Molten chloride salt electrolytes are promising for use as a working medium for the implementation of high-temperature technologies. Alkali metal chlorides are an aggressive environment in relation to structural materials. One of the possible methods of reducing the corrosion damage of a structural material is the method of oxygen passivation of the surface of a metal or alloy by introducing a certain amount of oxygen-containing additives into the melt. The article considers the effect of oxygen-containing impurities (lithium oxide and lithium hydroxide) on the corrosive behavior of a metal material — an alloy of the composition iron–cobalt–nickel. To assess the corrosion resistance of materials, gravimetric analysis, micro-X-ray spectral analysis (XRSA) of the surface and cross-section sections, and X-ray phase analysis (XRF) of the sample surface were used. The dependences of the corrosion rate of the material on the concentration of oxygen-containing additives Li2O and LiOH are presented. Based on the data set of gravimetric, MRSA and XRF data, it was found that 29NC alloy samples in the LiCl–KCl–nLi2O salt melt are not susceptible to corrosion, but in the LiCl–KCl–nLiOH melt, the speed of the 29NC alloy increases significantly due to the interaction of the LiOH additive with the most electronegative component of the alloy — iron.

The corrosion behavior ofEP-823 steel was studied during high-temperature treatment (HTT) with nitrogen. Itwas found that innitrogen attemperatures of 650–800°C, the steel issubject toonly slight surface corrosion. Itis shown that there isa slight change inthe surface composition and structure ofsteel, which does not have asignificant effect onthe reprocessing ofmodel SNF. Itis shown that onthe surface ofthe material, processes ofinteraction ofsome electronegative components offerritic-martensitic steel with components ofthe gas phase–nitrogen and impurity oxygen–occur with the formation ofinclusions ofnitride and oxide compounds ofchromium and manganese ofdifferent stoichiometric compositions. The process islimited bythe diffusion ofthese components from the volume ofthe alloy tothe surface. The corrosion rates ofEP-823 steel attemperatures of 650 and 800 ° Cwere 0.104 and 0.241 mm / year for 12 hours ofexposure, and 0.013 and 0.02 mm/year for 84 hours ofexposure, respectively. The nature ofthe destruction ofthe surface ofthe samples iscontinuous and uneven, localization ofcorrosion atthe boundaries ofsteel grains isclearly observed, which isassociated with the formation ofsecondary phases along the grain boundaries. Atthe temperature ofthe treatment, significant sensitization ofsteel occurs, chain-like precipitation ofsecondary phases isobserved along the grain boundaries, which leads tothe development ofintercrystalline corrosion. Conclusions are made about the change inthe structure ofthe material during high-temperature exposure and the nature ofthe corrosion damage ofthe material isdetermined; based onthe results ofX-ray fluorescence analysis, conclusions are made about the composition ofthe corrosion products ofEP-823 steel.

Molten alkali metal chlorides used in pyrotechnologies are aggressive corrosive agents. The high operating temperature of the process, the heterogeneity of the environment, and the significant corrosion activity of the molten salt necessitate both the search for stable structural materials and the development of methods for protecting the structural elements of high-temperature technological devices. Corrosion loss reduction techniques traditionally used in low temperature environments are not applicable at high temperatures. The article examines the influence of oxygen-containing impurities (lithium oxide and hydroxide) on the corrosion behavior of metallic nickel (grade NP1) – the main component of candidate structural alloys, a thermodynamically and structurally stable material in the melt for the process of electrolytic refining of spent nuclear fuel. A method for preparing the LiCl–KCl salt electrolyte and obtaining lithium oxide by thermal decomposition of anhydrous lithium hydroxide under vacuum is described, and the concentrations of impurities in the electrolyte and the synthesized lithium oxide are determined. An installation for conducting corrosion tests in an inert atmosphere of a glove box is presented. To assess the corrosion resistance of the material, the following were used: gravimetric analysis, X-ray diffraction analysis of the surface and cross-sectional sections, and X-ray diffraction analysis of the surface of the samples. The dependences of the corrosion rate of the material on the concentration of oxygen-containing additives Li2O and LiOH were obtained. Based on a combination of gravimetric, X-ray microspectral and X-ray phase analysis data, it was established that metallic nickel samples demonstrate high corrosion resistance in the studied melts with the introduction of Li2O and LiOH additives.

The equilibrium composition of the system (LiCl–KCl)eut + 1 mol E_Pb^2 + / . -0emPb^* and E_U^4 + / . -0emU^3 + ^* is only about 0.2 V. Therefore, PbCl2 is a weak oxidizing agent in this case. The difference between the E_Pb^2 + / . -0emPb^* and E_U^4 + / . -0emU^3 + ^* potentials increases with the temperature. Therefore, the equilibrium fraction of UCl4 increases with the temperature and excess PbCl2. For example, at 773 K and a threefold PbCl2 excess over stoichiometry, the average uranium valence is n = 3.27. At 973 K and the same PbCl2 excess, the average valence of uranium increases to n = 3.36. In addition, the reduction of UCl4 to UCl3 with metallic uranium in the molten LiCl–KCl eutectic is simulated taking into account the activity coefficients of the components. In the temperature range from 773 to 973 K, the reduction has no thermodynamic difficulties and proceeds completely. The HSC-9.9 software was used for the thermodynamic calculations.

The interaction between Gd2Zr2O7 and molten LiCl-Li2O (2 wt%) was studied for 24-52 h at 650-710 degrees C in an argon atmosphere. Gd2Zr2O7 is analyzed as a promising structural material for sensors used during pyrochemical reprocessing of spent nuclear fuel and for long-term storage or final disposal of high-level nuclear wastes. The chemical stability of Gd2Zr2O7 relative to the components of the LiCl-Li2O melt was thermodynamically evaluated. The surface morphology and structure of the samples before and after the experiment were analyzed using an X-ray diffractometer and scanning electron microscopy. The formation of a new Li+-doped phase based on Gd2Zr2O7 and the Gd2O3 evolution onto the material surface was revealed by the X-ray diffraction analysis (XRD). Changes in the microstructure of the samples confirm the presence of large particles in the surface layer corresponding to the Gd2O3 phase, which is in good agreement with the XRD data. A profilometer was used to measure the roughness of the ceramics. Presumably, the thickness of the lithium-doped Gd2Zr2O7 film, which is inhomogeneously distributed over the surface of the samples, was 3 mu m. Therefore, it was found that dense Gd2Zr2O7 (F) and Gd2Zr2O7 (P) ceramics can be used in LiCl-Li2O (2 wt %) as a structural material resistant to the high-temperature chemical attack.

The pyrochlore Gd1.55Li0.45Zr2O6.55 was prepared by the solution and solid-state methods. The introduction of lithium in the Gd-sublattice led to decrease in the lattice parameter a = 10.4830(8) Å in comparison with Gd2Zr2O7 (a =10.5346(2) Å). Monitoring of the lithium content in the sample during heat treatments showed a loss of lithium at temperatures above 1100 °C, so, to maintain the stoichiometry of lithium the low temperature sintering methods are required. The sample Gd1.55Li0.45Zr2O6.55 exhibited a predominant oxygen-ion transport over a wide range of temperatures. Although doping did not lead to an increase in the oxygen-ion conductivity compared to Gd2Zr2O7, it caused the suppression of the hole conductivity.

A mixed nitride U-Pu SNF pyrochemical reprocessing technology was suggested. It includes the following basic operations: dissolution of key components using the CdCl2 or PbCl2 oxidizer in chloride melt and their subsequent deposition as oxides. The obtained product, which is a mixture of actinides and rare-earth metals, may be additionally purified using hydrometallurgical processes (combined technology) or using pyrochemical methods. The mixture of actinides oxides is reduced to metal (“metallization”), additionally purified from fission products during electrorefining and then Am and Cm are separated using the potentiostatic electrolysis in order to obtain products that may be used for pure fuel fabrication.

Electrical conductivity of fluoride-chloride melts is discussed. The available experimental data for binary and ternary systems are analyzed and presented as a molar volume function.

Conductivity of molten mixtures KF-NaF-AlF3 is measured in the whole concentration range of the [NaF]/([KF] + [NaF]) ratio at the cryolite ratio CR = 1.3 and 1.5 in the temperature range from 800°C to liquidus temperature. Replacement of K+ cations by Na+ ones results in a considerable conductivity increase. Alumina solubility rises with temperature and cryolite ratio ([KF] + [NaF])/[AlF3] in the KF-NaF-AlF3 system, and decreases with sodium fluoride content. Regression equations obtained allow calculating alumina solubility and electrical conductivity in the KF-NaF-AlF3 system in the CR range from 1.3 to 3.0 depending on the concentration of the components and temperature.