Development of an inspection technique in opaque liquid-metal coolant is one of the important issues to ensure the safety of the liquid-metal fast breeder reactor (LMFBR). Performance tests of an under sodium viewer (USV), which was developed to detect an obstacle in the reactor vessel (RV) of the LMFBR Monju, have been carried out. Ultrasonic sensors and reflectors are located across the core inside Monju’s RV. The USV can detect an obstacle existing between the core top and the upper core structure bottom by identifying differences of echo signals. This paper describes the USV performance tests. In the tests, the reference echo signals under various conditions were accumulated, and the signal-to-noise ratio successfully exceeded the target value. Measured signals clearly differed with and without an obstacle. These experimental results show the performance of the USV for detecting an obstacle in a specified place.
