A Reactor Partially Melts Down at Three Mile Island

In the early morning hours of March 28, 1979, a major mechanical failure struck the Unit 2 reactor at the Three Mile Island Nuclear Generating Station near Harrisburg, Pennsylvania. Around 4:00 a.m., water pumps in the non-nuclear secondary cooling system shut down, halting the flow of water to the steam generators and causing the reactor's primary coolant loop to heat rapidly. As pressure mounted inside the cooling system, a pilot-operated relief valve opened as designed to release steam into a drainage tank. Eight seconds later, safety systems automatically inserted control rods into the reactor core to halt the nuclear chain reaction. Residual decay heat, however, continued to warm the primary coolant.

The emergency escalated when the pilot-operated relief valve failed to close after system pressure fell back to safe parameters. Although electrical power to the valve solenoid was automatically cut, the mechanical valve remained stuck open, allowing primary coolant to escape. Control room indicators did not reflect the true physical state of the valve. The dashboard light merely showed whether power was flowing to the solenoid, not whether the valve had sealed. Believing the unlit indicator meant the valve was shut, operating personnel failed to recognize that the reactor was undergoing a loss-of-coolant accident. Compounding the issue, two critical emergency feedwater lines were blocked by closed manual valves, preventing auxiliary water from replenishing the secondary system.