Separation of Lanthanides and Actinides in a Chloride Melt - Liquid Metal System: The Effect of Phase Composition

Pyrochemical processes employing high temperature molten salts can be used for treating spent nuclear fuels (SNF), especially with a high burn-up and after low cooling time. Before returning the fissile materials to the fuel cycle, fission products must be separated and the required separation factor depends on the nuclear reactor type. Alkali metal chlorides are normally considered as the working media for the pyrochemical reprocessing. Metallic or nitride SNFs can be anodically dissolved in the melt, whereas oxide fuels can be reduced to the metallic state and then subjected to the anodic dissolution. SNF arising from molten salt reactors can be directly used for the reprocessing. Rare earth elements (yttrium, lanthanum and lanthanides) represent an important group of fission products with the electrode potentials more negative than those of uranium and plutonium. During SNF dissolution these elements will also dissolve. Liquid metals can be effectively employed for the subsequent separation of the fissile materials from the fission products. The separation factor Θ depends on the electrode potentials (E*) of separating elements in the salt phase and their activity coefficients (γ) in the metallic alloy. If the separating metals form in the salt melt the ions in the same oxidation state (like actinides and lanthanides), the separation factor can be expressed as lgΘMe1/Me2 = [n·F / (2.303·R·T)]·(E*Me2 – E*Me1) + (lgγMe1 – lg γMe2). In the present study the effect of temperature, cationic composition of the alkali chloride based melts, and composition of the liquid metal were considered, and separation of uranium from neodymium was studied as an example. Gallium, indium, tin and their mixtures of various compositions were chosen as low melting metals. Thermodynamic and electrochemical properties of uranium and neodymium were determined between 573–1073 K thus allowing to model separation processes. Cationic composition of the salt phase has little effect on the separation factor, whilst the effect of the metallic phase nature is more pronounced. Efficiency of separation (Θ value) increases with lowering temperature. An example of thermodynamically possible Nd/U separation factors for various liquid metals and LiCl–KCl–CsCl eutectic melt is presented in the Fig. Here experimentally determined values of E* and γ were used for the calculations. To confirm the results of calculations, experiments on separating Nd and U were conducted. The experiments were performed under various conditions (e.g., static or with stirring the phases) and the experimentally achieved separation factors compared with the theoretical expectations. Figure 1