The fission gas bubble distribution in uranium dioxide from high temperature irradiated SGHWR fuel pins

The fission gas bubble distribution in uranium dioxide from SGHWR fuel pins irradiated in the UKAEA's Winfrith SGHWR has been studied, using transmission and scanning electron microscopy. The fuel pins had been irradiated to a burnup of approximately 1%, at temperatures in the range 400-23000C and at ratings of 25-46 watt/gram. The fission gas bubbles nucleate heterogeneously. The bubble size increases slightly with temperature and decreases with rating. The weak temperature dependence of the bubble size indicates an irradiation enhanced gas atom diffusion coefficient. There is no evidence of appreciable gas bubble mobility. The fraction of gas produced which is contained in the bubbles has a maximum value at approximately 16500C. Except in the restructuring regime the dominant mechanism leading to the release of gas by its transfer to grain boundaries is the diffusion of single atoms. This applies at all irradiation temperatures and implies that models of gas release developed for lower temperature AGR fuel can be applied to gas release in SGHWR fuel pins. (Auth.)