Critical Failure Strains for Incipient Crack Formation in the Oxide and Hydride Layers in the Outer Surface Region of Zircaloy Fuel Cladding Tubes

摘要

In order to elucidate the phenomena in the incipient crack formation and the failure process of fuel rods at power ramp, separate effect tests were made to obtain the critical strains for crack initiation of the oxide layer and the hydride layer, respectively. Data sets (AE signal, internal pressure, tube diameter, test temperature) were recorded at both room temperature and 290 ℃ during the pressurization of each specimen with the oxide and the hydride layers, and the morphologies of the cracks were also observed. Test results showed that the critical hoop strains for cracking of the oxide layer on the Zircaloy fuel cladding tubes were 0.21% at room temperature and 0.22%-0.24% at 290 ℃. At room temperature, hydride cracking started at a low strain level; however at 290℃, a relatively high critical strain of around 0.34% was detected. The scatter was qualitatively interpreted from the XRD analysis of the zirconium hydride layer, which showed mixed hydride compositions including brittle s-hydride as well as 5-hydride particles. It was found from the results of a series of tests on the combined effects with and without the oxide layer and with and without radial hydride formation that there were hardly any cracks in radial hydrides embedded in the tubes in the case without the oxide layer. The metallography observation results clearly depicted that the combined effects of the oxide cracking and the radial hydride formation generated an incipient crack, which would act as the trigger for delayed hydride cracking.