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 many consider 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. [5 Weird Things You Didn't Know About Chernobyl]

Where is Chernobyl?

The Chernobyl Nuclear Power Plant is located about 81 miles (130 kilometers) north of the city of Kiev, Ukraine and about 12 miles (20 km) south of the border with Belarus, according to the World Nuclear Association. It is made up of four reactors that 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 newly built city of Pripyat was the nearest town to the power plant at just under 2 miles away (3 km) and housed almost 50,000 people in 1986. A smaller and older town, Chernobyl, was about 9 miles (15 km) away and home to about 12,000 residents. The remainder of the region was primarily farms and woodland.

The power plant

The Chernobyl plant used four Soviet-designed RBMK-1000 nuclear reactors — a design that's now universally recognized as inherently flawed. RBMK reactors were of a pressure tube design that used an enriched U-235 uranium dioxide fuel to heat water, creating steam that drives the reactors' turbines and generates electricity, according to the World Nuclear Association.

In most nuclear reactors, water is also used as a coolant and to moderate the reactivity of the nuclear core by removing the excess heat and steam, according to the World Nuclear Association. But the RBMK-1000 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."

What happened?

The explosion occurred on April 26, 1986, during a routine maintenance check, according to U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Operators were planning on testing the electrical systems when they turned off vital control systems, going against the safety regulations. This caused the reactor to reach dangerously unstable and low-power levels.

Reactor 4 had been shut down the day before in order to perform the maintenance checks to safety systems during potential power outages, according to the Nuclear Energy Agency (NEA). While there is still some disagreement over the actual cause of the explosion, it is generally believed that the first was caused by an excess of steam and the second was influenced by hydrogen. The excess steam was created by the reduction of the cooling water which caused steam to build up in the cooling pipes — the positive-void coefficient — which caused an enormous power surge that the operators could not shut down.

The explosions occurred at 1:23 a.m. on April 26, destroying reactor 4 and initiating a booming fire, according to NEA. Radioactive debris of fuel and reactor components rained over the area while fire spread from the building housing reactor 4 to adjacent buildings. Toxic fumes and dust were carried by the blowing wind, bringing fission products and the noble gas inventory with it.

The Chernobyl nuclear power plant. (Image credit: Sergeev Kirill/Shutterstock)

Radioactive fallout

The explosions killed two plant workers — 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.

The initial fire was stifled by about 5 a.m., but the resulting graphite-fueled fire took 10 days and 250 firefighters to extinguish it, according to NEA. However, toxic emissions continued to be pumped into the atmosphere for an additional 10 days.

Most of the radiation released from the failed nuclear reactor was from fission products 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.

Evacuations of Pripyat commenced on April 27 — about 36 hours after the accident had occurred. By that time, many residents were already complaining about vomiting, headaches and other signs of radiation sickness. Officials closed off an 18-mile (30 km) area around the plant by May 14, evacuating another 116,000 residents. Within the next few years, 220,000 more residents were advised to move to less contaminated areas, according to the World Nuclear Association. [Images: Chernobyl, Frozen in Time]

Health effects

Twenty-eight of the workers at Chernobyl died in the first 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 levels raised concern in Sweden about three days later, scientists there were able to conclude the approximate location of the nuclear disaster based on radiation levels and wind directions, forcing Soviet authorities to reveal the full extent of the crisis, according to the United Nations.

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. Between 1991 and 2015, as many as 20,000 cases of thyroid cases were diagnosed in patients who were under the age of 18 in 1986, according to a 2018 UNSCEAR report. While there may still be additional cases of cancer that emergency workers, evacuees, and residents may experience throughout their lifetimes, the known overall rate of cancer deaths and other health effects directly 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 was likely too low to cause any problems, according to the World Nuclear Association. In 2000, 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.

Dead forest at the Chernobyl site. (Image credit: dreamstime)

Environmental impacts

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, according to the National Science Research Laboratory at Texas Tech University.

The damaged reactor was hastily sealed in a concrete sarcophagus intended to contain the remaining radiation, according to the NRC. However, there is ongoing intense scientific debate over how effective this sarcophagus has been and will continue to be into the future. An enclosure called the New Safe Confinement structure began construction in late 2006 after stabilizing the existing sarcophagus. The new structure, completed in 2017, is 843 feet (257 meters) wide, 531 feet (162 m) long, and 356 feet (108 m) tall and designed to completely enclose reactor 4 and its surrounding sarcophagus for at least the next 100 years, according to World Nuclear News.

Despite the contamination of the site — and the inherent risks in operating a reactor with serious design flaws — the Chernobyl nuclear plant continued operation to meet the power needs of Ukraine until its last reactor, reactor 3, was shut down in December of 2000, according to World Nuclear News. Reactors 2 and 1 were shut down in 1991 and 1996, respectively. Complete decommissioning of the site is expected to be completed by 2028.

The plant, the ghost towns of Pripyat and Chernobyl, and the surrounding land make up a 1,000-square-mile (2600 square kilometer) "exclusion zone," which is restricted to nearly everyone except for scientists and government officials.

Despite the dangers, several people returned to their homes shortly after the disaster, with some sharing their stories with news sources such as the BBC, CNN, and The Guardian. And in 2011, Ukraine opened up the area to tourists wanting to see the after-effects of the disaster firsthand.

Chernobyl today

Today, the region, including within the exclusion zone, is filled with a variety of wildlife that have thrived without interference from humans, according to National Geographic and the BBC. 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 power plant. Nonetheless, 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]

The area has recovered to some extent, but is far from returning to normal.. But in the areas just outside the exclusion zone, people are beginning to resettle. Tourists continue to visit the site, with visitation rates jumping 30-40% thanks to a new HBO series based on the disaster. And The catastrophe that occurred at Chernobyl resulted in a few significant changes for the nuclear industry: concern about reactor safety increased in eastern Europe as well as around the world; the remaining RBMK reactors were modified to reduce the risk in another disaster; and many international programs including the International Atomic Energy Agency (IAEA) and the World Association of Nuclear Operators (WANO) were founded as a direct result of Chernobyl, according to the World Nuclear Association. And around the globe, experts have continued researching ways to prevent future nuclear disasters.

Additional resources:

More about how water cools and moderates nuclear reactors from the International Atomic Energy Agency.

Find the latest news about the Chernobyl Nuclear Power Plant on their website.

Read answers to longstanding questions about the health effects of Chernobyl disaster according to the World Health Organization.

This article was updated on June 20, 2019 by Live Science Contributor Rachel Ross.