The Three Mile Island Unit 2 reactor, near Middletown, Pa., partially melted down on March 28, 1979. It was the most serious accident in U.S. commercial nuclear power plant operating history. Its aftermath brought about sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations.
All of these changes significantly enhanced U.S. reactor safety. The full story is available here.
U.S. NRC Blog 