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Small modular reactors

What makes a reactor a small modular reactor, where the IAEA draws the size lines, why size is judged per unit rather than per plant, which ones actually operate, and why their cost claims are not yet proven.

Guide · Updated

A small modular reactor, or SMR, is a fission reactor built smaller than a conventional power reactor and designed so that most of it can be made in a factory. The term describes a size and a way of building. It does not describe a technology: SMR designs include pressurised water, boiling water, gas-cooled, sodium, lead and molten-salt reactors. The reactor types guide explains each of those.

What “modular” means

The word is used in two senses, and designs usually lean on both.

  • Factory-built modules. Large reactors are mostly built on site, one-off, over many years. An SMR is meant to be made largely in a factory as modules sized to travel by road, rail or sea, then assembled on site. The aim is to move work from the construction site, where it is slow and hard to control, to a production line, where it can be repeated.
  • Several units in one plant. A power station can be built from several identical reactor units, added over time as demand grows. NuScale’s plant design uses up to 12 power modules of 77 MWe each, for up to 924 MWe. Kairos Power describes a standard plant of six reactors with a combined 450 MWe.

The size lines

The International Atomic Energy Agency (IAEA), the United Nations body for nuclear energy, describes SMRs as “advanced reactors with a power capacity of typically up to 300 MW(e) per unit, which is about one-third of the generating capacity of traditional nuclear power reactors and whose components and systems can be shop fabricated and then transported as modules”. Within that, it describes microreactors as “very small SMRs designed to generate electrical power of typically up to 10 MW(e)”. MWe means megawatts of electricity, as distinct from MWt, megawatts of heat.

Not everyone uses the same lines. The World Nuclear Association describes SMRs as “generally 300 MWe equivalent or less” and says reactors of “typically less than 20 MWe” are sometimes called microreactors. The word “typically”, which appears in the IAEA’s wording too, is doing real work: these are conventions, not legal categories.

This site uses the IAEA lines:

Class on this siteRated output per unit
Microreactorup to 10 MWe
Small modular reactorabove 10 and up to 300 MWe
Large reactorabove 300 MWe

Per unit, not per plant

Size is judged by the output of one reactor unit, not the whole station. The reason is that the claims made for SMRs, about factory building, transport, passive cooling and smaller safety zones, all depend on the size of a single reactor. A 12-module NuScale plant at 924 MWe is as large as many conventional stations, but each of its reactors is 77 MWe, and each one’s decay heat, pressure vessel and shipping weight are those of a 77 MWe unit.

The reverse also happens. Several designs sold as SMRs are rated above 300 MWe per unit. In the IAEA’s own 2024 SMR catalogue, the Rolls-Royce SMR is listed at 470 MWe, TerraPower’s Natrium at 345 MWe, Westinghouse’s AP300 at 330 MWe, Afrikantov OKBM’s floating VBER-300 at 325 MWe and Holtec’s SMR-300 at 320 MWe. This site classes each by its stated rating, so those five are large reactors here, whatever the marketing says.

How this site assigns a size class

The size class is derived, never typed in. It is computed each time the site is built from the electrical rating the developer has stated for one unit of the design, recorded as an electrical capacity measurement. Three rules follow.

  • A design with no stated rating has no size class. It is shown as “not stated”, not guessed.
  • For a reactor that has not been built, the rating is a design target. It is recorded as planned, never as achieved.
  • If a developer changes its rating, the class changes with it. Ratings do change. Oklo’s Aurora appears in the same 2024 IAEA catalogue at 1.5 MWe in one entry and at 15 and 50 MWe in another, and Oklo’s own filing for the quarter to 30 September 2024 describes it as producing up to 15 and 50 MWe. Oklo’s annual report for 2025 gives a range of 15 to 75 MWe, which is the rating this dataset now holds.

One caution: developers do not always say whether a rating is gross (at the generator) or net (after the plant’s own consumption). The difference can be large. The IAEA catalogue lists Holtec’s SMR-300 at 366 MWe gross and 320 MWe net, a gap of 46 MWe. It matters most for designs close to a boundary.

Operating and under construction

The IAEA’s 2024 catalogue describes 71 designs across its six parts. Very few have been built.

  • Akademik Lomonosov, Russia. A floating plant with two KLT-40S pressurised water reactors of 35 MWe each, moored at Pevek in Chukotka. According to the IAEA catalogue it was connected to the grid on 19 December 2019 and fully commissioned on 22 May 2020.
  • HTR-PM, China. A helium-cooled pebble-bed plant at Shidao Bay in Shandong province: two 250 MW thermal reactor modules feeding one 210 MWe turbine. Its first reactor reached criticality in September 2021 and was connected to the grid on 20 December 2021, and the plant reached initial full power on 9 December 2022.

Others are under construction or approaching operation:

  • ACP100 (Linglong One), China. A 125 MWe pressurised water reactor at Changjiang, Hainan, whose first concrete was poured in July 2021. The China National Nuclear Corporation (CNNC) completed a non-nuclear steam start-up test on 23 December 2025. Reuters reported in December 2025 that power generation was expected in the first half of 2026. This guide has not confirmed whether that date was met.
  • BREST-OD-300, Russia. A 300 MWe lead-cooled fast reactor under construction since 8 June 2021.
  • CAREM, Argentina. A 30 MWe pressurised water prototype near Zárate, listed by the IAEA catalogue as under construction. In September 2024 Argentina’s National Atomic Energy Commission (CNEA) put the building stage on hold for an engineering review, after contractors laid off workers for lack of government funding.
  • BWRX-300, Canada. Ontario Power Generation received a licence to construct one at Darlington on 4 April 2025.
  • Natrium, United States. TerraPower received a construction permit for Kemmerer Unit 1 in Wyoming on 9 March 2026. At 345 MWe it is a large reactor by this site’s rule, although it is widely described as an SMR.

In the United States, NuScale’s design approvals are the furthest advanced of any SMR developer’s in this dataset: the US Nuclear Regulatory Commission’s certification of its 50 MWe-per-module US600 design took effect on 21 February 2023, and the larger 77 MWe US460 received a standard design approval on 29 May 2025. Neither has been built. The licensing page lists these milestones with their docket references, and the licensing guide explains what each stage does and does not allow.

Why cost claims are unproven

The case for SMRs is mainly economic. SMR developers argue that factory production and repeated builds will bring costs below those of large reactors built one at a time. The IAEA’s catalogue puts it carefully: SMRs “show the promise of significant cost reduction through modularization and factory construction”.

The difficulty is that this is a promise about the tenth or hundredth unit, and no SMR design has yet been built as a series of factory-made units. Small reactors also give up economies of scale: a reactor vessel, a control room and a security force do not shrink in proportion to output. Whether factory learning wins out over that loss is the question the industry has not yet answered with built plants.

The one concrete American test so far did not reach construction. The Carbon Free Power Project, a NuScale plant planned by Utah Associated Municipal Power Systems (UAMPS) at Idaho National Laboratory and reduced to six 77 MWe modules (462 MWe), was terminated on 8 November 2023 because, in the partners’ words, “it appears unlikely that the project will have enough subscription to continue toward deployment”.

A cost per megawatt-hour quoted for an SMR that has not been built is a projection. The dataset records it as a company’s claim, and a reader should treat it as one.

Further reading on this site

Sources

Related pages

More in Modular reactors

  • Microreactors What microreactors are, how heat-pipe, gas-cooled and sodium-cooled designs work, what they are meant for, why most need HALEU or TRISO fuel, and where the leading projects actually stand.

All guides · Modular reactors