Pressure-temperature operating curves for nuclear reactor pressure vessels are based upon a lower-bound fracture toughness curve which bounds rapid load dynamic initiation and crack arrest fracture toughness data. The ASME Boiler and Pressure Vessel Code defines the reference toughness (KIR) curve as this lower bound, and this KIR curve was developed solely from unirradiated dynamic and arrest fracture toughness data from one heat of SA533B-1 steel (HSST Plate 02) and two heats of SA508-2 steel. The effects of radiation embrittlement on the shape and shift of the KIR curve to account for the increase in reference temperature is thought to be conservative, but this conservatism has not been fully verified. This study reviews available data from past dynamic and arrest toughness tests on irradiated vessel steels from test reactor irradiations and compares the data to the shifted KIR curve using the transition temperature shift approach detailed in Regulatory Guide 1.99, Revision 2. Dynamic initiation and crack arrest fracture toughness data are available from only a few irradiated large specimen tests (that is, test specimens with thicknesses greater than about 51 mm [2 in.]); small specimen tests (including precracked Charpy) are used for the other comparisons. The limited results indicate that the Regulatory approach for shifting the KIR curve is very conservative even when the Regulatory Guide 1.99, Revision 2 “margin term” is not used and a correction for fluence rate is ignored. No change in shape for the dynamic toughness and arrest data (in particular for low upper shelf materials) was observed.