No 2 (2023)

At the radiochemical plant of FSUE “PO “Mayak” in the process of processing spent nuclear fuel (SNF), about 170 tons/year of metal radioactive waste (MRW) are formed, mainly represented by fuel element shells structural materials (SM) containing residual amounts of radionuclides after fuel dissolution, and spent fuel assemblies (SFAS) sent to storage. It is possible to achieve compaction and decontamination of MRW by a method based on slag remelting in an induction furnace with a cold crucible. In order to establish the distribution of actinides and fission products (FP), thermodynamic slag remelting process modeling of the VVER-1000 reactor plant SM fuel rods shells and SFAS, experimental data obtained verification were carried out. The most probable distribution of actinides and FP by melting products is shown. Most of the curium and americium are concentrated in the metal – 99 and 94 wt %, respectively. The maximum extraction of uranium into the slag phase in the form of UO₂ dioxide is up to 40 wt %. The distribution of plutonium in the temperature range of 1500–2000°C over the slag (in the form of PuO and PuO_1.61 oxides) and metal phases occurs in almost equal proportions. It has been established that up to 99.78 wt % cesium, the content of europium and americium is 0.05 and 0.17 wt %, respectively.

Alloys based on titanium and aluminum with additions of niobium and rare-earth metals have unique mechanical and heat-resistant properties, and also it likely that such alloys would have increased corrosion resistance. The method of thermodynamic modeling using the HSC program was used to study a system with aluminum consumption varying in the range from 0 to 100% of the mass of the initial charge. The features of phase formation in Al–[50TiO₂–5Nb₂O₅–1Y₂O₃ (Gd₂O₃)] systems have been studied. The calculation of the heat balance of the process at 1600°C and 44% of Al was – 0.196 MJ per 1 kg of charge, which indicates the possibility of its occurrence with only the aluminothermic reactions. The reduction of titanium and niobium can proceed by reactions through the formation of their oxides of lower valency – TiO, NbO₂, NbO. The aluminothermic reduction of gadolinium is thermodynamically possible only at temperatures below 1200°C. The reduction of yttrium through the interaction of Y₂O₃ with aluminum with the formation of AlY, Al₂Y₃ AlY₂ compounds for the range of 1000–1800°C is thermodynamically impossible. The results of thermodynamic modeling of interactions correlated well with the data of differential thermal and X-ray phase analysis using STA 449 F3 Jupiter (NETZSCH) synchronous thermal analysis and XRD-7000 diffractometer (Shimadzu) with automatic program control, respectively. It was found that the process enters the active phase after the appearance of liquid aluminum and, apparently, is accompanied by exothermic effects with the formation of double and triple intermetallic compounds of aluminum with rare (Nb, Ti) and rare earth (Gd, Y) metals. Transformation of titanium dioxide and niobium pentoxide in the process of transformations is likely carried out through successive and parallel stages of formation of simple and complex oxides with low oxidation states. At the initial stages of the interaction of aluminum with oxides, niobium and titanium aluminides are mainly formed. At subsequent stages, the formation of more complex compounds is observed. At temperatures above 1300°C, ternary intermetallic compounds Al₄₃Nb₄Gd₆, Ti₄Al₂₀Gd and Ti₄Al₃Y₆, Al₃Ti, Al_0.23Nb_0.07Ti_0.7 are formed. Gadolinium and yttrium tend to form complex intermetallic compounds in such systems.

With the introduction of complex alloys and metals – reoxidizers into liquid steel, their waste is observed, or more precisely, oxidation by the gas phase of the furnace. To select the optimal composition of complex deoxidizers, it is necessary to know the physicochemical laws of this process, which are little studied. To study the kinetics of oxidation of metal melts, the method of continuous sample weighing is used, which is usually used in the study of high-temperature corrosion of solid metals. The mechanism of interaction of liquid metals with oxygen is similar in nature to high-temperature gas corrosion of solid metals. In both cases, adsorption of gas molecules on the metal surface, nucleation, and then growth of an oxide film take place. In this work, the kinetics of oxidation of Ca-Ge melts with atmospheric oxygen was studied using thermogravimetry, IR spectroscopy, and X-ray phase analysis. It is shown that germanium additions up to 33.3 at % increase the resistance of melts to oxidation. An increase in temperature contributes to an increase in the rate of oxidation of melts of the Ca–Ge system. The process of oxidation of the investigated melts obeys the parabolic law. The true rate of oxidation is on the order of 10^–4 kg · m^–2 · s^–1. The apparent activation energy of oxidation, depending on the composition of the alloys, is 39.8–526.7 kJ/mol. The products of melt oxidation are СaGe₄О₉ and GeО₂. The mechanism of the influence of germanium on the kinetics of oxidation of Ca-Ge melts has been established. The CaGe₄O₉ oxide plays a dominant role in the formation of a protective oxide film.

The aim of this paper is a physicochemical analysis of phase formation processes in the trinary oxide system CsVO₃–Cs₂MoO₄–Cs₂O. The given system was studied using a complex of methods of physico-chemical analysis, in particular, differential thermal analysis (DTA), visual polythermal analysis (VPTA), X-ray phase analysis (XRPA) and synchronous thermal analysis STA 409 PC Luxx by Netsch company. As a result, the nature of the phase reactions of the CsVO₃–Cs₂MoO₄–Cs₂O interaction in the melts of the system was revealed, the results of seventeen internal sections were studied on the basis of which the melting diagram of the system was constructed, the fields of crystallizing phases were outlined. It is established that in the triple oxide system, according to the thermochemical and structural analysis of topology and phase formation, an unlimited consistency of high-temperature modifications is realized.

At present, technologies are being developed for the regeneration of mixed nitride uranium-plutonium spent nuclear fuel (MNUP SNF) for the BREST-OD-300 reactor plant, including the use of a pyrochemical method of mild chlorination in alkali metal chloride melts to separate fuel from fuel rod claddings made from high radiation resistance of ferritic-martensitic steel EP-823. The paper gives the results of EP-823 static corrosion tests in KCl–LiCl and KCl–LiCl–nPbCl₂ molten salts at the temperature of 500 and 650°С during 24 h. Corrosion behaviour of EP-823 steel in non-oxidized and thermal air oxidized state with oxide film thickness up to ~12.5 µm has been investigated using neutron-activation analysis. EP-823 steel samples, irradiated in IVV-2M reactor up to neutron fluence of ~2.9 · 10¹⁷ n/cm², have been examined. It has been shown that corrosion impact of 2KCl–3LiCl and 2KCl–3LiCl–nPbCl₂ molten salts on EP-823 element corrosion is selective. It has been established that EP-823 steel in 2KCl–3LiCl molten salts of eutectic composition is highly corrosion-resistant. An increase in the test temperature and the introduction of PbCl₂ into the KCl–LiCl salt melt in the amount of one mole percent leads to an increase in the corrosion rate and the removal of steel corrosion products by almost two orders of magnitude. It has been established that oxide films on EP-823 steel surface does not restrain corrosion rate in 2KCl–3LiCl–nPbCl₂ molten salts. The values of the constants given in Table 6, make it possible to calculate the values of the average corrosion rates of EP-823 steel and its components (Fe, Cr, Mn) in molten salts 2KCl–3LiCl and 2KCl–LiCl–nPbCl₂ at various temperatures.