Mesoscale modelling of radioactive contamination formation in Ukraine caused by the Chernobyl accident

This work is devoted to the reconstruction of time-dependent radioactive contamination fields in the territory of Ukraine in the initial period of the Chernobyl accident using the model of atmospheric transport LEDI (Lagrangian-Eulerian DIffusion model). The modelling results were compared with available 137Cs air and ground contamination measurement data. The 137Cs atmospheric transport over the territory of Ukraine was simulated during the first 12 days after the accident (from 26 April to 7 May 1986) using real aerological information and rain measurement network data. The detailed scenario of the release from the accidental unit of the Chernobyl nuclear plant has been built (including time-dependent radioactivity release intensity and time-varied height of the release). The calculations have enabled to explain the main features of spatial and temporal variations of radioactive contamination fields over the territory of Ukraine on the regional scale, including the formation of the major large-scale spots of radioactive contamination caused by dry and wet deposition.     

Distribution of pre- and post-Chernobyl radiocaesium with particle size fractions of soils

The association of radiocaesium with particle size fractions separated by sieving and settling from soils sampled eight years after the Chernobyl accident has been determined. The three size fractions were: <2 microm, 2-63 microm and >63 microm. 137Cs in the soil samples was associated essentially with the finer size fractions, which generally showed specific activities 3-5 times higher than the bulk samples. Activity ratios of 134Cs/137Cs in the clay-sized fractions appear to be lower with respect to the corresponding values in bulk soil samples. This result indicates that some differences still exists in the particle size distribution between 137Cs originating from nuclear weapons, which has been in the soil for decades after fallout, and 137Cs coming from the Chernobyl accident, eight years after the deposition event. This behaviour could be related to "ageing" processes of radiocaesium in soils.     

Radionuclide migration in forest ecosystems--results of a model validation study

The primary objective of the IAEA's BIOMASS Forest Working Group (FWG) was to bring together experimental radioecologists and modellers to facilitate the exchange of information which could be used to improve our ability to understand and forecast radionuclide transfers within forests. This paper describes a blind model validation exercise which was conducted by the FWG to test nine models which members of the group had developed in response to the need to predict the fate of radiocaesium in forests in Europe after the Chernobyl accident. The outcomes and conclusions of this exercise are summarised. It was concluded that, as a group, the models are capable of providing an envelope of predictions which can be expected to enclose experimental data for radiocaesium contamination in forests over the time scale tested. However, the models are subject to varying degrees of conceptual uncertainty which gives rise to a very high degree of divergence between individual model predictions, particularly when forecasting edible mushroom contamination. Furthermore, the forecasting capability of the models over future decades currently remains untested.     

Geographical mapping and associated fractal analysis of the long-lived Chernobyl fallout radionuclides in Greece

Immediately after the Chernobyl accident, a soil sampling programme was undertaken in order to detect and quantitatively analyse the long-lived radionuclides in the Chernobyl fallout. Soil samples (1242 in number) of 1 cm thick surface soil were collected in Greece during the period from May-November 1986. The samples were counted and analysed using Ge detector set-ups. The 137Cs fallout data have already been analysed, mapped and published. In an attempt to improve this analysis and also to extent it to other fallout radionuclides, an in-house unix-based data base/geographical information system (DBGIS) was developed. Multifractal analyses of the deposition patterns have also been performed. In the present work, an analysis of the results of the deposition of 137Cs, 134Cs, 144Ce, 141Ce, 125Sb, 110mAg, 106Ru, 103Ru, 95Zr and 54Mn are presented together with relevant fractal analysis and three characteristic contour maps. The maximum detected values of the above-mentioned radionuclides were 149.5 +/- 0.1, 76.1 +/- 0.1, 32.9 +/- 0.2, 46 +/- 2, 4.56 +/- 0.02, 7.98 +/- 0.02, 79.1 +/- 0.4, 337 +/- 2, 20.1 +/- 0.2 and 3.02 +/- 0.02 kBq m-2, respectively. Furthermore, a statistical technique to compare contour maps was introduced and applied to explain the differences which appeared in the maps of the above-mentioned radionuclides.     

Contamination of Austrian soil with caesium-137

Austria ranks among the countries that have been most strongly affected by the Chernobyl fallout. The mean contamination with 137Cs is 21.0 kBq/m2, of which 18.7 kBq/m2 is due to the Chernobyl accident, whereas global fallout contributes 2.3 kBq/m2. Maximum values of total 137Cs contamination are nearly 200 kBq/m2. Total deposition of Chernobyl 137Cs on Austrian territory is 1.6 PBq or a fraction of around 2% of the 137Cs released from the reactor. 2115 measurements were used to draw the Austrian "caesium map". The geographical pattern of fallout distribution shows regional differences of contamination as high as 1:100.     

The FARMING approach: main results and perspectives of the French FARMING groups

The Chernobyl accident led to a long-term crisis that affected agriculture, food and living conditions in complex ways, and that modified the usual economical, political and social relations between public and private stakeholders at local, national and international level. The EC FARMING project aims to create a European stakeholder network to consider possible rehabilitation strategies for rural areas contaminated after a nuclear accident. The Institute of Patrimonial Strategies of the National Institute on Agronomy had responsibility for setting up the French stakeholder group. The objective was to develop favourable conditions and ways whereby stakeholders could deal with such a complex situation as a nuclear emergency and develop rehabilitation strategies. The results from the 3-year work programme on both strategic and technical aspects, showed an increasing commitment from the stakeholders that led to the building up of a common understanding of what a nuclear accident could be and how it could be dealt with.     

The marine impact of caesium-134 and -137 from the Chernobyl reactor accident

¹³⁴Cs and ¹³⁷Cs from the Chernobyl reactor accident were detected in UK shoreline seawater very quickly after activity from the accident reached this country. Concentrations were highest in areas adjacent to those where deposition over land was highest, but they declined quickly and did not reach radiologically significant levels in terms of public radiation exposure. Subsequently, the distribution in seawater was investigated further afield. Radiocaesium attributable to the Chernobyl accident was found to be widespread: it was readily distinguished from other sources by having a different ¹³⁷Cs:¹³⁴Cs ratio (about 2:1). Its presence was especially noticeable in northern UK waters rather than those to the south; much of the North Sea has been surveyed as well as the Norwegian Sea. Evidence of Chernobyl radiocaesium was found as far north as 70°N and in many of these areas, including the northern North Sea, it overshadowed the effect of BNFL (British Nuclear Fuels plc) Sellafield discharges, previously the main source of these radionuclides.     

Effects of non-human species irradiation after the Chernobyl NPP accident

The area affected by the Chernobyl Nuclear Power Plant accident in 1986 has become a unique test site where long-term ecological and biological consequences of a drastic change in a range of environmental factors as well as trends and intensity of selection are studied in natural settings. The consequences of the Chernobyl accident for biota varied from an enhanced rate of mutagenesis to damage at the ecosystem level. The review comprehensively brings together key data of the long-term studies of biological effects in plants and animals inhabiting over 20 years the Chernobyl NPP zone. The severity of radiation effects was strongly dependent on the dose received in the early period after the accident. The most exposed phytocenoses and soil animals' communities exhibited dose dependent alterations in the species composition and reduction in biological diversity. On the other hand, no decrease in numbers or taxonomic diversity of small mammals even in the most radioactive habitat was shown. In a majority of the studies, in both plant and animal populations from the Chernobyl zone, in the first years after the accident high increases in mutation rates were documented. In most cases the dose-effect relationships were nonlinear and the mutation rates per unit dose were higher at low doses and dose rates. In subsequent years a decline in the radiation background rate occurred faster than reduction in the mutation rate. Plant and animal populations have shown signs of adaptation to chronic exposure. In adaptation to the enhanced level of exposure an essential role of epigenetic mechanisms of gene expression regulation was shown. Based on the Chernobyl NPP accident studies, in the present review attempts were made to assess minimum doses at which ecological and biological effects were observed.     

137Cs in freshwater fish in Finland since 1986--a statistical analysis with multivariate linear regression models

The accident at the Chernobyl nuclear power plant in 1986 significantly elevated the 137Cs levels of fish in Finnish lakes. About 6200 fish samples from 390 lakes comprising 20 species have been analysed for 137Cs since 1986. The sizes of the lakes varied from a few hectares to about 1000 km2. Activity concentrations of 137Cs in fish still varied widely in 2003, from 16 to 6400 Bq/kg fresh weight. This paper presents the results of statistical analyses with multivariate linear regression models carried out on the empirical data collected since 1986. The statistical analysis resulted in separate models for two time periods describing temporal changes of 137Cs in fish. The explanatory variables were fish species with various feeding habits, the size class of the lake, municipal division, drainage area, time since the deposition and deposition level of the municipality. The calculated values for 137Cs in fish did not differ statistically significantly from the observed values in the validation data. The explanatory variables explained 58% (the first time period) and 72% (the second time period) of the total variability of 137Cs in fish.     

The comparison of generic model predictions with chernobyl fallout data on the transfer of radioiodine over the air-pasture-cow-milk pathway

Data have been collected on concentrations of ¹³¹I in air, vegetation and milk from numerous locations receiving Chernobyl fallout. Time-integrated concentrations derived from these data are used to compare predictions from generic models used for routine environmental radiological assessments. In general, the models markedly overestimated the transfer of ¹³¹I over the air-grass-cow-milk pathway. The reasons for this overestimate appear to be related to a combination of different factors, among which overestimation of the dry deposition velocity assumed for elemental ¹³¹I, overestimation of the interception and retention of wet-deposited ¹³¹I by pasture vegetation and overestimation of the cow's diet-to-milk transfer coefficient appear to be most important. The low transfer of Chernobyl ¹³¹I from air to milk indicates that the direct inhalation of contaminated air by humans may be more important in determining the ¹³¹I exposure to large populations than the consumption of contaminated cow's milk. Radiological assessments conducted prior to the Chernobyl accident have typically assumed that ¹³¹I exposure would be dominated by the consumption of milk. The consumption of milk, however, is still of dominant importance for the exposure of critical population subgroups composed of infants and small children.     

An analysis of the environmental mobility of radiostrontium from weapons testing and Chernobyl in Finnish river catchments

The mobility of radiostrontium within the Arctic environment and surrounding area has been studied by analysing the mobility of 90Sr in river catchments that are within Finland. The environmental mobility of 90Sr deposited by both nuclear weapons testing and the Chernobyl accident has been investigated in five Finnish river catchments. Different models assessing the time-dependent mobility of 90Sr have been evaluated. No significant differences were found between the mobility of 90Sr from nuclear weapons tests and from the Chernobyl accident. Model parameters obtained by fitting to the measurements of the deposition and runoff rates of the nuclear weapons test fallout gave predictions which were consistent with the mid- and long-term contamination by the Chernobyl fallout. A comparison of 90Sr with 137Cs showed that they had similar mobility on deposition but, as time passed, the relative mobility of 90Sr increased with respect to 137Cs over a period of 5-8 years. Once the relative migration of 90Sr with respect to 137Cs reached equilibrium, its runoff rate was, on average, approximately an order of magnitude greater than 137Cs.     

Fallout in the New York metropolitan area following the chernobyl accident

In the metropolitan New York area, maximum concentrations in air of radioactive aerosol and gaseous debris from the Chernobyl accident of April 1986 were much lower than those measured in Europe. The observed maxima were: for gaseous ¹³¹I, 23mBq m⁻³; for aerosol samples, 20mBq m⁻³ of ¹³¹I and 9·mBq m⁻³ of ¹³⁷Cs. The data suggest that little gas-to-particle transformation of iodine occurred during transport of the radioactive cloud from the Ukraine to New York. The ratios of ¹⁰³Ru and other refractories to ¹³⁷Cs were low in the first debris sampled, debris which probably was emitted from Chernobyl in late April during the early stages of the accident. In subsequent samples these ratios were higher, presumably because debris from the later, hotter stages of the fire had reached our sampling sites. A significant fraction (25–40%) of the deposition of ¹³¹I and ¹³⁷Cs into our samplers and on grass was by dry deposition. The total deposition of Chernobyl ¹³⁷Cs in the area was <1% of that already present in the soil from fallout from past nuclear weapon tests. The highest concentration of ¹³¹I measured in fresh milk was about 1.5 B1 liter⁻¹, <0.1% of the US action level. The dose to the thyroid of a six-month-old infant who had fresh milk as a sole food source would be about 70 μGy (7 mrad).     

Assessment of the consequences of the radioactive contamination of aquatic media and biota for the Chernobyl NPP cooling pond: model testing using Chernobyl data

The ‘Cooling Pond’ scenario was designed to test models for radioactive contamination of aquatic ecosystems, based on data from the Chernobyl Nuclear Power Plant cooling pond, which was heavily contaminated in 1986 as a result of the reactor accident. The calculation tasks include (a) reconstruction of the dynamics of radionuclide transfer and bioaccumulation in aquatic media and biota following the accident; (b) assessment of doses to aquatic biota; and (c) assessment of potential doses and radiation risks to humans from consumption of contaminated fish. Calculations for the Scenario were performed by 19 participants using 6 different models: LAKECO-B (Netherlands); LAKEPOND (Romania); POSOD (USA); WATER, GIDRO and ECOMOD-W (Russia). For all endpoints, model predictions were compared with the test data, which were derived from the results of direct measurements and independent dose estimates based on measurements. Most of the models gave satisfactory agreement for some portions of the test data, although very few participants obtained good agreement with all criteria for model testing. The greatest level of difficulty was with the prediction of non-equilibrium radioecological processes in the first year after the accident (1986). The calculations 5 for this scenario gave modellers a unique opportunity to test their models using an independent data base and to analyse the advantages and weaknesses of different model approaches. The use of post-Chernobyl data in such a scenario is also recommended for use in training students in the field of radioecology and environmental protection.     

Reconstruction of radioactive plume characteristics along Chernobyl's Western Trace

Using data obtained from 435 radiation sampling stations in the Red Forest, 1.5 km W if the Chernobyl Nuclear Complex, we reconstructed the deposition pathway of the first plume released by the accident, Chernobyl's Western Trace. The dimensions and deposition rates of the plume remain sharply defined 15 years after the accident. Assuming a uniform particle distribution within the original cloud, we derived estimates of plume dimensions by applying geometric transformations to the coordinates at each sample point. Our derived estimates for the radioactive cloud accounted for 87% of the variation of radioactivity in this region. Results show a highly integrated bell-shaped cross-section of the cloud of radiation, approximately 660 m wide and 290 m high, traveling at a bearing of 264 degrees from reactor IV. Particle sizes within Chernobyl's Western Trace were within the most dangerous range for inhaled aerosols (2-5 microm). Therefore, reconstruction of the dispersion of such particles is critical for understanding the aftermath of nuclear and biological aerosol releases.     

A review and test of predictive models for the bioaccumulation of radiostrontium in fish

Empirical relations between the ⁹⁰Sr concentration factor (CF) and the calcium concentration in freshwater aquatic systems have previously been determined in studies based on data obtained prior to the Chernobyl accident. The purpose of the present research is to review and compare these models, and to test them against a database of post-Chernobyl measurements from rivers and lakes in Ukraine, Russia, Belarus and Finland. It was found that two independently developed models, based on pre-Chernobyl empirical data, are in close agreement with each other, and with empirical data. Testing of both models against new data obtained after the Chernobyl accident confirms the models’ predictive ability. An investigation of the influence of fish size on ⁹⁰Sr accumulation showed no significant relationship, though the data set was somewhat limited.     

Uptake of dry-deposited radionuclides in Fucus—A field study after the Chernobyl accident

The dry deposition of various radionuclides emanating from the plume from the Chernobyl reactor accident was measured at two locations in southern Sweden at the beginning of May, 1986. Samples of Fucus, taken at or near these locations, were also analysed. No precipitation had fallen during the time between the accident and the time of sampling. The ratios between activity concentrations in Fucus and dry depositions on the water surface have been calculated. For any specific radionuclide, this ratio was found to be the same at the two locations, after differences in salinity and in uptake between F. vesiculosus and F. serratus had been taken into account.     

How long is long-term? reflections based on over 20 years of post-Chernobyl management in Norway

The radiation protection community has only recently started the important work of preparedness for long-term post-accidental management of radioactively contaminated areas, like for instance the EC projects STRATEGY, FARMING and EURANOS and the French authorities' CODIRPA and PAREX programmes. There are, however, different views concerning how long a long-term management might last. Based on the Norwegian and former Soviet Union experience after the Chernobyl accident, it is clear that a nuclear accident can entail decades of necessary management and rehabilitation of living conditions. The time period is dependent on a number of factors, e.g. amount of fallout, type of radionuclides, land use of contaminated area, number and density of people affected and available techniques and resources for implementing countermeasures. This paper discusses the management strategy implemented in Norway after the Chernobyl accident, the need for changing strategy over time and the important involvement of affected groups. Careful planning and reflections should be undertaken before actions are taken in the recovery phase, keeping in mind the possibility of decades with problems.     

Validation of I ecological transfer models and thyroid dose assessments using Chernobyl fallout data from the Plavsk district, Russia

Within the project “Environmental Modelling for Radiation Safety” (EMRAS) organized by the IAEA in 2003 experimental data of ¹³¹I measurements following the Chernobyl accident in the Plavsk district of Tula region, Russia were used to validate the calculations of some radioecological transfer models. Nine models participated in the inter-comparison. Levels of ¹³⁷Cs soil contamination in all the settlements and ¹³¹I/¹³⁷Cs isotopic ratios in the depositions in some locations were used as the main input information. 370 measurements of ¹³¹I content in thyroid of townspeople and villagers, and 90 measurements of ¹³¹I concentration in milk were used for validation of the model predictions.A remarkable improvement in models performance comparing with previous inter-comparison exercise was demonstrated. Predictions of the various models were within a factor of three relative to the observations, discrepancies between the estimates of average doses to thyroid produced by most participant not exceeded a factor of ten.     

Atmospheric transport patterns and possible consequences for the European North after a nuclear accident

The main purpose of this study is to examine possible impacts and consequences of a hypothetical accident at the Kola nuclear plant in north-west Russia on different geographical regions: Scandinavia, central Europe, European FSU and Taymyr. The period studied is 1991-1996. An isentropic trajectory model has been used to calculate forward trajectories that originated over the nuclear accident region. Atmospheric transport patterns were identified using the isentropic trajectories and a cluster analysis technique. From the trajectory model results, a number of cases were chosen for examination in detail using more complete transport models. For this purpose, the models MATHEW/ADPIC, DERMA and a newly developed FOA Random Displacement Model have been used to simulate the radionuclide transport and contamination in the case of a nuclear accident and their results have been compared with those of the trajectory modelling. Estimation of the long-term consequences for populations after an accident has been performed for several specific dates by empirical models and correlation between fallout and doses to humans on the basis of the Chernobyl accident exposures in Scandinavia.     

Variation in transfer factor of radiocaesium in bank voles (Clethrionomys glareolus) in clear cut and mature forest sites after the Chernobyl accident

Bank voles that were collected between 1986 and 2004 at sites in Chernobyl fallout areas of northern Sweden showed higher (137)Cs activity concentrations at the mature forest sites compared to clear cuts. This difference was not attributed to differences in ground deposition between sites but to differences in aggregated transfer rates to voles. Differences in transfer between forest types were evident for all years 1986-2004 but the change occurred at different rates in the two habitats. The apparent transfer factor between bilberry (Vaccinium myrtillus) and voles was positively related and indicated that a biomagnification was about 1.5 from vegetation to these small mammalian herbivores. The aggregated transfer factor to bank voles measured in the forest habitat, although starting at higher levels declined faster with time than clear cut sites and the differences between the forest habitat and the clear cut areas diminished with time. After the Chernobyl accident in 1986 the mean level in bank vole was 514Bq/kg fresh mass (SD=505) that increased to 1485Bq/kg (SD=881) in 1988. The activity concentration declined thereafter. The bank voles collected in similar habitats in 2004 contained on average 1022Bq/kg (SD=723). Still 18 years after the radionuclide fallout over Sweden high activity concentrations in voles could be found.