A study on molecular association in FLiNaK by high temperature mass spectrometry and Raman spectroscopy

Thermodynamics of coolant salt of MSRs, i.e., FLiNaK (eutectic mixture of LiF, NaF and KF) is important to understand the nature of its interaction with structural materials and to predict the long-term stability of reactor cooling channels. Here, we report thermodynamic activities of LiF, NaF and KF in molten FLiNaK measured employing a high temperature Knudsen Effusion Mass Spectrometer. The activity of LiF, NaF and KF varies from 0.174 to 0.135, 0.830 to 0.490 and 0.139 to 0.090, respectively (959 <= T/K <= 1007). The trend in temperature dependent activity suggests the stronger interaction between LiF-KF leaving behind NaF above 873 K. This was confirmed by measuring Delta Hmix of LiF, NaF and KF in FLiNaK using a high temperature calorimeter. The high temperature Raman spectroscopy of FLiNaK and eutectic mixtures of LiF-NaF, LiF-KF and NaF-KF corroborates the existence of stronger LiF-KF association compared to LiF-NaF and NaF-KF above melting point of FLiNaK.