Detailed visualization of radioactive hotspots inside the unit 1 reactor building of the Fukushima Daiichi Nuclear Power Station using an integrated Radiation Imaging System mounted on a Mecanum wheel robot

Figures & data

Figure 1. Front and back view of the setup used for radioactive hotspot investigation. The basic elements of iRIS, namely a Compton camera, 3D-LiDAR-based SLAM device, and survey meter, were mounted on a Mecanum wheel robot. The robot carried an overhead camera (for the operator to control the robot while viewing the video) and LED light to illuminate in the dark. All devices installed on the robot were covered with plastic bags to prevent radioactive contamination.

Table 1. Partial specifications of the Mecanum wheel robot used in the hotspot investigation.

Equipping functionality	Specification
Dimension	490 mm (W) × 642 mm (D) × 315 mm (H)
Weight	Approximately 60 kg (Excluding the battery)
Maximum speed	1.0 m/s (No load)
Maximum payload	80 kg
Loadable area	W308 mm × D520 mm
Wheel	Mecanum type Ø200
Drive system	4WD
Height of steps that can be overcome	40 mm (No load)
Battery	Lipo6s DC 22.2 V 8,000 mAh
Remote control	1. Operated from PC via Wi-Fi 2. Gamepad control via Bluetooth

Figure 2. Schematic of the top view of the investigation area inside Unit 1 R/B of FDNPS. The robot system travelled from the carry-in entrance of the R/B toward the front of the TIP room at the farthest point. The robot and Compton camera were operated wirelessly and remotely using Wi-Fi from a base station located in the carry-in entrance.

Figure 3. Dose-rate mapping results along the robot’s movement trajectory acquired by SLAM. (a) the robot system moved from the carry-in entrance toward the front of the TIP room, as shown in Figure 2. Panels (b) and (c) show the vicinity of the DHC system; observations were recorded near the DHC system in the direction of the arrows shown in the enlarged view in panel (a). Panel (d) shows the passageway in front of the TIP room indicated by the arrow in panel (a).

Figure 4. 2D imaging results obtained by the Compton camera near the U-shaped piping installed on the right side of the DHC system. The top panel shows a 3D model of the measurement area viewed from above, with each arrow indicating the approximate locations and measurement directions of the Compton camera. For each view, photographs acquired with the fisheye optical camera attached to the Compton camera (left) and the imaging results of the superimposed hotspot images (right) are shown.

Figure 5. Imaging results of the U-shaped piping on the right side of the DHC acquired with the Compton camera from a greater distance than in Figure 4.

Figure 6. 2D imaging results obtained by the Compton camera near the U-shaped piping installed on the left side of the DHC. The top panel shows a 3D model of the measurement area viewed from above, with each arrow indicating the approximate locations and measurement directions of the Compton camera. For each view, photographs acquired with the fisheye optical camera attached to the Compton camera (left) and the imaging results of the superimposed hotspot images (right) are shown.

Figure 7. 2D imaging results obtained by the Compton camera near the atmospheric control piping installed in the ceiling in front of the TIP room. The top panel shows a 3D model of the measurement area viewed from above, with each arrow indicating the approximate locations and measurement directions of the Compton camera. The artifact marked on the top panel is a point cloud generated by a robot separately used to monitor the Mecanum wheel robot equipped with the Compton camera.

Figure 8. The energy spectrum of the coincidence event between the scatterer and absorber of the Compton camera. This spectrum was acquired at view D in Figure 7.

Figure 9. 2D imaging results of the AC piping near the wall in front of the TIP room obtained with the Compton camera. The ceiling AC piping bends at the wall and extends perpendicular to the floor surface.

Figure 10. Schematic illustration of locating and visualizing a radiation source in 3D by combining a Compton camera and simultaneous localization and mapping (SLAM) device. These two devices are mounted on a robot, which can be operated remotely. The position and posture where the Compton camera detected gamma rays were estimated using the SLAM device. Generated Compton cones from multiple viewpoints are projected onto the 3D work environment model generated by SLAM, and the hotspot is identified at the intersection of the Compton cones. This figure is a modified version of Figure 3 from ref [9], which describes the principle of the iRIS.

Figure 11. Measurements were performed with the Compton camera across the AC piping in the ceiling, and (a) the reconstructed image of the hotspot was compared with (b) the dose-rate data acquired by a survey meter. Panel (b) is a rescaled figure of Figure 3(d). The artifact marked on the panel (a) is a point-cloud shape caused by a robot separately used to monitor the Mecanum wheel robot equipped with the Compton camera.

Figure A1. Photograph of the shielded gamma-ray sensor of the Compton camera used for hotspot detection inside Unit 1 R/B of FDNPS. This sensor setup is the same as that used to analyze the radioactive contaminated filter train inside the air conditioning room of Unit 2 R/B; this figure is a reprinted version of Figure A1 from ref [10].

Figure A2. Relationship between the incidence angle of 662-keV gamma rays into the gamma-ray sensor and the number of coincidence events detected. The number of detections is normalized with 1.0 for the 0° direction. The irradiation dose rate was 3 mGy/h at the ¹³⁷Cs irradiation facility. This figure is reproduced from ref [10].

Table A1. Performance of the gamma-ray sensor setup of the Compton camera used for hotspot investigation. This table is reproduced from ref [10].

	Performance	Notes
Detection efficiency for coincidence events	(1.1 ± 0.3) ×10 ^− 3%	The ^137Cs source was placed in the 0° frontal direction of the Compton camera’s FoV, and the irradiation dose rate was 3 mGy/h at the ^137Cs irradiation facility.
Angular resolution	14° (FWHM)	The distribution of reconstructed image in the measurement of the ^137Cs source. The ^137Cs source was placed in the 0° frontal direction of the Compton camera’s FoV, and the irradiation dose rate was 3 mGy/h at the ^137Cs irradiation facility.
Energy resolution	16% (FWHM)	Peak width of the 662 keV gamma ray in the coincidence spectrum. The ^137Cs source was placed in the 0° frontal direction of the Compton camera’s FoV, and the irradiation dose rate was 3 mGy/h at the ^137Cs irradiation facility.

Sato Y, Terasaka Y. Radiation imaging using an integrated Radiation Imaging System based on a compact Compton camera under unit 1/2 exhaust stack of Fukushima Daiichi Nuclear Power Station. J Nucl Sci Technol. 2022;59(6):677–687. doi: 10.1080/00223131.2021.2001391

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Sato Y, Terasaka Y. Radiation imaging of a highly contaminated filter train inside Fukushima Daiichi Nuclear Power Station unit 2 using an integrated Radiation Imaging System based on a Compton camera. J Nucl Sci Technol. 2023;60(8):1013–1026. doi: 10.1080/00223131.2022.2159894

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