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 re gion, using pellets which had been irradiated to a wide range of burnu ps (6-83 GWd/t; 10 GWd/t = 2.5 x 10(20) fissions/cm3) in light water r eactors. Dislocation density and volume fraction of intragranular bubb les 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 t o 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; a nd (3) coarsened intragranular bubbles generated by radiation-induced excess vacancies.