Three Mile Island

Nuclear power is a key part of American history that is still used today. The United States is the world's largest producer of nuclear power and nuclear energy provides about 20% of the power used in the United States.  But why, in a country that uses so much nuclear power, have no new nuclear plants been built since 1974?  The answer to that question is Three Mile Island.
The Three Mile Island nuclear power plant began construction in Pennsylvania in 1968, and the first of the two reactors began providing electricity in 1974.  The second reactor, however, caused a lot of issues.  From the time it began operating, the second reactor had many unscheduled shutdowns and the operators of the reactor actually falsified operational data on multiple occasions.
In the early morning of March 28th, things started to go wrong.  A valve in reactor 2 that provided water to the feedwater system closed.  The reactor shut down automatically (as it was supposed to) but human error and faulty decisions in the following hours would lead to disaster.  The staff at the plant didn't realize that the issue was a loss of cooling water and took actions that lead to even less cooling water flow.  This meant the reactor started to overheat and the nuclear fuel began to melt through the metal container that contained it.  The melting fuel seemed to be contained but a bubble of hydrogen built up in the reactor.  If this exploded, it would unleash large amounts of radioactive fuel.  Luckily, 3 days later, scientists determined that the bubble couldn't explode.
Three Mile Island didn't result in any loss of life or known health impacts to the community, but it did change the regulations and opinions around nuclear power in the US.  America as a whole went from being 69% for nuclear power to only 46% after the accident.  The antinuclear movement gained a lot of attention after the accident at Three Mile, though the movement also focuses on opposing nuclear weapons.  Three Mile Island also affected the state of nuclear power in the US, as no nuclear power plants commissioned after 1974 have been completed.  New federal rules and regulations surrounding nuclear power plants have made them more expensive and time-consuming to build and lower natural gas prices have further decreased the desire for new nuclear plants.
Three Mile Island's reactor 2 was permanently closed and cleaned up in a project that took 14 years.  Reactor One continued to operate until 2019 but dismantling the reactor will likely take at least 10 years.
Sources:
https://www.history.com/topics/1970s/three-mile-island
https://www.world-nuclear.org/information-library/country-profiles/countries-t-z/usa-nuclear-power.aspx
https://www.britannica.com/event/Three-Mile-Island-accident
https://americanhistory.si.edu/tmi/tmi11.htm
https://www.history.com/this-day-in-history/nuclear-accident-at-three-mile-island
https://www.britannica.com/topic/anti-nuclear-movement