Institute of Metals Division - The Immiscibility Limits of Uranium with the Rare-Earth Metals

The limits of miscibility in some of the uranium rare-earth alloy systems have been determined in the temperature range 1000°to 1250°C. The solubilities of lanthanum and cerium in uranium are greater than those of the remaining rare earths by a factor of more than two. The solubility of uranium is greater in cerium, braseodymium, and neodymium than in the other rare-earth metals studied. The values found in this study are in qualitative agreement with those which might be expected if the solubility rules of Hildebrand and Scott are applicable. AS interest in nuclear reactors intensifies, many new types of fuels are being suggested in attempts to improve the economics of some of the proposed reactor schemes. To remove some of the difficulties inherent in the use of solid-fuel elements and their reprocessing, many types of liquid-metal reactors have been suggested. One of the more attractive features of several of these reactor concepts is that they include a continuous or semicontinuous process for the extraction of fission products and "bred" fissionable materials from the fuel, utilizing immiscible metal extractants. This would enable a much higher burn-up of fissionable material to be achieved and would present a very attractive economic picture. Several studies have been reported on equilibrium systems in which there exists a high degree of immiscibility between uranium and another metal that might be used as an extractant in such a processing scheme.' Two of these systems in which a high degree of immiscibility exists are those of uranium with the two rare-earth metals, lanthanum, and cerium. Since the rare earths constitute a significant fraction of the fission products, their removal is of prime importance. It is reasonable to believe that this might be accomplished by equilibrating a rare-earth phase with the contaminated uranium fuel in the liquid state. In order to make a more complete study of those systems which would be of interest either as extractants in a liquid-liquid extraction process, or as fission products formed in the fuel, the alloy systems of uranium with lanthanum, cerium, praseodymium, neodymium, and samarium were studied in some detail in the temperature range 1000" to 1250°C; less detailed studies were made with the other rare earths. In addition to being of value to the reactor program, the data obtained in this study should be of help in making a study of the role played by the electronic structures of metals in determining the nature of metallic solutions. The unique electronic structures of the rare-earth elements make them particularly interesting in this respect. EXPERIMENTAL The usual procedure for a solubility determination was to seal equal volumes of uranium and the particular rare earth in a tantalum crucible under an atmosphere of helium; this crucible was then sealed in a stainless steel jacket in an atmosphere of helium. These samples were equilibrated by repeated inverting of the crucibles in a furnace for 15 min at the desired temperature, left in an upright position for 15 min to permit separation of the two phases, and then quenched under a stream of water. In some runs the temperature of the furnace was held 50' to 100°C above the desired quenching temperature while inverting in order to insure good mixing. However, it was found that above 1200°C the crucibles were subject to failure and for these runs the furnace temperature was not raised above the desired quenching temperature. A small amount of tantalum was dissolved in the uranium and the rare earths in these runs, a maximum of 3 wt pct in the uranium phase at 1250°C and up to 1 wt pct in the rare-earth phase at this temperature. On cooling, the major portion of this tantalum precipitated as primary tantalum crystals. Any residual tantalum would probably have a negligible effect on the mutual solubility of uranium and the rare earths in each other. Samples for analysis were cut from each phase with an abrasive cutting wheel; the region near the interface between the two metals was carefully avoided. In the case of the rare earths with melting points above 1250°C no solubility data were taken on the rare-earth phase since this phase could not have achieved equilibrium in a reasonable length of time. (For the same reason no data were taken on the uranium phase below its melting point of 1132°C.) Equilibrium appeared to have been reached in the uranium phase in these cases although the rare-earth phase had not melted. To verify this, samples were melted together in an arc furnace similar to that described by Kroll.2 These samples were sub-