America's worst accident at a civilian nuclear power plant occurred on March 28, 1979. Unbeknown to anyone, half the fuel melted in one of two nuclear reactors on Three Mile Island near Harrisburg, Pa. Large quantities of radioactivity leaked from the reactor, but most of it was contained. In all probability, no one received a harmful amount of radiation. The enormous damage to the reactor was revealed only years later when TV cameras and a specially developed ultrasonic, sonar-like imaging system looked inside the reactor vessel.
In recognition of the 25th anniversary of this event, the National Museum of American History devoted its History-in-the-News display case during spring 2004, to the accident in Unit 2 of the Three Mile Island nuclear power station (TMI-2). That exhibit featured the 1983 computer-controlled ultrasonic survey. It included the sonar probe and a precise topographic scale model of the upper third of the reactor's destroyed core that was prepared to present in visual form the numerical sonar data.