How a small modular reactor (SMR) works

A small modular reactor is, by IAEA convention, a reactor of up to 300 MWe per module: a tenth or less of a large plant. What sets it apart is not only the size but the way it is built: instead of a single ten-year civil works project, modules manufactured in series in a factory, transported by land or water and assembled on site. A plant can have one, four or twelve modules and grow one at a time as demand appears.

The idea is not new: naval reactors and the prototypes of the 1960s were small. What changed is the context. The last large plants in the West (Vogtle, Flamanville, Olkiluoto) finished years late and at double or triple the budget, private capital does not want to finance US$15 billion projects, and new customers appeared, data centers and industry, that need hundreds of firm megawatts rather than gigawatts. The SMR promises to cut construction risk: license a design once, repeat it many times and let learning bring the cost down.

Most designs are integral PWRs like the one in the diagram: NuScale VOYGR (77 MWe per module), China's Linglong One ACP100 (125 MWe), Russia's icebreaker-derived RITM-200 (55 MWe), Korea's SMART and Argentina's CAREM-25 (32 MWe). Others keep the loops of a classic PWR but compact: Rolls-Royce SMR (470 MWe, at the upper limit of the category), Westinghouse's AP300 and INVAP's ACR-300. GE Vernova Hitachi's BWRX-300 is a small boiling water reactor, with no steam generators.

Beyond light water the range widens. Liquid-metal-cooled fast reactors: Natrium (sodium, 345 MWe, with molten-salt thermal storage) and BREST-OD-300 (lead). High temperature with helium and TRISO fuel: HTR-PM and Xe-100. Molten salts: Kairos Hermes, with fluoride salt as coolant, and China's TMSR-LF1, with thorium dissolved in the salt. Floating on barges: KLT-40S and RITM-200M. And microreactors under 20 MWe, such as Westinghouse's eVinci and Oklo's Aurora, meant for mines, bases and remote communities.

In a conventional PWR the core, the steam generators and the pressurizer are separate equipment joined by pipes nearly a meter in diameter. In an integral PWR everything goes inside the same vessel: the core at the bottom, a central riser, helical-tube steam generators in the annulus and a steam dome at the top acting as pressurizer. In many designs even the control rod drive mechanisms go inside, and the possibility of a rod ejection disappears.

Only small penetrations cross the primary: the feedwater inlet and the steam outlet of the secondary. With no large piping there is no large-break loss-of-coolant accident, the scenario that sizes much of the safety of a classic PWR. The vessel ends up tall and narrow, 10 to 20 m depending on power, and fits on a truck or a barge: that is the condition for building it in series.

Water heats up in the core, loses density and rises through the riser. At the top it turns outward, flows down the annulus where the steam generators sit, gives up heat, cools and re-enters the core from below. The density difference between the hot and cold columns is enough as the only driver if there is height, hence the riser, and the power is moderate: NuScale, CAREM-25 and the BWRX-300 run without coolant pumps.

Designs that prioritize power or compactness add pumps: the ACP100 carries them mounted on the vessel; the RITM-200 and the Rolls-Royce SMR use forced circulation like a large PWR. In all of them, natural circulation is reserved for what matters: removing decay heat with the reactor shut down and no electricity. It is a trade-off between fewer components that can fail and more megawatts per vessel.

A shut-down reactor keeps producing decay heat: close to 6% of power one second after shutdown and 1% an hour later. In a large plant removing it depends on pumps and diesel generators. In an SMR the power is small and the surrounding mass of water is proportionally large, so the heat can be removed by gravity and convection for days or indefinitely.

The recipes repeat: emergency or isolation condensers that condense the vessel's steam in a pool and return the water under its own weight; modules submerged in a large pool, as in NuScale; compact steel containments, with a suppression pool and largely underground, that also need less concrete and resist an external impact better. Fewer things that can fail, more time to react and an emergency planning zone that in some cases shrinks to the plant perimeter.

The economic problem is simple to state: per kilowatt, a small reactor costs more than a large one, because the vessel, the containment and the staff do not shrink in proportion. The SMR's bet is to swap economies of scale for economies of series: the twentieth module has to come out much cheaper than the first. That only holds with repeat orders, and the first of each kind is not one.

NuScale showed it in 2023: its six-module project in Utah was canceled when the estimated cost rose to US$9.3 billion for 462 MWe, about US$89 per MWh even with subsidies, and the electric cooperatives did not complete their subscriptions. Even so, licensing moves on: NuScale holds the only NRC-certified design; Canada granted the West's first SMR construction license to the BWRX-300 in 2025; the United States issued construction permits to Kairos (2023) and Natrium (2026); the United Kingdom chose the Rolls-Royce SMR in 2025.

In 2026 there are only two SMRs in commercial operation, both outside the West: Russia's floating plant Akademik Lomonosov, with two KLT-40S units at Pevek since 2020, and China's helium-cooled pebble-bed HTR-PM at Shidaowan, commercial since December 2023. Hainan's Linglong One, the first commercial land-based integral PWR, finished cold testing in October 2025 and is starting up in 2026.

Behind them come the construction sites at Darlington, Kemmerer, Seversk and Yakutia, and some twenty designs in licensing. Argentina has a foot in each group, with the suspended construction of CAREM-25 and INVAP's ACR-300 in design: CAREM-25 and ACR-300 will have their own page. The detail, project by project, in the table below.