Counter-reactions

There are two types of self-stabilizing counter-reactions in the nuclear reactor: the moderating effect of the water in the reactor coolant system and the Doppler effect which has an impact on the absorption of neutrons by uranium 238.
These spontaneous phenomena allow the power of the core to be reduced quasi-instantaneously if its temperature increases, and to increase it if the temperature decreases. They allow the power to be stabilized at a constant value and to thwart any possible runaway events without external intervention.