Mechanisms of the formation of radioactive soil contamination in the waterstream zone from the tunnel in the area of underground nuclear tests at the Degelen site, Semipalatinsk test site

This study investigates the distribution of ¹³⁷Cs, ⁹⁰Sr, ²⁴¹Am, and ^239+240Pu in soil micro-aggregate fractions of various sizes in a waterstream area emerging through a tunnel cavity at the Degelen site of the Semipalatinsk Test Site, where an underground nuclear test was conducted. The objective was to identify radionuclides distribution patterns in soil micro-aggregate fractions and key mechanisms of radioactive soil contamination resulting from prolonged contact with contaminated water. Results show that radionuclides distribution among soil micro-aggregate fractions is uneven and depends on fraction size and composition. Mineral components (40-1000 μm sand particles) are significantly depleted in radionuclides, while organic matter (partially decomposed plant residues, humus) demonstrates high accumulation, especially of ^239+240Pu. A general trend was observed indicating that radionuclides concentration, especially ¹³⁷Cs, increases with decreasing particle size. However, this pattern is less expressed for ⁹⁰Sr, ²⁴¹Am, and ^239+240Pu due to their significant enrichment in organic matter. Thus, sorption processes in the "soil-water" system, increasing radionuclide accumulation as particle size decreases, and the formation of organic substances with uniformly high radionuclide content, which tends to level out typical sorption trends, are identified as the main mechanisms shaping radionuclide distribution in soil micro-aggregate fractions. Additionally, the presence of highly radioactive individual particles ("hot" particles) may locally enhance enrichment of particular fractions.