Shutdown is the state of a nuclear reactor when the fission reaction is slowed significantly or halted completely.
For reactivity, this is calculated in units of delta-k/k, where k is equal to the criticality of the reactor (essentially, how fast and controlled the nuclear fission reaction is).
[6] In a typical shutdown, regular levels of coolant are still required, and the fuel remains reasonably hot as it continues to react.
In a cold shutdown, the coolant system is typically lowered to pump water at atmospheric pressure, and the reactor vessel remains below 93 °C (200 °F).
When a reactor is in cold shutdown, the fuel and control rods can be safely removed and exchanged, and maintenance can be performed.