Small Modular Reactors Explained: How SMRs Work and Where Projects Stand in 2026
What small modular reactors are, how they differ from big nuclear plants and where US and Canadian SMR projects actually stand as of late 2026.
A small modular reactor is a nuclear reactor that typically produces up to about 300 megawatts of electricity per unit, with major parts built in factories and shipped to the site. SMRs aim to cut construction time and financial risk compared with large plants. In the US, TVA received the first commercial SMR construction permit in September 2026, with operation targeted for the 2030s.
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Electricity demand is climbing again in the US, driven by data centers, factories and electrification. Utilities and tech companies want power that runs around the clock without carbon emissions, and that has put small modular reactors, or SMRs, back in the headlines.
You’ve probably seen announcements about tech giants signing nuclear deals or a utility “approved to build” an SMR. What do those headlines actually mean, and when could these reactors produce power? This guide explains how SMRs work, how they differ from traditional plants and where real projects stand as of October 2026.
What is a small modular reactor?
A small modular reactor is a nuclear fission reactor that is smaller than a conventional plant and designed so that major components can be built in a factory and shipped to the site for assembly.
There is no single legal definition. The International Atomic Energy Agency and many US Department of Energy (DOE) materials describe SMRs as reactors producing up to about 300 megawatts of electricity (MWe) per unit. For comparison, many large US reactors produce roughly 1,000 MWe or more.
The 3 words in the name each matter:
- Small: lower power output per unit, so less radioactive material in each core and a smaller footprint.
- Modular: standardized units that can be added over time, from 1 reactor to several at the same site.
- Reactor: the same basic physics as any fission plant. Splitting atoms makes heat, heat makes steam, and steam spins a turbine.
SMRs vs microreactors
Microreactors are the smallest class. DOE describes them as factory-built, transportable reactors that typically produce about 20 megawatts of thermal energy or less. They target remote sites, military bases, mines and industrial users. Several microreactor designs reached criticality in DOE’s Reactor Pilot Program in mid-2026, which we cover below.
How SMRs differ from traditional nuclear plants
The case for SMRs rests on changing how nuclear plants are built and financed, not just their size.
| Feature | Traditional large reactor | Small modular reactor |
|---|---|---|
| Output per unit | Often about 1,000 MWe or more | Up to about 300 MWe |
| Construction | Mostly built on site, custom engineering | Major modules factory-built, then assembled |
| Upfront cost per project | Very large, multi-year capital commitment | Smaller per unit, though not cheap |
| Safety approach | Active systems with pumps and backup power | Many designs rely on passive cooling using gravity and natural circulation |
| Siting | Large site, usually near a big water source | Smaller footprint; some designs need less cooling water |
| Scaling | Fixed size from day 1 | Add units as demand grows |
| Track record | Decades of US operating experience | First US commercial units still years away |
Passive safety
Many SMR designs are built to shut down and cool themselves without operator action or outside power for an extended period. They use physics such as gravity-fed water, natural circulation and large water pools. That doesn’t eliminate risk, but it reduces reliance on pumps and diesel generators, which were central to past accidents.
Different coolants
Not every SMR uses water. Light-water SMRs, such as GE Vernova Hitachi’s BWRX-300 and NuScale’s design, build on technology US regulators know well. Advanced designs use other coolants:
- Sodium: TerraPower’s Natrium reactor uses liquid sodium and pairs it with molten salt energy storage.
- Molten fluoride salt: Kairos Power’s design uses salt as coolant with solid pebble fuel.
- High-temperature gas: X-energy’s Xe-100 uses helium and pebble fuel, aimed at industrial heat as well as power.
Some advanced designs need HALEU, a higher-enriched uranium fuel with limited commercial supply today, which is a schedule risk.
Key takeaway: SMRs use the same fission physics as large plants. The bet is on factory production, passive safety and smaller, repeatable projects that are easier to finance, and that bet has not yet been proven at commercial scale in North America.
Where US and Canadian SMR projects stand
Announcements are plentiful, so it helps to know the licensing steps. In the US, the Nuclear Regulatory Commission (NRC) typically issues a design approval or certification, then a construction permit for a specific site, then an operating license before fuel is loaded. A construction permit is a major step, but it doesn’t mean a reactor will run soon.
| Project | Design | Status as of October 2026 |
|---|---|---|
| TVA Clinch River, Tennessee | GE Vernova Hitachi BWRX-300, 300 MWe | NRC construction permit approved September 2026; operating license still needed |
| TerraPower Kemmerer Unit 1, Wyoming | Natrium sodium-cooled reactor, 345 MWe | NRC construction permit approved March 2026; completion targeted for 2030 |
| OPG Darlington, Ontario | BWRX-300, 4 units planned | Under construction; first unit targeted for grid connection by end of 2030 |
| NuScale US460 | Light-water SMR, 77 MWe per module | NRC standard design approval May 2025; no US plant under construction |
| Holtec Palisades, Michigan | SMR-300, 2 units planned | Construction permit application under NRC review |
The first US commercial SMR permit
On September 29, 2026, the NRC announced it had approved a construction permit for the Tennessee Valley Authority to build a 300 MWe BWRX-300 at its Clinch River site in Oak Ridge, Tennessee. The NRC called it the nation’s first commercial small modular reactor and said the 14-month review finished ahead of schedule. TVA still needs a separate operating license before it can load fuel.
Canada is building first
Ontario Power Generation began building the first of 4 BWRX-300 units at its Darlington site after receiving a construction licence from Canada’s nuclear regulator in 2025. OPG puts the cost of the 4-unit project at C$20.9 billion including interest, escalation and contingency, and expects the first unit on the grid by the end of 2030. Because TVA uses the same design, Darlington is the project to watch for real construction costs and schedules.
A lesson from NuScale
NuScale’s design was the first SMR certified by the NRC, and an uprated 77 MWe version received standard design approval in May 2025. Yet its first planned US plant, the Carbon Free Power Project in Idaho, was cancelled in November 2023 after it failed to sign up enough utility customers. Design approval alone doesn’t create a project; buyers willing to commit to the power price do.
Microreactors and the DOE pilot program
DOE’s Reactor Pilot Program set a goal, under a May 2025 executive order, of getting at least 3 test reactors to criticality by July 4, 2026. DOE reported that 4 had done so by early July 2026, including Aalo Atomics’ Aalo-X at Idaho National Laboratory. These were test and demonstration reactors, not commercial power plants.
Why tech companies are betting on SMRs
Data centers need steady power 24 hours a day, and big tech companies have climate pledges. That combination has made them some of the most visible SMR backers:
- Google and Kairos Power announced an agreement in October 2024 for a fleet of reactors totaling 500 MW by 2035.
- Amazon invested in X-energy in October 2024, with a stated aim of more than 5 GW of new projects by 2039.
These deals help fund early designs and signal demand, which matters after the NuScale lesson. They are long-dated commitments, though, and project timelines can slip. For the bigger picture on power and computing, see how AI data centers are reshaping energy demand.
The open questions: cost, waste and timing
SMRs face real hurdles, and honest coverage should name them.
- Cost: Smaller reactors lose some economies of scale. SMR advocates argue factory production and repeat builds will bring costs down, but that only happens after several units are built. First-of-a-kind projects usually cost the most.
- Fuel supply: Designs that need HALEU depend on new US enrichment capacity that is still being built.
- Waste: SMRs still produce spent fuel, and the US still has no permanent disposal site. Spent fuel is stored on site at plants today.
- Regulation: The NRC has been speeding up reviews, and it held webinars in October 2026 on a proposed overhaul of reactor regulations. Rules may change, so check the NRC’s site for current status.
- Timing: Even with permits in hand, the first US commercial SMRs are not expected to produce power until around 2030 or later.
For households, none of this changes your electric bill in the near term. If rising rates are your concern today, our guide on why electricity bills are rising explains the drivers.
How to follow SMR news without the hype
When you see an SMR headline, ask 5 questions:
- Which license step is it? Design approval, construction permit and operating license are very different milestones.
- Is there a named site and owner? A memorandum of understanding is not a project.
- Who is buying the power? Look for signed power purchase agreements or a utility rate case.
- Is construction underway? Concrete and a basemat beat renderings.
- What is the target date, and has it moved? Compare with earlier announcements.
Primary sources are the NRC’s news releases and docket pages, DOE’s Office of Nuclear Energy, and Canada’s Nuclear Safety Commission. If you’re interested in working in the field, see our green energy jobs guide, and look up unfamiliar terms in the glossary.
The bottom line
Small modular reactors are smaller fission reactors designed for factory construction, passive safety and step-by-step growth. In 2026 they moved from paper to permits: TerraPower and TVA hold US construction permits, Ontario is pouring concrete, and DOE’s test microreactors reached criticality. Commercial power from US SMRs is still several years away, and cost is the question that only finished projects can answer. Watch Darlington and Clinch River for the first real numbers, and pair the long-term nuclear story with near-term grid tools like virtual power plants.
Sources
- NRC: NRC Approves Construction of Nation’s First Small Modular Reactor (September 29, 2026) — TVA Clinch River construction permit, design, review time and operating license requirement
- DOE: NRC Issues Construction Permit for TerraPower’s Natrium Advanced Reactor — Kemmerer Unit 1 construction permit
- TerraPower: NRC Approves the Natrium Reactor Construction Permit — design size and project details
- Ontario Power Generation: Darlington SMR project — 4-unit cost estimate and 2030 grid target
- NRC: NRC Approves Standard Design for NuScale US460 — 77 MWe design approval in May 2025
- DOE: Department of Energy Celebrates Fourth Criticality Ahead of July 4th Goal — Reactor Pilot Program results
Frequently asked questions
What is a small modular reactor in simple terms?
A small modular reactor is a smaller nuclear power plant, usually up to about 300 megawatts per unit, designed so major parts can be built in a factory and assembled on site. It works like a conventional reactor, using fission to make steam and spin a turbine, but aims to be quicker to build and easier to finance.
Are small modular reactors safe?
SMRs are designed with strong safety features, and many use passive cooling that works without pumps, operators or outside power for an extended period. They also hold less nuclear fuel per unit than large reactors. They still must pass NRC safety reviews before construction and again before operation, and like any nuclear plant they produce radioactive waste that needs careful storage.
When will the first SMR operate in the US?
The first US commercial SMRs are not expected to produce power until around 2030 or later. TerraPower's Kemmerer plant in Wyoming targets completion in 2030, and TVA's Clinch River project received its construction permit in September 2026 but still needs an operating license. Test microreactors reached criticality in 2026, but they are not commercial power plants.
How much does a small modular reactor cost?
Costs for first-of-a-kind SMRs are high and still uncertain. Ontario Power Generation estimates C$20.9 billion for its 4-unit BWRX-300 project, including interest and contingency. TerraPower has described its Kemmerer project as costing up to about $4 billion. Developers expect costs to fall with repeat builds, but that has not yet been demonstrated.
What is the difference between an SMR and a microreactor?
The main difference is size. SMRs generally produce up to about 300 megawatts of electricity per unit and are meant for utility grids. Microreactors produce much less, typically about 20 megawatts of heat or less, and target remote communities, military bases, mines or single industrial sites. Some microreactors are designed to be transported as complete units.
Do small modular reactors produce nuclear waste?
Yes. SMRs produce spent nuclear fuel, just as large reactors do, and some advanced designs produce different waste forms that need their own handling plans. The US still lacks a permanent disposal site, so spent fuel is stored at plant sites in pools and dry casks. Waste management is part of every NRC license review.
Why are tech companies investing in nuclear power?
Tech companies need large amounts of steady, carbon-free electricity for data centers that run around the clock. Wind and solar vary with weather, so nuclear offers a complement. Google signed an agreement with Kairos Power in 2024 for up to 500 MW by 2035, and Amazon invested in X-energy the same year.
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Manishdai
Manishdai writes about the technology that is reshaping every industry — from AI agents and cloud platforms to healthtech, fintech and the IT companies building it all across the United States.
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