RECONSTRUCTION OF THE COMPOSITION OF THE CHERNOBYL RADIONUCLIDE FALLOUT AND EXTERNAL RADIATION ABSORBED DOSES TO THE POPULATION IN AREAS OFRUSSIA

The results of reconstruction of the radionuclide composition of the C hernobyl fallout in the territories of Russia is presented. Reconstruc tion has been carried out by means of statistical analysis of the gamm a spectrometry data on 2867 soil samples collected in the territories of Ukraine, Byelarus and Russia from 1986 to 1988. To verify the data, aggregated estimates of the fuel composition of the 4th block at the moment of the accident (available from the literature) have been used, as well as the estimates of activity released to the atmosphere. As a result, correlation and regression dependences have been obtained bet ween the activities of the radionuclides most contributing to the dose (Cs-137, Cs-134, I-131, Ba-140, La-140, Zr-95, Nb-95, Ru-109, Ru-106, Ce-141, Ce-144, Sb-125). Statistically significant regression relatio ns between different pairs of radionuclides (including analysis of the 'noise' contribution to the data) depending on the distance between t he point of sample collection and the power station are presented for the 'north-east track' -- the northern part of the 30 km zone and sout hern part of the Gomel 'district (Byelarus) and the Briansk, Kaluga, T ula and Orel districts (Russia). A methodology is also described for r econstructing space-time characteristics of the contamination of the t erritories by major dose-forming radionuclides released from the Chern obyl NPP 4th unit. This methodology is based on the results of modelli ng atmospheric dispersion of radionuclides released between the time o f the accident and 20 May 1986, on data from gamma spectrometry of soi l samples collected around the populated areas in 1986-1988, and on av ailable data for exposure dose rates in the populated areas. Based on the developed model of 'local effective precipitation volumetric conce ntrations and deposition rates of major radionuclides (including short -lived ones) are reconstructed and these are then used to derive a tim e dependence of exposure rate. Account is taken of vertical migration of radionuclides and attenuation by snow cover. The data obtained are further used to assess external absorbed doses from depositions and pa ssing radioactive plumes.