The chernobyl accident

The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in the Ukrainian SSR (now Ukraine). An explosion and fire released large quantities of radioactive contamination into the atmosphere, which spread over much of Western Russia and Europe. It is considered the worst nuclear power plant accident in history, and is one of only twoclassified as a level 7 event on the International Nuclear Event Scale. The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles, crippling the Soviet economy.[1]
The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant, which is near the town of Pripyat. Therewas a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite.[2] The resulting fire sent a plume of highly radioactive smoke fallout into the atmosphere and over an extensivegeographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union and Europe. From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine.[3][4] According to official post-Soviet data,[5][6] about 60% of the fallout landed in Belarus.
The accident raised concerns about the safety of the Sovietnuclear power industry, as well as nuclear power in general, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures.[7][notes 1]
Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. Thirty one deaths are directly attributed to theaccident, all among the reactor staff and emergency workers. A UNSCEAR report places the total confirmed deaths from radiation at 64 as of 2008. Estimates of the number of deaths potentially resulting from the accident vary enormously: the World Health Organization (WHO) suggest it could reach 4,000. On 26 April 1986, at 01:23 (UTC+3), reactor four suffered a catastrophic power increase, leading toexplosions in its core. This dispersed large quantities of radioactive fuel and core materials into the atmosphere[8]:73 and ignited the combustible graphitemoderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke, as the reactor had not been encased by any kind of hard containment vessel. The accident occurred during an experiment scheduledto test a potential safety emergency core cooling feature, which took place during the normal shutdown procedure.

The Three Mile Island accident was a partial core meltdown in Unit 2 (a pressurized water reactor manufactured by Babcock & Wilcox) of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg,United States in 1979.
The power plant was ownedand operated by General Public Utilities and Metropolitan Edison (Met Ed). It was the most significant accident in the history of the USA commercial nuclear power generating industry, resulting in the release of up to 481 P Bq (13 million curies) of radioactive gases, and less than 740 GBq (20 curies) of iodine-131.[1]
The accident began at 4 a.m. on Wednesday, March 28, 1979, with failures in thenon-nuclear secondary system, followed by a stuck-open pilot-operated relief valve (PORV) in the primary system, which allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident due to inadequate training and human factors, such as human-computer interaction...