CVD-COMPOSITES AND SALINE SOLUTIONS-MELTS: SIMILARITY AND DIFFERENCES

The general features and specific peculiarities of the thermodynamics of the processes occurring during the evaporation of CVD (Chemical Vapor Deposition) – composites based on germanium – metal chalcogenide systems and dissolution of poorly soluble compounds in salt melts are considered. The essence of both processes consists in the occurrence of exchange reactions between the initial components with the formation of highly volatile substances in the first case and highly soluble compounds in the second. Both processes are endothermic in their thermochemical essence, and their course is facilitated by the entropy component, the role of which increases with increasing temperature, deepening of the vacuum in the case of CVD composites, and dilution of the saline solution – melt. The peculiarities due to the difference between vacuum and salt melt in nature have also been established. If the interaction of molecules in a rarefied molecular vapor of evaporation products can be practically neglected, then in a salt solution-melt, as in a highly concentrated system, secondary reactions of complexation of ions and molecules of the dissolved compound and the main ions of the melt are characteristic. The latter factor significantly affects the solubility in the salt melt due to the shift in equilibrium. Kinetic factors, which differ significantly in both processes, are essential in the reverse reactions of condensation of a CVD‑composite vapor on a substrate and in the deposition of crystals during the crystallization of a solute from a salt melt. Due to the much higher rate of vapor condensation on the substrate, more significant vapor saturation is achieved and, accordingly, the nucleation rate than during crystallization of the salt melt. Therefore, the probability of nanostructuring or complete amorphization of a coating obtained from a CVD‑composite is much higher than for a salt solution-melt, in which the ability to form nanostructures is more limited.

SPECTROSCOPIC STUDY OF SALT MELTS OF THE SmF3-CeF3-NaCl-KCl SYSTEM

Redox interactions between the components of the SmF3-CeF3 and SmF3-CeF3-NaCl-KCl systems have been established by IR transmission spectroscopy, diffuse reflectance electron spectroscopy and luminescence spectroscopy. A significant decrease in the transparency in the IR range of the spectrum was found when passing from the first of the systems to the second, which is explained by an increase in scattering by ultramicrodispersed particles of fluorides in the salt melt. In both systems, the formation of a significant amount of Sm (II) and a decrease in the content of Sm (III) are observed. The change in the valence state of Samarium both during solid-phase heat treatment (1100 °C) and during holding in a salt melt at 700 °C is manifested in the disappearance of some absorption bands, the appearance of new bands, and a gypsochromic shift of the remaining bands. The luminescence spectra exhibit high-intensity emission bands in the 640–740 nm range, which correspond to 5d‑4f electronic transitions in Sm2+ ions. At the same time, the highest intensity is observed in the band corresponding to intracenter 5d‑4f electronic transitions in Ce3+ ions. Apparently, the Ce(IV) compound, formed as a result of the exchange reaction of complex fluoride with a salt melt, volatilizes with subsequent decomposition and does not affect the character of luminescence. On the whole, the luminescence intensity after treatment in the molten salt increases by several tens of times, which indicates a significant change in the radiation mechanism. The mechanism of redox reactions in the solid-phase state, as well as exchange processes in the salt melt and after its crystallization, is discussed. A significant role of solvation shells around particles of lanthanide fluorides in luminescence processes is assumed.

The Peculiarities of Crystallization of Lithium-Containing Granite Melt with High Water and Fluorine Contents in the Temperature Range of 800–400 °C and Pressure of 1 Kbar (According to Experimental Data)

The phase relations in the Si-Al-Na-K-Li-F-H-O model granite system are studied experimentally at T = 800, 700 °C and P = 1 and 2 kbar, as well as at T = 600, 550, 500 and 400 °C and P = 1 kbar and different water content from 2 to 50 wt.%. The initial composition was set in such a way that the composition of the resulting silicate melt was close to the granite eutectic. It is shown that in the presence of Li, two immiscible melts are formed in the system—an aluminosilicate (L) and a salt alkali-aluminofluoride (LF). It is shown that at Т = 800 °С, Р = 1 kbar and 2 kbar and water content > 10 wt. %, three phases are equilibrium in the system: L, LF, and fluid (Fl). Cryolite (Crl), which does not contain REE, begins to crystallize from the salt melt at 700 °C. Quartz (Qtz) crystallizes from the silicate melt at 600 °C and the equilibrium phases are L, LF, Crl, Qtz. At T = 500 °C Qtz, Na and K aluminofluorides and polylithionite crystallize from the aluminosilicate melt. The joint crystallization of Crl and Qtz is observed. Large crystals of cryolite and elpasolite are formed in both the salt and silicate melts. At the same time, the residual salt melt enriched in Li and REE is partially preserved. LF is completely crystallized at 400 °C, and L is in a metastable state. It is established that REE, Sc, Y and Li accumulate in the salt melt up to 500 °C with partition coefficients >> 1. REE and Sc enter into composition of the crystal phases at T = 500 °C and 400 °C. Sc partially isomorphically replaces Al. REE most often forms its own fluoride phases of the LnF3 type.

RADIATION AND PROTECTIVE PROPERTIES OF CONTAINERS FOR NPPS’ SALT MELT CONDITIONING IN UKRAINE

Considering that the problem of liquid radioactive waste management of Ukrainian nuclear power plants with WWER reactors is still unresolved, it is noted that the current level of scientific achievements provides grounds for creating a technology for the salt melt (SM) disposal without its processing. Since the exposure dose rate (EDR) for barrels with SM can significantly exceed the value of 5 mSv/h, the radiation-protective properties of the hypothetical packaging for conditioning of the salt melt formed by placing radioactive waste (RW) in a reinforced concrete container and its immobilization are considered. Exposure dose rate simulation was performed using the PHITS software package (Japan). The calculations were performed for a packaging consisting of 4 barrels of SM, placed in a universal protective container UZZK. Variants with different radionuclide composition of the SM, due to the duration of exposure of the SM – from 100% 137Cs to 137Cs-65%, 134Cs-15%, 60Co-20%, are considered. The specific concentration of radionuclides was taken from 5E7 to 1E9 Bq/kg. There are also 2 options for filling the voids: the first option involves filling the voids with the conditioning product of the bottom residue in the form of alkaline cement with a saline content of 25%, while the second option does not include radioactive salts in alkaline cement. The calculations showed the acceptability of the proposed conditioning of the salt melt by forming a package of 4 KRO-200 containers, universal reinforced concrete container UZZK TU U 29.2-26444970-005 and filling voids with the product of conditioning of the bottom residue in the form of alkaline cement with salinity -protective properties. A necessary condition for the implementation of practical measures for the conditioning of the salt melt accumulated in WWER reactors, followed by transfer to landfills is the corresponding changes in OSPU-2005 in terms of classification of the SM and its classification as solid radwaste.

SPECTROSCOPIC PROPERTIES OF SOLIDIFIED MELTS OF THE EuF3-CeF3-NaCl-KCl SYSTEM

The nature of the interaction in the EuF3-CeF3 system in the process of high-temperature (1050 °C) oxidation - reduction reaction was established by the methods of IR transmission spectroscopy, diffuse reflection spectroscopy and fluorescence spectroscopy. Here is a significant bathochromic shift to 480-485 nm band of blue luminescence of Eu(II) - containing phases, due to the 5d–4f electronic transitions, as well as the manifestation of orange-red luminescence of Eu(III) - containing phases due to 4f–4f electronic transitions in the range of 590–690 nm. There is a bathochromic shift of the IR bandwidth in the spectrum of the solidified salt melt as a result of dissolution of the fluoride system. Diffuse reflection spectra reveal changes in the composition of the phases that dissolve in the salt melt due to exchange reactions. The wide absorption band in the UV range gives way to a negative absorption band consisting of two peaks due to luminescence. The almost complete disappearance of the band of 4f–4f transitions in Eu(III) in the near-IR range of the spectrum is evidence of its entire reduction in the chloride melt to Eu(II). The character of the luminescence spectra of solidified salt melts also changes in comparison with the initial sample of the EuF3-CeF3 system, namely, the luminescence band of Ce3+ ions disappears, and the luminescence band of Eu2+ ions at 430–440 nm becomes narrow and highly intensive. The luminescence band of Eu3+ ions in the orange-red region of the spectrum disappears completely. Thus, Eu2+ ions become dominant in the formation of the spectral picture of the solidified salt melt, which is evidence of the completion of the redox process in the system.