Evaluation of IGSCC Growth Behavior of 316NG PLR Piping in BWR

Abstract

Through inspections of Japanese boiling water reactors (BWRs) from 2002, a high occurrence rate of Inter Granular Stress Corrosion Cracking (IGSCC) was confirmed in core shrouds and re-circulation (PLR) pipings made of low-carbon (L-grade) stainless steels (Okamura, et al., 2003), and the evaluation of IGSCC growth behavior especially in PLR pipe weld joints has been recognized as one of the urgent and important issues for applying the Japanese maintenance code for BWRs. Adjacent to the weldment of PLR pipe joints, work-hardened area due to weld shrinkage is generated, which enhances SCC growth rates in BWR environment (high purity, 288°C water). However, the effect of hardening on SCC growth rate has not been considered in the present flaw evaluation codes. On the other hand, detailed examinations on actual cracked PLR joints showed that SCC growth in the depth direction seems to be arrested or, at least, decelerated around the fusion boundary. Therefore, it is not appropriate to apply a enhanced growth rate of work-hardened materials to the entire SCC path in PLR weld joints. In this paper, a new concept to evaluate SCC growth considering observed characteristics of SCC behavior is proposed, followed by tentative analyses using latest crack growth rate data measured in specimens taken from the real-size pipe joint mock-ups.