Estimation of Annual Effective Dose in Namie Town, Fukushima Prefecture due to Inhalation of Radon and Thoron Progeny

抄録

Since Fukushima nuclear accident occurred in 2011, the Japanese government has aimed at reducing the annual effective dose due to high radiation exposure in contaminated areas. Not only the external exposure but also the internal exposure need to be considered for the estimation of the effective dose. In the worldwide average, the effective dose due to radon and thoron by inhalation is estimated to be around 50% from the exposure to natural radiation sources. In the present work, radon, thoron, and thoron progeny measurements have been carried out in 93 dwellings of Namie Town, Fukushima Prefecture. The three-successive-month measurement was conducted for four periods from 2017 to 2019 using a passive type radon-thoron discriminative monitor and a thoron progeny monitor to obtain their activity concentrations and equilibrium equivalent concentration and subsequently to calculate the effective dose. The results showed the method of annual indoor radon activity concentration and thoron progeny concentration to be 31 Bq m^-3 and 0.7 Bq m^-3, respectively. The annual effective doses due to inhalation of radon and thoron progeny using the latest dose conversion factors were estimated to be 1.9 mSv and 0.6 mSv, respectively, and 2.5 mSv in total.