UO2 dissolution in bicarbonate solution with H2O2: the effect of temperature

Abstract

Upon nuclear waste canister failure and contact of spent nuclear fuel with groundwater, the UO₂ matrix of spent fuel will interact with oxidants in the groundwater generated by water radiolysis. Bicarbonate (HCO₃⁻) is often found in groundwater, and the H₂O₂ induced oxidative dissolution of UO₂ in bicarbonate solution has previously been studied under various conditions. Temperatures in the repository at the time of canister failure will differ depending on the location, yet the effect of temperature on oxidative dissolution is unknown. To investigate, the decomposition rate of H₂O₂ at the UO₂ surface and dissolution of U^VI in bicarbonate solution (0.1, 1, 10 and 50 mM) was analysed at various temperatures (10, 25, 45 and 60 °C). At [HCO₃⁻] ≥ 1 mM, the concentration of dissolved U^VI decreased with increasing temperature. This was attributed to the formation of U^VI-bicarbonate species at the surface and a change in the mechanism of H₂O₂ decomposition from oxidative to catalytic. At 0.1 mM, no obvious correlation between temperature and U dissolution was observed, and thermodynamic calculations indicated this was due to a change in the surface species. A pathway to explain the observed dissolution behaviour of UO₂ in bicarbonate solution as a function of temperature was proposed.