The worst accident at a U.S. commercial power reactor occurred on March 28, 1979 at Three Mile Island (TMI) Nuclear Station in Pennsylvania.

As a result of equipment failures and human operation errors, the water level in the reactor core decreased to the point that the fuel was no longer submerged in water. Without the cooling normally provided by this water, the cladding and some of the fuel pellets melted. Large quantities of radioactive materials were released into the containment building.

The containment building performed as it was designed. The radioactive releases to the atmosphere that occurred during the TMI accident were very small and resulted primarily from leaks in systems that were required to operate during the course of the accident. These systems carried water that contained very large amounts of fission products outside the containment and some leaking could not be prevented.