Radioactivity estimation of radioactive hotspots using a Compton camera and derivation of dose rates in the surrounding environment

佐藤 優樹

Applied Radiation and Isotopes, 212, p.111421_1 - 111421_8, 2024/10

https://doi.org/10.1016/j.apradiso.2024.111421

At the Fukushima Daiichi Nuclear Power Station, radiation sources released in the accident were deposited on various equipment and building structures. During decommissioning, it is crucial to understand the distribution of radiation sources and ambient dose equivalent rates to reduce worker exposure and implement detailed work planning. In this study, the author introduces a method for visualizing radiation sources, estimates their radioactivity using a Compton camera, and derives the dose rate around the radiation sources. In the demonstration test, the Compton camera was used to visualize radioactive hotspots caused by Cs radiation sources deposited in the outdoor environment and estimated the radioactivity. Furthermore, the dose rate around the hotspots was calculated from the estimated radioactivity, which confirmed that the calculated dose rate correlated with the dose rate measured using a survey meter. This approach is novel, where a series of analyses were conducted using the Compton camera to visualize radioactive hotspots, estimate the radioactivity, and derive the dose rate in the surrounding environment.

Development of a radioactive substance detection system integrating a Compton camera and a LiDAR camera with a hexapod robot

佐藤 優樹; 角藤 壮*; 田中 孝幸*; 嶋野 寛之*; 諸橋 裕子; 畠山 知圭*; 中島 準作; 石山 正弘

Nuclear Instruments and Methods in Physics Research A, 1063, p.169300_1 - 169300_7, 2024/06

https://doi.org/10.1016/j.nima.2024.169300

A system for locating radioactive substances using a hexapod robot equipped with a Compton camera and light detection and ranging camera was developed, and its performance evaluation test was conducted at FUGEN, a nuclear facility owned by Japan Atomic Energy Agency. In the test, by projecting images of radioactive substances acquired with the Compton camera onto a three-dimensional model of the work environment acquired with a light detection and ranging camera, the locations where radioactive substances have accumulated and the dose rate is higher than the surrounding area were successfully visualized and identified.

Radioactivity estimation of multiple radiation sources using a Compton camera to investigate radioactively contaminated objects

佐藤 優樹

Applied Radiation and Isotopes, 203, p.111083_1 - 111083_9, 2024/01

https://doi.org/10.1016/j.apradiso.2023.111083

At the Fukushima Daiichi Nuclear Power Station (FDNPS), radioactive substances released during the accident were deposited on various equipment and building structures. During the decommissioning work, an investigation of the deposition of radioactive substances inside the contaminated equipment and structures can provide information on the cause and progression of the accident. This study introduces a quantitative evaluation method of radioactivity using a Compton camera, a type of gamma-ray imager, to investigate the deposition and contamination level of radioactive substances on contaminated objects at the FDNPS. Multiple Cs radiation sources with varying radioactivity were placed horizontally in one dimension within the field of view of the Compton camera, and a proof-of-principle study was conducted to evaluate the radioactivity of each source quantitatively.

Visualization software for radioactive contamination based on Compton camera: COMRIS

佐藤 優樹; 峯本 浩二郎*; 根本 誠*

Radiation Protection Dosimetry, 199(8-9), p.1021 - 1028, 2023/06

https://doi.org/10.1093/rpd/ncad106

Identifying and visualizing the radiation source location are important in reducing the exposure dose of workers at the decommissioning site of the Fukushima Daiichi Nuclear Power Station and in improving the radiation protection functions in other sites where radiation sources are handled. In this paper, we developed the COMpton camera for Radiation Imaging System (COMRIS) to identify and visualize the radiation source location in 3D using output data from a Compton camera and a simultaneous localization and mapping (SLAM) device as input data. Here, we presented COMRIS to visualize a Cs-radiation source in a dark environment using data acquired by a commercial Compton camera and a LiDAR-based SLAM device mounted on a robot as input data. The radiation source image obtained using the Compton camera was drawn on the 3D work environment model acquired by the SLAM device, and the radiation source location was visible in 3D.

ガンマ線画像スペクトル分光法による高放射線場環境の画像化による定量的放射能分布解析法(委託研究); 令和2年度英知を結集した原子力科学技術・人材育成推進事業

廃炉環境国際共同研究センター; 京都大学*

JAEA-Review 2022-027, 85 Pages, 2022/11

JAEA-Review-2022-027.pdf:5.72MB

日本原子力研究開発機構(JAEA)廃炉環境国際共同研究センター(CLADS)では、令和2年度英知を結集した原子力科学技術・人材育成推進事業(以下、「本事業」という)を実施している。本事業は、東京電力ホールディングス株式会社福島第一原子力発電所(1F)の廃炉等をはじめとした原子力分野の課題解決に貢献するため、国内外の英知を結集し、様々な分野の知見や経験を、従前の機関や分野の壁を越えて緊密に融合・連携させた基礎的・基盤的研究及び人材育成を推進することを目的としている。平成30年度の新規採択課題から実施主体を文部科学省からJAEAに移行することで、JAEAとアカデミアとの連携を強化し、廃炉に資する中長期的な研究開発・人材育成をより安定的かつ継続的に実施する体制を構築した。本研究は、研究課題のうち、平成30年度に採択された「ガンマ線画像スペクトル分光法による高放射線場環境の画像化による定量的放射能分布解析法」の平成30年度から令和3年度の研究成果について取りまとめたものである(令和3年度まで契約延長)。本課題は令和3年度が最終年度となるため4年度分の成果を取りまとめた。本研究では、ガンマ線イメージング装置であるETCCを1F廃炉事業での使用可能な可搬型装置でかつ高線量環境下で動作可能なように改良し、廃炉の現場に導入できるようにした。ETCCは世界初の光学カメラと同じ全単射撮像が可能なガンマ線カメラである。そのためETCCは一般的な定量的画像解析法を放射線にも適用可能とすることが可能で、1Fの廃炉に係る解決すべき6つの重点課題に革新的進展をもたらす。例えば廃炉工事で発生する汚染の飛散を3次元ガンマ線イメージングによりオンラインで検知、さらに拡散の動的な検知から精度の高い拡散予想を可能にする。それを実証するため令和元年に、1F敷地内1㎞四方を一度に画像モニタリングし、同時に100か所以上のスペクトル観測を実現し、今まで不明だったスカイシャインガンマ線スペクトル及び分布の計測に成功した。さらに京都大学複合原子力科学研究所原子炉建屋内の3次元線量計測を実施、微量なAr41の大気拡散を3次元の動画撮像に成功。廃炉を超え一般の原子炉での高精度な放射線3次元画像モニタリング及び放射能拡散予想システムが実現できることを示す画期的な成果が得られた。

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

佐藤 優樹; 寺阪 祐太

Journal of Nuclear Science and Technology, 59(6), p.677 - 687, 2022/06

https://doi.org/10.1080/00223131.2021.2001391

The Fukushima Daiichi Nuclear Power Station (FDNPS) went into meltdown after being hit by a large tsunami caused by the Great East Japan Earthquake on March 11, 2011. Measuring and understanding the distribution of radioactive contamination inside the FDNPS is essential for decommissioning work, reducing exposure to workers, and ensuring decontamination. This paper reports the visualization tests of radioactive contamination in the Unit 1/2 exhaust stack of the FDNPS using a compact Compton camera. Fixed-point measurements were conducted using only a Compton camera and moving measurements using an integrated radiation imaging system (iRIS) that combines a Compton camera with a simultaneous localization and mapping device. For the moving measurements, an operator carrying the iRIS acquires data continuously while walking in a passage near the stack. With both types of measurements, high-intensity contamination was detected at the base of the stack, and detailed three-dimensional (3D) visualization of the contamination was obtained from the moving measurement. The fixed-point measurements estimated the source intensity of the contamination from the reconstructed contamination image acquired by the Compton camera. Furthermore, workers can experience the work environment before actual work by importing a 3D structure model into a virtual reality system displaying the contamination image.

A Concept of mirror world for radioactive working environment by interactive fusion of radiation measurement in real space and radiation visualization in virtual space

佐藤 優樹

Physics Open (Internet), 7, p.100070_1 - 100070_8, 2021/05

https://doi.org/10.1016/j.physo.2021.100070

To understand radiation information, such as dose rate and the position of radioactive substances in a radioactive working environment in detail, the author proposes the construction of a mirror world of the environment. In the proposed mirror world, the work environment is reproduced in a virtual space, and the radiation information measured in a real space is projected onto the virtual space. Note that in addition to displaying the radiation information in the virtual space, the visualization result of the radiation information in the virtual space is used for decision-making in the real space. It is seen that the radiation measurement in the real space and visualization of measurement results in the virtual space always interact. In this report, the author introduces an example of building the mirror world based on radiation measurements performed in a laboratory using Cs as a radiation source. In the laboratory, the dose rate was measured by using a survey meter together with a device for the simultaneous localization and mapping based on three-dimensional (3D) light detection and ranging. The measured dose rate was mapped onto the work environment in the virtual space. Technologies developed by the author, for example, the 3D visualization of the radiation source based on an integration of the Compton camera and structure-from-motion technology and the virtual-reality experience technology of the work environment that displays the source image were also used. The technology to project the radiation-source image into the real space using augmented-reality is also introduced in this report.

Automatic data acquisition for visualizing radioactive substances by combining a gamma-ray imager and an autonomous mobile robot

佐藤 優樹; 峯本 浩二郎*; 根本 誠*; 鳥居 建男

Journal of Instrumentation (Internet), 16(1), p.P01020_1 - P01020_18, 2021/01

https://doi.org/10.1088/1748-0221/16/01/P01020

To reduce the exposure doses of workers and to establish decontamination plans, it is important to understand and visualize the distribution of radioactive substances at the Fukushima Daiichi Nuclear Power Station in Japan, where an accident occurred on the 11th of March, 2011. In this decommissioning work environment, radioactive substances adhered to various objects, such as rubble and equipment, and it was necessary to visualize the distribution of these contaminants in all three dimensions. The technology used to automatically and remotely acquire data to visualize the distribution of radioactive substances in three dimensions was useful for reducing the exposure dose of the workers and to shorten the survey time. We constructed an automatic data acquisition system that consisted of a Compton camera and a 3D-light detection and ranging sensor mounted on an autonomously moving robot. We also evaluated the system feasibility using radiation sources and succeeded in automatically acquiring the data required for visualizing the radiation sources. For this data acquisition, the operator did not need to operate the system after the measurements were started. The effects of the imaging parameters of the Compton camera and the accuracy of the self-position estimation of the system on the radiation-imaging accuracy are also discussed.

Visualization of radioactive substances using freely moving gamma-ray imager based on Structure from Motion

佐藤 優樹; 鳥居 建男

Proceedings of International Youth Nuclear Congress 2020 (IYNC 2020) (Internet), 4 Pages, 2020/05

The Fukushima Daiichi Nuclear Power Station (FDNPS), operated by Tokyo Electric Power Company Holdings, Inc., suffered a meltdown as a result of a large tsunami triggered by the Great East Japan Earthquake on March 11, 2011. We have been conducting demonstration tests for detection of radioactive hotspots inside the FDNPS buildings using a Compton camera, a kind of gamma-ray imager. In this work, we introduced the 3D visualization of radioactive substances by using combination of the Compton camera and optical camera based on Structure from Motion (SfM). By taking photographs of the experimental environment while freely moving, a 3D structural model of the environment can be reconstructed from the multiple photographs, and the movement trajectory of the optical camera can be estimated simultaneously using the SfM. Furthermore, the radioactive substances can be visualized by drawing an image of the radioactive substances on the 3D structural model using gamma-ray data acquired by the Compton camera. In the demonstration, we succeeded in visualizing a Cs-radiation source on the 3D structural model of the experimental environment while freely moving these devices. This technology is useful for making it easy to recognize radioactive substances in decommissioning work site such as the FDNPS.

Radiation imaging using a compact Compton camera mounted on a crawler robot inside reactor buildings of Fukushima Daiichi Nuclear Power Station

佐藤 優樹; 寺阪 祐太; 宇津木 弥*; 菊地 弘幸*; 清岡 英男*; 鳥居 建男

Journal of Nuclear Science and Technology, 56(9-10), p.801 - 808, 2019/09

https://doi.org/10.1080/00223131.2019.1581111

The Fukushima Daiichi Nuclear Power Station (FDNPS), operated by Tokyo Electric Power Company Holdings, Inc., went into meltdown in the aftermath of a large tsunami caused by the Great East Japan Earthquake of 11 March 2011. Measurement of radiation distribution inside the FDNPS buildings is indispensable to execute decommissioning tasks in the reactor buildings. We conducted a radiation imaging experiment inside the reactor building of Unit 1 of FDNPS by using a compact Compton camera mounted on a crawler robot and remotely visualized gamma-rays streaming from deep inside the reactor building. Moreover, we drew a radiation image obtained using the Compton camera onto the three-dimensional (3-D) structural model of the experimental environment created using photogrammetry. In addition, the 3-D model of the real working environment, including the radiation image, was imported into the virtual space of the virtual reality system. These visualization techniques help workers recognize radioactive contamination easily and decrease their own exposure to radiation because the contamination cannot be observed with the naked eye.