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Reaction of solutions of rare earth chlorides in NaCl–KCl melt with sodium orthophosphate was studied at 750 oC. The rare earths investigated included yttrium, lanthanum and all lanthanides apart from promethium. The initial PO43–: RE3+mole ratio varied from 0.3 to 10 and the excess of phosphate required for the complete conversion of RECl3to phosphate as well as phase composition of the phosphates produced were determined. At PO43–: RE3+mole ratios below five only normal REPO4phosphates were formed, while at the ratios above 5 double Na3RE(PO4)2and/or K3RE(PO4)2phosphates were produced. Precipitation of a mixture of rare earths in the proportion simulating their abundance in the spent nuclear fuel was also investigated. Particle size of the phosphates formed was determined and this varied from 0.1 to hundredths of microns. Increasing the initial PO43–: RE3+mole ratio resulted in increasing particle size.

Speciation and electrochemical properties of molybdenum(IV) and tungsten(IV) species were investigated in NaCl–CsCl, NaCl–KCl–CsCl and LiCl–KCl–CsCl eutectic based melts between 500–900 oC. Metallic tungsten dissolves anodically, yielding W(IV) ions that are prone to oxidation upon prolonged contact with silica. Electrode potentials of tungsten in fused chlorides were measured. Electronic absorption spectra of the melts containing the products of anodic dissolution of tungsten were recorded. Mo(IV) ions can be formed by reacting molybdenum dioxide with hydrogen chloride or by anodic oxidation of Mo(III) ions. Electronic absorption spectra of Mo(IV) species were recorded and Mo(III)/Mo(IV) red-ox potentials were estimated.

Particle size distribution of phosphates of 13 rare earth elements formed in NaCl–2CsCl eutectic based melts at 650 oC was determined. Sodium orthophosphate was used as a precipitant. Initial phosphate-to-rare earth molar ratio in the reaction mixture corresponded to 50 and 100 % degree of precipitation. Particles in the powders obtained after aqueous treatment of quenched melts tend to agglomerate and ultrasonic treatment was employed in the particle size measurements. Normally particles of the phosphates have a size of 0.1–100 μm. The position of the maxima in PSD curves depends on the nature of RE element. In addition, precipitation of mixtures of RE elements mimicking their content in spent nuclear fuels was investigated in NaCl–2CsCl (at 650 oC) and NaCl–KCl (at 750 oC) melts.

The behavior of lanthanide (samarium, europium and ytterbium) chloride species in NaCl-2CsCl eutectic based melts was studied between 823–1023 K employing cyclic voltammetry, high temperature electronic absorption spectroscopy and spectroelectrochemistry. Diffusion coefficients of the lanthanide chloro-species, formal standard red-ox potentials and thermodynamics of the formation of LnCl3from the corresponding dichlorides in the melts were determined. A series of spectroelectrochemical experiments was performed to study cathodic reduction of Ln(III) ions under potentiostatic conditions. Sm(III), Eu(III) and Yb(III) species in NaCl-2CsCl melts can be reduced by metallic zirconium and this reaction was investigated using high temperature spectroscopy.

Electronic absorption spectra of solutions of molybdenum pentachloride in fused alkali chlorides were studied between 450 and 850 oC. LiCl-KCl, NaCl-CsCl, NaCl-KCl-CsCl eutectics and NaCl-KCl equimolar mixture were used as solvents. Although the experimental spectra contain only one broad band in the visible region, resolving the spectra into overlapping bands revealed two individual components under the experimental band. The ratio of intensities of the constituent bands depends on temperature and they are attributed to the absorption of MoCl6-ion and MoCl5molecular species. Spectroscopic parameters of MoCl6-were determined and MoCl6-↔MoCl5equilibrium is discussed. Absorption spectra of gaseous MoCl5were also recorded.

The results of spectroscopic studies of Nb anodic dissolution, dissolution of NbCl5 and reaction of various niobium oxides with HCl in different chloride melts at 450-7500C are presented. Nb(III) species, NbCl63-, is the major product of niobium anodic dissolution at current densities up to 40 mA/cm2. Addition of NbCl5 to alkali chloride melts predominantly leads to the formation of Nb(V) species, NbCl6-. Upon increasing Nb(V) concentration NbCl6- tend to decompose to Nb(IV) and Cl2. Reaction of NbO and NbO2 with HCl proceeds via an intermediate oxygen-containing Nb(IV) complexes. Oxygen-free Nb(IV) ions, NbCl62-, were obtained by exchange reactions between metallic niobium and melts containing Bi(III), Ag(I) or Ni(II) chloro-ions. Electroreduction of Nb(III) on a tungsten working electrode below -1.45 V vs. Cl2/Cl- led to the formation of metallic niobium, no evidence of intermediate reduction products was obtained.