Chernobyl

On April 26, 1986 at 1:23 a.m., reactor number four at the Chernobyl plant, near Prypia in the Ukrainian Soviet Socialist Republic, suffered a massive, catastrophic power excursion (meltdown). This caused a steam explosion, followed by a second (chemical, not nuclear) explosion from the ignition of generated hydrogen mixed with air, which tore the top from the reactor and its building and exposed the reactor core.

The resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive geographical area, including the nearby town of Pripyat. The plume drifted over large parts of the western Soviet Union, Eastern Europe, Western Europe, and Northern Europe. Large areas in Ukraine, Belarus, and Russia had to be evacuated, with over 336,000 people resettled.

In the aftermath of the accident, 237 people suffered from acute radiation sickness, of whom 31 died within the first three months.  Most of these were fire and rescue workers trying to bring the accident under control, who were not fully aware of how dangerous exposure to the radiation in the smoke was.

It is considered the worst nuclear power plant accident in history and is the only level 7 event on the International Nuclear Event Scale.  Four hundred times more radioactive material was released than had been by the atomic bombing of Hiroshima.

The Chernobyl reactor is now enclosed in a large concrete sarcophagus, which was built quickly to allow continuing operation of the other reactors at the plant.

Official Chernobyl site
IAEA
Wikipedia