The Chernobyl Power Complex, lying about 130 km north of Kiev, Ukraine, consisted of four nuclear reactors of the RBMK-1000 design. On 25 April, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. The crew initiated an emergency shutdown in response. However the amount of power generated actually rocketed up and blew the seals on the reactor vessel.
Firefighters were on the scene within minutes to contain the blaze, with additional ground teams and helicopters arriving from as far away as Kiev in two hours. By 6:35am, the external fires were extinguished and only the inferno inside Unit 4 continued to burn, as it would for another two weeks. From the second to tenth day after the accident, 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. Many citizens reported feeling ill by morning, coughing and vomiting involuntarily. It would not be until 2:00pm the following day — nearly 37 hours after the explosion — that officials began moving citizens out of the area. In a matter of hours, the town was empty.
The morning of April 28th, workers nearly 700 miles away at the Forsmark Nuclear Power Plant in Sweden were stopped when trace amounts of radioactive material were found on their clothes. Swedish administrators discovered by midday that the isotopes were from another location. In time, the fallout would reach as far away as the mountainous regions of Scotland, exposing Europeans to harmful radiation largely without their knowledge — and, until the Swedes brought their information onto the national stage, without a Soviet admission of guilt.
Reference:
http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Chernobyl-Accident/
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