Small modular nuclear reactors

Also known as Small modular reactors, SMRs

Small modular reactors are fission reactors with a smaller output than conventional nuclear plants, intended to be built partly in factories. Regulators have licensed a first construction project in Canada and approved a design in the United States. The sources reviewed here give no verified operating or cost figures for these designs.

Pilot and early commercialDemonstratedReviewed 21 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

A small modular reactor, or SMR, is a nuclear reactor that produces far less power than a large nuclear plant. The idea is that many of its parts could be made in a factory and assembled on site, which might reduce the time and cost of building a plant.

The two designs in the regulatory record used here are rated at 300 megawatts electric (the GE Hitachi BWRX-300) and 77 megawatts electric (NuScale). The sources reviewed here do not give verified cost figures for either design, so whether a small reactor costs less per unit of power than a large one remains an open question.

Technical explanation

The Darlington project in Ontario is licensed for one GE Hitachi BWRX-300, a water-cooled reactor rated at 300 MWe, and the Canadian Nuclear Safety Commission (CNSC) issued the construction licence in April 2025. The licence runs until 31 March 2035. The project plan allows for up to four units on the same site, so the licence covers only the first reactor. The CNSC describes the BWRX-300 as using natural circulation, which means the coolant moves without a pump.

The US Department of Energy reported on 30 May 2025 that the Nuclear Regulatory Commission (NRC) had approved NuScale Power's uprated design. The approval covers a module rated at 77 MWe, which the DOE describes as the second SMR design the NRC has approved. A design approval is not the same as a licence for a specific plant, so each project still needs its own approval to build and operate.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

In April 2025 the Canadian Nuclear Safety Commission issued Ontario Power Generation a licence to construct one GE Hitachi BWRX-300 at the Darlington site, valid until 31 March 2035.

Sources: [1], [2] · Decision summary

The US Department of Energy reported on 30 May 2025 that the NRC had approved NuScale Power's 77 MWe reactor design, the second SMR design the agency has approved.

Sources: [3] · Article text

The CNSC describes the BWRX-300 as a water-cooled small modular reactor that uses natural circulation.

Sources: [2] · Project page

Proposed A design or plan that has not yet been built or tested.

The Darlington project plan allows for up to four reactors on the site, but the April 2025 licence covers one.

Sources: [2]

Limitations and open problems

  • The Darlington BWRX-300 is licensed for construction only, so no operating record exists yet for this design at that site.
  • Smaller output per unit means that several units are needed to match the output of one large plant.
  • Each project needs its own licence for a specific site, even when the reactor design has already been approved.
  • The sources reviewed here do not give verified cost figures for any SMR design.

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • Supplying heat and electricity to industrial sites, such as chemical works, that need both at once.
  • Replacing retired fossil fuel plants on existing grid sites, where the connections and cooling infrastructure may already exist.
  • Providing power to remote communities or mining sites that cannot easily connect to a large grid.

Product concepts: Factory-built reactor module for an isolated town

Timeline

  1. Feb 2020Mignacca and Locatelli publish a systematic review of SMR economics and finance in Renewable and Sustainable Energy Reviews. Sources: [4]
  2. 4 Apr 2025The Canadian Nuclear Safety Commission issues Ontario Power Generation a licence to construct one BWRX-300 at the Darlington site. Sources: [1], [2]
  3. 30 May 2025The US Department of Energy reports that the NRC has approved NuScale Power's 77 MWe reactor design, the second SMR design the agency has approved. Sources: [3]

Futures this could help bring about

AI illustration
How far away?

A permanent base on the Moon

Engineering prototypes · 2 of 6 steps done

Related breakthroughs

Glossary

Megawatt electric (MWe)
A unit of electrical power output. One megawatt is one million watts.
Construction licence
Permission from a national regulator to build a specific nuclear plant at a named site.
Standard design approval
A regulator's finding that a reactor design meets its safety requirements, which a later applicant can refer to when applying for a licence.
Co-siting
Building several reactor units on one site, so that they can share infrastructure.

Sources

  1. government report
    Commission authorizes Ontario Power Generation Inc. to construct 1 BWRX-300 reactor at the Darlington New Nuclear Project site
    Canadian Nuclear Safety Commission, 4 Apr 2025. Accessed 21 Oct 2026.
  2. government report
    Darlington New Nuclear Project
    Canadian Nuclear Safety Commission, 2025. Accessed 21 Oct 2026.
  3. government report
    NRC Approves NuScale Power's Uprated Small Modular Reactor Design
    U.S. Department of Energy, Office of Nuclear Energy, 30 May 2025. Accessed 21 Oct 2026.
  4. review article
    Economics and finance of Small Modular Reactors: A systematic review and research agenda
    Mignacca, Locatelli. Renewable and Sustainable Energy Reviews, Feb 2020. DOI: 10.1016/j.rser.2019.109519. Accessed 21 Oct 2026.

Revision history

1 change since this profile was created.

Changes on 21 Oct 2026
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