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Ukraine, April 1986: The reactor in Unit 4 of the Chernobyl nuclear power plant explodes, releasing a radioactive cloud that covers almost all of Europe. To date, Chernobyl is highly radioactive. A design flaw in the RBMK reactor is known to only a small group of experts prior to the accident, but KGB files show that Soviet intelligence officials were already concerned about the safety of the nuclear power plant at the time.