The catastrophe in Chornobyl resulted in radioactive emissions, which were occurring in different parts of the active zone for a significant period of time. These emissions mostly had different rates of the fuel burn-out and thus different radionuclide compositions. The overall activity of the substances that got into the environment was about 13 EBq (more than 300 μCi). The substances included significant amounts of analogues of biogenous elements such as potassium and calcium — 137Cs and 90Sr. The accident resulted in the contamination of more than 145 thousands of km2 of the territories of Ukraine, Belarus and Russia. The density of the contamination with 137Cs was over than 37 kBq/m2. Around five million people have suffered from the catastrophe in Chornobyl. By the 1st of January 2007 2,15 millions of people (including 460 thousands of children) lived on the contaminated territories, even on the territories with the intense radiological control (1,6 millions); 2 millions of these people have a status of victims of the Chornobyl catastrophe (1). In 25 years after the accident water and food remain the main sources for the long-term anthropogenic radionuclide (137Cs, 90Sr) to get into human body and form the dose of the inner contamination (2).
The levels of irradiation and the anthropogenesis factors of influence on the biota
The activity of the radionuclide emitted into the environment at the time of the accident was presumably 13935.89593 PBq; 21 years later – 68,426067 PBq (3).
Summing up the results of the long-term complex researches in the Zone allows us to evaluate the ponderability of the main migration routs of the radionuclide beyond the Zone (4):
- river flow (river Pripyat) - 85-95 % of all the radionuclide migrating beyond the limits of the Zone;
- aerial transference - 5-10 %;
- biogenous migration - 0.1-3 %;
- anthropogenic migration – approximately 0.0002 %.
Today the main source of anthropogenic radionuclide in the air on the whole territory of the country is the secondary wind uplift of the radioactive elements from the ground. Nevertheless the concentration of 137С in the air was significantly (in several orders) below the acceptable level during several last years. The intensity of the exposure dose of gamma-radiation (gamma-background) on most of the territory is within the acceptable levels, conditioned by the natural radioactive isotopes and cosmic radiation. For different territories this level is (depending on natural differences) from 5 to 21 mkR/h (5).
Presently the comparatively high levels of the ground water contamination (in some cases exceeding the acceptable levels of concentration in drinking water in tens and hundreds of times) are only observed within the limits of those radioactive waste depositories, built without special watertight geochemical and other engineering barriers. According to the prognoses, the ground waters will start to get unloaded into the river Pripyat not sooner that in 45 years. As for the ground water contamination, beyond the limits of Zona, in the first level of aquifer it mostly does not exceed 0,3 Bq•dm3, for both 137Cs and 90Sr. After these 25 years one may assert that the routs of radionuclide migrations within the aeration zone and water-inundated formations are slow and inertial, unlike their behavior in the air and surface waters.
It is known that one of the main consequences of the Chornobyl accident is the radionuclide contamination of almost 9% of agricultural land in Ukraine. It was shown in numerous radiation-ecological researches that the longest periods of effective half-eduction from the anthropogenic radionuclide can be found in the forest ecosystems. This is why, comparing to other types of landscape, forests are still the critical landscapes in terms of radionuclide movements by the trophic chains to people (6). A radiation inspection of the agricultural lands of more than 5 millions of hectares was conducted. Up to 80 % of inspected forest food products contain 137Cs above the acceptable levels.
Biologic consequences of Chornobyl catastrophe
The effects of radioactive contamination on biological objects are manifested on all levels of organization – from viruses to ecosystems. The most brightly such effects can be seen within the Chornobyl exclusion zone, where biological objects were critically affected by the radiation. Within time such acute manifestations as pine forests’ dying off (the “Red forest” effect) are altered by slow renovation of the biota, accompanied by the emergence of radiation-proof life forms; weakening of natural immunity towards virus infections and parasites; new mutation forms; growth and reproduction depression and genetic changes on a condition of long-term low dose radiation. However the absence of human intrusions contributed to the revival of natural functional ecosystems and of biodiversity increase, including the introduction of new species. Radiation effects, found on the cellular, organism and population levels, do not break the general picture of present floral and faunal complexes. Biotic objects on the territory of 30 km exclusion zone – plants, mushrooms, lower and high animals, microorganisms and viruses – undergo the continuous effect of ionizing radiation.
The increase of frequency of discovering viruses is observed on the radionuclide contaminated Zone territory, including far more frequent discovering of plants; viruses within the 30 km zone. During the first decade after the accident, significant changes in the microorganism species composition were spotted in different places. It was found that under the conditions of increased radioactive contamination level of substance, the bacteria living in it are mutating more quickly and so may form new radiation-proof forms. The forming of new mushroom cultures, enriched with melanin, was also observed.
The biota of the most contaminated water complexes is characterized by the high level of radioactive contamination. In the tissues of fishes both 137Cs and 90Sr accumulate as well as plutonium isotopes, causing the damage of reproductive tissues. On the cellular level spineless water animals are known to have increased levels of cells with chromosome aberrations (approximately 10 times more often comparing to the clean water complexes).
Both amphibians and rodents that live in the exclusion zone are known to accumulate radionuclide in bone tissues which results in dystrophic reorganizations of spongy bones and exfoliation of bone lamellas.
In the generations of cattle that in the year of the accident was exposed to the doses of approximately 0,8 Gy/year (of 137Cs), a decrease of fertility and an increase of death level of the newborn are observed. The changes in the genetic structure in the generations subjected to the low-dose radiation coincide with the reactions of a generation to the extreme influences of other (non-radiation) nature.
Thus, the main answer of an animal organism to the regular ionizing radiation is the search for new genetic combinations in future generations.
The main tree type in the forests of the exclusion zone is the Scots pine. The most extreme reaction of the plants to the radiation was the death of pines and fur trees also known today as the “Red forest”. The initial radiation doses were very high. This can be confirmed by the loss of some leafy species of trees (the birch and the Black alder). Average values of the doses, consumed by the trees were exceeding 170 Gy. In such areas the demise of the whole species of both plants and animals took place.
A peculiar natural area, with a great potential for renewal, evolved in the circumstances of anthropogenic radionuclide anomaly. All the stakeholders agree, that these vast unique territories (as well as the protected radiation-ecological area in Byelorussian Polesye) should be preserved.
During the last decade intensive processes of afforestation of meadows and follows are observed in Chornobyl zone.
The species saturation of the plant cover is rather high in the Zone. The Botanical institute of NAS Ukraine recorded presence of more than 40 different species, most of them – adventive and ruderal, new and uncharacteristic for this territory.
The sudden removal of an anthropogenic pressure on the exclusion territories activated natural mechanisms of self-renewal and revival of forest and marshy biogeocenoses, characteristic for Kyiv Polissya. Accordingly to the changes in the vegetation, changes the food potential for the herbivorous animals and for the higher rank consumers; a new specific structure of the animal world appears, while the species that traditionally accompany humans disappeared.
Presently in the exclusion zone the population of large ungulate mammals – moose, wild boar and European roe – many times exceeds those before the accident. The same is true for the European hare and murine rodents; as a result the number of beasts of prey – wolves, foxes, lynxes – also increased comparing to the years before the years before the accident. The closure of melioration systems and their overgrowth with shrubbery helps the increase of the population of the European beaver. The population of the wild boars is around 7 thousands of heads at present. Scientists believe that such a high index of fauna diversity attests favorable conditions for the existence of animals at the territory of Chornobyl exclusion zone. There are over 340 different species of land and water spinal animals. Among them: around 50 species of fish, 11 – of amphibians, 7 – of reptiles, around 200 species of birds and 70 – of mammals. During season migrations the Zone is visited by some 60 more species of birds. 37 of birds’ species, that live at the territory, are listed in the Red List of Ukraine (e.g. osprey, lesser spotted eagle, short-toed eagle, white-tailed eagle and red kite). The number of mammals increased significantly (around 66 different species found on the territory at present) and 16 of them are named in the Red List of Ukraine.
In order to revive and enrich the biodiversity of the local fauna, 20 Przevalsky’s horses were introduced. The environment turned out to be favourable for their effective reproduction in natural habitat; the horses effectively resist beasts of prey and their number gradually increases.
Comparing to 1986 and 1991 the radiological situation 25 years after the accident has improved. At the same time within the third zone (about 200 settlements) measures should still be taken to decrease the radiation doses that could be obtained via local food consumption. The territories within the exclusion zone (approximately 300 km2) will stay unfit for living for some next hundreds of years.
The radiation background has decreased by hundreds of times comparing to 1986. The preventive measures taken and the natural processes of self-clarification both resulted in the decrease of the radionuclide content in both the environment and agricultural products, which in its turn led to the decrease of the inner radiation of the population. The areas of contaminated land have significantly decreased. The areas where the level of contamination with 137Сs is comparable to that before the accident have increased twice in Ukraine, while the territory with high levels of contamination with 90 Sг has become two times smaller. The level and the scale of contamination of Ukrainian territories by the isotopes of plutonium haven’t changed significantly. The activity of 241Ат gradually increases at the expense of 241Рu decay, and the scale of its prevalence is comparable with the prevalence of plutonium isotopes (7).
(1) Radiological state of the highly contaminated territories (by the region). Edited by V.I. Holosha, ME Ukraine, Kyiv 2008.
(2) The same.
(3) 20 years after the Chornobyl catastrophe. A look into the future: Ukrainian national report .– K.: Atika, 2006.– p. 224.
(4) Bondarenko O.O. Exclusion zone – a factor of the radiation risk for the population. HIS – prophylactic medicine, №2, 2005, pp. 88-95.
(5) Petelin G.I., Zimin Y.I. , Tepikin V.E., Rybalka V.B., Pazukhin E.M. «The hot particles of Chernobyl nuclear fuel emissions in a retrospective assessment of the wreckage processes on the 4th block // Radiochemistry. – 2003. – V. 45, N 3. – pp. 278 - 281.
(6) Bulavik I.M. Objectivation of forestry amidst the radioactive contamination of Byelorussian Polesye …— Gomel , 1998. — p. 39.
(7) 20 years after the Chornobyl catastrophe. A look into the future: Ukrainian national report .– K.: Atika, 2006.– p. 224.
by Yaroslav Movchan