Influence of surface rearrangement on fission gases diffusion: how can round grains contribute to fission gas depletion in the rim?

The relationship between high burn-up structure (HBS) and fission gas release is discussed for high burn-up UO2 fuel in France. The influence of round sub-grains is emphasised. Their morphology is characterised with SEM and their gas content is measured with EPMA. It is clearly evidenced that the xenon has escaped the round sub-grains. A surface diffusion mechanism is proposed to explain the formation of round sub-grains, which should induce a release of fission gases. The consequences of such a mechanism are discussed. (authors)