Consider, now, that a large, sudden reactivity is inserted into the reactor while it is running at nominal power. In this case the time to be considered between neutron generations is the prompt time, i. e. for a PWR, rnf = 0.1ms. The power increase is too rapid for the cooling system to be efficient. The cooling system is no longer efficient and melt-down of the fuel elements may occur. Boiling of the water coolant leads to a strong decrease of the reactivity and to a limitation of the power surge. In other types of reactor like RBMK and fast reactors the decrease of the reactivity only occurs with fuel dissemination.

In conclusion, we see that the safety of critical neutron reactors depends strongly on the delayed neutron fraction of the fuel as well as on the tempera­ture coefficients, especially on the Doppler temperature coefficient. These strong dependences severely constrain the type of fuel which can be used in critical reactors in safe conditions. We shall see that hybrid reactors might be very helpful in alleviating these constraints.