Pyrochemical reprocessing of spent nuclear fuels in molten salts is currently considered as one of possible concepts for closing nuclear fuel cycle. The technology involves extracting fissile materials and fission products. After extracting uranium and plutonium the remaining melt needs to be purified from the remaining fission products (rare earth and alkaline earth elements) before it can be reused. Oxygen is a common impurity in the atmosphere above the melt. Reaction of oxygen with the melt can affect the speciation of dissolved elements and efficiency of separation processes. From the other side treating molten electrolyte with oxygen can be used for selective precipitation of certain elements, i.e. rare earths, providing a method of the melt purification. Samarium is one of the rare earth group fission products that remain in the melt. The aim of the present study was investigating the kinetics of interaction of oxygen with alkali chloride based melts containing samarium chloride.

The experiments were performed in the melts of industrial interest, i.e. based on LiCl–KCl and NaCl–KCl–CsCl eutectics and NaCl–KCl equimolar mixture, between 450 and 750 ^oC. Samarium is one of the rare earth metals that forms in chloride melts stable species in the oxidation states of +2 and +3. Sm(II)/Sm(III) couple provides a convenient mean of monitoring samarium concentration in the melt. In the present work cyclic and square wave voltammetry were employed for monitoring samarium behavior in the melts sparged with oxygen. The experiments were performed in three-electrode cells with platinum working, glassy carbon counter and silver chloride reference electrodes. Voltammograms corresponding to Sm(II)/Sm(III) redox couple were recorded after the melt was treated with oxygen for a certain period of time. An example of the evolution of square wave voltammograms during this process is shown in the Figure. The intensity of Sm(III)/Sm(II) reduction peak near –1 V gradually decreased reflecting lowering samarium concentration in the melt while another peak near –1.6 V appeared and gradually increased. Dependencies of peak intensities in cyclic and square wave voltammograms on time of oxygen sparging were used to calculate the reaction rates and determine kinetic parameters of the reactions.

Figure. Square wave voltammograms recorded in (NaCl–KCl–CsCl)–SmCl₃ melt (3 wt. % Sm) at 550 ^oC during oxygen sparging through the melt. Ag/AgCl reference electrode. Total time of oxygen sparging was 240 min. Arrows show the direction voltammograms changed.