Chernobyl disaster

2007 Schools Wikipedia Selection. Related subjects: Recent History

Coordinates: 51°23′23″N, 30°5′58″E

The Chernobyl disaster was an accident at the Chernobyl Nuclear Power Plant on April 26, 1986 at 01:23 a.m., consisting of an explosion at the plant and subsequent radioactive contamination of the surrounding geographic area. The power plant is located at 51°23′23″N, 30°5′58″E, near Pripyat, Ukraine, at the time part of the Soviet Union. It is regarded as the worst accident ever in the history of nuclear power. A plume of radioactive fallout drifted over parts of the western Soviet Union, Eastern and Western Europe, Scandinavia, the UK, Ireland 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.

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 the 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 or even impossible to attribute specifically to the accident. Dr Peter Boyle, director of the International Agency for Research on Cancer, put the discussion of the figures into perspective: "Tobacco smoking will cause several thousand times more cancers in the same (European) population." 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 9,000 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, no evidence of increases in the solid cancers and, possibly more significantly, none of the widely expected increases in leukemia have been found in the population.

The plant

The Chernobyl station (Чернобыльская АЭС им. В.И.Ленина – V.I. Lenin Memorial Chernobyl Nuclear Power Station) ( 51°23′14″N, 30°06′41″E) is located near the town of Prypiat, 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, no. 5 and 6, capable of producing 1 GW each, were under construction at the time of the accident.

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. The accident can be thought of as an extreme version of the SL-1 accident where the core of a reactor was destroyed (killing three men) spreading radioactivity through the inside of the building that SL-1 was in.

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 by the Soviet and Ukrainian governments.

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 void coefficient is a measurement of how the reactor responds to increased steam formation in the water coolant. Most other reactor designs produce less energy as they get hotter, because if the coolant contains steam bubbles, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power. Chernobyl's RBMK reactor, however, used solid graphite to slow down the neutrons, and neutron-absorbing light-water to cool the core. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing an RBMK reactor's temperature means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly produce a lot more energy if the temperature rises. This was counter-intuitive and unknown to the crew.

• A more significant flaw was in the design of the control rods that are 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 rest of the rod – the truly functional part which absorbs the neutrons and thereby halts the reaction – was made of boron carbide. With this design, when the rods are initially inserted into the reactor, the graphite ends displace some coolant. This greatly increases the rate of the fission reaction, since graphite is a more potent neutron moderator (a material that enables a nuclear reaction) and also absorbs much fewer neutrons than the boiling light water. Thus for the first few seconds of control rod activation, reactor's power output increases, rather than reducing as desired. This behaviour is counter-intuitive and was not known to the reactor operators.

• The water channels run through the core vertically, meaning that the water's temperature increases as it moves up and thus creates a temperature gradient in the core. This effect is exacerbated if the top portion turns completely to steam, since the topmost part of the core is no longer being properly moderated and reactivity greatly increases. By contrast, the CANDU reactor's water channels run through the core horizontally, with water flowing in opposite directions among adjacent channels. Hence, the core has a much more even temperature distribution.

• The operators were careless and violated plant procedures, partly due to their lack of knowledge of the reactor's design, and lack of experience and training. Several procedural irregularities also contributed to cause 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 safety systems, which was generally prohibited by the plant's published technical guidelines.

• 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.

The IAEA's 1986 analysis attributed the main cause of the accident to the operators' actions. But in January 1993, the IAEA issued a revised analysis, attributing the main cause to the reactor's design.

Test plan

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's reactors have a pair of diesel 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 results - 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, and the reactor electricity output had been gradually reduced to 50%, a regional power station unexpectedly went offline. The Kiev grid controller demanded to postpone the further reduction of output as electricity was needed to satisfy the evening peak demand. The plant director agreed and postponed the test to comply. The ill-advised safety test was then left to be run by the night shift of the plant, a skeleton crew who would be working Reactor 4 that night and the early part of the next morning.

At 11:00pm, April 25, the grid controller allowed the reactor shut-down to continue. The power output of reactor 4 was to be reduced from its nominal 3.2 GW thermal to 0.7 — 1.0 GW thermal in order to conduct the test at a safer, lower level of power. However, due to a delay in starting the experiment the reactor operators reduced the power level too rapidly, and the actual power output fell to 30 MW thermal. According to operators, the rapid fall in output was due to malfunctioning of one of the automatic power regulators. As a result of output decline, the concentration of the nuclear poison product xenon-135 increased (the xenon production rate: xenon loss rate ratio goes initially higher during a reactor power down). 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.

Fatal experiment

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 the momentum of the turbine generator drove them, 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."

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. 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.

The effects of the disaster

Long-term health effects

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 been 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.

Soviet authorities started evacuating people from the area around the Chernobyl reactor 36 hours after the accident. By May 1986, about a month later, all those living within a 30-kilometre (18 mile) radius of the plant—about 116,000 people—had been relocated. This region is often referred to as the Zone of alienation. However, radiation affected the area in a much wider scale than this 30 km radius.

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.

Farming

Cows

Jiří Hála's text book (Radioactivity, Ionizing Radiation and Nuclear Energy, ISBN 807302053) explains how cattle only pass a minority of the strontium, cesium, plutonium and americium they ingest to the humans who consume milk and meat. For instance, for milk if the cow has a daily intake of 1000 Bq of the following isotopes then the milk will have the following activities. The meat and milk of cows was identified as a radioactive biproduct because of the extremely high amount of radiation that it contained.

• ⁹⁰Sr, 2000 Bq m^-3
• ¹³⁷Cs, 5000 Bq m^-3
• ²³⁹Pu, 1 Bq m^-3
• ²⁴¹Am, 1 Bq m^-3

Soil

Jiří Hála's text book states that soils vary greatly in their ability to bind radioisotopes, the clay particles and humic acids can alter the distribution of the isotopes between the soil water and the soil. The distribution coefficient K_d is the ratio of the soil's radioactivity (Bq g^-1) to that of the soil water (Bq ml^-1). If the radioactivity is tightly bonded to by the minerals in the soil then less radioactivity can be absorbed by crops and grass growing on the soil.

• Cs-137 K_d = 1000
• Pu-239 K_d = 10000 to 100000
• Sr-90 K_d = 80 to 150
• I-131 K_d = 0.007 to 50

Food restrictions

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 carnivore fish 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 km² and 200,000 sheep. Only limited areas of Cumbria, South Western Scotland and Northern Wales are still covered by restrictions.

In Norway, the Sami people were affected by contaminated food. Their reindeer had been contaminated by eating lichens, which extract radioactive particles from the atmosphere along with their nutrients.

Fauna and vegetation

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.

Chernobyl after the disaster

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.

It is unclear how long the ceramic form will retard the release of radioactivity. By conservative estimates, there is 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.

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 makes 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.

Some signs of a criticality were observed in June 24, 1990 - July 1, 1990 inside room 304/3; to avoid any further nuclear fission reaction, a neutron poison was added to this room.

Consequences of further collapse

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 is known that fires can make the radioactivity mobile again.

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 Chernobyl Fund and the Shelter Implementation Plan

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 stabilization 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.

Dimensions:

• Span: 270 m
• Height: 100 m
• Length: 150 m

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.

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. 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. This report is not free of controversy, and has been accused of trying to minimize the consequences of the accident.

The TORCH report

In 2006 German Green MEP (member of the European Parliament) Rebecca Harms, commissioned two UK scientists for an alternate report (TORCH ,The Other Report on Chernobyl) in response to the UN report. The report included areas not covered by the Chernobyl forum report, and also lower radiation doses. 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 urged more research stating that large uncertainties made it difficult to properly asses the full scale of the disaster.

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. 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 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.

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.

Other studies and claims

• The Ukrainian Health Minister claimed in 2006 that more than 2.4 million Ukrainians, including 428,000 children, suffer from health problems related to the catastrophe. Psychological after-effects, as the 2006 UN report pointed out, have also had adverse effects on internally displaced persons.
• Another study alleged heightened mortality in Sweden.
• The UNSCEAR 2000 report on worldwide sources and effects of ionizing radiation, Volume II, Annex J is dedicated to exposures and effects of Chernobyl
• According to the Union Chernobyl, the main organization of liquidators, 10% of the 600,000 liquidators are now dead, and 165,000 disabled.
• The Abstract of the April 2006 International Agency for Research on Cancer report Estimates of the cancer burden in Europe from radioactive fallout from the Chernobyl accident stated "It is unlikely that the cancer burden from the largest radiological accident to date could be detected by monitoring national cancer statistics. Indeed, results of analyses of time trends in cancer incidence and mortality in Europe do not, at present, indicate any increase in cancer rates - other than of thyroid cancer in the most contaminated regions - that can be clearly attributed to radiation from the Chernobyl accident." However, while undetectable, they estimate, based on the linear no threshold model of cancer effects, that 16,000 excess cancer deaths could be expected from the effects of the Chernobyl accident up to 2065. Their estimates have very wide 95% confidence intervals from 6,700 deaths to 38,000.
• A report from the European Committee on Radiation Risk (a body sponsored by the European Green Party) claims that the World Health Organization, together with most other international and national health bodies, has marginalized or ignored, perhaps purposely, the terrible consequences of the Chernobyl fallout to protect the vested interests of the nuclear industry.
• The application of the linear no threshold model to predict deaths from low levels of exposure to radiation was disputed in a BBC (British Broadcasting Corporation) “Horizon” documentary, broadcast on 13 July 2006. It offered statistical evidence to suggest that there is an exposure threshold of about 200 millisieverts below which there is no increase in radiation-induced disease. Indeed it went further, suggesting that low exposures to radiation can have a protective effect. The program interviewed scientists who believe that the increase in thyroid cancer in the immediate area of the explosion had been over-recorded, and predicted that the estimates for widespread deaths in the long term would be proved wrong. It also noted that whilst most cancers can take decades to manifest, Leukemia manifests within a decade or so, and none of the previously expected peak of leukemia deaths has been found, and none is now expected.

French legal action

Since March 2001 400 lawsuits have been filed in France against 'X' 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 Minister Jacques 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.

Comparison with other disasters

The Chernobyl disaster caused a few tens of immediate deaths due to radiation sickness; thousands of premature deaths are predicted over the coming decades. Since it is often not possible to prove the origin of the cancer which causes a person's death, it is difficult to estimate Chernobyl's long-term death toll.

Other man-made disasters with very high death tolls include:

• The failure of the Banqiao Dam ( Henan, China, 1975) — 171,000 killed.
• The Bhopal disaster (India, 1984) — 15,000 killed.
• The Great Smog (London, England, 1952) — 12,000 killed.
• The Johnstown Flood ( Pennsylvania, United States, 1889) — 2,209 killed.

Chernobyl in the popular consciousness

The Chernobyl accident attracted a great deal of interest. Because of the distrust that many people had in the Soviet authorities (people both within and outside the USSR) a great deal of debate about the situation at the site occurred in the first world during the early days of the event. Due to defective intelligence based upon photographs taken from space, it was thought that unit number three had also suffered a dire accident.

In general the public knew little about radioactivity and radiation (they still know very little) and as a result their degree of fear was increased. It was the case that many professionals (such as the spokesman from the UK NRPB) were mistrusted by journalists who in turn encouraged the public to mistrust them.

It was noted in Chernobyl ten years on that different governments tried to set contamination level limits which were stricter than the next country. In the dash to be seen to be protecting the public from radioactive food, it was often the case that the risk caused by the modification of the nations' diet was greater and un-noticed.

As a result of all these events, "Chernobyl" has entered the public consciousness.

After the disaster, the American nuclear-powered aircraft carrier USS Enterprise, the first nuclear-powered capital ship, gained the unofficial nickname "Mobile Chernobyl."