The safe treatment and reliable immobilization of waste generated during the reprocessing of spent nuclear fuel remain critical challenges for nuclear power. This study evaluates the feasibility of a plasma-based route that converts liquid reprocessing residues into chemically stable metal-oxide powders and then immobilizes these products into durable matrices suitable for long-term storage. The approach combines thermodynamic modeling with laboratory-scale experiments. Modeling was used to determine adiabatic combustion temperatures and equilibrium phase compositions for water–salt–organic feeds under plasma exposure. Experiments with a high-frequency plasma generator confirmed that, under optimized conditions near 1200°C, organic constituents are completely oxidized, and finely dispersed oxides are formed. The resulting powders include simple and complex oxides of iron, molybdenum, zirconium, neodymium, cerium, strontium, and yttrium; the phase balance depends on the plasma-cooling regime. Post-processing by gravitational and magnetic separation improves powder recovery and purity. For final conditioning, the oxides were incorporated into chloride-based melts, yielding dense, chemically and thermally stable solid forms after solidification. These results demonstrate that plasma treatment can integrate waste destruction, oxidation, and immobilization within a single technological workflow, reducing external heat demand and enabling robust products for storage or further use. The findings provide an engineering basis for scaling plasma systems for radioactive-waste management with an emphasis on safety, efficiency, and sustainability.

A mathematical model is formulated to describe the plasma treatment of spent nuclear fuel (SNF) reprocessing wastes, which are fed into an air-plasma flow as dispersed aqueous–salt–organic compositions (ASOCs) containing ethanol or acetone as organic additives. The model describes the coupled kinetics of droplet heating and evaporation, interphase momentum exchange, and heat and mass transfer within a one-dimensional two-phase-flow framework, which is cast in dimensionless form to enable parametric analysis and scale-up. Thermochemical calculations are used to determine optimal ASOC formulations that provide an adiabatic combustion temperature of approximately 1500 K, thereby ensuring energetically efficient plasma operation and complete oxidation of the organic fraction. The influence of key operating parameters—initial plasma temperature and velocity, droplet size and injection velocity, and liquid-to-gas mass ratio—on the spatial extent of droplet evaporation is investigated numerically using a fourth-order Runge–Kutta integration scheme with controlled accuracy. The results show that, at temperatures above about 1500 K, the overall rate of plasma utilization is governed by solvent (water) evaporation, whereas further increases in inlet plasma temperature have only a weak effect on the length of the complete-evaporation zone. In contrast, droplet size and flow hydrodynamics exert a dominant influence: reducing the initial droplet diameter from 100 to 40 μm and decreasing the gas-flow velocity from 90 to 10 m s–1 lead to multi-fold reductions of the evaporation length, allowing complete evaporation within reactor lengths not exceeding 1 m. The resulting model serves as a physically sound and computationally efficient means of forecasting and refining droplet-evaporation dynamics in air-plasma reactors designed for processing wastes from spent nuclear fuel reprocessing and other liquid radioactive waste streams.

—Experimental modeling of the interaction of eclogitic and lherzolitic garnets with CO2 fluid was carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in platinum ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa and in the temperature range 950–1550 °C. It has been established that the main interaction processes at 6.3 GPa and 950–1250 °C are partial dissolution, recrystallization, and carbonation of garnet which lead to the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on the seed at ≥1250 °C were observed. In the temperature range 1350–1550 °C, the garnet underwent partial dissolution and recrystallization in CO2 fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the rims of crystals relative to the cores, and the presence of carbonate, kyanite, coesite, and CO2 inclusions. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism with the participation of CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental research, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate–carbonate–CO2, which simulates natural diamond formation media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation with the participation of CO2 fluid.

The central problems of using ceramic nuclear fuel fabricated from uranium dioxide enriched in uranium-235 are low thermal conductivity, high disposal costs, and limited natural reserves of uranium-235. A potential direction for the further development of nuclear power in Russia is the use of mixed uranium-plutonium dispersion fuel consisting of fissile materials (uranium and plutonium dioxides) uniformly distributed in a matrix of metal oxides, which have a high thermal conductivity and a low neutron absorption cross section. The methods used to obtain complex oxide compositions (sol-gel, separate preparation, mechanical mixing, and others) are multi-stage, do not provide a uniform distribution of phases, and are characterized by high energy and labor costs. This article proposes a plasma-chemical method of synthesizing nanosized complex oxide compositions in an air-plasma flow from dispersed water-organic nitrate solutions, which provides a significant reduction in energy consumption, a uniform distribution of phases, and the required phase composition.

The article discusses the process of plasma chemical synthesis of uranium-thorium oxide powders for a new generation dispersion nuclear fuel. In the course of research, the combustion parameters of the precursors were calculated. Precursors were water-organic nitrate solutions based on uranyl nitrate and thorium nitrate (fissile components), as well as magnesium nitrate (matrix material). The organic component of the solutions was acetone due to the sufficiently high calorific value and good mutual solubility. In the course of thermodynamic calculations, the optimal modes of processing of the water-organic nitrate solutions in low-temperature plasma were determined. These modes ensured the synthesis of oxide powders of the necessary stoichiometry without impurities of unoxidized carbon (soot). Experiments to obtain the samples of powders were carried out with the model solutions in which uranyl and thorium nitrates were replaced by neodymium and cerium ones, which are in the same group of the periodic table. The synthesis was carried out with the use of a plasma chemicalunit based on a high frequency torch plasmatron. The synthesized powders were subjected to a number of analyzes including electron microscopy, particle size analysis, X-ray phase analysis and BET analysis. The results showed that the powders can be classified as nanosized.

The use of the accident tolerant fuel is one of promising development trends of nuclear power. This fuel contains metal oxides uniformly distributed in the oxide matrix having a high thermal conductivity and small thermal neutron capture cross section. Multi-stage methods used to obtain oxide compounds, do not provide the uniform phase distribution and appropriate composition, and are energy- and labor-consuming. The proposed plasma-chemical synthesis of nanostructured oxide compounds utilizes aqueous solutions of organic nitrates providing minimization of process energy, uniform phase distribution and required composition.

A promising direction of advancement in nuclear energy is the use of uranium-thorium dispersion nuclear fuel, which consists of fissile metal (uranium, thorium) oxides evenly distributed in an oxide matrix having a high thermal conductivity and small neutron absorption cross-section. The methods of obtaining oxide composites (sol-gel process, separate production, mechanical mixing, and others) are multistaged, do not afford a uniform distribution of phases, and have high energy consumption. This article discusses the plasma- chemical synthesis of nanosized oxide composites in an air-plasma flow of dispersed aqueous-organic nitrate solutions with heat value equal to at least 8.4 MJ/kg, affording significant energy consumption reduction and uniform phase distribution with the required phase composition.

This paper presents the results of experimental studies on the process of plasmachemical synthesis of fuel oxide compositions for plutonium-thorium dispersion nuclear fuel. Precursors were simulated water-organic nitrate solutions, which have a lower calorific value near 8.4 MJ/kg. The precursors consisted of an organic component (acetone) and mixed water nitrate solutions including a matrix metal (magnesium, yttrium), samarium (instead of plutonium), and cerium (instead of thorium). The authors determined the compositions and modes of processing simulated solutions that provide plasmachemical synthesis of nanosized complex oxide powders imitating plutonium-thorium dispersion nuclear fuel.

The article is devoted to the study of irradiated graphite-bakelite paste formed during the restoration of graphite stack and the operation of uranium-graphite reactors. It was shown that irradiation of graphite-bakelite paste in the reactor leads to the formation of long-lived radionuclides and the accumulation of Wigner energy. The studies of graphite-bakelite paste extracted from the graphite stack of one of the UGR with a closed primary loop showed that the samples of graphite-bakelite paste are contaminated unevenly with 137Cs, 154Eu, 60Co, and 241Am radionuclides (fission and activation products and actinides). From the results of experiments on leaching of radionuclides from graphite-bakelite paste, it can be seen that radionuclides 241Am and 137Cs are more subjected to leaching. The estimation of the stored energy in the selected samples shows that the heat release begins at a temperature of 600–650 ◦C. The largest amount of stored energy is released at a temperature of 750 ◦C. Moreover, the magnitude of this quantity does not exceed 1–6 J/g. It was also shown that the development of the concept and program for decommissioning of UGR, the stack of which was restored using graphite-bakelite paste, requires additional studies to determine the potential danger of irradiated graphite-bakelite paste.

The article is dedicated to analyzing the concentration processes of the long-lived 36Cl radionuclide in irradiated graphite of uranium-graphite nuclear reactors. The 36Cl radionuclide is one of the most important isotopes for assessing the safety of radioactive-waste disposal. An analysis of the so-called Nugget effect due to the substantial heterogeneity of the 36Cl content in samples of irradiated graphite of nuclear reactors is carried out. In the same graphite elements, the difference in the 36Cl concentration reaches 100 times regardless of operational factors, such as the neutron flux and temperature. The article discusses the main processes that can affect 36Cl contamination in irradiated graphite. Particular attention is paid to determining the form of 36Cl in graphite, taking into account the features of its manufacturing and thermodynamic modeling of the equilibrium compositions of reaction products during the purification of unirradiated graphite in a gaseous medium.

Interactions of mantle silicates with subducted carbonates, sulfides, and sulfur-rich fluids are experimentally simulated in the olivine-ankerite-sulfur and olivine-ankerite-pyrite systems using a multi-anvil high-pressure split-sphere apparatus at 6.3 GPa and range of 1050–1550 °C. Recrystallization of Fe,Ni-bearing olivine and ankerite in a sulfur melt was found to be accompanied by sulfidation of olivine and carbonate, involving partial extraction of metals, carbon, and oxygen into the melt, followed by the formation of pyrite (±pyrrhotite), diopside, and Fe-free carbonates. The main features of metasomatic alteration of Fe,Ni-olivine by a reduced sulfur fluid include: (i) a zonal structure of crystals (Fe-rich core, Mg-rich rim); (ii) inclusions of pyrite and pyrrhotite in olivine; (iii) certain Raman spectral characteristics of olivine. At T > 1350 °C, two immiscible melts, a predominantly sulfur melt with dissolved components (or a Fe–Ni–S–O melt) and a predominantly carbonate one, are generated. The redox interaction of these melts leads to the formation of metastable graphite (1350–1550 °C) and diamond growth (1550 °C). The studied olivine-ankerite-sulfur and olivine-ankerite-pyrite interactions may be considered as the basis for simulation of metasomatic processes accompanied by the formation of mantle sulfides during subduction of crustal material to the silicate mantle.

The process of plasma-chemical synthesis from water-organic nitrate solutions of uranium and magnesium of fuel compositions including a matrix of magnesium oxide with a high thermal conductivity and low neutron absorption is investigated. The compositions of the solutions, including acetone (ethanol), as well as the regimes that providing the direct synthesis of "UO2-MgO" fuel compositions of different concentrations in air plasma are determined. The calculation of their thermal conductivity and comparison with experimental data are presented.

The paper is dedicated to the issues of accumulation of irradiated reactor graphite and possible options for handling it. There are described the advantages of the electrochemical method of decontamination of radioactive waste widely used in the nuclear industry. The use of this method to reduce the potential hazard of irradiated reactor graphite is proposed. The processes occurring during the removal of radioactive contamination from the surface of graphite radioactive waste are described. An experimental setup is presented to assess the possibility of using the method of electrochemical decontamination of irradiated graphite. The results of the determining the electrolyzer current-voltage curve and the dissolution rate of the electrodes made of irradiated graphite are presented. According to the results of experimental studies, data on the decontamination coefficients of irradiated graphite were obtained for various radionuclides (Co-60, Cs-137, Eu-154, Eu-152) in HNO3, H2SO4, H2O, H2O2, HNO3 thorn KMnO4, H2SO4 thorn KMnO4 under various process modes. The evaluation of temperature fields inside the electrolyzer was carried out. The dependence of the removal efficiency of long-lived C-14 radionuclide on the total mass loss of irradiated graphite was obtained. (C) 2019 Elsevier B.V. All rights reserved.

While lattice thermal conductivity is an important parameter for many technological applications, its calculation is a time-consuming task, especially for compounds with a complex crystal structure. In this paper, we solve this problem using machine learning interatomic potentials. These potentials trained on the density functional theory results and provide an accurate description of lattice dynamics. Additionally, active learning was applied to significantly reduce the number of expensive quantum-mechanical calculations required for training and increases reliability of the potential. The CoSb3 skutterudite was considered as an example, and the solution of the Boltzmann transport equation for phonons was compared with the Green-Kubo method. We demonstrated that accurate and reliable potentials can be obtained by performing just a few hundred quantum-mechanical calculations. The potentials reproduce not only the vibrational spectrum, but also the lattice thermal conductivity, as calculated by various methods.

The paper discusses questions dedicated accumulation of graphite radioactive waste generated during commissioning of uranium-graphite nuclear reactor. Authors determine character of contamination of irradiated graphite by different radionuclides (especially (137)cs, Co-60, Sr-90) using autoradiographic methods and by means of electron microscope (scanning electron microscope - analysis). It has been obtained images of polished section of irradiated graphite in back-scattered electrons. Authors suggest the method of thermal decontamination of irradiated graphite surface by noble gas flow for the purpose of potential hazard reduction of graphite radioactive waste. The developed mathematical model of thermal decontamination of irradiated graphite subject to Wigner energy release under radiation defect annealing has been demonstrated in paper. The article represents laws reflecting dependence of recession velocity of radioactive contamination of graphite radioactive waste and different parameters of process subject to Wigner energy release.

The paper proposes a mathematical model of the reactor, which describes the process of treatment of dispersed water-organic nitrate solutions of metals at air-plasma flow temperatures more than 1500 K. Regularities and a quantitative comparison were made to show the effect of the initial values of the air-plasma flow parameters (temperature, velocity) and the WONS droplet parameters (size, velocity) on the rate of their evaporation in the reactor. Process conditions were established that ensure a high evaporation rate of dispersed solutions in the air-plasma flow.

Irradiated graphite of uranium graphite reactors, which is a radioactive waste, is formed during the decommissioning of nuclear reactors.The paper presents the results

Article represents results on theoretical and experimental research of yttria and zirconia plasmachemical synthesis in air plasma from water-salt-organic mixtures "yttrium nitrate-water-acetone" and "zirconyl nitrate-water-acetone". On the basis of thermotechnical calculations the influence of organic component on lower heat value and adiabatic combustion temperature of water-salt-organic mixtures as well as compositions of mixtures providing their energy-efficient plasma treatment were determined. The calculations found the influence of mass fraction and temperature of air plasma supporting gas on the composition of plasma treatment products. It was determined the conditions providing yttria and zirconia plasmachemical synthesis in air plasma. During experiments it was being carried out the plasmachemical synthesis of yttria and zirconia powders in air plasma flow from water-salt-organic mixtures. Analysis of the results for obtained powders (scanning electron microscopy, X-ray diffraction analysis, BET analysis) confirm nanostructure of yttria and zirconia.

Experimental studies in the Fe3C–SiO2–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron–wustite buffer.