French methodology proposal for environmentally assisted fatigue assessment

Environmental assisted fatigue is a phenomenon that has been investigated over the past 30 years through many test campaigns in various laboratories around the world. It is receiving nowadays an increased level of attention as many nuclear plants will see their life-time extended to up to 60 years. To provide a set of rules for this significant life extension, international codes have already proposed code cases to formally integrate these effects. More particularly, the ASME code has based itself on the NUREG/CR-6909 [1] to elaborate the Code Case N-792 [2] and suggests a modification of the fatigue curve combined with a calculation of an environmental penalty factor, namely Fen, which multiplies the typical usage factor. In France, the EDF and AREVA objective is also to more explicitly integrate these effects in the RCC-M code basing itself on international methodologies but also results from French inhouse testing campaigns [3]. The approach in this paper is globally similar to the one in the ASME code: it will indeed consist in an update of the fatigue curve as well as the calculation of an environmental penalty factor. Nevertheless, the methodologies differ in their detailed implementation, as was already hinted in previous papers discussing the French methodology [4-5]. This paper will present the proposal as well as highlight the differences with the ASME methodology.