FISSION-GAS RELEASE DURING GRAIN-GROWTH IN A MICROSTRUCTURE WITH A DISTRIBUTION OF GRAIN SIZES

Fission gas diffusion from a microstructure with a distribution of gra in sizes undergoing grain growth is analyzed. Hillert's theory is used to describe the evolution in grain morphology, during which some grai ns enlarge while others shrink and may even disappear. Limiting cases of Booth release (when only diffusion occurs) and Malen release (when grain-boundary sweeping is dominant) are derived for a body with a dis tribution of grain sizes. The moving boundary diffusion equation that couples the diffusion and sweeping mechanisms is derived. The grain-si ze distribution is divided into 25 groups for which the diffusion equa tions are solved numerically for the average intragranular gas concent ration. The results are compared to existing single-grain-size models which, in all cases, overpredict the fractional release. Depletion of intragranular fission gas provides a source for migration via grain bo undaries to the free surface.