Abstract Alteration phases may influence both the dissolution of nuclear waste forms and release of radionuclides from the waste package environment. In the present study, UO 2 pellets serve as surrogates for commercial spent nuclear fuel, with the pellets being exposed to periodic drops of simulated groundwater at 90°C. Uranium release was very rapid between one and two years, resulting from grain boundary corrosion and spallation of micrometer-sized UO 2+ x particles from the sample surface. The development of a dense mat of alteration phases after two years apparently trapped loose particles, resulting in reduced rates of uranium release. The paragenetic sequence of alteration phases is similar to that observed in surficial weathering zones of natural uraninite deposits, with alkali and alkaline earth uranyl silicates being the long-term solubility-limiting phases for uranium. Results from this study and comparisons with natural analogue deposits suggest that the migration of fission products from altered spent fuel may be retarded by their incorporation in secondary uranium phases.