Technical Basis For Revised P-T Limit Curve Methodology

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ABSTRACT The heatup and cooldown processes for an operating nuclear plant are controlled by pressure-temperature (P-T) limit curves, which are developed based on fracture mechanics analysis. These limits are developed according to Appendix G of the ASME Boiler and Pressure Vessel Code, Section Xl, and incorporate a number of safety margins. A key safety margin is the lower bound fracture toughness curve, or K IA (equivalent to K IR ). Based on the work described in this technical basis paper, Section XI recently approved Code Case N-640 permitting the use of the lower bound static fracture toughness curve, K IC , for calculating operating P-T limit curves. The same change appears in Appendix G of Section XI in the 1999 Addenda. There are two lower bound fracture toughness curves available in Section Xl: K IA , which is a lower bound on all static, dynamic and arrest fracture toughness, and K IC , which is a lower bound on static fracture toughness only. Code Case N-640 changes the fracture toug...

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