Journal of Nuclear Fuel Cycle and Waste Technology

The morphology of the high burnup UO2 spent fuel, which was oxidized and annealed in a PIA (Post Irradiation Annealing) apparatus, has been observed. The high burnup fuel irradiated in Ulchin Unit 2, average rod burnup 57,000 MWd/tU, was transported to the KAERI';s PIEF. The test specimen was used with about 200 of the spent UO2 fuel fragment of the local burnup 65,000 MWd/tU. This specimen was annealed at 1400 for 4hrs after the oxidation for 3hrs to grain boundary using the PIA apparatus in a hot-cell. In order to oxidize the grain boundary, the oxidation temperature increased up to 500 and held for 3hrs in the mixed gas (60 He and 100 STD-air) atmosphere. The amount of 85Kr during the whole test process was measured to know the fission gas release behavior using the online system of a beta counter and a gamma counter. The detailed micro-structure was observed by a SEM to confirm the change of the fuel morphology after this test. As the annealing temperature increased, the fission products were observed to move to the grain surface and grain boundary of the UO2 matrix. This specimen was re-structured through the reduction process, and the grain sizes were distributed from 5 to 10.

Keywords

High burn-up fuel,oxidation and annealing test,post-irradiation,fission product,micro-structure