The Three Mile Island nuclear accident 40 years ago

DEFCON Warning System

Staff member

Washington (AFP) - The 1979 accident at the Three Mile Island power plant, the worst in US history, claimed no lives but provoked an outcry over the country's nuclear electricity program.

Caused by mechanical, design and human errors, the partial meltdown registered a five on the International Nuclear Event Scale that peaks at seven, the rate given to the Chernobyl (1989) and Fukushima (2011) disasters.

Here is a rundown of what happened on March 28, 1979 at one of the two reactors at the Three Mile Island Generating Station in eastern Pennsylvania.

- Early morning alert -

An alert was declared at 4:00 am after a fairly minor malfunction in the water reactor's cooling system that was linked to a filter.

Authorities quickly emphasized there had been no radioactive leaks.

The reactor in question, TMI-2, automatically shut down. However engineers did not know that a pilot-operated relief valve remained open, allowing coolant to escape.

A high pressure back-up system immediately pumped water back into the reactor. However technicians feared that too much pressure would build up and reduced the flow of water.

- Partial meltdown -

As sufficient cooling was prevented, the reactor core heated up; water evaporation exposed the fuel core and temperatures climbed even higher.

The reactor remained intact, though a subsequent investigation determined that about 45 percent of its core had suffered a meltdown.

Radioactive material called corium flowed to the bottom of the reactor core, where it was contained, which averted disaster.

Engineers finally realized they had to restore the high pressure flow of water.

They were able to gradually vent built-up radioactive gases to waste tanks.

Some radioactivity, however, escaped into the atmosphere through a system of filters that officials insisted retained the most dangerous elements.

-- MORE --