Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials (NUREG/CR-6909, Revision 1) - Draft Report for Comment
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Manuscript Completed: March 2014
Date Published: March 2014
Omesh Chopra1 and Gary L. Stevens
1 Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
G. Stevens, NRC Project Manager
Division of Engineering
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Codes V6069 & V6269
The ASME Boiler and Pressure Vessel Code provides rules for the design of Class 1 components of nuclear power plants. Figures I–9.1 through I–9.6 of Appendix I to Section III of the Code specify fatigue design curves for applicable structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. The existing fatigue strain–vs.–life (ε–N) data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of pressure vessel and piping steels. Under certain environmental and loading conditions, fatigue lives in water relative to those in air can be a factor of approximately 12 lower for austenitic stainless steels, approximately 3 lower for Ni-Cr-Fe alloys, and approximately 17 lower for carbon and low-alloy steels. In 2007, the original version of NUREG/CR-6909, which is the technical basis document for NRC Regulatory Guide 1.207, summarized the work performed at Argonne National Laboratory on the fatigue of piping and pressure vessel steels in LWR environments. In that document, the existing fatigue ε–N data were evaluated to identify the various material, environmental, and loading parameters that influence fatigue crack initiation, and to establish the effects of key parameters on the fatigue lives of these steels. The report presented fatigue life models for estimating fatigue lives as a function of material, loading, and environmental conditions, and described the environmental fatigue correction factor, Fen , for incorporating the effects of LWR environments into ASME Section III fatigue evaluations. The report also presented a critical review of the ASME Code Section III fatigue adjustment factors of 2 on stress (or strain) and 20 on life and assessed the possible conservatism in the choice of these adjustment factors.
This report provides updates and improvements to the environmental fatigue correction factor approach based on an extensive update to the fatigue ε–N data from testing and results available over the past decade since this report was first published. The updated expressions also address concerns from interested stakeholders related to: (a) the constants in the Fen expressions that result in Fen values of approximately 2 even when the strain rate is very high or the temperature is very low, (b) the temperature dependence of Fen for carbon and low-alloy steels, and (c) the dependence of Fen on water chemistry for austenitic SSs. The Fen methodology was validated by comparing the results of five different experimental data sets obtained from fatigue tests that simulate actual plant conditions with estimates of fatigue usage adjusted for environmental effects using the updated Fen expressions. The potential effects of dynamic strain aging on cyclic deformation and environmental effects are also discussed.