SMR types and what each one is for

Every reactor family is defined by what carries heat out of the core. Water, sodium, helium, molten salt or a sealed heat pipe. That one choice sets how hot the plant can run, what pressure it runs at, what fuel it needs and what jobs it is good for. The reactor types lesson explained the physics of each family at large scale. Here the question is narrower: in the small sizes, which designs exist and what are they for?

The families at a glance

Family	Named small designs	Fuel	Main selling point	Main open question
Light-water	NuScale US460 (VOYGR plants), GE Vernova Hitachi BWRX-300, Holtec SMR-300	Standard LEU, under 5% U-235	Decades of experience, existing fuel supply	Can small units beat big ones on cost?
Sodium-cooled fast	TerraPower Natrium, Oklo Aurora	HALEU metallic fuel	Low pressure, high temperature, can burn more of the fuel	HALEU supply; sodium reacts with water and air
Sodium-cooled thermal	Aalo-X	Uranium oxide (Aalo-1 spec)	Small, aimed at data centers	New design, first of its kind
Helium gas (HTGR)	X-energy Xe-100, Radiant Kaleidos, BWXT BANR and Pele	HALEU TRISO particles	Very hot output for industry; fuel that holds fission products at high temperature	TRISO and HALEU supply; bulky spent fuel
Salt-cooled, solid fuel	Kairos Hermes and Hermes 2	HALEU TRISO pebbles	Low pressure with high temperature	Salt chemistry and corrosion at commercial scale
Fuel dissolved in salt	Terrestrial Energy IMSR (Project Tetra), Natura MSR-1	Uranium in liquid salt	No fuel rods to melt; online fuel handling	Least operating history; liquid radioactive fuel to manage
Heat pipe	Westinghouse eVinci	HALEU	No pumps, very few moving parts	Scaling heat pipes; still in testing

One note on Oklo, because it trips people up. Aurora is often listed with heat-pipe microreactors, based on the small early version the NRC denied in 2022. The Aurora being built at Idaho National Laboratory is described by the NRC as up to 75 MWe and by ANS as a liquid-metal-cooled, metal-fueled fast reactor. Check which Aurora a document means.

Light-water SMRs: the familiar route

These are shrunken versions of the reactors that already make close to a fifth of U.S. electricity. NuScale puts each 77 MWe module in a large pool of water below grade, which serves as the heat sink in an accident. The NRC certified NuScale's earlier 50 MWe design by rule in January 2023 and approved the 77 MWe US460 in May 2025. The BWRX-300 is a boiling water design rated up to 300 MWe with passive safety systems and a 12 to 24 month refueling cycle, per GE Vernova. Holtec's SMR-300, about 340 MWe per unit, is planned in pairs at the Palisades site in Michigan.

Purpose: grid electricity, first and foremost. Water-cooled plants make steam at roughly 300 °C, fine for turbines and for low-temperature uses like district heating or desalination, too cool for most heavy industry. Their big advantage is boring in the best sense. The fuel is the same kind the existing fleet buys, the regulators know the physics and the supply chain exists. Utilities like TVA and OPG chose them for that reason.

Sodium-cooled reactors: heat storage and fast neutrons

Liquid sodium carries heat well and boils far above reactor temperatures, so the system runs near atmospheric pressure. That removes the need for a thick pressure vessel. Idaho's EBR-II ran on sodium for 30 years, and in 1986 tests it shut itself down with its pumps cut off, a result developers cite constantly (lesson 5 of the Nuclear Power course has the history).

Natrium's twist is storage. The 345 MWe reactor heats molten salt in tanks, and the plant can draw on that stored heat to push output to 500 MWe when demand peaks. That suits grids with a lot of solar, where demand on the rest of the system jumps at sunset. Oklo pitches Aurora for data centers and industrial customers who buy power directly. Aalo's stated plan is for Aalo-X to power a co-located data center.

The tradeoff: sodium burns in air and reacts violently with water, so steam generators and leak detection get extra design attention. And these designs need HALEU, which lesson 8 explains is scarce.

Helium gas reactors: industrial heat

Helium doesn't become radioactive easily and doesn't react chemically, and it can leave the core much hotter than water. Paired with TRISO fuel, poppy-seed kernels coated in carbon and silicon carbide, these plants can deliver steam hot enough for chemical plants. That is why Dow, a chemical company, is the first customer for X-energy's Xe-100. Its Long Mott Energy subsidiary applied for a construction permit for four units at Seadrift, Texas, in March 2025.

The same family dominates the microreactor end. Radiant's Kaleidos (1 MWe, helium-cooled, TRISO fuel) and BWXT's Pele and BANR designs are gas-cooled. The small ones are aimed at bases and remote sites.

Tradeoffs: helium must be kept from leaking at high temperature and pressure, and TRISO spent fuel takes up much more volume per unit of energy than conventional fuel (lesson 9).

Molten salt reactors: two very different ideas

"Molten salt reactor" covers designs that share a word and little else.

• Salt-cooled, solid fuel. Kairos Power circulates fluoride salt past TRISO pebbles. The salt runs hot at low pressure. Kairos is building the Hermes test reactor and broke ground on Hermes 2, aimed at 50 MWe, in April 2026, with Google and TVA tied to the output.
• Fuel dissolved in salt. The uranium is in the liquid. Terrestrial Energy signed a DOE agreement in January 2026 for Project Tetra, a 390 MWe molten salt plant at Texas A&M's RELLIS campus, and Natura Resources is working with Abilene Christian University on the MSR-1 research reactor, which holds an NRC construction permit.

Purpose: grid power and high-temperature heat. The fuel-in-salt branch has the least operating history of any family here. Oak Ridge ran the Molten Salt Reactor Experiment in the 1960s, and nothing commercial since.

Heat pipes and other microreactor ideas

A heat pipe is a sealed tube with a working fluid inside, usually a liquid metal for reactors. The fluid boils at the hot end near the fuel and condenses at the cool end, carrying heat with no pumps. Westinghouse's eVinci, rated 5 MWe and 15 MWt commercially, uses this approach. A zero-power criticality experiment on a scaled mock-up of its core, run at a federal criticality lab in Nevada, was reported in August 2026. Its 1 MWe prototype is slated for INL's DOME test bed in the 2027 to 2028 window.

Other approaches exist. DOE's MARVEL test reactor uses a sodium-potassium alloy moved by natural circulation. Deep Fission plans a 15 MWe reactor in a borehole in Kansas. Antares, Valar and Deployable Energy reached zero-power criticality with small test units in 2026.

Purpose by family

Job	Families that fit best	Why	Real example
Grid power	Light-water, sodium	Proven turbines; Natrium adds storage for peaks	TVA Clinch River, Darlington, Kemmerer
Industrial heat	Helium gas, salt	Hottest outlet temperatures	Dow's Long Mott plan with Xe-100
Data centers	All of them, in practice	Buyers want steady power more than a specific design	Amazon with X-energy, Google with Kairos, Meta with TerraPower and Oklo, Aalo's co-located plan
Remote sites	Microreactors: gas, heat pipe	Shipped whole, long time between refueling, replace diesel	Kaleidos and eVinci target markets
Military bases	Microreactors	Run when the grid is down	Army Janus: BWXT, Radiant, General Atomics, Antares, Westinghouse