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| Fig. 1: Cs-137 radiation distribution in the region surrounding the Chernobyl plant, 1996. From UNSCEAR Report, Annex D. [9] (Source: Wikimedia Commons) |
Exclusion zones surrounding the sites of nuclear disasters offer, perhaps, the fantasy that we can contain nuclear risk with lines on a map. In reality, material is constantly crossing the boundaries we draw, whether in the air or in the bodies of migrating animals. [1] Several studies of the Chernobyl Exclusion Zone (CEZ) have taken up this risk, from multiple angles - including the danger of forest fires causing the resuspension of radionuclides in the air, to be redeposited elsewhere.
What is the danger to surrounding communities? What about far off regions? How does one begin to evaluate such questions? Recent fires in the CEZ have provided measurements for how much resuspension happens under such conditions. Moreover, they allow us to understand how this resuspended material gets redistributed. This brief review will look at measurements of redistribution due to previous fires in the CEZ. It also works through an example calculation using a simple model to demonstrate how the physical conditions of the CEZ affect how we predict resuspension.
Our starting point is assessing how much radioactive material would become airborne during a fire. This is typically treated as a function of three core variables: the surface deposition density of radionuclides in the region that is burning; the area of land burned; and an emission factor representing what fraction of the deposited radionuclides are actually volatilised and lifted into the smoke plume. Together, these form the basic equation:
where Q is the total activity released (Bq), Cdep is the surface deposition density (Bq/m2), A is the area burned (m2), and e is the emission factor.
To make this more concrete, we can pose our problem in the context of 2015, when two major fires burned through the CEZ. Cs-137 is the only radionuclide we consider in this example, for three key reasons:
It is highly labile (mobile), making it more susceptible to being resuspended by fire.
It has a long half-life (~30 years), meaning it is still present in significant quantities in the CEZ.
It was deposited in large quantities after the 1986 Chernobyl disaster.
If we assume that Cs-137 deposition density was approximately uniform over the areas that burned, we can assign this an average value of Cs-137 deposition density from 2015: about 200 Bq/m2 according to regularly-made measurements. [2] Research also shows that under fire conditions, an emission factor e of 20% is reasonable. [2] Finally, over the course of the 2015 fires, a total of 14,939 hectares of forest and former agricultural areas burned: 9,241 hectares in the spring, and 5,968 hectares in the summer. [2] With these numbers, our equation becomes
This represents a large amount of radiation, and it is worth pausing to ask how robust this estimate is. A significant source of uncertainty in our calculation is the emission factor. In experimental fire studies conducted within the CEZ, Yoschenko et al. found that the fraction of Cs-137 released from burning forest litter was as low as 0.4% - far below the 20% value used by Evangeliou et al. [2,3] Other studies find the value to be as large as 40%. [3] Holding other values fixed, and varying e between these two values gives us a range of radiation emission between 0.12 TBq and 12 TBq. The large spread represents genuine physical uncertainty. Emission factors depend strongly on fire intensity, fuel moisture, and the depth of the burning, none of which are uniform. We can see a piece of this non-uniformity in Fig. 1, which shows large differences in radiation deposition between regions. This non-uniformity is also true on smaller scales, at the ecological level. [4]
Our simplified example allows us to understand where significant sources of uncertainty originate in studies of radionuclide resuspension. Tools which model these factors have been used to make measurements of the radiation that is released during such events. Evangeliou et al. identify that approximately 10.9 TBq of Cs-137 were released due to the 2015 fires. [2]
These combined releases were assessed in absolute terms as a Level 3 "Serious Incident" on the International Nuclear and Radiological Event scale. But the research also shows that the deposition of these radionuclides causes minimal risk to those outside the immediate CEZ zone. Evangeliou et al. show that across Europe, most of the distributed material resulted in deposition concentrations of 0.001 to 1 Bq/m2, with regions in the vicinity of the CEZ experiencing higher (new) deposition concentrations above 10 Bq/m2. [2] 1 Bq/m2 is well below the existing background across Europe left by the original fallout.
It is also instructive to compare the 2015 fires with those in 2020, which burned a significantly larger region of 870 km2 (roughly six times more than the 2015 fires). [5] However, Masson et al. conclude based on field measurements that the 2020 fires released only 0.7-1.2 TBq of Cs-137. [5] Across Europe, they measure the increase in airborne concentrations of Cs-137 to be primarily between 0.1 and 10 μBq/m3, and up to several hundred μBq/ m3 in the CEZ and Northern Ukraine. [5] (Note that this is airborne concentration, not surface deposition, of the Cs-137).
This comparison demonstrates an important point: the total activity released depends not only on the total area burnt, but on which areas burn. Radionuclides are not uniformly distributed through a burning area. Moreover, distance from the fire source, the intensity and size of the fire, and smoke plume characteristics all impact the release of radionuclides. [6] The duration of the burn matters as well, as the surface deposition density will change over time! [5] Climate uncertainty also means that the region is facing anincreased risk of fires, and the changed ecosystem of the CEZ means that it may be more at risk for fires. [1,6] This is not to mention climate and air modelling itself, which is notoriously complicated, trying to account for wind speed, plume height, turbulence, and precipitation. [7]
Evangenliou et al. confirm that, while fires in the CEZ do indeed resuspend radionuclides at detectable levels, the additional deposition across surrounding regions in Europe is minimal. [2] Other studies also affirm this result, with different methods showing low doses, negligible redistribution, and minimal risk. [8,3] Even the direct risk to firefighters is small: inhalation doses are low, even assuming no PPE, leaving the danger of the smoke itself as a primary risk. [8]
Thus while radioactive material can be transported outside the boundaries of the CEZ during a fire event, the risk due to such an incident is very low, as confirmed by research on both the 2015 and 2020 forest fires in the CEZ.
© Natascha Barac. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. Nuwer, "Forest Around Chernobyl Aren't Decaying Properly," Smithsonian Magazine, 14 Mar 14.
[2] N. Evangeliou et al., "Resuspension and Atmospheric Transport of Radionuclides Due to Wildfires Near the Chernobyl Nuclear Power Plants in 2015: An Impact Assessment," Sci. Rep. 6, 26062 (2016).
[3] V. I. Yoschenko et al., "Reswuspension and Redistribution of Radionuclides During Grassland and Forest Fires in the Chernobyl Exclusion Zone: Part I. Fire Experiments," J. Environ. Radioact. 86, 143 (2006).
[4] G. Paliouris et al., "Fire as an Agent in Redistributing Fallout 137Cs in the Canadian Boreal Forest," Sci. Total Environ. 160-161, 153 (1995).
[5] O. Masson et al., "Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020," Enviro. Sci. Technol. 55, 13834 (2021).
[6] A. A. Ager et al., "The Wildfire Problem in Areas Contaminated by the Chernobyl Disaster," Sci. Total Environ. 696, 133954 (2019).
[7] S. Hashimoto et al., "Dynamics of Radiocaesium Within Forest in Fukushima - Results and Analysis of a Model Inter-Comparison," J. Environ. Radioact. 238-239, 106721 (2021).
[8] N. A. Beresford et al., "Wildfires in the Chornobyl Exclusion Zone - Risks and Concequences," Integr. Environ. Assess. Manage. 17, 1141 (2021).
[9] "Sources and Effects of Ionizing Radiation, Vol II," United Nations Scientific Committee on the Effects of Atomic Raciation, 2011, Annex D.
