“PVRC’s Position on Environmental Effects on Fatigue Life in LWR Applications” W. Alan Van Der Slys & the Steering Committee on Cyclic Life and Environmental Effects (Sumio Yukawa, Chair; William J. O’Donnell, Member) WRC Bulletin 487, December 2003
Keywords: fatigue analysis; pressure boundary materials; environmental correction factor; low cycle fatigue; stainless steel; fatigue properties
This Bulletin report describes the activities of the PVRC Steering Committee on Cyclic Life and Environmental Effects (CLEE) and the PVRC Working Group S-N Data Analysis. This report presents the PVRC recommendations to the ASME Board on Nuclear Codes and Standards (BNCS) concerning needed modifications to the ASME fatigue analysis procedure. The proposed modifications will account for the effect of the environment on the fatigue properties of the pressure boundary materials.
The PVRC Committee has worked closely with, and received comments from investigators in Japan, Europe and America. Considering all well characterized, available data, PVRC has drawn the following major conclusions:
1) ASME Section III should adopt a procedure such as proposed in Section 7 of this report to apply an environmental correction factor, Fen, to life fractions calculated using the existing ASME S-N design curves when anticipated operating conditions are sufficiently severe that it is necessary to account for environmental effects.
2) ASME Section XI should adopt a procedure such as proposed in a draft code case in Section 7 of this report and apply the environmental correction factor, Fen, to life fractions calculated using the existing ASME S-N design curves when it is necessary to account for environmental effects.
3) The Fen models are shown to work well in predicting the effect of the coolant environments on the low cycle fatigue properties of stainless steel.
The low cycle fatigue information on stainless steel in air, collected by the PVRC to perform the evaluation, does not appear to support the ASME mean data line for stainless steel, and more data are needed to adequately understand behavior. The conclusions are based on two principles: 1) The environmental correction factors can be determined using equations developed either by Argonne National Laboratory or by MITI’s investigators in Japan. While these equations are somewhat different; in real situations, they are expected to give similar results, within the bounds of experimental error and operating uncertainties. 2) The factor of 20 on life, originally used in the development of the fatigue design curves to account for uncertainties, is adequate to account for reductions in fatigue life due to the environment under well controlled operating conditions.
Under those conditions, provision for further reductions in fatigue life due to the environment is not essential. The PVRC has reviewed the ASME Section III Fatigue Analysis procedure to determine what modifications are needed to take into account the effects of the coolant environment on the S-N fatigue properties. In performing this review, the PVRC evaluated the following areas:
1) The margins used in the development of the Section III procedure.
2) Laboratory data used in the development of the Section III procedure.
3) Laboratory fatigue data on smooth specimens in simulated reactor coolant environments.
4) Models to predict the S-N properties in Light Water Reactor (LWR) coolant environments of the pressure boundary materials.
5) Laboratory data on structural tests conducted in water environments.