Most fusion power-plant designs now focus on deuterium–tritium (D–T), a reaction between two hydrogen isotopes. Deuterium can be separated from ordinary water, including seawater. Tritium is radioactive, scarce and continually decays, so a future D–T plant would have to make replacement tritium from lithium in a surrounding breeding blanket, then extract, purify, store and recycle it. That complete fuel cycle—not finding deuterium—is the central supply challenge.
What fuel do fusion reactors use?
Fusion research is not limited to one fuel, and no commercial fusion reactor yet operates as a power station. However, the main near-term design point is D–T fusion. The International Atomic Energy Agency (IAEA) identifies deuterium–tritium as the primary focus of current fusion research because its reaction is comparatively attainable relative to other proposed fuels.
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Deuterium and tritium are hydrogen isotopes
Every hydrogen isotope has one proton. Deuterium has one proton and one neutron; tritium has one proton and two neutrons. Their chemical behavior is broadly hydrogen-like, but their nuclear masses change how they fuse. A D–T plasma can reach useful fusion conditions at lower temperatures and densities than the alternatives discussed in current research, which is why most experimental programs choose it.
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What the D–T reaction produces
In a D–T reaction, the two nuclei combine to form a helium-4 nucleus (an alpha particle) and a high-energy neutron. The helium remains charged and can help heat the plasma. The neutron is uncharged, so it leaves the plasma and deposits its energy in surrounding material. In a power plant, that neutron energy would be captured as heat and, crucially, used to help produce new tritium in lithium-containing material.
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The U.S. Department of Energy describes the reaction and its products in its fusion-reaction explainer. The same neutron that creates a materials and shielding burden is therefore also part of the proposed fuel-supply solution.
How D–T compares with other proposed fusion fuels
D–T is the leading option, not the only one. Research also examines deuterium–helium-3 (D–³He) and proton–boron-11 (p–¹¹B). The IAEA’s World Fusion Outlook 2023 explains that these alternatives require even hotter and denser plasmas. They should therefore be treated as research options rather than established commercial reactor fuels.
| Fuel option | Current position | Plasma requirement | Supply and engineering issue |
|---|---|---|---|
| Deuterium–tritium (D–T) | Primary focus of current fusion research | Comparatively attainable among the leading options | Needs a managed, radioactive tritium supply and a breeding-and-recycling cycle; produces energetic neutrons |
| Deuterium–helium-3 (D–³He) | Alternative under study | Hotter and denser than D–T | Would change the fuel-supply and neutron-management problem, but has not become a commercial reactor fuel |
| Proton–boron-11 (p–¹¹B) | Alternative under study | Hotter and denser than D–T | Could avoid some D–T fuel-cycle issues, yet remains a demanding plasma-physics and engineering proposition |
The meaningful comparison is not simply which fuel is abundant. Designers must weigh the temperature and density needed for fusion, the availability and handling of each isotope, whether radioactive tritium must be bred, the reaction products and neutron load, and whether the resulting heat can be converted economically.
Where does deuterium come from?
It is naturally present in water
Deuterium occurs naturally mixed with ordinary hydrogen in water. Isotope-separation processes concentrate it from that mixture; the starting material can be freshwater or seawater. No reactor has to manufacture deuterium through a nuclear reaction.
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The U.S. Department of Energy reports that about one in every 6,500 hydrogen atoms in seawater is deuterium; the DOE page does not state a publication year. You can read the DOE explanation at DOE Explains: Deuterium-Tritium Fusion Fuel.
ITER gives a different, source-specific way to express the same natural resource: 33 grams of deuterium per cubic metre of seawater. ITER does not state a year for that figure on its fuelling page. The two numbers should not be treated as duplicate measurements or combined into a new estimate; each belongs to the institution and wording that reports it.
Why deuterium is not the limiting fuel
Because water is widespread and deuterium is a naturally occurring fraction of its hydrogen, the deuterium resource is large compared with the quantities a reactor would consume. The practical work is separating and purifying the isotope to the specification required by the fuel system, not locating a geological deposit of deuterium. ITER’s fuel-supply FAQ likewise describes deuterium extraction from water as the readily scalable part of the D–T supply picture.
Where does tritium come from today?
A radioactive isotope with a short half-life
Tritium is radioactive and decays into helium-3. The DOE gives its half-life as about 12 years. The IAEA’s 2023 outlook specifies 12.3 years. These are consistent rounded and more precise descriptions, but neither means that all tritium disappears after one half-life: roughly half of a sample remains after one half-life, and the remainder continues decaying over subsequent intervals.
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Because tritium decays, inventories must be monitored and replenished. Storage is not a permanent stockpile solution, and a plant must account for decay while it handles, purifies and delivers the isotope.
Natural and historical sources
Cosmic-ray interactions make tritium naturally, but only in trace amounts. The DOE also lists some fission reactors and historical nuclear-weapons testing among tritium sources. Those pathways do not provide a simple, unlimited commercial supply.
Heavy-water fission reactors are the readily available source discussed by ITER
ITER identifies heavy-water fission reactors, including CANDU types, as the only readily available source in its explanation of present supply. In those reactors, tritium is a by-product associated with the heavy-water moderator and coolant systems. The supply is limited, and future fusion plants cannot assume that enough existing fission capacity will be available indefinitely. The ITER context and current-fuel discussion are set out on its fuelling page and fuel FAQ.
The U.S. Nuclear Regulatory Commission also describes tritium as a fuel that would need to be supplied and, for a D–T reactor, bred in the plant; see the NRC’s fusion FAQs. Existing inventories are therefore best understood as a bridge for experiments and early facilities, not as proof that a fleet of power plants already has a closed fuel supply.
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How a fusion reactor would make tritium from lithium
Lithium is not mined as tritium. Lithium is the feedstock from which a reactor would create tritium through a nuclear reaction driven by fusion neutrons.
The lithium-6 reaction
In one key breeding reaction, a neutron interacts with lithium-6:
lithium-6 + neutron → tritium + helium-4
The IAEA gives this reaction in its World Fusion Outlook 2023. A blanket around the fusion chamber would contain lithium in a form that lets neutrons enter, while also carrying heat away and protecting magnets and other external components.
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Three jobs for the breeding blanket
The blanket is not just a tritium-making tank. The IAEA identifies three linked functions:
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- Breed fuel: use fusion neutrons and lithium to create replacement tritium.
- Extract thermal energy: absorb neutron energy and transfer heat to a coolant and, ultimately, a power-conversion system.
- Shield external equipment: reduce the neutron exposure reaching magnets, structures and maintenance areas.
These functions compete for space, materials and operating conditions. A blanket that breeds enough tritium must also survive intense neutron irradiation, remove heat reliably and allow fuel to be extracted before it decays or becomes trapped in materials.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the complete fusion fuel cycle must do
A power plant needs more than an initial cylinder or reservoir of tritium. The U.S. Department of Energy describes an integrated fuel cycle that connects breeding, processing and reuse. A practical D–T plant would need to perform the following operations.
- Receive or start with an initial inventory. Early facilities would require externally supplied tritium because their blankets would not instantly provide a surplus. The starting inventory must be measured, protected and accounted for.
- Prepare deuterium and tritium for injection. The two isotopes must be purified and delivered in the form and flow rate the plasma system requires. Tritium handling must limit releases and prevent contamination of equipment.
- Inject fuel into the plasma. A fuelling system meters the isotopes into the hot plasma while maintaining the pressure and composition needed for stable operation.
- Capture exhaust and unburned fuel. Most injected fuel is not fused in a single pass. Exhaust-processing equipment must recover deuterium and tritium rather than discard them with other gases.
- Extract bred tritium from the blanket. Neutrons create tritium inside lithium-bearing blanket material. The plant must move that tritium into a controllable stream, even as the blanket is producing heat and absorbing radiation.
- Separate and purify the isotopes. Recovered exhaust, bred tritium and other hydrogen isotopes must be separated, cleaned and checked before reuse. This is isotope processing, not simply pumping gas from one vessel to another.
- Store and recycle fuel. Short-term storage buffers variations between production and plasma demand. Recycled deuterium and tritium then return to the fuelling system, while losses, decay and maintenance inventories are tracked.
The DOE’s description of fuel-cycle and tritium processing emphasizes that securing supply and producing surplus tritium are deployment requirements. A reactor that merely breeds enough tritium on paper, but cannot extract, purify or recycle it at operating speed, has not closed its fuel cycle.
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Blanket and fuel-cycle tests are still development work
ITER is testing breeding-blanket modules and fuel-cycle technologies. Those experiments are important because they expose materials, extraction systems and operating procedures to conditions closer to a reactor. They should not be described as proof that a commercial plant has already achieved self-sufficient tritium production. ITER’s fuelling information is available at iter.org/machine/supporting-systems/fuelling.
Self-sufficiency requires margin, not a one-time balance
A plant must replace tritium burned in the plasma, compensate for radioactive decay, account for material and processing losses, and maintain enough reserve to restart after outages. It also needs enough surplus to support additional reactors if a fleet is to expand. The DOE’s Fusion Energy Strategy 2024 identifies lithium-6 and isotope supply-chain capabilities as needs for future deployment.
Lithium supply is an industrial issue
Breeding depends on access to suitable lithium and, potentially, on managing the lithium-6 fraction needed for efficient production. That creates a supply-chain and enrichment challenge distinct from simply mining bulk lithium. The strategy document does not turn lithium-6 into a consumer commodity or guarantee a particular future supplier; it identifies the capability as something the fusion industry must develop.
Common misconceptions about fusion fuel
- “Fusion reactors run on seawater.” Seawater is a source of deuterium after isotope separation. A reactor does not feed raw seawater into its plasma.
- “Lithium contains tritium.” Lithium supplies the nuclei from which tritium can be bred when fusion neutrons drive the reaction. Mining lithium does not directly yield tritium.
- “Tritium is abundant because hydrogen is abundant.” Tritium is a radioactive, short-lived isotope and is naturally scarce; ordinary hydrogen abundance does not translate into a large tritium stock.
- “Any fusion reaction can use the same fuel system.” D–³He and p–¹¹B have different plasma requirements and would change the fuel-cycle and neutron-engineering problems. They are not interchangeable drop-in fuels for a D–T machine.
- “A breeding blanket has already solved the supply problem.” Blanket tests and design studies are steps toward a closed cycle. Commercial self-sufficient tritium production still requires integrated demonstration of breeding, extraction, purification, storage and recycling.
Why the fuel cycle matters to fusion power
Deuterium gives D–T concepts a broad natural resource, but tritium determines whether a reactor can operate continuously and whether a fleet can grow. The key engineering task is therefore a chain: obtain a limited initial inventory, burn D–T fuel, use the resulting neutrons to breed tritium from lithium, remove that tritium from the blanket, purify it, recover unburned fuel and feed the plasma again. Until that chain works with adequate margin, D–T fusion remains a promising fuel concept under development rather than a proven self-sustaining power technology.
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