Uranyl(V) species are normally unstable in solutions but are here shown to be stable in high-temperature chloride melts. Reactions leading to the formation of UO 2 Cl 4 3ions were studied, including thermal decomposition and chemical reduction of uranyl(VI) chlorospecies in various alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl, and NaCl-2CsCl) at 550-850 °C. Decomposition of UO 2 Cl 4 2species under reduced pressure, with inert gas bubbling through the melt or using zirconium getter in the atmosphere results in the formation of UO 2 Cl 4 3and UO 2 . Elemental tellurium, palladium, silver, molybdenum, niobium, zirconium, and hydrogen, as well as niobium and zirconium ions were tested as the reducing agents. The outcome of the reaction depends on the reductant used and its electrochemical properties: uranyl(VI) species can be reduced to uranyl(V) and uranium(IV) ions, and to uranium dioxide.

Speciation of tungsten and molybdenum in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 450 and 750 oC using electronic absorption spectroscopy. Only W(IV) and W(V) chloro- and oxychlorospecies can be stabilised under conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution of W metal at anodic current densities in the range of 0.005-0.1 A/cm2 produces only WCl6 2- ions that can be electrochemically oxidised to WCl6 -. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro-species into W(IV) oxychloro-species. In the presence of O2 WCl6 2- is oxidised into tungsten(V) species WOCl5 2-. Molybdenum forms Mo(III), (IV) and (V) species. MoCl6 2- complex ions in chloride melts can be obtained by reacting MoO2 with HCl, or by electrochemical oxidation of Mo(III) (MoCl6 3- → MoCl6 2-) on a glassy carbon electrode.

Reactions leading to the formation of uranyl(V) species in fused alkali chlorides were studied. The reactions investigated include thermal decomposition of uranyl(VI) chloride in alkali chloride melts, electrochemical and chemical reduction of UO 2 2+ ions in LiCl, 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl melts at 550-850 o C. Elemental tellurium, palladium, silver, molybdenum, niobium and zirconium were tested as the reducing agents. The outcome of the reaction depends on the electrochemical properties of the reductant.

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The reactions of uranyl(VI) containing chloride melts with molybdenum, niobium, zirconium and palladium were studied using high temperature electronic absorption spectroscopy. Depending on the nature of the added element uranium is reduced to uranyl(V) and uranium(IV) chloro-species and UO2. Palladium, niobium and zirconium can all be removed from a uranyl(VI)-containing melt using molybdenum metal and the melt can then be purified from Mo(III) ions by bubbling Cl2 gas. Such approach can be employed for removal a number of fission product elements from molten chloride baths during reprocessing spent oxide fuels.