Uranium VI fluoride

Chem., 40, 3516 (2001). Solvent Effects on Uranium(VI) Fluoride and Hydroxide Complexes Studied by EXAFS and Quantum Chemistry. 

After the outbreak of World War II in 1939, leading nuclear scientists realized that a new type of weapon was possible, based on atomic fission. A key requirement was the separation of the isotope of mass 235 (only 0.7%) present in natural uranium. By 1941, it was clear that the only practicable process was by gaseous diffusion of uranium(VI) fluoride, the sole compound of sufficient volatility. However, this attacked normal organic materials rapidly, being almost as reactive as elemental fluorine. 

The other major springboard for the fluorocarbon chemical industry was the "Manhattan Project to develop the atomic bomb. This required the large-scale production of highly corrosive elemental fluorine and uranium(VI) fluoride for the separation of the radioactive 235U isotope. Oils capable of resisting these materials were needed to lubricate pumps and compressors, and polymers were needed to provide seals. Peril uorinated alkanes and polymers such as PTFE and poly(chlorotrifluoroethylene) (PCTFE) proved to have the appropriate properties so practical processes had to be developed for production in the quantities required. In 1947 much of this work was declassified and was published in an extensive series of papers3 which described the fundamental chemistry on which the commercial development of various fluoro-organic products, especially fine chemicals, was subsequently based. 

Tungsten(VI) fluoride (WF6) and molybdenum(VI) fluoride (MoF6) are available commercially, and can be made by reaction of the metals with fluorine.4 In the case of uranium(VI) fluoride (UF6), a preparation that is claimed5 to be feasible in the laboratory uses uranium metal and chlorine trifluoride uranium(VI) fluoride is prepared6 commercially by the fluorination of uranium(IV) fluoride, itself prepared from an oxide and hydrogen fluoride. 

Uranium(VI) fluoride is a colorless solid which sublimes at 56.2°C it does not exist as a liquid at atmospheric pressure. It is soluble in l,l,2-trichloro-l,2,2-trifluorocthane (Freon 113). hydrogen fluoride, and chlorohydrocarbons, although it will react31 slowly with the latter it cleaves some ethers12-13 and reacts vigorously32 with carbon disulfide to give sulfur tetrafluoride, carbon tetrafluoride, bis(trifluoromethyl) disulfide, and bis(trifluoromethyl) trisulfide. 

In support of the older literature is the finding34 in 1966 that uranium(VI) fluoride and several hydrocarbons (CH4, C2H6, C3H8, benzene) react at 150-400°C over sodium, potassium... 

Uranium(VI) fluoride can react35 as a Lewis acid or an oxidizing agent with alcohols. In the vapor phase at 80 °C (note that this is considerably lower than the temperatures used in the examples given vide supra) it behaves as a Lewis acid and gives35 mixtures of the corresponding fluorides, alkenes and ethers (Table 1). 

Table 1. Reaction of Uranium(VI) Fluoride with Alcohols in the Vapor Phase (85 °C, 24 h)...

In solution (l,l,2-trichloro-l,2,2-trifluoroethane) the oxidizing power of uranium(VI) fluoride predominates with alcohols and they are converted13 into ketones and aldehydes, which then go on to acid fluorides. 

Treatment of acetone with uranium(VI) fluoride in the vapor phase at room temperature gives13 some acetyl fluoride, along with much polymeric material. Under the same conditions, cyclohexanone also produces an unidentified acid fluoride initially, but this is then consumed to give polymeric material. 

In solution, it is possible to obtain what may be a complex of uranium(VI) fluoride and cyclohexanone, but under more drastic conditions uranium(VI) fluoride behaves as a Lewis acid and leads to condensation products, none of them fluorinated.13... 

Acetophenone does not appear13 to react with uranium(VI) fluoride at room temperature in l,l,2-trichloro-l,2,2-trifluoroethane, but adamantanone at 0CC does, to produce13 two mono-fluoro- and one difluoroadamantanones however, there is a direct conflict in the literature as other workers12 have claimed that, under near-identical conditions, 2,2-difluoroadamantane is the product. 

Uranium(VI) fluoride in l,l,2-trichloro-l,2,2-trifluoroethane at 0°C oxidizes12 aldehydes to acid fluorides in moderate yields (e.g., heptanoyl fluoride from heptaldehyde in 47% yield, benzoyl fluoride from benzaldehyde in 40% yield) benzaldehyde gives methyl benzoate and benzaldehyde dimethyl acetal when the reaction mixture is worked up by quenching with methanol.13... 

The amino acids alanine and threonine have been treated40 with uranium(VI) fluoride both form gels from which alanine can be recovered unchanged while threonine is converted into 1-aminoacetone via oxidation of the secondary alcohol (> CHOH) moiety to a carbonyl group (> C = 0). 

Uranium(VI) fluoride intercalates into graphite to give29 nominal C12UF6, but some fluo-rination occurs even at room temperature, and at 800-2500°C carbon tetrafluoride is formed.41... 

F Chemical Shifts of the UFnCl(6-n) Series. Uranium hexafluoride, UF6, is perhaps the most studied and best characterized actinide molecule. The 19F NMR chemical shift (taken relative to CFCI3) of this molecule is presented in Table HI for the scalar Pauli and all-electron spin-orbit ZORA approaches. In this table, we have also collected results for the whole series of the related uranium (VI) fluoride chlorides, UFnCl(6-n), n =1 - 6. The 19F NMR chemical shifts of all of these molecules have been measured in CFCI3 solution (39). The solvent has also been used as internal standard. All mixed UFnCl(6-n) species have been characterized virtually definitely in these experiments (39). 

Uranium(VI) fluoride converts adamantanone into 2.2-difluoroadamantane in 41 % yield. With other nonenolizable ketones no reaction takes place. 

In nuclear power stations electrical current is produced from nuclear energy. Efficient operation requires provision of the nuclear power station with fuel elements and the disposal of spent materials. These operations are brought together in the nuclear fuel cycle, which embraces on the provision side the extraction and dressing of uranium ores to uranium concentrates, their conversion to uranium(VI) fluoride, enrichment of the uranium isotope from 0.7% in natural uranium to ca. 3%, the conversion of uranium(VI) fluoride into nuclear fuel and the production of fuel elements. 

The main task in the conversion of uranium concentrate ( yellow cake ) to UF5 is the purification of the concentrate and its conversion into a chemically suitable form for further processing or enrichment of the U-isotope for the different reactor types. Isotope enrichment proceeds in the gas phase via uranium(VI) fluoride, the only uranium compound which boils at low temperatures and is stable in the vapor phase. It is advantageous that fluorine only occurs naturally in a single isotope. 

Two processes are employed for the production of uranium(VI) fluoride, namely the wet and dry processes. In both processes uranium(IV) oxide and uranium(lV) fluoride are formed as intermediates. In the wet process the uranium(IV) oxide is produced from the uranium concentrate by way of uranyl nitrate, whereas in the dry process the uranium concentrate is directly reduced to uranium(IV) oxide. The methods of purification used are also different in the wet process the purification proceeds at the uranyl nitrate stage, by solvent extraction, whereas in the dry process the end product uranium hexafluoride is itself distillatively purified. 

The conversion of uranium(IV) fluoride to uranium(VI) fluoride proceeds exclusively with elemental fluorine. 

The wet process for uranium(VI) fluoride manufacture is utilized in the Kerr-McGee process, in which the reduction proceeds with a H2/N2-mixture from ammonia cracking and hydrofluorination is carried out in a two stage fluidized bed. British Nuclear Fuel Ltd and Eldorado Nuclear Ltd/Canada also use wet processes. 

Enrichment of the Sy-isotope from the 0.711% in natural uranium to ca. 4% can proceed by gas diffusion, with a gas centrifuge and with separation nozzles. The separation nozzle process is no longer important. Pure uranium(VI) fluoride is utilized. 

In the gas dijfusion process uranium(VI) fluoride is forced through a cascade of fine pore membranes connected directly in series. This process exploits the... 

The nozzle separation process utilizes the centrifugal forces which occur upon diversion of a gas stream. A gas stream of uranium(VI) fluoride, helium and hydrogen is directed along a curved wall and then split by a peeling off plate into two gas streams with enrichment of the heavier and lighter isotopes respectively. 

Reconversion of Uranium(VI) Fluoride into Nuclear Fuel... 

There are three processes which are industrially convert enriched uranium(VI) fluoride into sinterable uranium(IV) oxide two wet processes and one dry process. 

In the ADU process the uranium(VI) fluoride from the enrichment plant is first evaporated and hydrolyzed with water ... 

The IDR (Integrated Dry Route) process consists of reacting gaseous uranium(VI) fluoride with superheated steam, whereupon solid UO2F2 is formed, which is reduced with hydrogen to uranium(IV) oxide. This reaction can be carried out in both a fluidized bed reactor and in a rotary kiln, whereby the latter appears more suitable. 

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