The Chernobyl disaster was the worst nuclear accident in history. During a late-night safety test on April 26, 1986, at the Chernobyl Nuclear Power Plant in present-day Ukraine, a combination of design flaws and operator errors triggered a massive power surge, leading to a pair of explosions that released radioactive material across large parts of the Northern Hemisphere.
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Built to contain the remains of Reactor 4 after the nuclear disaster, the "shelter structure" was constructed from about 400,000 cubic meters of concrete—enough to fill 160 Olympic-sized swimming pools. Construction required vehicles and cranes modified with special shielding to protect workers.
The town's 50,000 residents were evacuated more than a day after the explosions at Chernobyl, leaving behind schools, apartments, and streets seemingly frozen in time. Most evacuees were resettled to Slavutych, about 50 kilometers (31 miles) east of the plant.
The nuclear disaster produced around 200 tons of nuclear fuel that mixed with the surrounding melted reactor core materials. As of 2025, corium has been naturally formed four other times in history: once during the Three Mile Island accident and three times at the Fukushima Daiichi plant disaster.
Due to energy shortages in Ukraine, Chernobyl's other undamaged reactors continued operating for years. Units 1 and 2 operated until 1996 and 1991, respectively, while Reactor 3 remained online until December 15, 2000, when Ukraine finally shut down the plant.
The initial explosion resulted from excessive steam pressure that had built up in a runaway manner due to design flaws and violations of safety protocols. Once exposed to atmospheric oxygen, the hot graphite in the control rods underwent a combustion reaction, resulting in a secondary explosion.
A reactor explosion during a safety test resulted in a series of evacuations and resettlements that displaced over 300,000 people and left areas uninhabitable for up to 20,000 years. The Soviet government initially tried to hide the disaster, but rising radiation levels were identified by detectors in Sweden.
Nuclear meltdowns occur when a reactor overheats and radioactive materials leach into the surrounding environment. While rare, the reasons for each occurrence—as detailed in this piece—can help establish new safety protocols for future reactors.
The 1986 Chernobyl meltdown, caused by a combination of poor safety design and human error, is the worst nuclear accident in history. At least three dozen people died in the direct aftermath, and experts believe thousands suffered premature cancer deaths from radiation exposure.
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