H2O2 and radiation induced dissolution of UO2 and SIMFUEL in HCO3- deficient aqueous solution

Understanding UO ₂ matrix dissolution is of major importance for the safety assessment of a future deep geological repository. Oxidative dissolution of UO ₂ 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 UO ₂ and SIMFUEL pellets in HCO 3 - free solutions by studying the dissolution of U (VI) and consumption of H ₂O ₂ over time. The results are compared with previous experiments performed in HCO 3 - solutions. The oxidative dissolution rate is higher for the UO ₂ pellet in HCO 3 - compared to the other systems. It is evident that the kinetics of the reaction with H ₂O ₂ is qualitatively different for SIMFUEL in comparison with pure UO ₂. For the UO ₂ 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 UO ₂ 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 UO ₂ and SIMFUEL pellets were investigated in pure water, and compared with HCO 3 - systems. Also here the dissolution rate of uranium is higher for UO ₂ 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.