The reaction of uranium dioxide with excess hydrogen chloride in alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl) has been studied between 450 and 750^◦C, and the reaction products were characterized by electronic absorption and X-ray absorption spectroscopy. Uranium( V), [UO₂Cl₄]³⁻, and uranium(IV), [UCl₆]²⁻, species were formed. They depended upon the temperature and the radius of the alkali cations present. Uranium(V) ions predominated in melts with small cations (LiCl and 3LiCl-2KCl).

Corrosion of nickel alloys in molten salts is a complex process dependent on many factors. The paper describes the influence of microstructural changes in several nickel-based alloys (Hastelloy^® G-35^®, VDM^® Alloy 59, KhN62M-VI, Hastelloy^® B-3^®) on the mechanism of their corrosion in molten fluoride salts. The corrosion experiments were performed in LiF–NaF–KF and (LiF–NaF–KF) + UF₄ melts at 550–750 °C. Formation of excess secondary phases along the grain boundaries of the alloys led to heterogeneity of the alloy microstructure. As a result of these changes, microgalvanic couples formed on the surface of the alloys, leading to the development of intergranular corrosion upon contact with molten electrolytes. Secondary phases acted as microanodes or microcathodes depending on a number of factors. Formation and composition of the secondary phases affected the depth and extent of the corrosion damage to nickel alloys.

Neptunium behaviour in an LiCl-KCl eutectic melt at 723 K was studied using spectroelectrochemistry. Cathodic reduction of neptunium(IV)-containing melts led to the formation of Np(III) ions and then neptunium metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into individual Gaussian bands. The nature of neptunium complex ions in the melt is discussed.

Speciation of tungsten in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 550 and 750^◦C using electronic absorption spectroscopy. OnlyW(IV) and W(V) chloro and oxychloro species can be stabilized under the conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution ofWmetal at anodic current densities in the range of 0.005 - 0.1 A/cm² produces only [WCl₆]²⁻ ions that can be electrochemically (on a glassy carbon anode) oxidized to [WCl₆]⁻. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro species intoW(IV) oxychloro species. In the presence of O₂ in the atmosphere [WCl₆]²⁻ is oxidized into the tungsten(V) species [WOCl₅]²⁻. Dissolution of tungsten hexachloride, WCl₆, in an NaCl-2CsCl melt initially yields [WCl₆]²⁻ (due to disproportionation). Reaction of metallic tungsten with Pd(II)-containing melts results in the formation of [WCl₆]²⁻ species. The main spectroscopic parameters of [WCl₆]²⁻ and [WCl₆]⁻ complex ions were calculated.