Irradiation-induced recrystallization in high burnup UO2 fuel

The formation mechanism of recrystallized grains and coarsened bubbles in the peripheral region of a high burnup UO2 pellet has been proposed on the basis of transmission electron microscopy (TEM) results. TEM observations were made on the pellet edge (local burnup: 100 GWd/t) of a fuel sample which had been irradiated to a pellet average burnup of 49 GWd/t in a BWR. A recrystallized grain region was found adjacent to an accumulation region having an extremely high density of dislocations. The dislocations were organized into subdivided grains with high angle boundaries which were regarded as the nuclei for recrystallization. Almost all the coarsened bubbles were surrounded by recrystallized grains. Small intergranular bubbles precipitated on the recrystallized grain boundaries and interconnected with each other, and then linked up with the coarsened bubbles. From these results, it was concluded that the coarsened bubbles were formed by sweeping out of small bubbles during grain growth on recrystallization. Finally an overall formation process for the unique microstructural change, the rim structure, was proposed. (orig.)