Technical Basis for Revised Flaw Acceptance Criteria Under IWB-3610 of ASME Section XI

This paper describes the change from the K_Ia to K_Ic in performing flaw evaluation for Class 1 ferritic steel components according to IWB-3610 and Appendix A of the ASME Boiler and Pressure Vessel Code, Section XI. The primary reason for making this change is to reduce the excess conservatism in the current flaw evaluation acceptance criteria. The Appendix A calculation methods can be used in accepting flaws detected as part of the plant inservice inspection program. The K_Ia and K_Ic reference curves represent two lower bound fracture toughness curves available in ASME Section XI. The K_Ia reference curve is a lower bound on all static, dynamic and arrest fracture toughness, whereas the K_Ic reference curve is a lower bound on static fracture toughness only. A similar change has already been implemented in the calculation of the heatup and cooldown pressure-temperature (P-T) limit curves, which are also based on fracture mechanics analysis. The P-T limits are developed according to Appendix G of Section XI, which employ similar methods to those of Appendix A. A key input parameter to the P-T calculations is the lower bound fracture toughness curve, K_Ic (prior to change in Appendix G, the lower bound curve K_IR was used, which was equivalent to K_Ia). Based on the work described in this technical basis paper, Section XI recently approved the change from K_Ia to K_Ic in IWB-3610. In addition, changes to the use of K_Ic for fracture initiation, changes were also made to IWB-3613, which provides acceptance criteria for flanges and other shell regions near structural discontinuities. These changes clarified the scope of the article as to what discontinuities are covered and a redefinition for the minimum temperature from RT_NDT + 60°F (RT_NDT + 33°C) to just RT_NDT. These changes are also discussed in this paper. The changes will appear in the 2005 addenda to the 2004 Code Edition for Section XI.