Abstract: In the event of an incident in the secondary cooling circuit of a nuclear reactor preventing cooling the water in the primary circuit of the steam generators (2) which are common to both circuits, an emergency stop instruction is issued in order to cause control and stop rods to fall into the core of the reactor. If this instruction is not executed, and if the reactor remains at a high power level, an instruction is issued to stop the primary pumps. A rapid increase in the temperature of the water in the core then slows down the nuclear reaction soon enough to prevent subsequent excess pressure from damaging the primary circuit. The invention is particularly applicable to electricity-generating nuclear power stations.