Development of USV Used in Underground Floors Surveying of the Contaminated Buildings at Fukushima Daiichi NPS

A vast amount of radioactive water was generated as a result of the nuclear incident at the Fukushima Daiichi Nuclear Power Station. Underground floors of the radioactive waste management facility were utilized as a temporary storage for contaminated water, and hundreds of sandbags containing zeolite particles were placed onto the floors to remove radioactivity from the water. As the decommissioning work has been ongoing for over a decade now, all zeolite sandbags must be removed, and the floors dried up. A survey was conducted in this highly irradiated environment to investigate the unknown recent conditions of the zeolite and floor structures. A remotely operated USV (Unmanned Surface Vehicle) named “ROV-boat”, modified from a reliable commercial off-the-shelf ROV (Remotely Operated Vehicle), was developed for this purpose. The ROV-boat was equipped with two cameras and a dosimeter installed on the body, with an external float for increased buoyancy, allowing it to move on the water surface to conduct a detailed survey of the environment. The prototype was evaluated and improved iteratively following an agile development approach. The ROV-boat was successfully developed by TEPCO (Tokyo Electric Power Company) several times in the summer of 2021, attaining valuable data for the future dry-up process.