Image Credit: Richard Hertzler/LNP/LancasterOnline, via Associated Press

Three Mile Island and Nuclear Energy: the Failure and the Lesson

On March 28, 1979, reactor TMI 2 at the Three Mile Island power plant in Middletown, Pennsylvania experienced a partial meltdown, which released unknown amounts of radiation into the surrounding area and damaged the reactor beyond repair. It also damaged something else beyond repair: the reputation of nuclear energy as a safe and affordable alternative to fossil fuels. However, engineers have now developed newer, safer nuclear technologies that can avoid accidents like that of Three Mile Island and propel the United States to a more carbon-efficient future.

In order to understand the Three Mile Island failure, one must first understand the construction of the reactor, which operated with two cooling loops. The first was a closed loop that took heat directly from the reactor and then passed it on to the secondary cooling system, which used the heat to boil water into steam, producing power by driving the steam turbines.

According to a report from the United States Nuclear Regulatory Commission, the accident at the Three Mile Island plant began with a pump failure in Reactor 2’s secondary cooling system, the system which relieved the heat from the primary loop. As a result, the primary cooling loop began to overheat, its water evaporated into steam, and a valve opened to release the steam pressure. Unfortunately, the valve got stuck open, allowing a significant amount of water to drain from the cooling system and leaving the coolant-starved reactor to overheat. Confused reactor operators failed to take proper actions to restore coolant to the primary cooling loop, and the reactor was left uncooled for hours on end. By the time coolant was restored, the reactor cores had already cracked and leaked detectable amounts of radiation into the surrounding area.

As a result of the Three Mile Island incident, public opinion of nuclear power changed dramatically. What had once been viewed as an almost-endless source of clean-air power was regarded as a dangerous source of radiation and a threat to public health. The popularity of less powerful but supposedly safer wind and solar energy sources soared, and no new nuclear reactors were ordered until 2012.

Contrary to this perception, there are still some very good reasons to believe that the future of energy is nuclear. For example, the Energy Information Administration predicts a 50% increase in worldwide energy demands by 2050, a staggering figure that could easily outstrip renewable sources such as solar and wind energy, increasing demand for more powerful energy sources. Meanwhile, wind energy has led to unforeseen environmental problems involving wildlife and noise.

Most significantly, however, new reactors have the potential to avoid the chain reaction of technical and operator failures that led to the accident at Three Mile Island. Some designs use molten sodium metal as coolant rather than water. Because sodium has a higher boiling point than water, this could avoid the steam pressure problems that drained the Three Mile Island reactor loop and led to an overheat. Other designs would use balls of uranium packaged in graphite rather than the traditional metal-clad cores, which would make a meltdown by overheating nearly impossible. With the lessons learned from Three Mile Island, the United States is ready to move forward to a safer, more powerful nuclear future.



ChoOct. 16, Adrian, et al. “Department of Energy Picks Two Advanced Nuclear Reactors for Demonstration Projects.” Science | AAAS, 16 Oct. 2020, Editors. “Nuclear Accident at Three Mile Island.” HISTORY, 21 Aug. 2018,

Lake, James A. “Next Generation Nuclear Power.” Scientific American, 26 Jan. 2009,

  1. S. Energy Information Administration. “EIA Projects Nearly 50% Increase in World Energy Usage by 2050, Led by Growth in Asia.”, 2016,

—. “U.S. Energy Facts Explained – Consumption and Production – U.S. Energy Information Administration (EIA).”, 7 May 2020,

United States Nuclear Regulatory Commission. “NRC: Backgrounder on the Three Mile Island Accident.”, 2001,



  1. Interesting article, Ethan! Great thoughts!

  2. Wow! this is so cool. i had no idea

  3. Daisy Pipit Page????

    This was really interesting! Gj Ethan! 🙂

  4. Wow, Interesting article. Great Job!

  5. Oh no! this shocking Great writing

  6. Mr. Joachim, TPS AP Env. Sci. Teacher

    Excellent article, Ethan! I spent a fair amount of one of my AP Environmental Science lectures discussing the accidents at Three Mile Island, Fukushima Daiichi, and Chenobyl. In comparison to Chernobyl and Fukushima, where the release of radiation were quite high, the accident at Three Mile Island resulted in a much smaller release of radioactivity.

    I like the way you ended the article regarding the new safety features that are found on newer reactors. Your article was written in a good, balance way. Keep up the good work!