Nanoprecipitates and Nanostructures in Spent UO₂ Fuel

Spent nuclear fuel experiences substantial alterations in both its microstructure and chemical composition during its time in a reactor. While some fission products remain integrated within the UO₂ matrix, other inert gases like xenon (Xe) and krypton (Kr), as well as certain 4d group metals (molybdenum, technetium, ruthenium, rhodium, and palladium), form nanoprecipitates within the UO₂ matrix. In certain areas of the UO₂ fuel, these Kr/Xe precipitates and metallic precipitates can also come together to create paired structures at the rim regions. Furthermore, as the burn-up levels increase, grain subdivision within the UO₂ occurs, giving rise to a high burn-up structure (HBS) characterized by the presence of nanograins. This research aims to provide an in-depth examination of the in situ microstructural changes in nanoprecipitates and HBS in spent UO₂ fuel when subjected to Kr or Xe ion irradiation. To enhance our understanding of these changes, this study employs transmission electron microscopy techniques alongside density-functional theory and phase field modeling.