Characterization of vacancy type defects in irradiated UO₂ and CeO₂

Nuclear fuels containing uranium oxide, plus fission and activation products, will continue to undergo radioactive decay and also fission after removal from the reactor. These nuclear materials will undergo exposure to heat and persistent irradiation by alpha particles and neutrons. Vacancies and larger open volume defects, sometimes filled by helium, may be created and lead to swelling. Positron annihilation Doppler Broadening spectroscopy was implemented to investigate damage due to 1 MeV Kr-ion irradiation of UO₂ and 3 MeV Au irradiation of CeO₂ (a surrogate for UO₂) and lanthanide-doped versions of these materials. Contrary to expectation, significant differences arose due to the doping atoms. In UO₂, the doping atom type resulted in varying concentrations of the same type of defect. In CeO₂, the defect type and concentration changed due to irradiation of differently doped material. Graphical abstract: Comparison of TRIM calculations to positron annihilation spectroscopy Doppler Broadening (PAS-DB) data vacancy signal S (both left axis) and PAS-DB of irradiated CeO₂ (right axis). The UO₂ sample was irradiated with 1 MeV Kr ions and the CeO₂ sample with 3 MeV Au ions. The solid lines are the simulation of positron data (grey for UO₂ from two differently prepared samples and green for CeO₂) based on best fits of simple damage profiles (red). The damage profile assumes stacks of layers with constant damage. In UO₂, the best fit damage shows layers of decreasing defect concentration (red). In contrast, in CeO₂, the damage is low near the surface and rises. These differences may have implications for the use of CeO₂ as simple non-radioactive analogue of UO₂-based nuclear fuels.[Figure not available: see fulltext.]