H2O2 and radiation induced dissolution of UO2 and SIMFUEL in deficient aqueous solution

Understanding UO2 matrix dissolution is of major importance for the safety assessment of a future deep geological repository. Oxidative dissolution of UO2 and SIMFUEL pellets have been extensively studied in HCO 3 - solutions, while less is known about systems with no or very low HCO 3 - concentrations. The aim of this work is to elucidate the oxidative dissolution of UO2 and SIMFUEL pellets in HCO 3 - free solutions by studying the dissolution of U (VI) and consumption of H2O2 over time. The results are compared with previous experiments performed in HCO 3 - solutions. The oxidative dissolution rate is higher for the UO2 pellet in HCO 3 - compared to the other systems. It is evident that the kinetics of the reaction with H2O2 is qualitatively different for SIMFUEL in comparison with pure UO2. For the UO2 pellet in pure water, the presence of a secondary phase (meta) studtite, on the surface of the pellet is confirmed by Raman spectroscopy. The kinetic impact of the secondary phase is evaluated in separate UO2 powder experiments. The (meta) studtite (surface) precipitation leads to a slower release of uranium into the solution. Numerical simulations using experimentally determined rate constants are used to evaluate a simple mechanism of surface precipitation. The numerical results are in fair agreement with the experimental observations given certain criteria. In addition, the γ-radiation induced dissolution of UO2 and SIMFUEL pellets were investigated in pure water, and compared with HCO 3 - systems. Also here the dissolution rate of uranium is higher for UO2 in HCO 3 - compared to pure water, while for SIMFUEL longer irradiation times are needed to observe any difference between pure and HCO 3 - containing water.