Key Facts

• The 1986 accident at the Chernobyl nuclear power plant in Ukraine, then part of the former Soviet Union, is the only accident in the history of commercial nuclear power to cause fatalities from radiation. It was the product of a severely flawed Soviet-era reactor design combined with human error. Key differences in U.S. reactor design, regulation and emergency preparedness make it highly unlikely that a Chernobyl-type accident could occur in the United States.
• Twenty-eight highly exposed reactor staff and emergency workers died from radiation and thermal burns within four months of the accident. Officials believe the accident also was responsible for nearly 7,000 cases of thyroid cancer among individuals who were under 18 years of age at the time of the accident. As of 2005, 15 children had died of thyroid cancer. Improved monitoring has been implemented to help ensure that thyroid cancer is detected early, when it is highly treatable.
• Most emergency workers and people living in contaminated areas received relatively low whole-body radiation doses, according to a United Nations study published in 2011. The study found no evidence of increases in solid cancers, decreased fertility or congenital malformations. However, there is “some evidence of a detectable increase” in leukemia and cataract risk among workers who received higher radiation doses when engaged in recovery at the site. Long-term health monitoring of these workers is ongoing.

What Happened

The accident, which occurred in the early morning of April 26, 1986, resulted when operators took actions in violation of the plant’s technical specifications. Operators ran the plant at very low power, without adequate safety precautions and without properly coordinating or communicating the procedure with safety personnel. 

The four Chernobyl reactors were pressurized water reactors of the Soviet RBMK design, or Reactor BolshoMoshchnosty Kanalny, meaning “high-power channel reactor.” Designed to produce both plutonium and electric power, they were very different from standard commercial designs, employing a unique combination of a graphite moderator and water coolant.

The reactors also were highly unstable at low power, primarily owing to control rod design and “positive void coefficient,” factors that accelerated nuclear chain reaction and power output if the reactors lost cooling water.

These factors all contributed to an uncontrollable power surge that led to Chernobyl 4’s destruction. The power surge caused a sudden increase in heat, which ruptured some of the pressure tubes containing fuel.

The hot fuel particles reacted with water and caused a steam explosion, which lifted the 1,000-metric-ton cover off the top of the reactor, rupturing the rest of the 1,660 pressure tubes, causing a second explosion and exposing the reactor core to the environment. The fire burned for 10 days, releasing a large amount of radiation into the atmosphere.

The Chernobyl plant did not have the massive containment structure common to most nuclear power plants elsewhere in the world. Without this protection, radioactive material escaped into the environment.

The crippled Chernobyl 4 reactor now is enclosed in a concrete structure that is growing weaker over time. Ukraine and the Group of Eight industrialized nations have agreed on a plan to stabilize the existing structure by constructing an enormous new sarcophagus around it, which is expected to last more than 100 years.

Officials shut down reactor 2 after a building fire in 1991 and closed Chernobyl 1 and 3 in 1996 and 2000, respectively.

Dealing with the Consequences

Soviet scientists reported that the Chernobyl 4 reactor contained about 190 metric tons of uranium dioxide fuel and fission products. An estimated 13 percent to 30 percent of this escaped into the atmosphere. Contamination from the accident scattered irregularly, depending on weather conditions. Reports from Soviet and western scientists indicate that Belarus received about 60 percent of the contamination that fell on the former Soviet Union. A large area in the Russian Federation south of Bryansk also was contaminated, as were parts of northwestern Ukraine.

Soviet authorities started evacuating people from the area around Chernobyl within 36 hours of the accident. In 1986, 115,000 local people were evacuated. The government subsequently resettled another 220,000 people.

However, the United Nations study found significant shortcomings in the Soviet Union’s implementation of countermeasures. “In the first few weeks, management of animal fodder and milk production (including prohibiting the consumption of fresh milk) would have helped significantly to reduce doses to the thyroid due to radioiodine,” according to the study. “There is no doubt that a substantial contributor to the excess incidence of thyroid cancer has been exposure to radioiodine released during the Chernobyl accident.”

While the Soviets’ initial countermeasures were deemed inadequate, over the next few years the government implemented extensive measures to protect the public. These measures included:

• decontaminating settlements
• removing substantial amounts of food from human consumption
• treating pasture
• providing clean fodder to farm animals.

“In part because of the countermeasures taken, the resulting radiation doses were relatively low … and should not lead to substantial health effects in the general population that could be attributed to radiation exposure from the accident,” the study concluded. The average radiation dose in “contaminated areas” was about equivalent to that from a computed tomography (CT) scan, according to the study.

A Safety Comparison with the U.S.

A 2004 report by the National Academy of Sciences (NAS) identified two important differences between the conditions that led up to the Chernobyl disaster and the U.S. nuclear energy program.

The first key difference is in how the plants are designed and built. All U.S. power reactors have extensive safety features to prevent large-scale accidents and radioactive releases. The Chernobyl reactor had no such features and was unstable at low power levels.

Second, federal regulations require extensive emergency preparedness planning for all U.S. nuclear energy facilities. NAS cited three factors:

• Stringent emergency preparedness plans. Even with the Chernobyl reactor’s poor design, officials could have averted many radioactive exposures to the population with an effective emergency response. Key personnel at all U.S. power reactors work with surrounding populations on an ongoing basis to prepare for an orderly and speedy evacuation in the unlikely event of an accident.
• Alert and notification. Chernobyl plant operators concealed the accident from authorities and the local population, and thus the government did not even begin limited evacuations until about 36 hours after the accident. In the United States, nuclear power plant operators are required to alert local authorities and make recommendations for protecting the public within 15 minutes of identifying conditions that might lead to a significant release—even if such a release has not occurred. The U.S. Nuclear Regulatory Commission posts resident inspectors at every nuclear power plant site to ensure the plants are following federal safety requirements.
• Protecting the food chain. Since authorities did not promptly disclose details of the Chernobyl accident, many people unknowingly consumed contaminated milk and food. This would not be the case in the United States. As it did following the Three Mile Island nuclear accident in 1979, the federal government would carefully monitor and test food and water supplies that potentially could become contaminated. Under existing federal programs and regulations, the government would quarantine and remove from public consumption any unsafe food or water. The accident at Three Mile Island caused the release of a small amount of radioactive material into the atmosphere, but it was too small to cause discernible health effects to the population living near the plant.