[en] Studies of the effects of neutron irradiation on fracture toughness properties of steels have generally included a minimum number of tests for each material condition. The present study attempts to apply statistical analyses, with multiple testing at selected temperatures, to assess the accuracy and reliability of the results. Fracture toughness test specimens were irradiated in the Bulk Shielding Reactor at Oak Ridge National Laboratory at 288/degree/C to target neutron fluences of 2 /times/ 10²³ neutrons/m² (>1 MeV). The materials were ASTM A533 grade B class 1 plate (HSST Plate 02) and four submerged-arc welds representing the current fabrication practice for nuclear pressure vessels. Both unirradiated and irradiated specimens were tested by two separate laboratories, and multiple tests were conducted at selected temperatures. Statistical analyses permitted the determination of material and test variability and an interlaboratory comparison. Behavior in both the transition and ductile-shelf regions was studied. The results demonstrated the relatively low radiation sensitivity of low copper/nickel welds and the qualitative agreement between Charpy impact and fracture toughness observations of toughness degradation