Micro modular reactors and microreactors, explained
A micro modular reactor is a nuclear reactor small enough to be built in a factory and delivered on a truck. The industry mostly calls them microreactors. Search engines also see the terms micro nuclear reactor, mini nuclear reactor and mini modular reactor, which describe the same thing.
A note on names. "MMR" is a registered trade mark used by one developer for its own design. We use "micro modular reactor" and "microreactor" as generic descriptions, and we do not describe our own design as an MMR.
How small is a microreactor?
There is no single legal cutoff, but the public definitions cluster tightly:
- The U.S. Energy Information Administration says microreactors, a subset of SMRs, generally have a capacity of 20 MW or less and can run on the grid, off the grid or as part of a microgrid.
- The World Nuclear Association says the smallest designs, with capacities typically less than 20 MWe, are sometimes called microreactors.
- The U.S. Department of Energy measures heat instead of electricity and says most would be able to produce 1-20 megawatts of thermal energy.
So a microreactor is roughly one to two orders of magnitude smaller than a conventional plant. For the next size up, see our guide to small modular reactors, and for a side-by-side comparison, SMR vs microreactor.
What makes a reactor a microreactor
DOE is clear that microreactors are not defined by their fuel form or coolant. It points to three features:
- Factory-fabricated. All components would be fully assembled in a factory and shipped to location.
- Transportable. The units are small enough to ship by truck, vessel, aircraft or railcar.
- Self-regulating. Simple, responsive designs that adjust themselves, need fewer specialised operators and use passive safety systems.
Where microreactors fit
Microreactors are not meant to replace grid-scale plants. The uses public sources describe include:
- Remote and diesel-dependent sites. Remote, rural U.S. communities that fly or truck in diesel to run generators.
- Military installations. EIA reports that the U.S. Air Force is planning a microreactor at Eielson Air Force Base in Alaska.
- Industrial heat and power. DOE lists district heating, water desalination and hydrogen production as non-electric uses.
- Co-located industrial and data-centre load. Some microreactor developers now target data-centre and industrial customers directly. See nuclear power for data centres.
Each of these is a site that needs firm power where it is, often behind the meter instead of through a long grid connection.
The fuel question
The biggest practical constraint on microreactors is fuel. 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, high-assay low-enriched uranium, enriched to between 5% and 20% U-235.
There is another route. WNA notes that with graphite or heavy-water moderation it is possible to run a power reactor on natural instead of enriched uranium, which is how Canada's CANDU fleet works. We compare the two paths in HALEU vs natural uranium.
Microreactor trade-offs
The strengths are clear: factory build, transport by road, small footprint and firm output. The open questions are the same as for SMRs, but sharper at small scale:
- Fuel supply. HALEU-dependent designs depend on an enrichment chain that is still being built.
- Unit economics. Fixed costs for security, licensing and staffing do not shrink in step with output.
- Fleet scale. One microreactor will not power a large industrial campus. Several units, or a mix with other generation, are needed.
- Used fuel. Fuel management on site and at end of life must be planned from the start.
Firm power first, then the reactor
A microreactor only helps once it is licensed and installed. BridgePower Nuclear pairs its nuclear programme with a thermal power block from USP&E, with 24+ years in power, 150+ power projects in 45+ countries, 2,000+ MW in stock and 400+ engineers and project managers. The thermal block runs grid-tied or behind the meter on gas, diesel, HFO or dual-fuel.
To discuss a site, contact our team.
Frequently asked questions
What is a micro modular reactor?
A micro modular reactor, usually shortened to microreactor, is the smallest class of small modular reactor. EIA describes microreactors as generally 20 MW or less. They are designed to be factory-built and moved by truck, rail or ship.
Is a mini modular reactor the same as a microreactor?
In everyday use, yes. Mini modular reactor, micro modular reactor and microreactor all describe the same very small, factory-built reactor class. MMR is also a registered trade mark used by one developer, so this page uses the generic terms.
How much power does a microreactor produce?
DOE says most designs would produce 1 to 20 megawatts of thermal energy. Definitions of the electrical ceiling vary: EIA uses 20 MW or less, WNA says typically less than 20 MWe, and NuScale describes up to about 50 MWe.
What fuel do microreactors use?
DOE says most microreactor designs will need fuel with a higher concentration of uranium-235 than today's reactors, which in practice means HALEU. Heavy-water-moderated designs can run on natural uranium instead.
Where would a microreactor be used?
Public sources point to remote communities, military bases, mines, industrial sites, disaster recovery and, increasingly, co-located industrial and data-centre loads.