Chernobyl: the worst accident in the history of nuclear power

26 April 1986

In the early hours of 26 April 1986, at 1:23 a.m., a catastrophe struck reactor No. 4 of the Chernobyl Nuclear Power Plant. The station stood near the town of Pripyat in the north of the Ukrainian SSR, close to the border with Belarus.

During a test being run as the reactor was powered down, its output went out of control. An explosion — followed by a second — tore off the reactor's heavy lid, weighing about a thousand tonnes, and destroyed the building. The graphite inside the reactor caught fire, and for days the blaze threw enormous quantities of radioactive material into the air. It is the worst accident in the history of nuclear power.

Why it happened

For a long time there was argument over who was to blame — the operators or the designers. Today it is accepted that the cause was a combination of factors.

• A dangerous reactor design. The RBMK-type reactor had features that, in certain conditions, could cause its power to surge sharply. It had no robust containment shell to hold back a release.
• The crew's actions and rule violations. The test was run at low power with some safety systems switched off, in a regime the procedures did not allow for.

The wider setting played a part too: a lack of "safety culture" and the secrecy of the Soviet system, in which the reactor's weaknesses were known but left uncorrected.

Silence and the first days

At first the authorities said nothing. Neither residents nor the world were told the scale of the accident. Pripyat (about 50,000 people) only began to be evacuated some 36 hours later — in the afternoon of 27 April.

The world learned of the disaster from the outside: on 28 April a Swedish nuclear plant detected raised radiation levels carried on the wind and demanded an explanation. Only then did the USSR officially admit the accident. That secrecy later became a symbol of how dangerous it is to hide the truth.

Liquidators and the exclusion zone

To deal with the aftermath, hundreds of thousands of people were brought in — the "liquidators." By various estimates, between 600,000 and 800,000 took part in the work. They put out the fire, cleared radioactive debris from roofs and built the shelter — and many received dangerous doses of radiation.

An exclusion zone with a radius of about 30 kilometres was created around the plant. In 1986 about 115,000 people were evacuated, and roughly 220,000 more were later relocated from Ukraine, Belarus and Russia. By the end of 1986 a concrete "sarcophagus" had been built over the destroyed reactor; in 2016–2019 a new protective arch was erected over it.

How many people died

This is the most contested question — and here it is important to be honest.

There is agreement that about 30 people (plant workers and firefighters) died in the first weeks: two at the moment of the accident, the rest from acute radiation syndrome. A further 134 people suffered acute radiation sickness.

The number of later deaths — from illnesses that radiation can cause years afterward — is estimated very differently. The authoritative international report by the UN and the IAEA (the Chernobyl Forum, 2005) estimated up to about 4,000 possible deaths over the lifetimes of the most exposed groups. Other organizations have cited figures several times higher (tens of thousands). There is no single, universally accepted number: the counting methods differ, and the debate continues to this day.

A documented effect is a rise in thyroid cancer among children, linked to radioactive iodine that entered the milk supply. Most such cases are treatable.

Consequences for the country

The radioactive cloud spread over part of Europe; Belarus was hit hardest, with about a fifth of its territory contaminated.

The accident undermined trust in the Soviet government. The secrecy around Chernobyl contradicted the glasnost ("openness") that Gorbachev had proclaimed. Gorbachev himself later said that Chernobyl may have played an even greater role in the collapse of the USSR than his reforms did.

The plant's other reactors kept operating; the last was shut down only in 2000. Pripyat remains an abandoned town to this day.

Related

• Sputnik 1: the start of the space age — the other side of Soviet science and technology.
• Shortages and queues — the strains of the late USSR, the years of the accident.

Sources

The facts in this article can be verified against authoritative sources:

• IAEA, "Chernobyl: The True Scale of the Accident": https://www.iaea.org/newscenter/pressreleases/chernobyl-true-scale-accident
• World Nuclear Association, "Chernobyl Accident 1986": https://world-nuclear.org/information-library/safety-and-security/safety-of-plants/chernobyl-accident
• Encyclopædia Britannica, "Chernobyl disaster": https://www.britannica.com/event/Chernobyl-disaster
• UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), "Chernobyl": https://www.unscear.org/unscear/en/chernobyl.html
• Wikipedia, "Chernobyl disaster": https://en.wikipedia.org/wiki/Chernobyl_disaster

Where estimates differ (especially on the number of later deaths), we give the range rather than a single figure.