At about 4 a.m. on March 28, 1979, a failure in the secondary cooling system of the Unit 2 reactor at the Three Mile Island nuclear station, on the Susquehanna River near Middletown, Pennsylvania, set in motion the most serious accident in the history of American commercial nuclear power.
The reactor shut down automatically, as designed, and a pilot-operated relief valve opened to relieve pressure in the primary system — then stuck open when it should have closed, draining coolant from the reactor for more than two hours. The control-room indicator showed only that the valve had been signalled to close, not its actual position, so the operators believed it was shut. Seeing the water level in the pressurizer rise, they cut back the emergency cooling water that was, in fact, the core’s only protection. The top of the core was uncovered, overheated and began to melt. For days afterward, confusion over a hydrogen gas bubble inside the reactor vessel and conflicting public statements turned an industrial accident into a national crisis.
The full extent of the damage was learned only years later, when the reactor was opened and defuelled in the 1980s: about 45 per cent of the core had melted, and some 20 tons of molten fuel had flowed into the bottom of the pressure vessel — which, remarkably, held. The containment building, the final barrier, contained almost all of the radioactivity released from the fuel.
The radioactive releases to the environment were small. The roughly two million people living in the area received an average dose of about one millirem — less than the dose from a chest X-ray — and the maximum dose to any member of the public was estimated at under 100 millirem. Study after study found no detectable health effects on plant workers or the public, and no one was killed or injured by radiation. The accident’s deepest impact was on public confidence in nuclear power, which never recovered.
President Carter appointed a commission under Dartmouth president John G. Kemeny, which reported in October 1979. Its central conclusion was that “the fundamental problems are people-related problems”: inadequate operator training, a confusing control room, complacent utility management, and a Nuclear Regulatory Commission preoccupied with licensing rather than safety. The reforms that followed reshaped the industry — the utilities created the Institute of Nuclear Power Operations to police their own performance, the NRC overhauled operator training, control-room design and emergency planning, and no new American reactor ordered after 1979 was completed for a generation.
Download the Kemeny Commission Report (1.7 MB file)