No abstract

No abstract

No abstract

In April 2020, the largest forest fire occurred in the Chornobyl Exclusion zone in its history. The results of modeling the atmospheric transport of radioactive aerosols raised into the atmosphere as a result of fires in forest and grass areas in the Exclusion zone, as well as in radioactively contaminated forests outside it in Kyiv and Zhytomyr regions are presented in the paper. To assess the consequences of forest fires, a set of models of lifting, atmospheric transport and deposition of radionuclides on the underlying surface LEDI, developed at the Institute for Safety Problems of Nuclear Power Plants of the National Academy of Sciences of Ukraine, was used. Calculations of the dynamics of the 137Cs activity concentration field in the surface air on a regional scale (in Ukraine) and on a local scale (within the Exclusion zone) were performed. According to the simulation results, the maximum values of the 137Cs activity in the surface air of Kyiv in some periods during April 4−20 could reach 2−4 mBq/m3, and the integral value of 137Cs activity in the air of Kyiv for the whole period was about 450 mBq⋅s/m3. The obtained results are generally consistent with the data of measurements of radioactive contamination of the nearground air in Kyiv and areas of the nuclear power plants in Ukraine. The analysis of the consistency of the simulation results with the data of measurements of the 137Cs activity concentration in the air in the Exclusion zone was performed. The main ways to improve the methodology for assessing the consequences of forest fires by modeling the atmospheric transport of radionuclides are identified.

The increased risk of thyroid cancer among individuals exposed during childhood and adolescence to Iodine-131 ( 131 I) is the main statistically significant long-term effect of the Chornobyl accident. Several radiation epidemiological studies have been carried out or are currently in progress in Ukraine, to assess the risk of radiation-related health effects in exposed populations. About 150,000 measurements of 131 I thyroid activity, so-called 'direct thyroid measurements', performed in May-June 1986 in the Ukrainian population served as the main sources of data used to estimate thyroid doses to the individuals of these studies. However, limitations in the direct thyroid measurements have been recently recognized including improper measurement geometry and unknown true values of calibration coefficients for unchecked thyroid detectors. In the present study, a comparative analysis of 131 I thyroid activity measured by calibrated and unchecked devices in residents of the same neighboring settlements was conducted to evaluate the correct measurement geometry and calibration coefficients for measuring devices. As a result, revised values of 131 I thyroid activity were obtained. On average, in Vinnytsia, Kyiv, Lviv and Chernihiv Oblasts and in the city of Kyiv, the revised values of the 131 I thyroid activities were found to be 10-25% higher than previously reported, while in Zhytomyr Oblast, the values of the revised activities were found to be lower by about 50%. New sources of shared and unshared errors associated with estimates of 131 I thyroid activity were identified. The revised estimates of thyroid activity are recommended to be used to develop an updated Thyroid Dosimetry system (TD20) for the entire population of Ukraine as well as to revise the thyroid doses for the individuals included in post-Chornobyl radiation epidemiological studies: the Ukrainian-American cohort of individuals exposed during childhood and adolescence, the Ukrainian in utero cohort and the Chornobyl Tissue Bank.

Objective. To reconstruct the 131I activity concentrations in air and 131I ground deposition densities from 26 April to 7 May 1986 from the radioactivity release after the Chornobyl accident in the settlements of Ukraine using the mesoscale radionuclides atmospheric transport model LEDI and meteorological information from the numerical weather forecast model WRF and to compare the obtained results with those calculated previously as well as with available measurements of 131I activity in soil. Object of research: the near-ground layer of the atmosphere and the surface of the territory of Ukraine radioactively contaminated as a result of the Chornobyl accident. Materials and methods of research. The dispersion of 131I in the atmosphere and deposition on the ground surface in Ukraine were calculated using the Lagrangian-Eulerian diffusion model LEDI. The detailed fields of meteorological parameters calculated using the mesoscale weather forecast model WRF, which was adapted for the territory of Ukraine, were used as input data for the LEDI model. Results. The 131I daily-average activity concentrations in the surface air and 131I daily ground deposition densities from 26 April to 7 May 1986 were calculated using the up-to-date mesoscale model of numerical weather forecast WRF for 30,352 settlements in entire Ukraine, including 1,263 settlements in Kyiv, 1,717 – in Zhytomyr and 1,570 – in Chernihiv Oblasts. Conclusions. The method of mathematical modeling of the atmospheric transport of the radionuclides is combination with the up-to-date mesoscale model of numerical weather forecast WRF is a useful tool for reconstruction of radioactive contamination of the air and the ground surface after the Chornobyl accident. Calculated in this study 131I activity concentrations in air and 131I ground deposition densities were used to reconstruct the thyroid doses due to 131I intake to the population of Ukraine. Key words: Chornobyl accident, Iodine-131, atmospheric transport model.

No abstract

<p>In April 2020, the largest forest fire occurred in the Chernobyl Exclusion Zone (ChEZ) in its history. The results of modeling the atmospheric transport of radioactive aerosols released into the atmosphere as a result of wildland fires in the ChEZ and around it are presented. The atmospheric transport model LEDI, developed at the Institute for Safety Problems of NPPs, and the Atmospheric Dispersion Module of the real -time online decision support system for offsite nuclear emergency RODOS, which development was funded by the EU, were used. The <sup>137</sup>Cs activity concentration in the surface air is calculated on a regional scale (in Ukraine) and a local scale (within the ChEZ). The <sup>137</sup>Cs activity in the surface air of Kyiv (115 km from the ChEZ borders) is found to have reached 2–4 mBq m<sup>−3</sup> during the period April 3–20. The modeling results are generally consistent with measured data pertaining to radioactive contamination in Kyiv, within the ChEZ, and areas around four operating nuclear power plants in Ukraine.</p><p>A method for estimating the radionuclide activity emissions during wildland fires in radioactively contaminated areas is proposed. This method is based on satellite data of the fire radiative power (FRP), the radionuclide inventory in the fire area, and an emission factor for radioactive particles. A method was applied for forest fires in the ChEZ in April 2020. Preliminary estimations of an emission factor are made using FRP values obtained from NASA's MODIS and VIIRS active fire products.</p><p>On April 16, 2020, a strong dust storm was observed in the ChEZ, which coincided with the period of intense wildland fires. The additional <sup>137</sup>Cs activity raised by the dust storm from burned areas in the meadow biocenoses was estimated to be about 162 GBq, i.e. up to 20% of the total activity emitted into the air during the entire period of forest fires on April 3-20, 2020. According to the modeling results, during April 16-17, the input of resuspension of radioactive particles due to a dust storm was up to 80-95% of the total <sup>137</sup>Cs activity in the surface air near the Chernobyl NPP. In Kyiv, this value decreased to only about 4%.</p><p>The total effective dose to the population of Kyiv during the fire period is estimated to be 5.7 nSv from external exposure and the inhalation of <sup>137</sup>Cs and <sup>90</sup>Sr, rising to 30 nSv by the end of 2020. This is about 0.003% of the annual permissible level of exposure of the population. A committed effective dose up to 200-500 nSv is estimated for the personnel of the Chernobyl NPP from the radioactive aerosol inhalation during the 2020 forest fires, which is not more than 0.05% of the established control levels of internal exposure for them.</p>