Chernobyl: 20 years of disaster

26th April 2006 marked 20 years of the Chernobyl Nuclear Power Plant accident, which remains world's worst nuclear disaster.

On 26 April 1986, at 1:23:44, reactor no. 4 of the Chernobyl nuclear power station exploded. One hundred times more radiation was released than by the atom bombs dropped over Hiroshima and Nagasaki

The sequence of events according to Chernobyl.info:

The accident in reactor no. 4 at the Chernobyl nuclear power station took place in the night of 25 to 26 April 1986, during a test. The operating crew planned to test whether the turbines could produce sufficient energy to keep the coolant pumps running in the event of a loss of power until the emergency diesel generator was activated.

In order to prevent the test run of the reactor being interrupted, the safety systems were deliberately switched off. For the test, the reactor had to be powered down to 25 per cent of its capacity. This procedure did not go according to plan: for unknown reasons, the reactor power level fell to less than 1 per cent. The power therefore had to be slowly increased. But 30 seconds after the start of the test, there was a sudden and unexpected power surge. The reactor's emergency shutdown (which should have halted the chain reaction) failed.

Within fractions of a second, the power level and temperature rose many times over. The reactor went out of control. There was a violent explosion. The 1000-tonne sealing cap on the reactor building was blown off. At temperatures of over 2000°C, the fuel rods melted. The graphite covering of the reactor then ignited. In the ensuing inferno, the radioactive fission products released during the core meltdown were sucked up into the atmosphere.

For more, go to National Geographic's documentary and an excellent photo-essay by Pixel Press. The accident is very well covered on the Green Peace website too.