Chernobyl Nuclear AccidentApril 26, 1986 — Pripyat, Ukraine

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In the early hours of April 26, 1986, during a safety test on Reactor No. 4 at the Chernobyl Nuclear Power Plant near Pripyat, Ukraine — then part of the Soviet Union — a sudden power surge and steam explosions destroyed the reactor core. The explosions and the fire in the reactor’s graphite moderator, which burned for days, threw huge quantities of radioactive material high into the atmosphere, and fallout spread across Ukraine, Belarus and much of Europe.

Two plant workers were killed in the explosions. Twenty-eight firefighters and plant staff died of acute radiation sickness in the weeks that followed. The city of Pripyat, home to some 49,000 people, was evacuated the day after the accident and never reoccupied, and an exclusion zone roughly 30 kilometers (19 miles) in radius remains in force around the plant.

The Soviet account presented to the International Atomic Energy Agency in 1986, summarized in the IAEA’s INSAG-1 report, placed the blame almost entirely on the plant’s operators. The IAEA’s revised report, INSAG-7 (1992), reached a different conclusion: the accident was rooted in the design of the RBMK reactor itself — including a positive void coefficient that made it dangerously unstable at low power, and control rods whose graphite tips briefly increased reactivity as they were inserted — compounded by a safety culture that allowed the test to proceed in a hazardous state.

The ruined reactor was entombed in a concrete sarcophagus built in 1986. In 2016 a giant steel arch, the New Safe Confinement, was slid into place over the old shelter to contain the remains for the next century.

Chernobyl reshaped nuclear safety around the world. It led to the creation of the World Association of Nuclear Operators, to international conventions on early notification of nuclear accidents, to design fixes on every remaining RBMK reactor, and to the IAEA safety-review system of which the INSAG reports form a part.