Three Mile Island is a nuclear power plant located in Dauphin County, Pennsylvania, on the eastern side of the state. It was the site of the Three Mile Island nuclear meltdown, which occurred on the morning of May 28, 1979. Although no one outside of the plant was harmed, the workers inside were exposed to high levels of radiation, and the public became worried that nuclear power was unsafe.
The story of Three Mile Island shows that nuclear incidents can be prevented, as long as the right safety measures are in place and the employees are trained properly. What happened on Three Mile Island was small in comparison to the Fukushima Disaster in Japan or the Chernobyl Incident in Russia. However, it still serves as a cautionary tale about how small mistakes and errors in judgment can add up to major disasters.
Construction on nuclear reactor Unit One (also called TMI-1) on Three Mile Island began in 1968 and went off without a hitch. On the other hand, Unit Two (TMI-2, built between 1969 and 1978) was plagued with issues. Not only were there delays in the construction process (which should have only taken six years, not nine), but once it started operating, it suffered a series of unscheduled shutdowns.
The workers operating Unit Two found themselves falsifying data that went to the Nuclear Regulatory Commission, to prevent the commission from simply shutting the reactor down for good.
On March 28, 1979, the pump that sent hot water to the reactor's steam generator failed for reasons still not entirely understood (though it may have been the fault of a valve stuck closed, or water that leaked into an air line). In response, the plant's turbine immediately shut down, expelling a huge plume of high-pressure steam.
Without the steam generator to feed it water, the feedwater pump in Unit Two also quit working. This particular pump was designed to provide a non-stop flow of water to the steam generators that kept the nuclear core of the plant cool.
As long as the nuclear core is kept cool, it works just fine. However, when it's allowed to heat up, a catastrophic chain of events begins, resulting in the nuclear core going "critical" and melting down. The result is a release of nuclear energy that spreads throughout the atmosphere. When the pump stopped working, the plant went into emergency shut-down mode, and the chances of a meltdown at Three Mile Island increased.
With the whole system shutting down, the pressure inside the reactor began to fall. Usually, this sudden drop in pressure would trigger the relief valve that was venting the steam to close again. This time, though, the valve was stuck open, so steam kept right on pouring out of the plant, leaving the water levels lower and lower.