A Study on the Stress-induced Hydrogen Diffusion in Zircaloy-4 cladding tube

The first commercial nuclear power plant (NPP), Kori #1, has been operating since 1978 and 24 of NPP is operating in recent. However, due to increase of NPP, interim storage and disposal of spent nuclear fuel (SNF) remains to be a social issue. Many countries such as Unite State, Canada, Germany, and Russia already have implemented dry storage of SNF and Korea also has stored SNF of pressurized heavy water reactor (PHWR) using dry storage. Unfortunately, fuel cladding contains much more hydrogen than before because reactor operating changed from low burn-up to high burn-up. Absorbed hydrogen is considered as one of major factors which degrade the fuel cladding integrity. Therefore, many researchers had conducted hydrogen related SNF degradation mechanism such as terminal solid solubility (TSS), hydride re-orientation and delayed hydride cracking (DHC). In particular, Puls, Shi, Sagat, Shmakov, McRae and Kim conducted research on hydrogen behavior in zirconium alloys. Many researchers including Puls claimed hydrogen diffusion is driven by temperature gradient, concentration gradient and stress gradient. Hydrogen is diffused by chemical potential gradient which constituted temperature, concentration and stress then hydrogen is precipitated as a hydride form when hydrogen concentration reaches TSS for precipitation. In contrast, Kim claimed hydrogen diffusion is driven by not stress gradient but temperature gradient and concentration gradient. He also claimed stress causes the precipitation of supersaturated hydrogen near crack tip and driving force of hydrogen migration is the concentration gradient between crack tip and bulk region. Hence many researchers including Kammenzind conducted experiments to investigate the relationship between stress gradient and hydrogen migration. But they did not find the relationship clearly. In this study, ring tensile tests were conducted under various thermal conditions such as isothermal conditions and thermal cycled conditions to investigate the relationship between stress gradient and hydrogen diffusion in Zircaloy-4 cladding. No hydrogen diffusion was observed under isothermal conditions, but hydride accumulation was observed under some of thermal cycled conditions. According to the results, it is estimated that there is some threshold condition to cause hydride accumulation and hydrogen is diffused by stress gradient when threshold condition is satisfied. This results is useful to understand the relationship between stress and hydrogen behavior. In particular, this result useful to predict DHC phenomenon and develop accurate model