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 Reconversion of Uranium(Ⅵ) Fluoride into Uranium(Ⅳ) Oxide
  • Reconversion of Uranium(Ⅵ) Fluoride into Uranium(Ⅳ) Oxide
  • There are three processes which are industrially convert enriched (Ⅵ) fluoride into sinterable uranium(Ⅳ) oxide: two wet processes and one dry process.

    In all processes criticality avoidance safety features have to be incorporated, as in general when enriched uranium is being processed, i.e. there must be measures for hindering an uncontrolled chain reaction, which occurs upon attaining the critical mass. This is dependent upon the degree of 235U- enrichment, the chemical form of the uranium, the degree of moderation and geometric dimensions. Thus by increasing the surface area of a vessel, the loss of neutrons to the surroundings is increased with the result that the neutron population necessary for achieving a critical condition is not attained. The diameter of apparatuses or the height of the filtration layer is thus limited and additional heterogeneous neutron poisons are incorporated into the vessel e.g. boron carbide or cadmium.

    Uranium(Ⅳ) Oxide by Wet Processes

    In the wet processes poorly soluble uranium compounds are precipitated, whereby the fluoride left in the filtrate is reacted with lime to fluorspar or processed to other inorganic fluorine compounds by fluorine processing companies.

    Ammonium diurunate (ADU) process: This process was developed in the USA in the 1950's and is currently still the most important process. However, the raw uranium(Ⅳ) oxide produced with this process contains up to 2% by weight of fluoride and hence requires aftertreatment before it is suitable for pressing into fuel pellets. This disadvantage is not shared by the other two processes. New reconversion plants do not, therefore, in the main, utilize the ADU-process.

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

    The UO2F2 solution is treated with ammonia, whereupon ammonium diuranate precipitates out, although not in a strictly stoichiometric composition:

    The precipitate is largely freed of fluoride ions after filtration by extraction or recrystallization. After drying at 200°C, the ammonium diuranate is reductively decomposed by a H2/H2O mixture at ca 500°C to U3O8, which is then reduced with hydrogen at 500 to 800°C to uranium(Ⅳ) oxide. The reductive decomposition to U3O8 and its reduction can be carried out in a single step e.g. in a rotary kiln. Since the uranium(Ⅳ) oxide formed can be pyrophoric, it is weakly reoxidized.

    Ammonium urunyl carbonate (A UC) process: This process was developed in the 1960's in the Federal Republic of Germany. It comprises the simultaneous feeding of uranium(Ⅵ) 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 properties of the product are significantly influenced by the precipitation conditions, which therefore have to be carefully controlled. The filtered and with a ammonium carbonate solution-washed product contains less than 0.5% by weight of fluoride.

    The ammonium uranyl carbonate is reductively converted to uranium(Ⅳ) oxide via uranium(Ⅵ) oxide in a fluidized bed kiln at 500 to 700°C by feeding in water as the fluidizing medium and using hydrogen or a H2/N2-mixture, from the cracking of ammonia, as the reducing agent. At this high temperature the fluoride content is further reduced to 100 ppm by the action of the water vapor. The resulting fine particulate uranium(Ⅳ) oxide can be pyrophoric and therefore on cooling is weakly reoxidized at temperatures below 100°C.
     Uranium(Ⅳ) Oxide by the Dry (IDR) Process

    The TDR (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.

    Manufacture of Uranium(Ⅳ) Oxide Pellets

    The uranium(Ⅳ) oxide produced by the above-described processes is used for the manufacture of uranium(Ⅳ) oxide sintered pellets. The uranium(Ⅳ) oxide is ground, pressed in e.g. hydraulic presses, then sintered at ca. 1700°C in the presence of hydrogen and thereby shrink to the desired density. The pellets (diameter 10 mm, tolerance ±10 to ±25 µm) are ground with a cylindrical grinder, then washed and dried.

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