Uranyl nitrate conversion

PNC developed a co-conversion technology utilizing the microwave heating direct denitration process (MH method) which converts plutonium nitrate and uranyl nitrate solution to MOX powder. Compared with the conventional method, it is a simple process and generates less liquid waste. 

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

Uranium is deposited widely in the Earth s crust, hence it has few ores, notably the oxides uraninite and pitchblende. The ores are leached with H2SO4 in the presence of an oxidizing agent such as NaClOs or Mn02, to oxidize all the uranium to the (+6) state as a sulfate or chloride complex. On neutralization with ammonia a precipitate of yellow cake , a yellow solid with the approximate composition (NH4)2U207 is formed. This is converted into UO3 on ignition at 300 °C. This can be purified further by conversion into uranyl nitrate, followed by solvent extraction using tributyl phosphate in kerosene as the extractant. 

Uranium solubility is increased even more in the nitrate microcosms, and one possible explanation is the conversion of acetate to CO2, coupled with nitrate reduction, which would give higher dissolved carbonate concentrations in the nitrate microcosms. However, PHREEQE modelling showed that the higher CO concentration would not greatly affect uranium speciation in solution and is therefore unlikely to account for the enhanced solubility. Alternatively, as nitrate is reduced to ammonium (NH/), which promotes cation exchange, this could lead to displacement of U02 from surface complexes, which are the predominant uranyl species on mineral surfaces. " ... 

In the production of uranium(lV) oxide in the wet process, the uranium concentrate is first converted into a uranyl nitrate solution with nitric acid. After the purification of the uranyl nitrate by solvent extraction, it can be converted into uranium(IV) oxide by two different routes either by thermal denitration to uranium(VI) oxide which is then reduced to uranium(IV) oxide or by conversion of uranyl nitrate into ammonium diuranate which is reduced to uranium(IV) oxide. Purification proceeds by extraction of the uranyl nitrate hydrate from the acidic solution with tri-n-butylphosphate in kerosene and stripping this organic phase with water, whereupon uranium goes into the aqueous phase. 

Laboratory and Pilot Plant Studies on Conversion of Uranyl Nitrate to Uranium Hexafluoride... 

Exxon Nuclear Company has conducted various development programs to support the design and licensing of a commercial nuclear fuel reprocessing plant. The uranium conversion portion of the reprocessing plant will use fluidized-bed processes for conversion of uranyl nitrate hexahydrate (UNH) to uranium hexafluoride (UF ). This paper describes the laboratory and pilot plant studies conducted at Oak Ridge National Laboratory (1) for Exxon Nuclear Company on the conversion of UNH to UF, and on the purification of UF. ... 

E. L. Youngblood, I. J. Urza, G. I. Cathers, Laboratory and Pilot-Plant Studies on the Conversion of Uranyl Nitrate Hexahydrate to UF by Fluidized-Bed Processes, ORNL/TM-5913 (June, 1977). 

In the Aquafluor process [G4] developed by the General Electric Company, most of the plutonium and fission products in irradiated light-water reactor (LWR) fuel are separated from uranium by aqueous solvent extraction and anion exchange. Final uranium separation and purification is by conversion of impure uranyl nitrate to UFg, followed by removal of small amounts of PuF , NpFg, and other volatile fluorides by adsorption on beds of NaF and Mgp2 and a final fractional distillation. A plant to process 1 MT/day of irradiated low-enriched uranium fuel was built at Morris, Illinois, but was never used for irradiated fuel because of inability to maintain on-stream, continuous operation even in runs on unirradiated fuel. The difficulties at the Morris plant are considered more the fault of design details than inherent in the process. They are attributed to the attempt to carry out aqueous primary decontamination, denitration, fluorination, and distillation of intensely radioactive materials in a close-coupled, continuous process, without adequate surge capacity between the different steps and without sufficient spare, readily maintainable equipment [G5, R8]. 

A.G. Winger, Sorption Processes, membrane processes - ion exchange, Eng. Progr., 1957, 53, 606 W.W. Schlutz, E.W. Neuvar, J.I. Carroll and R.E. Bums, Electrodialytic conversion of uranyl nitrate to uranic fluoride salts, Ind. Eng. Chem., 1958, 50, 1768. 

The uranium product solution, after evaporation, is suitable for precipitation of ammonium di-uranate and conversion to uranium oxide. The major proportion at the present time is evaporated to uranyl nitrate and converted to oxide by thermal denitration, and then to fiuoride, using a fluidization process. 

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