Uncertainties in the current standard practice used to define RTNDT for indexing the fracture toughness reference curve of irradiated materials by adding the shift in Charpy V-notch energy transition curves to the initial reference temperature are evaluated. Reference fracture toughness curves for irradiated materials are compared to the corresponding lower bound curves of experimental toughness data on neutron-irradiated Mn-Ni-Mo steels and associated welds. Data have been collected for fluence levels ranging from 1 to 13 ∙ 1023 n ∙ m-2 (E ⩾ 1 MeV) and an irradiation temperature of about 290°C. Current practice leads to toughness predictions ranging from realistic to overconservative for all cases investigated. Good correlations are found between the Charpy-V transition temperatures, defined at a 56-J average energy level or at 68-J minimum level, and the static or dynamic 100 MPa √m fracture toughness transition temperatures of irradiated base metals and welds. A direct indexing of the toughness reference curve to these Charpy-V transition temperatures for irradiated materials reduces the scatter and the overconservatism of the standard practice for base metals.