At about 4:00 a.m. on March 28, 1979, a failure in the main feedwater pumps of Unit 2 of the Three Mile Island nuclear power station in Pennsylvania precipitated the most notorious accident involving nuclear power in the United States. The failure of the feedwater pumps prevented steam generators from removing excess heat, resulting in a pressure increase in the nuclear portion of the plant. A relief valve opened to release the excess pressure, but failed to close when the pressure normalized, permitting coolant to stream through the valve and allow the nuclear reactor to overheat. This then combined with failures in instruments, which provided incorrect information to operators of the plant. The result was that the operators responded with actions that exacerbated the condition by reducing coolant flow even further. The result was a severe nuclear-core meltdown as the nuclear fuel reached temperatures that ruptured its protective cladding.