Chernobyl Nuclear Power Station Disaster


In 1986, a serious nuclear power generation accident occurred at the Chernobyl nuclear power plant in the Soviet Union (in the area that later became known as the independent country of Ukraine). On April 25–26, 1986, technicians attempted a poorly designed experiment, causing the chain reaction in the core to go out of control. The reactor's lid was blown off, and large amounts of radioactive material were released into the atmosphere. The resulting radioactive cloud contaminated an area half the size of Italy and exposed nearly 8,400,000 people in the present countries of Belarus, Ukraine, and Russia. Millions of others in Poland, Scandinavia, and throughout Western Europe were also affected. A partial meltdown of the nuclear reactor core also occurred. Initially the Soviet government denied that there had been an accident, but after Swedish monitoring stations reported abnormally high levels of wind-transported radioactivity, the government admitted the truth.

A satellite image of Chernobyl, Ukraine. Water is black and urban areas are white. Red and green rectangular fields surround the river and the lake that supplied water to cool the Chernobyl nuclear power plant (white, center).

A satellite image of Chernobyl, Ukraine. Water is black and urban areas are white. Red and green rectangular fields surround the river and the lake that supplied water to cool the Chernobyl nuclear power plant (white, center). On 26 April 1986, the nuclear reaction in Reactor number Four went out of control, leading to a meltdown and an explosion. Around 1,000 square kilometers of land were directly contaminated by radioactive fall-out of heavy elements released into the atmosphere by the explosion. Lighter materials were carried large distances and contaminated rain falling across Europe.
(Earth Satellite Corporation/Science Source)


On April 26, 1986, at 1:24 A.M., Unit 4 (an RBMK type of nuclear reactor) of the Chernobyl nuclear power plant exploded, releasing large amounts of radioactivity into the environment. The power station is located 9 miles (14.5 km) northwest of the town of Chernobyl, which had a population of 14,000, and less than 2 miles (3.2 km) from the town of Prypiat, with about 50,000 inhabitants. The explosion and its aftermath, including the manner in which the accident was handled, raised questions about the safety and future of nuclear power.

Officials at first denied that there had been a serious accident at the power plant. The government in Moscow was led to believe for several hours after the explosion and fire at Chernobyl that the reactor core was still intact. This delayed the evacuation for a critical period during which local citizens were exposed to high radiation levels. The evacuation of Chernobyl and local villages was spread out over eight days. Overall, in the spring and summer of 1986, 116,000 people were evacuated from the area surrounding the Chernobyl reactor to non-contaminated areas. Children in particular were a concern and were evacuated to the southern Ukraine, the Crimea, and the Black Sea coast. Another 230,000 people were relocated in following years. In July 2010 the Belarus government announced that it had decided to settle back many thousands people in over 2,000 ghost-villages in the Belarus affected by the Chernobyl fallout, from which 24 years earlier they and their forbears were removed.

At the time of the accident, and for several days thereafter, the winds carried the radioactive waste to the north. The radioactive cloud split into two plumes, one spreading west and then north through Poland, Germany, Belgium, and the Netherlands, and the other through Sweden and Finland. By the first of May, the wind direction changed and the radioactive fallout—at a diminished rate—went south over the Balkans and then west through Italy. Large areas of Europe were affected, and many farmers destroyed their crops for fear of contamination.

In the twenty years after the disaster, 600,000 soldiers, firemen, and other workers (referred to as liquidators) were sent voluntarily and involuntarily to the disaster site to physically remove the radiation. They razed contaminated buildings and cleared forests. They disposed of contaminated topsoil and irradiated materials. Plastic film was laid in some areas in an effort to contain radioactive dust. The most dangerous job was the one-minute shifts on the roof of an adjacent Chernobyl unit, shoveling radioactive debris into the unit that had exploded. After the accident, forests were cleared and large amounts of earth were removed in order to clean up radioactivity.

The Soviet Union and, after the breakup of the Soviet Union republics, Ukraine continued to operate the remaining three reactors at the Chernobyl power station. The Soviet government oversaw the cleanup of Chernobyl, including construction of a concrete containment dome. The government also promised to pay lifelong health and other expenses for those who worked on clean up and construction duty after the accident. After the collapse of the Soviet Union in 1991, the Ukrainian government took over maintenance of the Chernobyl site. Economic and political upheavals made it difficult for countries of the former Soviet Union to respond to the immediate public health problems. Fortunately international funds were provided to Belarus, Russia, and Ukraine to meet the health needs of those affected by the disaster.


There is controversy over how many people were actually killed and how many people's health was affected by the accident. The reactor explosion and subsequent radiation exposure caused 31 direct deaths (two from the explosions, one reportedly from coronary thrombosis [heart attack], and 28 firemen and plant personnel from acute radiation syndrome). The approximately 600 emergency workers who were on the site of the Chernobyl power plant during the night of the accident received the highest doses, mostly due to external irradiation (relatively uniform whole-body gamma irradiation and beta irradiation of extensive body surfaces) rather than through inhalation of radioactivity. Acute radiation sickness was confirmed in 134 of those emergency workers. However, the number of additional deaths due to the accident will likely never to be precisely known.

With respect to cancer risk from exposure to low doses of radiation from the Chernobyl accident, the US National Academy of Sciences BEIR VII Committee concluded in 2006 that there may be up to 4,000 additional cancer deaths among the three highest exposed groups over their lifetime (240,000 cleanup personnel [liquidators], 116,000 evacuees, and the 270,000 residents of the SCZs [strict control zones]). Since more than 120,000 people in these three groups may eventually die of cancer, the additional cancer deaths from radiation exposure correspond to 3–4 percent above the normal incidence of cancers from all causes.

The Chernobyl radiation may also have caused, or was expected to cause, cancers in Europe outside Belarus, the Russian Federation, and Ukraine. However, the average dose to these populations was much lower, and so the increase in cancer deaths is not likely to be detectable using national cancer statistics.


The Chernobyl accident resulted from several factors. Flaws in the initial engineering design of the nuclear power plant were to be compensated by the operators being required to follow a strict set of procedures. However, failure of the plant management to enforce these procedures was a factor in the occurrence of the disaster. Also, reactors were not enclosed with a containment structure and there was not an off-site emergency plan. Specifically three design drawbacks were noted at the Chernobyl nuclear power plant:

Another cause of the disaster was the decision of the plant engineers to conduct a risky experiment. Unit 4 of the Chernobyl nuclear power plant was to be shutdown for routine maintenance on April 25, 1986. At that time, the Chernobyl engineers wanted to test whether the plant's turbine generator—from its rotating inertia—could provide enough power to the reactor in case of a power shutdown. This experiment required disconnecting the reactor's emergency core cooling pump and other safety devices.

The series of critical events of the experiment, as described by Richard Mould in Chernobyl: The Real Story, are as follows: At 1:00 A.M. on April 25, power reduction was started in preparation for the experiment. At 1:40 A.M. the reactor's emergency core cooling system was turned off. At 11:10 P.M. power was further reduced, resulting in a nearly unmanageable situation. At 1:00 A.M. on April 26, power was increased in an attempt to stabilize the reactor; however, cooling pumps were operating well beyond their rated capacity, causing a reduction in steam generation and a fall in stream pressure. By 1:19 A.M., the water in the cooling circuit had approached the boiling point. At 1:23 A.M., the operators tried to control the reaction by manually pushing control rods into the core; however, the rods did not descend their full length into the reactor since destruction of the graphite core was already occurring. In 4.5 seconds, the power level rose two thousand fold. At 1:24 A.M., there was an explosion when the hot reactor fuel elements, lacking enough liquid for cooling, decomposed the water into hydrogen and oxygen. The generated pressures blew off the 1,000-ton concrete roof of the reactor, and burning graphite, molten uranium, and radioactive ashes spilled into the atmosphere. The reactor was designed without an external containment structure as is done with U.S. reactors.

In addition, the Chernobyl management had no effective regulatory agency to oversee and inspect procedures and order closure of the facility if required. Also in years prior to the accident there was a lack of information given the public of prior nuclear accidents, typical of the press censorship and news management occurring in the period before glasnost. The operators were not adequately trained nor were they themselves fully aware of prior nuclear power accidents or near accidents, which would have made them more sensitive to the dangers of a runaway reactor system.


Potassium iodide pills can reduce the impact of radioactive iodine-131 on the thyroid gland and thus can prevent thyroid damage or cancer. This protective effect occurs because potassium iodide blocks radioactive iodine from being absorbed by the thyroid gland. The thyroid cannot distinguish between radioactive iodine and potassium iodide, so once the potassium iodide is taken in and the thyroid gland becomes saturated with the compound, no more radioactive iodine or potassium iodide can be absorbed for the next twenty-four hours. However, the use of potassium iodide has one major limitation. To be of value, it must be taken prior to (or within hours of) exposure, for it cannot provide a protective effect once the thyroid gland has been damaged or destroyed by radioactive iodine.

Iodide was distributed to site personnel immediately after the accident and to residents of Pripyat six and a half hours after the accident. From April 28 to early May, potassium iodide was distributed to the remaining 90,000 persons within 30 km of the accident. Each person took a daily tablet until May 6. Physicians felt that the potassium iodide treatment also produced a positive psychological effect on the population.

Within six days of the Chernobyl radiation release, the Polish government distributed potassium iodide to about eighteen million people. As a result, Poland appears to have had no cancers attributable to Chernobyl. Administration of stable iodine in liquid form (as a solution of Lugol) was initiated in the northeastern part of Poland approximately 38 hours after the Chernobyl explosion (around midnight on April 28). Treatment was given for the next three days. This action was made possible because Poland had developed plans for implementing nuclear war emergency measures, which included strategic stores of stable iodine at sites all over the country The program was begun in the early 1970s, and each Polish pharmacy, hospital, and various other institutions had large supplies of iodine. At the time of the Chernobyl accident, Poland had enough iodine ready for use for approximately 100 doses for each Polish citizen.

Common diseases and disorders

The human health impacts of the Chernobyl accident can be described in terms of acute health effects (death, severe health impairment), late health effects (cancers), and mental health issues. There was also an impact on agriculture and the environment.

Immediate health effects

Acute health effects occurred among the plant personnel and the persons who intervened in the emergency phase to fight fires, to provide medical aid and to participate in immediate clean-up operations. A total of 31 people died as a consequence of the accident, and 134 people suffered various degrees of radiation sickness and acute radiation syndrome. No members of the general public suffered these kinds of effects.

Radiation sickness, known as acute radiation syndrome (ARS), is a serious illness that occurs when the entire or large parts of the body receive a high dose of radiation, usually over a short period of time. The first symptoms of ARS typically are nausea, vomiting, and diarrhea. These symptoms start within minutes to days after the exposure, last for minutes up to several days, and may come and go. Individuals usually look and feel healthy for a short time, after which they become sick again with loss of appetite, fatigue, fever, nausea, vomiting, diarrhea, and possibly even seizures and coma. This seriously ill stage may last from a few hours up to several months.

Persons with ARS typically also have some skin damage. This damage can start to show within a few hours after exposure and can include swelling, itching, and redness of the skin, similar to being sunburned. There also can be hair loss. As with the other symptoms, the skin may heal for a short time, followed by the return of swelling, itching, and redness days or weeks later. Complete healing of the skin may take from several weeks up to a few years, depending on the radiation dose the person's skin received.

The chance of survival for people with ARS decreases with increasing radiation dose. Most people who do not recover from ARS will die within several months of exposure. The cause of death in most cases is the destruction of the person's bone marrow, which results in infections and internal bleeding. For those who survive, the recovery process may last from several weeks up to two years.

Late health effects

In 2000, it was estimated that up to 1 percent of children exposed to the Chernobyl fallout had or would develop thyroid cancer. In Belarus, the Russian Federation, and Ukraine, the World Health Organization (WHO) reported in 2006 that nearly 5,000 cases of thyroid cancer had been diagnosed among children who were aged up to 18 years at the time of the accident. While a large number of these cancers resulted from radiation following the accident, intense medical monitoring for thyroid disease among the affected population also resulted in the detection of thyroid cancers at a sub-clinical level, and so contributed to the overall increase in thyroid cancer numbers observed. Even in children with advanced tumors, treatment was highly effective. It is expected that the increased incidence of thyroid cancer from Chernobyl will continue for many years, although the long-term numbers are difficult to predict.

Investigations suggest there may be a doubling of the incidence of leukemia among the most highly exposed Chernobyl liquidators. No such increase has been demonstrated among children or adults resident in any of the contaminated areas.

Mental health issues

An important health effect of the accident is the appearance of widespread psychological stress in the populations affected by the disaster. The severity of this phenomenon, which is mostly observed in the contaminated regions around Chernobyl, is a reflection of public fears about the unknowns of radiation and its effects, as well as mistrust of public authorities and official experts. The stress has been exacerbated by the disruption of the social networks and traditional ways of life caused by the accident and its long-term consequences. These stress-related effects have resulted in a general degradation of the health of the population living in the contaminated territories, as many of the observed illnesses are not typical of radiation exposure. These non-typical illnesses include diseases of the digestive and circulatory systems and other post-traumatic stress disorders such as sleep disturbance, headache, depression, anxiety, escapism, learned helplessness, unwillingness to cooperate, alcohol and drug abuse, and suicides.

Scientific and epidemiological research programs, some of them sponsored by international organizations such as the WHO, continue to be conducted to provide further insight into possible future health effects.

Agricultural and environmental effects

The impact of the accident on agricultural practices, food production and use, and other aspects of the environment has been more widespread than the direct health impact on humans. Although soil treatment and decontamination to reduce the accumulation of radioactivity in agricultural produce and cow's milk and meat was accomplished on a small scale, large areas of agricultural land are still excluded from use and are expected to continue to be so for a long time. Even in areas where agricultural and dairy production activities are conducted, the food produced is still subjected to strict controls and restrictions on distribution and use.

Contamination levels showed a decreasing trend for some time following the accident, but it appears that ecological stability has been reached. This is particularly true in forested areas. The decrease now appears to be following the decay period for 137 Cesium, which has a 30-year half-life.

Public health role and response

At the time of the accident, 444 workers were onsite. Hundreds of additional staff were called in for rescue operations, plant control, and firefighting. Medical personnel provided short-term and immediate responses, including onsite rescue and first aid to persons who were working at the plant at the time of the explosion. The onsite medical station, with decontamination facilities, a radiobioassay laboratory, ambulance transportation, and a holding facility, provided this initial response.

Hospital treatment of the victims was provided in Pripyat and Chernobyl at regional hospitals, which had facilities for emergency treatment of trauma, decontamination, and initial evaluation of radiation injuries. Definitive evaluation and treatment was accomplished in Moscow and Kiev hospitals. Overall, 203 victims were hospitalized.

Angelina Gustav, chief of the Institute of Biophysics in Moscow, and her colleagues oversaw the medical response and established procedures for the evaluation and triage of patients both at the site and in the regional hospitals. This strong central leadership minimized confusion and mismanagement.


Precautionary measures are still called for in some small areas of high radioactive contamination around Chernobyl, but in general, people living in contaminated areas are not likely to experience adverse health effects from radiation exposure and can safely live there.

Local communities still face obstacles, however. Because of the disruption caused by the nuclear accident, many areas suffer social and economic hardship. Communities that had earlier depended on farming and forestry are hindered by restrictions limiting commercial activity, and radiation stigma makes produce from the region difficult to market. Investment funds are scarce, and infrastructure is often lacking or neglected. Young people often leave the region to work elsewhere. All of these factors were exacerbated by the upheaval that followed the break-up of the Soviet Union.

In addition, government provision of a wide range of benefits to millions of people who were designated as having suffered from the Chernobyl accident had the unintended effect of creating a dependency syndrome. Combined with widespread (and often unfounded) fears about the health impact of the radiation from the accident, many citizens became despondent, passive, and helpless, in a state sociologists called Chernobyl victims syndrome. Because of these challenges, the UN changed from delivering emergency humanitarian assistance to focusing on social and economic development. People living in Chernobyl-affected areas need better economic opportunities, a restored sense of community self-reliance, and the information necessary to overcome fears associated with radiation and to address health threats arising from causes unrelated to Chernobyl.

The sarcophagus (the massive concrete structure constructed to provide confinement of the damaged nuclear fuel in order to reduce the likelihood of further releases of radioactivity to the environment) and the approximately 800 waste storage sites within and outside the 30-km exclusion zone around the reactor represent a series of potential sources of release of radioactivity that threaten the surrounding area and ground water. However, any such releases are expected to be very small in comparison with those from the 1986 Chernobyl accident, and their consequences would be limited to a relatively small area around the site. Studies are being conducted to develop a solution that would lead to the elimination of these sources of residual risk on the site.

Nuclear power station—
A thermal power station in which the heat source is a nuclear reactor. The heat produced is used to generate steam that drives a steam turbine connected to a generator that produces electricity.
Nuclear reaction—
A process, such as fission, fusion, or radioactive decay, in which the structure of an atomic nucleus is altered through release of energy or mass or by being broken apart.
Nuclear reactor—
A device to initiate and control a sustained nuclear chain reaction, which occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating series of these reactions. The nuclear chain reaction releases several million times more energy per reaction than any other chemical reaction.
The act of emitting radiation spontaneously. This is done by an atomic nucleus that, for some reason, is unstable; it wants to give up some energy in order to shift to a more stable configuration.

Unfortunately in the former Soviet block nations there are several nuclear reactors that are potentially as hazardous as Chernobyl but which must continue operation to maintain power requirements. However, their operational procedures are under constant review to avoid another accident.


See also Background radiation ; Leukemia ; Radiation ; Radiation exposure ; Radiation injuries .



Alexievich, Svetlana, and Keith Gessen. Voices from Chernobyl: The Oral History of a Nuclear Disaster. London: Picador, 2006.

Park, Chris C. Chernobyl: The Long Shadow. London: Routledge, 2012.


United Nations Development Programme. “United Nations and Chernobyl.” (accessed October 15, 2012).

World Health Organization. “Chernobyl Accident: An Overview of the Health Effects.” (accessed October 15, 2012).


Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy. NE, Atlanta, GA, 30341, (800) 232-4636,, .

World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, 2241 791 21 11, Fax: 2241 791 31 11,, .

Judith L Sims
Malcolm T. Hepworth

  This information is not a tool for self-diagnosis or a substitute for professional care.