Research on the Application of F_en for Environmentally Assisted Fatigue Evaluation of the Austenitic SS Pipe Under Combined Transient Loads

Accumulative test data indicate that the effects of the light water reactor (LWR) environment could cause the fatigue resistance of primary pressure boundary components materials to be significantly reduced. Environmentally assisted fatigue (EAF) is the abbreviation of the environmentally assisted fatigue. In 2007, Nuclear Regulatory Commission (NRC) issued RG. 1.207. It was updated in 2014. And, it requires that the effects of LWR environment on the fatigue life reduction of metal components should be considered for new design plants. And it suggests to use environmental correction factor, F_en, to account for EAF. NRC regulation (NUREG), NUREG/CR-6909 (NRC, 2013, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” U.S. Nuclear Regulatory Commission, Argonne, IL, Standard no. NUREG/CR-6909), presents the detail F_en calculation formula. F_en is a function of temperature, strain rate, dissolved oxygen level in water, and sulfur content of the steel. Accordingly, F_en calculation will present a comparatively conservative result. Depending on the experience of the primary pressure boundary piping transient operation, F_en varies during each transient. More uncertainty and confusion are raised during the application of the F_en method. The research work in this paper includes: first, the typical character of piping thermal transient is derived based on the existing experience. Second, small specimen EAF tests are conducted depending on the above derived combined loading characters. Then effort is taken to improve the application of the F_en method for the combined multitransient loading conditions. And the results are compared with those of the lowest instantaneous F_en method and equalization of the weighted F_en method. Finally, a designed test plan is presented.