An atomistic study of defect energetics and diffusion with respect to composition and temperature in γU and γU-Mo alloys

Uranium-molybdenum (U-Mo) alloys are promising candidates for high-performance research and test reactors, as well as fast reactors. The metastable γ phase, which shows acceptable irradiation performance, is retained by alloying U with Mo with specific quenching conditions. Point defects contribute to the atomic diffusion process, defect clustering, creep, irradiation hardening, and swelling of nuclear fuels, all of which play a role in fuel performance. In this work, properties of point defects in γU and γU-xMo (x = 7, 10, 12 wt.%) were investigated. Vacancy and self-interstitial formation energies in γU and γU-xMo were calculated with molecular dynamics (MD) simulations using an embedded atom method interatomic potential for the U-Mo system. Formation energies of point defects were calculated in the temperature range between 400 K and 1200 K. The vacancy formation energy was higher than the self-interstitial formation energy in both γU and γU-xMo in the evaluated temperature range, which supports the previous results obtained via first-principles calculations and MD simulations. In γU-xMo, the vacancy formation energy decreased with increasing Mo content, whereas the self-interstitial formation energy increased with increasing Mo content in the temperature range of 400 K to 1200 K. The self-diffusion and interdiffusion coefficients were also determined in γU-xMo as a function of temperature. Diffusion of U and Mo atoms in γU-xMo were negligible below 800 K. The self-diffusion and interdiffusion coefficients decreased with increasing Mo concentration, which qualitatively agreed with the previous experimental observations. Point defect formation energies, self-diffusion coefficients, and interdiffusion coefficients in γU-xMo calculated in the present work can be used as input parameters in mesoscale and engineering scale fuel performance modeling.