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Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone: part I. Fire experiments
Journal of Environmental Radioactivity 86 (2006) 143e163 www.elsevier.com/locate/jenvrad
Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone: part I. Fire experiments V.I. Yoschenko a, V.A. Kashparov a,*, V.P. Protsak a, S.M. Lundin a, S.E. Levchuk a, A.M. Kadygrib a, S.I. Zvarich a, Yu.V. Khomutinin a, I.M. Maloshtan a, V.P. Lanshin a, M.V. Kovtun a, J. Tschiersch b a
Ukrainian Institute of Agricultural Radiology (UIAR), Mashinobudivnykiv str.7, Chabany, Kyiv region 08162, Ukraine b GSF-National Research Center for Environment and Health, Institute of Radiation Protection, D-85764 Neuherberg, Germany Received 11 February 2005; received in revised form 8 August 2005; accepted 11 August 2005 Available online 5 October 2005
Abstract Controlled burning of experimental plots of forest or grassland in the Chernobyl exclusion zone has been carried out in order to estimate the parameters of radionuclide resuspension, transport and deposition during forest and grassland fires and to evaluate the working conditions of firemen. An increase of several orders of magnitude of the airborne radionuclide concentration was observed in the territory near the fire area. The resuspension factor for 137Cs and 90Sr was determined to range from 106 to 105 m1, and for the plutonium radionuclides from 107 to 106 m1 (related to the nuclides in the combustible biomass). These values are 2 orders of magnitude lower if they are calculated relatively to the total contamination density (including the nuclides in the soil). The radionuclide fallout along the plume axis is negligible in comparison to the existing contamination. However, the additional inhalation dose for firemen exposed in
* Corresponding author. Tel./fax: C380 44 266 45 02. E-mail addresses: [email protected] (V.A. Kashparov), [email protected] (J. Tschiersch). 0265-931X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2005.08.003
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the affected area can reach the level of the additional external irradiation in the period of their mission. The plutonium nuclides constitute the dominating contribution to the inhalation dose. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Biomass burning; Radioactive aerosol; Resuspension; Inhalation; Dose assessment
1. Introduction Wildland fires and other biomass burning are a major source of aerosol particles and gases in the atmosphere (Goldammer and Crutzen, 1993). Not only black carbon and organic material derives from pyrogenic emissions but also species such as K, Cl, SO4, NOx and heavy elements such as Cu and Zn (Andreae and Merlet, 2001; Yamasoe et al., 2000). Herbicide residues (McMahon and Bush, 1992) and radioactive iodine, caesium and chlorine were found in the smoke of biomass fires (Amiro et al., 1996). These trace substances may affect the surrounding of the fires and people in this area (e.g. forest workers and firemen). But even at about 17 km distance from a forest fire an increase of the airborne 137Cs concentration was measured (Garger et al., 1998). Therefore, the resuspension and redistribution of pollutants by biomass burning is not only a local problem. Inside the Chernobyl exclusion zone a huge amount of various radionuclides is deposited. Detailed contamination maps are published (Kashparov et al., 2001). Some of these nuclides were deposited during the Chernobyl accident directly to trees and bushes; others were transferred by root uptake from the contaminated soil to the vegetation. The exclusion zone is covered mostly (almost 2/3 of the total area) by forests and grasslands. It is estimated that several percents of the initially deposited radionuclides are contained in the biomass of the grasslands and even more in the biomass of the forests (see the characterization of the experimental plots in Section 3.1). These radionuclides currently fixed in the biomass may potentially be mobilized by wildland fires. The area is no longer cultivated and is mostly abandoned. Frequently wildland fires occur which are detected and combated late. The number of the detected fires varies in the range from 42 to 116 per year. The fires have already destroyed thousands of hectares of forest. The problem is expected to increase in future, because the absence of any measure for maintenance of the forests, meadows and peatlands will lead to an uncontrolled growth of the vegetation. The aim of the investigation presented here is to quantify the resuspension, transport and deposition of the relevant radionuclides during grassland and forest fires in Chernobyl exclusion zone. Firemen are exposed to these airborne radionuclides which may be inhaled. The potential dose due to inhalation and external radiation will be assessed. The experimental procedures are designed to accumulate an array of experimental data necessary for the validation of the models of radioactivity transport in the atmosphere during wildland fires. Previous studies on fire resuspension (Kashparov et al., 2000) had been performed outside the Chernobyl exclusion zone giving data only on 137Cs. Now, the focus of the investigation is on the potential impact of the radionuclides 90Sr, 238Pu and 239 C 240Pu.
2. Materials and methods In order to study the resuspension and transport of radionuclides in the atmosphere during wildland fires, the method of active fire experiments was applied, which implies the controlled burning of the
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selected experimental site under well-monitored conditions. Prior to the experiment, a complete radioecological description of the site was carried out, the background measurements were done and the sampling equipment items were installed according to the elaborated scheme of the experiment. 2.1. Characterization of the experimental sites The active experiments were performed on two grassland plots and one forest plot located in the Chernobyl zone. The sites were selected to satisfy the following criteria: highly and homogeneously contaminated area within the fuel trace of the radioactive fallout (contamination with 137Cs, 90Sr, 238Pu and 239 C 240Pu); uniform wide flat area allowing an undisturbed plume spreading; plot length of about 50e100 m (assuming 1 h stable meteorological conditions and a fire front speed of several m min1). Both grassland fires were carried out within the same experimental area near the former village Chistogalovka, at a distance of approximately 3 km west of the Chernobyl reactor site. The forest site is located 5 km WNW (280 ) of the Chernobyl reactor close to the former village Novoshepelichi. It is a cultivated pine forest surrounded by grassland. The age of the trees in the forest was 30 years. The trees were planted in rows with an inter-row distance of approximately 1.5 m. The distance between the trees in the rows varied from 0.5 to 3 m. The average quantity of trees per unit of the forest area was 0.275 m2. Due to the relatively high density of planting, the pine trees were rather thin (about 15 cm diameter at the height of 1.5 m) and the tree crowns were not well-developed. There was practically no understorey. A first evaluation of the spatial distribution of radioactivity within the plots was done by the measurement of the exposure dose rate (EDR) in the nodes of a 5-m regular mesh (grassland) and a 10-m mesh (forest) with the dosimeter DRG-01T (Tair SKS, Russia). The EDR values were measured at the height of 1 m above the ground surface, which means that each measurement result was related to the radionuclides located within a comparable large area (with a radius of about 10 m). The EDR values measured at the different plots are presented in Fig. 1, showing rather uniform g-fields. Samples of soil, natural litter and vegetation were collected before the experiments within the plots to be burnt. For the determination of the contamination density, 10 soil samples were collected from each plot using a cylindrical core driller of 3.7 cm diameter to the depth of 30 cm (grassland) and 20 cm (forest). Each sample consisted of 5 portions of soil that were sampled in the corners and in the center of a square 5 ! 5 m2. For the determination of the radionuclides in the soil profile, so-called layer sampling was carried out with a cylindrical core driller of 5 cm diameter which consists of two semi-cylinders. After the core has been drawn, one semi-cylinder is removed and the whole soil sample is accessible for further dividing into sub-samples of certain depths. In this case, each sample of a certain soil layer consisted of 3 portions sampled at various points in the corresponding depth. On each plot, 10 samples of natural litter were collected, each from the area of 20 ! 20 cm2. At the same points, grass samples were collected from areas of 40 ! 40 cm2 in plot #1 and 50 ! 50 cm2 in plot #2. In the forest, in addition, 10 representative samples of fallen tree branches were collected. Several representative trees were cut and divided into elements (timber, bark and needles) to determine the mass (and the radionuclide concentration). The biomass was determined in air-dry state. The characteristics of the three plots are summarized in Table 1. The activity analysis was performed for the following radionuclides: 137Cs, 90Sr, 238Pu and 239 C 240 Pu. 2.2. Activity measurements The activity concentration of 137Cs in all samples was measured with the low-background g-spectrometric complex ADCAM-300 (USA), which is equipped with the Ge-detector GEM-30185 (EG & G ORTEC, USA). The energy resolution of the detector is 1.78 keV for the 60Co-line at 1.33 MeV.
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a
meters 0
0
10
20
30
40
0
0
10
20
30
40
50
60
70
80
10
10 20 14.5
30
meters
16.0
meters
meters
b
50
10
20
9
30
13.0
8
40
40 11.5
7
50
50
10.0
6
c
4.0
3.6
3.2
2.8
0
20
40
2.4
meters
Fig. 1. Spatial distribution of the exposure dose rate (mGy h1), within the grassland plot #1 (a), grassland plot #2 (b) and forest plot (c).
The 90Sr activity concentration in the samples was measured using a standard radiochemical method, which is based on the extraction of the radiochemical pure 90Sr from the samples prepared by the oxalate method. The 90Sr activity was determined by radiometry of its progeny, 90Y, using the aeb-radiometer CANBERRA-2400 (USA). Atomic-absorption spectrometry was applied for the determination of the chemical yield of strontium. Pu isotopes were extracted from the samples and concentrated on metal plates by means of electrolytic deposition. Their activities were measured using the a-spectrometer SOLOIST (USA).
2.3. Determination of aerosol parameters During the experiments, aerosols in the lower troposphere (at 1 m height above the ground surface) were sampled in order to determine the airborne radionuclide concentrations at various distances from the source of release. For this purpose, the isokinetic aerosol samplers IPA (Antonov Air Plant, Ukraine) were utilized (Garger et al., 1997). In these samplers, the aerosol particles are sampled at the current wind
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Table 1 Characteristics of the plots used for the active fire experiments Area (m2) Land type Vegetation species Vegetation height (m) Biomass density (kg m2) Litter density (kg m2) Dose rate range (mGy h1)
Plot #1
Plot #2
Plot #3
3600 Wildland (grass) Elytrigia repens (L.) Nevski (85%) 0.5 0.4 0.7 10e16
5400 Wildland (grass) Elytrigia repens (L.) Nevski (85%) 0.2 0.3 1.25 6e10
8770 Forest Pinus silvestris 15 24 2.3 2e4
velocity on Petryanov filter tissue (Krzes´niak and Porstendo¨rfer, 1981). Assuming constant wind velocities during the experiment, the average airborne activity concentration AV of the considered radionuclide is calculated as follows: ÿ AV ZAf =V Bq m3
ð1Þ
where Af is the activity measured in the filter (Bq) and V is the air volume passed through the filter (m3). The deposition density of the radionuclides fallout from the radioactive fire plume was quantified with horizontal plates covered with Petryanov tissue and installed at 1 m height. The deposition density AS is calculated as follows: ÿ AS ZAp =SP Bq m2
ð2Þ
where Ap is the activity in the tissue on the plate (Bq) and SP is the surface area of the plate (m2). Horizontal tissue covered plates as deposition collectors provide standardized sampling for the determination of the deposition pattern. The Petryanov tissue entirely retains the deposited material and represents a sticky surface with a slightly higher roughness than a water surface, which is suggested for monitoring of radioactive fallout (Rosner and Winkler, 2001). The deposition velocity to this reference material is supposed to be in the same order of magnitude than to the grass surface but lower than to the adjacent trees and bushes. It is assumed that the radionuclides during the fire experiments were resuspended and transported mainly in particulate form. Such an assumption is based on the fact that the burning temperature is not high enough for an atomization of the radionuclides of Sr and Pu, and they can be transported with particles of ash, tar and water only. For Cs with the lowest boiling point, evaporation might be possible in the core of forest fires, but due to nucleation and condensation processes Cs-particles were formed rapidly. Radionuclide resuspension, transport and deposition depend on the aerosol particles size and density. In the present study, the dispersal composition of the radioactive aerosols was characterized using an impactor designed by the Institute of Biophysics of the Health Ministry, Russia (IBP impactor) and described by Frank et al. (1996). In this instrument aerosol particles are deposited on one of five cascades according to their aerodynamic diameter. The analysis of the radionuclides deposited on the cascades provides the activity size distribution. The sampling rate, however, is only 1.05 m3 h1. The high-volume Andersen sampler (model AH-600Z, USA) was utilized for the determination of the dispersal composition of radioactive aerosol at larger distances from the source of release. In this sampler, all particles are deposited on four cascades at an air sampling rate of 34 m3 h1.
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Other important parameters can be derived from the measured values of AV and AS. The term resuspension is used to mean the re-entrainment into the atmosphere of previously deposited radioactive material and is characterized by the resuspension factor R (Nicholson, 1988): ÿ RZAV =S m1
ð3Þ
where S is the primary areal activity density (Bq m2). By definition, the resuspension factor quantifies the wind-driven transfer from a homogeneously contaminated surface into the atmosphere and implies equilibrium between the resuspended and the deposited aerosol. These conditions are not achieved in the fire experiments and R should rather be considered as the airborne activity concentration normalized by the areal activity density (Wagenpfeil et al., 1999). R can either be normalized by the total contamination density or by the contamination density of the burning biomass only. R provides a means for inter-comparison between other published fire and surface release experiments. The deposition of the radionuclides can be described in terms of the deposition rate I as ÿ IZvg !AV ðzZ1 mÞ Bq m2 s1 ð4Þ or ÿ IZAS =t Bq m2 s1
ð5Þ
where vg is the deposition velocity (m s1), z is the vertical coordinate and t is the deposition period (s). If I is determined in the experiments using Eq. (5), vg can subsequently be derived from Eq. (4). Because the experiments were performed in the highly contaminated exclusion zone, the background values of AV and I were determined before the fire experiments. The values are given in Table 2. 2.4. Measurement of meteorological parameters During the experiments, the relevant meteorological parameters at the sites were determined. The air temperature T was measured using a mercury thermometer. To record the average wind velocity u during certain periods of time, the cup anemometer MS-13 was installed at the height of 2 m above surface. The anemometer provides the accurate measurement in the range of 1e20 m s1. The wind direction and wind velocity in gusts were determined using Tretyakov’s wind meter. The air pressure P was measured by the mercury cup barometer SRA, and the air humidity by the hygrometer M-19. The cloudiness was estimated visually according to Hrgian (1957). Data on the regional meteorological conditions as well as the data for the estimation of the atmospheric stability class in the local scale were presented by the Kiev Hydrometeorological Station and the Chernobyl meteorological station. These data were obtained by means of the aerial radar AVK Titan-1 and air balloon monitoring. All equipments were manufactured in the former USSR. Table 2 Background values of the airborne activity concentration AV and the deposition rate I of the considered radionuclides before the experiments 137
90
Cs
238
Sr
3
AV, !10 Bq m3
Plot #1 2.1 G 0.3 Plot #2 4.6 G 0.6 Plot #3 3.2 G 0.4
4
239 C 240
Pu
3
5
I, !10 AV,!10 Bq m2 s1 Bq m3
I, !10 AV, !10 Bq m2 s1 Bq m3
0.5 G 0.1 1.0 G 0.1 0.6 G 0.1
2.0 G 0.3 3.5 G 0.4 2.2 G 0.2
0.8 G 0.2 1.4 G 0.4 4.4 G 0.5
Pu
6
1.5 G 0.3 4.3 G 0.7 8.3 G 2.1
8
I, !10 AV, !106 2 1 Bq m s Bq m3
I, !107 Bq m2 s1
1.2 G 0.3 4.4 G 0.7 23 G 4
0.3 G 0.1 1.1 G 0.1 4.5 G 0.7
2.9 G 0.6 8.5 G 1.0 22 G 5
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According to the data of the Chernobyl and Kiev meteorological stations and to our observations, the lower tropospheric layer was neutral e slightly unstable (category C) during the two grassland fires, and it was stable (category E) during the forest fire. The other meteorological parameters are summarized in Table 3. 2.5. Fire characterization The experiments were carried out in favourable meteorological conditions. Dry weather facilitated the intensive fires, and as the wind direction remained stable during the experiments, the well-visible smoke plumes covered the air sampling area in the case of the grassland fires, and the vertical dimension of the plumes reached up to 10 m near the plots. The grassland fires caused the total combustion of the grass and the charring of the litter in the experimental plots. In the first experiment (plot #1), the fire front passed 60 m distance in 15 min, while the total period of burning was 30 min. In the second experiment (plot #2), the fire front velocity was higher e the 90 m distance was passed in 13 min, and the total period of burning was again 30 min. The forest fire can also be classified as an intensive one. It was mainly of the surface type (i.e. forest litter burning) with some periods of crown fire. The smoke plume arose 30e40 m above the burning area and was then transported over the sampling area. As a result of the fire, the forest litter was burnt and the tree bark was charred up to the height of 1.0e1.5 m. Some tree crowns were also burnt or damaged. The fire front has passed 100 m in about 30 min. The total duration of the active phase was about 90 min. 2.6. Scheme of the experiments The same principal scheme was used in all experimental runs. The sampling equipment was installed in several lines normal to the wind direction such that the smoke plume from the burning plot passed over the instruments. In order to cover the whole area of the plume transport, the far lines were approximately twice as long as the plot width. Along each line, the horizontal plates were installed uniformly, while the aerosol samplers and impactors were positioned in the center of the lines, at the plume axis. The inter-line distance in each experiment was approximately the same. The installation of the equipment and the performance of the experiment were done in steady wind direction conditions. After the completion of the preparation works, the plots were ignited along their leeward sides (along the 60-m side in the grassland plot #2). The most horizontal plates were deployed during the forest fire experiment (Fig. 2), 40 plates in total. Fewer horizontal plates were used during the grassland fires (18 in the first experiment and 13 in the second) because of the smaller spread of the plume. Also, the samplers were located closer to the grassland plots, at distances up to 150 m. Table 3 Meteorological parameters during the fire experiments Parameter
Plot #1
Plot #2
Plot #3
Time
10 OCT 2001, 15:00e16:00 5/0 Cie6/0 Ci, Ac 22.2 57 180e185 2 2e5 1004.9 C
18 APR 2002, 12:00e13:30 7/7 Cue8/8 Cu cong 16.2 33 315e350 2 2e7 1004.9 C
15 MAY 2003, 15:40e17:15 4/0 Cie8/0 Ci, Ac, Cu 26.4 50 110e170 3 4e7 996.5 F
Cloudiness Mean air temperature ( C) Air humidity (%) Wind direction at h Z 2 m ( ) Wind velocity, mean (m s1) Gusts (m s1) Atmospheric pressure (hPa) Diffusion category
150
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Horizontal plates IPA aerosol sampler
Forest plot
IBP impactor (3 units at each spot) Andersen aerosol sampler
0
50
100
meters
Fig. 2. Scheme of deployment of the sampling equipment during the forest fire experiment.
3. Results and discussion 3.1. Radionuclide inventories at the experimental plots The measured radionuclide activities in the samples from the experimental grassland plots and deduced characteristics are presented in Table 4. The uncertainty of each value is mainly determined by the heterogeneity of its spatial distribution within the site, in comparison, the analytical error is low. Assuming the full consumption of the combustible matter (grass and litter) in the plots, the maximal values of the radionuclide release into the atmosphere can be estimated as 1000 G 800 MBq of 137Cs, 700 G 500 MBq of 90Sr, 2.6 G 1.5 MBq of 238 Pu and 5.8 G 4.8 MBq of 239 C 240Pu during the fire on plot #1 and 640 G 390 MBq of 137 Cs, 570 G 300 MBq of 90Sr, 470 G 330 kBq of 238Pu and 1.1 G 0.7 MBq of 239 C 240Pu during the fire on pot #2. The radionuclide activities and deduced characteristics from the forest plots are presented Table 5. The caesium and plutonium activities in the soil are much higher than those in the combustible material, while for strontium the situation is different e a significant fraction of its total activity is located in the biomass. In the combustible material, about 70% of 137Cs and 90% of Pu are associated with the forest litter. For 90Sr, this amount reaches only about
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Table 4 Radionuclide characteristics of various compartments of the grassland plots: mass activity density C, contamination density S and total inventory TI
Plot #1 137 Cs
90
Sr
238
Pu
239 C 240
Pu
Plot #2 137 Cs
90
Sr
238
Pu
239 C 240
Pu
a
Soila
Grass
C, kBq kg1 S, MBq m2 TI, GBq
80 G 50 29 G 18 100 G 70
86 G 50 0.026 G 0.015 0.094 G 0.065
340 G 210 0.26 G 0.17 0.94 G 0.7
110 G 70
1.0 G 0.8
C, kBq kg1 S, MBq m2 TI, GBq
21 G 14 7.6 G 4.7 27 G 18
120 G 90 0.041 G 0.031 0.15 G 0.12
210 G 150 0.15 G 0.11 0.54 G 0.4
28 G 19
0.7 G 0.5
C, Bq kg1 S, kBq m2 TI, MBq
140 G 80 50 G 30 180 G 110
1.1 G 0.7 (4 G 3) ! 10-4 (1.4 G 1.1) ! 103
1000 G 500 0.72 G 0.4 2.6 G 1.5
180 G 110
2.6 G 1.5
C, Bq kg1 S, kBq m2 TI, MBq
280 G 160 100 G 60 360 G 200
2.4 G 1.6 (8 G 6) ! 104 (2.8 G 2.1) ! 103
2200 G 1500 1.6 G 1.2 5.8 G 4.8
370 G 210
C, kBq kg1 S, MBq m2 TI, GBq
14 G 9 5.1 G 3.1 27 G 17
49 G 21 0.014 G 0.006 0.07 G 0.03
85 G 5 0.11 G 0.07 0.57 G 0.36
28 G 17
0.64 G 0.39
C, kBq kg1 S, MBq m2 TI, GBq
7.3 G 6.0 2.7 G 2.2 15 G 12
62 G 19 0.018 G 0.005 0.10 G 0.01
81 G 35 0.087 G 0.053 0.47 G 0.29
16 G 12
0.57 G 0.30
C, Bq kg1 S, kBq m2 TI, MBq
57 G 33 20 G 12 110 G 60
0.17 G 0.13 (4.7 G 4.0) ! 105 (2.5 G 2.1) ! 104
130 G 87 0.087 G 0.060 0.47 G 0.33
110 G 60
0.47 G 0.33
C, Bq kg1 S, kBq m2 TI, MBq
110 G 65 40 G 27 210 G 150
0.44 G 0.33 (1.2 G 0.9) ! 104 (6.7 G 5.3) ! 104
270 G 190 0.19 G 0.14 1.1 G 0.7
210 G 150
1.1 G 0.7
Litter
Total
Total combustible
5.8 G 4.8
Upper 30-cm soil layer.
30%, and more than 40% are found in timber. Assuming that the forest litter is the primary combustible material during the forest fires, one may expect the same activities of 137Cs and 90 Sr, and much lower activities of the plutonium radionuclides in the resuspended material. 3.2. Airborne concentration, fallout and dispersal composition during the fires Averaged values of the airborne radionuclide concentrations during the fires at the various distances from the source of release are presented in Fig. 3. The samples were taken at 1 m height at the plumes axes. During the grassland fires, the airborne concentrations of 137Cs and 90Sr reached values of several Bq m3 near the source of release and decreased with the distance. The concentration of plutonium was generally 3 orders of magnitude lower and an increase with the distance was observed in plot #1. In plot #2, rather constant values were measured at greater distances after an initial decrease. This different behaviour of plutonium can be explained by differences in certain experimental parameters such as the initial heights of release or the deposition velocity (see below). During the forest fire experiment, in general, 1 order of magnitude lower activity concentrations for 137Cs and 90Sr were measured in comparison to the grassland fires. After an initial
152
Soila Biomass density, kg m2
Timber
Bark
1st year needles
2nd year needles
Forest litter Fallen branches
20.5 G 3.5
2.6 G 0.4
0.6 G 0.1
0.30 G 0.05
2.15 G 0.28
137
C, kBq kg1 S, kBq m2 TI, GBq
5.8 G 1.1 1700 G 300 14.9 G 2.6
0.83 G 0.20 17 G 7 0.15 G 0.06
5.1 G 1.4 13 G 6 0.12 G 0.05
57 G 15 34 G 15 0.30 G 0.13
6.9 G 1.9 2.1 G 0.9 0.018 G 0.008
64 G 17 137 G 53 1.2 G 0.5
90
C, kBq kg1 S, kBq m2 TI, GBq
3.7 G 0.9 1100 G 300 9.6 G 2.6
12.5 G 2.5 260 G 100 2.25 G 0.83
45.8 G 9.2 120 G 40 1.04 G 0.37
26.4 G 5.2 15.8 G 5.8 0.14 G 0.05
70 G 15 21 G 8 0.19 G 0.07
81 G 18 173 G 62 1.52 G 0.54
C, Bq kg1 S, Bq m2 TI, MBq
33 G 8 0.24 G 0.07 7.4 G 2.1 0.24 G 0.07 0.33 G 0.10 130 G 40 0.3 G 0.1 9800 G 2300 4.9 G 2.3 19 G 8 0.14 G 0.07 0.10 G 0.05 280 G 120 0.03 G 0.02 86 G 20 0.043 G 0.027 0.17 G 0.07 0.0013 G 0.0008 0.0009 G 0.0004 2.45 G 1.07 0.0003 G 0.0002
Cs
Sr
238
Pu
239 C 240
Pu
a
C, Bq kg1 S, Bq m2 TI, MBq
e
72 G 17 21 000 G 5000 184 G 44
Upper 20-cm layer of soil.
1.45 G 0.5 30 G 15 0.26 G 0.13
18 G 5 0.9 G 0.3 0.55 G 0.25 48 G 21 0.5 G 0.3 0.17 G 0.10 0.42 G 0.18 0.0047 G 0.0024 0.0015 G 0.0009
280 G 70 600 G 230 5.3 G 2.0
Total
Total combustible
0.11 G 0.03 1.1 G 0.2 0.12 G 0.06 0.0011 G 0.0005 16.7 G 3.3 1.8 G 0.7 16.6 G 3.5 1.8 G 0.9 0.016 G 0.008
1.9 G 0.6 0.2G0.1 0.0018 G 0.0011
14.8 G 4.5 5.2 G 1.9
89 G 21
2.7 G 1.2
190 G 46
6.0 G 2.3
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Table 5 Characteristics of biomass and radionuclides in various compartments of the forest plot: biomass density, mass activity density C, contamination density S and total inventory TI
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plot #1
AV
10
1
0.1 -50
0
50
100
distance from the plot, m
plot #2 10
AV
1
0.1
0.01 -50
0
50
100
150
200
distance from the plot, m
plot #3 10
AV
1
0.1
0.01 0
50
100
150
200
300
250
350
400
distance from the plot, m –
137
Cs
–
90
Sr
–
238
Pu
–
239+240
Pu
3
Fig. 3. Airborne activity concentrations, AV, during the fire experiments in Bq m for 137Cs and 90Sr, and in mBq m3 for plutonium. Vertical bars represent the measurement STD. The distance is calculated from the closest plot border, negative values indicate sample locations inside the plot; the background values of the airborne concentration are subtracted.
decrease in concentration over long distances a rather constant airborne concentration was observed for all nuclides. At the furthest sampling site the concentration increased again. The deposition patterns after the grassland fires are presented in Fig. 4. In general, the fallout density of the radionuclides during the grassland fires decreases with the distance from the source of release. For 137Cs and 90Sr, the fallout density can reach some hundreds Bq m2
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plot #1 1000
AS
100
10
1 -50
0
100
50
150
200
150
200
distance from the plot, m
plot #2 1000
AS
100 10 1 0.1 -50
0
100
50
distance from the plot, m –
137
Cs
–
90
Sr
–
238
Pu
–
239+240
Pu
Fig. 4. The deposition densities of radionuclides during the grassland fires, AS, in Bq m2 for 137Cs and 90Sr, and in mBq m2 for plutonium as a function of the distance from the plot border. Vertical bars represent the measurement STD.
near the burning plot, while for plutonium it is 3e4 orders of magnitude lower. During the forest fire, the fallout density of 137Cs and 90Sr reaches some tens Bq m2, and for plutonium these values are 2 orders of magnitude lower (Fig. 5). The plutonium fallout decreases with the distance from the source of release, while the higher values of the 137Cs and 90Sr fallout density were measured along the dominating wind direction during the experiment. It was observed that both, the airborne concentration and the fallout density have local maxima at relatively large distances from the source of release, especially for the plutonium radionuclides. Under the assumption that the radionuclides are released during the fires in different size fractions of the polydisperse aerosol, this observation may be explained as follows. The heavier fractions of the radioactive aerosol are released at lower height and have higher deposition velocities; therefore, they are mainly deposited closer to the plot. The measured airborne concentration and fallout density decrease with the distance until the area of deposition of the finer fractions of radioactive aerosols, which lifted up to a higher altitude and deposited with a smaller deposition velocity. The polydispersal nature of the released radioactive aerosol is confirmed by the results of the measurements of the cascades of the deployed impactors. During the grassland fires, the contribution of the giant particles (with AMAD O 25 mm) to the total airborne concentration of the radioactive aerosol is comparably high near the plot, and then it decreases with distance (Figs. 6
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155
plot #3, 137Cs and 90Sr
AS
100
10
1 0
100
200
300
400
500
600
500
600
distance from the plot, m
plot #3, Pu 1000
AS
100
10
1 0
100
200
300
400
distance from the plot, m –
137
Cs
–
90
Sr
–
238
Pu
–
239+240
Pu
Fig. 5. The deposition densities of radionuclides (averaged for each row of plates) during the forest fire, AS, in Bq m2 for 137Cs and 90Sr, and in mBq m2 for plutonium as a function of the distance from the plot border. Vertical bars represent the variation of values (STD) in each row.
and 7). The visual analysis of the sediments on the cascades collecting large particles showed that they were mainly composite ash, while the sediments on the cascades collecting finer particles were mainly composites of water drops, tars, small ash particles, etc. One should note that the dispersal composition of the various radionuclides was different and reflects the origin of each radionuclide. For instance, the percentage of 90Sr in the giant particles was the highest in both grassland experiments. Among the other studied radionuclides, 90Sr has the highest soil-to-plant transfer factor and, therefore, the biomass contamination with this radionuclide is high. In the grassland plots, the activity ratio grass/litter is the highest for 90Sr (Table 4). Taking into account that grass burning produces a huge amount of large ash particles, the 90Sr contamination of these large particles is reasonable. For plutonium and 137Cs, the contribution of litter in the total resuspended activity is higher (because of the lower contamination of grass). These nuclides are found in a higher proportion in the fine particle range. Therefore, mainly litter burning is indicated as being responsible for the production of the fine aerosol particles. The consequences of the differences in the contamination of the various biomass compartments (Table 5) can be seen in the forest fire data as well. In the dispersal composition of the forest fire aerosol (Fig. 8), a change can be observed with the distance from the plot. Close to the plot, the
V.I. Yoschenko et al. / J. Environ. Radioactivity 86 (2006) 143e163
156
activity fraction
a
1 0.8
137
Cs Sr 238 Pu 90
0.6
239+240
Pu
0.4 0.2 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
AMAD, µm
b activity fraction
1.2
0.8
0.4
0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
AMAD, µm Fig. 6. Dispersal composition of the radioactive aerosol during the grassland fire (plot #1): (a) at the border of the plot (0 m) and (b) 24 m far from the plot along the plume axis. Vertical bars represent the STD.
most nuclides were measured in the large particle range; in the far distance 90Sr is detected in the fine and giant particle ranges and the Pu nuclides still dominated in the large particle range. 3.3. Resuspension and deposition parameters The resuspension and deposition parameters were calculated according to Eqs. (3) and (5) from the measured airborne radionuclide concentration and deposition in the fire experiments. During the grassland fires, the resuspension factor R of 137Cs and 90Sr was determined to range from 106 to 105 m1, and R of the plutonium nuclides to range from 107 to 106 m1 (Table 6), if R is related to the burning biomass. If the contamination density AS in Eq. (3) includes the soil contamination as well, the calculated resuspension factors will be of 2 orders of magnitude lower. However, the experimental plots were covered with vegetation, and resuspension directly from the soil should be negligible. Therefore, the calculation of R relative to the biomass contamination seems to be reasonable. Furthermore, the grassland fires were carried out in very favourable conditions with intensive resuspension. Thus, the obtained values can be considered as an upper estimate for the Chernobyl zone conditions. During the forest fire, the resuspension factors of all radionuclides were in the range 107e 6 10 m1 if they were related to the biomass contamination, and 108e107 m1 if they were
V.I. Yoschenko et al. / J. Environ. Radioactivity 86 (2006) 143e163
activity fraction
a
157
0.7 0.6 0.5 0.4
137
Cs Sr 238 Pu 90
239+240
Pu
0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
13-25
>25
13-25
>25
AMAD, µm
activity fraction
b
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
AMAD, µm
activity fraction
c
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
AMAD, µm Fig. 7. Dispersal composition of the radioactive aerosol during the grassland fire (plot #2): (a) at the border of the plot (0 m) and (b) 50 m far from the plot along the plume axis; (c) 100 m far from the plot along the plume axis. Vertical bars represent the STD.
related to the total contamination (Table 6). In general, they were lower than those during the grassland fires, probably because of the absence of understorey in the forest. The forest litter was mainly formed by the fallen needles, while the meadow litter consisted of dry rests of grass. The highest resuspension factors were obtained only if the litter is taken into account (which contributed most to the fire burden, see Section 2.6). Especially for 90Sr, the difference is significant. The deposition rate and deposition velocity during the fire experiments are given in Table 7. In general, higher values of the deposition velocity were observed for 137Cs and 90Sr (up to 60 cm s1), while for plutonium mainly several cm s1 were obtained. In the fire plume different radionuclides were associated with aerosol particles of different size range. Because of the size dependence of the deposition process, as a consequence, the deposition rate and velocity
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158
a
0.8
activity fraction
0.7 0.6
137
Cs
90
Sr
239+240
0.5
Pu
238
Pu
0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
13-25
>25
AMAD, µm
activity fraction
b
0.6 0.5 0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
5.5-13
1.8-5.5
AMAD, µm
activity fraction
c
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 <1.1
1.1-2
2-3.3
3.3-7
>7
AMAD, µm Fig. 8. Dispersal composition of the radioactive aerosol during the forest fire (plot #3): (a) 37 m far from the plot along the plume axis; (b) 105 m far from the plot along the plume axis and (c) 492 m far from the plot along the plume axis. Vertical bars represent the STD.
may differ for various radionuclides. For instance, in Section 3.2 it was concluded that burning of litter (the biomass compartment with the highest plutonium contamination, Tables 4 and 5) is mainly responsible for the fine particle fire emission. In this size fraction high plutonium concentrations (especially in the grassland fires, Figs. 6 and 8) were measured. In contrast, higher airborne 137Cs/90Sr-activity concentrations were detected in the large particle range which could be responsible for the difference between the deposition velocities of plutonium and 137 Cs/90Sr. Both rate and velocity of aerosol deposition were lower during the forest fire in comparison to the grassland fires. Probably, this result is due to the lack of the highly contaminated giant particles (or fragments of burning material) in the smoke plume of the forest fire.
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159
Table 6 Resuspension factor R during the experiments for the considered radionuclides R related to
R, m1 137
90
238
239C240
Grassland fire, plot #1 (a) Combustible only (b) Total
(1.7 G 0.2) ! 105 (1.7 G 0.2) ! 107
(1.5 G 0.2) ! 105 (3.7 G 0.5) ! 107
(3.5 G 1.0) ! 107 (4.9 G 1.4) ! 109
(2.4 G 0.6) ! 107 (3.8 G 0.9) ! 109
Grassland fire, plot #2 (a) Combustible only (b) Total
(8.0 G 4.8) ! 106 (1.9 G 1.1) ! 107
(4.4 G 2.6) ! 106 (1.8 G 1.5) ! 107
(2.9 G 2.3) ! 106 (1.3 G 1.0) ! 108
(2.6 G 2.1) ! 106 (1.3 G 1.0) ! 108
Forest fire, plot #3 (a) Combustible only (b) Total (c) Litter only
(4.7 G 2.0) ! 107 (4.7 G 2.0) ! 108 (7.0 G 2.8) ! 107
(3.5 G 1.6) ! 107 (1.1 G 0.5) ! 107 (1.2 G 0.5) ! 106
(1.1 G 0.7) ! 106 (3.2 G 2.2) ! 108 (1.2 G 0.8) ! 106
(8.3 G 4.8) ! 107 (2.5 G 1.6) ! 108 (9.4 G 5.2) ! 107
Cs
Sr
Pu
Pu
R is given in relation to (a) the combustible inventory only and (b) the total inventory of the plot. For the forest fire, the litter inventory is given as reference as well (c).
In a previous study (Kashparov et al., 2000), resuspension factors of 108e107 m1 were determined for 137Cs during forest fires in the contaminated territories outside the 30-km Chernobyl zone. These values were calculated for the total contamination of the sites and are in good agreement with those obtained in the present study. Thus, for the typical pine forests of the Ukrainian Polessie the above values are confirmed as 137Cs resuspension factors during forest fires. Agricultural activities such as harrowing, fertilizing and ploughing may lead to enhanced resuspension in contaminated areas. Resuspension factors of the order of 107 m1 were determined for 137Cs resuspension during farming on dry soil in the Chernobyl area (Wagenpfeil et al., 1999). The comparison of resuspension by wildland fires and soil management indicates that only a relatively small fraction of the total areal contamination is available for re-entry into the atmosphere caused by the fires. 3.4. Radioecological impact of the fires Due to the transport and deposition of the radionuclides in the fire plumes, surfaces outside the fire plots will be contaminated. However, the neighbouring areas of the fire plots are initially contaminated to the same extend as the experimental areas. Therefore, the additional burden has to be related to the existing inventory. If the fallout density from the radioactive plume after the fires (Fig. 4) is compared to the existing contamination density before the fire (Tables 4 and 5), the additional contamination is low, even in the near vicinity of the burning plots; e.g., for plot #1, the additional contamination density of the grass after the fire would be about 1% 90Sr, 1% 137Cs, 7% 238Pu and 7% 239 C 240Pu of the contamination density before the fire if all newly deposited material is fixed on the grass. Thus, the wildland fires in the Chernobyl exclusion zone do not lead to a significant redistribution of contamination on the scale of the whole zone. However, if vegetation was grown next to plot #1 on uncontaminated soil with the same biomass density of 0.4 kg m2 (Table 1), the new vegetation would be contaminated with about 1 kBq kg1 90Sr, 500 Bq kg1 137Cs, 0.1 Bq kg1 238Pu and 0.2 Bq kg1 239 C 240 Pu after the fire according to the deposition density of Fig. 4. So, for decontaminated areas inside the zone, the secondary contamination by fire events might be a problem. Perhaps even more relevant from a radioecological point of view is the finding that the radionuclides in
160
X, m
137
90
Cs 2 1
I, Bq m Grassland 0 24 49 79
238
Sr
s
fire, plot #1 0.12 G 0.01 0.06 G 0.005 0.04 G 0.003 0.03 G 0.003
Grassland fire, plot #2 0 0.34 G 0.01 50 0.03 G 0.003 100 0.009 G 0.002 150 0.016 G 0.003 Forest fire, plot #3 57 (2.0 G 0.3) ! 103 102 (1.5 G 0.2) ! 103 210 (4.1 G 0.4) ! 103 294 (1.7 G 0.2) ! 103 365 (1.3 G 0.2) ! 103 483 (1.8 G 0.3) ! 103
1
vg, cm s
I, Bq m
239 C 240
Pu
2 1
s
1
Pu
2 1
vg, cm s
I, Bq m
s
1
vg, cm s
2 1
I, Bq m
s
vg, cm s1
2.6 G 0.2 4.0 G 0.5 5.3 G 0.5 3.1 G 0.3
0.2 G 0.01 0.15 G 0.01 0.11 G 0.01 0.06 G 0.006
16 G 1 22 G 2 57 G 6 21 G 4
(1.4 G 0.3) ! 105 (1.2 G 0.2) ! 105 (9.1 G 1.5) ! 106 (4.2 G 1.0) ! 106
6.3 G 1.4 5.3 G 0.9 2.2 G 0.4 0.1 G 0.03
(3.0 G .0.5) ! 105 (2.5 G 0.3) ! 105 (2.0 G 0.2) ! 105 (1.0 G 0.1) ! 105
9.5 G 1.2 6.6 G 0.8 1.1 G 0.1 0.1 G 0.01
35 G 3 34 G 7 13 G 3 22 G 6
0.3 G 0.01 0.02 G 0.002 0.01 G 0.001 0.006 G 0.001
60 G 6 15 G 2 9G1 12 G 3
(3.1 G 0.2) ! 105 (1.7 G 0.2) ! 105 (1.5 G 0.2) ! 105 (5.4 G 1.8) ! 106
12 G 2 28 G 7 17 G 6 7.0 G 3.5
(5.8 G .0.3) ! 105 (2.6 G 0.2) ! 105 (3.8 G 0.3) ! 105 (1.3 G 0.2) ! 105
11 G 1 21 G 4 26 G 7 8G3
1.3 G 0.3 1.0 G 0.4 1.9 G 0.8 2.7 G 2.3 0.2 G 0.07
(3.1 G 0.6) ! 106 (8.6 G 1.3) ! 106 (1.4 G 0.5) ! 105 (1.1 G 0.4) ! 106 (2.6 G 1.1) ! 106 (1.5 G 0.8) ! 106
2.7 G 1.4 4.2 G 1.7 7.8 G 5.3 4.7 G 4.5 0.35 G 0.19
(9.1 G 2.4) ! 106 (1.6 G 0.2) ! 105 (3.2 G 1.0) ! 105 (2.6 G 0.6) ! 106 (7.7 G 2.2) ! 106 (1.8 G 0.6) ! 106
1.4 G 0.6 5.2 G 1.8 4.1 G 1.5 3.1 G 2.1 0.29 G 0.14
2.6 G 0.4 1.2 G 0.2 2.8 G 0.5 1.9 G 0.4 0.6 G 0.1
(1.0 G 0.2) ! 103 (4.6 G 1.8) ! 104 (2.8 G 0.4) ! 103 (5 G 2) ! 104 (6 G 2) ! 104 (5 G 2) ! 104
X is the distance from the plot along the plume axis.
V.I. Yoschenko et al. / J. Environ. Radioactivity 86 (2006) 143e163
Table 7 Deposition rate, I, and deposition velocity, vg, during the experiments for the considered radionuclides
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161
the remaining ash are more concentrated and better soluble (Amiro et al., 1996). This means that in the water runoff (e.g. after fire fighting) the radionuclides can be more readily redistributed in the landscape. 3.5. Dose assessment The firemen in the exclusion zone are exposed to an additional irradiation due to the participation in fire suppression actions. In this section only this additional dose to the firemen is considered and referred to as the dose. The total dose is caused by external and internal irradiation. The radioactive contamination of soil and vegetation and the radionuclides in the passing fire plume has to be considered for the external exposure. The uptake by inhalation of radionuclides from the plume into the organism determines the dose of internal irradiation during the fire fighting. In the following, the total dose of a fireman working for 1 h at the burning plots is estimated. The dose from the radionuclides in the biomass and the soil was determined from the measured dose rate in air. The maximum exposure for a 1-h stay at the plots was 16 mSv in plot #1, 10 mSv in plot #2 and 4.2 mSv in plot #3 (Fig. 1). The external dose from the plume was calculated from the airborne radionuclide concentration and the corresponding dose coefficients (Gusev and Beliayev, 1991). The plutonium isotopes were neglected because of the short range of a-particles even in air. In Table 8 the calculated external dose from the radioactive plume is given for each plot; this part appears to be negligible in comparison to the external dose from soil and biomass. The inhalation dose is calculated using the dosimetric model of the human respiratory tract presented in Publication 66 of the ICRP (1994). According to the regulatory documents (Radiation Safety Norms of Ukraine, 1997), the dose absorbed in the whole body (effective equivalent dose, EED) during 50 years after the radionuclide intake is determined. The following assumptions and parameters of calculations are used: the subject of inhalation is an adult man at heavy work; the composition of the radioactive aerosol is polydisperse, fitting the measured distributions; the density of the aerosol is 1 g cm3; the solubility class of the aerosol particles is F (soluble); Table 8 Additional doses by irradiation of the firemen due to a 1-h stay in the fire zone Radionuclide Maximal airborne concentration, Bq m3 Plot #1
Plot #2
137
5
1
0.27
90
3
0.5
0.33
Cs
Sr
238
Pu
3
4
3.4 ! 10 2.5 ! 10 Pu 6.7 ! 103 5.1 ! 104 External (soil and vegetation) Total dose 239 C 240
Dose type Dose, Sv
Plot #3
Plot #1
4
4.6 ! 10 1.1 ! 103
External (cloud) Inhalation External (cloud) Inhalation Inhalation Inhalation
6.9 ! 10
Plot #2 10
1.4 ! 10
Plot #3 10
3.7 ! 1011
6 ! 108 1.2 ! 108 3.2 ! 109 1010 1.7 ! 1011 1.1 ! 1011 2.4 ! 107 7.1 ! 106 1.7 ! 105 1.6 ! 105 4.0 ! 105
4.1 ! 108 2.6 ! 108 5.3 ! 107 106 1.3 ! 106 2.8 ! 106 1 ! 105 4.2 ! 106 1.2 ! 105 8 ! 106
162
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the biokinetic models are those recommended by Publication 30 of the ICRP (1982); all progenies are taken into account according to the decay chains given in Publication 38 of the ICRP (1983). The results of the dose calculation due to a 1-h stay of the firemen in the experimental sites during the fire experiments are presented in Table 8. It can be seen that beside the dose of the external irradiation from soil and vegetation, a significant contribution to the total dose can be provided by the inhalation intake of the plutonium radioisotopes and their progenies. In some conditions this contribution can reach half of the total dose. Although this estimate is conservative and the total period of a fireman working in the fire area during a year is usually much smaller than the total period of his residence in the exclusion zone, the problem of protection of the firemen should be considered in more detail. At the same time, taking into account the sharp decrease of the airborne radionuclide concentration with the distance from the source of release (Fig. 3), it can stated that the inhalation component of the total dose (as well as the external irradiation from radionuclides in air) is not important for the personnel of the exclusion zone which is not involved in the fire fighting. One must note that the above statements are formulated for small fires, which result in a comparable low total release of radionuclides into the atmosphere. If the fire area is very large, the affected territory will also increase. In this case, the possibility of additional irradiation of the people in the Chernobyl area can be evaluated using the above estimates of relative release of radionuclides during the grassland or forest fires. 4. Conclusions The experiments performed in the Chernobyl exclusion zone made it possible to characterize the resuspension, transport and deposition of radionuclides during grassland and forest fires in well controlled conditions. The obtained values of the resuspension factor and the relative release of radionuclides are proposed for the estimation of the radioecological consequences of the wildland fires in this territory. In general, the fires impact a relatively small area downwind of the burning zone. Within this area, the airborne concentration of radionuclides raises several orders of magnitude as compared to its background level, which can result in a significant increase of the inhalation proportion of the total dose. The Pu radionuclides constitute the main contribution to the inhalation dose. The contribution of the wildland fires to the redistribution of radioactivity in/outside the Chernobyl zone can be considered as negligible. Taking into account that the experiments were done at previously characterized sites and were accomplished with the relevant meteorological records, the experimental data can be applied for the validation of transport models for fires (Yoschenko et al., submitted for publication). Modeling will give a more precise estimate of the relative release of radionuclides, which can be derived from the value of the source intensity for each radionuclide. Acknowledgements The presented investigations were carried out within the framework of the project #1992 funded by the Science and Technology Center in Ukraine (STCU). The authors kindly appreciate the support they met from the STCU, as well as the assistance of the administration of the exclusion zone in the organization of the experimental works.
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References Amiro, B.D., Sheppard, S.C., Johnston, F.L., Evenden, W.G., Harris, D.R., 1996. Burning radionuclide question: what happens to iodine, cesium and chlorine in biomass fires? Science of the Total Environment 187, 93e103. Andreae, M.O., Merlet, P., 2001. Emission of trace gases and aerosols from biomass burning. Global Biogeochemical Cycles 15, 955e966. Frank, G., Kashparov, V., Protsak, V., Tschiersch, J., 1996. Comparison measurements of a Russian standard aerosol impactor with several Western standard aerosol instruments. Journal of Aerosol Science 27, 477e486. Garger, E.K., Kashpur, V., Belov, G., Demchuk, V., Tschiersch, J., Wagenpfeil, F., Paretzke, H.G., Besnus, F., Holla¨nder, W., Martinez-Serrano, J., Vintersved, I., 1997. Measurement of resuspended aerosol in the Chernobyl area. Part I: discussion of instrumentation and uncertainty of measurement. Radiation and Environmental Biophysics 36, 139e148. Garger, E.K., Kashpur, V., Paretzke, H.G., Tschiersch, J., 1998. Measurement of resuspended aerosol in the Chernobyl area. Part II: size distribution of radioactive particles. Radiation and Environmental Biophysics 36, 275e283. Goldammer, J.G., Crutzen, P.J., 1993. Fire in the environment: scientific rationale and summary of results of the Dahlem workshop. In: Crutzen, P.J., Goldammer, J.G. (Eds.), Fire in the Environment. John Wiley & Sons, Chichester, pp. 1e14. Gusev, N.G., Beliayev, V.A., 1991. Radioactive Releases into the Biospxhere: A Manual. Book Company ‘‘Energoatomizdat’’, Moscow, 256 p. (in Russian). Hrgian, A.H., 1957. Atlas of Clouds. Book company ‘‘Hydrometeoizdat’’, Leningrad, 176 p. (in Russian). ICRP, 1982. Publication 30. Limits for intakes of radionuclides by workers. Annals of the ICRP 8. Pergamon Press, Oxford. ICRP, 1983. Publication 38. Radionuclide transformations: energy and intensity of emissions. Annals of the ICRP 11e 13. Pergamon Press, Oxford. ICRP, 1994. Publication 66. Human respiratory tract model for radiological protection. Annals of the ICRP 24. Pergamon Press, Oxford. Kashparov, V.A., Lundin, S.M., Kadygrib, A.M., Protsak, V.P., Levtchuk, S.E., Yoschenko, V.I., Kashpur, V.A., Talerko, N.N., 2000. Forest fires in the territory contaminated as a result of the Chernobyl accident: radioactive aerosol resuspension and exposure of firefighters. Journal of Environmental Radioactivity 51, 281e298. Kashparov, V.A., Lundin, S.M., Khomtinin, Y.V., Kaminsky, S.P., Levchuk, S.E., Protsak, V.P., Kadygrib, A.M., Zvarich, S.I., Yoschenko, V.I., Tschiersch, J., 2001. Soil contamination with 90Sr in the near zone of Chernobyl. Journal of Environmental Radioactivity 56, 285e298. Krzes´niak, J.W., Porstendo¨rfer, J., 1981. Experimental determination of the collection efficiency of the Petrianov type FPP 15-1.7 filter for sampling submicron atmospheric aerosols. Nukleonika 26, 669e679. McMahon, C.K., Bush, P.B., 1992. Forest worker exposure to airborne herbicide residues in smoke from prescribed fires in the southern United States. American Industrial Hygiene Association Journal 53, 265e272. Nicholson, K.W., 1988. A review of particle resuspension. Atmospheric Environment 22, 2639e2651. Radiation Safety Norms of Ukraine, 1997. Publication RSNU-97. Kiev (in Ukrainian). Rosner, G., Winkler, R., 2001. Nuclide-dependent local and collector surface effects in sampling of radioactive deposition to ground. Applied Radiation and Isotopes 55, 823e829. Wagenpfeil, F., Paretzke, H.G., Peres, J.M., Tschiersch, J., 1999. Resuspension of coarse particles in the region of Chernobyl. Atmospheric Environment 33, 3313e3323. Yamasoe, M., Artaxo, P., Miguel, A.H., Allen, A.G., 2000. Chemical composition of aerosol particles from direct emissions of vegetation fires in the Amazon Basin: water-soluble species and trace elements. Atmospheric Environment 34, 1641e1653. Yoschenko, V.I., Kashparov, V.A., Levchuk, S.E., Glukhovskiy, A.S., Khomutinin, Yu.V., Protsak, V.P., Lundin, S.M. Tschiersch, J. Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone. Part II. Modeling the transport process. Journal of Environmental Radioactivity, submitted for publication.
Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone: part I. Fire experiments V.I. Yoschenko a, V.A. Kashparov a,*, V.P. Protsak a, S.M. Lundin a, S.E. Levchuk a, A.M. Kadygrib a, S.I. Zvarich a, Yu.V. Khomutinin a, I.M. Maloshtan a, V.P. Lanshin a, M.V. Kovtun a, J. Tschiersch b a
Ukrainian Institute of Agricultural Radiology (UIAR), Mashinobudivnykiv str.7, Chabany, Kyiv region 08162, Ukraine b GSF-National Research Center for Environment and Health, Institute of Radiation Protection, D-85764 Neuherberg, Germany Received 11 February 2005; received in revised form 8 August 2005; accepted 11 August 2005 Available online 5 October 2005
Abstract Controlled burning of experimental plots of forest or grassland in the Chernobyl exclusion zone has been carried out in order to estimate the parameters of radionuclide resuspension, transport and deposition during forest and grassland fires and to evaluate the working conditions of firemen. An increase of several orders of magnitude of the airborne radionuclide concentration was observed in the territory near the fire area. The resuspension factor for 137Cs and 90Sr was determined to range from 106 to 105 m1, and for the plutonium radionuclides from 107 to 106 m1 (related to the nuclides in the combustible biomass). These values are 2 orders of magnitude lower if they are calculated relatively to the total contamination density (including the nuclides in the soil). The radionuclide fallout along the plume axis is negligible in comparison to the existing contamination. However, the additional inhalation dose for firemen exposed in
* Corresponding author. Tel./fax: C380 44 266 45 02. E-mail addresses: [email protected] (V.A. Kashparov), [email protected] (J. Tschiersch). 0265-931X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2005.08.003
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the affected area can reach the level of the additional external irradiation in the period of their mission. The plutonium nuclides constitute the dominating contribution to the inhalation dose. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Biomass burning; Radioactive aerosol; Resuspension; Inhalation; Dose assessment
1. Introduction Wildland fires and other biomass burning are a major source of aerosol particles and gases in the atmosphere (Goldammer and Crutzen, 1993). Not only black carbon and organic material derives from pyrogenic emissions but also species such as K, Cl, SO4, NOx and heavy elements such as Cu and Zn (Andreae and Merlet, 2001; Yamasoe et al., 2000). Herbicide residues (McMahon and Bush, 1992) and radioactive iodine, caesium and chlorine were found in the smoke of biomass fires (Amiro et al., 1996). These trace substances may affect the surrounding of the fires and people in this area (e.g. forest workers and firemen). But even at about 17 km distance from a forest fire an increase of the airborne 137Cs concentration was measured (Garger et al., 1998). Therefore, the resuspension and redistribution of pollutants by biomass burning is not only a local problem. Inside the Chernobyl exclusion zone a huge amount of various radionuclides is deposited. Detailed contamination maps are published (Kashparov et al., 2001). Some of these nuclides were deposited during the Chernobyl accident directly to trees and bushes; others were transferred by root uptake from the contaminated soil to the vegetation. The exclusion zone is covered mostly (almost 2/3 of the total area) by forests and grasslands. It is estimated that several percents of the initially deposited radionuclides are contained in the biomass of the grasslands and even more in the biomass of the forests (see the characterization of the experimental plots in Section 3.1). These radionuclides currently fixed in the biomass may potentially be mobilized by wildland fires. The area is no longer cultivated and is mostly abandoned. Frequently wildland fires occur which are detected and combated late. The number of the detected fires varies in the range from 42 to 116 per year. The fires have already destroyed thousands of hectares of forest. The problem is expected to increase in future, because the absence of any measure for maintenance of the forests, meadows and peatlands will lead to an uncontrolled growth of the vegetation. The aim of the investigation presented here is to quantify the resuspension, transport and deposition of the relevant radionuclides during grassland and forest fires in Chernobyl exclusion zone. Firemen are exposed to these airborne radionuclides which may be inhaled. The potential dose due to inhalation and external radiation will be assessed. The experimental procedures are designed to accumulate an array of experimental data necessary for the validation of the models of radioactivity transport in the atmosphere during wildland fires. Previous studies on fire resuspension (Kashparov et al., 2000) had been performed outside the Chernobyl exclusion zone giving data only on 137Cs. Now, the focus of the investigation is on the potential impact of the radionuclides 90Sr, 238Pu and 239 C 240Pu.
2. Materials and methods In order to study the resuspension and transport of radionuclides in the atmosphere during wildland fires, the method of active fire experiments was applied, which implies the controlled burning of the
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selected experimental site under well-monitored conditions. Prior to the experiment, a complete radioecological description of the site was carried out, the background measurements were done and the sampling equipment items were installed according to the elaborated scheme of the experiment. 2.1. Characterization of the experimental sites The active experiments were performed on two grassland plots and one forest plot located in the Chernobyl zone. The sites were selected to satisfy the following criteria: highly and homogeneously contaminated area within the fuel trace of the radioactive fallout (contamination with 137Cs, 90Sr, 238Pu and 239 C 240Pu); uniform wide flat area allowing an undisturbed plume spreading; plot length of about 50e100 m (assuming 1 h stable meteorological conditions and a fire front speed of several m min1). Both grassland fires were carried out within the same experimental area near the former village Chistogalovka, at a distance of approximately 3 km west of the Chernobyl reactor site. The forest site is located 5 km WNW (280 ) of the Chernobyl reactor close to the former village Novoshepelichi. It is a cultivated pine forest surrounded by grassland. The age of the trees in the forest was 30 years. The trees were planted in rows with an inter-row distance of approximately 1.5 m. The distance between the trees in the rows varied from 0.5 to 3 m. The average quantity of trees per unit of the forest area was 0.275 m2. Due to the relatively high density of planting, the pine trees were rather thin (about 15 cm diameter at the height of 1.5 m) and the tree crowns were not well-developed. There was practically no understorey. A first evaluation of the spatial distribution of radioactivity within the plots was done by the measurement of the exposure dose rate (EDR) in the nodes of a 5-m regular mesh (grassland) and a 10-m mesh (forest) with the dosimeter DRG-01T (Tair SKS, Russia). The EDR values were measured at the height of 1 m above the ground surface, which means that each measurement result was related to the radionuclides located within a comparable large area (with a radius of about 10 m). The EDR values measured at the different plots are presented in Fig. 1, showing rather uniform g-fields. Samples of soil, natural litter and vegetation were collected before the experiments within the plots to be burnt. For the determination of the contamination density, 10 soil samples were collected from each plot using a cylindrical core driller of 3.7 cm diameter to the depth of 30 cm (grassland) and 20 cm (forest). Each sample consisted of 5 portions of soil that were sampled in the corners and in the center of a square 5 ! 5 m2. For the determination of the radionuclides in the soil profile, so-called layer sampling was carried out with a cylindrical core driller of 5 cm diameter which consists of two semi-cylinders. After the core has been drawn, one semi-cylinder is removed and the whole soil sample is accessible for further dividing into sub-samples of certain depths. In this case, each sample of a certain soil layer consisted of 3 portions sampled at various points in the corresponding depth. On each plot, 10 samples of natural litter were collected, each from the area of 20 ! 20 cm2. At the same points, grass samples were collected from areas of 40 ! 40 cm2 in plot #1 and 50 ! 50 cm2 in plot #2. In the forest, in addition, 10 representative samples of fallen tree branches were collected. Several representative trees were cut and divided into elements (timber, bark and needles) to determine the mass (and the radionuclide concentration). The biomass was determined in air-dry state. The characteristics of the three plots are summarized in Table 1. The activity analysis was performed for the following radionuclides: 137Cs, 90Sr, 238Pu and 239 C 240 Pu. 2.2. Activity measurements The activity concentration of 137Cs in all samples was measured with the low-background g-spectrometric complex ADCAM-300 (USA), which is equipped with the Ge-detector GEM-30185 (EG & G ORTEC, USA). The energy resolution of the detector is 1.78 keV for the 60Co-line at 1.33 MeV.
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a
meters 0
0
10
20
30
40
0
0
10
20
30
40
50
60
70
80
10
10 20 14.5
30
meters
16.0
meters
meters
b
50
10
20
9
30
13.0
8
40
40 11.5
7
50
50
10.0
6
c
4.0
3.6
3.2
2.8
0
20
40
2.4
meters
Fig. 1. Spatial distribution of the exposure dose rate (mGy h1), within the grassland plot #1 (a), grassland plot #2 (b) and forest plot (c).
The 90Sr activity concentration in the samples was measured using a standard radiochemical method, which is based on the extraction of the radiochemical pure 90Sr from the samples prepared by the oxalate method. The 90Sr activity was determined by radiometry of its progeny, 90Y, using the aeb-radiometer CANBERRA-2400 (USA). Atomic-absorption spectrometry was applied for the determination of the chemical yield of strontium. Pu isotopes were extracted from the samples and concentrated on metal plates by means of electrolytic deposition. Their activities were measured using the a-spectrometer SOLOIST (USA).
2.3. Determination of aerosol parameters During the experiments, aerosols in the lower troposphere (at 1 m height above the ground surface) were sampled in order to determine the airborne radionuclide concentrations at various distances from the source of release. For this purpose, the isokinetic aerosol samplers IPA (Antonov Air Plant, Ukraine) were utilized (Garger et al., 1997). In these samplers, the aerosol particles are sampled at the current wind
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Table 1 Characteristics of the plots used for the active fire experiments Area (m2) Land type Vegetation species Vegetation height (m) Biomass density (kg m2) Litter density (kg m2) Dose rate range (mGy h1)
Plot #1
Plot #2
Plot #3
3600 Wildland (grass) Elytrigia repens (L.) Nevski (85%) 0.5 0.4 0.7 10e16
5400 Wildland (grass) Elytrigia repens (L.) Nevski (85%) 0.2 0.3 1.25 6e10
8770 Forest Pinus silvestris 15 24 2.3 2e4
velocity on Petryanov filter tissue (Krzes´niak and Porstendo¨rfer, 1981). Assuming constant wind velocities during the experiment, the average airborne activity concentration AV of the considered radionuclide is calculated as follows: ÿ AV ZAf =V Bq m3
ð1Þ
where Af is the activity measured in the filter (Bq) and V is the air volume passed through the filter (m3). The deposition density of the radionuclides fallout from the radioactive fire plume was quantified with horizontal plates covered with Petryanov tissue and installed at 1 m height. The deposition density AS is calculated as follows: ÿ AS ZAp =SP Bq m2
ð2Þ
where Ap is the activity in the tissue on the plate (Bq) and SP is the surface area of the plate (m2). Horizontal tissue covered plates as deposition collectors provide standardized sampling for the determination of the deposition pattern. The Petryanov tissue entirely retains the deposited material and represents a sticky surface with a slightly higher roughness than a water surface, which is suggested for monitoring of radioactive fallout (Rosner and Winkler, 2001). The deposition velocity to this reference material is supposed to be in the same order of magnitude than to the grass surface but lower than to the adjacent trees and bushes. It is assumed that the radionuclides during the fire experiments were resuspended and transported mainly in particulate form. Such an assumption is based on the fact that the burning temperature is not high enough for an atomization of the radionuclides of Sr and Pu, and they can be transported with particles of ash, tar and water only. For Cs with the lowest boiling point, evaporation might be possible in the core of forest fires, but due to nucleation and condensation processes Cs-particles were formed rapidly. Radionuclide resuspension, transport and deposition depend on the aerosol particles size and density. In the present study, the dispersal composition of the radioactive aerosols was characterized using an impactor designed by the Institute of Biophysics of the Health Ministry, Russia (IBP impactor) and described by Frank et al. (1996). In this instrument aerosol particles are deposited on one of five cascades according to their aerodynamic diameter. The analysis of the radionuclides deposited on the cascades provides the activity size distribution. The sampling rate, however, is only 1.05 m3 h1. The high-volume Andersen sampler (model AH-600Z, USA) was utilized for the determination of the dispersal composition of radioactive aerosol at larger distances from the source of release. In this sampler, all particles are deposited on four cascades at an air sampling rate of 34 m3 h1.
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Other important parameters can be derived from the measured values of AV and AS. The term resuspension is used to mean the re-entrainment into the atmosphere of previously deposited radioactive material and is characterized by the resuspension factor R (Nicholson, 1988): ÿ RZAV =S m1
ð3Þ
where S is the primary areal activity density (Bq m2). By definition, the resuspension factor quantifies the wind-driven transfer from a homogeneously contaminated surface into the atmosphere and implies equilibrium between the resuspended and the deposited aerosol. These conditions are not achieved in the fire experiments and R should rather be considered as the airborne activity concentration normalized by the areal activity density (Wagenpfeil et al., 1999). R can either be normalized by the total contamination density or by the contamination density of the burning biomass only. R provides a means for inter-comparison between other published fire and surface release experiments. The deposition of the radionuclides can be described in terms of the deposition rate I as ÿ IZvg !AV ðzZ1 mÞ Bq m2 s1 ð4Þ or ÿ IZAS =t Bq m2 s1
ð5Þ
where vg is the deposition velocity (m s1), z is the vertical coordinate and t is the deposition period (s). If I is determined in the experiments using Eq. (5), vg can subsequently be derived from Eq. (4). Because the experiments were performed in the highly contaminated exclusion zone, the background values of AV and I were determined before the fire experiments. The values are given in Table 2. 2.4. Measurement of meteorological parameters During the experiments, the relevant meteorological parameters at the sites were determined. The air temperature T was measured using a mercury thermometer. To record the average wind velocity u during certain periods of time, the cup anemometer MS-13 was installed at the height of 2 m above surface. The anemometer provides the accurate measurement in the range of 1e20 m s1. The wind direction and wind velocity in gusts were determined using Tretyakov’s wind meter. The air pressure P was measured by the mercury cup barometer SRA, and the air humidity by the hygrometer M-19. The cloudiness was estimated visually according to Hrgian (1957). Data on the regional meteorological conditions as well as the data for the estimation of the atmospheric stability class in the local scale were presented by the Kiev Hydrometeorological Station and the Chernobyl meteorological station. These data were obtained by means of the aerial radar AVK Titan-1 and air balloon monitoring. All equipments were manufactured in the former USSR. Table 2 Background values of the airborne activity concentration AV and the deposition rate I of the considered radionuclides before the experiments 137
90
Cs
238
Sr
3
AV, !10 Bq m3
Plot #1 2.1 G 0.3 Plot #2 4.6 G 0.6 Plot #3 3.2 G 0.4
4
239 C 240
Pu
3
5
I, !10 AV,!10 Bq m2 s1 Bq m3
I, !10 AV, !10 Bq m2 s1 Bq m3
0.5 G 0.1 1.0 G 0.1 0.6 G 0.1
2.0 G 0.3 3.5 G 0.4 2.2 G 0.2
0.8 G 0.2 1.4 G 0.4 4.4 G 0.5
Pu
6
1.5 G 0.3 4.3 G 0.7 8.3 G 2.1
8
I, !10 AV, !106 2 1 Bq m s Bq m3
I, !107 Bq m2 s1
1.2 G 0.3 4.4 G 0.7 23 G 4
0.3 G 0.1 1.1 G 0.1 4.5 G 0.7
2.9 G 0.6 8.5 G 1.0 22 G 5
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According to the data of the Chernobyl and Kiev meteorological stations and to our observations, the lower tropospheric layer was neutral e slightly unstable (category C) during the two grassland fires, and it was stable (category E) during the forest fire. The other meteorological parameters are summarized in Table 3. 2.5. Fire characterization The experiments were carried out in favourable meteorological conditions. Dry weather facilitated the intensive fires, and as the wind direction remained stable during the experiments, the well-visible smoke plumes covered the air sampling area in the case of the grassland fires, and the vertical dimension of the plumes reached up to 10 m near the plots. The grassland fires caused the total combustion of the grass and the charring of the litter in the experimental plots. In the first experiment (plot #1), the fire front passed 60 m distance in 15 min, while the total period of burning was 30 min. In the second experiment (plot #2), the fire front velocity was higher e the 90 m distance was passed in 13 min, and the total period of burning was again 30 min. The forest fire can also be classified as an intensive one. It was mainly of the surface type (i.e. forest litter burning) with some periods of crown fire. The smoke plume arose 30e40 m above the burning area and was then transported over the sampling area. As a result of the fire, the forest litter was burnt and the tree bark was charred up to the height of 1.0e1.5 m. Some tree crowns were also burnt or damaged. The fire front has passed 100 m in about 30 min. The total duration of the active phase was about 90 min. 2.6. Scheme of the experiments The same principal scheme was used in all experimental runs. The sampling equipment was installed in several lines normal to the wind direction such that the smoke plume from the burning plot passed over the instruments. In order to cover the whole area of the plume transport, the far lines were approximately twice as long as the plot width. Along each line, the horizontal plates were installed uniformly, while the aerosol samplers and impactors were positioned in the center of the lines, at the plume axis. The inter-line distance in each experiment was approximately the same. The installation of the equipment and the performance of the experiment were done in steady wind direction conditions. After the completion of the preparation works, the plots were ignited along their leeward sides (along the 60-m side in the grassland plot #2). The most horizontal plates were deployed during the forest fire experiment (Fig. 2), 40 plates in total. Fewer horizontal plates were used during the grassland fires (18 in the first experiment and 13 in the second) because of the smaller spread of the plume. Also, the samplers were located closer to the grassland plots, at distances up to 150 m. Table 3 Meteorological parameters during the fire experiments Parameter
Plot #1
Plot #2
Plot #3
Time
10 OCT 2001, 15:00e16:00 5/0 Cie6/0 Ci, Ac 22.2 57 180e185 2 2e5 1004.9 C
18 APR 2002, 12:00e13:30 7/7 Cue8/8 Cu cong 16.2 33 315e350 2 2e7 1004.9 C
15 MAY 2003, 15:40e17:15 4/0 Cie8/0 Ci, Ac, Cu 26.4 50 110e170 3 4e7 996.5 F
Cloudiness Mean air temperature ( C) Air humidity (%) Wind direction at h Z 2 m ( ) Wind velocity, mean (m s1) Gusts (m s1) Atmospheric pressure (hPa) Diffusion category
150
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Horizontal plates IPA aerosol sampler
Forest plot
IBP impactor (3 units at each spot) Andersen aerosol sampler
0
50
100
meters
Fig. 2. Scheme of deployment of the sampling equipment during the forest fire experiment.
3. Results and discussion 3.1. Radionuclide inventories at the experimental plots The measured radionuclide activities in the samples from the experimental grassland plots and deduced characteristics are presented in Table 4. The uncertainty of each value is mainly determined by the heterogeneity of its spatial distribution within the site, in comparison, the analytical error is low. Assuming the full consumption of the combustible matter (grass and litter) in the plots, the maximal values of the radionuclide release into the atmosphere can be estimated as 1000 G 800 MBq of 137Cs, 700 G 500 MBq of 90Sr, 2.6 G 1.5 MBq of 238 Pu and 5.8 G 4.8 MBq of 239 C 240Pu during the fire on plot #1 and 640 G 390 MBq of 137 Cs, 570 G 300 MBq of 90Sr, 470 G 330 kBq of 238Pu and 1.1 G 0.7 MBq of 239 C 240Pu during the fire on pot #2. The radionuclide activities and deduced characteristics from the forest plots are presented Table 5. The caesium and plutonium activities in the soil are much higher than those in the combustible material, while for strontium the situation is different e a significant fraction of its total activity is located in the biomass. In the combustible material, about 70% of 137Cs and 90% of Pu are associated with the forest litter. For 90Sr, this amount reaches only about
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Table 4 Radionuclide characteristics of various compartments of the grassland plots: mass activity density C, contamination density S and total inventory TI
Plot #1 137 Cs
90
Sr
238
Pu
239 C 240
Pu
Plot #2 137 Cs
90
Sr
238
Pu
239 C 240
Pu
a
Soila
Grass
C, kBq kg1 S, MBq m2 TI, GBq
80 G 50 29 G 18 100 G 70
86 G 50 0.026 G 0.015 0.094 G 0.065
340 G 210 0.26 G 0.17 0.94 G 0.7
110 G 70
1.0 G 0.8
C, kBq kg1 S, MBq m2 TI, GBq
21 G 14 7.6 G 4.7 27 G 18
120 G 90 0.041 G 0.031 0.15 G 0.12
210 G 150 0.15 G 0.11 0.54 G 0.4
28 G 19
0.7 G 0.5
C, Bq kg1 S, kBq m2 TI, MBq
140 G 80 50 G 30 180 G 110
1.1 G 0.7 (4 G 3) ! 10-4 (1.4 G 1.1) ! 103
1000 G 500 0.72 G 0.4 2.6 G 1.5
180 G 110
2.6 G 1.5
C, Bq kg1 S, kBq m2 TI, MBq
280 G 160 100 G 60 360 G 200
2.4 G 1.6 (8 G 6) ! 104 (2.8 G 2.1) ! 103
2200 G 1500 1.6 G 1.2 5.8 G 4.8
370 G 210
C, kBq kg1 S, MBq m2 TI, GBq
14 G 9 5.1 G 3.1 27 G 17
49 G 21 0.014 G 0.006 0.07 G 0.03
85 G 5 0.11 G 0.07 0.57 G 0.36
28 G 17
0.64 G 0.39
C, kBq kg1 S, MBq m2 TI, GBq
7.3 G 6.0 2.7 G 2.2 15 G 12
62 G 19 0.018 G 0.005 0.10 G 0.01
81 G 35 0.087 G 0.053 0.47 G 0.29
16 G 12
0.57 G 0.30
C, Bq kg1 S, kBq m2 TI, MBq
57 G 33 20 G 12 110 G 60
0.17 G 0.13 (4.7 G 4.0) ! 105 (2.5 G 2.1) ! 104
130 G 87 0.087 G 0.060 0.47 G 0.33
110 G 60
0.47 G 0.33
C, Bq kg1 S, kBq m2 TI, MBq
110 G 65 40 G 27 210 G 150
0.44 G 0.33 (1.2 G 0.9) ! 104 (6.7 G 5.3) ! 104
270 G 190 0.19 G 0.14 1.1 G 0.7
210 G 150
1.1 G 0.7
Litter
Total
Total combustible
5.8 G 4.8
Upper 30-cm soil layer.
30%, and more than 40% are found in timber. Assuming that the forest litter is the primary combustible material during the forest fires, one may expect the same activities of 137Cs and 90 Sr, and much lower activities of the plutonium radionuclides in the resuspended material. 3.2. Airborne concentration, fallout and dispersal composition during the fires Averaged values of the airborne radionuclide concentrations during the fires at the various distances from the source of release are presented in Fig. 3. The samples were taken at 1 m height at the plumes axes. During the grassland fires, the airborne concentrations of 137Cs and 90Sr reached values of several Bq m3 near the source of release and decreased with the distance. The concentration of plutonium was generally 3 orders of magnitude lower and an increase with the distance was observed in plot #1. In plot #2, rather constant values were measured at greater distances after an initial decrease. This different behaviour of plutonium can be explained by differences in certain experimental parameters such as the initial heights of release or the deposition velocity (see below). During the forest fire experiment, in general, 1 order of magnitude lower activity concentrations for 137Cs and 90Sr were measured in comparison to the grassland fires. After an initial
152
Soila Biomass density, kg m2
Timber
Bark
1st year needles
2nd year needles
Forest litter Fallen branches
20.5 G 3.5
2.6 G 0.4
0.6 G 0.1
0.30 G 0.05
2.15 G 0.28
137
C, kBq kg1 S, kBq m2 TI, GBq
5.8 G 1.1 1700 G 300 14.9 G 2.6
0.83 G 0.20 17 G 7 0.15 G 0.06
5.1 G 1.4 13 G 6 0.12 G 0.05
57 G 15 34 G 15 0.30 G 0.13
6.9 G 1.9 2.1 G 0.9 0.018 G 0.008
64 G 17 137 G 53 1.2 G 0.5
90
C, kBq kg1 S, kBq m2 TI, GBq
3.7 G 0.9 1100 G 300 9.6 G 2.6
12.5 G 2.5 260 G 100 2.25 G 0.83
45.8 G 9.2 120 G 40 1.04 G 0.37
26.4 G 5.2 15.8 G 5.8 0.14 G 0.05
70 G 15 21 G 8 0.19 G 0.07
81 G 18 173 G 62 1.52 G 0.54
C, Bq kg1 S, Bq m2 TI, MBq
33 G 8 0.24 G 0.07 7.4 G 2.1 0.24 G 0.07 0.33 G 0.10 130 G 40 0.3 G 0.1 9800 G 2300 4.9 G 2.3 19 G 8 0.14 G 0.07 0.10 G 0.05 280 G 120 0.03 G 0.02 86 G 20 0.043 G 0.027 0.17 G 0.07 0.0013 G 0.0008 0.0009 G 0.0004 2.45 G 1.07 0.0003 G 0.0002
Cs
Sr
238
Pu
239 C 240
Pu
a
C, Bq kg1 S, Bq m2 TI, MBq
e
72 G 17 21 000 G 5000 184 G 44
Upper 20-cm layer of soil.
1.45 G 0.5 30 G 15 0.26 G 0.13
18 G 5 0.9 G 0.3 0.55 G 0.25 48 G 21 0.5 G 0.3 0.17 G 0.10 0.42 G 0.18 0.0047 G 0.0024 0.0015 G 0.0009
280 G 70 600 G 230 5.3 G 2.0
Total
Total combustible
0.11 G 0.03 1.1 G 0.2 0.12 G 0.06 0.0011 G 0.0005 16.7 G 3.3 1.8 G 0.7 16.6 G 3.5 1.8 G 0.9 0.016 G 0.008
1.9 G 0.6 0.2G0.1 0.0018 G 0.0011
14.8 G 4.5 5.2 G 1.9
89 G 21
2.7 G 1.2
190 G 46
6.0 G 2.3
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Table 5 Characteristics of biomass and radionuclides in various compartments of the forest plot: biomass density, mass activity density C, contamination density S and total inventory TI
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plot #1
AV
10
1
0.1 -50
0
50
100
distance from the plot, m
plot #2 10
AV
1
0.1
0.01 -50
0
50
100
150
200
distance from the plot, m
plot #3 10
AV
1
0.1
0.01 0
50
100
150
200
300
250
350
400
distance from the plot, m –
137
Cs
–
90
Sr
–
238
Pu
–
239+240
Pu
3
Fig. 3. Airborne activity concentrations, AV, during the fire experiments in Bq m for 137Cs and 90Sr, and in mBq m3 for plutonium. Vertical bars represent the measurement STD. The distance is calculated from the closest plot border, negative values indicate sample locations inside the plot; the background values of the airborne concentration are subtracted.
decrease in concentration over long distances a rather constant airborne concentration was observed for all nuclides. At the furthest sampling site the concentration increased again. The deposition patterns after the grassland fires are presented in Fig. 4. In general, the fallout density of the radionuclides during the grassland fires decreases with the distance from the source of release. For 137Cs and 90Sr, the fallout density can reach some hundreds Bq m2
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plot #1 1000
AS
100
10
1 -50
0
100
50
150
200
150
200
distance from the plot, m
plot #2 1000
AS
100 10 1 0.1 -50
0
100
50
distance from the plot, m –
137
Cs
–
90
Sr
–
238
Pu
–
239+240
Pu
Fig. 4. The deposition densities of radionuclides during the grassland fires, AS, in Bq m2 for 137Cs and 90Sr, and in mBq m2 for plutonium as a function of the distance from the plot border. Vertical bars represent the measurement STD.
near the burning plot, while for plutonium it is 3e4 orders of magnitude lower. During the forest fire, the fallout density of 137Cs and 90Sr reaches some tens Bq m2, and for plutonium these values are 2 orders of magnitude lower (Fig. 5). The plutonium fallout decreases with the distance from the source of release, while the higher values of the 137Cs and 90Sr fallout density were measured along the dominating wind direction during the experiment. It was observed that both, the airborne concentration and the fallout density have local maxima at relatively large distances from the source of release, especially for the plutonium radionuclides. Under the assumption that the radionuclides are released during the fires in different size fractions of the polydisperse aerosol, this observation may be explained as follows. The heavier fractions of the radioactive aerosol are released at lower height and have higher deposition velocities; therefore, they are mainly deposited closer to the plot. The measured airborne concentration and fallout density decrease with the distance until the area of deposition of the finer fractions of radioactive aerosols, which lifted up to a higher altitude and deposited with a smaller deposition velocity. The polydispersal nature of the released radioactive aerosol is confirmed by the results of the measurements of the cascades of the deployed impactors. During the grassland fires, the contribution of the giant particles (with AMAD O 25 mm) to the total airborne concentration of the radioactive aerosol is comparably high near the plot, and then it decreases with distance (Figs. 6
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plot #3, 137Cs and 90Sr
AS
100
10
1 0
100
200
300
400
500
600
500
600
distance from the plot, m
plot #3, Pu 1000
AS
100
10
1 0
100
200
300
400
distance from the plot, m –
137
Cs
–
90
Sr
–
238
Pu
–
239+240
Pu
Fig. 5. The deposition densities of radionuclides (averaged for each row of plates) during the forest fire, AS, in Bq m2 for 137Cs and 90Sr, and in mBq m2 for plutonium as a function of the distance from the plot border. Vertical bars represent the variation of values (STD) in each row.
and 7). The visual analysis of the sediments on the cascades collecting large particles showed that they were mainly composite ash, while the sediments on the cascades collecting finer particles were mainly composites of water drops, tars, small ash particles, etc. One should note that the dispersal composition of the various radionuclides was different and reflects the origin of each radionuclide. For instance, the percentage of 90Sr in the giant particles was the highest in both grassland experiments. Among the other studied radionuclides, 90Sr has the highest soil-to-plant transfer factor and, therefore, the biomass contamination with this radionuclide is high. In the grassland plots, the activity ratio grass/litter is the highest for 90Sr (Table 4). Taking into account that grass burning produces a huge amount of large ash particles, the 90Sr contamination of these large particles is reasonable. For plutonium and 137Cs, the contribution of litter in the total resuspended activity is higher (because of the lower contamination of grass). These nuclides are found in a higher proportion in the fine particle range. Therefore, mainly litter burning is indicated as being responsible for the production of the fine aerosol particles. The consequences of the differences in the contamination of the various biomass compartments (Table 5) can be seen in the forest fire data as well. In the dispersal composition of the forest fire aerosol (Fig. 8), a change can be observed with the distance from the plot. Close to the plot, the
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activity fraction
a
1 0.8
137
Cs Sr 238 Pu 90
0.6
239+240
Pu
0.4 0.2 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
AMAD, µm
b activity fraction
1.2
0.8
0.4
0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
AMAD, µm Fig. 6. Dispersal composition of the radioactive aerosol during the grassland fire (plot #1): (a) at the border of the plot (0 m) and (b) 24 m far from the plot along the plume axis. Vertical bars represent the STD.
most nuclides were measured in the large particle range; in the far distance 90Sr is detected in the fine and giant particle ranges and the Pu nuclides still dominated in the large particle range. 3.3. Resuspension and deposition parameters The resuspension and deposition parameters were calculated according to Eqs. (3) and (5) from the measured airborne radionuclide concentration and deposition in the fire experiments. During the grassland fires, the resuspension factor R of 137Cs and 90Sr was determined to range from 106 to 105 m1, and R of the plutonium nuclides to range from 107 to 106 m1 (Table 6), if R is related to the burning biomass. If the contamination density AS in Eq. (3) includes the soil contamination as well, the calculated resuspension factors will be of 2 orders of magnitude lower. However, the experimental plots were covered with vegetation, and resuspension directly from the soil should be negligible. Therefore, the calculation of R relative to the biomass contamination seems to be reasonable. Furthermore, the grassland fires were carried out in very favourable conditions with intensive resuspension. Thus, the obtained values can be considered as an upper estimate for the Chernobyl zone conditions. During the forest fire, the resuspension factors of all radionuclides were in the range 107e 6 10 m1 if they were related to the biomass contamination, and 108e107 m1 if they were
V.I. Yoschenko et al. / J. Environ. Radioactivity 86 (2006) 143e163
activity fraction
a
157
0.7 0.6 0.5 0.4
137
Cs Sr 238 Pu 90
239+240
Pu
0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
13-25
>25
13-25
>25
AMAD, µm
activity fraction
b
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
AMAD, µm
activity fraction
c
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
AMAD, µm Fig. 7. Dispersal composition of the radioactive aerosol during the grassland fire (plot #2): (a) at the border of the plot (0 m) and (b) 50 m far from the plot along the plume axis; (c) 100 m far from the plot along the plume axis. Vertical bars represent the STD.
related to the total contamination (Table 6). In general, they were lower than those during the grassland fires, probably because of the absence of understorey in the forest. The forest litter was mainly formed by the fallen needles, while the meadow litter consisted of dry rests of grass. The highest resuspension factors were obtained only if the litter is taken into account (which contributed most to the fire burden, see Section 2.6). Especially for 90Sr, the difference is significant. The deposition rate and deposition velocity during the fire experiments are given in Table 7. In general, higher values of the deposition velocity were observed for 137Cs and 90Sr (up to 60 cm s1), while for plutonium mainly several cm s1 were obtained. In the fire plume different radionuclides were associated with aerosol particles of different size range. Because of the size dependence of the deposition process, as a consequence, the deposition rate and velocity
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158
a
0.8
activity fraction
0.7 0.6
137
Cs
90
Sr
239+240
0.5
Pu
238
Pu
0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
1.8-5.5
5.5-13
13-25
>25
13-25
>25
AMAD, µm
activity fraction
b
0.6 0.5 0.4 0.3 0.2 0.1 0 <0.7
0.7-1.8
5.5-13
1.8-5.5
AMAD, µm
activity fraction
c
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 <1.1
1.1-2
2-3.3
3.3-7
>7
AMAD, µm Fig. 8. Dispersal composition of the radioactive aerosol during the forest fire (plot #3): (a) 37 m far from the plot along the plume axis; (b) 105 m far from the plot along the plume axis and (c) 492 m far from the plot along the plume axis. Vertical bars represent the STD.
may differ for various radionuclides. For instance, in Section 3.2 it was concluded that burning of litter (the biomass compartment with the highest plutonium contamination, Tables 4 and 5) is mainly responsible for the fine particle fire emission. In this size fraction high plutonium concentrations (especially in the grassland fires, Figs. 6 and 8) were measured. In contrast, higher airborne 137Cs/90Sr-activity concentrations were detected in the large particle range which could be responsible for the difference between the deposition velocities of plutonium and 137 Cs/90Sr. Both rate and velocity of aerosol deposition were lower during the forest fire in comparison to the grassland fires. Probably, this result is due to the lack of the highly contaminated giant particles (or fragments of burning material) in the smoke plume of the forest fire.
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159
Table 6 Resuspension factor R during the experiments for the considered radionuclides R related to
R, m1 137
90
238
239C240
Grassland fire, plot #1 (a) Combustible only (b) Total
(1.7 G 0.2) ! 105 (1.7 G 0.2) ! 107
(1.5 G 0.2) ! 105 (3.7 G 0.5) ! 107
(3.5 G 1.0) ! 107 (4.9 G 1.4) ! 109
(2.4 G 0.6) ! 107 (3.8 G 0.9) ! 109
Grassland fire, plot #2 (a) Combustible only (b) Total
(8.0 G 4.8) ! 106 (1.9 G 1.1) ! 107
(4.4 G 2.6) ! 106 (1.8 G 1.5) ! 107
(2.9 G 2.3) ! 106 (1.3 G 1.0) ! 108
(2.6 G 2.1) ! 106 (1.3 G 1.0) ! 108
Forest fire, plot #3 (a) Combustible only (b) Total (c) Litter only
(4.7 G 2.0) ! 107 (4.7 G 2.0) ! 108 (7.0 G 2.8) ! 107
(3.5 G 1.6) ! 107 (1.1 G 0.5) ! 107 (1.2 G 0.5) ! 106
(1.1 G 0.7) ! 106 (3.2 G 2.2) ! 108 (1.2 G 0.8) ! 106
(8.3 G 4.8) ! 107 (2.5 G 1.6) ! 108 (9.4 G 5.2) ! 107
Cs
Sr
Pu
Pu
R is given in relation to (a) the combustible inventory only and (b) the total inventory of the plot. For the forest fire, the litter inventory is given as reference as well (c).
In a previous study (Kashparov et al., 2000), resuspension factors of 108e107 m1 were determined for 137Cs during forest fires in the contaminated territories outside the 30-km Chernobyl zone. These values were calculated for the total contamination of the sites and are in good agreement with those obtained in the present study. Thus, for the typical pine forests of the Ukrainian Polessie the above values are confirmed as 137Cs resuspension factors during forest fires. Agricultural activities such as harrowing, fertilizing and ploughing may lead to enhanced resuspension in contaminated areas. Resuspension factors of the order of 107 m1 were determined for 137Cs resuspension during farming on dry soil in the Chernobyl area (Wagenpfeil et al., 1999). The comparison of resuspension by wildland fires and soil management indicates that only a relatively small fraction of the total areal contamination is available for re-entry into the atmosphere caused by the fires. 3.4. Radioecological impact of the fires Due to the transport and deposition of the radionuclides in the fire plumes, surfaces outside the fire plots will be contaminated. However, the neighbouring areas of the fire plots are initially contaminated to the same extend as the experimental areas. Therefore, the additional burden has to be related to the existing inventory. If the fallout density from the radioactive plume after the fires (Fig. 4) is compared to the existing contamination density before the fire (Tables 4 and 5), the additional contamination is low, even in the near vicinity of the burning plots; e.g., for plot #1, the additional contamination density of the grass after the fire would be about 1% 90Sr, 1% 137Cs, 7% 238Pu and 7% 239 C 240Pu of the contamination density before the fire if all newly deposited material is fixed on the grass. Thus, the wildland fires in the Chernobyl exclusion zone do not lead to a significant redistribution of contamination on the scale of the whole zone. However, if vegetation was grown next to plot #1 on uncontaminated soil with the same biomass density of 0.4 kg m2 (Table 1), the new vegetation would be contaminated with about 1 kBq kg1 90Sr, 500 Bq kg1 137Cs, 0.1 Bq kg1 238Pu and 0.2 Bq kg1 239 C 240 Pu after the fire according to the deposition density of Fig. 4. So, for decontaminated areas inside the zone, the secondary contamination by fire events might be a problem. Perhaps even more relevant from a radioecological point of view is the finding that the radionuclides in
160
X, m
137
90
Cs 2 1
I, Bq m Grassland 0 24 49 79
238
Sr
s
fire, plot #1 0.12 G 0.01 0.06 G 0.005 0.04 G 0.003 0.03 G 0.003
Grassland fire, plot #2 0 0.34 G 0.01 50 0.03 G 0.003 100 0.009 G 0.002 150 0.016 G 0.003 Forest fire, plot #3 57 (2.0 G 0.3) ! 103 102 (1.5 G 0.2) ! 103 210 (4.1 G 0.4) ! 103 294 (1.7 G 0.2) ! 103 365 (1.3 G 0.2) ! 103 483 (1.8 G 0.3) ! 103
1
vg, cm s
I, Bq m
239 C 240
Pu
2 1
s
1
Pu
2 1
vg, cm s
I, Bq m
s
1
vg, cm s
2 1
I, Bq m
s
vg, cm s1
2.6 G 0.2 4.0 G 0.5 5.3 G 0.5 3.1 G 0.3
0.2 G 0.01 0.15 G 0.01 0.11 G 0.01 0.06 G 0.006
16 G 1 22 G 2 57 G 6 21 G 4
(1.4 G 0.3) ! 105 (1.2 G 0.2) ! 105 (9.1 G 1.5) ! 106 (4.2 G 1.0) ! 106
6.3 G 1.4 5.3 G 0.9 2.2 G 0.4 0.1 G 0.03
(3.0 G .0.5) ! 105 (2.5 G 0.3) ! 105 (2.0 G 0.2) ! 105 (1.0 G 0.1) ! 105
9.5 G 1.2 6.6 G 0.8 1.1 G 0.1 0.1 G 0.01
35 G 3 34 G 7 13 G 3 22 G 6
0.3 G 0.01 0.02 G 0.002 0.01 G 0.001 0.006 G 0.001
60 G 6 15 G 2 9G1 12 G 3
(3.1 G 0.2) ! 105 (1.7 G 0.2) ! 105 (1.5 G 0.2) ! 105 (5.4 G 1.8) ! 106
12 G 2 28 G 7 17 G 6 7.0 G 3.5
(5.8 G .0.3) ! 105 (2.6 G 0.2) ! 105 (3.8 G 0.3) ! 105 (1.3 G 0.2) ! 105
11 G 1 21 G 4 26 G 7 8G3
1.3 G 0.3 1.0 G 0.4 1.9 G 0.8 2.7 G 2.3 0.2 G 0.07
(3.1 G 0.6) ! 106 (8.6 G 1.3) ! 106 (1.4 G 0.5) ! 105 (1.1 G 0.4) ! 106 (2.6 G 1.1) ! 106 (1.5 G 0.8) ! 106
2.7 G 1.4 4.2 G 1.7 7.8 G 5.3 4.7 G 4.5 0.35 G 0.19
(9.1 G 2.4) ! 106 (1.6 G 0.2) ! 105 (3.2 G 1.0) ! 105 (2.6 G 0.6) ! 106 (7.7 G 2.2) ! 106 (1.8 G 0.6) ! 106
1.4 G 0.6 5.2 G 1.8 4.1 G 1.5 3.1 G 2.1 0.29 G 0.14
2.6 G 0.4 1.2 G 0.2 2.8 G 0.5 1.9 G 0.4 0.6 G 0.1
(1.0 G 0.2) ! 103 (4.6 G 1.8) ! 104 (2.8 G 0.4) ! 103 (5 G 2) ! 104 (6 G 2) ! 104 (5 G 2) ! 104
X is the distance from the plot along the plume axis.
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Table 7 Deposition rate, I, and deposition velocity, vg, during the experiments for the considered radionuclides
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161
the remaining ash are more concentrated and better soluble (Amiro et al., 1996). This means that in the water runoff (e.g. after fire fighting) the radionuclides can be more readily redistributed in the landscape. 3.5. Dose assessment The firemen in the exclusion zone are exposed to an additional irradiation due to the participation in fire suppression actions. In this section only this additional dose to the firemen is considered and referred to as the dose. The total dose is caused by external and internal irradiation. The radioactive contamination of soil and vegetation and the radionuclides in the passing fire plume has to be considered for the external exposure. The uptake by inhalation of radionuclides from the plume into the organism determines the dose of internal irradiation during the fire fighting. In the following, the total dose of a fireman working for 1 h at the burning plots is estimated. The dose from the radionuclides in the biomass and the soil was determined from the measured dose rate in air. The maximum exposure for a 1-h stay at the plots was 16 mSv in plot #1, 10 mSv in plot #2 and 4.2 mSv in plot #3 (Fig. 1). The external dose from the plume was calculated from the airborne radionuclide concentration and the corresponding dose coefficients (Gusev and Beliayev, 1991). The plutonium isotopes were neglected because of the short range of a-particles even in air. In Table 8 the calculated external dose from the radioactive plume is given for each plot; this part appears to be negligible in comparison to the external dose from soil and biomass. The inhalation dose is calculated using the dosimetric model of the human respiratory tract presented in Publication 66 of the ICRP (1994). According to the regulatory documents (Radiation Safety Norms of Ukraine, 1997), the dose absorbed in the whole body (effective equivalent dose, EED) during 50 years after the radionuclide intake is determined. The following assumptions and parameters of calculations are used: the subject of inhalation is an adult man at heavy work; the composition of the radioactive aerosol is polydisperse, fitting the measured distributions; the density of the aerosol is 1 g cm3; the solubility class of the aerosol particles is F (soluble); Table 8 Additional doses by irradiation of the firemen due to a 1-h stay in the fire zone Radionuclide Maximal airborne concentration, Bq m3 Plot #1
Plot #2
137
5
1
0.27
90
3
0.5
0.33
Cs
Sr
238
Pu
3
4
3.4 ! 10 2.5 ! 10 Pu 6.7 ! 103 5.1 ! 104 External (soil and vegetation) Total dose 239 C 240
Dose type Dose, Sv
Plot #3
Plot #1
4
4.6 ! 10 1.1 ! 103
External (cloud) Inhalation External (cloud) Inhalation Inhalation Inhalation
6.9 ! 10
Plot #2 10
1.4 ! 10
Plot #3 10
3.7 ! 1011
6 ! 108 1.2 ! 108 3.2 ! 109 1010 1.7 ! 1011 1.1 ! 1011 2.4 ! 107 7.1 ! 106 1.7 ! 105 1.6 ! 105 4.0 ! 105
4.1 ! 108 2.6 ! 108 5.3 ! 107 106 1.3 ! 106 2.8 ! 106 1 ! 105 4.2 ! 106 1.2 ! 105 8 ! 106
162
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the biokinetic models are those recommended by Publication 30 of the ICRP (1982); all progenies are taken into account according to the decay chains given in Publication 38 of the ICRP (1983). The results of the dose calculation due to a 1-h stay of the firemen in the experimental sites during the fire experiments are presented in Table 8. It can be seen that beside the dose of the external irradiation from soil and vegetation, a significant contribution to the total dose can be provided by the inhalation intake of the plutonium radioisotopes and their progenies. In some conditions this contribution can reach half of the total dose. Although this estimate is conservative and the total period of a fireman working in the fire area during a year is usually much smaller than the total period of his residence in the exclusion zone, the problem of protection of the firemen should be considered in more detail. At the same time, taking into account the sharp decrease of the airborne radionuclide concentration with the distance from the source of release (Fig. 3), it can stated that the inhalation component of the total dose (as well as the external irradiation from radionuclides in air) is not important for the personnel of the exclusion zone which is not involved in the fire fighting. One must note that the above statements are formulated for small fires, which result in a comparable low total release of radionuclides into the atmosphere. If the fire area is very large, the affected territory will also increase. In this case, the possibility of additional irradiation of the people in the Chernobyl area can be evaluated using the above estimates of relative release of radionuclides during the grassland or forest fires. 4. Conclusions The experiments performed in the Chernobyl exclusion zone made it possible to characterize the resuspension, transport and deposition of radionuclides during grassland and forest fires in well controlled conditions. The obtained values of the resuspension factor and the relative release of radionuclides are proposed for the estimation of the radioecological consequences of the wildland fires in this territory. In general, the fires impact a relatively small area downwind of the burning zone. Within this area, the airborne concentration of radionuclides raises several orders of magnitude as compared to its background level, which can result in a significant increase of the inhalation proportion of the total dose. The Pu radionuclides constitute the main contribution to the inhalation dose. The contribution of the wildland fires to the redistribution of radioactivity in/outside the Chernobyl zone can be considered as negligible. Taking into account that the experiments were done at previously characterized sites and were accomplished with the relevant meteorological records, the experimental data can be applied for the validation of transport models for fires (Yoschenko et al., submitted for publication). Modeling will give a more precise estimate of the relative release of radionuclides, which can be derived from the value of the source intensity for each radionuclide. Acknowledgements The presented investigations were carried out within the framework of the project #1992 funded by the Science and Technology Center in Ukraine (STCU). The authors kindly appreciate the support they met from the STCU, as well as the assistance of the administration of the exclusion zone in the organization of the experimental works.
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References Amiro, B.D., Sheppard, S.C., Johnston, F.L., Evenden, W.G., Harris, D.R., 1996. Burning radionuclide question: what happens to iodine, cesium and chlorine in biomass fires? Science of the Total Environment 187, 93e103. Andreae, M.O., Merlet, P., 2001. Emission of trace gases and aerosols from biomass burning. Global Biogeochemical Cycles 15, 955e966. Frank, G., Kashparov, V., Protsak, V., Tschiersch, J., 1996. Comparison measurements of a Russian standard aerosol impactor with several Western standard aerosol instruments. Journal of Aerosol Science 27, 477e486. Garger, E.K., Kashpur, V., Belov, G., Demchuk, V., Tschiersch, J., Wagenpfeil, F., Paretzke, H.G., Besnus, F., Holla¨nder, W., Martinez-Serrano, J., Vintersved, I., 1997. Measurement of resuspended aerosol in the Chernobyl area. Part I: discussion of instrumentation and uncertainty of measurement. Radiation and Environmental Biophysics 36, 139e148. Garger, E.K., Kashpur, V., Paretzke, H.G., Tschiersch, J., 1998. Measurement of resuspended aerosol in the Chernobyl area. Part II: size distribution of radioactive particles. Radiation and Environmental Biophysics 36, 275e283. Goldammer, J.G., Crutzen, P.J., 1993. Fire in the environment: scientific rationale and summary of results of the Dahlem workshop. In: Crutzen, P.J., Goldammer, J.G. (Eds.), Fire in the Environment. John Wiley & Sons, Chichester, pp. 1e14. Gusev, N.G., Beliayev, V.A., 1991. Radioactive Releases into the Biospxhere: A Manual. Book Company ‘‘Energoatomizdat’’, Moscow, 256 p. (in Russian). Hrgian, A.H., 1957. Atlas of Clouds. Book company ‘‘Hydrometeoizdat’’, Leningrad, 176 p. (in Russian). ICRP, 1982. Publication 30. Limits for intakes of radionuclides by workers. Annals of the ICRP 8. Pergamon Press, Oxford. ICRP, 1983. Publication 38. Radionuclide transformations: energy and intensity of emissions. Annals of the ICRP 11e 13. Pergamon Press, Oxford. ICRP, 1994. Publication 66. Human respiratory tract model for radiological protection. Annals of the ICRP 24. Pergamon Press, Oxford. Kashparov, V.A., Lundin, S.M., Kadygrib, A.M., Protsak, V.P., Levtchuk, S.E., Yoschenko, V.I., Kashpur, V.A., Talerko, N.N., 2000. Forest fires in the territory contaminated as a result of the Chernobyl accident: radioactive aerosol resuspension and exposure of firefighters. Journal of Environmental Radioactivity 51, 281e298. Kashparov, V.A., Lundin, S.M., Khomtinin, Y.V., Kaminsky, S.P., Levchuk, S.E., Protsak, V.P., Kadygrib, A.M., Zvarich, S.I., Yoschenko, V.I., Tschiersch, J., 2001. Soil contamination with 90Sr in the near zone of Chernobyl. Journal of Environmental Radioactivity 56, 285e298. Krzes´niak, J.W., Porstendo¨rfer, J., 1981. Experimental determination of the collection efficiency of the Petrianov type FPP 15-1.7 filter for sampling submicron atmospheric aerosols. Nukleonika 26, 669e679. McMahon, C.K., Bush, P.B., 1992. Forest worker exposure to airborne herbicide residues in smoke from prescribed fires in the southern United States. American Industrial Hygiene Association Journal 53, 265e272. Nicholson, K.W., 1988. A review of particle resuspension. Atmospheric Environment 22, 2639e2651. Radiation Safety Norms of Ukraine, 1997. Publication RSNU-97. Kiev (in Ukrainian). Rosner, G., Winkler, R., 2001. Nuclide-dependent local and collector surface effects in sampling of radioactive deposition to ground. Applied Radiation and Isotopes 55, 823e829. Wagenpfeil, F., Paretzke, H.G., Peres, J.M., Tschiersch, J., 1999. Resuspension of coarse particles in the region of Chernobyl. Atmospheric Environment 33, 3313e3323. Yamasoe, M., Artaxo, P., Miguel, A.H., Allen, A.G., 2000. Chemical composition of aerosol particles from direct emissions of vegetation fires in the Amazon Basin: water-soluble species and trace elements. Atmospheric Environment 34, 1641e1653. Yoschenko, V.I., Kashparov, V.A., Levchuk, S.E., Glukhovskiy, A.S., Khomutinin, Yu.V., Protsak, V.P., Lundin, S.M. Tschiersch, J. Resuspension and redistribution of radionuclides during grassland and forest fires in the Chernobyl exclusion zone. Part II. Modeling the transport process. Journal of Environmental Radioactivity, submitted for publication.