Flaw Tolerance Evaluation of PWR Reactor Coolant Loop Piping Cast Austenitic Stainless Steel Susceptible to Thermal Aging Embrittlement

Abstract As part of plant license renewal, the reactor coolant loop (RCL) cast austenitic stainless steel (CASS) is evaluated for thermal aging embrittlement. Thermal aging of CASS material results in a decrease in ductility, impact strength, and fracture toughness. Screening criteria can be used to determine whether certain components have experienced thermal aging. One such screening criteria is the calculation of Hull’s Equivalent factor per NUREG/CR-4513 to estimate delta ferrite concentration. The delta ferrite content can be compared with the susceptibility criteria provided in the license renewal report, NUREG-2191. Another screening criteria mentioned in the NUREG-2191 is the comparison of a calculated fracture toughness to 1450 in-lb/in2, where components with calculated fracture toughness below 1450 in-lb/in2 are regarded as having potentially significant thermal aging. Once the component is determined to be potentially susceptible to thermal embrittlement, structural integrity evaluation can be performed per fracture mechanics methodology. ASME Section XI Appendix C provides generic guidance for flaw evaluation of piping components. Fatigue crack growth calculations are performed to determine an initial allowable flaw size for the operating period of interest. This paper investigates three different topics used for the guidance in a typical flaw tolerance evaluation for CASS components, (1) susceptibility of CASS based on screening methodology, (2) fatigue crack growth, and (3) use of Z-factors in Appendix C limit load evaluation.