Fusion fuels, tritium and neutrons

Deuterium-tritium, deuterium-deuterium, deuterium-helium-3 and proton-boron-11 compared: what each reaction releases, where the fuel comes from, and why aneutronic is a relative term.

Guide · Updated 2026-09-26

A fusion fuel is a pair of light nuclei that can be made to join. The choice of fuel decides almost everything else about a machine: how hot the plasma must be, how much of the energy comes out as neutrons, what the walls must survive, and where the fuel comes from. This guide covers the four fuels that appear in this dataset.

Deuterium–tritium (D-T)

Deuterium is hydrogen with one neutron in its nucleus. Tritium is hydrogen with two. When they fuse they make a helium nucleus and a free neutron, and release 17.6 MeV:

the helium nucleus (also called an alpha particle) carries 3.5 MeV
the neutron carries 14.1 MeV, about 80% of the total

D-T fuses at a lower temperature than any other practical fuel, which is why every result on the gain page that came near breakeven used it: NIF’s target gain of 1.5 in December 2022 and JET’s plasma gain of 0.63 in October 1997. See deuterium–tritium.

The helium nucleus is charged, so a magnetic field holds it in the plasma, where it deposits its energy and helps keep the plasma hot. The neutron is not charged. It leaves the plasma at once and carries most of the energy into the surrounding structure. That is how a D-T power plant would collect its heat, and it is also the source of most of its engineering problems.

Tritium supply

Tritium is radioactive, with a half-life of 12.32 years, so about 5.5% of any stock decays each year (1 − 0.5^(1/12.32) ≈ 0.055). It does not accumulate in nature. ITER puts the world’s inventory at around 20 kilograms.

A D-T plant burns tritium quickly. Each reaction consumes one tritium nucleus and releases 17.6 MeV (2.82 × 10⁻¹² joules). One gigawatt of fusion power running for a year releases 3.16 × 10¹⁶ joules, which takes 1.12 × 10²⁸ reactions, or about 56 kg of tritium. The whole of today’s stated world inventory would run one such plant for about four months.

A D-T power plant must therefore make its own tritium. The plan in almost every design is a breeding blanket: a layer of lithium around the plasma. A neutron striking lithium-6 produces a tritium nucleus and a helium nucleus, releasing energy. A neutron striking lithium-7 also produces tritium, absorbs energy rather than releasing it, and frees a neutron that can go on to breed again.

The measure of success is the tritium breeding ratio: tritium made divided by tritium burned. It must exceed 1, with margin for losses, decay and stock for starting new plants. No fusion device has yet bred tritium at a ratio above 1 in operation. Breeding ratios in company and design documents are almost always modelled, and the dataset holds no measured value. ITER plans to test mock-up breeding blankets in its plasma. See tritium breeding ratio.

Deuterium–deuterium (D-D)

Deuterium alone can fuse, in two roughly competing ways. One produces tritium and a proton. The other produces helium-3 and a neutron. D-D needs a much hotter plasma than D-T to produce the same fusion power, but the fuel is abundant: ITER gives 33 grams of deuterium in every cubic metre of seawater, and no tritium breeding is needed. See deuterium–deuterium.

Many research machines run on deuterium only, including EAST, KSTAR and JT-60U, because it avoids handling tritium. Their results can be converted into what D-T would have given, but that conversion is a calculation. The benchmark files it as equivalent gain, marks it as simulated, and never ranks it beside measured D-T results.

Deuterium–helium-3 (D-He3)

Deuterium and helium-3 fuse to give a helium-4 nucleus and a proton. Both products are charged, so in principle their energy can be turned into electricity directly rather than through steam. See deuterium–helium-3.

Two problems stand in the way. Helium-3 is vanishingly scarce on Earth. And a plasma containing deuterium will also run D-D reactions, one branch of which releases a neutron, and the tritium from the other branch can then fuse with deuterium to give 14.1 MeV neutrons. A D-He3 plant makes fewer neutrons than a D-T plant, not none.

Helion Energy is the only organisation in the dataset filed under D-He3. In February 2026 Helion said its Polaris machine had produced measurable D-T fusion, and that D-He3 is planned for its future commercial operation.

Proton–boron-11 (p-B11)

A proton fusing with boron-11 produces three helium nuclei and no neutron in the main reaction. It is the fuel most often called aneutronic. See proton–boron-11.

The cost is temperature. p-B11 needs ion temperatures about nine times higher than D-T, and at least an order of magnitude more triple product. No aneutronic system has shown net energy.

“Aneutronic” is also relative. Side reactions in a boron plasma, and deuterium impurities, still produce some neutrons. Far fewer than D-T, but not zero, and a design still needs shielding.

Organisations in the dataset filed under p-B11: TAE Technologies, HB11 Energy, Marvel Fusion, LPP Fusion, Anubal Fusion and LINEA Innovations. The dataset holds no measured p-B11 gain for any of them. A net-energy date attached to a p-B11 plan is a roadmap, and the benchmark holds it as planned until a shot is measured. A plan that reaches p-B11 net energy without first showing it with D-T, the easier fuel, deserves particular scrutiny.

Neutrons

For D-T, neutrons are the main product. Counting them is often the most direct way to measure how much fusion took place, which is why the dataset has a neutron yield metric.

They also set the engineering agenda. A 14.1 MeV neutron passes straight through the magnetic field and into the surrounding structure, where over years it displaces atoms, weakens materials and makes some of them radioactive. Choosing materials that survive and that do not stay radioactive for long is an open field of work. This guide keeps to the fuel side; the neutron physics of blankets and materials is outside what the benchmark measures today.