The radial distribution of the fission products and the UO2 microstructure in a PWR fuel rod

Micro-gamma scanning with a high purity Ge detector (HPGe), a micro-collimator of 40.5 cm length and a slit of 0.5 mm x 0.5 mm was used to measure the radial distribution of the fission products; Cs-137, Cs-134 and Ru-106. No evidence of Cs-release was found in the fuel sample with 53.9 GWd/tU burn-up and the maximum local Cs-137 release rate across the pellet diameter was 29% in the fuel sample with 62.2 GWd/tU burn-up. Using the non-volatile isotope, Ru-106, Cs-137 release rate could be calculated. Ru-106 is regarded as an indicator of burn-up instead of Nd measured by EPMA. By making a comparison between Nd and Ru-106, it was found that Ru-106 was not released from the fuel pellet with 63.3 GWd/tU. Also pore morphology across the pellet radius was studied by the secondary electron microscopy confined by the carbon steel glove-box with a LaB6 filament. In the fuel sample with 61.7 GWd/tU burn-up, gas release channels formed along with grain boundary were detected at around r/r0=0.6 at which the onset point of Cs-137 release was regarded to be occurred by the micro-gamma scanning. The Cs-release mechanism is assumed to be related in the gas release channel. Xe-release would be estimated by using Cs-137 γ-ray intensity which is measured by a gamma tomography, a kind of non-destructive test in case of the high burn-up fuel. (author)