Radiation-induced grain subdivision and bubble formation in U₃Si₂ at LWR temperature

U₃Si₂, an advanced fuel form proposed for light water reactors (LWRs), has excellent thermal conductivity and a high fissile element density. However, limited understanding of the radiation performance and fission gas behavior of U₃Si₂ is available at LWR conditions. This study explores the irradiation behavior of U₃Si₂ by 300 keV Xe⁺ ion beam bombardment combining with in-situ transmission electron microscopy (TEM) observation. The crystal structure of U₃Si₂ is stable against radiation-induced amorphization at 350 °C even up to a very high dose of 64 displacements per atom (dpa). Grain subdivision of U₃Si₂ occurs at a relatively low dose of 0.8 dpa and continues to above 48 dpa, leading to the formation of high-density nanoparticles. Nano-sized Xe gas bubbles prevail at a dose of 24 dpa, and Xe bubble coalescence was identified with the increase of irradiation dose. The volumetric swelling resulting from Xe gas bubble formation and coalescence was estimated with respect to radiation dose, and a 2.2% volumetric swelling was observed for U₃Si₂ irradiated at 64 dpa. Due to extremely high susceptibility to oxidation, the nano-sized U₃Si₂ grains upon radiation-induced grain subdivision were oxidized to nanocrystalline UO₂ in a high vacuum chamber for TEM observation, eventually leading to the formation of UO₂ nanocrystallites stable up to 80 dpa.