SMR vs microreactor: what is the difference?
Small modular reactors and microreactors are often mentioned together, and the terms overlap. In short, a microreactor is the smallest kind of SMR. The differences in size, use, siting and fuel still matter a great deal to anyone planning a project.
The short answer
The U.S. Energy Information Administration puts it plainly. SMRs have a capacity of about 300 MW per unit or less, while microreactors, a subset of SMRs, generally have a capacity of 20 MW or less. So every microreactor is an SMR, but a typical SMR is ten or more times larger than a microreactor.
NuScale, an SMR developer, frames the relationship the same way: microreactors occupy the smallest end of the broader modular reactor landscape, while SMRs are usually designed for larger commercial and grid-scale roles.
Side by side
| Small modular reactor (SMR) | Microreactor | |
|---|---|---|
| Typical size | Up to about 300 MWe per unit | Generally 20 MW or less (some definitions up to about 50 MWe) |
| Typical role | Grid supply, large industry, repowering old plants | A single site, a small community or an off-grid load |
| Build | Factory-made modules, site assembly | Fully factory-assembled where possible |
| Transport | Large modules by road, rail or barge | Whole units by truck, rail, ship or air |
| Fuel | LEU for light-water designs; HALEU for many advanced designs | Most designs need HALEU, per DOE; some use other moderators |
| Public example | NuScale US460 (77 MWe per module); BWRX-300 (300 MWe) | Oklo Aurora at Eielson AFB (1 to 5 MW target) |
Sources: EIA, WNA, DOE, NuScale.
Size: where the line sits
There is no single legal cutoff, and the main sources differ slightly:
- EIA: microreactors generally 20 MW or less.
- World Nuclear Association: SMRs typically less than 300 MWe, and the smallest designs, typically less than 20 MWe, are sometimes called microreactors.
- NuScale: microreactors generally up to about 50 MWe, with many concepts well under 20 MWe.
- DOE: most microreactor designs would produce 1 to 20 MW of thermal energy.
The OECD Nuclear Energy Agency defines SMRs as having a power output between 10 MWe and 300 MWe. Note that a 10 MWe unit sits at the boundary of both classes.
Use: grid versus site
SMRs are mostly pitched for grid-scale roles. Examples are replacing retiring coal units, adding capacity to a utility system or serving large industrial parks. Ontario Power Generation's licence to construct a BWRX-300 at Darlington is a utility project of this kind.
Microreactors are aimed at a single load. NuScale lists remote communities, military installations, mining sites, research stations, and industrial locations that currently rely on diesel generation or limited grid access. DOE adds disaster recovery, district heating, desalination and hydrogen. The U.S. military is the most active early buyer.
Both classes are now pitched to data centres, which often need power behind the meter.
Build and transport
Both SMRs and microreactors rely on factory fabrication. The difference is how much is finished in the factory. DOE describes microreactors where all components of a microreactor would be fully assembled in a factory and shipped out to location, small enough to ship by truck, shipping vessel, airplane or railcar. Larger SMRs ship as modules or major components, with more civil work and assembly on site.
Fuel
Fuel is where the two classes often diverge, and it is easy to overlook. Light-water SMRs such as NuScale and the BWRX-300 use the low-enriched uranium the existing fleet already uses. DOE says most designs will require fuel with a higher concentration of uranium-235 that's not currently used in today's reactors. That fuel is HALEU, and DOE also says it is not yet available at commercial scale. Heavy-water or graphite moderation can allow natural uranium instead. We cover this in HALEU vs natural uranium.
Licensing and maturity
Larger SMRs are further along. China's HTR-PM entered commercial operation in December 2023. The NRC approved NuScale's US460 standard design in May 2025. Canada issued a licence to construct a BWRX-300 in April 2025. Microreactors are mostly at the test stage. INL's DOME test bed and the DOE pilot program target first criticality for test reactors, and commercial units still need full licences.
Economics in one paragraph
Smaller is not automatically cheaper per unit of energy. The Union of Concerned Scientists argues that smaller reactors will in general produce more expensive electricity than larger ones, because costs such as security do not shrink with size. It notes that some vendors have increased module sizes for this reason. Supporters answer that factory production and smaller financing steps change the equation. Both arguments remain to be proven at fleet scale.
Which one should you consider?
Start from the load, not the technology. A few questions narrow the choice quickly:
- How much firm power does the site need, and how fast does it grow? A single industrial site or campus phase in the tens of megawatts points toward smaller units that can be added in steps. A utility replacing a retiring plant points toward larger SMRs.
- Is the site on a strong grid? A site with grid access can sell or buy surplus. An isolated site needs on-site redundancy, which favours several smaller units over one large one.
- What fuel can the project secure? If the design needs HALEU, ask who supplies it and when.
- How will the reactor get there? Road limits, bridges and port access matter more for large modules than for truck-sized units.
- Is water available? Steam-cycle plants need cooling water. Designs with air-cooled or gas-turbine power conversion need far less.
- What powers the site before the reactor is licensed? For most projects this is the deciding question.
Where BridgePower Nuclear fits
Until the reactors arrive, BridgePower Nuclear powers sites with thermal generation from USP&E, backed by 24+ years in power, 150+ power projects in 45+ countries, and 2,000+ MW in stock.
Read more in our guides to small modular reactors and micro modular reactors, or contact our team.
Frequently asked questions
Is a microreactor an SMR?
Yes. EIA describes microreactors as a subset of SMRs that generally have a capacity of 20 MW or less. All microreactors are small modular reactors, but most SMRs are much larger than microreactors.
What is the size cutoff between an SMR and a microreactor?
There is no single legal cutoff. EIA uses 20 MW or less, WNA says typically less than 20 MWe, and NuScale says up to about 50 MWe, with many concepts well under 20 MWe.
Which is better, an SMR or a microreactor?
Neither is better in general. Larger SMRs suit grid-scale supply. Microreactors suit a single site or an off-grid load. The right choice depends on load size, site, fuel and licensing path.
Do microreactors and SMRs use the same fuel?
Not always. Light-water SMRs use the low-enriched uranium used today. DOE says most microreactor designs need higher-enrichment fuel, HALEU. Heavy-water-moderated designs can use natural uranium.