Fission vs Fusion: The Physics Behind Our Energy

We are, right now, deciding whether nuclear energy will be part of how we decarbonize the grid. Not in some abstract policy seminar—in utility boardrooms, in congressional budget negotiations, in the public imagination that shapes both. And the public imagination is, to put it gently, working with faulty source material.

That's the context I bring to T. Folse's recent video, in which the nuclear engineer sits with a StarTalk segment on fission versus fusion and annotates it in real time. Folse is generous with Neil deGrasse Tyson—more generous than I might be—but his corrections accumulate into something that matters beyond physics pedagogy. Misunderstanding how these reactions work, and what distinguishes them, is precisely how we end up having the wrong arguments about nuclear energy's role in decarbonization. Every time a politician conflates a reactor with a bomb, every time a news segment treats "critical" as synonymous with "catastrophic," we pay a cost in policy coherence. Folse's video is an attempt to collect that debt.

The scale problem, and why it matters for climate math

Start with the basics, because the basics are where the stakes hide.

Chemical reactions—burning coal, burning gas, burning anything—rearrange electron bonds. The typical energy released per molecule runs to a few electron volts. Nuclear reactions involve the nucleus itself, and the energy scales accordingly: uranium-235 fission releases around 200 million electron volts per reaction. That's not a rounding difference. That's roughly a million times more energy per event. Folse's shorthand: a single uranium fuel pellet—about 14 grams, fingertip-sized—contains roughly as much energy as a ton of coal. (Note: Folse uses a "pistachio" comparison in the video; the standard nuclear industry figure is approximately one fuel pellet to one ton of coal, though exact sizing varies by source and should be verified.)

I find this figure clarifying every time I encounter it, not because it makes nuclear energy automatically the right answer, but because it reframes the infrastructure question. A coal plant requires trains—literal freight trains, continuously—to feed its furnaces. A commercial reactor refuels every 18 to 24 months. When we debate the land use and supply chain footprint of the energy transition, this asymmetry should be part of the ledger.

Fusion, as Folse explains when Tyson gets to it, operates by a different mechanism entirely but arrives at an even more dramatic energy density. Where fission splits heavy atoms like uranium to release the binding energy difference, fusion merges light atoms—hydrogen isotopes, primarily—to form helium, and the mass of the product is slightly less than the sum of the parts. That missing mass becomes energy via E=mc². The sun converts roughly 600 million metric tons of hydrogen to helium every second, with approximately 4 million metric tons of that converted to pure energy. It has been doing this for about 4.6 billion years. (The efficiency figure Folse cites—approximately 0.7% mass-to-energy conversion for the proton-proton chain—is the standard figure; the video transcript records him saying 7%, which may be a transcription artifact or verbal slip and should be verified against the source before treating as authoritative.)

The reason we don't yet have fusion power plants is not that the physics is wrong. The physics is exactly as advertised. It's that recreating stellar conditions on Earth long enough to get more energy out than you put in is an engineering problem that has defied six decades of very serious effort. Folse is precise about this: the sun achieves fusion under gravitational pressure that no terrestrial reactor can replicate. Earth-based fusion requires temperatures far exceeding the sun's core temperature to compensate. "Quantum mechanics lets you cheat," Folse says of the quantum tunneling that makes solar fusion possible at temperatures that classical physics says should be insufficient—tunneling allows hydrogen nuclei to fuse even when they don't quite have enough energy to overcome their mutual electromagnetic repulsion. We don't get that cheat on Earth, at least not in the same form.

What the corrections reveal

The most useful thing about Folse's running commentary isn't the corrections themselves—it's what they're correcting for.

Tyson's segment was made for a mass audience riding an Oppenheimer wave, and it does what good science communication does: simplifies to illuminate. Folse's role is different. He's watching the same video and flagging every place where simplification slides into imprecision with downstream consequences.

Two corrections stand out. First, on spontaneous fission: Tyson implies that radioactivity essentially means atoms "splitting on their own," which Folse dismantles carefully. Most radioactive decay is alpha decay, beta decay, or gamma emission—none of which is fission. Fission specifically requires a neutron to strike a heavy nucleus, destabilize it, and trigger the split. Spontaneous fission exists but is rare and doesn't drive reactor operation. This matters because the distinction between "radioactive material" and "fissile material" is exactly what separates public fear from engineering reality. "Reactors are all about controlled criticality," Folse notes, walking through how one neutron per fission sustains a steady chain reaction, versus the prompt supercriticality—exponential neutron multiplication, cycles on the order of microseconds—that makes a bomb work. These are not the same phenomenon dressed up differently. They're governed by the same physics used in fundamentally opposite ways.

Second, on weapons: Tyson describes the World War II bombs as "just fission weapons," which is technically accurate but incomplete in a way Folse doesn't fully address in the clip. The Hiroshima bomb, Little Boy, was a uranium-235 fission device. The Nagasaki bomb, Fat Man, used plutonium-239—a different fissile material with different production requirements. (This distinction wasn't explicitly drawn in the portion of the video available; flagging for editorial review before publication.) Modern thermonuclear weapons are two-stage systems, using a fission primary to create the conditions for a fusion secondary. Understanding this architecture is what separates informed debate about nuclear nonproliferation from reflexive conflation of all things nuclear.

The systems question no one is asking loudly enough

Here is what I keep returning to as I write about the energy transition: we are asking nuclear technology to do something it has never been asked to do at the pace we now need.

Existing fission plants—the ones already on the grid—provide roughly 20% of U.S. electricity, and they do it around the clock regardless of whether the wind is blowing. That's not nothing. In a grid increasingly dependent on variable renewables, dispatchable carbon-free baseload has genuine value, and the fleet is aging out faster than we're replacing it. New reactor construction in the West has been expensive and slow; advanced reactor designs and small modular reactors are still largely in the demonstration phase. The economics are contested, the timelines are uncertain, and the waste question remains unresolved in most jurisdictions.

Fusion, meanwhile, is perpetually the energy source of the future—a joke so old it's become load-bearing in policy circles. The National Ignition Facility's December 2022 ignition milestone, where a fusion reaction produced more energy than the laser energy delivered to the target, was real and significant. It was also a proof of concept that sits several engineering generations away from a grid-connected power plant. The companies racing to commercialize fusion—Commonwealth Fusion, TAE Technologies, Helion, and others—are working on timescales measured in decades, not years. We need emissions reductions measured in years.

This is the tension that Folse's video, for all its careful physics, doesn't quite name: the gap between what we understand about nuclear energy and what we've built around it is not a knowledge problem. It's a policy, investment, and public trust problem. And public trust is shaped by whether people understand the difference between "critical" and "catastrophic," between spontaneous decay and induced fission, between a reactor and a bomb.

When Folse flags one of Tyson's simplifications, he frames it as precision for its own sake. I read it differently. Every imprecision in how we talk about nuclear technology costs something in the eventual reckoning. It costs it in the form of permit delays, canceled projects, and political constituencies organized around misunderstanding. The physics of fission and fusion are not in doubt. What remains genuinely open is whether we'll have enough clarity—scientific, political, institutional—to use what we know, in time, at scale.

Folse ends his video enthusiastic about the science. I find that enthusiasm clarifying and a little heartbreaking in equal measure.

Fact-check note: This article flags two figures requiring verification before publication—the mass-to-energy conversion efficiency for solar fusion (video transcript records 7%; standard figure is ~0.7%), and the precise sizing of the uranium fuel pellet comparison. The Hiroshima/Nagasaki bomb materials distinction was also not explicitly confirmed in the available transcript excerpt.

By Olivia Meng, Climate & Environment Correspondent