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A partial meltdown of a reactor at Three Mile Island in Pennsylvania was the worst accident ever at a commercial nuclear power plant in the U.S. What actually happened that day?
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The nuclear power plant at Three Mile Island (TMI), located about 10 miles south of Harrisburg, Pennsylvania, on the Susquehanna River, began operation in 1974. Its aspiration was to be among the examples of “clean” nuclear energy production across the country. However, it gained fame for another reason entirely.
In Pennsylvania and beyond, many people still recall the partial meltdown at TMI in 1979. Among the most serious engineering debacles in American history, the meltdown of TMI’s second reactor still holds the distinction of being the worst accident a U.S. nuclear power plant has ever suffered.
But nuclear power plants are enormously complicated. What actually happened that day, how significant was the damage, and was anybody harmed by radiation? As we look back on Three Mile Island’s meltdown, let’s look for some answers.
While Three Mile Island was the most serious nuclear power accident in U.S. history, the Chernobyl meltdown in Ukraine was a disaster of a different magnitude. This riveting MagellanTV documentary tells that story.
What Happened at Three Mile Island?
Nuclear reactors require complex cooling systems to function without melting, and such systems malfunctioned at TMI’s second reactor as it began to melt down at 4:00 a.m. on March 28, 1979. First, a power failure kept primary pumps from delivering water to steam generators that cooled the reactor core. This prompted internal turbines as well as the reactor to shut down – a common failsafe. (If the inside of your computer reaches 95 degrees Celsius/203 degrees Fahrenheit, it’ll shut down, too.)
As turbines slowed to a complete stop, steam pressure rose to a significant level. When this happens, a valve along coolant pipes opens automatically to relieve that pressure. On Three Mile Island, this valve opened properly, but it became stuck and couldn’t close itself.
The final and most preventable problem was a matter of diagnostics. Instruments in the control room indicated to workers that the second reactor’s pressure relief valve was closed. As generic alarms blasted throughout the plant and warning lights flashed, the workers actually didn’t know that the problem was caused by a sudden leakage of coolant.
Between misleading instruments and poor preparation, the workers quickly ran out of time to restart the cooling system and reduce temperatures. This resulted in a partial meltdown that released a small amount of radioactive material into the air.
What Happened After the Incident?
Thankfully, the accident didn’t injure or kill anybody. Studies over the years following the accident have reached conflicting conclusions regarding the incident's effect on public health in the area. Though the mishap was later dwarfed by the Chernobyl and Fukushima-Daiichi disasters, it remains the worst accident at a commercial nuclear power plant in the U.S.
Fukushima Daiichi Nuclear Power Station in 2007 (Credit: IAEA Image Bank, via Wikimedia Commons)
The silver lining is that the incident at Three Mile Island was a wake-up call for reactor operators across the country, resulting in the reformation of everything from operator training and general safety to emergency planning and engineering. The problem wasn’t that Three Mile Island’s partial meltdown was especially catastrophic, but that a single emergency had reduced operators to sitting ducks.
The accident poured fuel on the fire of a contemporary “No Nukes” movement in the United States and abroad. Led by celebrities and skeptical nuclear experts of the time, the movement may have succeeded in reducing the number of nuclear power plants built in the years after the TMI accident, but that reduction can also be attributed to the economics of building and maintaining the plants.
The second reactor at Three Mile Island remains shut down to this day. As it undergoes a long decommissioning phase, its fuel has been removed, and its coolant has been fully drained. Radioactive waste is in the process of being disposed of properly while remaining debris has been shipped to national laboratories for further study. As of 2019, the first reactor has also ceased operation. Two separate entities remain in charge of decommissioning the two reactors.
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Title Image credit: formulanone, via Wikimedia Commons
