chĭrnōˈbyēl, Ukr. Chornobyl, abandoned city, N Ukraine, near the Belarus border, on the Pripyat R. Ten miles (16 km) to the north, in the town of Pripyat, is the 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; more was released in the subsequent fire. Only after Swedish instruments detected fallout from the explosion did Soviet authorities admit that an accident had occurred. 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. The agricultural economies of E and N Europe were temporarily devastated, as farm products were contaminated by fallout. One Chernobyl reactor remained in operation until Dec., 2000, when the complex was shut down.

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