Today in 1979, reactor two at the Three Mile Island Nuclear Generating Station in Pennsylvania experienced a partial meltdown that is still considered one of the worst accidents in the history of nuclear power. The accident was rated a five out of seven on the International Nuclear Event Scale, meaning it was an "accident with wider consequences."
A perfect storm of circumstances lined up to precipitate the accident. It started with failures in the secondary system, and then a stuck-open valve in the primary system that led to a large volume of nuclear reactor coolant to hemorrhage. Plant operators mismanaged the crisis, not realizing what was happening. The operators' inadequate assessment of what was happening was exacerbated by flawed designs in the computer system that showed them unclear warnings in the control room.
Eventually, an operator overrode the reactor's automatic emergency cooling system because he didn't see a warning light that was installed in an obscure, hidden part of their control panel. The operator was under the false impression that there was an excess of coolant water in the reactor. The plant came frighteningly close to a full-scale meltdown, causing a short-term public crisis and a long-term crisis in public opinion about the safety of nuclear power.
Anti-nuclear activism was ascendant following the events described. The outcry led to more robust safety regulations for the nuclear industry and also foreshortened the nuclear power development boom that was occurring at the time.
Thankfully, subsequent studies showed that there was no increased risk of radiation-related illnesses around the Island. Total cost of the cleanup effort, which lasted until 1993, was about one billion dollars.
Nuclear power is still a hotly debated topic in American politics.