Chernobyl accident: retrospective and prospective estimates of external dose of the population of Ukraine.

Following the Chernobyl accident many activities were conducted in Ukraine in order to define the radiological impact. Considered here are gamma spectrometric analyses of soil-depth-profile samples taken in the years 1988-1999, gamma spectrometric measurements of radionuclide concentration in soil samples taken in 1986, and measurements of external gamma-exposure rate in air. These data are analyzed in this paper to derive a "reference" radionuclide composition and an attenuation function for the time-dependent rate of external gamma exposure that changes due to the migration of radiocesium into the soil column. An attenuation function for cesium is derived that consists of two exponential functions with half lives of 1.5 and 50 y. The dependencies of attenuation on direction and distance from the Chernobyl Nuclear Power Plant are also demonstrated. On the basis of these analyses the average individual and collective external gamma doses for the population of Ukraine are derived for 1986, 1986-2000, and 1986-2055. For the 1.4 million persons living in rural areas with 137Cs contamination of >37 kBq m(-2), the collective effective dose from external exposure is estimated to be 7,500 person-Sv by the end of 2000. A critical group of 22,500 persons who received individual doses of >20 mSv is identified for consideration of increased social and medical attention.