Chernobyl reactor 4 after the disaster, showing the extensive damage to the main reactor hall (image center) and turbine building (image lower left)
The early stages of construction of the sarcophagus.
The Chernobyl disaster occurred at 01:23 a.m. on April 26, 1986 at the Chernobylnuclear power plant in Pripyat, Ukraine within the Soviet Union. It is regarded as the worst accident in the history of nuclear power. Because there was no containment building, a plume of nuclear fallout drifted over parts of the western Soviet Union, Eastern and Western Europe, Scandinavia, the British Isles, and eastern North America. Large areas of Ukraine, Belarus, and Russia were badly contaminated, resulting in the evacuation and resettlement of over 336,000 people. About 60% of the radioactive fallout landed in Belarus, according to official post-Soviet data (quoting the "Committee on the Problems of the Consequences of the Catastrophe at the Chernobyl NPP: 15 Years after Chernobyl Disaster", Minsk, 2001, p. 5/6 ff., and the "Chernobyl Interinform Agency, Kiev und", and "Chernobyl Committee: MailTable of official data on the reactor accident") . The best general overviews of the event can be obtained from the NEA- these are Chernobyl ten years on and Chernobyl twenty years on. The TORCH report is considered less factually sound. According to the 2006 TORCH report, half of the radioactive fallout landed outside the three Soviet republics See also here for an animated Flash map of radioactive fallout caesium-137, produced by the French Institut de radioprotection et de sûreté nucléaire.
The accident raised concerns about the safety of the Soviet nuclear power industry, slowing its expansion for a number of years, while forcing the Soviet government to become less secretive. The now-independent countries of Russia, Ukraine, and Belarus have been burdened with continuing and substantial decontamination and health care costs of the Chernobyl accident. It is difficult to tally accurately the number of deaths caused by the events at Chernobyl, as Soviet-era cover-up made it difficult to track down victims. Lists were incomplete, and Soviet authorities later forbade doctors to cite "radiation" on death certificates. Most of the expected long-term fatalities, especially those from cancer, have not yet actually occurred, and will be difficult to attribute specifically to the accident ("tobacco smoking will cause several thousand times more cancer in the same population"BBC "Horizon" programme13 July2006). Estimates and figures vary widely. The 2005 report prepared by the Chernobyl Forum, led by the International Atomic Energy Agency (IAEA) and World Health Organization (WHO), attributed 56 direct deaths (47 accident workers, and nine children with thyroid cancer), and estimated that as many as 9000 people among the approximately 6.6 million most highly exposed, may die from some form of cancer (one of the induced diseases). . Nearly 20 years after the disaster, according to the Chernobyl Forum, none of the expected increases in leukemia have been found in the population, nor in the solid cancers.
The Chernobyl station (Чернобыльская АЭС им. '.И.Ленина " V.I. Lenin Memorial Chernobyl Nuclear Power Station) () is in the town of Pripyat, Ukraine, 18 km northwest of the city of Chernobyl, 16 km from the border of Ukraine and Belarus, and about 110 km north of Kiev. The station consisted of four reactors of type RBMK-1000, each capable of producing 1 GW of electric power (3.2 GW of thermal power), and the four together produced about 10% of Ukraine's electricity at the time of the accident. Construction of the plant began in the 1970s, with reactor no. 1 commissioned in 1977, followed by no. 2 (1978), no. 3 (1981), and no. 4 (1983). Two more reactors, nos. 5 and 6, capable of producing 1 GW each, were under construction at the time of the accident.
On Saturday April 26, 1986 at 1:23:58 a.m. reactor 4 suffered a catastrophic steam explosion that resulted in a fire, a series of additional explosions, and a nuclear meltdown.
Causes
There are two conflicting official theories about the cause of the accident. The first was published in August 1986 and effectively placed the blame solely on the power plant operators. The second theory, proposed by Valeri Legasov and published in 1991, attributed the accident to flaws in the RBMK reactor design, specifically the control rods. Both commissions were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and the Soviet and Ukrainian governments. Some independent experts now believe that neither theory is completely correct.
Another important factor contributing to the accident was that the operators were not informed about problems with the reactor. According to one of them, Anatoliy Dyatlov, the designers knew that the reactor was dangerous in some conditions but intentionally concealed this information. Contributing to this was that the plant's management was largely composed of non-RBMK-qualified personnel: the director, V.P. Bryukhanov, had experience and training in a coal-fired power plant. His chief engineer, Nikolai Fomin, also came from a conventional power plant. Dyatlov, deputy chief engineer of reactors 3 and 4, had only "some experience with small nuclear reactors", namely smaller versions of the VVER nuclear reactors that were designed for the Soviet Navy's nuclear submarines.
In particular: * The reactor had a dangerously large positive void coefficient. The RBMK reactor design used light water as its coolant. Coolant gives the operators some control over the speed of the reactions, controlling the reactor's energy output. If the coolant has bubbles (voids) from steam in it, these voids increase the amount of energy the reactor produces (because there is no liquid to attenuate neutrons). Without intervention, the reactor produces more energy, creating more voids, becoming harder to control. That the reactor design was dangerous at low power levels was counter-intuitive and unknown to the crew. * A more significant flaw was in the design of the control rods, inserted into the reactor to slow down the reaction. In the RBMK reactor design, the control rod end tips were made of graphite and the extenders (the end areas of the control rods above the end tips, measuring 1 m in length) were hollow and filled with water, while the balance of the control rod"the truly functional area, absorbing the neutrons and thereby halting the reaction"was made of boron carbide. For the initial few moments when control rods of this design are inserted into the reactor, coolant is displaced by the graphite ends of the rods. The coolant (water), a neutron absorber, is therefore replaced by graphite, a neutron moderator " that is, a material that enables the nuclear reaction, rather than slows the reaction down. For the first few seconds of control rod activation the rods increased the reactor's speed, rather than the desired effect of decreasing the reaction. This behavior is rather counter-intuitive and was not known to the reactor operators. Plus, the heat of ignition required for graphite is high, but it is extremely difficult to put the fire out afterwards. * The operators were careless and violated plant procedures, partly due to their lack of knowledge of the reactor's design flaws. Also, several procedural irregularities contributed to the cause of the accident. One was insufficient communication between the safety officers and the operators in charge of an experiment being run that night.
The operators switched off many of the reactor's safety systems, which was generally prohibited by the plant's published technical guidelines.
According to a Government Commission report published in August 1986, operators removed at least 204 control rods from the reactor core out of a total of 211, leaving seven (see Boris Gorbachev's article in Russian about the causes). The same guidelines (noted above) prohibit operation of the RBMK-1000 with fewer than 15 rods inside the core zone.
Events
During the daytime of April 25, 1986, reactor 4 was scheduled to be shut down for maintenance. It had been decided to use this occasion as an opportunity to test the ability of the reactor's turbine generator to generate sufficient electricity to power the reactor's safety systems (in particular, the water pumps) in the event of a loss of external electric power. This type of reactor requires water being continuously circulated through the core, as long as the nuclear fuel is present. Chernobyl reactors have a pair of deisel generators available as standby, but these do not activate instantaneously"the reactor was, therefore, to be used to spin up the turbine, at which point the turbine would be disconnected from the reactor and allowed to spin under its own rotational momentum, and the aim of the test was to determine whether the turbines in the rundown phase could power the pumps while the generators were starting up. The test was successfully carried out previously on another unit (with all safety provisions active) with negative result - the turbines did not generate sufficient power, but additional improvements were made to the turbines, which prompted the need for another test.
Prior to accident
As conditions to run this test were prepared during the daytime of April 25, a regional power station unexpectedly went offline. The Kiev grid controller demanded extra output from Chernobyl. The plant director agreed and cancelled the test to comply. The doomed safety test was then left to be run by the nightshift of the plant, a skeleton crew who would be working Reactor 4 that night and the early part of the next morning.BBC (British Broadcasting Corporation) Documentary entitled "Days That Shook The World" The power output of reactor 4 was to be reduced from its normal 3.2 GW thermal to 1 GW thermal in order to conduct the test at a safer, lower level of power. However, due to a delay in beginning the experiment the reactor operators reduced the power level too rapidly, and the actual power output fell to 30 MW thermal. As a result, the concentration of the nuclear poison product xenon-135 increased (the [xenon production rate]:[xenon loss rate] ratio goes initially higher during a reactor downpower). Though the scale of the power drop was close to the maximum allowed by safety regulations, the crew's management chose not to shut down the reactor, and to continue the experiment. Further, it was decided to 'shortcut' the experiment and raise power output to only 200 MW. In order to overcome the neutron absorption of the excess xenon-135, the control rods were pulled out of the reactor somewhat further than normally allowed under safety regulations. As part of the experiment, at 1:05 a.m. on April 26 the water pumps that were to be driven by the turbine generator were turned on; the water flow generated by this action exceeded that specified by safety regulations. The water flow increased at 1:19 a.m. " since water also absorbs neutrons, this further increase in the water flow necessitated the removal of the manual control rods, producing a very unstable and dangerous operating condition.
At 1:23:04 the experiment began. The unstable state of the reactor was not reflected in any way on the control panel, and it does not appear that anyone in the reactor crew was fully aware of any danger. Electricity to the water pumps was shut off, and as they were driven by the momentum of the turbine generator the water flow rate decreased. The turbine was disconnected from the reactor, increasing the level of steam in the reactor core. As the coolant heated, pockets of steam formed voids in the coolant lines. Due to the RBMK reactor-type's large positive void coefficient, the power of the reactor increased rapidly, and the reactor operation became progressively less stable and more dangerous. At 1:23:40 the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button that ordered a "SCRAM" " a shutdown of the reactor, fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. It is unclear whether it was done as an emergency measure, or simply as a routine method of shutting down the reactor upon the completion of an experiment (the reactor was scheduled to be shut down for routine maintenance). It is usually suggested that the SCRAM was ordered as a response to the unexpected rapid power increase. On the other hand, Anatoly Dyatlov, chief engineer at the nuclear station at the time of the accident, writes in his book:
"Prior to 01:23:40, systems of centralized control ... didn't register any parameter changes that could justify the SCRAM. Commission ... gathered and analyzed large amount of materials and, as stated in its report, failed to determine the reason why the SCRAM was ordered. There was no need to look for the reason. The reactor was simply being shut down upon the completion of the experiment." "лава 4. КАК ЭТО БЫЛО
Due to the slow speed of the control rod insertion mechanism (18–20 seconds to complete), the hollow tips of the rods and the temporary displacement of coolant, the SCRAM caused the reaction rate to increase. Increased energy output caused the deformation of control rod channels. The rods became stuck after being inserted only one-third of the way, and were therefore unable to stop the reaction. By 1:23:47 the reactor jumped to around 30 GW, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased, causing a large steam explosion. Generated steam traveled vertically along the rod channels in the reactor, displacing and destroying the reactor lid, rupturing the coolant tubes and then blowing a hole in the roof. Фатахов Алексей Чернобыль как это было - 2
To reduce costs, and because of its large size, the reactor had been constructed with only partial containment. This allowed the radioactive contaminants to escape into the atmosphere after the steam explosion burst the primary pressure vessel. After part of the roof blew off, the inrush of oxygen, combined with the extremely high temperature of the reactor fuel and graphite moderator, sparked a graphite fire. This fire greatly contributed to the spread of radioactive material and the contamination of outlying areas.
Contradictions among accounts
There is some controversy surrounding the exact sequence of events after 1:22:30 due to the inconsistencies between eyewitness accounts and station records. The version that is most commonly agreed upon is described above. According to this theory, the first explosion happened at approximately 1:23:47, seven seconds after the operators ordered the "SCRAM". It is sometimes claimed that the explosion happened 'before' or immediately following the SCRAM (this was the working version of the Soviet committee studying the accident). This distinction is important, because, if the reactor went critical several seconds after the SCRAM, its failure would have to be attributed to the design of the control rods, whereas the explosion at the time of the SCRAM would place the blame on the operators. Indeed, a weak seismic event, similar to a magnitude 2.5 earthquake, was registered at 1:23:39 in the Chernobyl area.
There are also contradicting reports on when and how many times the SCRAM was ordered. According to one of the articles, the button was pressed twice with a 2-second interval (which would imply the operator's state of panic in response to some unusual event), another claims it was pressed twice with a 7-second interval (the second time " after the explosion).
The plan of the test is also reported differently. According to some reports, the plan was to repeat the test multiple times at different loads on the turbine. If it were indeed so, the Dyatlov's claim "the reactor was shut down at the completion of the test" is misleading.
In January 1993, the IAEA issued a revised analysis of the Chernobyl accident, attributing the main root cause to the reactor's design and not to operator error. The IAEA's 1986 analysis had cited the operators' actions as the principal cause of the accident.
Radioactive release (source term)
In common with many other releases of radioactivity into the environment, the Chernobyl release was controlled by the physical and chemical properties of the radioactive elements present in the core. While the general population often has a morbid fear of plutonium, in common with bomb fallout the effects of the plutonium are almost entirely eclipsed by those of the fission products. For a more detailed discussion of the release of radioactivity from a power reactor please see fission products, nuclear fuel and Nuclear fuel and reactor accidents.
The external gamma dose for a person in the open near the Chernobyl site.
At different times after the accident, different isotopes were responsible for the majority of the external dose. The dose which has been calculated is from external gamma irradiation, for a person standing in the open. The gamma dose to a person in a shelter or the internal dose is harder to estimate.
Because the fission products page has a detailed discussion of the properties of those fission products which are most dangerous, only a short description of the radioisotopes released will be given here.
The release of the radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor. * All of the noble gases, including (krypton and xenon) contained within the reactor were released immediately into the atmosphere by the first steam explosion. * About 55% of the radioactive iodine in the reactor was released, as a mixture of vapour, solid particles and also in the form of organic iodine compounds. * Caesium and tellurium were released in aerosol form.
Two sizes of particles were released: the small particles were 0.3 to 1.5 micrometers (aerodynamic diameter) while the large were 10 micrometers in size. The larger particles contained about 80% to 90% of the released nonvolatile radioisotopes (95Zr, 95Nb, 140La, 144Ce and the transuranic elements (neptunium, plutonium and the minor actinides) embedded in a uranium oxide matrix.
The contributions made by the different isotopes to the dose (in air) caused in the contaminated area in the time shortly after the accident. Note that this image was drawn using data from the OECD report, the Korean table of the isotopes and the second edition of 'The radiochemical manual'.
Immediate crisis management
The scale of the tragedy was exacerbated by both the unpreparedness of local administrators and the lack of proper equipment. All but two dosimeters present in the reactor 4 building had limits of 1 milliröntgen per second. The remaining two had limits of 1000 R·s-1; access to one of them was blocked by the explosion, and the other one broke when turned on. Thus the reactor crew could ascertain only that the radiation levels in much of the reactor building were above 4 R·h-1 (true levels were up to 20,000 R·h-1 in some areas; lethal dose is around 500 R over 5 hours).
This allowed the chief of reactor crew, Alexander Akimov, to assume that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 4:30 a.m. were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most of them, including Akimov himself, died from radiation exposure in the three weeks following the accident.
Shortly after the accident, firefighters arrived to try to extinguish the fires. The first one to the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Vladimir Pravik, who died on May 9, 1986. They were not told how dangerously radioactive the smoke and the debris were. The fire was extinguished by 5 a.m., but many firefighters received high doses of radiation.
The explosion and fire threw into the air not just the particles of the nuclear fuel but also far more dangerous radioactive elements like caesium-137, iodine-131, strontium-90 and other radionuclides. The residents of the surrounding area observed the radioactive cloud on the night of the explosion. The cloud was noticeably glowing.
The government committee, led by Valeri Legasov, formed to investigate the accident arrived at Chernobyl in the evening of April 26. By that time two people were dead and 52 were hospitalized. During the night of April 26–April 27—more than 24 hours after the explosion—the committee, faced with ample evidence of extremely high levels of radiation and a number of cases of radiation exposure, had to acknowledge the destruction of the reactor and order the evacuation of the nearby city of Pripyat. In order to reduce baggage, the residents were told that the evacuation would be temporary, lasting approximately three days. As a result, Pripyat still contains personal belongings that can never be moved due to radiation. From eyewitness accounts of the firefighters involved before they died (as reported on the BBC television series Witness), one described his experience of the radiation as "tasting like metal", and feeling a sensation similar to that of pins and needles all over his face.
The water that had hurriedly been pumped into the reactor building in a futile attempt to extinguish the fire had run down underneath the reactor floor to the space underneath. The problem presented by this was that the smouldering fuel and other material on the reactor floor was starting to burn its way through this floor, and was being made worse by materials being dropped from helicopters, which simply acted as a furnace to increase the temperatures further. If this material had come into contact with the water, it would have generated a thermal explosion which would have arguably been worse than the initial reactor explosion itself, and would have, by many estimates, rendered land in a radius of hundreds of miles from the plant radioactive.http://www.discoverychannel.co.uk/battle_of_chernobyl/index.shtml
In order to prevent this, soldiers and workers (called "liquidators") were sent in as cleanup staff by the Soviet government. Two of these were sent in wet suits to open the sluice gates to vent the radioactive water, and thus prevent a thermal explosion.BBC.co.uk - The Chernobyl nightmare revisited These men, just like the other liquidators and firefighters that helped with the cleanup, were not told of the danger they faced and it is questioned whether they even returned to the surface before their death.
The worst of the radioactive debris was collected inside what was left of the reactor. The reactor itself was covered with bags with sand, lead and boric acid thrown off helicopters (some 5,000 tons during the week following the accident). By December 1986 a large concrete sarcophagus had been erected, to seal off the reactor and its contents.The Social Impact of the Chernobyl Disaster, 1988, p166, by David R. Marples ISBN 0333481984
Many of the vehicles used by the "liquidators" remain scattered around the Chernobyl area to this day.
A monument to the victims of the Chernobyl disaster at Moscow's Mitino cemetery, where some of the firefighters who battled the flames and later died of radiation exposure are buried. Photo by Mikhail Evstafiev
The nuclear meltdown produced a radioactive cloud which flew all over Europe. (page 3) The initial evidence that a major exhaust of radioactive material was affecting other countries came not from Soviet sources, but from Sweden, where on April 27 workers at the Forsmark Nuclear Power Plant (approximately 1100 km from the Chernobyl site) were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that led to the first hint of a serious nuclear problem in the western Soviet Union.
Contamination from the Chernobyl accident was not evenly spread across the surrounding countryside, but scattered irregularly depending on weather conditions. Reports from Soviet and Western scientists indicate that Belarus received about 60% of the contamination that fell on the former Soviet Union. However, the TORCH 2006 report stated that half of the volatile particles had landed outside Ukraine, Belarus and Russia. A large area in the Russian Federation south of Bryansk was also contaminated, as were parts of northwestern Ukraine.
In Western Europe, measures were taken including seemingly arbitrary regulations pertaining to the legality of importation of certain foods but not others. One commonly ridiculed contention was in France where some officials stated that the Chernobyl accident had no adverse effects — which was ridiculed as pretending that the radioactive cloud had stopped at the German and Italian borders.
Two hundred people were hospitalized immediately, of whom 31 died (28 of them died from acute radiation exposure) . Most of these were fire and rescue workers trying to bring the accident under control, who were not fully aware of how dangerous the radiation exposure (from the smoke) was (for a discussion of the more important isotopes in fallout see fission products). 135,000 people were evacuated from the area, including 50,000 from Pripyat, Ukraine. Health officials have predicted that over the next 70 years there will be a 2% increase in cancer rates in much of the population which was exposed to the 5–12 (depending on source) EBq of radioactive contamination released from the reactor. An additional ten people have already died of cancer as a result of the accident.
Soviet scientists reported that reactor 4 contained about 180"190 t of uranium dioxide fuel and fission products. Estimates of the amount of this material that escaped range from 5 to 30%, but some liquidators who have actually been inside the sarcophagus and the reactor shell itself " e.g. Mr. Usatenko and Dr. Karpan " state that not more than 5"10% of the fuel remains inside; indeed, photographs of the reactor shell show that it is completely empty. Because of the intense heat of the fire, much of the ejected fuel was lofted high into the atmosphere, with no containment building to stop it, where it spread.
The "liquidators" received high doses of radiation. According to Soviet estimates, between 300,000 and 600,000 liquidators were involved in the cleanup of the 30-km evacuation zone around the reactor, but many of them entered the zone two years after the accident.Chapter IV: Dose estimates, Nuclear Energy Agency, 2002
Long-term health effects
Map showing caesium-137 contamination in Belarus, Russia, and Ukraine. In curies by square meters (1 curie is 37 gigabecquerels (GBq) or 37 billion becquerels exactly).
Right after the accident, the main health concern involved radioactive iodine, with a half-life of eight days. Today, there is concern about contamination of the soil with strontium-90 and caesium-137, which have half-lives of about 30 years. The highest levels of caesium-137 are found in the surface layers of the soil where they are absorbed by plants, insects and mushrooms, entering the local food supply. However, in 2006 hedgehogs from the area, an insectivorous species seem to have absorbed little if any radioactive material, whilst rodents are strongly radiating (20 millisieverts per day), although seem to suffer no ill effects.
Some persons in the contaminated areas were exposed to large thyroid doses of up to 50 grays (Gy) because of an intake of radioactive iodine-131, a relatively short-lived isotope with a half-life of eight days, but which concentrates in the thyroid gland. This would have been absorbed from contaminated milk produced locally, particularly in children. Several studies have found that the incidence of thyroid cancer in Belarus, Ukraine and Russia has risen sharply, however there have barely more than a handful of deaths. Some scientists think that most of the increase is caused by greatly increased monitoring.
So far, no increase in leukemia in the general population is discernible.
Some scientists fear that radioactivity will affect the local population for the next several generations; however, there is little evidence so far of this.
The issue of long-term effects of Chernobyl disaster on civilians is controversial. Over 300,000 people were resettled because of the accident; millions lived and continue to live in the contaminated area. On the other hand, most of those affected received relatively low doses of radiation; there is little evidence of increased mortality, cancers or birth defects among them; and when such evidence is present, existence of a causal link to radioactive contamination is uncertain.
Aside from obstacles posed by Soviet policies during and after the catastrophe, scientific studies may still be limited by a lack of democratic transparency. In Belarus, Yuri Bandazhevsky, a scientist who questioned the official estimates of Chernobyl's consequences and the relevance of the official maximum limit of 1000 Bq/kg, has allegedly been a victim of political repression. He was imprisoned from 2001 to 2005 on a bribery conviction, after his 1999 publication of reports critical of the official research being conducted into the Chernobyl incident.
Food restrictions
An abandoned village near Prypiat, close to Chernobyl
In April 1986 several European countries, excluding France, had enforced food restrictions, most notably on mushrooms and milk. Twenty years after the catastrophe, restriction orders remain in place in the production, transportation and consumption of food contaminated by Chernobyl fallout, in particular caesium-137, in order to prevent them from entering the human food chain. In parts of Sweden and Finland, restrictions are in place on stock animals, including reindeer, in natural and near-natural environments. "In certain regions of Germany, Austria, Italy, Sweden, Finland, Lithuania and Poland, wild game, including boar and deer, wild mushrooms, berries and carnivorefish from lakes reach levels of several thousand Bq per kg of caesium-137", while "in Germany, caesium-137 levels in wild boar muscle reached 40,000 Bq/kg. The average level is 6800 Bq/kg, more than ten times the EU limit of 600 Bq/kg", according to the TORCH 2006 report. The European Commission has stated that "The restrictions on certain foodstuffs from certain Member States must therefore continue to be maintained for many years to come".
In the United Kingdom, under powers in the 1985 Food and Environment Protection Act (FEPA), Emergency Orders have been used since 1986 to impose restrictions on the movement and sale of sheep exceeding the limit of 1000 Bq/kg. This safety limit was introduced in the UK in 1986 based on advice from the European Commission's Article 31 group of experts. However, the area covered by these restrictions has decreased by 95% since 1986: while it covered at first almost 9000 farms and over 4 million sheep, as of 2006 it covers 374 farms covering 750 km 2 and 200 000 sheep. Only limited areas of Cumbria, South Western Scotland and Northern Wales are still covered by restrictions.
After the disaster, four square kilometres of pine forest in the immediate vicinity of the reactor went ginger brown and died, earning the name of the Red Forest, according to the BBC. Some animals in the worst-hit areas also died or stopped reproducing. Mice embryos simply dissolved, while horses left on an island 6 km from the power plant died when their thyroid glands disintegrated. Cattle on the same island were stunted due to thyroid damage, but the next generation were found to be surprisingly normal.
In the years since the disaster, the exclusion zone abandoned by humans has become a haven for wildlife, with nature reserves declared (Belarus) or proposed (Ukraine) for the area. Many species of wild animals and birds which were never seen in the area prior to the disaster, are now plentiful, due to the absence of humans in the area. Wildlife defies Chernobyl radiation, by Stefen Mulvey, BBC News
Controversy over fatality estimates
The majority of premature deaths caused by Chernobyl are expected to be the result of cancers and other diseases induced by radiation in the decades after the event. This will be the result of a large population (some studies have considered the entire population of Europe) exposed to relatively low doses of radiation increasing the risk of cancer across that population. It will be impossible to attribute specific deaths to Chernobyl, and many estimates indicate that the rate of excess deaths will be so small as to be statistically undetectable, even if the ultimate number of extra premature deaths is large. Furthermore, interpretations of the current health state of exposed population is subject vary. Therefore, estimates of the ultimate human impact of the disaster have relied on numerical models of the effects of radiation on health. Furthermore, the effects of low-level radiation on human health are not well understood, and so the models used, notably the linear no threshold model, are open to question. For evidence of increased congenital malformation and stillbirth rates see for example:http://www.ibis-birthdefects.org/start/cache/Congenital%20Malformations%20Stillborn.pdf
Given these factors, several different studies of Chernobyl's health effects have come up with substantially different conclusions and are the subject of considerable scientific and political controversy. The following section presents some of the major studies on this topic.
The Chernobyl Forum report
In September 2005, a draft summary report by the Chernobyl Forum, comprising a number of UN agencies including the International Atomic Energy Agency (IAEA), the World Health Organization (WHO), the United Nations Development Programme (UNDP), other UN bodies and the Governments of Belarus, the Russian Federation and Ukraine, put the total predicted number of deaths due to the accident at 4000 . This death toll predicted by the WHO included the 47 workers who died of acute radiation syndrome as a direct result of radiation from the disaster and nine children who died from thyroid cancer, in the estimated 4000 excess cancer deaths expected among the 600,000 with the highest levels of exposure. For full coverage see the IAEA Focus Page (op.cit.) and joint IAEA/WHO/UNDP September 5, 2005 press release Chernobyl: The True Scale of the Accident The full version of the WHO health effects report adopted by the UN, published in April 2006, included the prediction of 5000 additional fatalities from significantly contaminated areas in Belarus, Russia and Ukraine and predicted that, in total, 9000 will die from cancer among the 6.8 million most-exposed Soviet citizens .
The TORCH 2006 report
German GreenMEP (member of the European Parliament) Rebecca Harms, commissioned a report (TORCH ,The Other Report on Chernobyl) in 2006 in response to the UN report; it stated that:
"In terms of their surface areas, Belarus (22% of its land area) and Austria (13%) were most affected by higher levels of contamination. Other countries were seriously affected; for example, more than 5% of Ukraine, Finland and Sweden were contaminated to high levels (> 40,000 Bq/m2 caesium-137). More than 80% of Moldova, the European part of Turkey, Slovenia, Switzerland, Austria and the Slovak Republic were contaminated to lower levels (> 4000 Bq/m2 caesium-137). And 44% of Germany and 34% of the UK were similarly affected." (See map of radioactive distribution of Caesium-137 in Europe)
The IAEA/WHO and UNSCEAR considered areas with exposure greater than 40,000 Bq/m2; the TORCH report also included areas contaminated with more than 4000 Bq/m2 of Cs-137.
The TORCH 2006 report "estimated that more than half the iodine-131 from Chernobyl [which increases the risk of thyroid cancer] was deposited outside the former Soviet Union. Possible increases in thyroid cancer have been reported in the Czech Republic and the UK, but more research is needed to evaluate thyroid cancer incidences in Western Europe". It predicted about 30,000 to 60,000 excess cancer deaths and warned that predictions of excess cancer deaths strongly depend on the risk factor used; and predicted excess cases of thyroid cancer range between 18,000 and 66,000 in Belarus alone depending on the risk projection model TORCH report executive summary, op.cit., p.4 Furthermore it pointed out that many diseases have latencies such that it is very difficult to generate accurate estimates as early as 2006, stating that "most solid cancers have long periods between exposure and appearance of between 20 and 60 years. Now, 20 years after the accident, an average 40% increased incidence in solid cancer has been observed in Belarus with the most pronounced increase in the most contaminated regions." It also quoted the 2005 Forum's report, which documented preliminary evidence of an increase in the incidence of pre-menopausal breast cancer among women exposed at ages lower than 45 years. The TORCH report also stated that "two non-cancer effects, cataract induction and cardiovascular diseases, are well documented with clear evidence of a Chernobyl connection." Quoting the report, Nature wrote that: "it is well known that radiation can damage genes and chromosomes"; "the relationship between genetic changes and the development of future disease is complex and the relevance of such damage to future risk is often unclear. On the other hand, a number of recent studies have examined genetic damage in those exposed to radiation from the Chernobyl accident. Studies in Belarus have suggested a twofold increase in the germline minisatellite mutation rate". Concerning human minisatellite mutation rate after the Chernobyl accident, the Nature April 2006 article also quotes ò
Map of radioactive fallout caesium-137 after Chernobyl catastrophe. In kilobecquerels per square meter (kBq/m²). Copyright J.Smith and N.A. Beresford, "Chernobyl: Catastrophe and Consequences" (Praxis, Chichester, 2005). See also an animated map of radioactive fallout caesium-137, produced by the French [[Institut de radioprotection et de sûreté nucléaire
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Greenpeace
Greenpeace claimed contradictions in the Chernobyl Forum reports, quoting a 1998 WHO study referenced in the 2005 report, which projected 212 dead from 72,000 liquidators WHO Chernobyl report 2006 pdf. In its report, Greenpeace suggested there will be 270,000 cases of cancer attributable to Chernobyl fallout, and that 93,000 of these will probably be fatal, but state in their report that "The most recently published figures indicate that in Belarus, Russia and the Ukraine alone the accident could have resulted in an estimated 200,000 additional deaths in the period between 1990 and 2004." Blake Lee-Harwood, campaigns director at Greenpeace, believes that cancer was likely to be the cause of less than half of the final fatalities and that "intestinal problems, heart and circulation problems, respiratory problems, endocrine problems, and particularly effects on the immune system," will also cause fatalities. However concern has been expressed about the methods used in compiling the Greenpeace report.Wall Street Journal, 27 April 2006Spiegel, The Chernobyl body count controversy
The April 2006 IPPNW report
According to an April 2006 report by the German affiliate of the International Physicians for Prevention of Nuclear Warfare (IPPNW), entitled "Health Effects of Chernobyl", more than 10,000 people are today affected by thyroid cancer and 50,000 cases are expected. The report projected tens of thousands dead among the liquidators. In Europe, it alleges that 10,000 deformities have been observed in newborns because of Chernobyl's radioactive discharge, with 5000 deaths among newborn children. They also claimed that several hundreds of thousands of the people who worked on the site after the accident are now sick because of radiation, and tens of thousands are dead .
Map describing radioactive fallout of Caesium-137 in France in May 1986. In Bq/m2. Copyright CRIIRAD.
Since March 2001 400 lawsuits have been filed in France against persons unknown by the French Association of Thyroid-affected People, including 200 in April 2006. These persons are affected by thyroid cancer or goitres, and have filed lawsuits alleging that the French government, at the time led by Prime MinisterJacques Chirac, had not adequately informed the population of the risks linked to the Chernobyl radioactive fallout. The complaint contrasts the health protection measures put in place in nearby countries (warning against consumption of green vegetables or milk by children and pregnant women) with the relatively high contamination suffered by the east of France and Corsica. Although the 2006 study by the French Institute of Radioprotection and Nuclear Safety said that no clear link could be found between Chernobyl and the increase of thyroid cancers in France, it also stated that papillary thyroid cancer had tripled in the following years (includes Audio files, with an interview with Chantal Loire, president of the French Association of Thyroid-Affected People, as well as interviews with member of the CRIIRAD.
Far fewer people died as a result of the Chernobyl event than died at Hiroshima, even including those predicted by the WHO to die in the future. The radioactivity released at Chernobyl tended to be more long lived than that released by a bomb detonation. However Chernobyl released 890 times as much caesium-137 as the Hiroshima bomb, but it only released 87 times as much strontium-90 as the Hiroshima bomb. When the iodine-131 release is compared between the events (decay corrected to three days after the event) then Chernobyl only released 25 times as much as the Hiroshima bomb. When the xenon-133 release is compared between the events (decay corrected to three days after the event) then Chernobyl only released 31 times as much as the Hiroshima bomb. Hence it is not possible to draw a simple comparison between the two events.Sources of environmental radioactivity
A comparision of the gamma dose rates due to chernobyl and bomb fallout, these have been normalised to the same Cs-137 level.
The completed (but crumbling) sarcophagus surrounding Chernobyl reactor 4, viewed from the northwest.
Following the accident, questions arose on the future of the plant and its eventual fate. All work on the unfinished reactors 5 and 6 were immediately halted. However the trouble at the Chernobyl plant did not end with the disaster in reactor 4. The damaged reactor was sealed off and 200 metres of concrete was placed between the disaster site and the operational buildings. The Ukrainian government continued to let the three remaining reactors operate because of an energy shortage in the country. A fire broke out in reactor 2 in 1991; the authorities subsequently declared the reactor damaged beyond repair and had it taken offline. Reactor 1 was decommissioned in November 1996 as part of a deal between the Ukrainian government and international organizations such as the IAEA to end operations at the plant. On December 15th, 2000, then-President Leonid Kuchma personally turned off Reactor 3 in an official ceremony, effectively shutting down the entire plant. This transformed the Chernobyl plant from energy producer to energy consumer.
The need for future repairs
The sarcophagus is not an effective permanent enclosure for the destroyed reactor. Its hasty construction, in many cases conducted remotely with industrial robots, is aging badly. If it collapses another cloud of radioactive dust could be released. The sarcophagus is so badly damaged that a small earthquake or severe wind could cause the roof to collapse. A number of plans have been discussed for building a more permanent enclosure.
According to official estimates, about 95% of the fuel (about 180 tonnes) in the reactor at the time of the accident remains inside the shelter, with a total radioactivity of nearly 18 million curies (670 PBq). The radioactive material consists of core fragments, dust, and lava-like "fuel-containing materials" (FCM) that flowed through the wrecked reactor building before hardening into a ceramic form.
The radioactivity levels of different isotopes in the FCM, this has been back calculated by Russian workers to Aprl 1986
It is unclear how long the ceramic form will retard the release of radioactivity. By conservative estimates, there are at least four tons of radioactive dust inside the shelter. However, more recent estimates have strongly questioned the previously held assumptions regarding the quantity of fuel remaining in the reactor. Some estimates now place the total quantity of fuel in the reactor at only about 70% of the original fuel load, however the IAEA maintains that less than 5% of the fuel was lost due to the explosion. Moreover, some liquidators estimate that only 5"10% of the original fuel load remains inside the sarcophagus.
A photograph of one of the lava-flows formed by corium Fuel containing mass in the basement of the Chernobyl plant. 1 is the lava flow, 2 is concrete, 3 is a steam pipe and 4 is some electrical equipment
Water continues to leak into the shelter, spreading radioactive materials throughout the wrecked reactor building and potentially into the surrounding groundwater. The basement of the reactor building is slowly filling with water that is contaminated with nuclear fuel and is considered high-level radioactive waste. Though repairs were undertaken to fix some of the most gaping holes that had formed in the roof, it is by no means watertight, and will only continue to deteriorate.
The sarcophagus, while not airtight, heats up much more readily than it cools down. This is contributing to rising humidity levels inside the shelter. The high humidity inside the shelter continues to erode the concrete and steel of the sarcophagus.
Further, dust is becoming an increasing problem within the shelter. Radioactive particles of varying size, most of similar consistency to ash make up a large portion of the debris inside the shelter. Convection currents compounded with increasing intrusion of outside airflow are increasingly stirring up and suspending the particles in the air inside the shelter. The installation of air filtration systems in 2001 has reduced the problem, but not eliminated it.
The present shelter is constructed atop the ruins of the reactor building. The two "Mammoth Beams" that support the roof of the shelter are resting upon the structurally unsound west wall of the reactor building that was damaged by the accident. If the wall of the reactor building and subsequently the roof of the shelter were to collapse, then large amounts of radioactive dust and particles would be released directly into the atmosphere, resulting in a large new release of radiation into the environment.
A further threat to the shelter is the concrete slab that formed the "Upper Biological Shield" (UBS), and rested atop the reactor prior to the accident. This concrete slab was thrown upwards by the explosion in the reactor core and now rests at approximately 15° from vertical. The position of the upper bioshield is considered inherently unsafe, in that only debris is supporting it in a nearly upright position. The collapse of the bioshield would further exacerbate the dust conditions in the shelter, would probably spread some quantity of radioactive materials out of the shelter, and could damage the shelter itself.
The sarcophagus was never designed to last for the 100 years needed to contain the radioactivity found within the remains of reactor 4. While present designs for a new shelter anticipate a lifetime of up to 100 years, that time is minuscule compared to the lifetime of the radioactive materials within the reactor. The construction and maintenance of a permanent sarcophagus that can completely contain the remains of reactor 4 will present a continuing task to engineers for many generations to come.
Grass and forest fires
It has been reported by V.I. Yoschenko et. al., Journal of Environmental Radioactivity, 2006, 86, 143-163 that grass and forest fires can make the caesium, strontium, and plutonium become mobile in the air again. As an experiment fires were set and the levels of the radioactivity in the air down wind of these fires was measured.
The rate of delivery of radioactivity which has been made mobile by a grass fire.
A conceptual rendering of the New Safe Confinement to replace the aging sarcophagus.
The Chernobyl Shelter Fund was established in 1997 at the Denver G7 summit to fund the Shelter Implementation Fund. The Shelter Implementation Plan (SIP) calls for transforming the site into an ecologically safe condition through stabilisation of the sarcophagus, followed by construction of a New Safe Confinement (NSC). The original cost estimate for the SIP was US$768 million. The SIP is being managed by a consortium of Bechtel, Battelle, and Electricité de France, and conceptual design for the NSC consists of a movable arch, constructed away from the shelter to avoid high radiation, to be slid over the sarcophagus. The NSC will be the largest movable structure ever built, and is expected to be completed in early 2009.DimensionsSpan: 270 mHeight: 100 mLength: 150 m
The Chernobyl accident riveted international attention. Around the world, people read the story and were profoundly affected. As a result, "Chernobyl" has entered the public consciousness in a number of different ways.
