Aerial monitoring around TEPCO's Fukushima Daiichi Nuclear Power Station and development of radiation monitoring technology for unmanned airplanes in fiscal year 2023 (Contract research)

Futemma, Akira; Sanada, Yukihisa; Nakama, Shigeo; Sasaki, Miyuki; Ochi, Kotaro; Nagakubo, Azusa; Sawahata, Yoshiro*; Kawasaki, Yoshiharu*; Iwai, Takeyuki*; Hiraga, Shogo*; et al.

JAEA-Technology 2024-021, 232 Pages, 2025/03

JAEA-Technology-2024-021.pdf:25.79MB

The 2011 off the Pacific coast of Tohoku Earthquake on March 11, 2011, caused a tsunami that led to the TEPCO's Fukushima Daiichi Nuclear Power Station (FDNPS) accident, releasing a large amount of radioactive material into the surrounding environment. Since the accident, Aerial Radiation Monitoring (ARM) has been used to quickly and widely measure radiation distribution. As a commissioned project from the Nuclear Regulation Authority, the Japan Atomic Energy Agency (JAEA) has continuously conducted ARM around FDNPS using manned and unmanned helicopters. This report summarizes the monitoring results for fiscal year 2023, evaluates changes in dose rate from past results, and discusses the factors contributing to these changes. Additionally, an analysis considering terrain undulation was conducted to improve accuracy for converting ARM data into dose rate. Furthermore, a method to discriminate airborne radon progeny was applied for ARM results to evaluate its impact. Moreover, to perform wide-area monitoring more efficiently, we advanced the development of unmanned airplane monitoring technology.

Effects of different accident scenarios and sites on the reduction factor used for expressing sheltering effectiveness

Hirouchi, Jun; Watanabe, Masatoshi*; Hayashi, Naho; Nagakubo, Azusa; Takahara, Shogo

Journal of Radiological Protection, 45(1), p.011506_1 - 011506_11, 2025/03

https://doi.org/10.1088/1361-6498/adba6e

Public living in areas contaminated by nuclear accidents is exposed to radiation in the early phase and over the long term. Even under similar accident scenarios, radiation doses and sheltering effectiveness, which is one of the protective measures, depend on meteorological conditions and the surrounding environment. Radiation doses and sheltering effectiveness in the early phase of nuclear accidents are crucial information for the public as well as national and local governments planning a nuclear emergency preparedness. In this study, we assessed radiation doses and sheltering effectiveness at sites with nuclear facilities in Japan using the Off-Site Consequence Analysis code for Atmospheric Release accidents, which is one of the level-3 probabilistic risk assessment codes, for five accident scenarios: three scenarios from past severe accident studies, a scenario defined by the Nuclear Regulation Authority in Japan, and a scenario corresponding to the Fukushima-Daiichi Nuclear Power Station accident. The sheltering effectiveness differed by up to approximately 50% among the accident scenarios at the same sites and by approximately 20%50% among sites under the same accident scenario. Differences in the radionuclide composition among the accident scenarios and the differences in wind speeds among the sites primarily caused these differences in sheltering effectiveness.

Soil dust and bioaerosols as potential sources for resuspended Cs occurring near the Fukushima Dai-ichi Nuclear Power Plant

Ota, Masakazu; Takahara, Shogo; Yoshimura, Kazuya; Nagakubo, Azusa; Hirouchi, Jun; Hayashi, Naho; Abe, Tomohisa; Funaki, Hironori; Nagai, Haruyasu

Journal of Environmental Radioactivity, 264, p.107198_1 - 107198_15, 2023/08

https://doi.org/10.1016/j.jenvrad.2023.107198

One of the current major radiation exposure pathways from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident-fallout is inhalation of the re-suspended Cs occurring in air. While wind-induced soil particle resuspension has been recognized as a primary mechanism of Cs resuspension, studies following the FDNPP accident suggested that fungal spores can be a significant source of the atmospheric Cs particularly in the rural areas such as difficult-to-return zone (DRZ). To elucidate the relative importance of the two resuspension phenomena, we propose a model simulating resuspension of Cs as soil particles and fungal spores, and applied it to DRZ. Our model's calculation showed that soil particle resuspension was responsible for the surface-air Cs observed during winter-spring, but could not account for the higher Cs concentrations observed in summer-autumn. The higher concentrations in the summer-autumn were in general reproduced by implementing fungal spore Cs emission, that replenished low soil particle Cs resuspension in that period. According to our model's concept, Cs accumulation in fungal spores and high spore emission rate characterized by the rural environment were likely responsible for the abundance of spore Cs in the air. It was inferred that the influence of the fungal spores on the atmospheric Cs would last longer since un-decontaminated forests still exist in DRZ.

Dose assessment for residents in the residential environment considering the transfer of substances between indoor and outdoor after nuclear accidents, 5; Evaluation of resuspension coefficients based on experiments

Nagakubo, Azusa; Hayashi, Naho; Hirouchi, Jun; Matsui, Yasuto*; Yoneda, Minoru*; Takahara, Shogo

no journal, , 

In the residential environment after a nuclear power plant accident, the effects of diffusion and deposition of radionuclides have been considered for exposure due to the entry of outdoor resuspended radionuclides into the indoor environment. In practice, however, resuspension of deposited particles also occurs. In this study, the resuspension coefficient, defined as the ratio of the airborne concentration to the floor surface deposition density, was obtained experimentally to construct a resuspension model for indoor deposited particles. In the experiment, the resuspension coefficient was obtained by spreading test dust on the flooring of a family home and measuring the concentration of resuspended particles when disturbances such as the operation of air conditioning and human movement were applied. The results showed that the resuspension coefficient ranged from 1.0e-5 to 1.0 (m-1) depending on the type of disturbance and particle size. This work was supported by Research Project on the Health Effects of Radiation organized by Ministry of the Environment, Japan.

Dose assessment for residents in the residential environment considering the transfer of substances between indoor and outdoor after nuclear accidents, 1; Overview of research project

Takahara, Shogo; Nagakubo, Azusa; Hirouchi, Jun; Hayashi, Naho; Matsui, Yasuto*; Yoneda, Minoru*

no journal, , 

no abstracts in English

Dose assessment for residents in the residential environment considering the transfer of substances between indoor and outdoor after nuclear accidents, 3; Resuspension of indoor deposited particles

Nagakubo, Azusa; Hayashi, Naho; Matsui, Yasuto*; Hirouchi, Jun; Yoneda, Minoru*; Takahara, Shogo

no journal, , 

no abstracts in English

Dose assessment for residents in the residential environment considering the transfer of substances between indoor and outdoor after nuclear accidents, 4; Assessment of the sheltering effect in the early phase of nuclear accidents

Hirouchi, Jun; Watanabe, Masatoshi*; Hayashi, Naho; Nagakubo, Azusa; Matsui, Yasuto*; Yoneda, Minoru*; Takahara, Shogo

no journal, , 

no abstracts in English