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12 Things That Went Wrong To Cause The Three Mile Island Nuclear Meltdown

Updated September 23, 2021 20.9k views12 items

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. 

  • The Emergency Relief Valve Was Left Closed

    Photo: Public Domain / via Wikimedia Commons

    There were backup measures in place at Three Mile Island in case the feedwater pump malfunctioned. When the main feedwater pump failed, an emergency backup pump should have automatically turned on. However, the emergency pump's relief valve was stuck in a closed position.

    A few days prior to the meltdown, tests were done on the backup pump and it was found to be in good condition. However, in order to conduct those tests, the valve had to be manually opened and closed. When the valve is properly open, the backup pump kicks into gear. But since the valve was stuck in a closed position, the backup pump did not turn on, and any water near the core kept venting out, making the core even hotter. 

  • The Instruments In The Control Room Gave Off Incorrect Readings

    Photo: United States Department of Energy / via Wikimedia Commons

    The instrumentation in the control room failed to alert the operators of the growing problem. They could tell that the main pump had stopped, but believed that the backup pump was working properly. It didn't help that in the past, the emergency pumps had turned on in that reactor for no reason at all, so workers were conditioned to assume it wasn't anything serious. Even though the valve venting steam was stuck open, the control panel's confusing readings seemed to indicate it was closed. The operators had no idea that cooling water was still pouring out in the form of steam. There also wasn't any way to see how much water surrounded the nuclear core from the control room. 

  • The Staff Stopped The Flow Of Water To The Overheating Unit

    Photo: John G. Kemeny / via Wikimedia Commons

    Because of the faulty readings in the control room of Unit Two, workers thought that the situation was under control - the reactor was filling with steam, but they thought it was filled with water. So, in order to prevent the reactor core from getting flooded, they shut down the flow of water going to it. This just made the reactor core hotter, since it didn't have an adequate supply of water in the first place.

    According to a later report, there were two gauges that should have been checked before the water was turned off. An employee admitted to checking only one. Based on that particular reading, he thought the space was flooding with water instead of running dry.

    Mounting steam pressure then caused the coolant pumps to start vibrating, so plant workers turned the pumps off to keep them from getting damaged. "With no water flowing into the reactor and water and steam escaping the reactor, large portions of the reactor core became uncovered."

  • The Nuclear Fuel Pellets And The Tubes Holding Them Began To Melt

    The flow of water that cooled the Unit Two reactor core was stopped, allowing it to continue to heat up - exponentially, this time. The fuel rods, which held the nuclear pellets that fed the reactor and created the energy, started to melt. They were made of zirconium, a chemical and metal compound that melts at 3,371 degrees Fahrenheit. A full-scale meltdown had begun inside the reactor, though plant employees did not recognize the extent of the problem until much later.