The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO2 of the Oklo natural fission reactors
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Satoshi Utsunomiya
In spent nuclear fuel (SNF), the micrometer- to nanometer-sized epsilon phase (Mo-Ru-Pd-Tc-Rh) is an important host of 99Tc which has a long half life (2.13×105 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 (∼ 2.0 Ga) have been investigated using transmission electron microscopy (TEM). Because essentially all of the 99Tc has decayed to 99Ru, 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 UO2 matrix at the scale of 50−700 nm; froodite, PdBi2, with trace amounts of As, Fe, and Te, and palladodymite or rhodarsenide, (Pd,Rh)2As. The most abundant 4d-metal phase is ruthenarsenite, (Ru,Ni)As, which has a representative composition: As, 59.9; Co, 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 ∼5 μm). 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: (i) 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. (ii) All Mo and most of the Tc were released from the epsilon phase. Galena precipitated surrounding the 4d-metal phases. (iii) Once the uraninite matrix has dissolved, the epsilon nanoparticles were released and “captured” within alteration phases that are immediately adjacent to the uraninite.
© Oldenbourg Wissenschaftsverlag
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Articles in the same Issue
- Preface Migration 2005
- Solubility measurement of zirconium(IV) hydrous oxide
- Solubility of ThO2·xH2O(am) and the formation of ternary Th(IV) hydroxide-carbonate complexes in NaHCO3-Na2CO3 solutions containing 0−4 M NaCl
- Ion interaction (SIT) coefficients for the Th4+ ion and trace activity coefficients in NaClO4, NaNO3 and NaCl solution determined by solvent extraction with TBP
- Leaching of neptunium from garnet- and murataite-based ceramics
- Coprecipitation of thorium and lanthanum with UO2+x(s) as host phase
- Rare Earth element (REE) incorporation in natural calcite: Upper limits for actinide uptake in a secondary phase
- Protonation of D-gluconate and its complexation with Np(V) in acidic to nearly neutral solutions
- Complexing properties of α-isosaccharinate: thorium
- Determination of the thermodynamic quantities of complexation between Eu(III) and carboxylic acids by microcalorimetry
- Potentiometric investigations on the interaction of humic acid with Cu(II) and Eu(III) ions
- Investigation of complexation of thorium by humic acid using chemically immobilized humic acid on silica gel
- Correlation between X-ray chemical shift and partial charge in Tc(IV) complexes: Determination of Tc partial charge in TcnOy(4n-2y)+
- Oxidation and dissolution rates of UO2(s) in carbonate-rich solutions under external alpha irradiation and initially reducing conditions
- On the stability of Pu(III) at different pH under non-inert conditions
- Reduction behavior of uranium in the presence of citric acid
- Leaching of zirconolite ceramics under H+ and He2+ irradiation
- Reduction of plutonium(VI) in brine under subsurface conditions
- Theoretical first step towards an understanding of the uranyl ion sorption on the rutile TiO2(110) face: A DFT periodic and cluster study
- Cm(III) sorption onto γ-Al2O3: New insight into sorption mechanisms by time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure
- Modelling sorption data for the actinides Am(III), Np(V) and Pa(V) on montmorillonite
- Sorption of Cs, Ni, Pb, Eu(III), Am(III), Cm, Ac(III), Tc(IV), Th, Zr, and U(IV) on MX 80 bentonite: An experimental approach to assess model uncertainty
- The uptake of radium by calcium silicate hydrates and hardened cement paste
- Np(V)O2+ sorption on hydroxyapatite-effect of calcium and phosphate anions
- Uranium(VI) interaction with pyrite (FeS2): Chemical and spectroscopic studies
- Interaction of U(VI) with pyrite, galena and their mixtures: a theoretical and multitechnique approach
- Technetium sorption by stibnite from natural water
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- Sorption of cesium on granite under aerobic and anaerobic conditions
- Comparative study of cesium sorption on crushed and intact sedimentary rock
- The INE-Beamline for actinide research at ANKA
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- Interaction mechanisms of bacterial strains isolated from extreme habitats with uranium
- Biotransformation of plutonium complexed with citric acid
- Iodination of humic acid samples from different origins
- The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO2 of the Oklo natural fission reactors
- Rhenium migration at the Maqarin natural analogue site (Jordan)
- Classic adsorption isotherms incorporated in modern surface complexation models: Implications for sorption of actinides
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