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Radiation measurement for accelerating Fukushima's environmental recovery and decommissioning - 10 years after the Fukushima Nuclear Disaster

Session chair: Sato , Yuki (JAEA, Japan)
Shortcut: WS-01
Date: Sunday, 17 October, 2021, 7:00 AM - 11:00 AM
Room: WS-01
Session type: Workshop

The Fukushima Daiichi Nuclear Power Station (FDNPS), operated by Tokyo Electric Power Company Holdings, Inc., went into meltdown after the occurrence of a large tsunami caused by the Great East Japan Earthquake of March 11, 2011. Ten years have passed since the accident, and during that time, radiation measurement methods have been developed by groups in various countries to recover the environment in Fukushima Prefecture and to decommission the FDNPS. Dose rate mapping in an outdoor environment is being carried out through car-borne surveys and aerial monitoring, and these results are continuously provided to the national and local governments. On the other hand, in order to facilitate the decommissioning of the FDNPS, technologies such as dose rate monitoring by fiber type detectors in the containment vessel and visualization of the location of fuel debris by muon tomography have been developed. Visualization of radioactive substances using gamma-ray imagers and remote monitoring using a combination of robots and radiation detectors have also been implemented. In this workshop, the technologies developed for the environmental recovery of Fukushima and the decommissioning of FDNPS and their applications will be widely introduced, and the future prospects will be discussed.

Workshop organizers

  • Yuki SATO, Japan Atomic Energy Agency


Click on an contribution to preview the abstract content.

7:00 AM WS-01-01

Opening remarks & WS overview (#1587)

Y. Sato1

1 Japan Atomic Energy Agency, Collaborative Laboratories for Advanced Decommissioning Science, Tomioka Town, Futaba-gun, Japan



7:05 AM WS-01-02

Developments and applications of a carborne gamma-ray survey system, KURAMA-II (#1589)

M. Tanigaki1

1 Kyoto University, Institute for Integrated Radiation and Nuclear Science, Kumatori, Japan

KURAMA-II, a carborne gamma-ray survey system characterized by its compactness, autonomous operation, acquisition of pulse height spectrum data along with GPS data, has established its position as an effective method for the radiation monitoring method in the environment on a long-term basis. Periodical surveys in eastern Japan conducted by the Japanese government continue since 2011. A continuous carborne survey of radiation by KURAMA-II installed on local buses also continues in Fukushima prefecture as a collaboration among Fukushima prefecture, Kyoto University, and JAEA. These periodical and continuous measurement activities have resulted in the accumulation of a large amount of compilation of data, and many assessments of environmental radiation in Fukushima and surrounding have been realized.
The applications of KURAMA-II have begun to extend beyond just monitoring the radiation. A typical example is a tractor-based "robot," a tractor equipped with a gamma-ray detection system based on KURAMA-II, a hyperspectral sensing system, and a high-precision GNSS-based guidance system for a prompt and precise mapping of soil contamination and fertility evaluation in farmland near Fukushima Daiichi Power Plant. A five-year project has just begun this year. In this project, this robot will be applied to the evaluation of radioactive contamination of farmland in the "difficult-to-return" zones. Another trial is a low-power, compact KURAMA-II based on Sony Spresense and multi-hop LPWA. One of the expected applications of this KURAMA-II is the prompt establishment of radiation monitoring in nuclear accidents under extreme situations, e.g., a large-scale black-out caused by an earthquake.
The present status and prospects of KURAMA-II, including these trials, are introduced.

Keywords: radiometry, mapping, γ-ray, carborne survey, soil contamination, Spresense, Fukushima Daiichi nuclear power plant



Keywords: KURAMA-II
7:40 AM WS-01-03

Experience for Fukushima environmental radiation monitoring and application for preparedness of post-accident (#1590)

Y. Sanada1

1 Japan Atomic Energy Agency, Sector of Fukushima Research and Development, Fukushima, Japan

After the accident at the Fukushima Daiichi nuclear power plant (FDNPP), there was an increasing need to measure the distribution of radionuclides around the FDNPP. Environmental monitoring was carried out in collaboration with researchers and engineers from all over Japan in order to clarify the distribution of radionuclide deposits and ambient dose-rates in the environment. To this purpose, new technologies for radiation measurement have been developed after the accident. Carrying out air-borne radiation surveys using a manned helicopter, the Japan Atomic Energy Agency (JAEA) has developed and established an analysis method concurrently with the development of this survey method. In particular, because the background radiation level differs greatly between the East and West regions of Japan, the JAEA has developed a discrimination method for natural radionuclide and cosmic rays using the gamma energy spectra. In addition, aerial radiation monitoring using unmanned aerial vehicles (UAVs) and measurement technology for radioactive substances deposited on waterbeds have been developed, and these technologies have been applied to the monitoring of the environment. Nonetheless, the optimization of radiation measurement techniques and the establishment for logistics of nuclear disaster correspondence still faces many challenges. The rotor-type UAV, which was made in Japan and was applied to survey around FDNPP, is useful to quickly measure in a wide area where people cannot walk. Recently, some fixed-wing type UAVs whose duration time of flight is more than 10 hours are already sold around the world. The fixed-wing type UAV was necessary for the correspondence immediately after the accident, which includes the observation of the accidental situation, radiation plume tracking and so on. The ship-type unmanned surface vehicle whose maximum sailing time is 10 days ship is useful for monitoring of release contaminated water at post-accident. In this presentation, the experience and current situation for Fukushima environmental radiation monitoring will summarize and introduce the approaches and problems for preparedness of post-accident.

Keywords: Accident of the Fukushima Daiichi Nuclear Power Plant Environmental Radiation survey, unmanned aerial vehicles



Keywords: Fukushima
8:15 AM WS-01-04

Development of a fiber optic radiation monitor and a gamma camera applied to the investigation towards the decommissioning of the Fukushima Daiichi Nuclear Power Station (#1591)

T. Tadokoro1

1 Hitachi, Ltd., Research & Development Group, Hitachi, Japan

Measurement of dose rate distribution and radioactive contamination are important towards the decommissioning of the Fukushima Daiichi Power Station. For this purpose, Hitachi and Hitachi-GE have developed a fiber optic radiation monitor and a pinhole camera type gamma camera. We used neodymium-doped yttrium aluminum garnet crystal (Nd:YAG) as a light emission element of the fiber optic radiation monitor which emits light with the wavelength of 1,064 nm. The monitor have a capability for measuring dose rates from 10-2 Gy/h to 6.1x104 Gy/h using single photon counting method and it can be checked its measurement conditions using a semiconductor laser with the wavelength of 808nm. By installing the monitor on a shape shifting robot, we succeeded in measuring the dose rate distribution inside the 1F-1 PCV as a project of the International Research Institute for Nuclear Decommissioning funded by the FY2016 supplementary budget for the Project of Decommissioning and Contaminated Water Management. The pinhole camera type gamma camera can visualize the gamma-ray intensity distribution in real time under a high dose rate environment of 300mSv/h that is necessary to survey the entire area of the reactor building. We have developed it with the support of the New Energy Development Organization (NEDO), Japan. For the visualization of gamma-ray intensity distribution, an optical image is taken with an optical camera installed in parallel, and the gamma-ray intensity distribution and optical image are superimposed through software processing. In addition, from the spectral measurement of gamma-ray energy, the identification of radioactive nuclides and intensity distribution assumption for each specific nuclide are possible. An energy resolution of 2.3% and spatial resolution of 0.68 m at a distance of 5 m were confirmed.

Keywords: Fiber optic radiation monitor, Nd:YAG, Single photon counting, Gamma camera, Visualization, Gamma-ray intensity distribution



Keywords: Hitachi
8:50 AM WS-01-05


9:00 AM WS-01-06

Particle detectors to promote decommissioning of Fukushima Daiichi (#1592)

H. Miyadera1, N. Kume1

1 Toshiba Energy Systems & Solutions Corporation, Kawasaki, Japan

Various radiation detectors were developed to promote the decommissioning of Fukushima Daiichi. Among them, two unique detectors, muon scattering imaging and Alpha Camera, will be presented.
At Fukushima Daiichi, removal of nuclear debris is planned to start in the mid 2020’s. It is important to measure the amount of nuclear materials in the debris to preventing re-criticality and to reduce the storage cost. We have been working on applying muon scattering imaging [1] to estimate amount of nuclear materials in the debris. Since composition of each debris is different, we introduced deep learning technique to analyze muon scattering data to estimate the total amount of nuclear materials within the debris container.
Alpha contaminations are expected inside the reactor buildings of unit-1, unit-2 and unit-3, due to the nuclear fuel and fuel debris. It is important to specify locations of the alpha contaminations as well as their amounts to reduce the dose exposure of workers inside the reactor buildings, and to prevent the spreading of alpha radiations during the decommissioning process. A mobile alpha detection system, Alpha Camera, has been developed to detect alpha-induced UV lights from at a distance of a few meters [2]. When an alpha particle is emitted into the air, it excites nitrogen molecules, resulting in approximately 120 UV photons to be emitted within the wavelength of 300 to 400 nm range during its de-excitation process. A radiation shield can be attached to Alpha Camera when it is operated in elevated gamma-ray environments.
The Alpha Camera is a part of “Research and Development of Processing and Disposal of Solid Waste” project supported by the Ministry of Economy, Trade and Industry (METI).

[1] H. Miyadera et al., AIP Advances 3, 052133 (2013).
[2] N. Kume, H. Kuroda, and Y. Yoshimura, Proc. of The 23th International Conference on Nuclear Engineering (ICONE23), Chiba, Japan, May 2015.

Keywords: muon scattering, debris, Alpha Camera



Keywords: Fukushima
9:35 AM WS-01-07

Radiation imaging and source location in operating nuclear facilities (#1593)

M. J. Joyce1

1 Lancaster University, Department of Engineering, Lancaster, United Kingdom

Operating nuclear facilities (e.g., reactors, reprocessing plants and spent fuel stores etc.) can be distinct from legacy facilities due to the presence of mixed radiation fields comprising contributions of penetrating neutron and  radiations. These can arise from: sustained chain reactions derived largely from induced fission on 235U and 239Pu, specifically in the case of operating reactors; α,n reactions and spontaneous fission in minor actinides (e.g., 244Cm and 252Cf etc.). Sometimes these radiations can prevent access by people to such facilities and, where access is feasible, keeping exposure as low as practicable can favour the use of remote means for facilities assessment. Combined imaging of neutrons and γ rays can inform, and confirm, users and operators of radiation distributions, and also can reveal unexpected issues in operating plant to a much greater degree than isolated point measurements, and they can also complement spatial assessments by such techniques as LIDAR etc. In some circumstances, done remotely and achieved via a robot, this can also usefully remove the perturbation on the field of the person (especially in the case of neutrons) and, hypothetically, robots might achieve better results since they are more at ease with repetitive exercises, accuracy and precision and the need to sometimes complete operations in constrained environments. In this contribution, current methods and achievements in the context of radiation imaging and source location will be discussed, and then a description of the important challenges and opportunities that remain will be given.



Keywords: Radiation
10:20 AM WS-01-08

So far and future prospect: Radiation measurement technology for environmental recovery in Fukushima and decommissioning of the Fukushima Daiichi Nuclear Power Station (#1594)

T. Torii1

1 Fukushima University, Institute of Environmental Radioactivity, Fukushima, Japan

Ten years have passed since the Fukushima Daiichi Nuclear Power Station (FDNPS) accident caused by the Great East Japan Earthquake, and various efforts have been made in the field of radiation measurement. In the environment contaminated with radioactive materials due to the accident, various measurement methods such as airborne, carborne, and man-borne have been used to monitor the contamination of radioactive materials deposited on the ground surface and air dose-rate. Monitoring has been carried out from immediately after the accident until today. As a result, it has become clear that a dose-rate decrease has been occurring immediately after the accident over a wide area. Evaluation of the decreasing rate, defined as the effective half-life, is important for estimating the future status of air dose-rate.
On the other hand, contamination has been widely measured in the FDNPS site, in the building, and in the containment vessel, from ultra-high dose rates to relatively high dose areas. Various methods are used for them depending on the purpose. In particular, in order to identify locations with high dose rates contaminated highly by radioactive substances, research and development of gamma-ray visualization technology has been conducted, and imaging measurements using gamma cameras and their 3-D imaging have been carried out in various locations in/around the FDNPS site. In particular, JAEA performed 3D imaging of the radiation source distribution in the FDNPS site by combining a small Compton camera with a digital camera and 3D-LiDAR. New research is being conducted for this purpose. Furthermore, with the progress of decommissioning work, not only gamma-ray emitting nuclides but also alpha-ray emitters caused by nuclear fuel have been detected, and these are also measured in the reactor buildings of the FDNPS.
I will outline the current status and issues of these measurement technologies.



Keywords: Fukushima
10:45 AM WS-01-09

Closing remarks (#1595)

Y. Sato1

1 Japan Atomic Energy Agency, Collaborative Laboratories for Advanced Decommissioning Science, Tomioka Town, Futaba-gun, Japan



Keywords: Closing

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