Radiation Significance of the Sources of Carbon-14 Releases into the Atmosphere from the Mayak Production Association

The main parameters of the sources of natural (cosmogenic) and anthropogenic carbon-14 at the reactor, radiochemical, and isotope plants of the Mayak Production Association are considered. The results of monitoring the specific activity of ¹⁴С in the air and vegetation in 1965 and 1982–1986 in Metlino settlement by specialists of the Research Experimental Station (ONIS) of the Mayak Production Association are analyzed. The results of monitoring the content of ¹⁴С in organs and tissues of adult inhabitants of the three towns nearest to the enterprise (Ozyorsk, Kasli, Kyshtym), obtained in the 1980s by staff members of Branch no. 1 of the Institute of Biophysics (Biophysics Institute) independently of the ONIS specialists, are discussed. From the results of measuring the specific activity of ¹⁴С in the air in Metlino settlement, the intake of anthropogenic ¹⁴С by adult inhabitants with the food was estimated at 66 kBq/year, and the total content of ¹⁴C in the living body, at 10.4 kBq. These data agree well with the results obtained by Biophysics Institute for the population of Ozyorsk, Kasli, and Kyshtym. Carbon-14 was mainly taken up with the local foodstuffs (74% with cereals, 9% with meat products, and 9% with dairy products). The rate of the ¹⁴C production in the nuclear fuel and reactor core materials by reactions of reactor neutrons with nuclei of light elements (nitrogen, oxygen, carbon) and by ternary fission of ²³⁵U (²³⁹Pu) was calculated. In industrial uranium-graphite reactors (PUGR), the major part of ¹⁴C (about 90%) generated in unintended voids (cavities formed as a result of graphite distortion) during nitrogen purging of graphite blocks. The ¹⁴C activity accumulated in the spent nuclear fuel of the WWER-440 is determined by the content of nitrogen impurity in fuel assemblies and at the permissible limit (up to 0.012 wt %) can significantly exceed the ¹⁴C production from the main materials (²³⁵U and oxygen). The calculated estimates do not contradict the results of single measurements of intensity of ¹⁴C release from Mayak stacks. In the 1980s total intensity of ¹⁴C release (for all sources) into the atmosphere was 60−90 TBq/year.