JP2870728B

PROBLEM TO BE SOLVED: To provide an ultrasonic detecting method in which not only a small leak scale but also a large leak scale can be detected with high sensitivity and at an early stage. SOLUTION: A plurality of transmitting/receiving sensors 6 are installed in the circumferential direction on the outer face of a trunk part at a heat exchanger 1 so as to correspond to a plurality of chambers 11 which are partitioned and formed in the circumferential direction inside the trunk, a plurality of stages of the sensors are installed in the length direction, and an operation in which burst wave-shaped ultrasonic waves at the same frequency are transmitted respectively from transmitting parts at the respective transmitting/receiving sensors 6 in the circumferential direction so as to be received by receiving parts is repeated. In addition, after the operation has made a tour, the operation is repeated by using burst wave-shaped ultrasonic waves at different frequencies, the burst wave-shaped ultrasonic waves are passed through hydrogen air bubbles generated by jetting high-pressure vapor into liquid sodium flowing inside the trunk at the heat exchanger 1, attenuated burst wave-shaped ultrasonic waves are received by a reception part at any transmitting/receiving sensor, and a water leak is detected.