Pripyat was a Soviet new town of 49,000 people, built from scratch in the 1970s to serve the Chernobyl plant; the plant's workers and their families lived there. On the night of 25 April 1986, Reactor No. 4 was being shut down for scheduled maintenance, and a test was to be carried out during the shutdown: how long the rotational inertia of the turbine could power the emergency cooling pumps after a loss of external electricity. The test had been postponed for administrative reasons, and a less experienced night shift took over. The reactor entered a state of "xenon poisoning" at very low power; to regain control, the operators withdrew nearly all the control rods — and hidden design flaws of the RBMK reactor (graphite-tipped control rods, a positive void coefficient at low power) made this configuration extremely unstable. When the emergency-shutdown (AZ-5) button was pressed at 01:23:40, the graphite tips of the rods first accelerated the reaction; within seconds power jumped to hundreds of times nominal, fuel vaporised, a steam explosion shattered the reactor, and a second explosion (most likely hydrogen) blew the roof off. The exposed graphite fire burned for ten days; the atmosphere received more than a hundred times the radioactive material released at Hiroshima.

In the first hours, plant firefighters and operators tackled the fire without protective equipment, not knowing what they faced; in the following weeks at least 31 people died — 28 of acute radiation syndrome, two in the explosions themselves (this figure counts only direct, individually verified deaths). On the afternoon of 27 April, 36 hours after the accident, Pripyat was evacuated: 1,200 buses moved 49,000 people in three hours. The city emptied that day and never reopened. The Soviet government at first concealed the event; the first outside warning came on 28 April, when staff arriving at Sweden's Forsmark nuclear plant set off contamination alarms on their shoes. Moscow issued a one-paragraph statement that evening. On 14 May Gorbachev addressed the public on television for the first time — a speech later described as the "true beginning" of glasnost. Over the following months some 600,000 "liquidators" — soldiers, miners, builders — were drawn into cleaning the site and encasing the reactor in a concrete sarcophagus; the long-term health effects on them have run for decades.

The long-term legacy runs along two axes. Environmentally, large regions of Belarus, Ukraine and Russia, and less intensely much of Europe, were contaminated with caesium-137 and iodine-131; childhood thyroid cancer cases in Belarus and northern Ukraine rose roughly tenfold (WHO/UNSCEAR reports). The death toll is contested: the Chernobyl Forum (IAEA-WHO-UNDP, 2005) estimated about 4,000 future cancer deaths within the most exposed populations; broader models that include all of Europe (e.g. the TORCH report, Greenpeace) range from tens of thousands to about 200,000 — the most defensible range methodologically runs from several thousand to several tens of thousands. Politically, Gorbachev would later write that "the true cause of the collapse of the Soviet Union was perhaps Chernobyl." The accident shook confidence in Soviet technical superiority and in the system's handling of information; it gave rise to environmental movements in Belarus, Ukraine and Russia, effectively widened the limits of glasnost, and became one of the events that prepared the mental ground for the collapse of the USSR five years later, in 1991. The 2,600 km² exclusion zone — roughly the size of Luxembourg — remains closed to settlement; the one paradoxical beneficiary is wildlife: wolves, wild boar, deer and Przewalski's horse have reclaimed the territory.

Chernobyl shaped the nuclear-energy debate of the 21st century. After 1986 new reactor construction in Western Europe nearly halted; Germany cited Chernobyl alongside Fukushima (2011) when accelerating its phase-out. At the same time, the accident made "safety culture" an international engineering standard; the independent international mechanisms for nuclear oversight (IAEA safety standards, the 1994 Convention on Nuclear Safety) were built after Chernobyl. In 2016 the New Safe Confinement, a 1.5-billion-euro arch, was slid over the original sarcophagus; it has a design life of 100 years. On the Eon timeline, 26 April 1986 is the moment humanity saw the limits of its claim to have tamed the atom, and the moment the ideological self-assurance of the Cold War order first visibly cracked.