On March 28, 1979 at 4:00 AM, a minor malfunction occurred in the system which feeds water to the steam generators at the Three Mile Island (TMI) Unit 2 Nuclear Generating Station. This event led eventually to the most serious commercial nuclear accident in US history and fundamental changes in the way nuclear power plants were operated and regulated.
The accident itself progressed to the point where over 90% of the reactor core was damaged. The containment building in which the reactor is located as well as several other locations around the plant were contaminated. The subsequent cleanup and recovery effort is documented at this web page.
Those of you wishing to understand the accident are referred to the above link.
Despite the severity of the damage, no injuries due to radiation occurred.
The resulting contamination and state of the reactor core led to the development of a ten year cleanup. The accident and subsequent analysis of it changed the way nuclear power is used and regulated both here and abroad.