Internal thyroid doses to Fukushima residents—estimation and issues remaining

Enormous amounts of radionuclides were released into the environment following the disastrous accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March, 2011. Needless to say, it is of importance to grasp the exposure doses to the general populations living in the radiologically affected areas; however, there has been significant difficulty estimating the internal thyroid dose received by intake of short-lived radionuclides (mainly, 131I) because of the lack of early human measurements. An estimation by the National Institute of Radiological Sciences in FY 2012 was thus performed using a combination of the following three sources: thyroid measurement data (131I) for 1,080 children examined in the screening campaign, whole-body counter measurement data (134Cs, 137Cs) for 3,000 adults, and atmospheric transport dispersion model simulations. In this study, residents of Futaba town, Iitate village and Iwaki city were shown to have the highest thyroid equivalent dose, and their doses were estimated to be mostly below 30 mSv. However, this result involved a lot of uncertainties and provided only representative values for the residents. The present paper outlines a more recent dose estimation and preliminary analyses of personal behavior data used in the new method