On 26 April 1986, at 1:23 in the morning, Reactor Number Four at the Chernobyl Nuclear Power Plant in Soviet Ukraine exploded. The blast and the fire that followed released a cloud of radioactive material that drifted across much of Europe. It remains the worst nuclear power accident in history.
What went wrong
Engineers were running a safety test on the reactor — checking whether, in an emergency, the reactor could still power the cooling pumps after the main power was cut. The test was poorly designed, the operators were under pressure to complete it quickly, and safety protocols were ignored or overridden. The reactor's power surged wildly out of control.
There was an enormous steam explosion, followed almost immediately by a second, more powerful explosion. The 1,000-tonne steel roof of the reactor was blown off. The reactor core — now open to the atmosphere — was burning, spewing radioactive smoke high into the sky.
A nuclear reactor works a bit like a very carefully controlled kettle: the nuclear reaction heats water to make steam, which drives turbines to make electricity. Chernobyl's reactor was like a kettle where someone removed the safety valve, cranked the heat to maximum, and sealed the lid. The pressure built until the whole thing blew apart.
The immediate response
Firefighters were called to the site, most with no idea they were fighting a radioactive fire. Many received fatal doses of radiation in the first hours. The Soviet government initially tried to downplay the disaster, waiting 36 hours before ordering an evacuation of the nearby city of Pripyat, home to 50,000 people. Those people were told to take only what they needed for three days. Most never returned.
The exclusion zone
Over the following days and months, approximately 600,000 emergency workers — known as liquidators — were brought in to contain the disaster. They buried the remains of the reactor under a concrete shelter called the sarcophagus. An exclusion zone of 30 kilometres radius was established. It still exists today.
The nearby ghost town of Pripyat, with its abandoned fairground and crumbling tower blocks, is one of the most haunting places in the world — a snapshot of life frozen in April 1986.
The wider impact
Estimates of deaths caused by Chernobyl vary enormously — from the officially recognised 31 direct deaths to projections of tens of thousands of cancer deaths over subsequent decades. The disaster badly damaged the Soviet Union's credibility and economy, and many historians argue it accelerated the collapse of the USSR in 1991. The accident fundamentally changed public attitudes to nuclear power around the world — effects still felt in energy policy today.
On 26 April 1986, at 1:23 in the morning, something terrible happened. A reactor at the Chernobyl Nuclear Power Plant exploded. The plant was in a part of the world called Soviet Ukraine. The explosion and fire sent a cloud of dangerous radioactive material into the air. This cloud spread across much of Europe. It is still the worst nuclear power accident that has ever happened.
What went wrong
Engineers were doing a safety test on the reactor. They were checking if the reactor could still work in an emergency. The test was not planned well. The workers were being rushed to finish it quickly. Important safety rules were ignored. Suddenly, the reactor's power shot up wildly and went out of control.
A huge steam explosion happened. Almost straight away, a second bigger explosion followed. The reactor had a steel roof weighing 1,000 tonnes. The explosion blew it clean off. The inside of the reactor was now open to the sky. It was on fire and sending radioactive smoke high into the air.
A nuclear reactor works a bit like a very carefully watched kettle. The nuclear reaction heats water to make steam. The steam drives machines called turbines to make electricity. The Chernobyl reactor was like a kettle where someone took off the safety valve. Then they turned the heat up as high as it would go and pressed the lid down tight. The pressure kept building until the whole thing exploded apart.
The immediate response
Firefighters were called to the plant straight away. Most of them did not know the fire was radioactive. Many of them received dangerous amounts of radiation in the first few hours. This was fatal, meaning it would kill them. The Soviet government tried to make the disaster seem less serious than it was. They waited 36 hours before telling people to leave the nearby city of Pripyat. Around 50,000 people lived there. They were told to pack only enough things for three days. Most of them never came back.
The exclusion zone
Over the next days and months, around 600,000 emergency workers arrived to help. They were called liquidators. Their job was to stop the disaster from spreading further. They covered what was left of the reactor with a concrete shelter. This shelter was called the sarcophagus. A large safety area was set up in a circle around the plant. This circle reached 30 kilometres in every direction. It is still there today.
The city of Pripyat is now a ghost town. It has an abandoned fairground and crumbling blocks of flats. It looks like time stopped completely in April 1986. It is one of the strangest and saddest places in the world.
The wider impact
People disagree about how many deaths Chernobyl caused. The officially recognised number of direct deaths is 31. But some experts think tens of thousands of people may have later died from cancer caused by the radiation. The disaster made people trust the Soviet government much less. It also cost the country a huge amount of money. Many historians think it helped cause the Soviet Union to collapse in 1991. The accident also changed how people all around the world felt about nuclear power. Those feelings still affect decisions about energy today.