PURPOSE: To detect a void in a coolant by providing a means which generates a signal indicating the pressure of a coolant in a reactor, a means which adjusts the pressure, and a microprocessor means which compares signals with each other.

CONSTITUTION: When a reactor coolant pump 20 is operated, a coolant is circulated through a reaction vessel 14. A dynamic pressure transducer 28 detects the pressure fluctuation induced by the pump 20 or a high-pressure injection pump 22 and generates a dynamic signal indicating the pressure fluctuation. The dynamic signal is successively adjusted through a preamplifier 30, a signal adjusting amplifier 32, and appropriate band-pass filters 34 and 36 so that the signal can become suitable for the processing by means of a microprocessor 38. The processor 38 compares the pressure, temperature, and pressurizing level of a reactor coolant system 10 with the value of a void detecting signal and adjusts the signal generated from the transducer 28 in accordance with the operating condition of the system 10. Corrected void detecting signals can be displayed successively on a gauge 40 so as to indicate the occurrence of a void in the system 10.