What Is a Microreactor? And How Is It Different From an SMR?
Nuclear's smallest reactors are having a moment, and the vocabulary is finally catching up to the hardware.
Ask five people in the nuclear industry to define a microreactor, and you’ll get five confident, slightly different answers. That’s not because anyone is being sloppy. It’s because small modular reactor and microreactor grew up together, got used interchangeably for a decade, and only recently split into their own lanes as actual steel started showing up in Idaho, Alaska, and Tennessee. ⚛️
If you’ve followed the nuclear revival even loosely, you’ve probably seen both terms in the same headline, sometimes the same sentence. That’s fair, since a microreactor technically is a type of SMR. But treating them as synonyms glosses over what actually separates them: where these things go, how fast they show up, and who’s writing the checks. Let’s sort it out.
So what actually makes a reactor micro
The simplest way in is size, even though size alone doesn’t tell the whole story. Idaho National Laboratory’s Gateway for Accelerated Innovation in Nuclear program draws its line at 50 megawatts electric (MWe): anything below that, and often below 10 MWe, counts as a microreactor, while SMRs run roughly 50 to 300 MWe. The World Nuclear Association frames it a bit differently, defining SMRs generally as 300 MWe or less and reserving “microreactor” for the smallest designs, typically under 20 MWe. Wikipedia’s own breakdown lands somewhere in between: commercial SMRs deliver anywhere from 10 up to 300 MWe per module, and anything under 10 MWe usually earns the micro label.
None of these numbers match exactly, and that’s honestly the point. 🔬 Nobody has agreed on one hard cutoff, so the same reactor gets called a microreactor in one report and a “very small SMR” in another. What everyone does agree on is the scale difference from what’s already running. According to Idaho National Laboratory, microreactors are 100 to 1,000 times smaller than a conventional commercial reactor in electrical output, small enough to sit on a few acres and power a factory or a few thousand homes instead of an entire city. That’s not a rounding error, it’s a different category of machine. 📏
SMRs: roughly 50-300 MWe, with some designs stretching toward 600 MWe
Microreactors: generally under 20 MWe, frequently under 10 MWe
Today’s average U.S. reactor: around 1,000 MWe
Demonstration microreactors like Project Pele: as little as 1.5 MWe
So next time someone throws around either term, ask them what output they’re actually talking about, in MWe, not vibes. It clears up more confusion than any dictionary definition. Curious how your own back-of-napkin guess compares to the industry’s? 💡
It’s not just about megawatts
Here’s where the distinction gets more interesting than a spreadsheet of wattages. 🧭 According to Wikipedia’s entry on nuclear microreactors, microreactors are built specifically to be transportable, meaning they can be moved by road, rail, or air, and they typically fall in the 1 to 20 MWe range compared to the 20 to 300 MWe window for SMRs. That mobility is a design goal, not an afterthought. An SMR is still, in most cases, a permanent power plant, just a smaller and more factory-built one than the 1,000-plus-MWe giants utilities have built for decades. A microreactor is closer to a shippable appliance. 🚚
Most microreactor designs share a few traits that set them apart operationally, not just numerically:
They ship as a single, pre-assembled unit rather than being built piece by piece on site
Many run on high-assay low-enriched uranium (HALEU), fuel enriched higher than what today’s commercial fleet uses
Several use TRISO fuel, tiny ceramic-coated uranium particles that are famously hard to melt down
They’re designed to run for years, sometimes a full decade or more, without refueling
Many can island themselves entirely off the electric grid, powering a single facility on their own
That last point matters more than it sounds. Per the U.S. Energy Information Administration, microreactors, as a subset of SMRs, generally sit at 20 MW or less and can run tied to the grid, disconnected from it entirely, or feeding a tiny local grid of their own. That flexibility is exactly why the Pentagon, remote mining operations, and increasingly data center operators have started paying attention. Nobody’s proposing to truck a 300-MWe SMR into a forward operating base or a hospital parking lot. A microreactor, at least on paper, could actually get there. 🌍
Who’s actually building these things
The company rosters for SMRs and microreactors overlap, but they’re not the same list, and it’s worth knowing who’s playing in which category. On the SMR side, you’ve got designs like X-energy’s Xe-100, already slated for a demonstration at Dow’s Seadrift site in Texas, GE Vernova Hitachi’s BWRX-300, Holtec’s SMR-300, and Westinghouse’s AP300, all sized in the tens to hundreds of megawatts and built for permanent grid connection.
Microreactor developers are a different crowd, and a scrappier one. 🚀 Oklo is developing its liquid-metal-cooled Aurora powerhouse and, according to Utility Dive, expects its combined licensing approach with the Nuclear Regulatory Commission to cut initial licensing timelines by 50 to 85 percent compared with a typical application. That’s not a small claim in an industry famous for permitting delays. Radiant Industries is building Kaleidos, a roughly 1-MWe reactor sized to fit inside a single shipping container, with testing at Idaho National Laboratory’s DOME facility targeted for this year. Westinghouse’s eVinci and BWXT’s Project Pele round out the field, alongside newer entrants like NANO Nuclear’s Kronos and Valar Atomics. If you’re tracking the nuclear market professionally, SMRbrief Pro gives you the structured database to go deeper than any single article can. Who do you think crosses the finish line to actual commercial operation first? 📈
Oklo: Aurora, a liquid-metal-cooled fast reactor targeting commercial operation at Idaho National Laboratory
Radiant: Kaleidos, 1 MWe, shipping-container form factor, aimed at diesel generator replacement
Westinghouse: eVinci, the first microreactor with an NRC-approved instrumentation and control system
BWXT: Project Pele, 1.5 MWe, built for the Department of Defense
NANO Nuclear: Kronos micro modular reactor, with a proposed unit at the University of Illinois
Worth noting: none of these companies are generating meaningful revenue yet from an operating reactor. Every one of them is still pre-revenue on the hardware itself, funded by federal contracts, venture capital, and increasingly by hyperscalers hungry for power. That’s not a knock, it’s just where the industry actually stands in mid-2026. Skepticism and optimism can coexist here. 🤔
The military is writing the microreactor playbook first
If you want to see where microreactors are actually closest to running, skip the utility press releases and look at the Pentagon. 🪖 Project Pele, run by the Department of Defense’s Strategic Capabilities Office, broke ground at Idaho National Laboratory in 2024 to build a transportable 1 to 5 MWe reactor designed to fit inside four 20-foot shipping containers and travel by truck or cargo plane, according to the Department of Energy. BWXT delivered the initial HALEU fuel load to the lab in December 2025, with testing scheduled for 2027.
That’s just one program among several. The Air Force picked Eielson Air Force Base in Alaska to pilot a fixed microreactor and issued a notice of intent to award the contract to Oklo for a liquid-metal-cooled design. The Army’s newer Janus Program has already identified nine candidate installations for reactor siting. And the Advanced Nuclear Power for Installations program has cleared eight companies, including Oklo, BWXT, General Atomics, Kairos, Radiant, Westinghouse, and X-energy, to compete for base-power contracts. In February 2026, a Valar Atomics microreactor even made the trip from California to Utah aboard Air Force C-17s, proving the transport concept works in practice and not just in a slide deck. ✈️
None of these are commercial power plants yet in the way a utility-scale SMR eventually will be. They’re demonstrations, prototypes, and pilot programs, funded because the military needs resilient power at remote bases faster than a decade-long licensing process can normally deliver. 🔋 But that urgency is exactly why microreactors are likely to hit commercial reality before most SMRs do. Which of these programs do you think actually reaches full operation first: Pele, the Eielson pilot, or one of the Janus sites? 🎯