Various abnormal operating conditions are possible, some of which can rapidly damage the reactor, some only if the abnormal operation is protracted over a very long time. It is very important to ensure both a safe and reliable operation. In order to ensure plant reliability it is important to avoid unnecessary scrams while on the other hand it is necessary to protect the reactor from dangerous conditions. Scram signals are released when limiting values are exceeded in various quantities, of which the most important are: too high flux, too short period and too high gas outlet temperature.

Before these limits are reached it is possible to have alarms and power set back. A scram signal causes the control rods to be inserted in the core with their maximum speed.

The maximum temperature reached after an accident depends on the delay between the beginning of the abnormal condition and the introduction of the required negative reactivity to shut down the reactor.

After a scram the coolant mass flow must be reduced in order to avoid too rapid a cooling down of the reactor structures. The mass flow must be adjusted to the requirement posed by the decay heat.