Helium atoms generated via (n,a) reaction are considered to be one of the main reasons for the degradation of materials used in nuclear energy reactor systems. Through clustering He bubbles are formed leading to embrittlement. In this presentation, the nucleation of helium atoms in single crystal and the grain boundary systems studied by molecular dynamics (MD) simulations are given. The processes of athermal emission of self-interstitial atom (SIA) and the formation of dislocation loop by helium atoms in single BCC iron are firstly predicted. And furthermore, He clusters in the symmetrical grain boundaries (Σ3 {111}, Σ3 {112} and Σ5 {012}) and general nanograin boundaries including the high, low angle and non-ideal (close to Σ3) symmetrical grain boundaries are also investigated. The local atomic excess free volume (LAEFV) is found to be main parameter affecting the helium nucleation process in grain boundary system by comparing with results of helium in single crystal.
PSI Switzerland,