Developments in Flaw Evaluation for CANDU Reactor Zr-Nb Pressure Tubes

Abstract

Delayed hydride cracking (DHC) in Zr-2.5Nb alloy material is of interest to the CANDU (CANada Deuterium Uranium) 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 root. The hydrided region can then fracture to the extent that a crack forms, and is able to grow by the DHC crack growth mechanism. The existing CANDU blunt flaw DHC evaluation procedure is based on a threshold peak flaw-tip stress for DHC initiation that is independent of flaw geometry. Work is underway to improve the existing blunt flaw DHC evaluation procedure by developing a methodology that takes into account the effect of flaw geometry parameters. 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. This approach has been used successfully in other engineering applications. A description of how the process-zone methodology can be used to quantify the effect of flaw geometry on the threshold peak stress for DHC initiation is provided. A model that is suitable for actual engineering applications is described, and the engineering model illustrates the dependency of the threshold peak stress for DHC initiation on flaw geometry. Agreement between engineering model predictions and experimental results is reasonable. Various aspects of the DHC initiation problem that are being addressed, in addition to flaw geometry, are described.