Activity Coefficients and Solubility of Uranium in Liquid-Metal Pb–Sn Alloys

Abstract—The equilibrium potentials of liquid-metal U–M alloys (where М = Pb, Pb–Sn, and Sn) are measured in a U(М)_(dil)|NaCl–2CsCl–UCl₃||NaCl–2CsCl|C_(s), Cl_2(g) electrochemical cell using open-cercuit potentiometry. The electromotive force (emf) of a U|NaCl–2CsCl–UCl₃|M + UM_x galvanic cell, in which the liquid-metal alloy corresponds to the two-phase region and UM_x is an intermetallic compound, is measured to determine uranium solubility in the alloys. The experiments are carried out in a molten NaCl–2CsCl eutectic under an inert gas atmosphere. Reagents containing no impurities of moisture, oxygen, and their compounds are used in the experiments. All the main procedures are carried out in a dry glove box. The NaCl–2CsCl–UCl₃ melts are prepared in two stages. A NaCl–2CsCl–UCl₄ melt is prepared first by the chlorination of metallic uranium with gaseous chlorine, and then the prepared electrolyte is stored in contact with uranium to form a NaCl–2CsCl–UCl₃ solution. Uranium alloys with low-melting metals are prepared prior to experiment by the electrolysis of the NaCl–2CsCl–UCl₃ melt. The electrolysis time corresponds to conditions of one-phase or two-phase region formation. The high-temperature filtration of saturated alloys through a porous quartz filter is used to determine the uranium concentration in an alloy containing no intermetallic compounds. The dependences of the activity coefficients of γ-U in lead, tin, and Pb–Sn bimetallic alloy are detected. The temperature dependences of the emf of the galvanic cell are approximated by straight lines, and the values of emf depend on the nature of the alloy components and decrease in the order Sn > Pb–Sn > Pb. The solubility of uranium in the U–Pb, U–Pb–Sn, and U–Sn alloys is determined. The solubility of uranium is found to be low and almost independent of the alloy nature.