The flaw evaluation rules for ferritic vessels in IWB-3610, IWB-3620 and Appendix A of ASME Section XI are based on linear elastic fracture mechanics techniques and were developed primarily for the irradiated reactor pressure vessel beltline region and other low temperature carbon and low-alloy steel applications in which the material exhibits limited or no ductility prior to failure. There are situations in which ferritic steel components operate in the upper shelf temperature range and therefore exhibit significant ductility and increased flaw tolerance. Application of linear elastic fracture mechanics techniques to these cases can be very conservative. In order to address flaw evaluation of ferritic materials exhibiting upper shelf toughness and high ductility, the proposed Code Case N-749 of ASME Section XI was developed and is currently under committee review. This proposed Code Case provides alternate acceptance criteria for situations in which the component is operating in the upper shelf temperature range and therefore has adequate ductility to allow the use of elastic-plastic fracture mechanics techniques.

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