Uploaded on Sep 17, 2011
In the nighttime hours preceding the incident, the TMI-2 reactor was running at 97% of full power, while the companion TMI-1 reactor was shut down for refueling.
The chain of events leading to the partial core meltdown began at 4 am EST on March 28, 1979, in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor.

Workers were cleaning a blockage in one of the eight condensate polishers (sophisticated filters cleaning the secondary loop water), when, for reasons still unknown, the pumps feeding the polishers stopped. When a bypass valve did not open, water stopped flowing to the secondary's main feedwater pumps, which also shut down. With the steam generators no longer receiving water, they stopped and the reactor performed an emergency shutdown (SCRAM). Within eight seconds, control rods were inserted into the core to halt the nuclear chain reaction but the reactor continued to generate decay heat and, because steam was no longer being used by the turbine, heat was no longer being removed from the reactor's primary water loop.

Once the secondary feedwater 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 NRC rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This failure was later singled out by NRC officials as a key one, without which the course of events would have been very different.

Due to the loss of heat removal from the primary loop and the failure of the auxiliary system to activate, the primary loop pressure began to increase, triggering the pilot-operated relief valve (PORV) at the top of the pressurizer—a pressure active-regulator tank—to open automatically. The relief valve should have closed again 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 due to a mechanical fault. The open valve permitted coolant water to escape from the primary system, and was the principal mechanical cause of the true coolant-loss meltdown crisis that followed.

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