To calibrate the X-ray absorption spectroscopy measurements, FLiNaK salt specimens with known concentrations of NiF 2 were prepared by mixing NiF 2 powder (anhydrous, 99%, Strem Chemicals Inc) with the above FLiNaK salt in a certain ratio in glassy carbon crucibles (HTW Germany). The powders underwent several melting-freezing-grinding cycles to fabricate well-mixed samples of FLiNaK -NiF 2 with 400 ppm and 1200 ppm NiF 2 .