David Kirtley: Nuclear Fusion, Plasma Physics, and the Future of Energy

Lex Fridman introduces David Kirtley as a nuclear engineer, fusion expert and CEO of Helion Energy. He describes Helion as a company that has made fast progress toward building fusion reactors. He gives the stakes plainly: commercial fusion would supply most of our energy needs cleanly and safely, with virtually unlimited clean electricity. He is also clear about the difficulty. Hydrogen has to be heated to over 100 million degrees Celsius and held long enough for atoms to fuse.

That’s why the joke in the past has been that fusion is 30 years away, and always will be.

Fridman then sets fusion against fission, which today's nuclear power plants use to split heavy uranium atoms. Fusion combines light hydrogen atoms instead, the same reaction that powers the sun and stars. He lists the benefits as fuel from water, no long-lived radioactive waste, no carbon emissions, and inherent safety: a fusion reactor cannot melt down, and if something goes wrong it simply stops. These are the host's framing claims. This part of the conversation does not test them with evidence. He also takes a short detour on pronunciation. Citing a Sample Size subreddit post by pmgoodbeer, he says Americans prefer "fission" rhymed with "vision" while the U.K. and other countries rhyme it with "mission," and he picks the American version "because America."

On technology, Fridman says most fusion efforts have used tokamaks, giant donut-shaped magnetic containment chambers, while Helion uses pulsed magnetoinertial fusion. He promises Kirtley will cover the detailed physics and engineering later. His wider argument is historical: humanity has always been limited by energy scarcity, and agriculture, industrialization and the information age each came "in part" from new energy sources. On that view, solving commercial fusion would start an era of energy abundance. The intro ends with the usual sponsor notes and his welcome to Kirtley.

Kirtley opens with a cosmic framing. Fusion powers the stars, and it is ultimately where most of the energy we use on Earth comes from. Sunlight powers plants, plants become oil, and fossil fuels have run human civilization for the last 100 years. The irony he points out is that humans do not yet use fusion directly to make electricity.

So fusion is what powers the universe.

In his definition, fusion joins hydrogen and light isotopes of hydrogen and helium into heavier elements. The resulting nucleus is slightly lighter than the sum of its parts. He traces that to quantum mechanics and to the strong nuclear force that holds nuclei together. Through E=mc², the missing mass is released as energy. He adds that the real reactions are richer than this one-line summary.

And the actual reactions, I think, are a lot more interesting than simply it’s a little bit lighter, and therefore, energy is released.

Fission runs the other way. Kirtley describes uranium and plutonium as so heavy, with so many internal particles, that they are "barely held together at all": unstable, radioactive, and close to falling apart. Add a neutron, an uncharged subatomic particle, to a Uranium-235 or Plutonium-239 nucleus and it breaks into a whole spectrum of pieces. Together those pieces again weigh slightly less than the original nucleus, and the difference is released as energy.

He ties both processes to a well-known curve across the periodic table, with iron as the dividing point.

Iron is the magical point in between where lighter elements than iron fuse together, and heavier elements fission or are fissile and break apart and release energy.

Kirtley then follows the stellar life cycle. Our sun is an early-stage star burning hydrogen into helium. Later-stage stars fuse helium into carbon, and carbon into heavier elements still. He says this progression pushes Helion to think about more advanced fusion fuels than the simplest ones, the kinds seen in stars across the universe. He does not yet say which advanced fuels Helion has in mind.

Fridman zooms out and offers his own opinion: alien civilizations across hundreds of millions or billions of years are likely powered by fusion, as ours already is indirectly, with the sun as its power plant. He then asks Kirtley to explain E=mc².

Kirtley calls it a fundamental relationship found by Einstein while he was a patent clerk. He says it opened atomic physics, showed what happens inside the nucleus, and paved the way for later advances. The core idea is that particles we think of as mass are also energy, with a direct quantitative link between the two. He says all the energy released in atomic reactions follows from it.

His less familiar point is that the same holds for chemistry. When hydrogen and oxygen burn to form water, mass changes, but the change is tiny.

that change per atom and per molecule is actually so small that it’s extremely hard to measure, but it’s still there

He adds that energies in both atomic and chemical processes are measured in electron volts. This sets up the scale difference between chemical and nuclear energy, though he does not give specific per-reaction figures here.

Asked about fuels, Kirtley says humans do not make uranium and plutonium nuclei. They formed in the primordial universe through supernovae, the Big Bang and the first formation of matter, and we dig them out of the ground. He then qualifies this: most plutonium is actually made from uranium. He offers to discuss uranium enrichment but does not go into it in this section.

Fusion fuels, by contrast, are hydrogen species, primordial and the most common matter in the universe. Suns and stars are made of hydrogen and helium, and most atoms in the universe are still hydrogen. Fridman sums it up: fission fuel is already in the ground, while fusion fuel is everywhere. Kirtley agrees.

Helion's specific fuel is deuterium, a heavier hydrogen isotope. Ordinary hydrogen is one proton and one electron with an atomic mass of one. Deuterium has an atomic mass of two, because its nucleus holds a charged proton and an uncharged neutron. Kirtley stresses how common it is: it is in all water on Earth, in the water he is drinking, in his body, and in Coca-Cola. He calls it safe and clean.

His most concrete number in this section is an estimate, not a measurement. He says that if all of humanity ran on fusion at current electricity use, seawater would supply fuel for a very long time. The range is wide, spanning an order of magnitude, and it is tied to today's electricity demand rather than any future growth.

somewhere between 100 million years and a billion years of fuel in hydrogen and deuterium here on Earth.

When Fridman asks how that deuterium is stored, Kirtley says it is mostly just in water. Ordinary water is H₂O, while heavy water is D₂O, two deuteriums and an oxygen. In practice there is a mix, including HDO molecules that carry one hydrogen and one deuterium, plus other forms of hydrogen. His explanation trails off at that point.

This part of the interview is foundational. It sets out the mass-defect physics behind fusion and fission, the iron boundary, the cosmic origin of each fuel, and the abundance of deuterium. The strongest claims about fusion's benefits come from Fridman's introduction, not from evidence in the conversation. These include no long-lived radioactive waste, no possibility of meltdown, and the idea that fusion would solve most energy needs. Helion's own method, pulsed magnetoinertial fusion, is named but not yet explained. Neither is the engineering challenge of reaching more than 100 million degrees Celsius.

Kirtley's own numbers come with stated hedges. The fuel supply is something "we estimate," given as a range from 100 million to a billion years at current electricity use. His fission account is also qualified: he first says uranium and plutonium are dug from the ground, then adds that most plutonium is made from uranium. The transcript repeats several passages in later segments, so the same explanations of chemical mass change, fuel origins and Fridman's "fuel is everywhere" summary appear twice. In one of those repeats, the speaker label does not match the content.

David Kirtley says the fuel for fusion is hydrogen, and in particular deuterium, a heavier isotope of hydrogen. A typical hydrogen atom has one proton and one electron and an atomic mass of one. Deuterium has an atomic mass of two because its nucleus holds a proton, which is charged, and a neutron, which is not. Kirtley stresses how common it is. He says deuterium is in all water on Earth, including the water he was drinking during the interview, in his own body and in Coca-Cola. He calls it safe and clean, and describes it as one of the fundamental particles "born in the cosmos."

we would have somewhere between 100 million years and a billion years of fuel in hydrogen and deuterium here on Earth.

Kirtley presents this figure as an estimate. It assumes that all of humanity runs on fusion at today's level of electricity use. The range covers a factor of ten, and this part of the conversation does not show how it was calculated.

Lex Fridman asks how this deuterium is mostly stored. Kirtley answers that it is mostly in water, as a mixture. Ordinary water is H2O, with two hydrogens and one oxygen. Heavy water is D2O, with two deuteriums and one oxygen. He says real water is a mix that also contains HDO, a molecule with one hydrogen and one deuterium. His next sentence, about other forms of hydrogen, is cut off in the transcript.

Kirtley argues that the supply would hold up even if humanity used far more energy. He says that even "at much higher output power levels for humanity" there would still be "millions and millions of years of power." Fridman takes this further, speculating. In his view, if we ever started running out of hydrogen and helium, that would mean our technology was doing incredible things. That technology, he suggests, would probably let us spread into the universe and find fuel on other planets, since it "looks more and more likely" that many of them have water.

The transcript attributes segment-000028 to Fridman, though it reads like an explanation run together with a question. Its point is that chemical reactions also change mass. When hydrogen and oxygen burn and combine into water, the total mass changes. The change per atom or molecule is so small that it is extremely hard to measure, but it is real, it is the energy released, and it can be quantified. Energy in atomic and chemical processes is measured in electron volts. Fridman then checks the fuels for each process: uranium and plutonium for fission, hydrogen isotopes for fusion. Kirtley confirms this is "correct to say at today's power level."

Fridman asks how mass turns into energy (E=MC²) in fusion. Kirtley begins with the obstacle. Atomic nuclei carry electric charge, like charges repel, and so the electromagnetic force pushes fusing nuclei apart. To overcome this, fusion heats the fuel to very high temperatures, which he puts at "100 million degree temperatures." He explains that temperature is really kinetic energy: motion and velocity.

At those speeds, particles can get close despite the repulsion. Once they are within a distance comparable to the size of the nuclei themselves, the strong force takes over. The particles attract, combine and fuse. The heavier nucleus that results has slightly less total mass than the inputs, and that missing mass is released as energy according to E=MC².

fusion is hard and fission is easy

Fridman suggests that the temperature each reaction needs is one difference between fusion and fission. Kirtley agrees and widens the point. Fission happens at room temperature. Uranium and plutonium are so prone to splitting that adding a single neutron breaks a nucleus apart and releases energy. Put enough of that material together and it forms a chain reaction. Kirtley says fusion does not happen that way "at all." Even one fusion reaction requires overcoming the electromagnetic repulsion.

Kirtley uses the Sun as an example. It relies on gravitational confinement: the mass of the fuel itself pulls toward the Sun's center. That large force holds the fuel together so that it gets close enough and hot enough for long enough to fuse. Fridman notes that a fusion generator has to achieve confinement without the Sun's enormous gravity. Kirtley agrees, saying the Sun is vastly larger than Earth, and Fridman jokes "Yet." Kirtley's answer is that engineers can use a force the Sun cannot apply in this way, the electromagnetic force.

Kirtley stops the conversation to object to a word. Fusion machines have historically been called reactors, and he says "I don't think that's right." He adds that his team is careful about this terminology. To support the point, he reads the definition used by the Nuclear Regulatory Commission (NRC).

A nuclear reactor is an apparatus, other than an atomic weapon, designed or used to sustain nuclear fission in a self-supporting chain reaction.

The definition refers to fission, and fusion is a different reaction. The definition refers to a self-sustaining reaction. A reactor keeps going after you take your hands off it. Kirtley says fusion does not, "and we know because we have to do it every day, and it's really hard to do."

Kirtley compares fusion to natural gas. Nobody talks about a natural gas reactor, because when you stop adding fuel, it turns off, and he says fusion behaves the same way. That is why his team says generator: fuel goes in, electricity comes out, and stopping the fuel shuts the machine down. He also frames the word around what people want from fusion. In his words, humanity does not simply want "a set of reactions or even heat and energy". It wants electricity. His argument depends on the fission-specific wording of the NRC definition, and he presents it as his team's deliberate choice of words.

Fridman says one big benefit of the "linear design" Kirtley's team uses is that it produces electricity directly, as quickly as possible. He says some alternative designs add an intermediate step, and he leaves the technical details for later in the interview. He then asks for the high-level advantages of fusion as an energy source.

Kirtley gives two advantages. First, he says fusion releases its energy as charged particles, which he describes as "already in the form of electricity." Second, the reaction is hard to sustain and so turns itself off. That, he argues, makes fusion "fundamentally safe," which he calls a key requirement for any industrial system. He compares it to a car: turn the key off and you know the car will stop.

Fusion itself has electricity built into it without a lot of the steam or thermal system requirements.

In this part of the interview, both advantages are stated as principles. Kirtley gives no figures for conversion efficiency and no experimental results. How the linear design actually captures energy from the charged particles is left for later.

Fridman describes the contrast: fission is a chain reaction, so it is hard to shut off, and it works by boiling water into steam that spins turbines to make electricity. Kirtley explains the process in more detail. A reactor puts enough fissile material, uranium or plutonium, together that the unstable atoms crack open and release heat. The fragments, which he says span a whole range of different atoms and nuclei, are hot, and the splitting also releases neutrons. When the fuel is arranged correctly, those neutrons strike neighboring uranium nuclei, which split and release more heat and more neutrons.

Kirtley says fission reactors are designed for a balance. They must make enough neutrons to keep the reaction going, but not so many that it speeds up. The fuel typically sits in water, which serves as the cooling fluid, although he notes that some more advanced reactors use other fluids. The water absorbs both the heat and the extra neutrons, and it drives a steam turbine that produces electricity. The result is a complicated system of flowing liquids that balances heat. Kirtley says much of fission reactor design comes from managing that thermal balance: keeping the reaction going, preventing an uncontrolled chain reaction, and handling cooling and output.

Fridman then qualifies the comparison. He accepts that fusion is much safer because there is no chain reaction and it can simply be shut off. But he says that, as far as he understands, today's fission reactors are also very safe, and that public fear of them does not match the record.

if you just look empirically at the statistics, that fear is not justified by the actual safety data.

Fridman's safety question leads into Kirtley's answer about engineering safeguards and the role of operators, developed in the following section. Neither speaker supplies specific safety statistics at this point.

David Kirtley steps back from the reaction physics to what he says matters most, the power plant that makes electricity. Speaking as a nuclear engineer, he argues that modern fission reactors are engineered to be safe. As reactions speed up and the system heats, it is built to expand and cool down passively, and protection systems are in place on top of that.