Abstract
For the purpose to evaluate external radiation exposure of inhabitants who were living in settlements around the Semipalatinsk Nuclear Test Site, Kazakhstan, current levels of Pu-239,240 and Cs-137 contamination have been measured in soil samples taken around the test site. The trail of the radioactive plume could be clearly observed in the radioactivity distribution around several settlements. For example, the shape of the radioactive trail created by the first USSR atomic bomb test in 1949 was confirmed near Dolon village located 110 km from the ground zero. As a result of a Gaussian fitting of the radioactivity distribution, the center-line of the radioactive plume was supposed to have passed over about 2 km north of Dolon village. The initial deposition of Cs-137 due to the 1949 test was evaluated by subtracting the contribution of the global fallout from the measured values, and estimated to be 15 kBq m-2 at the center-line of the plume, while it was 7 ± 2 kBq m-2 in the Dolon village area. The deposition levels of fission products other than Cs-137 were estimated by considering their relative ratios of fission yield to Cs-137 as well as the fractionation effect during the transport-deposition process. Assuming a refractory/volatile fractionation factor of 5, external gamma-ray exposure was calculated at 1 m above ground from deposited radionuclides. A cumulative exposure of 350 ± 100 mGy was obtained for Dolon village, 80 % of which was delivered within one month after the deposition. Our cumulative exposure seems to be consistent with values of 440 - 480 mGy by other investigators based on TL measurement using brick samples in Dolon. Thus, local fallout contamination more than 50 years ago could be effectively reconstructed using the recent contamination data in soil.
