Nuclear power plant in Pennsylvania which experienced a partial (52%) meltdown of its reactor core and minor radiation leakage (about 0.005 rems) on March 28, 1979 just after 4:00am. A water pump in the secondary cooling system of the Unit 2 pressurized water reactor failed, and a relief valve jammed open, flooding the containment vessel with radioactive water. A backup pumping system was down for maintenance, so the temperature inside the reactor core rose quickly, rupturing fuel rods after the core ran without coolant for 40 minutes.
If the coolant were not replaced in time, the molten fuel in the reactor could have escaped its containment vessel and come into contact with the nearby radioactive water, causing a steam explosion and rupturing the reactor's containment dome. The radioactive steam could have escaped and caused a disaster like Chernobyl.