But then at 4:00 a.m. on Mar. 28, 1979 a cooling valve got stuck at the Three Mile Island Nuclear Generating Station's Unit 2.

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The pressure was slowly released through a valve into the cooling coils.

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That thinking changed on March 28, 1979, when a small valve stuck open, cooling water escaped and the reactor core of TMI's Unit 2 began to melt.

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