Conversion

Conversion is the process of manufacturing pure UF6 from the yellowcake generated by the mining and refining process. It requires the use of a combination of fluorine and HF in aqueous or gaseous form to fluorinate the oxide feed. There is one established process where this is carried out using fluorine gas alone. Purification is carried out along the way so that the conversion process will incorporate a number of stages.

There are five major providers of commercial conversion services, these being:

1 Rosatom/JSC TVEL (Russia)

2 Honey well/Converdyn (USA)

3 AREVA NC/Comurhex (France)

4 Cameco (Canada)

5 Westinghouse/Springfields Fuels Limited (SFL, UK)

All of these organisations have been operating successfully for many years, each using a different process. Figure 7.2 shows a schematic based upon the processes used by Cameco, AREVA and SFL, as while there are some differences amongst the three, they may easily be considered together.

In this process, the yellowcake is first dissolved in concentrated nitric acid to form uranyl nitrate in solution:

U3O8 + 8HNO3 ^ 3UO2(NO3)2 + 2NO2 + 4H2O [7.2]

Tributylphosphate (TBP) dissolved in a hydrocarbon diluent, such as kerosene, at a concentration of 20-25% is then mixed and agitated with the uranyl nitrate solution so that the uranium is extracted into the solvent phase. The uranium is separated from the aqueous phase with very high efficiency under the right chemical conditions, leaving the impurities behind in the aqueous phase. The uranium is then washed out of the solvent with fresh, dilute nitric acid or water to give a solution of purified uranyl nitrate. The kerosene does not have a chemical role in the extraction process, but serves to lower the density of the solvent making it easier to separate the aqueous and solvent phases. The solvent is not consumed within the process and may be recycled repeatedly, with some cleaning (typically an alkaline wash) to remove low concentrations of solvent degradation products and impurities that may be held up in the solvent. This solvent extraction process is employed with minor variations by Cameco and AREVA. SFL currently receives purified oxide from Cameco, although it has carried out purification in the past.

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7.2 Example uranium hexafluoride conversion process.

Following on from solvent extraction the uranyl nitrate is boiled down to give a high concentration solution of around 1100-1300 kg/m3 uranium. This concentrated solution is then fed into a high temperature denitration unit where the water is driven off and the nitrate decomposed to give a purified uranium trioxide (UO3) product according to the reaction:

UO2(NO3)2.6H2O ^ UO3 + NO2 + NO + O2 + 6H2O [7.3]

Cameco use pot denitrators for this operation, whereas SFL used a fluidised bed reactor at 300 °C before they started to take purified oxide from Cameco. The
process used by AREVA is a little different as it injects ammonia gas into the purified uranyl nitrate solution to generate an ammonium diuranate ((NH4)2U2O7) precipitate, with the precipitate calcined at high temperature to give a purified UO3 product. AREVA has announced, however, that a planned new facility (Comurhex II) will use the denitration scheme in preference to ammonia injection.

The next stage is to convert UO3 to the intermediary product, uranium tetrafluoride (UF4), which, at atmospheric pressure, is a solid up to 1036 °C. An initial reaction is carried out by adding hydrogen and reducing to uranium dioxide (UO2) at high temperature. Cameco use a fluidised bed reactor for this, SFL use a rotary kiln and AREVA a furnace but the process is essentially the same in each case. AREVA and SFL then react the UO2 with HF gas at high temperature in a rotary kiln to give UF4. Cameco uses a wet process for this operation, where the UO2 is reacted with aqueous hydrofluoric acid at 100 °C to generate the solid UF4, which is then dried and calcined to remove water of crystallisation prior to further fluorination. The reduction and fluorination reactions are given by

UO3 + H2 ^ UO2 + H2O [7.4]

UO2 + 4HF ^ UF4 + 2H2O [7.5]

In the final stage of the conversion process, UF4 is reacted with fluorine gas at high temperature to give the UF6 product.

UF4 + F2 ^UF6 [7.6]

The highly reactive fluorine gas is generated using electrochemical cells with graphite anodes. The cells contain molten potassium bifluoride (KHF2) salt as the electrolyte, which is continuously fed with anhydrous hydrogen fluoride gas. The hydrogen fluoride is split into its hydrogen and fluorine component elements with the fluorine fed to the UF6 production reactor.

Cameco and AREVA carry out the fluorination reaction in a flame reactor at a temperature of around 800-900 °C. SFL use a fluidised bed reactor at a much lower temperature of around 450 °C. The UF6 is produced as a gas and is collected and condensed as a solid. Its temperature is then raised to liquefy it. This drives off light gas impurities, allows sampling and provides a form that may be easily dispensed. The liquid UF6 is dispensed into transport containers in batches of 12.5 tonnes and allowed to cool and solidify, a process taking around 5 days. The UF6 is then shipped to the enrichment facility, where it is referred to as ‘feed’.

Converdyn use essentially the same fluorination method described above to generate UF6 from UO2; however, the method of converting yellowcake to UO2 is somewhat different. A dry process is used where the yellowcake is dried at temperature and crushed to a uniform size. This is then reacted with hydrogen gas directly to give the UO2 feed for fluorination, rather than going through an initial dissolution and purification procedure. From there the UO2 is reacted with HF gas in a fluidised bed reactor to produce UF4 and then fluorinated with fluorine gas in
a flame reactor to give the UF6 product. In the absence of an early purification process, an extra UF6 distillation stage is added prior to dispensing into transport containers.

Подпись: (NH4)4UO2(CO3)3 Подпись: UO2 + 3CO2 + 2NH3 + N2 + 2H2 + 3H2O Подпись: [7.7]

There are two conversion technologies in use in Russia which differ significantly from those used by Western companies. One is a wet process and the other is dry. In the wet process the yellowcake feed is first dissolved in nitric acid and the uranium extracted into a TBP/hydrocarbon mix. Instead of recovering the purified uranium in nitric acid the solvent is mixed with an ammonium hydrogen carbonate solution so that the uranium crystallises out as solid ammonium uranyl tricarbonate (AUTC, chemical formula (NH4)4UO2(CO3)3). The mix is cooled to reduce the solubility of AUTC then the solid crystals are filtered off. The filtered AUTC is then thermally decomposed. Various uranium oxides are formed at different temperatures with UO2 formed in the absence of oxygen and at temperatures of greater than 620 °C according to the reaction:

The UO2 is then dissolved in a mixture of hydrochloric and hydrofluoric acids. Chemical conditions are adjusted so that impurities can be separated as solids and uranium metal added to reduce any trace uranium (VI) to uranium (IV). Excess hydrofluoric acid is added after impurity separation, causing the uranium to precipitate out as hydrated UF4. This is separated, dried at 200-250 °C and then calcined at 450-500 °C in a hydrogen and HF atmosphere to provide the UF4 feed for fluorination, which is carried out with fluorine gas in a flame reactor. The temperature of the reactor at 1100 °C is higher than used by Western converters. The U3O8 feed from Russian mining operations has traditionally been produced to a higher purity than the standard specification used at Western facilities and if the feed is of sufficiently high purity then the U3O8 may be converted to UO2 directly by calcination without the need for solvent extraction and AUTC formation. The dissolution and fluoride precipitation process provides some purification of the uranium and is sometimes referred to as fluoride refining.

The other conversion process used in Russia is direct fluorination of U3O8 with fluorine gas in a flame reactor. The reaction for this is:

U3O8 + 9F2 ^ 3UF6 + 4O2 [7.8]

The same flame reactors that are used for fluorination of UF4 may be used with only minor modification although the reaction is more exothermic than for fluorination of UF4 so that the temperature in the reaction zone rises as high as 2000 °C. Extensive cooling is applied to the walls of the flame reaction vessel, maintaining it at a much lower temperature than found in the reaction zone and thereby preventing excessive corrosion. The U3O8 feed for this process must be of very high quality if the UF6 specification for enrichment is to be met and so the initial feed material must first be purified, This is carried out by dissolving the feed in nitric acid, purification using TBP solvent extraction, recovery as
uranyl nitrate, concentration of the solution and finally thermal denitration in a fluidised bed reactor. Reaction conditions are adjusted to form U3O8 rather than the UO3 produced by Western denitration processes according to the reaction:

3UO2(NO3)2.6H2O ^ U3O8 + 6NO2 + 2O2 +18H2O [7.9]

The U3O8 is then recovered and ground to a fine powder for feeding into the direct fluorination reactor.

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