Nuclear Fuel Production Chain: Ore to Fuel Assembly

Reference article · Updated 2026-11-06

Key facts

Mill concentrate (U3O8) typically contains more than 80% uranium; the ore it came from may hold as little as 0.1% (World Nuclear Association).
Enrichment raises U-235 from about 0.7% to typically 3.5-5% for light-water reactors.
A 1,000 MWe reactor needs roughly 24 t of enriched uranium per year, made from about 211 t U of natural uranium in the 4.5% case.
Fabrication plants are generally limited to 5% U-235 for criticality safety.

Simplified nuclear fuel cycle. Original schematic.

Five steps from rock to reactor core

The diagram above shows the front end of the fuel cycle. Uranium is extracted from ore bodies by open-pit, underground or in-situ methods, concentrated at a mill into a uranium oxide product, converted to a gas, enriched in the fissile isotope U-235, and finally made into ceramic fuel pellets in metal tubes. Each step is performed by different companies, often in different countries, which is why shipment of uranium between facilities is a standing feature of the industry. Some reactor types, such as pressurised heavy-water reactors, use natural uranium and skip conversion to UF₆ and enrichment; the page concentrates on light-water reactors, which make up most of the world fleet.

Mining and milling to U3O8

Ore grades vary widely; the World Nuclear Association notes that ore may contain as little as 0.1% uranium. Mining methods and the mines themselves are not covered in detail here; in-situ recovery is explained on the ISL page, and the waste left by conventional milling on the mill tailings page. At a conventional mill, ore is crushed and leached with acid or alkali, and the dissolved uranium is recovered and precipitated. The product is a concentrate commonly called yellowcake, sold as U₃O₈ and typically containing more than 80% uranium by mass. In-situ recovery operations produce the same type of concentrate from solution without conventional milling. The concentrate is the form in which most uranium is traded.

Conversion to uranium hexafluoride

Enrichment plants operating at present process uranium as a gas, and UF₆ is the only practical compound for that. At a conversion plant the concentrate is purified and turned into UF₆. The World Nuclear Association describes two industrial routes: a wet process using solvent extraction, in use at facilities such as those of Cameco, Orano and in China and Russia, and a dry process with fluorination and distillation, used at the ConverDyn plant in Metropolis in the United States. UF₆ is corrosive, especially when moist, is a gas when warm, and is shipped as a solid in thick-walled steel cylinders that weigh more than 15 tonnes when full. Specific plants are listed on the facilities page.

Enrichment

Natural uranium is about 0.7% U-235; most light-water reactors need 3.5-5%. Enrichment today uses gas centrifuges, in which UF₆ flows through cascades of rapidly spinning cylinders that gradually separate lighter U-235 from U-238. The stream with more U-235 becomes the product, and the other, depleted in U-235, becomes the tails. The work involved is measured in separative work units, calculated on the SWU calculator page. The World Nuclear Association reports centrifuge plants using roughly 40-50 kWh per SWU, against 2,400-2,500 kWh for gaseous diffusion, the older technology. Depleted tails are a by-product with their own stocks and uses (see depleted uranium).

Fabrication into pellets, rods and assemblies

Enriched UF₆ is converted to uranium dioxide powder, either by a dry or by a wet route. The powder is pressed into pellets, sintered at about 1,750 °C and machined to size. Pellets, just under 1 cm in diameter, are loaded into zirconium-alloy tubes that are pressurised with helium and welded shut; the rods are then assembled into a structural framework. According to the World Nuclear Association, a typical pressurised-water assembly is 4-5 m tall and weighs about half a tonne, and the core of an 1,100 MWe PWR holds about 193 assemblies, over 50,000 rods and about 18 million pellets. Fabrication is the last front-end step; assemblies leave the plant for the reactor.

Mass flows for one reactor year

The World Nuclear Association illustrates the scale for a 1,000 MWe light-water reactor. In its 4.5% case, about 24.3 t of enriched uranium (27.6 t as UO₂) is loaded per year, produced from about 211 t of natural uranium, equivalent to 249 t U₃O₈, at a tails assay of 0.22%. The ratios can be checked with the formulas on the material balance page: at a 0.25% tails assay, each kilogram of 4.5% product needs about 9.2 kg of natural uranium and about 6.9 SWU.

Stage	Form	Typical quantity per year, one 1,000 MWe reactor (4.5% case, WNA)
Mine output	U₃O₈	about 249 t
Conversion feed	Natural uranium as UF₆	about 211 t U
Enrichment product	Enriched uranium	about 24.3 t U
Fabrication output	UO₂ in assemblies	about 27.6 t

The result is that more than 85% of the natural uranium mass leaves enrichment as tails; at the shares in this example the figure is 211 minus 24.3 over 211, or about 88%. The fuel's cost structure is described on the fuel-cost page.

Frequently asked questions

What is yellowcake?

Yellowcake is the informal name for uranium oxide concentrate produced at a mill, typically sold as U3O8 containing more than 80% uranium by mass.

Why is uranium converted to UF6?

Gas-centrifuge and diffusion enrichment need a process gas. UF6 is the practical compound because it becomes a gas at modest temperatures.

How much natural uranium does a reactor use per year?

For a 1,000 MWe light-water reactor, the World Nuclear Association gives about 211 t of uranium (249 t U3O8) in its 4.5% enrichment case, with a 0.22% tails assay.

Do all reactors need enriched uranium?

No. Pressurised heavy-water reactors can run on natural uranium, so their fuel skips the conversion to UF6 and enrichment steps.

Why is fuel enrichment limited to about 5% in fabrication plants?

The World Nuclear Association states that fabrication plants are generally limited to 5% U-235 for criticality safety.