On March 28, 1979 the Three-Mile Island Unit-2 nuclear power plant experienced a feed system failure which prevented the steam generators from removing heat from the plant. The reactor automatically shutdown but, without the feed system to cool the primary, the pressure in the primary system (the nuclear portion of the plant) began to increase. In order to prevent that pressure from becoming excessive, a relief valve opened. The valve should have re-closed once the pressure dropped by a small amount, but it didn’t. The only indication available in the control room showed the valve in the closed position, but that indication was erroneous, representing only that the signal to close the valve (pressure below a set value) had been sent to the valve. Nothing in the system verified the actual valve position. This stuck-open valve caused the pressure to continue to decrease in the system (and ultimately provided a path for spewing thousands of curies of radioactive material into the atmosphere), but the false shut indication prevented the operators from taking actions to mitigate their severe loss of coolant accident.
The primary relief valve design had a history of sticking. That same valve had been involved in at least nine other minor incidents prior to the TMI incident. Most notably, eighteen months before TMI, a similar incident had occurred in another nuclear plant involving a loss of feed and rising temperatures shutting down the plant. In that incident, the plant was just starting up after a maintenance shutdown, so the power level and temperature of the system were not as dangerously high as at Three-Mile Island.