Thyroid dose reconstruction for the population of Belarus after the Chernobyl accident

An I-131 environmental transfer model - adapted for Belorussian conditions - was applied to estimate thyroid doses for different population groups. For this purpose the available data were analysed and the important radioecological parameters assessed, i.e. (a) the elimination rate of I-131 from grass due to weathering and growth dilution, (b) the initial interception of I-131 by vegetation, (c) the transfer coefficient for I-131 from grass to cow's milk, (d) the yield of pasture grass and (e) the milk consumption rate. Additionally, the influence of applied countermeasures has been taken into account, such as the interruption of locally produced milk consumption, and the appropriate correction factors have been estimated. As a result, the average age-dependent thyroid doses were assessed for the Belorussian population. The highest average doses in children (> 1 Gy) have been estimated for the Bragin, Khoiniki, Narovlia and Vetka raions of the Gomel oblast. The thyroid exposure tends to decrease from the south-eastern (closest to the Chernobyl nuclear power plant areas) to the northwestern part of the republic. When comparing the assessed thyroid doses with dose estimates derived from direct I-131 activity measurements in thyroids (for the locations with more than 15 direct measurements), the results agree fairly well. The model calculation may perhaps overestimate thyroid doses of the population residing in the settlements of the central and northern parts of Belarus, distant from the areas with direct measurements of I-131 activities in soil, grass and milk. These thyroid dose estimates may serve as a basis for further epidemiological studies and risk analyses.