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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.

Guide · Updated

A microreactor is a fission reactor small enough to be built in a factory and shipped whole, or nearly whole, to where its power is needed. The International Atomic Energy Agency (IAEA) describes microreactors as “very small SMRs designed to generate electrical power of typically up to 10 MW(e)”, and its 2024 catalogue gives their thermal power as typically up to 30 MW. The US Department of Energy describes microreactors by heat output instead, as producing 1 to 20 MW thermal. For comparison, the World Nuclear Association’s count of 441 operable power reactors with about 404 GWe of capacity gives an average of about 920 MWe per reactor.

This site classes a design as a microreactor when its developer’s stated rating for one unit is 10 MWe or less. The class is computed from that rating; a design with no stated rating has no class. The small modular reactors guide explains the size lines and why they are drawn per unit.

What they are for

The IAEA catalogue describes the intended market as “niche electricity and district heat markets in remote regions, mining, industries and fisheries that for decades have been served by diesel power plants”. Three uses recur in company and government material.

  • Remote sites. Mines, islands and northern communities that now burn diesel delivered by road, sea or air.
  • Military use. The US Department of Defense’s Project Pele aims to demonstrate a transportable reactor for military power.
  • Data centres and industry. Several developers now market microreactors, often in clusters, as on-site power for data centres and heat for industrial plants.

Heat-pipe, gas-cooled and liquid-metal designs

Heat-pipe reactors. A solid block, usually graphite, holds the fuel and is threaded with sealed heat pipes. Liquid sodium inside each pipe evaporates at the hot end, condenses at the cold end and runs back by capillary action. There are no pumps, no pressurised coolant and no coolant loop. Westinghouse’s eVinci is the best-documented example: 5 MWe from a 15 MW thermal core, with TRISO fuel enriched to 19.75%, designed to run eight or more full-power years before refuelling and to be transported, fully factory-assembled, in shipping containers by rail, barge and truck. See heat-pipe.

Gas-cooled reactors. Helium carries heat from a graphite core fuelled with TRISO particles, as in a larger high-temperature gas-cooled reactor, but at a far smaller scale. BWXT’s Project Pele reactor, rated at 1.5 MWe, is of this type.

Liquid-metal-cooled reactors. A few designs pump liquid sodium or another liquid metal through the core, as a larger liquid-metal-cooled reactor does. Idaho National Laboratory’s MARVEL research microreactor, rated at about 15 kWe, uses a liquid-metal coolant with a thermal (slowed) neutron spectrum and is listed by the IAEA as under construction.

Why the fuel is different

Most microreactors use one or both of two fuels that the existing reactor fleet does not.

HALEU. High-assay low-enriched uranium contains between 5% and 20% uranium-235, against the 3% to 5% of ordinary power reactor fuel. A small core loses a larger share of its neutrons out through its surface than a large one does, because a smaller object has more surface for its volume. To stay critical, and to keep running for years without refuelling, the fuel must contain more fissile material. eVinci’s 19.75% is just under 20%, the level at which uranium becomes classed as highly enriched.

TRISO. Tri-structural isotropic fuel particles, each about the size of a poppy seed, contain a uranium kernel sealed in layers of carbon and silicon carbide. The US Department of Energy reports that irradiated TRISO particles came through more than 300 hours of testing at temperatures up to 1,800 °C with no to minimal damage. Each particle acts as its own containment, which is the basis for designs that rely on the fuel rather than a large containment building.

HALEU is the binding constraint. The US Department of Energy says it “is not currently available from domestic suppliers”, and the World Nuclear Association names Russia’s Tenex as the only commercial supplier worldwide. The nuclear fuel guide sets out the supply position in detail.

Decay heat at small scale

A reactor keeps producing heat after shutdown from the decay of its fission products: about 6.5% of its previous power at the moment of shutdown, and about 1 to 1.5% after an hour. For eVinci’s 15 MW thermal core that is roughly 1 MW at shutdown (15 × 0.065) and 0.15 to 0.2 MW an hour later. For a large 3,000 MW thermal reactor the same fractions give about 195 MW and 30 to 45 MW. The smaller absolute figure is what makes it credible for a microreactor to shed its decay heat by conduction and natural airflow alone, with no pumps. Whether a particular design does so is a matter for its own analysis, not its size alone. The how fission reactors work guide explains decay heat and passive safety.

Project status

None of the power-producing microreactors named below had delivered electricity to a customer on the sources listed at the end of this guide. Figures from companies are their own claims.

  • Project Pele (BWX Technologies, for the US Department of Defense). A 1.5 MWe high-temperature gas-cooled reactor. Its full core of 40,000 TRISO fuel compacts was delivered to Idaho National Laboratory on 5 November 2025, while the reactor itself was being built at BWXT’s Lynchburg, Virginia facilities. BWXT plans to begin formal system testing as early as 2027 and to produce electricity at Idaho National Laboratory as soon as 2028. See BWX Technologies.
  • eVinci (Westinghouse). 5 MWe heat-pipe design, listed by the IAEA as under development. See Westinghouse.
  • Aalo Atomics. A sodium-cooled design. The company says its zero-power critical test reactor at Idaho National Laboratory first reached criticality on 3 July 2026, and describes Aalo-X, a planned 10 MWe plant on the same site, as its first integrated power plant. A zero-power reactor proves the core physics; it produces no electricity. See Aalo Atomics.
  • Radiant. Radiant’s website describes Kaleidos as a portable microreactor cooled by helium gas and fuelled with TRISO particles, producing “1MW electric”, and says “hundreds of units” could operate autonomously under central monitoring. This dataset does not yet hold that rating as a measurement, so Kaleidos has no size class here. See Radiant.
  • Oklo. Oklo’s Aurora appears in the IAEA’s 2024 catalogue as a 1.5 MWe microreactor, while Oklo’s own filing for the quarter to 30 September 2024 describes Aurora at up to 15 and 50 MWe, and its annual report for 2025 gives 15 to 75 MWe, all above the microreactor line. This dataset therefore classes Aurora as a small modular reactor. It is a sodium-cooled fast reactor, and Oklo’s first commercial deployment is at Idaho National Laboratory. See Oklo.
  • Others in this dataset. Antares Nuclear (sodium heat pipes, TRISO fuel), NANO Nuclear Energy (the KRONOS gas-cooled design and others), Jimmy (a gas-cooled reactor of 10 to 20 MW thermal for industrial heat, with no electrical rating) and Terra Innovatum (the SOLO gas-cooled design).

Microreactors have been demonstrated before at the smallest scale. KRUSTY, a heat-pipe space reactor producing a few kilowatts of heat, was tested at the Nevada National Security Site between November 2017 and March 2018. Scaling that up by a factor of more than a thousand, to megawatts, is the step the current projects are attempting.

The open questions

  • Cost. A microreactor gives up almost all economies of scale. The argument for it is that it competes with delivered diesel rather than grid electricity, and that factory production of many identical units will bring costs down. Neither has been shown with built units.
  • Fuel. Enough HALEU and TRISO for a fleet does not yet exist.
  • Operation. Many designs propose running with few or no staff on site and monitoring from a central control room. Whether regulators will accept that for a power reactor is still open.
  • Transport and return. A reactor shipped with fuel inside, and shipped back years later with used fuel inside, raises transport and security questions that a fixed plant does not.

Sources

Related pages

More in Modular reactors

  • 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.

All guides · Modular reactors