A study on the assessment criteria of DHC in PWR spent fuel

One of the major degradation mechanisms of PWR spent fuel cladding is delayed hydride cracking (DHC), which can activate within a relatively low temperature range. Its maximum crack growth rate is estimated to be within the range of 10^-7 m/s, at which the cladding can rupture in 1 to 2 years during dry storage. For this reason, it is essential to prevent DHC crack growth itself, but most of the existing DHC studies have been focused on the crack growth rate. In this regard, the values and characteristics of the threshold stress intensity factor (K_IH) for PWR Zircaloy-4 cladding were described. Based on the limited existing test results and analysis, the experimentally measured K_IH were compared with the predicted K_IH. The comparison shows that the predicted values are much lower than the measured ones and both theoretical estimation and measured values of K_IH increase as the temperature increases. The possibilities of degradation by DHC were also discussed during long-term dry storage. For a more reliable analysis, additional information on a pre-existing crack of spent fuel is needed