UO₂ is the most commonly used commercial nuclear fuel material. For this reason, the chemistry of UO₂ has been studied extensively for many decades. In this presentation, mechanistic aspects of radiation induced dissolution of UO₂ under conditions relevant for a geological repository for used nuclear fuel will be discussed. Particular focus will be put on the reactivity of H₂O₂ towards UO₂ since H₂O₂ has been shown to be the most important radiolytic oxidant under the expected repository conditions. Several studies have shown that H₂O₂ can undergo catalytic decomposition on oxide surfaces forming surface-bound hydroxyl radicals as an intermediate. On UO₂, this intermediate can react in two different ways: (1) oxidation of the surface and (2) reaction with H₂O₂. In other words, the two competing reactions have a common intermediate. In a system containing HCO₃^-/CO₃^2-, U(VI) will be readily dissolved once it is formed. This enables the formation of uranyl-peroxo-carbonato complexes in solution which, at higher uranyl concentrations, can dominate the peroxide speciation. These negatively charged complexes display a completely different reactivity towards UO₂ as well as other oxide surfaces and thereby change system quite dramatically. The impact of these complexes will be discussed in detail as well as some of the features of catalytic H₂O₂ decomposition on oxide surfaces.

In addition to the numerous studies performed on UO₂, similar studies of alternative fuel materials have been conducted in recent years. Here we will focus on UN, UC and U₃Si₂ and discuss the mechanism and kinetics of radiation induced dissolution of these materials.