Development of the unmanned surface vehicle for radiation monitoring at seabed; Observation of temporal change of radiocesium concentration in the sea bottom sediment at coastal zone of Fukushima

Sanada, Yukihisa   ; Miyamoto, Kenji*; Ochi, Kotaro   ; Matsuzaki, Koji*; Ogawa, Toshihiro*; Senga, Yasuhiro*

Seven years passed since Fukushima Dai-ichi Nuclear Power Station accident which was caused large amount of radionuclide release to the sea. Elucidation of behavior mechanism of radiocesium in the seabed is required for restarting fishing industry. We developed radiation detection system using the unmanned surface vehicle for in-situ measurement of radiocesium concentration in seabed sediment. This system is able to automatically navigate to measurement point and obtain the radiation data on the bottom sediment. The detector was calibrated by comparing the actual sediment samples. The periodical measurement off-shore the Fukushima Prefecture was performed using developed this system. As these results, distribution of radiocesium concentration was changed due to oceanographic condition. However, radiocesium inventory was tendency to decrease according to radiocesium half-life in measurement area. This system is effective for elucidation of behavior mechanism of radiocesium because it can easily measure the radiocesium concentration in the bottom sediment.