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 UO₂ ²⁺ forms stable uranyl peroxo-carbonato complexes in water containing H₂O₂ and HCO₃ ^-/CO₃ ^2-, yet this concept has largely been overlooked in studies where the oxidative dissolution of UO₂ is considered. In this work, we show that uranyl peroxo-carbonato 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 UO₂ ²⁺ dissolution will thus depend on the UO₂ ²⁺ concentration and becomes limited by the free H₂O₂ fraction. The rate of peroxide consumption and the subsequent UO₂ ²⁺ dissolution can be accurately predicted based on the first-order kinetics with respect to free H₂O₂, taking the initial H₂O₂ surface coverage into account.