March 28, 1979: Three Mile Island

As the accounts and grim reports continue to pour in from Japan let’s take a look back at the worst nuclear disaster in the history of The United States.

Photo: A cooling tower of the Three Mile Island nuclear power plant looms behind an abandoned playground. (Barry Thumma/AP)

In 1979, Equipment malfunction and human error lead to a partial reactor meltdown at the Three Mile Island nuclear power plant near Middleton, Pennsylvania. It is the most serious accident ever involving a U.S. commercial nuclear facility.

Things started going wrong at 4 a.m. when a malfunction caused the main feed-water pumps to stop running, leading to an overheating of the plant’s TMI-2 reactor. Although the reactor shut down automatically, a relief valve — which should have closed as the pressure decreased — failed to operate, causing coolant to leak out and the reactor’s core to overheat.

Operators were unaware of the leak and erroneously assumed that the core was properly cooled. They attempted to relieve the pressure on the core by reducing the flow of coolant — exactly the wrong thing to do. Overheating caused the zirconium tubes where the nuclear fuel pellets were stored to rupture, and the pellets began to melt.

Only the structural integrity of the containment building’s walls prevented Three Mile Island from becoming a Chernobyl-style catastrophe. There was no rupture, meaning all the damage was contained within the facility.

No deaths or injuries have ever been attributed to the Three Mile Island meltdown. But it did lead to tighter safety and design regulations and more rigorous oversight by the U.S. Nuclear Regulatory Commission.

Source: U.S. Nuclear Regulatory Commission.
Via: March 28, 2007.


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