The kinetics and mechanism of HO decomposition at the UO surface in bicarbonate solution.

In the event of nuclear waste canister failure in a deep geological repository, groundwater interaction with spent fuel will lead to dissolution of uranium (U) into the environment. The rate of U dissolution is affected by bicarbonate (HCO ) concentrations in the groundwater, as well as HO produced by water radiolysis. To understand the dissolution of UO by HO in bicarbonate solution (0.1-50 mM), dissolved U concentrations were measured upon HO addition (300 μM) to UO/bicarbonate mixtures. As the HO decomposition mechanism is integral to the dissolution of UO, the kinetics and mechanism of HO decomposition at the UO surface was investigated. The dissolution of UO increased with bicarbonate concentration which was attributed to a change in the HO decomposition mechanism from catalytic at low bicarbonate (≤5 mM HCO ) to oxidative at high bicarbonate (≥10 mM HCO ). Catalytic decomposition of HO at low bicarbonate was attributed to the formation of an oxidised surface layer. Second-order rate constants for the catalytic and oxidative decomposition of HO at the UO surface were 4.24 × 10 m s and 7.66 × 10 m s respectively. A pathway to explain both the observed UO dissolution behaviour and HO decomposition as a function of bicarbonate concentration was proposed.