more info on Chernobyl:
The accident at Chernobyl Unit 4, on 26 April 1986, did not occur during normal operation of the reactor. It happened during a test designed to assess the reactor's safety margin in a particular set of circumstances. The test, which had to be performed at less than full reactor power, was scheduled to coincide with a routine shut-down of the reactor.
The test
Nuclear power stations not only produce electricity, they also consume electricity, for example to power the pumps that circulate the coolant. This electricity is usually supplied from the grid. If the source of electricity should fail, most reactors are able to derive the required electricity from their own production. However, if the reactor is operating but not producing power, for example when in the process of shutting down, some other source of supply is required. Generators are generally used to supply the required power, but there is a time delay while they are started.
The test carried out at Chernobyl-4 was designed to demonstrate that a coasting turbine would provide sufficient power to pump coolant through the reactor core while waiting for electricity from the diesel generators. The circulation of coolant was expected to be sufficient to give the reactor an adequate safety margin.
Unit #4 of the Chornobyl nuclear complex, located at Pripyat, Ukraine, exploded in the early morning of 26 April 1986 at approximately 01:24:00 hrs Kyiv time. The explosion and the following graphite fire spewed some 185 million curies of deadly radioactive debris onto the city of Pripyat and the surrounding area. The present-day republics of Belarus, Ukraine and the Russian Federation were particularly severely contaminated by the radioactive dust spread by the air currents throughout eastern, northern and central Europe, and, indeed, around the world. Next to the atomic bombs dropped on Hiroshima and Nagasaki on the 6 and 9 August 1945, it remains (hopefully forever) the world's worst nuclear tragedy. Its consequences, which have already been enormous, shall remain with the planet earth for the next 250,000 years.
