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For near-term clean, firm electricity, a technically viable restart of an existing nuclear plant usually has the advantage. It can reuse the site, grid connection, cooling systems, licensed location and workforce base. Small modular reactors (SMRs) are a longer-term expansion option: their smaller units may reduce the capital committed to each project and could benefit from factory production, but those benefits depend on licensing, first-of-a-kind construction, supply chains, financing and repeated builds.
The choice is not “old nuclear versus new nuclear.” It is a comparison between a refurbishment project with known infrastructure and a new technology deployment whose commercial learning curve is still developing. Some sites may eventually do both.
What the two options actually are
Small modular reactors
An SMR is a nuclear reactor design intended to be built in modules, often with factory fabrication and serial deployment. “Small” describes the output of each unit relative to a conventional large reactor; “modular” describes the intended manufacturing and construction method. A project can add units in stages, which may suit a smaller grid or an industrial customer that cannot absorb a very large block of generation at once.
The International Energy Agency’s 2025 outlook expects first commercial SMR projects to start operation around 2030. It also says SMRs could reach cash-flow breakeven up to 10 years earlier than large reactors if modular construction and financing assumptions are achieved. Those are outlooks, not guaranteed dates or costs for every design. The commercial case depends on a design being licensed, a qualified fuel and component supply chain being available, a factory achieving repeatable production and enough units being ordered to create learning effects.
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Restarting an existing plant
A restart is not turning a switch back on. The owner must demonstrate that the plant can safely return to service under a restored or amended licensing basis. The work can include inspections, replacement of degraded equipment, testing, safety upgrades, updated analyses, staffing and operator training, fuel and spent-fuel arrangements, and regulatory approvals.
U.S. Nuclear Regulatory Commission guidance for Duane Arnold illustrates the process: the licensee must restore the plant’s licensing basis, return components to safe operating status and complete upgrades required for the proposed operating basis. A plant that retained recoverable infrastructure is a very different proposition from a site that has progressed deeply into decommissioning.
Which can deliver electricity sooner?
For a viable candidate, a restart generally has the shorter path because the site, interconnection, cooling-water systems, buildings and much of the operating organization already exist. The Electric Power Research Institute (EPRI) gives a typical recommissioning range of 2–5 years. That is a general industry estimate, not a promise for every plant; condition, licensing work, ownership and financing can push a project outside the range.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →SMRs must complete a new-reactor licensing and construction sequence. The IEA expects the first commercial projects around 2030, indicating that many deployments are still ahead of commercial operation. A design may have made substantial licensing progress and still face site permitting, manufacturing, construction, fuel qualification where applicable, commissioning and financing risks.
| Question | Restarting an existing plant | Building an SMR |
|---|---|---|
| Typical schedule evidence | EPRI cites 2–5 years for recommissioning | IEA expects first commercial projects around 2030 |
| Existing infrastructure | Can reuse a nuclear site, grid connection, cooling systems and workforce base when they remain recoverable | Requires a new deployment, although an existing nuclear site can reduce some siting and grid-integration work |
| Primary schedule uncertainty | Plant condition, licensing restoration, refurbishment scope, ownership and economics | Design certification, site licensing, first-of-a-kind construction, factory capacity, supply chains and financing |
The schedule comparison must distinguish a plant that was shut down but maintained from one that has been dismantled or allowed to deteriorate. EPRI estimates that only a small fraction of roughly 200 permanently shut-down reactors worldwide are viable restart candidates.
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Which option is cheaper?
There is no established rule that SMRs are cheaper than restarts. A fair comparison includes refurbishment, licensing, financing, fuel, spent-fuel and waste management, decommissioning obligations, transmission upgrades and wider system costs. Comparing an SMR’s projected overnight cost with a restart’s narrow equipment budget can produce a misleading result.
Economics of a restart
A restart can avoid major portions of greenfield construction because the plant already has a nuclear-grade site, interconnection and cooling arrangement. That does not make the work inexpensive. The owner may need extensive inspections, replacement components, digital or safety upgrades, new contracts, workforce rebuilding and regulatory analysis. A credible revenue contract or other market mechanism is often necessary to finance the work.
The OECD Nuclear Energy Agency’s 2025 study places long-term operation of existing nuclear plants among the options below USD 100 per megawatt-hour in its plant-level dataset. The figure is a dataset result, not a universal restart price, and it does not remove project-specific refurbishment or system costs.
Economics of an SMR
SMR cost reductions are conditional. Factory production, standardized designs, repeat orders and shorter construction periods could lower cost and reduce the amount of capital committed at one time. The first unit, however, may carry design, licensing, supply-chain and construction premiums before a repeat-build learning curve exists. Financing costs can dominate when a project takes years before revenue begins.
The IEA warns that long permitting and construction schedules can push breakeven for a new large reactor to 20–30 years after project start. SMRs are intended to shorten and stage construction, but the benefit must be demonstrated in actual projects rather than assumed from the design concept.
| Economic evidence | What it means |
|---|---|
| Below USD 100/MWh for long-term operation in the NEA’s 2025 plant-level dataset | Existing-plant operation can be among the lower-cost clean-firm options; the value is not a universal restart quote |
| Up to 10 years earlier cash-flow breakeven for SMRs than large reactors, in the IEA’s 2025 outlook | A conditional projection requiring modular construction and financing assumptions to hold |
| 20–30 years to breakeven for a new large reactor when timelines are long, according to the IEA | Schedule and cost of capital can matter as much as engineering cost |
How mature are the technologies?
Operating experience favors existing plants
An operating or previously operating reactor has an operating record, known systems and established maintenance and supplier relationships. Those advantages do not prove that every component remains fit for service; they give engineers and regulators a body of plant-specific evidence to evaluate.
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SMRs have licensing progress but limited commercial operation
SMR designs have made regulatory progress, but commercial operating experience remains limited. The NRC reported completing the NuScale US460 standard-design review in 22 months, two months early and 13% under budget. That result demonstrates that a particular design review can be completed efficiently; it does not establish the construction cost, schedule or operating performance of every SMR.
The key maturity questions are whether the design has a completed licensing path in the relevant country, whether its fuel and major components can be supplied at the required scale, whether a factory can produce consistent modules and whether multiple customers are willing to place repeat orders.
Site, grid and workforce fit
Existing nuclear sites can be strategically valuable. A restart may preserve a known transmission interconnection, cooling-water system, emergency planning arrangements, trained personnel and a nuclear regulatory history. Reusing those assets can reduce both schedule and interface risk, although each asset must be inspected and accepted for the proposed operating basis.
The U.S. Department of Energy identified 41 operating or retired U.S. sites with potential room for more than 60 gigawatts of large-reactor capacity, or about 95 gigawatts using smaller 600-megawatt advanced reactors. This is a siting potential assessment, not a list of approved projects or a guarantee that every site can host a restart or an SMR.
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SMRs can fit smaller grids and industrial loads because capacity can be added in increments. An existing plant generally returns a large unit at once, and its output is constrained by the unit’s original design and condition. A site that cannot support a large new reactor may still be suitable for smaller units, subject to licensing, cooling, seismic, safety and grid studies.
How the risk profiles differ
Restart risk
- Condition: hidden degradation, obsolete equipment or incomplete maintenance records can expand the refurbishment scope.
- Licensing: the owner must restore the licensing basis and satisfy the regulator that the proposed operating condition is safe.
- Commercial structure: the project needs an owner, workforce plan, fuel and waste arrangements and dependable revenue.
- Decommissioning status: work already performed during decommissioning may be difficult or costly to reverse.
- Scale: returning one large unit concentrates the project’s financial and operational exposure.
SMR risk
- Design certification and site licensing: a design can be technically promising while approvals remain incomplete in the target jurisdiction.
- First-of-a-kind execution: the first commercial project may encounter redesigns, construction rework or commissioning delays.
- Factory and supply chain: the economic model requires qualified suppliers and repeatable module production.
- Fuel and component qualification: some designs require new fuel or specialized components that must be demonstrated at commercial scale.
- Financing: revenue is deferred while licensing and construction proceed, and the learning curve may not arrive without multiple orders.
In short, a restart concentrates uncertainty in the plant’s condition, licensing restoration and economics. An SMR shifts more uncertainty into design approval, manufacturing, first-project construction, fuel and financing before repeat builds are proven.
Safety and regulatory questions
“Can an old plant be restarted safely?” is answered through a plant-specific regulatory process, not by its age alone. The licensee must show that structures, systems and components meet the applicable safety basis, that operating procedures and staffing are adequate, and that required upgrades have been completed. The NRC’s Duane Arnold guidance makes restoration of the licensing basis and safe component status explicit requirements.
SMRs also require rigorous licensing. Smaller output does not eliminate requirements for reactor safety, containment, emergency planning, security, quality assurance, radioactive-waste management and decommissioning. A design review can resolve the standard design, but the owner still has to license and build a specific facility.
For context, the NRC reported in 2024 that 59 plants and 96 reactor units had completed license-renewal environmental reviews and received renewed licenses. That record shows an established regulatory route for continued operation; it does not automatically qualify a permanently shut-down plant for restart.
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Examples of restart and hybrid strategies
Palisades, Michigan
The NRC approved major licensing actions required for the Palisades restart on July 24, 2025. DOE and Holtec have also discussed two SMR-300 units at the site. The example shows how an existing nuclear location can support a near-term restart while preserving a possible path for future new-build capacity. The two projects remain distinct: a restart must restore the existing plant’s licensing and equipment, while the SMR proposal would require its own licensing and construction work.
Duane Arnold, Iowa
Duane Arnold demonstrates why “restart” is a technical program rather than a switch. NRC requirements include restoration of the licensing basis, returning components to safe operating status and completing upgrades needed for the proposed operating basis.
Christopher M. Crane Clean Energy Center, Pennsylvania
The former Three Mile Island Unit 1 is identified as a restart candidate. DOE describes an 871-megawatt facility with an existing NRC license allowing generation through 2034. The case illustrates the potential value of a site with an existing license and large-unit infrastructure, while still requiring a plant-specific assessment of condition, economics and restart work.
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The most defensible process is to screen restart candidates first, then compare surviving candidates with SMRs and other new-build options on the same financial and system basis.
- Confirm physical condition. Review maintenance records, inspections, component condition, decommissioning activities and the cost and duration of required replacements.
- Map the licensing route. Determine what must be restored, amended or newly approved, and obtain a regulator-aligned schedule for safety reviews and inspections.
- Verify site and grid assets. Check transmission interconnection rights, cooling-water availability, seismic and environmental constraints, emergency planning infrastructure and the condition of buildings and support systems.
- Rebuild the operating organization. Identify licensed operators, maintenance staff, contractors, training capacity and the knowledge needed to operate the specific unit.
- Secure fuel, waste and ownership arrangements. A restart plan needs fuel procurement, spent-fuel management, radioactive-waste pathways, decommissioning responsibilities and a financially capable owner.
- Establish revenue. Test whether a power-purchase agreement, regulated recovery, capacity payment or other credible contract can support financing through the restart period.
- Compare like-for-like economics. Include refurbishment, licensing, financing, fuel, waste, decommissioning, transmission, capacity factor and system costs for a restart; use the same categories for SMRs.
- For SMRs, validate repeatability. Require evidence for design certification, factory capacity, qualified suppliers, fuel availability, construction schedule, first-unit contingency and the number of committed follow-on units.
- Assess portfolio fit. A large restart may deliver firm power sooner, while SMRs may add capacity incrementally or serve an industrial load. Model both against demand growth, grid flexibility and reliability needs.
When each option is the better fit
Favor a restart when
- The plant’s physical condition is recoverable and decommissioning has not removed critical systems.
- The licensing basis can be restored on a credible schedule.
- The site retains a valuable grid connection, cooling system and nuclear workforce base.
- A customer or market mechanism can support financing and the unit’s large output.
- The cost of refurbishment compares favorably with new-build alternatives after including financing and system costs.
Favor an SMR project when
- No restart candidate passes condition, licensing or economic screening.
- The grid or industrial customer needs capacity in smaller increments.
- A site can support new units and the project has a credible licensing and supply-chain plan.
- The owner can tolerate first-of-a-kind risk and has a path to repeat builds.
- Modular construction, standardized procurement and staged additions provide value that a single large restart cannot.
Use a combined strategy when
A safe restart can preserve near-term clean generation while a limited SMR program proves whether a design, factory and supply chain can deliver repeatably. Existing nuclear sites may provide grid, cooling and workforce advantages for later units, but each SMR still requires its own technical and regulatory case.
Questions to ask before approving either project
- What exact work remains, and which party has independently estimated its cost?
- What date is tied to a regulator-accepted licensing milestone rather than an announcement?
- Which costs are included: financing, fuel, waste, decommissioning, transmission and system integration?
- For an SMR, is the quoted price for a first unit or a mature series build?
- For a restart, what decommissioning steps have already occurred and can they be reversed?
- Who will employ and train the operating workforce?
- What happens if the schedule slips by two years or the refurbishment scope expands?
- Does the grid need one large block of power or smaller additions over time?
Bottom line
A viable restart is usually the faster route to clean firm power because it can reuse infrastructure that a new project must create. Its advantage disappears when a plant’s condition, licensing history, workforce, waste arrangements or economics make restoration impractical. SMRs offer a potentially more flexible and repeatable way to add capacity, but “SMRs are cheaper” remains a conditional expectation rather than an established fact. The strongest strategy in many regions is to preserve safe existing capacity while advancing a carefully limited number of SMR projects with transparent assumptions and measurable delivery milestones.
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