What happend on Three Mile Island?

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Three Mile Island was the location of a nuclear power plant that had a meltdown in 1979.  While no people died from the meltdown at the time it could not have been good for them.  A government report concluded that the projected number of fatalities due to cancer was one, that figure seems a little low to me.

The plant is on an island in the Susquehanna River in Pennsylvania, Three Mile Island to be exact.  The accident was the result of both technical malfunction and human error.  The feedwater pumps failed early one morning.  These pumps are used for cooling, when the water pumps failed due to a mechanical failure, the back up pumps should have taken over, unfortunately the backup pumps were out of commission due to a human error.

The value in the primary system was stuck open which resulted in a loss of pressure and coolant.  The indicator that would have told people that this valve was stuck open was not included in the original plans of the plant in order to save time.   The backup feedwater system had a value that was stuck closed, due to human error.  Eight minutes into the accident the value was discovered closed and it was immediately reopened.

The problem would have been solved right there is it were not the case that the first value was stuck open, and that no one knew it was stuck open.  The water was getting to the system but because the pressure was getting lower and lower due to the open valve, this caused another false indication that there was enough water in the system to cool it, the operator than turned the water flow off.  Since the water was still coming out of the valve an indicator assumed there was enough water, why else would the valve be open?

130 minutes after the accident began, the reactor core was exposed to the steam that was building as the temperature rose due to a lack of coolant.  Once that happened a reaction involving hydrogen and radioactive gases occurred.  The tank collecting discharge overfilled and radioactive coolant began to leak.  After about 250,000 gallons of radioactive coolant leaked out someone noticed that some holding tanks were excessively high and shut off the coolant venting.

After a full sixteen hours the primary pump was working again, but that was after a large part of the core had melted.  Most of the system was very radioactive, but it held itself together and did not lose its integrity, that is why the accident did not cause immediate significant harm.

Here’s a link to the Presidents Commission on the Accident at Three Mile Island.

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