Molten fluoride salts can be used as the fuel and coolant for molten salt reactors (MSR) and electrolytes for spent nuclear fuel (SNF) reprocessing. 46.5%LiF-11.5%NaF-42%KF (FLiNaK) melt is the prospective media for these purposes due to their desirable thermophysical and nuclear properties. Finding construction materials with sufficient corrosion and mechanical resistance is the most challenging task for practical implementation of MSR concept. In the present study the corrosion and mechanical properties of different types of construction materials were investigated. The materials included various low carbon Ni-Cr-Fe-Mo, Ni-Cr-Mo, and Ni-Mo alloys, and metals with relatively positive electrode potentials. The corrosion experiments were performed in FLiNaK melt at different temperatures (from 550 to 750 °C) in the specially designed stainless steel cells under high-purity argon atmosphere. Corrosion properties of studied materials were investigated under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. In a special series of experiments fluorides of typical fissile nuclides and fission products were added to the salt electrolyte to estimate the influence of the red-ox potential on the corrosion resistance of the materials. Corrosion rates were determined from the weight loss measurements and chemical analysis of quenched melts. Surface and microstructure of corroded samples was examined by various microscopic techniques. Mechanical properties of investigated materials were also studied at the ambient temperature and at 600 °C. Advantages and limitations of different types of construction materials were evaluated on the basis of data obtained. The effect of temperature on corrosion and mechanical properties of the studied materials were determined. Possible mechanisms of corrosion of various materials in fluoride melts were proposed. The alloys based on Ni-Cr-Mo and Ni-Mo systems, and molybdenum and its alloys were selected for further long-time tests under dynamic conditions to determine the resource of materials in contact with molten fluorides for MSR and SNF recycling technologies.

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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.

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.

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