Paper RADIATION DOSIMETRY FOR HIGHLY CONTAMINATED BELARUSIAN, RUSSIAN AND UKRAINIAN POPULATIONS, AND FOR LESS CONTAMINATED POPULATIONS IN EUROPE

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1 Paper RADIATION DOSIMETRY FOR HIGHLY CONTAMINATED BELARUSIAN, RUSSIAN AND UKRAINIAN POPULATIONS, AND FOR LESS CONTAMINATED POPULATIONS IN EUROPE André Bouville,* Illya A. Likhtarev, Lina N. Kovgan, Victor F. Minenko, Sergei M. Shinkarev, and Vladimir V. Drozdovitch*, ** Abstract The explosions at the Chernobyl Nuclear Power Plant (CNPP) in Ukraine early in the morning of 26 April 1986 led to a considerable release of radioactive materials during 10 d. The cloud from the reactor spread many different radionuclides, particularly those of iodine ( 131 I) and cesium ( 134 Cs and 137 Cs), over the majority of European countries, but the greatest contamination occurred over vast areas of Belarus, the Russian Federation and Ukraine. As the major health effect of Chernobyl is an elevated thyroid cancer incidence in children and adolescents, much attention has been paid to the thyroid doses resulting from intakes of 131 I, which were delivered within 2 mo following the accident. The thyroid doses received by the inhabitants of the contaminated areas of Belarus, Russia, and Ukraine varied in a wide range, mainly according to age, level of ground contamination, milk consumption rate, and origin of the milk that was consumed. Reported individual thyroid doses varied up to 40,000 mgy, with average doses of a few to 1,000 mgy, depending on the area where people were exposed. In addition, the presence in the environment of long-lived 134 Cs and 137 Cs has led to a relatively homogeneous exposure of all organs and tissues of the body via external and internal irradiation, albeit at low rates. Excluding the thyroid doses, the whole-body (or effective) dose estimates for the general population accumulated during 20 y after the accident ( ) range from a few millisieverts (msv) to some hundred msv with an average dose of 10 msv in the contaminated areas of Belarus, Russia, and Ukraine. In other European countries, both the thyroid and the effective doses are, on average, much smaller. Health Phys. 93(5): ; 2007 * Division of Cancer Epidemiology and Genetics, National Cancer Institute, 6120 Executive Boulevard (EPS 7094), Rockville, MD 20852; Ukrainian Radiation Protection Institute, ATS Ukraine and Scientific Center for Radiation Medicine, Academy of Medical Science of Ukraine, Melnikova 53, Kiev, Ukraine; Republican Scientific and Practical Center of Radiation Medicine and Human Ecology, Minsk, Belarus; State Research Centre Institute of Biophysics, Federal Medical-Biological Agency, 46 Zhivopisnaya Street, Moscow, , Russian Federation; ** International Agency for Research on Cancer; 150 cours Albert Thomas, Lyon, France. For correspondence contact: André Bouville, NCI, 6120 Executive Boulevard, EPS-7094, Bethesda, MD 20892, or bouvilla@mail. nih.gov. (Manuscript accepted 18 June 2007) /07/0 Copyright 2007 Health Physics Society 487 Key words: National Council on Radiation Protection and Measurements; Chernobyl; thyroid; iodine INTRODUCTION A VERY large number of scientific papers, books, reports, and conference proceedings have been published in Russian, English, and other languages on various aspects of the Chernobyl accident and of its consequences. References in the English literature include, for example, the two main reports related to the Chernobyl Forum (IAEA 2006; WHO 2006), two United Nations reports (UNSCEAR 1988, 2000), and many other publications (e.g., Mould 1988; Medvedev 1990; Moberg 1991; Kryshev 1992; Merwin and Balonov 1993; Nagataki 1994; Ilyin 1995; Burlakova 1996; EC 1996; IAEA 1996; Kelly and Shershakov 1996; NEA 1996, 2002; Dainiak et al. 1997; Yamashita and Shibata 1997; Vargo 2000; Yamashita et al. 2002; Vozianov et al. 2003; Alexakhin et al. 2004; Ivanov et al. 2004). Most of the material presented here is based on the four main reports prepared by the United Nations or its agencies (UNSCEAR 1988, 2000; IAEA 2006; WHO 2006), which represent a broad consensus among a variety of experts on the consequences of the Chernobyl accident. Complementary information on current and planned research also is given. The material that is presented in this paper includes estimates of: the activities released into the atmosphere; the activities deposited on the ground; and the individual and average doses received by various population groups. The populations considered are: (1) the approximately 120,000 members of the general public who were evacuated soon after the accident from settlements around the CNPP; (2) the people who resided in the heavily contaminated areas of Belarus, Russia, and

2 488 Health Physics November 2007, Volume 93, Number 5 Ukraine that were not evacuated; and (3) the populations of European countries other than Belarus, Russia, and Ukraine. A large number of radiation measurements (film badges, thermoluminescent dosimeters, wholebody counts, thyroid counts, etc.) were made to evaluate the radiation exposures of the first two population groups that are considered. As the major health effect of the Chernobyl accident is an elevated thyroid cancer incidence in children and adolescents, much attention has been paid to the thyroid doses resulting from intakes of 131 I, which were delivered within 2 mo following the accident. ACTIVITIES RELEASED INTO THE ATMOSPHERE The accident resulted from efforts to conduct a test on an electric control system, which allows power to be provided in the event of a station blackout. A steam explosion blew the core apart and destroyed most of the reactor building. Fuel, core components, and structural items were blown from the reactor hall onto the roof of adjacent buildings and on the ground around the reactor building. A major release of radioactive materials into the environment occurred as a result of this explosion. The period of intense releases lasted about 10 d, after which time the substantial efforts made to control those releases were met with success. Estimated core inventories at the time of the accident, and atmospheric releases of some of the radionuclides are presented in Table 1. From the radiological point of view, the releases of 131 I and 137 Cs, estimated to have been 1,760 and 85 PBq, respectively, are the most important to consider. Table 1. Estimated inventories and releases of some of the radionuclides involved in the Chernobyl accident on 26 April 1986 (UNSCEAR 2000). Radionuclide Radioactive half-life Core inventory (PBq) Activity released (PBq) Noble gases 85 Kr 10.7 y Xe 5.25 d 6,500 6,500 Volatile elements Te 3.26 d 4,200 1, I 8.04 d 3,200 1, I 20.8 h 4, Cs 2.06 y Cs 30.0 y Intermediate Sr 29.1 y Ru 39.3 d 3, Ru 368 d Refractory (including fuel particles) 95 Zr 64.0 d 5, Ce 284 d 3, Pu 24,065 y ACTIVITIES DEPOSITED ON THE GROUND The radionuclides released during the accident deposited with greatest density in the regions surrounding the reactor in the European part of the former Soviet Union. 137 Cs was chosen as reference radionuclide for the ground contamination because of its substantial contribution to the lifetime effective dose, its long radioactive half-life, and its ease of measurement. The three main contaminated areas, defined as those with 137 Cs deposition density 37 kbq m 2 (1 Ci km 2 ), are in Belarus, the Russian Federation, and Ukraine; they have been designated as the Central, Gomel-Mogilev- Bryansk, and Kaluga-Tula-Orel areas (Fig. 1). The Central area is within about 100 km of the reactor, predominantly to the west and northwest. The Gomel- Mogilev-Bryansk contamination area is centered 200 km to the north-northeast of the reactor at the boundary of the Gomel and Mogilev Oblasts of Belarus and of the Bryansk Oblast of the Russian Federation. The Kaluga- Tula-Orel area is located in the Russian Federation, 500 km to the northeast of the reactor. All together, territories from the former Soviet Union with an area of 150,000 km 2 were contaminated. As shown in Table 2, there was also appreciable 137 Cs contamination in ten other European countries, with a total contaminated area of 50,000 km 2. It is generally assumed that radioactive fallout in most of the highly contaminated areas, with the exception of the Central area, resulted from rainfall that occurred during the passage of the radioactive cloud. In the Central area, most of the fallout occurred via dry processes, i.e., in the absence of precipitation. THYROID DOSE ESTIMATION As a result of ground deposition of the radionuclides released during the accident, the pasture grasses and leafy vegetables covering the ground were contaminated. Cows and goats grazing on those pasture grasses transferred some of the contamination to their milk. For most people, the thyroid dose was essentially due to the consumption of fresh cow s milk contaminated with 131 I, minor pathways being the consumption of leafy vegetables and the inhalation of 131 I-contaminated air. Because the thyroid mass increases with age from birth to adulthood by a factor of 10, while the consumption rate of milk does not depend much on age, the average thyroid dose to adults is smaller than that to infants by a factor of 10. The doses to the thyroid were much greater than those to any other organ or tissue of the body; because of the short radioactive half-life of 131 I, the thyroid doses were essentially delivered within a few weeks after the accident.

3 Radiation dosimetry for highly contaminated and less contaminated populations A. BOUVILLE ET AL. 489 Fig. 1. Main contaminated areas in Belarus, the Russian Federation, and Ukraine with 137 Cs deposition density greater than 37 kbq m 2 (shaded in grey). The radiation symbol denotes the location of the CNPP. Although intakes of 131 I were responsible for most of the thyroid dose, there were also minor contributions due to intakes of radionuclides other than 131 I and to external irradiation from a number of radionuclides deposited on the ground. Within a few weeks following the accident, approximately 400,000 of those measurements (called thyroid counts or direct thyroid measurements ) were made in the main contaminated areas. As shown in Fig. 2 for the measurements made in Ukraine, almost all of the direct Thyroid doses from 131 I In Belarus, Russia, and Ukraine, the assessment of the individual thyroid doses from intakes of 131 I is based on the results of measurements of gamma radiation using detectors placed against the neck (Gavrilin et al. 1999). Table 2. Contaminated areas in European countries following the Chernobyl accident (UNSCEAR 2000). Area (km 2 ) in deposition density ranges Country kbq m ,480 kbq m 2 kbq m 2 1,480 kbq m 2 Russian Federation 49,800 5,700 2, Belarus 29,900 10,200 4,200 2,200 Ukraine 37,200 3, Sweden 12,000 Finland 11,500 Austria 8,600 Norway 5,200 Bulgaria 4,800 Switzerland 1,300 Greece 1,200 Slovenia 300 Italy 300 Moldova 60 Fig. 2. Temporal distribution of the number of direct thyroid measurements made in May June 1986, in Ukraine (Likhtarev et al. 2006).

4 490 Health Physics November 2007, Volume 93, Number 5 thyroid measurements were made between 10 and 60 d after the accident, that is, after the short-lived 133 I (half-life: 21 h) and 132 Te (half-life: 3.2 d) had substantially decayed and before 131 I (half-life: 8 d) decayed to negligible levels. The methods used to derive the thyroid dose from intake of 131 I vary according to the information available and the purpose of the dose assessment. For example, the thyroid doses for the entire population of Ukrainian children aged 0 18 y at the time of the accident have been estimated according to a three-level system of thyroid dose calculation (Likhtarev et al. 2006): at the first level, individual doses to the thyroid have been calculated for all persons with direct thyroid measurements, with a distinction made between two categories: (1) the individuals whose residence histories and dietary habits were obtained by means of personal interviews, and (2) the individuals for whom residence history and dietary habits were assumed on the basis of information for the population in the same area of residence at the time of the accident; at the second level of the system of dose estimation, group doses to the thyroid have been estimated for the population that resided at the time of the accident in locations (settlements) where direct thyroid measurements were carried out; and at the third level of the system of dose estimation, group doses to the thyroid have been estimated for the population that resided at the time of the accident in locations where no direct thyroid measurements were carried out. The degree of precision of the thyroid dose estimates decreases from the first to the third level in the system of dose estimation, and is highest for individuals with a direct thyroid measurement and a personal interview on residence history and dietary habits (first level, Category 1). Usually, those individuals were measured only once, so that the thyroid dose rate resulting from the 131 I activity is inferred for only one point in time. To calculate the thyroid dose, which is proportional to the time-integrated activity of 131 I in the thyroid, the variation with time of the 131 I activity has to be assessed. Models of environmental transfer and metabolism of 131 I are used to determine: (1) the relative rate of intake of 131 I, both before and after the measurement, taking into account the information on residence history and dietary habits obtained during the personal interview, and (2) the variation with time of the 131 I activity in the thyroid, taking into account the metabolism of 131 I in the body and its possible modification by the intake of prophylactic stable iodine (Likhtarov et al. 2005). In the framework of epidemiological studies conducted in Belarus and in Ukraine (Stezhko et al. 2004; Hatch et al. 2005; Tronko et al. 2006), the thyroid doses have been calculated in a stochastic mode, in order to provide an estimation of the uncertainties (Likhtarev et al. 2003). The results obtained for the 13,125 Ukrainian subjects (Table 3) illustrate the large variability of the geometric means of the individual thyroid doses, which range from 3 42,000 mgy, and the general decrease of the thyroid dose with increasing age up to 18 y. For a given individual, the probability distribution of the thyroid dose estimates was found to be Table 3. Distribution of the individual thyroid dose estimates (geometric means of the stochastic distributions) for the subjects according to age (Likhtarev et al. 2006). Percentage of subjects with geometric mean thyroid dose (mgy) in interval Age (y) Number of subjects ,000 1,000 5,000 5,000 10,000 10,000 20,000 20, ,

5 Radiation dosimetry for highly contaminated and less contaminated populations A. BOUVILLE ET AL. 491 approximately lognormal, with a geometric standard deviation usually in the range from 1.7 2, although some of the geometric standards deviations were estimated to be 5 (Likhtarev et al. 2006). The largest uncertainties are related to the derivation of the 131 I activity in the thyroid at the time of the direct thyroid measurement and to the variability of the thyroid mass (Likhtarev et al. 2003; Gavrilin et al. 2004). Thyroid doses have been calculated in a less precise manner for individuals with direct thyroid measurements for whom personal information on whereabouts and dietary habits has not been collected (first level, Category 2). In that case, the residence history and dietary habits are assumed on the basis of generic information for the population in the same area of residence at the time of the accident (Gavrilin et al. 2004). At the second level of the system of dose estimation, individual or group doses to the thyroid have been estimated for the population that resided at the time of the accident in locations (settlements) where a sufficient number of direct thyroid measurements were carried out. The estimation of the thyroid doses to the persons without direct thyroid measurement is based on the doses assessed to the persons with direct thyroid measurements. The uncertainties attached to these so-called passport doses are mainly due to the uncertainties on the consumption rates of milk and of other 131 I- contaminated foodstuffs, and to the variability of the thyroid mass when individual doses are considered (Gavrilin et al. 2004). At the third level of the system of dose estimation, individual or group doses to the thyroid are estimated for the population that resided at the time of the accident in locations where few or no direct thyroid measurements were carried out. The models of environmental transfer of 131 I that are used for that purpose can be divided into two classes: empirically-based models (e.g., Gavrilin et al. 1999; Zvonova et al. 2000); and process-driven models (e.g., Vlasov and Pitkevich 1999; Kruk et al. 2004). In that case, the main uncertainties in the thyroid dose estimates are due to the assessment of the 131 I deposition density in the location of interest, the transfer coefficient of 131 I from deposition to cow s milk, and, when individual doses are considered, the variability of the thyroid mass (Likhtarev et al. 2003; Gavrilin et al. 2004). Dose estimates For the large groups of population that are considered in this paper, a mixture of the methods indicated above was used to estimate the thyroid doses resulting from intakes of 131 I. The evacuees and the residents of the contaminated areas are considered in turn: Evacuees: Within a few weeks after the accident, 100,000 persons were evacuated from the most contaminated areas of Belarus and Ukraine. For the entire population of evacuees, the population-weighted average thyroid dose is estimated to be 470 mgy (Table 4). The thyroid doses varied according to the age, place of residence, and date of evacuation of the evacuees. For example, for the residents of Pripyat, who were evacuated essentially within 36 h after the accident, the population-weighted average thyroid dose is estimated to be 370 mgy, and to range from 275 mgy for adults to 1,000 mgy for young children. A detailed study of several tens of Pripyat evacuees who received their thyroid doses from inhalation only (Balonov et al. 2003) shows that taking stable iodine pills as early as possible after the accident led to a reduction in the thyroid dose from 131 Ibya factor of about 6 7; also, people who spent most of the time indoors before their evacuation received, on average, a thyroid dose from 131 I that was about half the corresponding dose received by the people who spent most of their time outdoors. Residents of the contaminated areas of Belarus, Russia, and Ukraine: Thyroid doses also have been estimated for population groups of the contaminated areas of Belarus, Russia, and Ukraine, which were not evacuated, as well as for the entire populations of the Table 4. Estimates of thyroid doses to the evacuees. Mean thyroid dose b (mgy) Population a Size of population b 0 7 y Adults Average Evacuees of 1986, including 119,056 1, Villages in Belarus 24,725 3, ,000 Pripyat town (Ukraine) 48, Chernobyl town (Ukraine) 13, villages in Ukraine 31,991 1, a In addition, 186 people were evacuated in August 1986 from four villages of Bryansk Oblast in the Russian Federation. b The values for Belarus were taken from UNSCEAR (2000). The Ukrainian results were calculated for the purposes of this paper.

6 492 Health Physics November 2007, Volume 93, Number 5 Table 5. Estimates of thyroid doses for the population of Belarus, Russia, and Ukraine. Mean thyroid dose a (mgy) Population Size of population a 0 7 y Adults Average Belarus Entire country 10,000, Gomel Oblast 1,680, Ukraine Entire country 51,000, Kyiv Oblast 1,900, Zhytomyr Oblast 1,500, Chernihiv Oblast 1,400, Kyiv City 2,500, Russian Federation Entire country 150,000,000 2 Bryansk Oblast 1,457, b Kaluga, Orel, Tula Oblasts 4,000, a The values for Belarus and Russia were taken from UNSCEAR (2000). The Ukrainian results were calculated for the purposes of this paper. b Children aged 0 5 y. three countries (Table 5). Average values depend to a large extent on the magnitude of the 131 I ground deposition, which varied greatly from region to region. In the relatively small country of Belarus, where a substantial fraction of the population resides in contaminated areas, the average dose received in Gomel Oblast was 220 mgy, while the country-average was 53 mgy. In Russia, where only a small fraction of the population resides in contaminated areas, the average thyroid dose received in Bryansk Oblast was 41 mgy, while the country-average was 2 mgy. The highest population-weighted average thyroid dose in Ukraine was found to be 81 mgy in Zhytomyr Oblast compared with the country-average thyroid dose of 19 mgy (Table 5). In each of the three Republics, individual thyroid doses exceeding 1,000 mgy have been estimated for the most exposed infants. Populations from other countries: Drozdovitch et al. (in press) have estimated the thyroid doses resulting from intakes of 131 I in almost all of the other European countries. Estimates of country-average thyroid doses for 1-y-old children range from mgy, while the corresponding values for adults are from mgy. Intercomparison of the 131 I dose assessment models Within the framework of a study conducted by International Agency for Research on Cancer (Cardis et al. 2005), an intercomparison study was organized to assess the adequacy of four models: two empiricallybased models (No. 1 and No. 2) and two process-driven models (No. 3 and No. 4). The thyroid doses from 131 I calculated by models were compared with the doses assessed from direct thyroid activity measurements in the settlement (reference doses). Results are shown in Fig. 3 for 1-y-old children. As can be seen in Fig. 3, the empirically-based models (No. 1 and No. 2) show a better agreement with the reference doses than the process-driven models (No. 3 and No. 4). Nevertheless, all models were in satisfactory agreement with the reference doses within the limits of uncertainties. Thyroid doses from radionuclides other than 131 I In addition to the thyroid dose resulting from intake of 131 I, three other contributions to the thyroid dose were quite small for the majority of people but relatively important for some individuals: Fig. 3. Comparison of the average thyroid doses from 131 I that were calculated by each model for different settlements with the doses derived from direct thyroid measurements.

7 Radiation dosimetry for highly contaminated and less contaminated populations A. BOUVILLE ET AL. 493 internal irradiation resulting from intakes of shortlived radioiodines ( 132 I, 133 I, and 135 I) and of short-lived radiotelluriums ( 131m Te and 132 Te); external irradiation resulting from the deposition of radionuclides on the ground and other materials; and internal irradiation resulting from intakes of long-lived radionuclides such as 134 Cs and 137 Cs. The contributions of these exposure pathways to thyroid doses received by subjects of an epidemiologic study of children from Belarus (Astakhova et al. 1998) have been evaluated and presented in two publications (Gavrilin et al. 2004; Minenko et al. 2006). On average over the entire country, the mean contributions to the thyroid doses due to intakes of short-lived radionuclides, external exposure, and intakes of long-lived radiocesiums were estimated to be 2, 1.8, and 0.95%, respectively. In Regions 1, 2, and 3 of Belarus (Fig. 4), where radionuclide deposition was highest, the contributions of radiocesium ingestion and external exposure were generally lower than those of the short-lived radioiodine isotopes and their precursors. In other areas, the contributions of these two pathways were comparable to those of the short-lived radioiodines (Table 6). The above values apply to the usual situation where the thyroid doses were essentially due to the consumption of 131 I-contaminated milk. A much higher contribution of the intake of short-lived radionuclides ( 133 I and 132 I) is obtained for the people who inhaled radioiodines soon after the accident and were then removed from the contaminated areas, as was the case for the Pripyat evacuees who were analyzed by Balonov et al. (2003): for the group of persons who did not employ stable iodine prophylaxis (oral intake of KI tablets), the mean contribution of 132 I to the thyroid dose is estimated to have been about 9% and that of 133 I 21%. In total, 30% of the internal thyroid dose to persons not taking KI came from short-lived radioiodines; while for the group of persons who took KI tablets on April, the contributions of short-lived radioiodines were significantly higher, e.g., 40% from 132 I and 14% from 133 I. Thus, more than half of the internal thyroid dose originated from short-lived radioiodines in that group. One should note, however, that the intake of KI tablets reduced the committed thyroid dose from 131 Iby an order of magnitude and the total thyroid dose from 131 I, Fig. 4. Location of the 10 regions in Belarus with relatively homogeneous radioactive fallout from the Chernobyl accident (Gavrilin et al. 2004).

8 494 Health Physics November 2007, Volume 93, Number 5 Table 6. Total thyroid dose estimates for subjects and contributions of secondary sources of radiation dose (Minenko et al. 2006). Percentages of total thyroid dose from sources other than 131 I Total thyroid dose (mgy) Short-lived iodines b 137 Cs, 134 Cs ingestion External exposure Region in Belarus a Median Range Median Range Median Range Median Range 1 2, , , , a The locations of the 10 regions are shown in Fig. 4. In each region, the radionuclide mix in the activity deposited on the ground is assumed to be homogeneous. b Includes 132 I, 133 I, 135 I, and precursor isotopes 131m Te and 132 Te. 132 I, and 133 I by a factor of about five, compared with the thyroid dose received by the Pripyat residents who did not take KI (Balonov et al. 2003). Current research Current research on the assessment of thyroid doses resulting from the Chernobyl accident focuses mainly on the improvement of the quality of the individual doses required in epidemiological studies carried out in Belarus, Russia, and Ukraine. This includes: a careful processing of the direct thyroid measurements: under some circumstances, the signal read by the detector was not essentially due to the presence of 131 I in the thyroid, but included non-negligible contributions from the contamination of skin and clothes, from gamma-emitter radionuclides in the body, and from the room background. A measurement against the shoulder or the liver of the body was often, but not always, made to account for those sources of background. Another difficulty was that correct procedures were not systematically applied when the direct thyroid measurements were made. Those problems arose mainly in Belarus, where a thorough reevaluation of the direct thyroid measurements is in progress; a detailed analysis of the available information on the thyroid masses: there is a varying degree of iodine deficiency in the regions of Belarus and Ukraine that were affected by the Chernobyl accident, thus implying that the average thyroid masses in a given age group varied from one region to another. However, the calculations of the thyroid dose have so far been made for the populations of all regions using the agedependent reference values recommended by the ICRP (1989). Because the thyroid dose is inversely proportional to the thyroid mass, it is important to determine with as much accuracy as possible the thyroid masses of individuals with direct thyroid measurements. However, there is some evidence (Zvonova 1989) that the thyroid dose to adults per unit intake of 131 I is relatively independent of the value of the thyroid mass, as a higher or lower thyroid mass is compensated by a corresponding higher or lower thyroidal uptake; therefore, for the children without direct thyroid measurements, it is possible that the thyroid dose is independent of the degree of iodine deficiency; the derivation of 131 I concentrations from total beta activities in milk: in Belarus, Russia, and Ukraine, a large number of milk samples were analyzed for their total beta activities. In recent years, efforts have been made in Belarus and in Russia to derive the 131 I concentrations in these milk samples from the total beta activities (e.g., Savkin et al. 2004). It seems, however, that the records of total beta activities for the main cities of Belarus that were affected by the accident (Minsk, Gomel, and Mogilev) are not available; for those cities, the 131 I concentrations in commercial milk will have to be inferred from the knowledge of the milk distribution network, which is limited; and efforts to reconstruct the pattern of 131 I deposition on the ground: the assessment of the 131 I deposition density on the ground is usually derived from the large number of available 137 Cs measurements and from the 131 I/ 137 Cs ratios that were measured in a limited number of locations. Because the 131 I/ 137 Cs ratios were found to have a wide variability, depending mainly on the variation with time of the 131 I/ 137 Cs ratio in the activity released from the reactor and on the type of deposition, the 131 I ground deposition densities, as well as the kinetics of deposition, are highly uncertain. For that reason, a

9 Radiation dosimetry for highly contaminated and less contaminated populations A. BOUVILLE ET AL. 495 model of atmospheric transport and deposition of 131 I was developed in Ukraine (Talerko 2005). WHOLE-BODY DOSE ESTIMATION Following the first few weeks after the accident when 131 I was the main contributor to the radiation exposures, doses were delivered at much lower dose rates by radionuclides with much longer half-lives. There was a transition period of a few months during the summer of 1986, when radionuclides with intermediate half-lives such as 95 Zr, 95 Nb, 103 Ru, 106 Ru, 141 Ce, and 144 Ce contributed to a varying extent to the doses from external irradiation. Since 1987, the doses received by the populations from the contaminated areas have resulted essentially from external exposure from 134 Cs and 137 Cs deposited on the ground and internal exposure due to contamination of foodstuffs by the same radionuclides. Other, usually minor, contributions to the long-term radiation exposures include the consumption of foodstuffs contaminated with 90 Sr and the inhalation of aerosols containing 239 Pu. Both external irradiation and internal irradiation due to 134 Cs and 137 Cs result in relatively uniform doses in all organs and tissues of the body. These doses from the Chernobyl accident have been reported either as whole-body doses (in milligrays) or as effective doses (in millisieverts) and, by convention, do not include the thyroid doses mainly due to 131 I intakes that were received during the first few weeks after the accident. For the purposes of this paper, effective doses, expressed in millisieverts, are considered to be numerically equal to whole-body doses, expressed in milligrays. Whole-body doses from external irradiation The basic model for the estimation of the wholebody doses from external irradiation resulting from activities deposited on the ground is the model for exposure above an open plot of undisturbed soil: the absorbed dose rate in air at time t at a height of 1 m above the soil surface is used to describe the radiation field. The value of this absorbed dose rate depends on the deposition densities of the different radionuclides, and also on such natural factors as the initial penetration of radionuclides in soil and their radioactive decay, vertical migration of long-lived radionuclides, and the presence of snow cover (IAEA 2006); the reference value of the absorbed dose rate in air is then modified to take into account the influence of the altered environment in the actual locations where people are exposed, whether indoors or outdoors; and finally, an age-dependent conversion coefficient is applied to determine the effective, or whole-body, dose from the absorbed dose in air. The measurement of monthly individual doses by means of thermoluminescent dosimeters, which were distributed to inhabitants of the most contaminated areas (e.g., Golikov et al. 1999), provided confirmation of the validity of the method of dose estimation. The spatial variation of the distribution, relative to 137 Cs, of the radionuclides deposited on the ground is Table 7. Radionuclide activities relative to 137 Cs activity used in calculations of thyroid doses from external exposure (Minenko et al. 2006). Radionuclide activity relative to 137 Cs activity at the time of the main deposition in the region Radionuclide a 4 5 a 6 a 7 8 a Zr Nb eq b eq eq eq eq eq eq eq eq eq 99 Mo Ru Ru m Te Te I I eq eq eq eq eq eq eq eq eq eq 133 I I Cs Cs Cs Ba La eq eq eq eq eq eq eq eq eq eq 141 Ce Ce Np a Deposition in this zone was primarily by wet processes. b eq denotes that the radionuclide activity was assumed to be in equilibrium with the precursor radionuclide.

10 496 Health Physics November 2007, Volume 93, Number 5 illustrated in Table 7 for different regions of Belarus (Fig. 4). Isotopes of refractory elements, like cerium and zirconium, were much more abundant in the 30 km zone around the Chernobyl reactor site (indicated as Region 1 in Table 7), which is the closest to the reactor site, than in the city of Minsk (indicated as Region 8 in Table 7), which is far from the reactor site, as the refractory elements became progressively depleted, in comparison to 137 Cs, along the path of the radioactive cloud. A similar result was obtained for short-lived radionuclides like 133 I and 135 I, due to radioactive decay. The variation with time of the dose rate from external irradiation, normalized to a unit deposition density of 137 Cs, is presented for the same Regions 1 and 8 of Belarus in Fig. 5. As expected, the normalized dose rate is higher in Region 1 than in Region 8 because of the greater contributions of the isotopes of refractory radionuclides and of the short-lived radionuclides in Region 1. Dose estimates The whole-body, or effective, dose estimates from external irradiation that have been reported for various population groups are as follows: Evacuees: the estimated arithmetic mean effective doses are estimated to be 30 msv for the Belarusian evacuees and 20 msv for the Ukrainian evacuees (UNSCEAR 2000). Those values are at least 10 times smaller than the corresponding thyroid doses. Residents of the contaminated areas of Belarus, Russia, and Ukraine: whole-body or effective doses from external irradiation also have been estimated for the populations of the contaminated areas of Belarus, Russia, and Ukraine that were not evacuated. Average doses for the time period from are 5 msv in Belarus, 4 msv in Russia, and 8 msv in Ukraine. For residents of a given locality, doses are greater for individuals who spent most of their time outdoors in undisturbed areas, such as forests and uncultivated fields; doses are also greater by a factor of in rural areas than in urban areas. There is also a slight age dependency as the dose conversion coefficients are 30% greater for young children than for adults. Because doses have been delivered at a varying rate since 1986, and will continue to be delivered in the next several decades, it is of interest to compare the doses in relative terms over different time periods: typically 25% of the lifetime effective dose (defined as being the dose delivered from ) will have been caused Fig. 5. Normalized dose rate from external exposure to 137 Cs and to all radionuclides in two regions with predominantly wet and dry deposition (Minenko et al. 2006).

11 Radiation dosimetry for highly contaminated and less contaminated populations A. BOUVILLE ET AL. 497 Table 8. Measured 137 Cs body burdens for adults in the Belarusian population (Minenko et al. 2006). 137 Cs body-burden (kbq) a,b Year Cities c Towns Rural settlements 1986 No estimate (367) (3,328) (16) (117) (4,498) (20) (606) (3,266) (1,405) (5,283) (10,652) (3,551) (5,989) (8,232) (11,458) (2,994) (4,617) (7,155) (5,264) (5,921) (1,266) (4,679) (5,414) (2,001) (5,488) (5,098) (530) (361) (392) a Values of the mean the standard deviation of all results for the year. b Number of measurements shown in parentheses. c Cities (Gomel, Mogilev, and Mozyr) are located in areas with 137 Cs soil contamination 37 kbq m 2. by the radiation exposure during 1986; corresponding values for , , and are 40%, 15%, and 20%, respectively (IAEA 2006). In other words, about two-thirds of the lifetime effective dose had been delivered in 1995 and only one-fifth remains to be delivered in the next 50 y. Populations from other countries: Drozdovitch et al. (2007) have estimated the effective doses resulting from both external and internal irradiation for the populations of almost all other European countries. Estimates of country-average effective doses for the time period range from msv. Whole-body doses from internal irradiation Whole-body doses from internal irradiation result mainly from the ingestion of foodstuffs contaminated with 134 Cs and 137 Cs. During the first few weeks after the accident, the contamination of the vegetation was due to direct deposition of the surfaces above the ground. Later on, contamination of the vegetation, and, consequently, of milk and meat, resulted from root absorption. The whole-body doses from internal irradiation are the products of: (1) the location- and time-dependent concentrations of 134 Cs and 137 Cs in the foodstuffs of interest, which are essentially milk, meat, and potatoes; (2) the consumption rates of those foodstuffs; and (3) the conversion coefficients from intake to effective doses for 134 Cs and 137 Cs. The large numbers of measurements of radiocesium contents in the body and of radiocesium concentrations in milk are very helpful to estimate the whole-body doses with a reasonable degree of variability and to explain the spatial and temporal variabilities of the intakes of radiocesium. For example, it is shown in Table 8 that the 137 Cs body burdens in Belarus that were measured between 1987 and 1995 are systematically higher in rural areas than in towns and in cities, reflecting the larger consumption rate of milk in rural areas and the effect of the limitation of the contamination of commercial milk (Minenko et al. 2006). Following the first few weeks after the accident, the concentrations of 134 Cs and 137 Cs in milk and other foodstuffs showed a wide variability, depending on the type of soil. For most types of soil, the concentrations in milk, however, decreased rapidly with time in 1986 and have decreased much more slowly since However, forest products such as wild game, mushrooms, and berries do not show a rapid decrease with time. On average, the consumption rate of milk is greater than that of any other foodstuff. The comparison of the measured whole-body burdens of radiocesium and of the concentrations in milk showed that there was a voluntary reduction in the consumption rate of contaminated milk after the accident. Forest products are usually consumed in small amounts and do not greatly influence the average dose over a population group. The conversion coefficients from intake to effective dose are, as a first approximation, age independent for the long-lived radiocesiums, as the shorter residence time in the body of children, as compared to adults, is compensated with a smaller body mass. The variation with time of the cumulative wholebody dose from internal irradiation, normalized to a unit ground deposition density of 137 Cs, is illustrated in Fig. 6; the influence of the transfer coefficient from ground deposition density to milk concentration and of the age of the population groups is clearly shown (Minenko et al. 2006). Dose estimates The whole-body, or effective, dose estimates from internal irradiation that have been reported for various population groups are as follows:

12 498 Health Physics November 2007, Volume 93, Number 5 Fig. 6. Normalized cumulative doses from ingestion of radiocesiums by children in areas with intermediate [ (Bq L 1 ) per (kbq m 2 )] and high [ 1.0 (Bq L 1 ) per (kbq m 2 )] soil-to-milk transfer factors (Minenko et al. 2006). Evacuees: the arithmetic mean effective doses are estimated to be about 6 msv for the Belarusian evacuees and 14 msv for the Ukrainian evacuees (UNSCEAR 2000). Those values are at least two times smaller than the corresponding effective doses from external irradiation. Residents of the contaminated areas of Belarus, Russia, and Ukraine: effective doses from internal irradiation also have been estimated for the populations of the contaminated areas of Belarus, Russia, and Ukraine that were not evacuated. Average doses for the time period from 1986 to 1995 are 3 msv in Belarus, 2.5 msv in Russia, and 4 msv in Ukraine (UNSCEAR 2000). These values are somewhat smaller than the corresponding values for external irradiation that were obtained for residents of the contaminated areas in Belarus, Russia, and Ukraine. As was the case for external irradiation, it is of interest to compare the doses from internal irradiation in relative terms over different time periods: for most types of soil, 30 to 60% of the lifetime effective dose, for the time period, will have been caused by the radiation exposure during 1986, and about 95% of the dose had been delivered by 2005; however, in peat bogs, only 8% of the lifetime dose was delivered in 1986 and 12% remained to be delivered in the next 50 y (IAEA 2006). In other words, in comparison to external irradiation, more of the dose from internal irradiation was delivered in 1986 and less remains to be delivered in the future. The absolute values of the effective doses from both external and internal irradiation vary in a relatively wide range, but it is estimated that only about 100,000 persons, out of the approximately five million people residing in the contaminated areas, currently receive 1 msv annually (IAEA 2006). Populations from other countries: as indicated above, Drozdovitch et al. (2007) have estimated the effective doses resulting from both external and internal irradiation for the populations of almost all other European countries. Estimates of country-average whole-body doses for the time period range from msv. SUMMARY OF DOSE ESTIMATES The average thyroid and effective dose estimates for the population groups that have been considered are summarized in Table 9. The thyroid doses were received

13 Radiation dosimetry for highly contaminated and less contaminated populations A. BOUVILLE ET AL. 499 Table 9. Summary of average dose estimates for several population groups (UNSCEAR 2000; IAEA 2006; Drozdovitch et al. 2007). Belarus Russia Ukraine a Other European countries Evacuees: Population (millions) Thyroid dose (mgy) 1,000 NC b 331 NA c Effective dose (msv) 36 NC 30 NA Contaminated areas: Population (millions) NC Thyroid dose (mgy) 220 d 41 e 143 NC Effective dose f (msv) NC Entire country: Population (millions) Thyroid dose (mgy) Effective dose f (msv) NC NC NC a The values for Ukraine were calculated for the purposes of this paper. b NC Not considered. c NA Not applicable. d Gomel Oblast, with a population of 1.7 million. e Bryansk Oblast, with a population of 1.5 million. f Effective dose for the time period. essentially during the first few weeks after the accident, whereas the effective doses, calculated here for the time period, have been delivered at a low rate and will continue to be delivered, at an even lower rate, over the next several decades. Although the two sets of dose estimates (thyroid and effective) usually were not calculated for exactly the same population groups, it is clear that the thyroid dose was, in general, much greater than the effective dose. CONCLUSION Very large quantities of radioactive materials were released during the Chernobyl accident. Consequently, vast areas of Belarus, Russia, and Ukraine were contaminated and radionuclides were detected in practically every country of the northern hemisphere. From the radiological point of view, the most important radionuclides were 131 I, which delivered doses to the thyroid during the first few weeks after the accident, and two long-lived isotopes of cesium, 134 Cs and 137 Cs, which are responsible for the whole-body, or effective, doses that have been delivered at low rate to the population and that will continue to be delivered during the next several decades. The thyroid doses were mainly due to the consumption of 131 I-contaminated milk. The dose estimation is based on 400,000 direct thyroid measurements that were conducted in Belarus, Russia, and Ukraine. The thyroid doses were found to vary in a wide range according to age, location of residence, and milk consumption rate. Average thyroid doses to large population groups range from a few to 1,000 mgy. In addition to the consumption of 131 I-contaminated milk, usually small contributions to the thyroid dose were caused by the consumption of 131 I-contaminated leafy vegetables, by the inhalation of 131 I-contaminated air, as well as by intakes of short- ( 133 I and 132 Te) and long-lived ( 134 Cs and 137 Cs) radionuclides, and by external irradiation from radionuclides deposited on the ground. The effective doses arise in approximately equal parts from external and internal irradiation. By convention, the effective doses exclude the thyroid doses that were received soon after the accident. The determination of the effective doses from internal irradiation was greatly facilitated by the numerous measurements of 137 Cs in milk, in other foodstuffs, and in the human body, while the measurement of external exposures by means of personal dosimeters was very helpful to identify the main factors influencing the variability of the external doses. The effective doses, averaged over population groups, are typically found to be times smaller than the thyroid doses. In the contaminated areas of Belarus, Russia, and Ukraine, they are 10 msv on average for the time period. Because doses have been delivered at a varying rate since 1986, and will continue to be delivered in the next several decades, it is of interest to compare the doses in relative terms over different time periods. With regard to external irradiation, typically 25% of the lifetime effective dose, taken to correspond to the time period, will have been caused by the radiation exposure during 1986; corresponding values for , , and are 40%, 15%, and 20%, respectively. In comparison to external irradiation, more of the dose from internal irradiation was delivered in 1986, and less

14 500 Health Physics November 2007, Volume 93, Number 5 remains to be delivered in the future. Consequently, more than 80% of the lifetime dose has been delivered by 2005, and 20% remains to be delivered, at a low rate, over the next 50 y. The absolute values of the effective doses from both external and internal irradiation vary in a relatively wide range, but it is estimated that only 100,000 persons, out of the 5 million people residing in the contaminated areas, currently receive 1 msv annually. REFERENCES Alexakhin RM, Buldakov LA, Gubanov VA, Drozhko YeG, Ilyin LA, Kryshev II, Linge II, Romanov GN, Savkin MN, Saurov MM, Tikhomirov FA, Kholina YuB. Large radiation accidents: consequences and protective countermeasures. Moscow: Izdat Publisher; Astakhova LN, Anspaugh LR, Beebe GW, Bouville A, Drozdovitch VV, Garber G, Gavrilin YI, Khrouch VT, Kuvshinnikov AV, Kuzmenkov YN, Minenko VP, Moshchik KF, Nalivko AS, Robbins J, Shemiakina EV, Shinkarev SM, Tochitskaya SI, Waclawiw MA. Chernobyl-related thyroid cancer in children of Belarus: a case-control study. Radiat Res 150: ; Balonov M, Kaidanovsky G, Zvonova I, Kovtun A, Bouville A, Luckyanov N, Voillequé P. Contribution of short-lived radioiodines to thyroid doses received by evacuees from the Chernobyl area estimated using early in vivo activity measurements. Radiat Protect Dosim 105: ; Burlakova EB. Consequences of the Chernobyl catastrophe: human health. Moscow: Center for Russian Environmental Policy; Cardis E, Kesminiene A, Ivanov V, Malakhova I, Shibata Y, Khrouch V, Drozdovitch V, Maceika E, Zvonova I, Vlasov O, Bouville A, Goulko G, Hoshi M, Abrosimov A, Anoshko YA, Astakhova L, Chekin S, Demidchik E, Galanti R, Ito M, Korobova E, Lushnikov E, Maksiutov M, Masyakin V, Nerovnia A, Parshin V, Parshkov E, Piliptsevich N, Pinchera A, Polyakov S, Shabeka N, Suonio E, Tenet V, Tsyb A, Yamashita S, Williams D. Risk of thyroid cancer following 131 I exposure in childhood. J Natl Cancer Inst 97: ; Dainiak N, Schull WJ, Karkanitsa L, Aleinikova OA. Radiation injury and the Chernobyl catastrophe. Miamisburg, OH: AlphaMed Press; Drozdovitch V, Bouville A, Chobanova N, Filistovic V, Ilus T, Kovacic M, Malatova I, Moser M, Nedveckaite T, Volkle H, Cardis E. Radiation exposure to the population of Europe following the Chernobyl accident. Radiat Protect Dosim 123: ; European Commission and the Belarus, Russian and Ukrainian Ministries on Chernobyl Affairs, Emergency Situations and Health. The radiological consequences of the Chernobyl accident. Proceedings of the First International Conference. Luxembourg: European Commission; Report EN 16544; Gavrilin YuI, Khrouch VT, Shinkarev SM, Krysenko NA, Skryabin AM, Bouville A, Anspaugh L, Straume T. Chernobyl accident: reconstruction of thyroid dose for inhabitants of the Republic of Belarus. Health Phys 76: ; Gavrilin Y, Khrouch V, Shinkarev S, Drozdovitch V, Minenko V, Shemiakhina E, Ulanovsky A, Bouville A, Anspaugh L, Voillequé P, Luckyanov N. Individual thyroid dose estimation for a case-control study of Chernobyl-related thyroid cancer among children of Belarus. Part 1: 131 I, short-lived radioiodines ( 132 I, 133 I, 135 I), and short-lived radiotelluriums ( 131m Te and 132 Te). Health Phys 86: ; Golikov V, Balonov M, Erkin V, Jacob P. Model validation for external doses due to environmental contaminations by the Chernobyl accident. Health Phys 77: ; Hatch M, Ron E, Bouville A, Zablotska L, Howe G. Cancer following the accident at the Chernobyl Nuclear Power Plant. Epidemiol Rev 27:56 66; Ilyin LA. Chernobyl: myth and reality. Moscow: Megapolis; International Atomic Energy Agency. One decade after Chernobyl. Summing up the consequences of the accident. Proceedings of an International Conference. Vienna: IAEA; IAEA/STI/PUB 1001; International Atomic Energy Agency. Environmental consequences of the Chernobyl accident and their remediation: twenty years of experience. Report of the Chernobyl Forum Expert Group Environment. Vienna: IAEA; International Commission on Radiological Protection. Agedependent doses to members of the public from intake of radionuclides. New York: Elsevier Science; ICRP Publication 56, Part 1, Ann. ICRP 20(2); Ivanov V, Tsyb A, Ivanov S, Pokrovsky V. Medical radiological consequences of the Chernobyl catastrophe in Russia. Estimation of radiation risks. St. Petersburg: NAUKA; Kelly GN, Shershakov VM. Environmental contamination, radiation doses and health consequences after the Chernobyl accident (radiation protection dosimetry). Ashford, UK: Ramtrans Publishing; Kruk JE, Prohl G, Kenigsberg JL. A radioecological model for thyroid dose reconstruction of the Belarus population following the Chernobyl accident. Radiat Environ Biophys 43: ; Kryshev II. Radioecological consequences of the Chernobyl accident. Moscow: Nuclear Society International; Likhtarev I, Minenko V, Khrouch V, Bouville A. Uncertainties in thyroid dose reconstruction after Chernobyl. Radiat Protect Dosim 105: ; Likhtarev I, Bouville A, Kovgan L, Luckyanov N, Voillequé P, Chepurny M. Questionnaire- and measurement-based individual thyroid doses in Ukraine resulting from the Chornobyl nuclear reactor accident. Radiat Res 166: ; Likhtarov I, Kovgan L, Vavilov S, Chepurny M, Bouville A, Luckyanov N, Jacob P, Voillequé P. Post-Chernobyl thyroid cancers in Ukraine. Part 1. Estimation of thyroid doses. Radiat Res 163: ; Medvedev Z. The legacy of Chernobyl. New York: W.W. Norton & Company; Merwin S, Balonov M. The Chernobyl papers. Volume I: Doses to the Soviet population and early health effects studies. Richland, WA: Research Enterprises; Minenko VF, Ulanovsky AV, Drozdovitch VV, Shemiakina EV, Gavrilin YI, Khrouch VT, Shinkarev SS, Voillequé PG, Bouville A, Anspaugh LR, Luckyanov N. Individual thyroid dose estimates for a case-control study of Chernobylrelated thyroid cancer among children of Belarus. Part II. Contributions from long-lived radionuclides and external radiation. Health Phys 90: ; Moberg L. The Chernobyl fallout in Sweden. Results from a research programme on environmental radiology. Stockholm: Swedish Radiation Protection Institute; 1991.