Thoughtventions Reactor Relevant Molten Salt Production

The work of Huajian Liu, et. [1] is a good example of the production and use of a relatively standard reactor relevant molten salt: LiF–NaF–KF. They made their own salt as described in their paper, being unable to obtain the salt commercially. Their process is as follows:

Their starting materials were 29.2 kg LiF (99.9%) purity),11.7 kg NaF (99% purity), and 59.1 kg anhydrous KF (99.9% purity) resulting an a FLiNaK salt, with a composition 46.5 mol% LiF, (2 g $96 - Sigma Aldrich), 11.5 mol% NaF (5g $51), 42 mol% KF (5g $104). The salt was then mixed and pre-melted at 580 ° C in a UNS N02201 alloy vessel. Nickel Alloy 201 is a wrought commercially pure Nickel with a maximum carbon level of 0.02%. The mixture was then purged with pure H2, and the molten FLiNaK salt was flowed to other vessels for processing the mixture using H2 and HF. H2 /HF was bubbled through the salt to remove oxygen-containing and sulfur-containing impurities. Finally, the molten FLiNaK salt was transferred into another vessel and processed by H2 to remove metallic ions. Impurities in the salt were monitored in a separate vessel. If the purity of molten salt was not as specified, it was processed again.

Purification of the fluoride salt was achieved by H2 /HF gas mixture sparging followed by hydrogen sparging and filtering. Other non-corrosive fluorination reagents were also reported for purification of fluoride salts. If the salt is controlled at a slightly reducing redox potential, by controlling the UF 4 /UF 3 ratio at a desired value, the corrosion of the candidate alloys, such as Hastelloy N and 316SS, is expected to be minimized.

Safety Comments. The materials used in this process are all hazardous in varying degrees. The base materials are toxic and generally corrosive to standard containment materials when heated; care must be used for the basic salts used. Processing the salts with bubbled H2/HF is significantly more hazardous. HF is perhaps the most hazardous common acid available; it will attack almost all common materials and quickly burn skin. H2 is highly flammable and will diffuse through many materials to create combustible mixtures. This salt fabrication is best performed in a closely monitored, safe commercial facility – especially not by students in an academic environment.

Production at Thoughtventions.

Component Chemical Sourcing. Single component chemicals of varying purities can be obtained from standard chemical supply companies such as Sigma-Aldrich or Fischer Scientific. Subtask work with respect to component chemical will be to determine which impurities and what level of impurities are appropriate to the relevant molten salts. Molten salt corrosion is sometimes dominated by impurities, rather than the salts themselves (e.g. oxides [2]) Another corrosion mechanism that is not of concern for this program is the dissolution of alloy components into pure molten salts (e.g. [3])

Nickel alloys such as used in the example are those recommended and used in industrial molten salt facilities. The alloys exhibit minimal corrosion and can be fabricated relatively easily. However, in a facility such as that proposed here, dedicated to producing salts, more corrosion resistant ceramics such as sapphire and alumina can and will be used.

Mixing. Mixing will be performed in a tilt-sloshing vessel. The vessel will be mounted in a furnace with an axle for the slosh mixing as shown in Figure 1. One end is sealed, while the other is open for adding solid salt components at room temperature and removing/pouring out the mixed liquid liquid salt. The molten salt container shown will be fabricated with TvU’s high temperature alumina tube bonding procedure. Mixing will take place in TvU’s inert gas glove box.

Purification. Purification methods will be investigated to some extent to be able to use less expensive base materials. The contaminant will determine the appropriate purification method. For instance Mg impurities can be removed from a chloride salt using an electrolysis process as described by [4].

Impurity Corrosion. As of 2018 the available data for corrosion of materials in fluoride salts is limited, and even less data is available for chloride salts. The purification of chloride salt is not as established as that of fluoride salt, and the oxide impurity in the chloride salts is more difficult to be removed by HCl sparging. CCl4 gas sparging was found to be much more effective than the HCl sparging. To make high quality, impurity free chloride salts, a standardized purification process is desired. Current (2018) results on the corrosion of materials in chloride salts are not consistent, probably due to different experimental conditions and the unknown impurity in the tested chloride salts. For example, the effects of chromium content in the alloy and temperature on the corrosion of materials in the chloride salts are not clear. An improved understanding on chloride salt corrosion mechanism is needed. To select the most suitable materials for long-term service in chloride fast reactors, further systematic study is needed.

Commercial Product Final Form. Very common molten salts such and potassium and sodium nitride salts are sold in granular form. The easiest way to provide a room temperature molten salt is to make it as a block, heating the interface to its container so that it can easily be removed. Powders are usually made by crushing or grinding, but in the case of the salts produced in this program these techniques are a major avenue for contamination. A much better way to produce the molten salts for shipping in this program is to pour the melt into small diameter half cylinder molds, from which the cool solid can be cut up into sections for standard amount shipping.

1. Huajian Liu, et. al., “Study on the Mechanism of Failure in the LiF–NaF–KF Molten-Salt-Purifying Processing System,” Frontiers in Materials, Volume 9, Article 839538, (2022)

2. P. F. Tortorelli, P. S. Bishop, J. R. DiStefano, “Selection of Corrosion-Resistant Materials for Use in Molten Nitrate Salts” ORNL TM-11162 (1989)

3. H.L. Chan, E. Romanovskaia, V. Romanovski, D. Sur, M. Hong, P. Hosemann, and J.R. Scully, Corrosion Electrochemistry of Chromium in Molten FLiNaK Salt at 600 °C, J. Electrochemical Society, 170, 081502 (2023)

4. W. Ding, J. Gomez-Vidal, A. Bonk, T. Bauer, “Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level,” https://www.sciencedirect.com/science/article/pii/S0927024819302065 (2019)

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Last updated: July 2025