© Collective of Authors, 1990

UDC 614.7:621.039(497.2)

V. F. Markelova, G. Dichev, N. Todorov

Radioecological Monitoring of Environmental Media in the Vicinity of the Kozloduy Nuclear Power Plant

Cooperation among the member countries of the Council for Mutual Economic Assistance (CMEA) promotes the formulation and successful implementation of national nuclear power development programs based on the construction of nuclear power plants with VVER-440 and VVER-1000 reactors designed in the USSR. By 2000, the role of nuclear power in the energy sectors of the CMEA member countries will increase considerably [5].

The Kozloduy nuclear power complex has been built in the People’s Republic of Bulgaria. It consists of two nuclear power plants of the same name, each equipped with two VVER-440 reactors, and NPP-III, with a VVER-1000 reactor.

In accordance with a joint work program of the Institute of Biophysics of the USSR Ministry of Health and the Kozloduy NPP, systematic determinations of the volumetric activity of radionuclides in environmental media have been carried out since 1982 as part of the effort to address one of the principal tasks of radioecological monitoring in the NPP region [1]. The experimental data accumulated make it possible to assess the radiological situation in the vicinity of the Kozloduy NPP in relation to radioactive discharges and emissions during plant operation.

The radionuclide composition of the principal environmental media was investigated by radiochemical and gamma-spectrometric methods [6], using semiconductor spectrometers equipped with germanium–lithium detectors, at the sampling points shown in the diagram and tables.

Tritium activity in environmental samples was measured using liquid scintillation techniques, as the simplest and most…

Layout of sampling locations

Map labels: Romania; Danube River; Lom; Kozloduy; Oryahovo; Harlets; Glozhene; Ogosta River; nuclear power plant; USSR; sampling points 1, 1a, 2, 3, 3a, 4, and 5; N, S, W, E.

…providing sufficient sensitivity. Samples of 8–10 mL were mixed with 8–10 mL of Lumogel and counted in a liquid scintillation counter (models 1215 and 1718, “Rack Beta”) for 5 h. Samples of a standard tritium solution and “dead water” were measured in the same way. The background was 5–6 counts/min, and the counting efficiency was 15–20%. The detection limit at a confidence level of 95% was approximately 5 Bq/L.

Vegetation and food samples were analyzed using the methods described in the literature [7, 10]. The analysis took into account that tritium is present in vegetation and food products as a constituent of water and organic matter. These biological samples were analyzed using a low-background scintillation counter. The background (1 mL of sample and 10 mL of ZhS-8) was 3 counts/min, and the counting efficiency was 39%. The minimum detectable activity for a counting time of 5 h was 18.5 Bq/L.

The results were statistically processed by computer in accordance with GOST standards 11.002—737 and 8.207—76 and the data in [4].

As is known, water discharged from nuclear power plants contains various radionuclides at low activity levels [2]. At the Kozloduy nuclear power plant, as at other nuclear power plants, liquid discharges consist primarily of surplus process water. Because of the low activity of the discharged water, radiochemical analysis is performed periodically and gamma-spectrometric analysis continuously on water from the monitoring tanks before discharge. The concentration of radioactive substances in the surplus water is generally low and is determined by \(^{137}\mathrm{Cs}\) (\(11.5\cdot10^{-2}\) Bq/L), \(^{60}\mathrm{Co}\) (\(5.2\cdot10^{-2}\) Bq/L), \(^{110m}\mathrm{Ag}\) (\(9.6\cdot10^{-2}\) Bq/L), and also Tr (\(5.7\cdot10^{-4}\) Bq/L), whose activity is attributable to the lack of sufficiently effective methods for its removal before discharge.

Systematic monitoring of radioactivity in the vicinity of the plant in 1982–1985 made it possible to detect reliably only \(^{90}\mathrm{Sr}\), \(^{137}\mathrm{Cs}\), and tritium among the artificial radionuclides in samples of water, bottom sediments, and algae (Table 1).

Analysis of the results shows that during this period the \(^{90}\mathrm{Sr}\) content of water samples from the monitoring locations varied from 5.6 to 11.3 mBq/L, whereas that of \(^{137}\mathrm{Cs}\) ranged from 3.4 to 10.0 mBq/L. These weighted-average concentrations are extremely low and close to those currently measured throughout the Danube [8]. It proved impossible to establish a statistically significant elevation in the content of these radionuclides at any sampling location above the level attributable to global contamination of the Danube’s waters.

Table 1

Weighted mean volumetric activity of radionuclides in environmental media in the vicinity of the Kozloduy nuclear power plant during 1982–1985 (\(M\pm m\))

The concentrations of these nuclides in bottom sediments range from 1.1 to 1.9 Bq/kg for \(^{90}\mathrm{Sr}\) and from 2.0 to 5.5 Bq/kg for \(^{137}\mathrm{Cs}\), whereas background concentrations are 0.37–1.85 Bq/kg for \(^{90}\mathrm{Sr}\) and 0.74–11.0 Bq/kg for \(^{137}\mathrm{Cs}\) [3].

The data on the accumulation of radionuclides by aquatic vegetation (1.0–23.8 Bq/kg for \(^{90}\mathrm{Sr}\) and 1.4–5.8 Bq/kg for \(^{137}\mathrm{Cs}\)) show that aquatic vegetation should be regarded as an indicator of radioactive contamination of Danube water.

Total activity, an indicator used in a rapid method for determining radioactive contamination, remains at practically the same level from year to year in all types of samples analyzed, and the slight fluctuations are associated with natural seasonal phenomena. These results are comparable with those of measurements made by an international expedition organized by Atomenergo in autumn 1978 [11].

Observations of radionuclide concentrations in the tissues of fish inhabiting the plant’s discharge and intake canals indicate that these concentrations are generally characteristic of levels attributable to global fallout and do not exceed those in other parts of the Danube.

Table 2 presents the concentration factors (CFs) for \(^{90}\mathrm{Sr}\) and \(^{137}\mathrm{Cs}\) in various components of the Danube aquatic system in the vicinity of the nuclear power plant. These values agree well with published data [8, 12] and allow the minimum, maximum, and weighted mean CFs to be identified. These provide 95% reliability in predicting human radiation doses when the factors are used for dosimetric assessments of the importance of food chains in determining the radiation environment around the Kozloduy nuclear power plant.

Using the data obtained, internal radiation doses were assessed for residents who use Danube water for drinking, as well as doses received through the fish food chain, using the method described in [9]. The weighted mean effective equivalent dose during the observation period due to \(^{137}\mathrm{Cs}\) in Danube water and fish was approximately 0.016 and 0.13 µSv/year, respectively. The collective radiation dose to the population in the vicinity of the nuclear power plant

Table 2

Concentration factors for radionuclides in environmental media

Table 3

Tritium activity concentrations in aquatic vegetation \((M\pm m)\)

…at the Kozloduy nuclear power plant from liquid discharges of \(^{137}\mathrm{Cs}\) was in the range of 0.0017–0.0052 person-Sv/year, and the contribution of other radionuclides did not appreciably change these values. The collective radiation dose to the population from the natural radiation background in this region of Bulgaria was approximately 18 person-Sv/year, more than 10,000 times the dose from liquid discharges.

It should be noted that \(^{134}\mathrm{Cs}\) and \(^{137}\mathrm{Cs}\), in a ratio of 1:2, as well as \(^{106}\mathrm{Ru}\) and other radionuclides, were reliably detected in environmental samples investigated in 1986–1987 at the specified sampling points. This presence is associated with the deposition of radioactive products on the ground as a result of the Chernobyl nuclear power plant accident.

An analysis of data on tritium content in the Danube near the plant over several years showed that the weighted-average activity concentrations were of the same order of magnitude, varying between 6 and 12 Bq/L. Elevated tritium concentrations were observed at distances of 6 km in the intake canal and 500 m and 6.5 km in the discharge canal from the nuclear power plant. These increases are attributable to the release of excess water into the discharge canal and its entry into the flow supplying the intake canal. Further dilution results in lower tritium concentrations. During the observation period, an attempt was made to establish a correlation between tritium content and the total activity of the discharged water. The combined results showed that the ratio of tritium concentration to total activity in water samples was around 100, whereas the average value of this ratio at the control point was 40, with a \(^{40}\mathrm{K}\) concentration in Danube water of \(120\pm12\) mBq/L [8].

The radiobiological significance of tritium is considered relevant to the assessment of radiation doses to humans associated with nuclear production. This radionuclide, released mainly in the form of tritium-enriched water, is readily soluble in water. Its high degree of dilution reduces the average concentration and resulting dose to a very low level.

Table 4

Tritium activity concentration in foodstuffs

At present, despite the large number of publications [13, 15], there is no rigorous mathematical model describing the behavior of tritium in natural ecosystems and its migration through biological chains, including humans.

For radiation dose assessment, it is generally assumed that released tritium moves through the environment with water and that the radiation dose from various sources can be estimated from the activity concentration of tritium in water from those sources and the total amount of water consumed by humans. At the same time, measurements [14] show that the ratio of organically bound tritium (OBT) to hydrogen in plants can be 2–5 times higher than in free water. This is explained by the incorporation of tritium into organic matter primarily through photosynthesis in green plants.

At the present stage of nuclear power development, it is necessary both to calculate the radiation doses received by the population from tritium in the region of a nuclear power plant and to establish permissible limits for its release into the environment. To this end, in the vicinity of the Koz… nuclear power plant,

…duy,” studies to determine tritium levels in environmental biological samples have been conducted since 1986. These studies assumed that 80% of the water in biological samples consists of a transit (free) fraction, while the remaining 20% is associated with hydrogen and tritium incorporated into biological structures.

As shown in Table 3, the tritium inventory in algae and higher aquatic vegetation relative to water and soil during the growing season, at maximum plant biomass and under the prevailing climatic conditions, ranges from 60 to 180 Bq/L. These variations are determined primarily by differences in the tissues of the biological samples and by the uptake of large amounts of tritium oxide during the period of accelerated growth, when tritium fallout is at its maximum. In individual samples, levels of this nuclide differ little between the free and organic fractions.

The dietary analysis included foods identified as critical (potatoes, milk, and dairy products); it also accounted for consumption of leafy greens, vegetables, and melons during the summer–autumn period, which is particularly significant for the Bulgarian population.

Tritium contamination of plant-derived foods occurs directly through irrigation and atmospheric radioactive fallout onto their surfaces, as well as through uptake from the soil into tissues via the root system along the chain water—air—soil—plant foods—humans.

As shown in Table 4, the tritium content of the free fraction of foods and of drinking water is at the same level, namely tens of becquerels per liter, whereas the tritium concentration in milk and dairy products is approximately 10 times higher.

Analysis of potatoes collected in different years showed that the ratio of organically bound tritium (OBT) to the free fraction averages 1:2, while OBT accounts for 55% of the total activity. The observed tritium concentrations in leafy greens (green onions, parsley, and dill) sampled during the spring–autumn period range from tens to hundreds of becquerels per liter and can be used as indicators of its content in other highly organized plants.

The data obtained for the main components of the diet, which constitutes one link in the chain of tritium transfer to the human body, make it possible to estimate the individual equivalent dose and compare it with the dose determined from data on tritium oxide content in bodily excretions (urine).

The dose assessment did not take into account inhalation as a route of tritium intake or the organically bound fraction. It was also assumed that the tritium concentration in free water in the remaining, unanalyzed dietary products was the same as in drinking water.

The mean weighted dose received by the Bulgarian population is \(1.3\pm0.6\) and \(3.2\pm1.3\) µSv/year [2] for children (up to 10 years of age) and adults, respectively, living within 5–200 km of the nuclear power plant. These estimates, based on actual tritium levels in bodily excretions of \(50\pm20\) and \(120\pm50\) Bq/L, exceed the doses calculated from intake by factors of 3 and 8, respectively.

Thus, the radiological conditions in the Danube River aquatic ecosystem in the vicinity of the Kozloduy nuclear power plant are attributable to radionuclides from global fallout. Direct measurements of environmental samples reveal no discernible influence from the plant’s discharges and emissions. The radiation dose from liquid discharges is tens of thousands of times lower than that from natural background radiation. Monitoring \(^{90}\mathrm{Sr}\) and \(^{137}\mathrm{Cs}\) requires even more sensitive methods for measuring low activity levels.

The tritium activity concentrations observed in environmental biological samples primarily represent background values and the contribution from the operating nuclear power plant units. The radioactivity of plant foods is close to the level in Danube water, except in leafy greens (parsley, green onions, and dill), potatoes, and other highly organized plants that, owing to their tissue structure, can accumulate tritium in free and organic fractions. The tritium content of milk (33–100 Bq/L) corresponds to its content in plant biomass.

The continuous entry of anthropogenic tritium into the biosphere calls for continued study of its migration in ecological systems using modern scintillation spectrometry methods. This will make it possible to assess the contribution of this nuclide to the radiation dose received by people living in the nuclear power plant region and to establish a permissible level for tritium discharge into the environment.

The mean weighted dose calculated from tritium content in bodily excretions is approximately 3 times higher for children and 8 times higher for adults than the dose estimated from actual tritium oxide levels in the diet, without accounting for OBT or tritium entering with inhaled air.

Overall, the observed activity concentrations of artificial radionuclides and the radiation doses are far below permissible limits.

References

1. Gedeonov, L. I., Gusev, D. I., Ivanova, L. I., et al. // Radioactive Contamination of Water Systems of International Importance. AEA-C-42/352. Vienna, 1982. pp. 567–572.

1. Ionizing Radiation: Sources and Biological Effects. UNSCEAR Report to the UN General Assembly for 1982. New York, 1982.
2. Investigation of Natural Radioactivity and Background Radiation at the Kozloduy Nuclear Power Plant. Sofia, 1968. P. 56.
3. Maleta, Yu. S., and Tarasov, V. V. Mathematical Methods of Statistical Analysis in Biology and Medicine. Moscow, 1981.
4. Ovchinnikov, F. Ya., Gost, K., Solovyev, V. A., et al. International Cooperation of CMEA Member Countries in Nuclear Power Engineering. Moscow, 1985.
5. Methodological Recommendations for Sanitary Monitoring of Radioactive Substances in Environmental Media. Edited by A. N. Marey and A. S. Zykova. Moscow, 1980.
6. Morozova, G. K., Rovinsky, F. Ya., and Shusharina, N. M. In: Radiochemical Analysis of Environmental Media. Moscow, 1972. Pp. 34–37.
7. Summary Report on the Radiation Situation in the Danube River during 1981–1985. CMEA Standing Commission on Atomic Energy, ORB Series No. 20. Moscow, 1987. P. 31.
8. Assessment of Individual and Collective Radiation Doses to the Population Due to Radioactive Contamination of the Danube River. CMEA Standing Commission on Atomic Energy, ORB Series No. 14. Moscow, 1982.
9. Rovinsky, F. Ya., Iokhelson, S. V., and Yushkan, E. I. Methods for Analyzing Environmental Pollution. Moscow, 1978. Pp. 187–210.
10. Summary Report on the Results of a Scientific Expedition by Specialists from CMEA Member Countries to Study Radioactivity in the Danube River. ORB No. 10. Moscow, 1980. P. 144.
11. Bioaccumulation Factor for Radionuclides in Freshwater Biota. Environmental Science Division Publication No. 783, ORNL-5002. 1975. P. 221.
12. Behaviour of Tritium in the Environment. Vienna, 1979.
13. Dinner, P. I., Gorman, D. J., and Spencer, F. S. In: Symposium on Tritium Technology in Fission Fusion and Isotopic Applications: Proceedings. Dayton, 1980.
14. Gerber, G. B. // Studiecent kernenera. BLC/RAPP/. 1984. No. 563. Pp. 60–88.

Received September 12, 1988.