Elena V. Nikolaeva

Molten systems based on alkali metal halides with lithium oxide additives are promising as a working medium on pyrochemical reprocessing of nuclear waste. A mixture of alkali metal fluorides of eutectic composition (FLiNaK) is a suggested solvent due to the high solubility of actinide oxides, low viscosity, high boiling points, low vapor pressure and resistance to radiation damage. Thermal analysis, XRD, Raman spectroscopy and thermodynamic simulations were used to obtain evidences on the phase equilibria and liquidus points of the system (0.85 FLiNaK?0.15 CeF3)?Li2O, containing up to 8.8 mol. % lithium oxide. The solubility of lithium oxide in the fluoride melt FLiNaK? ?CeF3 and the thermodynamic parameters of dissolution were obtained. The eutectic point (the Li2O content is 3.1 mol. %, Tm = 489 ??) and two peritectic points (lithium oxide content are 3.2 and 4.2 mol. %, and liquidus points are 497 and 549 ??, respectively) were found. Thermodynamic simulation results show an exothermic effect due to interaction between lithium oxide and fluoride melt. The interaction product oxyfluoride CeOF was detected by XRD analysis and Raman spectroscopy.

The mechanism and kinetic parameters of the interaction of the FLiNaK–CeF₃ melt with water vapors and oxygen in the air atmosphere were determined using Raman and IR spectroscopy, XRD analysis, and thermodynamic modeling of processes. The presence of the 4CeF_3(solution) + 6H₂O _(gas) + O_2(gas) = 4CeO_2(solid) + 12HF_(gas) reaction, which disturbs the fluoride melt homogeneity, was verified in situ by Raman spectroscopy adopted for high-temperature, chemically aggressive fluoride systems. Based on the obtained spectral data, the type of the kinetic equation, order, and rate constant of the chemical reaction were determined. The concentration of cerium dioxide was found to increase linearly in time and a zero reaction order with respect to CeO₂ was detected. The change in the concentration of CeO₂ over time at T = 510 °C is described by the equation C = 0.085t; the reaction rate constant is 0.085 mol. %∙min⁻¹. The obtained kinetic parameters may be used to model emergencies related with the depressurization of the coolant circuit or the working area of the molten salt reactor.

The electrical conductivity of LiCl?GdCl3 molten systems with the gadolinium chloride additions ranging from 0 to 23 mol % was measured depending on both the temperature and concentration of GdCl3. The molar conductivity of the molten GdCl3?LiCl system is calculated taking into account the assumption of additivity of the molar volume of the mixture. The obtained temperature dependencies can be approximated by Arrhenius-type equation. The effective activation energy, Ea, increased with the GdCl3 content. The liquidus temperatures of the studied systems were determined by differential scanning calorimetry. The high-temperature Raman spectra of LiCl?GdCl3 chloride melts were recorded. In addition, the conductivity of 0.77LiCl?0.23GdCl3 molten system with 1 mol % of Gd2O3 was measured. The investigation demonstrates that the addition of gadolinium oxide results in a decrease of the conductivity of the chloride molten system and growth of its liquidus temperature

The structure, conductivity, melting points and caloric melting effects of the (Li2CO3-Na2CO3-K2CO3)eut melt/MgO nanopowder heterogeneous system with MgO concentration up to 70 vol % have been investigated. A wide variety of methods (DSC, XRD, BET, high resolution scanning electron microscope, AC impedance method, IR and Raman spectroscopy) were used to evaluate samples and research. It is revealed that at the values of effective thickness of the salt phase interlayer between MgO particles below 100 nm there is an abrupt decrease in the melting points of the salt and the normalized phase transition enthalpy of the heterogeneous system. The activation energy of the electrical conductivity rises as the values of effective thickness of the melt phase interlayer between MgO particles decreases. The study established the lack of any chemical interaction between MgO and carbonate melt at 400-600°С. In situ Raman spectroscopy of the (Li2CO3-Na2CO3-K2CO3)eut melt/MgO nanopowder systems revealed the solvation of solid particles by salt-melt ions.

The electrical conductivity of GdCl ₃ chloride melts with the gadolinium oxide (Gd ₂ O ₃ ) additions ranging from 0 to 5 mol % was measured depending on both the temperature and concentration of Gd ₂ O ₃ . The obtained temperature dependencies can be approximated by Arrhenius-type equation. The calculated effective activation energy values of electrical conductivity increase with the increasing concentration of Gd ₂ O ₃ above 2.5 mol %. A decrease in the electrical conductivity of the chloride melts when adding the gadolinium oxide according to the Raman spectra data can be explained by the presence of [OGd ₃ Cl _n ] oxychloride associates.

The electrical conductivity of 0.507KCl–0.493YbCl ₃ and 0.5YbCl ₃ –0.25KCl–0.25NaCl chloride melts with the ytterbium oxide (Yb ₂ O ₃ ) additions ranging from 0 to 3.15 mol% was measured depending on both the temperature and concentration of Yb ₂ O ₃ . The liquidus temperatures of the studied systems containing from 0 to 3.35 mol.% Yb ₂ O ₃ were determined by DSC method and the electrical conductivity measurements. The Yb ₂ O ₃ additions increase the liquidus temperatures of the systems under study. The high-temperature Raman spectra of chloride (YbCl ₃ –KCl–NaCl) and oxide-chloride (YbCl ₃ –KCl–NaCl–Yb ₂ O ₃ ) melts were recorded. In the oxide-chloride melt an additional band was detected in the region of 460 cm ⁻¹ corresponding to stretching-vibrations of the Yb–O bond in [OYb ₃ Cl _n ] oxychloride associates. There was no evidence of vibrational bands related to ytterbium oxide. Such changes in the Raman spectrum points to the dissociation of ytterbium oxide and modifications to the local structure of the chloride melt.

The temperature dependence of the electrical conductivity of the molten eutectic GdCl₃(0.515)–KCl(0.485) containing up to 3.3 mol % gadolinium oxide is studied. The temperature dependences of the conductivity of the molten GdCl₃–KCl–Gd₂O₃ system are approximated by second-order polynomials. The formation of gadolinium oxychlorides is confirmed by spectral studies of the solidified melt.

The electrical conductivity of molten ternary alkali carbonate eutectic, coexisting with MgO particles, has been investigated. The conductivity was measured by the AC impedance method. The apparent activation energy ΔEa increased with the MgO content. This fact can be attributed to the effect of the solid phase. The specific conductivity of those systems could not be described using the Maxwell model over the solvation process of the carbonate ions on the particles of the magnesium oxide.

The electrical properties of composite electrolytes (suspensions) composed of α-Al₂O₃ powder and molten carbonate eutectic (Li₂CO₃–Na₂CO₃–K₂CO₃)_eut or molten carbonate–chloride mixture 0.72(Li₂CO₃–Na₂CO₃–K₂CO₃)_eut–0.28NaCl have been investigated by AC impedance method. This system shows a dependence of the electrical conductivity upon the temperature and the α-Al₂O₃ content. The specific electrical conductivity of the α-Al₂O₃/(Li₂CO₃–Na₂CO₃–K₂CO₃)_eut system can be adequately described by the Maxwell equation for two-phase heterogeneous materials. The regression equation for the dependence of the specific conductivity of the α-Al₂O₃/(Li₂CO₃–Na₂CO₃–K₂CO₃)_eut composite on the aluminium oxide concentration and temperature was obtained.