Conversion and Deconversion
- Uranium enrichment requires uranium as uranium hexafluoride, which is obtained from converting uranium oxide to UF6.
- Conversion plants are operating commercially in Canada, China, France, Russia and the USA.
- Deconversion of depleted UF6 to uranium oxide or UF4 is undertaken for long-term storage of depleted uranium in more stable form.
Uranium leaves the mine as the concentrate of a stable oxide known as U3O8 or as a peroxide. It still contains some impurities and prior to enrichment has to be further refined before or after being converted to uranium hexafluoride (UF6), commonly referred to as 'hex'. Both processes are normally included in the step between the mine and enrichment plant – referred to as 'conversion'.
Estimated world primary conversion capacity and UF6 production 2022
| Company | Country | Location | Licensed capacity (tU) | UF6 production (tU) |
| Cameco | Canada | Port Hope | 12,500 | 10,600 |
| CNNC (est.)* | China | Lanzhou & Hengyang | 15,000 | 10,500 |
| ConverDyn | USA | Metropolis | 7000 | 0 |
| Orano | France | Pierrelatte, Malvesi |
15,000 |
8900 |
| Rosatom (est.) | Russia | Seversk | 12,500 | 12,000 |
| Total | 62,000 | 42,000 |
World Nuclear Association Nuclear Fuel Report (2023 edition)
* Estimated licensed capacity and 2022 production. The Nuclear Fuel Report assumes that China will develop its conversion capacity sufficient to supply the needs of the domestic reactor fleet.
Conversion process
The main, 'wet' process, is used by Cameco in Canada, by Orano in France, at Lanzhou in China and Seversk in Russia. For the wet process, the concentrate is first dissolved in nitric acid. The resulting clean solution of uranyl nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined in a fluidized bed reactor to produce UO3 (or UO2 if heated sufficiently).
Alternatively, the uranyl nitrate may be concentrated and have ammonia injected to produce ammonium diuranate, which is then calcined to produce pure UO3.
Crushed U3O8 from the dry process and purified uranium oxide UO3 from the wet process are then reduced in a kiln by hydrogen to UO2:
U3O8 + 2H2 ===> 3UO2 + 2H2O ΔH = -109 kJ/mol
or UO3 + H2 ===> UO2 + H2O ΔH = -109 kJ/mol
This reduced oxide is then reacted in another kiln with gaseous hydrogen fluoride (HF) to form uranium tetrafluoride (UF4), though in some places this is made with aqueous HF by a wet process:
UO2 + 4HF ===> UF4 + 2H2O ΔH = -176 kJ/mol
The tetrafluoride is then fed into a fluidized bed reactor or flame tower with gaseous fluorine to produce uranium hexafluoride, UF6. Hexafluoride ('hex') is condensed and stored.
UF4 + F2 ===> UF6
Removal of impurities takes place at each step.
The alternative, 'dry' process is used in the USA. In the dry process, uranium oxide concentrates are first calcined (heated strongly) to drive off some impurities, then agglomerated and crushed. At Converdyn’s US conversion plant, U3O8 is first made into impure UF6 and this is then refined in a two-stage distillation process.
UF6, particularly if moist, is highly corrosive. When warm it is a gas, suitable for use in the enrichment process. At lower temperature and under moderate pressure, the UF6 can be liquefied. The liquid is run into specially designed steel shipping cylinders which are thick walled and weigh over 15 tonnes when full. As it cools, the liquid UF6 within the cylinder becomes a white crystalline solid and is shipped in this form.
The siting, environmental and security management of a conversion plant is subject to the regulations that are in effect for any chemical processing plant involving fluorine-based chemicals.
Secondary sources of conversion supply
Secondary supply of equivalent conversion services includes UF6 material from commercial and government inventories, enricher underfeeding, and DU tails recovery. Uranium and plutonium recycle effectively adds to this. All these were estimated at 26,000 tU in 2013 but with the end of the Russian HEU supply to the USA, they are now much less – estimated to be about 9000 tU in 2021. By 2030 they are expected to be less than 5000 tU.
Depleted uranium and deconversion
Up to 90% of the original uranium feed ends up as depleted uranium (DU), which is stored long-term as UF6 or preferably, after deconversion, as U3O8, allowing HF to be recycled. It may also be deconverted to UF4, which is more stable, with much higher temperature of volatilization. DU stockpiles were estimated to be 1.2 million tU in 2021.
The largest deconversion plant is the 20,000 t/yr one run by Orano at Tricastin, France, and over 300,000 tonnes has been processed here. The technology has been sold to Russia. Two plants have been built by Uranium Disposition Services (UDS) at Portsmouth and Paducah, USA, with capacities of 13,500 and 18,000 t/yr respectively.
Uranium Deconversion Plants
| Operator | Location | Capacity tU/yr |
| Orano | Tricastin, France | 20,000 |
| Richland, Washington, USA | small | |
| Urenco ChemPlants | Capenhurst, UK | 10,000 |
| Mid America Conversion Services | Portsmouth, Ohio, USA | 13,500 |
| Paducah, Kentucky, USA | 18,000 | |
| INIS Fluorine Products | Hobbs, New Mexico, USA | 6500 (construction on hold) |
| Tenex | Zelenogorsk, Russian Federation | 10,000 |
Russia’s W-ECP deconversion plant is at Zelenogorsk Electrochemical Plant (ECP) in Siberia. The 10,000 tU/yr deconversion (defluorination) plant was built by Tenex under a technology transfer agreement with Orano, so that depleted uranium can be stored long-term as uranium oxide, and HF is produced as a by-product. The W-ECP plant is similar to Orano’s W2 plant at Pierrelatte in France and has mainly west European equipment. It was commissioned in December 2009 and to January 2021 had processed 100,000 tonnes of depleted uranium. TVEL is building a second unit, W-ECP2, with equipment from Orano Projects in France. This was to double the plant's capacity from 2023, although as of October 2024 it was not clear if the project was still progressing.
In the UK, Urenco ChemPlants has built the 10,000 t/yr Tails Management Facility (TMF) at Capenhurst to treat Urenco's European inventory of depleted uranium, which is stored there. The total investment was about £1 billion. It enables the recycling of about 5000 tonnes of HF per year for industrial use. In November 2023 Urenco announced that it had selected Bechtel to support a proposed expansion of the facility.
These use essentially a dry process, with no liquid effluent. It is the same as that used for the enriched portion, albeit at a scale of 20,000 tU/yr in the one plant.
The UF6 is first vaporized in autoclaves with steam, then the uranyl fluoride (UO2F2) is reacted with hydrogen at 700°C to yield an HF product for sale to converters and U3O8 powder which is packed into 10-tonne containers for storage.
A 6500 t/yr plant in the USA was bought by International Isotopes (INIS) to be set up in New Mexico, and this has been licensed, but development was put on hold in 2013.
UF6 + 2H2O ==⇒ UO2F2 + 4HF
3UO2F2 + 2H2O + H2 ===> U3O8 + 6HF
The INIS plant formerly in Idaho uses a slightly different deconversion followed by fluorine extraction process (FEP), on a toll basis. The deconversion plant had been used to produce DU metal for the military and was purchased by INIS for the New Mexico project. In this, the depleted UF6 is first vapourised in autoclaves and hydrogen is added to give depleted UO2 and anhydrous UF4 which is the main product for sale. The FEP then involves reacting some UF4 with silica to give silicon fluoride (SiF4) as a commercial co-product.
Ownership title is normally transferred to the enricher as part of the commercial deal. It is sometimes considered as a waste, though only for legal or regulatory reasons and in the USA, but usually it is understood as a long-term strategic resource which can be used in a future generation of fast neutron reactors. Any much more efficient enrichment process would also make it into an immediately usable resource to supply more U-235. Enrichment companies with ownership of large amounts of depleted uranium are quite clear that their stocks are a significant asset, though Urenco speaks of deconversion being for long-term storage “prior to geological disposal”.
Notes & references
General references
World Nuclear Association, The Nuclear Fuel Report, Global Scenarios for Demand and Supply Availability 2023-2040 (September 2023)