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

Radionuclides discharged into the atmosphere during a month-long period due to the accident at the Fukushima Dai-ichi Nuclear Power Plant in March 2011 were dispersed and deposited to the surface over a wide-range area. It is important to reproduce and analyze the atmospheric dispersion and deposition processes for assessment of the radiological dose to the public resulting from this accidental release. To address this issue, regional-scale atmospheric dispersion simulations of ¹³¹I and ¹³⁷Cs from March 12 to April 30, 2011 were carried out by using a computer-based nuclear emergency response system, WSPEEDI-II. We used the source term estimated by combination of environmental monitoring data and atmospheric dispersion simulations assuming unit release rate (1 Bq h^-1). The simulation generally reproduced the distributions of daily and monthly surface depositions over land in eastern Japan, while there were some discrepancies between calculations and measurements. The analysis of atmospheric dispersion and deposition suggests that the present distribution of a large amount of ¹³⁷Cs surface deposition in eastern Japan observed by airborne monitoring was produced primarily by the following four events: dry deposition in the northeastern coastal area of Miyagi Prefecture on March 12; wet and dry depositions in Fukushima Prefecture and in the northern part of the Kanto region on March 15-16; wet deposition in Iwate and Miyagi Prefectures on March 20; and wet deposition in the Kanto region on March 21-23. In the simulation results, the ratio of wet deposition to the total varied widely depending on the influence by the particular event.