Fast reactivity insertion

Consider, now, that a large, sudden reactivity is inserted into the reactor while working at nominal power. In this case the time to be considered between neutron generations is the prompt time, i.e., for a PWR, $\tau_{nf}$=0.1 ms. The power increase is too fast for the cooling system to be efficient any more. The cooling system is no more 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 reactors like RMBK and fast reactors the decrease of the reactivity only occurs due to the fuel dissemination.

In conclusion we see that the safety of critical neutron reactors depend strongly on the delayed neutrons 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 enough conditions. We shall see that hybrid reactors might be very helpful in alleviating these constraints.