Overview of AFCEN Non-Linear Benchmark in China – Improving Rules for Vessel Ratcheting Evaluation

The French nuclear standard, RCC-M 2017 edition, has seen the inclusion of an Appendix on non-linear calculation methods: the Appendix ZC. This non-mandatory Appendix encompasses a set of rules and guidelines which can help the designer perform non-linear analyses. It includes the definition of models adequate to use, fatigue Ke factor optimization methods, etc... In fatigue analyses per the RCC-M B-3200 paragraph, rules on thermal ratcheting limits are exposed in paragraph B-3234.8. These rules aim at limiting any thermal ratcheting of the structure and the criteria exposed are sometimes difficult to meet, even though there is no threat to safe operation of the concerned equipment. Non-linear analyses in this case could be used to help meet the criteria but would need extra rules to be defined: these are considered to be added to the Appendix ZC. In early 2019, an international benchmark effort was launched on this topic. The case is a typical nozzle geometry subjected to both thermal and pressure transients. In China, some main nuclear entities have taken part in this benchmark effort: NPIC, SNPI, CNPDC and CNPE, along with the coordination of EDF and Yuansuan. • let participants get familiar with the benchmark and performing linear elastic calculations along with simplified RCC-M code evaluation methods. The outcome is a participant comparison of the respect or not of the code criteria. • perform an analysis based first on one’s own parameter identification and an Armstrong-Frederick model (with 1 and 2 parameters) and then perform the comparison with a common set of parameters. • use a defined cyclic stress-strain curve including full hysteresis loops to derive a common identification methodology and compare results. This paper aims at providing the public with a progress of the benchmark case (using ANSYS and code_aster software), the Chinese participants findings, and recommendations for the structural integrity community on the first two bullet points above.