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 i s available on the kinetics of high temperature release of helium prec ipitated into bubbles. Recently, based on the many observations showin g that bubble coarsening during annealing is induced initially in regi ons nearest thermal vacancy sources (e.g. grain boundaries or surfaces ), a qualitative model has been suggested in which the vacancy gradien t between the vacancy source and initially overpressurised bubble conc entrations must lead to directed bubble migration up the gradient towa rds the source. It is clear that this gives a potential for gas releas e, either directly for surfaces, or indirectly for grain boundaries. T he present paper extends work on this mechanism by discussing quantita tive aspects of bubble movement in vacancy gradients and using compute r simulations to provide information on the parameters of importance i n the gas release mechanism. Among these are the helium content and th e associated local swelling, while release kinetics and temperature ar e primarily controlled by self-diffusion parameters.