An atomistic study of fundamental bulk and defect properties in [formula omitted]-uranium

Abstract

Alpha-uranium plays an important role in the performance and structure evolution of metallic fuels under irradiation. It is a highly anisotropic material that demonstrates a complex microstructural response to irradiation dependent on the irradiation temperature, and the mechanisms that control this behavior are not well understood. In this work, fundamental bulk properties and energetic and thermodynamic properties of single point defects and di-defects in α-uranium are determined using molecular dynamics over the range of 400 K - 750 K. The constant pressure heat capacity, total volumetric thermal expansion, anisotropy of the thermal expansion, and single vacancy and single interstitial formation energies all agree well with previous experimental and ab initio results, lending support to the di-defect formation and binding energies and the diffusivity results. We find that the diffusion of vacancies and di-vacancies is strongly anisotropic, while interstitial and di-interstitial diffusion is slightly anisotropic, and the most rapidly migrating species changes depending on the temperature. The formation energy of vacancies and di-vacancies increases slightly with temperature, while the formation energy of interstitials and di-interstitials increases strongly with temperature. In addition, di-vacancies are very loosely bound over the entire temperature range, with an average binding energy of 0.017 eV. Conversely, di-interstitials are strongly bound (0.64 eV) until approximately 600 K, above which the binding energy decreases significantly, with an average binding energy of 0.31 eV. These results provide valuable new insights into the possible mechanisms of the complex irradiation damage behavior of α-uranium.