PURPOSE: To prevent a decline in the water level of a steam drum and to secure the thermal tolerance of a reactor by causing a recycling pump to make a trip only when the insertion of a control rod is confirmed.

CONSTITUTION: When a reactor scram signal is issued from a safety protection circuit 10, a control rod not shown in the figure is inserted into a reactor core and neutrons are absorbed. The neutron flux in the reactor is always measured and a signal corresponding to the quantity of the neutron flux from a neutron flux instrumentation circuit 14, neutron flux signal from a setter 15, and the reactor scram signal from the circuit 10 incorporate AND logic. Then a signal for opening a breaker 13 for the power supply to a recycling pump is issued form this recycling pump controller 12 only when a recycling signal is issued. When the reactor scram signal is generated and the control rod is inserted into the core, the recycling pump is caused to make a trip so as to prevent the reduction of voids, suppress a decline in the water level of a steam drum, and secure the thermal tolerance of the reactor.