Cold shutdown

In the "cold shutdown" state, as in the "hot shutdown" state, the reactor is subcritical, i.e. no nuclear fission takes place. In a pressurised water reactor, the primary system is depressurised and the temperature of the cooling water is below boiling point.

A cold shutdown is performed when the operator needs to open the reactor pressure vessel for maintenance purposes or to refuel, or when a repair is required that can only be carried out in this state (for example, for reasons of radiation protection). When a reactor is in cold shutdown, fuel and control rods can be safely removed and replaced, and maintenance work can be carried out. However, after a reactor has been in cold shutdown, more time and energy is required to restart the chain reaction than if the reactor had been in hot shutdown.