Definition of Pressurized Thermal Shock:

One safety issue which is a long-term problem brought on through the aging of nuclear facilities is PTS (pressurized thermal shock).  Pressurized thermal shock is the shock experienced through a thick-walled vessel because of the combined stresses from a rapid temperature and/or pressure modified. Nonuniform temperature distribution and subsequent differential expansion and contraction are the causes of the stresses included. As the facilities get sequence in terms of full power operating years then the neutron radiation causes a modification in the ductility of the vessel material, making it more susceptible to embrittlement. Therefore, if an older reactor vessel is cooled rapidly at high pressure then the potential for failure through cracking rising greatly.