Investigation of the Fatigue Behavior of Austenitic Stainless Steels and Their Welds for Reactor Internals Under Combined Low Cycle (LCF), High Cycle (HCF) and Very High Cycle (VHCF) Operational Loading Conditions

The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability of nuclear power plants. Reactor internals are subjected to thermo-mechanical fatigue induced by operational temperature transients and to flow induced vibrations. The resulting complex loading collectives induce low cycle (LCF), high cycle (HCF) and even very high cycle (VHCF) fatigue and their interaction. The existing methodological gaps within the current fatigue assessment approach are to be closed. Design code fatigue analyses use defined loads and frequencies of occurrence (specified or measured). High cycle and very high cycle fatigue loadings are not explicitly considered except for endurance limit studies of reactor internals. Furthermore, design fatigue curves in the applicable international design codes were extended by extrapolation from originally 106 up to 1011 load cycles. However, the existing data base for load cycles equal to or above 107 is still highly insufficient. The cyclic deformation behavior of the material in question (austenitic stainless steel 1.4550) is different depending on the fatigue regime respectively the applied load or deformation amplitude. While the LCF behavior is already well investigated and the basic behavior in the HCF regime is fairly well known the VHCF cyclic deformation behavior has not been characterized in sufficient detail so far. As a consequence, the real damage accumulation of variable amplitude combinations consisting of LCF- and HCF/VHCF loads is still widely unknown. A new cooperative R&D project of MPA Stuttgart, TU Kaiserslautern and Framatome GmbH addresses the existing gaps of knowledge presented above and has recently been launched. The scheduled first phase of the project will entail the following key items: • Substantiation of the threshold strain amplitude εa ≥ 0.1% for the consideration of Environmentally Assisted Fatigue (EAF) conditions in the HCF regime; • Basic characterization of the HCF and VHCF fatigue behavior at relevant operational temperatures in air at 106 − 1010 load cycles; • Fatigue behavior at variable amplitude loading (combination of LCF / HCF and LCF / VHCF); • Fatigue behavior of welds in the region of high numbers of load cycles (HCF regime > 105 load cycles and VHCF > 107); • Validation of the existing design and assessment procedures (base material and welded material); • Development of a fatigue assessment methodology for reactor internals under consideration of the transient endurance limit and damage accumulation effects. A later second phase of the project will concentrate on the following items: • Examination of the fatigue behavior of welded specimens and representative components; • Consolidation of the design process from laboratory specimen to real structures and components; • Examination of the operational loading characteristics of reactor internals with respect to dominant loadings; • Probabilistic consideration of the influence of fatigue assessment on the plant risk (core damage). The project structure will be discussed in detail in the paper.