Small modular reactors (SMRs) are nuclear reactors that are generally defined as producing up to 300 megawatts of electricity (MWe) each. They are designed to be built in factories and assembled on site, allowing utilities to add units as demand grows. The 300 MWe threshold is used by the World Nuclear Association and in peer-reviewed research, though some definitions stretch it to 600 MWe.
While often grouped together, SMRs cover a wide range of technologies. These include light water reactors, fast neutron reactors, graphite-moderated high-temperature gas reactors and molten salt reactors (also discussed for thorium fuel). More than 100 designs are in development, but only a handful are operating or under construction. For context, each EPR unit at Hinkley Point C generates about 1,630 MWe.
How it works
The core principle of an SMR is modularity. Instead of building massive components on site, the reactor vessel and primary systems are fabricated in controlled factory environments. These modules are then transported to the site and assembled. This approach aims to reduce construction time and lower financial risk by allowing developers to build one unit at a time, scaling up as the local grid requires more power.
Safety is another defining feature. Many SMR designs use passive safety systems. These rely on natural physical principles, such as gravity, natural circulation and convection, to cool the reactor core without requiring external power or human intervention. For instance, the BWRX-300 is a natural-circulation design. Developers say this reduces the complexity of emergency systems and the scope for human error during an incident.
However, “modular” does not always mean “small” in absolute terms. The Rolls-Royce SMR, for example, is a 470 MWe pressurised water reactor. It is small only relative to gigawatt-scale plants, not by the common 300 MWe definition.
Real-world examples
As of October 2026, no Western SMR is in operation.
Operating plants
- Akademik Lomonosov (Russia): A floating plant at Pevek, Chukotka. It houses two KLT-40S reactors of 35 MWe each (32 MWe net). It connected to the grid in December 2019 and entered commercial operation in May 2020.
- HTR-PM (China): Located at Shidaowan, Shandong. This plant uses two high-temperature gas-cooled reactor modules to drive a single turbine of about 200 MWe (210 MWe turbine, 150 MWe net). It reached commercial operation in December 2023, and a district heating system supplying about 1,850 households was added in March 2024. China is also building a larger version, the HTR-PM600 (600 MWe per unit), at Xin’an.
Under construction
- Linglong One (China): The ACP100 at Changjiang, Hainan. Construction started in July 2021 and the plant is in commissioning, with commercial operation expected in 2026. As of October 2026 there was no public report of first criticality or grid connection.
- Darlington (Canada): Ontario Power Generation is building four BWRX-300 units of about 300 MWe each. The final investment decision came in May 2025 and the first reactor basemat was placed in April 2026; OPG plans commercial operation of the first unit in October 2030.
Contracted, not yet built
- Wylfa (UK): The UK government has chosen Wylfa, Anglesey, for Rolls-Royce SMRs. The contract signed in April 2026 covers three 470 MWe units (about 1.4 GW in total). A final investment decision is expected in 2029, with power to the grid in the mid-2030s. The plant is to be called Gwyndod Power Station.
What the numbers say
The economics of SMRs are still being tested. While manufacturers claim lower upfront costs, early construction data shows high first-of-a-kind costs.
At the Darlington site in Canada, the first unit is expected to cost C$6.1 billion, plus about C$1.6 billion for systems shared by all four units. The total project envelope is C$20.9 billion for roughly 1,200 MWe of capacity, or very approximately C$17 billion per GW. For a rough sense of scale, Sizewell C in the UK has a target cost of about £38 billion (2024 prices) for 3.2 GW; the currencies and price bases differ, so this comparison is indicative only.
In the UK, the government allocated £2.6 billion in the 2025 Spending Review for the Wylfa contract and wider programme delivery. The National Wealth Fund is providing a facility of up to £599 million to Rolls-Royce SMR. The Welsh Secretary cited “over £2.5 billion”; both are government funding figures, not the build cost, which is unpublished. The project is expected to create up to 3,000 jobs at peak construction.
Limitations / honest assessment
Several important issues remain unresolved.
Cost and commercial risk No Western SMR has yet produced operating cost data. The cancellation of NuScale’s Carbon Free Power Project in the US in November 2023 shows the commercial risk: not enough utilities signed up, after the project’s 2022 power price estimate rose to $90–100/MWh. In the UK, Holtec scaled back and delayed its planned South Yorkshire factory after Rolls-Royce SMR was chosen.
Waste and fuel A 2022 peer-reviewed study in PNAS found that water-cooled, molten salt and sodium-cooled SMR designs would increase the volume of nuclear waste needing management and disposal by factors of 2 to 30 compared with a large 1,100 MWe pressurised water reactor, mainly because smaller cores leak more neutrons. Developers dispute such conclusions, so this is one study’s finding, but SMRs cannot be assumed to produce less waste than large reactors.
Additionally, many advanced (non-light-water) SMR designs require High-Assay Low-Enriched Uranium (HALEU), enriched to 5–20%. Only Russia and China produce HALEU at scale. Centrus Energy began demonstration-scale production in the US in October 2023, but supply chains are not yet established.
Timelines SMRs are not quick to build. The Rolls-Royce SMR entered the UK’s Generic Design Assessment (GDA) in 2022. With a final investment decision expected in 2029 and power to the grid in the mid-2030s, it will take more than a decade from the start of regulatory assessment to first power. Darlington’s first unit is planned to take about five years from construction licence to operation. Both exceed earlier claims that SMRs could be installed within three to five years.
The UK angle
SMRs are one part of UK plans to replace an ageing fleet. Four of the five existing nuclear stations, the advanced gas-cooled reactor (AGR) stations, are due to close by March 2030. In 2025 nuclear supplied 12.2% of UK electricity (see the live GB electricity mix for today’s picture). The government’s target is up to 24 GW of nuclear capacity by 2050, about 25% of electricity.
The Wylfa project, with three Rolls-Royce units providing about 1.4 GW, is a small part of this 24 GW target and will not arrive before the mid-2030s. Great British Energy – Nuclear (GBE-N) was also tasked with finding sites for another large, Hinkley-scale project, reporting by autumn 2026.
The ONR and the Environment Agency issued GDA statements for GE Vernova Hitachi’s BWRX-300 in December 2025 and Holtec’s SMR-300 in March 2026. These were GDA statements, not the full Design Acceptance Confirmations granted to large designs. The Rolls-Royce SMR is in the final step of the UK’s GDA, which began in July 2024; no Design Acceptance Confirmation had been announced as of October 2026. Separately, the government granted “regulatory justification” for the design in March 2026. For the wider trade-offs, see the pros and cons of nuclear energy.
Frequently asked questions
What does SMR stand for?
SMR stands for small modular reactor. In hydrogen production it can also mean steam methane reforming, but in discussions of nuclear power it refers to the reactor.
How big is a small modular reactor?
SMRs are generally defined as 300 MWe or less per unit, though some definitions extend to 600 MWe. Microreactors are smaller, typically under about 20 MWe. The Rolls-Royce SMR is 470 MWe, so it is small only relative to gigawatt-scale plants.
Are there any small modular reactors operating today?
Yes, but not in the West. Russia’s Akademik Lomonosov has been in commercial operation since 2020, and China’s HTR-PM since 2023. China’s Linglong One is in commissioning, with commercial operation expected in 2026, but no grid connection had been reported as of October 2026.
Where will the UK’s first SMRs be built?
At Wylfa on Anglesey, North Wales. The contract covers three Rolls-Royce SMR units; a final investment decision is expected in 2029, with power to the grid expected in the mid-2030s.
What are the disadvantages of small modular reactors?
Key disadvantages include unproven costs, first-of-a-kind construction risks and research findings of larger nuclear waste volumes. They also face long regulatory timelines and, for some advanced designs, reliance on scarce HALEU fuel.
How much does an SMR cost?
Costs vary by design and location. The first Darlington unit in Canada is estimated at C$6.1 billion, plus C$1.6 billion of shared systems; the four-unit project is C$20.9 billion. In the UK, the government allocated £2.6 billion for the Wylfa contract and wider programme, but no build cost per unit has been published.