To help predict the behavior of a nuclear pressure vessel undergoing pressurized thermal shock, values of the arrest toughness are required. The purpose of the work described here is to show how toughness values can be calculated from the thickness reduction (TR) of the tested specimens and then used as “lower-bound” estimates. In the NIST wide-plate crack-arrest test program, 16 single edge-notched tension specimens were fractured, using the 26.7 MN universal testing machine. The first series of tests used HSST A533 Grade B Class 1 quenched and tempered steel, while the second series used a low upper-shelf 21/4 Cr-1 Mo steel. For each specimen the TR was measured on the two halves of the broken specimen and a contour map was constructed. The thickness reduction, TR, along the crack propagation plane can be related to the toughness, K, by the relationship K2 = E ∙ σY ∙ TR, where E is Young's modulus and σY an estimate of the tension effective yield strength adjusted for temperature, strain-rate, and constraint. The results show good agreement with values of K for arrest and reinitiation computed from a finite-element generation-mode analysis. Therefore, the indication of plastic work rate provided by the thickness reduction near the fracture plane is a useful preliminary assessment of the fracture toughness.