This paper introduces and discusses the commonly used methods for calculating environmental penalty factors (Fen) to be used in fatigue assessment of stainless reactor components. Three alternative methods for determining Fen factors are introduced and instructed in the JSME S NF1-2009 Code in a language providing a designer the guidance needed for reliable and repeatable EAF assessment. Following these instructions together with the equations and parameters given in the NUREG/CR-6909 it is possible to obtain same Fen factors as by careful reading of the NUREG/CR-6909 alone. The NUREG/CR-6909, Rev. 1 contains notable amounts of valuable information, but instructions for application can be ambiguous or conflicting. An example related to the modified strain rate approach for determining Fen factors is provided.

All international codes used for design, operation and inspection of NPP primary circuit pressure boundaries are rooted to the ASME Boiler and Pressure Vessel Code, Section III, Nuclear Vessels, 1963. Article 4, N-415 “Analysis for cyclic operation” instructed calculation of stress intensities for fatigue transients and provided two design curves for basic material types. Different codes such as ASME, RCC-M, KTA, PNAE and JSME have much in common, but partial deviations exist. In 2007 the US NRC Regulatory Guide 1.207 endorsed a methodology for accounting the environmental effects. It was mainly based on extensive work in Japan and the Argonne National Laboratory. The final report of ANL, NUREG/CR-6909 became a major reference and subject of criticism. However, the first approach for environment assisted fatigue (EAF) written in ‘code language’ was published in Japan and a regulatory requirement for consideration of EAF both for operating reactors and new designs appeared first in Finland. This paper discusses challenges in management of fatigue and the evolving state-of-the-art in different codes, standards, rules and assumptions. The roots and current status of fatigue curves and design criteria applied in Finnish NPP’s are explained.

The design fatigue curves applied for safety class 1 components in NPP’s are based on an experimentally determined strain-life fatigue data, the resulting ε-N reference curves and transferability margins. To account for detrimental effects of coolant water environment, penalty factors (Fen) are obtained by comparing fatigue lives in air and environment. To avoid bias in comparison, the same fatigue testing procedures according to ASTM E606 would be preferred in both environments. However, modified EAF test procedures were developed when direct control of strain in the specimen gauge section was not possible. This paper introduces and discusses commonly used experimental approaches for conducting (directly or remotely) strain-controlled fatigue tests in simulated LWR coolant water environments. At or near gauge section remotely controlled and tubular specimens have been successfully used to determine penalty factors for fatigue usage in environment, but variation and uncertainties related to experimental methods used for collecting the data cannot be completely excluded. The challenges and solutions adopted during latest 20 years in Finland are explained.

The 1963 ASME Section III design fatigue curve was generically applicable to a range of stainless steels in low cycle fatigue. In high cycle fatigue the behavior is sensitive to factors such as material strength, which raises questions about the applicability of the post-2009 ASME III stainless steel design fatigue curve as the generic and mandatory requirement up to 1011 cycles. In response, international codes and standards are investigating and incorporating opportunities for alternative design curves. Long-term Finnish research supports and application of risk-informed inspection and long-term operation strategies benefit from addressing material-specific behavior in fatigue management. In the ongoing study, stainless steel material batches representative of the EPR reactor in Olkiluoto have been selected for experimental fatigue research. This paper presents the first results on a 12″ schedule 140 Z2CND1812 N2 Class 1 pipe, including tensile properties and low cycle fatigue at room temperature.

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