Nuclear fuels: uranium, MOX, HALEU, thorium

Behind arguments about nuclear power stations sits a quieter, more strategic issue: nuclear fuels-how they are produced, where the raw materials are found, and which countries are likely to command supply chains over the coming decades.

Nuclear fuels (LEU, MOX, HALEU, TRISO and thorium): what keeps reactors running today

Most operating reactors rely on a small family of isotopes. Each option comes with its own engineering trade-offs and geopolitical implications.

Uranium: the workhorse the grid still depends on

Although uranium ore is not rare, only a tiny fraction of natural uranium is directly useful for most reactors. About 0.72% is uranium‑235 (U‑235)-the fissile isotope that sustains a chain reaction-while the bulk is uranium‑238 (U‑238), which largely acts as ballast in many current reactor designs.

To make the material suitable for mainstream reactors, industry increases the U‑235 proportion through enrichment until it typically reaches about 3–5%. This product is called low enriched uranium (LEU), and it supplies the overwhelming majority of pressurised water reactors and boiling water reactors across the world.

LEU underpins global nuclear generation: it is proven, widely standardised, and supported by an end‑to‑end industrial system spanning mining through to storage of spent fuel.

A major strength of LEU is how established it is: regulators have long experience with it, reactor operators are trained around it, and a supply chain controlled by relatively few firms has been refined over many decades.

MOX: turning “waste” plutonium back into fuel

Mixed oxide fuel (MOX) is made by taking plutonium recovered from spent fuel and combining it with depleted uranium. In practical terms, a material once treated mainly as a disposal burden is re-framed as an energy resource with strategic value.

In a closed fuel cycle, using MOX can reduce natural uranium requirements by roughly 20%. France is often cited as the reference case: its industrial recycling approach routinely loads MOX assemblies into part of its reactor fleet.

The appeal is clearest for countries anxious about long-run uranium prices. However, MOX also brings difficult chemistry, stringent safeguards, and higher initial costs.

HALEU: the coming fuel for small modular reactors

HALEU-short for High Assay Low Enriched Uranium-sits between conventional LEU and weapons-grade material, with enrichment in the range of 5–20% U‑235.

That band is particularly well suited to many small modular reactors (SMRs) and a number of Generation IV concepts. By packing more fissile atoms into the fuel, designers can shrink the core and extend operation for longer periods between refuelling.

What HALEU offers developers: longer fuel cycles, more compact reactors, and fewer outages-precisely the performance profile many next‑generation designs are aiming for.

The constraint is not physics but production capacity. Only a limited number of facilities can make HALEU at scale today, and several are in Russia-a dependency that is prompting concern in Western capitals planning rapid SMR deployment.

TRISO: fuel engineered not to melt

TRISO (tristructural‑isotropic) is best understood not as a conventional pellet, but as a micro‑engineered particle. Each tiny kernel of uranium is enclosed in multiple layers of ceramic and carbon, forming a protective, onion‑like structure around the fuel.

These particles can withstand temperatures above 1,600°C while still retaining fission products. That capability makes TRISO attractive for high‑temperature gas‑cooled reactors, where designers pursue “walk‑away safety”-meaning that even severe incidents struggle to compromise the fuel itself.

The downside is the manufacturing challenge. Producing millions (or billions) of particles to extremely tight tolerances is complex, and that difficulty is reflected in cost.

Thorium: the slow‑burn challenger

By itself, thorium‑232 (Th‑232) is not fissile. Inside a reactor, though, it can absorb a neutron and, through subsequent transformations, become uranium‑233 (U‑233)-a fissile isotope with broadly comparable behaviour to U‑235.

Because they hold large thorium resources, India and China treat this pathway as a long-term strategic option. Both are investing heavily in molten‑salt reactors and other designs intended to operate on thorium‑based fuel cycles.

Thorium is not a quick fix: it is a slower‑moving alternative that could significantly alter fuel security in the latter half of the century.

Advocates point to thorium’s abundance and the possibility of generating fewer long‑lived waste components. Sceptics counter that the necessary industrial ecosystem-fuel fabrication through to reprocessing-would need to be created largely from the ground up.

Energy density that bends intuition

At the nuclear scale, energy density defies everyday comparison. A typical fission releases about 200 MeV (million electronvolts) per event, which corresponds to close to 80 million megajoules per kilogram of fuel.

By contrast, coal contains around 24 megajoules per kilogram. On a mass basis, fission is therefore on the order of 10 million times more energetic than burning coal.

• 1 kg of uranium fuel: can keep a city supplied with electricity for days
  
• 1 kg of coal: can be consumed within minutes in a power station boiler

Even among fissile options, the differences influence design choices:

Isotope	Energy per fission (MeV)	Average neutrons released	Typical use
Uranium‑235	~193	~2.45	Conventional thermal reactors
Plutonium‑239	~199	~2.9	Fast reactors, MOX
Uranium‑233	~191	~2.5	Thorium‑based cycles

That higher neutron yield from plutonium helps explain why it is so desirable for fast breeder reactors, which can be configured to create more fissile material than they consume.

Reserves and geopolitics: who owns the atoms?

Uneven uranium, better‑spread thorium

Estimated recoverable uranium resources are about 7.9 million tonnes. Annual demand is currently around 69,000 tonnes, and could more than double by 2040 if the much‑discussed nuclear resurgence becomes reality.

Australia has the largest uranium reserves, with Kazakhstan and Canada also holding major shares. Yet Kazakhstan leads actual production, supplying over 40% of global mine output via its state-backed champion Kazatomprom.

Mining and enrichment are becoming politically charged in much the same way that gas pipelines were in the 2000s.

Thorium is estimated at roughly 6.3 million tonnes of resources. It is three to four times more abundant in the Earth’s crust than uranium and is distributed more evenly. India, the United States, and Australia all hold substantial deposits, lowering the chance that a single supplier could dominate the market if thorium reactors scale up.

Open, closed and alternative cycles: what happens to spent fuel?

Open cycle: use once, store for the long term

A number of countries-including the United States-continue to operate an open fuel cycle. Spent fuel is first cooled in pools and then transferred into dry casks for extended storage, without chemical reprocessing.

A gigawatt‑scale pressurised water reactor operating for one year produces around 28.8 tonnes of highly radioactive spent fuel, as well as large quantities of residues from mining and milling.

This pathway simplifies the industrial chain, but it also leaves future generations responsible for managing long‑lived materials for centuries or longer.

Closed cycle: recycle and shrink the waste footprint

Countries such as France and Russia pursue closed cycles, where spent fuel is chemically processed to separate uranium and plutonium. The recovered plutonium is used to make MOX, and the recovered uranium can be re‑enriched or kept for potential use in future fast‑reactor programmes.

Reprocessing can reduce the volume of final high‑level waste by around a factor of four. The trade-off is that what remains is more heat‑generating in the near term, and reprocessing facilities must be run under strict safeguards to manage proliferation concerns.

Thorium cycle: fewer long‑lived nasties

A key argument for thorium is that it can reduce the creation of minor actinides-elements that persist for hundreds of thousands of years and dominate the long‑term radiotoxicity profile of conventional waste streams.

There is also an important complication for misuse. U‑233 bred from thorium commonly contains traces of uranium‑232 (U‑232), which produces intense gamma radiation. That contamination makes any hypothetical military handling far more difficult, a characteristic valued by many non‑proliferation specialists.

Who holds the key pieces of the fuel market?

Mining and enrichment as strategic choke points

On the mining side, Kazatomprom, Cameco (Canada), and Orano (France) form an informal group of heavyweight suppliers. In enrichment, the field is tighter still.

Rosatom and its subsidiary Tenex account for a large share of global enrichment capacity-often quoted at about 40–50%. The European consortium Urenco holds roughly 30%, while Orano provides a smaller but meaningful additional slice.

Replacing Russian enrichment will be slow and difficult: new centrifuge capacity takes years to build, not months.

Fabrication and advanced fuels

For fuel fabrication, Western suppliers such as Westinghouse and Framatome provide LEU fuel assemblies to reactor fleets across Europe and Asia. For MOX, France’s Melox facility operated by Orano remains one of the few industrial‑scale manufacturing sites.

In the United States, companies including Centrus and BWXT are pushing to deliver HALEU for SMRs and other advanced reactors. Without that fuel, many high-profile “reactors of the future” risk delays for a very mundane reason: there may be no qualified supply available when needed.

Two practical realities often overlooked: conversion and fuel qualification

Before enrichment can even begin, uranium concentrate must be converted into forms suitable for the front end of the fuel cycle (commonly as uranium hexafluoride for enrichment, and then into uranium dioxide for fabrication). These intermediate steps can become bottlenecks in their own right, especially when countries try to re-shore or diversify supply chains quickly.

Equally, new fuels are not deployed simply because they exist on paper. Whether the target is HALEU, TRISO, or novel MOX variants, qualification involves irradiation testing, post‑irradiation examination, licensing work, and factory learning curves. Those timelines can stretch across years, influencing how quickly new reactor concepts can move from demonstration to fleet-scale rollout.

Beyond fission: how fusion frames the debate

Investors with a long horizon often wonder whether fusion will make today’s fuel questions irrelevant. For the moment, that remains speculative.

Fusion uses hydrogen isotopes-deuterium and tritium-rather than uranium or plutonium. The flagship deuterium–tritium reaction releases about 17.6 MeV per event and, by mass, can yield roughly four times the energy of fission fuels.

However, tritium supply is itself a major hurdle. It must be bred from lithium in specialised blankets around the plasma, and no commercial system has yet demonstrated reliable, self‑sustaining tritium production.

ITER, the huge experimental reactor being built in southern France, is intended to show that fusion can produce more energy than it consumes. Even under optimistic assumptions, widespread commercial fusion before the 2040s would be a stretch-meaning fission fuels are likely to remain central for some time.

Key concepts readers often ask about

Actinides, toxicity and time scales

People often ask what makes nuclear waste most hazardous. Much of the near‑term risk comes from fission products, many of which decay substantially over a few centuries. The long-duration challenge is driven by heavy elements known as actinides-including plutonium, americium, curium and others.

Closed fuel cycles, along with future fast reactors, aim to burn or transmute a greater share of these actinides, shortening the period during which waste requires extreme isolation. Thorium cycles may also contribute by producing fewer of these elements in the first place.

What an HALEU‑fuelled grid could look like

Some analysts outline futures where dozens-or even hundreds-of HALEU‑fuelled SMRs are deployed near industrial clusters, supporting renewables while also supplying high‑temperature heat for hydrogen production or district heating. With refuelling intervals of 8–15 years, such reactors could reduce the operational churn typical of today’s large units.

That vision comes with its own vulnerabilities: more individual sites to secure, more movements of specialised fuel, and a heavy reliance on a still‑developing HALEU supply chain. Governments considering HALEU‑centred strategies therefore have to weigh not only cost and carbon objectives, but also long‑term resilience and security of supply.