Gas release processes for high concentrations of helium bubbles in metals

Although many aspects of helium behaviour in metals relevant to fusion reactor technology have been studied, relatively little information is available on the kinetics of high temperature release of helium precipitated into bubbles. Recently, based on the many observations showing that bubble coarsening during annealing is induced initially in regions nearest thermal vacancy sources (e.g. grain boundaries or surfaces), a qualitative model has been suggested in which the vacancy gradient between the vacancy source and initially overpressurised bubble concentrations must lead to directed bubble migration up the gradient towards the source. It is clear that this gives a potential for gas release, either directly for surfaces, or indirectly for grain boundaries. The present paper extends work on this mechanism by discussing quantitative aspects of bubble movement in vacancy gradients and using computer simulations to provide information on the parameters of importance in the gas release mechanism. Among these are the helium content and the associated local swelling, while release kinetics and temperature are primarily controlled by self-diffusion parameters.