Background thumb|upright=2.2|A simplified schematic diagram of the TMI-2 plant In the night hours before the incident, the TMI-2 reactor was running at 97% power while the companion TMI-1 reactor was shut down for refueling. The main chain of events leading to the partial core meltdown on Wednesday, March 28, 1979, began at 4:00:36 a.m. EST in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor.
Background thumb|upright=2.2|A simplified schematic diagram of the TMI-2 plant In the night hours before the incident, the TMI-2 reactor was running at 97% power while the companion TMI-1 reactor was shut down for refueling. The main chain of events leading to the partial core meltdown on Wednesday, March 28, 1979, began at 4:00:36 a.m. EST in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor.