Microstructure of α-U and δ-UZr2 phase uranium-zirconium alloys irradiated with 140-keV He+ ion-beam

Abstract Ion-beam irradiation has been conducted to demonstrate and assess solid phase dependence of fission gas bubble nucleation and growth in metallic nuclear fuels. Differential formation and growth of irradiation-induced voids and bubbles were observed from as-cast uranium-zirconium (U–Zr) alloys including 0.1 and 40 wt% zirconium. The U–Zr alloys were irradiated with 140-keV He + ions with fluences ranged from 1 × 10 14 ions/cm 2 to 5 × 10 16 ions/cm 2 . Several larger spherical bubbles ( In-situ mergence and leakage of the bubbles in the uranium-rich alloy was also evident during the observation. In contrast, numerous number of small (∼6 nm) angular voids, or low pressure bubbles, were found in an irradiated δ-UZr 2 phase alloy (U–40Zr), in which the radiation-induced nanostructure was stationary throughout the observation. This study may reaffirm higher radiation-susceptibility of the α-U phase than the δ-UZr 2 phase, which could strengthen the needs of separate accounting of solid phase effects for metallic fuel performance modeling particularly regarding the fuel restructuring phenomena, such as fission gas swelling and constituent redistribution, since conventional models are commonly adopted homogeneous medium assumption, not in accordance with the results of post-irradiation examinations of used metallic nuclear fuel in which the formation of several concentric phase zone was distinct corresponding to a steep radial temperature gradient in the fuel pin and multiple isotherm lines in the binary phase diagram of U–Zr alloy system.