Effects of Hydride Rim on the Fracture Strain of Zircaloy-4 using Ring Compression Test

Abstract

Currently, the wet storage of spent nuclear fuel in Korea are going to be saturated, and the specific policy for the spent fuel management has not been decided yet. Therefore, dry storage, which are part of interim storage, are emerging as realistic alternatives. There are various factors that threaten the integrity of spent nuclear fuel, but among them, the influence of hydrides is considered as a major factor. Many researchers have studied the mechanism of spent fuel deterioration caused by hydrogen such as terminal solid solubility, hydride reorientation and delayed hydride cracking. Especially, in this study, ‘Hydride rim’ was focused among the various issues related in hydrogen. Hydride rim is the phenomenon that hydrides are concentrated on outer surface of cladding. During operating nuclear power plant, there is a temperature gradient between outer and inner surface of cladding tube because the nuclear fuel is located in the inner surface of cladding and coolant is located in the outer surface. The hydrogen which is picked-up from coolant to cladding is precipitated as hydrides in Zr-matrix by temperature gradient. These hydrides are brittle, so the cladding with hydride rim is vulnerable to external shocks. For this reason, the study about hydride rim is needed using ring compression tests which can simulate pinch-type loading during transportation of spent nuclear fuel. The main objective of this study is comparing the cladding with hydride rim and the cladding with uniformly hydrogen distribution. The results indicated that the hydride rim should be considered when we performed experiments with hydride. The fracture strain of specimens with hydride rim is lower than the specimens with uniform. The morphologies are shown that the crack initiated at 3 and 6 o’clock because the hydrides are concentrated on outer surface of cladding. In other words, it indicates that hydride rim deteriorate ductility of cladding. In addition, there are two main factors that the hydride rim affects a loss of ductility: increasing hydrogen content and/or hydride rim thickness. The results represents that as hydrogen contents in cladding increases, the fracture strain decreases. Furthermore, as the hydride rim thickness increases, the fracture strain decreases.