The Difference Between Fission and Fusion — And Why It Matters for SMRs
Every SMR operating or under construction today runs on fission — but fusion is coming for the crown.
If you follow nuclear energy closely, you’ve probably noticed something frustrating: the words “fission” and “fusion” get swapped, blurred, or casually conflated in headlines all the time. A politician promises “nuclear power” and means a uranium reactor. A tech CEO announces fusion investment and Twitter/X loses its mind. Meanwhile, the rest of us are left wondering: are these even related? Are they different? And what does any of it mean for the small modular reactor boom happening right now?
The short answer is yes, they’re related. And no, they’re not the same. And the distinction matters enormously, especially as billions of dollars pour into SMR development and a handful of fusion startups race to rewrite the rules entirely.
Let’s break it down properly.
What fission actually is
Fission is the process that powers every nuclear reactor on Earth right now. 🔬 It’s also the process that powered the first atomic bomb. These two facts live in awkward proximity to each other, which is part of why public opinion on nuclear energy has been complicated for decades.
The physics is elegant in a violent sort of way. A neutron slams into a heavy, unstable atom — almost always uranium-235 or plutonium-239 — and the nucleus can’t hold itself together. It splits. The split releases energy and kicks out two or three more neutrons, which then hit neighboring atoms, which then split, releasing more neutrons. This is a chain reaction, and if you let it run unchecked, you get a bomb. But in a reactor, you control it. Engineers insert control rods that absorb excess neutrons, keeping the reaction at a steady, manageable rate.
That heat — and there is a lot of it — boils water into steam. The steam spins a turbine. The turbine generates electricity. It’s basically the same process as a coal plant, except the heat comes from nuclear splitting rather than burning carbon.
Here’s what fission gives you:
Proven, deployable technology that’s been generating electricity since the 1950s
A reliable, carbon-free power source that runs 24/7 regardless of weather
Long-lived radioactive waste, some of which remains hazardous for thousands of years
A chain reaction that, if the control systems fail, can spiral badly — as Chernobyl and Fukushima proved
Fuel (uranium-235) that requires mining, enrichment, and careful handling before it’s usable
⚡ Every SMR currently in development or operation runs on fission. The NuScale VOYGR, TerraPower’s Natrium, GE Hitachi’s BWRX-300, Rolls-Royce’s UK SMR — all of them. Fission is the technology of the nuclear present.
Do you already have a clear mental model of fission and fusion? Or has the terminology always felt a bit slippery? Either way, stick with me — the contrast coming next is the key to understanding what the nuclear industry might look like in 2035.
What fusion actually is
Fusion is the opposite process. Literally the opposite. 🌞 Instead of breaking a heavy atom apart, fusion pushes two light atoms together until they merge into a heavier one. The sun does this every second of every day, fusing hydrogen into helium at temperatures of around 15 million degrees Celsius at its core. No one engineered the sun; gravity handled the confinement problem.
On Earth, the preferred fuel combination is deuterium and tritium — both isotopes of hydrogen. Deuterium is remarkably abundant; it’s in ordinary seawater. Tritium is rarer and slightly radioactive, but it has a short half-life and can be produced inside a fusion reactor itself. When deuterium and tritium fuse, they produce helium and a fast-moving neutron, and they release roughly four times more energy per unit of mass than uranium-235 fission, according to nuclear physicist analyses.
What fusion doesn’t produce is a long-lived radioactive nightmare. The primary byproduct is helium — the same inert gas that makes balloons float. The reactor walls get activated by neutron bombardment over time, but this material is low-level radioactive waste that decays in decades, not millennia.
The inherent safety case for fusion is also genuinely different from fission. Because fusion is:
Not based on a chain reaction, there’s no runaway cascading meltdown risk
Extremely sensitive to plasma conditions — too cold, too hot, wrong magnetic field, and the reaction simply stops
Described by the IAEA as self-terminating if something goes wrong
Reliant on continuous external heating and magnetic confinement, which must be actively maintained
🧪 The challenge isn’t making fusion safe — it already is. The challenge is making it work at all, and then making it economically viable. Plasma at 100 million degrees Celsius needs to be held in place by magnetic fields in a machine we’ve only recently gotten good at building. That’s an engineering problem of staggering complexity.
The key differences that actually matter
Okay, so fission splits, fusion joins. Got it. But for anyone thinking seriously about nuclear energy’s trajectory, the differences worth paying attention to go beyond the basic physics. 🔑
Energy density is the first big one. Fusion reactions, per kilogram of fuel, produce far more energy than fission. The Fusion Industry Association notes that fusion produces an enormous amount of energy relative to the tiny quantity of fuel consumed. Less fuel in, more energy out.
Fuel supply is the second. Uranium-235 — the fission fuel that actually works in reactors — is rare. It makes up less than 1% of natural uranium, so the rest has to be enriched, which requires special centrifuge facilities and careful international oversight. Deuterium, fusion’s primary fuel, is effectively inexhaustible; it’s found in every litre of ocean water on the planet.
Waste is the third and arguably most politically charged difference:
Fission generates high-level radioactive waste that must be safely stored for up to one million years — a fact that no country has fully solved yet
Fusion produces low-level activated materials from the reactor walls, which decay in decades and can potentially be recycled
Fusion’s main output is helium, which isn’t radioactive at all
Safety profile is the fourth. Fission’s chain reaction means a real, if manageable, risk of runaway events. Modern third-generation reactors — and SMRs in particular — use passive safety systems that rely on gravity and natural convection rather than active operator intervention. That’s a genuine improvement. But it’s not the same as fusion’s physics-based safety, where the reaction self-terminates the moment conditions slip even slightly.
None of this means fission is bad technology. It’s produced carbon-free electricity for 70 years. The point is that fusion, if and when it arrives, changes the entire calculus.
Why every SMR today is a fission machine — and what that means
Here’s something the breathless fusion coverage often glosses over: no commercial fusion reactor exists anywhere on Earth. Not one. The SMR industry — which is attracting record investment, regulatory attention, and Big Tech partnerships right now — is 100% fission. 🏭
According to the U.S. Energy Information Administration, SMRs are defined as nuclear fission reactors with a power output below 300 megawatts electric, built using modular factory-fabricated components. The modularity is the innovation. The fission is not.
What makes fission SMRs genuinely interesting, despite the familiar physics, is what they do differently from large conventional reactors:
Factory-built modules reduce construction time from the decade-plus typical of large plants to potentially 24 to 36 months
Passive safety features — like NuScale’s natural convection cooling — eliminate the need for operator action during emergencies
Smaller footprints allow SMRs to replace retiring coal plants, power AI data centers, or serve remote communities
Modular scaling lets operators add capacity incrementally, rather than committing to a single enormous build
In June 2025, TerraPower broke ground on its Natrium reactor in Kemmerer, Wyoming. X-energy closed a $700 million funding round to advance its Xe-100 gas-cooled reactor. NuScale announced a deal with the Tennessee Valley Authority for up to 6 gigawatts of SMR capacity. The industry is real, moving, and entirely powered by uranium splitting.
For now.
Fusion SMRs are coming — but “coming” is doing a lot of work in that sentence
The fusion industry insists it is not perpetually 20 years away anymore. They may be right, or they may be saying what venture capitalists need to hear. 🚀 The truth is probably somewhere in between.
Commonwealth Fusion Systems, spun out of MIT and backed by Google, Nvidia, and Bill Gates, has raised over $2 billion and signed a power purchase agreement with Google for 200 megawatts from its first ARC plant in Virginia. That plant, targeting 400 megawatts of total capacity, is projected to deliver electricity in the early 2030s. The company’s key innovation is high-temperature superconducting magnets, which allow for smaller, stronger tokamaks than anything built before.
TAE Technologies, the oldest private fusion company at 27 years old, has raised $1.3 billion and is constructing its Copernicus reactor to demonstrate net energy gain before 2030. Its commercial plant, Da Vinci, targets grid delivery in the early 2030s. In early 2025, TAE unveiled a simplified plasma control method that its team said brings commercial fusion materially closer.
Helion Energy, backed by Sam Altman, has already signed a power purchase agreement with Microsoft and targets its first fusion generator deployment in 2028 — which would make it the first commercial fusion plant in history, if the timeline holds.
Are these projections reliable? That’s the honest question. Here’s what the Fusion Industry Association’s 2025 Global Fusion Industry Report says, based on data from 53 companies across more than a dozen countries: fusion is being engineered, tested, and piloted for commercialization in this decade. That’s a shift in tone from where the industry was five years ago. It doesn’t guarantee success, but it’s not science fiction either.
What would a fusion SMR actually offer, compared to a fission SMR? The list is attractive:
No long-lived radioactive waste — a genuine political and practical relief
Effectively unlimited fuel from seawater
Physics-based safety with no chain reaction to contain
Potentially even smaller and more modular designs as the technology matures
The gap between today’s fission SMRs and tomorrow’s fusion plants is not just a technology gap. It’s a gap in what we mean when we say “nuclear energy.” Fission gives us a tool we already know how to use, getting smaller and smarter. Fusion offers something categorically different — if the engineering holds.
Given the trillions of dollars at stake in the global energy transition, and the specific pressure from AI data centers demanding clean, always-on power, the question isn’t whether fusion will arrive. It’s whether it arrives in time to matter for the decisions being made right now about which fission SMRs to build, where, and how many.
What’s your read on the timeline? Do you think fusion plants will be operating at commercial scale before the first wave of fission SMRs reaches the end of their operating lives — or will fission own the nuclear century?