239/240Pu species in radioactive fallout at Semipalatinsk test site (STS)

A complex study on the character of radionuclides distribution in radioactive fallouts of the south-east (thermonuclear) trace was carried out using field and laboratory radiological researches. The south-east trace resulted from thermonuclear test conducted on 12.08.53. The yield of the charge was 400 kt. The selection of the site was based on the fact that this direction had maximal territory contamination within STS. The contamination trace of 137Cs stretches for more than 120 km and partially extends beyond the border of STS. To make a detailed study of radionuclide contents in radioactive particles of radioactive fallouts in the thermonuclear trace, the territories at a distance of 2, 30, 60, 90 and 120 km from the ground zero were investigated. Determination of quantitative and qualitative composition of man-made radionuclides in the inspected territory made it possible to assess the degree of radioactive contamination of soil, and to detect the outstanding features of the contamination. The obtained data allow determining the dependence of radionuclide content in soil samples upon the distance from the ground zero. At 30 km distance the activity concentrations of 239/240Pu, 241Am and 137Cs in soil decreased 30 times. A particle analysis of the collected samples was performed. The results obtained give evidence that the character of the radionuclide distribution among particle-size fractions of soil depends upon the distance from the source of contamination. At 30 km distance from the ground zero the 137Cs and 239/240Pu content in the soil with grain size less than 100 μm is about 10 % and 20 %, respectively, with the increasing distance the content increases up to 80 %. To assess the 239/240Pu biological availability and geochemical mobility, the water-soluble, exchangeable, mobile, acid-soluble and adsorbed plutonium forms in the soil samples collected at different distance from the ground zero were determined. Water-soluble form of 239/240Pu is within <1-5 % from the total content in the samples collected near the ground zero. However, at 120 km distance from the ground zero Pu solubility sharply increases to 33 % of the total plutonium activity in the sample. Data on relative plutonium contents in exchangeable, mobile, acid-soluble and adsorbed forms illustrate the difference in plutonium mobility and migration ability depending on distances from the ground zero. Thus, a great migration ability of 239/240Pu at significant distance from the ground zero resulted in high content of Pu in a water-soluble condition (colloid form) and in exchangeable, mobile conditions as well. With the larger distance from the ground zero the basic part of radioactivity concentrates in small-size fractions. It is possible to expect an increase of radionuclides solubility degree; this in its turn increases the risk of quicker radionuclide entry into soil-plant-animal-man food chain through vegetation. (author)