What it will take to restart decommissioned US nuclear plants. A primer

One of the most jubilant convenings of the US nuclear power industry in the last few years occurred in Covert Township, Michigan on a snowy day this March. There, policymakers, stakeholders, and hundreds of union workers gathered to celebrate the announcement that Palisades Nuclear Power Plant would be restarted—the first of two reactors, Palisades in Michigan and Three Mile Island Unit 1 in Pennsylvania, being revived.

The decision is not just unusual for the nation’s nuclear energy sector, it is unprecedented. Never has a fully shut down US nuclear reactor, on its way to decommissioning, been restarted. That Palisades and Three Mile Island Unit 1 (TMI-1) are promising to make power again—two reactors that were shut down for market reasons, not technical failures—also promises to make history.

The push to revive these two reactors comes from a couple of key drivers.

In the United States, nuclear energy is back in the spotlight because it is the country’s single largest source of carbon-free energy at a time when reducing carbon emissions is increasingly becoming urgent. At the same time, demand is on the rise for reliable, always-on electricity to undergird the stability of the US power grid while supporting the expansion of computational and industrial applications.

With a growing necessity to add hundreds of carbon-free megawatts to the grid, the question now is: What does it take to bring a nuclear plant back online after decommissioning has begun? The answer is a series of technical and regulatory tasks that are individually routine at many nuclear power plants, but, taken together, will amount to a historic undertaking.

What is decommissioning anyhow? Reactors routinely shut down and restart as part of their fuel cycle. Every reactor in the United States goes offline for about a month every year or two to refuel, inspect components, replace parts, and conduct maintenance. That is because nuclear reactors are precision machines that are required to undergo inspection and maintenance on a schedule calibrated to enable regulatory compliance, maximize economic efficiency, and optimize long-term performance. After each outage, these reactors return to criticality—that is, nuclear fuel sustains a fission chain reaction under normal reactor operations—until it is time to refuel again.

Decommissioning is distinct from these routine outages. It involves multiple stages starting with planning, followed by permanent cessation of operations, used fuel management steps, and then dismantlement of buildings, structures, systems, and components. Decommissioning is a process that, according to the International Atomic Energy Agency (IAEA), typically takes around 15 to 20 years and involves close coordination between the US Nuclear Regulatory Commission (NRC), utilities, contractors, and the community involved. That is why, for a reactor that has already entered decommissioning, restarting operations can get complicated.

Many of the tasks that will need to be undertaken to restart a reactor are the direct result of actions taken before the reactor even shuts down. During the planning period, while the reactor is operated with an intent to decommission, operators may prioritize necessary repairs for safety while deferring maintenance that only serves to optimize plant performance beyond the scheduled end date. Such deferred maintenance may mean that regulations require refurbishment or replacement of those components, including structures, pipes, and pumps, needed to restart the reactor.

In addition, as soon as the operator submits a letter that operations have permanently ceased, the license under 10 CFR part 50—the portion of the Code of Federal Regulations relating to nuclear facilities operation—no longer allows reactor operation. So, before a reactor can restart operations, the licensing basis must be restored, and the NRC must formally approve a return to operations.

Finally, once decommissioning and dismantlement activities begin, the physical status of the plant, reactor, and its components is altered. Large components may have been sold for reuse elsewhere and critical systems may be irreversibly disabled. As those activities proceed, the work necessary to reverse course increases.

Rewinding Palisades’ decommissioning process. The Palisades nuclear power plant, shut down in May 2022, is now undergoing a significant overhaul. Not long after Holtec International took ownership of the plant in June 2022 from its then-owner and operator, Entergy, the company announced its intention to get it back online as soon as possible—as early as 2025.

As a first step, Holtec and the NRC must thoroughly inspect the whole plant, with a particular focus on the reactor pressure vessel and cooling systems. The cooling towers, pumps, and piping systems all need to be inspected for corrosion, scaling, or blockages. And then there is the turbine. Turbines can be sensitive to long periods of inactivity, which risks damaging the bearings and blades. At Palisades, the turbines will need to be inspected—and likely refurbished—to ensure they can convert the reactor’s thermal output into electricity efficiently and safely.

These inspections will be similar to inspections required at existing plants under consideration for subsequent license renewals, which may allow them to operate for up to 80 years. Such inspections include the use of non-destructive ultrasonic and radiographic testing methods to detect any small flaws or corrosion that might have developed and repair them before the reactor can be licensed for operation again. At Palisades, some such inspections and repairs are already underway. In a public letter this September, the NRC noted further analysis or repair—or both—of multiple steam generator tubes would be needed, and Holtec is subsequently preparing to fix instances of stress corrosion cracking and wear in the equipment.

Then, there is the used fuel.

When the Palisades plant shut down, it had decades’ worth of used nuclear fuel in the spent fuel storage pool. The first step in decommissioning saw that fuel moved to a dry cask storage area. On-site dry storage is beneficial for the prospect of a restart because it allows the pool to have ample space to accommodate new and used fuel for decades after the reactor restarts, facilitating plant operation and reducing maintenance costs.

Awakening Three Mile Island Unit 1. To be clear, no one is planning to restart Three Mile Island Unit 2 (TMI-2), which melted down in 1979. It is TMI-1 that Constellation Energy, which purchased the reactor in 1999, announced will be undergoing refurbishment. Despite being adjacent to Unit 2, TMI-1 was undamaged by the 1979 accident and remained fully functional. Even as TMI-2 underwent defueling and decontamination procedures and the core debris was shipped offsite, Unit 1 operated safely and reliably for four decades until it ceased operations in 2019.

Three Mile Island Unit 1 has been shut down a few years longer than Palisades, but the extent of decommissioning already undertaken is not significantly different between the two plants. At TMI-1, the decommissioning process started with initial used fuel management actions until 2022 when it entered a dormancy period (a period during which the reactor remains under close surveillance as the radioactivity of the internal structures decays naturally, and after which active decommissioning and dismantling operations can resume at optimal safety conditions and cost-effectiveness) slated to last until 2073. During the current dormancy period, many electrical, ventilation, and other systems have been reconfigured and de-energized, but larger-scale, irreversible dismantlement has largely been on hold.

As at Palisades, the used fuel at TMI-1 was moved out of the pool to dry cask storage between 2019 and 2022, leaving the pool empty and therefore ready to potentially accept new fuel. Of course, to become operational again, this pool may need to be refurbished: Its cooling, filtration, and other systems will need to be inspected, and the structural integrity of the pool must be confirmed before a restart.

Even though the dormancy phase has forestalled large-scale dismantlement at TMI-1, Constellation Energy will still need to rewind those decommissioning steps that have been undertaken already. Since the plant’s pumping, safety, and other critical systems were powered down, a long list of inspections and testing procedures will need to be undertaken before reactivating major systems including all cooling systems, control systems, and safety mechanisms that ensure the reactor can operate safely.

As part of that work, they’ll need to replace some items too, including the main power transformer. Without it, TMI-1 will not be able to send any electricity to the grid, even if the reactor is up and running. As at Palisades, the turbine too will need to be refurbished or replaced.

Regulatory and workforce challenges. This is where things will get tricky: For both Palisades and TMI-1 to restart, they’ll need to navigate a somewhat uncharted regulatory landscape with NRC involvement at every step of the way.

For Palisades, Holtec has begun submitting regulatory requests, developing detailed safety reports, and facilitating NRC inspections. All of this—and more— will be necessary to fully reestablish the licensing basis across the plant, from the integrity of the plant’s systems to performance during startup tests. The same goes for Three Mile Island Unit 1.

At Palisades, regulatory submissions, inspections, and public meetings have been underway in earnest since 2023. In November 2023, the NRC even established a dedicated panel of the agency’s experts to help map their regulatory approach to reactor restart. Its charter describes a scope aiming to facilitate timeliness, identify potential challenges, and proactively resolve issues in the process.

Reconnecting the reactors to the grid will have its share of challenges too. Both plants must continue to closely coordinate with regulators, regional grid operators, utilities, and customers to ensure that they can safely rejoin the electrical grid system without causing power instability.

Recruiting and training staff to refurbish and run these plants will also need significant effort. While many of the original staff at Palisades have returned for this restart, most personnel who ran TMI-1 until 2019 have moved on. Either way, Holtec and Constellation will need to rebuild their workforces. But this task is not about hiring just any workers. Nuclear operators must be highly trained and certified by the NRC for their specific plant, an effort that involves extensive training and simulation exercises.

Bringing these nuclear plants back online will require a talented pool of technicians and trades workers, such as welders and electricians certified in nuclear-specific codes and standards. The concentrated need for these workers at a plant under construction is a major takeaway from the recent completion of Vogtle Units 3 and 4 in Waynesboro, Georgia. Even though Palisades and TMI-1 will require only a fraction of the workers needed for new reactor build (According to Bechtel, Vogtle pulled over 9,000 skilled workers from 48 states at peak construction), these two plants would need to tap into a new generation of workers with recent plant construction knowledge.

Are these one-time projects or the first of many? As the projects at Palisades and TMI-1 proceed, they will provide invaluable experience for future such cases in the United States and other countries with aging reactors. In the United States, however, there may not be many other shutdown plants that could reasonably be restarted.

While Florida-based NextEra is reportedly considering a potential restart of Duane Arnold’s boiling water reactor in Iowa, most recently shuttered reactors are probably too far into their decommissioning process to be restarted cost-effectively. At a certain point, refurbishment costs exceed the price of building a new reactor. Reaching that inflection point might even happen earlier in the decommissioning process if new reactor builders manage to minimize costs by locating them at existing nuclear plant sites or former coal plant sites where infrastructure and transmission capacity already exist.

The story of Palisades and TMI-1 is more than just about the revival of two retired plants. It is a test case for the future of the US nuclear industry and how it can help meet the growing needs for cleaner forms of electricity generation. If successful, the efforts to bring these plants back online safely and effectively will revitalize the workforce and supply chain needed to pave the way for new nuclear projects in the United States. Most importantly, if they succeed, they will demonstrate that nuclear energy can still play a crucial role in the clean energy transition.