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1.INTRODUCTION

This paper provides an up to date evaluation of the environmental effects of the accident which occurred on 26 April 1986 at the Chernobyl nuclear power plant. Even though it is now nearly 20 years after the accident, there are still many conflicting reports and rumours concerning its consequences.

For this reason, the Chernobyl Forum was initiated by the IAEA, in cooperation with the WHO, the UNDP, the FAO, the UNEP, the UN-OCHA, the UNSCEAR, the World Bank and the Governments of Belarus, the Russian Federation and Ukraine. The Chernobyl Forum was established as a series of managerial and expert meetings with the purpose of (a) generating authoritative consensual statements on the health effects attributable to radiation exposure arising from the accident and the environmental consequences induced by the released radioactive materials; (b) providing advice on remediation and special health care programmes; and (c) suggesting areas where further research might be required.

The working practices of the Chernobyl Forum have been described by Dr. B. Bennett in his Introductory Remarks. The summary of the environmental consequences of the accident presented in this paper is based on the results of two years of operation of the Forum’s Expert Group on Environment. The Expert Group on Environment included 35 scientists (whose names are listed in the Acknowledgements section of this paper); persons involved were from the three more affected countries of Belarus, the Russian Federation and Ukraine, and from the international community of persons who have performed environmental work related to the deposition of Chernobyl debris. The latter included a large number of scientists from European countries, including the Scandinavian countries, France, Germany and the United Kingdom.

Members of the Expert Group on Environment met seven times in Vienna in order to carry out their work. The work of the Expert Group on Environment is being published by the International Atomic Energy Agency [1]; the full report is nearly 200 pages long and provides extensive information. In this paper, only brief summaries will be presented of the releases of radioactive materials, the distribution of these materials, the countermeasures that were employed to mitigate the effects, the doses that were received by the persons residing in the more affected territories, the effects upon biota, and issues relevant to the environmental safety of the shelter over the damaged reactor and radioactive waste produced by the accident.

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2.RELEASES

The accident occurred shortly after midnight on 26 April 1986 at Unit 4 of the Chernobyl nuclear power plant. The cause of the accident was human error, and the accident occurred after prolonged operation of the reactor in a nondesign configuration. One or more steam explosions blew the reactor apart with an initial large release of radioactive materials. The graphite within the now exposed core of the reactor caught fire and burned for ten days despite heroic efforts to extinguish the blaze. Releases of radioactive material continued for this time period; the radionuclide content of the releases varied markedly with time and temperature of the burning reactor.

Major releases included radioactive gases, condensed aerosols and a large amount of fuel particles. The total release of radioactive substances was about 14 EBq1 (as of 26 April 1986), which included 1.8 EBq of 131I, 0.085 EBq of 137Cs and other Cs isotopes, 0.01 EBq of 90Sr and 0.003 EBq of Pu radioisotopes. The noble gases contributed about 50% of the total release of activity. The best estimates of the releases of representative and important radionuclides are shown in Table 1.

3.CONTAMINATION

3.1. Radionuclide deposition

Large areas of Europe were affected to some degree by the radionuclide mixture released from the Chernobyl accident. An area of more than 200 000 km2 in Europe was contaminated with radiocaesium (above 37 kBq of 137Cs/m2) of which 71% was in the three more affected countries, Belarus, the Russian Federation and Ukraine. The deposition was highly heterogeneous; it was strongly influenced by the presence or absence of rain during passage of contaminated air masses. For convenience in mapping deposition, 137Cs was chosen as a representative radionuclide, because it was easy to measure and was of radiological significance. Most of the strontium and plutonium radioisotopes were deposited close (less than 100 km) to the reactor, due to their being contained within larger particles.

1 1 1 EBq = 1018 Bq.

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FIG. 1. Surface-ground deposition of 137Cs throughout Europe as a result of the Chernobyl accident [6].

TABLE 2. AREAS CONTAMINATED AT 137Cs-SOIL DEPOSITION OF >37 kBq/m2 (1 Ci/km2) [1].

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Two atlases of contaminated areas have been published [6, 7]; one example of the maps produced is given in Fig. 1 [6]. In Table 2, countries with the larger areas of land considered to be contaminated, which was defined at the time as areas contaminated with >37 kBq/m2 (1 Ci/km2) of 137Cs are indicated. In the three more contaminated countries of Belarus, the Russian Federation and Ukraine, more than five million persons lived in such areas.

Much of the release was comprised of radionuclides with short physical half-lives; long lived radionuclides were released in smaller amounts. Thus, many of the radionuclides released by the accident have already decayed. The releases of radioactive iodines caused concern immediately after the accident. After the initial period, 137Cs became the nuclide of greatest radiological importance, with 90Sr being of lesser importance. For the first few years, 134Cs was also important. Over the longer term (hundreds to thousands of years), the only radionuclides anticipated to be of interest are the plutonium isotopes and

241Am.

Airborne radionuclides from the Chernobyl release contaminated urban, agricultural, aquatic, and forest areas.

3.2. Urban environment

In urban areas, open surfaces such as lawns, parks, streets, roads, squares, building roofs and walls became contaminated. Under dry conditions, trees, bushes, lawns and roofs were contaminated; under wet conditions, horizontal surfaces, such as soil plots, lawns etc. received the higher contaminations. Particularly high 137Cs-activity levels were found around houses where rain had transported radioactive materials from roofs to the ground. The deposition in urban areas in the nearest city of Pripyat and surrounding settlements could have initially given rise to substantial external radiation doses, but this was partially averted by the evacuation of the people.

Due to wind and rain, and human activities, including traffic, street washing and cleanup, surface contamination by radioactive material was reduced significantly in inhabited and recreational areas during 1986 and afterwards, as shown in Fig. 2. One of the consequences of these processes was the secondary contamination of sewage systems and resulting sludge storage.

At present, in most of the settlements subjected to radioactive contamination, the dose rate in air above solid surfaces has returned to the pre-accident background level. An elevated dose rate in air remains mainly over undisturbed soil in gardens, kitchen gardens and parks.

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FIG. 2. Typical distribution of 137Cs on different surfaces within settlements in 1986 and 14 years after deposition of the Chernobyl fallout. (a) Dry deposition; (b) Wet deposition [8].

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3.3. Agricultural environment

In the early phase, the direct surface deposition of many different radionuclides dominated the contamination of agricultural plants and of animals consuming them. The release and deposition of radioiodine isotopes caused the most immediate concern, but the problem was confined to the first two months, as shown in Fig. 3, because of the short physical half-life of eight days of the most important iodine isotope. The radioiodine was rapidly transferred to milk at a high rate in the Russian Federation, Ukraine and Belarus, and led to significant thyroid doses to those consuming milk, especially children.

After the early phase of direct contamination, uptake of radionuclides through plant roots from soil became increasingly important and showed strong time dependences. Radioisotopes of caesium (137Cs and 134Cs) were the nuclides which led to the greater problems, and, after the decay of 134Cs, 137Cs remains to cause problems in some Belarus, Russian Federation and Ukrainian areas. In addition, 90Sr causes problems in the near field of the reactor, but at longer distances the deposition levels were too low to be of radiological significance. Other radionuclides, such as plutonium isotopes and 241Am, were present at very low deposition levels and did not have significant transfer from soil through roots to plants.

In general, there was a substantial initial reduction in the transfer of radionuclides to vegetation and animals in the first few years due to weathering, physical decay, migration of radionuclides down the soil column and reductions in radionuclide bioavailability in soil; this reduction is indicated in Fig. 4. On average, the levels of 137Cs contamination were about 100 times less than initial values after about ten years. This decrease with time was much more rapid than that due to radioactive decay alone. However, in the last decade, there has been little further obvious decline and long term effective half-lives have been difficult to quantify.

The radiocaesium concentrations in foodstuffs after the early phase were influenced not only by deposition levels, but also by soil types, management practices and types of ecosystems. Major and persistent problems in the affected areas occurred in extensive agricultural systems in soils with high organic content and where animals grazed in unimproved pastures which were not ploughed or fertilized. Effects were particularly notable for rural residents in the former Soviet Union who were commonly subsistence farmers with privately owned dairy cows.

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FIG. 3. Changes with time in 131I activity concentrations in (a) cow milk and (b) leafy vegetables, in different regions of France, May–June 1986 [9].

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FIG. 4. Changes with time of 137Cs concentrations in grain and potatoes, as measured in contaminated areas of the Bryansk region (Bq/kg) [10].

3.4. Forest environment

Following the Chernobyl accident, vegetation and animals in forests and mountain areas showed particularly high uptake of radiocaesium, with the highest recorded 137Cs concentrations being found in forest products, due to the persistent recycling of radiocaesium in forest ecosystems. Particularly high 137Cs-activity concentrations have been found in mushrooms, berries and game, and these high levels have persisted since the accident; this is demonstrated in Fig. 5. Therefore, the relative importance of forests in contributing to the radiation exposures of the populations of several affected countries has increased with time. This can be expected to continue for several decades to come.

The high transfer of radiocaesium in the pathway lichen–reindeer meat–humans was demonstrated again after the Chernobyl accident in arctic

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FIG. 5. The average concentration of 137Cs in moose in one hunting area in Sweden, based on approximately 100 animals/a [11].

and subarctic areas of Europe. The Chernobyl accident led to contamination of reindeer meat in Finland, Norway, the Russian Federation and Sweden, and caused significant problems for the Sami people.

The use of timber and associated products only makes a small contribution to the exposure of the general public, although wood ash can contain high amounts of 137Cs and could potentially give rise to higher doses than other uses of wood. 137Cs in timber is of minor importance, although doses to persons in the wood pulp industry are of concern.

Widespread forest fires in 1992 caused increased air-activity concentrations, but not to a significant extent. The possible radiological consequences of forest fires have been much discussed, but these are not expected to cause any problems of radionuclide transfer from contaminated forests, except, possibly, in the nearest surroundings of a fire.

3.5. Aquatic environment

Radionuclides from Chernobyl contaminated surface-water systems, not only in areas close to the site, but also in many other parts of Europe. The initial contamination of water was due primarily to direct deposition of radionuclides onto the surface of rivers and lakes, and was dominated by short lived radionuclides (most importantly 131I). In the first few weeks after the accident, activity

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FIG. 6. Annually averaged (a) 137Cs and (b) 90Sr activity concentrations in the water of the Kiev Reservoir (Vishgorod) and the Kahovka Reservoir of the Dnieper cascade [12].

concentrations in drinking water from the Kiev Reservoir were of particular concern.

The contamination of water bodies decreased rapidly during the weeks after fallout through dilution, physical decay and absorption of radionuclides by catchment soils, as shown in Fig. 6. For lakes and reservoirs, the settling of suspended particles to the bed sediments also played an important role in reducing levels in water.

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