Fatigue Flaw Tolerance Evaluation of Pressurizer Surge Line for Plant License Renewal Applications

Abstract

Evaluation of metal fatigue is one of the identified Time Limited Aging Analyses (TLAA) described in the License Renewal Applications for nuclear power plants. A related area is the evaluation of Environmentally Assisted Fatigue (EAF) as required by the Nuclear Regulatory Commission (NRC) in the United States. The focus is on fatigue sensitive components in power plants selected based on anticipated loading conditions and service experience. The pressurizer surge line was identified as a representative component for the evaluation of EAF for nuclear power plants of both older and newer vintage. A particular location of interest on the surge line is the surge line hot leg nozzle to pipe weld. Typical resolution of EAF concerns in a License Renewal Application requires the incorporation of large Environmental Fatigue Multipliers (Fen) in determining the cumulative usage factors for austenitic stainless steel components. Consequently, the 60-year projected cumulative usage factor at this weld location of interest has the potential to exceed the ASME Section III Code allowable limit of 1.0. For components that fail to demonstrate that the predicted cumulative usage factor is less than 1.0, ASME Section XI Non-Mandatory Appendix L provides guidance for evaluating the component’s fitness for service. The approach is based on a fatigue flaw tolerance evaluation and the implementation of an inspection strategy to demonstrate that growth of a postulated flaw would remain below the allowable flaw size for the component of interest. NUREG/CR-6934 establishes a technical basis for improvements to the initially published version of Appendix L requirements. These improvements have been incorporated in the 2008 Addenda to ASME Section XI Code. One of the improvements is the use of Equivalent Single Crack (ESC) aspect ratio to account for the effects of multiple fatigue crack initiations and the linking of these cracks as they grow to form a single long crack with large aspect ratio. Since the NRC in the United States has not yet endorsed the use of 2008 Addenda to the ASME Section XI Code which contains the latest version of Appendix L, fatigue flaw tolerance analysis is performed in accordance with the requirements in NUREG/CR-6934 for the current license renewal applications. This paper demonstrates the use of the fatigue flaw tolerance approach in accordance with the requirements in NUREG/CR-6934. Typical thermal stratification loading and thermal transients including the use of ESC aspect ratios are considered in the fatigue flaw tolerance evaluation. The results are used to demonstrate fitness for service for a typical 14-inch Schedule 160 pressurizer surge line when the predicted cumulative usage factor exceeds 1.0 for 60 years of plant operation.