Post-irradiation annealing test of high burnup UO2 fuel

To investigate the behavior of the fission gases in a high burnup UO2 fuel, PIA (Post Irradiation Annealing) test was performed. The highest burnup fuel, which was irradiated in Ulchin Unit 2, was transported to KAERI for the PIE. Non-destructive and destructive post irradiation test such as burnup analysis, pellet density change and detailed microstructure analysis by optical microscopy, SEM and EPMA were performed to confirm the fuel characteristics. By using the controlled oxidation technique, fission gas inventory could be distinguished grain boundary from grain matrix. From the preliminary test results, preferential oxidation of grain boundary was detected and confirmed by SEM image which showed separation of grain boundary. In addition, by the temperature elevation and longer oxidation time, Sharp re-increase of Kr-85 release was measured which released from grain matrix and confirmed by SEM also. The first PIA test result shows that about 40% to total residual fission gases were released from grain boundary. (author)