Research Article |
Corresponding author: Nataliya V. Shchukina ( shchukina@nrer.ru ) Academic editor: Yury Korovin
© 2023 Oleg K. Vlasov, Irina A. Zvonova, Nataliya V. Shchukina, Sergey Yu. Chekin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Vlasov OK, Zvonova IA, Shchukina NV, Chekin SYu (2023) Radioecological modeling of the 131I activity dynamics in the pasture vegetation of Mazovia in the year of the Chernobyl accident: Reconstruction, verification, reliability assessments. Nuclear Energy and Technology 9(4): 245-252. https://doi.org/10.3897/nucet.9.116654
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A radioecological model, which is a system of linear differential equations describing the dynamics of the transport of 137Cs and 131I radionuclides along the food chain after their release into the atmosphere after the Chernobyl accident, was used to reconstruct “instrumental” data of the 131I activities in the grass pastures in the central part of Mazovia. Four atmospheric models were used for the reconstruction: direct calculation, homogeneous cloud – inhomogeneous rainfall, inhomogeneous cloud – homogeneous rainfall, and a model with recalculation of the 137Cs and 131I activities in the atmosphere. The “instrumental” data were reconstructed based on data from direct measurements of the 131I activity in lawn grass. It has been shown that the direct calculation and homogeneous cloud models lead to a better agreement of the calculated and reconstructed “instrumental” data than the inhomogeneous cloud model. The arithmetic mean ratio of the calculated and reconstructed “instrumental” data lie in a range of 0.84 to 0.95 for the direct calculation and homogeneous cloud models, and in a range of 1.7 to 3.0 for the inhomogeneous cloud model. The mean geometric deviation for all models is constant and equal to 1.7. Instrumental and reconstructed “instrumental” data show a significant decrease in the specific activity of 131I in grass due to its wash-off by continuous rainfall, both during rainfall and after most of the deposition takes place. Due to this effect, the coefficient of the 131I retention on grass in the form of the maximum activity ratio to the 137Cs deposition density decreases from 34 to 1.4 m2/kg while it increases from 1 to 29 kBq/m2 as the result of the rainfall growth from 0 to 40 mm.
Chernobyl accident, IAEA’s EMRAS project, Warsaw scenario, agro-radioecological simulation model, radioactive cloud model, 137Cs deposition densities, atmospheric 131I forms, radioecological model verification
This paper is the fifth one in a series of papers (
The purpose of the study is to implement the method for recalculating instrumental data on the dynamics of the radionuclide activity in grass vegetation of one type for vegetation of other types, specifically from lawn grass to pasture grass, to create a database for the dynamics of reconstructed instrumental and calculated data on the specific activities of 131I in the cultivated pasture grass in the milk districts (the districts hereinafter) in Mazovia based on the example of the Warsaw Scenario data, and to estimate the statistical parameters of the calculated data ratios to the reconstructed instrumental data on the dairy cow green fodder in the above districts. The conclusions made earlier in (
The dynamics of the 131I specific activity in the cultivated pasture grass in the milk districts in Mazovia is studied based on a radioecological simulation model after radiation accidents with release of products into the atmosphere (
The Warsaw Scenario data for the ‘iodine group’ of the IAEA’s EMRAS project are used as the initial data for the calculation model (
Dynamics of the 131I forms in the atmosphere in the central part of Mazovia –the relative iodine fractions for the main fallout period of 2.5 to 5 days after the accident: 1 – aerosol form; 2 – interpolation (1); 3 – gaseous form; 4 – interpolation (3), 5 – organic form, 6 – interpolation (5).
Ecological time parameters (1–3) and daily average air temperature (4, 5):1 – beginning of spring vegetation; 2 – beginning of the dairy cattle pasture season (t_pastc = 6 days); 3 – beginning of radioactive fallout (q_dep0 = 2.4 days); 4 – weather data; 5 – smoothed data (Mazovia, vegetation – cultivated pasture grass).
Direct and its instrumental data on the 131I activity in lawn grass (Gr1, Gr1_1) with weather data on rainfall at the Warsaw Observ Astr weather station, the Warsaw Area (Rain1), and reconstructed “instrumental” data for pasture grass in the county Ostrolecki (Gr2, Gr2_1) with rainfall at the Myzsyniec weather station, the Ostroleka Area (Rain2): Gr1 – instrumental data; Gr1_1 – exponential interpolation Gr1; Gr2 – reconstructed “instrumental” data; Gr2_1 – nonlinear interpolation (Gr2).
The scenario presents the following set of initial data for 1986:
The dynamics of the 131I activity in grass was reconstructed based on the following four models of the atmosphere:
The following system of milk and dairy food production exists in Mazovia: the farms are united into districts; each district has its own center for receiving milk from the farms which includes milk receiving tanks; tank trucks further deliver milk to dairy plants united into milk areas (the areas hereinafter). All these scenarios are presented for two areas: the Warsaw Milk Area and the Ostroleka Milk Area. Milk samples were taken from tanks at the milk receiving centers in the districts and from vessels at the farms where milk from all cows was received.
There are two visible peaks in the dynamics of the 131I and 137Cs activities in the atmosphere (see Fig.
Given that the integrals of the volume activity of 131I increased to 92% and the activity of 137Cs increased to 96% for the period of the first peak, this period, 2.4–5.3 days, was accepted as the time of main fallouts.
In the main fallout period, rainfall took place at two weather stations: Warszawa Obserw Astr (0.8 mm) and Brinow (7.3 mm). There was no rainfall in the period of peak 1 at all other weather stations. The maximum activity of 131I in the atmosphere in the period of peak 2 was an order of magnitude smaller and amounted to 15 Bq/m3. According to weather data, heavy rainfall with a daily intensity of 10 to 15 mm was recorded between May 13 and 18 at all weather stations in both areas that captured the beginning of peak 2 (see Fig.
The dynamics of the 13II activity in the pasture vegetation of the milk farms was reconstructed using data on the 137Cs fallout densities for the settlements where they were located, and, for districts, using the average values of the 137Cs fallout within them.
At the initial stage, the data from measuring the specific activity of 131I in lawn grass at the Warszawa Obserw Astr weather station using the method proposed in
The instrumental data on the specific activities of 131I in pasture grass for all farms and districts in both areas were reconstructed based on the direct calculation model, and for the Ostroleka Area, additionally, based on the model with recalculation of the 137Cs and 131I activities in the atmosphere.
The basis for such recalculation was a comparison of data on the mutual location of the Warsaw Area and the Ostroleka Area in the central part of Mazovia and the Chernobyl NPP.
Mazovia is at a comparatively close distance from the ChNPP. Therefore, quite expectable is a non-uniform distribution of the ChNPP emission products in the atmosphere over comparatively large portions of its territory, such as spatially separated Warsaw Area and Ostroleka Area. It can be added that the dimensions of these areas are two times and a half to three times smaller than the dimensions of Mazovia as such and seven to nine times smaller than their distances from the ChNPP. This permits to suggest that, on the one hand, differences are possible in the values of the radionuclide activities in the atmosphere over the territory of each area and that, on the other hand, their spatial distribution is uniform. This suggestion was verified (
The results from reconstructing the activities of 131I in the pasture grass in the Ostrolecki district of the Ostroleka Area using the cloud recalculation model (Fig.
The results of calculating the dynamics of the 131I activity in the pasture grass in the Piski district for the atmosphere model with the cloud recalculation (Ostroleka Area) are shown in Fig.
Dynamics of reconstructed “instrumental” and calculated data of the 131I specific activity in cultivated pasture grass and rainfall in the Piski district, the Ostroleka Area (exponential reduction rates for instrumental (lm = 0.19 day–1) and calculated (lc = 0.21 day–1) data for the 131I specific activity in grass). Data options for the 131I activity in pasture grass: Gr1 – reconstructed “instrumental” data for the cloud with recalculation; Gr2 – nonlinear interpolation of Gr1; Gr3 – reconstructed “instrumental” data for the cloud with instrumental data; Gr4 – direct calculation model; Gr5 – heterogeneous cloud model; Gr6 – homogeneous cloud model. Rainfall: Rain1 – weather data; Rain2 – reconstruction in an interval of 2.4 to 5.5 days after the accident for the cloud with recalculation.
The average density of the 137Cs fallout in this district (3.8 kBq/m2) practically coincides as well with the recalculated density of dry fallout in this area (3.24 kBq/m2). Therefore, the calculation results based on all three models for this district: direct calculation (calculated to instrumental data ratio, μ1 = 0.88), homogeneous cloud (μ2 = 1.06), and heterogeneous cloud (μ3 = 0.92), coincide with an accuracy of up to 12% and are close to the ideal value of 1.
The figure also presents the results of a similar direct calculation for the 131I activity in grass for the cloud without recalculation of the 137Cs and 131I activities in the atmosphere (Fig.
There is an effect observed in all calculated and reconstructed instrumental data with a significant daily rainfall (10 to 20 mm) for an increased reduction rate of the 131I specific activity in pasture grass as compared with its value for time periods without rainfall smaller than (6–13) days and larger than 20 days. A more detailed option of considering this effect is presented for the Brinow bis farm in Fig.
Reduction rates of the 131I specific activity in grass taking into account and without taking into account the rainfall after the main fallout end
Data | Rainfall not taken into account (1) | Rainfall taken into account (2) | (2) / (1) |
---|---|---|---|
Measurement | 1.15 | 1.25 | 1.09 |
Calculation | 1.17 | 1.39 | 1.19 |
Calculation / Measurement | 1.02 | 1.11 | 1.09 |
Dynamics of reconstructed “instrumental” and calculated data of the 131I specific activity in the cultivated pasture grass taking into account and without taking into account the rainfall after 10 days: GrM – (“instrumental” data for the homogeneous cloud); GrС1 – weather data on rainfall intensity (Rain1) (“instrumental” data - homogeneous cloud); GrC2 – weather data on rainfall intensity (calculation - heterogeneous cloud); GrC2R0 – weather data – Rain0 (calculation - heterogeneous cloud); Rain1 – rainfall intensity - weather data); Rain0 – effective rainfall intensity, homogeneous cloud model.
These present reconstructed instrumental data (Gr1), their nonlinear interpolation (Gr2) and calculated data for the heterogeneous cloud models taking into account (Gr3) and without taking into account the rainfall after the main fallout end (Gr4). The reduction rates of the calculated and reconstructed instrumental data on 131I in pasture grass during calculations taking into account and without taking into account the rainfall after the end of the main fallout for times larger than six days given in Table
Of interest are the results of comparing the calculations for the 131I activities in grass, the 137Cs fallout densities and rainfall for settlements based on the direct calculation model (Fig.
Farm | Brinow bis | Model | σdep, kBq/m2 | Rainfall*, mm | m | Qmax, kBq/kg |
---|---|---|---|---|---|---|
Weather station | Brinow | Direct calculation | 6.92 | 7.3 | 0.87 | 54.8 |
σdep | 22.7 | Heterogeneous cloud | 22.7 | 7.3 | 2.49 | 155.9 |
Rainfall*, mm | 7.3 | Homogeneous cloud | 23.6 | 30 | 0.46 | 29.4 |
This result is the direct reflection of the physics of the activity delay on the vegetation leaf surface during fallout with rainfall. As continuous rainfall increases, the rate of the rainwater deposition on the vegetation leaf surface decreases rapidly, and stops entirely, starting from its particular value, due to all newly fallen moisture flowing down on the ground. Such dynamics is confirmed by scenario calculations in which the rainfall for the main fallout period increased from 0 to 40 mm (Fig.
Scenario option for dependences of calculated data on the 131I specific activity in the cultivated pasture grass at the Brinow bis farm, the Warsaw Area, on rainfall of 0 to 40 mm in the main fallout period (beginning of radioactive fallout q_dep0 = 2.4 days; beginning of rainfall q_rain0 = 2.5 days; end of rainfall q_rain1 = 5.5 days).
Here, the time to achieving the maximum activity decreases monotonously with the rainfall growth, but exceeds all the same the time for the 131I second activity maximum to occur in the atmosphere.
More detailed data in Fig.
Scenario calculation for the dependence of time and maximum activity of 131I on grass as a ratio of its maximum activity to the density of fallout from rainfall taking into account (Time1, Activity1) and without taking into account (Time2, Activity2) the self-cleaning of grass in the main fallout period (beginning of radioactive fallout q_dep0 = 2.4 days; beginning of rainfall q_rain0 = 2.5 days; end of rainfall q_rain1 = 5.5 days). Brinow bis farm, Warsaw Area.
The time of the 131I activity maximum in grass is small, but it decreases continuously as rainfall grows: from 5.1 days in the event of dry fallout to 4.5 days with a rainfall of 40 mm, but, all the same, more time of the 131I second activity maximum in the atmosphere remains (Fig.
The dependence of the 131I maximum activity in grass on the amount of rainfall for the main fallout period has the maximum with a rainfall of 10 to 15 mm. It follows from the data in Fig.
The coefficient of delay (ratio of the maximum 131I specific activity in grass to the 137Cs fallout density) is also decreasing rapidly from 34 to 1.4 m2/kg as the rainfall grows from 0 to 40 mm.
It follows from the radionuclide transport physics that there is a directly proportional dependence between the maximum radionuclide activities in all its series elements of the food chain. With this taken into account, disparity dCMgrf in the form of the ratio of the calculated specific activities of 131I to instrumental data in pasture grass within the region of their maximum values can be used to adjust the calculated data for milk based on the relation:
qCcormilk (t) = qCmilk (t) / dCMgrf, (1)
where qCmilk (t) and qCcormilk (t) are the direct and adjusted calculated specific activities of radionuclides in milk.
The same procedure can be applied as well to adjusting the calculated data for the dynamics of the radionuclide intake into the human body along the food chain and estimating its internal exposure doses.
Since the series of instrumental data for the specific activities of 131I in lawn grass was obtained only in the interval of its activity decrease, quantity dCMgrf for pasture grass was estimated based on two data samples:
The proximity to unity of the absolute values of the mean geometrical ratio of calculated data to instrumental data makes it possible to evaluate in option A the capabilities of the simulation model for reproducing the dynamics of instrumental data in their entire time interval. In option B, the values of this ratio, with their significant differences from unity, can be used as adjusting factors when reconstructing the milk contamination dynamics and further, along the chain, when reconstructing the human internal exposure doses.
The calculation results show that the calculated activities of 131I in pasture grass, based on the direct calculation and homogeneous cloud models, agree well with the instrumental data recalculated from perennial lawn grass (Table
Mean geometrical values of σG and standard deviation sG for the calculated data ratio to instrumental data on the 131I specific activities in pasture grass for the entire data time sample
Milk area | Warsaw Area | Ostroleka Area | |||
---|---|---|---|---|---|
Cloud model | Direct data | Direct data | Data with recalculation | ||
Vegetation | Lawn grass | Pasture grass | Pasture grass | Pasture grass | |
μG | Direct calculation | 0.88 | 0.84 | 0.92 | 0.89 |
Heterogeneous cloud | 1.66 | 2.74 | 2.99 | 1.08 | |
Homogeneous cloud | 0.86 | 0.89 | 0.98 | 0.95 | |
σG | Standard deviation | 1.69 | 1.65 | 1.69 | 1.69 |
Therefore, the direct calculation and homogeneous cloud models lead to a fairly satisfactory agreement of the calculated and instrumental data on the specific activity of 131I in pasture grass recalculated from perennial lawn grass, with the geometrical mean values of the average and standard geometrical deviations equaling 0.8 and 2 respectively.
Recalculation of actual instrumental data for the specific activities of 131I from perennial lawn grass to reconstructed instrumental data for cultivated pasture grass leads to a better agreement with the calculated data than recalculation from annual lawn grass.
The instrumental data reduction rate, equal to 0.21 day–1, practically coincides with the calculated data reduction rate for perennial lawn grass equal to 0.23 day–1.
The dependence of the maximum activity of 131I in grass on the amount of rainfall for the main fallout period reaches the maximum with a rainfall of 10 to 15 mm. The time for the 131I activity maximum to occur is small but decreases continuously as the rainfall grows from 5.1 days in the event of dry fallout to 4.5 days with a rainfall of 40 mm.
The coefficient of the 131I delay on grass (ratio of the 131I maximum specific activity to the 137Cs fallout density) decreases from 34 to 1.4 m2/kg as the rainfall in the main fallout period increases from 0 to 40 mm.
For perennial lawn grass, the spread of the calculation/measurement ratios for the entire series of reconstructed instrumental data for both areas is the same and lies in a range of 1.0 to 1.3.
For the series of instrumental data in the region of their maximum values, the range of the calculation/measurement ratios is close to unity (0.7 to 0.8), and this ratio is much smaller than unity and is equal to 0.4 on the average for the annual lawn grass option.
The standard mean geometrical deviation of the calculated data ratio to reconstructed instrumental data for all calculation models is practically constant and lies in a range of 1.6 to 1.7.
It is proposed that the obtained values of the calculated data ratio to instrumental data for the specific activities of 131I in pasture grass in the region of their maximum values be used to adjust the dynamics of the calculated 131I activities in milk in the central part of Mazovia.
The authors are grateful to Pawel Krajewski (Warsaw, Poland), the iodine team leader under the IAEA’s EMRAS project and the author of the Warsaw Scenario, for efficiently organizing the team’s activities, in the course of which our radioecological simulation model was modified using the bank of unique.