The 5 types of small modular reactors, explained
Water, salt, gas, and metal all boil down to the same job, moving heat, but the differences decide who wins the next decade of nuclear power
Ask five nuclear engineers to name their favorite reactor and you will start a genuinely heated argument. 🔥 That is not a metaphor problem, it is the whole industry in miniature. Small modular reactors are not one technology wearing five outfits. They are five fundamentally different machines competing for the same job: making steady, carbon-free electricity in a box small enough to ship on a truck. Some use water. Some use molten salt. One uses liquid metal that would look at home in a blacksmith’s forge. Understanding which is which matters more than ever, because in 2026 this stopped being a science-fair debate and became a multibillion-dollar race with tech giants, utilities, and governments all picking sides.
The stakes are real. <cite index=”2-1”>The U.S. is leading the world in small modular nuclear reactor development, with 28 siting announcements as of 2026, more than the next four countries combined</cite>. So let’s break down the five reactor families actually competing for that ground, what makes each one tick, and who is betting big on them. ⚡
Light water reactors: the safe, boring, winning bet
Light water reactors, or LWRs, are the reactor world’s equivalent of a Toyota Camry. Not flashy, but everyone trusts them because they have logged decades on the road. 🚗 These designs use ordinary water as both coolant and moderator, the same basic physics running inside nearly every large power plant on Earth today.
That familiarity is the whole selling point. <cite index=”19-1,10-1”>Light water-cooled SMR designs are typically smaller versions of existing large reactor designs that use hydrogen in water as a moderator to slow neutrons and increase the odds of a fission event, and in most cases they run on the same low-enriched uranium fuel already used in U.S. reactors</cite>. Regulators already know how to evaluate this physics, which shaves years off the approval process compared to anything more exotic.
NuScale Power is the clearest example, with its VOYGR design already carrying NRC design approval, the first SMR to hit that milestone
Deep Fission is pursuing a borehole-based light water design meant to skip most above-ground construction entirely
China’s Linglong One (ACP100), a 125 MWe demonstration unit, is on track to become the world’s first land-based commercial SMR
The tradeoff is that light water designs cap out around 300 MWe, and they still need active cooling systems and pressurized components, which is exactly the complexity SMRs were supposed to shrink. Still, being first through licensing counts for a lot, and NuScale is already doing front-end engineering work with Romania’s RoPower on that country’s first SMR plant. 📈 If you want the full breakdown of who’s actually shipping product versus who’s still in the slide-deck phase, SMRbrief Pro tracks every licensing milestone as it lands.
High temperature gas-cooled reactors: heat that does double duty
Swap water for helium and you get a completely different animal. High temperature gas-cooled reactors (HTGRs) run hot, really hot, often above 750°C, which opens doors that water-cooled designs simply cannot reach. 🌡️
These reactors typically use TRISO fuel, tiny uranium kernels wrapped in ceramic and carbon layers that are individually almost indestructible, paired with graphite as a moderator. The payoff is versatility. Because HTGRs can deliver both electricity and industrial-grade process heat, they are a natural fit for the unglamorous but massive markets of steel, cement, hydrogen production, and desalination, not just the power grid. 🏭
X-energy’s Xe-100 delivers 80 MWe per module and is headed for a four-unit demonstration at Dow’s Seadrift plant in Texas, with construction expected to begin in 2026
Nano Nuclear Energy is developing a 15 MWe HTGR under a build-own-operate model
China’s HTR-PM, a pebble-bed design, is one of only two SMRs currently operational anywhere in the world
Amazon has also put money behind X-energy, including backing a four-unit Xe-100 project with Energy Northwest in Washington state. That is not a small tech company hedging its bets, that is a hyperscaler betting on process heat and grid power from the same reactor. The industrial-heat angle is probably the most underrated part of this whole category, honestly. Most coverage fixates on electricity, but decarbonizing a cement kiln is arguably the harder, more valuable problem. 💡
Molten salt reactors: liquid fuel, low pressure
This is where things get genuinely strange, in a good way. Molten salt reactors (MSRs) use, well, molten salt, either as a coolant around solid fuel or, in some designs, as the fuel itself, dissolved directly into the liquid. <cite index=”16-1”>This distinguishes molten salt reactors from designs that use liquid metal, gas, or water as coolants</cite>.
Why bother with something this unconventional? <cite index=”17-1”>Molten salt reactors use molten fluoride salts as primary coolant, at low pressure</cite>, which sidesteps a lot of the engineering headaches that come with keeping water pressurized at high temperatures. And because the fuel can be liquid, <cite index=”16-1”>MSRs can be refueled while operating, essentially performing online reprocessing, while conventional reactors have to shut down for refueling</cite>.
Terrestrial Energy’s Integral Molten Salt Reactor (IMSR) is a 195 MWe design moving through Canadian regulatory pre-licensing, with roughly $950 million committed
Moltex is pursuing the Stable Salt Reactor with a partnership tied to New Brunswick Power and Point Lepreau
Kairos Power, technically a fluoride salt-cooled variant, has a 500 MW deal with Google, one of the biggest single commitments in the sector
Canada in particular has become an MSR hub. It has the regulatory infrastructure, the workforce, and the political will, and it is not chasing headlines the way some U.S. players are. 🌍 The catch with MSRs is that almost none of them have run commercially at scale yet, so cost and reliability numbers are still mostly projections rather than track record.
Fast neutron reactors: skipping the slow-down step
Every reactor discussed so far relies on a moderator to slow neutrons down, because slow neutrons split uranium atoms more reliably. Fast neutron reactors throw that assumption out. <cite index=”13-1”>They use high-energy neutrons to split atoms rather than the slower thermal neutrons used in most commercial power plants, and they’re typically cooled with liquid metal, usually sodium or lead</cite>.
Ditching the moderator has real advantages. <cite index=”13-1”>Liquid metal-cooled fast reactors extract more energy from their fuel and reduce the amount of long-lived waste generated</cite>, which means these designs can burn fuel types, including some reprocessed waste, that conventional reactors cannot touch. ♻️
TerraPower’s Natrium pairs a 345 MWe sodium-cooled reactor with molten salt energy storage, is under construction in Kemmerer, Wyoming, and has an 8-plant, 2.8 GW deal with Meta
Oklo’s Aurora uses liquid-metal cooling in a compact 15-75 MWe design, holds a site use permit from the Department of Energy, and just locked in domestic HALEU fuel supply from Centrus for up to five units in southern Ohio
Newcleo is pursuing a lead-cooled fast reactor, having relocated its headquarters from the UK to Paris while chasing an Italian demonstration by the end of 2026
Oklo, in particular, has become the poster child for this category on Wall Street. Its market capitalization sits near $11 billion even before its first reactor achieves criticality, which tells you how much investors are pricing in future data-center demand rather than current output. 🚀 Whether that bet pays off depends heavily on whether the domestic HALEU supply chain, still dominated by companies like Centrus, can scale fast enough to feed all these fast reactors at once.
Heavy water and other niche designs
Beyond the big four sit a handful of other approaches worth a mention, mostly variants and hybrids rather than wholly separate categories. <cite index=”11-1”>The broader universe of SMR types also includes heavy water reactors</cite>, which use deuterium oxide instead of ordinary water as a moderator, a lineage that traces back to Canada’s CANDU reactors. Microreactors, generally defined as anything under 10 MWe, blur the line further, since some use light water internals while others borrow gas or metal cooling from the categories above. 🔬
Heavy water designs: better neutron economy, can run on natural uranium, but face a smaller commercial pipeline today
Microreactors: sub-10 MWe units aimed at remote sites, military bases, and mining operations rather than grid-scale power
Hybrid concepts: some developers are blending fast-spectrum physics with molten salt coolant, chasing benefits from both categories at once
None of these is likely to dominate the SMR market outright, but they matter because they show the field is still genuinely experimental. Nobody has locked in a winning formula yet. 🧬
So which type actually wins?
Honestly? Probably more than one. Light water designs will likely dominate the first wave of deployments simply because regulators already understand them. Gas-cooled and molten salt designs make more sense for industrial customers who need heat, not just electrons. Fast reactors are the ones chasing the biggest, splashiest data-center deals right now, Meta, Google, Amazon are all in somewhere on this list, which says a lot about where near-term capital is flowing. 📊
What is your read? Are you betting on the tortoise, light water’s regulatory head start, or the hare, fast reactors and their eye-popping hyperscaler contracts? I would genuinely like to hear pushback on this, because the honest answer is nobody, including people running these companies, knows for certain which technology wins the next decade. If you want to go deeper than any single article can, including deal-by-deal financing data and regulatory timelines across all five categories, that is precisely what SMRbrief Pro was built to provide.
One more wrinkle worth watching: fuel. Nearly every advanced design above, gas-cooled, molten salt, fast, leans on HALEU, and <cite index=”30-1”>at present only Russia and China have infrastructure to produce it at scale</cite>, with domestic U.S. supply still ramping. Track that bottleneck, via sources like the U.S. Energy Information Administration or the World Nuclear Association, and you will have a pretty good early signal for which reactor types actually get built this decade versus which stay stuck on the drawing board. For deeper background on the underlying physics, Wikipedia’s SMR overview is a solid starting point.