Cooldown Stress Profile:

Plant temperature transients who have the widest potential for causing thermal shock involve excessive plant cooldown and heatup, plant scrams, plant pressure excursions outside of normal pressure bands, and loss of coolant accidents (LOCAs). Within PWRs (pressurized water reactors), the two transients which could cause the most severe thermal shock to the reactor pressure vessel are the LOCA along with subsequent injection of  ECCS (emergency  core cooling system) water and  a severe rise in  the primary-to- secondary heat transfer.

Figure: Cooldown Stress Profile