What happened in the accident at Three Mile Island?
On March 28, 1979, the Three Mile Island (TMI) Nuclear Power Station near Harrisburg, Pennsylvania, experienced this country’s most serious accident in a commercial nuclear power plant. TMI’s Unit #2 suffered a loss-of-coolant accident in which the reactor’s primary cooling system failed. Operators made a relatively minor accident more serious by cutting off backup systems. This error caused the water level to drop low enough to uncover all but about 2 feet of the reactor’s 12-foot-long fuel assemblies. Without cooling water surrounding the fuel, its temperature exceeded 5,000 degrees Fahrenheit. This caused melting and damage to a large part of the reactor core. As a result of this damage, radioactive material normally confined to the fuel escaped into the reactor’s cooling water system. It was several hours before the operators understood the size of the problem. By then, the core was damaged. The accident at TMI was a serious commercial reactor accident from an economic standpoint.
