Grain boundary influence on displacement cascades in UO₂: A molecular dynamics study

The influence of grain boundaries on the primary damage state created by a recoil nucleus in UO₂ matrix is studied here by molecular dynamics simulations. This study is divided in two steps: (1) the study of the structural properties of several symmetrical tilt boundaries for different misorientation angles ranging from 12.7 deg. to 61.9 deg.; and (2) the study of displacement cascades near these grain boundaries. For all the grain boundaries studied, the structure around the interface up to about 2 nm presents a perturbed but stable fluorite lattice. The type of defect at the interface depends directly on the value of the misorientation angles. For the small angles (12.7 deg. and 16.3 deg.) the interface defects correspond to edge dislocations. For higher misorientation angles, a gap of about 0.3 nm exists between the two halves of the bicrystal. This gap is composed of Schottky defects involving numerous vacancies along the interface. About 10 keV displacement cascades were initiated with an uranium projectile close to the interface. In all the cases, numerous point defects are created in the grain boundary core, and the mobility of these defects increases. However, cascade morphologies depend strongly on the grain boundary structure. For grain boundaries with edge dislocations, the evolution of the displacement cascades is similar to those carried out in monocrystals. On the other hand, cascades initiated in grain boundaries with vacancy layer defects present an asymmetry on the number of displaced atoms and the number of point defects created