Uranyl carbonate, ammonium

AUC [Ammonium uranyl carbonate] A process for converting uranium hexafluoride into uranium dioxide for use as a nuclear reactor fuel. The hexafluoride vapor, together with carbon dioxide and ammonia, are passed into aqueous ammonium carbonate at 70°C, precipitating ammonium uranyl carbonate ... 

Uranium Trioxide, Uranic Oxide, Uranic Anhydride, or Uranyl Oxide, UO3, is obtained when uranic acid, ammonium diuranate, or ammonium uranyl carbonate is heated to a temperature not exceeding 300° Cd When uranyl nitrate is similarly heated the product always contains basic nitrate, but if this product is heated in a current of oxygen at 500° C. it yields pure uranium trioxide. ... 

Ammonium Uranyl Carbonate, (NH4)4U0,(C03)3.2H20, is obtained by dissohnng freshly precipitated ammonium uranate in a solution of ammonium carbonate at 60° to 70° C. or by treating a soluble uranyl salt with ammonium carbonate and ammonia. In either case a yellow solution is formed which on evaporation deposits yellow monoclinic ery> stals of the double carbonate. The same compound results on mixing concentrated solutions of ammonium carbonate (3 molecules) and uranyl nitrate (1 molecule) at 50° to 55° C.,... 

Uranium is extracted from pitchblende, essentially UgOg. The ore is washed, then fused with sodium carbonate and sodium nitrate. From the mass, dilute sulphuric acid extracts uranyl sulphate, UO2SO4. The addition of ammonium carbonate enables ammonium uranyl carbonate, (NH4)4U02 (003)3, to be crystallised which, on ignition, yields pure U3O3. 

Ammonium uranyl carbonate (AUC) process This process was developed in the 1960 s in the Federal Republic of Germany. It comprises the simultaneous feeding of uranium(Vl) fluoride, carbon dioxide and ammonia into an aqueous ammonium carbonate solution at 70°C, whereupon tetra-ammonium tricarbonato-dioxo-uranate (ammonium uranyl carbonate) precipitates out ... 

The UO2 powder obtained by this process is readily granulated and sintered, without the addition of binders, to controlled densities. Pore size distribution can be controlled by addition of UsOg. The pellets have high stability (no further densification takes place during operation). The pellet density in the French (dci) process is controlled by the cold compaction pressure in forming the green compacts. The ammonium uranyl carbonate (AUC) process in Germany (KWU) produces UO2 powder that flows freely as... 

The diuranate is then reduced to UO2 with hydrogen at 820°C. In the third, AUC (ammonium uranyl carbonate) process, developed in West Germany by Nukem, streams of gaseous UF, CO2, and NH3 are fed batchwise into demineralized water, whereby (NH4)4U02 (003)3 is precipitated. The AUC is converted batchwise to UO2 by contacting it with steam and hydrogen at 500°C in a fluidized bed, with recovery of CO2 and NH3. Subsequently, steam at 650°C is supplied to the fluidized bed to reduce fluorine content to 50 to 60 ppm by pyrohydrolysis. 

Another often used reaction sequence begins with the formation and precipitation of ammonium uranyl carbonate (AUC) by reaction of UF with water, CO2 and NH3, see Figure 21.2. The AUC is reduced to UO2 by reduction in a fluidized bed using a mixture of hydrogen, nitrogen and steam and cooled to room tenqierature in a mixture of air and nitrogen. 

Another production method used for the conversion of the UF-a to UO2 is the ammonium uranyl carbonate (AUC) process. The form of the AUC precipitate is probably (NHa) [UOz(COa)3]s. This compound is calcined to UOior UjOa, which is then reduced to UOa. 

Uranyl carbonate complexes, like sodium uranyl tricarbonate, Na4[U02(C03>3], that is obtained when uranium ore is leached with sodium carbonate solutions and ammonium uranyl carbonate (AUC), (NH4)4[U02(C03)3l, that is used to precipitate the uranium in the UCF, are important in the NFC. These carbonates serve to purify the uranium from several metals (like Fe, Al, Cr, Ni, and other metals) that are precipitated as hydroxides or oxycarbonates, as well as aUcaline-earth elements. These purification methods utilize the effect of the ammonium carbonate concentration on the solubility of uranium. Upon heating of AUC to 300°C-500°C, it decomposes to UO3, ammonia, CO2, and water and at temperatures of 700°C-800°C, without air, UO2 may be formed (the ammonia serves as the reducing agent). The solubility of AUC decreases markedly in the presence of ammonium carbonate, for example, from 119.3 g L" at 50°C without ammonium carbonate to 0.5 g L" with 35% ammonium carbonate (Galkin 1966). The carbonate complexes also play a role in biological systems and affect clearance by the blood after exposure to uranium compounds. 

The starting material for the fabrication of UO2 standard fuel is enriched UFe, which is supplied by the enrichment plant in pressure cylinders (the preceding steps of fabrication starting from uranium ore have been described, among others, by Peehs, 1996). From this material, the UO2 compound is made either by a wet or by a dry process. Among the wet processes, the ADU (ammonium diuranate) process is mainly used, in which UFe gas is hydrolyzed in aqueous ammonia solution to form ammonium diuranate. Besides ADU, the AUC (ammonium uranyl carbonate) process is applied in which UFe is hydrolyzed in an aqueous solution... 

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