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Chernobyl accident

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The nuclear power plant at Chernobyl.

The Chernobyl accident occurred on April 26, 1986, at the Chernobyl nuclear power plant in Ukraine (then part of the Soviet Union). It is regarded as the worst nuclear accident in the history of nuclear power. It produced a plume of radioactive debris that drifted over parts of the western Soviet Union, Eastern Europe, and Scandinavia. Large areas of Ukraine, Belarus, and Russia were contaminated, resulting in the evacuation and resettlement of roughly 200,000 people. About 60% of radioactive fallout landed in Belarus. 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-separate countries of Russia, Ukraine, and Belarus have been burdened with continuing and substantial costs for decontamination and health care because of the Chernobyl accident. It is difficult to tally accurately the number of deaths caused by the events at Chernobyl — estimates range from hundreds to hundreds of thousands. The issue is still heavily disputed, and the long-term effects are not yet completely understood.

Contents

The plant

The Chernobyl station is situated at the settlement of Pripyat, Ukraine, 11 miles (18 km) northwest of the city of Chernobyl, 10 miles (16 km) from the border of Ukraine and Belarus, and about 70 miles (110 km) north of Kiev. The station consisted of four reactors, each capable of producing 1 GW of electric power (3.2 gigawatts 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 No. 6, also capable of producing 1 gigawatt each) were under construction at the time of the accident.

The four plants were designed as a type of reactor called RBMK-1000.

The accident

On Saturday, April 26, 1986, at 1:23:58 a.m. local time, the fourth reactor of the Chernobyl power plant—known as Chernobyl-4—suffered a catastrophic steam explosion that resulted in a fire, a series of additional explosions, and a nuclear meltdown.

Causes

The catastrophe is attributed to a flawed reactor design and mistakes made by the plant operators. Another important factor contributing to the accident was the fact that the operators were not informed about some problems with the reactor. According to one of them, Dyatlov, the designers knew that the reactor is dangerous in some conditions but intentionally concealed this information.

In particular,

  • The reactor had a dangerously large positive void coefficient (see below). Even worse, at low power output, this positive void coefficient was not compensated by other factor, which made the reactor unstable and dangerous. Such a behaviour (i. e. that the reactor is dangerous at low powers) was counterintuitive and unknown to the crew.
  • A more significant flaw of the reactor was in the design of the control rods. The control rod extenders were partially hollow; when the control rods were inserted, for the first few seconds coolant was displaced by the hollow fragments of the rods. Since the coolant (water) is a neutron absorber, the reactor's power output actually went up. This behavior of the reactor upon the insertion of control rods was also counterintuitive and not known to the reactor operators.
  • The reactor was dangerous at low powers, however there was no any alert about this.
  • The operators were careless and violated the procedures, partly due to their unawareness of the reactor flaws.
  • The operators switched off many safety systems, however it seems that the regulations did not prohibit this.

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.

Events

On April 25, 1986, the number 4 reactor was scheduled to be shut down for routine maintenance. It was 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. The reactor was to be used to spin up the turbine, at which point the turbine would be disconnected from the reactor and allowed to spin under the force of its own rotational inertia. The power output of the reactor was to be reduced from its normal capacity of 3.2 GW to 700 MW in order to conduct the test at a safer, low power. Due to a delay in beginning the experiment, however, the reactor operators reduced the power level too rapidly, and the actual power output fell to 30 MW. As a result, the concentration of the neutron absorbing fission product xenon-135 increased; this product is typically consumed in a reactor under higher power conditions. Though the scale of the power drop was close to the maximum allowed by safety regulations, the crew chose not to shut down the reactor and to continue the experiment. Further, it was decided to 'shortcut' the experiment and raise power output only to 200 MW. In order to overcome the neutron absorption of the excess xenon-135, the control rods were pulled out of the reactor somewhat farther than normally allowed under safety regulations. As part of the experiment, at 1:05 AM on April 26, the water pumps which 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 highly unstable and dangerous operating condition.

At 1:23:04 A.M., 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 aware of danger. Electricity to the water pumps was shut off, and as they were driven by the inertia 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 in the coolant lines. The particular design of the RBMK graphite moderated reactor at Chernobyl has a large positive void coefficient, which means that the power of the reactor increases rapidly in the absence of the neutron-absorbing effect of water, and in this case, the reactor operation becomes progressively less stable and more dangerous. At 1:23:40 A.M. the operators pressed the "Accident Defense" button that ordered a "scram"—full insertion of 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 on Chernobyl nuclear station at the time of the accident, writes in his book (http://rrc2.narod.ru/book/gl4.html):

"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, displacing and destroying the reactor lid, rupturing the coolant tubes and then blowing a hole in the roof.

To reduce costs, and because of its large size, the reactor was 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 ultimate contamination of outlying areas.

There is some controversy surrounding the exact sequence of events after 1:22:30 local time 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. This event could have been caused by the explosion or could have been completely coincidental. The situation is complicated by the fact that the "scram" button was pressed more than once, and the person who actually pressed it died 2 weeks after the accident from radiation poisoning.

Immediate crisis management

The scale of the tragedy was exacerbated by the incompetence of local administration and lack of proper equipment. All but two dosimeters present in 4th reactor building had limits of 1 milliroentgen per second. The remaining two had limits of 1000 R/s; access to one of them was blocked by the explosion, and the other one broke when turned on. Thus the reactor crew could only ascertain that the radiation levels in much of the reactor building were above 4 R/hour (true levels were up to 20,000 roentgen per hour in some areas; lethal dose is around 500 roentgen 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. local time were dismissed under the pretext that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building till 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 during the three weeks following the accident.

Shortly after the accident, firefighters arrived to try to extinguish the fires. 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 government committee, formed to investigate the accident, arrived at Chernobyl in the evening of April 26. By that time two people were dead and fifty-two were hospitalized. During the night of April 26–27 — 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 limit the scale of the disaster, the Soviet government sent in workers to try to clean up. Many "liquidators"—members of the army and other workers—were sent in as cleanup staff; most were not told anything about the danger. Effective protective gear was unavailable. The worst of the radioactive debris was collected inside what was left of the reactor. The reactor itself was covered with sandbags thrown off helicopters (some 5,000 tons during the week following the accident). A large steel sarcophagus was hastily erected to seal off the reactor and its contents.

Immediate results

203 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. 135,000 people were evacuated from the area, including 50,000 from the nearby town of 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 10 individuals have already died of cancer as a result of the accident.

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.

Soviet scientists have reported that the Chernobyl Unit 4 reactor contained about 190 metric tons of uranium dioxide fuel and fission products. Estimates of the amount of this material that escaped range from 13 to 30 percent.

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. But a large area in the Russian Federation south of Bryansk was also contaminated, as were parts of northwestern Ukraine.

Chernobyl was a secret disaster at first. The initial evidence that a major nuclear accident had occurred came not from Soviet sources, but from Sweden, where on April 27 workers at the Forsmark nuclear power plant (approximately 1100km 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.

Short-term effects

Workers and liquidators

The workers involved in the recovery and cleanup after the accident received high doses of radiation. In most cases, these workers were not equipped with individual dosimeters to measure the amount of radiation received, so experts can only estimate their doses. Even where dosimeters were used, dosimetric procedures varied. Some workers are thought to have been given more accurate estimated doses than others. According to Soviet estimates, between 300,000 and 600,000 people 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. (Estimates of the number of "liquidators"—workers brought into the area for accident management and recovery work—vary; the World Health Organization, for example, puts the figure at about 800,000; Russia lists as liquidators some people who did not work in contaminated areas). In the first year after the accident, the number of cleanup workers in the zone was estimated to be 211,000, and these workers received an estimated average dose of 165 millisievert (16.5 rem).

Civilians

Some children in the contaminated areas were exposed to high thyroid doses up to 50 gray (Gy) because of an intake of radioactive iodine, a relatively short-lived isotope, from contaminated local milk. Several studies have found that the incidence of thyroid cancer among children in Belarus, Ukraine and Russia has risen sharply. The IAEA notes "1800 documented cases of thyroid cancer in children who were between 0 and 14 years of age when the accident occurred, which is far higher than normal" but fails to note the expected rate. The childhood thyroid cancers that have appeared are of a large and aggressive type, and if detected early, can be treated. Treatment entails surgery followed by iodine-131 therapy for any metastases. To date, such treatment appears to have been successful in all diagnosed cases.

Late in 1995, the World Health Organisation linked nearly 700 cases of thyroid cancer among children and adolescents to the Chernobyl accident and among these some 10 deaths are attributed to radiation. However, the rapid increase in thyroid cancers detected suggests that some of it at least is an artifact of the screening process. Typical latency time of radiation-induced thyroid cancer is about 10 years; but the increase in childhood thyroid cancers in some regions was observed as early as 1987. Presumably, either the increase is unrelated to the accident or the mechanisms behind it are not well enough understood.

So far no increase in leukaemia is discernible, but this is expected to be evident in the next few years along with a greater, though not statistically discernible, incidence of other cancers. There has been no substantiated increase attributable to Chernobyl in congenital abnormalities, adverse pregnancy outcomes or any other radiation-induced disease in the general population either in the contaminated areas or further afield.

Longer-term 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. Recent tests (ca. 1997) have shown that caesium-137 levels in trees of the area are continuing to rise. There is some evidence that contamination is migrating into underground aquifers and closed bodies of water such as lakes and ponds (2001, Germenchuk). The main source of elimination is predicted to be natural decay of caesium-137 to stable barium-137, since runoff by rain and groundwater has been demonstrated to be negligible.

Global effect

The IAEA notes that, while the Chernobyl accident released as much as 400 times the radioactive contamination of the Hiroshima bomb, it was 100 to 1000 times less than the contamination caused by atmospheric nuclear weapons testing in the mid-20th century. One can argue that while the Chernobyl accident was a local disaster, it was not a global one.

Effect on the natural world

According to reports from Soviet scientists at the First International Conference on the Biological and Radiological Aspects of the Chernobyl Accident (September 1990), fallout levels in the 10 km zone around the plant were as high as 4.81 GBq/m². The so-called "Red Forest" of pine trees killed by heavy radioactive fallout lay within the 10 km zone, immediately behind the reactor complex. The forest is so named because in the days following the accident the trees appeared a deep red hue as they died due to extremely heavy radioactive fallout. In the post-disaster cleanup operations, a majority of the 4 km² forest was bulldozed and buried. The site of the Red Forest remains one of the most contaminated areas in the world. However, astonishingly, it has proved to be a fertile habitat for many endangered species.

Evacuation

Map showing Cesium-137 contamination in , , and
Enlarge
Map showing Cesium-137 contamination in Belarus, Russia, and Ukraine

Soviet authorities started evacuating people from the area around Chernobyl within 36 hours of the accident. By May 1986, about a month later, all those living within a 30 km (18 mile) radius of the plant—about 116,000 people—had been relocated.

According to reports from Soviet scientists, 28,000 km² (10,800 mile²) were contaminated by caesium-137 to levels greater than 185 kBq/m². Roughly 830,000 people lived in this area. About 10,500 km ² (4,000 mile²) were contaminated by caesium-137 to levels greater than 555 kBq/m². Of this total, roughly 7,000 km² (2,700 square miles) lie in Belarus, 2,000 km² (800 square miles) in the Russian Federation and 1,500 km (580 square miles) in Ukraine. About 250,000 people lived in this area. These reported data were corroborated by the International Chernobyl Project.

Comparison with other disasters

The Chernobyl accident was a unique event, on a scale by itself. It was the first time in the history of commercial nuclear electricity generation that radiation-related fatalities occurred. (note: an accident at the Japanese Tokaimura nuclear fuel reprocessing plant on September 30, 1999, resulted in the radiation related death of one worker on December 22 of that same year)

Long-term effects on civilians

The issue of long-term effects of Chernobyl disaster on civilians is highly controversial. The number of people whose life was affected by the accident is enormous. Over 300,000 people were resettled because of the accident; around 600,000 participated in the cleanup; 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.

An increased incidence of thyroid cancer among children in areas of Belarus, Ukraine and Russia affected by the Chernobyl accident has been firmly established as a result of screening programs and, in the case of Belarus, an established cancer registry. The findings of most epidemiological studies must be considered interim, say experts, as analysis of the health effects of the accident is an ongoing process.

Epidemiological studies have been hampered in the former Soviet Union by a lack of funds, an infrastructure with little or no experience in chronic disease epidemiology, poor communication facilities and an immediate public health problem with many dimensions. Emphasis has been placed on screening rather than on well-designed epidemiological studies. International efforts to organize epidemiological studies have been slowed by some of the same factors, especially the lack of a suitable scientific infrastructure.

The activities undertaken by Belarus and Ukraine in response to the accident—remediation of the environment, evacuation and resettlement, development of uncontaminated food sources and food distribution channels, and public health measures—have overburdened the governments of those countries. International agencies and foreign governments have provided extensive logistic and humanitarian assistance. In addition, the work of the European Commission and World Health Organization in strengthening the epidemiological research infrastructure in Russia, Ukraine and Belarus is laying the basis for major advances in these countries' ability to carry out epidemiological studies of all kinds.

A 2002 Nuclear Energy Agency report identified significant long-term effects of the accident from non-radiological origins. The anxiety and stress of living in affected areas has had a severe psychological impact on the population. The resettlement of inhabitants away from areas where they have lived all their lives has also had psychological effects by disrupting family and social networks and placing people in areas where they may be resented by the existing population.

Wildlife

In marked contrast to the human cost, the evacuation of the area surrounding the plant has created a lush and unique wildlife refuge. It is unknown whether fallout contamination will have any long-term adverse effect on the flora and fauna of the region, as plants and animals have significantly different and varying radiologic tolerance compared with humans. However, it seems that the biodiversity around the massive radiation spill has increased due to the removal of human influence (see the first hand account of the wildlife preserve (http://www.nsrl.ttu.edu/chernobyl/wildlifepreserve.htm) below). There are reports of mutations in some plants in the area, leading to unsubstantiated tales of a "forest of wonders" containing many strangely mutated plants. Also, the area is reported to be silent, suggesting that birds have not yet re-colonised it.

Chernobyl after the accident

The trouble at the Chernobyl plant itself did not end with the disaster in Reactor No. 4. 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 No. 2 in 1991; the authorities subsequently declared the reactor damaged beyond repair and had it taken offline. Reactor No. 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. In November 2000, Ukrainian President Leonid Kuchma personally turned off the switch to Reactor No. 3 in an official ceremony, effectively shutting down the entire plant.

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, means it is aging badly, and if it collapses, another cloud of radioactive dust could be released. A number of plans have been discussed for building a more permanent enclosure, but they have so far been hampered by corruption. Most of the money donated by foreign countries and contributed by Ukraine has been squandered by inefficient distribution of construction contracts and overall management, or simply stolen.

About 95% of the fuel in the reactor at the time of the accident remains inside the shelter, with a total radioactivity of nearly 18 million curies. 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. By conservative estimates, there is at least four tons of radioactive dust inside the shelter.

Water continues to leak into the shelter, spreading radioactive materials throughout the wrecked reactor building and into the surrounding groundwater. The high humidity inside the shelter continues to erode the concrete and steel of the sarcophagus.

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 $768 million. The SIP is being managed by a consortium of Bechtel, Battelle, and Electricit 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.

Chernobyl in the popular consciousness

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.

Political outcome

The Chernobyl accident was clearly a major disaster, and it received worldwide media attention. Public awareness of the risks of nuclear power increased significantly. Organizations, both pro- and anti-nuclear, have made great efforts to sway public opinion. Casualty figures, reactor safety estimates, and estimates of the risks associated to other reactors differ greatly depending on which position is favored by the author of any given document. For example, the UN scientific committee on the effects of radiation has publicly criticized the UN office on humanitarian affairs with respect to some of its publications. The true facts of the affair are therefore rather difficult to uncover.

Chernobyl and the Bible

Because of a controversial translation of "chernobyl" as wormwood, an urban myth started among English-speaking Christians that the Chernobyl accident was mentioned in the Bible:

And the third angel sounded, and there fell a great star from heaven, burning as it were a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters; and the name of the star is called Wormwood: and the third part of the waters became wormwood; and many men died of the waters, because they were made bitter. — Revelation 8:10-11

The story appears to have originated—or at least spread to the West—with a New York Times article by Serge Schmemann (Chernobyl Fallout: Apocalyptic Tale, July 25, 1986) in which an unnamed "prominent Russian writer" was quoted as claiming the Ukrainian word for wormwood was chernobyl.

The name of the city comes from the Ukrainian word for mugwort (Artemisia vulgaris), which is "chornobyl". The word is a combination of chornyi (чорний, black) and byllia (билля, grass blades or stalks), hence it literally means black grass or black stalks.

Computer virus

The CIH computer virus was popularly named "the Chernobyl virus" by many in the media, after the fact that the v1.2 variant activated on April 26 of each year: the anniversary of the Chernobyl accident. However, this is simply because of a coincidence with the virus author's birthday.

Related articles

External links

General Information

Event & technical analysis

Witness accounts (before and after)

Photography/Videography

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