BUBBLE NUCLEATION IN GRAIN INTERIOR AND ITS INFLUENCE ON HELIUM ACCUMULATION AT GRAIN-BOUNDARIES

The production of helium and hydrogen is relatively fast in the structural materials used for fusion devices, so that at elevated temperatures helium can accumulate at grain boundaries and cause embrittlement. The density of microscopic cavities formed in the grains during irradiation, which moderated the gas flux to the boundaries, can vary considerably under high gas production conditions. Thus the mechanisms responsible for the nucleation of voids and gas bubbles become very relevant and must be discussed. A diffusion calculation is made of the flux of helium atoms to a grain boundary, taking account of the denuded zones. The calculated accumulation of helium is in good agreement with the measured gas content of grain boundaries in a bulk irradiated material. This demonstrates that a high density of gas bubbles in a material can protect the boundaries from excessive deposition of gas.