S. Levchuk

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.

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.

Soil Contamination with Fuel Component of Chernobyl Radioactive Fallout

Radionuclide activity ratios in the fuel component of the Chernobyl fallout are reestimated on the basis of new experimental data, and maps of the density of contamination of the Chernobyl 30-km zone with 154Eu, 238Pu, 239+240Pu, and 241Am as of January 1, 2000 are compiled. The total radionuclide inventories in the top 30-cm horizon of the soil of the 30-km zone (minus the NPP service area, cooling pond, and radioactive waste disposal sites) on January 1, 2000 were estimated to be (Bq): 90Sr 7.7×1014, 137Cs 2.8×1015, 154Eu 1.4 × 1013, 238Pu 7.2 × 1012, 239+240Pu 1.5 × 1013, and 241Am 1.8 × 1013, which makes up to 0.4-0.5% of the total amount of these radionuclides produced in the 4th block of CNPP. This value is lower by a factor of 3 than that generally accepted so far. The radionuclide inventories in other objects of the 30-km zone and beyond it are also estimated. The total amounts of radionuclides fallen out with fuel particles beyond the NPP service area are reestimated. The resulting value (1.5±0.5% of the total produced in the reactor by the accident time) is lower by half as compared to the previous estimates. Two thirds of these amounts are found on the Ukrainian territory.

Dissolution of Particles of Irradiated Nuclear Fuel in the Temporary Storages of Radioactive Waste in Chernobyl Zone: Sources for Radionuclides Migration

Results of the ongoing international study (i.e., Chernobyl Pilot Site Project) on characterization of the surface waste disposal site in the Chernobyl zone are presented. The reported experimental work includes determination of physico-chemical forms of radionuclides inside the waste storage (trench) and the data on kinetics of the fuel particles (FP) dissolution in natural conditions. In order to characterize the trench source term, the parameters of the FP dissolution kinetics in the trench were estimated and the model of the 90Sr mobile forms dynamics was created.

Application of the Method of Repeated Mixing to Non-Uniformly Contaminated Bulky Samples

The method of repeated mixing was applied to soil samples containing hot particles. The results were interpreted using a developed mathematical formulation, which describes the frequency distribution of results in the presence of one or more hot particles in a sample, the statistical characteristics such as mathematical expectancy and dispersion, and includes an approach to estimate the activity of a hot particle without its separation from the sample. It was shown that by application of the method to a limited number of repeated mixings/measurements, the estimated activity of hot particles can be referred as a value determining the uncertainty of the results at a given level of confidence. For instance: for 5 mixings/measurements in a 103 cm3 vessel, the difference between the actual activity of a sample and the averaged result with a probability of about 64% does not exceed the estimated value of the hot particle activity. The probability is increasing with increasing number of mixings (to about 84% for the same uncertainty range at 10 mixings). For a fixed number of mixings the probability is increasing with increasing uncertainty range. The probability is increasing with decreasing size of the measuring vessel, but in many situations small samples can not be a representative subject of study.

Formation of Radioactive Aerosol Particles During the Wildland Fires in Chernobyl Zone and their Radioecological Impact

Controlled grassland and forest fires have been carried out at the experimental plots in the Chernobyl zone in order to determine the parameters of the radioactive aerosols resuspension, transportation and deposition under well-described conditions. Radionuclide airborne concentrations, resuspension factors, deposition intensity and velocities and dispersal composition of the radioactive aerosols were measured. Doses to firemen from inhalation of radio-nuclides were calculated. A mathematical model for the initial plume rise was developed, while the further transportation of radioactivity in the atmosphere was described by the Gauss model. A Monte-Carlo algorithm was applied for modeling the radioactive aerosols resuspension/transportation in order to take into account the stochastic nature of the plume rise input parameters. In the model exercises the total radioactivity releases from the burning plots were estimated.