Local-scale atmospheric dispersion of radionuclides discharged during the Fukushima Dai-ichi Nuclear Power Plant accident

To understand how the high dose rate zones were created during the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident on March 2011, the atmospheric dispersion of radionuclides during the period from 12 to 17 March was reproduced by using a computer-based nuclear emergency response system, WSPEEDI-II. For the simulation, release rates estimated by combining environmental monitoring data with atmospheric dispersion simulations under the assumption of a unit release rate (1 Bq h^-1) was used. Overall, the spatial pattern of ¹³⁷Cs deposition and the increases in the air dose rates observed at the monitoring posts around FNPP1 were reproduced by WSPEEDI-II. The high-concentration plumes discharged during the afternoon on March 12 after the hydrogen explosion of Unit 1 and late at night on March 14 flowed to the northwest and south–southwest directions of FNPP1, respectively, and caused a large amount of dry deposition along their routes. The simulation indicated that air dose rates significantly increased in the south–southwest region of FNPP1 by dry deposition of the high-concentration plume discharged from the night of March 14 to the morning of March 15. A large part of current high dose rate zones in Fukushima Prefecture was explained by simulated wet and dry deposition due to major releases on 15 March. The highest dose rate zones to the northwest of FNPP1 were created by a significant deposition of radionuclides discharged from FNPP1 during the afternoon.