The Three Mile Island Accident – Caused by a Light!

The Three Mile Island accident was the worst nuclear power plant accident that has ever happened in the US history. The fact took place on March 28th, 1979, in the state of Pennsylvania.

Everything began when a pressure valve failed to close in the Unit-2 reactor. This caused cooling water (containing radiation) to escape into other buildings, while the core of the reactor started to greatly overheat. Following that, emergency cooling pumps went into action to prevent th rise in temperature in the reactor.

an animated plant diagram showing the series of events which caused the TMI accident - as found in this article's Summary of Events section

However, a huge human mistake occurred, which led to the accident. 

The nuclear power plant had a control panel with a light that showed the status of the relief valve which prevented the reactor from overheating. If the light was on, the valve was open. If the light was off, the valve was closed. Or at least that was what was thought.

Actually, the person who designed the control panel programmed the light to go off once the system had sent the signal to close the valve – which didn’t guarantee that the valve had been closed. Simply put, the system had sent a signal to close the valve, but a malfunction occurred and the valve did not close, while at the same time the control panel light went off,  misleading the staff to believe that the valve had been closed. Everything because of a light.

After thinking that the valve was closed, the employees turned off the emergency cooling system and the reactor, but at that point, residual heat was still being released and a partial meltdown had occurred. The contaminated water released dangerous levels of xenon-135 and krypton-85 gases around the plant.

A series of precautions were taken to protect the plant staff and fortunately there were no casualties. However, thousands of people were evacuated from the surrounding region and a nationwide media panic was made. Moreover, this accident greatly tarnished the nuclear power image in the people’s eyes.

This shows that huge accidents can begin with really simple and small things. In this case, the lack of understanding of the functioning of the light in the control panel lead to the wrong assumption that the relief valve had been closed.

This could have been avoided if the team who designed and programmed the control panel functions had carefully explained to the operating staff how each of the commands work. In addition, it’s clear that this nuclear power plant did not have a proper Safety Management System installed for their operations.

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