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

• Irradiation-induced microstructures of U–Zr alloys were observed using TEM.
• Larger spherical bubbles (<∼70 nm) were revealed from α-U phase alloy (U–0.1Zr).
• Small angular voids/low pressure bubbles were found in δ-UZr2 phase alloy (U–40Zr).
• Higher radiation-susceptibility of α-U, compared to δ-UZr2, phase is reaffirmed.
• Solid phase effects on fission gas bubble swelling is experimentally demonstrated.

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 × 1014 ions/cm2 to 5 × 1016 ions/cm2. Several larger spherical bubbles (<∼70 nm) were revealed from an irradiated single α-U phase alloy (U–0.1Zr) in which observed bubbles were mobile under 200-keV electron beam of transmission electron microscope (TEM). 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-UZr2 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-UZr2 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.

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