What happened at Chernobyl? The 1986 disaster explained

What happened at Chernobyl was the worst accident in the history of nuclear power. Early on 26 April 1986, during a safety test, reactor 4 of the Chernobyl plant in Soviet Ukraine surged out of control and exploded, and its graphite core burned for about ten days. Radioactive material spread across Europe, some 350,000 people were eventually moved from their homes, and the death toll is still argued over.

In short

• A flawed Soviet reactor design and operators breaking their own rules combined in a test that sent power soaring in seconds.
• Two workers died in the explosions and 28 more within weeks from radiation sickness; the long-term toll from cancer is estimated, not counted, and estimates range from about 4,000 to over 90,000.
• The clearest health effect is thousands of thyroid cancers in people exposed as children, mostly treatable; fear and displacement did lasting harm too.

What went wrong that night?

The plant, about 110 kilometers north of Kyiv, ran four RBMK-1000 reactors, a Soviet design that used graphite to slow the neutrons and water to carry away heat. The test on unit 4 was meant to check a safety idea: if the plant lost outside power, could the turbines, as they spun down, keep the cooling pumps running for the minute or so before the backup diesel generators started?

The RBMK had a dangerous quirk the operators had not been properly told about. In most reactors, if the cooling water boils into steam, the reaction slows. In the RBMK at low power, it could speed up, because water absorbs neutrons and steam doesn't. Engineers call this a positive void coefficient. At low power, more steam meant more power, which meant more steam.

Time, 26 April 1986	What happened
The day before	The test is delayed for hours at the grid's request; the reactor sits at half power, building up neutron-absorbing xenon
Just after midnight	Power falls far too low; to raise it, operators pull out almost all the control rods, against the rules
About 1:23 a.m.	The test begins; the pumps slow as the turbine runs down, and the water in the core starts turning to steam
Seconds later	Power rises; the operators press the emergency shutdown button
Moments later	Graphite tips on the descending rods briefly add power at the bottom of the core; power spikes to many times its maximum
Within seconds	A steam explosion tears loose the reactor's 1,000-tonne cover plate; a second explosion follows, scattering burning graphite and fuel

The shutdown rods were the final flaw. As the World Nuclear Association explains, a design peculiarity meant that inserting them caused a power surge before they slowed the reaction. Operators had also switched off automatic safety systems for the test. Early Soviet accounts blamed the staff; later reviews, including an international one in 1992, put more weight on the design.

It was not a nuclear explosion like a bomb's. A reactor's fuel can't detonate that way; the blasts were steam and, probably, gas, but they threw the core open to the sky.

How did the Soviet Union respond?

Firefighters arrived within minutes, many without protective gear, and put out the roof fires by dawn. Several of them received fatal doses. The town of Pripyat, home to about 45,000 people, most of them plant workers and their families, was not evacuated until the afternoon of 27 April, about 36 hours after the explosion. Residents were told to pack for three days. Few ever went back.

The outside world found out from Sweden. On the morning of 28 April, workers at a Swedish nuclear plant more than 1,000 kilometers away set off radiation alarms, and the contamination was traced not to their plant but to the east. Only that evening did Soviet television read a short statement. The secrecy reflected how the Soviet Union handled bad news, and Chernobyl later became a byword for it under Mikhail Gorbachev's new policy of openness.

Over the following months, about 116,000 people were evacuated from a 30-kilometer zone around the plant, and more later. Helicopters dropped sand, lead and boron onto the burning core. Hundreds of thousands of "liquidators", soldiers, miners and workers from across the Soviet Union, cleaned up the site and built a concrete and steel shelter, the "sarcophagus", over the ruined reactor by November 1986.

How many people died because of Chernobyl?

The count depends on what is counted, and that is why figures differ so widely.

Group	Figure	Source
Killed by the explosions	2	World Nuclear Association
Died of acute radiation syndrome within weeks	28, of 134 confirmed cases	UNSCEAR, via the World Nuclear Association
Deaths directly attributed to radiation by mid-2005	Fewer than 50	WHO, 2005
Possible eventual cancer deaths among the 600,000 most exposed	Up to 4,000	WHO, 2005
Possible eventual cancer deaths, including wider contaminated areas	Up to 9,000	WHO, as reported in 2006
Estimated fatal cancers, in a report commissioned by Greenpeace	About 93,000	Greenpeace, 2006

The larger numbers are projections. Cancer is common, so a few thousand extra deaths among millions of people can't be picked out one by one; they are estimated from the doses received and the known risk per dose. The WHO's 4,000 covers about 600,000 people: 200,000 emergency workers from 1986 and 1987, 116,000 evacuees and 270,000 residents of the worst-hit areas. Greenpeace's figure, reported by MercoPress and others in April 2006, drew on national health statistics instead, and its critics say it counts deaths that radiation can't be shown to have caused.

Still argued

Whether Chernobyl's toll is in the thousands or the tens of thousands is a real dispute, not a settled fact with a dissenting fringe. The UN agencies' figures rest on dose estimates and a cautious model of risk; critics, including some scientists in Belarus and Ukraine, argue that the official doses were too low and that illnesses besides cancer were missed. What both sides accept is that the deaths can't be counted directly. The UN's Scientific Committee on the Effects of Atomic Radiation concluded that, apart from the thyroid cancers and the most exposed workers, "there is no evidence of a major public health impact attributable to radiation exposure 20 years after the accident".

The clearest effect is thyroid cancer. Radioactive iodine from the fire settled on pasture, passed into cows' milk, and collected in children's thyroid glands. A 2018 UNSCEAR report counted about 20,000 thyroid cancers diagnosed from 1991 to 2015 in people who were 18 or younger at the time, and estimated that about a quarter were probably caused by radiation, with a wide range of uncertainty. Most were treated successfully; by 2005, 15 had been fatal.

The WHO also called the mental-health impact "the largest public health problem unleashed by the accident to date". Fear of radiation, sometimes exaggerated, did damage of its own: displacement, anxiety and fatalism, and, according to the World Nuclear Association, abortions that some doctors in Europe advised even though the doses were far too low to harm a fetus.

What is Chernobyl like now?

The other three reactors kept running for years; the last closed in December 2000. A huge steel arch, the New Safe Confinement, was slid over the old sarcophagus in November 2016 to contain the ruin for the next century while it is taken apart. The 30-kilometer exclusion zone remains closed to residents, though some elderly people returned on their own and the zone has drawn tourists and scientists.

War reached it too. Russian forces occupied the site from 24 February to 31 March 2022. On 14 February 2025, a drone struck the arch's roof, leaving a hole about 6 meters across; radiation levels outside stayed normal, according to the World Nuclear Association.

What did Chernobyl change?

Chernobyl reshaped how the world thinks about nuclear power. The remaining RBMK reactors were modified to remove the flaws that caused the accident, and international safety reviews and reporting became the norm. Several countries slowed or halted new plants. Some historians, and Gorbachev himself, have argued that the disaster and its cover-up hastened the end of the Soviet Union, five years later, by showing its citizens how much their government had hidden.

Like the Titanic, Chernobyl is remembered as a disaster that a confident system thought impossible, and a lesson about what happens when warnings are not passed on. The 2019 television series Chernobyl brought the story to millions; it is drama, which compresses events and invents characters, so the record above is the place to check it against.

Check yourself

• Explain the positive void coefficient in one sentence, and why it mattered at low power.
• Why do estimates of Chernobyl's death toll range from fewer than 50 to more than 90,000?
• What was the clearest health effect, and how did the radioactive iodine reach people?

Sources

• Chernobyl accident 1986 — World Nuclear Association, updated 2026
• Chernobyl: the true scale of the accident — World Health Organization, 2005
• The Chernobyl accident — United Nations Scientific Committee on the Effects of Atomic Radiation
• Chernobyl controversy: 9,000 or 100,000 cancer deaths? — MercoPress, 2006