Atmospheric dispersion simulations for estimating early internal doses to residents in the Fukushima Daiichi nuclear power station accident

Enormous amounts of radionuclides were released into the environment in the Fukushima Daiichi nuclear power station accident by the Great East Japan Earthquake on 11 March 2011. Needless to say, estimating the internal dose of residents in affected area is very important to investigate the effects on health caused by radionuclides, especially having short half-life such as 131I has a half-life of about 8days. However, the dose estimation using the human measurement data is very difficult because of limited measurement data available. In this study, in order to examine the feasibility of the estimation method using atmospheric dispersion simulation, the internal dose due to 131I and 137Cs was estimated using inhalation intake amounts that were calculated from time series air concentration maps generated by a World-wide version of System for Prediction of environmental Emergency Dose Information (WSPEED-II). The maps by atmospheric dispersion simulation almost cover eastern Japan including Fukushima prefecture (690km x 960km) and include the air concentration data from March 12 to April 30, 2011. The spatial and time resolution of the map is 3km wide and 1 hour, respectively. The integrated air concentration map of 131I until March 31 demonstrated that areas where thyroid equivalent doses to 1-y-old children were significantly larger than 10 mSv were limited to the Hamadori district (mainly costal area) of Fukushima prefecture. The validation of this method was confirmed by comparison with those obtained by the screening campaign on the thyroid exposure to children in Kawamata town, Iitate village and Iwaki-city. The comparison was also made for groups of evacuees from 20 km-radius of the station. From the comparison results, although the doses by simulation were 2 to 6 times higher than those by the screening campaign on thyroid exposure to children., it was suggested that the estimation method using the simulation results would be a good indicator for estimating the internal dose the residents that have no the human measurement data. For further validation, the comparison will be made for persons having both their measurement data and information on individual behaviour.International Conference on Individual Monitoring of Ionising Radiation 2015（iM2015