F.A. Tikhomirov

Main investigation results on the forest radioecology in the Kyshtym and Chernobyl accident zones.

As a result of the long-term studies of radionuclide migration in forest ecosystems in zones of radioactive contamination after the Kyshtym and Chernobyl accidents, the following trends were revealed: (1) High retention capacity of stand canopy with respect to radioactive fallout. This leads to high doses absorbed by apical and leaf meristems, beta-radiation giving the main part of the dose; (2) Fast self-decontamination of crowns during the growth period and relatively slow decontamination in the phase of physiological rest, regardless of amount of atmospheric precipitation. The rate of crown decontamination determines the value and duration of radiation stress on woody plants; (3) Accumulation not less than 95% of the total radionuclide amount in the forest litter 1-2 years after the cessation of radioactive fallout; (4) Relatively slow migration of strontium and cesium radionuclides along the forest soil profile; (5) High capacity of the forest when serving as a biogeochemical barrier to the routes of horizontal and vertical radionuclide migration and export out of the zone of initial contamination, including migration into the river water; (6) Considerable difference between strontium and cesium when migrating in forest soils and in the soil-plant system; (7) Broad variations in transfer factors for uptake of cesium-137 from soil into forest plants depending on the plant species and soil type. The primary radiobiological effects connected with irradiation of organisms are considered and secondary disturbances due to changes of ecological bonds between the components of irradiated forest ecosystem are discussed.

Association of Chernobyl-derived 239 + 240Pu, 241Am, 90Sr and 137Cs with organic matter in the soil solution

To investigate the extent of association of fallout radionuclides with soil organic matter, gel filtration was applied to the soil solution obtained from the three top horizons AOf, AOh and AOh + A1A2 of a forest soil within the 10 km zone of the nuclear reactor at Chernobyl/Ukraine. In the five fractions isolated (fraction 1: nominal molecular weight Mw ⩽ 2000, fraction 2: Mw = 1300-1000, fraction 3: Mw = 800, fraction 4: Mw = 400 daltons, fraction 5: inorganic compounds), 239 + 240Pu, 238Pu, 241Am, 90Sr and 137Cs were determined. For that purpose, an efficient method for the simultaneous determination of the actinides and 90Sr was developed. The data show that plutonium and americium are associated mainly with the high molecular fraction 1 and to a much smaller percentage also with the fraction 2. While the differences between plutonium and americium were rather small in the top two horizons, americium in the third soil layer is present to some extent also in the fractions 3, 4, and 5. Strontium-90 from the AOf horizon is associated almost exclusively with fraction 4. In the other two soil layers, however, this radionuclide is present essentially only in fraction 5 (inorganic compounds). Caesium-137 from the soil solution of the AOf horizon is associated essentially only with the fraction 3, but in the deeper layers progressively also with all other fractions. Thus, in the third layer, 137Cs is distributed almost uniformly between all five fractions. Because the mobility and biological availability of these radionuclides will depend on their association with soil organic matter, the present data suggest that the determination of only the total concentration of a radionuclide in the soil solution might not be sufficient to interpret or predict adequately the fate of radionuclides in the soil.

Forests and forestry: radiation protection measures with special reference to the Chernobyl accident zone

A large proportion of the area contaminated by the Chernobyl accident in the former USSR is forested and has presented unique problems when considering appropriate post-contamination management and clean-up techniques. These problems are related to the forests role as both a source and sink for radioactive contamination. Although it has been suggested that resuspension from forested areas may provide a secondary source of contamination to adjacent land, data collected after the Kyshtym and Chernobyl accidents suggest that forest ecosystems may also be effective in limiting the further spread of contamination away from the point of initial deposition and that this effect will increase over time. Such evidence serves to highlight the importance of these ecosystems in influencing the behaviour of radionuclides immediately after their release to the environment. Management practices for forested areas adopted since 1986 are described and a critical appraisal is presented of engineering-based countermeasures implemented over the initial post-accident period. These were intended to remove large quantities of contaminated materials from the forest environment. However, it is suggested that the natural processes of self-decontamination of trees and forest floor litter layers are sufficiently rapid and efficient to necessitate radical alterations to the technology based approaches adopted in the first 2 years after the Chernobyl accident.

Dynamics of 137Cs in the forests of the 30-km zone around the Chernobyl nuclear power plant

Dynamics of the 137Cs content in the components of the forests in the 30-km zone around the Chernobyl nuclear power plant (NPP) in 1986-1994 are associated mainly with such factors as the size of radioactive particles in the fallout, ecosystem humidification and soil type, tree age. The influence of particle size was especially noticeable between 1986-1987 and was displayed by low biological availability of radionuclides in the near part of the zone (within the 10-km radius circle around the NPP) in comparison with more distant regions (within the 30-km radius circle). Later, the expression of this influence decreased and transfer factor (the ratio of 137Cs content in overground phytomass to the soil contamination density) became approximately the same for all plots with similar ecological and fallout characteristics. Humidity of landscape and soil type determined the velocity of radionuclide vertical migration in the soil and 137Cs biological availability. These parameters were maximum for the hydromorphic soils of wet landscapes enriched in organic substance and poor clayey minerals. Differences of 137Cs accumulation in overground phytomass of trees caused by tree age are displayed in the higher 137Cs concentration in structural parts of young trees as compared with old ones.

Decontamination methods for reducing radiation doses arising from radioactive contamination of forest ecosystems — a summary of available countermeasures

Abstract This paper provides a short summary of discussions within the REACT ‘Forest Ecosystem’ working group. It considers the applicability and effectiveness of specific countermeasures designed to reduce radiological dose commitments to human populations resulting from the contamination of forested areas, either through occupancy or the use of forest products contaminated with radioactive materials. The results of the groups deliberations are presented as a summary table of countermeasures which are considered to be potentially feasible in forest environments.

Vertical radionuclide transfer by infiltration water in forest soils in the 30-km Chernobyl accident zone

Vertical intrasoil flow within the 30-km zone of the Chernobyl nuclear power plant (ChNPP) was investigated by a lysimetric method in 1989-1990. The regularity of radionuclide migration within the flow has been found to be dependent on the contamination density, the type of radionuclide, and the type of ecosystem and depth.

Principles and practices of countermeasures to be carried out following radioactive contamination of forest areas

Abstract An earlier article [1] featured a comprehensive list of specific measures aimed at reducing radiation levels within forest ecosystems. All these measures were assessed globally in terms of their expected effectiveness and practical limitations. This study considers the same measures but focuses on differences between three widely distributed forest ecosystems in Europe: Mediterranean calcicolous evergreen forest, temperate mesotrophic deciduous forest and boreal acidophilous coniferous forest. From this analysis, it emerges that the relatively wide range of countermeasures which, in theory, could be applied to forest ecosystems is much more limited when considering specific ecosystems and the variation between ecosystems.

The effects of acute irradiation on a forest biogeocenosis; experimental data, model and practical applications for accidental cases

Abstract The effects of acute irradiations of a mixed pine and birch forest in spring and autumn with a high power point-type gamma radiation source (1180 TBq 137Cs) have been described. Radiation dose relationships for numerous response reactions of woody and herbaceous plants (growth and development of organs of woody plants, cytogenetical, physiological and biochemical changes in trees, reproductive potential of plants, damage and dying off of the forest as a biogeocenosis on the whole) have been calculated. Post-radiation recovery of the forest was investigated. Changes involving the secondary reactions related to radiation damage and death of the trees are presented. A model for radiation damage of forests has been designed. Examples are given on the usage of this model in the description of radiation effects in forests in the event of accidental releases of radionuclides into environment.

Lysimetrical study of radionuclides in the forests around the Chernobyl nuclear power plant

Vertical intrasoil water within the 30 km zone of ChNPP was studied by lysimetrical method during 1989–1991. Explorations were carried out in different ecosystems where the sum of total accumulation deposition of the radionuclides (Ce-144; Cs-134, 137; Ru-106; Sr-90) ranged from 90 M Bq/m2 to 0·5 M Bq/m2. It was shown that 0·01–0·6% of the total amount of radionuclides was lost from the 20–30 cm layer of soil every year depending on the type of radionuclide, type of ecosystem and the location of study plot. Main absorption of the radionuclides from the intrasoil flow generally took place in the 5–10 cm layer. Radionuclides were not absorbed by the soil in proportion to their ratio in lysimetrical water leached through the forest litter. The mobility of radionuclides in the intrasoil flow can be represented by the following series: Sr-90 > Ru-106 > Cs-134, 137 > Ce-144. Sr-90 and Cs-137 are absorbed by soil more intensively than their chemical macroanalogs—Ca and K.

Chemical forms of γ-emitting radionuclides in soils adjacent to the Chernobyl NPP

Abstract Samples of sandy forest soils, meadow sandy-peat soil and meadow sandy-loam soil were taken at different sites within a 30-km zone around the Chernobyl NPP (ChNPP). The samples were extracted with water and a 0.1-N solution of ammonium acetate. The extracts were measured for γ-radionuclides and stable cation content. The content of all mobile forms of the radionuclides present in the 0–10-cm soil layer accounts for 0.5-5% of the total radionuclide content in this layer, depending on the type of radionuclide and soil. Water soluble forms of the radionuclides were found in the 0–5-cm layer only. Exchangable radionuclide forms were represented, as a rule, by radiocaesium in both the 0–5- and 5–10-cm layers. Content of Cs-137 exchangeable forms in the organic-mineral horizon were roughly inversely proportional to the sum of stable exchangeable cations and organic matter content. Forest vegetation takes up a significant share of the mobile forms of radiocaesium from the soils.