The uranium concentrate we followed in the previous part contains uranium in a solid chemical form. To become conventional fuel for light water reactors, the material must meet two further, distinct requirements. First, it must be prepared in a form suitable for isotope separation. Then, the proportion of uranium-235 within the uranium itself must be increased.
Conversion fulfils the first requirement, and enrichment the second. Conversion changes the compound in which the uranium is bound; enrichment changes the ratio between its isotopes. Keeping these two changes distinct helps us understand why several industrial stages lie between the concentrate and the finished fuel.
In this part, we follow the route from natural uranium to enriched uranium hexafluoride, UF₆, and the transition to fuel fabrication. This is the typical route for conventional uranium fuel used in pressurised water and boiling water reactors. Other reactor designs may use different fuels and preparation processes.
Chemical formula and isotopic composition
The formula U₃O₈ describes a uranium oxide with a specific ratio of uranium to oxygen. The formula alone does not tell us what proportion of the uranium is uranium-235. The same applies to UF₆: the formula tells us that one uranium atom is bonded to six fluorine atoms in the molecule, but it does not specify which isotope that uranium atom is.
Uranium-235 and uranium-238 are isotopes of the same element. Both have 92 protons in the nucleus, but they differ in the number of neutrons and therefore in mass. Their chemical properties are very similar, so ordinary purification of a uranium compound does not achieve isotopic enrichment.
As explained in Part 2, yellowcake is a collective term for uranium concentrates of different compositions. U₃O₈ is a common form and a useful example when describing the fuel cycle, but a facility must know the actual composition of the material it receives. The concentrate still needs further processing to reach the required purity and chemical form.
Conversion prepares the material for isotope separation
On the route to UF₆, uranium concentrate undergoes further purification and chemical conversion. Removing impurities and preparing the uranium compound are related tasks whose sequence depends on the technology used. Intermediate forms may include various uranium oxides and uranium tetrafluoride, UF₄.
Industrial processes also differ in how the stages are distributed between facilities. Cameco, for example, produces high-purity uranium trioxide, UO₃, during refining at Blind River. This becomes the feed material for subsequent conversion. In Orano’s process, the concentrate is purified and converted to UF₄ at Malvési, while conversion to UF₆ is completed at the Tricastin site.
The purpose of this route is to produce UF₆ suitable for enrichment. Throughout conversion, the uranium retains its natural isotopic composition. The term “converted uranium” therefore indicates that it has undergone a chemical transformation; it does not, by itself, mean that the uranium has been enriched.
Why we use uranium hexafluoride
A gas centrifuge requires uranium in a gaseous compound. UF₆ is suitable because it can be brought into the gas phase under appropriate temperature and pressure conditions. The uranium oxide in the concentrate is therefore first chemically converted to a fluoride.
At normal ambient temperature, UF₆ in a transport cylinder is predominantly solid. To feed it into the enrichment process, it is heated under controlled conditions and brought into the gaseous state. For shipment, the material is collected again and cooled in purpose-built cylinders. Its physical state therefore depends on where it is in the process and on the conditions at that stage.
The gas contains, among others, UF₆ molecules with uranium-235 and UF₆ molecules with uranium-238. Their structure is the same, but their masses differ slightly because of the different masses of the uranium atoms. Centrifugation exploits this difference.
It is therefore essential to distinguish the chemical conversion from oxide to fluoride from a change in the physical state of the same UF₆. Heating solid UF₆ to form a gas does not, by itself, enrich the uranium.
Why enriched uranium is needed
Natural uranium contains approximately 0.7 per cent uranium-235. Most of the remainder is uranium-238, with uranium-234 also present in trace amounts. Uranium-235 can undergo fission after absorbing a slow neutron and therefore plays an important role in sustaining the chain reaction in a light water reactor.
In a light water reactor, ordinary water slows neutrons down, making them more likely to cause further fissions, but it also absorbs some of them. Some neutrons are absorbed by other materials or escape from the core. The design must therefore ensure that enough neutrons remain to sustain the chain reaction. A higher proportion of uranium-235 increases the probability that a neutron will cause fission in the fuel.
Conventional fresh uranium fuel for light water reactors typically contains about 3 to 5 per cent uranium-235. The required proportion must be matched to the core design and the intended use of the fuel. This range is typical of conventional fuels; fuel designs with higher enrichment also exist.
Heavy water absorbs substantially fewer neutrons than ordinary water. This is an important reason why CANDU heavy water reactors can use natural uranium. The need for enrichment is therefore also linked to the choice of moderator, the material that slows neutrons down.
The enrichment percentage refers to the mass fraction of uranium-235 in the total mass of uranium. When we specify 4 per cent enrichment, the percentage applies to the uranium, not to the total mass of UF₆ including the fluorine. Even after enrichment to this level, most of the uranium is still uranium-238.
How a gas centrifuge separates isotopes
In rapidly rotating UF₆ gas, the heavier molecules containing uranium-238 are relatively more concentrated towards the outer edge, while the lighter molecules containing uranium-235 are relatively more concentrated towards the axis. This creates a small difference in isotopic composition between the two portions of gas withdrawn from the process.
Separation in any single step is partial. Both streams still contain both main isotopes, but in slightly different proportions. Several connected stages therefore form a cascade, in which the separation effect is repeated until the required product composition is reached. The material remains in the chemical form of UF₆ throughout.
The process redistributes isotopes that are already present. During enrichment, uranium-238 does not turn into uranium-235, and no new uranium-235 nuclei are created. Their proportion increases in the product stream and decreases in the other stream.
Gaseous diffusion was also historically important. It exploited differences in the rates at which molecules passed through porous barriers, but required substantially more electricity to achieve a comparable degree of separation. Gas centrifuges dominate modern industrial enrichment.
Enrichment produces two output streams
An enrichment facility receives UF₆ feed and produces an enriched product and a depleted stream. In conventional production of low enriched uranium from natural uranium, most of the uranium feed mass ends up in the depleted stream. The full quantity received therefore does not continue along the fuel fabrication route.
| Material stream | Chemical form during separation | Uranium-235 content |
| Natural uranium feed | UF₆ | Approximately 0.7% |
| Enriched product | UF₆ | Higher than in the feed; approximately 3 to 5% for conventional LWR fuel |
| Depleted stream | UF₆ | Lower than in the feed |
This comparison applies to the route described here, using natural uranium feed. Percentages indicate the mass fraction of uranium-235 in uranium. LWR stands for light water reactor.
The term tails refers to the depleted stream from enrichment. Mill tailings, discussed in Part 2 of the series, are the residues left by hydrometallurgical ore processing. These materials arise at different facilities and have different compositions. Depleted uranium is also distinct from spent fuel: on this route, it has not yet been used in a reactor.
What happens to depleted uranium
Depleted uranium contains less uranium-235 than natural uranium. It remains radioactive, and its handling must also take account of the chemical properties of the compound in which it is stored.
Depleted UF₆ can be stored in purpose-built cylinders. For longer-term management, it can be chemically converted to a more stable uranium oxide, such as U₃O₈. This removal of fluorine and conversion to another compound is called deconversion. Urenco uses this process at its Tails Management Facility.
Under certain conditions, depleted uranium can be re-enriched to recover some of the remaining uranium-235. The decision depends on its composition, available capacity and economics. Material intended for further use and material classified as waste follow different management routes. Both require planned and controlled handling.
Safety also includes chemical hazards
Conversion and enrichment involve radioactive material as well as substances with significant chemical hazards. UF₆ reacts with moisture to form, among other products, hydrogen fluoride, HF, and uranyl fluoride, UO₂F₂. The safety of such a facility therefore cannot be assessed solely from the radiation level near a cylinder.
Keeping substances contained within the process system, detecting leaks, controlling ventilation and handling cylinders correctly all help limit the potential for exposure. Maintenance and other work must also account for possible equipment contamination and the generation of waste and releases.
Preventing inadvertent criticality is important when working with enriched uranium. Equipment and procedures must maintain a subcritical state, in which the fission chain reaction is not self-sustaining. Requirements are determined for the actual material composition and conditions at the facility.
The IAEA therefore addresses the safety of conversion and enrichment facilities throughout their entire lifetime. Design, staff competence, maintenance, checks on equipment condition and emergency preparedness work together to control the hazards.
The material must remain traceable
Both material and its documentation move between facilities. The mass, chemical and isotopic composition, and identity of each shipment must be verifiable. Product quality must be confirmed before the material is accepted by the next stage of the fuel cycle.
Alongside quality and safety checks, the use of nuclear material is also subject to verification. Under the applicable agreements, IAEA safeguards serve to verify compliance with commitments to peaceful use. They include verification of records and measurements and, depending on the safeguards approach, the use of seals and surveillance. Their purpose differs from that of radiation protection, even though both concern the same material.
From enriched UF₆ to fuel fabrication
Enriched UF₆ is the product of one industrial stage and the feed material for the next. For conventional oxide fuel used in light water reactors, it must be converted to uranium dioxide, UO₂. The chemical conversion itself does not change the required uranium enrichment.
The prepared UO₂ powder is then formed and sintered into ceramic fuel pellets. These are inserted into fuel rods, which are assembled into fuel assemblies. At this stage, ceramic density, dimensions and manufacturing quality become critical alongside composition.
The next part will follow this fabrication process: how powder becomes a fuel pellet, and how individual components become a fuel assembly ready for use in a reactor.
Short glossary
Conversion
The chemical transformation of uranium into a form suitable for the next stage of the fuel cycle. In the route described here, the product is UF₆.
Isotope
One of the forms of an element, distinguished by the number of neutrons in the nucleus. Isotopes of the same element have the same number of protons.
Enrichment
Increasing the proportion of uranium-235 through isotope separation. The enrichment level is usually expressed as the mass fraction of uranium-235 in uranium.
Low enriched uranium or LEU
Uranium with a uranium-235 content above the natural level and below 20 per cent by mass. LEU stands for low enriched uranium.
Cascade
A connected system of separation stages in which the effect of isotope separation builds up progressively.
Depleted uranium
Uranium with a uranium-235 content below that of natural uranium. During enrichment, it is produced in the depleted stream, known as tails.
Deconversion
The chemical conversion of UF₆ into another form, such as a uranium oxide for fuel fabrication or further management of depleted uranium.
Subcriticality
A state in which the fission chain reaction is not self-sustaining.
Image credits
Cover photograph: a cascade of gas centrifuges used to produce enriched uranium. The photograph shows the U.S. Department of Energy gas centrifuge plant in Piketon, Ohio, in 1984. Source: Wikimedia Commons, U.S. Department of Energy, public domain. The photograph has been cropped and graphically adapted for use as the cover image.
Sources
NRC. Uranium Conversion. Chemical conversion of concentrate, UF₆ handling and chemical hazards.
Cameco. Uranium 101: Refining & Conversion. Refining, intermediate chemical forms and different fuel routes.
Orano. Tricastin. An industrial example of separate conversion and enrichment stages.
NRC. Uranium Enrichment. The physical principle of centrifugation and gaseous diffusion.
NRC. Frequently Asked Questions About Gas Centrifuge Enrichment Plants. Isotopic proportions, material streams and safety.
U.S. Department of Energy. Uranium Enrichment, Explained. 4 November 2025. The purpose of enrichment, fuel types and subsequent fabrication.
Urenco. Our role in nuclear fuel. In particular, the section How does Urenco enrich uranium? Physical states of UF₆, product verification and material records.
Urenco. Urenco ChemPlants. Management of depleted UF₆ and conversion to U₃O₈.
NRC. Deconversion of Depleted Uranium. Properties of depleted uranium and the purpose of deconversion.
IAEA. Safety of Conversion Facilities and Uranium Enrichment Facilities. Safety Standards Series No. SSG-5 (Rev. 1), 2023. In particular, Chapters 2, 5 and 8.
IAEA. IAEA Safeguards Glossary 2022 Edition. International Nuclear Verification Series No. 3 (Rev. 1). Concepts relating to nuclear material safeguards and verification of its use.
IAEA. Design of a nuclear reactor. Training material, Module 4. The relationship between the moderator and the choice of fuel.
Cameco. Uranium 101: Fuel Manufacturing. The transition from UO₂ powder to fuel pellets and fuel assemblies.
depleted uranium enriched uranium from ore to energy gas centrifuges isotope separation nuclear fuel nuclear fuel cycle U₃O₈ UF₆ uranium concentrate uranium conversion uranium enrichment uranium hexafluoride uranium-235 uranium-238
Last modified: October 17, 2026
