PURPOSE: To enable stable core operation by detecting a core unstable factor such as a recirculation pump trip and limiting the operation area of selection control rod insertion, etc.

CONSTITUTION: When one of low-voltage detection signals are generated by recirculation pump motor bus low-voltage detectors 12A and 12B, an OR circuit 16 outputs a signal. On the output side of the circuit 16, a delay circuit 13 is installed and outputs a nuclear reactor operation area limitation request signal 18 with time delay. A nuclear reactor core unstableness monitor part 11 generates the signal 18. Selection switches 14 are installed in control rod driving operation devices 10A and 10B and only when the switches 14 are placed at selection positions, the condition of an AND circuit 17 is satisfied and an output signal 9A is sent out. Thus, the signal 18 is inputted to the selection control rod driving controller to insert a selected control rod 1; and the output of the nuclear reactor is reduced and the nuclear reactor is prevented from entering the unstable area.