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EXECUTIVE SUMMARY

Chapter I. THE SITE AND ACCIDENT SEQUENCE
  • The site
  • The RBMK-1000 reactor
  • Events leading to the accident
  • The accident
  • The graphite fire

Chapter II. THE RELEASE, DISPERSION AND DEPOSITION OF RADIONUCLIDES

  • The source term
  • Atmospheric releases
  • Chemical and physical forms
  • Dispersion and deposition
  • Within the former Soviet Union
  • Outside the former Soviet Union

Chapter III. REACTIONS OF NATIONAL AUTHORITIES

  • Within the former Soviet Union
  • Outside the former Soviet Union

Chapter IV. DOSE ESTIMATES

  • The liquidators
  • The evacuees from the 30-km zone
  • Doses to the thyroid gland
  • Whole-body doses
  • People living in the contaminated areas
  • Doses to the thyroid gland
  • Whole-body doses
  • Populations outside the former Soviet Union

Chapter V. HEALTH IMPACT

  • Acute health effects
  • Late health effects
  • Thyroid cancer
  • Other late health effects
  • Other studies
  • Psychological effects
  • Within the former Soviet Union
  • Outside the former Soviet Union

Chapter VI. AGRICULTURAL AND ENVIRONMENTAL IMPACTS

  • Agricultural impact
  • Within the former Soviet Union
  • Within Europe
  • Environmental impact
  • Forests
  • Water bodies

Chapter VII. POTENTIAL RESIDUAL RISKS

  • The Sarcophagus
  • Radioactive waste storage sites

Chapter VIII. LESSONS LEARNED

  • Operational aspects
  • Scientific and technical aspects

EXPLANATION OF TERMS

LIST OF ACRONYMS

Chapter IV

DOSE ESTIMATES

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 to amount up to 800,000 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.

About 135,000 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 7. It is clear that tellurium-132 played a major role in the first week after the accident, and that, after one month, the radiocaesiums (caesium-134 and caesium-137) became predominant. Subsequently, however, caesium-134 decayed to levels much lower than those of caesium-137, which became after a few years the only radionuclide of importance for practical purposes. It is usual to refer to caesium-137 only, even when the mix of caesium-134 and caesium-137 is meant, because the values for the constituents can be easily derived from those for caesium-137.



Figure 7. Relative contribution of gamma radiation from individual radionuclides to the absorbed dose rate in air during the first several months after the Chernobyl accident (Go93)




With regard to internal doses from inhalation and ingestion of radionuclides, the situation is similar: radioiodine 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 (caesium-134 and caesium-137) again became predominant, and, after a few years, caesium-137 became the only radionuclide of importance for practical purposes, even though strontium-90 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 caesium-137 deposition levels greater than 37 kBq/m2. About 4 million people live in those areas. Of special interest are the inhabitants of the spots with caesium-137 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.

Early after the accident, the All-Union Dose Registry (AUDR) was set up by the Soviet Government in 1986 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 of 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 caesium-137 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 800,000, 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 140 of these patients received whole-body doses from external irradiation in the range 1-2 Gy, that 55 received doses between 2 and 4 Gy , that 21 received between 4 and 6 Gy, and that the remaining 21 received doses between 6 and 16 Gy. In addition, it was estimated from thyroid measurements that the thyroid dose from inhalation would range up to about 20 Sv, with 173 individuals in the 0-1.2 Sv range and five workers with thyroid doses greater than 11 Sv (UN88).

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. Several hundreds of thousands of individuals participated in this work. Initially, 50 per cent 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.

There are only fragmented data on the doses received by the 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 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 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.

It is interesting to note that a small special group of 15 scientists who have worked periodically inside the sarcophagus for a number of years have estimated accumulated whole-body doses in the range 0.5 to 13 Gy (Se95a). While no deterministic effects have been noted to date, this group may well show radiation health effects in the future.

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 80,000 people were evacuated in the following days and weeks from the contaminated areas.

Information relevant for the assessment of the doses received by these people have been obtained by 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).

Doses to the thyroid gland

The iodine activity in thyroid glands of evacuees was measured. More than 2,000 measurements of former inhabitants of Pripyat had sufficient quality to be useful for dose reconstruction (Go95a). A comparative analysis with the questionnaire responses showed that thyroid doses were mainly due to inhalation of iodine-131. Average individual doses and collective doses to the thyroid are shown in Table 3 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 3. Average doses to the thyroid gland and collective thyroid doses to the evacuees from Pripyat (Go95a).
Year of birth Number of
people
Average individual
dose (Sv)
Collective dose
(person-Sv)
1983 - 1986 2,400 1.4 3,300
1971 - 1982 8,100 0.3 2,400
< = 1970 38,900 0.07 2,600


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 iodine-131 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 tellurium-132/iodine-132, caesium-134 and caesium137 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.

People living in the contaminated areas

Doses to the thyroid gland

The main information source for the dose reconstruction is the vast amount of iodine activity measurements in thyroid glands. In Ukraine 150,000 measurements, in Belarus several hundreds of thousands of measurements 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.

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. An estimate (Ba94) of the dose distribution among these children is shown in Table 4. 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-Sv (Ri94). 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).


Table 4. Distribution of thyroid doses to children (0-7 years) in the Gomel oblast of Belarus (Ba94).
Thyroid dose (Sv) Number of children Collective dose
(person-Sv)
0 - 0.3 15,100 2,300
0.3 - 2 13,900 11,500
2 - 10 3,100 13,700
10 - 40 300 4,700


Evaluations of questionnaires on food consumption rates in the period May/June 1986 and measurements of food contamination showed iodine-131 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 contaminations 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 again collecting more mushrooms than they were in the first years after the accident .

Table 5 summarises a recent 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 5. Distribution of external and total whole-body doses during 1986-89, to inhabitants of contaminated areas (caesium137 activity per unit area > 555 kBg/mІ) (Ba94)
Whole-body
dose (mSv)
External exposure Total exposure
No. of
persons
Collective dose
(person-Sv)
No. of
persons
Collective dose
(person-Sv)
5 - 20
20 - 50
50 - 100
100 - 150
150 - 200
> 200
132,000
111,000
24,000
2,800
530
120
1,700
3 ,500
1,600
330
88
26
88,000
132,000
44,000
6,900
1,500
670
1,200
4,200
3,000
820
250
160
Total270,0007,300273,0009,700


Populations outside the former Soviet Union

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 (iodine-131 and caesium-137) were detected in most countries of the Northern hemisphere. However, the doses to the population were in most places much lower than in the contaminated areas of the former Soviet Union; they reflected the deposition levels of caesium-137 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, iodine-131 was the main contributor to the dose, via ingestion of milk (Ma91). Infant thyroid doses generally ranged from 1 to 20 mSv in Europe, from 0.1 to 5 mSv in Asia, and were about 0.1 mSv in North America. Adult thyroid doses were lower by a factor of about 5 (UN88).

Later on, caesium-134 and caesium-137 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 mSv in Europe, from 0.005 to 0.1 mSv in Asia, and of the order of 0.001 mSv 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:

  • Liquidators - Hundreds of thousands of workers, estimated to amount up to 800,000, were involved in clean-up operations. The most exposed, with doses of several grays, were the workers involved immediately after the beginning of the accident and the scientists who have performed special tasks in the sarcophagus. The average doses to liquidators are reported to have ranged between 170 mSv in 1986 and 15 mSv in 1989.

  • Evacuees - More than 100,000 persons were evacuated during the first few weeks following the accident. The evacuees were exposed to internal irradiation arising from inhalation of radioiodines, especially iodine-131, and to external irradiation from radioactivity present in the cloud and deposited on the ground. Thyroid doses are estimated to have been, on average, about 1 Sv for small children under 3 years of age and about 70 mSv for adults. Whole-body doses received from external irradiation prior to evacuation from the Ukrainian part of the 30-km zone showed a large range of variation with an average value of 15 mSv.

  • People living in contaminated areas of the former Soviet Union - About 270,000 people live in contaminated areas with caesium-137 deposition levels in excess of 555 kBq/m2. Thyroid doses, due mainly to the consumption of cow's milk contaminated with iodine-131, were delivered during the first few weeks after the accident; children in the Gomel region of Belarus appear to have received the highest thyroid doses with a range from negligible levels up to 40 Sv and an average close to 1 Sv for children aged 0 to 7. Because of the control of foodstuffs in those areas, most of the radiation exposure since the summer of 1986 is due to external irradiation from the caesium-137 activity deposited on the ground; the whole-body doses for the 1986-1989 time period are estimated to range from 5 to 250 mSv with an average of 40 mSv. In areas without food control, there are places, such as the Rovno region of Ukraine, where the transfer of caesium137 from soil to plant is very high, resulting in doses from internal exposure being greater than those from external exposure.

  • Populations outside the former Soviet Union - The radioactive materials of a volatile nature (such as iodine and caesium) that were released during the accident spread throughout the entire northern hemisphere. The doses received by populations outside the former Soviet Union were relatively low, and showed large differences from one country to another depending mainly upon whether rainfall occurred during the passage of the radioactive cloud.


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