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Most fusion power-plant designs now under study use deuterium–tritium (D–T), two hydrogen isotopes. Deuterium can be separated from ordinary water, including seawater. Tritium is radioactive, decays in about 12 years, and is scarce; today’s readily available supply is mainly a by-product of heavy-water fission reactors. A future D–T plant is expected to make replacement tritium by sending fusion neutrons into lithium in a surrounding breeding blanket, then extracting, purifying and recycling that tritium. Closing that entire fuel cycle remains a major engineering task.
The short answer: D–T is the leading fusion fuel
Deuterium (D) and tritium (T) are isotopes of hydrogen. A deuterium nucleus contains one proton and one neutron; tritium contains one proton and two neutrons. When they fuse, they produce a helium-4 nucleus (an alpha particle) and a high-energy neutron. The reaction’s energy is split between those products: the charged helium helps heat the plasma, while the neutron carries energy into the reactor structures and blanket. The neutron can also be used to make more tritium from lithium.
The D–T reaction is the main focus of current fusion research because it reaches useful fusion rates at lower plasma temperatures and densities than the other fuel combinations being seriously studied. The International Atomic Energy Agency (IAEA) identifies D–T as the primary near-term focus, while deuterium–helium-3 and proton–boron-11 require more demanding plasma conditions. D–T is therefore the leading candidate for early fusion power plants, not a guarantee that every future reactor will use it.
The U.S. Department of Energy explains that the D–T reaction produces helium and a neutron. ITER’s fuelling overview likewise describes D–T as the practical starting point for large experimental machines.
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What deuterium is and where it comes from
A naturally occurring hydrogen isotope
Deuterium is stable: it does not undergo radioactive decay. It occurs naturally alongside ordinary hydrogen (protium) in water. Producing fusion fuel does not mean manufacturing deuterium from scratch; an industrial plant separates the deuterium that is already present in water and concentrates it to the required purity.
Seawater is the largest accessible reservoir
Because every ocean contains water, seawater represents a very large potential deuterium resource. The DOE states that about one in 6,500 hydrogen atoms in seawater is deuterium (the source page does not state a publication year). ITER gives a related but separately measured figure of 33 grams of deuterium per cubic metre of seawater (the ITER page does not state a publication year). These figures describe the same natural abundance in different ways; they should not be treated as identical measurements or as a guaranteed extraction yield.
Industrial isotope plants use chemical-exchange, distillation or related separation processes to enrich heavy hydrogen from water. The exact process depends on scale, energy costs and the purity specification. A fusion facility would receive purified deuterium as a controlled fuel stream rather than pump seawater directly into its plasma.
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DOE’s deuterium–tritium fuel explainer and ITER’s fuel FAQ both describe deuterium as abundant in ordinary water and seawater. Abundance of the raw isotope is not the same as having a complete reactor fuel system: separation, purification, storage and delivery still have to be engineered.
What tritium is and where today’s supply comes from
Radioactive, short-lived and naturally scarce
Tritium is radioactive hydrogen. The DOE gives its half-life as about 12 years; the IAEA’s World Fusion Outlook 2023 gives 12.3 years. After one half-life, half of a quantity of tritium has decayed; after another half-life, half of what remains decays. A reactor must therefore replace tritium continuously rather than rely indefinitely on an initial inventory.
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Cosmic-ray interactions create tiny amounts of tritium naturally in the atmosphere. The quantities are only trace-level. The DOE also lists some fission reactors and historical nuclear-weapons testing among tritium sources. For present fusion planning, the important point is that tritium is not naturally available in the way deuterium is.
Heavy-water fission reactors are the current readily available source
ITER identifies heavy-water fission reactors, including CANDU-type reactors, as the only readily available source in its fuel-supply discussion. Tritium is produced there as a by-product of neutron interactions with the reactor’s heavy-water moderator or coolant and can be recovered during maintenance and processing. Existing inventories and production capacity are limited, and those reactors are not designed to supply every future fusion plant.
This distinction matters: lithium mining does not yield tritium directly. Lithium is the feedstock from which a fusion plant would make tritium through a nuclear reaction. The DOE and IAEA describe current supply constraints and future breeding requirements in their fuel-cycle material.
For background on sources and handling requirements, see the U.S. Nuclear Regulatory Commission’s fusion FAQs, the DOE isotope overview and ITER’s explanation of fuel availability.
How a D–T reactor makes replacement tritium
The lithium-6 breeding reaction
In a proposed power plant, the fusion plasma sits inside a blanket containing lithium. The high-energy neutrons from D–T fusion enter that blanket. The IAEA gives the key breeding reaction as:
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lithium-6 + neutron → tritium + helium-4
The newly formed tritium must then be removed from the blanket material and returned to the fuel system. Blanket designs may use different lithium-bearing solids, liquids or coolant arrangements, but all must capture enough neutron energy to make tritium while surviving intense radiation and heat.
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Three jobs for the breeding blanket
The blanket is not merely a tritium factory. The IAEA identifies three linked functions:
- Breed fuel: convert lithium and fusion neutrons into replacement tritium.
- Extract heat: absorb neutron and other radiation energy and transfer it to a coolant for electricity generation.
- Shield components: reduce neutron exposure to magnets, vessel structures and other equipment outside the blanket.
A blanket that performs one function well but fails another would not make a practical power plant. Its design must balance breeding performance, heat removal, shielding, materials durability and the ability to extract tritium without excessive losses.
The complete fusion fuel cycle, step by step
A commercial D–T plant would need an integrated system rather than a one-time delivery of fuel. The DOE describes the required operations as a fuel cycle that links plasma fuelling, exhaust treatment, tritium breeding and recycling.
- Receive and qualify deuterium and tritium. Incoming isotopes are measured for composition, impurities and inventory. Tritium accounting is especially important because it is radioactive, decays continuously and must be contained.
- Prepare the plasma fuel. Deuterium and tritium are metered, mixed and injected into the vacuum vessel. The plant must control the amount entering the plasma while minimizing the quantity held in pipes and storage.
- Fuse the fuel. D–T reactions create helium and neutrons. Helium remains in the plasma long enough to help sustain heating, while neutrons leave the magnetically confined plasma and deposit energy in the blanket.
- Process the exhaust. Not every fuel atom fuses. Exhaust systems remove helium “ash,” unburned deuterium and tritium, and impurities eroded from plasma-facing materials. Hydrogen isotopes are separated so usable fuel can be recovered.
- Breed tritium in the blanket. Neutrons interact with lithium, including lithium-6, to create fresh tritium. The blanket also captures heat and provides shielding.
- Extract and purify bred tritium. Tritium must be separated from lithium-bearing material, coolant and other gases, then purified to fuel quality. The extraction technology must work repeatedly under radiation, heat and chemical constraints.
- Store and deliver fuel. Short-term storage buffers the difference between breeding, processing and plasma demand. Pumps, valves, sensors and containment systems return the right isotope mixture to the injector.
- Measure the inventory and recycle. Operators track tritium in the plasma, exhaust plant, blanket, storage and waste streams. Recovering unburned fuel and minimizing hold-up are necessary to keep the system supplied.
DOE’s fuel-cycle and tritium-processing description emphasizes that breeding, extraction, purification, storage, fuelling, exhaust processing and recycling must operate as one system. A plant that breeds tritium in principle but cannot recover it quickly or cleanly would still fall short of fuel self-sufficiency.
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Why tritium self-sufficiency is difficult
The reactor must make more than it burns
To replace the tritium consumed by fusion, a blanket must produce enough to cover the burn itself plus radioactive decay, processing losses, leaks, start-up inventories and tritium retained in materials. The exact surplus requirement depends on plant design and operating conditions. The official sources do not establish a demonstrated commercial breeding ratio, so claims that a power reactor has already closed the cycle would be premature.
Extraction and processing are as important as neutron physics
Making tritium inside lithium is only one step. It has to be released from the blanket, transported through a controlled system, separated from other hydrogen isotopes and delivered back to the plasma. Every stage introduces delay and potential inventory. The fuel plant must also cope with deuterium–tritium mixtures, helium ash and impurities without losing excessive fuel.
Supply must cover the period before breeding works
Early D–T machines need an initial tritium inventory. Existing heavy-water-reactor supplies are limited, and tritium decays while it is stored. Future deployment therefore requires both a reliable starting stock and a blanket system capable of producing surplus tritium for the next machine. The DOE’s Fusion Energy Strategy 2024 identifies lithium-6 and isotope supply chains as development needs.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How alternative fusion fuels compare
D–T is favored for near-term development, but researchers also study reactions that could reduce or eliminate the need to breed radioactive tritium. Those alternatives are not commercially established reactor fuels and demand more extreme plasma conditions.
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| Fuel combination | Fuel availability and handling | Reaction and neutron implications | Development position |
|---|---|---|---|
| Deuterium–tritium (D–T) | Deuterium is abundant in water; tritium is radioactive, scarce and currently supplied mainly by heavy-water fission reactors. Future plants must breed tritium from lithium. | Produces helium-4 and a high-energy neutron. The neutron enables lithium breeding but creates substantial blanket, shielding and materials demands. | Leading focus of current fusion research and the main candidate for early power plants. |
| Deuterium–helium-3 (D–³He) | Requires helium-3, whose practical supply is not established in the cited sources. | Requires hotter and denser plasma conditions than D–T; neutron production and fuel-cycle details differ from D–T. | Research alternative, not a commercially established fuel. |
| Proton–boron-11 (p–¹¹B) | Uses hydrogen and boron isotopes, but the plasma requirements are significantly more demanding. | Requires hotter and denser plasma conditions than D–T; the cited sources do not establish a commercial reactor design. | Research alternative, not a commercially established fuel. |
The IAEA discusses these alternatives in its World Fusion Outlook 2023. Avoid interpreting “less tritium” as “easier”: higher temperature and density requirements can create their own confinement, heating and engineering obstacles.
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What current experiments have—and have not—shown
ITER is testing breeding-blanket modules and fuel-cycle technologies. These experiments are intended to validate components and operating methods, not to prove that a commercial power station already has a self-sufficient tritium cycle. A demonstration machine can test neutron interactions, extraction concepts or fuel processing without operating the full set of systems required for continuous net power production.
The IAEA’s tritium-breeding information describes the blanket functions and lithium reaction, while ITER’s fuelling page explains the role of D–T and planned breeding work. Together, they show why “the reactor can make tritium” is an incomplete statement: the plant must breed, extract, purify, store, deliver and recycle it with enough margin.
Common misconceptions about fusion fuel
- “Fusion runs on seawater.” Seawater contains deuterium, but a reactor uses separated and purified isotopes, not untreated seawater.
- “Lithium is the fuel.” Lithium is a blanket feedstock for breeding tritium. The plasma fuel in the leading design is still deuterium and tritium.
- “Tritium is unlimited because reactors can make it.” A future blanket is intended to make replacement tritium, but complete breeding and recycling remain under development and require a starting inventory.
- “All fusion concepts use D–T.” D–T is the leading near-term option; D–³He and proton–¹¹B are studied alternatives with more demanding plasma conditions.
- “A half-life of 12 years means tritium disappears in 12 years.” Half-life means half the atoms decay in that period. The remainder continues decaying over subsequent half-lives.
What the fuel question means for future power plants
Deuterium supply is comparatively straightforward because water contains a large natural reservoir. Tritium is the limiting isotope: it is radioactive, decays during storage and is available today only in limited quantities. For D–T fusion to scale beyond experiments, a plant must combine a dependable initial tritium supply with a lithium blanket that breeds a surplus, a high-recovery extraction system and disciplined recycling.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThat is why lithium-6 enrichment, blanket materials, tritium processing and isotope-accounting systems are central technology priorities. The DOE’s 2024 strategy identifies these supply-chain and fuel-cycle capabilities as needs for future deployment. The fuel itself is only one part of the challenge; the ability to move, recover and replace it continuously determines whether a D–T plant can operate as a power station.
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