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Molten salts (MS) can be used for nuclear fuel reprocessing (NFR) and as a working media in molten salt nuclear reactors (MSNR). However, application of MS in NFR and MSNR technologies is limited by the problem of finding suitable corrosion resistance materials. In the present work the results of examining structure and properties of a new prospective alloy are presented. Existing corrosion-resistant alloys are normally manufactured for an intended application in aggressive aqueous-based media, whereas high-temperature alloys are mostly designed for aerospace industry where high mechanical strength under extreme conditions is required. On the basis of the comprehensive studies of corrosion resistance of different types of stainless steels and nickel-based superalloys we concluded that a new alloy with a “nickel–chromium–molybdenum” matrix is required for molten salts applications. In the present work the results of the new alloy fabrication process and detailed examination of the alloy’s properties are presented. The requirements for the alloy composition were determined on the basis of thermodynamic calculations (Thermocalc Software AB was used to simulate the phase content) and previously obtained experimental data on metals and alloys corrosion in molten chlorides and sensibility of these materials to the intergranular corrosion. Carbon content in the material should be maintained at a minimal level. The influence of other elements presence on stability of the γ-phase austenite matrix was analyzed. The alloy of the required composition was produced by induction vacuum melting in several stages. First a low carbon Ni–Mo master-alloy was manufactured. Then this master alloy was further melted and chromium and microelements added. Sheet, tube, rod and forged samples were fabricated. Mechanical and thermophysical properties, susceptibility to the intergranular corrosion of this new nickel alloy in the “as received” state were investigated. The results obtained correlated well with the properties of typical nickel-based superalloys with austenite structure. In a special series of experiments thermodynamic stability of the fabricated alloy was studied. No formation of carbide phases was observed. Phase structure of the excessive intermetallic phases was characterized. The constructed “time – temperature – precipitation” diagram allowed to determine maximum temperature and time of alloy’s operation in contact with molten salts. The phase stability of the alloy designed exceeded that of a variety of other nickel-based superalloys. The corrosion resistance of the material was studied in molten KCl-AlCl 3 and 3LiCl-2KCl electrolytes at 450–750 °C. Chloroaluminates are prospective media for the second loop of MSNFR and acidic KCl-AlCl 3 electrolytes are very corrosive compare to other chloride mixtures. From the other hand, 3LiCl-2KCl system is a prospective medium for NFR technologies. To model the worst possible conditions for the alloy, the electrolytes containing excess aluminum chloride were selected for the present investigation. Time of exposure varied from 6 to 1000 h, and the samples were tested in the “as received” state as well as after typical technological manipulations (bending, welding, heat treatment, etc .). Metallographic analysis showed that the alloy was subjected to slow frontal corrosion up to 550 ºC (Figure). Experimentally determined corrosion rates were below 50μm/year depending on the melt composition. The surface of the corroded samples was slightly depleted in chromium. Formation of secondary phases was not observed. Excessive sigma phases along the grain boundaries were formed after 100 h exposure at 650 ºC in the surface layer and after 512 h in the bulk of the material. Formation of the secondary phases in the surface layer was caused by selective chromium leaching and degradation of the nickel-based solid solution. Prolonged contact of the material with the high-temperature melt led to unavoidable decomposition of thermodynamically metastable nickel-based austenite. Such behavior correlates very well with the constructed “time – temperature – precipitation” diagram. Using the alloy at such temperatures (650 o C and above) becomes undesirable due to possible development of integranular corrosion. Figure. Microstructure of the new alloy after 100 h exposure in KCl–AlCl 3 melt at 450 (a, b); 550 (c, d) and 650 °С (e, f) Figure 1

The applicability of the active criteria for estimating the metastability of austenite for predicting the operating capacity of high-alloy corrosion-resist

The corrosion of Hastelloy S, Hastelloy X, Haynes 230, Hastelloy N, Hastelloy G35 and Hastelloy C2000 alloys was studied in KCl–AlCl 3 melts at 550 ° С . The rates and the mechanisms of corrosion of the studied materials were determined. The processes taking place during the interaction between alloys and chloroaluminate melts were investigated.

Interaction between metallic uranium and rare earth oxides, i.e. neodymium oxide, were investigated using thermodynamic modeling and experimental exposures at 1300–1500 °C. It was shown that metallic uranium partly reduced neodymium oxide resulting in formation of uranium dioxide film that prevented a uniform ingot formation. Increasing temperature led to higher degree of reduction and further complicated the rare earth oxide separation from uranium metal. Direct smelting cannot be used for fissile materials extraction from a mixture of metallic and oxide components obtained during nuclear fuel reprocessing.

The corrosion resistance of nickel-based Hastelloy G-35 alloy was investigated in molten chloride salts and aggressive aqueous solutions. The corrosion rates of the alloy in NaCl–KCl, 3LiCl–2KCl, AlCl3–KCl melts at 750 °C and in boiling sulfuric acid solution in the presence of iron(III) sulphate were determined. Mechanism of the corrosion depended on the oxidizing ability of the chloride melts. Intergranular corrosion dominated in the weakly oxidizing melts and general corrosion prevailed in the highly corrosive molten salts. The presence of oxygen in the system led to a significant increase in the rate of corrosion and a change in the mechanism of corrosion.

Bismuth–lithium alloys are considered primary candidates for the reductive extraction step in the on-line reprocessing of molten salt reactor fuel. The corrosion behavior of molybdenum-based alloys and Hastelloy® B-3 alloy (taken for comparison) was examined here in a liquid Bi–Li (5 mol.%) alloy at 650 °C. MoW10, MoW30, and TZM corrosion-resistant alloys were studied as prospective construction materials for holding liquid bismuth–lithium alloy. Rates of corrosion were determined by the gravimetric method as well as by chemical analysis of corrosion products formed in liquid-phase Bi–Li alloy. The microstructure and chemical composition of samples of the materials and Bi–Li alloys containing the corrosion products after the tests were determined using inductively coupled plasma–atomic emission spectroscopy, X-ray fluorescence analysis, scanning electron microscopy, and energy dispersive spectroscopy. TZM molybdenum-based alloy corrodes in the bismuth-lithium alloy due to the formation of a zirconium–bismuth intermetallic compound, which passes into the liquid phase. The corrosion rates of MoW10, MoW30, and TZM alloys at 650 °C were 16, 16, and 23 µm/year, respectively. Hastelloy® B-3 alloy, despite its high molybdenum content, was subjected to severe corrosion in liquid Bi–Li alloys due to dissolution of nickel in liquid bismuth. The corrosion rate of this alloy was 14 mm/year.