As part of the Westinghouse APD irradiation effects program on structural materials, fracture toughness specimens including both Charpy V impact tests (transition temperature approach) and wedge opening loading (WOL) specimens (fracture mechanics approach) were irradiated. The reactor vessel materials investigated were two heats of SA302 Grade B including the ASTM reference correlation monitor material. The shift in the nil-ductility transition temperature (NDTT) based on Charpy impact tests and the change in fracture toughness based on WOL tests were determined. The neutron doses were 2.7 × 10¹⁰ to 1.25 × 10²⁰ n/cm² > 1 Mev at 550 F. Because of transition temperature shifts, stresses would be limited in the temperature range of 220 to 405 F during normal startup and shutdown procedures for reactor vessels. However, fracture mechanics tests showed that flaws greater than 9 in. in depth under stresses of 10,000 psi would not limit operating procedures at temperatures above 300 F.