In the primary circuit of PWR, main components composed of ferritic steel, such as the pressure vessel or the pressurizer, are connected to the piping system made of austenitic steel with weld joints. In this paper, we are considering a narrow gap weld made of Inconel 52, typical of the new EPR™ design. Thus, there are two interfaces between the different materials, that is to say Ferritic steel/Inconel 52 and Inconel 52/austenitic steel. For safety purposes, the demonstration of the fracture resistance of this area is necessary. In that frame, a conventional defect is postulated at one of the interfaces and the fracture resistance of the structure is evaluated. To do so, the J parameter is determined and then compared to the material toughness. However, calculation of the J parameter for a crack located at an interface is usually difficult to evaluate depending on the Finite Element (F.E.) code used. As a matter of fact, this configuration is close to the limit of validity of the J formulation since the crack is surrounded by different materials. In general, most of studies are conducted with a conventional defect parallel and close to the interface. This configuration is usually used because the domain of validity of the J parameter is well known in that case. Nevertheless, it is important to be able to evaluate the J parameter for a crack located on the interface since it is more representative of a defect due to the welding process and also easier to mesh. This paper focuses on that particular case and evaluates the validity of the J parameter for such a location of the conventional defect. Using F.E.M. with Systus® and Castem®, the J parameter is calculated for a piping structure containing a narrow gap weld and a defect at the ferritic steel/Inconel 52 interface and around this area. The structure is submitted to different types of loading such as internal pressure, axial force, or thermal shocks. All the different results allow concluding about the continuity of the J parameter through the interface and its numerical validity. Also, it gives interesting information about the size of the integration path to use and the adapted mesh size around the crack.

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