Solubility and oxidation state of uranium under anoxic conditions (N-2 atmosphere)

The UO2-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 UO2 pellets in deionized water, in NaHCO3 solutions, and in two different synthetic groundwaters under anoxic conditions (N-2) at 25 degrees 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, U4O9, in all solutions. The dominant oxidation state was the U(VI) state.