Kinetic Effects of H2O2 Speciation on the Overall Peroxide Consumption at UO2-Water Interfaces

The interfacial radiation chemistry of UO2 is of key importance in the development of models to predict the corrosion rate of spent nuclear fuel in contact with groundwater. Here, the oxidative dissolution of UO2 induced by radiolytically produced H2O2 is of particular importance. The difficulty of fitting experimental data to simple first-order kinetics suggests that additional factors need to be considered when describing the surface reaction between H2O2 and UO2. It has been known for some time that UO22+ forms stable uranyl peroxo-carbonato complexes in water containing H2O2 and HCO3-/CO32-, yet this concept has largely been overlooked in studies where the oxidative dissolution of UO2 is considered. In this work, we show that uranyl peroxocarbonato complexes display little to no reactivity toward the solid UO2 surface in 10 mM bicarbonate solution (pH 8-10). The rate of peroxide consumption and UO22+ dissolution will thus depend on the UO22+ concentration and becomes limited by the free H2O2 fraction. The rate of peroxide consumption and the subsequent UO22+ dissolution can be accurately predicted based on the firstorder kinetics with respect to free H2O2, taking the initial H2O2 surface coverage into account.