Atmospheric Behavior, Deposition, and Budget of Radioactive Materials from the Fukushima Dai-Ichi Nuclear Power Plant

To understand the atmospheric behavior of radioactive materials emitted from the Fukushima Dai-Ichi nuclear power plant (FDNPP) after the nuclear accident that accompanied the great East Japan earthquake and tsunami on 11 March 2011, we simulated the spatial and temporal variations of 131I and 137Cs around the FDNPP for the period of March 11 to April 30, 2011, by using a chemical transport model (CMAQ). The model domain covered most of the Tohoku region and the Kanto region of Japan (700 × 700 km2) at a 3 km grid resolution and a 34 layer vertical structure with a surface layer thickness of about 60 m. We calculated meteorological fields by using the Weather Research and Forecasting Model (WRF) version 3.1 driven by the three-dimensional meteorological fields from the Japan Meteorological Agency Meso-Scale Model datasets. The model and the simulation conditions are basically same as Morino et al. (2011) except for grid resolution. Emission data from the FDNPP were taken from Terada et al. (2012). The model reproduced the fundamental feature of observed spatial distribution of total deposition of 137Cs in East Japan by the airborne monitoring survey by MEXT (Ministry of Education, Culture, Sports, and Science and Technology, Japan). Additionally, the model roughly reproduced the spatiotemporal variations of deposition rates of 131I and 137Cs over 15 prefectures in Japan (60–400 km from the FDNPP) and the temporal variations of atmospheric concentration measured at two sites in the Kanto region, although there were some discrepancies between the simulated and observed data, most likely due to uncertainties in the treatment of emission, transport, and deposition processes in the model. Budget analysis indicated that approximately 12% of 131I and 25% of 137Cs were deposited over land in Japan, and the rest was deposited to the ocean or transported out of the model domain. Most of the radioactive materials emitted from the FDNPP were deposited or transported out of the model region within a few days and did not stay in the atmosphere in the model domain. Even though there are still large uncertainties in this simulation, agreement between the observed and simulated results indicates the validity of the model, and the atmospheric transport and deposition model developed by this study will be an important tool for understanding the behavior of radionuclides emitted from the FDNPP and for estimating the exposure to radiation.