BUBBLE DIFFUSION TO GRAIN-BOUNDARIES IN UO2 AND METALS DURING ANNEALING - A NEW APPROACH

In modelling fission gas release from UO2 during postirradiation annealing, it is well known that the removal of intragranular gas to grain boundaries is orders of magnitude faster than predictions from bubble diffusion experiments. In this paper, this difficulty is attacked by building on previous quite fundamental observations, both in UO2 and metals, to produce a new mechanism of gas release. A vital component in this mechanism is the arrival of thermal vacancies from grain boundaries which must impose a large directed motion of bubbles up the concentration gradient. Initial modelling produces the neat result that the flow of bubbles to the boundaries is controlled primarily by the self-diffusion parameters. A brief comparison with some selected UO2 gas release results suggests that the implications of the model deserve to be fully explored while the potential effects in metals should also be recognised. In addition, the model can resolve conflicting UO2 bubble mobility data in the literature.