In the early morning hours of March 28, 1979, a mechanical failure at the Unit 2 reactor at Three Mile Island began a chain reaction of events that led to the worst accident in the history of U.S. nuclear power industry.
Three Mile Island Nuclear Generating Station is located on an island in the Susquehanna River, south of Harrisburg, Pa. The plant had been operating for five years, Unit 2 for only one year, when at approximately 4 a.m., the main feed water pumps stopped running, which prevented steam generators from removing heat from the reactor. The reactor automatically shut down, but pressure in the nuclear portion of the plant began to increase. To prevent excessive pressure buildup, a pilot-operated relief valve opened, but failed to close again once the pressure decreased. Because of confusing and contradictory instruments, operators at the plant were unaware that the valve was stuck open, and as a result, radioactive cooling water drained out and caused the core of the reactor to overheat. During the early stages of the accident, the core suffered a partial meltdown as about one-half of the fuel pellets melted, but it did not breach the walls of the containment building.