Effect of LWR Water Environments on the Fatigue Life of Reactor Materials (NUREG/CR-6909, Revision 1) – Final Report

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Publication Information

Manuscript Completed: January 2017
Date Published: May 2018

Prepared by:
Omesh Chopra¹ and Gary L. Stevens²

¹ Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439

² Electric Power Research Institute
1300 West W.T. Harris Boulevard
Charlotte, NC 28262

Appajosula Rao, NRC Project Manager

NRC Job Codes V6069 and V6269

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME 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, "Rules for Construction of Nuclear Facility Components," of the ASME Code specify fatigue design curves for applicable structural materials. However, the ASME Code design curves do not explicitly address the effects of light-water reactor (LWR) water environments. The existing fatigue strain vs. life (ε–N) data illustrate potentially significant effects of LWR water environments on the fatigue resistance of pressure vessel and piping steels. Under certain environmental and loading conditions, the fatigue lives of reactor components in water relative to those in air can be a factor of approximately 12 lower for austenitic stainless steels, approximately 3 lower for nickel-chromium-iron alloys, and approximately 17 lower for carbon and low-alloy steels. The original version of NUREG/CR-6909, "Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials—Final Report," issued February 2007, which is the technical basis document for Regulatory Guide 1.207, "Guidelines for Evaluating Fatigue Analyses Incorporating the Life Reduction of Metal Components Due to the Effects of the Light-Water Reactor Environment for New Reactors," issued March 2007, summarizes the work performed at Argonne National Laboratory on the fatigue of piping and pressure vessel steels in LWR environments. That document evaluates the existing fatigue ε–N data 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 presents fatigue life models for estimating fatigue lives as a function of material, loading, and environmental conditions and describes the environmental fatigue correction factor (F_en) for incorporating the effects of LWR environments into ASME Code 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 F_en 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 (1) the constants in the F_en expressions that result in F_en values of approximately 2 even when the strain rate is very high or the temperature is very low, (2) the temperature dependence of F_en for carbon and low-alloy steels, and (3) the dependence of F_en on water chemistry for austenitic stainless steels. The F_en methodology was validated by comparing the results of five different experimental datasets obtained from fatigue tests that simulate actual plant conditions with estimates of fatigue usage adjusted for environmental effects using the updated F_en expressions. The potential effects of dynamic strain aging on cyclic deformation and environmental effects are also discussed.