On April 26, 1986, an accident occurred at Unit 4 of the nuclear power station at Chernobyl, Ukraine, in the former USSR. The accident, caused by a sudden surge of power, destroyed the reactor and released massive amounts of radioactive material into the environment.

After the accident, access to the area in a 30-kilometer (18-mile) radius around the plant was closed, except for persons requiring official access to the plant and to the immediate area for evaluating and dealing with the consequences of the accident and operation of the undamaged units. The population evacuated from the most heavily contaminated areas numbered approximately 116,000 in 1986 and another 230,000 people in subsequent years (Source: UNSCEAR 2000, IAEA 2005).

Pripyat, the town near Chernobyl where most of the workers at the plant lived before the 1986 accident, was evacuated several days after the accident, because of radiological contamination.
It was included in the 30-km Exclusion Zone around the plant and is closed to all but those with authorized access.