Nano-crystallization induced by high-energy heavy ion irradiation in UO2

Advanced microstructure investigations of the high-burnup structure (HBS) in UO₂ produced by high-dose 84 MeV Xe ion irradiation are reported. Spark plasma sintered micro-grained UO₂ was irradiated to 1357 dpa at 350 °C. The characteristic nano-grains and micro-pores of the HBS were formed. The grain size and grain boundary misorientation distributions of the HBS were measured using transmission electron microscopy based orientation imaging microscopy. Grain polygonization due to accumulation of radiation-induced dislocations was found to be the mechanism of nano-crystallization. The morphology of Xe bubbles was quantitatively investigated. This study provides crucial references for advanced fuel performance modeling of high-burnup UO₂.