Flaws found during in-service inspection of Zr-2.5Nb pressure tubes include fuel bundle scratches, debris fretting flaws, fuel bundle bearing pad fretting flaws and crevice corrosion flaws. These flaws are volumetric and blunt in nature. Crack initiation from these in-service flaws can be caused by the presence of hydrogen in operating pressure tubes and resultant formation of hydrided regions at the flaw tips during reactor heat-up and cool-down cycles.

There are two known hydride induced crack initiation mechanisms. One is delayed hydride cracking (DHC) that refers to the situations in which hydride formation and fracture occur at the same applied loads. The second crack initiation mechanism is known as overload crack initiation in a hydrided region. In this case, crack initiation occurs in the course of reactor transients when the applied stress acting on hydrided regions at flaw tips exceeds the stress at which these hydrided regions have been formed. The process-zone model was previously established to predict DHC initiation. There has been recent work to extend the process-zone model for predicting overload crack initiation in hydrided regions ahead of flaw tips. Efforts were made to identify appropriate modeling approaches for the two sequential and distinct physical stages of overload crack initiation: formation of hydrided regions with thermal cycles under constant load, and crack initiation from the pre-existing, fixed hydrided regions ahead of flaw tips under rising load (i.e. overload). The established process-zone methodology is used to model formation of hydrided regions at flaw tips with thermal cycles under constant load. A Weibull stress approach is proposed to predict fracture of the preexisting, flaw-tip hydrided regions during overload. This paper describes the extended process-zone modeling framework for overload crack initiation.

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