Validation of a fuel particle dissolution model with samples from the Red Forest within the Chernobyl exclusion zone.

The contamination in the near exclusion zone of the Chernobyl nuclear power plant (ChNPP) with 90Sr, 238-240Pu and 241Am is associated with irradiated nuclear fuel particles. Fit for purpose models enabling long term prediction of mobility and bioavailability of particle-associated radionuclides are crucial for radiation protection of humans and the environment, as well as for planning of remediation measures and future land use. In the present work, a dynamic fuel particle dissolution model developed in 1999-2002 is described and validated using data based on sampling in 2015. The model is based on the characterization of the radionuclide source term of the waste in a shallow sub-surface radioactive waste storage, trench #22, in the Chernobyl Pilot Site (CPS) located in the Red Forest, 2.5 km west of the ChNPP, as well as the description of physical and chemical properties of the fuel particles and subsequent radionuclide leaching into the soil solution. Transformation rate constants of the fuel particle dissolution model related to topsoil, radioactive waste trench and submerged materials, and drained cooling pond sediments, should largely control the mobility and bioavailability of radionuclides (e.g., solubility in the soil, migration to groundwater and transfer to plants). The predicting power of the Chernobyl fuel particle dissolution model with respect to radionuclide leaching dynamics was validated using samples from the same experimental site, showing that predicted particle leaching and subsequent mobility and bioavailability were within 46 ± 3% of the observed data. Therefore, linking source- and release-scenario dependent characteristics of radioactive particles to their potential weathering can provide information that can be implemented in impact assessments models for existing contaminated areas as well as for future events.

Effects of radiation on radial growth of Scots pine in areas highly affected by the Chernobyl accident.

The effects of radiation on radial growth of Scots pine (Pinus sylvestris L.) affected by the Chernobyl accident were studied at five sites with different deposition levels. The study sites were chosen along a high gradient of depositions at the distance of a few hundred meters in the forests with similar forest characteristics. Air dose rates varied at the sites from the background values up to 30 µGy h-1 as of December 1, 2016. Scots pine trees exposed to sub-lethal doses of 8.6-13.2 Gy at the "Red Forest" site did not demonstrate deviations in formation of annual rings 30 years after the Chernobyl accident compared to trees with lower doses and control trees. Variation with time in annual rings thickness did reveal that the effect of radiation in trees growing at the sites with different contamination not detectable in 1986 and 1987. Conversely, the effects were clear observed in a later period in 1989-1991, i.e. 3-5 years after the accident. Until 2000, there were statistically significant differences in the annual rings growth rates of Scots pine trees exposed to external absorbed dose rates of 4.4-6.7 mGy h-1 as estimated for June 1, 1986 (or 19.5-30.0 µGy h-1 as of December 1, 2016) compared with the trees of the other sites studied. The results comply with the conclusions from research with acute pine exposure conducted in the Kyshtym area in 1975-1984.

Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone: part II. Modeling the transport process.

To predict parameters of radionuclide resuspension, transport and deposition during forest and grassland fires, several model modules were developed and adapted. Experimental data of controlled burning of prepared experimental plots in the Chernobyl exclusion zone have been used to evaluate the prognostic power of the models. The predicted trajectories and elevations of the plume match with those visually observed during the fire experiments in the grassland and forest sites. Experimentally determined parameters could be successfully used for the calculation of the initial plume parameters which provide the tools for the description of various fire scenarios and enable prognostic calculations. In summary, the model predicts a release of some per thousand from the radionuclide inventory of the fuel material by the grassland fires. During the forest fire, up to 4% of (137)Cs and (90)Sr and up to 1% of the Pu isotopes can be released from the forest litter according to the model calculations. However, these results depend on the parameters of the fire events. In general, the modeling results are in good accordance with the experimental data. Therefore, the considered models were successfully validated and can be recommended for the assessment of the resuspension and redistribution of radionuclides during grassland and forest fires in contaminated territories.

Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone: part I. Fire experiments.

Controlled burning of experimental plots of forest or grassland in the Chernobyl exclusion zone has been carried out in order to estimate the parameters of radionuclide resuspension, transport and deposition during forest and grassland fires and to evaluate the working conditions of firemen. An increase of several orders of magnitude of the airborne radionuclide concentration was observed in the territory near the fire area. The resuspension factor for (137)Cs and (90)Sr was determined to range from 10(-6) to 10(-5) m(-1), and for the plutonium radionuclides from 10(-7) to 10(-6) m(-1) (related to the nuclides in the combustible biomass). These values are 2 orders of magnitude lower if they are calculated relatively to the total contamination density (including the nuclides in the soil). The radionuclide fallout along the plume axis is negligible in comparison to the existing contamination. However, the additional inhalation dose for firemen exposed in the affected area can reach the level of the additional external irradiation in the period of their mission. The plutonium nuclides constitute the dominating contribution to the inhalation dose.

Territory contamination with the radionuclides representing the fuel component of Chernobyl fallout.

The data obtained through a series of experiments were used to specify the correlation of activities of the fuel component radionuclides of Chernobyl fallout and to create the maps of the 30-km Chernobyl zone terrestrial density of contamination with 154Eu, 238Pu, 239+240Pu and 241Am (on 01.01.2000). In the year 2000, total inventories of the fuel component radionuclides in the upper 30-cm soil layer of the 30-km Chernobyl zone in Ukraine (outside the ChNPP industrial site, excluding the activity located in the radioactive waste storages and in the cooling pond) were estimated as: 90Sr--7.7 x 10(14) Bq; 137Cs--2.8 x 10(15) Bq; 154Eu--1.4 x 10(13) Bq; 238Pu--7.2 x 10(12) Bq; 239+240Pu--1.5 x 10(13) Bq; 241Am--1.8 x 10(13) Bq. These values correspond to 0.4-0.5% of their amounts in the ChNPP unit 4 at the moment of the accident. The current estimate is 3 times lower than the previous widely-cited estimates. Inventories of the fuel component radionuclides were also estimated in other objects within the 30-km zone and outside it. This allowed more accurate data to be obtained on the magnitude of a relative release of radionuclides in the fuel particles (FP) matrix during the Chernobyl accident outside the ChNPP industrial site. It amounts to 1.5+/-0.5% of these radionuclides in the reactor, which is 2 times lower than the previous estimates. Two-thirds of the radionuclides release in the FP was deposited on the territory of Ukraine.

Autoradiographical methods for the assessment of radionuclides in hot particles on filter samples.

Three different autoradiographical methods were tested for detection, localisation and quantification of radionuclides in hot particles deposited on air filters. With all methods, hot particles could successfully be localised on air filters. Two methods, a X-ray film technique and a digital autoradiography technique using a microchannel array detector, have been examined with particles of known activity and with particles on air filter samples of the Chernobyl area to be able to quantify the 90Sr activity in the hot particles consistently. The results obtained in this study suggest that the digital autoradiography system is a very useful tool having a high efficiency with a low measurement uncertainty and allowing short exposure times for extensive measurement tasks.

Soil contamination with 90Sr in the near zone of the Chernobyl accident.

Representative large-scale soil sampling on a regular grid of step width about 1 km was carried out for the first time in the near zone of the Chernobyl accident (radius 36 km). An integrated map of terrestrial 90Sr contamination density in the 30 km exclusion zone (scale 1:200,000) has been created from the analysed samples. Maps of the main agrochemical characteristics of the soils, which determine the fuel particle dissolution rates and the contamination of vegetation, were produced. The total contents of 90Sr on the ground surface of the 30 km zone in Ukraine (without the reactor site and the radioactive waste storages) was about 810 TBq (8.1 x 10(+14) Bq) in 1997, which corresponds to 0.4-0.5% of the Chernobyl reactor inventory at the time of the accident. This assessment is 3-4 times lower than previous estimates.

[Radio-ecological and hygienic assessment of consequences of forest fires in the areas polluted during the Chernobyl accident]. / Radioékologicheskaia i gigienicheskaia otsenka posledstvii lesnykh pozharov na territorii, zagriaznennoi v rezul'tate Chernobyl'skoi avarii.

Retransfer of radionuclides on the condensation trails of Chernobyl radioactive fallouts during forest fires has been experimentally evaluated and their mathematical transfer model verified. It has been shown that radionuclide retransfer will make no great impact on additional pollution of an area even under the most unfavourable conditions. The contribution of convective and non-convective components of transfer to the formation of a radioactive aerosol concentration field has been assessed. Time course of changes in the concentration of radioactive aerosol and its dispersive composition are shown in different phases of fire and at different distance from its source.

Kinetics of fuel particle weathering and 90Sr mobility in the Chernobyl 30-km exclusion zone.

Weathering of fuel particles and the subsequent leaching of radionuclides causes 90Sr mobility in Chernobyl soils to increase with time after deposition. Studies of 90Sr speciation in soils collected in 1995 and 1996 from the Chernobyl 30-km exclusion zone have been used to calculate rates of fuel particles dissolution under natural environmental conditions. Results show that the velocity of fuel particle dissolution is primarily dependent on the physico-chemical characteristics of the particles and partially dependent on soil acidity. Compared to other areas, the fuel particle dissolution rate is significantly lower in the contaminated areas to the west of the Chernobyl reactor where deposited particles were presumably not oxidized prior to release. The data have been used to derive mathematical models that describe the rate of radionuclide leaching from fuel particles in the exclusion zone and changes in soil-to-plant transfer as a function of particle type and soil pH.

[Radionuclide inhalation during agricultural work in the regions polluted by the Chernobyl accident]. / Ingaliatsionnoe postuplenie radionuklidov pri selskokhoziaistvennykh rabotakh na territoriiakh, zagriaznennykh v rezultate Chernobylskoi avarii.

With pollution of agricultural fields by plutonium at a level of more than 3.7 kBg/m2 and cesium more than 7.4 MBq/m2 radionuclide levels in tractor cabin can exceed the permissible level. However field work being seasonal, annual level of inhaled radionuclides does not exceed the permissible limit. Actually the equivalent dose for the lungs does not exceed just few mSv under conditions of agricultural production.