The Three Mile Island accident was the partial meltdown of reactor number 2 of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania and subsequent radiation leak that occurred on March 28, 1979. It was the most significant accident in U.S. commercial nuclear power plant history.
In the night time hours preceding the incident, the TMI-2 reactor was running at 97% of power, while the companion TMI-1 reactor was shut down for refueling. The main chain of events leading to the partial core meltdown began in the early morning, in TMI-2’s secondary loop, one of the three main water/steam loops in a pressurized water reactor.
The initial cause of the accident happened eleven hours earlier, during an attempt by operators to fix a blockage in one of the eight condensate polishers, the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and impurities in the water accumulating in the steam generators and increasing corrosion rates in the secondary side.
Blockages are common with these resin filters and are usually fixed easily, but in this case the usual method of forcing the stuck resin out with compressed air did not succeed. The operators decided to blow the compressed air into the water and let the force of the water clean the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument air line. This would eventually cause the feed-water pumps, condensate booster pumps and condensate pumps to turns off, which would in turn cause a turbine trip.
With the steam generators no longer receiving feed-water, heat and pressure increased in the reactor coolant system, causing the reactor to perform an emergency shutdown. Within eight seconds, control rods were inserted into the core to halt the nuclear chain reaction. The reactor continued to generate decay heat and because steam was no longer being used bu the turbine, heat was no longer being removed from the reactor’s primary water loop.
Once the secondary feed-water pumps stopped, three auxiliary pumps activated automatically. However, because the valves had been closed for routine maintenance, the system was unable to pump any water. The closure of these valves was a violation of a key Nuclear Regulatory Commission rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by NRC officials as a key failure.
The loss of heat removal from the primary loop and the failure of the auxiliary system to activate caused the primary loop pressure to increase, triggering the pilot-operated relief valve at the pressurizer- a pressure active regulator tank to open automatically. The relief valve should have closed when the excess pressure had been released, and electric power to the solenoid of the pilot was automatically cut, but the relief valve stuck open because of mechanical fault. The open valve permitted coolant water to escape from the primary system and was the principle cause of the primary coolant system depressurization and partial core disintegration that followed.
The application of human factor principles may have averted the situation. In the filed of human factors, we often times speak about expectancy. These are an expectations about the way a system should work. We also speak about visibility, which is how transparent a system is to the operator, specifically in reference to the operators understanding of the system’s underlying processes. Problems arise when these expectancy are broken, yet the user is unaware of a change.
Three Mile Island’s control panel had a light showing the valve of concern was closed. In fact the operators were so used to this that they looked for very possible other solution, even though this valve being stuck open was an obvious cause of the initial problems. Not only that, but further downstream information that would very well have hinted to the operators that something was seriously wrong was either hidden from view due to bad interface design or was not something the operators were trained on. The field of human factors works hard to do its best to prevent events like this from ever happening again, through the application of usability, design and training.