Liquid metals are proposed as working media for pyrochemical reprocessing irradiated nuclear fuels and separating remaining or newly formed fissile materials (uranium and plutonium) from fission product elements (including rare earths). Separation factors can be estimated from the thermodynamic properties of the elements in the metallic alloys. Normally each element is studied individually and their possible interaction in the working media is neglected. The present work was aimed at determining basic thermodynamic properties of multicomponent liquid metal alloys containing uranium and a rare earth element, and comparing them with the thermodynamics of the constituent subsystems to establish the validity of modeling the properties of the mixed systems on the basis of known data for the individual ones. Two gallium-based solvent alloys were selected for the study, i.e. Ga–In (21.4 wt.% In) and Ga–Al (1.6 wt. % Al) eutectic mixtures. The quaternary systems investigated were La–U–(Ga–In) and La–U–(Ga–Al) and the measurements were performed between 573–1073 K. Solubility of lanthanum and uranium in the said systems were determined by taking the samples and analyzing the composition of the liquid metal phase saturated with La and U and equilibrated with the solid intermetallic compounds (IMCs). Composition and structure of the IMCs was determined by X-ray diffraction analysis. The following equations describe temperature dependencies of lanthanum and uranium solubility (in atomic fraction): lgXLa (La–U–Ga–In) = 2.431 – 3714 / T lgXU (La–U–Ga–In) = 2.081 – 4724 / T lgXLa (La–U–Ga–Al) = 2.468 – 3687 / T lgXU (La–U–Ga–Al) = 0.619 – 2862 / T Thermodynamic activity of the elements of interest was determined employing the electromotive force measurements technique and uranium and lanthanum activity coefficients were obtained by subtracting the temperature dependencies of solubility from those of activity. The following results were obtained: lgγLa (La–U–Ga–In) = 1.678 – 4506 / T lgγU (La–U–Ga–In) = 3.543 – 11789 / T lgγLa (La–U–Ga–Al) = 3.070 – 11824 / T lgγU (La–U–Ga–Al) = 3.041 – 6078 / T The data obtained here for the quaternary alloys were compared with the results of previous measurements for the corresponding ternary La–(Ga–In), U–(Ga–In), La–(Ga–Al) and U–(Ga–Al) systems.

Behavior of uranium and rare earth elements in liquid metal systems

254th National Meeting (2017)

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