Visualization of radiocesium distribution in surface layer of seafloor around Fukushima Daiichi Nuclear Power Plant.

Large quantities of volatile radionuclides were released into the atmosphere and the hydrosphere following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March, 2011. Monitoring of radiocesium in sediment is important for evaluating the behavior of radiocesium in the environment and its effect on aquatic organisms. In this study, the radiocesium distribution in the surface sediment around the FDNPP was visualized as a radiocesium concentration map using periodical survey data from a towed gamma-ray detection system. The uncertainty of the radiocesium map was evaluated via comparison with a large amount of sediment core sample data. The characteristics of the radiocesium distribution were examined considering the seafloor topography and a geological map, which were obtained via acoustic wave survey. The characteristics of the formation of 137Cs anomaly at the estuaries were analyzed using a contour map of 137Cs concentration combined with water depth. Validation of the created map showed that it was comparable with actual sediment core samples. The map generated using the towed radiation survey depicted the 137Cs concentration distribution as the position resolution of a 1 km mesh. Finally, the 137Cs concentration decreased with time in consideration of such uncertainty.

Distribution map of natural gamma-ray dose rates for studies of the additional exposure dose after the Fukushima Dai-ichi Nuclear Power Station accident.

The information on the absorbed dose rate which is derived from natural radionuclides is needed to evaluate additional exposure dose. However, there is inadequate positional resolution and precision for such data around Fukushima Dai-ichi Nuclear Power Station (FDNPS). In this study, we created a map of the absorbed dose rate that is derived from natural radionuclides based on several airborne radiation monitoring data. The reliability and accuracy of the created map was verified by comparison with the many in-situ measurements on the ground. To evaluate the effectiveness of this study, the effective half-lives of the ambient dose rate at residential areas of Fukushima Prefecture were assessed by discriminating these absorbed dose rate of the natural background from the results of a periodic dose rate survey by local government. The results of the distribution of natural background absorbed dose rates are expected to contribute to the evaluation of the additional exposure dose after the FDNPS accident.

Evaluation of ecological half-life of dose rate based on airborne radiation monitoring following the Fukushima Dai-ichi nuclear power plant accident.

Airborne radiation monitoring was conducted in order to evaluate the influence of radionuclides emitted by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident throughout Japan. Carrying out airborne radiation monitoring using manned and unmanned helicopters, the we have developed and established an analysis method concurrently with the development of this monitoring method. In particular, because the background radiation level differs greatly between East and West regions of Japan, we have developed a discrimination method for natural radionuclide and cosmic rays using the gamma energy spectra. The reliability of the airborne radiation monitoring data was validated through comparison with large amounts of ground measurement data. The ecological half-lives of short and long components for decline of the ambient dose equivalent (air dose rate) were 0.61 years and 57 years, respectively, based on the results of air dose rate of airborne radiation monitoring using manned helicopter. These results indicate the importance of airborne monitoring to evaluate and predict the radiation exposure of residents.

Development and application of a method for discriminating the influence of radon progenies in air from aerial radiation monitoring data.

The influence of gamma-rays from natural nuclides (particularly the radon progenies, 214Pb and 214Bi) must be excluded from aerial radiation monitoring (ARM) data to accurately estimate the deposition of artificial radionuclides. A method for discriminating the influence of the radon progenies in air from the ARM data was developed. Two types of detectors with different crystal sizes were installed in a helicopter. The gamma-ray responses of these detectors were simulated using EGS5. The influence of the radon progenies in air was excluded using the relation between the count rates of six NaI (Tl) detectors and a LaBr3 detector. The discrimination method was applied to the ARM data obtained from around the Sendai and Fukushima Dai-ichi Nuclear Power Stations. To verify the validity of the discrimination method, the dose rate estimated from the ARM data was compared with the dose rate measured using a NaI survey meter at a height of 1 m above the ground. The application of the discrimination method improved the dose rate estimation, showing the validity of the discrimination method.

Development of an Analytical Method for Estimating Three-Dimensional Distribution of Sediment-Associated Radiocesium at a Reservoir Bottom.

After the Fukushima Daiichi Nuclear Power Station accident, the distributions of sediment-associated radiocesium were 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 at the bottoms of 64 ponds in Fukushima during 2014-2016 by using a NaI(Tl) scintillation detector. For visualizing horizontal distribution, the correlation between detector counting rate and radiocesium concentration of the bottom sediment was confirmed. In estimating vertical distribution, the depth profile of sediment-associated radiocesium was found to be correlated to the intensities of scattered and photo peaks. Good agreement was observed between the results of in situ spectrometry and core sampling. These results indicate that the developed method is suitable for understanding the behavior of radiocesium and determining whether decontamination of reservoirs is required.

Evaluation of ecological half-life of dose rate based on airborne radiation monitoring following the Fukushima Dai-ichi nuclear power plant accident.

Airborne radiation monitoring was conducted in order to evaluate the influence of radionuclides emitted by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident throughout Japan. Carrying out airborne radiation monitoring using manned and unmanned helicopters, the we have developed and established an analysis method concurrently with the development of this monitoring method. In particular, because the background radiation level differs greatly between East and West regions of Japan, we have developed a discrimination method for natural radionuclide and cosmic rays using the gamma energy spectra. The reliability of the airborne radiation monitoring data was validated through comparison with large amounts of ground measurement data. The ecological half-lives of short and long components for decline of the ambient dose equivalent (air dose rate) were 0.61 years and 57 years, respectively, based on the results of air dose rate of airborne radiation monitoring using manned helicopter. These results indicate the importance of airborne monitoring to evaluate and predict the radiation exposure of residents.

Altitudinal characteristics of atmospheric deposition of aerosols in mountainous regions: Lessons from the Fukushima Daiichi Nuclear Power Station accident.

To understand the formation process of radiologically contaminated areas in eastern Japan caused by the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, the deposition mechanisms over complex topography are the key factors to be investigated. To characterize the atmospheric deposition processes of radionuclides over complex mountainous topography, we investigated the altitudinal distributions of the radiocesium deposited during the accident. In five selected areas, altitudinal characteristics of the air dose rates observed using airborne surveys were analyzed. To examine the deposition mechanisms, we supplementarily used vertical profiles of radiocesium deposition in each area calculated in the latest atmospheric dispersion model. In southern Iwate, the vertical profile of the observed air dose rate was uniform regardless of altitude. In western Tochigi, the areas with the highest levels of contamination were characteristically distributed in the middle of the mountains, while in southern Fukushima, the areas with the highest contamination levels were enhanced near the summits of mountains. In central Fukushima, high air dose rates were limited to the bottoms of basin-like valley. In the region northwest of FDNPS, the air dose rate was the highest at the bottom of valley topography and decreased gradually with altitude. The simulation results showed that calculated wet deposition and observed vertical profiles of total deposition were similar in areas of southern Iwate and northwest of FDNPS qualitatively, suggesting that the dominant deposition mechanism was wet deposition. In contrast, the atmospheric dispersion model failed to reproduce either the timing of precipitation events or vertical profiles of radiocesium deposition in three other areas. Although it was difficult to elucidate the deposition mechanisms in these areas due to uncertainties of the present model results, potential mechanisms such as cloud water deposition were still proposed based on circumstantial evidences of limited meteorological data during the early stage of the accident.