Radiation-induced microstructural change in high burnup UO₂ fuel pellets

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.