Methodology for estimating the emission of radionuclides into the atmosphere from wildfires in the Chernobyl Exclusion Zone

Abstract

<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>

Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020

“…Our 137 Cs source term estimation is fully consistent with that proposed by Ukrainian researchers: 690 GBq as of April 20 or 660 to 945 GBq until the end of the blaze event. , These estimates are based on a very different method combining land cover and vegetation features, radionuclide distribution in the ecosystem, biomass burning emission factors, and fire satellite detections. Evangeliou & Eckhardt indicated that their model underestimated measurements by about 70%, which means that in their study, the average modeled concentration was almost half of the average measured concentration .…”

“…Evangeliou & Eckhardt indicated that their model underestimated measurements by about 70%, which means that in their study, the average modeled concentration was almost half of the average measured concentration . Based on observed airborne concentrations, our estimated average 90 Sr emission (480 GBq) is much higher than those estimated by Protsak et al (13.5 GBq) and Evangeliou & Eckhardt (51 GBq). , These two 90 Sr assessments combined estimations of the residual 90 Sr biomass contamination and satellite detections and used a 90 Sr emission factor of 0.2% (i.e., 25-fold lower than for 137 Cs, based on their research). Our 238 Pu estimate is also consistent with that of Evangeliou & Eckhardt .…”

“…Two main fire areas were identified during this period: in the Polisske district and in the Kopachi-Chistogalovka-CNPP cooling pond (<12 km from CNPP). Daily information about burned areas including vegetation cover, contamination density, and radionuclide emissions were then published by the Ukrainian Hydrometeorological Institute (UHMI) . According to the UHMI, 870 km 2 were burned in total, including 65 km 2 in proximity to the CNPP and 20 km 2 on the left bank of the Pripyat river.…”

“…A similar situation had already occurred from the end of August to early September 2002 with a slight increase in airborne 137 Cs concentrations in the western European atmosphere . The follow-up of the April 2020 wildfire situation was scrutinized on a daily basis by the UHMI and regularly commented by the SCK-CEN , and the IRSN. − In total, about 1,100 137 Cs results were gathered throughout Europe including already published data from the International Monitoring System (IMS) in support to the Comprehensive Nuclear-Test-Ban Treaty Organization. , This collection, which also includes a hundred values for 90 Sr, 238 Pu, and 239+240 Pu from Ukraine, corresponds to the most comprehensive available dataset related to that event (see Tables S6 and S7, SI). The purpose of this study is to investigate both the RN source terms and the additional exposure to RNs for firefighters and inhabitants of Kiev.…”

“…For the April 2020 fire event, the UHMI used a 137 Cs emission factor of 5% because of the exceptional intensity of the crown fires. As of April 20, a total of 690 GBq of 137 Cs would have been released into the atmosphere . The 137 Cs source term was later re-evaluated including emission from April 20 to 30, leading to a release between 600 and 860 GBq for the CEZ, as well as 60 to 85 GBq for the Zhytomyr region .…”