Chernobyl Reactor 4 exploded because of a design flaw — 500,000 liquidators, $700B cost

On 26 April 1986, Reactor 4 at the Chernobyl Nuclear Power Plant in Ukraine exploded during a safety test. The RBMK-1000 reactor had a fundamental design flaw: a positive void coefficient that caused reactivity to increase as coolant turned to steam. The test was treated as an electrical experiment, not a nuclear safety test, and the reactor's emergency core cooling system was disabled. When operators pressed the emergency scram button, the graphite-tipped control rods initially increased reactivity, causing a power surge to 30,000 MW — ten times normal.

The explosion killed two workers instantly and ejected radioactive material across Europe. 28 firefighters and plant workers died of acute radiation syndrome within three months. Over 500,000 liquidators were deployed for cleanup. The nearby city of Pripyat was abandoned permanently. The disaster contaminated vast areas of Ukraine, Belarus, and Russia, forcing the permanent resettlement of 350,000 people.

The total cost of the disaster is estimated at $700 billion, making it the most expensive disaster in history. The Soviet government initially covered up the accident, which was only discovered when Sweden detected elevated radiation levels. The disaster led to the worldwide reassessment of nuclear safety and the eventual shutdown of all RBMK reactors. The New Safe Confinement, a €1.5B structure to contain the destroyed reactor, was completed in 2018.