Conversion and Deconversion

(Updated January 2019)

  • Uranium enrichment requires uranium as uranium hexafluoride, which is obtained from converting uranium oxide to UF6.
  • Conversion plants are operating commercially in the USA, Canada, France, Russia and China.
  • 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'.

Conversion plants are operating commercially in the USA, Canada, France, Russia and China. The main new plant is Areva’s Comurhex, operating between two sites in France. China’s capacity is expected to grow considerably through to 2025 and beyond to keep pace with domestic requirements.

World Primary Conversion Capacity 

Company Location Nameplate capacity 
(tonnes U/yr as UF6)
Approx capacity
utilisation
Capacity
utilisation
tU/yr
Cameco Port Hope, Canada 12,500 50% 6250
TVEL (Rosatom) SGCE Seversk, Russia 18,000 100% assumed 18,000
Areva Pierrelatte, France 15,000 70% 10,500
ConverDyn Metropolis, USA

7000

100% 7000
CNNC Lanzhou, China* 5000 80% 4000
IPEN Brazil 100 70% 70
World Total   57,600   45,820

World Nuclear Association Nuclear Fuel Report 2017; World Nuclear Association information paper on China's Nuclear Fuel Cycle.

* Information on China's conversion capacity is uncertain. An additional 9000 t/yr plant is reported to be under construction at Lanzhou, as well as a 3000 tU/yr plant at Hengyang in Hunan.

Conversion process

The main, 'wet' process, is used by Cameco in Canada, by Areva 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 fluidised 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 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 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 – an estimated 10,000 tU in 2017. By 2030 they are predicted to be less than 9000 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 volatalisation. To early 2007, about one-quarter of the world's 1.5 million tonnes of DU had been deconverted. 

The main deconversion plant is the 20,000 t/yr one run by Areva NC 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. A 6500 t/yr plant is being built at New Mexico in the USA by International Isotopes (INIS). In the UK, Urenco ChemPlants has built a 15,000 t/yr plant.

Uranium Deconversion Plants

Operator Location Capacity tU/yr
Areva Tricastin, France 20,000
  Richland, Washington, USA small
Urenco ChemPlants Capenhurst, UK 15,000
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 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 tU/yr was planned for 2012, with subsequent increase to 6000 tU/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 tU/yr 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, 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 2017-2035 (September 2017)

 

 


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