Three Mile Island
The initial failure was in pumps bringing water in for the cooling system, which stopped for unknown reasons early in the morning of March 28, 1979. With no water being fed in, the water system turbine automatically shut down. To release the pressure now building in the primary cooling system, a pressurizer valve opened. The valve should have closed automatically once a safe pressure was reached, but it did not.
With the valve remaining open, so much coolant escaped that the reactor began to overheat. The plant operators were confused by the partial information available to them: there was no indication that the valve was open, nor could they directly monitor the amount of water in the cooling system. Since the pressurizer was still reporting high pressure, they inferred that there must still be high pressure throughout the cooling system, and drained even more coolant from it deliberately. The result was that reactor became so hot that the fuel pellets began to melt.
Having finally diagnosed the real problem, the core was cooled to a reasonable temperature by the evening of March 28. By March 30, there were new concerns about a large hydrogen bubble which had formed inside the reactor and seemed to be in danger of breaching the containment system. However, by April 1, the size of the bubble had been greatly reduced and it was deemed to no longer be a threat.
The reactor involved was not opened until 1984, after extensive analysis of the accident. The nuclear fuel was removed over a period from 1985 to 1990, followed by slow evaporation of contaminated water until August 1993. As of December 1993, the status of TMI-2 is "post-defueling monitoring storage".
The accident at TMI-2 released very little radiation, and there appear to have been no long-term health effects in the surrounding communities. However, the level of anxiety caused by the events is often cited as a reason that no nuclear power plants have been built in the United States since then.