Effects of hydride morphology on the embrittlement of Zircaloy-4 cladding

Abstract

Spent nuclear fuel claddings discharged from water reactors contain hydrogen up to 800 wppm depending on the burn-up and power history. During long-term dry storage, the cladding temperature slowly decreases with diminishing decay heat and absorbed hydrogen atoms are precipitated in Zr-matrix according to the terminal solid solubility of hydrogen. Under these conditions, hydrides can significantly reduce cladding ductility and impact resistance, especially when the radial hydrides are massively present in the material. In this study, the effects of hydride morphology on the embrittlement of Zircaloy-4 cladding were investigated using a ring compression test. The results show that circumferentially hydrided Zircaloy-4 cladding is brittle at room temperature but its ductility is regained substantially as the temperature goes above 150 degrees C. On the other hand, radially hydrided cladding remains brittle at 150 degrees C and micro-cracks developed in the radial hydrides can act as crack propagation paths. Fracture energy analysis shows that ductile to brittle transition temperature is low in between 25 degrees C and 100 degrees C in the former case, whereas it lies in between 200 degrees C and 250 degrees C in the latter case.