Reduction of actinide carbides

The yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

Actinide metals with very low vapour pressures (U,Np,Pu) have been obtained by metallothermic reduction of their carbides by non-volatile reductants (Ta, W) (3) ... 

Carbides of the Actinides, Uranium, and Thorium. The carbides of uranium and thorium are used as nuclear fuels and breeder materials for gas-cooled, graphite-moderated reactors (see Nuclearreactors). The actinide carbides are prepared by the reaction of metal or metal hydride powders with carbon or preferably by the reduction of the oxides uranium dioxide [1344-57-6] UO2 tduranium octaoxide [1344-59-8], U Og, or thorium... 

The free energies of formation of the transition metal carbides are somewhat more negative than the free energies of formation of the actinide carbides. To facilitate separation of the actinide metal from the reaction products and excess transition metal reductant, a transition metal with the lowest possible vapor pressure is chosen as the reductant. Tantalum metal and tantalum carbide have vapor pressures which are low enough (at the necessary reaction temperature) to avoid contamination of the actinide metal by co-evaporation. 

Actinide carbides are prepared by carbothermic reduction of the corresponding dioxides according to the reaction ... 

In practice, a mixture of actinide dioxide and graphite powder is first pelletized and then heated to 2275 K in vacuum in a graphite crucible until a drop in the system pressure indicates the end of CO evolution. The resulting actinide carbide is then mixed with tantalum powder, and the mixture is pressed into pellets. The reduction occurs in a tantalum crucible under vacuum. At the reduction temperature, the actinide metal is vaporized and deposited on a tantalum or water-cooled copper condenser. 

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

Molten-Tin Process for Reactor Fuels (16). Liquid tin is being evaluated as a reaction medium for the processing of thorium- and uranium-based oxide, carbide, and metal fuels. The process is based on the carbothermic reduction of UO2 > nitriding of uranium and fission product elements, and a mechanical separation of the actinide nitrides from the molten tin. Volatile fission products can be removed during the head-end steps and by distilling off a small portion of the tin. The heavier actinide nitrides are expected to sink to the bottom of the tin bath. Lighter fission product nitrides should float to the top. Other fission products may remain in solution or form compounds with... 

Due to its drawbacks (difficult preparation of water-free starting material, neutron emission from ( a,n) reactions, presence of non-volatile impurities in the product), methods involving vaporisation of the actinide metal after reduction of a compound (oxide, carbide) are preferred. If the vapour pressure of the reductant and that of the actinide compound are markedly lower than that of the metal formed, the latter can be removed from the reaction mixture via the vapour phase and condensed in high purity ... 

The tantalothermic reduction of PuC and UC requires reaction temperatures of 1700 and 1900°C, respectively. The starting carbides are prepared by carboreduction of their oxides too low a CO partial pressure during carboreduction may lead to actinide losses by evaporation. 

All of the actinide elements are metals with physical and chemical properties changing along the series from those typical of transition elements to those of the lanthanides. Several separation, purification, and preparation techniques have been developed considering the different properties of the actinide elements, their availability, and application. Powerful reducing agents are necessary to produce the metals from the actinide compounds. Actinide metals are produced by metallothermic reduction of halides, oxides, or carbides, followed by the evaporation in vacuum or the thermal dissociation of iodides to refine the metals. 

With Ta as the reductant, the actinide starting material may instead be the carbide, previously prepared by the carbo-reduction of the oxide. This process is an attractive alternative for producing Pu and Cm because in the preparation of the carbide from the oxide a large number of impurities are eliminated by vaporization. Table 19.4 presents a brief summary of commonly used preparation methods. 

The choice of the starting compound of the actinide (oxide or carbide) and of the reductant is determined by the vapour pressure of the actinide metal. For a given temperature, the vapour pressures of the actinide metals (Fig. 2) span a ratio of more than 10 . As the vapour pressure of La is similar to those of Ac, Cm, Pu, only the more volatile... 

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