Chernobyl Accident

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The Chernobyl accident in 1986 was the result of a flawed reactor design that was operated with inadequately trained personnel and without proper regard for safety.
The resulting steam explosion and fire released at least five percent of the radioactive reactor core into the atmosphere and downwind.
28 people died within four months from radiation or thermal burns, 19 have subsequently died, and there have been around nine deaths from thyroid cancer apparently due to the accident: total 56 fatalities as of 2004.
An authoritative UN report in 2000 concluded that there is no scientific evidence of any significant radiation-related health effects to most people exposed. This was confirmed in a very thorough 2005-06 study.
The April 1986 disaster at the Chernobyl nuclear power plant in the Ukraine was the product of a flawed Soviet reactor design coupled with serious mistakes made by the plant operators in the context of a system where training was minimal. It was a direct consequence of Cold War isolation and the resulting lack of any safety culture.
NB: "Chernobyl" is the well-known Russian name for the site; "Chornobyl" is preferred by Ukraine.

The accident destroyed the Chernobyl-4 reactor and killed 30 people, including 28 from radiation exposure. A further 209 on site and involved with the clean-up were treated for acute radiation poisoning and among these, 134 cases were confirmed (all of whom apparently recovered). Nevertheless 19 of these subsequently died from effects attributable to the accident. Nobody off-site suffered from acute radiation effects. However, large areas of Belarus, Ukraine, Russia and beyond were contaminated in varying degrees.
The Chernobyl disaster was a unique event and the only accident in the history of commercial nuclear power where radiation-related fatalities occurred.* However, its relevance to the rest of the nuclear industry outside the then Eastern Bloc is minimal.
* There have been fatalities in military and research reactor contexts, eg Tokai-mura.
The accident
On 25 April, prior to a routine shut-down, the reactor crew at Chernobyl-4 began preparing for a test to determine how long turbines would spin and supply power following a loss of main electrical power supply. Similar tests had already been carried out at Chernobyl and other plants, despite the fact that these reactors were known to be very unstable at low power settings.
A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. As flow of coolant water diminished, power output increased. When the operator moved to shut down the reactor from its unstable condition arising from previous errors, a peculiarity of the design caused a dramatic power surge.
The fuel elements ruptured and the resultant explosive force of steam lifted off the cover plate of the reactor, releasing fission products to the atmosphere. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to burst into flames.
There is some dispute among experts about the character of this second explosion. The graphite - there was over 1200 tonnes of it - burned for nine days, causing the main release of radioactivity into the environment. A total of about 14 EBq (1018 Bq) of radioactivity was released, half of it being biologically-inert noble gases. See also appended sequence of events.
Some 5000 tonnes of boron, dolomite, sand, clay and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles.