The expolsion at Chernobyl was the worst nuclear accident to date.
In the early morning hours of April 26, 1986, the Chernobyl nuclear power plant in Ukraine (formerly part of the Soviet Union) exploded, creating what has been described as the worst nuclear disaster the world has ever seen.
Even after many years of scientific research and government investigation, there are still many unanswered questions about the Chernobyl accident — especially regarding the long-term health impacts that the massive radiation leak will have on those who were exposed.
Where is Chernobyl?
Located about 81 miles (130 km) north of the city of Kiev, Ukraine, and about 12 miles (20 km) south of the border with Belarus, the four reactors at the Chernobyl Nuclear Power Plant were designed and built during the 1970s and 1980s. A manmade reservoir, roughly 8.5 square miles (22 sq. km) in size and fed by the Pripyat River, was created to provide cooling water for the reactor.
The nearest town to the power plant was the newly built city of Pripyat, which housed almost 50,000 people in 1986, according to the World Nuclear Association. A smaller town, Chornobyl, was home to about 12,000 residents. The remainder of the region was primarily farms and woodland.
The Chernobyl plant used four Soviet-designed RBMK-1000 nuclear reactors — a design that's now universally recognized as inherently flawed. RBMK reactors use enriched U-235 uranium fuel to heat water, creating steam that drives the reactors' turbines and generates electricity.
In most nuclear reactors, where water is used as a coolant and to moderate the reactivity of the nuclear core, as the core heats up and produces more steam, the increase in steam bubbles or "voids" in the water reduces the reactivity in the nuclear core. This is an important safety feature found in most reactors built in the United States and other Western nations.
But not in the RBMK-1000, which used graphite to moderate the core's reactivity and to keep a continuous nuclear reaction occurring in the core. As the nuclear core heated and produced more steam bubbles, the core became more reactive, not less, creating a positive-feedback loop that engineers refer to as a "positive-void coefficient."
The day before the Chernobyl nuclear disaster, plant operators were preparing for a one-time shutdown to perform routine maintenance on reactor number 4. In violation of safety regulations, operators disabled plant equipment including the automatic shutdown mechanisms, according to the U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).
At 1:23 a.m. on April 26, when extremely hot nuclear fuel rods were lowered into cooling water, an immense amount of steam was created, which — because of the RBMK reactors' design flaws — created more reactivity in the nuclear core of reactor number 4. The resultant power surge caused an immense explosion that detached the 1,000-ton plate covering the reactor core, releasing radiation into the atmosphere and cutting off the flow of coolant into the reactor.
A few seconds later, a second explosion of even greater power than the first blew the reactor building apart and spewed burning graphite and other parts of the reactor core around the plant, starting a number of intense fires around the damaged reactor and reactor number 3, which was still operating at the time of the explosions.
The explosions killed two plant workers, who were the first of several workers to die within hours of the accident. For the next several days, as emergency crews tried desperately to contain the fires and radiation leaks, the death toll climbed as plant workers succumbed to acute radiation sickness.
Most of the radiation released from the failed nuclear reactor was from iodine-131, cesium-134 and cesium-137. Iodine-131 has a relatively short half-life of eight days, according to UNSCEAR, but is rapidly ingested through the air and tends to localize in the thyroid gland. Cesium isotopes have longer half-lives (cesium-137 has a half-life of 30 years) and are a concern for years after their release into the environment.
On April 27, the residents of Pripyat were evacuated — about 36 hours after the accident had occurred. By that time, many were already complaining about vomiting, headaches and other signs of radiation sickness. Officials eventually closed off an 18-mile (30 km) area around the plant; residents were told they would be able to return after a few days, so many left their personal belongings and valuables behind.
Twenty-eight of the workers at Chernobyl died in the four months following the accident, according to the U.S. Nuclear Regulatory Commission (NRC), including some heroic workers who knew they were exposing themselves to deadly levels of radiation in order to secure the facility from further radiation leaks.
The prevailing winds at the time of the accident were from the south and east, so much of the radiation plume traveled northwest toward Belarus. Nonetheless, Soviet authorities were slow to release information about the severity of the disaster to the outside world. But when radiation alarms began to go off at a nuclear plant in Sweden, authorities were forced to reveal the full extent of the crisis.
Within three months of the Chernobyl accident, a total of 31 people died from radiation exposure or other direct effects of the disaster, according to the NRC, UNSCEAR and other sources. More than 6,000 cases of thyroid cancer may eventually be linked to radiation exposure in Ukraine, Belarus and Russia, though the precise number of cases that are directly caused by the Chernobyl accident is difficult (if not impossible) to ascertain.
Surprisingly, the overall rate of cancer deaths and other health effects related to Chernobyl's radiation leak is lower than was initially feared. "The majority of the five million residents living in contaminated areas … received very small radiation doses comparable to natural background levels (0.1 rem per year)," according to an NRC report. "Today the available evidence does not strongly connect the accident to radiation-induced increases of leukemia or solid cancer, other than thyroid cancer."
Some experts have claimed that unsubstantiated fear of radiation poisoning led to greater suffering than the actual disaster. For example, many doctors throughout Eastern Europe and the Soviet Union advised pregnant women to undergo abortions to avoid bearing children with birth defects or other disorders, though the actual level of radiation exposure these women experienced were too low to cause any problems. Even the United Nations published a report on the effects of the Chernobyl accident that was so "full of unsubstantiated statements that have no support in scientific assessments," according to the chairman of UNSCEAR, that it was eventually dismissed by most authorities.
Shortly after the radiation leaks from Chernobyl occurred, the trees in the woodlands surrounding the plant were killed by high levels of radiation. This region came to be known as the "Red Forest" because the dead trees turned a bright ginger color. The trees were eventually bulldozed and buried in trenches.
The damaged reactor was hastily sealed in a concrete sarcophagus intended to contain the remaining radiation: How effective this sarcophagus has been — and will continue to be into the future — is a subject of intense scientific debate. Plans to construct a safer and more permanent containment structure around the reactor have yet to be implemented.
Despite the contamination of the site — and the inherent risks in operating a reactor with serious design flaws — the Chernobyl nuclear plant continued operation for many years, until its last reactor was shut down in December of 2000. The plant, the ghost towns of Pripyat and Chernobyl, and a large area surrounding the plant known as the "zone of alienation" are largely off-limits to humans.
There are, however, exceptions: A few hundred former residents of the area have returned to their former homes, despite the risks of radiation exposure. Scientists, government officials and other personnel are allowed on the site for inspections and other purposes. And in 2011, Ukraine opened up the area to tourists who want to see firsthand the after-effects of the disaster.
The region today is widely known as one of the world's most unique wildlife sanctuaries. Thriving populations of wolves, deer, lynx, beaver, eagles, boar, elk, bears and other animals have been documented in the dense woodlands that now surround the silent plant. Only a handful of radiation effects, such as stunted trees growing in the zone of highest radiation and animals with high levels of cesium-137 in their bodies, are known to occur. [Infographic: Chernobyl Nuclear Disaster 25 Years Later]
But that's not to suggest that the area has returned to normal, or will at any point in the near future. Because of the long-lived radiation in the region surrounding the former Chernobyl Nuclear Power Plant, the area won't be safe for human habitation for at least 20,000 years.