In this study the diffusion of uranium dissolved from uranium oxide fuel was studied experimentally in compacted sodium bentonite (Wyoming bentonite MX-80). The parameters varied in the study were the density of bentonite, the salt content of the solution and the redox conditions. In the studies with non-saline water of total dissolved solids about 300 ppm, uranium was both in aerobic and anaerobic experiments as anionic complexes and followed the anionic diffusion mechanism. Anion exclusion decreased effective diffusion coefficients, especially in more dense samples. In the studies with saline water of total dissolves solids about 35000 ppm, uranium appeared in the aerobic experiments probably as cationic complexes and followed the cationic diffusion mechanism. Uranium in the saline, anaerobic experiment was probably U(OH)4 and followed the diffusion mechanism of neutral species.
The U0 2 -matrix solubility is a critical parameter for predicting the stability of spent nuclear fuel under disposal conditions. This paper presents the results obtained from the dissolution experiments of unirradiated sintered polycrystalline U0 2 pellets in deionized water, in NaHCO, solutions, and in two different synthetic groundwaters under anoxic conditions (N 2 ) at 25 °C. The oxidation state of uranium in solution was determined experimentally using a method based on the separation of the tetravalent and hexavalent states by ion-exchange chromatography in HCl medium. The uranium contents of each fraction were analysed by ICP-MS.In order to study the controlling phase for the concentration of uranium at steady state in the solutions, a comparison of the experimental data was made with the solubilities calculated with the geochemical code EQ3/6. The measured concentrations were at the level of the solubilities of the mixed valence oxide, U 4 0 9 , in all solutions. The dominant oxidation state was the U(VI) state.
Article:Myllykyla, E., Lavonen, T., Stennett, M. et al. (3 more authors) (2015) Solution composition and particle size effects on the dissolution and solubility of a ThO2 microstructural analogue for UO2 matrix of nuclear fuel. Radiochimica Acta, 103 (8
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We report on the spectroscopic analysis of several samples relevant to the processes governing the behavior of oxidized uranium species in groundwater solutions under anoxic conditions. Both Fe samples with different times of exposure to the U(IV) solution and Fe metal-solution inetrfaces in the liquid cell ex-situ and in-situ, respectively. Resonant inelastic soft x-ray scattering is shown to be sensitive to the chemical state of uranium. The measurements were performed at a number of energies of the primary photon beam across the U 5d absorption edge. The results unambiguously indicate the reduction of U(VI) to U(IV) on the Fe surface.
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