Small Modular Reactors Explained: How They Work and Who Is Building Them

A small modular reactor (SMR) is a nuclear fission reactor with a capacity of up to 300 megawatts of electricity per unit. That is about one-third the capacity of a conventional reactor. An SMR's systems and components can be assembled in a factory. That is how the International Atomic Energy Agency (IAEA) describes an SMR. As of early October 2026, no commercial SMR is operating in the United States or Canada. But regulators in both countries have approved construction of the first units.

What is a small modular reactor?

The name describes three features. Small means a fraction of the size of a conventional plant. Modular means systems and components can be assembled in a factory and transported as a unit. Reactor means it uses nuclear fission, the splitting of heavy atoms such as uranium, to make heat.

Output is given in megawatts electric (MWe), the electrical power a plant can deliver. The IAEA treats microreactors, which typically generate up to 10 MWe, as a subset of SMRs.

How do small modular reactors work?

SMRs rely on the same chain reaction as existing plants. A neutron splits a uranium atom, which releases energy and more neutrons that split further atoms. Designs differ mainly in the coolant that carries heat away from the core.

Light-water designs use ordinary water. NuScale's US460 groups six 77 MWe modules into a 462 MWe plant. The US Nuclear Regulatory Commission (NRC) issued a standard design approval for it in May 2025. GE Vernova Hitachi's BWRX-300 is a 300 MWe boiling-water reactor.

Other designs use different coolants. TerraPower's Natrium is a 345 MWe sodium-cooled fast reactor with molten salt energy storage. The storage can temporarily raise output to 500 MWe. Both figures are above the IAEA's 300 MWe line. The Department of Energy calls Natrium an advanced reactor. X-energy's Xe-100 is a high-temperature gas-cooled reactor rated at 80 MWe per module. It is fueled by graphite pebbles containing coated particles known as TRISO.

Who is building small modular reactors?

Completion dates below are targets, not guarantees.

• Ontario Power Generation, Darlington, Canada. The Canadian Nuclear Safety Commission announced on April 4, 2025 that it would issue a license to construct one BWRX-300. OPG says it applied for an operating license in March 2026. It says it plans to connect the unit to the grid by the end of 2030.
• TerraPower, Kemmerer, Wyoming. The NRC announced its approval of a construction permit on March 4, 2026. Its staff issued the permit to a TerraPower subsidiary on March 9. The Department of Energy called it the first NRC construction permit for a commercial non-light-water power reactor. It said the project is expected to be completed in 2030.
• Tennessee Valley Authority, Clinch River, Tennessee. The NRC approved a construction permit for a BWRX-300 on September 29, 2026, after a 14-month review. It called the project the nation's first commercial SMR. TVA had not announced a construction start date, World Nuclear News reported.
• X-energy's Xe-100, Long Mott, Texas. Long Mott Energy applied on March 31, 2025, to build four modules totaling 320 MWe. The NRC's project page was last updated on September 14, 2026. It lists a final safety evaluation targeted for November 2026 and no date yet for a permit decision.

A US construction permit does not allow a plant to run. The NRC says TVA will need a separate operating license before loading fuel. The Department of Energy says TerraPower must apply for one too.

Projects can fail or slip. A six-module NuScale plant was planned in Idaho for Utah Associated Municipal Power Systems. It was terminated in November 2023 after the partners said it was unlikely to win enough subscribers. Outside North America, the IAEA says Russia's floating Akademik Lomonosov plant, with two 35 MWe units, began commercial operation in May 2020. China's 125 MWe Linglong One was slated in January for commercial operation in the first half of 2026, World Nuclear News reported. The outlet said it was still in pre-commissioning tests in mid-September.

Are small modular reactors safe?

The IAEA says proposed SMR designs are generally simpler than existing reactors and often rely on passive safety. It describes passive safety as systems that use natural circulation, convection and gravity to shut down the reactor without operator action or outside power.

Each design must still satisfy a regulator, and most have no operating record. Three questions remain open.

Waste. A 2022 Stanford-led study in the journal PNAS examined designs from NuScale, Toshiba and Terrestrial Energy. It concluded they would produce 2 to 30 times the waste volume of conventional reactors.

Fuel. The Department of Energy says most US advanced reactor designs need high-assay low-enriched uranium, enriched to between 5% and 20% uranium-235. In December 2024, it described that fuel as not yet available at commercial scale.

Cost. The IAEA says SMRs have yet to show in practice that they can compete on cost.

What to watch

Two regulatory markers come next. One is the NRC's Long Mott safety evaluation, targeted for November 2026. The other is the Canadian regulator's decision on OPG's operating license application. After that, the test is whether TerraPower, TVA and OPG keep to schedule and budget.