BasicsFour coolants decide almost everything about a small modular reactor: water, helium gas, liquid sodium and molten salt. 🔥 The coolant sets the operating temperature, the fuel, the safety case and, in the end, how fast a regulator can say yes. ⚙️ Pick the coolant and the rest of the design follows.
This guide sorts the field into those four families, with one real project per family and its current regulatory status. The small modular reactor label covers very different machines, and the coolant is the quickest way to tell them apart. 📌 Status checks draw on filings and regulator pages dated through mid-2026, and anything that may have moved since is flagged.
Light water: the familiar one
Light-water designs use ordinary water to carry heat away and slow the neutrons down. 💧 It is the same basic physics as the reactors that already supply most of the world’s nuclear power, so regulators know what to look for. That is why this family is furthest along in the West. 🏗️
GE Vernova Hitachi BWRX-300: a 300 MWe boiling water reactor. Canada’s CNSC issued Ontario Power Generation a construction licence in April 2025, valid through March 31, 2035. OPG filed for an operating licence on March 25, 2026, and the first unit targets completion by the end of 2030.
NuScale VOYGR: 77 MWe pressurized water modules, up to 12 per plant (924 MWe). The NRC approved the 77 MWe standard design in May 2025. The ENTRA1 and TVA deal for 6 GW, or 72 modules, still had no definitive power purchase agreement in NuScale’s second-quarter 2026 results.
Rolls-Royce SMR: a 470 MWe three-loop pressurized water reactor, picked for three units at Wylfa, Wales. The UK’s Generic Design Assessment was due to finish in August 2026. A Design Acceptance Confirmation from the ONR was not confirmed in the sources found, so check the regulator’s page.
Darlington has the concrete. NuScale has the paper. 🧾 Which of those two proves the economics first? Hold that thought, because the other three families make a different bet.
Helium and pebbles: high-temperature gas
Gas-cooled designs swap water for helium, which never boils and never corrodes anything. 🎈 The fuel is TRISO, tiny uranium kernels wrapped in ceramic layers and packed into tennis-ball-sized graphite pebbles. The payoff is heat hot enough to make industrial steam, not just electricity. 🏭
China’s HTR-PM at Shidao Bay, the first modular high-temperature gas-cooled reactor, entered operation in December 2023. The leading Western design is the X-energy Xe-100.
Project: four Xe-100 reactors at Dow’s Seadrift site in Texas, run through Long Mott Energy LLC.
Regulator: the NRC docketed the construction permit application in May 2025 for an 18-month review. In May 2026 it issued an environmental assessment with a Finding of No Significant Impact.
Timeline: X-energy’s 10-Q filing expects review completion by late 2026, with construction in 2027 and operations in the early 2030s.
Risk: if Dow does not proceed, X-energy has no obligation to keep funding the project and would need a new partner under its DOE agreement.
Liquid sodium and the battery trick
Sodium is a metal that melts at a bit under 100 °C and carries heat extremely well. 🧂 It runs at near-atmospheric pressure, so the vessel does not need the thick walls a water reactor does. These are fast reactors, meaning the neutrons are not slowed down. ⚡
TerraPower’s Natrium is a 345 MWe sodium-cooled fast reactor with a molten-salt storage tank bolted on. The tank lets output climb to 500 MW when demand peaks. 🔋 NRC commissioners voted in March 2026 to approve the construction permit for Kemmerer Unit 1 in Wyoming, the first of its kind in nearly 10 years. The permit allows building only, not operating.
NRC staff flagged some remaining areas of uncertainty in the design, which is normal for a first-of-a-kind plant.
TerraPower says construction starts within weeks, with commissioning aimed at 2030.
Meta agreed to help fund two more Natrium units totaling 690 MW, with delivery as early as 2032.
Molten salt: the odd one out
The name misleads. In Kairos Power’s KP-FHR, the salt is the coolant, not the fuel. 🧪 The fuel is solid TRISO pebbles that float in a fluoride salt. Like helium, the salt runs at low pressure, so there is no steam explosion scenario to design around. 🛡️
Kairos is building in steps. Hermes 1 is a 35 MWt test reactor in Oak Ridge, Tennessee, that will not make electricity. The NRC extended its construction deadline to April 30, 2029, though Kairos expects to finish in 2028. 📅 Hermes 2 broke ground April 17, 2026, with two 35 MWt reactors and a planned 50 MWe output under a TVA and Google power agreement. Kairos targets operations by 2030, per ANS reporting.
Coolant: fluoride salt, low pressure
Fuel: solid TRISO pebbles
First power: Hermes 2, around 2030
How the four compare on the calendar
Ranked by regulatory maturity, light water leads, sodium has the newest permit, gas has a permit in review, and salt is still proving its test reactor. 🏁 Every family has a first-of-a-kind risk. Light water carries cost risk. The other three carry technology and fuel-supply risk. 💰 Every project, deal, company, and regulatory event covered here is logged, verified, and searchable in SMRbrief Pro. 🔎
Fastest to power: BWRX-300 at Darlington, around 2030
Newest construction permit: Natrium, March 2026
Next permit decision: Xe-100 at Long Mott, late 2026
Longest road: Kairos Hermes 2, around 2030
The honest read is that nobody knows yet which family wins on cost. 🤔 Which one are you betting reaches the grid first, and which would you bet against?