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Chernobyl nuclear power station, site of the worst nuclear reactor disaster in history. On Apr. 25, 1986, during an unauthorized test of one of the plant's four reactors, engineers initiated an uncontrolled chain reaction in the core of the reactor after disabling emergency backup systems. On Apr. 26, an explosion ripped the top off the containment building, expelling radioactive material into the atmosphere. The reactor core was sealed off by air-dropping a cement mixture, but not before eight tons of radioactive material had escaped. Twenty firefighters died immediately from overexposure to radioactivity, while hundreds suffered from severe radiation sickness. Pripyat, Chernobyl, and nearby towns were evacuated. People who lived near the plant in Ukraine and Belarus at the time have seen a greatly increased incidence of thyroid cancer, and genetic mutations have been discovered in children later born to exposed parents. Ukraine has estimated that as many as 8,000 people died as a result of the accident and during its cleanup.