The acidity of salt
If you have ever eaten a salt and vinegar potato chip or put tangy ketchup on your fries, you are familiar with the pairing of acidity and salt. In addition to being flavors that pack a punch when combined, acidity and salt are two basic chemistry concepts that can be curiously related. The definition of an acid has to do with how something behaves, whereas salt describes what something is. Given this, we can describe salt itself as acidic or basic. This seems reasonable if we add water to the mix—you could dissolve some salt in water and then measure the pH. However, researchers from the Fundamental Understanding of Transport Under Reactor Extremes (FUTURE) EFRC are interested in these concepts where water is notably absent—that is, in a molten salt nuclear reactor. Here, molten salt is used to transfer heat produced by nuclear reactions to a power cycle for electricity generation. In a reactor, materials experience high temperatures, radiation damage, high stresses, and contact with corrosives. To understand the fundamentals of corrosion in molten salts, FUTURE scientists are going back to the basics of chemistry, seeking to understand the acidity of molten salts and its effect on corrosion.
The basics of acid and salt
In chemistry, an acid is a substance that can accept a pair of electrons. This theory (the Lewis theory of acids) provides a broader definition than the Bronsted-Lowry theory, which states that an acid is a substance that gives H+ to another molecule. The acidity scale you might be familiar with, pH, is primarily an indicator of a solution’s ability to accept or donate H+. The name ‘pH’ refers to the “potential of hydrogen” since pH is quantified by the concentration of H+ ions in a solution. However, if acids are defined by their acceptance of electrons rather than their ability to donate H+, one can consider alternative acidity scales based on electron pair acceptors other than H+. For example, we can measure acidity based on the concentration of F- ions in a solution since F- is a Lewis base (it donates electrons). This is known as fluoroacidity (pF) and is especially relevant when the liquid of interest is a molten fluoride salt.
In the case of molten salt reactors, the acidic liquid of interest is not some standard laboratory acid, but molten salt. Salt, heated to temperatures at which it is liquid, is used in advanced nuclear reactors to transport heat from the reactor core to a turbine to generate electricity. A salt, by definition, is a compound made up of an assembly of ions. These could be sodium and chloride ions which season your potato chips, or beryllium and fluoride ions which are suited for use in a nuclear reactor. When melted, these ions dissociate from their matrix and are free to float about and interact with each other, feeling each other’s ionic charges and interacting as they move. The ions’ interactions depend on the salt’s elemental composition and dictate the liquid’s molecular structure; the ions can exist within clumps or independently. Figure 1 shows a snapshot model of liquid 2LiF-BeF2 salt. The Li, Be, and F ions are no longer ordered as they were in the solid salt, but they strongly interact with each other and form a clumpy melt structure and, therefore, a viscous melt. Molten salts composed of different ions will have different tendencies to form clumps—2LiF-BeF2 tends to be more polymer-like compared to other molten fluorides. Yet, in any molten fluoride salt, there exist some independent F- floating around at a given point in time. By measuring how much independent F- there is, we determine the salt’s fluoroacidity.
Fluoroacidity and corrosion
As explained, fluoroacidity is related to the salt’s structure. A salt’s structure affects its viscosity and also influences its corrosive behavior. FUTURE materials scientists at the University of California, Berkeley have modeled how chromium, a metal which is preferentially corroded from structural alloys, behaves in melts of differing fluoroacidities. During corrosion, chromium metal in the reactor’s piping or vessel, for example, has its electrons taken from it by some impurity in the melt. The process of losing electrons is called oxidation and is central to corrosion. The resulting chromium ion (Cr2+ or Cr3+) can then dissolve in the salt melt, creating defects in the reactor’s piping or vessel.
In the past, fluorobasic salts (which contain more independent F-) were generally understood as more corrosive than acidic salts because they can incorporate chromium ions by surrounding them with the independent F- and thus helping them to dissolve. Melts with fewer independent F- were believed to be less corrosive because there would not be enough available F- to surround chromium and keep it stable in the melt. However, this study’s simulations showed that clumpy salts with fewer independent F- (fluoroacidic melts) can solvate (or dissolve) chromium by incorporating it into the clumps (see Fig. 2)—abundant independent F- are not needed. This finding supports prior experimental evidence that both highly fluorobasic and highly fluoroacidic melts are corrosive.
Further, considering fluoroacidity to be simply a measure of F- concentration might be too simplistic. The concentration of independent F- is really dictated by the ions and ion clumps in equilibrium with the independent F-. In fluoride melts, F- is the base, and other ions in the melt (such as Na+, Be2+, or clumps of these) behave as acids, each to a different extent depending on how strongly they interact with F-. By measuring acidity in terms of acidic behavior (e.g., how many F- act as bridges like in Fig. 2), this study found a different ranking of fluoroacidity compared to a quantification simply based on the number of independent F-. Thus, for a fuller picture of fluoroacidity and its effect on corrosion, one must look at the equilibrium of both acidic and basic ions.
Basic research, big applications
In summary, FUTURE researchers are developing the language for understanding molten fluoride salts, using old-school chemistry concepts to support the development of next-generation technologies. The fluoroacidity and structure of salt might seem like minute details when it comes to the large-scale development of advanced nuclear reactors, but understanding this chemistry has important implications for answering the crucial questions of corrosion and salt flow dynamics which are essential for reactor design. Understanding and predicting corrosion and changes in salt viscosity are critical for the safety and performance of nuclear reactors, and the concept of fluoroacidity is valuable to addressing these questions.
Winner, Nicholas, et al. "Ab-initio simulation studies of chromium solvation in molten fluoride salts." Journal of Molecular Liquids 335 (2021): 116351. https://doi.org/10.1016/j.molliq.2021.116351
This research was supported as part of FUTURE (Fundamental Understanding of Transport Under Reactor Extremes), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). The simulations made use of computing resources from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1548562. HW also acknowledges the support of the University of California, Berkeley, Chancellor’s Fellowship.