The NSSS heatup from Cold Shutdown (MODE 5) to Hot Standby (MODE 3) is performed by reactor coolant pumps which are very powerful (they can consume up to 6 MW each) and therefore its work together with a decay heat can be used for heating the primary coolant before a reactor startup. To operate the reactor coolant pumps, reactor coolant system pressure must be increased to satisfy net positive suction head requirements. Reactor coolant pumps are started sequentially. Primary plant heatup rate is limited to about 30°C per hour in order to minimize internal stress in the material of the pressure vessel, primary piping and other components.
Heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Large components such as flanges, the reactor vessel head, and even the reactor vessel itself are the limiting components. Usually the most limiting component will set the heatup and cooldown rates.