On the role of grain boundary diffusion in fission gas release

It is generally believed that thermal fission gas release from LWR fuel occ urs mainly via interconnected grain boundary bubbles. Grain boundary diffus ion is not considered to be a significant mechanism. We investigated this s upposition 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 ir radiated to burnups exceeding similar to 20 MWd/kg, gas atoms will be trapp ed after a migration distance equal to the size of a grain or less. This su pports the supposition for medium-to-high burnups. However, the above-menti oned model is inapplicable for trace-irradiated specimens. In our second an alysis, 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 latt ice and grain boundary diffusion unimpeded by intergranular traps. The anal ysis also provided rough estimates of the grain boundary diffusion coeffici ent in UO2. (C) 2001 Elsevier Science B.V. All rights reserved.