A nuclear and radiation accident is defined by the International Atomic Energy Agency (IAEA) as "an event that has led to significant consequences to people, the environment or the facility." Examples include lethal effects to individuals, large radioactivity release to the environment, or reactor core melt."[4] The prime example of a "major nuclear accident" is one in which a reactor core is damaged and significant amounts of radioactivity are released, such as in the Chernobyl disaster in 1986.

The impact of nuclear accidents has been a topic of debate practically since the first nuclear reactors were constructed in 1954. It has also been a key factor in public concern about nuclear facilities.[5] Some technical measures to reduce the risk of accidents or to minimize the amount of radioactivity released to the environment have been adopted. Despite the use of such measures, human error remains, and "there have been many accidents with varying impacts as well near misses and incidents".
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A nuclear disaster could take several forms. The most obvious would be a meltdown at a nuclear reactor plant. Though the plant could not explode, the results of such a disaster would very likely be the release of massive amounts of radiation and radioactive material into the environment. And it would take hundreds of years to decay to anything near "safe" levels. Cleaning it up is out of the question, as exemplified by the Chernobyl disaster. In the Ukraine event, the reactor actually caught fire and burned. Prypiat is a ghost town. In the case of Three Mile Island, the meltdown was contained within the reactor vessel and the containment building, but there were some large releases of steam that was heavily laced with the radioactive debris of spent fuel fission fragments. These radioactive materials, which would normally be contained inside fuel elements, were released into the primary coolant when a loss-of-coolant accident overcame the plant. 
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