POSTIRRADIATION FISSION-GAS RELEASE FROM HIGH BURN-UP UO2 FUEL ANNEALED UNDER OXIDIZING CONDITIONS

In post-irradiation fission gas release experiments on UO2 fuel, large increases in effective diffusion coefficients have been reported when anneals have been carried out under oxidising conditions. These condi tions affect in some way the processes involved in the diffusive phase of gas release associated with the important step of intragranular fi ssion gas transfer to the grain boundaries. The purpose of this paper is to extend a recent model involving fission gas bubble movement to p ropose a possible explanation. A central part of the model concerns th e importance of grain boundaries as a dominant vacancy source. Under n ormal conditions, boundary concentrations are due to thermal vacancy p roduction but under oxidising conditions, it is suggested that if fast oxygen diffusion takes place along grain boundaries then the subseque nt oxidation reaction at the boundary has the potential to cause a hug e enhancement of the local vacancy level. As will be demonstrated, an acceleration in the rate of fission gas bubble movement to the grain b oundaries will result. A comparison of model calculations with a few s elected literature results suggests that the mechanism has more than s ufficient strength to give the large effects observed. (C) 1997 Elsevi er Science B.V.