Atmospheric discharge and dispersion of radionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident. Part I: Source term estimation and local-scale atmospheric dispersion in early phase of the accident.

The atmospheric release of (131)I and (137)Cs in the early phase of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident from March 12 to 14, 2011 was estimated by combining environmental data with atmospheric dispersion simulations under the assumption of a unit release rate (1 Bq h(-1)). For the simulation, WSPEEDI-II computer-based nuclear emergency response system was used. Major releases of (131)I (>10(15) Bq h(-1)) were estimated when air dose rates increased in FNPP1 during the afternoon on March 12 after the hydrogen explosion of Unit 1 and late at night on March 14. The high-concentration plumes discharged during these periods flowed to the northwest and south-southwest directions of FNPP1, respectively. These plumes caused a large amount of dry deposition on the ground surface along their routes. Overall, the spatial pattern of (137)Cs and the increases in the air dose rates observed at the monitoring posts around FNPP1 were reproduced by WSPEEDI-II using estimated release rates. 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.