At the Fukushima Daiichi Nuclear Power Plant (NPP) in Japan, investigations of NPP conditions are conducted before retrieval of fuel debris to achieve the decommissioning of NPP. In this study, we focused on the ultrasound measurement technique for the investigation, since ultrasound techniques can overcome environmental issues such as a cloudy, dark and highly radiation dose environment. However, ultrasound is strongly attenuated in the air even if the distance is short. We attempted to apply

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... arametric sound source technology, which is expected to make the measurable distance longer for this problem. Parametric sound source is characterized by higher directivity and lower attenuation. Therefore, it is expected that shape measurement using a parametric sound source can measure long distances. In this paper, as the fundamental study on ultrasound sensing for measurement application inside of a nuclear reactor using this technique, we investigated the measurable length and conducted shape measurement for the simplified shape sample of the simulated fuel debris. In addition, we tried to measure the sample under mist environment, which is difficult to visualize using optical measurements such as camera. Moreover, we developed the small ultrasonic measurement device for carrying by robot transportation system.