HALEU vs natural uranium: what fuel choice means for SMRs
Fuel is the least discussed and most decisive choice in reactor design. Most advanced small modular reactors and microreactors need a fuel called HALEU that is not yet made at commercial scale. A smaller group of designs avoid that dependency by running on natural uranium, which needs no enrichment.
Enrichment, in one paragraph
Uranium as mined is mostly U-238, with a small fraction of U-235, the isotope that sustains a chain reaction in most reactors. The World Nuclear Association describes natural uranium as having the same elemental composition as when it was mined (0.7% U-235, over 99.2% U-238). Enrichment raises the U-235 share, commonly to 3.5% to 5% for today's light-water reactors.
What HALEU is, and why designers want it
DOE defines HALEU as enriched between 5% and less than 20%. It says HALEU is required for most U.S. advanced reactors to achieve smaller designs that get more power per unit of volume, and that it allows longer operating cycles, increased efficiencies and better fuel utilisation. For a compact core, higher enrichment is the natural route to more power from less space.
DOE's microreactor guidance makes the same point. Most designs will require fuel with a higher concentration of uranium-235 that's not currently used in today's reactors. See micro modular reactors.
The HALEU supply problem
The catch is supply. DOE says the majority of advanced designs will require a fuel that isn't yet available at a commercial scale, and that without it the deployment of many advanced nuclear technologies may be in jeopardy. DOE describes three supply routes:
- Down-blending existing material. Recovering highly enriched uranium from used research-reactor fuel and blending it down.
- A centrifuge demonstration. DOE partnered with Centrus to run 16 advanced centrifuges at Piketon, Ohio.
- A commercial supply chain. Through its HALEU Availability Program, DOE awarded its first contracts in 2024.
World Nuclear Association adds that HALEU also needs specific fuel-cycle infrastructure, new or modified regulations and new or modified transport containers. We cover trade-offs in disadvantages of small modular reactors.
How natural uranium reactors work
A reactor can run on natural uranium if it slows neutrons very efficiently. WNA explains that if graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium.
The best-known example is Canada's CANDU, a pressurised heavy-water reactor (PHWR). It has a long operating history. Learn more on our CANDU reactor page.
The trade-offs of natural uranium
Natural uranium is not free of drawbacks:
- Heavy water. The moderator must be heavy water or graphite. Heavy water is a specialist material with its own supply and handling needs.
- Used fuel volume. WNA notes that the PHWR produces more energy per kilogram of mined uranium than other designs, but also produces a much larger amount of used fuel per unit output.
- Core size. Lower enrichment generally means a larger core for a given output.
The advantages are equally concrete. There is no dependence on enrichment, let alone HALEU. The fuel supply chain for natural uranium is established.
Questions to ask a reactor vendor about fuel
Fuel risk is project risk. Before relying on any SMR or microreactor schedule, ask:
- What enrichment does the first core need? LEU, HALEU or natural uranium, and at what U-235 percentage.
- Who fabricates the fuel, and is that line built?
- Is there a signed supply agreement for the first cores and reloads?
- Which transport packages are licensed for this fuel?
- How is used fuel stored on site, and for how long?
- What happens to the schedule if fuel is late?
Where BridgePower Nuclear fits
Until the reactors arrive, BridgePower Nuclear serves sites with thermal power from USP&E, backed by 24+ years in power, 150+ power projects in 45+ countries and 2,000+ MW in stock.
For a comparison of reactor classes, read small modular reactors or SMR vs microreactor, or contact our team.
Frequently asked questions
What is HALEU?
High-assay low-enriched uranium is uranium enriched to between 5% and less than 20% U-235. Today's commercial reactors use fuel enriched up to 5%.
What is natural uranium?
Natural uranium has the same isotopic make-up as when it was mined: about 0.7% U-235 and over 99.2% U-238. It can be used as reactor fuel without enrichment if the reactor uses heavy water or graphite as its moderator.
Why do advanced reactors need HALEU?
DOE says HALEU is required for most U.S. advanced reactors to achieve smaller designs that get more power per unit of volume, and allows longer operating cycles and better fuel utilisation.
Is HALEU commercially available?
Not yet at commercial scale. DOE says most advanced designs require a fuel that is not yet available at commercial scale, and is supporting supply through down-blending, a centrifuge demonstration and its HALEU Availability Program.
What are the downsides of natural uranium fuel?
Natural uranium reactors need an efficient moderator such as heavy water, which adds its own supply and handling. WNA also notes that PHWRs produce a much larger amount of used fuel per unit of output than enriched-fuel designs.