BRIEFING NOTE

DATE: 21/04/2021

Large fires in the Chernobyl region
Origins and environmental consequences
Since the nuclear accident that occurred on 26 April 1986 on Reactor 4 at the Chernobyl NPP,
the environment near the plant has been very heavily contaminated over areas ofapproximately 2,600 km2 in Ukraine and 2,400 km2 in Belarus that are still prohibited today(exclusion zones).
These areas, which were partially cultivated or forested at the time of the disaster, have largely
been abandoned. As a result, the vegetation and forests have continued to grow and develop
to the detriment of cultivated areas. Despite the efforts, made by the countries most affectedby the fallout from the accident, to limit this phenomenon, the state of the forest isdeteriorating, giving way to the accumulation of dead plants and densifying biomass. This
deterioration of the forest, together with climate change marked by a more intense and earlier
dry season, increases the risk of fire, and certain agricultural practices (burning) intended toeliminate dead vegetation in the spring, regularly cause fires. Arson, electrical accidents, andlightning (although to a lesser extent) are the other main causes of fire.

Local and regional fires and plumes
The ecosystem close to the Chernobyl NPP has regularly suffered from major fires: about ten
since 1992. The magnitude of these fires depends on several factors including, as mentioned
above, vegetation density, shrub and tree species (heather, pine, birch, oak, aspen, alder, beech,etc.), land use, meteorological conditions (humidity, drought, wind, and precipitation), and thecapacity to fight against their spread. The surface areas concerned vary from one fire toanother (from a few tens of km2 to nearly 900 km2, as in 2020).
2002 was a particularly damaging year for the wilderness areas (forests and peatlands) in theMoscow region and, especially, in Ukraine. During the summer of 2002, over 60,000 hot spots
were detected in a vast area encompassing Eastern Europe, Belarus, Ukraine, and the westernpart of the Russian Federation. According to the State Emergency Service of Ukraine (SESU),
the first three months of 2020 were characterised by a 30% increase in the number and severityof fires. It was precisely in April 2020 that the worst forest fires ever recorded in the Ukrainian
region around the Chernobyl NPP occurred.

In temperate regions of the northern hemisphere (latitudes between 30 and 60°), the generalcirculation of tropospheric air masses is west to east. Therefore, the smoke plumes thatdevelop in the eastern part of Europe generally tend to disperse towards the east and theEurasian continent. However, in high-pressure conditions, continental air masses can movetowards Western Europe. The combination of a high-pressure situation and a major fire incontaminated areas remains relatively rare but has repeatedly caused the dispersion of an ash
plume containing artificial radioactive material detectable in traces in several Europeancountries, as was the case in August and September 2002, March 2003, early August 2010,
and, more recently, in April 2020.

The great fire of April 2020
At the beginning of April 2020, forest fires started to rage in the north of Ukraine, in particularin the area most contaminated by the fallout from the accident at the Chernobyl NPP. The fires
spread for about four weeks, penetrating the Chernobyl Exclusion Zone (CEZ) coming close tothe NPP and radioactive waste storage facilities. The fires were eventually extinguished bysustained rainfall which significantly helped firefighters contain the fire. According to theUkrainian Hydrometeorological Institute (UHMI), approximately 870 km2 (87,000 ha) weredestroyed, of which 65 km2 in the immediate vicinity of the NPP and another 20 km2 on the leftbank of the Pripyat river. Forest fires can emit large quantities of artificial radionuclides (137Cs,90Sr, plutonium isotopes, and 241Am) into the air. These radionuclides are mainly contained insurface soil layers, forest litter, and, to a lesser extent, in biomass.

Figure 1: Satellite image of the fires in the Chernobyl region on 9 April 2020. Photo credit: NASAEarth Observatory images, Lauren Dauphin. Use of MODIS data from NASA, EOSDIS/LANCE
andGIBS/Worldview.
https://earthobservatoryr.nasa.gov/images/146561/fires-burn-innorthern-ukraine.

Figure S2: Cumulative representation of active fire detections from MODIS and VIIRS satellite
data between 1 April and 1 May 2020. Data source: NASA/FIRMS (Fire Information forResource Management System). The Chernobyl nuclear power station’s location is marked
with a yellow star.

Estimation of the quantities of remobilised radionuclides
The duration of the episode (> 4 weeks) caused abundant plumes which generally drifted
eastwards but also several times towards southern and western Europe. The radioactivity ofthese plumes was widely measured, in particular that of cesium-137 which emits radiation thatis more easily detectable than other artificial radionuclides like Pu, Sr, and Am. Based on the1,200 measurements of airborne 137Cs concentrations in Europe communicated by nineteenof its European counterparts, the IRSN was able to evaluate, using atmospheric dispersion
models (inverse modelling), the quantity of 137Cs emitted by the fires (source term). This wasestimated to be around 0.7 to 1.2 TBq for all outbreaks. As no concentration of 90Sr or
plutonium was measured in the air outside of Ukraine, the same methodology could not be
used to estimate the source term of these radionuclides. However, around forty
measurements of 90Sr activity concentrations and around thirty of plutonium in the air, carriedout in the Chernobyl exclusion zone, were reported. The ratio of these concentrations to thoseof Cs-137 measured in the same area as well as the use of characteristic ratios of theChernobyl accident made it possible to extrapolate the orders of magnitude of the amount ofSr, Pu, and Am remobilised by the fires. A 90Sr source term two times weaker than that of 137Cs,
around 480 GBq (range between 345 and 612 Gbq), was estimated. For plutonium isotopes
and 241Am, the source terms estimated by the IRSN were respectively 1.5 GBq for 238Pu and239
Pu, 2.2 GBq for 240Pu, 59.0 GBq for 241Pu and 21.7 GBq for 241Am.

Assessment of radiological consequences

The IRSN carried out a dosimetric evaluation on two categories of people: firefighters whofought the fires in the exclusion zone and the inhabitants of Kiev (about 100 km to the south).
The inhalation dose was assessed taking into account a deliberately aggravating approach
including the following assumptions: 1) the firefighters did not have respiratory protection 2)
a total working time in the exclusion zone of around one hundred hours for each of them (tendays at the rate of ten working hours per day), 3) a respiratory rate of 3 m3/h corresponding tovery intense physical effort during firefighting (CIPR 66), 4) all the emitted aerosols in therespirable fraction. The atmospheric concentrations considered were mainly estimated using
the measurements taken on filters sampled in the field for which 137Cs, 90Sr, 238Pu, and 239+240Pu
were simultaneously measured. The estimates of activity concentrations of 241Pu and 241Am
were deduced from the 241Pu/239+240Pu ratio characteristic of the fallout from the Chernobyl
accident, taken from literature1, taking into account the radioactive relationship between 241Pu
and 241Am.
From the highest concentrations measured in the air (around 1 Bq.m-3 for 137Cs and 90Sr, 1
mBq.m-3 for 238Pu), the dose potentially received by a firefighter due to the inhalation ofradioactive smoke would have been at most 170 microSievert (µSv) in total, of which around
80% was solely due to 241Am. This result is probably overestimated since it was also assumed,
for the purposes of the calculation, that the maximum concentrations observed over very shortperiods (half an hour, for example) lasted a hundred hours. A more realistic dose assessment
can be made based on the spatially and temporally averaged concentrations found within theexclusion zone during the fires. In this case, the resulting dose induced by inhalation of all theartificial radionuclides considered in this study over a period of a hundred hours would be 1.3µSv (or 0.0013 mSv).
Despite the relative significance of the maximum 137Cs concentration measured in the vicinityof the fires (0.2 Bq/m3), dosimetric evaluations indicate that the contribution of 137Cs to theinhalation dose remains negligible compared to that due to 90Sr, Pu isotopes, and especially
241
Am. In addition, most of the dose (all exposure routes combined) is attributable to external
exposure by radionuclides deposited in the environment near the Chernobyl NPP in April andMay 1986. This external exposure results from the presence of firefighters in this highly
contaminated area and is not directly related to the fires. Based on data published in theliterature, external exposure due to the contaminated environment is between 0.1 and 1 mSv
for a presence of around a hundred hours in the Chernobyl exclusion zone. By way ofcomparison, the current limit imposed by Ukrainian regulations for personnel working in theChernobyl exclusion zone is 3 mSv/year.
The dosimetric impact of the fires for Kiev inhabitants, taking into account the inhalation ofparticles and the consumption of foodstuffs contaminated by atmospheric deposition ofradioactive particles, remained very low. The effective dose for an adult has been estimatedat 0.150 µSv resulting mainly from inhalation (0.1 µSv for inhalation from 1 to 22 April 2020
and 0.050 µSv for ingestion over a period of one year after deposit). Americium 241 alone
represented about 75% of this inhalation dose, followed by plutonium isotopes (~ 25%), thecontribution of 137Cs and 90Sr being around 1%. To calculate the ingestion dose, the main
agricultural products considered included seasonal vegetables (April), dairy products, andmeat. A daily consumption of 500 g of leafy vegetables was used as the worst-case scenario.
As confirmed by Talerko et al. (2021), the dose induced by external exposure to the fire plume(immersion) was negligible compared to internal exposure via inhalation. Total exposure
therefore remained well below the annual public exposure limit of 1 mSv according to theUkrainian radiation protection standards for the public as an added effective dose (NRBU-97,2000) or compared to the global annual average exposure of 2.4 mSv induced by naturalradiation (UNSCEAR, 2000).
1

The activity concentrations obtained were: 1 mBq/m3 for 239Pu, 1.5 mBq/m3 for 240Pu, 44 mBq/m3 for 241Pu,
and 12 mBq/m3 for 241Am

In the rest of Europe and particularly in France, the dosimetric impact by inhalation andingestion of foodstuffs on which minute amount of radionuclides from these fires may havebeen deposited was negligible.

For more information:
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https://www.irsn.fr/fr/actualites_presse/actualites/pages/20200505_ni-tchernobylukraine-incendies-zone-exclusion-synthese-des-consequences.aspx#Masson O. et al. (2021) Europe-wide atmospheric radionuclide dispersion byunprecedented wildfires in the Chernobyl nuclear power plant region, April 2020.
Environ. Science and Technol.
Protsak V. P., Wojciechowicz O. V., Laptev G. V. (2020). Estimation of radioactive
source term dynamics for atmospheric transport during wildfires in Chernobyl zone inspring
2020.
UkrainianHydrometeorologicalInstitute,
Kiev.
https://uhmi.org.ua/msg/fire2020/analytical.pdf.
Tabachnyi, L.; Kokot, I., Results of consequences of the fires on the radioactive

contamination territories of Kyiv and Zhytomyr regions according
Hydrometeorological radiometrical network data. IAEA: Vienna, 2020.
toТаlerko M., Коvalets I., Lev T., Igarashi Y., Romanenko O. Simulation study ofradionuclide atmospheric transport after wildland fires in the Chernobyl Exclusion Zonein April 2020. Atmos. Pollut. Res. 12, (3), 193-204.
Gueibe, C.; De Meutter, P.; Delcloo, A.; Verheyen, L.; Bruggeman, M.; Camps, J. Forest
fires in the surroundings of Chernobyl: follow-up of potential radiological aspects. 3004-2020.
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CIPR 66. Human Respiratory Tract Model for Radiological Protection. Annals of theInternational Commission on Radiological Protection. ICRP Publication 66. 1994, Ann.
ICRP 24 (1-3).
NRBU-97/D-2000, 2000. Radiation Safety Standards of Ukraine. Ministry of Health ofUkraine, Kyiv (en Ukrainien).
UNSCEAR. “Sources and Effects of Ionizing Radiation”, United Nations ScientificCommittee on the Effects of Atomic Radiation (UNSCEAR). 2000. New York, United
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Tschiersch, J., Resuspension and redistribution of radionuclides during grassland andforest fires in the Chernobyl exclusion zone: part I. Fire experiments. J. Environ.
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