"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 water flow 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 the power to surge dramatically.
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. The graphite burned for nine days, causing the main release of radioactivity into the environment. A total of about 12 x 1018 Bq of radiation was released.
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."
I can't believe that was already 15 years ago, yesterday. I still remember it happening. I remember how we (all the way in Holland) couldn't eat spinage or use tealeaves for months. And ofcourse the horrible footage all over the news. Also a scarry thought: similar, but modifed reactors continue to operate in Russia and Lithuania. For more info check, The Uranium Institute | Nuclear Industry Issues | Chernobyl.