A thermodynamic formalization was developed for description of the nucleation and growth of He bubbles in metals during irradiation. The proposed formalization was available for evaluating both microstructural changes in fusion first wall materials where He was produced by (n,α) nuclear transmutation reactions, and those in fusion diverter materials where He particles with low energy were directly implanted. The calculated nucleation barrier was significantly reduced by the presence of He, showing that a He bubble with an appropriate number of He atoms depending on bubble size could nucleate without any large nucleation barriers, even at a condition where an empty void has very large nucleation barriers without He. With the proposed thermodynamic formalization, the nucleation and growth process of He bubbles in Fe during irradiation was simulated by using the kinetic Monte Carlo technique. This showed the nucleation path of a He bubble in (NHe, NV) space as a function of temperature and the concentration of He in the matrix; where NHe and NV were the numbers of He atoms and vacancies contained in the He bubble. It was found that the bubble growth rates depended upon the nucleation path, and suggested that 2 different mechanisms operated in bubble growth. One of them was controlled by vacancy diffusion and the other was controlled by interstitial He diffusion.

Nucleation Path of Helium Bubbles in Metals during Irradiation. K.Morishita: Philosophical Magazine, 2007, 87[7], 1139-58