Faulted and Perfect Loop Evolution in Single Crystal Thorium Dioxide under High-Temperature Proton Irradiation

Nuclear fuels experience a pronounced microstructural evolution leading to reduction in thermal conductivity, changes in mechanical properties, and impacting their ability to retain fission products in operational environments, significantly affecting reactor's performance. Such changes result primarily from accumulation of fission products in the form of solid precipitates and inert gas bubbles, and fuel restructuring upon accumulation of radiation-induced defects. Particularly within the realm of radiation damage, the formation and evolution of faulted loops play a critical role in subsequent evolution of dislocations, grain subdivision and recrystallization degrading fuel's physical properties. This research focused on the dynamics of faulted and perfect loops, alongside the unfaulting process, in single crystal ThO₂ subjected to proton-irradiation at doses of 0.42, 0.62, and 0.74 dpa at 1000 °C. Transmission electron microscopy characterization and cluster dynamics modeling were employed to uncover the mechanisms of nucleation and growth, and unfaulting of faulted loops, which are crucial in driving the evolution of defects. In this study, our findings suggested that the critical diameter for faulted loop unfaulting was approximately 2 nm. Moreover, the rate of unfaulting was found to scale linearly with the dislocation loop density and quadratically with the loop radius from the modeling results, highlighting the intricate dependence of defect evolution on microstructural parameters.