Three Mile Island Nuclear Energy is Coming Back

Three Mile Island’s Unit 1 Reactor is Coming Back Into Service, and new Plants are Being Planned

Julian Whitecar, Staff Writer • October 6, 2024

On September 21st, 2024, Constellation, an energy company, announced plans to restart Three Mile Island Unit 1 (TMI-1) following an agreement with Microsoft. Microsoft plans on using energy from TMI Unit 1 to power its AI data centers.

Constellation plans to pour 1.6 billion dollars into the plant’s Unit 1 Pressure Water Reactor to get it back online. Unit 1 was not affected by the Three Mile Island incident in 1979. Unit 2 will continue to be decommissioned by Energy Solutions.

According to Data Center Dynamics, Unit 1 was in service until 2019 where it was shut down for economic reasons, nearly 30 years after Unit 2 melted down. Unit 2 has been in active decommissioning since 1979 (Moss, Sept. 20).

Reactor Vessel Head (Not TMI) (U.S. NRC)

What Happened in 1979?

Three Mile Island used Pressure Water Reactors. These reactors are normally very safe and are used all around the country. On March 28th, 1979, a feedwater pump that supplied water to the reactor ‘tripped’ and stopped providing water to the core.

This resulted in a built up of pressure, opening a ‘block valve’ that removed the pressure, but allowed water to leave the cooling loop. (Seen in orange below) Normally the block valve would have closed after the pressure was back to normal, but it was stuck open due to lack of maintenance, preventing any water from staying in the core cooling loop.

In short, this one valve not closing prevented the fuel from getting cooling flow, and eventually, the fuel partially melted (World Nuclear Association, 2022).

Diagram of a Pressure Water Reactor, such as TMI-2. (U.S. NRC) See more about PWR’s here.

Should we be Worried?

In short, no. The meltdown of Three Mile Island’s Unit 1 resulted in 0 deaths, and the only real damage was to the core. All significant radiation was contained within the reactor containment structure. Some argue that there was significant stress among the public which may have played a role in the history of the accident.

Are New Plants Coming to Ohio?

The energy company Oklo wants to build a new nuclear plant in Ohio. Currently, there are 3 nuclear plants in Ohio, including the Piqua Nuclear Plant, which was decommissioned in 1966, and the Davis-Beese and Perry Nuclear plant who are both operating. Oklo is currently seeking federal approval from the U.S. NRC to build these plants (Singer, May 22, 2023). This plant will provide 30 MW of clean energy to the grid (Oklo, 2023).