Determination and Sensitivity Analysis of Target Flaw Sizes for Performance Demonstration of Cast Austenitic Stainless Steel Pipe by Probabilistic Fracture Mechanics

Abstract

The material of primary coolant pipe of nuclear power plants is cast austenitic stainless steel (CASS), which has a coarse crystal structure and its anisotropy causes low permeability of ultrasonic waves and low detectability. Also, the material strength and fracture toughness of CASS are affected by thermal aging depending on ferrite content, chemical compositions and casting process. The ASME Section XI Code Case N-838 provides the target flaw sizes for performance demonstration (PD) examination system of ultrasonic testing (UT) based on probabilistic fracture mechanics (PFM) analysis incorporated a thermal aging model. In this study, the target flaw sizes for Japanese pressurized water reactor (PWR) plants were investigated by using the PFM analysis code “PREFACE.” Thermal aging prediction models for tensile strength and fracture toughness of CASS were incorporated into the PREFACE Code depending on not only ferrite content and chemical compositions but also the casting process, centrifugal casting, or static casting. Those parameters were input as probabilistic variables, which were based on the material database from the Japanese PWR plants. The PFM analysis revealed that static CASS pipes have smaller target flaw sizes than centrifugal CASS pipes and the obtained target flaw sizes were very close to those in ASME Code Case N-838.