The Detailed Fatigue Calculation (DFC) of nuclear power plant components is part of the three staged approach to lifetime assessment and lifetime management of the AREVA Fatigue Concept (AFC). It is applied for fatigue relevant components showing high usage factors by application of the Simplified Fatigue Evaluation (SFE) and/or the Fast Fatigue Evaluation (FFE), e.g. the preceding stages of the AFC. The quality of the fatigue lifetime assessment essentially depends on one hand on the fatigue model assumptions and on the other hand on the load data as the basic input. In the case of nuclear power plant components thermal transient loading is most fatigue relevant.
The issue of qualified determination of the real operating loads is crucial and closely connected to the fatigue monitoring strategy to be applied in the plant. As regards fatigue monitoring, two possible basic approaches are practiced: global and local concepts.
The first mentioned relies on signals from the standard plant instrumentation in connection with transfer functions whereas the second one requires additional measurement sections at fatigue relevant locations. As a compensation to the additional instrumentation effort, the application of a local fatigue monitoring strategy paves the way of delivering continuously (at a frequency of 1 Hz) realistic load data. Disposal of these data constitutes the first step within the flowchart of fatigue assessment. The according methods of qualified processing of these data are discussed in detail. The processing of arbitrary operational load sequences and the derivation of representative model transients are essential steps. Appropriate cycle counting approaches and the consideration of Environmentally Assisted Fatigue (EAF) by way of Fen-factors are addressed in this context.
Within the fatigue evaluation model the appropriate consideration of cyclic plasticity effects and the identification of fatigue damaging events are central modules. Plasticity correction is equally proposed in a staged approach by the application of established and more advanced Ke-factors within the simplified elasto-plastic analysis, the application of direct methods (such as the simplified theory of yield zones) and the general elasto-plastic analysis based on appropriate material models.
These three stages are characterized by increasing calculation effort and (usually) decreasing degree of conservatism. Their application is case dependent. Additionally, the general elasto-plastic analysis entails a cycle-by-cycle ratcheting check based on an appropriate material model as part of the detailed fatigue check (not elaborated in detail in this paper).
The application of the integrated AFC approach is explained by way of a representative example.