V. Kashparov

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.

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.

Particle-associated Chernobyl fall-out in the local and intermediate zones

Abstract 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.

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.

Forest fires in the territory contaminated as a result of the Chernobyl accident: radioactive aerosol resuspension and exposure of fire-fighters

Abstract Studies were carried out to investigate the processes of resuspension and redistribution of radionuclides by fire in the territories contaminated as a result of the Chernobyl accident. In this set of experiments, the dispersed radioactive aerosol composition, the values of airborne radioactive aerosol concentrations, the resuspension factor, the resuspension rate, the deposition flux and the deposition velocity have been obtained for the different phases of a fire and at various distances from the fire. In the active phase of a fire, the airborne concentrations of radionuclides increase by several orders of magnitude relative to the background value. The resuspension factor for the active phase of a fire was assessed as 10 −7 – 10 −8 m −1 , while the value of the resuspension rate had a 10 −10 s −1 order of magnitude at a deposition velocity of 1– 2 cm s −1 . The additional terrestrial contamination due to a forest fire can be estimated as a value in the range 10 −4 –10 −5 of its background value. As recommended by ICRP, the human respiratory tract model was applied for calculation of the Effective Equivalent Dose (EED) to firemen. The dose coefficient for radioactive aerosol inhalation was estimated at 1.5×10 −8 Sv ( Bq m −3 h ) −1 .

Impact of Scots pine (Pinus sylvestris L.) plantings on long term 137Cs and 90Sr recycling from a waste burial site in the Chernobyl Red Forest

Plantings of Scots pine (Pinus sylvestris L.) on a waste burial site in the Chernobyl Red Forest was shown to greatly influence the long term redistribution of radioactivity contained in sub-surfaces trenches. After 15 years of growth, aboveground biomass of the average tree growing on waste trench no.22 had accumulated 1.7 times more (137)Cs than that of trees growing off the trench, and 5.4 times more (90)Sr. At the scale of the trench and according to an average tree density of 3300 trees/ha for the study zone, tree contamination would correspond to 0.024% of the (137)Cs and 2.52% of the (90)Sr contained in the buried waste material. A quantitative description of the radionuclide cycling showed a potential for trees to annually extract up to 0.82% of the (90)Sr pool in the trench and 0.0038% of the (137)Cs. A preferential (90)Sr uptake from the deep soil is envisioned while pine roots would take up (137)Cs mostly from less contaminated shallow soil layers. The current upward flux of (90)Sr through vegetation appeared at least equal to downward loss in waste material leaching as reported by Dewiere et al. (2004, Journal of Environmental Radioactivity 74, 139-150). Using a prospective calculation model, we estimated that maximum (90)Sr cycling can be expected to occur at 40 years post-planting, resulting in 12% of the current (90)Sr content in the trench transferred to surface soils through biomass turnover and 7% stored in tree biomass. These results are preliminary, although based on accurate methodology. A more integrated ecosystem study leading to the coupling between biological and geochemical models of radionuclide cycling within the Red Forest seems opportune. Such a study would help in the adequate management of that new forest and the waste trenches upon which they reside.

Twenty years’ application of agricultural countermeasures following the Chernobyl accident: lessons learned

The accident at the Chernobyl NPP (nuclear power plant) was the most serious ever to have occurred in the history of nuclear energy. The consumption of contaminated foodstuffs in affected areas was a significant source of irradiation for the population. A wide range of different countermeasures have been used to reduce exposure of people and to mitigate the consequences of the Chernobyl accident for agriculture in affected regions in Belarus, Russia and Ukraine. This paper for the first time summarises key data on countermeasure application over twenty years for all three countries and describes key lessons learnt from this experience.

Chronic irradiation of Scots pine trees (Pinus sylvestris) in the Chernobyl exclusion zone: dosimetry and radiobiological effects.

To identify effects of chronic internal and external radiation exposure for components of terrestrial ecosystems, a comprehensive study of Scots pine trees in the Chernobyl Exclusion Zone was performed. The experimental plan included over 1,100 young trees (up to 20 y old) selected from areas with varying levels of radioactive contamination. These pine trees were planted after the 1986 Chernobyl Nuclear Power Plant accident mainly to prevent radionuclide resuspension and soil erosion. For each tree, the major morphological parameters and radioactive contamination values were identified. Cytological analyses were performed for selected trees representing all dose rate ranges. A specially developed dosimetric model capable of taking into account radiation from the incorporated radionuclides in the trees was developed for the apical meristem. The calculated dose rates for the trees in the study varied within three orders of magnitude, from close to background values in the control area (about 5 mGy y−1) to approximately 7 Gy y−1 in the Red Forest area located in the immediate vicinity of the Chernobyl Nuclear Power Plant site. Dose rate/effect relationships for morphological changes and cytogenetic defects were identified, and correlations for radiation effects occurring on the morphological and cellular level were established.

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.

Inhalation of radionuclides during agricultural work in areas contaminated as a result of the Chernobyl reactor accident

Abstract 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 − and the 137 Cs deposit exceeded 7.4 MBq m −2 , the levels of these radionuclides in the operators 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.