The NSSS heat up 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. Reactor coolant system pressure must be increased to satisfy net positive suction head requirements to operate the reactor coolant pumps. Reactor coolant pumps are started sequentially. The primary plant heat-up rate is limited to about 30°C per hour to minimize internal stress in the material of the pressure vessel, primary piping, and other components.
Heat up, and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heat up 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 heat up and cool down rates.