Evaluation of the Minimum Temperature Requirements for the RPV Closure Head Flange Region in 10 CFR 50 Appendix G

Title 10 of the Code of Federal Regulations, Part 50 Appendix G (10CFR50 App. G) prescribes the pressure-temperature (P-T) limits for light water power reactors by incorporating American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code, Section XI, Appendix G (ASME App. G) by reference. The P-T limits in ASME App. G are based on the fracture toughness of the vessel limiting material accounting for irradiation effects. The fracture toughness of the limiting material is determined based on the plane strain fracture toughness, KIc, indexed to the material reference temperature, RT NDT. Prior to the 2000 Addenda of ASME App. G, fracture toughness was based on the arrest fracture toughness, KIa, but the approval of Code Case N-640 [1] in 1998 changed the fracture toughness basis to KIc. In addition to the P-T limits determined by ASME App. G, 10CFR50 App. G sets additional minimum temperature requirements that limit the operating pressure based on other factors such as the minimum unirradiated reference temperature, RTNDT(u), of the material in the closure head flange region. This paper presents an analysis of the closure head flange region minimum temperature requirements based on both KIa and KIc. Recommendations for potential alternative minimum temperature requirements are made for normal operation and hydrostatic test conditions.