連携計測による線源探査ロボットシステムの開発研究(委託研究); 令和5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; 東北大学*

JAEA-Review 2025-011, 74 Pages, 2025/08

JAEA-Review-2025-011.pdf:5.31MB

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、英知を結集した原子力科学技術・人材育成推進事業を実施している。本事業は、東京電力ホールディングス福島第一原子力発電所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。本研究は、令和3年度に採択された研究課題のうち、「連携計測による線源探査ロボットシステムの開発研究」の令和3年度から令和5年度分の研究成果について取りまとめたものである。本研究は、ガンマ線の飛来方向を検出可能な検出器を開発し、これを搭載した複数のロボットの連携により、単一センサーでは得られない広視野・迅速・安価な放射線源探査を実現するロボットシステム(Cooperative Operation Robot system for RAdiation Source Exploration: CORRASE、コラッセ)を開発することを目的とする。令和5年度は、これまでの研究で得られた成果を結集させて線源探査に関する実証試験を中心に研究を実施した。ガンマ線の飛来方向を検出可能な指向性検出器として、8個のBGOシンチレータと遮蔽体を用いた多面体型指向性検出器を製作した。この検出器とIMU(Inertial Measurement Unit)、LiDAR(Light Detection And Ranging)を多脚ロボットに搭載したシステムを3組構築し、連携計測による線源探査実験を行った。7.85.3mの部屋に障害物及び模擬放射能汚染源として10MBqのCs密封線源を配置して線源探査実験のための未知環境とした。本システムを用いて環境地図の作成、探索計画の立案、放射線カウントのヒートマップ作成、計算された最適観測配置からの線源イメージングを行った。得られた環境地図と線源イメージを融合させて表示することに成功し、3台のロボットシステムの連携により模擬放射能汚染源の位置の特定に成功した。以上の成果から、本研究では広視野・迅速・安価な放射線源探査を実現するロボットシステムの開発に成功し、当初の目標は達成されたと結論できる。

Feasibility study of reactor radiation photon spectroscopy in Fugen for nuclear decommissioning

冠城 雅晃; 宮本 勇太; 森 教匡; 岩井 紘基; 手塚 将志; 黒澤 俊介*; 田川 明広; 高崎 浩司

Journal of Nuclear Science and Technology, 62(3), p.308 - 316, 2025/03

https://doi.org/10.1080/00223131.2024.2417786

Nuclear decommissioning has recently accelerated, particularly following the accident at Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Holdings. -ray/X-ray (radiation photon) spectroscopy provides information on the types of radionuclides with radiation photon emissions. Radiation photon spectroscopy in a control rod guide tube positioned at the center of Fugen was conducted. Fugen is a prototype advanced thermal reactor with 165 MWe electric power generation that is being decommissioned. The dose rates measured in a control rod guide tube positioned at the center of the reactor were 4.1 - 9.1 Gy/h. The dose rate considerably increased at a position close to a tank that contained Co caused by the radioactivation of stainless steel. Radiation photon spectroscopy was performed without radiation shielding, identifying Co with an energy resolution better than 5.4% at 1333 keV and Nb with an energy resolution better than 5.9% at 871 keV.

Mobile radiation measurement system by multiple small gamma-ray detectors for radioisotope detection and identification supporting responders in the field of nuclear detection and nuclear security

木村 祥紀; 山口 知輝

Progress in Nuclear Science and Technology (Internet), 7, p.60 - 66, 2025/03

https://doi.org/10.15669/pnst.7.60

Rapid and precise detection or identification of artificial radioisotopes is one of the challenging issues in the field of nuclear detection and nuclear security. Although many handheld instruments capable of automated radioisotope detection and identification using small gamma-ray detectors have been recently employed in the field of nuclear security, the performance of such instruments is suffered from the limitation on cost of their detectors in many cases due to limited efficiency or limited energy resolution of the small size detectors. In this paper, a mobile radiation measurement system using multiple small gamma-ray detectors for radioisotope detection and identification has been proposed. The performance of the proposed mobile system for radioisotope identification has been tested for measured gamma-ray spectra of artificial radioisotopes and nuclear materials by CZT detector and CsI(Tl) detector. By analyzing the combined spectrum to be output by the proposed system, significant improvements in the minimum detection time for peaks from radioisotopes and lower detection limits for nuclear materials have been confirmed, comparing to the spectra measured by individual detectors. The authors have also discussed the optimization of detector combinations in terms of detector cost and improvement of sensitivity performance by performing simulations for several types of gamma-ray detectors.

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.

A Segmented Total Energy Detector (sTED) optimized for (n,) cross-section measurements at n_TOF EAR2

Alcayne, V.*; 木村 敦; 他134名*

Radiation Physics and Chemistry, 217, p.111525_1 - 111525_11, 2024/04

https://doi.org/10.1016/j.radphyschem.2024.111525

The neutron time-of-flight facility n_TOF at CERN is a spallation source dedicated to measurements of neutron induced reaction cross-sections of interest in nuclear technologies, astrophysics, and other applications. Since 2014, Experimental ARea 2 (EAR2) is operational and delivers a neutron fluence of neutrons per nominal proton pulse, which is 50 times higher than the one of the other experimental area. The high neutron flux at EAR2 results in high counting rates in the detectors that challenged the previously existing capture detection systems. For this reason, a Segmented Total Energy Detector (sTED) has been developed to overcome the limitations in the detector's response, by reducing the active volume and by using a photo-multiplier (PMT) optimized for high counting rates. This paper presents the main characteristics of the sTED, including energy and time resolution, response to -rays, and provides details for the Pulse Height Weighting Technique (PHWT). The sTED has been validated to perform neutron-capture cross-section measurements in EAR2 in the neutron energy range from thermal up to at least 400 keV.

Measurement of the prompt fission -rays from slow neutron-induced fission of U with STEFF

Wright, T.*; 原田 秀郎; 木村 敦; 他121名*

European Physical Journal A, 60(3), p.70_1 - 70_11, 2024/03

https://doi.org/10.1140/epja/s10050-024-01277-8

The average energy and multiplicity of prompt -rays from slow neutron-induced fission of U have been measured using the STEFF spectrometer at the neutron time-of-flight facility n_TOF. The individual responses from 11 NaI scintillators were corrected for multiple -ray interactions, prompt fission neutrons and background counts before being deconvolved to estimate the emitted spectrum of prompt fission -rays. The results give an average -ray energy of 1.71(5) MeV and multiplicity of 2.66(18) considering -rays emitted within the energy range from 0.8 to 6.8 MeV. The n_TOF data has a slightly larger and smaller than other recent measurements, however the product of the two is in agreement within the quoted uncertainties.

Development of a DDA+PGA-combined non-destructive active interrogation system in "Active-N"

古高 和禎; 大図 章; 藤 暢輔

Nuclear Engineering and Technology, 55(11), p.4002 - 4018, 2023/11

https://doi.org/10.1016/j.net.2023.07.015

An integrated neutron interrogation system has been developed for non-destructive assay of highly radioactive special nuclear materials, to accumulate knowledge of the method through developing and using it. The system combines a differential die-away (DDA) measurement system for the quantification of nuclear materials and a prompt gamma-ray analysis (PGA) system for the detection of neutron poisons which disturb the DDA measurements; a common D-T neutron generator is used. A special care has been taken for the selection of materials to reduce the background gamma rays produced by the interrogation neutrons. A series of measurements were performed to test the basic performance of the system. The results show that the DDA system can quantify plutonium of as small as 20~mg and it is not affected by intense neutron background up to 4.2~TBq and gamma ray of 2.2~TBq. As a result of the designing of the combined system as a whole, the gamma-ray background counting rate at the PGA detector was reduced down to s even with the use of the D-T neutron generator. The test measurements show that the PGA system is capable of detecting less than 1~g of boron compound and about 100~g of gadolinium compound in~30 min. This research was implemented under the subsidy for nuclear security promotion of MEXT.

Identification of depth location of a radiation source by measurement from only one direction using a Compton camera

佐藤 優樹

Applied Radiation and Isotopes, 195, p.110739_1 - 110739_9, 2023/05

https://doi.org/10.1016/j.apradiso.2023.110739

In the decommissioning work environment of a nuclear power plant, it is essential to accurately identify the location of radiation sources to reduce worker doses and develop decontamination plans. In this regard, gamma-ray imagers are promising devices in the visualization of the location of radiation sources. However, they can only estimate the direction in which the radiation source is located and not the distance to the source. A previous study has reported a method to identify the three-dimensional coordinates of the radiation source by measuring from multiple viewpoints with a gamma-ray imager. In some cases, the many pieces of object located at the actual decommissioning site limit the accessible area, making it difficult to measure the target area from multiple viewpoints. Thus, the present study devised and demonstrated a method of estimating the distance to the radiation source by measuring from only one direction using a Compton camera, a type of gamma-ray imager. The proposed method considered the reconstructed image intensity to be proportional to the inverse square of the distance to radiation source. This approach will be useful in accurately locating radiation sources in environments with limited workable areas, such as the broken reactor buildings inside the Fukushima Daiichi Nuclear Power Station.

Advances and new ideas for neutron-capture astrophysics experiments at CERN n_TOF

Domingo-Pardo, C.*; 木村 敦; 他135名*

European Physical Journal A, 59(1), p.8_1 - 8_11, 2023/01

https://doi.org/10.1140/epja/s10050-022-00876-7

This article presents a few selected developments and future ideas related to the measurement of (n, ) data of astrophysical interest at CERN n_TOF. The MC-aided analysis methodology for the use of low-efficiency radiation detectors in time-of-flight neutron-capture measurements is discussed, with particular emphasis on the systematic accuracy. Several recent instrumental advances are also presented, such as the development of total-energy detectors with -ray imaging capability for background suppression, and the development of an array of small-volume organic scintilla tors aimed at exploiting the high instantaneous neutron-flux of EAR2.

Accurate estimation of spectral density of LCS gamma-ray source

Omer, M.; 静間 俊行*; 羽島 良一*; 小泉 光生

第43回日本核物質管理学会年次大会会議論文集(インターネット), 3 Pages, 2022/11

Gamma-rays originated from laser Compton scattering (LCS) are convenient photon sources for nondestructive interrogation of nuclear materials. LCS can be used with nuclear resonance fluorescence (NRF) and X-ray fluorescence (XRF), the two of which are considered photon-based active interrogation techniques. However, an accurate estimation of the incident LCS -ray flux is crucial. The -ray flux is customarily measured using high purity germanium (HPGe) detectors, usually calibrated using standard point-like radioactive -ray sources. These standard sources are entirely different from LCS beams in terms of detection geometry. Therefore, the calibration process must be corrected to meet the LCS beam conditions. Here, we demonstrate how to implement the required corrections and provide experimental validation of these corrections.