Ilya B. Polovov

Behaviour of Rare Earth Elements in Molten Salts in Relation to Pyrochemical Reprocessing of Spent Nuclear Fuels

Electrochemical Society, Phys. Anal. Electrochem. Div.;Electrochemical Society, Electrodeposition Division;Electrochemical Society, High Temperature Materials Division;Electrochemical Society, Battery Division;Electrochemical Society, Energy Technology Division

Behavior of Molybdenum Chloro-Species in Alkali Chloride-Based Melts: Implications for Spent Nuclear Fuel Treatments

Molten salts can be used for the electrowinning and electrorefining of molybdenum, as well as for molybdenum electroplating. Coherent molybdenum deposits can be produced from alkali chloride or chloridemolybdate baths. Molybdenum is also present in significant amounts in spent nuclear fuels and high temperature molten salts, including alkali chlorides, can be employed as reaction media for pyrochemical reprocessing. Designing or optimizing a technological process involving molybdenum requires detailed knowledge of the element’s properties, speciation and behavior in molten salts.

Effect of Melt Composition on the Reaction of Uranium Dioxide with Hydrogen Chloride in Molten Alkali Chlorides

The reaction of uranium dioxide with excess hydrogen chloride in alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl) has been studied between 450 and 750◦C, and the reaction products were characterized by electronic absorption and X-ray absorption spectroscopy. Uranium( V), [UO2Cl4]3−, and uranium(IV), [UCl6]2−, species were formed. They depended upon the temperature and the radius of the alkali cations present. Uranium(V) ions predominated in melts with small cations (LiCl and 3LiCl-2KCl).

Spectroelectrochemical Study of Neptunium in Molten LiCl-KCl Eutectic

Neptunium behaviour in an LiCl-KCl eutectic melt at 723 K was studied using spectroelectrochemistry. Cathodic reduction of neptunium(IV)-containing melts led to the formation of Np(III) ions and then neptunium metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into individual Gaussian bands. The nature of neptunium complex ions in the melt is discussed.

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X-ray Diffraction, and Density Functional Theory Calculations

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K5Sc3F14 was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF4, Li3ScF6, KSc2F7, and Na3ScF6 compounds were studied in detail from solid-state 19F and 45Sc NMR experiments. The 45Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The 19F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the 19F and 45Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state 45Sc NMR spectroscopy.

Speciation of Rhenium in Chloride Melts: Spectroscopic and Electrochemical Study

Speciation of rhenium in high-temperature alkali chloride-based melts was studied using electronic absorption and IR spectroscopy of molten salts and diffuse reflectance spectroscopy of quenched melts. Rhenium was added to the melts by anodic dissolution of the metal (at anodic current densities of 0.005 - 0.05 A/cm2), by reacting Re and ReO2 with Cl2 and HCl, and by dissolving K2[ReCl6]. The melts included 3LiCl-2KCl and NaCl-2CsCl eutectics, an NaCl-KCl equimolar mixture, and pure NaCl, KCl and CsCl between 450 and 850 ◦C. Rhenium was present in the melts as Re(IV) hexachloro-ions, [ReCl6]2−; no evidence of species containing rhenium in oxidation states below four was obtained. The kinetics of [ReCl6]2− disproportionation in molten alkali chlorides were investigated, and the IR spectra of [ReO4]− ions in molten CsCl-CsI and CsI were measured for the first time.

Corrosion resistance of nickel-based alloys in salt and metal melts containing REE

The corrosion resistance of Hastelloy G-35 and VDM® Alloy 625 nickel alloys was studied in a wide temperature range (750–1100 °C) in fused LiCl, CaCl2, NaCl–KCl, LiF mixtures containing REE. The rates and the mechanisms of the corrosion of the materials studied were determined. The processes taking place during the interaction between alloys and melts were investigated.

High-temperature corrosion of metals in the salt and metallic melts containing rare earths

A complex of independent methods was employed to study the corrosion resistance of molybdenum, zirconium, tantalum and tungsten in chloride, chloride-fluoride and fluoride-oxide melts based on LiCl, CaCl2, NaCl– KCl, LiF, and containing rare earths. Tests were conducted for 30 h at 750–1050 °C. The metals showed excellent corrosion resistance in fused chlorides (the corrosion rates were below 0.0005 g/(m2 h). Despite the presence of chemically active fluoride ions in the chloride-fluoride melts, the metals studied also showed very low corrosion rates, except molybdenum, for which the rate of corrosion was 0,8 g/(m2 h). The corrosion resistance of tantalum was considerably reduced in the fluoride-oxide melts; the corrosion rate was over 1 g/(m2 h) corresponding to the 8-th grade of stability and placing tantalum to the group of “low stability” materials.

Special Features of Formation of Excess Phases During Aging of Corrosion-Resistant High-Alloy Austenitic Alloys Based on Fe And Ni

The applicability of the active criteria for estimating the metastability of austenite for predicting the operating capacity of high-alloy corrosion-resistant alloys based on iron and nickel in a single-phase preliminarily quenched condition in a temperature range of up to 500 – 650°C is analyzed. The temperature and time ranges of formation of excess phases are determined and fragments of C-curves are plotted. The effect of preliminary cold plastic deformation on the kinetics of aging and on the morphology of segregation of particles of second phases is studied.

Corrosion behavior of austenitic steels and their components in niobium-containing chloride melts

The mechanism of corrosion of austenitic steels 12Kh18N10T, 10Kh17N13M2T, and 03Kh17N14M3 and metals Cr, Fe, Ni, and Mo in a NaCl-KCl-NbCln (n = 3.5, Nb content is 5 ± 0.1 wt %) melt at 750°C is studied. The metal and steel corrosion rates under these conditions are determined. The character of material fracture and the mechanisms of material corrosion are found.