Reference Fracture Toughness Curves for Irradiated Pressure Vessel Steels

Recently developed statistically-based fracture toughness reference curve procedures (Charpy V-notch and precracked Charpy methods) have been applied to unirradiated and irradiated plate and weld material from the HSST program and the Maine Yankee reactor surveillance program. The results have been compared to the reference procedure based upon the ASME Code and NRC regulations (K_IR curve) and to actual measurements of fracture toughness. It was found that the fracture toughness measurements were closely grouped around the mean property curve predicted by the Charpy V-notch method. The bounds for each method were similar for HSST Plate 02, but differed considerably for the Maine Yankee material. It was found that the K_IR approach overstated the shift in the temperature of the property transition for the Maine Yankee reactor material. However, the low temperature fracture toughness was predicted to be lower than predicted by the K_IR approach. These conclusions are subject to some reservations because of the poor quality of some of the referencing data, and because the precracked Charpy and Charpy V-notch reference curve procedures are still being developed and tested. Reasons for the differences between the current K_IR method as compared to the proposed new methods have been traced to shortcomings in current procedures.