Three Mile Island: What Happened
On March 28, 1979, a cooling circuit pump in the non-nuclear section of Three Mile Island's second station (TMI-2) malfunctioned, causing the reactor's primary coolant to heat and internal pressure to rise. Within seconds, the automated response mechanism thrust control rods into the reactor and shut down the core. An escape valve opened to release pressure but failed to close properly. Control room operators only saw that a "close" command was sent to the relief valve, but nothing displayed the valve's actual position.[1] With the valve open, coolant escaped through the pressurizer, sending misinformation to operators that there was too much pressure in the coolant system. Operators then shut down the water pumps to relieve the "pressure."
Operators allowed coolant levels inside the reactor to fall, leaving the uranium core exposed, dry, and intensely hot. Even though inserting control rods halted the fission process, the TMI-2 reactor core continued to generate about 160 megawatts of "decay" heat, declining over the next three hours to 20 megawatts.[2] Approximately one-third of the TMI-2 reactor was exposed and began to melt.
By the time operators discovered what was happening, superheated and partially radioactive steam built up in auxiliary tanks, which operators then moved to waste tanks through compressors and pipes. The compressors leaked. The steam leakage released a radiation dose equivalent to that of a chest X-ray scan, about one-third of the radiation humans absorb in one year from naturally occurring background radiation.[3] No damage to any person, animal, or plant was ever found.[4]