Fatigue Crack Flaw Tolerance in Nuclear Power Plant Piping - A Basis for Improvements to ASME Code Section XI Appendix L (NUREG/CR-6934)
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Manuscript Completed: November 2006
Date Published: May 2007
S.R. Gosselin, F.A. Simonen, P.G. Heasler,
Pacific Northwest National Laboratory
Richland, WA 99352
D.A. Jackson and W.E. Norris, NRC Project Managers
Division of Fuel, Engineering and Radiological Research
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code Y6604
Appendix L of the ASME Section XI Boiler and Pressure Vessel Code as adopted in 1996 provides non-mandatory guidelines for the nuclear power industry to evaluate the serviceability of components that are adversely subjected to fatigue stresses. Appendix L uses a damage tolerance approach to establish an examination strategy to ensure that components operate reliably between successive inspections. Experience has identified the need for two main improvements: 1) updating requirements for postulated flaw depths by incorporating insights gained from industry and U.S. Nuclear Regulatory Commission (NRC) performance demonstration data and 2) accounting for the initiation and coalescence of multiple fatigue cracks observed in the field. The work described in this report provides a technical basis for improvements to the requirements of Appendix L.
Using industry Performance Demonstration Initiative (PDI) fatigue crack detection data, Pacific Northwest National Laboratory (PNNL) developed a matrix of statistically based probability of detection (POD) curves that considered various nondestructive evaluation performance factors. The improved POD curves, developed for ferritic and for wrought stainless steel materials, are presented in this report. These results showed that very good nondestructive evaluation (NDE) performance could be expected when Appendix L postulated flaw depths are based on the existing Section XI flaw acceptance standards. Probabilistic fracture mechanics calculations comparing best-estimate leak probabilities obtained from both the new performance-based POD curves and previous probabilistic fracture mechanics models are presented.
A review of service experience showed that a crack growth model was needed to address 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 flaws used by ASME Section XI for damage tolerance calculations. Fracture mechanics calculations established aspect ratios for equivalent single cracks (ESC) between the extremes of a 6:1 ratio and a full circumferential crack that can be used in ASME Section XI Appendix L flaw tolerance assessments. The computations presented in this report consider two materials (stainless steel and low-alloy steel), five nominal pipe (sizes 50.8 mm(2 in.) Schedule 80 and 160, 154.6 mm (6 in.) Schedule 160, 254 mm (10 in.) Schedule 160, and 558.8 mm (22 in.) Schedule 160), five cyclic membrane-to-gradient stress ratios (0.0, 0.10, 0.25, 1.0, and 3.0), and a wide range of primary loads. Calculations identify the ESC aspect ratio associated with the hypothetical reference flaw depth assumptions in Appendix L. Examples of computations that apply the proposed ESC model of the revised Appendix L also are presented.
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