Hot shutdown
In the "hot shutdown" state, as in the "cold shutdown" state, the reactor is subcritical, i.e. no nuclear fission takes place. In contrast to "cold shutdown", a pressurised water reactor in "hot shutdown" mode has operating pressure in the primary system and the temperature is in the range of the operating levels. The exact pressure and temperature values can vary depending on the reactor type.
As the decay heat of the reactor core alone is not sufficient over longer periods of time to reach and maintain corresponding temperatures and pressures, the cooling water of the primary system is additionally heated during "hot shutdown" by the frictional heat of the reactor coolant pumps and by heating elements in the so-called pressuriser.
Normally, a reactor that was in "cold shutdown" is first transferred to "hot shutdown"; at certain pressure and temperature levels, it can then be transferred to a critical condition (a self-sustaining chain reaction is established).