Small modular reactor (SMR)
Also known as: SMR, small modular nuclear reactor
A small modular reactor (SMR) is a nuclear reactor that produces up to about 300 megawatts of electricity, roughly a third of a typical large reactor or less, and is designed to be built from standardized, factory-made components.
Supporters promise lower upfront costs, faster construction, and flexible siting. While SMRs have attracted huge interest from governments and tech companies, the promise of cheap, quick nuclear power still has to be proven on real construction sites.
Small modular reactor (SMR), explained at three levels
At home
Traditional nuclear plants are giant, custom-built projects. SMRs are smaller nuclear reactors designed to be built more like products, with many parts made in a factory and assembled on site.
At work
SMRs could provide steady, low-carbon electricity for grids, industrial sites, or remote communities. Their smaller size means lower upfront investment per project, though cost per megawatt is still uncertain.
At home
Many SMR designs use 'passive safety' features: they can cool themselves using natural forces like gravity and convection, without pumps or power, if something goes wrong. That's meant to reduce the risk of accidents.
At work
SMR designs range from light-water reactors similar to today's plants (BWRX-300, NuScale) to advanced designs using molten salt, high-temperature gas, or sodium cooling. Some need special fuel called HALEU, which has limited supply outside Russia. Licensing timelines and first-of-a-kind costs are major factors.
At home
Big reactors got big for a reason: economies of scale. SMRs bet that mass production can beat that advantage. In my opinion, that only works if the same design gets built many times, and the first few will almost certainly cost more than promised.
At work
Evaluate SMR proposals on fleet commitments (multiple units drive learning), supply chain readiness, fuel availability, regulatory approvals, and waste management plans. Microreactors under about 20 MW target remote mines and northern communities that rely on diesel. Watch whether announced data-centre deals translate into binding contracts and construction.
Real-world examples
At home
Thomas lives near Bowmanville, Ontario, and has watched construction begin at the Darlington site. He attended a community information session where OPG explained how the new reactor would differ from the larger units already operating there.
At work
Carlos is an energy planner for a mining company in northern Canada that runs on diesel. His team is studying microreactors as a future replacement, while noting that none are yet commercially operating in the country.
How it compares
| feature | Small modular reactor (SMR) | Large nuclear reactor |
|---|---|---|
| Output | Up to about 300 MW | About 1,000 MW or more |
| Construction approach | Factory-made modules | Custom site construction |
| Upfront cost per project | Lower | Very high |
| Track record | Limited | Decades of operation |
SMRs trade proven scale for the hope of faster, repeatable builds.
For analysts
SMR announcements are plentiful, but signed construction contracts are rare. I'd distinguish memorandums of understanding from firm orders when assessing any SMR claim, and ask who carries the cost-overrun risk.
History & origin
Small reactors have powered submarines and ships since the 1950s. Civilian SMR designs multiplied in the 2000s and 2010s. NuScale's design became the first SMR to receive US Nuclear Regulatory Commission design approval in 2023, but its first planned plant in Utah was cancelled that November after costs rose. Russia's floating Akademik Lomonosov began commercial operation in 2020. In Canada, Ontario Power Generation chose GE Hitachi's BWRX-300 for its Darlington site; the Canadian Nuclear Safety Commission issued a construction licence in April 2025, OPG made its final investment decision in May 2025, and the first unit is targeted for commercial operation around the end of 2029, with four units planned. Tech companies, including Google and Amazon, signed SMR deals in 2024 to power data centres. In my view, Darlington is one of the most important tests of whether SMRs can deliver on budget.