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What actually happened at Three Mile Island?

On March 28, 1979, the Three Mile Island nuclear power plant in Harrisburg, Pennsylvania, experienced a partial meltdown of its reactor core. (A meltdown is when the fuel core of a nuclear reactor melts and releases dangerous amounts of radioactivity.) Radioactive steam escaped into the atmosphere and radioactive water leaked from the plant into the Susquehanna River.

The accident occurred just after 4:00 a.m., when a water pump in the secondary cooling system of the Unit 2 pressurized water reactor failed. A relief valve jammed open, flooding the containment vessel with radioactive water. A backup system for pumping water was shut down for maintenance. Temperatures inside the reactor core rose and fuel rods ruptured. A partial (52 percent) meltdown of the radioactive uranium core occurred because the core was almost entirely uncovered by coolant for 40...

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