Chernobyl reactor. Aerial view of the remains of the core of Reactor 4 at the Chernobyl Nuclear Power Station, Ukraine, being covered. The covering was built to prevent further release of radioactive contamination from the damaged reactor. The accident at Chernobyl occurred on 26 April 1986, when the reactor overheated during a low-power experiment. Over a billion billion becquerels of radioactive nuclides were released into the atmosphere, forcing 100,000 people to be evacuated from the area. Chernobyl's three other reactors were returned to power within a year.