On March 28, 1979, an accident at Unit 2 of the Three Mile Island nuclear station near Harrisburg, Pennsylvania made the issue starkly and alarmingly real. As a result of a series of mechanical failures and human errors, the accident (researchers later determined) uncovered the reactor's core and melted about half of it. The immediate cause of the accident was a pressure relief valve that stuck open and allowed large volumes of reactor coolant to escape. The reactor operators misread the signs of a loss-of-coolant accident and, for several hours, failed to take action to cool the core. Although the plant's emergency cooling systems began to work according to design, the operating crew decided to reduce the flow from them to a trickle. By the time that the nature of the accident was recognized and the core was flooded with coolant, the reactor had suffered irreparable damage.
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