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Chernobyl Nuclear Accident

3. How has the environment been affected by the Chernobyl accident?

  • 3.1 To what extent have urban areas been contaminated?
  • 3.2 To what extent have agricultural areas been contaminated?
  • 3.3 To what extent have forests been contaminated?
  • 3.4 To what extent have water bodies been contaminated?
  • 3.5 How did radiation affect plants and animals?

The source document for this Digest states:

The report of the Expert Group on environmental consequences covers the issues of radioactive release and deposition, radionuclide transfers and bioaccumulation, application of countermeasures, radiation-induced effects on plants and animals as well as dismantlement of the Shelter and radioactive waste management in the Chernobyl Exclusion Zone.

Release and Deposits of Radioactive Material

Release and Deposits of Radioactive Material

Major releases of radionuclides from unit 4 of the Chernobyl reactor continued for ten days following the April 26 explosion. These included radioactive gases, condensed aerosols and a large amount of fuel particles. The total release of radioactive substances was about 14 Ebq5 [1 EBq = 1018 Bq (Becquerel)], including 1.8 EBq of iodine-131, 0.085 EBq of 137Cs, 0.01 EBq of 90Sr and 0.003 EBq of plutonium radioisotopes. The noble gases contributed about 50% of the total release.

More than 200 000 square kilometres of Europe received levels of 137Cs above 37 kBq m-2. Over 70 percent of this area was in the three most affected countries, Belarus, Russia and Ukraine. The deposition was extremely varied, as it was enhanced in areas where it was raining when the contaminated air masses passed. Most of the strontium and plutonium radioisotopes were deposited within 100 km of the destroyed reactor due to larger particle sizes.

Many of the most significant radionuclides had short physical half-lives. Thus, most of the radionuclides released by the accident have decayed away. The releases of radioactive iodines caused great concern immediately after the accident. For the decades to come 137Cs contamination will continue to be of greatest importance, with secondary attention to 90Sr. Over the longer term (hundreds to thousands of years) the plutonium isotopes and americium-241 will remain, although at levels not significant radiologically.

Source & ©: UN Chernobyl Forum
 Chernobyl's Legacy: Health, Environmental and Socio-economic Impacts (2006)
Forum Expert Group Report: Environmental Consequences, p.21-22

3.1 To what extent have urban areas been contaminated?

The source document for this Digest states:

Radionuclides deposited most heavily on open surfaces in urban areas, such as lawns, parks, streets, roads, town squares, building roofs and walls. Under dry conditions, trees, bushes, lawns and roofs initially had the highest levels, whereas under wet conditions horizontal surfaces, such as soil plots and lawns, received the highest levels. Enhanced 137Cs concentrations were found around houses where the rain had transported the radioactive material from the roofs to the ground.

p23-2-small

The deposition in urban areas in the nearest city of Pripyat and surrounding settlements could have initially given rise to a substantial external dose. However, this was to a large extent averted by the timely evacuation of residents. The deposition of radioactive material in other urban areas has resulted in various levels of radiation exposure to people in subsequent years and continues to this day at lower levels.

Due to wind and rain and human activities, including traffic, street washing and cleanup, surface contamination by radioactive materials has been reduced significantly in inhabited and recreational areas during 1986 and afterwards. One of the consequences of these processes has been secondary contamination of sewage systems and sludge storage.

At present, in most of the settlements subjected to radioactive contamination as a result of Chernobyl, the air dose rate above solid surfaces has returned to the background level predating the accident. But the air dose rate remains elevated above undisturbed soil in gardens and parks in some settlements of Belarus, Russia and Ukraine.

Source & ©: UN Chernobyl Forum
 Chernobyl's Legacy: Health, Environmental and Socio-economic Impacts (2006)
What is the scope of urban contamination?, p.22-23

3.2 To what extent have agricultural areas been contaminated?

The source document for this Digest states:

The levels radioactivity of agricultural plants and plant-consuming animals was dominated by surface deposits of radionuclides

In the early months after the accident, the levels radioactivity of agricultural plants and plant-consuming animals was dominated by surface deposits of radionuclides. The deposition of radio iodine caused the most immediate concern, but the problem was confined to the first two months after the accident because of fast decay of the most important isotope, 131I.

The radioiodine was rapidly absorbed into milk at a high rate leading to significant thyroid doses to people consuming milk, especially children in Belarus, Russia and Ukraine. In the rest of Europe increased levels of radioiodine

After the early phase of direct deposit, uptake of radionuclides through plant roots from soil became increasingly important. Radioisotopes of caesium (137Cs and 134Cs) were the nuclides which led to the largest problems, and even after decay of 134Cs (half-life of 2.1 years) by the mid-1990s the levels of longer lived 137Cs in agricultural products from highly affected areas still may require environmental remediation. In addition, 90Sr could cause problems in areas close to the reactor, but at greater distances its deposition levels were low. Other radionuclides such as plutonium isotopes and 241Am did not cause real problems in agriculture, either because they were present at low deposition levels, or were poorly available for root uptake from soil.

In general, there was a substantial reduction in the transfer of radionuclides to vegetation and animals in intensive agricultural systems in the first few years after deposition, as would be expected due to weathering, physical decay, migration of radio nuclides down the soil and reductions in bioavailability in soil, see Fig. 5. However, in the last decade there has been little further obvious decline, by 3-7 percent per year.

FIG. 5. Reduction with time of 137Cs activity concentration in milk produced in private and collective farms of the Rovno region of Ukraine with a comparison to the temporary permissible level (TPL).

The radiocaesium content in foodstuffs was influenced not only by deposition levels but also by types of ecosystem and soil as well as by management practices. The remaining persistent problems in the affected areas occur in extensive agricultural systems with soils with a high organic content and animals grazing in unimproved pastures that are not ploughed or fertilized. This particularly affects rural residents in the former Soviet Union who are commonly subsistence farmers with privately owned dairy cows.

In the long term 137Cs in milk and meat and, to a lesser extent, 137Cs in plant foods and crops remain the most important contributors to human internal dose. As 137Cs activity concentration in both vegetable and animal foods has been decreasing very slowly during the last decade, the contribution of 137Cs to internal dose will continue to dominate for decades to come. The importance of other long lived radionuclides, 90Sr, plutonium isotopes and 241Am, in terms of the human dose will remain insignificant.

Currently, 137Cs activity concentrations in agricultural food products produced in areas affected by the Chernobyl fallout are generally below national and international action levels. However, in some limited areas with high radionuclide contamination (parts of the Gomel and Mogilev regions in Belarus and the Bryansk region in Russia) or poor organic soils (the Zhytomir and Rovno regions in the Ukraine) milk may still be produced with 137Cs activity concentrations that exceed national action levels of 100 Bq per kilogram. In these areas environmental remediation may still be warranted.

Source & ©: UN Chernobyl Forum
 Chernobyl's Legacy: Health, Environmental and Socio-economic Impacts (2006)
How contaminated are agricultural areas?, p.23-25

3.3 To what extent have forests been contaminated?

The source document for this Digest states:

Vegetation and animals in forests and mountain areas have shown particularly high uptake of radiocaesium

Following the accident vegetation and animals in forests and mountain areas have shown particularly high uptake of radiocaesium, with the highest recorded 137Cs levels found in forest food products. This is due to the persistent recycling of radiocaesium particularly in forest ecosystems.

Particularly high 137Cs activity concentrations have been found in mushrooms, berries, and game, and these high levels have persisted for two decades. Thus, while the magnitude of human exposure through agricultural products has experienced a general decline, high levels of contamination of forest food products have continued and still exceed intervention levels in many countries. In some areas of Belarus and Russia, consumption of forest foods with 137Cs dominates internal exposure. This can be expected to continue for several decades.

Therefore, the relative importance of forests in contributing to radiological exposures of the populations of several affected countries has increased with time. It will primarily be the combination of downward migration in the soil and the physical decay of 137Cs that will contribute to any further slow long term reduction in contamination of forest food products.

The high transfer of radiocaesium in the pathway lichen-to-reindeer meat-to-humans has been demonstrated again after the Chernobyl accident in the Arctic and sub-Arctic areas of Europe. The Chernobyl accident led to high contamination of reindeer meat in Finland, Norway, Russia and Sweden and caused significant problems for the indigenous Sami people.

Source & ©: UN Chernobyl Forum
 Chernobyl's Legacy: Health, Environmental and Socio-economic Impacts (2006)
What is the extent of forest contamination?, p.25

3.4 To what extent have water bodies been contaminated?

The source document for this Digest states:

Initial levels were due primarily to direct deposition of radionuclides on the surface of rivers and lakes

Radioactive material from Chernobyl resulted in levels of radioactive material in surface water systems in areas close to the reactor site and in many other parts of Europe. The initial levels were due primarily to direct deposition of radionuclides on the surface of rivers and lakes, dominated by short lived radionuclides (primarily 131I). In the first few weeks after the accident, high activity concentrations in drinking water from the Kyiv Reservoir were of particular concern.

Levels in water bodies fell rapidly during the weeks after fallout through dilution, physical decay and absorption of radionuclides to catchment soils. Bed sediments are an important long term sink for radioactivity.

Initial uptake of radioiodine to fish was rapid, but activity concentrations declined quickly, due primarily to physical decay. Bioaccumulation of radiocaesium in the aquatic food chain led to significant activity concentrations in fish in the most affected areas, and in some lakes as far away as Scandinavia and Germany. Because of generally lower fallout and lower bioaccumulation, 90Sr levels in fish were not significant for human doses in comparison to radiocaesium, particularly since 90Sr is accumulated in bone rather than in edible muscle.

In the long term, secondary inputs by runoff of long lived 137Cs and 90Sr from soils continues (at a much lower level) to the present day. At the present time, activity concentrations both in surface waters and in fish are low, see Fig. 6. Therefore, irrigation with surface water is not considered to be a hazard.

FIG. 6. Averaged 137Cs activity concentrations in non-predatory (Bream, upper histogram) and predatory (Pike, lower histogram) fish from Kyiv reservoir (UHMI 2004).

While 137Cs and 90Sr levels in water and fish of rivers, open lakes and reservoirs are currently low, in some "closed" lakes with no outflowing streams in Belarus, Russia and Ukraine both water and fish will remain contaminated with 137Cs for decades to come. For example, for some people living next to a "closed" Kozhanovskoe Lake in Russia, consumption of fish has dominated their total 137Cs ingestion.

Owing to the large distance of the Black and Baltic Seas from Chernobyl, and the dilution in these systems, activity concentrations in sea water were much lower than in freshwater. The low water radionuclide levels combined with low bioaccumulation of radiocaesium in marine biota has led to 137Cs levels in marine fish that are not of concern."

Source & ©: UN Chernobyl Forum
 Chernobyl's Legacy: Health, Environmental and Socio-economic Impacts (2006)
How contaminated are the aquatic systems?, p.26

3.5 How did radiation affect plants and animals?

The source document for this Digest states:

Irradiation from radionuclides released from the accident caused numerous acute adverse effects on the plants and animals living in the higher exposure areas, i.e., in localized sites at distances up to 30 kilometres from the release point. Outside the Exclusion Zone, no acute radiation-induced effects in plants and animals have been reported. The response of the natural environment to the accident was a complex interaction between radiation dose and radiosensitivities of the different plants and animals. Both individual and population effects caused by radiation-induced cell death have been observed in biota inside the Exclusion Zone as follows:

  • Increased mortality of coniferous plants, soil invertebrates and mammals; and
  • Reproductive losses in plants and animals.

No adverse radiation-induced effect has been reported in plants and animals exposed to a cumulative dose of less than 0.3 Gy during the first month after the accident.

Following the natural reduction of exposure levels due to radionuclide decay and migration, biological populations have been recovering from acute radiation effects. As soon as by the next growing season following the accident, population viability of plants and animals had substantially recovered as a result of the combined effects of reproduction and immigration from less affected areas. A few years were needed for recovery from major radiation-induced adverse effects in plants and animals.

A white-tailed eagle chick observed recently in the Chernobyl Exclusion Zone
FIG. 8. A white-tailed eagle chick observed recently in the Chernobyl Exclusion Zone. Before 1986, these rare predatory birds have been hardly found in this area (Sergey Gaschak, 2004).

Genetic effects of radiation, in both somatic and germ cells, have been observed in plants and animals of the Exclusion Zone during the first few years after the Chernobyl accident. Both in the Exclusion Zone, and beyond, different cytogenetic anomalies attributable to radiation continue to be reported from experimental studies performed on plants and animals. Whether the observed cytogenetic anomalies in somatic cells have any detrimental biological significance is not known.

The recovery of affected biota in the exclusion zone has been facilitated by the removal of human activities, e.g., termination of agricultural and industrial activities. As a result, populations of many plants and animals have eventually expanded, and the present environmental conditions have had a positive impact on the biota in the Exclusion Zone. Indeed, the Exclusion Zone has paradoxically become a unique sanctuary for biodiversity.

Source & ©: UN Chernobyl Forum
 Chernobyl's Legacy: Health, Environmental and Socio-economic Impacts (2006)
What were the radiation-induced effects on plants and animals?, p.29-30


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