What was the Chernobyl Disaster?

On 30 April 1986, the world’s worst nuclear disaster goes public, days after the event. What exactly happened at Reactor Number Four at Chernobyl's nuclear power plant, and why?

Chernobyl nuclear power plant a few weeks after the disaster

When an ill-conceived experiment at Chernobyl nuclear power station went wrong on 26 April 1986, the consequences were catastrophic.

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Technicians on Reactor Number Four at the Soviet plant, in Ukraine, hoped to ascertain whether the reactor turbine could power the cooling pumps, in case of electrical failure. They did this by running the reactor on low power but disabling emergency safety systems – including the automatic shutdown.

The increasingly unstable reactor overheated but tests weren’t cancelled, regulations were ignored and mistakes piled up until 1.23am, when a chain reaction in the core caused a power surge and meltdown. The reactor exploded, sending flames and radioactive material 300 metres into the sky.

What followed was a tragic and costly cover-up. Firefighters weren’t informed of the radiation, so were exposed to fatal doses, while the evacuation of the nearby city of Pripyat didn’t begin for 36 hours. It was only after monitoring stations in Sweden (620 miles away) picked up high radioactivity in the air that the accident was made public on Russian news.

The radiation was contained by early May, but at extreme risk to the workers who built a concrete-and-steel ‘sarcophagus’ over the reactor.

In the immediate aftermath, 32 people perished due to radioactivity, but countless more died later as radiation blew as far as Russia, France and Italy.

An ‘exclusion zone’ extended nearly 19 miles from the station, but that couldn’t stop the poisoning of wildlife, a drastic rise in cancer cases and worldwide fear that nuclear power was far from safe.

Jonny Wilkes is a freelance writer specialising in history

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This content first appeared in the April 2016 issue of BBC History Revealed