r-process. Furthermore, it is discussed that the probability of triple fission could be high for A>260 and have an effect on the formation of the abundances of heavy nuclei. Fission after beta-delayed neutron emission is discussed as well as different aspects of the influence of fission upon r-process calculations.

Panov, I.V. [University of Basel, Klingelbergstr. 82, CH-4056 Basel (Switzerland) and Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya 25, 117259 Moscow (Russian Federation)]. E-mail: igor.panov@itep.ru; Kolbe, E. [University of Basel, Klingelbergstr. 82, CH-4056 Basel (Switzerland); Pfeiffer, B. [Institute fuer Kernchemie, Fritz-Strassman-Weg 2, D-55128 Mainz (Germany); Rauscher, T. [University of Basel, Klingelbergstr. 82, CH-4056 Basel (Switzerland); Kratz, K.-L. [Institute fuer Kernchemie, Fritz-Strassman-Weg 2, D-55128 Mainz (Germany); Thielemann, F.-K. [University of Basel, Klingelbergstr. 82, CH-4056 Basel (Switzerland)

2005-01-24

157

Calculations of fission rates for r-process nucleosynthesis  

International Nuclear Information System (INIS)

Fission plays an important role in the r-process which is responsible not only for the yields of transuranium isotopes, but may have a strong influence on the formation of the majority of heavy nuclei due to fission recycling. We present calculations of beta-delayed and neutron-induced fission rates, taking into account different fission barriers predictions and mass formulae. It is shown that an increase of fission barriers results naturally in a reduction of fission rates, but that nevertheless fission leads to the termination of the r-process. Furthermore, it is discussed that the probability of triple fission could be high for A>260 and have an effect on the formation of the abundances of heavy nuclei. Fission after beta-delayed neutron emission is discussed as well as different aspects of the influence of fission upon r-process calculations

158

Thermal annealing of fission tracks in synthetic apatites  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Synthetic analogues of poor-silicated natural apatites have been doped with uranium. These minerals have been irradiated with a thermal neutron dose in the aim to induce the 235U fission and to obtain a fission track population. Thermal annealing experiments have been performed on the fission track population and allow us to compare the ability of the synthetic minerals to anneal such nuclear damages with their natural analogues. The thermal of the fission tracks in the synthetic minerals nee...

Carpena, J.; Lacout, Jean-louis

2010-01-01

159

Crystal chemistry and radiation-induced amorphization of P-coffinite from the natural fission reactor at Bangombe Gabon  

International Nuclear Information System (INIS)

Phosphorous-rich coffinite, U(Si,P)O4.H2O, from the natural nuclear reactor at Bangombe, Gabon (depth 12.25 m), has been examined as an important primary mineral and alteration product of uraninite under reducing conditions. Based on electron microprobe analyses (EMPA) and textural relationships, two distinct types of coffinite have been identified: (1) P-coffinite-(i) [with P2O5 and (REE+Y)2O3 as high as 9.84 and 8.66 wt%, respectively] replaces uraninite and has a chemical formula of (U0.84±0.05 Y,REE0.12±0.02Ca0.10±0.02Th0.003±0.002= )?1.07±0.06 (Si0.41±0.07P0.34±0.03S0.08±0.05)#SIGMA= #0.84±0.05O4 and; (2) coffinite-(ii), lacks uraninite inclusions [with P2O5 and (Y+REE)2O3 up to 1.45 and 1.79 wt%, respectively] and has a chemical formula of (U0.78±0.02Ca0.05±0.003Y,REE0.03±0.01Th= 0.002±0.001)?0.87±0.02(Si1.02±0.= 02P0.06±0.01)?1.08±0.01O4. The EMPA elemental maps reveal a homogeneous distribution P,Si,Nd, and U in P-coffinite-(i). Charge-balance calculations indicate that S substitution for Si in the coffinite structure results in the enhancement of the incorporation of P and REEs. High EMPA totals (95-100 wt%), suggess (95-100 wt%), suggest that water is not an essential component of the coffinite structure. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveal the decomposition of single crystals of uraninite into 50 nm grains during the alteration process to P-coffinite-(i). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns confirm that P-coffinite-(i) has the coffinite structure. However, P-coffinite-(i) has a large amorphous fraction probably due to radiation damage caused by alpha-decay events. The calculated cumulative dose for both types of coffinite varies from 1.6-1.9 * 1018 (?-decay events/mg), which is equivalent to 134.2-161.5 displacement per atom (dpa) that have accumulated most probably during the past 800 million years. (authors)

160

Energy released in fission  

International Nuclear Information System (INIS)

The effective energy released in and following the fission of U-235, Pu-239 and Pu-241 by thermal neutrons, and of U-238 by fission spectrum neutrons, is discussed. The recommended values are: U-235 ... 192.9 ± 0.5 MeV/fission; U-238 ... 193.9 ± 0.8 MeV/fission; Pu-239 ... 198.5 ± 0.8 MeV/fission; Pu-241 ... 200.3 ± 0.8 MeV/fission. These values include all contributions except from antineutrinos and very long-lived fission products. The detailed contributions are discussed, and inconsistencies in the experimental data are pointed out. In Appendix A, the contribution to the total useful energy release in a reactor from reactions other than fission are discussed briefly, and in Appendix B there is a discussion of the variations in effective energy from fission with incident neutron energy. (author)

 
 
 
 
161

Synthesis report on the relevant diffusion coefficients of fission products and helium in spent nuclear fuels  

International Nuclear Information System (INIS)

This document corresponds to the deliverable D2 of the Work Package 1 of the 'Spent Fuel Stability under repository conditions' (SFS) European project. It constitutes a synthesis report on relevant diffusion coefficients of fission products and helium in spent nuclear fuels at high and low temperatures. Coefficients corresponding to thermally activated diffusion were reviewed from literature data for O, U (self-diffusion coefficients), fission gases and other fission products. Data showed that thermal diffusion was irrelevant at temperatures expected in repository conditions. The occurrence of diffusion enhanced by alpha self-irradiation was studied through different theoretical approaches. A 'best estimate' value of the alpha self-irradiation diffusion coefficient, D (m2.s-1), is proposed. It is extrapolated from enhanced diffusion under irradiation observed in reactor and would be proportional to the volume alpha activity in the spent nuclear fuel, Av (Bq.m-3) as: D/Av ? 2.10-41 (m5)The migration of stable Pb in Oklo's uraninites was studied in order to validate the proposed diffusion coefficient. The obtained value is one order of magnitude higher than the theoretical proposed value. As for He behaviour in spent nuclear fuel, a few data are today available in open literature. The document will be completed as soon as new experimental results are available. (authors)

162

Thorium-uranium fission radiography  

Science.gov (United States)

Results are described for studies designed to develop routine methods for in-situ measurement of the abundance of Th and U on a microscale in heterogeneous samples, especially rocks, using the secondary high-energy neutron flux developed when the 650 MeV proton beam of an accelerator is stopped in a 42 x 42 cm diam Cu cylinder. Irradiations were performed at three different locations in a rabbit tube in the beam stop area, and thick metal foils of Bi, Th, and natural U as well as polished silicate glasses of known U and Th contents were used as targets and were placed in contact with mica which served as a fission track detector. In many cases both bare and Cd-covered detectors were exposed. The exposed mica samples were etched in 48% HF and the fission tracks counted by conventional transmitted light microscopy. Relative fission cross sections are examined, along with absolute Th track production rates, interaction tracks, and a comparison of measured and calculated fission rates. The practicality of fast neutron radiography revealed by experiments to data is discussed primarily for Th/U measurements, and mixtures of other fissionable nuclei are briefly considered.

Haines, E. L.; Weiss, J. R.; Burnett, D. S.; Woolum, D. S.

1976-01-01

163

HAC and fission reactors  

International Nuclear Information System (INIS)

In the fission process, newly formed fission products undergo hot atom reactions due to their energetic recoil and abnormal positive charge. The hot atom reactions of the fission products are usually accompanied by secondary effects such as radiation damage, especially in condensed phase. For reactor safety it is valuable to know the chemical behaviour and the release behaviour of these radioactive fission products. Here, the authors study the chemical behaviour and the release behaviour of the fission products from the viewpoint of hot atom chemistry (HAC). They analyze the experimental results concerning fission product behaviour with the help of the theories in HAC and other neighboring fields such as radiation chemistry. (Auth.)

164

Fifty years of nuclear fission: Nuclear data and measurements series  

Energy Technology Data Exchange (ETDEWEB)

This report is the written version of a colloquium first presented at Argonne National Laboratory in January 1989. The paper begins with an historical preamble about the events leading to the discovery of nuclear fission. This leads naturally to an account of early results and understanding of the fission phenomena. Some of the key concepts in the development of fission theory are then discussed. The main theme of this discussion is the topography of the fission barrier, in which the interplay of the liquid-drop model and nucleon shell effects lead to a wide range of fascinating phenomena encompassing metastable isomers, intermediate-structure effects in fission cross-sections, and large changes in fission product properties. It is shown how study of these changing effects and theoretical calculations of the potential energy of the deformed nucleus have led to broad qualitative understanding of the nature of the fission process. 54 refs., 35 figs.

Lynn, J.E.

1989-06-01

165

Fifty years of nuclear fission: Nuclear data and measurements series  

International Nuclear Information System (INIS)

This report is the written version of a colloquium first presented at Argonne National Laboratory in January 1989. The paper begins with an historical preamble about the events leading to the discovery of nuclear fission. This leads naturally to an account of early results and understanding of the fission phenomena. Some of the key concepts in the development of fission theory are then discussed. The main theme of this discussion is the topography of the fission barrier, in which the interplay of the liquid-drop model and nucleon shell effects lead to a wide range of fascinating phenomena encompassing metastable isomers, intermediate-structure effects in fission cross-sections, and large changes in fission product properties. It is shown how study of these changing effects and theoretical calculations of the potential energy of the deformed nucleus have led to broad qualitative understanding of the nature of the fission process. 54 refs., 35 figs

166

Dynamics of Coulomb fission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A general formalism is described for the treatment of Coulomb fission, within the framework of the semiquantal theory. We develop a model for the fission probabilities of levels excited in Coulomb excitation. This model contains penetration of the double-humped fission barrier, competition from gamma and neutron emission, and the spreading of the collective states into noncollective compound states. For 74184W + 92238U, the fission probability at ?c.m.=180° is increased by a factor of 3...

Kruse, Hans; Pinkston, W. T.; Greiner, Walter; Oberacker, Volker

2006-01-01

167

Neutrons and fission  

International Nuclear Information System (INIS)

The contribution that neutrons have made in the last decade to a better understanding of fission is reviewed with stressing the most recent aspects of this contribution which have been quite important. Special emphasis is put on the double-humped fission barrier situation, the macroscopic-microscopic approach to the calculation of fission barriers is presented together with the general consequences of double-humped barrier shapes; a comparison is also made between calculated and measured fission barrier parameters. More specific properties of neutron-induced fission relevant to the barrier shape are discussed. Fission induced by resonance neutrons is examined with special attention given to properties connected to the Bohr theory of fission exit channels and to intermediate structure effects. The existence and interpretation of other structure effects, in the near-threshold fission cross sections, are discussed. Some results on fission dynamics are presented: for the spontaneous fission half-lives of even-even nuclei, taking account of more accurate calculations of mass inertia parameter, and for damping effects in the low energy fission of 240Pu. Emphasis is put mainly on the fission mechanism for actinide nuclei and how it varies with the nucleus, its excitation energy and the quantum numbers J, ?, and K of the fissioning state. In contrast, very little attention is paid to a detailed description of the fission properties of a given nuclear state: for this reason, fission induced by thermal neutrons is ignored here, and the subject of neutrons emitted in fission is not treated as such

168

Fission of metallic clusters  

International Nuclear Information System (INIS)

The fission of sodium clusters is discussed using Nuclear Physics method. After presenting the Liquid Drop Model for spherical jelly clusters, we introduce shell corrections and compare the obtained energies with self-consistent quantal results. Fission is studied evaluating Q-values and fission barriers, with the shell correction method and the Two Center Shell Model. The threshold of stability with respect to fission is predicted within the Liquid Drop Model. (author)

169

Physics and chemistry of fission  

International Nuclear Information System (INIS)

Full text: In the pleasant and hospitable atmosphere of the Kernforschungsanlage Juelich in the Federal Republic of Germany, the IAEA symposium on the Physics and Chemistry of Fission took place. Almost 200 scientists attended, 154 abstracts were submitted, and 57 papers presented, but more important than the numbers was the quality of the contributions and the progress reported at the symposium. The neutron was discovered almost 50 years ago; 40 years ago the idea of nuclear fission was born. Since then, a number of laboratories have worked hard to explain the phenomenon of fission One would expect that by now scientists would know exactly what happens in a nucleus before and during the process of fission, particularly as there are hundreds of power and research reactors in operation, and fission of uranium isotopes is the basis of their functioning. At first glance, fission seems a simple process: a neutron hits and penetrates the uranium nucleus which becomes excited, i.e. has a surplus of energy. One way to get rid of this energy is for the nucleus to split into two parts; additional products of this process are energy and more neutrons. Nature, however, seems to dislike such straightforward explanations. In the case of fission, scientists have observed a number of phenomena which disagree with a simple model. Sometimes, a nucleus will split into two parts without being 'attacked' by a neutron; this spontaneous fission opens up a new line of fission research and sns up a new line of fission research and several contributions at the symposium reported on sophisticated experiments designed to unravel some of its specific details. Sometimes, a fissioning nucleus will emit another particle: ternary fission has become a powerful tool for studying the properties of nuclei during the fission process. For the scientist, it is fascinating to observe how the nucleus behaves during fission. They invent models which are supposed to reproduce the most probable course of events leading to fission. In one of these models, the nucleus is imagined to be a very small drop of liquid; the theorist then devises many schemes that lead to its splitting into two smaller drops. It is surprising how many detailed features theorists can predict, based on such simple models, and how well these predictions have been confirmed by experiments. The symposium summarized the progress in this field, and indicated how many intricate details can be introduced into a simple liquid drop model to give better agreement with experimental results. Step by step, a picture is emerging and being continuously improved, coming closer and closer to the truth. However, the liquid drop model has several competitors of which the most prominent is the shell model. This model assumes that the constituents of the nucleus are sorted into well defined energy levels, or shells; the distribution of protons and neutrons in these shells, and their movements from one shell to another, can provide an explanation for many experimentally observed facts New theoretical results drawn from these models were reported at the symposium, together with some efforts to combine different theoretical concepts, thereby trying to create a unified picture of nuclear fission. The nucleus is too small for the scientist to be able to observe directly what is happening inside it. There is really only one way for an experimenter to study a process in a nucleus, he must measure the energies of the particles coming out of it. In the case of fission, these 'messengers' can be the fission fragments, i.e. nuclei of elements that are being created from the splitting atom, but they can also be the neutrons or gamma rays which emerge during and after the fission process. Several sessions in the symposium were devoted to reports of such studies. With the help of sophisticated electronics systems, or complicated radiochemical methods, experimenters have measured the energies of fission fragments, neutrons, gamma and X-rays, and other particles emerging from fissioning nuclei, with great accuracy. By putting t

170

Comparisons of theoretical and experimental neutron spectra, 115In(n,n') and fission rates, in the centre of three spherical natural uranium and iron shell configurations, located at BR1  

International Nuclear Information System (INIS)

Three spherical configurations of iron and uranium shells have been studied. The configurations were a 1-cm thick natural uranium shell, a 1-cm thick natural uranium shell with an inner 7-cm thick iron shell and a 1-cm thick natural uranium shell with an inner iron shell of 14-cm thickness. For the measurements, the shells were located at the centre of a hollow cavity, 100-cm in diameter, in the vertical graphite thermal column of the BR1 reactor. The central neutron spectra were calculated by means of the DTF-IV code, using the 208-group KEDAK-3 library, and by means of the ANISN code, using the 171-group VITAMIN-C library. Central neutron spectra, measured by the proton-recoil and 6Li(n,?)t spectrometry techniques, are compared to the theory between ? 100 keV and 5 MeV. Mean fission cross-sections of 240Pu, 237Np, 234U, 235U, 236U and 238U were deduced from the calculations. Their ratios with respect to 238U are compared to measurements made with NBS dual fission chambers. (Auth.)

171

Solvent extraction of some fission products using tetracycline as a complexing agent : dependence on the ph of the aqueous phase and on the nature of some inorganic anions  

International Nuclear Information System (INIS)

The behavior of tetracycline as a complexing agent in solvent extraction studies is presented. The extraction curves for the fission products 90Sr, 140Ba, 99Mo, sup(99m)Tc, 95Zr, 95Nb, 103Ru and also for U have been determined for the extraction system tetracycline-benzyl alcohol. The extraction dependence on the pH of the aqueous phase as well as on the kind of electrolyte present was examined. As a practical application, the possiblity of using the tetracycline-benzyl alcohol system for separation of the fission products present in a mixture of them, as well as for the separation of uranium from those elements, was tested. (Author)

172

Binary fission in damped rotating polytropes  

International Nuclear Information System (INIS)

The fission sequences of Gingold and Monaghan are recomputed using a significantly larger number (800) of particles in the smoothed-particle-hydrodynamic treatment. The results confirm the general mode of fission found earlier for polytropes with n = 0.5. An interesting three-pointed 'star fish' structure is now apparent before fission and a low-mass third object is formed. The nature of the disruption of n = 1.5 polytropes is clarified and discussed in relation to the problem of binary star formation. (author)

173

Nuclear friction and lifetime of induced fission  

International Nuclear Information System (INIS)

Induced nuclear fission is described as a transport process of the fission degree of freedom over the fission barrier. The lifetime of the event is defined in terms of the probability of finding the nuclear system in the potential well corresponding to the ground-state deformation. This definition appears as a natural generalization to nonstationary transport processes of the usual expression for the lifetime. Using the conservation law for the current we relate the lifetime to the time-integrated escape rate across the collective potential barrier

174

Fast and slow fission  

International Nuclear Information System (INIS)

Measurements of alpha particle induced fission of actinide nuclei and fission of the composite system 170Yb formed in 12C and 20Ne bombardments both show significantly greater neutron emission prior to fission than is consistant with current statistical models. Implications of these results are discussed in the context of possible extreme models: (1) the enhancement of fission at low excitation energies due to shell effects; (2) the inhibition of fission at high excitations due to a limiting of the fission width; and (3) the possibility of significant neutron emission during the descent from saddle to scission. In addition the apparent incompatability between current models of incomplete fusion processes and the analysis of light heavy ion induced fission which ignore incomplete fusion is discussed

175

Status of geological studies undertaken in France on the Oklo deposit since the Libreville Symposium  

International Nuclear Information System (INIS)

The questions posed since the Libreville symposium can be grouped under four headings: origin of the deposits and the over-concentrations; degree of burial reached by the series; temperatures attained during the reactions, and effects of the reactions; perturbations subsequent to the reactions. These questions have been the subject of multidisciplinary investigations, including conventional petrographic and mineralogical investigations (Cogema, Fontenay-aux-Roses and the Centre for Sedimentology and Geochemistry of the Surface, Strasbourg), a study of fluid inclusions (Centre for Petrographic and Geochemical Research, Nancy), studies of organic material (French Institute of Petroleum, Rueil, and Cerchar, Verneuil-en-Halatte) and a study of fission traces (Rene Barnas Laboratory, Orsay). (author)

176

Fission level densities  

International Nuclear Information System (INIS)

Fission level densities (or fissioning nucleus level densities at fission saddle deformations) are required for statistical model calculations of actinide fission cross sections. Back-shifted Fermi-Gas Model, Constant Temperature Model and Generalized Superfluid Model (GSM) are widely used for the description of level densities at stable deformations. These models provide approximately identical level density description at excitations close to the neutron binding energy. It is at low excitation energies that they are discrepant, while this energy region is crucial for fission cross section calculations. A drawback of back-shifted Fermi gas model and traditional constant temperature model approaches is that it is difficult to include in a consistent way pair correlations, collective effects and shell effects. Pair, shell and collective properties of nucleus do not reduce just to the renormalization of level density parameter a, but influence the energy dependence of level densities. These effects turn out to be important because they seem to depend upon deformation of either equilibrium or saddle-point. These effects are easily introduced within GSM approach. Fission barriers are another key ingredients involved in the fission cross section calculations. Fission level density and barrier parameters are strongly interdependent. This is the reason for including fission barrier parameters along with the fission level densities in the Starter File. The recommended file is maslov.dat - fission barrier parameters. Recent version of actinide fission barrier data obtained in Obninsk (obninsk.dat) should only be considered as a guide for selection of initial parameters. These data are included in the Starter File, together with the fission barrier parameters recommended by CNDC (beijing.dat), for completeness. (author)

177

Thermal annealing of fission tracks in synthetic apatites  

International Nuclear Information System (INIS)

Synthetic analogues of poor-silicated natural apatites have been doped with uranium. These minerals have been irradiated with a thermal neutron dose in the aim to induce the 235U fission and to obtain a fission track population. Thermal annealing experiments have been performed on the fission track population and allow us to compare the ability of the synthetic minerals to anneal such nuclear damages with their natural analogues. The thermal of the fission tracks in the synthetic minerals need more time and/or higher temperature to reach the same annealing rate as in the natural analogues. The alpha damage present in the natural analogues seems to enhance the thermal annealing of fission tracks.

178

Dynamics of fluctuations for compound nucleus fission and fast fission  

International Nuclear Information System (INIS)

Dynamical calculations of the fluctuations associated with the collective variables involved in fission are presented without invoking the local harmonic approximation. The calculated mass distribution for compound nucleus-fission and fast fission are qualitatively compared. (orig.)

179

Intermediate energy nuclear fission  

International Nuclear Information System (INIS)

Nuclear fission has been investigated with the double-kinetic-energy method using silicon surface barrier detectors. Fragment energy correlation measurements have been made for U, Th and Bi with bremsstrahlung of 600 MeV maximum energy. Distributions of kinetic energy as a function of fragment mass are presented. The results are compared with earlier photofission data and in the case of bismuth, with calculations based on the liquid drop model. The binary fission process in U, Yb, Tb, Ce, La, Sb, Ag and Y induced by 600 MeV protons has been investigated yielding fission cross sections, fragment kinetic energies, angular correlations and mass distributions. Fission-spallation competition calculations are used to deduce values of macroscopic fission barrier heights and nuclear level density parameter values at deformations corresponding to the saddle point shapes. We find macroscopic fission barriers lower than those predicted by macroscopic theories. No indication is found of the Businaro Gallone limit expected to occur somewhere in the mass range A = 100 to A = 140. For Ce and La asymmetric mass distributions similar to those in the actinide region are found. A method is described for the analysis of angular correlations between complementary fission products. The description is mainly concerned with fission induced by medium-energy protons but is applicable also to other projectiles and energies. It is shown that the momentum and excitation energy distributions of cascade residuals leading to fission can be extracted. (Author)

180

Fission fragment angular distributions  

International Nuclear Information System (INIS)

A statistical scission model (SSM) is developed and applied to fragment angular distributions from alpha- and heavy-ion-induced fission. Although the formal equations for fragment angular distributions in the statistical scission and transition state models are of the same structure, the variances in the distribution of angular momentum projections on the fission direction are established at very different stages of the fission process in the two models. The SSM predicts angular distributions in reasonable agreement with those measured for heavy-ion-induced fission of some reaction systems where the fission barrier was vanished or is very small relative to the nuclear temperature. For a number of these systems, the transition state model (TSM) is inapplicable. The SSM, however, predicts the variance to have too weak an energy dependence, possibly indicating dynamical effects. Inclusion of pre- and post-scission particle emission and asymmetric fission does not remove this discrepancy. The TSM gives a better representation of fragment angular distributions from alpha-particle-induced fission of several targets (low-spin systems) as a function of fissility parameter. The very large experimental variances, observed for systems with large spin and no fission barrier, are inconsistent with the concepts of fast or preequilibrium fission, occurring on a time scale too short for relaxation of the tilting mode. Fragment spins from ?-ray multiplicities are compared with predictions of the two above models

 
 
 
 
181

Fission products experimental programme  

International Nuclear Information System (INIS)

The 'Fission Products (FPs)' experimental programme was carried out from 1998 to 2004 in CEA/Valduc Apparatus B in the framework of the IRSN-AREVA NC Common Interest Program dealing with 'burnup credit' studies. It aims at compensating for the lack of critical experiments with FPs in the literature and validating a selection of 6 stable, non-volatile, fission products representing half of the irradiated fuel absorption of all fission products: 103Rh, 133Cs, 143Nd, 149Sm, 152Sm and 155Gd. This paper provides a more exhaustive approach of the keff results, uncertainties and tendencies associated with the different steps of the 'Fission Products' programme. (authors)

182

Is channeling of fission tracks taking place?  

CERN Document Server

A single crystal of natural zircon which is sliced to have (010) basal plane and thinned by ion thinning is electron microscopically observed after slow neutron irradiation to ascertain whether channeling of the nuclear fission fragments is taking place or not. A fairly large number of the induced fission tracks are recognized at low magnification images where a considerable number of them are parallel to low-index lattice planes such as 100, 001, 101, 301, 103 though their directions changed some time up to several degrees. High resolution images of fission tracks often show a variety of zigzag passing of the tracks along low-index lattice planes in atomistic level. The rate of the tracks which are parallel to these low-index lattice planes is fairly high as about 45%, which strongly suggests that channeling of the fission tracks is taking place.

Yada, K

1999-01-01

183

The nuclear fission  

International Nuclear Information System (INIS)

The nuclear fission process considering initially the formation of compound nucleus and finishing with radioactive decay of fission products is studied. The process is divided in three parts which consist of the events associated to the nucleus of intermediate transitional state, the scission configuration, and the phenomenum of post scission. (M.C.K.)

184

Fission gas detection system  

Science.gov (United States)

A device for collecting fission gas released by a failed fuel rod which device uses a filter to pass coolant but which filter blocks fission gas bubbles which cannot pass through the filter due to the surface tension of the bubble.

Colburn, Richard P. (Pasco, WA)

1985-01-01

185

Muon-induced fission  

International Nuclear Information System (INIS)

A review of recent experimental results on negative-muon-induced fission, both of 238U and 232Th, is given. Some conclusions drawn by the author are concerned with muonic atoms of fission fragments and muonic atoms of the shape isomer of 238U. (author)

186

Fission Xenon on Mars  

Science.gov (United States)

Fission Xe components due to Pu-244 decay in the early history of Mars have been identified in nakhlites; as in the case of ALH84001 and Chassigny the fission gas was assimilated into indigenous solar-type Xe. Additional information is contained in the original extended abstract.

Mathew, K. J.; Marti, K.; Marty, B.

2002-01-01

187

Fission Spectrum Related Uncertainties  

Energy Technology Data Exchange (ETDEWEB)

The paper presents a preliminary uncertainty analysis related to potential uncertainties on the fission spectrum data. Consistent results are shown for a reference fast reactor design configuration and for experimental thermal configurations. However the results obtained indicate the need for further analysis, in particular in terms of fission spectrum uncertainty data assessment.

G. Aliberti; I. Kodeli; G. Palmiotti; M. Salvatores

2007-10-01

188

Study of hypernuclei fission  

International Nuclear Information System (INIS)

This work is about PS177 experience made on LEAR machine at CERN in 1988. The annihilation reaction of anti protons on a target of Bismuth or Uranium is studied. Lambda particles are produced by this reaction, in the nucleus in 2% of cases 7.1 10-3 hypernuclei by stopped antiproton in the target are produced. The prompt hypernucleus fission probability of uranium is 75% and that of Bismuth 10%. The mass distribution of fission fragments is symmetrical ((? the excitation energy of the nucleus is very high). If the nucleus hasn't fissioned, the non-mesonic lambda decay, gives it an energy of 100 MeV, what allows to fission later. This fission is delayed because the hypernucleus lifetime is 1.3+0.25-0.21 10-10 sec for Bismuth

189

Fission energy surfaces and ternary fission  

International Nuclear Information System (INIS)

Complete text of publication follows. A new parametrization, able to describe typical shapes occuring in the binary, ternary, and multicluster (1) accompanied fission processes has been developed. It specifies the nuclear shape in terms of seven smoothly joined portions of quadratic surfaces of revolution. In this way we can reproduce even four aligned fragments in touch at the most probable (axially symmetric) scission configuration. Our knowledges about fission phenomena were substantially enriched during the last years, as a consequence of experiments (2) performed by using triple ? coincidences in a modern large array of ?-ray detectors (GAMMASPHERE). A particularly interesting feature, observed (3,4) in 10Be accompanied cold fission of 252Cf is related to the width of the light particle ?-ray spectrum. For example, the 3.368 MeV ? line of 10Be, with a lifetime of 125 fs is not Doppler-broadened, as it should be if it would be emitted when 10Be is in flight (taking about 1 ns to reach the detector). A plausible suggestion was made, that the absence of Doppler broadening is related to a trapping of 10Be in a potential well of nuclear molecular character. We would like to extend to cold ternary fission the unified approach (5) of cold binary fission, cluster radioactivity, and ?-decay. A three-center phenomenological model, able to explain qualitatively the recently obtained experimental results (3,4) concerning the quasimolecular stage of a light-particle accompanied fission process has been developed. It was derived from the liquid drop model under the assumption that the aligned configuration, with the emitted particle between the light and heavy fragment, is reached by increasing continuously the separation distance, while the radii of the heavy fragment and of the light particle are kept constant. In such a way, a new minimum of a short-lived molecular state appears in the deformation energy at a separation distance very close to the touching point. This minimum allows the existence of a short-lived quasi-molecular state, decaying into the three final fragments. The half-lives of some quasimolecular states which could be formed in the 10Be and 12C accompanied fission of 252Cf are roughly estimated to be the order of 1 ns, and 1 ms, respectively. (author)

190

Nucleon-induced fission at intermediate energies  

Energy Technology Data Exchange (ETDEWEB)

The absence of a satisfactory theoretical description to predict isotope yields as well as the need for experimental fragment mass and charge distributions at intermediate-energies form the motivation of this work. Like the objects under study, the research presented in this thesis consists two main parts. Part 1 concerns an activation experiment that has been performed at the 'Kernfysisch Versneller Instituut' (Nuclear Physics Accelerator Institute) in Groningen, Netherlands, using the AGOR cyclotron. Fission product yields have been measured resulting from 190 MeV proton-induced fission of {sup nat}W, 197Au, {sup nat}Pb, 208Pb and 232Th. In Chapter 2 the experimental set up is discussed, followed in Chapter 3 by a description of the data analysis. The results on the reconstructed mass yields and the total fission cross sections are presented in Chapter 4. Part 2 is of a theoretical nature. The objective is to compute fission product mass yields from intermediate-energy nucleon-induced reactions. In the approach presented here, two stages can be distinguished. In the first stage the fission cross section is determined for the various fissioning isotopes as a function of their excitation energy in competition with other processes like pre-equilibrium decay and particle evaporation. ALICE-91 is a nuclear reaction code that takes care of this first stage. The second stage consists of constructing the total fission-fragment mass and charge distributions from the different contributions of all the equilibrated fissioning systems. Hence, a model is needed that gives a prediction for the fission-product mass yields in a large range of mass, charge, and excitation energy of the fissioning nucleus. For this purpose, the multi-modal random neck-rupture model by Brosa is extended with temperature-dependent shell and pairing corrections and a temperature-dependent LDM. The combination of ALICE-91 and the modified Brosa approach is used for the analysis of the experiments given in the first part of this thesis as well as other available experimental results. Chapter 5 is dedicated to the extension of the Brosa model. The multi-chance fission treatment in ALICE-91 is discussed in Chapter 6. Chapter 7 contains the coupling between ALICE-91 and the revised Brosa model as well as the results obtained in this manner. Finally, a summary and outlook can be found in Chapter 8. Appendix A contains a list explaining all the abbreviations used in this manuscript. Parts of this work have already been published in journals and conference proceedings. refs.

Duijvestijn, M.C

2000-09-29

191

Study on fission blanket fuel cycling of a fusion-fission hybrid energy generation system  

Science.gov (United States)

This paper presents a preliminary study on neutron physics characteristics of a light water cooled fission blanket for a new type subcritical fusion-fission hybrid reactor aiming at electric power generation with low technical limits of fission fuel. The major objective is to study the fission fuel cycling performance in the blanket, which may possess significant impacts on the feasibility of the new concept of fusion-fission hybrid reactor with a high energy gain (M) and tritium breeding ratio (TBR). The COUPLE2 code developed by the Institute of Nuclear and New Energy Technology of Tsinghua University is employed to simulate the neutronic behaviour in the blanket. COUPLE2 combines the particle transport code MCNPX with the fuel depletion code ORIGEN2. The code calculation results show that soft neutron spectrum can yield M > 20 while maintaining TBR >1.15 and the conversion ratio of fissile materials CR > 1 in a reasonably long refuelling cycle (>five years). The preliminary results also indicate that it is rather promising to design a high-performance light water cooled fission blanket of fusion-fission hybrid reactor for electric power generation by directly loading natural or depleted uranium if an ITER-scale tokamak fusion neutron source is achievable.

Zhou, Z.; Yang, Y.; Xu, H.

2011-10-01

192

Fiftieth anniversary of nuclear fission  

International Nuclear Information System (INIS)

Few discoveries have influenced mankind and few discoveries have had such an intricate history as that of nuclear fission. Thus it is natural that it is remembered and commemorated in many places on its fiftieth anniversary. The author participated in the early phases in Rome and later in the USA and he knew well most of the principals, except Fritz Strassmann. He gives a short outline; a detailed history would require many hours and would be laborious to follow in detail. He begins the story with the first neutron bombardment of uranium

193

The dependence of the fission cross-sections for heavy nuclei on neutron energy in the ''plateau'' region  

International Nuclear Information System (INIS)

The paper presents a wide set of experimental data on fission probability and fast neutron cross-sections for Pa-Cf nuclei in the first ''plateau'' region (En = 1-7 MeV). Energy dependences of the fission cross-sections, fission probabilities and compound nucleus formation cross-sections are compared. Some conformity to natural laws governing the fission probability energy dependence as function of nucleon composition of fissionable nuclei is identified. 4 figs, 1 tab

194

The influence of fission dose on fission gas atom trapping  

International Nuclear Information System (INIS)

The trapping of fission gas atoms in uranium dioxide irradiated to low doses was examined. It was possible to interpret the experimentally deduced trapping in terms of fission gas atoms being immobilised at very small vacancy clusters produced by fission fragment damage. Using this model experimental evidence for the thermal solubility of fission gas was investigated and was shown to be in good accord with the SINGAR fission gas release model and its representation of thermal solubility. (author)

195

Fission fragment angular distributions  

International Nuclear Information System (INIS)

The first fission fragment angular distributions were observed by Winhold et al. in 1952 for the photofission of thorium. They found angular distributions peaked towards 90 degrees with respect to the ?-beam direction. Corresponding angular distribution measurements were soon reported for fission induced by neutrons and charged particles (see Vaz and Alexander and Bond and references therein). The customary theory gives for all fragments the same angular distribution independent of their masses, yields, and kinetic energies. This subject will be treated in Section II. Recently, measurements by Wilke and Wilke et al. for photofission and by Gokhberg et al. for neutron-induced fission showed mass dependences of the fission fragment angular distributions. If these measurements are confirmed, then the contemporary theory has to be extended. Such a possible direction of extension is proposed in Section III. 16 refs., 1 fig

196

Fission products experimental programme  

Energy Technology Data Exchange (ETDEWEB)

The 'Fission Products (FPs)' experimental programme was carried out from 1998 to 2004 in CEA/Valduc Apparatus B in the framework of the IRSN-AREVA NC Common Interest Program dealing with 'burnup credit' studies. It aims at compensating for the lack of critical experiments with FPs in the literature and validating a selection of 6 stable, non-volatile, fission products representing half of the irradiated fuel absorption of all fission products: {sup 103}Rh, {sup 133}Cs, {sup 143}Nd, {sup 149}Sm, {sup 152}Sm and {sup 155}Gd. This paper provides a more exhaustive approach of the k{sub eff} results, uncertainties and tendencies associated with the different steps of the 'Fission Products' programme. (authors)

Leclaire, N. [Institut de Radioprotection et de Surete Nucleaire IRSN, BP 17, 92262 Fontenay-aux-Roses Cedex (France); Anno, J. [Commissariat a l' Energie Atomique CEA, Institut de Protection et de Surete Nucleaire (CEA/IPSN) (France); Girault, E. [Commissariat a l' Energie Atomique CEA, Centre de Valduc, 21120 Is-Sur-Tille (France); Letang, E. [Institut de Radioprotection et de Surete Nucleaire IRSN, BP 17, 92262 Fontenay-aux-Roses Cedex (France)

2006-07-01

197

Characteristics of Coulomb fission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Within an extended semiquantal theory we perform large-sized coupled-channel calculations involving 260 collective levels for Coulomb fission of 238U. Differential Coulomb fission cross sections are studied as a function of bombarding energy and impact parameter for several projectiles. In the Xe + U case, total cross sections are also given. We find a strong dependence on projectile charge number, PCF(180°)?(Zp)6 in the region 50?Zp?92 for a fixed ratio E/ECoul, which migh...

Oberacker, Volker; Greiner, Walter; Kruse, Hans; Pinkston, William T.

2006-01-01

198

Potentials of fissioning plasmas  

International Nuclear Information System (INIS)

equilibrium of the generation and destruction of such actinides at fission when being fed back into the reactor. An equilibrium of the generation and destruction of such actinides can thus be achieved. These characteristics and the unique radiative properties led to the expectation that the gas-phase fission reactor could feature improved safety, safeguarding and economy, in addition to new technologies such as processing, photochemistry and the transmission of power over large distances in space

199

Natural occurring radioactive substances. Vol. 1  

International Nuclear Information System (INIS)

Naturally occurring radioactive substances produced by cosmic rays of those of terrestrial origin are surveyed. The different radioactive decay series are discussed. Special emphasis is given to the element radium as regards its properties and distribution in different environmental samples. The properties of naturally occurring k-40 and its distribution in different natural media are also outlined. Induced radionuclides which are formed as a result of the interaction of cosmic rays with the constituents of the atmosphere are mentioned. In this respect the intensity of natural background radiation and the dose at different locations and levels is surveyed. Some regions of exceptionally high radioactivity which result in high exposure rates are mentioned. Monazite deposits and water springs are mentioned in some detail. The Oklo phenomenon as a natural reactor is also discussed. 8 tabs

200

Modernizing the fission basis  

International Nuclear Information System (INIS)

The first TUNL-LANL-LLNL collaborative experiment under the program of high precision measurements of neutron induced product yields as a function of incident neutron energy was carried out at the 10-MV FN Tandem Accelerator at the Triangle Universities Nuclear Laboratory. The activation measurements were performed at 9 MeV neutrons produced from the D(d,n)3He reactions on 235U and 238U targets. The main objective of the experiment was to demonstrate the capability to measure characteristic ? rays from specific fission products and to study the background from room scattered neutrons. The neutron flux was measured using the different monitor foils such us Al, In and Au. In addition, time of flight measurements were performed using a neutron-monitor (NE 213) and two 3He counters. After this successful run, the second experiment under this program was completed in September 2011 with 9 MeV neutron beam using the dual fission chamber for 235U, 238U and 239Pu targets. The dual fission chamber design permits simultaneous exposure of absolute fission fragment emission rate and fission activation foils. The results will be coupled with theoretical analysis to provide a more fundamental understanding of the fission process. (author)

 
 
 
 
201

Singlet exciton fission photovoltaics.  

Science.gov (United States)

Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption. PMID:23611026

Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

2013-06-18

202

Induction of nuclear fission by high-voltage application  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In nuclear power generation, fissile materials are mainly used. For example, $U^{235}$ is fissile and therefore quite essential for use of nuclear energy. However, the material $U^{235}$ has very small natural abundance less than 1 %. We should seek possibility of utilizing fissionable materials such as $U^{238}$ because natural abundance of such fissionable materials is generally much larger than fissile ones. In this paper, we show that thermal neutrons with vanishing kine...

Maruyama, Hirokazu

2007-01-01

203

Nuclear waste criticality analysis. Final report, 1 July 1995--30 June 1996  

International Nuclear Information System (INIS)

The natural reactors that occurred in Gabon, Africa over 2 billion years ago present an interesting analog to the underground repositories proposed around the world for the long-term storage of high-level spent nuclear fuel. Many articles have been written concerning the low migration rates of actinides and fission products from the Oklo reactor sites, but Oklo also presents researchers with an opportunity to discover the conditions that led to nuclear criticality in uranium oxides with low enrichments. A computer model was developed to predict the conditions that were necessary to lead to criticality in the Oklo reactors. Critical core dimensions and infinite multiplication factors are presented as a function of time, the porosity of the host rock, and the water and uranium content of the sandstone deposits at Oklo

204

The fission TPC project  

International Nuclear Information System (INIS)

New high-precision fission experiments have become a priority within the nuclear energy community due to a growing, world wide, interest in nuclear reactors. In particular, the design of the next generation reactors requires a reduction in the errors on a number of cross section measurements. Most of the required nuclear data has been measured over the last 50 years, although improvements in the accuracy of the data appear unlikely with the current technology. A potential breakthrough is the deployment of a detector developed within the particle physics community called the Time Projection Chamber (TPC). A group of 6 universities and 3 national laboratories have undertaken the task of building the first TPC for this purpose. In this talk I will present the fission TPC concept, and why we think we can make an improvement on 50 years of fission study. (author)

205

Current position on fission product behavior  

International Nuclear Information System (INIS)

The following phenomena are treated and modeled: fission product release from fuel, both in-vessel and ex-vessel; fission product deposition in the primary system, fission product deposition in the containment, and fission product revolatization

206

From fission to multifragmentation  

International Nuclear Information System (INIS)

Heavy ion collisions are known to have different reaction mechanisms depending on the beam energy. While binary fission is the most likely outcome at energies about 1-2 MeV/A, at intermediate energies (say above 5 MeV/A) several medium size fragments are usually produced. Here the authors report on a study on how this change of mechanisms takes place. They use the Bohr-Wheeler transition state method of binary fission generalized to higher breakup modes. They also contrast the kinematical observables obtained under a sequential production of fragments with those of a simultaneous multifragmentation with the hope of identifying the breakup mechanism

207

Low energy nuclear fission  

International Nuclear Information System (INIS)

In these lectures the liquid drop model of fission is presented and some of its predictions compared with experiment. The liquid drop analogy allows to define in a rather simple and intuitive way a number of useful concepts and possible observables. It is shown how a synthesis of the liquid drop model and of the shell model can be made using the Strutinsky shell averaging procedure. Some experimental data related to the existence of shape isomers are presented and discussed. We conclude by discussing some aspects, both experimental and theoretical, of fission dynamics

208

Low energy nuclear fission  

International Nuclear Information System (INIS)

In these lectures we present the liquid drop model of fission and compare some of its prediction with experiment. The liquid drop analogy allows to define in a rather simple and intuitive way a number of useful concepts and possible observables. We then discuss, using the example of the oscillator model, the generality of shell effects. We show how a synthesis of the liquid drop model and of the shell model can be made using the Strutinsky shell averaging procedure. Some experimental data related to the existence of shape isomers are presented and discussed. We conclude by discussing some aspects, both experimental and theoretical, of fission dynamics

209

Uranium transport around the reactor zone at Okelobondo (Oklo). Data evaluation with M3 and HYTEC  

International Nuclear Information System (INIS)

yer in contact with upwardly oxidising waters. M3 and HYTEC show a gain of manganese rich minerals downstream the reactor. A comparison of the U and Mn plots for M3 deviation and HYTEC results showed an almost mirror behaviour. The U transport stops when the Mn gain increases. Thus, HYTEC and M3 modelling predict that a possible reason for not having U transport up to the surface in Okelobondo is due to an inorganic trap which may hinder the uranium transport. The two independent modelling approaches can be used to complement each other and to better understand the processes that can take place in nature. This provides the opportunity to assess the necessary tools for site investigations, data evaluation and helps to trace the reactions and to identify the hydro-geo-chemical system. Thus, we can build reliable tools which can be used to assess the performance of possible waste repository sites

210

Theory of macroscopic fission dynamics  

International Nuclear Information System (INIS)

Recent advances in the theory of macroscopic fission dynamics are outlined with special emphasis on the diffusion model with realistic inertia and friction parameters, used to describe the fission fragment distributions. (orig.)

211

Microscopic Description of Induced Fission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Selected aspects of the description of neutron-induced fission in 240Pu in the framework of the nuclear energy density functional theory at finite temperature are presented. In particular, we discuss aspects pertaining to the choice of thermodynamic state variables, the evolution of fission barriers as function of the incident neutron energy, and the temperatures of the fission fragments.

Schunck, N.

2013-01-01

212

Measurement of fission cross sections  

International Nuclear Information System (INIS)

235U fission cross sections were measured for 0.5-2.2MeV neutrons. Three spectra are given: the energy spectrum of the fission fragments detected, the energy sum spectrum of these fragments, and the energy sum spectrum for correlated fission fragments

213

Status of fission yield measurements  

International Nuclear Information System (INIS)

Fission yield measurement and yield compilation activities in the major laboratories of the world are reviewed. In addition to a general review of the effort of each laboratory, a brief summary of yield measurement activities by fissioning nuclide is presented. A new fast reactor fission yield measurement program being conducted in the US is described

214

Fission product filter  

International Nuclear Information System (INIS)

The fission product filter for He consists of a winding body composed of two corrugated metal sheets simultaneously wound on a core laterally reversed. It is inserted into an enclosing tube and held at top and bottom by a star-shaped yoke. (orig./PW)

215

Investigation of nuclear fission  

International Nuclear Information System (INIS)

Historical aspects of studies on nuclear fission physics, performed in the Institute of Physics and Power Engineering during more than 40 years are briefly described. Investigations were initiated by demands in nuclear data for fast reactors and turned into one of the most important sections of nuclear physics. 50 refs

216

Radiation Detection from Fission  

Energy Technology Data Exchange (ETDEWEB)

This report briefly describes the neutrons and gamma rays emitted in fission, briefly discusses measurement methods, briefly discusses sources and detectors relevant to detection of shielded HEU in sealand containers, and lists the measurement possibilities for the various sources. The brief descriptions are supplemented by reference.

Mihalczo, J.

2004-11-17

217

Fission rate sensitivities and fission fragment ranges for uranium and thorium-bearing materials irradiated with 252 Cf neutron source  

International Nuclear Information System (INIS)

The induced fission reactions of 235 U (n, f), 238 U (n,f) and 232Th (n, f) based on the activation of natural uranium and thorium in samples with 252Cf, neutron source, have been evaluated. The obtained results are the average of three samples from each test material (SRM 2710 Montana soil and Zirconia). The prepared samples have been configured in the so-called compensated - beam geometryin which both gamma spectrometry jointly with solid state nuclear track detector techniques, revealed the fission rate sensitivity of 4.64 x 107 fission per gram per second per neutron (F.h1. S1.n1), for zirconia as obtained by gamma activity arising from 134 I fission product (as a good monitor which is produced only from fissions), and the related value for SRM 2710 soil is 5.22 x 108 (F.g1 S1 .n1). while the obtained values of the mean fission fragment ranges in SRM 2710 and zirconia as estimated from the fission track densities (by CR-39) and fission rates (by gamma spectrometry) are 3.97 x 10 3 g. cm2 respectively which reveal good agreement with experiments and calculations done hitherto

218

Fission modes of mercury isotopes  

CERN Document Server

Recent experiments on beta-delayed fission in the mercury-lead region and the discovery of asymmetric fission in $^{180}$Hg [1] have stimulated renewed interest in the mechanism of fission in heavy nuclei. Here we study fission modes and fusion valleys in $^{180}$Hg and $^{198}$Hg using the self-consistent nuclear density functional theory employing Skyrme and Gogny energy density functionals. We show that the observed transition from asymmetric fission in $^{180}$Hg towards more symmetric distribution of fission fragments in $^{198}$Hg can be explained in terms of competing fission modes of different geometries that are governed by shell effects in pre-scission configurations. The density distributions at scission configurations are studied and related to the experimentally observed mass splits.

Warda, M; Nazarewicz, W

2012-01-01

219

Clusterization in Ternary Fission  

Science.gov (United States)

This lecture notes are devoted to the new kind of ternary decay of low excited heavy nuclei called by us "collinear cluster tri-partition" (CCT) due to the features of the effect observed, namely, decay partners fly away almost collinearly and at least one of them has magic nucleon composition. At the early stage of our work the process of "true ternary fission" (fission of the nucleus into three fragments of comparable masses) was considered to be undiscovered for low excited heavy nuclei. Another possible prototype—three body cluster radioactivity—was also unknown. The most close to the CCT phenomenon, at least cinematically, stands so called "polar emission", but only very light ions (up to isotopes of Be) were observed so far.

Kamanin, D. V.; Pyatkov, Y. V.

220

Fission fragment angular distributions  

International Nuclear Information System (INIS)

Recently a Letter appeared (Phys. Rev. Lett., 522, 414(1984)) claiming that the usual expression for describing the angula distribution of fission fragments from compound nuclear decay is not a necessarily valid limit of a more general expression. In this comment we wish to point out that the two expressions arise from distinctly different models, and that the new expression as used in the cited reference is internally inconsistent

 
 
 
 
221

Fission track dating  

International Nuclear Information System (INIS)

A review is made of the basic priciples of the Fission Track (FT) method and the available informations on fission track stability in minerals. The latter implies that the FT ages calculated with the usual age equation is an 'apparent' age often geochronologically meaningless due to the partial geological fading of the spontaneous 238U fission tracks. Significant ages can however be obtained, which take into account partial fossil track retentivity, either (i) by applying a 'correction procedure' to the apparent FT age, using an experimentally determined, relationship between the mean etchable track lenght and the etchable track density with increasing track annealing conditions, or (ii) by thermally treating the samples before FT measurements, as in the model age methods developed by the author's group. Three kinds of FT model ages are described:the Isochronal Plateau Age (ICPA), Isothermal Plateau Age (ITPA) and Isochrone type of model ages. These model ages present the advantages over previous procedures of FT dating of a better precision and a control of the geological closing temperatures and track identification. (Author)

222

Isotopic yields of fission fragments from transfer-induced fission  

International Nuclear Information System (INIS)

It is proposed to use transfer-induced fission in inverse kinematics coupled to the large acceptance spectrometer VAMOS to identify in atomic and mass number the complete distribution of the fission fragments. The measure of the kinetic properties of the transfer partner allows for determining precisely the excitation of the fissioning system. For the first time, the isotopic yields of the heavy and light fragments may be measured as a function of the excitation energy in neutron-rich actinides. (authors)

223

On the initial stage of fission. Parity effects in fission  

International Nuclear Information System (INIS)

The experimental data on the fission of a number of even-even nuclei in (t,pf ), (d,pf ), and (n, f ) reactions, as well as the data on the fission-neutron and ?-ray yields in (d,pf ) and (n, f ) reactions are systematically analyzed. It is shown that the fission of nuclei from states with negative parity is inhibited even for E*--8.0 MeV, and most probably occurs only after ?-ray emission

224

The evaluation of fission product yields for 238U fission  

International Nuclear Information System (INIS)

The cumulative fission yields for 45 fission product nuclides of 238U fission induced by neutron of fast reactor spectrum and high energy (around 14.0 MeV) neutron have been evaluated. The results were compared with the ENDF/B-6, JENDL-3/FY, CENDL-FY (86) and JEF-2/FY. It was found that in the total 68 evaluated values, the most of present results (88 percent in total) are in agreement with at least one of the main fission yield libraries within the quoted error limits

225

Ternary fission of uranium fissioning isomers, excited by neutrons  

International Nuclear Information System (INIS)

Spontaneously fissioning isomers (SFI) of uranium have been excited in the reactions 236,238U(n, n') at 4.5 MeV mean neutron energy. An electrostatic accelerator with pulsed beam and the time analysis of events have been used. The fission fragments were detected by the scintillation method. Detection of long-range particles was carried out by the ionization method. The relative probability of nuclear fission through the SFI states has been measured: (1.30±0.01)x10-4 (236U) and (1.48±0.02)x10-4 (238U). The decay half-lifes of the isomers have been determined: 121±2 ns (236U SFI) and 267±13 ns (238U SFI). Studying the ternary fission of the uranium SFI have been found that the probability of the process is one ternary per 163±44 binary fissions of 236U SFI and one ternary per 49±14 binary fissions of 238U SFI. Essential increase of the uranium SFI ternary fission probability as compared to the case of ternary fission of nuclei in nonisomeric states may be attributed to a specific nucleon configuration of uranium fissionable isotopes

226

Ternary fission of neutron induced uranium fissioning isomers  

International Nuclear Information System (INIS)

Spontaneously fissioning isomers (SFI) of uranium were produced in 236,238U(n, n') reactions at 4.5 MeV mean neutron energy. A pulsed beam electrostatic accelerator was used and a time analysis of fission events was applied. Fission fragments were detected by a scintillation method, and long-range fission particles were detected by an ionization technique. Relative fission probabilities through the SFI state for both the nuclei were determined as (1.30±.01).10-4 in 236U and (1.48±.02).10-4 in 238U case. The SFI half-lives were determined as 121±2 ns and 267±13 ns respectively. As a result of uranium SFI ternary fission study it was observed that one ternary per 163±44 binary fissions of 236mfU and one ternary per 49±14 binary fissions of 238mfU took place. These figures seem to be rather high as compared with nonisomeric ternary fission probability data, and tentative effect of a particular nucleon configuration in the uranium SFI is supposed. (orig.)

227

Temporal evolution of the diffusion process and variability of a natural reactor  

Energy Technology Data Exchange (ETDEWEB)

Full text: The Earth's magnetic field is one of the evidences of an energy source in the inner core of the Earth. Conventional energy sources such as transformation of potential energy into heat, latent heat of crystallization, remaining heat of the early Earth's formation, or based on the decay of heavy nuclei have the disadvantage of operating for a relatively short period of time and present a slow variability in only one direction in the time. The magnetic field, however, presents variability in its intensity, reversing its direction irregularly at certain time intervals. Nowadays, the most interesting proposal for explication of the Earth's dynamo is based in a nuclear fission reactor at the inner core. As initially proposed by Kuroda, the possibility of the existence of naturals reactors in remote times, when the percentage of U{sup 235} in natural uranium was higher, it acquired credibility with the discovery of one these reactors in 1972 at Oklo in the Gabon Republic. Herndon, in 1979, then suggests the existence in the Earth's core of a regenerator reactor (i.e. a reactor burning and producing U{sup 235}) as an energy source for the terrestrial dynamo. If this reactor operates with a variable power, then the flux of antineutrinos it produces is also variable and, in principle, it could be measured on Earth's surface, and this could serve to corroborate this hypothesis. In this work, from a simple model of neutron diffusion, we analyzed some arguments which lead us to expect a variable power for the reactor. The neutrons density, which is created through introduction of a sudden source at the system, is obtained by solving the diffusion equation time-dependent in one-dimension. Next, we analyzed the criticality conditions of the system. The cumulative effect of several sudden sources independent of each other is calculated. After, the cases in which the system changes of the critical condition for the subcritical condition are examined. Finally, we extend the equations for the three-dimensional case and we find the solutions for a spherical reactor. From these solutions, the proposed geo-reactor is analyzed. (author)

Amorim, Bruna Cristina; Aquino, Verisssimo Manuel de; Iwamoto, Hiromi [Universidade Estadual de Londrina (UEL), PR (Brazil)

2011-07-01

228

Low-energy ternary fission  

International Nuclear Information System (INIS)

With the detector system DIOGENES thermal neutron induced and spontaneous ? particle associated fission and spontaneous nuclear tripartition into three fragments of similar masses has been investigated. DIOGENES is a concentric arrangement of toroidal angular position sensitive ionization chambers and proportional counters to measure the kinetic energies and relative angular distributions of the three reaction products of ternary fission. For ?-particle accompanied fission some of the many possible ? particle fission-fragment parameter correlations will be discussed. For nearly symmetric low-energy nuclear tripartition new upper limits are presented. Former experimental results which pretended evidence for so called true ternary fission could be explained by charged-particle associated fission with a light particle in the mass range of 13 < A < 23

229

New developments in nuclear fission  

International Nuclear Information System (INIS)

A few recently observed new effects in nuclear fission are reviewed from the experimental side and theoretical explanations are indicated: (i) The structures in the mass yields and in the maximum fragment kinetic energies in cold fission are due to Coulomb effects; (ii) Mass-symmetric fission of even-even nuclei does not proceed by splitting the nucleus into two equal parts and the fragments are differently deformed; (iii) The dip in the total fragment kinetic energy at symmetry is due to a different symmetric fission barrier; (iv) In analogy to the cold compact fission events with highest kinetic energy, also those events of lowest kinetic energy but highest deformation are cold at the scission point; (v) Cold fission proceeds close to the saddle point. (orig.)

230

Nuclear fission yesterday and today  

International Nuclear Information System (INIS)

After an outline of the experiments which led to the discovery of nuclear fission by O. Hahn and F. Strassmann in December 1938, the present state of knowlege of the fission process is reviewed. Important aspects of this process such as the double-humped shape of the fission barrier have been recognized only during the last decade. Other aspects such as the behavior of the fissioning nucleus between saddle point and scission point and the asymmetry of the mass division are not yet fully understood. In addition to further experimental and theoretical studies of the fission process itself, work on nuclear reactions between heavy nuclei may lead to better insights since these reactions, like the fission process, reveal the dynamical behaviour of nuclear matter. (orig.)

231

Magnetic influence on fission chambers  

International Nuclear Information System (INIS)

On large tokamak devices neutron measurements are performed by means of fission chambers can be affected because of the presence of high magnetic fields (E * B drift). A study has been carried out of the magnetic field influence on the fission chamber performances. By means of code, the drift properties of the electrons have been evaluated at different values of crossed E and B fields. The application of incore fission chambers on large tokamak devices results feasible

232

Fission hindrance in hot nuclei  

International Nuclear Information System (INIS)

The role of dynamics in fission has attracted much interest since the discovery of this process over fifty years ago. However, the study of the dynamical aspects of fission was for many years hampered by the lack of suitable experimental observables against which theoretical calculations could be tested. For example, it was found that the total kinetic energy release in fission can be described equally well by very different dissipation mechanisms, namely the wall formula, that is based on the collisions of the nucleons with the moving wall of the system, as well as a bulk viscosity of the nuclear matter. Although early theoretical work suggested that the fission process may be described as a diffusion process over the fission barrier, this was largely forgotten because of the success of a purely statistical model which instead of enumerating the ultimate final states of the process argues that the fission rate is determined at the open-quote transition state close-quote as the system traverses the fission saddle point. It was therefore significant when Gavron showed that the transition state model was unable to describe the number of neutrons emitted prior to scission at high excitation energy in reactions of 16O+142Nd. Subsequent experimental work using different methods to measure the fission dissipation/viscosity has confirmed these initial observations. It was therefore very surprising when Moretto in recent publications concluded that their analysis of fission excitation functions obtained with a and ? and 3He induced projectiles was perfectly in accord with the transition state model and left no room for fission viscosity. In this paper we'll show that Moretto's analysis is flawed by assuming first chance fission only (in direct contradiction to the experimental observation of pre-scission neutron emission in heavy-ion induced fission), and reveal why the systematics presented by Moretto looked so convincing despite these flaws

233

Fission barriers of light nuclei  

International Nuclear Information System (INIS)

Experimental fission excitation functions for compound nuclei /sup 52/Fe, /sup 49/Cr, /sup 46/V, and /sup 44/Ti formed in heavy-ion reactions are analyzed in the Hauser-Feshbach/Bohr-Wheeler formalism using fission barriers based on the rotating liquid drop model of Cohen et al. and on the rotating finite range model of Sierk. We conclude that the rotating finite range approach gives better reproduction of experimental fission yields, consistent with results found for heavier systems

234

Nature  

Science.gov (United States)

Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance, and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public. Nature publishes more articles than any other multidisciplinary journal, and retains its position as the most cited weekly science journal. The site provides free access to news stories in the latest issue; access to research articles, and to the Nature archive, is by subscription.

235

Natural repository analogue program. Progress report, January 1-March 30, 1982  

International Nuclear Information System (INIS)

Lead and uranium isotopic abundances in rocks from the Oklo mine show large deficiencies of radiogenic lead in the mineralized regions and enormous excesses of this element outside the uraniferous zones. A fracture lined with secondary minerals and its host rock from distances as far as approx. 13 meters away contain lead that was deposited contemporaneously. The isotopic composition of lead in these samples varies systematically as a function of distance from the fracture. This regularity may reflect the nature of the processes that transported lead from the ores and deposited it in the surrounding rocks

236

Fission track dating of obsidians  

International Nuclear Information System (INIS)

Fission track dating is applied to 16 obsidian artifacts from an archaeological excavation in Cayonu Tepesi at southeastern Anatolia. Since obsidian samples are rich in bubbles and inclusions, only the six most compact samples were found to be usable for fission track measurements. Size studies of etched fossil and induced fission tracks showed that no strong thermal effects occurred during the history of these glasses so that no fading correction has been applied to the computed ages. The fission track ages obtained range between 7.51 m.y. and 20.89 m.y. It may be stated that these obsidian samples are from the two different origins. (author)

237

Hidden systematics of fission channels  

Science.gov (United States)

It is a common procedure to describe the fission-fragment mass distributions of fissioning systems in the actinide region by a sum of at least 5 Gaussian curves, one for the symmetric component and a few additional ones, together with their complementary parts, for the asymmetric components. These components have been attributed to the influence of fragment shells, e.g. in the statistical scission-point model of Wilkins, Steinberg and Chasman. They have also been associated with valleys in the potential-energy landscape between the outer saddle and the scission configuration in the multi-channel fission model of Brosa. When the relative yields, the widths and the mean mass-asymmetry values of these components are fitted to experimental data, the mass distributions can be very well reproduced. Moreover, these fission channels are characterised by specific values of charge polarisation, total kinetic energy and prompt-neutron yields. The present contribution investigates the systematic variation of the characteristic fission-channel properties as a function of the composition and the excitation energy of the fissioning system. The mean position of the asymmetric fission channels in the heavy fragment is almost constant in atomic number. The deformation of the nascent fragments at scission, which is the main source of excitation energy of the separated fission fragments ending up in prompt-neutron emission, is found to be a unique function of Z for the light and the heavy fragment of the asymmetric fission channels. A variation of the initial excitation energy of the fissioning system above the fission saddle is only seen in the neutron yield of the heavy fragment. The charge polarisation in the two most important asymmetric fission channels is found to be constant and to appreciably exceed the macroscopic value. The variation of the relative yields and of the positions of the fission channels as a function of the composition and excitation energy of the fissioning system obey a hidden systematics that can be explained by the number of states in the vicinity of the outer fission barrier as a function of mass asymmetry, if the potential is constructed as the sum of the macroscopic contribution of the compound nucleus and empirically determined fragment shells. This hidden systematics also explains the transition from asymmetric to symmetric fission around 226Th and around 258Fm.

Schmidt, Karl-Heinz; Jurado, Beatriz

2013-12-01

238

Hidden systematics of fission channels  

Directory of Open Access Journals (Sweden)

Full Text Available It is a common procedure to describe the fission-fragment mass distributions of fissioning systems in the actinide region by a sum of at least 5 Gaussian curves, one for the symmetric component and a few additional ones, together with their complementary parts, for the asymmetric components. These components have been attributed to the influence of fragment shells, e.g. in the statistical scission-point model of Wilkins, Steinberg and Chasman. They have also been associated with valleys in the potential-energy landscape between the outer saddle and the scission configuration in the multi-channel fission model of Brosa. When the relative yields, the widths and the mean mass-asymmetry values of these components are fitted to experimental data, the mass distributions can be very well reproduced. Moreover, these fission channels are characterised by specific values of charge polarisation, total kinetic energy and prompt-neutron yields. The present contribution investigates the systematic variation of the characteristic fission-channel properties as a function of the composition and the excitation energy of the fissioning system. The mean position of the asymmetric fission channels in the heavy fragment is almost constant in atomic number. The deformation of the nascent fragments at scission, which is the main source of excitation energy of the separated fission fragments ending up in prompt-neutron emission, is found to be a unique function of Z for the light and the heavy fragment of the asymmetric fission channels. A variation of the initial excitation energy of the fissioning system above the fission saddle is only seen in the neutron yield of the heavy fragment. The charge polarisation in the two most important asymmetric fission channels is found to be constant and to appreciably exceed the macroscopic value. The variation of the relative yields and of the positions of the fission channels as a function of the composition and excitation energy of the fissioning system obey a hidden systematics that can be explained by the number of states in the vicinity of the outer fission barrier as a function of mass asymmetry, if the potential is constructed as the sum of the macroscopic contribution of the compound nucleus and empirically determined fragment shells. This hidden systematics also explains the transition from asymmetric to symmetric fission around 226Th and around 258Fm.

Schmidt Karl-Heinz

2013-12-01

239

Fission barrier of 210Po  

International Nuclear Information System (INIS)

Statistical model analysis of fission and evaporation residue cross-sections along with pre-fission neutron multiplicities (?pre) data for 12C+198Pt system yielded fission barriers much smaller (?13 MeV) than those (?21 MeV) obtained for same compound nuclei from the analysis of light ion induced reactions. In the present study, the fission excitation functions for p+209Bi, ?+206Pb, 12C+198Pt and 18O+192Os systems leading to the same compound nucleus has been investigated

240

Fusion-fission type collisions  

International Nuclear Information System (INIS)

Three examples of fusion-fission type collisions on medium-mass nuclei are investigated whether the fragment properties are consistent with fission from equilibrated compound nuclei. Only in a very narrow band of angular momenta the data fulfill the necessary criteria for this process. Continuous evolutions of this mechanism into fusion fission and into a deep-inelastic process and particle emission prior to fusion have been observed. Based on the widths of the fragment-mass distributions of a great variety of data, a further criterion for the compound-nucleus-fission process is tentatively proposed

 
 
 
 
241

Measurement of Fission Product Yields from Fast-Neutron Fission  

Science.gov (United States)

One of the aims of the Stockpile Stewardship Program is a reduction of the uncertainties on fission data used for analyzing nuclear test data [1,2]. Fission products such as 147Nd are convenient for determining fission yields because of their relatively high yield per fission (about 2%) and long half-life (10.98 days). A scientific program for measuring fission product yields from 235U,238U and 239Pu targets as a function of bombarding neutron energy (0.1 to 15 MeV) is currently underway using monoenergetic neutron beams produced at the 10 MV Tandem Accelerator at TUNL. Dual-fission chambers are used to determine the rate of fission in targets during activation. Activated targets are counted in highly shielded HPGe detectors over a period of several weeks to identify decaying fission products. To date, data have been collected at neutron bombarding energies 4.6, 9.0, 14.5 and 14.8 MeV. Experimental methods and data reduction techniques are discussed, and some preliminary results are presented.

Arnold, C. W.; Bond, E. M.; Bredeweg, T. A.; Fowler, M. M.; Moody, W. A.; Rusev, G.; Vieira, D. J.; Wilhelmy, J. B.; Becker, J. A.; Henderson, R.; Kenneally, J.; Macri, R.; McNabb, D.; Ryan, C.; Sheets, S.; Stoyer, M. A.; Tonchev, A. P.; Bhatia, C.; Bhike, M.; Fallin, B.; Gooden, M. E.; Howell, C. R.; Kelley, J. H.; Tornow, W.

2014-09-01

242

Spatial distribution of thorium fission rate in a fast spallation and fission neutron field: An experimental and Monte Carlo study  

Science.gov (United States)

The Energy plus Transmutation (EpT) set-up of the Joint Institute for Nuclear Research (JINR), Dubna, Russia is composed of a lead spallation target surrounded by a blanket of natural uranium. The resultant neutron spectrum is a combination of spallation and fission spectra, modified by a reflective external layer of polyethylene and an internal absorbing layer of cadmium. The EpT set-up was irradiated with a beam of 4 GeV deuterons from the Nuclotron Accelerator at JINR. The spatial distribution of thorium fission rate within the assembly was determined experimentally, using a fission track detector technique, and compared with Monte Carlo predictions of the MCNPX code. Contributions of neutrons, protons, deuterons, photons and pions to total fission were taken into account. Close agreement between the experimental and calculated results was found.

Borger, J. J.; Hashemi-Nezhad, S. R.; Alexiev, D.; Brandt, R.; Westmeier, W.; Thomauske, B.; Adam, J.; Kadykov, M.; Tiutiunnikov, S.

2012-02-01

243

Tip model of cold fission  

International Nuclear Information System (INIS)

Cold fission is defined to be the limiting case of nuclear fission where virtually all of the available energy is converted into the total kinetic energy of the fragments. The fragments have, therefore, to be born in or at least close to their respective ground states. Starting from the viewpoint that cold fission corresponds to most compact scission configurations, energy constraints have been exploited to calculate minimum tip distances between the two nascent fragments in binary fission. Crucial input parameters to this tip model of cold fission are the ground-state deformations of fragment nuclei. It is shown that the minimum tip distances being compatible with energy conservation vary strongly with both the mass and charge fragmentation of the fission prone nucleus. The tip distances refer to nuclei with equivalent sharp surfaces. In keeping with the size of the surface width of leptodermous nuclei, only configurations where the tip distances are smaller than a few fm may be considered as valid scission configurations. From a comparison with experimental data on cold fission this critical tip distance appears to be 3.0 fm for the model parameters chosen. Whenever the model calculation yields tip distances being smaller than the critical value, a necessary condition for attaining cold fission is considered to be fulfilled. It is shown that this criterion allows to understand in fair agreement with experiment which mass fragmentations are susceptible to lead to cold fission and which fragment-charge divisions are the most favored in each isobaric mass chain. Being based merely on energy arguments, the model cannot aim at predicting fragment yields in cold fission. However, the tip model proposed appears well suited to delineate the phase space where cold fission phenomena may come into sight. (orig.)

244

Mechanism of nuclear fission (Part I)  

International Nuclear Information System (INIS)

The authors' 1941 paper on fission is translated. Bohr-Wheeler theory is used. After a general discussion of fission fragments and energy balance concepts, the neutron induced fission of uranium is considered

245

Nuclear dissipation, fission probability and neutron multiplicity prior to fission  

International Nuclear Information System (INIS)

We discuss the effects of nuclear dissipation of fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratio asub(f)/asub(n) of the level density parameters for a given strength of the reduced dissipation coefficient ?. These low energy constraints on asub(f)/asub(n) and ? balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of 158Er formed in the reaction 16O+142Nd at 207 MeV we show that due to transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis. (orig.)

246

Fission throughout the periodic table  

International Nuclear Information System (INIS)

The dualistic view of fission and evaporation as two distinct compound nucleus processes is substituted with a unified view in which fission, complex fragment emission, and light particle evaporation are seen as different aspects of a single process. 47 refs., 22 figs

247

Biological danger of fission products  

International Nuclear Information System (INIS)

Basic aspects of biological danger of fission products formed during nuclear weapons tests and by nuclear enterprise releases are considered. Impact of internal and external irradiation on animals and man, toxicity of nuclear products fission depending on their age, preventive measures and urgent treatment in case of radiational injuries are discussed. 3 tabs

248

Progress in fission product nuclear data  

International Nuclear Information System (INIS)

This is the tenth issue of a report series on Fission Product Data, which informs us about all the activities in this field, which are planned, ongoing, or have recently been completed. The types of activities included are measurements, compilations and evaluations of: fission product yields (neutron induced and spontaneous fission), neutron reaction cross sections of fission products, data related to the radioactive decay of fission products, delayed neutron data of fission products, lumped fission product data (decay heat, absorption, etc.). There is also a section with recent references relative to fission product nuclear data

249

Decay modes in spontaneous fission  

CERN Document Server

Spontaneous fission (SF) is considered to be the choice reaction for studying the influence of shell and pairing effects in fission in general, and in particular their impact on the mass and energy distributions of fission fragments. For the time being some 35 SF reactions have been analysed in detail for elements ranging from Pu up to Rf. Going from the lighter to the heavier actinides both, the distributions of fragment mass (or charge) and of total kinetic energy undergo dramatic changes. It is observed in experiment, however, that these distributions may be well described as a superposition of a few fission modes, each with its own characteristic mass an energy pattern. The experimental modes are traced in theory to fine structures in the potential energy surface of a fissioning nucleus, provided shell and pairing corrections to the basic liquid drop model are accounted for.

Gönnenwein, F

1999-01-01

250

Fission track temperature age theory  

International Nuclear Information System (INIS)

As a consequence of thermal fading of fission tracks in minerals the fission track dating method can be used to obtain a temperature (cooling) age, if it is possible to determine the temperature associated with a measured fission track age. Based on the concept of a minimum measurable fission track length, lsub(min), we have recently solved for apatite the differential annealing equation dl/dt = -?t taking into account that the annealing coefficient, ? = ?(T) depends also on the fission track reduction p/p0. This calculation yielded an improved age-temperature relationship. We could demonstrate in this study, however, that to a good approximation it is sufficient instead to use an annealing coefficient, where p/p0 is substituted by a constant (high) degree of reduction. In the present study we have used this approximation to calculate age-temperature relationships for a number of minerals. (author)

251

The fission-track analysis: An alternative technique for provenance studies of prehistoric obsidian artefacts  

CERN Document Server

Comparison of fission-track parameters - age and track densities - is an alternative tool for correlating obsidian artefacts with their potential natural sources. This method was applied by different fission-track groups in various regions and results were compared with those obtained using the more popular provenance identification techniques based on chemical composition studies. Hundreds of analyses prove that fission-track dating is a complementary technique which turns out to be very useful, specially when the chemical composition does not fully discriminate different sources. Archaeologically significant results were obtained applying the fission-track analysis in various regions of earth.

Bellot-Gurlet, L; Dorighel, O; Oddone, M; Poupeau, G; Yegingil, Z

1999-01-01

252

Fission of heavy and superheavy nuclei at low excitation energies  

CERN Document Server

The talk presents the results of an investigation of the main characteristics (mass and energy distributions of fission fragments and multiplicity of neutrons) of the fission of the nuclei of sup 2 sup 2 sup 0 Ra, sup 2 sup 2 sup 6 Th, sup 2 sup 5 sup 6 No, sup 2 sup 7 sup 0 Sg, sup 2 sup 8 sup 6 112 produced in reactions with ions of sup 1 sup 8 O, sup 2 sup 2 Ne and sup 4 sup 8 Ca at energies close to and essentially below the Coulomb barrier. The data obtained show that the form of the mass and energy distributions of the fission fragments of sup 2 sup 2 sup 6 Th and sup 2 sup 7 sup 0 Sg is accounted for by the multimodal nature of the fission. In addition, for sup 2 sup 2 sup 6 Th, a new phenomenon was established: there is a significant difference between the numbers of prescission neutrons for symmetric and asymmetric fission modes. It was found that, for the low-energy fission of the nucleus of sup 2 sup 8 sup 6 112, the mass distribution of the fragments is of a clear-cut asymmetric form, contrary to ...

Itkis, M G; Hanappe, F; Itkis, Y M; Kelic, A; Kondratev, N A; Kozulin, E M; Oganessian, Yu T; Pokrovsky, I V; Prokhorova, E V; Rudolf, G; Rusanov, A Ya; Stuttgé, L

1999-01-01

253

Fission fragment angular distribution in heavy ion induced fission  

Directory of Open Access Journals (Sweden)

Full Text Available   We have calculated the fission fragment angular anisotropy for 16O + 232Th,12C + 236U , 11B + 237 Np , 14 N + 232 Th , 11B + 235U , 12C + 232Th systems with the saddle point statistical model and compared the fission fragment angular anisotropy for these systems. This comparison was done with two methods a without neutron correction and b with neutron correction. Also we studied normal and anomalous behavior of the fission fragment angular anisotropy. Finally, we have predicted the average emitted neutron from compound nuclei considering the best fit for each system.

S. Soheyli

2006-06-01

254

Fission fragment angular distribution in heavy ion induced fission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

  We have calculated the fission fragment angular anisotropy for 16O + 232Th,12C + 236U , 11B + 237 Np , 14 N + 232 Th , 11B + 235U , 12C + 232Th systems with the saddle point statistical model and compared the fission fragment angular anisotropy for these systems. This comparison was done with two methods a) without neutron correction and b) with neutron correction. Also we studied normal and anomalous behavior of the fission fragment angular anisotropy. Finally, we have predicted the avera...

Soheyli, S.; Ziaeian, I.

2006-01-01

255

Fission product yields in 238Np thermal fission  

International Nuclear Information System (INIS)

Relative cumulative yields for 23 products in the range A=131-155 thermal fission of short-lived odd-odd 238Np nuclide are determined, using mass spectrometric method. The curve of mass yield in the region of heavy product peak is plotted, the absolute yields of the above-mentioned masses and the main parameters of the heavy product peak are evaluated. Irregularities in the curve of fission product mass distribution, observed in the ranges of mass yields A=134-136, A=137-140, A=144 and A=151-152, are explained in the framework of the Wilkins-Steinberg-Chasman statistical model of fission

256

The use of recoil for the separation of uranium fission products  

International Nuclear Information System (INIS)

The recoil distance of fission fragments in U3O8 is about 8 microns. By using highly diluted suspensions of uranium oxide particles having dimension much smaller than this figure (mean diameter 0,5 micron), we were able to study the re-adsorption of fission products on uranium oxide. Separation results have been studied as a function of the nature of the irradiation medium (solid or liquid) and the separation medium, of particle size and of concentration of particles in the dispersing medium. Decay curves can be used to discriminate between 239Np and mixed fission products. Most of the 239Np is found in the U3O8 particles. The location of fission products in solid dispersing media has been determined, fission products being found always inside the dispersing medium particles. The results obtained can be applied to the rapid separation of short-lived fission products from a uranium-free starting material. (author)

257

Fission product behavior  

International Nuclear Information System (INIS)

During the TMI-2 accident, radioactive fission products were released from the fuel to the reactor coolant. Radionuclides were subsequently transported from the reactor primary system to the Reactor Building sump and into the Auxiliary Building. The residual radionuclide distributions have been studied to gain insight into the physical and chemical processes operative during and soon after the accident. Isotopes of (a) the radioactive noble gases (Kr and Xe), (b) radioiodine, (c) radiocesium, and (d) radiostrontium are of principal concern. The analyses of post-accident fluid and solid samples have identified 60--70 percent of the inventories of each of the four radionuclide categories. Except for the noble gases, the entire inventories are contained in the plant. The observed distributions indicate differences between the behavior of iodine and cesium, either during or after the accident

258

Pulsed fission/fusion hybrid engines  

International Nuclear Information System (INIS)

Research into high-thrust, high-specific impulse rocket engines using energy from nuclear reactions which has been conducted at this organization will be discussed. The engines are all conceptual in nature, yet are within the realization of conventional or near-term technology. The engine concepts under study at Foundation, Inc. are designed to obviate or minimize these negative effects of the ORION scheme. By using non-chemical triggers to initiate a non-breakeven fusion reaction at the core of a target composed of both fission and fusion fuel, it should be possible to employ the fusion neutrons thus produced to begin a fission reaction in U-235 or Pu-239. Since the density of the target can be increased by as much as a factor of 250 through compression of the pellet, the amount of fission material necessary to produce a critical mass can be greatly reduced. (This also means that the amount of fission products produced for a giventhrust level is also reduced from the ORION levels.) Coupling this eeffect to the large number of 14 MeV fusion neutrons produced early in the compression process and subsequently to the heating of some additional fusion fuel surrounding the critical mass leads to the very efficient burnup of the target. This insures both high yield from the target as well as low cost per MJ energy released. Finally, the use of such small pellets allows the scale of the energy released to be tailored to a level usable in rocket engines of a few tens of tons thrust level. (orig.)

259

Dynamical features of nuclear fission  

International Nuclear Information System (INIS)

It is now well established that the transition-state theory of nuclear fission due to N. Bohr and J.A. Wheeler underestimates several observables in heavy-ion induced fusion-fission reactions. Dissipative dynamical models employing either the Langevin equation or the Fokker-Planck equation following the work of Kramers have been developed for fission of heavy nuclei at high excitations (T ? 1MeV or higher). The dissipation strength, an ingredient in the above theories, is usually treated as an adjustable parameter to reproduce experimental data. In the present talk, the physical picture underlying the dissipative fission dynamics is presented. The work mainly concentrates upon the Kramers' prescription for including dissipation in fission dynamics. The results of a statistical model analysis of the pre-scission neutron multiplicity data from the reactions 19F+ 194,196,198Pt using Kramers' fission width are discussed in some detail. Various aspects of Kramers' approach with numerical simulations using Langevin equation in the context of nuclear fission are compared

260

Transmission measurements of 235U fission fragments in Mg, Al, Fe, Ni, Ag  

International Nuclear Information System (INIS)

Transmission measurements of the fission fragments arising in 235U thermal-neutron-induced fission were performed by irradiating a back-to-back fission chamber in the thermal column of the RB-2 reactor of Montecuccolino (Bologna) Italy. The back-to-back fission chamber is a gas-flow counter utilizing argon containing 2 per cent nitrogen. The electrode spacing was 10 mm and the gas pressure was adjusted slightly above ambient pressure. The operating voltage was 100 V positive applied to each anode. The source of fission fragments consists of a thin layer of natural uranium (?0.2mg/cm2) evaporated under vacuum on a Pt disc. This uranium deposit is located in one of the two fission chambers of the counter. Irradiations were made in a neutron flux of ? 109 cm-2 s-1. During irradiation the fission fragments, which a r e able to penetrate a thin sheet of variable thickness (1-10 mg/cm2 ) of the element investigated, are directly counted. By repeating the measurements with different thickness of the sheets, one obtains a transmission curve of the 235U fission fragments for each element investigated. Another natural uranium deposit located in the second fission chamber acts as a monitor for the neutron flux for the different sorts of irradiation. From the transmission curves it is possible to derive the relative effective stopping power of the different targets, and the ranges of the fission fragments in the different elements investigated. Actually the ranges a r e obtained by coupling the measured relative stopping powers with the absolute value of the range of 235U fission fragment in Al, measured by Segre and Wiegand, by an independent experiment using the activation technique. The results obtained a r e compared with theoretical calculations concerning the loss of energy of fission fragments in material. (author)

 
 
 
 
261

ICON, Reactor Operation Fission Products Inventory Calculation  

International Nuclear Information System (INIS)

1 - Nature of physical problem solved: Determines complete fission product inventory for a reactor after a particular history of operating conditions taking into account the structure of the individual decay chains and also the effects of neutron capture. 2 - Method of solution: Each chain is fitted to a generalised decay scheme and the simultaneous differential equations, whose coefficient matrix is lower triangular, are solved. 3 - Restrictions on the complexity of the problem: In its present form up to 100 reactor cycles may be examined but this may be readily increased

262

The spectroscopy of fission fragments  

International Nuclear Information System (INIS)

High-resolution measurements on ? rays from fission fragments have provided a rich source of information, unobtainable at the moment in any other way, on the spectroscopy of neutron-rich nuclei. In recent years important data have been obtained on the yrast- and near yrast-structure of neutron-rich fission fragments. We discuss the scope of measurements which can be made on prompt gamma rays from secondary fission fragments, the techniques used in the experiments and some results recently obtained. (author)

263

Fission at intermediate nucleon energies  

Science.gov (United States)

In the present work Monte Carlo calculations of fission of actinides and pre- actinides induced by protons and neutrons in the energy range from 100 MeV to 1 GeV are carried out by means of a recent version of the Liège Intranuclear Cascade Model, INCL++, coupled with different evaporation-fission codes, in particular GEMINI++ and ABLA07. Fission model parameters are adjusted on experimental (p, f) cross sections and used to predict (n, f) cross sections, in order to provide a theoretical support to the campaign of neutron cross section measurements at the n_TOF facility at CERN.

Lo Meo, S.; Mancusi, D.; Massimi, C.; Vannini, G.; Ventura, A.

2014-07-01

264

Cluster aspects of nuclear fission  

International Nuclear Information System (INIS)

Experimental fission data for nuclei in the mass region A = 210 to A = 258 are discussed from a unified point of view using cluster representations of nuclei. It is shown that these data can be described as resulting from a superposition of two different modes, one determined by energetically favoured cluster correlations, the other due to liquid-drop effects only. For all nuclei lighter than fermium the above distinction coincides with that of symmetric and asymmetric fission. Contrary to that in fermium-isotopes, both symmetric and asymmetric fission are associated with energetically favoured cluster correlations at the fermi surface. (orig.)

265

Intrinsic energy partition in fission  

Directory of Open Access Journals (Sweden)

Full Text Available The intrinsic energy partition between two complementary fission fragments is investigated microscopically. The intrinsic excitation energy of fission fragments is dynamically evaluated in terms of the time-dependent pairing equations. These equations are corroborated with two conditions. One of them fixes the number of particles and the other separates the pairing active spaces associated to the two fragments in the vicinity of the scission configuration. The excitation energy in a wide distribution of fission fragments is calculated for the 234U parent nucleus.

Mirea M.

2013-03-01

266

Gamma Radiation from Fission Fragments  

International Nuclear Information System (INIS)

The gamma radiation from the fragments of the thermal neutron fission of 235U has been investigated, and the preliminary data are presented here with suggestions for further lines of research and some possible interpretations of the data. The data have direct bearing on the fission process and the mode of fragment de-excitation. The parameters measured are the radiation decay curve for the time interval (1 - 7) x 10-10 sec after fission, the photon yield, the total gamma ray energy yield, and the average photon energy. The last three quantities are measured as a function of the fragment mass

267

Fission and deep spallation characteristics in relativistic nuclear collisions  

International Nuclear Information System (INIS)

Numbers of emitted fast charged particles associated with heavier nuclear fragments are employed as a graphic means of differentiating fission and spallation contributions to target fragment emission in relativistic nuclear collisions. The violent nature of the deep spallation mechanism is observed directly

268

Fission barriers of exotic nuclei  

International Nuclear Information System (INIS)

Using available experimental data on fission barriers and ground-state masses a detailed study on the predictions of different models concerning the isospin dependence of saddle-point masses is performed. (authors)

269

Dynamics of cold fission processes  

International Nuclear Information System (INIS)

The neutron-rich 264 Fm should be the best cold-fissioning nucleus, owing to the strong shell effect of the doubly magic fragments 132 Sn. The optimum cold-fission path in the plane of the two independent shape coordinates, the separation distance R and of the radius of the light fragment R2, is determined. Werner-Wheeler inertia tensor and Yukawa-plus-exponential deformation energy are used to calculate the action integral. Microscopic shell and pairing correction ar based on the asymmetric two-center single-particle model. Different variation laws R2 = R2 (R), for the fission trajectory were tested: polynomials of 2nd, 3rd and 4th degree and an exponential function. The partial half-life for cold fission is estimated to be several orders of magnitude shorter than that of ?-decay. (authors)

270

?-delayed fission of 180Tl  

Science.gov (United States)

The detailed analysis of the ?-delayed fission data of 180Tl is presented. The experiment was performed by producing a pure beam of 180Tl by means of highly selective resonance laser ionization followed by mass separation with the ISOLDE (CERN, Geneva) isotope separator. A surprising asymmetric mass distribution of fission fragments from 180Hg, the daughter of 180Tl ? decays, was observed. Here, the energy calibration of the silicon detectors, which is crucial for a proper determination of the fission fragments’ energy and mass split, is presented and the total kinetic energy and its dependence on the mass split ratio is discussed. A more precise ?-delayed fission probability P?DF(180Tl)=3.2(2)×10-3% was deduced.

Elseviers, J.; Andreyev, A. N.; Huyse, M.; Van Duppen, P.; Antalic, S.; Barzakh, A.; Bree, N.; Cocolios, T. E.; Comas, V. F.; Diriken, J.; Fedorov, D.; Fedosseev, V. N.; Franchoo, S.; Ghys, L.; Heredia, J. A.; Ivanov, O.; Köster, U.; Marsh, B. A.; Nishio, K.; Page, R. D.; Patronis, N.; Seliverstov, M. D.; Tsekhanovich, I.; Van den Bergh, P.; Van De Walle, J.; Venhart, M.; Vermote, S.; Veselský, M.; Wagemans, C.

2013-10-01

271

Advanced Space Fission Propulsion Systems  

Science.gov (United States)

Fission has been considered for in-space propulsion since the 1940s. Nuclear Thermal Propulsion (NTP) systems underwent extensive development from 1955-1973, completing 20 full power ground tests and achieving specific impulses nearly twice that of the best chemical propulsion systems. Space fission power systems (which may eventually enable Nuclear Electric Propulsion) have been flown in space by both the United States and the Former Soviet Union. Fission is the most developed and understood of the nuclear propulsion options (e.g. fission, fusion, antimatter, etc.), and fission has enjoyed tremendous terrestrial success for nearly 7 decades. Current space nuclear research and technology efforts are focused on devising and developing first generation systems that are safe, reliable and affordable. For propulsion, the focus is on nuclear thermal rockets that build on technologies and systems developed and tested under the Rover/NERVA and related programs from the Apollo era. NTP Affordability is achieved through use of previously developed fuels and materials, modern analytical techniques and test strategies, and development of a small engine for ground and flight technology demonstration. Initial NTP systems will be capable of achieving an Isp of 900 s at a relatively high thrust-to-weight ratio. The development and use of first generation space fission power and propulsion systems will provide new, game changing capabilities for NASA. In addition, development and use of these systems will provide the foundation for developing extremely advanced power and propulsion systems capable of routinely and affordably accessing any point in the solar system. The energy density of fissile fuel (8 x 10(exp 13) Joules/kg) is more than adequate for enabling extensive exploration and utilization of the solar system. For space fission propulsion systems, the key is converting the virtually unlimited energy of fission into thrust at the desired specific impulse and thrust-to-weight ratio. This presentation will discuss potential space fission propulsion options ranging from first generation systems to highly advanced systems. Ongoing research that shows promise for enabling second generation NTP systems with Isp greater than 1000 s will be discussed, as will the potential for liquid, gas, or plasma core systems. Space fission propulsion systems could also be used in conjunction with simple (water-based) propellant depots to enable routine, affordable missions to various destinations (e.g. moon, Mars, asteroids) once in-space infrastructure is sufficiently developed. As fuel and material technologies advance, very high performance Nuclear Electric Propulsion (NEP) systems may also become viable. These systems could enable sophisticated science missions, highly efficient cargo delivery, and human missions to numerous destinations. Commonalities between NTP, fission power systems, and NEP will be discussed.

Houts, Michael G.; Borowski, Stanley K.

2010-01-01

272

Binary fission origin of the moon  

International Nuclear Information System (INIS)

The major arguments for and against the binary fission model of lunar origin are reviewed. Unresolved problems include: (1) how the protoearth acquired sufficient angular velocity to fission, and (2) how the earth-moon system lost its excess angular momentum after fission. Despite these uncertainties, the compositional similarities between the earth's mantle and the bulk moon suggest that the fission model is worth considering. The proposed sequence of events in the formation of the moon by binary fission is given. 54 references

273

Theory of nuclear fission: a review  

International Nuclear Information System (INIS)

General properties of nuclear fission are reviewed and related to our present knowledge of fission theory. For this purpose the basic reasons for the shape of the fission barriers are discussed and their consequences compared with experimental results on barrier shapes and structures. Special emphasis is put on the asymmetry of the fission barriers and mass-distributions and its relation to the shells of the nascent fragment shells. Finally the problem of calculating fission cross sections is discussed

274

Hidden systematics of fission channels  

Digital Repository Infrastructure Vision for European Research (DRIVER)

It is a common procedure to describe the fission-fragment mass distributions of fissioning systems in the actinide region by a sum of at least 5 Gaussian curves, one for the symmetric component and a few additional ones, together with their complementary parts, for the asymmetric components. These components have been attributed to the influence of fragment shells, e.g. in the statistical scission-point model of Wilkins, Steinberg and Chasman. They have also been associated with valleys in th...

Schmidt Karl-Heinz; Jurado Beatriz

2013-01-01

275

The microscopic theory of fission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Fission-fragment properties have been calculated for thermal neutron-induced fission on a $^{239}\\textrm{Pu}$ target, using constrained Hartree-Fock-Bogoliubov calculations with a finite-range effective interaction. A quantitative criterion based on the interaction energy between the nascent fragments is introduced to define the scission configurations. The validity of this criterion is benchmarked against experimental measurements of the kinetic energies and of multipliciti...

Younes, W.; Gogny, D.

2009-01-01

276

Composition of fission product mixtures  

International Nuclear Information System (INIS)

The report gives a compilation of the isotopic composition and specific activity of fission products produced by thermal neutron fission of U-235 and Pu-239. The composition was computed using an electronic computer ES 1040. The presentation comprises data of 27 elements at the end of a neutron irradiation of 3 years and after a time interval of 1, 4 and 10 years after the end of irradiation. (author)

277

Measurement of fission cross sections  

International Nuclear Information System (INIS)

The absolute fission cross sections were measured for 235U and 238U with 14.6MeV neutrons and 241Am with neutrons between 0.5 and 3MeV, the ratio of 238U and 235U fission cross sections for neutrons between 2 and 7MeV was also measued. Results obtained are compared with previous values and ENDF data

278

Fission profile in 256Fm  

International Nuclear Information System (INIS)

The nuclear fission is one of the most challenging quantum many-body problems, due to the difficulty of finding a proper formulation of the evolution from a compound nucleus to fission fragments. The optimum trajectories, the associated shapes and energy in the fissioning nuclei at high angular momentum are studied. Hence the effect of angular momentum in the fission profile of a nucleus 256Fm is looked in to. Here the results for 256Fm, which was one among the most studied nuclei in various approaches are discussed. An independent-particle shell model with the mean field represented by the deformed Woods-Saxon potential is utilized. The main development in this work is the extension of Cassinian oval calculations to study the rotating nuclei. The properties of a fissioning nuclei at fixed spin is achieved by tuning the rotation frequency to get the desired angular momentum. The details about the calculations and the evolution of potential energy surfaces are explained in literature. The effect of spin in the fission profile is shown

279

Fission fragment angular distributions and fission cross section validation  

International Nuclear Information System (INIS)

The present knowledge of angular distributions of neutron-induced fission is limited to a maximal energy of 15 MeV, with large discrepancies around 14 MeV. Only 238U and 232Th have been investigated up to 100 MeV in a single experiment. The n-TOF Collaboration performed the fission cross section measurement of several actinides (232Th, 235U, 238U, 234U, 237Np) at the n-TOF facility using an experimental set-up made of Parallel Plate Avalanche Counters (PPAC), extending the energy domain of the incident neutron above hundreds of MeV. The method based on the detection of the 2 fragments in coincidence allowed to clearly disentangle the fission reactions among other types of reactions occurring in the spallation domain. I will show the methods we used to reconstruct the full angular resolution by the tracking of fission fragments. Below 10 MeV our results are consistent with existing data. For example in the case of 232Th, below 10 MeV the results show clearly the variation occurring at the first (1 MeV) and second (7 MeV) chance fission, corresponding to transition states of given J and K (total spin and its projection on the fission axis), and a much more accurate energy dependence at the 3. chance threshold (14 MeV) has been obtained. In the spallation domain, above 30 MeV we confirm the high anisotropy revealed in 232Th by the single existing data set. I'll discuss the implications of this finding, related to the low anisotropy exhibited in proton-induced fission. I also explore the critical experiments which is valuable checks of nuclear data. The 237Np neutron-induced fission cross section has recently been measured in a large energy range (from eV to GeV) at the n-TOF facility at CERN. When compared to previous measurements, the n-TOF fission cross section appears to be higher by 5-7 % beyond the fission threshold. To check the relevance of n-TOF data, we simulate a criticality experiment performed at Los Alamos with a 6 kg sphere of 237Np. This sphere was surrounded by enriched uranium 235U so as to approach criticality with fast neutrons. The simulation predicts a multiplication factor keff in better agreement with the experiment (the deviation of 750 pcm is reduced to 250 pcm) when we replace the ENDF/B- VII.0 evaluation of the 237Np fission cross section by the n-TOF data. We also explore the hypothesis of deficiencies of the inelastic cross section in 235U which has been invoked by some authors to explain the deviation of 750 pcm. The large distortion that should be applied to the inelastic cross sections in order to reconcile the critical experiment with its simulation is incompatible with existing measurements. Also we show that the ?-bar of 237Np can hardly be incriminated because of the high accuracy of the existing data. Fission rate ratios or averaged fission cross sections measured in several fast neutron fields seem to give contradictory results on the validation of the 237Np cross section but at least one of the benchmark experiments, where the active deposits have been well calibrated for the number of atoms, favors the n-TOF data set. These outcomes support the hypothesis of a higher fission cross section of 237Np. (author)

280

Fission product gamma spectra  

International Nuclear Information System (INIS)

The fission product gamma spectra of 233U, 235U, and 239Pu were measured at 12 cooling times following 20,000-s irradiations in the thermal column of the Omega West Reactor. The mean cooling times ranged from 29 s to 146,500 s. The total gamma energies were obtained by integrating over the energy spectra, and both the spectra and the total energies are compared with calculations using the CINDER-10 code and ENDF/B-IV data base. The measured and calculated gamma spectra are compared in a series of figures. The measured total gamma energies are approx. 14% larger than the calculated energies during the earliest counting period (4 s to 54 s cooling time). For 235U, the measured and calculated total gamma energies are nearly the same after 1200 s cooling time, and the measurements are 2% to 6% lower at longer cooling times. For 239Pu, the measured and calculated total gamma energies are nearly the same at cooling times longer than 4,000 s, and for 233U this condition prevails at cooling times longer than 10,000 s. 39 figures, 4 tables

 
 
 
 
281

Fusion-fission hybrid blankets of thorium metal piles  

International Nuclear Information System (INIS)

Use of fission fertile materials in fusion reactor blankets, usually called fission-fusion hybrid reactors, has been discussed by several authors just after 'de-classification of fusion'. Our understanding at present, however, still wait a break-through. With an aim, production of nuclear fuels, candidates of fertile materials may be natural U, depleted U, and Th. The paper presents major findings obtained in these years by the collaboration between us. Specifically experimental systems for reaction cross-sections, and measurement of reaction rates by the foil-activation method. (author)

282

Fission-track stability in zircons under geological conditions  

International Nuclear Information System (INIS)

The stability behavior of fission tracks in zircon under natural annealing conditions has been studied. The samples have been obtained from the drill core of basement rocks (1664 to 3330 m depth) of Urach (Southwest Germany). Using the external detector method, Zircon fission-track ages between 158 and 327 Myr were found. The ages decrease steadily downhole. For a cooling rate of 1degC/1 Myr the closure temperature for zircon has been estimated to 210 +- 20degC. A geometry factor of 1.47 +- 4.7% (4?-zircon/2?-muscovite) has been determined relevant to the dating procedure used in this work. (author)

283

Measurement of fission gas release from irradiated nuclear fuel elements  

International Nuclear Information System (INIS)

A fission gas measurement system for the analysis of released gases from MOX and PHWR fuels has been designed, fabricated and commissioned in the hot cells of Post Irradiation Examination Division of Bhabha Atomic Research Centre, Mumbai. The system was used for the measurement of fission gases released from natural UO2 fuels and ThO2 fuels from PHWRs. The burnups of these fuels ranged from 2 GWD/TeU to 15 GWD/TeU. Some of the results from PHWR fuel elements from Kakrapar Atomic Power Station are presented in the paper, to highlight the utility of the system. (author)

284

An analytical procedure for calculating the fast fission factor  

International Nuclear Information System (INIS)

Processes occurring in fast reactors or in thermal reactors due to fast neutrons are treated by multigroup method. This is needed because of the complexity of operator representing the inelastic scattering on heavy nuclei. This paper shows and analytical procedure for solving the slowing-down equation in the fuel. This procedure is applied to calculate the fast fission spectrum and number of reactions in the natural uranium fuel element. The method for calculating the fast fission factor in the isolated fuel element is shown as well

285

Chemical reactions of fission products with ethylene using the gas jet technique  

International Nuclear Information System (INIS)

An understanding of the nature of the chemical reactions taking place between fission products and their carrier gases, and the designing of a fast separation procedure were the purposes of this investigation. Chemical reactions of short-lived (less than one minute half-life) fission products with carrier gases lead to various chemical species which can be separated in the gas phase. The Gas Jet Facility at the Ford Nuclear Reactor was used to study the yields of volatile selenium and bromine fission products of 235U using a semi-automatic batch solvent extraction technique. Heptane and water were used as organic and inorganic solvents. A carrier gas mixture of ethylene to pre-purified nitrogen (1 : 3) was used to sweep the fission products from the target to the chemistry area for analysis. The results indicated that the volatile selenium products generated by the interaction of selenium fission fragments with ethylene were predominantly organic in nature (84%), possibly organoselenides. The selenium values were used to resolve the fractions of the bromine nuclides, which come from two major sources, viz., directly from fission and from the beta-decay of selenium. The data showed that the fractions of independent bromine fission products in the organic phase were much lower compared to selenium; the bromine values range from 10 to 22% and varied with mass number. Results indicated that the bromine products were inorganic in nature, as possibly hydrogen chlrganic in nature, as possibly hydrogen chloride. ((orig.))

286

Probability of fission of radium and actinium near threshold  

International Nuclear Information System (INIS)

The natures of the structures in the fission probabilities of 227Ra and 227Ac near threshold are studied. The cross sections for fission of 226Ra by neutrons have been measured with improved energy resolution and decreased background. The data obtained confirm the existence of a plateau-shaped irregularity in the 227Ra fission probability. Special attention is devoted to calculation of the spectrum of transition states on the basis of the superfluid model with use of the single-particle spectra of a deformed potential. It is shown that the irregularities in the fissility of 227Ra and 227Ac are the result of the properties of the spectra of transition states in the framework of the one-hump barrier model. Here no hypotheses are required regarding the existence of quasistationary states in the third potential minimum

287

Reference Reactor Module for the Affordable Fission Surface Power System  

International Nuclear Information System (INIS)

Surface fission power systems on the Moon and Mars may provide the first US application of fission reactor technology in space since 1965. The requirements of many surface power applications allow the consideration of systems with much less development risk than most other space reactor applications, because of modest power (10s of kWe) and no driving need for minimal mass (allowing temperatures 2-fueled, liquid metal-cooled fission reactor coupled to free-piston Stirling converters. The reactor shielding approach utilizes both in-situ and launched shielding to keep the dose to astronauts much lower than the natural background radiation on the lunar surface. One of the important 'affordability' attributes is that the concept has been designed to minimize both the technical and programmatic safety risk

288

Quasi-fission and fusion-fission competition in 32S + 184W reaction  

International Nuclear Information System (INIS)

The angular distribution of fission fragments for the 32S+184W reaction at center-of-mass energies of 118.8, 123.1, 127.3, 131.5, 135.8, 141.1 and 144.4 MeV were measured. The experimental fission excitation function is obtained. The fragment angular anisotropy is found by extrapolating the fission angular distributions. The measured fission cross sections are decomposed into fusion-fission, quasi-fission and fast-fission contributions by the dinuclear system model. The total evaporation residue and fusion-fission excitation functions are calculated in the framework of the advanced statistical model. (authors)

289

Fission product solvent extraction  

International Nuclear Information System (INIS)

Two main objectives concerning removal of fission products from high-level tank wastes will be accomplished in this project. The first objective entails the development of an acid-side Cs solvent-extraction (SX) process applicable to remediation of the sodium-bearing waste (SBW) and dissolved calcine waste (DCW) at INEEL. The second objective is to develop alkaline-side SX processes for the combined removal of Tc, Cs, and possibly Sr and for individual separation of Tc (alone or together with Sr) and Cs. These alkaline-side processes apply to tank wastes stored at Hanford, Savannah River, and Oak Ridge. This work exploits the useful properties of crown ethers and calixarenes and has shown that such compounds may be economically adapted to practical processing conditions. Potential benefits for both acid- and alkaline-side processing include order-of-magnitude concentration factors, high rejection of bulk sodium and potassium salts, and stripping with dilute (typically 10 mM) nitric acid. These benefits minimize the subsequent burden on the very expensive vitrification and storage of the high-activity waste. In the case of the SRTALK process for Tc extraction as pertechnetate anion from alkaline waste, such benefits have now been proven at the scale of a 12-stage flowsheet tested in 2-cm centrifugal contactors with a Hanford supernatant waste simulant. SRTALK employs a crown ether in a TBP-modified aliphatic kerosene diluent, is economically competitive with other applicable separation processes being considered, and has been successfully tested in batch extraction of actual Hanford double-shell slurry feed (DSSF)

290

Neutron dosimetry in fission-track analysis  

International Nuclear Information System (INIS)

This paper reports a series of irradiation experiments performed to illustrate the effects of neutron energy on fission-track age determinations. These experiments reveal that errors of several percent can be introduced into an age determination when fluence monitors which contain U in non-natural isotopic ratios and (or) contain Th are used in the irradiations, or when age standards and samples of unknown age are irradiated at different conditions (i.e. with neutrons having different energy spectra). To avoid errors in age determinations it is recommended that all irradiations be performed at identical conditions, preferably with a highly thermalized neutron flux, or that fluence monitors which do not contain Th and which contain U in natural abundances be used in the age-determination process. (author)

291

Compact fission counter for DANCE  

Energy Technology Data Exchange (ETDEWEB)

The Detector for Advanced Neutron Capture Experiments (DANCE) consists of 160 BF{sub 2} crystals with equal solid-angle coverage. DANCE is a 4{pi} {gamma}-ray calorimeter and designed to study the neutron-capture reactions on small quantities of radioactive and rare stable nuclei. These reactions are important for the radiochemistry applications and modeling the element production in stars. The recognition of capture event is made by the summed {gamma}-ray energy which is equivalent of the reaction Q-value and unique for a given capture reaction. For a selective group of actinides, where the neutron-induced fission reaction competes favorably with the neutron capture reaction, additional signature is needed to distinguish between fission and capture {gamma} rays for the DANCE measurement. This can be accomplished by introducing a detector system to tag fission fragments and thus establish a unique signature for the fission event. Once this system is implemented, one has the opportunity to study not only the capture but also fission reactions. A parallel-plate avalanche counter (PPAC) has many advantages for the detection of heavy charged particles such as fission fragments. These include fast timing, resistance to radiation damage, and tolerance of high counting rate. A PPAC also can be tuned to be insensitive to {alpha} particles, which is important for experiments with {alpha}-emitting actinides. Therefore, a PPAC is an ideal detector for experiments requiring a fast and clean trigger for fission. A PPAC with an ingenious design was fabricated in 2006 by integrating amplifiers into the target assembly. However, this counter was proved to be unsuitable for this application because of issues related to the stability of amplifiers and the ability to separate fission fragments from {alpha}'s. Therefore, a new design is needed. A LLNL proposal to develop a new PPAC for DANCE was funded by NA22 in FY09. The design goal is to minimize the mass for the proposed counter and still be able to maintain a stable operation under extreme radioactivity and the ability to separate fission fragments from {alpha}'s. In the following sections, the description is given for the design and performance of this new compact PPAC, for studying the neutron-induced reactions on actinides using DANCE at LANL.

Wu, C Y; Chyzh, A; Kwan, E; Henderson, R; Gostic, J; Carter, D; Bredeweg, T; Couture, A; Jandel, M; Ullmann, J

2010-11-06

292

Contribution to the study of nuclear fission  

International Nuclear Information System (INIS)

The author proposes an overview of his research activity during the past fifteen years and more particularly that dealing with nuclear fission. The first part reports works on nucleus physics at the scission via the investigation of ternary fission (experimental procedure, influence of fission modes, influence of resonance spin, influence of excitation energy of the fissioning nucleus, emission probabilities, energy spectra of ternary alphas and tritons, emission mechanism). The second part reports measurements and assessments of neutron-induced fission cross sections. The third part reports the investigation of some properties of fission products (efficiencies, branching ratios of the main delayed neutron precursors)

293

The latest progress of fission track analysis  

International Nuclear Information System (INIS)

Fission track analysis as a new nuclear track technique is based on fission track annealing in mineral and is used for oil and gas exploration successfully. The west part of China is the main exploration for oil and gas. The oil and gas basins there experienced much more complicated thermal history and higher paleotemperature. In order to apply fission track analysis to these basins, following work was be carried out: 1. The decomposition of grain age distribution of zircon fission tracks. 2. Study on thermal history of Ordos basin using zircon fission track analysis. 3. The fission track study on the Qiang Tang basin in tibet

294

Change over from compound nuclear fission to quasi-fission  

Directory of Open Access Journals (Sweden)

Full Text Available Fission fragment mass distribution has been measured in two reactions to populate compound nucleus 246Bk. Both the target nuclei were deformed. However, entrance channel mass asymmetry of the two systems was on the either side of the Businaro Gallone mass asymmetry parameter. Near the Coulomb barrier, at similar excitation energies, the width of the fission fragment mass distribution was found to be significantly different for the 14N+232Th reaction compared to the 11B+235U reaction. The entrance channel mass asymmetry was found to play a significant role in deciding the fusion process.

Bhattacharya P.

2010-03-01

295

Fission Yields in the Iodine Region  

International Nuclear Information System (INIS)

Independent yields of all iodine isotopes from l118 to I134 except I122 (short-lived), I127(stable) and I129(long-lived) resulting from irradiating natural uranium with 590-MeV and 19-GeV protons have been measured. In addition, cross-sections (mostly cumulative) of many xenon and tellurium isotopes have been obtained. In the experiments extensive use has been made of an electromagnetic isotope separator, constructed at CERN for nuclear reaction studies, by which the iodine (or tellurium) isotopes were separated from samples chemically isolated from the irradiated targets. In the study of xenon isotopes the uranium target was heated in a small oven connected to the separator via a cold trap to stop unwanted activities. After the separation, the activity of the samples was measured by counting methods. In certain cases (I118, I119, I120,I121, Xe118, Xe119, Xe120, Xe121) spectroscopic investigations were performed to provide information for the conversion of the counting data to absolute counting-rates needed for the determination of the fission yields. The experiments show a significant shift in the isotopic cross-section distribution when the 19-GeV results are compared with those obtained at the lower irradiation energy. The yields far out ai the neutron-deficient wing increase considerably whereas the other part of the distribution is depressed. A comparison with spallation data indicates that the neutron-deficient part of the distribution might result from the spallation of uranium. Spallation gives negligible yields in the iodine regional 590 MeV, but at 19 GeV these cross-sections a re expected to be much larger. The competition with spallation decreases the probability for fission, and consequently the yields of the fission products will decrease. (author)

296

Experimental approach to fission process of actinides  

Energy Technology Data Exchange (ETDEWEB)

From experimental views, it seems likely that the mechanism of nuclear fission process remains unsolved even after the Bohr and Weeler`s study in 1939. Especially, it is marked in respect of mass distribution in unsymmetric nuclear fission. The energy dependency of mass distribution can be explained with an assumption of 2-mode nuclear fission. Further, it was demonstrated that the symmetrical fission components and the unsymmetrical ones have different saddle and fission points. Thus, the presence of the 2-mode fission mechanism was confirmed. Here, transition in the nuclear fission mechanism and its cause were investigated here. As the cause of such transition, plausible four causes; a contribution of multiple-chance fission, disappearance of shell effects, beginning of fission following collective excitation due to GDR and nuclear phase transition were examined in the condition of excitation energy of 14.0 MeV. And it was suggested that the transition in the nuclear fission concerned might be related to phase transition. In addition, the mechanism of nuclear fission at a low energy and multi-mode hypothesis were examined by determination of the energy for thermal neutron fission ({sup 233,235}U and {sup 239}Pu) and spontaneous nuclear fission ({sup 252}Cf). (M.N.)

Baba, Hiroshi [Osaka Univ., Toyonaka (Japan). Faculty of Science

1997-07-01

297

Fission products measurements in the SLOWPOKE 2 reactor at the University of Toronto  

International Nuclear Information System (INIS)

During 1995 and 1996 new measurements on fission and activation products in the water and air of the SLOWPOKE 2 nuclear reactor at the University of Toronto were performed. Due to the age of the fuel, small quantities of fission products are released during normal operation of the reactor. Low concentrations of fission and activation products can be measured in the water and air of the reactor. Studying the behaviour of fission and activation products in their natural conditions inside the reactor is very useful, both for improving the understanding of their real chemical and physical properties and for the development and evaluation of analytical methodologies. Adsorption and desorption of noble gases on charcoal can play an important role during any release of fission and activation products from containment to the atmosphere following a reactor accident. Experiments conducted at the SLOWPOKE 2 Reactor permitted the study of these phenomena

298

Effects of pre-fission neutron emission on the fission fragment angular distributions in heavy-ion induced fission  

International Nuclear Information System (INIS)

In view of the recent observation of significant pre-fission neutron multiplicities in heavy-ion induced fusion-fission reactions, we have carried out calculation of fragment anisotropies taking into account the effect of pre-fission neutron emission to make a better comparison of the standard saddle point statistical model (SSPSM) with the experimental results. (author). 10 refs., 2 figs

299

Fission at intermediate neutron energies  

Science.gov (United States)

In the present work, as a theoretical support to the campaign of neutron cross section measurements at the n_TOF facility at CERN[1], Monte Carlo calculations of fission induced by neutrons in the energy range from 100 MeV to 1 GeV are carried out by means of a recent version of the Liege Intranuclear Cascade Model, INCL++[6], coupled with different evaporation-fission codes, such as Gemini++[7] and ABLA07[8]. Theoretical cross sections are compared with experimental data obtained by the n_TOF collaboration and perspectives for future theoretical work are outlined.

Lo Meo, S.; Mancusi, D.; Massimi, C.; Vannini, G.; Ventura, A.

2014-09-01

300

Search for Singlet Fission Chromophores  

Energy Technology Data Exchange (ETDEWEB)

Singlet fission, in which a singlet excited chromophore shares its energy with a ground-state neighbor and both end up in their triplet states, is of potential interest for solar cells. Only a handful of compounds, mostly alternant hydrocarbons, are known to perform efficiently. In view of the large number of conditions that a successful candidate for a practical cell has to meet, it appears desirable to extend the present list of high performers to additional classes of compounds. We have (i) identified design rules for new singlet fission chromophores and for their coupling to covalent dimers, (ii) synthesized them, and (iii) evaluated their performance as neat solids or covalent dimers.

Havlas, Z.; Akdag, A.; Smith, M. B.; Dron, P.; Johnson, J. C.; Nozik, A. J.; Michl, J.

2012-01-01

 
 
 
 
301

Photo fission of actinide targets  

International Nuclear Information System (INIS)

The photofission cross-section of 232Th, 238U and 237Np was measured using Bremsstrahlung radiation in the energy range 7.4 to 9.2 MeV from microtron at Mangalore University. In the present work SSNTD lexan polycarbonate films were used to detect the fission tracks. The experimental photofission cross-section measurements were compared with the results obtained from the theoretical predictions of EMPIRE-II code for various potential barrier parameters and analytical fission barrier formula. (author)

302

The VERDI fission fragment spectrometer  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The VERDI time-of-flight spectrometer is dedicated to measurements of fission product yields and of prompt neutron emission data. Pre-neutron fission-fragment masses will be determined by the double time-of-flight (TOF) technique. For this purpose an excellent time resolution is required. The time of flight of the fragments will be measured by electrostatic mirrors located near the target and the time signal coming from silicon detectors located at 50 cm on both sides of the target. This conf...

Frégeau M.O.; Bry? T.; Gamboni Th.; Geerts W.; Oberstedt S.; Oberstedt A.; Borcea R.

2013-01-01

303

Ceramic Hosts for Fission Products Immobilization  

Energy Technology Data Exchange (ETDEWEB)

Natural spinel, perovskite and zirconolite rank among the most leach resistant of mineral forms. They also have a strong affinity for a large number of other elements and including actinides. Specimens of natural perovskite and zirconolite were radioisotope dated and found to have survived at least 2 billion years of natural process while still remain their loading of uranium and thorium . Developers of the Synroc waste form recognized and exploited the capability of these minerals to securely immobilize TRU elements in high-level waste . However, the Synroc process requires a relatively uniform input and hot pressing equipment to produce the waste form. It is desirable to develop alternative approaches to fabricate these durable waste forms to immobilize the radioactive elements. One approach is using a high temperature process to synthesize these mineral host phases to incorporate the fission products in their crystalline structures. These mineral assemblages with immobilized fission products are then isolated in a durable high temperature glass for periods measured on a geologic time scale. This is a long term research concept and will begin with the laboratory synthesis of the pure spinel (MgAl2O4), perovskite (CaTiO3) and zirconolite (CaZrTi2O7) from their constituent oxides. High temperature furnace and/or thermal plasma will be used for the synthesis of these ceramic host phases. Nonradioactive strontium oxide will be doped into these ceramic phases to investigate the development of substitutional phases such as Mg1-xSrxAl2O4, Ca1-xSrxTiO3 and Ca1-xSrxZrTi2O7. X-ray diffraction will be used to establish the crystalline structures of the pure ceramic hosts and the substitution phases. Scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDX) will be performed for product morphology and fission product surrogates distribution in the crystalline hosts. The range of strontium doping is planned to reach the full substitution of the divalent metal ions, Mg and Ca, in the ceramic host phases. The immobilization of rear earth (lanthanide series) fission products in these ceramic host phases will also be studied this year. Cerium oxide is chosen to represent the rear earth fission product for substitution studies in spinel, perovskite and zirconolite ceramic hosts. Cerium has +3 and +4 oxidation states and it can replace some of the trivalent or tetravalent host ions to produce the substitution ceramics such as MgAl2-xCexO4, CaTi1-xCexO3, CaZr1-xCexTi2O7 and CaZrTi2-xCexO7. X-ray diffraction analysis will be used to compare the crystalline structures of the pure ceramic hosts and the substitution phases. SEM-EDX analysis will be used to study the Ce distribution in the ceramic host phases. The range of cerium doping is planned to reach the full substitution of the trivalent or tetravalent ions, Al, Ti and Zr, in the ceramic host phases.

Peter C Kong

2010-07-01

304

Fission Matrix Capability for MCNP Monte Carlo  

Science.gov (United States)

We describe recent experience and results from implementing a fission matrix capability into the MCNP Monte Carlo code. The fission matrix can be used to provide estimates of the fundamental mode fission distribution, the dominance ratio, the eigenvalue spectrum, and higher mode forward and adjoint eigenfunctions of the fission neutron source distribution. It can also be used to accelerate the convergence of the power method iterations and to provide basis functions for higher-order perturbation theory. The higher-mode fission sources can be used in MCNP to determine higher-mode forward fluxes and tallies, and work is underway to provide higher-mode adjoint-weighted fluxes and tallies. Past difficulties and limitations of the fission matrix approach are overcome with a new sparse representation of the matrix, permitting much larger and more accurate fission matrix representations. The new fission matrix capabilities provide a significant advance in the state-of-the-art for Monte Carlo criticality calculations.

Brown, Forrest; Carney, Sean; Kiedrowski, Brian; Martin, William

2014-06-01

305

Characterization of Samples with Spontaneously Fissioning Isotopes  

International Science & Technology Center (ISTC)

Development of the Method for Characterization of the Samples, Containing Spontaneously Fissioning Radionuclides, by Measuring Fission Products Gamma-Radiation (for the System of NM Control and Accountability of the Federal State Unitarian Enterprise "PA"Mayak")

306

Progress in fission product nuclear data  

International Nuclear Information System (INIS)

This is the 12th issue of a report series on Fission Product Nuclear Data (FPND) which is published by the Nuclear Data Section (NDS) of the IAEA. The purpose of this series is to inform scientists working on FPND, or using such data, about all activities in this field which are planned, ongoing, or have recently been completed. The type of activities included are: measurements, compilations and evaluations of fission product yields (neutron induced and spontaneous fission), neutron reaction cross sections of fission products, data related to the radioactive decay of fission products, delayed neutron data of fission products and lumped fission product data (decay heat, absorption etc.). The first part of the report consists of unaltered original contributions which the authors have sent to IAEA/NDS. The second part contains recent references relative to fission product nuclear data, which were not covered by the contributions submitted, and selected papers from conferences

307

Neutronics for critical fission reactors and subcritical fission in hybrids  

Science.gov (United States)

The requirements of future innovative nuclear fuel cycles will focus on safety, sustainability and radioactive waste minimization. Critical fast neutron reactors and sub-critical, external source driven systems (accelerator driven and fusion-fission hybrids) have a potential role to meet these requirements in view of their physics characteristics. This paper provides a short introduction to these features.

Salvatores, Massimo

2012-06-01

308

Anomalous anisotropies of fission fragments in near- and sub-barrier fusion-fission reactions  

International Nuclear Information System (INIS)

Systematic summary on the anomalous anisotropies of fission fragments in near- and sub-barrier fusion-fission reactions is presented. A new version of the preequilibrium fission model is put forward to explain the anomalous anisotropies of fission fragments, except the bump of anomaly. Further study is still needed

309

Nuclear fission in covariant density functional theory  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The current status of the application of covariant density functional theory to microscopic description of nuclear fission with main emphasis on superheavy nuclei (SHN) is reviewed. The softness of SHN in the triaxial plane leads to an emergence of several competing fission pathes in the region of the inner fission barrier in some of these nuclei. The outer fission barriers of SHN are considerably affected both by triaxiality and octupole deformation.

Afanasjev A.V.; Abusara H.; Ring P.

2013-01-01

310

Cold (neutronless) ? ternary fission of 252Cf  

International Nuclear Information System (INIS)

The phenomenon of cold (neutronless) alpha ternary fission in spontaneous fission of 252Cf was experimentally observed by triple gamma coincidence technique with Gammasphere with 72 gamma-ray detectors. Correlated pairs of 36Kr-60Nd, 38Sr-58Ce, 40Zr-56Ba, 42Mo-54Xe, 44Ru-52Te, and 46Pd-50Sn were observed to be associated with ? ternary fission of 252Cf. Yields of cold ? ternary fission were extracted. copyright 1998 The American Physical Society

311

Process for treating fission waste. [Patent application  

Science.gov (United States)

A method is described for the treatment of fission waste. A glass forming agent, a metal oxide, and a reducing agent are mixed with the fission waste and the mixture is heated. After melting, the mixture separates into a glass phase and a metal phase. The glass phase may be used to safely store the fission waste, while the metal phase contains noble metals recovered from the fission waste.

Rohrmann, C.A.; Wick, O.J.

1981-11-17

312

Nuclear fission in covariant density functional theory  

Directory of Open Access Journals (Sweden)

Full Text Available The current status of the application of covariant density functional theory to microscopic description of nuclear fission with main emphasis on superheavy nuclei (SHN is reviewed. The softness of SHN in the triaxial plane leads to an emergence of several competing fission pathes in the region of the inner fission barrier in some of these nuclei. The outer fission barriers of SHN are considerably affected both by triaxiality and octupole deformation.

Afanasjev A.V.

2013-12-01

313

Fission distribution measurements of Atucha's fuel pellets with solid state track detectors  

International Nuclear Information System (INIS)

Distribution of fissions in a UO2 rod has been measured by means of solid state detectors. Mica muscovite and Makrofol-N detectors were used in the experiment. The merits of mica muscovite relative to the Makrofol-N for the detection of fission fragments have been verified. However both fission track detectors closely agree (0,5%) in the final fission distribution of the UO2 rod. Sensitivity of the detectors shows to be linear in the range between 50.000and 360.000 fission tracks per square centimeter. Due to the high spatial resolution this method is better than any other technique. Determination were made in UO2 pellets similar to the fuel element of the Atucha reactor. The average fission rate in the rod has been measured within 0,8% error, and provides an accurate determination for the distribution of fissions in the rod wich is needed for the determination of energy liberated per fission in the natural uranium rod.(author)

314

Determination of fission yields in the fast neutron induced fission of 238U, 237Np and 243Am using fission track etch-cum gamma spectrometry  

International Nuclear Information System (INIS)

The absolute fission yields of 39 fission products in pure 238U(99,9997 atom percent), 36 fission products in 237Np and 30 fission products in 243Am (99.998 atom percent) were determined in the fast neutron induced fission, employing fission track etch-cum gamma spectrometry. The fissions were induced by a well defined reactor neutron spectrum which was measured by using threshold detectors. 5 refs, 3 tabs

315

Implication of fission yields data to fission process and their relevance to ADS  

International Nuclear Information System (INIS)

Cumulative and independent fission yields of various fission products have been determined in the neutron induced fission of 229,232Th, 232,233,235,238U, 237Np,238,239,240,241Pu, 241,243Am and 244,245Cm as well as in the spontaneous fission of 244Cm and 252Cf. The fission yields data were used to carry out mass and charge distribution studies to understand the fission mechanism. Besides this, the data of fast neutron induced fission of minor long-lived actinides were discussed from the point of fast reactor design and their importance to ADS. (author)

316

Origin of empirical threshold for dissipative fission  

International Nuclear Information System (INIS)

The recent observation that the empirical threshold for fission hindrance correlates with the ratio of the threshold temperature to the temperature dependent fission barrier height is interpreted in terms of Bohr-Wheeler theory. A total fission delay of (2--4) times 10 sup -2--- s is deduced from the empirical value of this ratio

317

Origin of empirical threshold for dissipative fission  

Science.gov (United States)

The recent observation that the empirical threshold for fission hindrance correlates with the ratio of the threshold temperature to the temperature dependent fission barrier height is interpreted in terms of Bohr-Wheeler theory. A total fission delay of (2-4) times 10 sup -2- s is deduced from the empirical value of this ratio.

Vandenbosch, Robert

1994-11-01

318

Hybrid nuclear cycles for nuclear fission sustainability  

International Nuclear Information System (INIS)

Nuclear fission can play and must play an important role in paving the road to Energy Sustainability. Nuclear Fission does not produce CO2 emissions, and it is already exploited at commercial level with the current NPP (Nuclear Power Plants). Most of them are based on LWR reactors, which have a very good safety record. It must be noted, however, that all LWR (including the advanced or evolutionary ones) have some drawbacks, particularly their very poor efficiency in exploiting the natural resources of nuclear fuels. In this paper, an analysis is presented on how to maximize the energy actually generated from the potential contents of fission natural resources. The role of fertile-to-fissile breeding is highlighted, as well as the need of attaining a very high safety performance in the reactors and other installations of the fuel cycle. The proposal presented in this paper is to use advanced and evolutionary LWR as energy producing reactors, and to use subcritical fast assemblies as breeders. The main result would be to increase by two orders of magnitude the percentage of energy effectively exploited from fission natural resources, while keeping a very high level of safety standards in the full fuel cycle. Breeders would not be intended for energy production, so that safety standards could rely on very low values of the thermal magnitudes, so allowing for very large safety margins for emergency cooling. Similarly, subcriticality would offer a very large margin for not to reach prompt criticality in any event. The main drawback of this proposal is that a sizeable fraction of the energy generated in the cycle (about 1/3, maybe a little more) would not be useful for the thermodynamic cycle to produce electricity. Besides that, a fraction of the generated electricity, between 5 and 10 %, would have to be recirculated to feed the accelerator activating the neutron source. Even so, the overall result would be very positive, because more than 50 % of the natural resources could be exploited with such a cycle, using very safe reactors. This percentage is much higher than the actual value for the once-through cycle (0.5 %) and the value for multiple Pu recycling in the MOX scheme (1 %). Moreover, thorium could also be exploited through fertile conversion into U-233 in the subcritical breeders. The separation between energy production (to be done in LWR) and nuclear breeding (to be done in subcritical hybrids) presents a scenario with very appealing safety features and a high potential for an efficient utilization of all natural resources of uranium and thorium, that account for 1024 J, i.e., 25 Gtoe, which is 35,000 times as large as the annual production of Nuclear Energy nowadays, and about 2,500 times as large as the total annual energy consumption all over the globe

319

Weathering processes at the natural nuclear reactor of Bangombe (Gabon). Identification and geochemical modeling of the retention and migration mechanisms of uranium and rare earth elements  

International Nuclear Information System (INIS)

The natural nuclear fission reactor of Bangombe (Gabon) was discovered in 1985. It is located 30 km SE of the uranium Oklo ore deposit which is well-known for its reactors discovered in 1972. In contrast to the latter ones, the reaction zone of Bangombe is situated close to the surface and therefore has been affected by supergene weathering processes. The purpose of this study was to characterize the main effects related to these alteration processes on the rocks surrounding the Bangombe reactor zone as well as to determine the major mechanisms, influencing the migration and retention of U and REE in this geological system. The different approaches considered in this study comprise mineralogical and geochemical investigations, mass balance calculations, sequential extraction experiments as well as thermodynamic simulations. It could be shown that the present rock and mineral assemblages result from a complex and multi-stage history during which the rocks were affected by diagenetic, hydrothermal, tectonic and recent alteration processes. Multiple transformations led to the setting of different horizons characterized by very specific physico-chemical conditions and mineral associations. It has been shown that in the various units of the weathering sequence, the mechanisms and mineral phases determining the U and REE migration/retention behaviour are quite different and highly dependent on the physico-chemical conditions prevailing in the ambient environment. Apart froming in the ambient environment. Apart from residual and neo-formed clays, especially amorphous and crystalline Fe- and Mn oxides and oxy-hydroxides, phosphates as well as minor phases such as carbonates and heavy minerals were identified to play an important role in REE and U mobility. The acquisition of hydrodynamic data enabled to simulate water-rock interaction and mass-transfer processes which have been produced during the alteration of the Bangombe reactor zone. Thermodynamic simulations showed that elevated U-concentrations downstream the reactor zone at present-days may be simply explained by local re-equilibration of the aquifer forming pelites (containing primary and secondary U-minerals) with the ambient weathering solution. (authors)

320

How spontaneous fission was discovered  

International Nuclear Information System (INIS)

The 70th anniversary of the discovery of spontaneous fission by the young Russian physicists Konstantin A. Petrzhak and Georgii N. Flerov is commemorated. The situation in the 1940s is described and the activities of the 2 scientists, including their involvement in the development of the A-bomb, is outlined. (P.A.)

 
 
 
 
321

Spontaneous fission of rutherfordium isotopes  

International Nuclear Information System (INIS)

We have found several new spontaneous fission (SF) activities and suggested assignments for some of them to rutherfordium (element 104) isotopes. Their half-lives and production cross sections have been measured by collecting recoils on a moving nickel or steel tape and transporting them past stationary mica track detectors at known velocities. The following tentative assignments are based on several cross bombardments and comparisons between experimental and calculated production cross sections: 256Rf(9 +- 2 ms), 257Rf(3.8 +- 0.8 s, 14 +- 9% SF), 258Rf(13 +- 3 ms), 259Rf(3.4 +- 1.7 s, 9 +- 3% SF), 260Rf(21 +- 1 ms), and 262Rf(47 +- 5 ms). Presently we are unable to assign several other new spontaneous fission activities with half-lives of approx.15 ms, approx.22 ms, approx.100 ms (Z260Rf, in reactions in which we expected to produce this isotope. Our measurements support a shift in the spontaneous fission half-life systematics at element 104, first proposed by Flerov and Oganessian et al. and later predicted by Randrup et al. and Baran et al. and attributed to the disappearance of the second hump of the double-humped fission barrier and a weakening of the 152-ission barrier and a weakening of the 152-neutron subshell

322

Fine structure in superasymmetric fission  

Science.gov (United States)

The influence of the Landau-Zener effect on the fine structure of cluster decay phenomenon is emphasized. Estimations of hindrance factors are reported for the 14C emission from 223Ra. The half-lives are computed considering the cluster decay as a superasymmetric fission process. The level scheme is evaluated with the superasymmetric two-center shell model.

Mirea, M.; Clapier, F.

1997-12-01

323

Energetics of the fission process  

Energy Technology Data Exchange (ETDEWEB)

The mass asymmetry of fragments from nuclear fission of heavy nuclei is reviewed. While mass asymmetry is a common and well-known phenomenon for low-energy fission of the lighter actinides, more recent experiments have demonstrated that, for the heaviest actinides, the mass distribution switches to a symmetric one. On the other hand, it has been discovered that, though for fissioning nuclei with mass numbers A< or [approx]225 the mass distribution is basically symmetric, an asymmetric component is clearly to be identified for nuclei down to the Pb-region. In the absence of a generally accepted dynamical theory of fission, the above experimental findings are discussed in terms of static energy considerations. Triggered from the outset by the structure of the potential energy surface at the saddlepoint, the energy balance at the scission point between the available energy (Q-value) of the reaction and the Coulomb and deformation energy of the nascent fragments is shown to steer the characteristics of the fragment mass distributions. (orig.)

Goennenwein, F. (Physikalisches Inst., Univ. Tuebingen (Germany))

1994-09-01

324

Energetics of the fission process  

International Nuclear Information System (INIS)

The mass asymmetry of fragments from nuclear fission of heavy nuclei is reviewed. While mass asymmetry is a common and well-known phenomenon for low-energy fission of the lighter actinides, more recent experiments have demonstrated that, for the heaviest actinides, the mass distribution switches to a symmetric one. On the other hand, it has been discovered that, though for fissioning nuclei with mass numbers A< or ?225 the mass distribution is basically symmetric, an asymmetric component is clearly to be identified for nuclei down to the Pb-region. In the absence of a generally accepted dynamical theory of fission, the above experimental findings are discussed in terms of static energy considerations. Triggered from the outset by the structure of the potential energy surface at the saddlepoint, the energy balance at the scission point between the available energy (Q-value) of the reaction and the Coulomb and deformation energy of the nascent fragments is shown to steer the characteristics of the fragment mass distributions. (orig.)

325

Nuclear fission in reactor instrumentation  

International Nuclear Information System (INIS)

The limitation in designing a high neutron sensitive fission counter comes from the alpha induced pulse pile-up with increase in coated quantity of 235U. The thicker coating reduces the number of fission fragments reaching the active volume and increase in coated surface area gives rise to higher capacitance resulting in severe alpha pile-up reducing the effective neutron sensitivity. The short resolving time (small capacitance) and small charge collection time (smaller electrode spacing) are the required features to overcome the problems associated with alpha pulse pile-up for achieving higher sensitivities. One such detector configuration using large number of electrodes connected in series as transmission line will achieve both the features. The adjacent signal electrodes are connected to each other through inductive elements thus each electrode constitutes a lumped-element transmission-line. The processing of pulses from each end of the transmission line to a time coincidence gate can improve discrimination. This configuration of fission detector can achieve sensitivity of about 6 cps/nv for detector of length 1 m and diameter 70 mm. The talk gives an overview of the fission detectors in reactor application and the recent developments

326

Brownian shape dynamics in fission  

Directory of Open Access Journals (Sweden)

Full Text Available It was recently shown that remarkably accurate fission-fragment mass distributions are obtained by treating the nuclear shape evolution as a Brownian walk on previously calculated five-dimensional potentialenergy surfaces; the current status of this novel method is described here.

Randrup Jørgen

2013-12-01

327

Calculation of fission fragment yields at low and intermediate energy fission  

International Nuclear Information System (INIS)

The model for fission fragment mass distributions and results of calculations for low energy fission and fission induced by intermediate energy nucleons is presented. Formation of mass distributions is considered as a result of oscillations on mass asymmetry degree of freedom in the potential well calculated in the macroscopic-microscopic approach. For intermediate energy fission the distribution of fissioning nuclei is taken into account with detailed reaction calculations including direct, preequilibrium and statistical reaction stages. (authors)

328

Sustainable and safe nuclear fission energy technology and safety of fast and thermal nuclear reactors  

CERN Document Server

Unlike existing books of nuclear reactor physics, nuclear engineering and nuclear chemical engineering this book covers a complete description and evaluation of nuclear fission power generation. It covers the whole nuclear fuel cycle, from the extraction of natural uranium from ore mines, uranium conversion and enrichment up to the fabrication of fuel elements for the cores of various types of fission reactors. This is followed by the description of the different fuel cycle options and the final storage in nuclear waste repositories. In addition the release of radioactivity under normal and possible accidental conditions is given for all parts of the nuclear fuel cycle and especially for the different fission reactor types.

Kessler, Günter

2012-01-01

329

The decay modes of heavy excited nuclei: from binary fission to multifragmentation  

International Nuclear Information System (INIS)

The decay modes of heavy excited nuclei formed in dissipative collisions for the systems Ar+Au at 60 MeV/u and Kr+Au at 43 MeV/u are studied. A transition from binary fission towards multi-fragment emission is observed around 3 MeV/u excitation energy, independently of the considered system. The charge distributions of the three target-emitted fragments suggest a smooth change from fission events (as a ''natural'' continuity of binary fission) to ''residue protection'' then to ''symmetric ternary fragmentation'' events when the excitation energy increases from 3 MeV/u to 5 MeV/u. (authors). 2 figs

330

Spontaneous fission. A many-body approach  

Energy Technology Data Exchange (ETDEWEB)

We propose new model to calculate the fission phenomena in tunnel region. By the Vlasov equation and the imaginary time method, we could calculate actinides nuclear fission. This method makes possible to describe unified the motion of fission inside and outside of potential wall. The potential energy and mass parameters can be calculated by no means of the special model. The freedom of internal motion are calculated automatically both collective and a particle motion. Accordingly, particle released during fission process can be calculated. The kinetic energy of fragment after fission was very agreeable with the calculation results. (S.Y.)

Iwamoto, Akira; Bonasera, A. [Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment

1997-03-01

331

Binary and ternary fission within statistical approach  

International Nuclear Information System (INIS)

Binary fission observables are described within the scission-point model by calculating the potential energy with the dinuclear system model. For the description of ternary fission we extend the scission-point model of binary fission. The ternary system with a light nucleus between two heavy fragments is assumed to be formed from the binary configuration near to the scission point. Charge number distributions, mean total kinetic energies and neutron multiplicity distributions are treated and compared with the experimental data for binary fission of actinides and for ternary fission of 252Cf accompanied by different light charged particles. (authors)

332

Brownian motion model of the fission process  

International Nuclear Information System (INIS)

Fission process is considered as a multi-dimensional Brownian motion. The Fokker-Planck equation is solved and the fission probability in the steady state approximation is calculated. The results are compared with Bohr--Wheeler theory. A numerical calculation for the fission of 236U shows the capability of the model in taking the effect of viscosity into account. From the available estimate of the fission viscosity, it is estimated that such effects can reduce the fission probability by 20% to 40%

333

Progress in fission product nuclear data  

International Nuclear Information System (INIS)

The purpose of this series is to inform scientists working on Fission Product Nuclear Data, or using such data, about all activities in this field which are planned, ongoing, or have recently been completed. This report consists of reproductions of essentially unaltered original contributions which the authors have sent to IAEA/NDS. The types of activities being included in this report are measurements, compilations and evaluations of: fission product yields; neutron cross-section data of fission products; data related to ?-, ?-decay of fission products; delayed neutron data; and fission product decay-heat

334

Fission of rapidly rotating fluid systems  

International Nuclear Information System (INIS)

Progress in understanding of the problem of fissional breakup of an equilibrium fluid system through the growth of dynamical distortional instabilities caused by rapid rotation is reviewed. Classic results on equilibrium states, stability, and routes to fission are reviewed, and the equilibrium states of uniformly rotating incompressible fluids, uniformly rotating polytropes, differentially rotating polytropes, rotating isothermal gas clouds, and nonisentropic fluids are discussed in detail. The Bardeen, Friedman, Schutz, and Sorkin (1977) criterion for linear stability in fission modes is considered along with the generic instability of rotating bodies and linear stability of fission modes in hydrodynamic simulations of fission are described in detail

335

Analysis of fissionable material using delayed gamma rays from photofission  

Energy Technology Data Exchange (ETDEWEB)

The energetic gamma-ray spectra from the fission products of photofission have been investigated to determine whether photofission can identify heavily shielded fissionable material. Target samples of natural thorium, 93% enriched /sup 235/U, natural uranium, and 93% enriched /sup 239/Pu were irradiated with bremsstrahlung gamma rays produced by 10-MeV electrons from a small linear accelerator. The gamma-ray spectra for each of the four isotopes studied reveals a distinctive intensity distribution. For example, the intensity ratio of the pair of gamma rays at 1436 keV (/sup 138/Cs) and 1428 keV (/sup 94/Sr) is 1.9 for /sup 235/U, 2.4 for /sup 238/U, 1.7 for /sup 232/Th and 1.4 for /sup 239/Pu. 6 refs., 2 figs., 1 tab.

Hollas, C.L.; Close, D.A.; Moss, C.E.

1986-09-01

336

Fission dynamics at low excitation energy  

CERN Document Server

The origin of mass asymmetry in the fission of uranium at a low excitation energy is clarified by a trajectory analysis of the Langevin equation. The positions of the peaks in the mass distribution of fission fragments are mainly determined by fission saddle points originating from the shell correction energy. The widths of the peaks, on the other hand, result from a shape fluctuation around the scission point caused by the random force in the Langevin equation. We found that a random vibration in the oblate direction of fissioning fragments is essential for the fission process. According to this picture, fission does not occur with continuous stretching in the prolate direction, similarly to that observed in starch syrup. This is expected to lead to a new viewpoint of fission dynamics and the splitting mechanism.

Aritomo, Y

2013-01-01

337

A fission fragment detector for correlated fission output studies  

Science.gov (United States)

A digital data acquisition system has been combined with a double Frisch gridded ionization chamber for use at both moderated and unmoderated neutron sources at the Los Alamos Neutron Science (LANSCE) facility. The high efficiency of the instrument combined with intense LANSCE beams and new acquisition system permits fission output measurements across 11 orders of magnitude incident neutron energy. The acquisition and analysis system is presented along with the first in-beam performance tests of the setup.

Mosby, S.; Tovesson, F.; Couture, A.; Duke, D. L.; Kleinrath, V.; Meharchand, R.; Meierbachtol, K.; O'Donnell, J. M.; Perdue, B.; Richman, D.; Shields, D.

2014-09-01

338

Space Fission Propulsion System Development Status  

Science.gov (United States)

The world's first man-made self-sustaining fission reaction was achieved in 1942. Since then fission has been used to propel submarines, generate tremendous amounts of electricity, produce medical isotopes, and provide numerous other benefits to society. Fission systems operate independently of solar proximity or orientation, and are thus well suited for deep space or planetary surface missions. In addition, the fuel for fission systems (enriched uranium) is virtually non-radioactive. The primary safety issue with fission systems is avoiding inadvertent system start. Addressing this issue through proper system design is straight-forward. Despite the relative simplicity and tremendous potential of space fission systems, the development and utilization of these systems has proven elusive. The first use of fission technology in space occurred 3 April 1965 with the US launch of the SNAP-10A reactor. There have been no additional US uses of space fission systems. While space fission systems were used extensively by the former Soviet Union, their application was limited to earth-orbital missions. Early space fission systems must be safely and affordably utilized if we are to reap the benefits of advanced space fission systems. NASA's Marshall Space Flight Center, working with Los Alamos National Laboratory (LANL), Sandia National Laboratories, and others, has conducted preliminary research related to a Safe Affordable Fission Engine (SAFE). An unfueled core has been fabricated by LANL, and resistance heaters used to verify predicted core thermal performance by closely mimicking heat from fission. The core is designed to use only established nuclear technology and be highly testable. In FY01 an energy conversion system and thruster will be coupled to the core, resulting in an 'end-to-end' nuclear electric propulsion demonstrator being tested using resistance heaters to closely mimic heat from fission. Results of the SAFE test program will be presented. The applicability of a SAFE-powered electric propulsion system to outer planet science missions will also be discussed.

Houts, M.; Van Dyke, M. K.; Godfroy, T. J.; Pedersen, K. W.; Martin, J. J.; Dickens, R.; Williams, E.; Harper, R.; Salvail, P.; Hrbud, I.

2001-01-01

339

production of some long-lived fission -product radionuclides  

International Nuclear Information System (INIS)

the thesis entitled: production of some long-lived fission-product radionuclidescomprises three chapters; introduction , experimental, and results and discussion. chapter 1, introduction, includes brief accounts on nuclear fission, chemistry, radiochemistry and nuclear chemistry of uranium, periodic table and 235 U-fission products, chemical processing for separation of some 235 U-fission products (including radioiodine, radio ruthenium, radiocesium, radio molybdenum, and radio-zirconium and -niobium), some beneficial applications of 235 U-fission products (including 137Cs/137mBa and 99Mo/99mTc radioisotope generators, sealed sources, nuclear batteries, and miscellaneous applications). chapter 2, experimental, includes description of the chemicals and solutions as well as specifications of the equipment's used. it includes also description of processes related to the target preparation, irradiation , and dissolution, preparation of 6-tungstocerate(IV), 6-WCe, gel matrix, separation methods of radioiodine, radio ruthenium, radiocesium, radio molybdenum, and radio-zirconium and -niobium from the aged and/or hot irradiated natural-abundance UO3 targets,some beneficial applications of 235U-fission products (including 137Cs/137m Ba and 99Mo/99mTc radioisotope generators as well as 99Mo/99mTc-137Cs/137mBa dual radioisotope generator) and quality control investigations of the product radionuclides(including elution profiles of generator-produced radionuclides, separation and elution yields, recovery yield, radionuclidic purity, radiochemical purity, chemical purity, and ph-value). chapter 3, results and discussion.. the uranium targets, consisted of 4*0.025 g of natural-abundance UO3, were irradiated in ETRR-2 research reactor-egypt for 4 h at a thermal neutron flux of 1x1014 n.cm-2.s-1. thereafter, they were cooled for 10 d (hot sample) or for ? 2.5 y(aged sample) before alkali/acid dissolution process to obtain hot and aged fission-product (F P) feeding solutions, respectively. these solutions were submitted to gamma-ray spectrometric analysis

340

DSP Algorithms for Fission Fragment and Prompt Fission Neutron Spectroscopy  

International Nuclear Information System (INIS)

Digital signal processing (DSP) algorithms are in high demand for modern nuclear fission investigation due to importance of increase the accuracy of fissile nuclear data for new generation of nuclear power stations. DSP algorithms for fission fragment (FF) and prompt fission neutron (PFN) spectroscopy are described in the present work. The twin Frisch-grid ionization chamber (GTIC) is used to measure the kinetic energy-, mass- and angular distributions of the FF in the 252Cf(SF) reaction. Along with the neutron time-of-flight (TOF) measurement the correlation between neutron emission and FF mass and energy is investigated. The TOF is measured between common cathode of the GTIC and the neutron detector (ND) pulses. Waveform digitizers (WFD) having 12 bit amplitude resolution and 100 MHz sampling frequency are used for the detector pulse sampling. DSP algorithms are developed as recursive procedures to perform the signal processing, similar to those available in various nuclear electronics modules, such as constant fraction discriminator (CFD), pulse shape discriminator (PSD), peak-sensitive analogue-to-digital converter (pADC) and pulse shaping amplifier (PSA). To measure the angle between FF and the cathode plane normal to the GTIC a new algorithm is developed having advantage over the traditional analogue pulse processing schemes. Algorithms are tested by comparing the numerical simulation of the data analysis of the 252Cf(SF) reaction with data available from literature.

 
 
 
 
341

A brief history of the ''Delayed'' discovery of nuclear fission  

International Nuclear Information System (INIS)

This year marks the Fiftieth Anniversary of the discovery of Nuclear Fission. In the early 1930's, the neutron was discovered, followed by the discovery of artificial radioactivity and then the use of the neutron to produce artificial radioactivity. The first experiments resulting in the fission of uranium took place in 1934. A paper which speculated on fission as an explanation was almost immediately published, yet no one took it seriously not even the author herself. Why did it take an additional five years before anyone realized what had occurred? This is an abnormally long time in a period when discoveries, particularly in nuclear physics, seemed to be almost a daily occurrence. The events which led up to the discovery are recounted, with an attempt made to put them into their historical perspective. The role played by Mendeleev's Periodic Table, the role of the natural radioactive decay chain of uranium, the discovery of protactinium, the apparent discovery of masurium (technetium) and a speculation on the reason why Irene Curie may have missed the discovery of nuclear fission will all be discussed. 43 refs

342

JEFF-3T. Decay data and fission yield libraries  

International Nuclear Information System (INIS)

Comprehensive decay-data and fission-yield libraries provide important input to a wide range of nuclear physics codes for nuclear applications. A new initiative has begun under the auspices of the NEA/OECD to generate improved data sets that will constitute the JEFF-3 libraries in ENDF-6 format, primarily for nuclear power, fuel reprocessing and waste management needs. Various sources of decay data have been accessed in order to assemble these files: NUBASE, ENSDF, UKPADD-6 and UKHEDD-2. Efforts have also focused on the evaluation of decay data for a number of important short-lived fission products, so that artificial adjustments to some of the relevant decay data and fission yields are not required to accommodate a previous lack of such data. Fission yields were adopted from UK evaluations recently undertaken to create the UKFY3 library. Decay-data files for 3 755 nuclides have been prepared, including sets of data for the stable nuclides (i.e. mass, natural abundance, spin and parity). Problems in the assignment of ENDF material numbers were addressed, while format and consistency tests were made using CHECKR and FIZCON, respectively. The assembly processes are discussed and reviewed, and the contents of the JEFF-3T starter libraries are described. (author)

343

Monte Carlo based toy model for fission process  

Science.gov (United States)

There are many models and calculation techniques to obtain visible image of fission yield process. In particular, fission yield can be calculated by using two calculations approach, namely macroscopic approach and microscopic approach. This work proposes another calculation approach in which the nucleus is treated as a toy model. Hence, the fission process does not represent real fission process in nature completely. The toy model is formed by Gaussian distribution of random number that randomizes distance likesthe distance between particle and central point. The scission process is started by smashing compound nucleus central point into two parts that are left central and right central points. These three points have different Gaussian distribution parameters such as mean (?CN, ?L, ?R), and standard deviation (?CN, ?L, ?R). By overlaying of three distributions, the number of particles (NL, NR) that are trapped by central points can be obtained. This process is iterated until (NL, NR) become constant numbers. Smashing process is repeated by changing ?L and ?R, randomly.

Kurniadi, R.; Waris, A.; Viridi, S.

2014-09-01

344

Nuclear Dissipation from Fission Time  

International Nuclear Information System (INIS)

Fission times, pre-scission neutron multiplicities and GDR pre-scission ?-ray multiplicities measured for uranium or thorium nuclei formed with temperatures T ? 1.8 MeV have been compared with calculations performed with CDSM2, a two-dimensional dynamical model combined with a statistical one. Among the three experimental approaches considered, fission times give access to the most precise pieces of information on nuclear dissipation at high excitation energy. For the temperature range under consideration, an agreement between the model and data is achieved if one-body dissipation is used with a strength factor kred ? 0.45 ± 0.10 applied to the wall term for the mononuclear configuration. (authors)

345

Sexual differentiation in fission yeast  

DEFF Research Database (Denmark)

The regulation of sexual reproduction in yeast constitutes the highest level of differentiation observed in these unicellular organisms. The various ramifications of this system involve DNA rearrangement, transcriptional control, post-translational modification (such as protein phosphorylation) and receptor/signal processing. A few basic similarities are common to both fission and budding yeasts. The wiring of the regulatory circuitry, however, varies considerably between these divergent yeast groups.

Egel, R; Nielsen, O

1990-01-01

346

Fission Data and Nuclear Technology  

International Nuclear Information System (INIS)

Accurate nuclear data for fissile nuclei are required not only by reactor designers, but also by reactor physicists for the interpretation of integral experiments, e.g. studies of the change of reactivity with irradiation. Some of the requests that have been made for such fission data, and the reasons behind them, are discussed, along with the progress that has been made towards their fulfilment. An attempt is made to outline those areas where better data are required. (author)

347

The VERDI fission fragment spectrometer  

Science.gov (United States)

The VERDI time-of-flight spectrometer is dedicated to measurements of fission product yields and of prompt neutron emission data. Pre-neutron fission-fragment masses will be determined by the double time-of-flight (TOF) technique. For this purpose an excellent time resolution is required. The time of flight of the fragments will be measured by electrostatic mirrors located near the target and the time signal coming from silicon detectors located at 50 cm on both sides of the target. This configuration, where the stop detector will provide us simultaneously with the kinetic energy of the fragment and timing information, significantly limits energy straggling in comparison to legacy experimental setup where a thin foil was usually used as a stop detector. In order to improve timing resolution, neutron transmutation doped silicon will be used. The high resistivity homogeneity of this material should significantly improve resolution in comparison to standard silicon detectors. Post-neutron fission fragment masses are obtained form the time-of-flight and the energy signal in the silicon detector. As an intermediary step a diamond detector will also be used as start detector located very close to the target. Previous tests have shown that poly-crystalline chemical vapour deposition (pCVD) diamonds provides a coincidence time resolution of 150 ps not allowing complete separation between very low-energy fission fragments, alpha particles and noise. New results from using artificial single-crystal diamonds (sCVD) show similar time resolution as from pCVD diamonds but also sufficiently good energy resolution.

Frégeau, M. O.; Bry?, T.; Gamboni, Th.; Geerts, W.; Oberstedt, S.; Oberstedt, A.; Borcea, R.

2013-12-01

348

The VERDI fission fragment spectrometer  

International Nuclear Information System (INIS)

The VERDI time-of-flight spectrometer is dedicated to measurements of fission product yields and of prompt neutron emission data. Pre-neutron fission-fragment masses will be determined by the double time-of-flight (TOF) technique. For this purpose an excellent time resolution is required. The time of flight of the fragments will be measured by electrostatic mirrors located near the target and the time signal coming from silicon detectors located at 50 cm on both sides of the target. This configuration, where the stop detector will provide us simultaneously with the kinetic energy of the fragment and timing information, significantly limits energy straggling in comparison to legacy experimental setup where a thin foil was usually used as a stop detector. In order to improve timing resolution, neutron transmutation doped silicon will be used. The high resistivity homogeneity of this material should significantly improve resolution in comparison to standard silicon detectors. Post-neutron fission fragment masses are obtained form the time-of-flight and the energy signal in the silicon detector. As an intermediary step a diamond detector will also be used as start detector located very close to the target. Previous tests have shown that poly-crystalline chemical vapour deposition (pCVD) diamonds provides a coincidence time resolution of 150 ps not allowing complete separation between very low-energy fission fragments, alpha particles and noise. New results from using artificial single-crystal diamonds (sCVD) show similar time resolution as from pCVD diamonds but also sufficiently good energy resolution. (authors)

349

The VERDI fission fragment spectrometer  

Directory of Open Access Journals (Sweden)

Full Text Available The VERDI time-of-flight spectrometer is dedicated to measurements of fission product yields and of prompt neutron emission data. Pre-neutron fission-fragment masses will be determined by the double time-of-flight (TOF technique. For this purpose an excellent time resolution is required. The time of flight of the fragments will be measured by electrostatic mirrors located near the target and the time signal coming from silicon detectors located at 50 cm on both sides of the target. This configuration, where the stop detector will provide us simultaneously with the kinetic energy of the fragment and timing information, significantly limits energy straggling in comparison to legacy experimental setup where a thin foil was usually used as a stop detector. In order to improve timing resolution, neutron transmutation doped silicon will be used. The high resistivity homogeneity of this material should significantly improve resolution in comparison to standard silicon detectors. Post-neutron fission fragment masses are obtained form the time-of-flight and the energy signal in the silicon detector. As an intermediary step a diamond detector will also be used as start detector located very close to the target. Previous tests have shown that poly-crystalline chemical vapour deposition (pCVD diamonds provides a coincidence time resolution of 150 ps not allowing complete separation between very low-energy fission fragments, alpha particles and noise. New results from using artificial single-crystal diamonds (sCVD show similar time resolution as from pCVD diamonds but also sufficiently good energy resolution.

Frégeau M.O.

2013-12-01

350

Fission properties for r-process nuclei  

CERN Document Server

We present a systematics of fission barriers and fission lifetimes for the whole landscape of super-heavy elements (SHE), i.e. nuclei with Z>100. The fission lifetimes are also compared with the alpha-decay half-lives. The survey is based on a self-consistent description in terms of the Skyrme-Hartree-Fock (SHF) approach. Results for various different SHF parameterizations are compared to explore the robustness of the predictions. The fission path is computed by quadrupole constrained SHF. The computation of fission lifetimes takes care of the crucial ingredients of the large-amplitude collective dynamics along the fission path, as self-consistent collective mass and proper quantum corrections. We discuss the different topologies of fission landscapes which occur in the realm of SHE (symmetric versus asymmetric fission, regions of triaxial fission, bi-modal fission, and the impact of asymmetric ground states). The explored region is extended deep into the regime of very neutron-rich isotopes as they are expec...

Erler, J; Loens, H P; Martínez-Pinedo, G; Reinhard, P -G

2011-01-01

351

Experimental techniques for fission data measurements  

International Nuclear Information System (INIS)

Progresses in the development of experimental techniques or fission data measurements are reviewed briefly. This review comprises techniques for the preparation of special compound nuclei leading to fission (fission entrance) as well as experimental techniques which permit the measurement of the diversified characteristics of the emitted radiations in fission (fission exit). The latter developments are only considered when also other parameters than yield, mass, and energy of fission fragments are determined. Ionization chambers developed at CBNM are described in more detail. A simple ionization chamber with Frisch grid was used to determine fission layer characteristics, e.g. the number of fissile nuclei of a sample with an accuracy of smaller than 0.3 %. A twin ionization chamber is described which has an advantageous 2 x 2? solid angle for fission fragment detection, a timing jitter of less than 0.7 ns, an energy resolution of smaller than 500 keV for fission fragments, and an angular resolution of ?cos? < 0.005. Also the nuclear charge distribution of the fragments can be determined. A pulse pile-up rejection circuit was developed, which reduces pulse pile-up by more than a factor 30. This detector is well suited for correlation measurements between fission fragment parameters, like mass and total kinetic energy, and the characteristics of the different radiations emitted from the fragments. This type of ionization chamber was successfully used in several experiments and some results are shown to demonstrate its capabilities. (author)

352

Status of fission yield evaluations  

Energy Technology Data Exchange (ETDEWEB)

Very few yield compilations are also evaluations, and very few contain an extensive global library of measured data and extensive models for unmeasured data. The earlier U.K. evaluations and US evaluations were comparable up to the retirements of the primary evaluators. Only the effort in the US has been continued and expanded. The previous U.K. evaluations have been published. In this paper we summarize the current status of the US evaluation, philosophy, and various integral yield tests for 34 fissioning nuclides at one or more neutron incident energies and/or for spontaneous fission. Currently there are 50 yield sets and for each we have independent and cumulative yields and uncertainties for approximately 1100 fission products. When finalized, the recommended data will become part of the next version of the US Evaluated Nuclear Data File (ENDF/B-VI). The complete set of data, including the basic input of measured yields, will be issued as a sequel to the General Electric evaluation reports (better known by the authors' names: Rider - or earlier - Meek and Rider). 16 references.

England, T.R.; Rider, B.F.

1983-01-01

353

Fission fusion hybrids- recent progress  

Science.gov (United States)

Fission-fusion hybrids enjoy unique advantages for addressing long standing societal acceptability issues of nuclear fission power, and can do this at a much lower level of technical development than a competitive fusion power plant- so it could be a nearer term application. For waste incineration, hybrids can burn intransigent transuranic residues (with the long lived biohazard) from light water reactors (LWRs) with far fewer hybrid reactors than a comparable system within the realm of fission alone. For fuel production, hybrids can produce fuel for ˜4 times as many LWRs with NO fuel reprocessing. For both waste incineration or fuel production, the most severe kind of nuclear accident- runaway criticality- can be excluded, unlike either fast reactors or typical accelerator based reactors. The proliferation risks for hybrid fuel production are, we strongly believe, far less than any other fuel production method, including today's gas centrifuges. US Thorium reserves could supply the entire US electricity supply for centuries. The centerpiece of the fuel cycle is a high power density Compact Fusion Neutron Source (major+minor radius ˜ 2.5-3.5 m), which is made feasible by the super-X divertor.

Kotschenreuther, M.; Valanju, P.; Mahajan, S.; Covele, B.

2012-03-01

354

Terracentric Nuclear Fission Reactor: Background, Basis, Feasibility, Structure, Evidence, and Geophysical Implications  

CERN Document Server

The background, basis, feasibility, structure, evidence, and geophysical implications of a naturally occurring Terracentric nuclear fission georeactor are reviewed. For a nuclear fission reactor to exist at the center of the Earth, all of the following conditions must be met: (1) There must originally have been a substantial quantity of uranium within Earth's core; (2) There must be a natural mechanism for concentrating the uranium; (3) The isotopic composition of the uranium at the onset of fission must be appropriate to sustain a nuclear fission chain reaction; (4) The reactor must be able to breed a sufficient quantity of fissile nuclides to permit operation over the lifetime of Earth to the present; (5) There must be a natural mechanism for the removal of fission products; (6) There must be a natural mechanism for removing heat from the reactor; (7) There must be a natural mechanism to regulate reactor power level, and; (8) The location of the reactor or must be such as to provide containment and prevent ...

Herndon, J Marvin

2013-01-01

355

Study of actinides fission induced by multi-nucleon transfer reactions in inverse kinematics  

International Nuclear Information System (INIS)

The study of actinide fission encounters two major issues. On one hand, measurements of the fission fragment distributions and the fission probabilities allow a better understanding of the fission process itself and the discrimination among the models of nuclear structure and dynamics. On the other hand, new measurements are required to improve nuclear data bases, which are a key component for the design of new generation reactors and radio-toxic waste incinerators. This thesis is in line with different French and American experimental projects using the surrogate method, i.e. transfer reactions leading to the same compound nuclei as in neutron irradiation, allowing the study of fission of actinides which are inaccessible by conventional techniques, whereas they are important for applications. The experiment is based on multi-nucleon transfer reactions between a 238U beam and a 12C target, using the inverse kinematics technique to measure, for each transfer channel, the complete isotopic distributions of the fission fragments with the VAMOS spectrometer. The work presented in this dissertation is focused on the identification of the transfer channels and their properties, as their angular distributions and the distributions of the associated excitation energy, using the SPIDER telescope to identify the target recoil nuclei. This work of an exploratory nature aims to generalize the surrogate method to heavy transfers and to measure, for the first time, the fission probabilities in inverse kinematics. The obtained results are compared with available direct kinematics and neutron irradiation measurements. (author)

356

Uncertainties and credibility building of safety analyses. Natural analogues  

International Nuclear Information System (INIS)

The substance of natural analogues and their studies is defined as a complementary method to laboratory and in-situ experiments and modelling. The role of natural analogues in the processes of development of repositories is defined, mainly in performance assessment of repository system and communication with public. The criteria for identification of natural analogues which should be evaluated in the phase of initiation of new studies are specified. Review part of this report is divided to study of natural analogues and study of anthropogenic and industrial analogues. The main natural analogue studies performed in various countries, in different geological setting, with various aims are characterized. New results acquired in recently finished studies are included: Palmottu (2nd phase of project financed by European Commission), Oklo (results of research financed also by European Commission), Maqarin (3rd phase) and other information obtained from last meetings and workshops of NAWG. In view of the fact that programmes of development of deep repositories in Czech and Slovak Republics are interconnected, the natural analogues studies carried out in the Czech republic are incorporated in separate chapter - study of uranium accumulation in Tertiary clays at Ruprechtov site and study of degradation of natural glasses. In final part the areas of natural analogue studies as an integral part of development of deep geological repository are proposed along with characterizationy are proposed along with characterization of broader context and aspects of realization of these studies (international cooperation, preparation and evaluation of procedures, communication with public). (author)

357

The resonance neutron fission on heavy nuclei  

International Nuclear Information System (INIS)

A new approach to the description of the fission, similar to the well-known reaction theory and based on the helicity representation for the exit fission channels, is briefly summarized. This approach allows one to connect the multimodal fission representation with A. Bohr's concept of the fission transition states and to obtain formulae for the partial and differential fission cross sections. The formulae are used for analysis of the angular anisotropy of fragments in the neutron resonance induced fission of aligned 235U nuclei and of the P-even angular forward-backward and right-left correlations of fragments oe the P-odd correlations caused by the interference of s- and p-wave neutron resonances

358

MODELING AND FISSION CROSS SECTIONS FOR AMERICIUM.  

Energy Technology Data Exchange (ETDEWEB)

This is the final report of the work performed under the LANL contract on the modeling and fission cross section for americium isotopes (May 2004-June 2005). The purpose of the contract was to provide fission cross sections for americium isotopes with the nuclear reaction model code EMPIRE 2.19. The following work was performed: (1) Fission calculations capability suitable for americium was implemented to the EMPIRE-2.19 code. (2) Calculations of neutron-induced fission cross sections for {sup 239}Am to {sup 244g}Am were performed with EMPIRE-2.19 for energies up to 20 MeV. For the neutron-induced reaction of {sup 240}Am, fission cross sections were predicted and uncertainties were assessed. (3) Set of fission barrier heights for each americium isotopes was chosen so that the new calculations fit the experimental data and follow the systematics found in the literature.

ROCHMAN, D.; HERMAN, M.; OBLOZINSKY, P.

2005-05-01

359

Collective spectra along the fission barrier  

Directory of Open Access Journals (Sweden)

Full Text Available Discrete and continuous spectra of fissioning nuclei at the humps of fission barriers (Bohr transition states and in the intermediate wells (superdeformed and hyperdeformed states play a key role in the calculation of fission cross sections. A theoretical evaluation of the collective parts of the spectra is possible within the framework of the dinuclear system model, which treats the wave function of the fissioning nucleus as a superposition of a mononucleus configuration and two–cluster configurations in a dynamical way, permitting exchange of upper–shell nucleons between clusters. The impact of theoretical spectra on neutron–induced fission cross sections and, in combination with an improved version of the scission–point model, on angular distribution of fission fragments is evaluated for plutonium isotopes of interest to nuclear energy applications.

Pigni M. T.

2012-12-01

360

Fission dynamics for three decay modes  

International Nuclear Information System (INIS)

Available as short communication only. An investigation of the optimum fission trajectories in a tri-dimensional configuration space with respect to elongation, necking-in and mass-asymmetry is carried out. The paths are obtained by solving the two associated Euler-Lagrange equations. Calculations for alpha-decay, 28 Mg -radioactivity and cold fission with the light fragment 100 Zr of 234 U are performed. The optimal fission asymmetry kept nearly constant during the decay process. A different situation appears in the case of Mg -radioactivity and cold fission were in the first stage the decay goes through diamond-like shapes. For cold fission the neck radius decreases in the vicinity of the scission point. A general increase of the radius of the larger fragment before reaching the final value is characteristics for cluster emission while in cold fission a small decrease is observed. (Author)

 
 
 
 
361

Fission properties of the BCPM functional  

CERN Document Server

We explore the properties of the Barcelona Catania Paris Madrid (BCPM) energy density functional concerning fission dynamics. Potential energy surfaces as well as collective inertias relevant in the fission process are computed for several nuclei where experimental data exists. Inner and outer barrier heights as well as fission isomer excitation energies are reproduced quite well in all the cases. The spontaneous fission half lives $t_{\\textrm{\\textrm{SF}}}$are also computed using the standard semiclasical approach and the results are compared with the experimental data. A reasonable agreement with experiment is found over a range of 27 orders of magnitude but the theoretical predictions suffer from large uncertainties associated to the values of the parameters entering the spontaneous fission half life formula. The impact that increasing the pairing correlations strengths has in the spontaneous fission half lives is analyzed and found to be large in all the nuclei considered. Given the satisfactory descripti...

Giuliani, Samuel A

2013-01-01

362

Fission-track dating using object-based image analysis  

International Nuclear Information System (INIS)

Full text: Geological dating with the help of fission track analysis is based on a time-consuming counting of the spontaneous and induced tracks in the minerals. Fission tracks are damage trails in minerals caused by fast charged particles, released in nuclear fission. In this study the 950;-method is used for fission-track dating. In order to determine the age, spontaneous tracks in the apatite and induced tracks in the muscovite external detector have to be counted. The automatic extraction and identification would not only improve the speed of track counting and eliminate the personal factor. Pixel values alone are not enough to distinguish between tracks and background. Traditional pixel based approaches are therefore inefficient for fission track counting. Image analysis based on objects, which include shape, texture and contextual information is a more promising method. A procedure for automatic object - based classification is used to extract the track objects. Resolving the individual tracks in a multi-track object is based on morphological operations. The individual track objects are skeletonized and the number of individual tracks in the object is counted by processing the skeletons. To give the right fission track age, there has to be a calibration of every single user manually counting the tracks. We calibrate the automatic approach for counting in the same way. Durango apatite standard samples are used to determine the 950;- and Z-calibration factor. The e the 950;- and Z-calibration factor. The automatic approach is useful for counting tracks in apatite standards and induced tracks in muscovite external detectors where the quality and quantities of the etched tracks is high. Muscovite detectors irradiated against glasses can also be used to determine the thermal neutron fluence, which is necessary to determine an absolute age. These images are of high quality and free of disturbing background irregularities. Here the automatic approach is a practical alternative. However for natural samples of small grain size, low track-numbers and background irregularities, the implementation is questionable. The algorithm for the automatic extraction and counting of fission tracks in standard samples of Durango Apatite and muscovite external detectors is shown to be self-consistent. (author)

363

Two-mode fission. Experimental verification and characterization of two fission-modes  

International Nuclear Information System (INIS)

Experimental verification and characterization of the two-mode fission are reviewed. The presence of two independent deformation-paths in low energy fission of actinides is demonstrated by studying correlation among saddle-point configurations, scission-point configurations, and mass-yield distributions; the elongated scission configuration is related with the fission process that goes over a higher threshold energy and results in a symmetric mass-division mode, while the compact scission configuration with the process that experiences a lower threshold ends up with an asymmetric mass-division mode. Based on an extensive systematic analysis of scission properties in a wide range of actinide fission, the bimodal fission observed in the spontaneous fission of the heavy actinides is interpreted as the result of the presence of two fission paths, namely, the ordinary asymmetric fission path and a strongly shell-influenced symmetric mode. (orig.)

364

Two-mode fission. Experimental verification and characterization of two fission-modes  

Energy Technology Data Exchange (ETDEWEB)

Experimental verification and characterization of the two-mode fission are reviewed. The presence of two independent deformation-paths in low energy fission of actinides is demonstrated by studying correlation among saddle-point configurations, scission-point configurations, and mass-yield distributions; the elongated scission configuration is related with the fission process that goes over a higher threshold energy and results in a symmetric mass-division mode, while the compact scission configuration with the process that experiences a lower threshold ends up with an asymmetric mass-division mode. Based on an extensive systematic analysis of scission properties in a wide range of actinide fission, the bimodal fission observed in the spontaneous fission of the heavy actinides is interpreted as the result of the presence of two fission paths, namely, the ordinary asymmetric fission path and a strongly shell-influenced symmetric mode. (orig.)

Nagame, Y. [Japan Atomic Energy Agency, Tokai, Ibaraki (Japan). Advanced Science Research Center; Nakahara, H. [Tokyo Metropolitan Univ., Hachioji, Tokyo (Japan). Dept. of Chemistry

2012-07-01

365

International conference on fifty years research in nuclear fission  

International Nuclear Information System (INIS)

These proceedings contain extended abstracts of the papers presented at the named conference. They deal with static properties of fission, instrumentation for fission studies, fission in compound-nucleus reactions, fission dynamics, fission-like heavy ion reactions, and fusion reactions. See hints under the relevant topics. (HSI)

366

Maximum entropy approach to nuclear fission processes  

International Nuclear Information System (INIS)

The conservation of the number of nucleons is shown to be an important constraint that governs the nuclear fission process. Both cold and energy-rich fission processes are analyzed. Particular attention is paid to the role of pairing effects in governing the fine structure in the mass and charge distributions. Symmetric and asymmetric fission processes are studied using a maximal entropy procedure. The interpretation of the results in terms of the Planck potential of the nucleons is discussed. (orig.)

367

Fission of nuclei far from stability  

International Nuclear Information System (INIS)

The secondary-beam facility of GSI provided the technical equipment for a new kind of fission experiment. Fission properties of short-lived neutron-deficient nuclei have been investigated in inverse kinematics. The measured element distributions reveal new kinds of systematics on shell structure and even-odd effects and lead to an improved understanding of structure effects in nuclear fission. Prospects for further experimental studies are discussed. (orig.)

368

Search for unusual nuclei in fission products  

Energy Technology Data Exchange (ETDEWEB)

A search is carried out for various rare events in spontaneous fission of /sup 252/Cf: ..beta.. electrons with unusually high energy, positron activity, and fission events with anomalously large prompt ..gamma..-ray energy. The existence of such phenomena could be related to the appearance of unusual nuclei, particularly superheavy nuclei, in the fission process. It is found that the relative probability of these events does not exceed (3--7) x 10/sup -6/.

Aleshin, V.I.; Bakalyarov, A.M.; Balysh, A.Y.; Borovoi, A.A.; Ketov, S.N.; Kopeikin, V.I.; Lebedev, V.I.; Mikaelyan, L.A.; Kheruvimov, A.N.; Shkol' nik, K.D.

1977-11-01

369

Search for unusual nuclei in fission products  

International Nuclear Information System (INIS)

Various rare events were searched for in spontaneous fission of 252Cf, viz: ? electrons with unusually high energy, positron activity and fission events with abnormally high energy of the a prompt ? radiation. Existence of such phenomena cound be related to appearance of ''unusual'', in particular of superhigh-density, nuclei during the fission. The relative probability of such events was found to be not higher than (3-7)x10-6

370

Fission of multielectron bubbles in liquid helium  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The stability of multielectron bubbles (MEBs) in liquid helium is investigated using the liquid-drop model for fissioning nuclei. Whereas a critical positive pressure can make the bubble unstable against fissioning, a small negative pressure suffices to introduce a restoring force preventing any small deformation of the bubble to grow. We also find that there exists an energy barrier making MEBs metastable against fissioning at zero pressure. The results obtained here overco...

Tempere, J.; Silvera, I. F.; Devreese, J. T.

2002-01-01

371

Nuclear fission with a Langevin equation  

International Nuclear Information System (INIS)

A microscopically derived Langevin equation is applied to thermally induced nuclear fission. An important memory effect is pointed out and discussed. A strong friction coefficient, estimated from microscopic quantities, tends to decrease the stationary limit of the fission rate and to increase the transient time. The calculations are performed with a collective mass depending on the collective variable and with a constant mass. Fission rates calculated at different temperatures are shown and compared with previous available results. (author) 23 refs.; 7 figs

372

Fission cross section measurements for minor actinides  

International Nuclear Information System (INIS)

The main task of this work is the measurement of fast neutron induced fission cross section for minor actinides of 238Pu, 242mAm, 243,244,245,246,247,248Cm. The task of the work is to increase the accuracy of data in MeV energy region. Basic experimental method, fissile samples, fission detectors and electronics, track detectors, alpha counting, neutron generation, fission rate measurement, corrections to the data and error analysis are presented in this paper. (author)

373

Fission barriers and half-lives  

International Nuclear Information System (INIS)

We briefly review the development of theoretical models for the calculation of fission barriers and half-lives. We focus on how results of actual calculations in a unified macroscopic-microscopic approach provide an interpretation of the mechanisms behind some of the large number of phenomena observed in fission. As instructive examples we choose studies of the rapidly varying fission properties of elements at the end of the periodic system. 31 refs., 10 figs

374

Fission-product retention in HTGR fuels  

International Nuclear Information System (INIS)

Retention data for gaseous and metallic fission products are presented for both Triso-coated and Biso-coated HTGR fuel particles. Performance trends are established that relate fission product retention to operating parameters, such as temperature, burnup, and neutron exposure. It is concluded that Biso-coated particles are not adequately retentive of fission gas or metallic cesium, and Triso-coated particles which retain cesium still lose silver. Design implications related to these performance trends are identified and discussed

375

Fission-energy release for 16 fissioning nuclides. Final report  

International Nuclear Information System (INIS)

Results are presented of a least-squares evaluation of the components of energy release per fission in 232Th, 233U, 235U, 238U, 239Pu, and 241Pu. For completeness, older (1978) results based on systematics are presented for these and ten other isotopes of interest. There have been recent indications that the delayed energy components may be somewhat higher than those used previously, but the LSQ results do not seem to change significantly when modest (approx. 1 MeV) increases in the total delayed energy are included in the inputs. Additional measurements of most of the energy components are still needed to resolve remaining discrepancies

376

Future Scenarios for Fission Based Reactors  

Science.gov (United States)

The coming century will see the exhaustion of standard fossil fuels, coal, gas and oil, which today represent 75% of the world energy production. Moreover, their use will have caused large-scale emission of greenhouse gases (GEG), and induced global climate change. This problem is exacerbated by a growing world energy demand. In this context, nuclear power is the only GEG-free energy source available today capable of responding significantly to this demand. Some scenarios consider a nuclear energy production of around 5 Gtoe in 2050, wich would represent a 20% share of the world energy supply. Present reactors generate energy from the fission of U-235 and require around 200 tons of natural Uranium to produce 1GWe.y of energy, equivalent to the fission of one ton of fissile material. In a scenario of a significant increase in nuclear energy generation, these standard reactors will consume the whole of the world's estimated Uranium reserves in a few decades. However, natural Uranium or Thorium ore, wich are not themselves fissile, can produce a fissile material after a neutron capture ( 239Pu and 233U respectively). In a breeder reactor, the mass of fissile material remains constant, and the fertile ore is the only material to be consumed. In this case, only 1 ton of natural ore is needed to produce 1GWe.y. Thus, the breeding concept allows optimal use of fertile ore and development of sustainable nuclear energy production for several thousand years into the future. Different sustainable nuclear reactor concepts are studied in the international forum "generation IV". Different types of coolant (Na, Pb and He) are studied for fast breeder reactors based on the Uranium cycle. The thermal Thorium cycle requires the use of a liquid fuel, which can be reprocessed online in order to extract the neutron poisons. This paper presents these different sustainable reactors, based on the Uranium or Thorium fuel cycles and will compare the different options in term of fissile inventory, capacity to be deployed, induced radiotoxicities, and R&D efforts.

David, S.

2005-04-01

377

Fission-track dating of obsidians  

International Nuclear Information System (INIS)

Fission-track dating is applied to 16 obsidian artifacts from an archaeological excavation at Cayonu Tepesi in southeastern Anatolia. Since obsidian samples are rich in bubbles and inclusions, only the six most compact samples were found to be usable for fission-track measurements. Size studies of etched fossil- and induced-fission tracks showed that no strong thermal effects occurred during the history of these glasses so that no fading correction has been applied to the computed ages. The fission-track ages obtained range between 8.95 Myr and 25.14 Myr. It may be stated that these obsidian samples are from two different origins. (author)

378

Attachment of gaseous fission products to aerosols  

International Nuclear Information System (INIS)

The circumstances under which gaseous fission products may be attached to aerosols are examined. A simple model is assumed for the interaction between the fission product molecules and the aerosol, in which the probability of attachment of a molecule on any encounter is given by a constant ?, the sticking probability. It is concluded that when gaseous fission products are released in the presence of a population of particles, particularly small particles, there is a strong possibility that they will attach rapidly to those particles. In such circumstances gaseous fission product transport would be governed by the transport properties of the particles. (author)

379

Theoretical Description of the Fission Process  

Energy Technology Data Exchange (ETDEWEB)

The main goals of the project can be summarized as follows: Development of effective energy functionals that are appropriate for the description of heavy nuclei. Our goal is to improve the existing energy density (Skyrme) functionals to develop a force that will be used in calculations of fission dynamics. Systematic self-consistent calculations of binding energies and fission barriers of actinide and trans-actinide nuclei using modern density functionals. This will be followed by calculations of spontaneous fission lifetimes and mass and charge divisions using dynamic adiabatic approaches based on the WKB approximation. Investigate novel microscopic (non-adiabatic) methods to study the fission process.

Witold Nazarewicz

2003-07-01

380

Correlated prompt fission ?-ray data measurements  

International Nuclear Information System (INIS)

The OECD-NEA has published in its recent high priority data request list a demand for new and precise data on prompt fission ?-ray emission for the standard actinide isotopes 235U and 239Pu in view of their importance for the development of future nuclear fission applications and for a responsible handling of nuclear waste during an a-priori assessment of the fission-fragments' heat production and toxicity. Prompt fission ?-rays, together with prompt neutrons, represent very powerful probes of the nuclear fission process near the scission point. They provide information to better understand how the total excitation energy available in the fissioning system gets transferred to intrinsic excitation in the fragments. Prompt fission ?-rays should preferably be known as a function of fission-fragment mass and excitation energy, but existing experimental data date back to the early 1970s for the above mentioned isotopes. We have performed a detailed feasibility study on novel lanthanum-halide and cerium-bromide detectors with a particular focus on their corresponding time and energy resolution, essential parameters to arrive at a set-up with high detection efficiency in conjunction with a high neutron/?-ray separation power. New prompt fission ?-ray spectra from the reactions 252Cf(SF) and 235U(nth, f) are presented.

 
 
 
 
381

Fission - track age of the Marjalahti Pallasite  

International Nuclear Information System (INIS)

Full text: Investigation of fossil charged-particle tracks in various mineral phases of extraterrestrial samples is a powerful method for research the early stages of the solar system. Over geological time, meteorites crystals have accumulated a record of tracks produced by heavily charged energetic particles from both internal (spontaneous fission of 238U and some other extinct isotopes) and external sources (galactic cosmic rays with Z>20). The fortunate fact that meteorite grains can accumulate latent and very long-lived tracks since soon after the end of nucleosynthesis in the solar nebula enables one to decode their radiation history and to detect any thermal events in the meteorite cosmic history by revealing these tracks through suitable etching procedures. Only a few minerals in meteorites (mainly phosphates) contain small amount of uranium; the fact that 238U undergoes fission with fission-decay constant ?f?8.2x10-17 yr-1 allows one to use this isotope as a chronometer. By measuring the U concentration in the crystals (by reactor irradiation) and the density of the spontaneous-fission tracks it is relatively easy to calculate the 'fission-track age' if 238U is the main source of fission tracks. However the fission-track dating of extraterrestrial samples compared with the terrestrial ones has some peculiar features due to presence of a number of other potential track sources except the spontaneous fission of 238U, such as the spontaneous fission of presently extinct 244Pu, heavy nuclei of cosmic rays and induced fission by cosmic ray primaries. Only tracks from the spontaneous fission of U and Pu are suitable for fission-track dating. The competing effects of these fissioning elements, whose half-lives differ by a factor of ?50, form a basis for a fission-track chronology for samples older than ? 4.0 Gyr. Over small intervals in time (? few x108 yr ) the track density from spontaneous fission of 238U is nearly constant. However, the contribution from 244Pu doubles every 82 Myr providing a very sensitive measure of the age of a studied sample. The results of the determination of the fission-track age of the Marjalahti pallasite (stony-iron meteorite) are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U content determination in whitlockites allowed us to estimate the contributions of all possible track sources to the total track density and to calculate a value of the model fission-track age. It was found out that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); induced fission of U and Th by cosmic rays; spontaneous fission of 238U; spontaneous fission of extinct short-lived 244Pu nuclei presented in significant quantities in the early solar system. The initial ratio (244Pu/238U)0 at the time of the pallasite parent body formation (taken as 4.6x109 yr) was estimated as 0.015. A great track density attributed to the extinct 244Pu testified to the high value of the fission-track age. The model fission-track ages of (4.37± 0.02)x109 yr for the Marjalahti pallasite was calculated. The comparison of the represented data with petrographic analyses allowed us to interpret a value of the fission-track age as the time of the last intensive shock/thermal event in the cosmic history of the pallasite. (author)

382

Baby fission chambers; Etude de chambres a fission miniatures  

Energy Technology Data Exchange (ETDEWEB)

The present report is intended, on the one band, as a study of the main types of fission chambers produced to date, and on the other, to deal more generally with this type of detector. Originally, it was with a view to the charting of neutron scatter in 'Proserpine' that the authors undertook the study of these chambers. During the course of the task, it was considered worth tbe trouble of developing its scope to include a more general application: neutron scatter measurement of various energy neutrons within a reduced volume with slight local disturbance. (author) [French] Le present rapport se propose, d'une part, d'exposer les principales realisations de chambres a fission, d'autre part de faire une mise au point a caractere plus general sur ces detecteurs. Au depart, c'est surtout en vue des mesures de densite neutronique dans 'Proserpine' que les auteurs ont etudie ces chambres; au cours de la mise au point, il a paru interessant de developper leur etude pour des applications plus generales: mesures de densites de neutrons de differentes energies dans un element de volume tres reduit et avec faible perturbation locale. (auteur)

Guery, U.; Tachon, J. [Commissariat a l' Energie Atomique, Saclay (France). Centre d' Etudes Nucleaires

1957-07-01

383

The phebus fission product project  

Science.gov (United States)

A new facility is being built at the Phebus test reactor in Cadarache, France, for investigations into phenomena of fuel damage and fission product (FP) release under severe power reactor accident conditions, as part of a large international research program. Phebus FP simulates core, cooling system and containment of an accidented reactor by appropriate scaled-down experimental components. The test fuel, with 25 to 30 GWd/t burnup, is re-irradiated in situ and then overheated up to UO 2 melting. Fission products and other aerosols are swept through the primary pipework into the containment vessel, by hot steam and hydrogen. Experimental instrumentation and posttest analyses will enable the following main phenomena to be studied: structural material and fuel dislocation, final fuel state; release, chemical form and transport/depletion of fission products in the facility, particularly aerosol physics, including nonfission product material and iodine chemistry in terms of volatile species formation through radiolysis, reactions with organic material, aerosol-vapor reactions, etc. Design and development of equipment and experimental procedures are supported by modeling and code calculations with the scope of predicting the experimental sequence, on one hand, and to prepare code validation through the results, on the other hand. More than 25 organisation from Europe and overseas, collaborate in the scientific and technological development of the Phebus FP program. The first in-pile test is planned for spring 1993, and five subsequent experiments are scheduled to follow in yearly intervals. This paper describes facility and support activities, and highlights a number of nuclear materials aspects involved.

von der Hardt, P.; Tattegrain, A.

1992-06-01

384

Fission tracks diameters in glasses  

International Nuclear Information System (INIS)

Standard glass microscope slides have been irradiated with fission fragments from the uranium. The etching track conditions have been the same for the series, having changed the etching time only for each specimen. For each glass, a minimum of 250 measurements of the tracks diameters have been made, the distributions of which are the bimodal type. Diameters-etching dependence with time is roughly lineal. Energy determinations have been made with the help of the diameters-energy relations. The calculated values agree very well with the know ones. (author)

385

Neutron emission prior to fission  

International Nuclear Information System (INIS)

Neutron emission in the 158Er composite system is studied in order to investigate particle emission with energy spectrum and angular distribution in excess of statistical model predictions. Data are analyzed using a modified statistical model which incorporates effects due to nuclear dissipation, and also calculates neutron emission during the descent from the saddle to the scission point. Calculations consider the Kramers effect and the Transient effect. It is concluded that a detailed interpretation of enhanced neutron emission preceding fission in compound nucleus reactions is possible, and that an upper limit may be set on the reduced nuclear dissipation coefficient. 5 refs., 2 figs

386

Fission track grochronology of India  

International Nuclear Information System (INIS)

The fission track ages of cogenetic/co-existing minerals namely garnet, muscovite and apatite from three mica belts i.e., Bihar, Rajasthan, Nellore of peninsular India and Himalayan region, coupled with the corresponding closing temperatures of the minerals have been used to reveal the thermal and uplift histories of these regions. The data show that the extra-peninsular part of the subcontinent during Himalayan orogenic cycle (upper cretaceous-tertiary) witnessed the highest cooling and uplift rates in comparison to the older cycles in peninsular India. (author)

387

Fission track geochronology of India  

Science.gov (United States)

The fission track ages of cogenetic/co-existing minerals namely garnet, muscovite and apatite from three mica belts i.e., Bihar, Rajasthan, Nellore of peninsular India and Himalayan region, coupled with the corresponding closing temperatures of the minerals have been used to reveal the thermal and uplift histories of these regions. The data show that the extra-peninsular part of the subcontinent during Himalayan orogenic cycle (upper cretaceous-tertiary) witnessed the highest cooling and uplift rates in comparison to the older cycles in peninsular India.

Nagpaul, K. K.

1981-11-01

388

Least squares analysis of fission neutron standard fields  

International Nuclear Information System (INIS)

A least squares analysis of fission neutron standard fields has been performed using the latest dosimetry cross sections. Discrepant nuclear data are identified and adjusted spectra for 252Cf spontaneous fission and 235U thermal fission fields are presented

389

Effect of inertia parameters on static fission path  

International Nuclear Information System (INIS)

Properties of static (minimum potential) fission path in the formalism of Hofmann are investigated. It is pointed out that the inertial parameters greatly affect the fission path and hence the penetrability. The difficulty of determining fission path is discussed

390

Gabon's natural reactors: nature shows how to contain radioactive waste  

International Nuclear Information System (INIS)

When the Okla ''natural'' fission reactors were discovered in 1972, investigators were soon surprised at how little several of the fission products or U-235 had moved in the two billion years since they were active. Studies underway in several countries are seeking to determine whether these ancient reactors can serve as time-tested analogs for the modern processes needed for a nuclear waste repository. (author)

391

Analysis of fission excitation functions and fission barrier heights of preactinide nuclei  

International Nuclear Information System (INIS)

Following a recently proposed method of analysis of fission excitation functions of nuclei in the mass region A approximately 200, alpha-induced fission cross sections of a number of nuclei in this mass region have been analysed. For the nuclei studied, the analysis indicates the presence of single-particle effects at the fission transition state shape. Consequently, while the analysis yields fission barrier heights in agreement with the results of earlier works, the derived values of the macroscopic (liquid drop model) part of the fission barrier heights are found to be significantly lower than the predictions of frequently used mass formulae in literature. (orig.)

392

Fission  

International Nuclear Information System (INIS)

Progress is reported in the areas of radiation physics; radiation dosimetry and radiation biophysics; microdosimetry of internal sources; dosimetry of internal emitters; real-time measurement of Pu in air at below-MPC levels; analytical techniques for measurement of 99Tc in environmental samples; and radiation instrumentation--radiological chemistry

393

Spectroscopy of selected fission fragments  

Energy Technology Data Exchange (ETDEWEB)

The spectroscopy of nuclei produced as fragments in the fission process has been undertaken using the EUROGAM II {gamma}-ray multidetector array. The first experiment involved a spontaneously fissioning {sup 248}Cm source and produced neutron-rich nuclei. The data analysis concentrated on the odd-A Ce isotopes and the present contribution details the structure of {sup 151}Ce which results from the strong coupling of the odd neutron to the core. The results of a preliminary analysis of the yrast structure of {sup 138}Te will also be given. In a second experiment performed at the VIVITRON accelerator in Strasbourg, nuclei on the neutron-rich side of the valley of stability were produced via the {sup 28}Si + {sup 176}Yb reaction at 145 MeV bombarding energy. The level schemes of {sup 99}Mo, {sup 101}Tc and {sup 103}Ru have been extended to high spins ({approximately} 20h). Two new high lying structures in {sup 101}Tc are explained with the help of cranked shell model calculations.

Hoellinger, F.; Schulz, N.; Gall, B. J. P.; Bentaleb, M.; Courtin, S.; Lubkiewicz, E.; Durell, J. L.; Jones, M. A.; Leddy, M.; Phillips, W. R.; Smith, A. G.; Urban, W.; Varley, B. J.; Deloncle, I.; Porquet, M.-G.; Wilson, A.; Ahmad, I.; Morss, L. R.; Kutsarova, T.; Minkova, A.; Duprat, J.; Sergolle, H.; Gautherin, C.; Lucas, R.

1999-10-22

394

Quark bags and their fission  

International Nuclear Information System (INIS)

The aim of this work is to gain more insight into the physics of hadrons and to develop a theoretical framework for the treatment of hadronic fission. The description of hadronic matter in terms of quantum chromodynamics, which is generally accepted as the fundamental theory of strong interactions, is intimately connected with the concept of colour confinement. Since this phenomenon is a property of the vacuum state, much effort has been invested into trying to understand it in analogy with the occurence of the condensed ground state in the theory of superconductivity. In Chapter II, we therefore go back to non-relativistic physics, discuss the methods used there, and report in Chapter III on various attempts to apply these methods in quantum field theories, in particular in connection with the so-called 'dynamical symmetry breaking'. Although the fundamental problem of the 'true' vacuum state has not yet been solved, its solution has been anticipated in the so-called bag models which are discussed in great detail in Chapter IV. In Chapter V, we finally seek to understand the microscopic process of fission of hadrons. (orig./HSI)

395

Phebus FP: fission product behaviour  

International Nuclear Information System (INIS)

The ongoing Phebus FP programme is the centrepiece of a wide international co-operation investigating, through a series of six integral in-pile experiments, key-phenomena involved in the progression of a postulated severe accident in a Light Wate Reactor (LWR). The Phebus facility offers the capability to study the degradation of real core material, from the early phase of cladding oxidation and hydrogen production up to the late phase of melt progression and molten pool formation. The subsequent release of fission products and structural materials is also experimentally studied, including their transport in the cooling system, and their deposition in the containment, under representative physicochemical conditions. The volatility of iodine in the containment is in particular receiving a special interest in the first experiments, as large uncertainties related to its modelling subsist. FPT-0 and FPT-1, performed respectively in December 1993 and July 1996, have reached very advanced states of degradation, comparable to what was observed in TMI-2, and generated a wealth of results on core degradation and fission product behaviour in particular, pool formation was obtained for a temperature well below the melting point of (U, Zr) O2 and volatile forms of iodine were detected in the containment much earlier than expected. The resulting database is used to develop and validate the computer codes used to assess the safety of the currently operating plants, to check the efficiency of accident management procedures and also support the design of future plants as EPR. (orig.)

396

Fission wire plane chambers (WPC)  

International Nuclear Information System (INIS)

Wire Plane Chamber (WPC) with multiple grid planes was found to be useful for detection of alpha particles with argon fill gas. Using this concept, studies have been carried out using wire plane chamber for neutron detection using 235U as the neutron converter material. The experimental studies have been done using 252Cf source mounted on the cathode plate inside the WPC. WPC is found to be operated at low working voltages and with better pulse shape features: faster and narrower pulse shapes better suited for high count rate applications. Present grid configuration is found to be superior to conventional ionization chamber of similar dimensions in performance. This is achieved as the average distance traveled by the charge particle cloud from the ionization track to reach the nearest electrode has been reduced as compared to two electrode ionization chamber of similar size. The capacitance of the WPC is considerably reduced with presently used multi anode wire plane electrodes so that large area fission chambers in parallel plate or cylindrical geometry can be realized. The results obtained with this novel fission WPC are described for the first time. (author)

397

Options for Affordable Fission Surface Power Systems  

International Nuclear Information System (INIS)

Fission surface power systems could provide abundant power anywhere on the surface of the moon or Mars. Locations could include permanently shaded regions on the moon and high latitudes on Mars. To be fully utilized, however, fission surface power systems must be safe, have adequate performance, and be affordable. This paper discusses options for the design and development of such systems. (authors)

398

Fission stability diagram of 240Pu  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have used an axially symmetric deformed Thomas-Fermi model to evaluate the fission barrier of 240Pu as a function of the quadrupole moment Q2 for different values of the angular momentum L and temperature T. The fission stability diagram of this nucleus is investigated.

Garcias, Francisca; Barranco Go?mez, Manuel; Wio, Horacio S.; Ngo?, Christian; Nemeth, J.

1989-01-01

399

Fission cross section calculations for Pa isotopes  

International Nuclear Information System (INIS)

Based on the recently measured cross-section values for the neutron-induced fission of 231Pa and our experience gained with other isotopes, new self consistent neutron cross section calculations for n+231Pa have been performed up to 30 MeV. The results are quite different to the existing evaluations, especially above the first chance fission threshold. (authors)

400

Methods for determination of fission gases  

International Nuclear Information System (INIS)

This paper describes procedures for determination of fission gases by measuring the quantity of released and remaining gases in the fuel after irradiation. Experiments are described for determining the quantity of fission gases release from fuel during irradiation in the reactor as well as the pressure of gases in fuel cladding. Principles of gamma scintillation spectroscopy, mass spectrometry and gas chromatography are included

 
 
 
 
401

Radiative Neutron Capture In Fissionable Nuclei  

International Nuclear Information System (INIS)

Because of the great success of the (n, ?) method in nuclear structure studies, it is desirable to extend such measurements to fissionable target nuclei. A suitable spectrometer for investigating the high-energy radiative capture spectrum without significant interference from the fission process has been installed at the Karlsruhe research reactor FR- 2. The instrument is briefly discussed. First preliminary results are presented. (author)

402

Independent Isotopic Fission Yield Studies with Jyfltrap  

Science.gov (United States)

A technique to determine the independent isotopic fission yields utilizing the ability of a Penning trap to unambiguously identify the isotopes based on their mass has recently been developed at the IGISOL facility in the University of Jyväskylä. The analysis of proton induced fission of 238U at 25 MeV energy has been finalized. The results will be compared with theoretical models.

Penttilä, H.; Elomaa, V.-V.; Eronen, T.; Gorelov, D.; Hakala, J.; Jokinen, A.; Kankainen, A.; Karvonen, P.; Kolhinen, V. S.; Moore, I. D.; Rahaman, S.; Rinta-Antila, S.; Rissanen, J.; Rubchenya, V.; Saastamoinen, A.; Sonoda, T.; Weber, C.; Äystö, J.

2014-09-01

403

Detector instrumentation for nuclear fission studies  

International Nuclear Information System (INIS)

New kind of detector telescopes have been developed for energy measurement and nuclear charge identification of particles in transfer and fission experiments. An experimental program using the General Purpose Scattering Chamber (GPSC) and National Array of Neutron Detector (NAND) for performing fission experiments with Tandem-LINAC accelerator system at IUAC is underway from last few years. New developments in detector instrumentation has been carried out. Multi-wire Proportional counter based (MWPC) time of flight (TOF) spectrometers have been developed for investigating fission mass distribution and Neutron Multiplicity experiments. For generating absolute timing, experiments with TOF system based on start-stop detector has also been developed. To extract fission cross-section, a detection system based on hybrid telescopes has been developed for extracting fission angular distribution. To investigate pre and post-scission charged particle multiplicity in fission process, a charged particle array, based on CsI(TI) scintillators coupled to photodiode, has been developed for detecting light charged particles (protons and alphas) in coincidence with fission fragments. Recent developments in detector development and its nuclear instrumentation for carrying out fission experiments will be discussed

404

Progress in fission product nuclear data  

International Nuclear Information System (INIS)

This is the ninth issue of a report series on Fission Product Nuclear Data (FPND) which is published by the Nuclear Data Section (NDS) of the International Atomic Energy Agency (IAEA). The purpose of this series is to inform scientists working on FPND, or using such data, about all activities in this field which are planned, ongoing, or have recently been completed. The main part of this report consists of unaltered original contributions which the authors have sent to IAEA/NDS. The present issue contains also a section with some recent references relative to fission product nuclear data, which were not covered by the contributions submitted. The types of activities being included in this report are measurements, compilations and evaluations of: Fission product yields (neutron induced and spontaneous fission); Neutron reaction cross sections of fission products; Data related to the radioactive decay of fission products; Delayed neutron data of fission products; and lumped fission product data (decay heat, absorption etc.). The eighth issue of this series has been published in July 1982 as INDC(NDS)-130. The present issue includes contributions which were received by NDS between 1 August 1982 and 25 June 1983

405

Fission ?-ray data measurements - a challenging endeavour  

International Nuclear Information System (INIS)

The OECD has published in its high priority data request list a demand for new data on prompt fission ?-ray emission for the standard actinide isotopes 235U and 239Pu in view of their importance for the development of future nuclear fission applications and a responsible handling of nuclear waste with respect to heat production and toxicity. Prompt fission ?-rays can be used to study the configurations of fission fragments very close to the scission point and to better understand how the total excitation energy available in the fissioning system gets transferred to intrinsic excitation in the fragments. They should preferably be known as a function of fission-fragment mass and excitation energy. Existing experimental data, however, were obtained in the 1970s for the above mentioned isotopes. In order to arrive at new and precise correlated ?-ray emission data the problem of efficient neutron/?-ray separation has to be solved. This is usually achieved by means of time-of-flight and the pulse-shape discrimination technique and requires excellent timing resolution of the measurement system. Additionally, high detection efficiency is required. The talk discusses present activities on fission ?-ray measurements with a particular emphasis on state-of-the art fission-fragment and ?-ray detectors.

406

Fission gas behavior in safety experiments  

International Nuclear Information System (INIS)

This book examines fission product release from power reactors during reactor safety experiments. Topics considered include modes and mechanisms of gas precipitation and migration, results and analysis of recent transient in pile experiments, modelling of fission gas behavior, and a review of knowledge and future work

407

Progress in fission product nuclear data  

International Nuclear Information System (INIS)

This is the seventh issue of a report series on Fission Product Nuclear Data (FPND) which is published by the Nuclear Data Section (NDS) of the International Atomic Energy Agency (IAEA). The purpose of this series is to inform scientists working on FPND, or using such data, about all activities in this field which are planned, ongoing, or have recently been completed. The present issue contains also a section with some recent references relative to fission product nuclear data, which were not covered by the contributions submitted. The types of activities being included in this report are measurements, compilations and evaluations of: fission product yields (neutron induced and spontaneous fission); neutron reaction cross sections of fission products; data related to the radioactive decay of fission products; delayed neutron data of fission products; and lumped fission product data (decay heat, absorption etc.). The sixth issue of this series has been published in June 1980 as INDC(NDS)-113/G+P. The present issue includes contributions which were received by NDS between 1 August 1980 and 25 May 1981

408

Volatile fission products: a thermochemical approach  

International Nuclear Information System (INIS)

Since the TMI accident the investigation of the chemical properties of volatile fission products is strongly intensified. Thermochemical considerations can give an important contribution to this since they allow, in a relatively simple way, for calculations of complicated equilibrium situations in the fission materials and the outcome of an accident. (Auth.)

409

Consistent dynamical and statistical description of fission  

International Nuclear Information System (INIS)

The research survey of consistent dynamical and statistical description of fission is briefly introduced. The Channel theory of fission with diffusive dynamics based on Bohr channel theory and Fokker-Planck equation, and Kramers-Modified Bohr-Wheeler expression according to Strutinsky method given by P. Frobrich et al. are compared and analyzed

410

Correlation measurements of fission-fragment properties  

Directory of Open Access Journals (Sweden)

Full Text Available For the development of future nuclear fission applications and for a responsible handling of nuclear waste the a-priori assessment of the fission-fragments’ heat production and toxicity is a fundamental necessity. The success of an indispensable modelling of the fission process strongly depends on a good understanding of the particular mechanism of scission, the mass fragmentation and partition of excitation energy. Experimental observables are fission-fragment properties like mass- and energy-distributions, and the prompt neutron as well as ?-ray multiplicities and emission spectra. The latter quantities should preferably be known as a function of fragment mass and excitation energy. Those data are highly demanded as published by the OECD-NEA in its high priority data request list. With the construction of the double (v, E spectrometer VERDI we aim at measuring pre- and post-neutron masses directly and simultaneously to avoid prompt neutron corrections. From the simultaneous measurement of pre- and post-neutron fission-fragment data the prompt neutron multiplicity may then be inferred fully correlated with fragment mass yield and total kinetic energy. Using an ultra-fast fission event trigger spectral prompt fission ?-ray measurements may be performed. For that purpose recently developed lanthanum-halide detectors, with excellent timing characteristics, were coupled to the VERDI spectrometer allowing for a very good discrimination of fission ?-rays and prompt neutrons due to their different time-of-flight.

Oberstedt A.

2010-10-01

411

Qualitative specific features of nuclear fission dynamics  

International Nuclear Information System (INIS)

Influence of the attainment of a statistical equilibrium on compound nucleus decay by means of fission or neutron emission was investigated. Analitic expressions for the probabilities of distribution in time and for the pre-equilibrium fission fragments spectra were obtained. They were determined by character of initial state, excitation energy and shell structure of compound nucleus

412

Progress in fission product nuclear data  

International Nuclear Information System (INIS)

This is the eighth issue of a report series on Fission Product Nuclear Data (FPND) which is published by the Nuclear Data Section (NDS) of the International Atomic Energy Agency (IAEA). The purpose of this series is to inform scientists working on FPND, or using such data, about all activities in this field which are planned, ongoing, or have recently been completed. The main part of this report consists of unaltered original contributions which the authors have sent to IAEA/NDS. Therefore, the IAEA cannot be held responsible for the information contained nor for any consequences resulting from the use of this information. The present issue contains also a section with some recent references relative to fission product nuclear data, which were not covered by the contributions submitted. The types of activities being included in this report are measurements, compilations and evaluations of: Fission product yields (neutron induced and spontaneous fission); Neutron reaction cross sections of fission products; Data related to the radioactive decay of fission products; Delayed neutron data of fission products; and lumped fission product data (decay heat, absorption etc.). The seventh issue of this series has been published in July 1981 as INDC(NDS)-116. The present issue includes contributions which were received by NDS between 1 August 1981 and 15 June 1982

413

A Computer Code For Calculation Of Fission Product Concentrations For Time Following Fission Burst  

International Nuclear Information System (INIS)

Precise information of the variation of fission product concentration for time after a fission burst is necessary for safety designs and operations of nuclear power reactors, fuel storage, transport flasks, and for spent fuel management and processing. In this study, a computer code has been developed for exact analysis of the buildup and decay of fission products for time following a fission burst. In which, a new numerical measure to solve the linear and nonlinear decay chains was introduced. The analytical data of fission product concentration, obtained by using the present code, was applied to calculate aggregate fission product decay heat from neutron fission of 235U, 238U, 233U, 239Pu, 241Pu and 232Th. (author)

414

Cumulative fission yield of Ce-148 produced by thermal-neutron fission of U-235  

International Nuclear Information System (INIS)

Cumulative fission yield of 148 cesium isotopes and some other fission products produced by thermal-neutron fission of 235 uranium is determined by Germanium/Lithium spectroscopic methods. The measuremets were done at Tsing-Hua open pool reactor using 3 to 4 mg of 93.15% enriched 235 uranium samples. Gamma rays are assigned to the responsible fission products by matching gamma rays energies and half lives. Fission rate is calculated by fission track method. Cumulative fission yields of 148 cesium, 90 krypton, 130 iodine, 144 lanthanum, 89 krypton, 136 xenon, 137 xenon and 140 cesium are calculated. This values are compared with previously predicted values and showed good agreement. 21 Ref

415

Theoretical Description of the Fission Process  

International Nuclear Information System (INIS)

Advanced theoretical methods and high-performance computers may finally unlock the secrets of nuclear fission, a fundamental nuclear decay that is of great relevance to society. In this work, we studied the phenomenon of spontaneous fission using the symmetry-unrestricted nuclear density functional theory (DFT). Our results show that many observed properties of fissioning nuclei can be explained in terms of pathways in multidimensional collective space corresponding to different geometries of fission products. From the calculated collective potential and collective mass, we estimated spontaneous fission half-lives, and good agreement with experimental data was found. We also predicted a new phenomenon of trimodal spontaneous fission for some transfermium isotopes. Our calculations demonstrate that fission barriers of excited superheavy nuclei vary rapidly with particle number, pointing to the importance of shell effects even at large excitation energies. The results are consistent with recent experiments where superheavy elements were created by bombarding an actinide target with 48-calcium; yet even at high excitation energies, sizable fission barriers remained. Not only does this reveal clues about the conditions for creating new elements, it also provides a wider context for understanding other types of fission. Understanding of the fission process is crucial for many areas of science and technology. Fission governs existence of many transuranium elements, including the predicted long-lived superheavy species. In nuclear astrophysics, fission influences the formation of heavy elements on the final stages of the r-process in a very high neutron density environment. Fission applications are numerous. Improved understanding of the fission process will enable scientists to enhance the safety and reliability of the nation's nuclear stockpile and nuclear reactors. The deployment of a fleet of safe and efficient advanced reactors, which will also minimize radiotoxic waste and be proliferation-resistant, is a goal for the advanced nuclear fuel cycles program. While in the past the design, construction, and operation of reactors were supported through empirical trials, this new phase in nuclear energy production is expected to heavily rely on advanced modeling and simulation capabilities.

416

Theoretical Description of the Fission Process  

Energy Technology Data Exchange (ETDEWEB)

Advanced theoretical methods and high-performance computers may finally unlock the secrets of nuclear fission, a fundamental nuclear decay that is of great relevance to society. In this work, we studied the phenomenon of spontaneous fission using the symmetry-unrestricted nuclear density functional theory (DFT). Our results show that many observed properties of fissioning nuclei can be explained in terms of pathways in multidimensional collective space corresponding to different geometries of fission products. From the calculated collective potential and collective mass, we estimated spontaneous fission half-lives, and good agreement with experimental data was found. We also predicted a new phenomenon of trimodal spontaneous fission for some transfermium isotopes. Our calculations demonstrate that fission barriers of excited superheavy nuclei vary rapidly with particle number, pointing to the importance of shell effects even at large excitation energies. The results are consistent with recent experiments where superheavy elements were created by bombarding an actinide target with 48-calcium; yet even at high excitation energies, sizable fission barriers remained. Not only does this reveal clues about the conditions for creating new elements, it also provides a wider context for understanding other types of fission. Understanding of the fission process is crucial for many areas of science and technology. Fission governs existence of many transuranium elements, including the predicted long-lived superheavy species. In nuclear astrophysics, fission influences the formation of heavy elements on the final stages of the r-process in a very high neutron density environment. Fission applications are numerous. Improved understanding of the fission process will enable scientists to enhance the safety and reliability of the nation’s nuclear stockpile and nuclear reactors. The deployment of a fleet of safe and efficient advanced reactors, which will also minimize radiotoxic waste and be proliferation-resistant, is a goal for the advanced nuclear fuel cycles program. While in the past the design, construction, and operation of reactors were supported through empirical trials, this new phase in nuclear energy production is expected to heavily rely on advanced modeling and simulation capabilities.

Witold Nazarewicz

2009-10-25

417

Spontaneous fission in the heavy elements  

International Nuclear Information System (INIS)

Investigations of mass and kinetic-energy distributions from spontaneous fission have been extended in recent years to an isotope of element 104 and, for half-lives, to an isotope of element 108. The results were surprising in that spontaneous fission half-lives turned out to be much longer than expected and mass and kinetic-energy distributions were found to abruptly shift away from those of the lighter actinides, showing two modes of fission. These new developments caused a re-evaluation of the understanding of the fission process, bringing an even deeper appreciation of the role played by nuclear shell effects upon spontaneous fission properties. (author) 22 refs.; 14 figs

418

Fission dynamics at low excitation energy. 2  

CERN Document Server

The mass asymmetry in the fission of U-236 at low excitation energy is clarified by the analysis of the trajectories obtained by solving the Langevin equations for the shape degrees of freedom. It is demonstrated that the position of the peaks in the mass distribution of fission fragments is determined mainly by the saddle point configuration originating from the shell correction energy. The width of the peaks, on the other hand, results from the shape fluctuations close to the scission point caused by the random force in the Langevin equation. We have found out that the fluctuations between elongated and compact shapes are essential for the fission process. According to our results the fission does not occur with continuous stretching in the prolate direction, similarly to that observed in starch syrup, but is accompanied by the fluctuations between elongated and compact shapes. This picture presents a new viewpoint of fission dynamics and the splitting mechanism.

Aritomo, Y; Ivanyuk, F A

2014-01-01

419

Fission at sub-barrier energies  

International Nuclear Information System (INIS)

Fission fragment angular distributions have been measured for the reaction 16O + 232Th at beam energies of 78.2, 82.2, 86.6, and 100.6 MeV. In this experiment, it was possible to determine the relative contributions of fragments from both the fission decay of the 248Cf compound nuclei, formed in complete fusion reactions between the target and the projectile, and the sequential fission decay of nuclei in the Th region, populated in inelastic scattering and/or transfer reactions on the Th target. This separation was made possible by using flight-time measurements. It was found that fragments from fission of 248Cf showed a somewhat smaller angular anisotropy as compared with earlier measurements in which the sequential