CC BY
11APPLYING THE EARTH REMOTE SENSING DATA TO ASSESS THE RELEASE OF RADIONUCLIDES IN CASE OF FIRES IN THE RADIOACTIVELY CONTAMINATED
TERRITORIES OF UKRAINE
Mahlovana T.,
State Ecological Academy of Postgraduate Education and Management
Dolin V.,
State Institution "Institute of Environmental Geochemistry of the National Academy of Sciences of Ukraine"
Kopytin D.
Cherkasy Institute of Fire Safety named after Chornobyl Heroes of National University of Civil Defence of
Ukraine
DOI: 10.5281/zenodo.7479816
ABSTRACT
The article presents an analysis of the consequences of forest fires in March-April 2020 in the radioactively contaminated territory of the Narodychi district (Zhytomyr region). Based on satellite remote sensing data, the calculation of the total burned area and the amount of radionuclides that entered the surface layer as a result of fires is presented. Estimated nature of the values of individual exposure doses of the population of radioactively contaminated territories is given.
Keywords: forest fires, radioactively contaminated territories, forest ecosystems, radioactive contamination, migration of radionuclides, smoke aerosols, dose loads.
Formulation of the problem
Accidental releases of radionuclides, nuclear weapons testing, improper methods of handling radioactive substances, disposal of radioactive waste, forest fires in radioactively contaminated areas, in particular near critical infrastructure facilities, lead to radioactive contamination of soils, and can be viewed as global forms of soil degradation [1].
Soil contamination with radionuclides is an important source of danger to the economy, environment and public health.
The ability of soil to immobilize radionuclides is the main factor controlling the concentration of activity available to biota, and acts in conjunction with numerous external factors. Soil texture and structure, its mineral composition, organic components, redox potential and pH, climate change and soil management are the most important factors for radionuclide migration [1].
Analysis of recent research and publications
The accident at the Chernobyl nuclear power plant has changed life drastically not only in the territories adjacent to it, but also in the society itself. The release of technogenic radionuclides into the environment, which were carried over long distances by air masses and surface runoff in the first days and weeks after the tragedy, created an acute problem of radioactively contaminated territories. Forest ecosystems acted as the main barrier within a large area of radionuclide spread, effectively absorbing radioactive particles with a predominant content of 137Cs and 90Sr long-lived radionuclides with a half-life of about 30 years [2].
The largest number of the most contaminated forest ecosystems (excluding the 30-kilometer zone) are located in the territory of the Zhytomyr region. There are forest areas where any economic activity is prohibited: 32.4 thousand hectares, and where mandatory ra-dioecological control over forestry products has been introduced (66.7 thousand hectares) [2].
Currently, the forest ecosystems, both in the exclusion zone itself and in the adjacent territories, are a
long-term source of radiation and environmental hazard.
Radionuclides that have migrated into the soil are firmly fixed, and carried away into the biological cycle, which makes it possible to predict the spatial and temporal dynamics of the radiation status in these territories in the absence of crisis phenomena such as dust storms, forest fires, tornadoes, heavy rains, earthquakes.
The statistics of forest fires over the past years [36] testifies to their scale and duration both in the exclusion zone itself and in the adjacent territories. For the period from 1993 to 2020, more than 1,800 forest fires of various types and intensity occurred in the territory of the Chernobyl exclusion zone, on an area of more than 100 thousand hectares of territories contaminated with radionuclides, with an estimated stand of dry radioactively contaminated timber of approximately 25 mln
m3.
The forest fires unprecedented in number, scale and duration were observed in the Chernobyl exclusion zone in 2015-2016 and in the spring of 2020 [7], with the fires in the spring of 2020 being the largest in the radioactively contaminated territories [5]. These fires led to a significant migration of radionuclides, an increase in the specific volumetric activity of radionu-clides in the surface air layer by 2-3 orders of magnitude [5]. The total area of fires on the territory of Ukraine in 2020 was more than 2,500 sq.km, and 35% of the annual CO2 equivalent in 2020 was due to the loss of tree cover in the Zhytomyr region [5].
As a result, this led to a deterioration in the radiation and environmental state of the surrounding areas and an increase in the risk of inhalation and oral exposure of the population and personnel involved in fire fighting.
These phenomena violate the integrity of the forest ecosystem and its barrier function, thus affecting vegetation, soil, hydrochemical, geochemical, and thermal balances, as well as contributes to secondary radi-
oactive contamination of relatively clean areas, increasing the dose load. The fires in the near exclusion zone, where insoluble highly active particles of radioactive elements are preserved in the soil, pose the greatest danger [8-9]. Because of fires, vegetation dies, its root system no longer absorbs radionuclides in the root layer of the soil, and mobile forms of radionuclides, while interacting with the components of the soil complex, are redistributed along the soil profile due to vertical migration processes. This leads to a change in the migration mobility of 137Cs and 90Sr in the upper soil layer [3]. Moreover, radioactive particles are formed because of wood ashing products and forest leaf litter that have aerodynamic properties and that are carried by wind currents over significant distances (over 100 km).
Thus, forest fires in the radioactively contaminated territories of Ukraine is a social and environmental problem that requires an enhancement of the existing risk management system and the use a complex of short-term and long-term measures to minimize risks.
The purpose of this study is to assess the risks of increasing individual exposure doses for persons involved in fire extinguishing and population in case of forest fires in radioactively contaminated areas using geoinformation technologies.
Research methods
The satellite remote sensing data of two types of sensors were used: VIIRS (Visible Infrared Imaging Radiometer Suite) and MODIS (Moderate Resolution Imaging Spectroradiometer) [10].
VIIRS is installed on two satellites: the US N0AA-20 non-geosynchronous environmental satellite, which is part of the Joint Polar Satellite System for weather forecasting and climate monitoring, and the Suomi NPP satellite, which belongs to the US National Oceanic and Atmospheric Administration. VIIRS fire layer shows active detection and thermal anomalies to study the spatial and temporal spread of fire, and search for hot spots to identify the source of smoke pollution.
VIIRS data makes it possible to detect fires in real time. The VIIRS Fire and Thermal Anomalies product is available on the NASA/NOAA shared Suomi-Na-tional Polar-orbiting Partners (S-NPP) and N0AA-20 (JPSS-1) satellite. Remote monitoring tools of the MODIS type are installed on the Terra and Aqua (EOS AM-1) satellites, transnational research satellites in a sun-synchronous orbit around the Earth, which are controlled by NASA [10]. MODIS data show active fire detection centres and thermal anomalies to identify the source of air pollution from smoke, which can adversely affect human health.
The MODIS Fire and Thermal Anomalies product is available from the Terra (MOD14) and Aqua (MYD14) satellites, as well as the combined Terra and Aqua (MYD14) product. The Terra orbit (MYD14) passes the equator from north to south in the morning, and Aqua (MYD14) orbit passes the equator from south to north in the afternoon, which increases the scan density and the reliability of fire parameters estimation. Thermal anomaly data from the Aqua and Terra satellites (MODIS spectroradiometer) are downloaded using free NASA reference materials at regular intervals.
Thematic layers of maps [11] and geoinformation tools were used to calculate the areas of fires, and integrate the territory by contamination density. The areas burned during fires were taken into account for two main categories of landscapes: forests and meadows. All areas covered with woody vegetation (coniferous and deciduous) were categorized as forests.
The areas of water bodies, roads and other techno-genic facilities were not taken into account for calculation of fire areas.
Integral releases of radionuclides were calculated as the product of the spatial density of radionuclide fallout (Bqm-2) by the area of burned out territories (sq.m), based on MODIS data using release factors [12]. With regard to radionuclide release factors, the studies [13] report that about 20% of labile radionuclides can be redistributed after fires into the surface atmosphere from soil, and up to 40-100% from biomass [14-15]. The studies [16-17] demonstrate that the release factors are 4 to 10%. Under laboratory conditions, 137Cs release factors for biomass have been established, which are 1 to 2.5% [18]. As for 90Sr and trans-uranium elements (TUE), there is a limited number of measurements of release factors from biomass combustion [9, 17], which demonstrate that resuspension has little effect on the level of radioactive contamination of the surface layer of the atmosphere and biogeocenoses.
The estimate of carry-over during fires for 137Cs -5% and for 90Sr and EPu - 0.2% of the radioactivity reserve in combustible material (conservative approach) [12] was used for calculations of long-range migration (more than 10 km).
The calculation of the volume of radioactive combustion products into the atmosphere was carried out according to the formula [12]:
Ai(MBq)=S(km2)-106-As1(kBq/m2)-K1cenosis-n1/103(1),
where Ai(MBq) is the activity of the i-th radionu-clide released into the air during fire; S(km2) is area of burning territory; Asi(kBq/m2) is the contamination density of the burning territory; K'cenosis is a coefficient linking the reserves of radionuclides in the combustion material depending on the biocenosis and density of soil contamination (137Cs=0.11; 90Sr=0.28; £Pu=0.03); ni is the fraction of the migration of the i-th radionuclide from the fuel material into the atmosphere.
The estimated effective dose from the inhalation of radioactive materials was carried out according to the methods [19] for three groups of the population: children, adults, personnel of fire departments:
Ef
inh _
i E
inh
m*? vbm
(1),
where I is the respiratory rate for population groups (2.5710-4 m3/s - adults; 5.08 - 7.62 10-4 m3/s fire extinguishing participants under extensive load;
1.77 10-4 m3/s children under 10 years old); ^ is the effective inhalation dose coefficient of radionuclide m 137 Cs for adults: 4.610-9 Sv Bq-1 (adults); 3.7- 10-9 Sv Bq-1 (children under 10 years old); for 1 ^m particles; Am is the surface density of radionuclide m on the ground (Bqm-2); Vbm is the volumetric deposition rate of the radionuclide m for specific weather and surface conditions (m/s).
Research results
The Narodychi district (Zhytomyr region) was chosen as the study area, which, both in terms of the area of territories contaminated with radionuclides and in terms of the levels of exposure of the population, is one of the most affected by the accident at the Chernobyl nuclear power plant [2].
In recent years, the doses of internal exposure of the residents of the Zhytomyr region have significantly decreased, but the highest levels of internal exposure, at present, are recorded in the settlements of Narodychi, Ovruch, Luhyny districts [20]. The main part of the dose of internal exposure to the residents of the settlements of the Narodychi district is caused due to the consumption of local radioactively contaminated products
of household and forestry [20], which determined this territory as the object of study, taking into account possible radiation dose loads as a result of forest fires in radioactively contaminated territories and when using forestry products.
As a result of a forest fire that occurred on April 3, 2020 near the village of Zvezdal Narodychi district) [21], followed by the spread of fire to the territory of Narodychi, Davydky, Klesiv, Denysovhychi, Dytiatky, Korohod, Kotovske forestries, the total area of the fire was calculated using MODIS products (Fig. 1), which was up to 41.06 sq.km.
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b v fi./ ■. /j^ii-S iftf '' " an '' - ^ - ■
•fc
*" iRena 1 • i
- r ^M,. - H n '
¡Si 1 . mi C i Mm* p wM fS
♦v.
** " J
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Fig.1. Formation offire beds (03/04/2020) based on MODIS products
The analysis of burned out areas was carried out according to open access information according to NASA tools (Fig. 2, 3), which makes it possible to determine the aerosol index, calculated based on the density of smoke and suspended particles in the vertical column of the atmosphere and indicating the presence of smoke and aerosol releases due to biomass combustion to a height of 1 to 3 km.
Values of 5.0 indicate high concentrations of aerosols that may impair visibility or affect human health. The OMPS Aerosol Index Layer is a scientific parameter of the Ozone Mapping Profiler Suite (OMPS)/National Polar-orbiting Partnership (NPP) (OMPS-NPP) L2 NM Aerosol Index Swath orbital V2, available from the OMPS Nadir-Mapper tool on Suomi National Satellite of the Polar-orbiting Partnership (NPP Suomi).
Fig. 2. Burned out areas according to NASA tools Narodychi district (05/04/2020)
The Ozone and Mapping and Profiler Suite (OMPS) tools can be used to detect and track ozone and other atmospheric constituents, including SO2, NO2, formaldehyde, and ultraviolet-absorbing aerosols such as dust, volcanic ash and biomass smoke. The aerosol
index is derived from the normalized emissions using two pairs of wavelengths at 340 and 378.5 nm. The sensor resolution is 50 km. The image resolution is 2 km, and the temporal resolution is daily-based.
KJochky
Noryntsi Latashi Staryl
_ . ' ixuvyi •
Dorohyn z^tela
Lugovyky 'Zirka sto^C' Zelena
Rahivka ' ? Marianlvka
Mlachivka
Slara Shkneva Markivka
Fig. 3. Formation of fires (20/04/2020) according to NASA product
Based on the parameters of satellite remote sens- to fires in March-April 2020 was calculated, which is ing, as well as WorldView calculation products, the up to 108.12 sq.km (Fig. 4). area of burned territories of the Narodychi district due
Fig. 4. Total area offires in the territory of the Narodychi district in March (red) and April (yellow) 2020.
Furthermore, the total area of territories damaged by fires in March-April 2020 is approximately 600 sq.km [5] (Fig. 5).
Fig. 5. Territories damaged by fire in March (red) and April (yellow) 2020
Based on the data on radioactive contamination of the territories of the Narodychi district [11], the volume of radionuclides that entered the surface layer of the atmosphere due to fires in the territories of the Narodychi district in March-April 2020 was calculated (Fig. 4), which, according to conservative calculations, is up to 110.3693 GBq, while the total value of 137Cs release
due to fires in 2020 in the Chernobyl exclusion zone and adjacent territories is 690 GBq [22].
The estimated effective dose from the inhalation of radioactive materials was carried out according to the methods [19] for three groups of the population: children, adults, personnel of fire departments.
The obtained data are given in Table 1.
Table 1.
Volumes of the calculated effective dose of exposure for the population of the Narodychi district and those
Population groups Effective dose, D, [p.Sv]
Adults 3.50 to 5.10
Children (up to 10 years old) 1.30 to 1.50
Fire extinguishing participants 7.70 to 11.30
The results obtained correlate well with the calcu- State Emergency Service of Ukraine when extinguish-lated data [5], as well as with the data on the levels of ing a fire in April-May 2020, and the assessment of the radionuclides intake into the body of rescuers of the internal exposure dose due to this intake by measuring
the contents of incorporated gamma radionuclides on whole-body radiation meters [23].
The presented methods make it possible to quantify the predicted values of the effective dose of internal exposure for the population, due to the release of radionuclides into the environment as a result of fires. Although the risk of exceeding the annual effective dose limits [24] for small fires is negligible, such data are important for informing the public about additional radiation doses.
It should be noted that the accuracy of determining the numerical values of individual exposure doses for the population of radioactively contaminated territories depends on many factors that may not be fully taken into account in the simulation.
For instance, it should be taken into account that near the source of the fire, the activity of radionuclides in the air is maximum, and the contribution to the total effective dose is made by trans-uranium elements [25]; furthermore, the active concentrations of radionuclides in smoke particles are much higher compared to the original vegetation, so prolonged exposure to smoke enhances the inhalation of radionuclides associated with smoke particles [26], which determines the availability of anti-radiation protection for fire extinguishing participants.
The inhalation intake of radionuclides (especially, the isotopes of the transuranium series) can affect the change in the average annual equivalent doses not only of the fire extinguishing participants, but also of the people residing near the source of the fire [27].
Nonetheless, the main role in the environmental hazard for the population of radioactively contaminated territories is played by the oral component, namely, forest products, which are renewed after fires.
The deposited forms of radionuclides in the ecosystem turn into mobile forms due to fires, which enter the food chains after the forest commodities are restored [2].
The manifestation of the influence of forest fires on the formation of dose loads of the population of the territory should be expected approximately 2 years after the forest fire [2], as a result of the restoration of forest products under conditions of increased mobility of radionuclides in forest soils, which leads to food chain contamination.
Since the lifetime of a radioactive smoke cloud in the lower troposphere is less than a week and is about a month in the upper troposphere, this creates a radiation hazard to public health even in relatively remote regions.
As a result of the passage of a radioactive cloud of forest fire smoke over the settlements, their residents are exposed to the following radiation hazards:
- external exposure due to smoke plume;
- external exposure to nuclides deposited from the smoke plume onto the parts of the environment;
- internal exposure due to inhalation of radioactive particles (smoke and dust aerosols);
- internal exposure due to the intake of radionu-clides into the human body with food (forest commodities, medicinal herbs, mushrooms, berries, water, milk, etc.).
Conclusion
1. Forest fires act as catalysts for dangerous changes in the parameters of ecosystem radioactive contamination. Taking into account the predicted climate change, one should expect an increase in radiation risks to the human living environment associated with forest fires in radioactively contaminated territories.
2. Based on the data of geoinformation technologies, a possibility is demonstrated for assessing the formation of an individual effective dose of internal exposure for three groups of the population: children, adults, personnel of fire departments.
3. Increasing the effectiveness of measures to prevent and minimize the negative effects of forest fires requires the introduction of modern web platforms that combine the application of geoinformation technologies, remote sensing data for their timely detection and a comprehensive assessment of life risks, including in the territories of increased radiation and environmental hazard. This can help to enhance the reliability of forecasting and the performance of forest fire prevention by transferring modern knowledge between various entities involved in fire fighting, as well as for assessing the risks to the health of the population and fire extinguishing participants in the territories contaminated with ra-dionuclides.
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