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In the mid 1970s, America's worst nuclear power plant disaster occurred at Three Mile Island when operators misread computer control panels. Prior to the mishap at Three Mile Island's Unit 2 (TMI-2), Lockheed Space and Missiles Corp. conducted a survey that found that very little human engineering was applied to existing control boards. The survey quotes the following comment from one designer as typical: "I have no pride of authorship in the layout of these boards. The client has to live with them. Nobody here cares that much. The Nuclear Regulatory Commission (NCR) is only interested in knowing whether or not there is a certain function covered on the boards -- either in front or in back."

At TMI's Unit 2, the reactor drain tank indicators, which could have told the operators that the electromatic relief valve was leaking, were behind the control panel and their significance was not recognized. The most repetitive phrase heard by the survey investigators during their interviews with panel designers was "client preference," a polite way of saying that the utilities got exactly what they ordered.

Lawrence Kanous of Detroit Edison had similar comments. "There is," he said, "little gut-level appreciation of the fact that plants are indeed man-machine systems. Insufficient attention is given," he asserted, "to the human side of such systems, since most designers are hardware-oriented."

One industry study divided the control panel problems into those that can be altered without changing the boards and those that will require completely new control boards.

The following improvements, the study indicated, can be made without physically changing the control boards:

1. Eliminate hunt-and-peck among a lot of controls by the use of clearly defined summary labels and taped lines of demarcation.
2. Color-code the boards or use control handles of different shapes.
3. Attach color bands for different limit conditions to each meter and replace poorly marked meter scales.
4. Add additional chart recorders where those currently in use are overloaded and largely illegible.
5. Replace maintenance tags with lucite panels or magnetic markers, so they don't obscure controls.

Remaining Problems

1. Nonfunctional grouping of controls and instruments.
2. Reversed-image duplications of control boards. This was apparently done to save costs in cutting different lengths of cable, but it violates a basic principle of human engineering to have two identical control boards mirror images of one another.
3. Inaccessible instruments. An operator sometimes has to stand on a chair to read a meter.
4. Inaccessible controls. To handle some emergency operations, an operator must run back and forth across the control room.
5. Unnecessarily large controls that increase panel size.
6. Hundreds of confusing annunciator warnings during an accident. Warning lights went off at TMI-2 but it was impossible for the operators to tell which light belonged to which indicating instrument. This cannot be remedied without major rebuilding of the present control boards.

The exact sequence of events at Three Mile Island may never be repeated if only because pressurized water reactor operators by now have received simulator training for that precise sequence. But other accident scenarios are possible.

Discussion Questions

1. What might the designers of the control panel at Three Mile Island have done differently to improve feedback to the operators?
   
   
   
   
2. Who was responsible for the control panel design at Three Mile Island?
   
   
   
   

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