The formation mechanism of a unique microstructure, the rim structure, in high burnup UO2 fuels has been elucidated by transmission electron microscopy (TEM). Specimens were prepared from the fuel peripheral region, using pellets which had been irradiated to a wide range of burnups (6-83 GWd/t; 10 GWd/t = 2.5 × 1020 fissions/cm3) in light water reactors. Dislocation density and volume fraction of intragranular bubbles increase with burnup. Low angle boundaries begin to form above 30-40 GWd/t. The TEM images and selected area electron diffraction (SAD) analyses of the rim structure observed in the 83 GWd/t fuel show: (1) sub-divided grains, 20-30 nm in size, with high angle boundaries due to the accumulation of an extremely high density of sub-boundaries; (2) recrystallized grains, 50-200 nm in size, adjacent to the sub-divided grain region, which are induced by the stored energy of the matrix; and (3) coarsened intragranular bubbles generated by radiation-induced excess vacancies.
!!!All Science Journal Classification (ASJC) codes