The morphology and crystal structure of hydrides in CANDU Zr-2.5Nb pressure tubes

The morphology and crystal structure of zirconium hydrides in CANDU Zr-2.5Nb pressure tubes were investigated by transmission electron microscopy and electron diffraction pattern. The purpose of this study is to examine the precipitation behavior of zirconium hydrides. The hydride precipitates were observed on the two sections (longitudinal-normal (LN) and transeverse-normal(TN) sections) from the Zr-2.5Nb pressure tube. As a result of this study, The hydride habit plane, related to the crack growth direction of DHC (delayed hydride cracking), was {1017 }Zr plane. The orientation relationship between the hydride and the Zr matrix on the LN section was found to be : [1210)Zr //(110)H, (0001)Zr // (111)H. The morphology and growth pattern of hydrides on the TN section is classified two cases : one is long hydride grown along the α-Zr grain, the other is thin needle type hydride across the α-Zr grain. Based on the analysis of MBD (micro beam diffraction) patterns on each hydride, the difference in the precipitation pattern of hydrides is discussed