Investigation of Extended Defect Evolution in UO₂ and ThO₂ during Ion Irradiation and Post-Irradiation Annealing

A rate theory model is applied to analyze the kinetics of extended defect evolution revealed by in-situ transmission electron microscopy characterization during ion irradiation and post-irradiation annealing of uranium dioxide (UO₂) and thorium dioxide (ThO₂). The objective is to investigate mechanisms governing dislocation loop growth, unfaulting of Frank loops, and extended defect coarsening. In-situ characterization of krypton ion irradiated ThO₂ reveals that loop growth is limited by the mobility of cation interstitial and loop nucleation is influenced by the mobility of both cation and anion interstitials. Similar conclusion has been obtained from ex-situ characterization of proton irradiated ThO₂. Additionally, we determined that during in-situ experiments utilizing TEM lamellae it is important to include surface as a sink for both extended and point defects. Observed extended defect coarsening under in-situ annealing is best described by coalescence mechanism resulting from migration of extended defects. This detailed understanding of extended defect evolution allows to improve assessment of physical properties important for nuclear fuel performance analysis. The established procedure also allows to infer point defect concentration, which are more impactful in determining several physical properties, such as thermal conductivity and atomic diffusion.