Chapter IV
Dose estimates*

Conclusions
(Conclusions will open in a pop-up window)

The exposure of the population as a result of the accident resulted in two main pathways of exposure. The first is the radiation dose to the thyroid as a result of the concentration of radioiodine and similar radionuclides in the gland. The second is the whole-body dose caused largely by external irradiation mainly from radiocesium.

The absorbed dose to the whole body is thought to be about 20 times more deleterious, in terms of late health effects incidence, than the same dose to the thyroid (IC90).

The population exposed to radiation following the Chernobyl accident can be divided into four categories: (1) the staff of the nuclear power plant and workers who participated in clean-up operations (referred to as "liquidators"); (2) the nearby residents who were evacuated from the 30-km zone during the first two weeks after the accident; (3) the population of the former Soviet Union, including especially the residents of contaminated areas; and (4) the population in countries outside the former Soviet Union.

A number of liquidators, estimated at up to 600 000 (civilian and military according to laws promulgated in Belarus, the Russian Federation and Ukraine) took part in mitigation activities at the reactor and within the 30-km zone surrounding the reactor. The most exposed workers were the firemen and the power plant personnel during the first days of the accident. Most of the dose received by the workers resulted from external irradiation from the fuel fragments and radioactive particles deposited on various surfaces. Of particular interest are the 226 000 recovery operation workers who were employed in the 30-km zone in 1986-1987, as it is in this period that the highest doses were received. The remainder of the recovery operation workers, who generally received lower doses, amounted to about 400 000 (UN00).

About 116 000 people were evacuated during the first days following the accident, mainly from the 30-km zone surrounding the reactor. Prior to evacuation, those individuals were exposed to external irradiation from radioactive materials transported by the cloud and deposited on the ground, as well as to internal irradiation essentially due to the inhalation of radioactive materials in the cloud.

The relative contributions to the external whole-body dose from the main radionuclides of concern for that pathway of exposure and during the first few months after the accident are shown in Figure 8. It is clear that 132Te played a major role in the first week after the accident, and that, after one month, the radiocaesiums (134Cs and 137Cs) became predominant. Subsequently, however, 134Cs decayed to levels much lower than those of 137Cs, which became after a few years the only radionuclide of importance for practical purposes. It is usual to refer to 137Cs only, even when the mix of 134Cs and 137Cs is meant, because the values for the constituents can be easily derived from those for 137Cs.

With regard to internal doses from inhalation and ingestion of radionuclides, the situation is similar: radioiodine (¹³¹I) was important during the first few weeks after the accident and gave rise to thyroid doses via inhalation of contaminated air, and, more importantly, via consumption of contaminated foodstuffs, mainly cow's milk. After about one month, the radiocaesiums (134Cs and 137Cs) again became predominant, and, after a few years, 137Cs became the only radionuclide of importance for practical purposes, even though 90Sr may in the future play a significant role at short distances from the reactor.

Among the population of the former Soviet Union, it is usual to single out the residents of the contaminated areas, defined as those with 137Cs deposition levels greater than 37 kBq/m2. About 5 million people live in such areas. Of special interest are the inhabitants of the spots with 137Cs deposition levels greater than 555 kBq/m2. In those areas, called "strict control zones", protection measures are applied, especially as far as control of consumption of contaminated food is concerned. In 1998, 42 554 measurements were performed in the Federal Republic of Russia, and the national authorities are planning to maintain such controls beyond 2000 (Bo99). In 1986, shortly after the accident, the All-Union Dose Registry (AUDR) was set up by the Soviet Government to record medical and dosimetric data on the population groups expected to be the most exposed: (1) the liquidators, (2) the evacuees from the 30-km zone, (3) the inhabitants of the contaminated areas, and (4) the children of those people. In 1991, the AUDR contained data on 659 292 persons. Starting from 1992, national registries in Belarus, Russian Federation, and Ukraine replaced the AUDR.

Outside the former Soviet Union, the radionuclides of importance are, again, the radioiodines and radiocaesiums, which, once deposited on the ground, give rise to doses from ingestion through the consumption of foodstuffs. Deposited radiocaesium is also a source of long-term exposure from external irradiation from the contaminated ground and other surfaces. Most of the population of the Northern hemisphere was exposed, in varying degrees, to radiation from the Chernobyl accident. The 137Cs deposition outside the former Soviet Union ranged from negligible levels to about 50 kBq/m2.

The liquidators

Most of the liquidators can be divided into two groups: (1) the people who were working at the Chernobyl power station at the time of the accident viz. the staff of the station and the firemen and people who went to the aid of the victims. They number a few hundred persons; and (2) the workers, estimated to amount up to 600 000, who were active in 1986-1990 at the power station or in the zone surrounding it for the decontamination, sarcophagus construction and other recovery operations.

On the night of 26 April 1986, about 400 workers were on the site of the Chernobyl power plant. As a consequence of the accident, they were subjected to the combined effect of radiation from several sources: (1) external gamma/beta radiation from the radioactive cloud, the fragments of the damaged reactor core scattered over the site and the radioactive particles deposited on the skin, and (2) inhalation of radioactive particles (UN88).

All of the dosimeters worn by the workers were over-exposed and did not allow an estimate of the doses received. However, information is available on the doses received by the 237 persons who were placed in hospitals and diagnosed as suffering from acute radiation syndrome. Using biological dosimetry, it was estimated that 41 of these patients received whole-body doses from external irradiation in the range 1-2 Sv, that 50 received doses between 2 and 4 Sv, that 22 received between 4 and 6 Sv, and that the remaining 21 received doses between 6 and 16 Sv. In addition, it was estimated from thyroid measurements that the thyroid dose from inhalation would range up to about 20 Gy, with 173 individuals in the 0-1.2 Gy range and seven workers with thyroid doses greater than 11 Gy (UN88). Internal exposure of those workers was mainly due to ¹³¹I and shorter-lived radioiodines, the median value of the ratio of the internal thyroid dose to the external effective dose was estimated to be 0.3 Gy per Sv. The doses resulting from intakes of other radionuclides was estimated to about 30 mSv for the early months following the accident and 85 mSv for committed dose (UN00).

The second category of liquidators consists of the large number of adults who were recruited to assist in the clean-up operations. They worked at the site, in towns, forests and agricultural areas to make them fit to work and live in. About 600 000 of individuals participated in this work. Initially, about 240 000 of those workers came from the Soviet armed forces, the other half including personnel of civil organisations, the security service, the Ministry of Internal Affairs, and other organisations. The total number of liquidators has yet to be determined accurately, since only some of the data from some of those organisations have been collected so far in the national registries of Belarus, Russia, Ukraine and other republics of the former Soviet Union (So95). Also, it has been suggested that, because of the social and economic advantages associated with being designated a liquidator, many persons have contrived latterly to have their names added to the list. To day the total number of recovery operation workers recorded in the registries appears to be about 400 000 well below the figure of 600 000, which corresponds to the number of people who have received special certificates confirming their status as liquidators. The workers were all adults, mostly males aged 20-45 years.

There are only fragmented data on the doses received by these liquidators. Attempts to establish a dosimetric service were inadequate until the middle of June of 1986, until then, doses were estimated from area radiation measurements. The doses to the recovery operation workers who participated in mitigation activities within two months after the accident are not known with much certainty. The liquidators were initially subjected to a radiation dose limit for one year of 250 mSv. In 1987 this limit was reduced to 100 mSv and in 1988 to 50 mSv (Ba93). The registry data show that the average recorded doses in the three national registries decreased from year to year, being about 170 mSv in 1986, 130 mSv in 1987, 30 mSv in 1988 and 15 mSv in 1989 (Se95a). It is, however, difficult to assess the validity of the results as they have been reported since these statistics indicates that the dose is known for only 52% of workers for the period 1986-1989, with a lower percentage, 45% for the first year. Moreover uncertainties associated with dose estimations are assessed to be up to 50% for individual dosimetry, (if the dosimeter was correctly used), up to a factor 3 to 5 for group and time-and-motion dosimetry However, the doses do not seem to have been systematically overestimated, because biological dosimetry performed on limited number of workers produced results compatible with physical dose estimates.

It is interesting to note that a small special group of 672 scientists from the Kurchatov Institute who have worked periodically inside the sarcophagus for a number of years have initially estimated accumulated whole-body doses in the range 0.5 to 13 Gy (Se95a). These dose estimates had been reestimated. Recorded and calculated doses available for 501 workers show that more than 20% of them received doses between 0.05 and 0.25 Sv and about 5% of them received doses between 0.25 and 1.5 Sv (Sh97) Additional analysis by mean of FISH technique for three of them resulted in doses 0.9, 2.0 and 2.7 Sv (Sh00) While no deterministic effects have been noted to date, this group may well show radiation health effects in the future.

More than sixteen years after the accident, comparisons between the different techniques of dosimetry confirm the effectiveness of the chromosome aberration technique, but indicate that some new methods recommended by some scientists, such as fluorescent in situ hybridisation (FISH), do not appear to be a sufficiently sensitive or specific to allow the estimation of doses for the majority of recovery operation workers (Li98).

The evacuees from the 30-km zone

Immediately after the accident monitoring of the environment was started by gamma dose rate measurements. About 20 hours after the accident the wind turned in the direction of Pripyat, gamma dose rates increased significantly in the town, and it was decided to evacuate the inhabitants. About 20 hours later the 49 000 inhabitants of Pripyat had left the town in nearly 1 200 buses. About a further 67 000 people were evacuated in the following days and weeks (in fact, until September) from the contaminated areas (a number of 86 000 people given in the NEA's 1996 report (NE95b) was not substantiated).

Information relevant for the assessment of the doses received by these people have been obtained by 30 000 responses of the evacuees to questionnaires about the location where they stayed, the types of houses in which they lived, the consumption of stable iodine, and other activities (Li94). The average effective dose from external irradiation was estimated to be 17 mSv, with individual values varying from 0.1 to 380 mSv (Li94). This value is concordant with the absorbed dose of 20 mGy estimated by Electron Spin resonance (ESR) measurements of sugar and exposure rate calculations (Na94).

The main source of uncertainty in the estimation of the average effective doses from external irradiation is the assessment of the activity ratios of 132Te and ¹³¹I to 137Cs in the deposition.

The iodine activity in thyroid glands of evacuees was measured. More than about 5 000 measurements of former inhabitants of Pripyat had sufficient quality to be useful for dose reconstruction (Go95a). A comparative analysis with the questionnaire responses of 10 000 evacuees showed that thyroid doses were mainly due to inhalation of ¹³¹I. Average individual doses and collective doses to the thyroid are shown in Table 6 for three age groups. Individual doses in the age classes were distributed over two orders of magnitude. The main factor influencing the individual doses was found to be the distance of the residence from the reactor.

Table 6. Average doses to the thyroid gland and collective thyroid doses to the evacuees from Pripyat (Go95a)

Assessments of the doses to the thyroid gland of the evacuees from the

30-km zone (Li93a) showed similar doses for young children as those for the Pripyat evacuees. Exposures to adults were higher. These high doses were due to a greater consumption of food contaminated with ¹³¹I among those evacuated later from the 30-km zone.

Whole-body doses

The whole-body doses to the evacuees were mainly due to external exposure from deposited 132Te/132I, 134Cs and 137Cs and short lived radionuclides in the air. Measurements of the gamma dose rate in air were performed every hour at about thirty sites in Pripyat and daily at about eighty sites in the 30-km zone. Based on these measurements and using the responses to the questionnaires, whole-body doses were reconstructed for the 90 000 persons evacuated from the Ukrainian part of the 30-km zone (Li94). There was a wide range of estimated doses with an average value of 15 mSv. The collective dose was assessed to be 1 300 person-Sv. The 24 000 persons evacuated in Belarus might have received slightly higher doses, since the prevailing wind was initially towards the north.

The estimates of collective doses for the populations that were evacuated in 1986 from the contaminated areas of Belarus, Russia and Ukraine was about 3 800 man Sv for effective dose and 25 000 man Gy for thyroid doses (UN00). Most of the collective doses were received by the populations of Belarus and Ukraine.

Because the distributions of iodine tablets was done with a one-week delay and because only part of the population was covered, the averted collective thyroid dose from ingestion of contaminated milk was about 30% of the expected collective thyroid dose from that pathway while the thyroid doses from inhalation remained unchanged.

People living in the contaminated areas

Areas contaminated by the Chernobyl accident have been defined with reference to the background level of 137Cs deposition caused by the atmospheric weapons tests, which when corrected for radioactive decay to 1986, is about 2-4 kBq m-2. considering variations about this level, it is usual to specify the level of 37 kBq m-2 as the area affected by the Chernobyl accident.

Approximately 3% of the European part of the former USSR was contaminated with 137Cs deposition density greater than 37 kBq.m-2. Many people continue to live in these contaminated territories, although efforts have been made to limit their doses, 4 400 000 inhabitants were living in areas with a 137Cs contamination ranging from 37 to 185 kBq.m-2, 580 000 in areas 185-555 kBq.m-2. Areas of 137Cs deposition density greater than 555 kBq km-2 were designated as areas of strict control. In these areas, preventive measures have been successfully maintained annual effective dose below 5 mSv. Because of extensive migration, the number of people living in these areas under strict control was about 193 000 people in 1995, down from 273 000 in 1986-1987.

In the first few months, because of the significant release of the short-term ¹³¹I, the thyroid was the most exposed organ, the main route of exposure was cow-milk pathway. During the first year after the accident, doses from external irradiation arose from ground deposition of radionuclides with half-lives of one year or less only in areas close to the reactor, but the radiocesiums deposition was the greater contributors in more distant areas only one month after the accident. Over the following years, the doses received by the populations living in contaminated areas have come essentially from external exposure due to 134Cs and 137Cs deposited on the soil and internal exposure due to contamination of foodstuffs by these two isotopes.

A very large number of measurements have been done in the three republics. The publications prepared for regulatory purposes, tend to over estimate the average doses that were received during the years 1986-1990.

Figure 8. Contributions of radionuclides to the absorbed dose rate in air in a contaminated area of the Russian Federation during the first several months after the Chernobyl accident
(pdf format, 7 kb)

Doses to the thyroid gland

The main information source for the dose reconstruction is the vast number of iodine activity measurements of thyroid glands. In Ukraine 150 000 measurements, in Belarus several hundreds of thousands of measure-ments and in the Russian Federation more than 60 000 measurements were performed in May/June 1986. Some of the measurements were performed with inadequate instrumentation and measurement conditions and are not useful for dose assessment purposes. Using these measurements, the thyroid dose for people who lived in areas where direct thyroid measurements were done within a few weeks after the accident are being reconstructed using available data on ¹³¹I and ¹³⁷Cs deposition.

The influence of having taken stable iodine for prophylactic purposes has usually not been taken into account in the determination of thyroid doses (except for the evacuees from Pripyat, iodine prophylaxis was not effective in reducing the doses substantially as it was done too late).

The large variability of individual doses makes estimates of dose distributions difficult and current dose estimates are still subject to considerable uncertainties, especially in areas where only a few activity measurements in the thyroid were performed. Children in the Gomel oblast (region) in Belarus received the highest doses. The distribution of estimated individual doses in the thyroid of 0-7 years children is shown in Table 7.

Table 7. Distribution of estimated individual doses in the thyroid of 0-7 years old children in Gomel and Mogilev contaminated districts

For the total population of Belarus, the average dose to the thyroid is 0.9 to 1 Gy for 0-7-year-old children and 0.3 Gy for the total population giving collective doses of 34 000 and 134 000 man Gy respectively. (Il91) For the populations of the three republics, the collective thyroid doses are roughly estimated to 550 000, 200-300 000, 390 000 man Gy for Belarus, Russian Federation and Ukraine respectively (UN00). The average thyroid dose received by the populations of the three republics is estimated to be 7 mGy and exceeding 1 Gy for the most exposed children (UN00). In the eight most contaminated districts of Ukraine where thyroid measurements were performed, the collective dose to this age group was about 60 000 person-Sv and for the whole population about 200 000 person-Sv (Li93). In the Russian Federation the collective dose to the whole population was about 100 000 person-Sv (Zv93).

The thyroid doses are about two times greater in rural areas than in urban aeras.

An estimate of the dose distribution among children from Gomel oblast is shown in Table 8. For the whole Belarus the collective thyroid dose to children (0 to 14 years) at the time of the accident was assessed to be about

170 000 person-Gy (Ri94). To day the UNSCEAR report give an estimation of 34 000 man Gy to 0-7 year old children (UN00).

Table 8. Distribution of thyroid doses to children (0-15 years)
in the Gomel oblast of Belarus (from UN00)

In some Russian villages average doses exceeded 1 Gy, and individual doses exceeded 10 Gy.

Limited information is available on in utero thyroid doses. In a study in 250 children, born between may 1986 and February 1987 in Belarus, thyroid doses were estimated to range up to 4.3 Gy, with 135 children exposed to less than 0.3 Gy, 95 children between 0.3 and 1 Gy, and 20 children with doses greater than 1.Gy (Ig99).

Evaluations of questionnaires on food consumption rates in the period May/June 1986 and measurements of food contamination showed ¹³¹I in milk as the major source for the thyroid exposure of the population living in the contaminated areas. However, in individual cases the consumption of fresh vegetables contributed significantly to the exposure.

Whole-body doses

Two major pathways contributed to the whole-body doses of the population in contaminated areas, the exposure to external irradiation from deposited radionuclides (Iv95) and the incorporation into the body of radio-caesium in food.

The external exposure is directly related to the radionuclide activity per unit area and it is influenced by the gamma dose rates in air at the locations of occupancy. Forestry workers and other workers living in woodframe houses received the highest doses.

Most of the higher contaminated areas are rural and a large part of the diet is locally produced. Therefore, the uptake of caesium by the plants from the soil is a deciding factor in the internal exposure. These are regions with extraordinarily high transfer factors, as the Rovno region in Ukraine, where even moderate soil contamination led to high doses. In order of decreasing magnitude of transfer factors these regions are followed by regions with peaty soil, sandy podzol (acidic infertile forest soil), loamy podzol, and chernozem which is rich black soil.

In the first years after the accident the caesium uptake was dominated practically everywhere by the consumption of locally produced milk (Ho94). However, later mushrooms began to contribute significantly in many settlements to the caesium incorporation for two reasons. First, the milk contamination decreased with time, whereas the mushroom contamination remained relatively constant. Second, due to changes in the economic conditions in the three republics, people are collecting more mushrooms than they were in the first years after the accident.

The normalised lifetime doses for urban and rural populations of the three republics is now estimated to range from 42 to 88 μSv per kBqm^-2 of ¹³⁷Cs, 60% being received during the first 10 years. These values are lower than the first estimates, because they are more realistic and take account of, for example, the vertical migration of cesium in soils. During the first 10 years after the accident, average effective doses in theses areas ranged from 5mSv in Russian urban areas to 11 mSv in the rural areas of Ukraine. The variability of dose distribution could be represented by a log-normal distribution with a geometric standard deviation of 1.54. The decontamination measures had a limited impact on members of population. It was expected that less than 15% of the dose could be averted for the general population, and only 35% for school children. The total averted collective dose attributable to decontamination procedures was estimated to 1 500 mSv for the first four years.

The distribution of the collective dose from external irradiation by region and dose interval are presented in Table 9 and 10.

Table 9. Estimated collective effective dose to the populations of contaminated areas (1986-1995) excluding thyroid dose

Table 10. Distribution of estimated total effective doses received by the populations of contaminated areas (1986-1995) excluding thyroid dose

Table 11 summarises an estimate of whole-body doses to people living in the higher contaminated areas. On average, external irradiation was by far the highest contributor to the total population exposure (Er94). However, the highest doses to individuals were produced by the consumption of food from areas with high transfers of radionuclides.

Table 11. Distribution of external and total whole-body doses during 1986-89, to inhabitants of contaminated areas (¹³⁷Cs activity per unit area >555 kBq/m²) (Ba94)

The total collective effective dose received during the first 10 years after the accident by the approximately 5.2 million people living in the contaminated areas of Belarus, the Russian federation and Ukraine is estimated to be 24 200 man.Sv. This means, as ten years represents 60%, that the lifetime collective dose from external irradiation would be 40 300 man.Sv (UN00).

Internal doses were 5 500 man Sv for Belarus, 5 000 man.Sv for the Russian Federation an 7 900 man.Sv for Ukraine. As 10 years represents 90%, the lifetime total for the three republics would be 20 400 man.Sv, corresponding to an average individual lifetime effective dose of 3.9 mSv [UN00].

Total collective thyroid doses in Belarus, the Russian Federation, and the Ukrain, respectivly, were estimated to be 550 000 250 000 and 740 000 man Gy.

The total of about 60 700 man Sv for external and internal doses corresponds to an average individual lifetime effective dose of 12 mSv, excluding thyroid collective dose delivered during the first year. This is estimated to be 1 500 000 man Gy in total for the three countries.

Even though the releases of radioactive materials during the Chernobyl accident mainly affected the populations of Belarus, Russia and Ukraine, the released materials became further dispersed throughout the atmosphere and the volatile radionuclides of primary importance (¹³¹I and ¹³⁷Cs) were detected in most countries of the Northern hemisphere. However, population doses were, in most places, much lower than in the contaminated areas of the former Soviet Union; they reflected the deposition levels of ¹³⁷Cs and were higher in areas where the passage of the radioactive cloud coincided with rainfall. Generally speaking, however, and with a few notable exceptions, the doses decreased as a function of distance from the reactor (Ne87).

During the first few weeks after the accident, ¹³¹I was the main contributor to the dose, via ingestion of milk (Ma91). Infant thyroid doses generally ranged from 1 to 20 mGy in Europe, from 0.1 to 5 mGy in Asia, and were about 0.1 mGy in North America. Adult thyroid doses were lower by a factor of about 5 (UN88).

Later on, ¹³⁴Cs and ¹³⁷Cs were responsible for most of the dose, through external and internal irradiation (Ma89). The whole-body doses received during the first year following the accident generally ranged from 0.05 to 0.5 mGy in Europe, from 0.005 to 0.1 mGy in Asia, and of the order of 0.001 mGy in North America. The total whole-body doses expected to be accumulated during the lifetimes of the individuals are estimated to be a factor of 3 greater than the doses received during the first year (UN88).

In summary

A large number of people received substantial doses as a result of the Chernobyl accident:

*Special thanks to Dr. André Bouville, of the US National Cancer Institute, for his verification of the facts in this chapter.

Next Chapter: Health Impact

Home - About NEA- Publications -Activities -Data Bank - About this site - What's New

© 2008 Organisation for Economic Co-operation and Development