On March 28, 1979, the reactor’s fuel core at Three Mile Island #2 became uncovered because the reactor coolant water level dropped below the top of the fuel core and more than one-third of the fuel melted.
Inadequate instrumentation and training programs at the time hampered operators’ ability to respond to the accident.
The accident was accompanied by communications problems that led to conflicting information available to the public, contributing to the public’s fears.
A very small amount of radiation was released from the plant. The releases were not serious and were not health hazards. This was confirmed by thousands of environmental and other samples and measurements taken during and after the accident.
The safety provisions designed into the reactor system worked. The containment building contained the radioactive material as designed. Despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained the damaged fuel.
The average radiation dose to people living within 10 miles of the plant was eight millirem, and no more than 100 millirem to any single individual. Eight millirem is about equal to a chest X-ray, and 100 millirem is about a third of the average background level of radiation received by U.S. residents in a year.
What did not Happen
There was no “China Syndrome,” the supposed scenario in which a nuclear plant accident creates a mass of molten reactor fuel so hot that it burns though a steel reactor vessel, through the plant’s reinforced concrete foundation, into the Earth and exits through the opposite side of the planet.
There were no injuries or detectable health impacts from the accident beyond the initial stress.
Several health studies found there were no long-term adverse effects on the health of the population living around TMI.
Applying the accident’s lessons produced important, continuing improvement in the performance of all nuclear power plants worldwide, emergency response preparations, and communication with the public.
The accident also fostered better understanding of fuel melting, including improbability of a “China Syndrome” meltdown breaching the reactor vessel or the containment building.