Interstitial Loop Unfaulting and Extended Defect Evolution in Uranium Dioxide and Thorium Dioxide

The evolution of dislocation loops and lines, as well as fission gas bubbles, plays an important role in nuclear fuel behaviour. Their evolution has significant implications for fission gas release and fuel restructuring. In this presentation, we discuss the utilization of transmission electron microscopy (TEM) to image defects in irradiated UO2 and ThO2. TEM imaging is done under various conditions, including in-situ characterization under ion beam irradiation and in-situ annealing of pre-irradiated samples, as well as their ex-situ counterparts. To interpret our experimental observations, we employ cluster dynamics simulations to describe the evolution of extended defects. Our analysis reveals various evolution mechanisms, including the growth of interstitial dislocation loops by point defect absorption, the unfaulting of Frank loops, as well as the coalescence of dislocation loops and bubbles.
Post-irradiation characterization of proton-irradiated samples indicates that at low irradiation doses, faulted dislocation loops are homogeneously nucleated and grow via absorption of point defects. Irradiation with heavier krypton ions allows the characterization of extended defects at higher doses, where results suggest that dislocation loops undergo the unfaulting process and exhibit coalescence. While voids are not very common under proton irradiation, krypton ion irradiation produces gas bubbles. In general, post-irradiation annealing results in the further evolution of extended defects, where loops, voids, and bubbles coarsen. While initial krypton ion irradiation experiments only provided an indication of the coalescence of the extended defects as a coarsening mechanism, more recent in-situ annealing experiments provide further evidence of the coalescence of extended defects. These results provide a means for validating some of the assumptions made to describe microstructure evolution in irradiated nuclear fuels.