In-Situ TEM study of microstructural evolution in proton irradiated single crystal UO2 under high-temperature annealing

Understanding microstructural changes in nuclear fuels under different irradiation conditions is important as it directly affects thermal, oxidation and mechanical properties and thereby reactor safety and longevity. This work focuses on the effect of temperature on the evolution of extended defects in uranium dioxide. In-situ transmission electron microscopy (TEM) annealing of proton irradiated UO 2 was performed at different temperatures: 900 °C, 1100 °C and 1300 °C, 1 hour for each temperature. Microstructural evolution in terms of dislocation loops and voids were captured using in-situ TEM during annealing at different temperatures. Post-annealing at each temperature, detailed characterization using rel-rod dark field, bright field, and underfocus-overfocus imaging in TEM were used to identify faulted loops, perfect dislocation loops, and voids, respectively. Dislocation loops showed migration, disappearance, coalescence and loop-line interactions which significantly contributed to the recovery process during annealing at temperatures of 1100 °C and 1300 °C. Small voids were observed at 900 °C (diameter ∼ 0.5–1.5 nm) and grew rapidly during 1300 °C annealing (diameter ∼ 1–3 nm). Void growth was attributed to vacancy absorption, Ostwald ripening and void coalescence mechanisms. Extensive void growth was unique to in-situ TEM annealing due to free surface effect as ex-situ annealing confirmed negligible TEM resolvable (> 0.5 nm) voids at 1300 °C. The dislocation loops and voids behavior during in-situ TEM annealing were compared with rate theory and cluster dynamics predictions.