Vol 2019, No 8 (2019)

More than 100 power reactors, reactors for plutonium production, and research reactors are presently available in the world. In these reactors, graphite is used as a reflector, moderator, and cladding of fuel elements. Flameless combustion is a promising method for reducing the amount of solid radioactive waste. The method is based on the oxidation of solid radioactive waste in oxide–carbonate melts and makes it possible to reduce the radioactive graphite volume considerably. Thermodynamic simulation and analysis of the oxidation of radioactive graphite in molten Na₂CO₃–K₂CO₃–Sb₂O₃ in a carbon dioxide atmosphere are performed using the TERRA software package. The thermodynamic simulation is conducted at a pressure of 1 atm and initial and final temperatures of 273 and 3273 K, respectively. The step of temperature changing is 100 K. Based on these data, the distributions of elements between condensed and gas phases are examined. The simulation results show that carbon disappears at a temperature of 873 K. Heating of the system up to 1073 K leads to the evaporation of condensed antimony compounds. Heating of the system up to 1673 K leads to the evaporation of condensed potassium, sodium, chlorine, uranium, and cesium compounds. Heating of the system up to 2273 K leads to the evaporation of condensed nickel compounds. Heating of the system up to 2573 K leads to the evaporation of condensed calcium, plutonium, beryllium, strontium, americium, and europium compounds. Only a vapor–gas phase is observed at temperatures above 2573 K.

The amorphization and solidification of binary and multicomponent transition-metal-based melts are considered. The presolidification structural changes and the atomic dynamics in metallic melts and other liquid substances that occur during forced quenching and deep supercooling are analyzed and compared as functions of their composition, the atomic size, the character of interatomic bonds, and the presence of eutectics and compounds in phase diagrams. The hydrogenation of a melt changes presolidification processes, affecting the atomic configuration, the modulation of a frequency spectrum, and the fluctuations of band structure (which determines the character of vitrification).

A theory is developed to describe the nonstationarity growth of spherical crystals in supercooled melts and supersaturated solutions. The first two corrections to the fundamental contribution to the law of crystal growth rate are determined using differential series, the expansion of unknown functions in a small parameter, and the integral Laplace–Carson transform. These corrections are shown to substantially change the growth rate of spherical crystals in metastable liquids.

The electrochemical behavior of nickel in the K₂WO₄–Na₂WO₄ (1 : 1)–WO₃ (35 mol %) and K₂WO₄–Na₂WO₄ (1 : 1)–WO₃ (50 mol %) polytungstate melts was studied by the cyclic voltammetry. Based on a comparison of the obtained cyclic voltammograms with the data of SEM, EDS and XRD conclusions were drawn about the sequence of processes proceeding on the nickel substrate. Equations of the electrolysis products formation are proposed.

The combined aluminothermic reduction of titanium and tantalum and of titanium and vanadium from their pentoxides is theoretically and experimentally simulated. The sequence and features of the formation of compounds in the Al–TiO₂–Ta₂O₅ and Al–TiO₂–V₂O₅ systems are established.

The effect of temperature on the corrosion of nickel (N1) in the range from 500 to 800°C during testing in the salt melt of the eutectic mixture of lithium and potassium chlorides with the addition of lanthanum trichloride in the amounts from 0.5 to 2 mol % is studied. The nickel corrosion rate increases with temperature, and lanthanum chloride additives decrease the corrosion rate at 500–650°C (inhibition of the shielding type due to the subsequent chemical reaction). The corrosion potential of nickel is approximately –0.5 V relative to the silver chloride electrode at 500°C, insignificantly decreases with temperature, and is independent of the concentration of lanthanum chloride. To specify the mechanism of corrosion damage, current–voltage curves are recorded at lanthanum chloride contents of 0.5 and 2 mol % and sweep speeds of 10 and 20 mV/s. X-ray diffraction analysis and X-ray spectral microanalysis are used. The sweep speed exerts no effect on the irreversible electrochemical oxidation process, and the addition of lanthanum chloride increases the current density in the anodic region tenfold or more. A comparison of the corrosion rates obtained by gravimetric and chemical analytical methods suggests an electrochemical corrosion mechanism.

The liquidus temperatures of the ternary mutual KF–KCl–KI system with KF/KCl molar ratios of 0.8 and 2 are determined by thermal analysis of cooling curves and differential scanning calorimetry. The dependences of the liquidus temperature on the potassium iodide content for the [45KF–55KCl, mol %]–KI and [66KF–34KCl, mol %]–KI melts are determined. The results are generalized in the form of two sections of the ternary mutual KF–KCl–KI system. The data obtained by thermal analysis agree with the data obtained by differential scanning calorimetry and thermogravimetry. Synchronous thermal analysis confirms that the KF–KCl–KI system is characterized by the 25KF–34KCl–41KI (mol %) eutectic with a melting temperature of 750 K.

The densities of the salt melts of the GdCl₃–NaCl and GdCl₃–KCl systems are determined by the dilatometric method over the entire concentration range and in a wide temperature range. The molar volumes of the systems and their deviations from additivity are calculated. All electrolyte composition dependences are nonlinear. The results can be used in the manufacture of gadolinium and its compounds and for the refinement of the structures of the molten salt systems under study.