Retrieval of fuel debris is one of the most urgent issues regarding the decommissioning of Fukushima Daiichi Nuclear Power Plant (NPP). A retrieval of fuel debris requires a measurement system which can be applied in the severe environment to investigate inside the Reactor Pressure Vessel (RPV) and the Control Rod Drive (CRD) housing. In this study, an air-coupled ultrasonic measurement is proposed because of the feasibility of its application in the severe environment such as the high-radiation atmosphere. However, compared with using it in water or solids, there are some difficulties with applying the air-coupled ultrasonic technique. Therefore, we employed a point-focused transducer which can converge ultrasonic energy in a small area. As a fundamental study, this paper focuses on the experimental assessment of the potential feasibility and property of the developed air-coupled ultrasonic imaging. Firstly, to obtain the property and characteristics of the point-focused transducer, the sound field of the transducer was measured. The sound field showed the convergence rate, acoustic intensity and the beam width of the transducer. Secondly, in order to improve Signal to Noise ratio (SN ratio), the ultrasonic imaging method using a frequency correlation was designed taking the feeble ultrasonic echoes from complex shape object into consideration. Finally, the shape of a test specimen, which imitates a fuel debris, was measured with the developed imaging system. The ultrasonic image of the specimen showed a good agreement with a photo. In conclusion, the imaging using air-coupled point-focused ultrasound was developed to successfully visualize a mock fuel debris from a relatively short distance of 40 mm – 100 mm.

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