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What we learnt from the Three Mile Island accident

by Maygen Jacques

22 June 2020 15:24pm

President Jimmy Carter touring the TMI-2 control room on April 1, 1979, with NRRDirector Harold Denton, Governor of Pennsylvania Dick Thornburgh and James Floyd, supervisor of TMI-2 operations

A brief history lesson before we get to our main message:

The Three Mile Island accident happened in 1979. A reactor within a nuclear generating station in Pennsylvania had a partial meltdown, causing a leak, and subsequently gaining the title of most significant accident in US commercial nuclear power plant history.

A series of errors and equipment malfunctions, mixed with some
questionable instrument readings, resulted in loss of reactor coolant, overheating of the core, damage to the fuel, and some release outside the plant of radioactive gases.

The operators of the nuclear power plant made NO attempt to close a critical valve that could have prevented this. Why? Despite the valve being stuck open, a status indicator on the control panel suggested the valve was closed. In fact, the status light didn’t indicate whether the valve was open or closed, only whether it was powered or not. The status indicator thus gave false evidence of a closed valve.

‘Press here’ to avoid a nuclear catastrophe

When the control room operators were unable to interpret the meaning of the light correctly, they could not diagnose the problem for several hours. By this time, major damage had occurred.

Don Norman, explains: “The control room and computer interfaces at Three Mile Island could not have been more confusing if they tried.”

So, really, the design of a simple on/off button (and status indicator) can save lives. When things go wrong, we don’t point the finger at poor design (which we should), rather we blame the people (“user error”). In fact, over 90% of industrial accidents are blamed on the people involved.

We should really stop and think about this.

Don concluded:

“Pinning the blame on the person may be a comfortable way to proceed, but why was the system ever designed so that a single act by a single person could cause calamity? Worse, blaming the person without fixing the root, the underlying cause does not fix the problem: the same error is likely to be repeated by someone else.”


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