Three Mile Island Nuclear Accident Begins

Context

By the late 1970s, pressurized-water reactors supplied a growing share of U.S. electricity, with designs emphasizing multiple redundant safety systems and automatic shutdown features. The Three Mile Island Nuclear Generating Station, operated by Metropolitan Edison, consisted of two such units on the Susquehanna River; Unit 2 had entered commercial service only months earlier in December 1978. Federal oversight rested with the Nuclear Regulatory Commission, whose rules required plants to maintain auxiliary feedwater capability and to train operators for loss-of-coolant scenarios.

What Happened

At approximately 4:00 a.m. on March 28, 1979, a blockage in the condensate polisher system caused the main feedwater pumps in the non-nuclear secondary loop to stop, cutting heat removal from the steam generators. The reactor automatically scrammed, but decay heat continued. The pilot-operated relief valve on the pressurizer opened to relieve rising primary-system pressure yet failed to close when pressure dropped, allowing coolant to escape into the reactor coolant drain tank. Control-room instruments showed only that the valve’s solenoid had de-energized, leading operators to believe the valve had reseated.

Aftermath

Operators, misled by pressurizer level indications and lacking direct confirmation of valve position, throttled emergency core-cooling flow and later secured the reactor-coolant pumps to prevent vibration. With insufficient cooling, portions of the core overheated and partially melted. Radioactive gases and iodine escaped into the containment and, to a limited extent, the environment; by midday, low-level releases prompted Pennsylvania officials to advise evacuation of pregnant women and young children within five miles. No immediate injuries or fatalities occurred among workers or the public.

Legacy

The accident prompted the NRC to mandate improved instrumentation, simulator-based operator training, fitness-for-duty programs, and stronger emergency-planning requirements. It also led to the formation of the Institute of Nuclear Power Operations and slowed the licensing and construction of new reactors in the United States for more than three decades. Internationally, the event became a reference point for safety-culture reforms and remains classified as an International Nuclear Event Scale Level 5 accident.