Iodine-Induced Stress Corrosion of Zircaloy Fuel Cladding: Initiation and Growth

Abstract

Because of its impact on nuclear power plant reliability, fuel cladding failure induced by pellet cladding interaction is a problem of major concern. It is usually analyzed as stress corrosion cracking caused by volatile fission products like iodine. In order to analyze the mechanisms of iodine stress corrosion in Zircaloy-4 cladding tubes, a series of tests have been carried out on recrystallized and stress-relieved cladding from different origins. The tubes were pressurized in the presence of iodine (1.5 mg cm−2 Zry) at 350°C, and the time to failure was measured versus applied hoop stresses. In order to be able to analyze independently initiation and growth, a special procedure has been developed to induce a small fatigue crack (⋍200 µm) in the inner wall of some specimens before the pressurization test. After failure of the specimen, the initial fatigue crack and the subsequent stress corrosion crack are clearly separated. Using linear elastic fracture mechanics analysis, those measurements allowed us to determine the stress corrosion average crack growth rates and the threshold stress intensity factor for crack growth KIscc. The effect of loading path has been analyzed during slow tensile tests (5 µm s−1) performed under iodine vapor at 350°C along the axial and circumferential orientations. Differences in time to fracture and loss of ductility for the two orientations can be explained in terms of crack growth along preferred crystallographic planes or grain boundaries that were confirmed by SEM fracture surface observation.