旧ソ連セミパラチンスク核実験場周辺集落の環境放射能汚染 : サルジャール、カラウル及びカイナル集落とパブロダール州南部の集落

More than 450 nuclear atomic explosions including atmospheric, aboveground and underground nuclear tests had been conducted by the former Soviet Union (USSR) at the Semipalatinsk Nuclear Test Site (SNTS) over the past 40 years. Since 1994, we have concentrated our energies on assessing the human health and environmental impacts of nuclear testing. Especially, residual long-lived radionuclides such as 137Cs and Pu isotopes have been measured for soil samples from various areas. The studies focused on the evaluation of radioactive contamination in the settlements where the Semipalatinsk historical cohort resided. In October 2005, detailed soil sampling was carried out around Dolon settlement to estimate the width and position of center-axis of the fallout plume associated with the first USSR atomic bomb on 29 August 1949, and provided useful information on the efforts to estimate radiation dose in Dolon settlement as reliably as possible. Such studies were, next, devoted more time and attention to the Sarzhal and Karaul settlements located in the southeastern direction from the SNTS. The local fallout from the fist thermonuclear event on 12 August 1953 (30 m in height, 400 kt in scale) heavily affected those settlements.
Here, laying stress on Sarzhal and Karaul settlements, the accumulated levels and spatial distribution of soil contamination by 137Cs and 239,240Pu have been excessively investigated since 2007. Besides, the radionuclides 236U and 237Np derived from bomb materials themselves were also measured together with the induced radionuclides, 152Eu and 60Co, in some soil samples. Based on those data obtained, radiation doses in air in two settlements were first reconstructed. As a result, adsorbed doses in air in case of the evacuation (non-evacuation) for inhabitants in Sarzhal and Karaul settlements were evaluated to be 20〜100 mGy (210〜530 mGy) and 10〜75 mGy (120〜380 mGy), respectively, as our tentative estimate for 1-year cumulative external doses mainly due to the local fallout from the first USSR hydrogen bomb test in August 1953.
Apart from the above researches, in the Kainar village, one of the villages where the historical cohort resides, located south of the SNTS, and in the Pavlodar District located in the adjust north direction of the SNTS in which data were very limited, accumulated levels of 137Cs and Pu isotopes in soils have been measured for evaluating 137Cs and Pu inventories and their spatial distributions.
The inventories of 137Cs found out both inside and outside of the Kainar village were in a wide range of 130〜7,500 Bq/m2. A clear shape similar to a Gaussian function along the center-axis of the radioactive plume, which has been found in Dolon, Sarzhal and Karaul settlements, was not observed in their spatial distributions, indicating a wider and uniformly close contamination of this region.
On the other hand, in Pavlodar District, the 137Cs inventories ranged from 1,000 to 3,000 Bq/m2 for soil samples. As a whole, Pavlodar areas that the surveys have been conducted were found not to be contaminated so highly, compared with the northeastern areas including Dolon settlement contaminated heavily by the first nuclear atomic explosion in August 1949. The obtained data were also compared with all the data from other areas where we have surveyed until now. The obtained data will be useful for future residents' exposure dose assessment