Session: CS-17-03 Fatigue Evaluation Method

Paper Number: 106439

106439 - EPR Piping Material Study: Effects of Sampling Location and Temperature on Low Cycle Fatigue in Air 

Satisfactory fatigue design of nuclear power plant Class 1 components relies on limiting usage factors in order to retain margins for crack initiation. From a calculation perspective, this is achieved by combining stress analysis results with an applicable design fatigue curve, such as found in ASME III, RCC-M, JSME or KTA rules. The 1963 ASME III design curve for stainless steels was intended for low cycle fatigue (LCF) (<<10^6 cycles) and adopted internationally in the other major codes and standards. For LCF, the Coffin-Manson fatigue model, which was adjusted by Langer for use with total strain rather than plastic strain, was generically found suitable for primary circuit structural materials.

In ASME III, the fatigue design curve for stainless steels was replaced and extended with a new model for use up to 10^11 cycles in 2009. The experimental data and derivation of transferability factors on the design fatigue curve were reported in technical basis document NUREG/CR-6909. Particularly the use of both low and high cycle fatigue data, ranging from room to operating temperature, in the same LCF regression model, can be criticized for having an influence on the large mismatch between the original and current best-fit regression curves behind the design curves.

When combined with environmentally-assisted fatigue penalty factors, usage factor limits with the current ASME III design curve are more difficult to fulfil. This often necessitates adoption of flaw tolerance for long-term continued operation. For more than a decade, industry and research organizations have proposed other candidate fatigue curves. For example, the German KTA standard adopted unique design curves for stabilized grades in 2013. The French RCC-M code intends to adopt a modification of the ASME III design curve tailored for a restricted selection of permitted material grades. The JSME in Japan is adopting fully revised design curves, which account for material strength at temperatures. Lastly, the Fatigue Action Plan for Working Groups of ASME III standards committee contains ongoing works aiming to introduce new material grade specific fatigue curves.

Long-term research in Finland aims to support application of Risk-Informed Inspection and LTO strategies, which benefit from addressing material-specific behavior in fatigue management. The first experimental air fatigue results on stainless steel 12” schedule 140 Z2CND1812 N2 (≈316L) class 1 pipe material, which is representative for the EPR reactor in Olkiluoto, were reported in PVP2022-84007. This paper follows up our progress, investigates detail effects of material sampling location (within the pipe component) and test temperature on fatigue behavior in air. The results are discussed in the context of current codified practices.

Presenting Author: Tommi Seppänen VTT Technical Research Centre of Finland Ltd.

Presenting Author Biography: To be added

Authors:

Tommi Seppänen VTT Technical Research Centre of Finland Ltd.
Jouni Alhainen VTT Technical Research Center of Finland Ltd.
Esko Arilahti VTT Technical Research Center of Finland Ltd.
Jussi Solin VTT Technical Research Center of Finland Ltd.
Rami Vanninen TVO Oyj