[en] Most of the territory of the Chernobyl Exclusion Zone (CEZ) is covered by forest. Forest of CEZ have accumulated a significant part of the radioactive release and for many years have served as a barrier to the non spreading of the radionuclide contamination outside the CEZ. According to the classification of wildfire danger, the forests of CEZ belong to high, above average and medium classes, making cases of wildfires as quite common. Poor, sod-podzolic soils of Ukrainian Polesye contribute to the entry the activity of 90Sr and 137Cs in plant biomass. During wildfires some of the radionuclides contained in combustion products of biomass are emitted into the atmosphere. Biologically important radionuclides such as 90Sr, 137Cs, plutonium isotopes and 241Am bound to fine aerosols - combustion products - can be transported with atmospheric flows over the long range, causing secondary radioactive fallout and forming additional inhalation dose loads on the population. Lack of the actual information on the source term (rate of emission of radionuclides) does not allow reliable modeling of the radiological impact of wildfires. To address this issue, we have proposed a methodology that allows for operational assessments of the dynamics of radionuclide emissions into the atmosphere from wildfires in the CEZ. The basic parameters for the calculations are cartographic data on the density of radionuclide contamination of the territory of the CEZ; classification of the territory of the CEZ according to the distributive features of forests and meadows; classification of CEZ forests according to taxa characteristics to estimate amount of stored fuel biomass (kg/m2); experimental data on the transfer of radionuclides from soil to the main components of biomass for the calculation of radionuclide inventory in fuel biomass (Bq/m2). Thus, for meadows the main fuel component is grass turf, while for forest these are litter, wood, bark and pine needles. experimental data on emission factors of radionuclides from fuel biomass. Implementation of the proposed algorithm in the form of GIS application makes it possible to assess the dynamics of radionuclide emission into the atmosphere by delineation the fire areas on the CEZ map. The NASA WorldView interactive mapping web application can be used to estimate the temporal and spatial characteristics of the wildfire while it is being developed. The contouring of the area affected by fire is carried out according to the analysis of the cluster of thermal points. Also, operational contouring of wildfire can be carried out using data delivered from unmanned aerial vehicles. The application of the proposed algorithm for the analysis of the dynamics of 137Cs emissions into the atmosphere from the April 2020 wildfire showed a good agreement with the data reported by various authors who used the method of inverse simulation. Improving the accuracy of calculations according to the proposed algorithm can be done by rectifying radionuclide emission factors and taking into account fire intensity data, which in turn can affect both the radionuclide emission factor and the degree of burnout of plant biomass.

[en] Radioactive contamination of territories of Ukrainian and Byelorussian Polesye as a result of ChNPP accidental release is characterized by high level of un-homogenity of fallout properties (physico-chemical properties, radionuclide composition etc.), density of the territory contamination by long-lived radionuclides. On the other hand, the soil-plant cover of contaminated territory is presented by large set of soils, characterized by contrast physico-chemical and water-physical properties. Peculiarities of the behavior of different radionuclides, represented initially by various components of radioactive fallout, in soils, as a first link of migration chains are considered

[en] The Chornobyl accident in 1986 led to radioactive contamination of surface water bodies (SWB) in the Chornobyl Exclusion Zone (ChEZ), including lakes (Azbuchyn, Glyboke, Yanov crawl etc) and the Cooling Pond (CP). An abundance in fallout the dispersed fuel particles (FPs) was a specific feature of the accident, comprising the debris of irradiated nuclear fuel in different states of uranium oxidation mixed with construction materials. Contamination of SWB by 90Sr and transuranic isotopes was mainly because of FPs. Experimental studies on the behaviour of FPs in soils and aquatic systems have shown that main factors controlling release of radionuclides outside FPs are: composition of the matrix, state of initial oxidation and oxidation properties of the environment. FPs behavior in SWB has not been sufficiently studied, though limited data suggest contrasting differences to terrestrial environments. Mainly is because of the different oxidation properties of soil and bottom sediment, creating better conservation conditions for FPs in sediment. In case when in SWB bed become dry and exposed, as is the case in the CP after decommissioning, an intensive processes of primary soil formation begin on exposed areas. This later forced dissolution of FPs, and hence radionuclides transition into more mobile forms followed by release to groundwater and surface runoff. We have developed convenient method for identifying Chornobyl FPs based on radiography, which comprises the exposure of X-ray film by spreading over a thin-layered dry solid sample. Processing X-ray films and image analysis makes it possible to estimate the size of FPs, as well as dispersion and distribution of radionuclide activity within the FPs of different size fractions. This also facilitates picking up single FPs to carry out extended individual analysis by EDXRF, SEMs etc. The radiography method was used to estimate the chemical resistance of FPs after sequential leaching to predict the behaviour of radionuclides in natural field conditions. Overall, more than 120 samples were collected from bottom sediments cores taken from different SWB in ChEZ and analyzed by radiography. The results obtained show that from 70 to 90% of activity of radionuclides in sediments are associated with FPs. One gram of sediment contains several 10s to several 100s of individual FPs, while there is significant spatial heterogeneity of FPs density over the territory of ChEZ. The majority of FPs are less than 3 microns and their contribution to total activity was estimated as minor. The main contribution to activity (>70%) comes from particles with a diameter of more than 10 microns and, accordingly, mobility of radionuclides will be determined by processes of destruction and leaching of radionuclides from particles of these size. Chemically resistant FPs are of 3-5 microns, and the highest concentration of fuel particles is typical for SWB located in close proximity to the ChNPP. Up to 7% of the activity remains associated with FPs after being treated with strong 8M nitric acid indicating that 90Sr, transuranic and, partly, 137Cs are confined in chemically very stable particles and may not be mobilized under natural conditions for many decades.

[en] Fuel and condensed particle components of the Chernobyl fall-out are differentiated and the constitution of the fuel component is displayed on a map of the area to about 60 km from the reactor. The nuclear physics related properties of the fuel ''hot particles'' are discussed. The possibility of a long-term increase in the bioavailability of Sr and Pu in soils due to dissolution of these hot particles is commented upon. (author)

[en] Since the 1986 Chornobyl accident transport of radionuclides by Pripyat River shares more than 90% of the annual total flux of radioactivity coming out the Chornobyl Exclusion Zone (ChEZ). 90Sr was the main contributor to this flux. In course of time destruction of the accidentally dispersed ^fuel particles leads to increase of mobile, e.g. water-soluble, forms of 90Sr on territories affected by, while fixation of 137Cs in soil is reflected by significant reduction in 137Cs aquatic transport outside the ChEZ. Heavily contaminated floodplain of the Pripyat River, located in vicinity of ChNPP upstream of Yaniv Bridge up to Ukrainian - Belorussian border, was considered as a “hotspot” with highest risks to the Pripyat and Dnipro water contamination due to recurrent flooding of these territories. This was evidently elucidated after the 1991 ice jam event when drastic increase of 90Sr in water was observed. The dikes splitting leftbank floodplain from the Pripyat river channel were constructed in 1992-1993. Yet, it is still important to quantify the amount of 90Sr that can be washed off the floodplain due to potential dike breakage caused by the extreme floods. Key parameters used to describe status of radionuclide in reaching equilibrium in soil-water system are the distribution coefficient (Kd) and kinetic rate that is reciprocal of typical time scale of desorption processes. These parameters subsequently were estimated in 1991 on the basis of batch experiment carried out with the soil monoliths sampled from the Pripyat floodplain ( Laptev and Voistekhovich, 1991). Results were used in the 2D model COASTOX for justification the construction of protecting dikes (Zheleznyak at al., 1992). To analyze current ability of 90Sr to be washed off the floodplain, soils monoliths were collected in 2020. The experimental studies of the soil cores collected from same location as the monoliths allowed to estimate mobile speciation of 90S and calibrate mass-exchange parameters. Amount of the readily exchangeable forms of 90Sr in soils significantly increased from 10-30% in the first years after the accident up to 65-75% as to 2020. Results of field and laboratory studies were used for simulation the scenarios of 90Sr washing off the floodplain during the dikes breaks on the basis of contemporary version of COASTOX model, that includes the parallel algorithms for numerical solution of the model equations on the unstructured computational grids for multi CPU and GPU systems. Approaches for the modelling of the secondary release of 90Sr due the rapid destruction of “fuel particles” are considered. Taking into account two concurrent processes - decrease of amount of 90Sr in uppermost soil layer due to decay and downward vertical migration (1), amid increased amount of exchangeable forms of 90Sr (2), one could project subsequent increasing of 90Sr in Pripyat and Dnipro river waters downstream the source in case of the dike breakage scenarios. On the other hand, computer simulation suggests that the maximal values of the 90Sr concentrations expected to be not higher than the measured ones during the high floods events after the accident.

[en] Standard approaches to the construction of maps of radioactive contamination do not provide errors in map data, so such maps do not, in fact, guarantee the accuracy of the map information. In this paper, based on the fact that the characteristics of radioactive contamination at a particular point in the territory have a lognormal probability distribution, a methodology for creating maps with a guaranteed confidence level of the provided information has been proposed and tested. There are considered two ways of creating maps, based on the results of ^direct measurements of radioactive contamination characteristics and in the combination of ^direct and ⁱndirect measurements of values statistically related to the mapping characteristic. The approaches and use of kriging methods proposed in the article allow to create maps with a given level of confidence and, accordingly, to take into account the risks caused by the uncertainty of measurements of radioactive contamination characteristics and uncertainty of their approximation. (author)

[en] Cooling Pond (CP) of the Chornobyl Nuclear Power Plant (ChNPP) is one of the most radioactively contaminated large water bodies over the globe. During the active phase of the ChNPP accident, radionuclides got into the CP in result of atmospheric deposition, release of highly contaminated water from system of accidental cooling, and water used to extinguish the fire. In the years after the accident, the contamination was distributed in the CP due to currents. For this period, three types of hydrological conditions dominated in the CP. Initially, the currents were forced by the cooling system of the ChNPP, which caused a circular movement of water. After the decommissioning of the ChNPP, the natural circulation took place in the CP. Starting from the end of 2014, when pumps that continuously fed the CP with water from the Prypiat River were shutdown, a gradual decrease of water level began. Now the water level has dropped by about 6 m leading to the transformation of the whole reservoir into several small lakes and redistribution of radionuclides in them. The objectives of the study were to calibrate models, which were customized for the CP, using data for the whole post-accident period including data collected during the drawdown period by the joint efforts of Ukrainian and Japanese researchers, and then to provide model based predictions of the future radionuclide concentrations in new water bodies. During field studies that were carried out in November 2020, the current state of radioactive contamination of the CP was investigated. Samples of water, suspended and bottom sediments and biota were taken in 9 closed or semi-closed water bodies formed after partial drying of the CP. Concentrations of Cs-137 and its distribution in dissolved and particulated forms were measured in the laboratory. For simulations, the modeling system that consists of the 3D model of thermohydrodynamics and radionuclide transfer THREETOX and the box model POSEIDON-R was created. The THREETOX model was used for the obtaining currents in the CP for each type of hydrological conditions. The POSEIDON-R model was applied for the long-term simulations of the changes of activity concentration in the water, bottom sediments and biota starting from the 1986. The system of boxes in the POSEIDON-R model includes shallow and deep-water boxes. It was built in such a way that after the water level in the CP fell, the calculations were performed only in deep-water boxes. Fluxes of water between boxes were calculated based on currents from the THREETOX model. Seasonal changes in distribution coefficient Kd describing the partition of Cs-137 concentration between water and sediments were also taken into account. Calculated concentrations of Cs-137 in water and bottom sediments agree well with measurements for all boxes and for entire modeling period. It has been shown that POSEIDON-R model is able to reproduce changes in the concentrations of Cs-137 in freshwater fish occupying different levels the food chain. Scenarios for the potential changes of Cs-137 concentrations were considered by the variation of basic parameters.

[en] The paper presents a methodology for prompt mapping of radioactively contaminated areas. The efficiency of obtaining cartographic information is achieved by using correlation dependences between the characteristics of radioactive contamination obtained during various radiological surveys of contaminated areas. The method of spatial interpolation based on regression-kriging is used to formalize the data on radioactive contamination. This method allows combining the information resulting from direct measurements of density of territory contamination by radionuclides we are interested in with the information contained in other characteristics of radioactive contamination received at the surveyed area (for example, ER). Such an approach allows more accurate mapping of ¹³⁷Cs deposition density and concentration of its activity in the root layer of soil, as well as significant reduction of the time and cost to survey the mapped area (sampling, sample preparation and measurement of samples). This, in turn, accelerates and reduces the cost for mapping of radioactively contaminated territories (fields, lands, sites). The methodology has been tested on the fields contaminated by radionuclides resulting from the Chornobyl accident in the Narodychi, Polissia and Ivanivka Districts and has proved its performance and efficiency. Comparison and analysis of the obtained maps of radioactive contamination of the territory show that in the conditions of limited amount of data on direct measurements and time limit, the use of correlation dependences between the characteristics of radioactive contamination of soil is often the only possible way to increase the information content and accuracy of the obtained cartographic information. (author)

[en] Radionuclide concentrations have been determined inside and outside the cabs of tractors operated on soils that are typical of the 30 km exclusion zone around the Chernobyl nuclear power plant. It was found that when the total plutonium deposit exceeded 3.7 kBq m^{- 2} and the ¹³⁷Cs deposit exceeded 7.4 MBq m^{- 2}, the levels of these radionuclides in the operator's cabin could exceed the maximum permissible air concentrations. However, due to the seasonal nature of work, the quantities of these radionuclides inhaled would not exceed the annual limit on intake. Dose to the lungs caused by the inhalation of hot particles has been addressed by either including or neglecting spatial dose distribution. The levels of risk of carcinogenic changes in cells of lung tissue calculated according to each of the two approaches have been shown to be of the same order of magnitude. (author)

[en] A series of deposition measurements have been undertaken in 1988 around a decontaminated village close to the Chernobyl nuclear power plant. The results indicate that the spread of contamination into decontaminated areas via resuspension processes is likely to be slow. Deposition rates within the village were higher than those a short distance away and might reflect the importance of traffic-generated resuspension. Agricultural operations in the area were found to have a measurable effect on the spread of contamination up to a few hundred metres, but were unlikely to result in a major regional spread of contamination. (author)