Radiation doses received by adult Japanese populations living outside Fukushima Prefecture during March 2011, following the Fukushima 1 nuclear power plant failures

Data on the concentration of radionuclides in air for March following the reactor failures at the Fukushima NPP were available for Takasaki, Chiba and Tokyo. Gamma dose data for the same/close locations and for fixed locations in other prefectures were also obtained. Gamma dose data was used to calculate the cumulative gamma dose, during 2 weeks (15th–28th March) following the power plant failures. Corresponding doses were calculated for sites in other Japanese prefectures – except Fukushima Prefecture, for which equivalent monitoring data was not published. For Takasaki, Chiba and Tokyo air concentration data and ICRP dose coefficients were used to calculate inhalation committed effective doses (CED, E50) and thyroid equivalent doses (H) for adult members of the public. Average ratios of gamma dose to inhalation CED and inhalation CED from iodine isotopes to thyroid equivalent dose, determined for Takasaki, Chiba and Tokyo, were then used to predict these quantities for sites in other prefectures. Cumulative gamma dose profiles were used to identify dose increments that could be attributed to Fukushima releases within 11 prefectures (excluding Fukushima Prefecture). The most impacted of these were located in Gunma, Ibaraki, Tochigi and Saitama Prefectures – to the south of Fukushima – and in Miyagi Prefecture – to the north of Fukushima. Calculated total doses ranged from 16 μSv in Shizuoka (Shizuoka) to 400 μSv in Ibaraki (Mito). The total doses calculated for the major population centres of Tokyo and Chiba were 97 μSv and 80 μSv, respectively. For all prefecture locations the largest calculated contribution to total dose, during the period of assessment, was from inhalation (∼80%). Estimated thyroid equivalent doses ranged from 5.9 mSv in Ibaraki to 200 μSv in Shizuoka. All total doses calculated were probably overestimates – since no allowances were made for shielding and shelter during the passage of radioactive clouds. Minor contributions to dose from the ingestion of contaminated food and water were not calculated.