Reassuring safety through research
Properties of non-replaceable component materials, such as pressure vessels, may change considerably during the lifetime of a nuclear power plant. For example, embrittlement is a phenomenon whereby the material toughness is lowered by the effect of radiation exposure or due to accidental lowering of temperatures. Hence the structural integrity analyses of the reactor power vessel need to be updated from time to time with information on fracture toughness or faults in the structure. During the Fifth Framework Programme, the FRAME project was supported by the AMES network to research into irradiation embrittlement, which is currently estimated from Charpy-V impact test data. As Charpy-V impact toughness is in many respects a different material property than fracture toughness, scientists' understanding of embrittlement may have been up until now a biased one. FRAME was primarily an experimental project, where fracture toughness transition temperature was measured under unirradiated and irradiated conditions for different materials, including steel. Furthermore, trend curves were derived by in essence mathematically fitting candidate functions to the measured fracture toughness data. These provided a description of the embrittlement response in terms of material chemistry and neutron fluence, which could be compared with published models based on Charpy-V test data. Fracture toughness-based trend curves were not available before the FRAME project, because of the insufficient size of the experimental data required. If variation of material chemistry inherent in the large surveillance database could be minimised, the establishment of even more generic trend curves was proven to be feasible.
