Efficient development of nuclear power generation requires and effective use of available uranium mineral resources requires closing nuclear fuel cycle (NFC). Molten salts can be used in various parts of NFC, including nuclear fuel manufacture and reprocessing, power generation (in molten salt reactors), etc. Pyrochemical methods of nuclear fuel production and spent fuel reprocessing were mostly developed for fused halide (mostly chloride) media; considerably less attention was paid to other possible melts (sulfate, nitrate, carbonate, etc.). Work in chloride melts is normally performed under an inert atmosphere and requires careful selection of construction materials with sufficient corrosion resistance to molten chlorides.

Alkali metal molybdates represent an interesting alternative to chlorides. Molybdate melts have high thermal and radiation stability, sufficient electric conductivity and acceptable viscosity. In addition anodic reaction in fused molybdates results in evolution of oxygen, rather than chlorine in chloride melts. Therefore work with molybdate melts can be performed in the oxygen containing atmosphere, e.g. ambient air. Lowering working temperatures can be achieved by using alkali molybdate mixtures rather than individual salts. Addition of molybdenum oxide further lowers melting point and increases solubility of uranium species.

The present work was devoted to studying the effect of temperature and melt composition on the cathodic product deposited from uranium containing molybdate melts. The experiments were performed in the melts based on the mixtures of lithium and potassium molybdates (1–x)(0.605Li₂MoO₄–0.395K₂MoO₄)–xMoO₃, where x was varied from 0.3 to 0.5. Uranium was added to the melt as UO₂MoO₄, and concentration of uranyl molybdate was maintained around 15 wt. %. Working temperature was varied between 550–850 ^oC. Electrochemical behavior of uranium species was studied by cyclic voltammetry, chronopotentiometry and cathodic polarization. Electrolysis was performed in galvanostatic mode on platinum cathode with the initial current density varied between 0.05–1 A/cm². Depending on the experimental conditions the cathodic deposits consisted of UO_2+x, U₄O_9–y and MoO₂ phases. Melt composition had a noticeable effect on the cathodic deposit morphology and examples of appearance of the electrodes after electrolysis are presented in the Figure. Phase composition and oxygen-to-uranium ratio were determined for the products obtained. Conditions allowed obtaining uranium oxide with minimal molybdenum contamination were determined, and bulk electrolysis was conducted in the melts containing 27 wt. % UO₂MoO₄.

Figure. Cathodic deposits obtained from 0,625(0,605Li₂MoO₄–0,395K₂MoO₄)–0,375MoO₃ (left) and 0,5(0,605Li₂MoO₄–0,395K₂MoO₄)–0,5MoO₃ (right) based melts containing 15 wt. % UO₂MO₄ at 550 ^oC and initial cathodic current density of 0.1 A/cm².