The Effect of Plasticity on Process-Zone Predictions of DHC Initiation at a Flaw in CANDU Reactor Zr-Nb Pressure Tubes

Abstract

Delayed Hydride Cracking (DHC) in Zr-2.5 Nb alloy material is of interest to the CANDU industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU reactor pressure tube. The material is susceptible to DHC when there is diffusion of hydrogen atoms to the flaw, precipitation of hydride platelets, and development of a hydrided region at the flaw tip. The hydrided region could then fracture to the extent that a crack forms, and is able to grow by the DHC crack growth mechanism. A procedure for evaluating DHC initiation at a blunt flaw that takes into account the effect of flaw geometry has been developed. This methodology can, in principle, be applied to any flaw that is found during in-service inspection. The methodology is based on representing the stress relaxation due to hydride formation, and crack initiation, by a process zone. The authors have presented predictions of DHC initiation on the basis of elastic conditions in the surrounding zirconium alloy matrix material outside of the process zone in papers presented at the 1999, 2000 and 2001 ASME PVP Conferences. In the present paper, the effects of plasticity in the surrounding zirconium alloy matrix material have been incorporated into a simple theoretical process-zone model for DHC initiation, as well as into an engineering process-zone model that is suitable for evaluation of in-service flaws. The resulting process-zone model calculations and comparisons with DHC initiation experiments demonstrate that DHC initiation predictions can be overly-conservative if plasticity is not taken into account.