Chernobyl Disaster (1986): The World's Worst Nuclear Accident

The Chernobyl disaster was an explosion and fire at reactor No. 4 of the Chernobyl Nuclear Power Plant in Soviet Ukraine on 26 April 1986. It released large amounts of radioactive material over Ukraine, Belarus, Russia and much of Europe, forced the permanent evacuation of the town of Pripyat and left a vast exclusion zone. Rated at the maximum level 7 on the International Nuclear Event Scale, it remains the most serious accident in the history of nuclear power.

Background

The Chernobyl plant, officially the V. I. Lenin Nuclear Power Plant, stood about 100 kilometres north of Kyiv, close to the border with Belarus. Its four working reactors were of the Soviet RBMK-1000 type, a large graphite-moderated design cooled by water. Pripyat, a new town built for plant workers and their families, lay about 3 kilometres away.

The RBMK design had serious weaknesses that were not widely understood by those who operated it. At low power it could become unstable, because steam bubbles in the cooling water increased the reactor’s activity rather than reducing it, a feature known as a positive void coefficient. Its control rods also had graphite tips, which meant that when they were first inserted they could briefly increase power instead of shutting the reactor down.

The night of 26 April

The accident occurred during a safety test. Engineers wanted to check whether the slowing turbines could supply enough electricity to keep the cooling pumps running during the gap between a power cut and the start of the backup diesel generators. The test had been postponed for several hours at the request of the regional grid, so it was carried out by the night shift.

During the preparations the reactor’s power fell much lower than planned. To raise it again, operators withdrew most of the control rods, leaving the reactor in a highly unstable state that broke the plant’s own operating rules. The test began at about 1:23 a.m. As the cooling flow dropped, power started to climb.

When the shift pressed the emergency shutdown button, known as AZ-5, the descending rods triggered a sudden power surge. Within seconds, at 1:23:44 a.m. according to the standard reconstruction, one or two powerful explosions tore apart the reactor and blew off its heavy upper lid. Burning fragments of fuel and graphite were scattered around the site, and the exposed graphite core caught fire.

Two plant workers died that night. Valery Khodemchuk, a pump operator, was killed in the explosion and his body was never recovered; Vladimir Shashenok died of his injuries hours later. Firefighters, many with no protective equipment, put out the fires on the roofs of the neighbouring buildings by early morning, preventing the disaster from spreading to reactor No. 3.

The Soviet response

At first, officials at the plant and in Moscow did not grasp that the reactor had been destroyed. A government commission led by deputy prime minister Boris Shcherbina, with the chemist Valery Legasov as its leading scientific member, arrived on 26 April. Radiation readings soon made the scale of the disaster impossible to deny.

The town of Pripyat, with about 49,000 residents, was evacuated on the afternoon of 27 April, about 36 hours after the explosion. People were told to take only essentials for a short absence and were carried away in a column of more than a thousand buses. An exclusion zone with a radius of 30 kilometres was declared in early May, and around 116,000 people were evacuated in 1986.

To smother the burning core, helicopters dropped thousands of tonnes of sand, clay, lead and boron onto the reactor. The fire and the release of radioactivity continued for about ten days before being brought largely under control in early May.

The Soviet government kept silent at first. On 28 April workers at Sweden’s Forsmark nuclear plant detected radioactive particles on their clothes, and Swedish authorities traced them to the Soviet Union. Only that evening did the Soviet news agency TASS issue a brief statement. The May Day parade in Kyiv went ahead on 1 May, and General Secretary Mikhail Gorbachev did not address the nation on television about the accident until 14 May.

The liquidators and the sarcophagus

The clean-up was carried out by people known as liquidators: soldiers, reservists, miners, engineers and workers brought in from across the Soviet Union. Their number is usually estimated at around 600,000 over several years, although figures vary by source, and the most heavily exposed worked in 1986 and 1987.

Their tasks included clearing radioactive debris from the roof of the reactor building, often by hand after remote-controlled machines failed, as well as decontaminating villages, burying topsoil and building a cooling slab beneath the reactor. Miners dug a tunnel under the building for this purpose.

By November 1986 a massive concrete and steel structure, officially called the Shelter Object and popularly the “sarcophagus”, had been built around the ruined reactor. It was meant as a temporary solution. The other three reactors returned to service, and the plant continued to generate electricity until the last unit was shut down on 15 December 2000.

Health and environmental consequences

The health effects of Chernobyl have been studied intensively and remain debated. According to the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 134 plant workers and emergency responders developed acute radiation syndrome, and 28 of them died in 1986. Several more died in later years from causes that may or may not have been linked to radiation.

The clearest long-term effect has been thyroid cancer among people who were children or adolescents in 1986, caused mainly by radioactive iodine in milk. UNSCEAR reported almost 20,000 cases between 1991 and 2015 in Belarus, Ukraine and the most affected regions of Russia, and estimated that roughly a quarter of them were attributable to radiation. Most cases were treated successfully.

Estimates of eventual cancer deaths vary widely. A 2005 assessment by the Chernobyl Forum, which included the World Health Organization and the International Atomic Energy Agency, projected up to about 4,000 deaths among the 600,000 most exposed people. Other studies that include wider populations across Europe give figures in the tens of thousands, and such numbers are very uncertain.

The social and economic damage was enormous. In total about 350,000 people were eventually resettled from contaminated areas. Belarus was especially hard hit, as much of the fallout landed on its territory. Large areas of farmland and forest were contaminated, and fears about radiation caused lasting psychological stress among affected communities.

Investigation and blame

In July 1987 a Soviet court convicted the plant director Viktor Bryukhanov, chief engineer Nikolai Fomin and deputy chief engineer Anatoly Dyatlov, who had supervised the test, and sentenced each to ten years in a labour camp. Three other officials received shorter sentences.

The first official Soviet report, presented by Legasov to the International Atomic Energy Agency in Vienna in August 1986, placed the main blame on operator error. A revised IAEA report in 1992, known as INSAG-7, gave much more weight to the flaws in the RBMK design and to a poor safety culture across the Soviet nuclear industry. Legasov, troubled by the failures he had witnessed, took his own life in April 1988.

Myths and facts

Chernobyl was a steam and chemical explosion followed by a fire, not a nuclear explosion like that of an atomic bomb. A nuclear power reactor cannot explode in that way, but the accident released far more long-lived radioactive material than the bombs dropped on Hiroshima and Nagasaki.

The exclusion zone is often described as a lifeless wasteland. In fact, with people gone, wolves, elk, bison and many other animals have thrived there, although scientists still disagree about the effects of radiation on individual animals and plants. Popular dramatisations, such as the 2019 television series, have also simplified or invented some scenes, so they should not be treated as historical records.

Legacy

Chernobyl damaged trust in the Soviet system at a time when Gorbachev was promoting glasnost, or openness. The early secrecy and later revelations fed public anger, particularly in Ukraine and Belarus, and Gorbachev himself later suggested that the accident contributed to the collapse of the Soviet Union.

Internationally, the disaster led to new conventions on early notification of nuclear accidents and on mutual assistance, both adopted in 1986, and to modifications of the remaining RBMK reactors. Several countries slowed or abandoned nuclear power programmes. In November 2016 a huge arched steel structure, the New Safe Confinement, was slid over the old sarcophagus to contain the ruins for at least a century. In February and March 2022 the site was occupied for several weeks by Russian forces during the invasion of Ukraine.

Timeline

• 1986, 26 April, 1:23 a.m.: Reactor No. 4 explodes during a safety test.
• 1986, 27 April: Pripyat is evacuated, about 36 hours after the explosion.
• 1986, 28 April: Radiation is detected in Sweden; Soviet media admit an accident has occurred.
• 1986, 14 May: Gorbachev addresses the Soviet public about the disaster on television.
• 1986, November: The concrete and steel sarcophagus around reactor No. 4 is completed.
• 1987, July: Three senior plant managers are sentenced to ten years in prison.
• 2000, 15 December: The plant’s last working reactor is shut down.
• 2016, November: The New Safe Confinement is moved into place over the ruined reactor.

Why it matters

Chernobyl showed how design flaws, rule-breaking and a culture of secrecy could combine into a catastrophe affecting millions of people across borders. It shaped attitudes to nuclear power for decades, alongside the Three Mile Island accident before it and the Fukushima Daiichi nuclear disaster after it. It also became part of the story of the dissolution of the Soviet Union.