Chernobyl, Ukraine (51°16N, 30°13' E), a small city on the Pripiat River in the former U.S.S.R., is site of the worst nuclear accident in history that occurred on April 26, 1986. The accident was the the third largest single exposure to radiation of a substantial population, the two largest being the radiation from the atomic bombs dropped Hiroshima and Nagasaki, Japan, at the end of World War II in August, 1945. The accident had significant and wide-ranging health impacts that continue to be monitored and assessed. The accident also produced a significant international response whose effectiveness is the subject of debate. The Chernobyl accident also generated significant debate about the safety of nuclear power plants.
The fourth nuclear reactor of V.I. Lenin Nuclear Power Plant, located about twenty-five kilometers (km) upstream of the city, was scheduled to be shut down for routine maintenance. On April 25, 1986, prior to the shutdown, the reactor crew at Chernobyl-4 began preparing for a test to determine how long turbines could spin and continue to supply power following loss of the primary electrical power supply. Similar tests had already been carried out at Chernobyl and other plants, despite the fact that these reactors were known to be very unstable at low power settings.
A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. As flow of coolant water diminished, power output increased. When the operator moved to shut down the reactor from its unstable condition arising from previous errors, a peculiarity of the design caused a dramatic power surge.
The sudden increase in temperature caused part of the fuel core to rupture; fuel particles reacted with the water creating a steam explosion that destroyed the reactor core. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to burst into flames.
There is some dispute among experts about the character of this second explosion. The graphite burned for nine days, causing the main release of radioactivity into the environment. A total of 12-14 EBq (1018 becquerels) of radioactivity was released, half of it being biologically-inert noble gases.
The cloud from the burning reactor spread numerous types of radioactive materials, especially iodine and cesium radionuclides, over much of Europe. Radioactive iodine-131 (131I), most significant in contributing to thyroid doses, has a short half-life (8 days) and largely disintegrated in the first few weeks of the accident. Radioactive cesium-137 (137C), which contributes to both external and internal doses, has a much longer half-life (30 years) and is still measurable in soils and some foods in many parts of Europe. The greatest deposits of radionuclides occurred over large areas of the Soviet Union surrounding the reactor in what are now the countries of Belarus, the Russian Federation and Ukraine.
An estimated 350 000 emergency and recovery operation workers, including army, power plant staff, local police and fire services, were initially involved in containing and cleaning up the accident in 1986–1987. Among them, about 240 000 recovery operation workers took part in major mitigation activities at the reactor and within the 30-km zone surrounding the reactor. Later, the number of registered “liquidators” rose to 600,000, although only a small fraction of these were exposed to high levels of radiation.
More than five million people live in areas of Belarus, Russia and Ukraine that are classified as "contaminated" with radionuclides due to the Chernobyl accident. Amongst them, about 400,000 people lived in more contaminated areas classified by Soviet authorities as areas of strict radiation control. Of this population, 116,000 people were evacuated in the spring and summer of 1986 from the area surrounding the Chernobyl power plant (designated the “Exclusion Zone”) to non-contaminated areas. Another 220,000 people were relocated in subsequent years.
Fifty tons of radioactive dust were dispersed over 140,000 square miles of Belarus, Ukraine, and Russia, and 4.9 million people were estimated to have been exposed to radiation.
Within a few days or weeks, the accident had caused the deaths of 30 plant employees and firemen (including 28 deaths that were due to radiation exposure), brought about the evacuation of about 116,000 people from areas surrounding the reactor during 1986, and the relocation, after 1986, of about 220,000 people from what are now Belarus, the Russian Federation, and Ukraine. Extensive areas of those nations were contaminated, and trace deposition of released radionuclides was measurable in all countries of the northern hemisphere. Stratospheric interhemispheric transfer may also have led to some environmental contamination in the southern hemisphere.
In addition, about 240,000 workers called “liquidators” were called upon in 1986 and 1987 to take part in major mitigation activities at the reactor and within the 30-km zone surrounding the reactor; residual mitigation activities continued until 1990. All together, about 600,000 persons were employed as “liquidators.”
From the radiological point of view, 131I and 137Cs are the most important radionuclides to consider because they were responsible for most of the radiation exposure received by the general population. The release of 137Cs is estimated to be 85 PBq, equal to about 30% of the amount in the reactor core, and that of 131I is estimated to be 1,760 PBq, about 50% of the core inventory.
The area affected was large because of the dispersion of small particles into the upper atmosphere and duration of the release. However, changes in wind direction and rainfall throughout the 10-day period resulted in an unevenly distributed significant deposition of radionuclides, mainly over Belarus, Ukraine, and a part of Russia. The doses outside the former Soviet Union were low, and varied depending upon whether rainfall occurred during the passage of the radioactive cloud.
People were exposed to both internal and external radiation. The major routes of human exposure to radiation were from ingestion of cow's milk contaminated with iodine-131 (resulting in internal exposure), contact with gamma/beta radiation from the radioactive cloud, and contact with cesium-137 deposited on the ground (resulting in external exposure).
Human exposure to radiation
Three population categories were exposed from the Chernobyl accident:
- Emergency and recovery operation workers who worked at the Chernobyl power plant and in the exclusion zone after the accident;
- Inhabitants evacuated from contaminated areas; and
- Inhabitants of contaminated areas who were not evacuated.
With the exception of the on-site reactor personnel and the emergency workers who were present near the destroyed reactor during the time of the accident and shortly afterwards, most of the recovery operation workers and people living in the contaminated territories received relatively low whole-body radiation doses, comparable to background radiation levels accumulated over the 20 year period since the accident.
The highest doses were received by emergency workers and on-site personnel, in total about 1,000, during the first days of the accident, ranging form 2 to 20 Gray (GY), which was fatal for some of the workers. One Gy is a joule per kilogram (J/kg). The absorbed dose in a human body of more than one gray may cause acute radiation syndrome (ARS) as happened with some of the Chernobyl emergency workers.
The doses received by recovery operation workers, who worked for short periods during four years following the accident ranged up to more than 500 millisieverts (mSv), with an average of about 100 mSv according to the State Registries of Belarus, Russia, and Ukraine. Effective doses to the persons evacuated from the Chernobyl accident area in the spring and summer of 1986 were estimated to be of the order of 33 mSv on average, with the highest dose of the order of several hundred mSv.
For comparison, annual natural background doses of humans worldwide average 2.4 mSv, with a typical range of 1–10 mSv. Lifetime doses due to natural radiation would thus be about 100–700 mSv. Radiation doses to humans may be characterized as low-level if they are comparable to natural background radiation levels of a few mSv per year.
Ingestion of food contaminated with radioactive iodine did result in significant doses to the thyroid of inhabitants of the contaminated areas of Belarus, Russia, and Ukraine. The thyroid doses varied in a wide range, according to age, level of ground contamination with 131I, and milk consumption rate. Reported individual thyroid doses ranged up to about 50 Gy, with average doses in contaminated areas being about 0.03 to few Gy, depending on the region where people lived and on their age. The thyroid doses to residents of Pripyat city located in the vicinity of the Chernobyl power plant, were substantially reduced by timely distribution of stable iodine tablets. Drinking milk from cows that ate contaminated grass immediately after the accident was one of the main reasons for the high doses to the thyroid of children, and why so many children subsequently developed thyroid cancer.
The general public has been exposed during the past twenty years after the accident both from external sources (137Cs on soil, etc.) and via intake of radionuclides (mainly, 137Cs) with foods, water and air. The average effective doses for the general population of ‘contaminated’ areas accumulated in 1986–2005 were estimated to be between 10 and 30 mSv in various administrative regions of Belarus, Russia and Ukraine. In the areas of strict radiological control, the average dose was around 50 mSv and more. Some residents received up to several hundred mSv. It should be noted that the average doses received by residents of the territories ‘contaminated’ by Chernobyl fallout are generally lower than those received by people who live in some areas of high natural background radiation in India, Iran, Brazil and China (100–200 mSv in 20 years).
The vast majority of about five million people residing in contaminated areas of Belarus, Russia and Ukraine currently receive annual effective doses from the Chernobyl fallout of less than 1 mSv in addition to the natural background doses. However, about 100,000 residents of the more contaminated areas still receive more than 1 mSv annually from the Chernobyl fallout. Although future reduction of exposure levels is expected to be rather slow, i.e. of about 3 to 5% per year, the great majority of dose from the accident has already been accumulated.
The accident had significant and wide-ranging health impacts that continue to be monitored and assessed.
Acute Radiation Syndrome mortality
The number of deaths due to acute radiation syndrome (ARS) during the first year following the accident is well documented. According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), ARS was diagnosed in 134 emergency workers. In many cases the ARS was complicated by extensive beta radiation skin burns and sepsis. Among these workers, 28 persons died in 1986 due to ARS. Two more persons had died at Unit 4 from injuries unrelated to radiation, and one additional death was thought to have been due to a coronary thrombosis. Nineteen more have died in 1987–2004 of various causes; however their deaths are not necessarily — and in some cases are certainly not — directly attributable to radiation exposure. Among the general population exposed to the Chernobyl radioactive fallout, however, the radiation doses were relatively low, and ARS and associated fatalities did not occur.
Other health effects
In 1990, four years after the Chernobyl accident, an increase in thyroid cancer was found in children exposed to fallout from the accident. Two years later, the first reports in the Western literature of an increase in childhood thyroid cancer (CTC) in Belarus were published. In 2000, about 2,000 cases of thyroid cancer had been reported in those exposed as children in the former Soviet Socialist Union, and in 2005, the number was estimated at 4,000; the latest estimate for the year 2056 ranges from 3,400 to 72,000. The effects are not limited by national borders; Poland has recorded cases in spite of a rapid precautionary distribution of stable iodine. The causative agent, 131I, was detected in many European countries with as yet unknown effects. Interestingly, a significant increase in leukemia has not been reliably reported in the three most affected countries.
This dramatic contrast between the two incidents is in part due to the different types of radiation exposure, but both show that the effects of massive exposures to radiation are immensely complex. In comparing the health effects after Chernobyl with those after the atomic bombs, it must be remembered that apart from workers in or close to the power plant, the Chernobyl accident involved mainly exposure to radioactive isotopes, and the atomic bombs primarily involved direct exposure to gamma rays and neutrons. Because of the prominence given to thyroid carcinoma after Chernobyl, less attention has been given to whole-body exposure from the ingestion and inhalation of all isotopes, together with the shine from the radioactive cloud and deposited radioactivity. Consideration of the health effects of Chernobyl must take into account both tissue-specific doses due to isotope concentration and whole-body doses.
The most prominent tissue-specific dose is that to the thyroid, largely from 131I, with a smaller contribution from short-lived isotopes of iodine. For many in the 30-km zone (135,000), there were relatively high absorbed doses to other organs as well as the thyroid until evacuation, and for those living in the contaminated areas around the 30-km zone (5 million), relatively high dose rate exposure (days to weeks) was followed by prolonged (years) exposure to a low dose rate. This exposure was a complex mixture of external radiation and internal emitters. For others living farther from the accident, in Western Europe, for example, their average exposure was equivalent to an additional ≤ 50% of average annual natural background level of radiation. About 600,000 liquidators assisted with the cleanup. Those working at the site shortly after the accident (200,000) received substantial doses. For all of these groups, estimates of numbers of fatal cancers can be derived from the collective doses. However, such estimates depend on the assumed risk coefficient, but of the order of 60,000 such fatalities in total can be estimated, based on the collective dose estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), less than half of which would derive from the declared contaminated areas. A more recent estimate of the numbers of fatal cancers based on a collective dose of less than half the UNSCEAR estimate gives a central value of 16,000 (95% confidence interval, 7,000–38,000).
The explosion of Unit 4 at the Chernobyl nuclear power plant left the nuclear reactor destroyed, with about 96% of the irradiated nuclear fuel inventory of the reactor of unit 4 exposed to the atmosphere. This amounted to about 180 metric tons of uranium with a total radioactivity 7 x 1017 Bq. In an attempt to prevent the escape of additional radiation, the Ukrainians built a concrete sarcophagus over the unit. Construction of the sarcophagus, called an "ukrytie", or shelter, by the Ukrainians, began in May 1986 and was completed in November of the same year.
In addition to the approximately 180 metric tons of fuel or fuel-containing materials, scientists identified 64,000 cubic meters of radioactive building materials, 10,000 metric tons of metal structures, and 800-1,000 metric tons of radioactive water in the destroyed unit.
Due to the high radiation fields, portions of the sarcophagus were erected using remote construction methods. Additionally, without complete information on the strength of the original building, its structural integrity could not be gauged.
The strucutal integrity of the sarcophagus has become a concern in recent years. In 2005, work was begun to repair the sarcophagus after experts warned it was so old it could collapse. Considerable financial backing for the project came from abroad. Repair plans include adding a second shelter around the old one. “Shelter 2” is a huge 19,800-ton steel arch designed to be assembled nearby, then slid into place on rails to minimize workers’ radiation exposure. The sarcophagus is designed to last at least 100 years, providing improved conditions for further stabilization work and eventual cleanup of radioactive debris isolated inside.
Shutdown of the Chernobyl Facility
The agreement to build the Chernobyl power plants dates from 1966 when six 1,000 megawatt electrical (MWe) nuclear reactors were planned.
- Unit 1, which began production in 1977, ceased operation in November 1996. In December 1997, the decision was made to decommission the facility.
- Unit 2, which was first connected to the electric grid in December 1978, ceased operation in 1991 after it was damaged in a fire. The Ukrainian national authorities decided to permanently close this plant in March 1999.
- Unit 3 began power generation in 1981 but suffered many shutdowns for maintenance, inspections and repairs. The Ukrainian national authorities decided to permanently close this plant in 2000.
- Unit 4 was the site of the 1986 accident.
- Units 5 and 6 were under construction at the site at the time of accident, but were never finished.
Impact on reactor design and safety
Leaving aside the verdict of history on its role in melting the Soviet iron curtain, some very tangible practical benefits have resulted from the Chernobyl accident . The main ones concern reactor safety, notably in eastern Europe. (The U.S. Three Mile Island accident in 1979 had a significant effect on western reactor design and operating procedures. While that reactor was destroyed, all radioactivity was contained – as designed – and there were no deaths or injuries.).
While no-one in the West was under any illusion about the safety of early Soviet reactor designs, some lessons learned have also been applicable to western plants. Certainly the safety of all Soviet-designed reactors has improved vastly. This is due largely to the development of a culture of safety encouraged by increased collaboration between East and West, and substantial investment in improving the reactors.
Modifications have been made to overcome deficiencies in all the RBMK reactors still operating. In these, originally the nuclear chain reaction and power output would increase if cooling water were lost or turned to steam, in contrast to most Western designs. It was this effect which caused the uncontrolled power surge that led to the destruction of Chernobyl-4.
All of the RMBK reactors have now been modified by changes in the control rods, adding neutron absorbers and consequently increasing the fuel enrichment from 1.8 to 2.4% uranium-235 (235U), making them very much more stable at low power. Automatic shut-down mechanisms now operate faster, and other safety mechanisms have been improved. Automated inspection equipment has also been installed. A repetition of the 1986 Chernobyl accident is now virtually impossible, according to a German nuclear safety agency report.
Since 1989 over 1,000 nuclear engineers from the former Soviet Union have visited Western nuclear power plants and there have been many reciprocal visits. Over 50 twinning arrangements between East and West nuclear plants have been put in place. Most of this has been under the auspices of the World Association of Nuclear Operators, a body formed in 1989 which links 130 operators of nuclear power plants in more than 30 countries.
Many other international programs were initiated following Chernobyl. The International Atomic Energy Agency (IAEA) safety review projects for each particular type of Soviet reactor are noteworthy, bringing together operators and Western engineers to focus on safety improvements. These initiatives are backed by funding arrangements. The Nuclear Safety Assistance Coordination Centre database lists Western aid totaling almost US$1 billion for more than 700 safety-related projects in former Eastern Bloc countries. The Nuclear Safety Convention is a more recent outcome.
In 1998 an agreement with the U.S. provided for the establishment of an international radioecology laboratory inside the exclusion zone.
- Baverstock, Keith and Dillwyn Williams, 2006. The Chernobyl Accident 20 Years On: An Assessment of the Health Consequences and the International Response. Environmental Health Perspectives, 114(9):1312-1317. doi:10.1289/ehp.9113.
- Chernobyl Forum: 2003–2005, Chernobyl’s Legacy: Health, Environmental and Socio-economic Impacts and Recommendations to the Governments of Belarus, the Russian Federation and Ukraine.
- Mahoney, M.C. et al., 2004. The Chernobyl childhood leukemia study: a background and lessons learned. Environmental Health: A Global Access Science Source, 3(12). doi:10.1186/1476-069X-3-12.
- Nuclear Energy Agency, 2002. Chernobyl: Assessment of Radiological and Health Impacts.
- World Nuclear Association, 2006. Chernobyl Accident, Nuclear Issues Briefing Paper 22, March 2006.
- United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), The Chernobyl accident.