The radionuclides in sediments and soils evaluated in this study are listed in Table 3.1. In addition to the nuclides listed in Table 3.1, several shorter-lived ra-dionuclides are evaluated in Chapter 5 to estimate doses from consumption of con-taminated fish. In general, levels for the gamma-emitting radionuclides were deter-mined using results of gamma spectrometry analyses of sediment and soil samples performed by previous investigators. To estimate contamination levels where no direct sampling took place, exposure rate data were used in conjunction with the relative ratios of radionuclides based on nearby samples and published external
Table 3.1. Radionuclides evaluated in the present study.
aEvaluated only for the contaminated fish pathway.
exposure dose conversion factors. In addition, contamination levels were estimated from exposure dose rate data using the radionuclide distribution of nearby sam-ples and dose conversion factors from external exposure to nuclides. Data on90Sr contamination, which is a pure beta emitter, were practically nonexistent for Kras-noyarsk. Therefore the environmental concentrations for this radionuclide were assumed to be the same as those for137Cs (see discussion in Section 4.2.2). Ad-equate data on alpha-emitting nuclides were also generally lacking for the Yenisei River, and therefore these nuclides were not included in the current analysis.
The primary release pathway for 137Cs and 90Sr was probably accidental re-leases of reprocessing waste from the radiochemical plants. This same pathway is likely the cause of releases of ruthenium, uranium, plutonium, and other transuranic radionuclides. The radionuclides60Co, 152Eu, and154Eu are activated corrosion products that were probably discharged with water used to cool the once-through reactors at these two sites. Contamination levels for other activation products, in-cluding22Na,24Na,51Cr,54Mn,56Mn,59Fe,56Co,65Zn,76As,144Ce, and156Eu, were also reported in some radiological surveys. However, because the once-through reactor designs ceased operations in 1990 at the SCC and in 1992 at the MCC (Bradley, 1997), the release of these short-lived radionuclides (all with half-lives of less than one year) has decreased significantly. Only releases from open-loop cooling of the control rods of the dual-purpose reactors result in continued release of these short-lived radionuclides.
Based on available data concerning plutonium production at the SCC and the MCC and on data from similar reactors at the Hanford site in the United States, Bradley (1997) suggests that 75–80% of the decay-adjusted radioactivity released to the environment may result from63Ni (half-life = 100 years). This radionuclide, a weak beta emitter, has not been reported in the environment at either site, which is no indication that it is not present. However, assuming the relative releases of
radionuclides provided by Bradley and using the exposure scenarios discussed in Section 3.4, the contribution of the dose from 63Ni is approximately 15% of that provided by90Sr, indicating a relatively low dose contribution even if the radionu-clide is present in large amounts.
Data related to levels and locations of specific radionuclides were based on site reports and analysis of data in the literature. For the Yenisei River, the pri-mary sources of information on existing contamination were site data provided for this study, summaries of radiological surveys of the river made in 1990–1991 (Khizhnyak, 1995; Kosmakov, 1996), and a 1995 river sampling expedition by a joint US/Russian team of investigators (Phillips et al., 1997). Selected results of the 1990–1991 expedition have been published by Bradley (1997) and Robinson and Volosov (1996), so that combining the data from those reports with data pro-vided by the site gives the most complete picture to date, outside Russia, of the existing contamination in the river valley. The data of the joint US/Russian expe-dition are considered a limited independent verification of Russian data in the few areas of overlap.
For the Tom River below the SCC, the primary sources of data were the monitoring activities of the SCC (Andreev et al., 1994) and the research activi-ties of off-site organizations such as Roskomgidromet (1991), Goskomecologia of the Tomsk Oblast (1996), and Tomsk Spravka (1994). Other contamination data were provided by Rikhvanov (1997), Lyaschenko et al. (1993), Zubkov (1997), Arkhangelskii et al. (1996), and Rikhvanov (1994). Summaries of some of these references were provided by the site contacts, and some references were summa-rized by Bradley (1997). According to the available data, surprisingly low levels of radioactivity were reported in the Tom River relative to the Yenisei; possible reasons for this situation are discussed in Chapter 5.
The primary measure for reporting contamination data for river bottom sed-iments and floodplain soil samples was surface contamination density (in curies per square kilometer, Ci/km2). Less frequently, the data were reported in terms of concentrations [e.g., microcuries per kilogram (Ci/kg) dry weight of sediment or soil]. Typical contamination profiles by depth were reported for the Yenisei River but not for the Tom River.
Results of aerogamma surveys of the Yenisei and Tom Rivers were used to estimate the length of contamination along the river channel. Widths of contami-nated plots were estimated for the Yenisei using statistical contamination data from a 13-km reach of that river. For the Tom, assumptions about the widths of contam-ination were made using expert judgement based on the topography of the flood-plains. The contamination data for the radionuclides of interest were converted to soil mass concentration values by assuming a mixing depth of 20 centimeters (cm) and a bulk soil density of 1,800 kilograms per cubic meter (kg/m3).