The current approach in evaluating the Pressurized Water Reactor (PWR) inlet and outlet nozzle corner regions with respect to plant heat-up and cool-down pressure-temperature limit curves contains a number of conservatisms. These conservatisms include postulation of a large ¼ thickness flaw at the nozzle corner region and use of RTNDT (reference nil-ductility temperature) or an estimation of RTNDT. The paper herein discusses generic fracture toughness of nozzle forging material SA-508 Class 2 for use with postulated smaller surface flaws in developing pressure-temperature limit curves for nozzle corners for nuclear power plant operations.
ASME Appendix G uses the lower bound KIC curve, which has inherent margin since RTNDT is a conservative method for locating the KIC curve. RTNDT is based on the drop weight test, which is a crack arrest transition temperature measurement, and the Charpy impact test, which is a blunt notch impact test. These data are conservatively bounded by the KIC curve, which is a lower bound crack initiation toughness curve. In contrast, the master curve method is based on an initiation transition temperature fracture toughness test technique per ASTM E1921. The master curve index temperature (T0) provides a more accurate measure of the material fracture toughness than KIC indexed with RTNDT. Since many of the nuclear pressure vessels were fabricated to ASME Code editions prior to 1972, RTNDT was not measured for the nozzles. In many cases, RTNDT has been estimated. Therefore, for this work, the fracture toughness was generically established based on conservative T0 measurements of 22 representative forgings with a margin of two standard deviations to ensure a conservative lower bound toughness using ASME Appendix G, G-2110.
The properties of a forging are better near the surface due to the faster cooling rate during heat treatment. The difference in reactor pressure vessel fracture toughness was established for forgings near the surface at the postulated flaw location as allowed by ASME Section III, NB-2223.2 relative to the traditional ¼ thickness location. The near-surface forging toughness was conservatively determined through evaluation of 31 near-surface and approximate ¼ thickness location fracture toughness measurements.