1. THYROID DOSES FROM INCORPORATED 131I FOR RESIDENTS
OF THE ORYOL OBLAST

 

 

The worst accident in the history of the nuclear industry occurred early in the morning of 26 April 1986 at about 1’ 23’’ during a drop in reactor power for scheduled maintenance, as an experiment was conducted to improve one of the safety systems at the Chernobyl NPP. As a result of a breach of the installation control guidelines, as well as shortcomings in design of control devices and nuclear and physical characteristics of the reactor, a thermal explosion occurred leading to a total destruction of the reactor core, damage to the building of the 4th unit of the Chernobyl NPP and environmental release of radioactivity of tens of millions of Curies. After   ten days the rate of release of radioactivity was reduced by a factor thousand.  This may be due to the intense efforts involving specialists of different ministries and agencies, units from the Ministry of Defence and the Ministry of Interior or merely the complete consumption of the burning graphite and the drop of the molten fuel through the floor to the rooms below.  The Ministries later  proceeded to contain the consequences of the accident.

Dynamics of radioactivity depositions after the accident [1, 2]. The first plumes on 26th April, of radioactive materials resulting from the Chernobyl accident were westward. Then, on 27th April the wind direction changed to the northwest and on 28th  April eastward (Fig. 1.1, [1]).

 

 

Fig. 1.1. Dispersion of the radioactivity plumes from the Chernobyl reactor [1].   (Note: The Oryol oblast is the region surrounding the town of Oryol (shown as Oriel) at the right hand (eastern) edge of this map.)

 

Area contamination [1, 2]. The radioactive fission products, fuel particles and structural materials released from the damaged core were dispersed by air currents over hundreds and thousands of kilometers. The three main contaminated areas were designated the Central, Gomel-Mogilev-Bryansk and Kaluga-Tula-Oryol areas. The Kaluga-Tula-Oryol area is located 500 km from the reactor and contamination came there from the same radioactive cloud that caused contamination in the Gomel-Mogilev-Bryansk area as a result of the rainfall on 28th -29th  April. The 137Cs deposition density, however, was lower in this area, generally less than 600 kBq/m2.

Behavior of radionuclides in the environment [3]. The environmental behavior of radionuclides depends on their physical and chemical properties, precipitation (dry or wet) and environmental characteristics. The main contributors to the internal radiation dose for the populations of the contaminated areas were 137Cs and 131I deposited on the ground surface and subsequently injected or inhaled

During the year immediately following the accident the leading processes controlling radionuclide intake for humans were inhalation, and contamination of leaf vegetables and pasture grass eaten by grazing cows and goats with resulting contamination of milk. The radioactivity deposited onto vegetation stayed there for about two weeks and was then transferred to the underlying surface and soil. The specific activity in plants declines over time due to vegetation self-cleaning under the action of natural factors such as wind and rainfall, increase in clean biomass and natural decay of radionuclides. The Chernobyl accident occurred at the end of April. In central Russia this is the time of spring growth and a grazing season. Important regional factors, which determined the extent of contamination of leaf vegetables and pasture grass, as well as the beginning of cattle grazing, were the weather conditions in April and May 1986. The transfer of 131I from soil to food is quite a lengthy process and if food is consumed later then two months after fallout, it does not play any significant role because the 131I half-life is only eight days.

Populations of the contaminated regions [1-3]. There was a considerable release of short-lived 131I and the radiation risk for the public during the first few months was primarily associated with deposition of radioactive isotopes of iodine on the ground after the accident. At that point, the thyroid gland was most exposed and its exposure primarily resulted from the food chain: pasture-cow-milk. Another major entryway into the body was consumption of perennial leaf vegetables (sorrel). The smallest contributor to the thyroid dose was inhalation. The bans on consumption of milk from cows that grazed on contaminated meadows and pastures and iodine prophylaxis (which unfortunately not implemented in a timely manner in many areas) made it possible to prevent overexposure of some of the public. However, at many populated points radioactive iodine did cause thyroid exposure in children above the permissible levels.

Agroclimatic conditions in the spring-summer 1986 in the contaminated areas. The depositions of the Chernobyl radioactivity on the ground was in early spring and caused air-borne contamination of the above-ground vegetation which by that time was well-developed. Then included winter crops, perennial leaf vegetables (sorrel), perennial meadow grass, natural and semi-natural pastures. Four major types of vegetation can be identified which contributed to human intake of iodine and cesium in the contaminated areas after entering the food chain in 1986. There were green fodder on semi-natural pastures (public cattle) and natural pastures (private cattle), perennial leaf vegetables (sorrel) and annual greens (lettuce, parsley, onion). For other crops the leading contamination pathway was transfer of radionuclides from soil and resuspension.

For assessing the influence of weather conditions of spring 1986 on region-specific contamination of vegetation and time of start of cattle grazing a study of agroclimatic conditions of the spring-summer 1986 in the Oryol region was performed. For this purpose a database was assembled with weather conditions from March to July 1986, as observed at all meteorological stations of the network in the European part of the former USSR (mean daily temperatures and precipitation with time step of 1 day). Then using GIS technology these data were interpolated to 276 nodes of a regular grid superimposed on the territory of the Oryol region. The air mean daily temperatures in the Oryol region for three dates: start of depositions of the Chernobyl radioactivity on 28 April, the cold weather peak on 5 May and on 4 June are shown in Fig. 1.2.

Then the territory of the oblast was divided into three regions in which the monthly variations of mean daily temperatures was uniform. Figure 1.3 shows the pattern of temperature regions within the Oryol oblast. As a next step for the whole oblast and each temperature region separately time dependences of mean daily and effective biological temperatures averaged over all nodes of regular grids were generated (Figs. 1.4-1.6).

Fig. 1.2. Mean daily temperature in the Oryol oblast on 3 dates.

 

Fig. 1.3. “Uniform” temperature regions within the Oryol oblast.

 

 

Days after  accident

 

Effective Temperature °C

 

 Oryol oblast

 

Fig. 1.4. The mean daily air temperature in the third temperature region of the Oryol oblast.

X axis - time after the accident, days; Y axis - temperature, °C;
the black point is the beginning of the spring growing of pasture vegetation.

 

 


Days after accident

 

Effective Temperature °C

 

 

 

 

  Oryol oblast

 

Fig. 1.5. Mean daily effective air temperature in the third temperature region of the Oryol oblast.

X axis - time after the accident, days; Y axis - effective temperature, °C; the black point is
the beginning of milk cattle grazing and consumption of greens (sorrel, perennial onion).

Days after accident

 

Effective Temperature °C

 

 Oryol oblast

 

Fig. 1.6. Mean daily effective air temperature in the third temperature region of the Oryol oblast.

X axis - time after the accident, days; Y axis - effective temperature, °C;
the diamond shows the time of sowing of perennial greens; square - sprouts of perennial greens;
black point - beginning of consumption of perennial greens (parsley, dill, annual onion).

 

 

As can be seen from the data in Fig. 1.3-1.6, the weather conditions in the spring 1986 in the Oryol oblast were such that the spring growth of pasture grass and perennial plants started as early as late March-early April and, in fact, the grazing season had begun and 4-5 days before the accident people started consuming perennial greens from their kitchen gardens. As a result, when the accident occurred and the radioactivity deposited, all three pathways were involved: intake via inhalation, consumption of contaminated milk and greens. Table 1.1 includes radiological parameters for the Oryol region in comparison with three other severely contaminated oblasts of the Russian Federation. As is seen, from the standpoint of contamination of food at the time it began  to be consumed, considering  the worst consequences of the 131I unit deposition from the accident  were observed in the Oryol oblast.

 

Table 1.1. Radioecological parameters (mean values for four contaminated oblasts of the Russian Federation).

 

Model parameters

Oblast

Bryansk

Kaluga

Oryol

Tula

Temporal parameters, days after accident

Fallout time, start/end

2.8/3.8

3.5/4.3

3.3/4.2

3.5/4.3

Beginning of consumption of contaminated greens (annual/perennial)

4/26

4/31

4/27

4/31

Beginning of consumption of contaminated fodder by cattle (private/public)

4/3

11/8

3/3

11/8

Beginning of consumption of contaminated milk by population (rural/urban)

5/5

12/10

4/5

12/10

Contamination at the beginning of consumption, kBq/kg
with the 131I deposition density of 1 kBq/m2 at the time of the accident

Greens (perennial/annual)

0.19/0.0014

0.046/0.0001

0.25/0.0015

0.06/0.0004

Vegetation of pastures (semi-natural/natural)

0.17/0.19

0.043/0.047

0.22/0.25

0.006/0.006

Cow’s milk (public/private)

0.09/0.07

0.023/0.019

0.012/0.01

0.03/0.025

 

 

Radiation situation in the territory of the contaminated oblasts. As was mentioned above (fig. 1-1), the fallout in each of the territories of the Russian Federation contaminated after the Chernobyl accident originated from the same radioactive cloud. Unlike the fall out in Belarus and Ukraine, the fallout in the areas of the RF was one-time and mainly occurred on 27th - 28th April. It should be stressed that the ratio of 131I to 137Cs in the contaminated areas of the RF did not vary significantly.  The mix of the radionuclides was relatively uniform. Since the  137Cs deposition density was the easiest to measure of the radiation parameters it was therefore the most important.

The scale and complexity of the accident and the incompleteness of the data about radioactivity release and deposition necessitated efforts to study and describe the situation. This work is being continued in Roshydromet, Ministry of Emergency, Ministry of Health and other institutions, both in Russia and abroad. In particular, SPA “Typhoon” (Obninsk) has performed extensive reconstruction of the temporal and spatial pattern of radioactivity dispersion and deposition after the Chernobyl accident.

The reference book [4] contains data on the radioactive contamination with 137Cs, 90Sr and 239+240Pu in the territory of the Russian Federation as a result of the Chernobyl accident. These data were put together using results of the surveys conducted by SPA “Typhoon” from 1986 to 1999, were evaluated by experts and entered into the database. The tables show mean deposition densities as of 1 January 2000, as well as minimal and maximum values acquired in the surveys. There is also a table showing the distribution of settlements by 137Cs contamination levels. These data, however, are not available for all population points of the Oryol oblast. Open circles in Fig. 1.7 show places for which data are available [4], and closed circles show places for which data are unavailable.

 

 

 

Fig. 1.7. Completeness of 137Cs density deposition data for the Oryol oblast
available to SPA “Typhoon”.

 

 

The maps of “The Atlas of radioactive contamination of the European part of Russia, Belarus and Ukraine” [5] include only outlines of 137Cs deposition densities rather than specific measurements and therefore the atlas cannot be used directly in radioecological studies (Fig. 1.8).  The reconstruction of 137Cs deposition densities for all settlements in the Oryol oblast for which official data were unavailable was performed using the method of two-dimensional interpolation in the environment GIS Manifold based on the GIS-technology developed in the RNMDR specifically for this purpose [6].

 

 

 

 

Fig. 1.8. Map of 137Cs density deposition in the territory of the Oryol oblast from reference 5.

 

 

The summary map of 137Cs deposition densities in the Oryol oblast at the settlement level is shown in Fig. 1.9. These data were used to estimate mean deposition densities. Figure 1.10 shows a map of mean deposition densities of 137Cs in the Oryol oblast. The mean is at the “rayon” level.

 

 

Fig. 1.9. Map of reconstructed 137Cs contamination density in the populated points
of the Oryol oblast.

 

Fig. 1.10. Map of reconstructed 137Cs deposition density in the Oryol oblast

(mean for each rayon)

 

The data from the database with 137Cs deposition densities for all settlements and rayons of the Oryol oblast were used to calculate 131I deposition densities based on an official methodology [7] and data on radionuclide composition of the Chernobyl fallout. The summary maps of 131I deposition of the Oryol oblast are shown in Fig. 1.11 and 1.12. These data were used for estimating thyroid doses and risks for those exposed as children in the case-control studies.

 

 

Fig. 1.11. Map of reconstructed 131I deposition density in the populated points
of the Oryol oblast.

 

 

 

Fig. 1.12. Map of reconstructed 131I deposition density in the Oryol oblast.

(mean for each rayon)

 

 

Thyroid doses for the population of the Oryol oblast. Official information on thyroid doses among residents of the Oryol oblast can be found in the recent reference book [8]. It contains thyroid doses for 6 age groups who lived in 1986 in the settlements with 137Cs deposition densities more than 37 kBq/m2. The doses in the reference book were estimated based on the official guidelines on thyroid dose reconstruction [9] (hereinafter Guidelines).

Methodologically the Guidelines draw on the dose models for residents of the Russian Federation areas affected by the Chernobyl contamination. Parameters of the dosimetric models for residents of different age in the Guidelines have been inferred using field radiation measurements, data about cattle grazing practices and local milk consumption in the contaminated areas of Russia. For determination of absorbed dose the key were results of 45 thousand measurements of 131I among residents of the four worst contaminated oblasts of Russia (Bryansk, Kaluga, Oryol and Tula) and more than 5 thousand local milk measurements for the same oblasts performed in May-June 1986.

The gaps in the data in the reference book [9] for the Oryol oblast are illustrated in Fig. 1.13. Thyroid doses for residents of those settlements of the Oryol oblast for which no official data are available (settlements with 137Cs deposition densities less than 37 kBq/m2) were estimated using the linear regression relationships as a function of 137Cs deposition density as described in the Guidelines and Figures 1.14-1.16 show results of calculating thyroid doses (children + adolescents and adults) which were then used to estimate risk of developing thyroid diseases.

 

 

 

 

 

 

 

Fig. 1.13. Completeness of official thyroid dose data in the reference book [9]
 for the population of the Oryol oblast.

 

 

 

 

Fig. 1.14. Thyroid doses for children and adolescents less than 17 years of age
at exposure in the settlements of the Oryol oblast.

 

 

 

 

 

 

Fig. 1.15. Mean rayon thyroid doses for children and adolescents less than 17 years of age
at exposure in the settlements of the Oryol oblast.

 

 

 

 

Fig. 1.16. Mean rayon thyroid doses for adults at exposure
in the settlements of the Oryol oblast.

 

 


References

 

1.     International Journal of Radiation Medicine. UNSCEAR 2000 Report to the General Assembly, Special Issue, 2-4 (6-8), Annex J. Exposures and effects of the Chernobyl accident, 2000.

2.     Bulletin of atomic energy. Central research institute of control, economics and information, March-April (3/4) 2001 (in Russian).

3.     Report of the UNSCEAR 2002, Annex J, Exposures and effects of the Chernobyl accident. - Moscow: RADECON, 2001 (in Russian).

4.     International Atomic Energy Agency. Present and future environmental impact of the Chernobyl accident. IAEA-TECDOC, 2000.

5.     Data on radioactive contamination with 137Cs, 90Sr and 239+240Pu of the populated points of the Russian Federation: Reference book. - Obninsk: Federal Service on Hydrometeorology and Environmental Monitoring, SPA “Typhoon”, 2000 (in Russian).

6.     Atlas of radioactive contamination of the European part of Russia, Belarus and Ukraine. Compiled by the Institute of Global Climate and Ecology of Roshydromet and RAS under scientific guidance of academician Yu.Izrael. - M.: Federal Service of Geodesy and Cartography of Russia, 1998 (in Russian).

7.     Vlasov O.K., Shishkanov N.G., Chekin S.Yu., Ivanov V.K., Godko A.M., Shchukina N.V. Programming package for reconstruction of internal whole body and thyroid doses for the public based on GIS-technology and agroecological simulation models. International symposium devoted to 15 years of the Chernobyl accident, 21-25 April 2001, Kiev (in Russian).

8.     Reconstruction of mean effective dose accumulated over 1986-1995 for residents of populate points of the Russian Federation affected by the radioactive contamination as a result of the Chernobyl accident in 1986: Guidelines 2.6.1.579.96. - Moscow, 1996 (in Russian).

9.     Balonov M.I., Zvonova I.A., Bratilova A.A., Vlasov O.K., Shchukina N.V. Comparison of thyroid doses for persons of different age residing in 1986 in the settlements of the Bryansk, Tula, Oryol and Kaluga region contaminated by radionuclides after the Chernobyl accident: Reference book, official edition. - Moscow: Ministry of Health, 2002 (in Russian).

10.   Ramzaev P.V., Balonov M.I., Zvonova I.A., Bratilova A.A., Tsyb A.F., Vlasov O.K., Pitkevich V.A., Stepanenko V.F., Shishkanov N.G., Ilyin L.A., Gavrilin Yu.I. Reconstruction of thyroid dose from iodine radioisotopes for the residents of the populated points in the Russian Federation affected by radioactive contamination as a result of the Chernobyl accident in 1986: Guidelines 2.6.1.1000-00. – Moscow: State Sanitary and Epidemiological Regulation in the Russian Federation, Ministry of Health of Russia, 2001 (in Russian).