The effect of oxidized UO₂ on iodine-induced stress corrosion cracking of fuel sheathing

In this study, axially-slotted Zircaloy-4 rings prepared from CANDU-type sheathing were stressed and inserted into glass-blown tubes with iodine crystals. Iodine quantities per total Zircaloy-4 surface area ranged from 4.2±0.2 mg cm^-2 to 35.5±0.7 mg cm^-2 . Glass tubes were evacuated of air, sealed, and inserted into a tube furnace at 350^oC to simulate the temperature of the fuel-to-sheath gap. The extent of specimen corrosion was assessed by observing stress corrosion cracking failures or when no cracking occurred, by measuring specimen deflections. Results show that the corrosion resistance of Zircaloy-4 to iodine species, which is Zircaloy texture dependent, increased when additives of: i) oxygen gas; and ii) hyperstoichiometric UO₂ with dried graphite were included in the glass ampoule. The latter suggests a chemical role of CANLUB in mitigating stress corrosion cracking in fuel sheathing. (author)