An assessment of the safety factors and uncertainties in the fatigue rules of the RCC-M code through the benchmark with the en-13445-3 standard

The RCC-M code [1] is a well recognized international code and provides rules for the design and the construction of mechanical equipment for pressurized water reactors. It is used today for the nuclear industry exclusively, in countries such as France, South Africa and China and it is the basis for the design of the UK EPR to be built in Hinkley Point. The RCC-M code's fatigue rules emanate from the ASME Boiler and Pressure Vessel Code and are hence very similar, albeit they have evolved in their own way over time to include some R&D results and other evolutions. These rules are published by AFCEN which involves a wide range of international organizations from the nuclear industry such as Apave, Areva, Bureau Veritas, CEA, DCNS, EDF, EDF Energy, ONET-MHI, Rolls-Royce and Westinghouse. The EN-13445-3 [3] is a European standard which is mostly in use today in the conventional industry. Its fatigue rules are a compilation of rules from various national European codes, such as the German AD-Merkblatt, the British Standards, the Eurocodes for civil works and the French CODAP. The rules for fatigue are compiled in Chapters 17 and 18 of EN-13445-3 and have been the result of the work of contributors from major European organizations from the nuclear, oil and gas, chemical and mechanical industries: these include, among others, Areva, the Linde Group, CETIM, TÜV, and the TWI (The Welding Institute). Since the beginning of 2015, AFCEN has created a technical Working Group (WG) on the topic of fatigue with the objective of identifying the Safety Factors and Uncertainties in Fatigue analyses (SFUF) and of potentially proposing improvements in the existing fatigue rules of the code. Nevertheless, the explicit quantification of safety factors and uncertainties in fatigue is an extremely difficult task to perform for fatigue analyses without a comparison to the operating experience or in relation to another code or standard. Historically, the approach of the code in fatigue has indeed been to add conservatism at each step of the analyses which has resulted in a difficult quantification of the overall safety margin in the analyses. To fulfill its mission, the working group has deemed necessary to lead a benchmark with the EN-13445-3 standard given its wide use through other industries. Two cases were identified: either the comparison with EN-13445-3 is possible and in this case, the identification of safety factors and uncertainties is performed in relation to this standard; either the comparison is not possible, in which case the overall conservatism of the RCC-M code is evaluated in relation with operating experience, test results, literature, etc⋯ This paper aims at describing the overall work of the group and focuses more specifically on the results obtained through the benchmark with the EN-13445-3 standard.