The uranium hexafluoride (UF₆) is a compound of the Uranium. Its industrial use is related to the Cycle of nuclear fuel (proceeded which produces fuel for the nuclear reactors and the nuclear weapons). Its chemical Synthèse, which intervenes after the Extraction of uranium, provides then the entry to the process of enrichment.

Properties

The structure of the solid was described parJ.H. Levy, J.C Taylor and A.B Waugh. In this article, the neutron Diffraction was used to determine the structure of UF₆, MoF₆ and WF₆ to 77 K.

Properties

UF₆ is solid with room temperature, gas with normal pressure and 56.4 °C and liquid under 1.5 atmospheres and with 64 °C. It has a Point triples with 64°C (147°F, 337 K) and with a pressure slightly higher than the atmospheric pressure.

It forms gray solid crystals in the Normal conditions of temperature and pressure (CNTP).

In addition to its radioactivity due to uranium, it is a highly toxic product, which reacts violently with water. Into wet atmosphere or in the presence of water, it is transformed into fluoride of uranyl (UO2F2) and acid hydrofluoric (HF). The transformation is immediate and violent and is accompanied by emission of abundant opaque, irritating and suffocating smoke of HF.

The product is corrosive for the majority of metals. It reacts slightly with the Aluminum, forming a fine layer of AlF₃ which resists then corrosion (Passivation).

Chemical characteristics

It was assembled that the uranium hexafluoride is a Oxydant and a Acide of Lewis who can bind to a Fluorure, for example the reaction of fluoride of Cuivre with the uranium hexafluoride in the Acétonitrile is considered to form Cu₂.5MeCN.

It is interesting to note that the uranium fluorides (VI) Polymère S containing of the Cation S organics were isolated and characterized by Diffraction of x-rays.

Other uranium fluorides

The uranium pentafluoride (UF₅) and the nonafluorure of diuranium (U₂F₉) were characterized by C.J. Howard, J.C Taylor and A.B. Waugh.

The uranium trifluoride was characterized by J. Laveissiere.

The structure of UOF₄ was described by J.H. Levy, J.C. Taylor and P.W. Wilson.

All the other uranium fluorides are nonvolatile solids which are polymeric of coordination.

Industrial application

Process of synthesis

Two stages are necessary to the synthesis:

• the refining:
• # the ore of pulverized uranium - U₃O₈, or “Yellowcake” - is dissolved in the nitric Acid , providing a solution of Nitrate of uranyl UO₂ (NO₃) ₂.
• # possibly Filtration,
• # the nitrate of pure uranyl is obtained by Extraction by solvent, with a solution of TBP.

This stage makes it possible to obtain a uranyl nitrate UO₂ (NO₃) ₂ of great purity (> 99.95%).

• conversion into itself:

• # precipitation of uranyl nitrate by the Ammonia gas to obtain Diuranate of ammonium (NH₄) ₂U₂O₇ (DUA),
• # calcination of the ammonium diuranate, towards 400 °C, to produce UO₃,
• # reduction of UO₃ by the Hydrogen to obtain from UO₂,
• # hydrofluoration of UO₂ by the hydrofluoric Acid HF in a furnace to produce Tetrafluoride of uranium UF₄,
• # oxidation by the Fluor of UF₄ in an engine with flame finally results in obtaining from UF₆.

In addition to its use in the enrichment, the uranium hexafluoride was used in a process of advanced reprocessing developed in Czech Republic. In this process, the Fuel nuclear worn oxide is treated with fluorine to form a mixture of fluorides. This last is then distilled to separate the various types of materials.

Conversion into uranium oxide

After enrichment, the uranium hexafluoride is converted into Oxyde of uranium (UO2) for its nuclear applications.

Conversion into UO2 can be done by dry roads (most frequently) or by wet process. ; Dry roads

• the UF6 are vaporized by heating in a drying oven and put in the presence of overheated steam.
• the UF6 is hydrolized in UO2F2 between 250-300°C: UF6 + 2:2 O - - - > UO2F2 + 4HF Dr. - 113 kJ/mole
• compound UO2F2 is then tiny room towards 700-800°C by hydrogen, producing UO2 in the form of powder: UO2F2 + H2 - - - > UO2 + 2HF + 14,2 kJ/mole

The output is higher than 99,5%. ; Wet process This process presents the disadvantage of producing more effluents than the dry roads, which has a more important environmental impact. More flexible, on the other hand, it is often used for the recovery of the fissile materials in the rejects and waste.

The stages consist of a treatment of the UF6 to the steam and of obtaining, successively, UO2F2, uranium salts, ammonium diuranate, UO3 and UF4. This process connects dissolution in nitric medium, purification by solvent in pulsated column, ammoniacal precipitation and reduction under hydrogen.

Storage in tanks

In the United States, approximately 95% of depleted uranium produce until now is stored in the form of uranium hexafluoride, (D) UF₆, in steel gas tanks on parks in the open air near the factories of enrichment. Each tank contains up to 12,7 tons of UF₆. In the United States only, 560000 tons of impoverished UF₆ were stored in 1993. In 2005,686500 tons are contained in 57122 storage tanks located at Portsmouth in the Ohio, Oak Ridge in the Tennessee and Paducah in the Kentucky. The long-term storage of DUF₆ presents environmental, medical risks and of safety because of its chemical instability. When UF₆ is in contact with humid air, it reacts with the water contained in the air to produce UO₂F₂ (fluoride of uranyl) and HF (hydrofluoric acid) which are both very soluble and toxic. The storage tanks must be regularly inspected to seek traces of corrosion or escapes. The estimated lifespan of a steel tank is measured in decades.

There were several accidents implying uranium fluoride in the United States. The US government started to convert DUF₆ into solid uranium oxide for a long-term storage. Such a storage of the whole of the stock of DUF₆ could cost between 15 million and 450 million dollars.

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