The Three Mile Island nuclear accident began
On this day · 28 March 1979
A stuck valve and a string of misread signals melted half a reactor core and froze American nuclear power for a generation.
At about 4 a.m. on March 28, 1979, a partial meltdown began at the Three Mile Island Unit 2 reactor near Middletown, Pennsylvania. It became the most serious accident in U.S. commercial nuclear power history.
The trouble started small: a pressure relief valve stuck open and failed to reseat, quietly draining coolant from the core. Instruments gave operators a misleading picture, and in the confusion they cut back the very cooling water the reactor needed. Over the next hours more than half the core overheated and melted.
Crucially, the molten fuel never breached its containment, and offsite radiation releases were tiny, with no detectable health effects on workers or the public.
The damage to public confidence was another matter. The accident triggered sweeping reforms in operator training, emergency planning, and control-room design, and it helped stall new U.S. reactor orders for decades.
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