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An accurate analysis of local-scale atmospheric dispersion of radioactive materials is important for safety and consequence assessments and emergency responses to accidental release from nuclear facilities. It is necessary to predict the three-dimensional distribution of the plume in consideration of turbulent effects induced by individual buildings and meteorological conditions. In this study, first, we conducted with meteorological observations by a Doppler LiDAR and simple plume release experiments by a mist-spraying system at the site of Japan Atomic Energy Agency. Then, we developed a framework for prediction system of local-scale atmospheric dispersion based on a coupling of large-eddy simulation (LES) database and on-site meteorological observation. The LES-database was also created by pre-calculating high-resolution turbulent flows in the target site at mean wind directions of class interval 10°. We provided the meteorological observed data with the LES-database in consideration of building conditions and calculated the three-dimensional distribution of the plume with a Lagrangian dispersion model. Compared to the instantaneous shots of the plume taken by a digital camera, it was shown that the mist plume transport direction was accurately simulated. It was concluded that our proposed framework for prediction system based on a coupling of LES-database and on-site meteorological observation is effective.

The behaviors of atmospheric 129I and the global 129I cycle remain incompletely understood because the spatiotemporal resolution of monitoring is insufficient, and few measurement-based models have been reported. This study aims to quantitatively understand the global 129I budget. When quantifying, we conducted global atmospheric 129I dispersion simulations covering from the period 2007 to 2010. To achieve this goal, the present study newly incorporates the iodine chemical processes of two gas-phase chemical reactions, six photolysis reactions, and two heterogeneous reactions into an existing atmospheric 129I transport model (GEARN–FDM). In addition to the aerial release of 129I from nuclear fuel reprocessing facilities, the model includes the volatilization processes of 129I compounds from the Earth’s surface. The net 129I exchange fluxes from the atmosphere to the Earth’s oceans and land were estimated to be 5.3 GBq/y and 18.0 GBq/y, respectively. The global 129I emission from the oceans was estimated as 7.2 GBq/y, and nearly half of the emission totals are emitted from the English Channel (3.2 GBq/y). In addition, the global 129I emissions from land are estimated to be 1.7 GBq/y. The remarkable 129I emission levels from land mainly appear in Europe, Russia, and North America, and the emission distribution is impacted by the activities of past and ongoing nuclear fuel reprocessing facilities. The total 129I emissions from the oceans and land are lower than the 129I emissions from the model-included nuclear fuel reprocessing facilities (23.3 GBq/y), and show that the aerial releases from the nuclear fuel reprocessing facilities in operation are still important 129I sources.

The third model intercomparison project (MIP) for investigating the atmospheric behavior of atmospheric caesium-137 (137Cs) emitted from Fukushima Daiichi Nuclear Power Plant (FDNPP) (3rd FDNPP-MIP), Japan, in March 2011, was conducted. A finer horizontal grid spacing (1 km) was used than in the previous FDNPP-MIP (2nd FDNPP-MIP, Sato et al., 2018; 3 km) to evaluate the models’ performance for high-concentration events measured near FDNPP. Nine of the models used in the 2nd FDNPP-MIP were also used in the 3rd FDNPP-MIP, and all models used identical source terms and meteorological fields. The performance of the models was evaluated through a comparison with observational data. Our analyses indicated that most of the observed high atmospheric 137Cs concentrations (plumes) were reasonably well simulated by the models, and the good performance of some models improved the performance of the multimodel, highlighting the advantage of using a multimodel ensemble. The analyses also confirmed that the use of a finer grid resolution resulted in the meteorological field near FDNPP being better reproduced in the 3rd FDNPP-MIP, and the performance of the models was better than that of the 2nd FDNPP MIP. The good representation of the wind field resulted in the reasonable simulation of the narrow distribution of high deposition amount to the northwest of FDNPP and the reduction of the overestimation in deposition amount over the area to the south of FDNPP compared to the 2nd FDNPP MIP. In contrast, the performance of the models in simulating plumes observed over the Nakadori area, the northern part of Gunma, and the Tokyo Metropolitan Area (TMA) was slightly worse than in the 2nd FDNPP-MIP.

The long-lived radioactive iodine ($^{129}$I) is a useful geochemical tracer in the atmospheric environment. We recently observed clear seasonal trends in air concentration and deposition of $^{129}$I in Japan. Using these data, we developed a global atmospheric $^{129}$I transport model to reveal key processes for the global atmospheric $^{129}$I cycle. The model generally reproduced the observed seasonal change in air concentration and deposition of $^{129}$I in Japan, and the global distribution of $^{129}$I concentration in rain as presented in past literature. Numerical experiments changing the intensity of anthropogenic and natural sources were conducted to quantify the impact of anthropogenic sources on the global $^{129}$I cycle. The results indicated that the atmospheric $^{129}$I from the anthropogenic sources was deposited in winter and can be accumulated mainly in the northern part of Eurasia. In contrast, the atmospheric $^{129}$I from the natural sources dominated the deposition in summer. These results suggested that the re-emission process of $^{129}$I from the Earth's surface may be important as a secondary impact of $^{129}$I in the global-scaled environment. Furthermore, although wet deposition dominated the total deposition in the Northern hemisphere, dry deposition regionally and seasonally contributed to the total deposition over arctic and northern part of Eurasia in winter, suggesting that the dry deposition may play a key role in the seasonal change of $^{129}$I deposition in the Northern hemisphere high latitudes., 長寿命放射性ヨウ素($^{129}$I)は、大気環境における放射性核種の有用な地球化学トレーサである。本研究では、$^{129}$Iの大気濃度および沈着の観測を実施し、観測データから大気濃度および沈着の明瞭な季節変動を得た。さらに、大気中の$^{129}$I循環を支配する要因を明らかにすることを目的として、得られた観測データを用いて、移流、乱流拡散、大気沈着、光化学、ガス粒子変換、核燃料再処理工場からの$^{129}$Iの排出、海洋および陸域からの$^{129}$Iの揮発の各物理・化学過程を考慮した全球ヨウ素輸送モデルを開発した。全球ヨウ素輸送モデルは、我々が観測した$^{129}$Iの大気濃度および沈着の季節変動、そして既往文献の$^{129}$Iの降水中濃度の全球分布を良好に再現した。開発した全球ヨウ素輸送モデルを用いて人為起源と自然起源の$^{129}$Iインベントリの強度を変化させる数値実験を実施し、地球全体の$^{129}$I循環に対する人為起源の$^{129}$Iの影響を評価した。その結果、冬季においては、人為起源の$^{129}$Iが主にユーラシアの北部に沈着する可能性があることが示された。一方で、夏季においては、自然起源の$^{129}$Iが北半球中高緯度の沈着に支配的であった。これらの結果は、地球表面からの$^{129}$Iの再飛散過程が全球規模での$^{129}$I循環に重要であることを示唆している。さらに、冬季のユーラシア北部や北極域においては局所的に乾性沈着が寄与しており、乾性沈着が環境中の$^{129}$Iの季節変化に重要な影響を及ぼすことが示唆された。

In this study, we developed an atmospheric dispersion model based on the finite difference method (GEARN-FDM) to simulate long-range radiological dispersion. The model includes a mass conservation and monotonic advection scheme and a horizontal diffusion scheme based on the deformation of the resolved horizontal wind. Meteorological fields in calculating dispersion were simulated by the Advanced Research Weather Research and Forecasting (WRF) model. ERA-interim reanalysis was used for the WRF nudging calculation. By using tracer gas concentrations from the European tracer experiment, the GEARN-FDM performance could be tested. The results indicated high performance with factors of 2 and 5 of 37% and 72%, respectively. To investigate the impact of horizontal diffusivity on the concentration distribution, we examined an additional simulation run using GEARN-FDM with a constant value of horizontal diffusivity. In comparison with the additional run, the plume calculated in the original GEARN-FDM run was locally and temporally diluted due to large horizontal diffusivity in a mountainous region. Therefore, geographical heterogeneity of horizontal diffusivity may affect regional-scale atmospheric dispersion simulations.