PNNL

Abstract

Section XI of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code has worked since the early 1990’s to develop guidelines for the nuclear power industry to evaluate the serviceability of components that are subject to fatigue stresses. Results of this work formed the basis for a non-mandatory Appendix L that became part of the 1996 Addenda to the 1995 Edition of Section XI [1]. A key part of this appendix was the introduction of a damage tolerance based examination strategy designed to assure that the component will operate reliability between subsequent inspections. Since the 1996 introduction of Appendix L into ASME Section XI, the industry and NRC have raised concerns regarding the initial flaw tolerance procedure. New data on the flaw detection capabilities has become available, and these data show that ultrasonic inspections can detect flaws much smaller than assumed during the original development of Appendix L. This information allows for significantly smaller sizes for postulated flaws. A second area of needed improvement was related to the growth of fatigue cracks. A crack growth model was needed to address component fatigue failures for which several fatigue cracks can initiate at multiple sites and then link together to form a single fatigue crack that is much longer than the standard 6:1 aspect ratio reference flaw typically used by ASME Section XI for damage tolerance calculations. In 1999 the ASME Working Group on Operating Plant Criteria (WGOPC) re established the Task Group on Operating Plant Fatigue Assessments (TGOPFA) to address these concerns. This paper summarizes the research results, supporting computations, and technical bases for TGOPFA recommended Appendix L improvements. A detailed sample calculation is presented for a PWR charging nozzle-to-pipe weld.