The Chernobyl disaster was a well-known nuclear accident of catastrophic proportions that occurred on 26 April 1986, at the Chernobyl Nuclear Power Plant in Ukraine (then in theUkrainian Soviet Socialist Republic, part of the Soviet Union). It is considered the worst nuclear power plant accident in history and is the only level 7 event on the International Nuclear EventScale.
The disaster occurred on 26 April 1986, 1:23 A.M., at reactor number four at the Chernobyl plant, near the town of Pripyat, during an unauthorized systems test. A sudden power output surge tookplace, and when an attempt was made at an emergency shutdown, a more extreme spike in power output occurred which led to the rupture of a reactor vessel as well as a series of explosions. This eventexposed the graphite moderator components of the reactor to air and they ignited; the resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive area, including Pripyat.The plume drifted over large parts of the western Soviet Union, and much of Europe. As of December 2000, 350,400 people had been evacuated and resettled from the most severely contaminated areas ofBelarus, Russia, and Ukraine. According to official post-Soviet data, up to 70% of the fallout landed in Belarus.
Following the accident, Ukraine continued to operate the remaining reactors atChernobyl for many years. The last reactor at the site was closed down in 2000.
The accident raised concerns about the safety of the Soviet nuclear power industry as well as nuclear power in general, slowingits expansion for a number of years while forcing the Soviet government to become less secretive about its procedures.
Russia, Ukraine, and Belarus have been burdened with the continuing andsubstantial decontamination and health care costs of the Chernobyl accident. A 2006 report prepared by the Chernobyl Forum, led by the World Health Organization (WHO) states, "Among the 134 emergency...