PNNL

Abstract

Stability and diffusion of small defect clusters containing alloying or fission atoms (“F” is used to represent both atoms) in an a-uranium (U) crystal are investigated using first-principles calculations. Results indicate that different stable defect structures are observed for an interstitial defect and a cluster (FnVm) containing different alloying or fission atoms and vacancies. An interstitial defect is found to be most stable either at the center of a pyramid pentahedron or in a mixed dumbbell configuration. The pyramid pentahedron is reported for the first time. Formation energy of a small FnVm cluster depends on the distribution of vacancies. Based on these stable defects, new migration energies and paths have been explored in this work. Anisotropic migration of an interstitial defect by jumping between nearest centers of two pyramid pentahedrons or jumping between two mixed U-Zr dumbbells has been suggested. Furthermore, a new migration-rotation mechanism has been explored for the first time for a FV2 cluster, resulting in a 3D diffusion. Finally, the effect of substitutional Pu atoms on the migration of a FV2 cluster suggests that optimizing the Pu concentration may prohibit the diffusion of some alloying or fission atoms, increasing the safe performance of metallic fuels.