Post-irradiation fission gas release from high burn-up UO2 fuel annealed under oxidising 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 conditions affect in some way the processes involved in the diffusive phase of gas release associated with the important step of intragranular fission gas transfer to the grain boundaries. The purpose of this paper is to extend a recent model involving fission gas bubble movement to propose a possible explanation. A central part of the model concerns the importance of grain boundaries as a dominant vacancy source. Under normal conditions, boundary concentrations are due to thermal vacancy production but under oxidising conditions, it is suggested that if fast oxygen diffusion takes place along grain boundaries then the subsequent oxidation reaction at the boundary has the potential to cause a huge enhancement of the local vacancy level. As will be demonstrated, an acceleration in the rate of fission gas bubble movement to the grain boundaries will result. A comparison of model calculations with a few selected literature results suggests that the mechanism has more than sufficient strength to give the large effects observed. (C) 1997 Elsevier Science B.V.