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Setting the scene
By 1986, the Soviet Union operated 15 RBMK nuclear reactors, a design unique to the USSR that used graphite moderation and water cooling. The RBMK had a dangerous positive void coefficient—if cooling water turned to steam, reactivity increased, creating a feedback loop. The design also lacked a full containment dome. On April 26, 1986, at the Chernobyl Nuclear Power Plant in Ukraine, operators were conducting a safety test on Reactor 4 to see if the turbine could power cooling pumps during a power outage. The test was delayed, and the inexperienced night shift took over.
What happened
At 1:23 a.m., operators violated safety protocols, disabling automatic shutdown systems and withdrawing control rods. The reactor became unstable, power surged to 100 times normal, and steam explosions blew the 1,000-ton reactor lid off. A second explosion exposed the graphite core to air, which ignited. The fire burned for 10 days, releasing radioactive material equivalent to 400 Hiroshima bombs. Soviet authorities initially concealed the disaster, not evacuating nearby Pripyat (population 49,000) until 36 hours later. The radiation cloud spread across Europe, detected in Sweden on April 28.
Why it still matters
Chernobyl was the worst nuclear disaster in history, killing 31 people immediately and causing an estimated 4,000-16,000 eventual cancer deaths. It contaminated 100,000 square kilometers and created a 1,000-square-mile exclusion zone still largely uninhabited. The disaster exposed Soviet incompetence and secrecy, accelerating glasnost (openness) under Gorbachev and contributing to the USSR's collapse. It led to major changes in nuclear safety worldwide and effectively ended new nuclear construction in the West for decades. The Chernobyl sarcophagus, built to contain the reactor, was replaced with a new confinement structure in 2016. The disaster remains a cautionary tale about technological hubris and the importance of safety culture.
Background
A large exclusion zone remains around the site.