Actinide metals vapor pressures

Fig. 1. Vapor pressures and some melting points of actinide metals Pa- Es. O, melting point.

The reductant metal must have the following properties (1) the free energy of formation of the oxide of the reductant has to be more negative than that of the actinide oxide and (2) the vapor pressure of the reductant metal needs to be smaller by several orders of magnitude than that of the actinide metal. This difference in vapor pressure should be at least five orders of magnitude to keep the contamination level of the co-evaporated reductant metal in the product actinide metal below the 10 ppm level. 

Actinide metals with lower vapor pressures (Th, Pa, and U) cannot be obtained by this method since no reductant metal exists which has a sufficiently low vapor pressure and a sufficiently negative free energy of formation of its oxide. For the large-scale production of U, Np, and Pu metals, the calciothermic reduction of the actinide oxide (Section II,A) followed by electrorefining of the metal product is preferred (24). In this process the oxide powder and solid calcium metal are vigorously stirred in a CaCl2 flux which dissolves the by-product CaO. Stirring is necessary to keep the reactants in intimate contact. 

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. 

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. 

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

Proceeding from thorium to plutonium along the actinide series, the vapor pressure of the corresponding iodides decreases and the thermal stability of the iodides increases. The melting point of U metal is below 1475 K and for Np and Pu metals it is below 975 K. The thermal stabilities of the iodides of U, Np, and Pu below the melting points of the respective metals are too great to permit the preparation of these metals by the van Arkel-De Boer process. 

Table XI gives the room-temperature, atmospheric pressure crystal structures, densities, and atomic volumes, along with the melting points and standard enthalpies of vaporization (cohesive energies), for the actinide metals. These particular physical properties have been chosen as those of concern to the preparative chemist who wishes to prepare an actinide metal and then characterize it via X-ray powder diffraction. The numerical values have been selected from the literature by the authors.

Plutonium-noble metal compounds have both technological and theoretical importance. Modeling of nuclear fuel interactions with refractory containers and extension of alloy bonding theories to include actinides require accurate thermodynamic properties of these materials. Plutonium was shown to react with noble metals such as platinum, rhodium, iridium, ruthenium, and osmium to form highly stable intermetallics. Vapor pressures of phases in these systems were measured by the Knudsen effusion technique. Use of mass spectrometer-target collection apparatus to perform thermodynamic studies is discussed. The prominent sublimation reactions for these phases below 2000 K was shown to involve formation of elemental plutonium vapor. Thermodynamic properties determined in this study were correlated with corresponding values obtained from theoretical predictions and from previous measurements on analogous intermetallics. 

Metals. Kruglikh, et al. (104) measured saturated vapor pressures of erbium, samarium, and ytterbium by the Knudsen effusion method, and standard (average) sublimation entropies of 18.4, 20.7, and 25.6 cal./(gram atom °K.) were derived. Nesmeyanov, et al. (146) studied the vapor pressure of yttrium by an integral variant of the effusion technique. Similar studies at higher temperatures by Herrick (70) on samarium metal have been interpreted in good accord by both first and second law methods. Ideal gas thermodynamic functions have been derived from 100 °K. to 6000°K. at 100° intervals for both actinide and lanthanide elements by Feber and Herrick (45). 

In summary, we have tried to describe the most recent state of understanding for the cohesive energies of the actinide metals. New results show there are still some surprises, even beyond the f-bonded early actinides. Vapor pressure measurements on Ra, Ac, and E are planned, along with completion of the Bk studies. The Pa and Pa-oxygen systems will obviously require extensive work. 

Es vapor pressures will probably be studied as Raoult s-Law evaporations from liquid metal solvent. This will complete the vapor pressure measurements possible on the actinide metals, as there are no isotopes stable enough beyond Es. 

For metal fuel fabrication, the actinide metals are alloyed in an injection casting furnace that melts, mixes the alloy and injects the molten metal into quartz molds. After quick cooling, the quartz mold is removed from the metal pin, which is cut to length and undergoes quality assurance measurements. These pins are placed into new fuel cladding that contains a small amount of metallic sodium, which provides a thermal bond in early irradiation in the nuclear reactor. These fuel elements are welded closed and are ready for the reactor. Recent research in this area has focused on modifying the process to minimize the volatization of americium, which is a key component in U/TRU recovered for fast reactors and has a high vapor pressure. 

The A//298 WrtS calculated to be 196.23 1.26 kjmol , and AS298 was derived to be 80.54 JK mol . The estimated boiling point for the metal is 1745 K. Nugent et al. [81] had estimated the heat of sublimation to be 163 kJ mol and David et al. [82] had predicted a value of 197 kJmol . The vapor pressure of californium metal is intermediate between that of samarium metal (trivalent) and of europium metal (divalent) [80]. The data show that the californium metal was clearly trivalent up to 1026 K, and that it is one of the most volatile actinide metals. Its high volatility precludes bulk vaporization studies above 1073 K by the Knudsen technique. No evidence by mass spectrometry was obtained in this latter work for the presence of CfO. 

Thermodynamic measurements that have been made on the actinide metals are low- and high-temperature heat capacities, properties of phase transitions, and vapor pressures. At least one of these measurements has been made on each element through einsteinium unfortunately, however, none has been made on actinium, so that even its enthalpy of vaporization must be estimated. As of the time of writing (February 1986) vapor-pressure measurements have been made through einsteinium [22] and low-temperature heat-capacity measurements through americium [23] by innovative microscale methods, innovative microscale methods have been applied to determine vapor pressures and very low-temperature heat capacities of transplutonium actinides. 

The actinide oxide is reduced with La or Th and the resulting actinide metal is distilled from the reactant-reductant mixture and collected. The vapor pressure of the reductant metal must be several orders of magnitude less than the vapor pressure of the actinide metal. 

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