Self-diffusion near symmetrical tilt grain boundaries in UO_2 matrix:A molecular dynamics simulation study

摘要

Molecular dynamics simulations have been carried out to study the influence of grain boundaries in stoichiometric UO2 on uranium and oxygen self-diffusions over a large range of temperature varying from 300 K to 2100 K. The study was carried out on two symmetrical tilt grain boundaries, R5 and R41, which have respectively two different atomic structures. Firstly, the study of the temperature effect on the grain boundary core structure is presented. With the raise of temperature, the grain boundary core growswith an increase of disorder. Secondly, self-diffusion near both grain boundaries is studied. It has been found that grain boundaries accelerate the uranium and oxygen self-diffusion rates over several nanometres from the grain boundary interface. Uranium and oxygen self-diffusion are anisotropic, with a high acceleration along the grain boundary interface. Using the self-Van Hove correlation functions, hopping mechanisms were identified for R41 in all directions while for R5 hopping mechanism takes place along the grain boundary interface and random diffusion appears in the perpendicular direction of the grain boundary plane.