Flaw behavior trends associated with pressurized thermal shock (PTS) loading of pressurized water reactor (PWR) pressure vessels have been under investigation at Oak Ridge National Laboratory (ORNL) for approximately twelve years. During that time, eight thermal shock experiments with thick-walled steel cylinders were conducted as a part of the investigations. These experiments demonstrated, in good agreement with linear elastic fracture mechanics (LEFM), crack initiation and arrest, a series of initiation/arrest events with deep penetration of the wall, long crack jumps without significant dynamic effects at arrest, arrest in a rising stress-intensity-factor (K_I) field, extensive surface extension of an initially short and shallow flaw, and warm prestressing with K_I ⩽ 0. This information was used in the development of a fracture mechanics model that is being used extensively in the evaluation of the PTS issue.