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Journal Articles

Assessment of ambient dose equivalent rate distribution patterns in a forested-rugged terrain using field-measured and modeled dose equivalent rates

Yasumiishi, Misa*; Masoudi, P.*; Nishimura, Taku*; Ochi, Kotaro; Ye, X.*; Aldstadt, J.*; Komissarov, M.*

Radiation Measurements, 168, p.106978_1 - 106978_16, 2023/11

In this study, we surveyed air dose rates using hand-held and backpack-type scintillators in a forest of deciduous and evergreen trees in Fukushima Prefecture, Japan. The effects of topographic features on air dose rates were examined using multivariate adaptive regression splines (MARS) against five selected topographic parameters. The air dose rates were distributed unevenly in the forest, and air dose rates varied by more than 1$$mu$$Sv/h as a function of time, likely owing to ground wetness etc. The effect of different topographic parameters varied between survey dates. The MARS model predictions with all topographic parameters yielded an R$$^{2}$$ of 0.54 or higher. To discuss whether the effect of topography on air dose rates and soil contamination levels is consistent, air dose rates measured in the field were compared with those estimated from the depth profile of radiocesium in soil. Most air dose rates estimated from the soil samples were in the range of field measurements.

Journal Articles

Compton camera measurements of radiation distribution images at Hamadori, Fukushima Prefecture

Kawahara, Rika*; Ochi, Kotaro; Yamaguchi, Katsuhiko*; Torii, Tatsuo*

Hoshasen (Internet), 48(2), p.43 - 48, 2023/04

The accident at the Fukushima Daiichi NPP resulted in a large amount of radioactive material being dispersed into the surrounding environment. It is important to understand the radiation distribution in the area, which is still located in the difficult-to-return zone. In this study, a Compton camera and an optical camera were combined to produce a three-dimensional distribution map of source locations and their intensities. Two locations in the difficult-to-return zone (Okuma town) were surveyed: a park parking lot and the area around a private house. The distribution of air dose rates was determined by measurements with a survey meter with a wand-type GPS (Gamma Plotter H manufactured by Nippon Radiation Engineering Co. Measurements were taken with a Compton camera (H420, H3D, USA) at several different positions and angles towards the hotspots. The software (Application Programming Interface Example) allowed real-time confirmation of the radiation incident information (incident time, energy, x, y, z) obtained by the Compton camera. By photographing hotspots in high-dose areas from different positions and angles, the location of the radiation sources and their spatial extent could be confirmed.

Journal Articles

Validation study of ambient dose equivalent conversion coefficients for radiocaesium distributed in the ground; Lessons from the Fukushima Daiichi Nuclear Power Station accident

Ochi, Kotaro; Funaki, Hironori; Yoshimura, Kazuya; Iimoto, Takeshi*; Matsuda, Norihiro; Sanada, Yukihisa

Radiation and Environmental Biophysics, 61(1), p.147 - 159, 2022/03

 Times Cited Count:1 Percentile:49.94(Biology)

Journal Articles

Joint environmental radiation survey by JAEA and KAERI around the Fukushima Daiichi Nuclear Power Plant; Performance of mobile gamma-ray spectrometry using backpack and carborne survey platforms

Ji, Y.-Y.*; Ochi, Kotaro; Hong, S. B.*; Nakama, Shigeo; Sanada, Yukihisa; Mikami, Satoshi

Health Physics, 121(6), p.613 - 620, 2021/12

According to the implementing arrangement between JAEA (Japan Atomic Energy Agency) and KAERI (Korea Atomic Energy Research Institute) in the field of the radiation protection and environmental radiation monitoring, the joint measurement has been conducted to assess the radioactive cesium deposition in the ground around the Fukushima Daiichi Nuclear Power Plants (FDNPP). First, mobile gamma-ray spectrometry using backpack survey platform was conducted to assess the distribution of dose rate around specific three survey sites. The carborne survey using gamma-ray spectrometers, as loading inside a vehicle, was successfully conducted to compare measured dose rates in routes from site to site and verify evaluation methods including the attenuation correction.

Journal Articles

${it In situ}$ spectrometry of terrestrial gamma rays using portable germanium detectors in area of 80 km radius around the Fukushima Daiichi Nuclear Power Plant

Mikami, Satoshi; Tanaka, Hiroyuki*; Okuda, Naotoshi*; Sakamoto, Ryuichi*; Ochi, Kotaro; Uno, Kiichiro*; Matsuda, Norihiro; Saito, Kimiaki

Nihon Genshiryoku Gakkai Wabun Rombunshi, 20(4), p.159 - 178, 2021/12

In order to know the background radiation level where the area affected by the Fukushima Daiichi Nuclear Power Plant accident in 2011, terrestrial gamma rays had been measured by using portable germanium detectors repeatedly from 2013 through 2019, at 370 locations within 80 km radius area centered on the Fukushima Daiichi Nuclear Power Plant. Radioactive concentrations of Uranium 238, Thorium 232, Potassium 40 and kerma rates in air due to terrestrial gamma rays were obtained at those locations based on the method of ICRU report 53. Averaged concentrations of $$^{238}$$U, $$^{232}$$Th and $$^{40}$$K were 18.8, 22.7, 428 Bq/kg, respectively, and kerma rate in air over the area was found to be 0.0402 $$mu$$Gy/h. The obtained kerma rates in air were compared to those reported in literatures. It was confirmed that the data were correlated with each other, and were agreed within the range of their uncertainty. This is because the kerma rate in air due to terrestrial gamma rays is depend on geology. The similar trend to previous findings was observed that the kerma rate in air at locations geologically classified as Mesozoic era, Granite and Rhyolite were statistically significantly higher than the others.

Journal Articles

The Effect and effectiveness of decontaminating a pond in a residential area of Fukushima

Katengeza, E. W.*; Ochi, Kotaro; Sanada, Yukihisa; Iimoto, Takeshi*; Yoshinaga, Shinji*

Health Physics, 121(1), p.48 - 57, 2021/07

 Times Cited Count:1 Percentile:16.97(Environmental Sciences)

Special reconstruction and revitalization bases were designated in Fukushima's difficult-to-return zones by the Japanese government as targets of intensive decontamination to drastically lower air dose rates and enable residents to return. A pond amidst residences of one of these bases was targeted for decontamination and this study aimed at evaluating the effect and effectiveness of the decontamination by decontamination factor, air dose rate reduction factor, and the additional annual effective dose to residents. Air dose rates were measured in-situ with KURAMMA-II while soil core samples were collected and measured for radioactivity in the laboratory by gamma spectrometry. Lower decontamination factors were observed for more deeply distributed radiocesium soil profiles whereas areas covered with gravel demonstrated the largest reduction in air dose rates. Decontamination effectively lowered the radiocesium inventory and air dose rates by 51% and 37-91% respectively. Moreover, the additional annual effective dose to the public changed from 1.7$$pm$$0.79 mSv to 1.2$$pm$$0.57 mSv because of decontamination representing a dose aversion of 29%. These findings demonstrate how decontaminating ponds in residential areas can help to further lower the external exposure.

Journal Articles

Fixed point observations and characterization of radioactive caesium in Tama river

Hagiwara, Kenta*; Ochi, Kotaro; Koike, Yuya*

Radioisotopes, 70(4), p.227 - 237, 2021/05

Behavior of radioactive caesium in river water and sediment in Tama river was investigated during 2012-2016. Concentrations of radioactive caesium in river water and sediment were decreased with time exponentially. Concentrations of suspended radioactive caesium, and radioactive caesium in sediment temporarily increased when sediments were resuspended due to rain. On the other hand, dissolved radioactive caesium was not easily impacted by this factor. Sequential extraction, elemental and crystal phase analysis were performed on the sediments and examined the chemical state of radioactive caesium as well as the adsorption mechanism. Radioactive caesium in sediment was present in a stable chemical form, and there is possibility that radioactive caesium was incorpolated in biotite.

Journal Articles

Performance of in situ gamma-ray spectrometry in the assessment of radioactive cesium deposition around the Fukushima Daiichi Nuclear Power Plant

Ji, Y.-Y.*; Ochi, Kotaro; Hong, S. B.*; Nakama, Shigeo; Sanada, Yukihisa; Mikami, Satoshi

Radiation Physics and Chemistry, 179, p.109205_1 - 109205_11, 2021/02

 Times Cited Count:6 Percentile:73.26(Chemistry, Physical)

In situ gamma-ray spectrometry using diverse survey platforms has been conducted in contaminated areas with several dose rate levels around the Fukushima Daiichi Nuclear Power Plant (FDNPP). Six survey sites, including two evacuation zones around the FDNPP, were selected for ground-based gamma-ray spectrometry using HPGe (high purity Ge) and LaBr$$_{3}$$(Ce) detectors to assess the radioactive cesium deposition in the ground. The diverse levels of radioactivity of $$^{137}$$Cs were then distributed to six survey sites from 30 to 3000 kBq m$$^{-2}$$ in the measurement period of October 2018. A method to directly calculate the depth profile using in situ measurement was introduced so as to have representation over a wide area, and the results were successfully compared with those of sample analysis at one point in the site.

Journal Articles

Simulation analysis of the Compton-to-peak method for quantifying radiocesium deposition quantities

Malins, A.; Ochi, Kotaro; Machida, Masahiko; Sanada, Yukihisa

Proceedings of Joint International Conference on Supercomputing in Nuclear Applications + Monte Carlo 2020 (SNA + MC 2020), p.147 - 154, 2020/10

JAEA Reports

Development of measurement system of radioactive plume using unmanned airplane in the fiscal year 2018 (Contract research)

Sanada, Yukihisa; Ochi, Kotaro; Ishizaki, Azusa

JAEA-Research 2020-006, 60 Pages, 2020/07

JAEA-Research-2020-006.pdf:4.84MB

At the accident of nuclear facilities, a prediction of the behavior of released radioactive plume is indispensable to make a decision on a refuge plan of inhabitants. Currently, prediction system which is based on atmospheric dispersion simulation has been implemented as a tool of the atomic energy disaster prevention. However, the direct measurement method of the radioactive plume has not existed. In this study, some component technologies were developed for the establishment of direct measurement methods of radioactive plume using unmanned aerial vehicle whose technological innovation is remarkable. In addition, the spray test using mock aerosol was conducted to obtaining the deposition rate to the airplane body. The algorism of making a flight plan was developed based on a prediction model of the radioactive plume. This report summarized the outcome of the last year of the three-year plan.

Journal Articles

The Ecological half-life of radiocesium in surficial bottom sediments of five ponds in Fukushima based on ${it in situ}$ measurements with Plastic Scintillation Fibers

Katengeza, E. W.*; Sanada, Yukihisa; Yoshimura, Kazuya; Ochi, Kotaro; Iimoto, Takeshi*

Environmental Science; Processes & Impacts, 22(7), p.1566 - 1576, 2020/07

 Times Cited Count:8 Percentile:50.99(Chemistry, Analytical)

Journal Articles

Student and Young researcher's view of research on health physics and environment science

Miwa, Kazuji; Terasaka, Yuta; Ochi, Kotaro; Futemma, Akira; Sasaki, Miyuki; Hirouchi, Jun

Nihon Genshiryoku Gakkai-Shi ATOMO$$Sigma$$, 61(9), p.687 - 691, 2019/09

This report summarizes the contents of the session of the Health Physics and Environment Science Division, which was held in Atomic Energy Society of Japan 2019 Spring Meeting. In this session, six students and young researchers who engaged in the field of nuclear energy and radiation gave a lecture about health physics and environmental science research through their expertise. After the all presentations end, we took discussion time about the issues and future development in this field with all attendees. In this report, we summarized each lecture outline and discussion contents.

JAEA Reports

Development of measurement system of radioactive plume using unmanned airplane in the fiscal year 2017 (Contract research)

Sanada, Yukihisa; Nishizawa, Yukiyasu*; Ochi, Kotaro; Yuki, Yoichi*; Ishizaki, Azusa; Osada, Naoyuki*

JAEA-Research 2018-009, 48 Pages, 2019/01

JAEA-Research-2018-009.pdf:14.77MB

At the accident of nuclear facilities, a prediction of the behavior of released radioactive plume is indispensable to make a decision on a refuge plan of inhabitants. Currently, prediction system which is based on atmospheric dispersion simulation has been implemented as a tool of the atomic energy disaster prevention. However, the direct measurement method of the radioactive plume has not existed. In this study, some component technologies were developed for the establishment of direct measurement methods of radioactive plume using unmanned aerial vehicle whose technological innovation is remarkable. In addition, the spray test using mock aerosol was conducted to obtaining the deposition rate to the airplane body. The algorism of making a flight plan was developed based on a prediction model of the radioactive plume. This report summarized the outcome of the second year of the three-year plan.

Journal Articles

Comparison of airborne and ground-based tools used for radiation measurement in the environment

Ochi, Kotaro; Sasaki, Miyuki; Ishida, Mutsushi*; Sanada, Yukihisa

Progress in Nuclear Science and Technology (Internet), 6, p.103 - 107, 2019/01

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, a large amount of radionuclides was spread out all over the world. In our previous study, we developed the aerial radiation monitoring technique using unmanned helicopter for investigating the dose rate derived deposited radionuclides over wide area. In addition, many monitoring techniques were developed for investigating the local distribution of radionuclides using unmanned aerial vehicle, handheld instrument and car within small area. Distinction of these methods depends on desirable position resolution of dose rate. However, the comparison method of the measurement result between different methods is not established. In this study, we attempted to evaluate the some methods of airborne and ground radiation measurement in same extended farm.

Journal Articles

Evaluation of ecological half-life of dose rate based on airborne radiation monitoring following the Fukushima Dai-ichi Nuclear Power Plant accident

Sanada, Yukihisa; Urabe, Yoshimi*; Sasaki, Miyuki; Ochi, Kotaro; Torii, Tatsuo

Journal of Environmental Radioactivity, 192, p.417 - 425, 2018/12

 Times Cited Count:26 Percentile:68.96(Environmental Sciences)

Journal Articles

Development of the unmanned surface vehicle for radiation monitoring at seabed; Observation of temporal change of radiocesium concentration in the sea bottom sediment at coastal zone of Fukushima

Sanada, Yukihisa; Miyamoto, Kenji*; Ochi, Kotaro; Matsuzaki, Koji*; Ogawa, Toshihiro*; Senga, Yasuhiro*

Kaiyo Riko Gakkai-Shi, 24(2), p.9 - 18, 2018/12

Seven years passed since Fukushima Dai-ichi Nuclear Power Station accident which was caused large amount of radionuclide release to the sea. Elucidation of behavior mechanism of radiocesium in the seabed is required for restarting fishing industry. We developed radiation detection system using the unmanned surface vehicle for in-situ measurement of radiocesium concentration in seabed sediment. This system is able to automatically navigate to measurement point and obtain the radiation data on the bottom sediment. The detector was calibrated by comparing the actual sediment samples. The periodical measurement off-shore the Fukushima Prefecture was performed using developed this system. As these results, distribution of radiocesium concentration was changed due to oceanographic condition. However, radiocesium inventory was tendency to decrease according to radiocesium half-life in measurement area. This system is effective for elucidation of behavior mechanism of radiocesium because it can easily measure the radiocesium concentration in the bottom sediment.

Journal Articles

Development of an analytical method for estimating three-dimensional distribution of sediment-associated radiocesium at a reservoir bottom

Ochi, Kotaro; Urabe, Yoshimi*; Yamada, Tsutomu*; Sanada, Yukihisa

Analytical Chemistry, 90(18), p.10795 - 10802, 2018/09

 Times Cited Count:2 Percentile:7.2(Chemistry, Analytical)

After the Fukushima Daiichi Nuclear Power Station accident, the distributions of sediment-associated radiocesium have been investigated to evaluate the dispersion and accumulation of radiocesium in the reservoir field. To develop an analytical method for measuring the horizontal and vertical distributions of radiocesium on a wide scale, we obtained 253 $$gamma$$-ray spectra using a NaI(Tl) scintillation detector at the bottom of 64 ponds in Fukushima during 2014-2016. The depth profile of sediment-associated radiocesium was found to be correlated with intensities of scattered and photo peaks. In parallel, core sediments were collected in same ponds to validate the estimates. Good agreement was observed between the results of in situ spectrometry and core sampling. These results indicated that our developed method would be a good approach for understanding the behavior of radiocesium and determining whether decontamination of reservoirs is required.

Journal Articles

Evaluation of physicochemical properties of radioactive cesium in municipal solid waste incineration fly ash by particle size classification and leaching tests

Fujii, Kengo*; Ochi, Kotaro; Obuchi, Atsushi*; Koike, Yuya*

Journal of Environmental Management, 217, p.157 - 163, 2018/07

 Times Cited Count:32 Percentile:77.7(Environmental Sciences)

After the Fukushima Daiichi-Nuclear Power Plant accident, environmental recovery was a major issue because a considerable amount of municipal solid waste incineration (MSWI) fly ash was highly contaminated with radioactive cesium. In this study, MSWI fly ash was sampled in Fukushima Prefecture, and the physicochemical properties of radioactive cesium in MSWI fly ash were evaluated by particle size classification and the Japanese leaching test No. 13 (JLT-13). The results obtained from the classification of fly ash indicated that the activity concentration of radioactive cesium and the content of the coexisting matter (i.e., chloride, potassium, and sylvite) temporarily change in response to the particle size of fly ash. X-ray diffraction results indicated that water-soluble radioactive cesium exists as CsCl because of the cooling process and that insoluble cesium is bound to the inner sphere of amorphous matter.

Journal Articles

Estimation of the vertical distribution of radiocesium in soil on the basis of the characteristics of $$gamma$$-ray spectra obtained via aerial radiation monitoring using an unmanned helicopter

Ochi, Kotaro; Sasaki, Miyuki; Ishida, Mutsushi*; Hamamoto, Shoichiro*; Nishimura, Taku*; Sanada, Yukihisa

International Journal of Environmental Research and Public Health, 14(8), p.926_1 - 926_14, 2017/08

 Times Cited Count:4 Percentile:22.93(Environmental Sciences)

After the Fukushima Daiichi Nuclear Power Plant accident, the vertical distribution of radiocesium in soil has been investigated to better understand the behavior of radiocesium in the environment. The typical method used for measuring the vertical distribution of radiocesium is troublesome because it requires collection and measurement of the activity of soil samples. In this study, we established a method of estimating the vertical distribution of radiocesium by focusing on the characteristics of $$gamma$$-ray spectra obtained via aerial radiation monitoring using an unmanned helicopter. In this method, the change in the ratio of direct $$gamma$$ rays to scattered $$gamma$$ rays at various depths in the soil was utilized to quantify the vertical distribution of radiocesium. The results show a positive correlation between the abovementioned and the actual vertical distributions of radiocesium measured in the soil samples.

Oral presentation

Estimation of depth profile of radiocesium in soil based on characteristics of $$gamma$$-ray spectra obtained by airborne radiation monitoring

Ochi, Kotaro; Sasaki, Miyuki; Ishida, Mutsushi*; Sato, Tomohiko*; Hamamoto, Shoichiro*; Nishimura, Taku*; Sanada, Yukihisa

no journal, , 

A large amount of radiocesium ($$^{134}$$Cs & $$^{137}$$Cs) were released into the atmosphere as a result of 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. To estimate the impact of the accident to the environment, dose rate around FDNPP have been measured by MEXT. Dose rate nearby FDNPP, however, still stay high even though decontamination work was carried out, which means it is necessary to develop an effective decontamination method promptly. Information of depth profile of radiocesium in soil is required to realize it, though, most of measurement methods have trouble due to collection and measurement of soil samples. As we have developed the radiation measurement techniques using unmanned aerial vehicle to measure dose rate distribution over wide areas for years, we attempt to establish the estimation method of depth profile of radiocesium in soil based on characteristics of $$gamma$$-ray spectra obtained by airborne radiation monitoring in this paper.

49 (Records 1-20 displayed on this page)