Interactions Between Stressed Zirconium Alloys and Iodine at 300°C

Various zirconium alloys have been exposed to iodine vapor at 300°C under static tensile stress and marked differences in their cracking behaviors were observed. Some alloys that resisted cracking in iodine before irradiation became susceptible after receiving neutron fluences exceeding 2 × 10²⁴n/m², (E > 1 MeV) in the proximity of UO₂ fuel but not after irradiation to higher fluences in air. Control tests showed that fission products adhering to or implanted into the surfaces of irradiated tubing did not cause cracking in the absence of deliberately added iodine. Experiments confirmed the strong influence of residual stresses on the cracking of cold drawn unirradiated tubes under static applied stress. Treatments that decreased the tensile residual stresses (roller straightening, shot peening, and irradiation to a low fluence of fast neutrons) reduced the likelihood of stress corrosion cracking in iodine vapor. After Zircaloy has reacted chemically with iodine, the Zircaloy picks up hydrogen rapidly. It was also observed that hydrided Zircaloy has a higher resistance to iodine-induced cracking than unhy-drided Zircaloy. Surface coatings of graphite and baked poly-dimethyl-siloxane grease that protect Zircaloy from iodine attack have been tested and are discussed here.