"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 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 partial meltdown that followed."
The relief valve PORV may have got stuck because of the mechanical fault engendered by the massive bending moment surge wave of the world's dams that crossed the TMI half an hour(330 AM local time,830 UTC) earlier:
The giant surge wave of the world's dams was equivalent to a strong damquake of magnitude 8.8 MM:
The hydrological year June 1978 to May 1979 was a particularly stressful year for the earth, with 15 major earthquakes occurring, 6 of them in the wet season: