Retrospective studies on radioactive contamination in Sarzhal and Karaul settlements in the Southern areas of the Semipalatinsk nuclear test site

More than 450 nuclear explosions were carried out in the Semipalatinsk Nuclear Test Site (SNTS) in Kazakhstan over a period of the past 40 years (1949-1989). Several hundred thousand peoples who were living in settlements around the SNTS have been impacted by long-term low-dose radiation from the close-in fallout. Considerable efforts have been devoted to assessing health and environmental impacts of nuclear testing. Since 1994, we have investigated the radiological situation in and around the SNTS, and measured long-lived radionuclides such as 137Cs and plutonium (Pu) isotopes, mainly for soil samples from various areas. In order to evaluate external and internal radiation doses for inhabitants, with greater precision, information on spatial distribution of fallout is indispensable, which made the center-axis and width of radioactive plume which passed over settlements very important. In this paper, detailed soil samplings were carried out around Sarzhal and Karaul villages contaminated by the USSR's first hydrogen bomb in 1953, by using the similar method performed previously at Dolon village heavily contaminated by the USSR's first nuclear weapon test in 1949. In 2007-2008, soil samples were performed at 38 locations (Sarzhal) and 21 locations (Karaul) along a line perpendicular to the supposed center-axis of radioactive plume. At each sample location, three soil cores were taken up to a depth of 30 cm using a stainless steel pipe (4.7 cm in diameter). As a result of 137Cs and 239,240Pu measurements in Sarzhal, the variation of their inventories was considerably large even at nearly the same location. Nevertheless, peak-like distributions similar to a Gaussian function were observed. At the plume center-axis, 137Cs and 239,240Pu inventories at Sarzhal were expected to be ranging from 10,000 to 15,000 and 2,000 to 3,000 Bq/m2, respectively. In Karaul settlement, although nearly the same trend as Sarzhal was found, the distributions of 137Cs and 239,240Pu inventories were lower in maximum, and looked widely. The 137Cs inventory at the plume center-axis was around 5,000 Bq/m2. Other interesting point was that 152Eu and 60Co, neutron-induced radionuclides, were detected in Sarzhal and Karaul soil samples. Now, the effort to estimate radiation doses for people in Sarzhal and Karaul is continued.