Atmospheric discharge and dispersion of radionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident, 1; Source term estimation and local-scale atmospheric dispersion in early phase of the accident

Katata, Genki; Ota, Masakazu  ; Terada, Hiroaki   ; Chino, Masamichi; Nagai, Haruyasu  

The source term of I and Cs discharged in early phase of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident was estimated by combining environmental data with atmospheric dispersion simulations of a computer-based nuclear emergency response system, WSPEEDI-II. Major releases were estimated when air dose rates increased after the hydrogen explosion at Unit 1 during the afternoon on 12 March and during the midnight on 14 March. The high-concentration plumes discharged during these periods caused large deposition to the northwest and south-southwest directions of FNPP1, respectively. The increases of air dose rates observed at the monitoring posts were overall 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 midnight on 14 March to the morning on 15 March.