In order to establish a program to preclude the pellet-cladding interaction (PCI) failure of nuclear fuel cladding in the normal operation conditions, experiments on iodine-induced stress corrosion cracking (SCC) of Zircaloy-4 cladding tube were carried out to elucidate mechanisms of SCC, which is the main cause of PCI failure. Apparatus for internal pressurization of a tube with constant stress and slow strain rate tester with constant strain rate were used as the experimental equipments, and the experimental variables were chosen the iodine concentrations in the range of 0-4 mg/㎠, the stresses of 250-550 MPa, the strain rates of $10^{-7}-10^{-4}/sec$ and the test temperatures of 300-400℃. The dependence of SCC characteristics on each variable was studied as followings.
First, SCC experiments on Zircaloy-4 cladding tube were undertaken using the internal pressurization apparatus in order to study the effects of iodine concentration on the SCC susceptibility. The time-to-failure, failure strain and strain rate due to SCC were measured as a function of the iodine concentration. Apparent activation energy for SCC was calculated from the temperature dependence of SCC, and fractographic interpretation was made throug scanning electron microscopic(SEM) examination. The results suggest that the iodine concentration has influence on the crack propagation step to increase the propagation rate of SCC, promoting the SCC susceptibility. It is found that the critical iodine concentrations mean the phenomenal values obtained from the iodine concentration dependence of SCC susceptibility, not the critical values to determine the occurrence of SCC. Effects of iodine concentration on the SCC behavior of Zircaloy were discussed from the viewpoint of the crack initiation and propagation processes.
Then, internal pressurization tests of tubes were performed on Zircaloy-4 tube in order to study effects of both stress and strain rate on the SCC susceptibility. The time-to-failure due to SCC was related to the n-th power on stress, and stress exponent varied from 19 to 6 with increasing test temperature in the range of 330-400℃. The failure strain due to SCC was a parabolic function of strain rate in the range of $10^{-7}-10^{-4}/sec$, and showed a minimum at the intermediate strain rate. The strain rate was more influential in determining the SCC susceptibility than stress, and there was the threshold strain rate below which SCC could not occur. The SCC susceptibility decreased with increasing test temperature. The results suggest that the film rupture should be involved in the overall SCC process of Zircaloy in order to adsorb the corrodant on the fresh metal surface at the crack tip, and that the adsorption of corrodant reduces the fracture energy of Zircaloy to propagate SCC easily.
Constant elongation rate tests(CERT) were undertaken using the ring specimen of Zircaloy-4 cladding tube in order to study the strain rate dependence of SCC. The CERT results were consistent with the results obtained from the internal pressurization tests, indicating that both film rupture and adsorption steps should be of importance in the SCC process of Zircaloy. SEM observations suggest that plastic deformation might be the key factor leading to initiation of SCC at a potential site of surface.
