Thermal recovery mechanisms of UO2 lattices by defect annihilation

Abstract

Previous investigations on the thermal recovery of irradiated uranium dioxide crystals defects related to each recovery stage have been estimated based on indirect observations such as activation energy calculations of ion migration during crystal lattice recovery. In this study, thermal recovery mechanisms for fission- and alpha C-particle-irradiated uranium dioxide lattices were investigated by atomistic simulation methods. Isochronal and isothermal annealing methods were applied to the irradiated samples. Following reported procedures, the activation energies were calculated by molecular dynamics simulations and compared with empirical data, which showed good agreement with the experimental results. The migration energy barrier and the recovery energies of the obstruction type defects were calculated using molecular dynamics simulations and density functional theory. The irradiated crystal recovery stages are also discussed. Contrary to the reported predictions, direct, visual observation of the simulated data show that fission- and alpha-particle-irradiated crystals are not thermally recovered via the same mechanism. All the three recovery stages of the fission-irradiated uranium dioxide crystal are attributed to oxygen interstitials, while the first, second, and third recovery stages of the alpha-particle-irradiated crystals are attributed to oxygen-uranium, uranium, and uranium interstitial defects respectively. Finally, in a previous study of the present author it was shown that fission- and alpha-particle induced lattice swelling of uranium dioxide occurs due to obstruction-type defects. In this study, the main driving force of thermal recovery of irradiated crystals was once more observed as obstruction-type defects, which initiated all other types of defect recovery at each lattice recovery stage. (C) 2017 Elsevier B.V. All rights reserved.