Activation energetics of actinide diffusion in UO2 from first-principles calculations

Abstract

Diffusion of actinides in uranium dioxide plays an important role in determining thermodynamic and mechanic properties of the material. Activation energies of Th, U, Np, and Pu diffusion in uranium dioxide were systematically studied using first-principles calculations. The generalized gradient approximation and projector-augmented wave methods with on-site Coulomb repulsive interaction were applied within Density Functional Theory and Plane Wave framework. Two diffusion paths, one along the lattice 〈110〉 direction and the other along the lattice 〈100〉 direction, were examined in the face-centered cubic UO2 structure. The results show that the 〈110〉 path has lower migration energy than the 〈100〉 path. Under the assumption of a vacancy-assisted jump diffusion mechanism, the major contribution to the activation energy is the migration energy, followed by the vacancy formation energy and vacancy binding energy, where the last has the lowest contribution. However, differences in the activation energies among different actinides stem from both the migration and vacancy binding energies, both of which decrease with atomic number. While discrepancies between the absolute values of the calculated and experimentally observed activation energies remain, this study shows a correlation between activation energy and atomic number and an asymptotic relation between activation energy and ionic radius of the actinides. The present study suggests that the migration of the actinides through the uranium dioxide lattice is closely correlated to the number of 5f electrons and the size of the diffusing atoms.