On the night of April 25, 1986, during a planned maintenance shutdown at the Chernobyl power plant in northern Ukraine, one of the four reactors overheated and began to burn. As plant engineers scrambled to regain control of it, they thought for a moment that there had been an earthquake. In fact, a buildup of steam had propelled the two-hundred-ton concrete top of the reactor’s casing into the air, with masses of radioactive material following close behind when the core exploded. The plant workers had been assured again and again of the safety of the “peaceful atom,” and they couldn’t imagine that the reactor had exploded. Firefighters rushed to the scene without special equipment or a clear understanding of the potential risks; they had not been trained to deal with a nuclear explosion, because such training would have involved acknowledging that an explosion was possible.
Source : The Chernobyl Syndrome
