Exploring the Change in Redox Reactivity of UO₂ Induced by Exposure to Oxidants in HCO₃^- Solution

Understanding the possible change in UO₂ surface reactivity after exposure to oxidants is of key importance when assessing the impact of spent nuclear fuel dissolution on the safety of a repository for spent nuclear fuel. In this work, we have experimentally studied the change in UO₂ reactivity after consecutive exposures to O₂ or γ-radiation in aqueous solutions containing 10 mM HCO₃^-. The experiments show that the reactivity of UO₂ toward O₂ decreases significantly with time in a single exposure. In consecutive exposures, the reactivity also decreases from exposure to exposure. In γ-radiation exposures, the system reaches a steady state and the rate of uranium dissolution becomes governed by the radiolytic production of oxidants. Changes in surface reactivity can therefore not be observed in the irradiated system. The potential surface modification responsible for the change in UO₂ reactivity was studied by XPS and UPS after consecutive exposures to either O₂, H₂O₂, or γ-radiation in 10 mM HCO₃^- solution. The results show that the surfaces were significantly oxidized to a stoichiometric ratio of O/U of UO_2.3 under all the three exposure conditions. XPS results also show that the surfaces were dominated by U(V) with no observed U(VI). The experiments also show that U(V) is slowly removed from the surface when exposed to anoxic aqueous solutions containing 10 mM HCO₃^-. The UPS results show that the outer ultrathin layer of the surfaces most probably contains a significant amount of U(VI). U(VI) may form upon exposure to air during the rinsing process with water prior to XPS and UPS measurements.