Conversion and Deconversion
(Updated April 2014)
- Plants to convert uranium oxide to UF6 are operating commercially in several countries.
- Conversion to UF6 is achieved by a dry fluoride volatility process in the USA, while all other converters use a wet process.
- 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 being converted to uranium hexafluoride (UF6), commonly referred to as 'hex'.
Conversion plants are operating commercially in USA, Canada, France, UK, Russia and China. The main new plant is Areva’s Comurhex, operating between two sites in France.
Conversion of uranium oxide to UF6 is achieved by a dry fluoride volatility process in the USA, while all other converters use a wet process.
World Primary Conversion capacity
(tonnes U as UF6)
|Cameco, Port Hope, Ont, Canada
|Cameco, Springfields, UK
|JSC Enrichment & Conversion Co (Atomenergoprom),
Irkutsk & Seversk, Russia
|Comurhex (Areva), Malvesi (UF4) & Tricastin (UF6), France
|Converdyn, Metropolis, USA
|CNNC, Lanzhou, China
WNA Global Nuclear Fuel Market report 2011 & 2013
* Operating capacity estimated at 15,000 tU/yr
After initial refining of U3O8 (or peroxide), which may involve the production of uranyl nitrate, uranium trioxide is reduced in a kiln by hydrogen to uranium dioxide. This is then reacted in another kiln with hydrogen fluoride (HF) to form uranium tetrafluoride (HF4). The tetrafluoride is then fed into a fluidised bed reactor with gaseous fluorine to produce UF6. The alternative wet process involves making the tetrafluoride from uranium oxide by a wet process, using aqueous HF.
Some secondary supplies, from downblended high-enriched uranium or re-enriched tails (see below) may be supplied or already exist in the form of UF6. Recycled uranium from reprocessing plants needs to be converted so that it can be enriched.
Chemistry of Conversion
In the dry process, uranium oxide concentrates are first calcined (heated strongly) to drive off some impurities, then agglomerated and crushed.
For the wet process, the concentrate is dissolved in nitric acid. The resulting 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 fluidised bed reactor to produce UO3 (or UO2 if heated sufficiently).
Purified 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 deltaH = -109 kJ/mole
or UO3 + H2 ===> UO2 + H2O deltaH = -109 kJ/mole
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 deltaH = -176 kJ/mole
The tetrafluoride is then fed into a fluidised 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 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.
Depleted uranium and deconversion
Depleted uranium (DU) 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 volatalisation. To early 2007, about one quarter of the world's 1.5 million tonnes of DU had been deconverted. World deconversion capacity was about 60,000 t/yr at end of 2010.
The main deconversion plant is the 20,000 t/yr one run by Areva NC at Tricastin, France, though 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. A 6500 t/yr plant is being built at New Mexico in the USA by International Isotopes (INIS). In the UK, Urenco ChemPlants is building a plant and expects 2016 start-up.
Russia’s W-ECP deconversion plant is at Zelenogorsk Electrochemical Plant (ECP) in Siberia. The 10,000 t/yr deconversion (defluorination) plant was built by Tenex under a technology transfer agreement with Areva NC, 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 Areva’s W2 plant at Pierrelatte in France and has mainly west European equipment. It was commissioned in December 2009.
Russia is also building a plant at Angarsk to deconvert UF6 to UF4, recovering some HF in the process. Capacity of 2000 t/yr was planned for 2012, with subsequent increase to 6000 t/yr.
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 tonnes per year in the one plant.
The UF6 is first vapourised 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.
UF6 + 2H2O ==⇒ UO2F2 + 4HF
3UO2F2 + 2H2O + H2 ===> U3O8 + 6HF
The INIS plant 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. 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.