During plant operation, the walls of reactor pressure vessels (RPVs) are exposed to neutron radiation, resulting in localized embrittlement of the vessel steel and weld materials in the core area. If an embrittled RPV had a flaw of critical size and certain severe system transients were to occur, the flaw could propagate very rapidly through the vessel, resulting in a through-wall crack and challenging the integrity of the RPV. The severe transients of concern, known as pressurized thermal shock (PTS) events, are characterized by a rapid cooling of the internal RPV surface in combination with repressurization of the RPV. Advancements in its understanding and knowledge of materials behavior, its ability to model realistically plant systems and operational characteristics, and its ability to better evaluate PTS transients to estimate loads on vessel walls led the U.S. Nuclear Regulatory Commission to realize that the analysis conducted in the course of developing the PTS Rule in the 1980s contained significant conservatisms.