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<p>Most of the territory of the Chernobyl Exclusion Zone (CEZ) is covered by forest. Forest of CEZ have accumulated a significant part of the radioactive release and for many years have served as a barrier to the non spreading of the radionuclide contamination outside the CEZ.</p><p>According to the classification of wildfire danger, the forests of CEZ belong to high, above average and medium classes, making cases of wildfires as quite common.</p><p>Poor, sod-podzolic soils of Ukrainian Polesye contribute to the entry the activity of <sup>90</sup>Sr and <sup>137</sup>Cs in plant biomass. During wildfires some of the radionuclides contained in combustion products of biomass are emitted into the atmosphere. Biologically important radionuclides such as <sup>90</sup>Sr, <sup>137</sup>Cs, plutonium isotopes and <sup>241</sup>Am bound to fine aerosols - combustion products - can be transported with atmospheric flows over the long range, causing secondary radioactive fallout and forming additional inhalation dose loads on the population.</p><p>Lack of the actual information on the source term (rate of emission of radionuclides) does not allow reliable modeling of the radiological impact of wildfires. To address this issue, we have proposed a methodology that allows for operational assessments of the dynamics of radionuclide emissions into the atmosphere from wildfires in the CEZ.</p><p>The basic parameters for the calculations are</p><ul><li>cartographic data on the density of radionuclide contamination of the territory of the CEZ;</li> <li>classification of the territory of the CEZ according to the distributive features of forests and meadows;</li> <li>classification of CEZ forests according to taxa characteristics to estimate amount of stored fuel biomass (kg/m<sup>2</sup>);</li> <li>experimental data on the transfer of radionuclides from soil to the main components of biomass for the calculation of radionuclide inventory in fuel biomass (Bq/m<sup>2</sup>). Thus, for meadows the main fuel component is grass turf, while for forest these are litter, wood, bark and pine needles.</li> <li>experimental data on emission factors of radionuclides from fuel biomass.</li> </ul><p>Implementation of the proposed algorithm in the form of GIS application makes it possible to assess the dynamics of radionuclide emission into the atmosphere by delineation the fire areas on the CEZ map. The NASA WorldView interactive mapping web application can be used to estimate the temporal and spatial characteristics of the wildfire while it is being developed. The contouring of the area affected by fire is carried out according to the analysis of the cluster of thermal points. Also, operational contouring of wildfire can be carried out using data delivered from unmanned aerial vehicles.</p><p>The application of the proposed algorithm for the analysis of the dynamics of <sup>137</sup>Cs emissions into the atmosphere from the April 2020 wildfire showed a good agreement with the data reported by various authors who used the method of inverse simulation. Improving the accuracy of calculations according to the proposed algorithm can be done by rectifying radionuclide emission factors and taking into account fire intensity data, which in turn can affect both the radionuclide emission factor and the degree of burnout of plant biomass.</p>

From early April 2020, wildfires raged in the highly contaminated areas around the Chernobyl nuclear power plant (CNPP), Ukraine. For about 4 weeks, the fires spread around and into the Chernobyl exclusion zone (CEZ) and came within a few kilometers of both the CNPP and radioactive waste storage facilities. Wildfires occurred on several occasions throughout the month of April. They were extinguished, but weather conditions and the spread of fires by airborne embers and smoldering fires led to new fires starting at different locations of the CEZ. The forest fires were only completely under control at the beginning of May, thanks to the tireless and incessant work of the firefighters and a period of sustained precipitation. In total, 0.7−1.2 TBq 137 Cs were released into the atmosphere. Smoke plumes partly spread south and west and contributed to the detection of airborne 137 Cs over the Ukrainian territory and as far away as Western Europe. The increase in airborne 137 Cs ranged from several hundred μBq•m −3 in northern Ukraine to trace levels of a few μBq•m −3 or even within the usual background level in other European countries. Dispersion modeling determined the plume arrival time and was helpful in the assessment of the possible increase in airborne 137 Cs concentrations in Europe. Detections of airborne 90 Sr (emission estimate 345−612 GBq) and Pu (up to 75 GBq, mostly 241 Pu) were reported from the CEZ. Americium-241 represented only 1.4% of the total source term corresponding to the studied anthropogenic radionuclides but would have contributed up to 80% of the inhalation dose.

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