Nuclear accident at Three Mile Island (1979)
The most serious event at a United States Nuclear Plant began on March 28, 1979. A breakdown occurred outside of the reactor which resulted in a disruption in the cooling of the reactor. A series of events occurred that resulted in the normal cooling process further breaking down so that ultimately the nuclear fuel heated to the point that they melted through the metal containing rods. While this was the most dangerous type of nuclear accident, it remained contained within the reactor chamber so that the risk of a wide-spread release of radioactive contamination was avoided. A controlled release of some radiation occurred, and the health of the surrounding population has been monitored. (image of Three Mile Island from Wikimedia Commons)
This event is similar to what is currently occurring at the Japanese nuclear power plants. The earthquake of March 11 and the subsequent tsunami caused a power outage that interrupted the cooling of the nuclear reactor. Related explosions at the plants have exposed portions of the containment structures designed to keep the radioactivity confined, although it appears that the integrity of the containment structures themselves remain intact. There is added risk that the spent fuel pools where old radioactive fuel rods are stored after use are also at risk of overheating. These spent fuel pools are outside of the heavy concrete reactor containment buildings so are more vulnerable to a release of radioactivity if something goes wrong.