PURPOSE: To achieve a greater safety with the control of the flow rate of a nuclear reactor by detecting the generation of a foreign metal while determining the sound source position thereof and absolute amplitude value thereof to calculate an impingement energy from a detector to the sound source position and the absolute amplitude value.

CONSTITUTION: A nuclear reactor pressure vessel 1 having sensors 21W2m is provided with amplifiers 31W3m, full-wave rectifier circuits 41W4m, detection circuits 51W5m, comparators 61W6m, an OR circuit 7, a transient recorder 8, a microcomputer 9, a flow rate adjustment signal generator 10, a display unit 11 and a recorder 12 while a calibration coefficient is set at a specified value. When a foreign metal causes an impingement sound, a waveform is recorded into the transient recorder 8 in a time-siries sequence and taken with the microcomputer 9 to calculate an impingement energy from the sound source position determined from the difference in the rising time of the waveform, the miximum amplitude value and the calibration modulus.