The Background to Light Water Reactor Environment Fatigue Evaluation Methods for Austenitic Stainless Steel and Their Application to Sample Pressurised Water Reactor Components

There is now a large amount of small cylindrical specimen test data, which indicates that in a Light Water Reactor (LWR) environment, compared to that in air, the fatigue life of stainless steel is significantly reduced. The current ASME III design fatigue curve does not explicitly include factors to account for a LWR environment. Using the available cylindrical specimen test data, methods for accounting for a LWR environment in fatigue assessments have been proposed in NuReg/CR-6909 and in two American Society of Mechanical Engineers (ASME) code cases. One of the code cases (N-792) uses a penalty factor (F_en) approach, similar to that in NuReg/CR-6909, another (N-761) utilizes a set of environmentally corrected fatigue curves. A third code case, which is still under development, uses a flaw tolerance approach. In this paper the background to the methods for correcting for a LWR environment in fatigue calculations is presented. The safety margin present in the ASME fatigue design methodology is discussed and a short review of civil nuclear plant operating and geometrical features testing experience provided. The NuReg/CR-6909 and ASME code case N-761 methods are applied to a number of ASME III Class 1 austenitic stainless steel components, and the cumulative usage factors calculated compared with those obtained using the ASME 2007 design fatigue curve. An objective of the paper is to highlight some of the issues arising out of applying the newly proposed methods to reactor plant components.