The Reopening of the Three Mile Island Nuclear Power Plant

On September 20, 2024, Microsoft announced that they had reached an agreement to reopen the Three Mile Island nuclear power plant. As the site of the worst nuclear accident on U.S. soil—a partial reactor meltdown in 1979—the plant has a legacy that prompts safety concerns now that it has been reopened, even with modern technology.

The Three Mile Island plant generates nuclear power, which relies on nuclear fission to generate energy. Nuclear fission begins when a neutron, commonly from an element like californium, collides with a larger atom. Californium is spontaneously fissile, meaning it can split apart and release neutrons on its own. The collision between the neutron and the atom causes the atom to gain energy, become excited, and destabilize. This instability makes the atom split into two smaller atoms, leading to a sustained chain reaction that releases heat. 

While elements like thorium and plutonium are expensive fuel sources, U-235—an isotope of uranium—is commonly used as fuel for nuclear reactors because it is both efficient and relatively accessible. The arrangement of particles in the nucleus of U-235 is unstable, so when it collides with a neutron, it is easily split into smaller atoms, making it ideal for sustaining nuclear fission. Furthermore, uranium is relatively abundant, with a concentration of 2.8 parts per million in the Earth’s crust and traces found in rocks worldwide.

Control rods are made of metals whose nuclei easily absorb neutrons. These metals, like boron, cadmium, hafnium, indium, and silver, are used to control fission rates. U-235 is turned into fuel pellets, which are inserted into control rods. The control rods are then submerged in water. The heat from fission in the rods boils the water, creating steam. This steam spins turbines, producing electricity.

The Three Mile Island nuclear power plant is home to two reactors: TMI 1 and TMI 2. On March 28, 1979, at 4:00 a.m., TMI 2’s main feedwater pumps—essential for cooling the reactor core—failed after water infiltrated an instrument air line. This led to a dangerous rise in heat and reactor pressure. Operators opened a valve to release pressure and then closed it after pressure levels returned to normal. However, a faulty indicator failed to show that the valve was stuck open. Large amounts of coolant escaped, preventing effective heat dissipation from the control rods. 

In another section of the reactor, flashing lights indicated extremely high water levels. Following their training, operators shut the water valves off. The temperatures in the already hot reactor core soared, resulting in a partial meltdown. The meltdown destroyed about 45 percent of the control rods in the reactor and released radiation within a five-mile radius. Although the radiation released from the meltdown was six times weaker than that of an X-ray machine, the incident triggered widespread panic and opposition to nuclear energy. After the partial meltdown of TMI 2, TMI 1 ran until 2019, when it was shut down due to rising costs.

The plant’s history caused a wave of concern, with people worrying about a repeat of the partial meltdown incident. However, significant advancements in safety regulations and technology have drastically reduced such risks. Following the 1979 incident, regulations for nuclear power plants were tightened, and operator training became far more rigorous. Modern nuclear reactors are equipped with advanced technology like electromagnetic flow sensors to monitor coolant circulation and neutron flux sensors to monitor the fission rates. Reactors also utilize ceramic-matrix composites and other materials engineered to withstand higher temperatures and radiation, enhancing reactor safety and resilience. Rebuilding the plant will have financial benefits. 

While construction can only happen after extensive inspection, construction will create over 600 jobs, including positions for reactor operators, nuclear engineers, and maintenance technicians, boosting the local economy. Furthermore, the plant will provide energy for over 800,000 homes. For Microsoft, reopening the plant offers a reliable, cost-effective energy source that supports the company’s growing data centers and artificial intelligence development. Beyond the economic benefits, nuclear energy is one of the most eco-friendly power sources available, aligning with global efforts to combat climate change. With companies like Google—which are constructing six of their own nuclear plants—reopening the Three Mile Island nuclear power plant signals a new era for nuclear energy in the United States.