Ab-Initio Calculation of Spectral Absorption Coefficients in Molten Fluoride Salts with Metal Impurities

Molten salts have been proposed as coolants for numerous advanced reactor designs. It is envisioned that these reactors, both fluoride-salt–cooled high-temperature reactors and molten-salt–fueled reactors will operate at high temperatures, where the radiative heat transfer properties of the salts may be required for accurate heat transfer analysis. Experimental challenges have prevented the measurement of absorption coefficients in most salts. In an attempt to fill this gap in data, the Vienna Ab-Initio Simulation Package is used in the present research to calculate the absorption coefficient resulting from photoelectric interactions in numerous molten salts. Ab-initio molecular dynamics is used to generate the amorphous structures of a variety of salts. The pure halide salts LiF, FLiNaK, and FLiBe, are shown to be optically clear through a wide portion of the electromagnetic spectrum. Conversely, the transition metal fluoride salt KF-ZrF₄ is shown to be substantially opaque. As chromium is a known impurity of concern from the corrosion of steels in reactor environments, the effect on absorption of low levels of chromium in an otherwise transparent salt is investigated and found to significantly increase absorption at relevant wavelengths.