During normal operation the control systems maintain a balance between power generation (and hence reactivity control) and heat removal (leading to electricity generation). Key parameters required for control and protection are monitored by the control and protection systems. In general the functions of control and protection are designed to be independent and are often delivered by separate systems. The protection system is designed to monitor and detect deviations from normal operation, which are beyond the capability of the control system to correct. Small deviations can be corrected by the control system but larger ones will lead to reactor trip (or in US terminology, reactor scram) and the rapid insertion of control rods as well as the initiation of engineered safety systems to ensure core cooling and the confinement of radioactivity.

In PWRs the control and shutdown rods are released and fall under gravity. In BWRs the control rods are rapidly inserted hydraulically. Although these systems are very reliable, diverse means of shutting down the reactor are usually provided based on the injection of dissolved absorbers. In PWRs this is generally achieved by injection into the circulation loops while in BWRs the standby liquid control system injects into the inlet plenum of the reactor vessel.

When a large plant is tripped the loss of generation will disturb the grid and, although the grid connections are designed to cope with this, there is the possibility that local protection will trip the line resulting in a loss of offsite power to the plant. All nuclear power plants are therefore designed so their safety systems can deal with faults on the assumption that offsite power is lost simultaneously. To enable this emergency generators are provided, which will start automatically and power electrically driven safety systems. To provide redundancy it is usual to have between two and four divisions of emergency electrical power, each with their own diesel generator.