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The microstructure investigations of a U 3 Si 2 specimen implanted by 84 MeV Xe ions at 450∘C are reported here. In the region that corresponds to the highest irradiation dose, U 3 Si 2 was found to decompose into a Si-enriched nanocrystalline USi matrix phase and a U-enriched amorphous inclusion phase. Density functional theory (DFT) calculations were used to help understand the decomposition mechanism. Xe bubbles of different morphologies were observed in USi nano-grains, on USi grain boundaries, and phase boundaries. Original U 3 Si 2 micro-grains were preserved on both sides of the irradiation damage peak region within the sample, implying a radiation dose threshold for decomposition of approximately 150 dpa at 450∘C. In the preserved U 3 Si 2 region beneath the irradiation damage peak, where significant amount of Xe ions were deposited, a monomodal size distribution of intragranular Xe bubbles with an average size of 2.68 nm formed. In both decomposed and preserved U 3 Si 2 , the size of Xe bubbles was found to be lower than 50 nm. Based on this observation, the fission gas behavior of U 3 Si 2 is controllable and free of run-away swelling despite the occurrence of decomposition at this irradiation temperature. [ABSTRACT FROM AUTHOR]

Advanced microstructure investigations of the high-burnup structure (HBS) in UO 2 produced by high-dose 84 MeV Xe ion irradiation are reported. Spark plasma sintered micro-grained UO 2 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 UO 2 . [ABSTRACT FROM AUTHOR]

The microstructure modifications of a high-energy Xe implanted U 3 Si 2 , a promising accident tolerant fuel candidate, were characterized and are reported upon. The U 3 Si 2 pellet was irradiated at Argonne Tandem Linac Accelerator System (ATLAS) by an 84 MeV Xe ion beam at 300 °C. The irradiated specimen was then investigated using a series of transmission electron microscopy (TEM) techniques. A dense distribution of bubbles were observed near the range of the 84 MeV Xe ions. Xe gas was also found to accumulate at multiple types of sinks, such as dislocations and grain boundaries. Bubbles aggregated at those sinks are slightly larger than intragranular bubbles in lattice. At 300 °C, the gaseous swelling strain is limited as all the bubbles are below 10 nm, implying the promising fission gas behavior of U 3 Si 2 under normal operating conditions in light water reactors (LWRs). [ABSTRACT FROM AUTHOR]