Post-irradiation examinations focused on fuel integrity of spent BWR-MOX and PWR-UO2 fuels stored for 20 years

Post-irradiation examinations (PIEs) of spent BWR-MOX and PWR-UO₂ fuel rods irradiated in commercial LWRs and stored for 20 years were carried out to evaluate fuel integrity during storage. Average burn-up of five fuel rods of the BWR-MOX fuel was about 20 MWd/kgHM and that of the PWR-UO₂ fuel was 58 MWd/kgHM. The PIE items included: (a) visual inspection of the cladding surface, (b) puncture test, (c) ceramographic observation on the pellet and cladding, (d) pellet density, (e) electron probe microanalysis of the pellet, (f) cladding tensile test, (g) hydrogen content and hydride orientation in the cladding, and (h) hydrogen redistribution in the cladding under temperature gradient. The PIE results showed no marked difference in the visual inspection, fission gas release, oxide layer thickness, pellet microstructure, and cladding mechanical properties or hydride orientation after storage. The result of the hydrogen redistribution experiment showed that hydrogen migration had little effect on the fuel integrity during dry storage. Hydrogen migration on the fuel rod for 40 years of storage was evaluated using the heat of transport obtained in the hydrogen redistribution experiment and calculated result showed that hydrogen migration had little effect on the fuel integrity during dry storage.