On the role of grain boundary diffusion in fission gas release

It is generally believed that thermal fission gas release from LWR fuel occurs mainly via interconnected grain boundary bubbles. Grain boundary diffusion is not considered to be a significant mechanism. We investigated this supposition by two methods; first, by assessing the distance a gas atom can migrate in a grain boundary containing perfectly absorbing traps. For areal number densities and fractional coverages by the traps observed in fuel irradiated to burnups exceeding approximately 20 MWd/kg, gas atoms will be trapped after a migration distance equal to the size of a grain or less. This supports the supposition for medium-to-high burnups. However, the above-mentioned model is inapplicable for trace-irradiated specimens. In our second analysis, we examined Xe release from trace-irradiated UO₂. The measurements indicated that the liberation involves more than only lattice diffusion at the specimen surface, and that the data are consistent with sequential lattice and grain boundary diffusion unimpeded by intergranular traps. The analysis also provided rough estimates of the grain boundary diffusion coefficient in UO₂.