Chernobyl

Type: Nuclear and radiation accident
Cause: Reactor design and operator error
Outcome: INES Level 7 (major accident)
Deaths:
2 killed by debris (including 1 missing) and 28 killed by acute radiation sickness.
15 terminal cases of thyroid cancer, with varying estimates of increased cancer mortality over subsequent decades.
~4,000–16,000 deaths from long-term effects.

Chernobyl Disaster

On 26 April 1986, reactor 4 of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukraine, exploded. With dozens of direct casualties and thousands of health complications stemming from the disaster, it is one of only two nuclear accidents rated at the maximum severity on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident, which happened in Japan.

The response involved more than 500,000 personnel and cost an estimated 18 billion rubles (about $85 billion in 2026). It remains the most destructive nuclear disaster and the most expensive disaster in history, with an estimated cost of US$700 billion. The disaster occurred during a test to simulate the cooling of the reactor during a serious accident in blackout conditions. The operators carried out the test following an accidental drop in reactor power. Upon shutting down the reactor in such conditions, pervasive design flaws led to a power surge. The reactor components ruptured and lost coolant, and the resulting steam explosions and meltdown destroyed the reactor building. This was followed by a reactor core fire that spread radioactive contaminants across the Soviet Union and Europe.

The Soviet government established a 10 kilometres (6.2 mi) exclusion zone 36 hours after the accident, initially evacuating around 49,000 people. This was later expanded to 30 kilometres (19 mi), resulting in the evacuation of approximately 68,000 more people. The government did not publicly acknowledge the disaster until two days after the explosion, when elevated radiation levels were detected in Sweden.

Following the explosion, which killed two engineers and severely burned two others, an emergency operation began to put out the fires and stabilize the reactor. Of the 237 workers hospitalized, 134 showed symptoms of acute radiation syndrome (ARS); 28 of them died within three months. Over the next decade, 14 more workers (nine of whom had ARS) died of various causes mostly unrelated to radiation exposure. It is the only instance in commercial nuclear power history where radiation-related fatalities occurred.

As of 2005, 6000 cases of childhood thyroid cancer occurred within the affected populations (15 of them fatal), "a large fraction" being attributed to the accident. Long-term death estimates range from up to 4,000 in Ukraine, Belarus, and Russia (per the United Nations) to 16,000 in total across Europe.

Pripyat was abandoned and replaced by the purpose-built city of Slavutych. The Chernobyl Nuclear Power Plant sarcophagus, completed in December 1986, reduced the spread of radioactive contamination and provided radiological protection for the crews of the undamaged reactors. In 2016–2018, the Chernobyl New Safe Confinement was constructed around the old sarcophagus to enable the removal of the reactor debris, and clean-up is scheduled for completion by 2065.