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The aim of the work is the radiation-hygienic assessment of 90Sr and  137Cs content in fish of the Ob-Irtysh river system and the study of the basic laws of the radionuclides accumulation in the ichthyofauna of these rivers. To perform this task, long-term results of radioecological studies of fish from the Techa, Irtysh and Ob rivers for the period 2004-2016 were used. Fish as a food product was evaluated according to two criteria: a) permissible levels of radionuclides specific activity (SanPiN 2.3.2.1078-01); b) using the indicator of conformity В and uncertainty of its definition ΔВ (GOST 32161-2013 and GOST 32163-2013). It is shown that a higher content of radionuclides is observed in the ichthyofauna of the Techa river (1379.1 Bq/kg for 90Sr and 41.9 Bq/kg for 137Cs). On the Ob and Irtysh rivers, the average specific activity of radionuclides in fish was significantly lower and slightly changed during the surveyed area: for 90Sr in the range of 6.0 ÷ 8.1 Bq/kg (mean 6.8), for 137Cs – 0.6 ÷ 1.9 Bq/kg (mean 1.3). Assessment for compliance with radiation safety criteria using the indicator of compliance and its uncertainty (В+ΔВ) confirmed the unsuitability of use for food purposes by the radiation factor of all studied fish species from the Techa river (45÷55 >1). Fish from all other studied areas of the Ob-Irtysh river system can be used for food without restrictions (0,06÷0,53 < 1). The distribution of 90Sr and 137Cs in the ichthyofauna of the Ob-Irtysh river system part over a 2400 km is presented in the form of empirical regression models. The models describe a sharp decrease in the radionuclides specific activity in fish in the Techa and Iset rivers by two orders for 90Sr (from 2000 to 20 Bq/kg), and by one order for 137Cs (from 40 to 2 Bq/kg). With a high degree of reliability (R2>0.86 for 90Sr and R2>0.92 for 137Cs), the presence of power relationships between the content of radionuclides in the ichthyofauna and their content in water was shown, with adequacy confirmed by Fisher’s F-criteria. This can be used for a preliminary assessment of the radionuclides level in fish based on the results of measurements of the average annual concentrations of these radionuclides in water.

Relevance. In connection with the development of nuclear energy, many aquatic ecosystems have been exposed to radioactive substances. Fish, as an element of biota, is capable to accumulate radionuclides. However, fish is a traditional food. The control of the technogenic radionuclides accumulation level in the ichthyofauna is an important link in ensuring human security.Intention. To analyze the long-term data on the content of long-lived technogenic radionuclides 90Sr and 137Cs in the ichthyofauna of the Ob-Irtysh river system for the period from 2004 to 2016 and Beloyarsky pond for the period from 1977 to 2018.Methodology. Fish as a food product was assessed according to two criteria: a) permissible levels of specific activity of radionuclides (SanPiN 2.3.2.1078-01); b) using the indicator of conformity B and the uncertainty of its definition В (GOST 32161-2013 and GOST 32163-2013).Results and Discussion. Fish of all species that live in the river Tetcha, is not suitable for food use according to the criteria of SanPiN 2.3.2.1078-01, GOST 32161-2013 and GOST 32163-2013. The fish of the Ob-Irtysh river system, the habitat of which is located outside the Tetcha, meets the requirements for fish products. In the period from 1977 to 1989 in the Beloyarsk pond the accumulations of technogenic radionuclides in fish in quantities exceeding sanitary and hygienic standards were possible. Currently, the fish of the Beloyarsky pond fully complies with the sanitary and hygienic requirements for the radiation factor and is safe for human consumption.Conclusion. In the ponds exposed to the atomic energy enterprises, it is necessary to continuously monitor the content of long-lived technogenic radionuclides in fish and assess their amount in accordance with the requirements of SanPiN 2.3.2.1078-01 and using the conformity indicator В and the uncertainty of its determination В.

Relevance. New units commissioning at the operating nuclear power plants may be associated with an increase in the radiation load on environment and human beings. The 4th Unit BN-800 was commissioned at the Beloyarsk nuclear power plant (BNPP) in 2016. The Beloyarsk artificial lake created in 1959–1963 serves as a cooling pond. A radioecological study of the cooling pond was performed in 2014 before the BN-800 Unit commissioning. Its results were published in the issue No. 2 of the present journal in 2016.Intention. The objective of the work is to show (comparing with the results of the previous research) the effect of the BN-800 Unit commissioning on the cooling pond radioecological state.Methodology. The samples of water and macrophytes (Potamogeton pectinatus, Ceratophyllum demersum, Spirogyra, Cladophora) were examined at the reference points of the cooling pond.Results and Discussion. It was found that 137Cs decreased in the water by 1.4–5.9 times in all reference points (including the 4th NPP Unit discharge channel) compared to 2014. Potamogeton pectinatus (from the 4th NPP Unit discharge channel) had 3.1 times less 137Cs and 1.4 times less 90Sr than macrophytes from the Pyshma river area below the dam.Conclusion. The BN-800 Unit commissioning was not associated with additional radiation load on the artificial lake. It does not deleteriously affect the natural purification processes after the 1st and 2nd BNPP Units decommissioning. The further preservation of the Beloyarsk artificial lake natural resources assumes a systematic radioecological monitoring to determine the level of long-lived radionuclides in main natural components of the cooling pond.Sampling from the cooling pond, sample preparation, 137Cs and 90Sr concentration measurement in water and aquatic plants as well as interpretation of results are performed in the framework of the State Task of the Institute of Plant and Animal Ecology, Ural Branch of the Russian Academy of Sciences.