Industry’s First NRC Approved Appendix L Flaw Tolerance Evaluation to Manage Fatigue in a Surge Line

Appendix L flaw tolerance was incorporated into ASME Section XI as an alternative approach for addressing fatigue and to justify period extended operation (PEO) of nuclear power plants to 60-years or beyond. A flaw tolerance analysis consists of performing a baseline inspection of the component to verify absence of a flaw, and then determining that a postulated flaw, initially sized at the detection threshold of the inspection, will not grow to the critical crack size by the time of the next inspection. For license renewal, the U.S. Nuclear Regulatory Commission (NRC) expects consideration of reactor water environmental effects in the postulated crack growth laws for any Appendix L evaluation. However, these environmental effects for austenitic stainless steel components have not yet been incorporated into the ASME Code. This paper describes the overall techniques that were used to successfully perform a flaw tolerance evaluation for a pressurizer surge line piping system. This was the first flaw tolerance evaluation to have been submitted and approved by the NRC as an acceptable approach for managing environmentally-assisted fatigue in the period of extended operation.