Three Mile Island was not a nuclear disaster 

The working cooling towers of Exelon Generation Unit 1 in the foreground emit water vapor. The dormant cooling towers are from Unit 2, which was permanently damaged in the 1979 accident. Photo courtesy of Wikimedia Commons

In 1979, the Three Mile Island nuclear power plant, located just outside of Harrisburg, Pennsylvania, experienced a meltdown. This was the worst nuclear accident in the history of the United States. However, it is not widely known that there was never a disaster at Three Mile Island.  

To understand what really happened, it is necessary to explain, in broad strokes, how nuclear power is made and how meltdowns occur. In the core of a reactor, uranium fuel rods are held suspended in water and undergo fission. They heat the core water, which is then pumped into a separate heat exchange chamber to boil a different water source. This generates steam, which spins a turbine to create electricity. Control rods can be inserted into the reactor to reduce the reaction rate by attracting neutrons; this can completely stop the reaction in a crisis. This is necessary to prevent the reactor from getting too hot, as a buildup of pressure from excess steam in the core could cause an explosion. The threat of radiation occurs if the fuel gets too hot and melts, sinking to the bottom of the reactor and eating away at it. This allows the radioactive material to escape and melt through the floor, as occurred at Chernobyl.  

The inciting incident at Three Mile occurred when the pump used to transport water to the steam generators of Core 2 malfunctioned, which caused heated water to accumulate inside. Immediately a valve automatically opened to relieve pressure and the control rods were inserted. At this point, fission stopped in the reactor and there should have been no threat of meltdown. However, the cooling valve did not close when pressure returned to its normal levels, causing the core water to escape as steam. Operators did not understand they were losing water due to confusing readings. Again the reactor did what it was designed to do and emergency coolant was pumped into the chamber. But operators were worried about the added water causing too much pressure and cut back the flow. This single decision starved the reactor of coolant. Had the emergency systems not been prevented from doing their job, it is likely there would have been no threat of overheating.  

A little over two hours after the initial incident, the emergency valve was closed and the issue seemed to have been resolved. However, at this point there were dangerously low levels of water in the reactor, and the cores were exposed. Radiation readings began to climb as water boiled away. For hours after the fact, and through a tumultuous media frenzy, the plant grappled with cooling the core. During this tense process, hydrogen exploded inside the chamber, causing an audible bang. Almost 16 hours after the valve had gotten stuck, cooling and circulation was finally restored to a partially melted down core.  

In 1979, the Three Mile Island nuclear power plant, located just outside of Harrisburg, Pennsylvania, experienced a meltdown. In the modern era, nuclear energy is the most efficient source of electricity in the U.S. and should be more relied on. Photo courtesy of Wikimedia Commons

Throughout this frenzy, increasingly higher emergencies were issued to the public, who were largely in the dark as to what was happening. Studies investigating media coverage of the event showed that 67% of statements given about the future of nuclear power after the fact were negative. While outlets reassured the public of their safety as well, the doubt they had experienced during the incident lingered, and nuclear approval ratings plummeted

It was sensationalized that the plant had exposed those in the area to dangerous radiation. While it was true that a small amount of gas was released accidentally, it had been filtered before escaping the facility, and the amount of radiation experienced by those near Three Mile Island was negligible. The normal background radiation that an average person would experience in a year near the facility was measured at 100 to 125 millirems. Three Mile Island emitted a dose of one millirem. This is slightly less than the amount of radiation emitted by a dental x-ray, at 1.5 millirem. 

It felt necessary to detail these events in some clarity because there is so much confusion surrounding this incident, even in the present. The accident was directly responsible for serious reforms regarding training and emergency indicators, and contributed to a lasting public mistrust of nuclear power. At the time, it was a symbol of nuclear disaster second only to Hiroshima in the public consciousness. However, the events at Three Mile Island largely prove the safety of the plants we have designed. Even decades ago, the reactor’s programming was adept in preventing a true disaster, and human error exacerbated the problem at every turn. It was human error which had caused the control panel to be poorly designed and which caused those working at the plant not to trust their instruments. When hydrogen exploded in the chamber, the walls of the core were thick and durable enough to prevent rupture, in spite of the fact that it was under immense heat and pressure. Even after hours of duress, the reactor only experienced a partial meltdown, and insulated those in the vicinity from lethal radiation. 

Revisiting this incident is important in the modern era. Nuclear energy is the most efficient source of electricity in the U.S., with plants operating at full capacity 93% of the time. It is more dependable than any other carbon-free energy source at our disposal. Fossil fuels, while familiar, are not renewable and are the largest contributor to rising global temperatures and ecological devastation. In order to solve these crises, we should seriously consider more reliance on nuclear power, the safety and efficacy of which has come a long way since Three Mile Island. The systems we have designed are much more reliable than our predecessors were led to believe, and could play a key role in cooling of a different kind. 


  1. Maybe you should do some more research on how the people in the area were affected before writing this article. Numerous people in the area ended up with cancer. Teenagers ended up having thyroid cancers. The government came to properties and cut down our fruit trees because they were contaminated with radiation.

  2. This revisionist story is poorly researched and appears to be a cut and paste of nuclear industry propaganda which was undoubtedly used as a source. Radiation monitors went off-scale on the North-East border of the plant and the off-scale readings were dismissed as instrumentation failure instead of indicating of a plume of gaseous radioactive isotopes including tritium, iodine 131, and krypton 85. Some researchers concluded approximately one MILLION more deaths than forecast were seen in the North-Eastern US when reviewing epidemiological data 20 years after the “accident”. Timing of the event leads to speculation of potential sabotage, but we’ll never know for sure. The exact timing sticks as a milepost however.

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