Modeling study of the atmospheric transport of radioactivity after wildland fires and a dust storm in the Chernobyl Exclusion Zone in April 2020

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 ¹³⁷Cs activity concentration in the surface air is calculated on a regional scale (in Ukraine) and a local scale (within the ChEZ). The ¹³⁷Cs activity in the surface air of Kyiv (115 km from the ChEZ borders) is found to have reached 2–4 mBq m⁻³ 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.

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.

On April 16, 2020, a strong dust storm was observed in the ChEZ, which coincided with the period of intense wildland fires. The additional ¹³⁷Cs 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 ¹³⁷Cs activity in the surface air near the Chernobyl NPP. In Kyiv, this value decreased to only about 4%.

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 ¹³⁷Cs and ⁹⁰Sr, 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.