H2O2 and radiation induced dissolution of UO2 and SIMFUEL pellets

Dissolution of the UO2 matrix is of major importance in the safety assessment of a future deep repository for spent nuclear fuel. The aim of this work is to elucidate if the observed differences in dissolution rates between SIMFUEL and UO2 can be attributed to differences in oxidant reactivity towards these two materials. To elucidate this, the oxidative dissolution of U(VI) and consumption of H2O2 have been studied for UO2 and SIMFUEL pellets under N2 and H2 atmosphere. The H2O2 and U(VI) concentrations have been measured as a function of reaction time. In addition, γ-radiation induced dissolution UO2 and SIMFUEL pellets have been studied. The experiments show that while the reactivity of the two types of pellets towards H2O2 is almost identical and in good agreement with the previously determined rate constant for the reaction, the dissolution rates differ considerably. The significantly lower rate of dissolution of the SIMFUEL pellet is attributed to an increased fraction of catalytic decomposition of H2O2. The radiation chemical experiments reveal a similar but less pronounced difference between the two types of pellets. This implies that the relative impact of the radiolytic oxidants in radiation induced UO2 dissolution differs between a pure UO2 pellet and SIMFUEL.