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Kinjo, Katsuki*; Fujibayashi, Hiroki*; Matsumura, Hiroki*; Hori, Fumiya*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; et al.
Science Advances (Internet), 9(30), p.2736_1 - 2736_6, 2023/07
Times Cited Count：17 Percentile：91.30(Multidisciplinary Sciences)
continuous air monitor for the measurement of highly radioactive alpha-emitting particulates (
-aerosols) under high humidity environmentTsubota, Yoichi; Honda, Fumiya; Tokonami, Shinji*; Tamakuma, Yuki*; Nakagawa, Takahiro; Ikeda, Atsushi
Nuclear Instruments and Methods in Physics Research A, 1030, p.166475_1 - 166475_7, 2022/05
Times Cited Count：4 Percentile：54.12(Instruments & Instrumentation)In the long-lasting decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), the dismantling of nuclear fuel debris (NFD) remaining in the damaged reactors is an unavoidable but significant issue with many technical difficulties. The dismantling is presumed to involve mechanical cutting, generating significant concentrations of particulates containing -radionuclides (-aerosols) that pose significant health risk upon inhalation. In order to minimize the radiation exposure of workers with -aerosols during the dismantling/decommissioning process at 1F, it is essential to monitor the concentration of -aerosols at the point of initial generation, i.e. inside the primary containment vessels (PCV) of the damaged reactors. Toward this end, an 
monitoring system for -aerosols ( alpha air monitor: IAAM) was developed and its technical performance was investigated under the conditions expected for the actual environments at 1F. IAAM was confirmed to fulfill four technical requirements: (1) steady operation under high humidity, (2) operation without using filters, (3) capability of measuring a high counting rate of -radiation, and (4) selective measurement of -radiation even under high radiation background with 
/
-rays. IAAM is capable of selectively measuring -aerosols with a concentration of 3.3 
10
Bq/cm
or higher without saturation under a high humid environment (100%-relative humidity) and under high background with /-radiation (up to 100 mSv/h of -radiation). These results demonstrate promising potential of IAAM to be utilized as a reliable monitoring system for -aerosols during the dismantling of NFD, as well as the whole long-lasting decommissioning of 1F.
Honda, Fumiya*; Kinase, Sakae
no journal, ,
no abstracts in English
Kinase, Sakae; Honda, Fumiya*
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Kinase, Sakae; Honda, Fumiya*
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no abstracts in English
Honda, Fumiya*; Kinase, Sakae
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no abstracts in English
Tsubota, Yoichi; Honda, Fumiya; Nakagawa, Takahiro; Kawasaki, Takashi; Ikeda, Atsushi; Tamakuma, Yuki*; Tokonami, Shinji*; Momose, Takumaro
no journal, ,
A large number of radioactive aerosols, especially alpha particles, are expected to be generated during fuel debris retrieval on Fukushima - Daiichi (1F) site and decommissioning of nuclear fuel cycle facilities. For conventional alpha dust monitor, 1-channel of silicon surface barrier detector (SSBD) is often used as a detector which has not so high limit of count rates. Moreover, since radioactive aerosol collected on a filter paper are measured by a detector, it is difficult to calculate an airborne concentration at the sampling site because a conventional dust monitor measures an integrated radioactivity on a filter paper. In this work, we developed a prototype of alpha dust monitor aiming at an in-situ measurement of high concentration airborne alpha particles. The efficiency for U
O
checking source was 80.8%. For high count rate test using 3 MBq of 
Am checking source, 2.1 10
cpm was achieved which was more than 10 times higher compared to an conventional alpha dust monitor using SSBD. For follow-up test to the airborne concentration of alpha particles using radon chamber, counting rate of prototype alpha dust monitor was smoothly followed with respect to the radon concentration in a radon chamber.
Tsubota, Yoichi; Honda, Fumiya; Nakagawa, Takahiro; Kawasaki, Takashi; Tamakuma, Yuki*; Tokonami, Shinji*; Momose, Takumaro
no journal, ,
A large number of radioactive aerosols, especially alpha particles, are expected to be generated during fuel debris retrieval on Fukushima - Daiichi (1F) site and decommissioning of nuclear fuel cycle facilities. In the case of conventional alpha dust monitor, silicon surface barrier detector (SSBD) is often used as a detector which has not so high limit of count rates. Moreover, SSBD is not suitable for high humidity environment. In this work, we developed a prototype of alpha dust monitor aiming at an in-situ measurement of high concentration of airborne alpha particles. Approx. 20M cpm of counting rate was achieved which was more than 10 times higher compared to a conventional alpha dust monitor using SSBD. For follow-up test, counting rate of prototype alpha dust monitor was smoothly followed with respect to the airborne concentration of alpha particles.
Honda, Fumiya; Isozaki, Kohei; Nabatame, Satoru
no journal, ,
no abstracts in English
Okada, Toyofumi; Shibanuma, Tomohiro; Honda, Fumiya; Komeno, Akira; Kikuno, Hiroshi
no journal, ,
Am is generated by beta-decay of Pu. In the case of treating MOX, it is important for shielding about 60keV gamma-ray emitted by Am. Also getting dose rate data of MOX containing Am is valuable. In this work, we measured gamma-ray dose rate from MOX containing Am by changing thickness of shielding materials and range between MOX and measuring instruments. Also we confirmed that calculating analysis is useful for evaluation of shielding material's performance.
Honda, Fumiya; Isozaki, Kohei; Nabatame, Satoru
no journal, ,
no abstracts in English
Tsubota, Yoichi; Honda, Fumiya; Tokonami, Shinji*; Tamakuma, Yuki*; Nakagawa, Takahiro; Ikeda, Atsushi
no journal, ,
no abstracts in English
Tsuji, Tomoya; Kowatari, Munehiko*; Kataoka, Noriaki*; Tanimura, Yoshihiko; Honda, Fumiya
no journal, ,
no abstracts in English
dust monitor for high-dose environments based on machine learning of track informationTsubota, Yoichi; Laffolley, H.; Tsuji, Tomoya; Honda, Fumiya; Nakamura, Hiroki; Suzuki, Masahiro
no journal, ,
Remote monitoring of radioactive dust in high-dose environments, such as the Fukushima Daiichi Nuclear Power Station (1F), is essential for ensuring the safety of decommissioning work. JAEA has prototyped "DICAS-OS" as a remote dust measurement instrument. Premised on mounting onto a quadruped robot, the device was lightened (approx. 3 kg) and equipped with an dust monitor using a semiconductor pixel detector and auxiliary measurement devices (particle sensor, ultrasonic anemometer), enabling unified continuous remote monitoring of dust information. However, due to its shield-free design, high ambient dose rates result in measurement noise. To address this, we developed the machine learning system "DICAS-ML". Based on the characteristics of radiation track data captured by the semiconductor pixel detector, it uses the LightGBM algorithm to discriminate rays and subtracts the influence of ambient dose, making it possible to accurately evaluate the radioactivity of the dust. Through the combination of both systems, we successfully conducted demonstration measurements of airborne radioactive material concentration and wind conditions inside the 1F Unit 3 Reactor Building in January 2025, confirming their effectiveness.
Honda, Fumiya*; Kinase, Sakae
no journal, ,
Tsubota, Yoichi; Yoshida, Masato; Honda, Fumiya; Tokonami, Shinji*; Nakagawa, Takahiro; Kawasaki, Takashi
no journal, ,
In order to measure the airborne concentration in air in real time, a dust monitor to measure directly the alpha-particle concentration in a flat-type flow path was developed and its performance was evaluated. The performance of the prototype dust monitor was evaluated in a high-humidity environment, and its counting rate was more than ten times higher than that of a conventional dust monitor.
Yoshida, Masato; Kawasaki, Kohei; Aita, Takahiro; Tsubota, Yoichi; Kikuchi, Ryo*; Honda, Fumiya
no journal, ,
no abstracts in English
Honda, Fumiya*; Kinase, Sakae
no journal, ,
no abstracts in English
Honda, Fumiya; Tsubota, Yoichi; Tamakuma, Yuki*; Tokonami, Shinji*; Ikeda, Atsushi; Nakagawa, Takahiro
no journal, ,
A large number of radioactive aerosols, especially alpha particles, are expected to be generated during the fuel debris retrieval on the Fukushima Daiichi Nuclear Power Station. From the viewpoint of preventing contamination of the surrounding environment and the vicinity of the human-access area, it is important to measure the activity concentration of airborne radioactive substances inside the primary containment vessels (PCVs). In particular, it is necessary to monitor the concentration of particulates containing -nuclides (-aerosols), which have high effective dose coefficients upon inhalation. This presentation reports the development of an in-situ alpha air monitor (IAAM) for direct measurement of -aerosols by combining a flat-type flow path (FFP), an air heater, a ZnS scintillator, and a multi anode photomultiplier tube. The monitor should operate under high humidity with the maximum counting rate of approx. 2.110 cpm. To achieve the two requirements, the monitor was designed to keep the air sufficiently dry without overheating the detector, and to reduce the detection of coarse particles. This study also conducted a basic performance test using the developed IAAM with a modified FFP. As a result, we could keep the humidity of the air less than 30%RH by heating the inlet of the FFP to 80 
C. In addition, by placing the FFP in a vertical position and installing a bend at the air intake port, coarse particles were reduced approx. 1/2-1/3. These achievements enable the monitor to measure -aerosols more precisely in the viewpoint of internal exposure assessment.
