The safety assessment of nuclear pressure vessels and piping requires a quantitative estimation of defect growth by stable and unstable manner during service. This estimation is essential for determining whether the defect detected during inspection should be repaired or whether the size of the defect even after its expected growth is small enough to leave the integrity of the vessel unaffected.
The most important stable defect growth mechanism is that of environmentally assisted cyclic crack growth. Recent results indicate that it is markedly affected by sulfur content and/or manganese sulfide morphology and distribution. This implies that an essential improvement in component safety has been gained by currently applied steelmaking practices, which result in extra low sulfur content, generally below 0.01 wt%, and in round shape and small size of inclusions, through, e.g., calcium treatment, hence considerably reducing the effect of environment on crack growth rate. This further implies that the ASME Section XI reference curves for environmentally accelerated cyclic crack growth are conservative for steels produced by current steelmaking practices.
The ASME Section XI applies predominantly linear elastic fracture mechanics to assess the effects of cracks on the integrity of nuclear power plant components. Unstable linear elastic fracture often propagates by cleavage mechanism. The cleavage fracture process has recently been shown to be of statistical nature in both ferritic and bainitic steels. The carbide size distribution plays a dominant role in controlling the fracture toughness of these steels. A cleavage fracture model has been developed, based on carbide induced cleavage fracture in ferritic and bainitic steels, which can be used to estimate the expected value and probability limits of fracture toughness. This method has been utilized to evaluate the conservatism of the ASME reference fracture toughness curve. For this purpose a microstructural analysis was carried out for the HSST-02 plate material, with which a large amount of KIc data has previously been generated for reference curve purpose. The result of the statistical evaluation indicates that based on the 95% survival probability limit some parts of the ASME reference fracture toughness curve are unconservative.