Alloying uranium with molybdenum allows stabilization of uranium ?-phase at low temperatures. This makes U-Mo alloys attractive for manufacturing high density nuclear fuels. One of the potential methods for reprocessing irradiated metallic fuels is pyrochemical technology employing fused alkali chlorides as working media. Electrochemical processes are used for separating spent fuel components and designing a feasible and effective process relies on the detailed knowledge of the electrochemical properties of all the elements present in the fuel. Although electrochemical behavior of molybdenum in molten salts was studied in the past, the information concerning Mo electrochemical properties in chloride melts is scarce and often contradictory. The present work was aimed at comprehensive study of molybdenum electrochemical properties and behavior in alkali chloride melts. Electrochemical techniques were complimented by in situ high temperature spectroscopy to monitor molybdenum speciation.The experiments were conducted in individual alkali chlorides and their mixtures up to 1123 K. Mo(III) is the lowest oxidation state present in the melts in equilibrium with molybdenum metal. No evidence of soluble Mo(II) species was obtained. Molybdenum formal standard electrode potentials increase with temperature and decrease with mean radius of the solvent salt cation, e.g.E*Mo3+/Mo=(�1.146+3�10-4�T) � 0.005 V (NaCl-KCl)E*Mo3+/Mo=(�1.670+8�10-4�T) � 0.005 V (NaCl-2CsCl)E*Mo3+/Mo=(�1.320+6�10-4�T) � 0.005 V (LiCl-4KCl-2CsCl)where the potential values are given vs. Cl2/Cl- reference electrode. Mo(III) species are prone to disproportionation and the rate of this reaction was determined spectroscopically.Electrochemical oxidation of Mo(III) proceeds in two stages Mo(III)?Mo(IV)?Mo(V). Mo(IV) chloro-species are stable in 3LiCl-2KCl below 773 K and NaCl-2CsCl below 873 K. Formal Mo(IV)/Mo(III) red-ox potential in NaCl-2CsCl melt at 823 K is �(0.83 � 0.04) V.Molybdenum(V) in chloride melts is present as a mixture of MoCl6- ions and dissolved MoCl5, the ratio of these forms depends on temperature and melt cationic composition.

Electrochemical properties of molybdenum in alkali chloride melts

247th National Meeting (2014)

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