Molten Salt Composition Measurement

Molten Salt Composition Diagnostics. The optical transparency of molten salt lead to a preference for optical sensors to measure and monitor the composition of molten salts at temperature. For this reason Thoughtventions' efforts will center efforts on spectroscopic composition detection. Spectroscopy of molten salts has been widely studied, aided by the natural transparency of molten salts [e.g. 1, 2, 3]. A spectrometer will be purchased by Thoughtventions for this purpose. Thoughtventions already possess a wide variety of light sources as well as transparent materials (primarily sapphire, but a wide variety of quartz) to perform the optical examination at temperature through Thoughtventions' various transparent furnaces and furnace windows.

The corrosiveness of the salts imply the need for the most inert optical materials; sapphire and diamond. Sapphire will primarily be used at Thoughtventions as a result of 1) Thoughtventions' experience with sapphire, 2) Thoughtventions' capabilities for bonding sapphire to other materials, and 3) The lower cost cost of polished sapphire compared with diamond.

Optical Composition Measurement. A spectrometer combined with an appropriate light source will be used to measure salt optical absorption from which molten salt composition can be deduced. A variety of light sources that can be used (listed from cheap to expensive) are halogen filament lights, arc lamp/flashlamps, diode lasers, gas lasers, dye lasers, quantum cascade lasers and broadband continuous or pulsed lasers. ORNL has already done extensive work on the optical properties of molten salts. [e.g. 4] A recent (2021) massive report on the Radiative Heat Transport and Optical Characterization of High Temperature Molten Salts [5] is a comprehensive source on this subject for this project.

Vessels for Molten Salt Optical Diagnosis at Temperature. Composition measurement work will use a variety of sapphire cells that have been made at Thoughtventions. A simple molten salt cell rated for 1000 C has already been prepared for this effort. It consists of a a polished sapphire tube gold bonded to a thick sapphire window. This cell was mounted in a tube furnace, filled with a nitrate molten salt granules, and the salt was melted in the cell without any leaks. Although Thoughtventions has the technology to make a sapphire to metal seal, it is easier to simply join an alumina tube to sapphire (with close thermal expansion coefficients) to form a container, or to alumina tubes.

Optical cells for molten salt research have been built and used elsewhere. [e.g.4,6] Sapphire cells have already been used to study molten salts. [e.g. 4]

Long Pass Cell for Low-Level Impurity Measurements. The figure shows a cross-section drawing of a long pass sapphire cell that will be used for low concentration impurity measurements. The cell is made from an alumina tube with a sapphire window bonded (with Thoughtventions' gold bonding technique) on either end. Salt (powder or liquid) can be introduced into the cell through a small hole in both the furnace and the top of the tube containing the molten salt. The sapphire windows used are cheap sapphire wafers that can be unbonded for replacement if fogged. Very low level concentrations use mirrors to create a multipass absorption cell coupled with sensitive detectors.

Impurity Detection. The standard issue for detection of multiple components if a mixture is to separate/subtract out the various absorption band set. There are standard records of almost every possible chemical, together with computer routines to optimize separation of the band sets when there are multiple components. Optical absorbances of elements and compounds are readily available, although perhaps not at elevated temperature, which can slightly shift and broaden absorbances.

TvU’s Optical Access Furnaces. Furnace equipment used at Thoughtventions for optical composition measurement will be either transparent furnaces or windows in standard furnaces. Transparent tube furnaces that are standard at Thoughtventions can house a closed-end sapphire tube filled with molten salt. Thoughtventions has made, tested, and sold transparent furnaces operating at temperatures up to 1200 C. Gold coated tubes reflect infrared heat, while transmitting visible light. Transparent furnaces without gold coatings can be operated up to 600-700 C. Furnaces with gold coatings and air cooling will operate to 950 C, whereas vacuum insulated transparent furnaces can be operated to 1200 C. Furnaces with inner diameters (IDs) compatible with the sapphire tube cell are standard Thoughtventions designs. Smaller IDs allow less insulation to be used, which makes achieving higher internal temperatures easier. Two and three zone Thoughtventions furnaces are also standard. An alumina tube pedestal, internally insulated, will support the cell at the center of the furnace hot zone. Note that coarser apparatus for testing the optical properties of molten salt mixtures have been built elsewhere (e.g. [7]) and extensive work has been done in them. Thoughtventions also has an assortment of glass, quartz, sapphire, and gold coated infrared reflecting windows. Thoughtventions has also fabricated and sold furnace equipment that allows microscopic sample inspection at 1000 C.

Apparatus Test Procedure: 1) Fill the sapphire container with the solid salt components, 2) Add to the end of the mixing chamber in the mixing furnace, 3) Heat the furnace to the target melting temperature, 4) Mix and temperature soak the mixture a standard Thoughtventions temperature controller, 5) Pour into the product receptacle for further examination.

Sapphire Windows for Molten Salt Optical Analysis. The resistance of α-Al2O3 to the attack of various chemicals and elements is described in [8]. The order of increasing stability against chemical attack for some refractory oxides on the basis of free energy is as follows: ½ ZrO2, 1/3 α-Al2O3, BaO, MgO, BeO, CaO. From heat content, entropy, and heat capacity data it was concluded that sintered alumina would not contain molten calcium. However, experimentally this is not true.

Sapphire is significantly less reactive than alumina. The reactivity of sapphire at elevated temperature changes in oxidizing, vs. inert vs. reducing atmospheres. It is important to note that sapphire surfaces are chemically different from the bulk material. The critical difference in terms of interaction of sapphire with molten salts is the perfection of the single crystal. Most sapphire surfaces, even those superpolished to nominally atomic roughness, are not perfect. The importance of this is that corrosion attacks surface imperfections. Therefore, to improve chemical resistance, it is desirable to use a crystal with fewer defects (i.e. polishing residue) and a precision polishing finish.

Properly prepared, atomically smooth sapphire surfaces are much more resistant to chemical corrosion. Furthermore, since many the properties of sapphire are orientation dependent, the corrosion of a sapphire surface is dependent on the crystal plane that is exposed. The corrosion resistance of a C-plane surface is very different from an R plane or a mixed plane of the basic hexagonal crystal structure of sapphire. By measuring the corrosion rate of sapphire under phosphoric acid, the corrosion rate changes by a factor of 7, depending on crystal surface orientation. [9]

The corrosion process in general is determined by a number of surface phenomena. The atomic structure near the free surface is characterized by a surface relaxation (shortening of the interplane distances is the subsurface atomic grids), and reconstruction (variation of symmetry in the subsurface layers), typical for crystals with covalent bonding. These phenomena increase the surface energy proportionally to the reticular density of the crystallographic planes. As a result, the best configuration for minimal corrosion would be a sapphire cell window where the face of the window in contact with the molten salt would be an S {1010} crystal plane. Most fabricated sapphire windows are C-plane windows.

1. J. Li and P.K. Dasguptaa, “A simple instrument for ultraviolet-visible absorption spectrophotometry in high temperature molten salt media,” Rev. Sci. Instrum., 71, 6 (2000)

2. A.M. Lines, et. al. “Evaluation of optical techniques for molten salt reactor materials control and accounting,” Pacific Northwest National Laboratory Report PNNL-30360 (2020)

3. N. L. Alukera, M. E. Herrmannb, and I. M. Suzdaltsevac, “A Spectroscopic Study of Nitrate and Nitrite Salts and Their Aqueous Solutions,” Optics and Spectroscopy, 2019, 127, 6, pp. 991–996 (2019)

4. J.F. Bowning, J. Seo, et. al., “A high temperature cell for investigating interfacial structure on the molecular scale in molten salt/alloy systems,” Rev. Sci. Instrum., Vol. 92, p. 123903, (2021)

5. M Anderson, R Scarlat, MA Kats, M Trujillo, W Derdeyn, “Radiative Heat Transport and Optical Characterization of High Temperature Molten Salts,” OSTI NEUP Project #17-13232 Final Report (2021)

6. Liu, Y. Liu, & T. Su, “An Instrument Established for the High Temperature Measurement of Ultraviolet-Visible Absorption Spectra of Molten Fluoride Salt Behaving As Coolant in the Molten Salt Reactor,” Paper # ICONE26-82013, 26 Intnl. Conf. On Nucl. Eng. Jul (2018)

7. L.R. Brock, “The wettability of sapphire, polycrystalline alumina, and quartz by molten metal halide salts,” J. Phys. & Chem. of Solids, Vol.66, Nos. 2–4, p. 484-487, (2005)

8. Alumina as a Ceramic Material, W.H. Gitzen, Ed. Published by the American Ceramic Society. (1970)

9. L.A.Lytvynov, T.V.Druzenko, V.G.Potapova, A.B.Blank, “Corrosion Resistance Of Sapphire Surface,” ResearchGate publication.

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Last updated: July 2025