Anisotropic delayed hydride cracking velocity of CANDU Zr-2.5Nb pressure tubes

摘要

Delayed hydride cracking (DHC) tests were conducted on CANDU(1) Zr-2.5Nb tubes containing hydrogen with a microstructure of elongated alpha-Zr and semi-continuous beta-Zr at temperatures ranging from 100 to 300 degrees C. DHC velocity (DHCV) was found to be around two times higher in the axial direction than that in the radial direction even on the same cracking plane. According to Kim's DHC model that DHCV at temperatures lower than 300 C is governed mainly by hydrogen diffusion and to a lesser extent by the hydrogen concentration gradient at the crack tip, we suggest that the enhanced DHCV in the axial direction arises from a faster hydrogen diffusion in the axial direction with a semi-continuous distribution of beta-Zr. Evidence to this suggestion is provided by Levi's experiment that Zr-2.5Nb tubes or plates with a fully discontinuous beta-Zr due to annealing at 400 degrees C for 1000 h or 650 degrees C for 9 h has no anisotropic DHCV with the orientation. Hence, we conclude that Kim's DHC model is plausible. An anisotropic DHCV of irradiated CANDU Zr-2.5Nb tubes is discussed based on their microstructural evolution with the neutron fluence and operating temperatures. (c) 2007 Elsevier B.V. All rights reserved.