Releases of Radionuclides to Surface Waters at Krasnoyarsk-26 and Tomsk-7
AbstractDuring the Cold War, production and testing of nuclear weapons in the United States and the Soviet Union led to major releases of radioactive materials to the environment. Although large studies have begun to clarify the magnitude and impact of releases in the United States, only since Perestroika, has information become available to begin an evaluation of the significance of releases to the environment in the former Soviet Union (FSU). The Radiation Safety of the Biosphere (RAD) Project at the International Institute for Applied Systems Analysis (IIASA), begun in 1995, is currently evaluating the radiation legacy of the nuclear weapons complex in the FSU. Because the three sites of Chelyabinsk-65 (Mayak Production Association - MPA), Tomsk-7 (Siberian Chemical Combine), and Krasnoyarsk-26 (Mining and Chemical Combine - MCC) account for the vast majority of the radioactive materials released to the environment in the FSU, these sites are the focus of RAD's studies. Contamination of such sites has resulted from normal and emergency atmospheric releases (such as the 1993 tank explosion at Tomsk-7), discharge of radioactively contaminated waste and cooling waters into rivers, spills and leaks, and deep-well injection disposal of liquid radioactive waste. This study is limited to the impact of past discharges of radioactive materials to the Yanisei River at the MCC and the Tom River at the SCC. Future studies are planned to asses the significance of deep-well injection of wastes at the MCC. This report draws on data ranging from published reports by Western scientists to unpublished data from the sites and affected regions to compile an initial picture of the currently most contaminated portions of these two rivers and to make a preliminary estimate of the potential doses. The report also considers two hypothetical scenarios. The first scenario examines the potential for redistribution of existing contamination by a major flood and the significance of the dose resulting from such an event. The second scenario considers a release of radioactively contaminated sediments from the surface storage basins into the adjacent river with an estimate of the resultant doses. This movement of the contaminated particles is based on an original, unvalidated model. Thus the results, based on incomplete data, provide insight into the magnitude of the problems that might occur but should not be used to determine regulatory compliance or degree of cleanup required. The results of the study indicate that some areas of the Yanisei River floodplain and island system are significantly contaminated. Conservative estimates of the maximum potential annual dose along the Yanisei are in the range of 5-15 millisieverts (mSv) per year. However, conservative estimates of the potential doses along much of the river are near or below the commonly accepted annual dose limit of 1 mSv per year. Contamination is mainly limited to relatively small areas, particularly in deposition zones around islands and depressions in floodplains that trap contaminated sediments during floods. Contamination is lower in the Tom River; only within the first few kilometers of the discharge point is there a potential for exceeding the 1 mSv annual dose limit. Doses along most of the river are substantially below this level. In addition, data on contamination of fish were available for the Tom River: based on conservative estimates, annual doses from fish consumption of up to 3 mSv are possible. This dose is primarily due to short-lived activity released from the control systems of the reactor at the SCC. At both sites, the discharge of radioactive material into the adjacent river has been significantly reduced by the shutdown of the single-pass reactors; at the MCC, additional reductions have resulted from reduced processing rates at the site's reprocessing plant. Based on the results of the hypothetical scenarios, there is no significant potential for extensive contamination downstream from the plants from existing contamination along the floodplains and islands. The resulting contamination would well below background levels and would be essentially undetectable. The increase in the annual dose resulting from such an event is likely to be less than 100 microsieverts in the Yenisei River, and substantially less in the Tom River. During a flood, the majority of contaminated sediments resuspended by the higher flows would remain in suspension for long distances, resulting in a more uniform distribution of radioactive material further downstream. However, release of highly contaminated sediments from the surface storage basins could result in high contamination levels, particularly near the release point. Because accurate data on the characterization of a hypothetical release were not available, the authors assumed unit releases of a relatively small fraction of the contamination in these ponds. However, even these relatively limited releases resulted in high levels of contamination. Higher releases would likely result in higher levels of contamination, and a large-scale pulse release could result in annual doses exceeding the 1 sievert for tens of kilometers along the river if emergency responses were not carried out. It is important to note that discharges into the rivers are not only pathways for radiological contamination at these sites. Contamination has resulted from routine and emergency atmospheric releases of radioactivity (most probably from a 1993 high-level waste tank explosion at Tomsk), wastes have been injected underground at both sites, and there are likely to be significant areas of contaminated soil within the territory of each site. These releases are not evaluated in the current report. This report is therefore an initial step in evaluating the legacy of nuclear weapons production in the FSU.
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