Integrity and Life Assessment of Cast Duplex Stainless Steel Elbows Used in the Primary Loops of PWRs

Type CF8M cast duplex stainless steels used in the primary loop elbows of PWRs can be affected by thermal aging embrittlement at service temperature, which is around 300°C. This mechanism, resulting from the microstructural evolution of the ferrite, can reduce their fracture toughness properties. In addition, it is necessary to consider manufacturing quality and the possible occurrence of casting defects, such as shrinkage cavities. In a context of life extension, it is important to assess the margins for crack initiation and crack propagation instability. This paper reports the present integrity and life expectancy assessment methodologies as carried out by EDF. The French approach is based on engineering code RSE-M guidelines and French regulation requirements on NPPs in operation. This work is supported by an extensive R&D program on one hand and on field experience analysis on the other hand. It is shown how R&D and engineering tools complement each other. This paper details the three main topics of the life assessment methodology: - Fracture toughness estimates are made with predictive formulae based on chemical composition and aging conditions. These formulae are supported by a data base which is regularly up-dated with new measurements. - An inventory of manufacturing quality is drawn up and the most severe defects are characterized regarding mechanical concerns. - Fracture mechanics analyses are performed using both engineering simplified methods and classical finite element analysis. The calculated J parameter is then compared with the estimated fracture toughness of the material. Margin coefficients are included in the calculation process as required by the French regulation and code. Finally, this evaluation enables the utility to: - Identify the sensitive parts regarding the aging of cast components, - Perform a monitoring and a maintenance program as necessary.