Chernobyl Unit 4 reactor about

The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

Everyone s worst fears about nuclear power became a reality in tlie later part of April 1986. A large Soviet reactor - unit number 4 at Chernobyl, 80 utiles nortli of Kiev, and only 3 years old blew out and burned, spewing radioactive debris over much of Europe. Radiation levels increased from Sweden to Britain, tlirough Poland, and as far soutli as Italy. The damage caused to tlie environment far surpassed tlial due to tlie accident at Tliree Mile Island. 

Nuclear accidents at Three Mile Island, Pennsylvania, in 1979 and Chernobyl, Ukraine, in 1986 had a devastating effect on public opinion in the United States and, to a smaller degree, elsewhere in the world. At Three Mile Island only about 50 curies of radiation were released to the environment and there were no casualties. The explosion at Chernobyl was a very different story. About 100 million curies were released, leading to at least 31 fatalities. Moreover, 135,000 people were permanently evacuated from the region surrounding the reactor. Since then, all the other reactors at Chernobyl—three, in addition to the one that exploded—have been permanently shut down. 

The specific events that led to the Chernobyl accident began on Friday, April 25, 1986. Very early in the morning ( 1 am) control room operators at Unit 4 reactor began a planned power reduction that reduced core power by about 50 % down to 1600 MWt (US NRC, 1987). By the afternoon of the 25 , the Central Electricity Board requested that no further power reductions occur at the plant to ensure that local power needs were met. At approximately this same time, as part of the test, the emergency core cooling systems were shut down (US NRC, 1987). 

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