Detailed source term estimation and atmospheric dispersion analysis for the Fukushima Dai-ichi Nuclear Power Plant accident

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

We estimated a detailed time trend of atmospheric releases during the accident by coupling additionally obtained monitoring data of air dose rate near the plant, parameters for the reactor events, and atmospheric dispersion simulation by WSPEEDI-II. The modified WSPEEDI-II using the newly estimated source term well reproduced local and regional patterns of air dose rate and surface deposition obtained by airborne observations. The results suggested that the major release occurred in the following periods during March 2011: afternoon on the 12th, midnight on the 14th, morning and night on the 15th, and morning on the 16th. Our dispersion simulations revealed that the highest radioactive contamination areas were created from 15th to 16th by complicated interactions among rainfall, plume movements, and phase properties and release rates.