The method of processing and the results of measurements of 131I content in the thyroids of Russian people performed in May-June 1986 are presented. The contribution of radiation from Cs radionuclides in the human body was taken into account in the processing of measurement data with an SRP-68-01 device. The greatest individual 131I content was found in the thyroids of inhabitants of the Bryansk region, up to 250-350 kBq, and in the Tula and Orel regions, up to 100 kBq. The average 131I thyroid activity in the middle of May 1986 reached 80 kBq for inhabitants of some settlements in the Bryansk region, 5-8 kBq in the Tula region and 5 kBq in the Orel region.

INTRODUCTION The Chernobyl accident has provoked radioactive contamination upon such extensive territories that the monitoring of short-lived iodine isotopes and estimation of their influence upon people were performed with a delay or not completely. For solving the problem of dose estimation for thyroid exposure with iodine radionuclides among the population of contaminated territories, it was necessary to analyze retrospectively the existing data and to develop a model of the thyroid dose reconstruction. The reconstruction procedure for the average for a settlement thyroid dose in Russians has been worked out basing on the following data received in May-June 1986: 44 thousand measurements of I-131 content in the thyroid of inhabitants; 2000 gamma-spectrometric measurements of milk samples in Tula region; about 3500 measurements of total beta-activity in milk samples from Bryansk, Kaluga and Orel regions; over 100 radiochemical analysis of milk samples for I and Cs radionuclides’ content; about 14000 thousand polls of inhabitants and local administration authorities upon the regime of behavior, milk consumption, cattle pasture dates and countermeasures in May 1986; the data of Roshydromet upon Cs-137 contamination of soil in settlements. The individual dose estimations by direct measurements of I-131 in the thyroid have been accepted as the basis for dose reconstruction.. In the absence of direct measurements, to reconstruct the thyroid dose, the measurement results of I concentration in milk of local produce and empirical dependencies between the dose in thyroid of inhabitants and the I concentration in milk were used. For those settlements where only the level of soil contamination with long-lived Cs radionuclide can be used, the empirical dependence of I concentration in milk on Cs in soil was used for recalculation in the dependence of the average dose on the Cs soil contamination density in the settlement. According to the analysis of meteorological conditions for transfer of radioactive substances released from the destroyed reactor, contamination of the central part of Russia took place due to depositions from one radioactive cloud (1-3). 10 Russian regions were subjected to radioactive contamination, beginning from the Bryansk region in the west of Russia (a part of the “Bryansk – Belorussia spot”), and to the east from it. Radionuclide ratios in depositions on these territories were approximately the same. Therefore, for them we may use the methods for assessment of thyroid doses in inhabitants obtained on the basis of the data of radiation monitoring in 1986 in four most contaminated regions. In the report, we present substantiation of the technique for reconstruction of doses in thyroid of inhabitants of Russia due to the Chernobyl accident, approved by theAll-Russia Scientific Commission on Radiation Protection as the official method for assessment of radioiodine isotopes exposure consequences after the Chernobyl accident.

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.

131 I fallout on the territories contaminated after the ChNPP accident has a significant effect on estimation of absorbed doses in the thy- roid in inhabitants of these regions. Individual absorbed doses in the 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 131 I activities and absorbed doses in the thyroid of inhabitants of these re- gions reflects differences in character and intensity of radio- active 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 incorpo- rated 131 I. The average value of this correction is about 0.1 Ci and it varies depending on the age of a person under exami- nation. The average absorbed doses in the 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 than in the Kaluga region. The paper presents an analysis of the relationship be- tween the estimated absorbed doses in the thyroid and the 137 Cs contamination density. It has been shown that the statistical hypothesis concerning the linear relationship between the aver- age thyroid dose and 137 Cs contamination density seems to be un- justified for the residents of the contaminated areas of the Kaluga and Bryansk regions.