Influence of dynamics of 131I fallout due to the ChNPP accident on value of absorbed doses in thyroid for population of Bryansk and Kaluga regions of Russia

Proposed is a model for estimating absorbed doses in thyroid using the time-dependencies between the volume concentration in the surface air layer and density of 131I fallout on soil obtained by modelling atmospheric transport of 131I. Using of Bryansk and Kaluga regions of the Russian Federation as an example it has been shown that the dynamics of 131I fallout on the territories contaminated after the ChNPP accident has a significant effect on estimation of absorbed doses in thyroid in inhabitants of these regions. Individual absorbed doses in thyroid of inhabitants of some settlements in Bryansk and Kaluga regions contaminated due to the ChNPP accident have been estimated from the results of direct radiometry of thyroid carried out in May-June 1986. Verification and comparison of data of thyroid radiometry for contaminated territories of Bryansk and Kaluga regions has also been conducted. The relation between estimated 131I activities and absorbed doses in thyroid of inhabitants of these regions reflects differences in character and intensity of radioactive contamination of the territories under consideration. The results indicate that adjustment of the values is required and this, as a rule, lowers estimated activities of incorporated 131I. The average value of this correction is about 0,1 µCi and it varies depending on the age of a person under examination. The average absorbed doses in thyroid for different age groups of people living in the contaminated points is higher by a factor of 5 to 15 in the Bryansk region in comparison with the Kaluga region. The paper presents analysis of the relationship between the estimated absorbed doses in thyroid and 137Cs contamination density. It has been shown that the statistical hypothesis concerning the linear relationship between the average thyroid dose and 137Cs contamination density seems to be unjustified for the residents of the contaminated areas of the Kaluga and Bryansk regions.