Lower-bound initiation toughness of A533-B (UNS #K 12539) reactor grade steel was determined over the temperature range from 0 to 57°C by using a modified-Charpy specimen. The lower-bound measurements were attained by utilizing the following procedures: (1) dynamic loading, (2) modification of the geometry of the specimen, and (3) axial precompression of the notch. The paper describes in detail the key features of the modified geometry, the method of precompressing the specimens, and the strain gage procedure. The dynamic initiation toughness KId, which correlates with the lower-bound toughness, was determined by analyzing straintime records from the specimen. The results from a fractographic analysis were correlated with those from the strain-time analysis. An empirical correlation was developed relating KId to the energy absorbed (Ecv) during the fracture of the specimen. Finally, the lower-bound toughness from this study compared favorably with KIa and KId measurements from the same material, established in other programs.