Abstract
To obtain the necessary information on fuel debris and water leakages during the decommissioning task of the Fukushima Daiichi Nuclear Power Plant, an ultrasonic-based method was proposed for future internal investigation of the primary containment vessel (PCV). In this article, we describe the rotatable winch mechanism and visual localization method, which were used to aid the investigation. We used the rotatable winch mechanism to adjust the height and orientation of the ultrasonic sensor and localized the robot with cameras to localize the sensor, to provide assisting information for data combination. We studied the feasibility of the conventional visual odometry method for application to the situation and performed localizing accuracy evaluation experiments with a mobile robotic platform prototype. The results showed that the visual odometry method could generate intuitive bird’s-eye-view maps, and provided an average error rate of 35 mm/1500 mm, which met the required maximum error rate of 100 mm/1500 mm for the grating movement. Experiments were also conducted with adjustable parameter ranges that could provide the required accuracy.
Highlights
The decommissioning of the Fukushima Daiichi Nuclear Power Plant (NPP) is an urgent national problem in Japan
We propose a new robotic system that uses ultrasonic sensors to detect fuel debris and water leakages, so that the robustness of camera-based underwater inspection is improved
We focused on the feasibility and localizing accuracy of a modified visual odometry method for the autonomous localization of a robot moving on the grating floor, for deployment in future investigations
Summary
The decommissioning of the Fukushima Daiichi Nuclear Power Plant (NPP) is an urgent national problem in Japan. The emergency cooling process of the reactor core failed due to loss of power, seriously damaging the cores of reactors No 1, 2, and 3. 200 tons of fallen nuclear fuel debris from the damaged cores, which contain melted nuclear fuels and structural materials, are estimated to remain in each nuclear reactor. Water leakages within the damaged containment container caused outflow of polluted water, which hindered. This article proposes a new robotic system that uses ultrasonic sensors to detect fuel debris and water leakage outside the pedestal in the PCV. The mobile robot moves on the grating floor, which is located above the pedestal. The winch mechanism on the mobile robot deploys the ultrasonic sensor through the grating lattice.
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