<P> Globally, there have been at least 99 (civilian and military) recorded nuclear power plant accidents from 1952 to 2009 (defined as incidents that either resulted in the loss of human life or more than US $50,000 of property damage, the amount the US federal government uses to define nuclear energy accidents that must be reported), totaling US $20.5 billion in property damages . Property damage costs include destruction of property, emergency response, environmental remediation, evacuation, lost product, fines, and court claims . Because nuclear power plants are large and complex, accidents on site tend to be relatively expensive . </P> <P> The 1979 Three Mile Island accident in Pennsylvania was caused by a series of failures in secondary systems at the reactor, which allowed radioactive steam to escape and resulted in the partial core meltdown of one of two reactors at the site, making it the most significant accident in U.S. history . </P> <P> The world's worst nuclear accident has been the 1986 Chernobyl disaster in the Soviet Union, one of two accidents that has been rated as a level 7 (the highest) event on the International Nuclear Event Scale . Note that the Chernobyl disaster may have scored an 8 or 9, if the scale continued . The accident occurred at the Chernobyl Nuclear Power Plant after an unsafe systems test led to a rupture of the reactor vessel and a series of steam explosions that destroyed reactor number four . The radioactivity plume spread to the surrounding city of Pripyat and covered extensive portions of Europe with traces of radioactivity, leaving reindeer in Northern Europe and sheep in portions of England unfit for human consumption . A 30 kilometres (19 mi) "Zone of alienation" has been formed around the reactor . </P> <P> At least 57 accidents have occurred since the Chernobyl disaster, and over 56 nuclear accidents have occurred in the USA . Relatively few accidents have involved fatalities . </P>

The most damaging nuclear power plant accident occured at