Calibration samples were prepared by mixing FLiNaK powder with known concentrations of NiF 2 and were encapsulated in quartz capillaries, as described in Section 2.1. XANES spectra of the samples were measured at room temperature. The BN cells (Figs. and) that we used to contain molten salts during corrosion experiments significantly attenuated the beam. Therefore, for calibration measurements, it was necessary to place an empty BN cell cover in front of the specimens to match the corrosion experiments. For the fluorescence-mode XANES, the BN cell was placed close to a 45 • angle with respect to the incoming beam, to maximize the fluorescence signal. This provides an optimal pathway for X-rays to penetrate through the BN cell, cause the Ni in the salt to fluoresce, and allow the fluorescence signal to penetrate back through the cell to reach the detector. The X-ray beam was centered on the sample and scanned in the vertical direction to find a region of uniform Ni signal intensity to perform measurements. A PIPS detector (described in Section 2.2) tuned to handle higher intensities was used to measure the fluorescence of the sample. Each specimen was measured ten times and the individual spectra were averaged. The measurements of these samples with varying NiF 2 concentrations are presented in Fig. .