• Most of the 495 commercial nuclear power reactors operating or under construction in the world today require uranium ‘enriched’ in the U-235 isotope for their fuel.
• The main commercial process employed for this enrichment involves gaseous uranium in centrifuges. An Australian process based on laser excitation is under development in the USA.
• Prior to enrichment, uranium oxide must be converted to a fluoride so that it can be processed as a gas, at low temperature.
• From a non-proliferation standpoint, uranium enrichment is a sensitive technology needing to be subject to tight international control.
… Enrichment processes require uranium to be in a gaseous form at relatively low temperature, hence uranium oxide from the mine is converted to uranium hexafluoride in a preliminary process, at a separate conversion plant.
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.
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
Company – Nameplate Capacity (tonnes U as UF6)
Cameco, Port Hope, Ont, Canada – 12,500
Cameco, Springfields, UK – 6000
JSC Enrichment & Conversion Co (Atomenergoprom), Irkutsk & Seversk, Russia – 25,000*
Comurhex (Areva), Pierrelatte, France – 14,500
Converdyn, Metropolis, [Illinois] USA – 15,000
CNNC, Lanzhou, China – 3000
IPEN, Brazil – 90
After initial refining, 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. 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…
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.