Evolution of Process-Zone-Based Hybrid Model for Resistance to Crack Initiation Due to Hydrided Region Overloads in CANDU Pressure Tubes

Abstract Zr-2.5%Nb pressure tubes in CANDU nuclear reactors are susceptible to formation of hydrides. Canadian nuclear standards require evaluations of in-service flaws in pressure tubes for potential crack initiation due to hydrided region overload events that occur when the applied stress exceeds the hydride formation stress. Dual process-zone approach to hydride-assisted crack initiation was adapted by the authors to establish a closed-form functional framework for modeling the resistance of pressure tube material to overload crack initiation. In this process-zone-based framework, the overload resistance during pressure transients is correlated with the material resistance to initiation of delayed hydride cracking under constant applied pressure during sustained reactor operation and varies with flaw geometry and hydride formation conditions. Over the years, three versions of a hybrid predictive model have been developed on the basis of this modeling framework, with each subsequent version incorporating the effects of additional factors affecting the overload resistance. The most recent version included the beneficial effect of temperature, with the overload resistance predicted to increase with temperature at an increasing rate that depends on flaw geometry and hydride formation conditions. Notably, all three versions are probabilistic models that are directly suitable for use in probabilistic evaluations of pressure tube flaws.