As a determinant of the associated health risks, the behavior of radionuclides in natural ecosystems needs to be better understood. Therefore, the activity concentration of various long-lived radionuclides released due to the Chernobyl accident, and the corresponding contributions to the whole-body dose rate, was studied as a function of time in mammalian indicator species inhabiting the natural forest ecosystems of Belarus, the bank vole (Clethrionomys glareolus) and the yellow-necked mouse (Apodemus flavicollus). The activity concentrations of 137Cs, 134Cs, 90Sr, 238Pu, 239,240Pu, 241Pu and 241Am in soil and in animals were measured at five monitoring sites with different ground deposition of radionuclides at different distances from the destroyed reactor. The observed temporal pattern of the radionuclide activity concentration in the studied animal populations reflects the changes in biological availability of these isotopes for biota, mostly due to fuel particle destruction and appearance of dissolved and exchangeable forms of radionuclides. The time course of 134+137Cs activity concentrations in animal populations appeared as a sequence of increase, peak and decrease. Maximal levels of radiocesium occurred 1-2 years after deposition, followed by an exponential decrease. Concentrations of incorporated 90Sr increased up to the tenth year after deposition. The activity concentrations of transuranic elements (238Pu, 239,240Pu, 241Pu and 241Am) were much lower than those of the other radionuclides, in the studied animals. A considerable activity of 241Am in animals from areas with high levels of contamination was firstly detected 5 years after deposition, it increased up to the tenth year and is expected to increase further in the future. Maximal values of the whole-body absorbed dose rates occurred during the year of deposition, followed by a decrease in the subsequent period. Generally, this decrease was monotonic, mainly determined by the decrease of the external gamma-ray dose rate, but there were exceptions due to the delayed maximum of internal exposure. The inter-individual distributions of radionuclide concentrations and lifetime whole-body absorbed doses were asymmetric and close to log-normal, including concentrations and doses considerably higher than the population mean values.

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The principal sources of plutonium and americium entering the environment are tests of nuclear weapons, the accident at the Chernobyl nuclear power station, and discharges from radiochemical factories. It is necessary to monitor the contamination of soil these produce from the levels caused by global radioactive fallout. The 241pu content in plutonium contaminating soil can vary by an appreciable factor (see Table I). This determines the contribution of 241Anl to the total c~ activity of the soil or of a sample prepared from it.The constancy of the ratio Apu/AAm on contaminated territory is of great importance for the introduction of -r/x-ray spectrometric methods. The number of analyses required for its determination depends on this. According to data in [4][5][6] plutonium is present in soil in various forms: water-insoluble, exchange, mobile, etc. The ratios between these forms differ depending on the content of organic matter, the pH of the medium, the composition of the soil minerals, and other factors. The rate of migration of plutonium is influenced by the conditions of the landscape geochemistry and the intensity of the microbiological processes.In soil samples selected from the accident zone at the Chernobyl nuclear power station, the presence of radionuclides was discovered in particles of destroyed nuclear fuel, aggregates of fuel particles with reactor graphite, and bituminous carbon and secondary radioactive particles [7]. Fuel particles are almost insoluble in aqueous solutions, but as they decompose the radionuclides pass into a mobile state. According to data from natural investigations of the migration of plutonium in soil following an accident in the South Urals in 1957, the rate of such migration and the nature of the plutonium distribution hardly differ from those established in the accident zone at the Chernobyl nuclear power station [8].Although the behavior of americium in natural media is less well studied than that of plutonium, it is considered that the laws governing the migration of plutonium also characterize that of americium [9] According to data in [10] which were obtained in a zone adjacent to the Chernobyl nuclear power station, the ratio between the concentrations of 241Am and 239pu + 24~ in the soil corresponded to their average ratio in the reactor fuel at the time of the accident, i.e.. no influence of migration was discovered. However, one cannot completely ignore the possible influence of migration on the ratio Ar,u/AAm and one must include a determination of this ratio for several samples from different sections when planning to investigate new territories.A radiochemical method is normally used for quantitatively determining plutonium in the environment. This includes the stages of selecting and preparing a sample, splitting it up, initially extracting and subsequently separating the plutonium from other elements, preparing a thin sample, and measuring its tx radiation in counter or spectrometric regimes. The plutonium from the soil samples is extracted either by leachin...

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