Technical Basis for JSME Code Case on Stress Corrosion Crack Growth Rates of Stainless Steels in PWR Environments

In 2020, an intergranular crack was detected near the welds of austenitic stainless-steel pipe in a pressurizer spray line of PWR plant in Japan (Ohi Unit 3). While no clear evidence has been found to identify the root cause of the crack initiation, it is thought that the crack propagated by SCC. In response to this operating experience, more than 900 similar welds in Japanese PWR plants have been inspected by UT and no indications have been detected so far. Currently, The JSME Rules on Fitness for Service do not specify SCC CGR for a stainless-steel pipe in PWR primary water environments. In preparation for further detection of SCC, a code case for SCC CGR for a stainless-steel in PWR primary water environments was proposed to JSME Code and Standards Committee in 2023 and approved by “Main committee on Power Generation Facility Codes” on December 2024. The public review was completed in February 2025, and the publication process is commenced in the spring of 2025. The proposed code case is based on the EPRI MRP-458 “Materials Reliability Program: Stress Corrosion Crack Growth Rates in Stainless Steels in PWR Environments,” published in 2022, which establishes the SCC CGR curves for stainless-steels in PWR primary water environments. While Japanese PWR primary system pipe utilizes type 316 stainless-steel, MRP-458 develops SCC CGR curves based on SCC CGR data for both type 316 and type 304. However, it is reported that the SCC CGR for type 304 and type 316 are equivalent in PWR environments. Therefore, MRP-458 has been deemed applicable in Japanese PWR. Furthermore, whereas MRP-458 recommends a 75th percentile curve that envelops 75% of the SCC CGR data, this code case utilizes the 50th percentile curve. The rationale for this choice is that JSME “Codes for Nuclear Power Generation Facilities - Rules on Fitness-for-Service for Nuclear Power Plants - JSME S NA1” has very conservative requirements for successive inspections for detected cracks, leading to the conclusion that the use of the 50th percentile curve will still ensure structural integrity. This paper aims to elucidate the technical basis considered in the development of this code case.