Nuclear fuel explained
Where reactor fuel comes from, what enrichment does, where the lines between LEU, LEU+, HALEU and HEU fall, how TRISO, MOX and thorium fuels differ, and why HALEU supply limits advanced reactors.
Guide · Updated
A reactor’s fuel decides more about it than almost any other choice: how big the core is, how long it runs between refuellings, what it leaves behind, and whether there is a supply chain to feed it. This guide covers the main fuels in use and in development, and the supply constraint facing most new designs.
Uranium and its two isotopes
Natural uranium is more than 99% uranium-238. About 0.72% is uranium-235, and that small fraction is what matters: uranium-235 is fissile, meaning a slow neutron can split it and keep a chain reaction going. Uranium-238 is fertile: it rarely fissions, but when it absorbs a neutron it turns, through two short radioactive decays, into plutonium-239, which is fissile.
Only reactors that waste very few neutrons can run on natural uranium. Heavy-water reactors such as CANDU do. Light-water reactors, the great majority of the world fleet, need more uranium-235 than nature provides.
Enrichment
Enrichment raises the share of uranium-235. The uranium is converted to a gas, uranium hexafluoride, and spun in fast-rotating centrifuges that separate the slightly lighter uranium-235 from uranium-238. One stream leaves richer in uranium-235 (the product) and one poorer (the depleted uranium, or tails).
The effort is measured in separative work units (SWU). Modern centrifuges use about 40 to 50 kWh of electricity per SWU; the gaseous diffusion plants they replaced used about 2,400 to 2,500 kWh. The effort rises steeply with the target enrichment. Using the standard separative work formula, with natural uranium at 0.711% uranium-235 by weight and tails at 0.25% (an assumed, typical figure):
| Product enrichment | Natural uranium needed per kg of product | Separative work per kg of product |
|---|---|---|
| 4.5% (typical power reactor fuel) | about 9.2 kg | about 6.9 SWU |
| 10% | about 21 kg | about 19 SWU |
| 19.75% (top of the HALEU range) | about 42 kg | about 41 SWU |
These are calculated figures, not measured ones. The point they show is the scale: a kilogram of 19.75% fuel needs about six times the enrichment work and four and a half times the natural uranium of a kilogram of ordinary fuel. A reactor may need less of it, but the enrichment industry has to be built for it.
The enrichment categories
| Name | Uranium-235 content | Used in |
|---|---|---|
| Natural uranium | about 0.72% | Heavy-water reactors |
| Low-enriched uranium (LEU), conventional | up to 5% (typically 3 to 5%) | Almost all power reactors today |
| LEU+ | 5% to 10% | Proposed for existing light-water reactors |
| High-assay low-enriched uranium (HALEU) | 5% to less than 20% | Most advanced and small reactor designs |
| Highly enriched uranium (HEU) | 20% or more | Naval reactors, some research reactors, weapons |
Two points about these lines. First, formally everything below 20% is low-enriched; “LEU+” and “HALEU” are names for parts of that range, not separate legal categories, and the two overlap: the World Nuclear Association lists LEU+ as 5 to 10% and HALEU as 5 to 19.75%. Second, 20% is the international boundary between low-enriched and highly enriched uranium, and highly enriched uranium is subject to much stricter controls. That is why so many advanced designs specify 19.75%: as much uranium-235 as possible while staying under the line.
On this site, LEU means below 5% and HALEU means 5 to 20%, so each company’s fuel tag says which supply chain it depends on.
Fuel forms
Oxide pellets. Uranium dioxide ceramic pellets, stacked in zirconium-alloy tubes and bundled into assemblies. This is the fuel of nearly every power reactor in operation.
Metal fuel. Uranium alloyed with zirconium, used in sodium-cooled fast reactors. EBR-II in Idaho ran on metallic fuel, with rods containing 10% zirconium, from 1964 to 1994, and TerraPower’s Natrium design specifies a metallic uranium alloy of up to 19.75% enrichment.
TRISO. Tri-structural isotropic particles: a uranium kernel about the size of a poppy seed, sealed in layers of carbon and silicon carbide. Many are pressed together into pebbles or cylindrical compacts. The US Department of Energy reports that irradiated TRISO particles came through more than 300 hours of testing at temperatures up to 1,800 °C with no to minimal damage, and reached about three times the burnup (the share of fuel used before it is removed) of current light-water fuel. Used in gas-cooled and salt-cooled reactors. See TRISO.
Fuel dissolved in salt. In a molten-salt reactor, the fuel is a fluoride or chloride dissolved in the coolant. There is no solid fuel element at all.
MOX
Mixed oxide fuel, MOX, is made from plutonium recovered by reprocessing used reactor fuel, mixed with depleted uranium. Commercial MOX typically contains 7 to 11% plutonium. According to the World Nuclear Association, 44 reactors have used MOX since 1972, 22 of them in France, and a reactor can load up to about half its core with MOX, with slight modifications, without changing its operating characteristics. Russia’s BN-800 fast reactor uses MOX. It reuses plutonium rather than requiring fresh enrichment, but depends on a reprocessing plant. See MOX.
Thorium
Thorium is about three times more abundant than uranium in rocks and soils. Thorium-232 is fertile, not fissile: it must absorb a neutron and become uranium-233 before it can sustain a chain reaction, so a thorium reactor needs a starting charge of fissile material. Thorium fuel has been used in the Peach Bottom (1967 to 1974) and Fort St Vrain (1976 to 1989) gas-cooled reactors in the United States, the Shippingport light-water breeder reactor (1977 to 1982), and Germany’s AVR and THTR. China’s TMSR-LF1 research reactor began running with thorium in its fuel salt in October 2024.
The main practical drawback is that uranium-233 bred from thorium carries traces of uranium-232, whose decay products emit strong gamma radiation. That makes fuel fabrication harder and more expensive. See thorium.
The HALEU supply constraint
Of the 64 reactor developers in this dataset, 21 have HALEU recorded as a fuel, either on the company’s own record (12) or on one of its reactor designs. Supply has not kept up.
- The World Nuclear Association names Russia’s Tenex as currently the only commercial HALEU supplier worldwide, and notes that the United States banned imports of Russian uranium in May 2024.
- The US Department of Energy states that HALEU “is not currently available from domestic suppliers, and gaps in supply could delay the deployment of advanced reactors”, and estimates that domestic demand “could reach 50 metric tons per year by 2035”.
- Centrus Energy’s demonstration cascade at Piketon, Ohio, had produced and delivered over 920 kg of HALEU to the Department of Energy by mid-2025, according to the World Nuclear Association. That is under 2% of the Department of Energy’s 2035 demand estimate for a single year (0.92 ÷ 50 = 1.8%).
- Urenco is building a HALEU facility at Capenhurst in the UK, targeting production by 2031, and Orano is developing a centrifuge enrichment plant, Project IKE, at Oak Ridge, Tennessee.
This is a real constraint, but it should not be overstated. It is a matter of building enrichment capacity, which is known technology, along with the plants that convert enriched uranium into fuel and the containers licensed to ship it, rather than of any physical shortage of uranium. The question for any HALEU-fuelled design is whose enrichment plant will supply its first cores, and when.
Companies in this dataset working on uranium enrichment include Centrus Energy, Urenco, Orano (whose Georges Besse II plant at Tricastin it calls the largest enrichment plant in Europe), Silex Systems (laser enrichment, licensed to Global Laser Enrichment) and General Matter. Quantum Leap Energy, the nuclear-fuel subsidiary of ASP Isotopes, says its technologies should be able to enrich uranium with further development. TRISO-X and BWX Technologies fabricate TRISO fuel.
Further reading on this site
- How fission reactors work
- Reactor types explained
- Microreactors, the designs most dependent on HALEU and TRISO
- Supply chain
Sources
- Wikipedia, Enriched uranium (natural abundance, LEU, HEU and HALEU boundaries, SWU and centrifuge energy use)
- US Department of Energy, What is high-assay low-enriched uranium (HALEU)?
- US Department of Energy, HALEU Availability Program
- World Nuclear Association, High-assay low-enriched uranium (HALEU)
- US Department of Energy, TRISO particles: the most robust nuclear fuel on Earth
- World Nuclear Association, Mixed oxide fuel (MOX)
- World Nuclear Association, Thorium
- Orano
- General Matter, Mission
- Quantum Leap Energy
- Silex Systems
- World Nuclear Association, Nuclear Power Reactors (PHWR fuel)
- IAEA, Small Modular Reactor Technology Catalogue 2024 (Natrium fuel)
- Wikipedia, Experimental Breeder Reactor II
- Wikipedia, TMSR-LF1
- American Nuclear Society, Project Pele progress: BWXT delivers fuel to INL (TRISO fabrication by BWXT)
- Enrichment table: calculated on this page with the standard value function V(x) = (2x − 1) ln(x / (1 − x)), feed 0.711%, tails 0.25%.
Related pages
- Low-enriched uranium · glossary
- High-assay low-enriched uranium · glossary
- TRISO particle fuel · glossary
- Mixed oxide fuel · glossary
- Thorium · glossary
- Fuel enrichment · glossary
- Fuel cycle · glossary
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