Sample records for clarkeite

  1. X-ray diffraction and geochemical studies on uranium minerals from Jogipalle Pegmatite, Nellore Schist Belt, Andhra Pradesh: paragenetic implications

    International Nuclear Information System (INIS)

    The uranium ore sample used in this study occurs as hand - pickable lumps and grains in the Jogipalle pegmatite, Nellore Schist Belt, Andhra Pradesh. Powder X-ray diffraction (XRD) studies on separated uranium minerals (UMs) have revealed the presence of both primary (uraninite) and secondary (ianthinite, clarkeite, curite and â- uranophane) uranium minerals, which are mostly characterised by their sharply-defined reflections. The crystallographic parameters of various UMs are: Uruniuite-1 and 2 unit cell dimension (a0) =5.4758 and 5.4422 Å and unit cell volume (V) = 164.08 and 161.18 Å3; clarkeite a0=3.9473 Å, b0=3.9473 Å, c0 =17.6835 Å, α =β =900, γ=1200, V=238.628 Å3I:curite a0=12.6292 Å, b0 =13.2035 Å, c0=8.3646 Å, V = 1394.81 Å3; and β-uranophane a0= 13.9481Å, bo=15.4688 Å. C0 =6.6362 Å, α =γ =9090, β =91.30, V =1430.90 Å3. Out of two, one uraninite has ao of 5.4758 Å, which is more than the value given for the uraninite standard (5.4645 Å), suggesting its anomalous nature and formation of uraninite (primary) under high temperature condition (∼500-550℃). In contrast, another uraninite has a0=5.4422 Å, reflecting its oxidized nature. It, thus, suggests that after their formation, the uraninites have been subjected to oxidation leading to the formation of secondary uranium minerals (SUMs) with a relict core of black mineral (uraninite) encircled by successive zones of SUMs, namely. black (ianthinite, in traces), orange (clarkeite-curite) and yellow (β - uranophane). Based on available mineralogical data, the inferred paragenetic sequence of the investigated uranium minerals is: Uranium oxide (primary uraninite) > uranium oxide (altered uraninite) > uraniumoxide hydrate (ianthinite) > sodium-potassium uranium oxide (clarkeite) - lead-uraniumoxide hydrate (curite) > calcium uranyl silicate hydroxide hydrate ((β -uranophane). Uraninite-I contains high U3O8,(74.25%), ThO2 (7.96%), PbO (7.73%) and rare earth elements (16214 ppm

  2. Thermodynamics of Uranyl Minerals: Enthalpies of Formation of Uranyl Oxide Hydrates

    Energy Technology Data Exchange (ETDEWEB)

    K. Kubatko; K. Helean; A. Navrotsky; P.C. Burns


    The enthalpies of formation of seven uranyl oxide hydrate phases and one uranate have been determined using high-temperature oxide melt solution calorimetry: [(UO{sub 2}){sub 4}O(OH){sub 6}](H{sub 2}O){sub 5}, metaschoepite; {beta}-UO{sub 2}(OH){sub 2}; CaUO{sub 4}; Ca(UO{sub 2}){sub 6}O{sub 4}(OH){sub 6}(H{sub 2}O){sub 8}, becquerelite; Ca(UO{sub 2}){sub 4}O{sub 3}(OH){sub 4}(H{sub 2}O){sub 2}; Na(UO{sub 2})O(OH), clarkeite; Na{sub 2}(UO{sub 2}){sub 6}O{sub 4}(OH){sub 6}(H{sub 2}O){sub 7}, the sodium analogue of compreignacite and Pb{sub 3}(UO{sub 2}){sub 8}O{sub 8}(OH){sub 6}(H{sub 2}O){sub 2}, curite. The enthalpy of formation from the binary oxides, {Delta}H{sub f-ox}, at 298 K was calculated for each compound from the respective drop solution enthalpy, {Delta}H{sub ds}. The standard enthalpies of formation from the elements, {Delta}H{sub f}{sup o}, at 298 K are -1791.0 {+-} 3.2, -1536.2 {+-} 2.8, -2002.0 {+-} 3.2, -11389.2 {+-} 13.5, -6653.1 {+-} 13.8, -1724.7 {+-} 5.1, -10936.4 {+-} 14.5 and -13163.2 {+-} 34.4 kJ mol{sup -1}, respectively. These values are useful in exploring the stability of uranyl oxide hydrates in auxiliary chemical systems, such as those expected in U-contaminated environments.

  3. Characterization of Solids in Residual Wastes from Underground Storage Tanks at the Hanford Site, Washington, U.S.A

    International Nuclear Information System (INIS)

    Solid phase physical and chemical characterization methods have been used in an ongoing study of residual wastes from several single-shell underground waste tanks at the U.S. Department of Energy's Hanford Site in southeastern Washington State. Because these wastes are highly-radioactive dispersible powders and are chemically-complex assemblages of crystalline and amorphous solids that contain contaminants as discrete phases and/or co-precipitated within oxide phases, their detailed characterization offers an extraordinary technical challenge. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS) are the two principal methods used to characterize solid phases and their contaminant associations in these wastes. Depending on the specific tank, numerous solids (such as eejkaite; Na2U2O7; clarkeite; gibbsite; boehmite; dawsonite; cancrinite; Fe oxides such as hematite, goethite, and maghemite; rhodochrosite; lindbergite; whewellite; nitratine; and several amorphous phases) have been identified in residual wastes studied to date. Because many contaminants of concern are heavy elements, SEM analysis using the backscattered electron (BSE) signal has proved invaluable in distinguishing phases containing elements, such as U and Hg, within the complex assemblage of particles that make up each waste. XRD, SEM/EDS, and synchrotron-based methods provide different, but complimentary characterization data about the morphologies, crystallinity, particle sizes, surface coatings, and compositions of phases in the wastes. The impact of these techniques is magnified when each is used in an iterative fashion to help interpret the results from the other analysis methods and identify additional, more focused analyses

  4. Formation of stable uranium(VI) colloidal nanoparticles in conditions relevant to radioactive waste disposal. (United States)

    Bots, Pieter; Morris, Katherine; Hibberd, Rosemary; Law, Gareth T W; Mosselmans, J Frederick W; Brown, Andy P; Doutch, James; Smith, Andrew J; Shaw, Samuel


    The favored pathway for disposal of higher activity radioactive wastes is via deep geological disposal. Many geological disposal facility designs include cement in their engineering design. Over the long term, interaction of groundwater with the cement and waste will form a plume of a hyperalkaline leachate (pH 10-13), and the behavior of radionuclides needs to be constrained under these extreme conditions to minimize the environmental hazard from the wastes. For uranium, a key component of many radioactive wastes, thermodynamic modeling predicts that, at high pH, U(VI) solubility will be very low (nM or lower) and controlled by equilibrium with solid phase alkali and alkaline-earth uranates. However, the formation of U(VI) colloids could potentially enhance the mobility of U(VI) under these conditions, and characterizing the potential for formation and medium-term stability of U(VI) colloids is important in underpinning our understanding of U behavior in waste disposal. Reflecting this, we applied conventional geochemical and microscopy techniques combined with synchrotron based in situ and ex situ X-ray techniques (small-angle X-ray scattering and X-ray adsorption spectroscopy (XAS)) to characterize colloidal U(VI) nanoparticles in a synthetic cement leachate (pH > 13) containing 4.2-252 μM U(VI). The results show that in cement leachates with 42 μM U(VI), colloids formed within hours and remained stable for several years. The colloids consisted of 1.5-1.8 nm nanoparticles with a proportion forming 20-60 nm aggregates. Using XAS and electron microscopy, we were able to determine that the colloidal nanoparticles had a clarkeite (sodium-uranate)-type crystallographic structure. The presented results have clear and hitherto unrecognized implications for the mobility of U(VI) in cementitious environments, in particular those associated with the geological disposal of nuclear waste.

  5. Residual Waste from Hanford Tanks 241-C-203 and 241-C-204. 2. Contaminant Release Model

    International Nuclear Information System (INIS)

    Fusion analyses, water leaches, selective extractions, empirical solubility measurements, and thermodynamic modeling were used with results from solid-phase characterization studies [see companion paper (1)] to determine total concentrations, contaminant-phase associations, and develop contaminant release models for residual sludge from single-shell underground waste tanks 241-C-203 and 241-C-204 at the U.S. Department of Energy?s Hanford Site in southeastern Washington state. U and Tc are primary contaminants of concern because of their long half-lives and their generally high mobility in oxidizing soil and groundwater environments. Uranium release was determined to be controlled by two phases; ?ejkaite [Na4(UO2)(CO3)3] and poorly crystalline Na2U2O7 [or clarkeite Na[(UO2)O(OH)](H2O)0-1] which were identified in C-203 and C-204 sludge samples (1). U release was determined to occur in three stages from these phases. In the first stage, U release will be controlled by the solubility of ?ejkaite, which is suppressed by high concentrations of sodium released from dissolution of NaNO3 in the residual sludges. Equilibrium solubility calculations indicate the U released during this stage will have a maximum concentration of 0.021 M. When all the NaNO3 has dissolved from the sludge, the solubility of the remaining ?ejkaite will increase to 0.28 M. After ?ejkaite has completely dissolved, the maximum concentration of U released is expected to be controlled by the solubility of Na2U2O7 at a concentration of 3.0 ? 10-5 M. For Tc, a significant fraction of its concentration in the residual sludge was determined to be relatively insoluble (20 wt% for C-203 and 80 wt% for C-204). Because of the low concentrations of Tc in these sludge materials, the characterization studies did not identify any discrete Tc solids phases. Therefore, release of the readily soluble fraction of Tc was assumed to be controlled by the solubility of NaTcO4 at 7.1 M. Selective extraction results