Corrosion of Metallic Materials in the Molten FLiNaK

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.