204 publications from this institution
Corrosion resistance of ceramic and carbon materials was studied in molten salt systems based on LiF–NaF–KF mixture in the temperature range 550–750°C. The rates of corrosion of silicon nitride, boron nitride, carbon-carbon composites, and high density graphite in the fluoride electrolyte were determined. The mechanisms of materials degradation after the contact with molten FLiNaK are proposed, and the effect of the salt media and the temperature on the mechanical properties of ceramic and carbon materials is considered.
Molten chlorides can be used for niobium electrowinning and electrorefining. Understanding niobium electrochemical properties and speciation in chloride melts is essential for designing or optimizing the industrial electrochemical process. Apart from the technological value, the information on the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of the heterophase electrochemical reactions. In the present study a variety of electrochemical techniques (polarization curves measurements (PCM), chronoamperometry (CA), chronopotentiometry (CP), linear (LVA), cyclic (CVA) and square-wave voltammetry (SWVA), impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. In addition the most valuable and reliable information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra, EAS) methods. The results of spectroelectrochemical experiments showed that the major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm 2 were niobium(III) species, NbCl 6 3- . In the potentiostatic regime of anodic dissolution of niobium metal niobium(III) complex ions were formed at potentials below -1.15 V vs. chlorine reference electrode. The polarization curves characterizing the anodic dissolution of niobium consist of three or more waves. The first wave is associated with reaching limiting current density of niobium ionization according to the three-electron scheme. Further increase of applied current leads to the oxidation of Nb(III) to Nb(IV) ions. These conclusions were confirmed by determined values of potentials of niobium or indifferent electrodes in the equilibrium with the melts containing Nb(III) and Nb(IV) ions. When studying anodic dissolution of niobium we found that at high current densities there is sudden increase of the ohmic part of voltage drop sometime accompanied by increase of the anode potential. As a result stationary and non-stationary polarization dependencies exhibit unstable behavior. This is explained by the salt passivation, SP. The product of the SP of niobium is unstable NbCl 4 , which decomposes to NbCl 3.13 and volatile NbCl 5 . The results of electrochemical experiments (PCM, CVA, LVA, CA, CP and SWVA) showed that soluble niobium(V) species were formed during Nb(IV) electrooxidation and represented the highest oxidation state of niobium in NaCl-KCl melts. We noticed that peak potential value (both cathodic and anodic) on CV changes with scan rate, whereas the i p /ν 1/2 value remains constant. The formation of Nb(V) species was confirmed by the disappearance of absorption bands in the visible part of the spectrum due to Nb(V) species formation during elecrooxidation of niobium-containing melts at potentials up to -0.5 V. The quasireversibility of Nb 4+ ↔Nb 5+ reaction was explained by the instability of niobium(V) species. Niobium(IV) ions were formed when potentiostatic electrolysis of NbCl 3 -NaCl-KCl melts (prepared by anodic dissolution of niobium metal) was performed at -(1.0÷0.8) V vs. chlorine reference electrode on a glassy-carbon anode, Fig. 1. The electroreduction of niobium on a tungsten electrode in NbCl 3 -NaCl-KCl melts at potentials below -1.45 V led to the formation of metallic niobium (Fig. 2), no evidence of any intermediate product formation was obtained. Linear voltammetry measurements showed that intermediate products can be formed during elecroreduction of NaCl-KCl-NbCl n (n=3.8-4.1) melts. Most likely the electroreduction of Nb(IV) results in the formation of niobium(III) ions. The shape of cyclic voltammograms recorded in niobium containing melts depended on the scan rate. Similar sets of cyclic voltammograms were obtained at different temperatures and niobium concentrations. SWVA measurements performed on different electrodes also confirmed that there is more than one step of electroreduction. The obtained results were explained by overlapping of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0).
A combined technological scheme for the reprocessing of a mixed nitride uranium–plutonium spent fuel, which consists of pyrochemical operations and h
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A new method for manufacturing aluminum–gadolinium master alloy was designed. It is based on an exchange reaction between metallic aluminum and gadolinium fluoride. The structure on the synthesized alloys with different gadolinium content was investigated. Al and Al3Gd were the main phases present in the alloys. Mechanical and thermophysical properties of the master-alloys obtained were measured.
The electrochemical and spectroscopic properties of tellurium were studied between 823 and 1123 K in alkali chloride melts based on 3LiCl-2KCl, NaCl-2CaCl and NaCl-KCl mixtures using potentiometry, cyclic voltammetry, anodic and cathodic polarization and absorption spectroscopy. Tellurium electrode potential values were measured at Te concentrations between 0.0006 and 0.008 mole fraction. Only Te(II) ions are present in chloride melts in the equilibrium with tellurium. Reactions of Te and Ag2Te with Cl2 and HCl were studied spectroscopically in NaCl-2CsCl melt at 823 K.
Corrosion resistance of molybdenum, tantalum, and nickel-based alloys was studied in molten salt systems based on LiF–NaF–KF mixture in the temperature range 550–750°C. The rates of corrosion of these materials were determined, the nature of the degradation of the surface of materials established, and the effect of the salt medium and temperature on the corrosive properties of metals and nickel-based alloys considered.
Beryllium-containing halide melts can be used for the electrolytic manufacturing and refining of beryllium and for reprocessing spent nuclear fuel. Data on
The corrosion resistance of ferritic (08Kh17T) and ferritic-martensitic (12Kh13) type steels was investigated at 750 0C in NaCl-KCl-VCl2 melts. The mechanism of corrosion of 12Kh13 ferritic-martensitic steel in NaCl-KCl-VCl2 melts includes two parallel processes: formation of new excessive phases inducing Fe3C (σ-phase)|melt|steel microgalvanic pairs, and reaction of most electronegative steel components (Mn and Cr) with V3+ ions formed due to disproportion of V2+. Contacting samples of 08Kh17T with NaCl-KCl-VCl2 melts resulted in no structural changes in the steel samples. In this case the mechanism of corrosion involves only one stage: disproportion reaction of V(II) with the formation V-Fe alloy and V3+ ions and further oxidation of chromium in the steel by V(III). It was shown, that the presence of a suitable reducing agent in the contact with the melt (for example, metallic vanadium) can prevent the steel destruction.
The fused halides can be used for nuclear fuel reprocessing and as working media for molten salt nuclear reactors. However practical implementation of such technologies is limited by the problem of finding suitable corrosion resistant materials capable of long-term working in contact with molten salts. From the economical point of view the application of stainless steels as construction materials for the molten salt media is one of the most prospective ways. In the present study the corrosion behaviour of metallic iron, nickel, chromium, molybdenum, AISI 316L and 12Kh18N10T (analogue of AISI 321) austenitic steels was studied in NaCl–KCl–UCl 3 (1 wt.% U) melts at 750 ºC. Gravimetric measurements served as a basis for estimating corrosion resistance of studied materials. Quenched melts were analyzed using ICP-AES method. The surface of the alloy samples after corrosion tests was characterised using XRD, SEM and Х-ray microanalysis. The foils of corroded materials were examined by TEM. The corrosion rates of studied metals in NaCl–KCl–UCl 3 melt decrease in the following order: Cr>Fe>Ni>Mo, which correlates well with the formal standard potentials of metals in chloride melts indicating electrochemical nature of the corrosion processes. Molybdenum is the only corrosion resistant metal in the uranium-containing chloride melts and it can be used as a construction material for molten salt reactors. The major products of stainless steel corrosion in chloride melts are iron, chromium and manganese species. The surface of the corroded samples of austenitic stainless steels was depleted in manganese and chromium and enriched in nickel and molybdenum. Intergranular corrosion was observed for all types of austenitic steels. In terms of intensity and depth of the corrosion layer in NaCl–KCl–UCl 3 the studied steels can be ranged in the order AISI 321>AISI 316L that correlates with the amount of excessive phases formed (figure). Figure 1
Molten salt nuclear fast nuclear reactor (MSNFR) is one of the prospective design in frame of Generation IV concept. This technology is also required for reprocessing of spent nuclear fuel (SNF) in molten salts to extract valuable components. Recycling SNF consists of extraction uranium and plutonium with the disposal of minor actinides and fission products. These stages need the materials that retain their corrosion and mechanical properties for long time under the influence of high temperatures, radiation fields and contact with molten salts. Various steels and alloys were widely studied to assess their use under such extreme conditions. Alternative materials include ceramics, composite and carbon materials, which have high corrosion resistance in various media and can be used up to 1000 °C. In the present work the corrosion and mechanical properties of nitride ceramics (Si 3 N 4 and BN) and carbon-based materials (carbon-carbon composite material (C/C) and high-density carbon) were studied in a molten mixture of lithium, sodium, and potassium fluorides (FLiNaK) in the temperature range of 550–750 °C under inert atmosphere. Corrosion tests were performed under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. It was found that carbon-containing materials (C/C and high-density carbon) showed high corrosion resistance in the melt. C/C samples had lower corrosion rates (less than 0,1 mm/year in the entire temperature range), while the high-density carbon showed better mechanical properties. The main disadvantage of these materials is their impregnation by molten salt due to relative high porosity. However, C/C composite and high-density carbon are promising structural materials for salt media based on FLiNaK in the selected temperature range. Tested nitride ceramics (BN, Si 4 N 3 ) demonstrated relatively low corrosion resistance in fluoride systems compare to carbon-containing and metallic materials. The corrosion rates of these materials in FLiNaK exceeded the value of 1 mm/year. The plastic properties of nitride ceramics were also poor. Possible application of such materials for MSNFR and SNF reprocessing is under further investigation.
