Small Modular Reactors, Explained: How SMRs Work and Which Projects Are Furthest Along in 2026
Small modular reactors (SMRs) are nuclear plants under 300 MWe built from factory-made modules. Six projects are now in construction or licensing worldwide.
A small modular reactor (SMR) is a nuclear fission reactor with an electrical output below 300 megawatts (MWe), built substantially from factory-made components rather than constructed piece by piece on site. The International Atomic Energy Agency (IAEA) uses this power threshold and the modular-construction criterion to define the category. As of mid-2026, no SMR is yet operating commercially, but several projects have moved from design review into construction or licensing in the United States, Canada, the United Kingdom, Romania, and Estonia.
What counts as a small modular reactor?
The International Atomic Energy Agency classifies SMRs as reactors rated below 300 MWe that apply modular design principles to individual components or to the entire reactor assembly. In the United States, the Nuclear Regulatory Commission (NRC) applies a narrower version of this definition: it reserves the “SMR” label for light-water reactors under 300 MWe, and classifies non-light-water designs — such as sodium-cooled or high-temperature gas reactors — as “advanced reactors” instead.
The concept is not new. Proposals for small, transportable nuclear power units date to the 1950s. The term “small modular reactor” became standard usage in policy circles after then US Energy Secretary Steven Chu described SMRs as “America’s new nuclear option” in 2010, and it has been used with increasing frequency since the 2020s as utilities and governments have sought lower-cost alternatives to large single-unit nuclear plants.
How SMRs differ from conventional nuclear plants
The core distinction is manufacturing method. Conventional nuclear plants are built largely on site, with components fabricated to a custom design for that single project. SMR designs shift as much fabrication as possible into a factory, then ship finished or near-finished modules to the site for assembly. Rolls-Royce describes its SMR design as approximately 90% factory-built, which the company says reduces on-site construction work and the schedule risk associated with it.
Most current SMR designs also incorporate passive safety systems, intended to bring the reactor to a safe shutdown state using gravity, natural convection, or stored pressure rather than external power or active operator intervention. Scalability is the third distinguishing feature: rather than sizing a single reactor to match expected demand, an operator can add modules incrementally, as NuScale’s design does with configurations of four, six, or twelve 77 MWe modules.
Which SMR projects are furthest along in 2026?
Six projects account for most of the confirmed construction and licensing activity as of mid-2026.
GE Vernova Hitachi BWRX-300, Darlington, Ontario, Canada. The Canadian Nuclear Safety Commission issued a licence to construct in April 2025 for a 300 MWe BWRX-300 unit at the existing Darlington nuclear site. It is the first SMR construction project under way in North America, with completion targeted for 2029–2030.
TerraPower Natrium, Kemmerer, Wyoming, United States. The Nuclear Regulatory Commission approved a construction permit for TerraPower’s sodium-cooled Natrium design in March 2026 — the NRC’s first construction permit approval for any commercial reactor in nearly a decade, and its first for a non-light-water design in more than 40 years. Construction began on site on April 24, 2026, at the location of a retiring coal plant. The design combines a 345 MWe reactor with molten-salt heat storage that can boost output to 500 MWe for brief periods. TerraPower expects construction to run through 2030; the project is supported by the US Department of Energy’s Advanced Reactor Demonstration Program.
NuScale Power US460, United States. The NRC approved Standard Design Approval for NuScale’s uprated US460 design on May 29, 2025 — a configuration of six 77 MWe modules producing approximately 460 MWe combined. NuScale remains the only SMR developer with an NRC-certified design.
Doicești, Romania. Romanian state nuclear operator Nuclearelectrica took a final investment decision on February 13, 2026, to build six NuScale modules (462 MWe total) at a former coal plant site in Doicești, roughly 90 kilometers northwest of Bucharest. Romania’s prime minister has put the project cost at USD 6–7 billion; the US Export-Import Bank has approved a USD 98 million loan for pre-project services, with potential further support of up to USD 1 billion from the US International Development Finance Corporation and USD 3 billion from the Export-Import Bank. Nuclearelectrica targets commercial operation of the first module in 2033.
Wylfa, Wales, United Kingdom. Great British Energy–Nuclear selected the Rolls-Royce SMR design as the UK’s chosen SMR technology on June 10, 2025. On November 13, 2025, the UK government named Wylfa, on Anglesey, as the site for three 480 MWe Rolls-Royce SMR units, a combined 1,440 MWe capacity the government says is enough to power about 1.5 million homes. The government pledged £2.5 billion for the programme over the current three-year spending period, subject to final agreement. On April 13, 2026, Rolls-Royce SMR signed a two-stage contract with Great British Energy–Nuclear covering site-specific design work, site-build preparation at Wylfa, and authorization to order long-lead-time equipment. No construction completion date has been set.
Estonia. Estonian utility Fermi Energia is pursuing two BWRX-300 units (600 MW combined), the same design under construction at Darlington. WINSS has previously covered Estonia’s move to explore small modular reactors for its green transition. Since that report, Fermi Energia signed an advisory-services agreement with TVO Nuclear Services and, in December 2025, launched a pre-feasibility study with South Korean government backing and technical input from Samsung C&T; results are due in April 2026. Estonia’s Fermi Energia currently projects site confirmation in 2027–2029, a construction-permit application in 2029, construction starting in 2031, and the first unit operating in the second half of 2035.
Cost and timeline uncertainty
None of the projects above has yet delivered power commercially, so cost figures remain project-specific estimates rather than confirmed outcomes. The International Energy Agency (IEA) publishes SMR construction cost projections by market through 2040, showing wide variation depending on region and how many units of a given design have already been built — a “first-of-a-kind” premium that developers expect to fall on subsequent units. The Romanian and UK projects illustrate this uncertainty directly: both remain in pre-construction phases with cost ranges rather than fixed contract prices, and neither has a confirmed date for first power.
The OECD’s cost benchmark for nuclear power
Cross-country nuclear cost comparisons are tracked by the OECD’s Nuclear Energy Agency (NEA), whose “Projected Costs of Generating Electricity” report, produced jointly with the International Energy Agency, is the standard reference for comparing nuclear generation costs against other technologies across OECD and partner countries. Because none of the SMR projects above has completed construction, current NEA cost benchmarks still rely primarily on conventional large-reactor data; SMR-specific comparative figures will only be possible once projects like Darlington and Kemmerer report final costs.
What remains unconfirmed
No SMR design has completed construction and entered commercial operation as of mid-2026. The BWRX-300 at Darlington and the Natrium reactor at Kemmerer are the two projects furthest along physically, but both are targeting completion around 2029–2030. Final costs, construction schedules, and the number of additional orders each design will attract remain open questions that will only be settled as these first projects are completed.
Sources: International Atomic Energy Agency, Nuclear Regulatory Commission, Great British Energy–Nuclear selected the Rolls-Royce SMR design, “Projected Costs of Generating Electricity”
Featured image: photo by Wolfgang Weiser on Pexels (free Pexels license).
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I specialize in sustainability education, curriculum co-creation, and early-stage project strategy. At WINSS, I craft articles on sustainability, transformative AI, and related topics. When I’m not writing, you’ll find me chasing the perfect sushi roll, exploring cities around the globe, or unwinding with my dog Puffy — the world’s most loyal sidekick.
