Enrichment levels

The scale

Natural uranium is already a usable fuel, for some reactors

Uranium as mined is 0.711% U-235 and 99.3% U-238. That is enough to sustain a chain reaction if the moderator absorbs few enough neutrons, which is why 46 operating heavy water reactors and the graphite-moderated Magnox designs before them ran with no enrichment plant anywhere in their supply chain. Every other family needs the U-235 fraction raised, and that is a separate industry with four significant participants.

Why 20% is the line that matters

The 20% figure is not a physical threshold. It is a safeguards convention: below it, material is treated as impractical for a weapon without further enrichment, and it is designated low-enriched. Above it, the material is highly enriched uranium, and the licensing, physical protection, transport and international reporting requirements all change.

That is why HALEU, at 5% to 20%, is defined the way it is. It sits as high as the material can go while remaining LEU. Designers want it because more fissile material per unit volume means a smaller core, longer intervals between refuelling, and in fast-spectrum designs the ability to sustain a reaction at all.

What the extra enrichment costs

Enrichment is not linear. Raising a kilogram of uranium to 19.75% takes far more than four times the work of raising it to 4.4%, because each additional percentage point is separated from an ever more depleted stream.

Computed from the standard enrichment equations at a 0.22% and 0.25% tails assay respectively. A kilogram of HALEU takes 5.8 times the separative work of a kilogram of ordinary fuel, and that is before the licensing and transport requirements above 10% U-235, which no published price reflects. See what a SWU is for where these numbers come from.

The proliferation arithmetic

The reason enrichment capacity is watched as closely as it is comes out of the same equations. Taking natural uranium all the way to 90% takes about 208 SWU per kilogram of product. Taking it only as far as ordinary 4.4% reactor fuel, and then feeding that fuel back into the cascade, leaves just 47 SWU per kilogram to go.

In other words, about 78% of the total separative work is already spent by the time uranium becomes ordinary civil reactor fuel. This is a published figure in the nonproliferation literature rather than anything sensitive, and it is the whole reason enrichment plants, not reactors, are the facilities under safeguards scrutiny.

Where each class actually turns up

• LEU below 5% is what essentially the entire operating fleet runs on. Every unit on the fleet list with an enrichment figure of 3-5% is here.
• HALEU is required by 12 of the 26 designs on the SMR tracker. Western production capacity for it is tiny: the facilities on this site are American Centrifuge Plant, Piketon, BWXT TRISO facility, Gillette, General Matter, Idaho National Laboratory EBR-II downblending, Nuclear Fuel Services, Erwin, Savannah River Site HEU downblending, TRISO-X Fuel Facility (TX-1), Oak Ridge, Urenco Capenhurst HALEU line. See the HALEU stage page.
• HEU is used in 15 of the 24 naval propulsion programmes where the enrichment class is on record. A submarine core enriched high enough never needs refuelling in the hull's life, which is an operational advantage no civil reactor has a reason to want.
• Weapons-grade material is covered under nuclear weapons stockpiles, kept deliberately separate from the civil fleet pages on this site.