25-26 April 1986 Engineers on the evening shift at Chernobyl's number four reactor began an experiment to see whether the cooling pump system could still function using power generated from the reactor under low power should the auxiliary electricity supply fail. At 2300 control rods, which regulate the fission process in a nuclear reactor by absorbing neutrons and slowing the chain reaction, were lowered to reduce output to about 20% of normal output required for the test. However, too many rods were lowered and output dropped too quickly, resulting in an almost complete shutdown. Safety systems disabled Concerned by possible instability, engineers began to raise the rods to increase output. At 0030 the decision was taken to carry on. By 0100 power was still only at about 7%, so more rods were raised. The automatic shutdown system was disabled to allow the reactor to continue working under low power conditions. The engineers continued to raise rods. By 0123, power had reached 12% and the test began. But seconds later, power levels suddenly surged to dangerous levels. Overheating The reactor began to overheat and its water coolant started to turn to steam. At this point it is thought that all but six control rods had been removed from the reactor core - the minimum safe operating number was considered to be 30. The emergency shutdown button was pressed. Control rods started to enter the core, but their reinsertion from the top displaced coolant and concentrated reactivity in the lower core. Explosions With power at roughly 100 times normal, fuel pellets in the core began to explode, rupturing the fuel channels. At about 0124, two explosions occurred, causing the reactor's dome-shaped roof to be blown off and the contents to erupt outwards. As air was sucked in to the shattered reactor, it ignited flammable carbon monoxide gas causing a reactor fire which burned for nine days. Because the reactor was not housed in a reinforced concrete shell, as is standard practice in most countries, the building sustained severe damage and large amounts of radioactive debris escaped into the atmosphere. Firefighters crawled onto the roof of the reactor building to fight the blaze while helicopters dropped sand and lead in an effort to quell the radiation.
In der Nacht vom 25. auf den 26. April 1986 wird im Block IV des Atomkraftwerkes von Tschernobyl ein Sicherheitstest gestartet. Der Bedienungsmannschaft unterläuft dabei ein Fehler. Sie schaltet daraufhin das Sicherheitssystem ab. Sekunden später kommt es zum größten anzunehmenden Unfall (GAU) in der Geschichte der Atomkraft. Der Reaktor explodiert und setzt radioaktive Stoffe in einer solchen Menge frei, dass ganz Europa die Auswirkungen zu spüren bekommt. Der Name der Stadt Tschernobyl wurde seitdem zum Synonym für die Gefahren der Kernenergie. Die Folgen sind bis heute sichtbar. Die Erinnerung daran ist vielen Menschen noch lebendig, die diese Zeit bewusst erlebt haben. Über die Schlussfolgerungen wird aber noch heute diskutiert.

Während die WHO und die IAEA die Zahl der Toten durch die Katastrophe von Tschernobyl auf etwa 4.000 unter den drei am meisten betroffenen Gruppen (Liquidatoren, Evakuierte und Bewohner der hochkontaminierten Zone) schätzt, kommt Greenpeace auf ganz andere Zahlen. Aufgrund eigener Berechnung kommt die Organisation auf 93.000 zusätzliche Krebstodesfälle. Hinzu kämen noch weitere 270.000 zusätzliche Krebserkrankungen.