Abstract
To reduce uncertainty in thyroid dose estimation, residents’ radiation protection behavior should be reflected in the estimation. Screening data of body surface contamination provide information on exposure levels during evacuation. Our purpose is to estimate thyroid equivalent doses based on body surface contamination levels using a new methodology. We obtained a record of 7,539 residents/evacuees. Geiger-Mueller survey meter measurement value in cpm was translated into Bq/cm2 according to the nuclides densities obtained by measuring clothing from two persons by germanium γ-spectrometer. The measurement value of body surface contamination on head was adjusted by a natural removal rate of 15 hours and radionuclides’ physical half-life. Thyroid equivalent dose of 1-year-old children by inhalation was estimated by two-dimensional Monte Carlo simulation. The proportions of evacuees/residents with measurement value in cpm of Namie and Minamisoma groups were higher than those of other groups during both periods (p<0.01, Kruskal-Wallis). During 12-14 March period, 50 and 95 percentiles of thyroid equivalent doses by inhalation were estimated as 2.7 and 86.0 mSv, respectively, for Namie group, and 4.2 and 17.2 mSv, respectively, for Minamisoma group, 0.1 and 1.0 mSv, respectively, for Tomioka/Okuma/Futaba/Naraha group, and 0.2 and 2.1 mSv, respectively, for the other group. During 15- 17 March period, 50 and 95 percentiles of thyroid equivalent doses by inhalation were 0.8 and 15.7 mSv, respectively, for Namie group, and 1.6 and 8.4 mSv, respectively, for Minamisoma group, 0.2 and 13.2 mSv, respectively, for Tomioka/Okuma/Futaba/Naraha group, and 1.2 and 12.7 mSv, respectively, for the other group. It was indicated that inhalation dose was generally higher in Namie and Minamisoma groups during 12-14 March than those during 15-17 March might reflect different self-protective behavior to radioactive plumes from other groups.
Highlights
The risk of childhood thyroid cancer is a major concern after the nuclear accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) in March 2011 [1]
During 12-14 March periods, the levels of measurement values in cpm were almost same between Namie and Minamisoma groups, but statistically higher than those of Tomioka/Okuma/Futaba/Naraha group and the other group (p
It was noted that the contamination levels in the other group, that included Kawamata, Iitate, Katurao villages located north-west direction from FDNPS, increased and became comparable to the levels in Namie and Minamisoma groups (Figure 2)
Summary
The risk of childhood thyroid cancer is a major concern after the nuclear accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) in March 2011 [1]. The UNSCEAR’s estimates of settlement-averaged thyroid doses were two to five times higher than the corresponding values based on direct measurements. This discrepancy might be, at least in part, due to the difficulty in estimating temporal and spatial nuclides concentrations in the air by ATDM and individual selfprotective behavior during evacuation. Most residents living in the Fukushima Prefecture were screened for their body surface contaminations by Geiger-Mueller (GM) survey meter [5,6]. We introduced a new methodology for estimating thyroid doses based on the levels of body surface contamination
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