Session: CS-08-02 The Warren H. Bamford Memorial Symposium on Recent Developments in ASME Codes and Standards: ASME Code Section XI Activities-2

Paper Number: 152337

152337 - Technical Basis for Jsme Code Case on Stress Corrosion Crack Growth Rates of Stainless Steels in Pwr Environments 

Abstract: 

In 2020, an intergranular crack was detected near the welds of austenitic stainless steel piping in the pressurizer splay 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 stress corrosion cracking (SCC). In response to this operating experience, ultrasonic inspections for the similar welds has been conducted for domestic PWR plants and no indications have been detected so far.
Currently, JSME Fitness For Service Code do not specify crack growth rate (CGR) correlation for stainless steel piping in PWR primary water environments. In preparation for further detection of SCC in the future, SCC CGR correlation for stainless steel component in PWR environments have been proposed as Code Case for JSME FFS code. This paper explains the technical basis of the Code Case, which is being established based on EPRI MRP-458 "Stress Corrosion Crack Growth Rates in Stainless Steels in PWR Environments". The main points are summarized as follows:
1.    This curve may be used only on non-irradiated austenitic stainless steel components due CGR database of MRP-458.
2.    MRP-458 reports that there is no significant difference in SCC growth rates between type 304 and type 316 stainless steels in PWR environments. In Japanese PWRs, type 316 stainless steel is mainly used in the primary piping, therefore, it is deemed applicable to Japanese plants.
3.    The data used to determine MRP-458 CGR curve is based on test results of cold worked stainless steels, while the test results of weld HAZ are excluded. However, it is determined to be applicable to HAZ as well, based on evidence that SCC crack growth rates exhibit similar distributions for cold-worked materials and HAZ materials of the corresponding hardness.
4.    While MRP-458 recommends a 75th percentile curve for SCC growth rate curve from a conservatism perspective, JSME will adopt a 50th percentile curve in conjunction with a previously established SCC growth rate curve, such as Ni alloy SCC growth rate in PWR. According to JSME's "Codes for Nuclear Power Generation Facilities Rules on Fitness for Service for Nuclear Power Plants (JSME S NA1)," when a crack is detected, first follow-up inspection is to be performed within one-tenth of the allowable operating period based on crack propagation evaluations, thus maintaining conservatism with the 50th percentile curve.

Presenting Author: Toshihiko Sato Mitsubishi Heavy Industries, Ltd.

Presenting Author Biography: Toshihiko Sato joined Mitsubishi Heavy Industries in 2003 and now manager of Maintenance Strategy Planning Groupe. He was in charge of maintenance proposals for PWR plants in Japan. After that, he was involved in the design of PWR Class 1 components, such as reactor vessels and pressurizers, and has since moved on to his current position.
He is an observer member of Flaw Evaluation Working Group of JSME Codes and Standards Committee .

Authors:

Toshihiko Sato Mitsubishi Heavy Industries, Ltd.
Toru Oumaya The Kansai Electric Power Co., Inc.
Takumi Terachi Institute of Nuclear Safety System, Incorporated (former position:The Kansai Electric Power Co., Inc.)
Koji Fujimoto Mitsubishi Heavy Industries, Ltd.
Yuichiro Nomura Mitsubishi Heavy Industries, Ltd.
Yoshihiro Kikuchi Mitsubishi Heavy Industries, Ltd.
Taisuke Hirochi Mitsubishi Heavy Industries, Ltd.
Yusuke Ozaki Mitsubishi Heavy Industries, Ltd.