The phenomenology of fracture in A533B pressure vessel steel was established by examining fractured and partially fractured round bar tensile and Charpy specimens. In the upper shelf region (above 200°F, 366°K) fracture occurs by the nucleation, growth, and coalescence of voids. Quantitative data establishing the void kinetics were obtained by counting and measuring fracture surface dimples and voids below the fracture surface. A computational fracture model is being constructed based on the observed fracture phenomenology and experimental void kinetics data. The model, written as a subroutine for use in finite difference computer codes, is intended to generate fracture toughness data that can be used to calculate critical loads in reactor components.