EPMA and SEM of fuel samples from PWR rods with an average burn-up of around 100 MWd/kgHM

Samples from the high and low power regions of two fuel rods with average burn-ups of 89.5 and 97.8 MWd/kgHM were examined. Electron probe microanalysis (EPMA) was used to measure the radial distributions of Xe, Cs and Nd in the UO2 fuel matrix, and scanning electron microscopy (SEM) was used to study the change in the UO2 microstructure across the fuel pellet radius. EPMA showed that a large fraction of the xenon created had been released from the fuel matrix of the high power samples at all radial positions. In these samples, SEM revealed the presence of recrystallised grains at intermediate radial positions where thermal fission gas release had previously occurred. Evidence of recrystallisation was found throughout the pellet cross-sections of the low power samples. It is concluded that recrystallisation of the fuel grains at intermediate radial positions is mainly responsible for the marked increase in fission gas release to the rod free volume at burn-ups above 80 MWd/kgHM.