Molten salt is exactly what it sounds like—simply the melted contents of your saltshaker. The descriptor “molten” may evoke images of volcanic lava, but once the salt is melted, it is typically not hot enough for it to glow. Depending on the salt’s purity and composition, it often looks and flows like water. There are more types of salt than your saltshaker’s sodium chloride, however. A “salt” is simply a compound made up of an assembly of ions. A variety of salts, both chlorides and fluorides, are of interest for use in nuclear reactors. Molten salts may serve as the coolant and/or fuel in the design of next-generation nuclear reactors, allowing for safer and more efficient nuclear energy. It is critical to understand how these salts behave, especially in the extremes of a nuclear reactor environment, so two Energy Frontier Research Centers (EFRCs) are addressing the questions of salt chemistry and containment materials properties with both experimental and theoretical studies. Scientists of the Molten Salts in Extreme Environments (MSEE) and Fundamental Understanding of Transport Under Reactor Extremes (FUTURE) EFRCs are studying the fundamentals of corrosion and materials irradiation, laying the groundwork for the realization of advanced nuclear reactors.
The Basics of Molten Salt-Cooled and -Fueled Reactors
Molten salts have heat conductivities and viscosities similar to water1, so they can be substituted for water in a reactor core, such as in light water reactors (LWRs). The coolant’s job, by definition, is to transport the heat generated in the reactor core (by the breaking apart of uranium atoms) to a heat exchanger, providing the heat for a thermodynamic cycle to generate electricity. As shown in the typical molten salt reactor (MSR) design presented in Figure 1, this heat is transferred through several loops—the salt in the reactor core would provide heat to another salt which ultimately heats steam or supercritical CO2 to turn a turbine. Unlike water, however, molten salts remain liquid at high temperatures, so a molten salt system does not experience the high pressure required in water-cooled systems to keep the water coolant in a liquid state, offering a notable safety advantage. In addition, molten salts are stable at higher temperatures compared to water, and higher temperature means higher efficiency for the thermodynamic cycles for electricity generation.
In addition to serving as the coolant, some advanced reactor designs use molten salts as a fuel carrier, creating a liquid-fueled reactor. The name “molten salt reactor” typically refers to a reactor that is both salt-cooled and salt-fueled, such as the reactor in Figure 1. In these reactors the uranium or thorium fuel itself is dissolved in the salt, meaning there is no solid fuel or fuel holder that may crack and release radioactive material. This design allows for clever shut-down safety mechanisms. For example, if the reactor overheats, the fuel salt can simply drain through a freeze plug (a safety release valve made of frozen salt) into a large holding container, with the fuel spread out such that it is not possible to sustain a nuclear chain reaction. Additionally, liquid fuels allow for chemical reprocessing of the fuel for separation of actinides and rare earth elements, which is a valuable advantage in a technological field plagued by questions of waste management. However, these MSR designs have challenges as well as advantages. Liquid fuel means that radioactive materials are moving through the primary salt loop, complicating maintenance and requiring thorough chemistry control. Overall, the technological challenges are notable, but these advanced reactors offer considerable safety and efficiency improvements compared to LWRs due to their walk-away-safe design and high operating temperatures.3
The Effort to Actualize MSRs
The concept of an MSR is not new. The idea was introduced and demonstrated at Oak Ridge National Laboratory in the 1950s and 60s, but research was discontinued as funding shifted to other nuclear technologies4. The success of the Molten Salt Reactor Experiment was not forgotten, and interest in the commercial development of reactors that use molten salts has arisen with the pressing need for baseline clean energy. Researchers within the MSEE and FUTURE EFRCs are addressing the fundamental science questions needed to make these reactors a commercial-scale reality.
A molten salt-cooled and/or -fueled nuclear reactor is a harsh environment for structural materials. High temperatures, radiation damage, mechanical stress, and corrosive fluids all pose considerable technological challenges. Characterization of the structure and thermal properties of molten salts is important for predicting and understanding corrosion and the solubility of corrosion products within the salt. In a system where containment is crucial for safety, corrosion must be well-managed. MSEE researchers are addressing the challenge of understanding corrosion through studies of atomic-scale phenomena within the bulk salt using both modeling and experimentation. In a recent publication, MSEE scientists compared computed and measured X-ray scattering data for structure functions (based on X-ray scattering parameters) of LiCl, NaCl, and KCl salt melts5. The work sought to understand chloride exchange for the alkali cations (Li+, Na+, K+) in the study, noting relations between atomic-scale phenomena and macroscopic properties such as viscosity, and found good agreement between melt simulations and experimental data for the structure functions. This work is valuable for its predictions on the important driving forces of transport within molten salts. While MSEE’s research is primarily on chloride salts, FUTURE focuses on fluorides, investigating the coupled effects of corrosion and irradiation and how materials are damaged. FUTURE researchers contributed to a recent publication that showed proton irradiation of Ni-Cr alloys slowed down corrosion in molten LiF/NaF/KF/EuF3.6 They propose this surprising result is due to increased diffusion within the irradiated metal. The authors suggest that the disorder in the metal structure caused by the bombardment of protons means that more metal atoms are knocked out of their place and diffuse. Atoms prefer to diffuse to interfaces within the metal, and these interfaces are also where corrosion usually occurs. Thus, there exists a possible “self-healing” mechanism where the zones depleted by corrosion can be replenished by increased diffusion to the same zone.
A Promising Salty Future
The research conducted by MSEE and FUTURE extends well beyond the highlights presented here. With diverse teams of computational modelers and experimentalists, the MSEE and FUTURE EFRCs are contributing greatly to the fundamental understanding of molten salts and their interaction with materials in irradiated environments. The value of salt chemistry and corrosion studies extends even beyond MSR applications; molten salts have other impactful energy applications such as fusion reactors or concentrated solar power. With an abundant need for clean energy, the motivation for studying molten salts is clear. The research conducted by MSEE and FUTURE is laying the fundamental groundwork for next-generation nuclear energy. And with the ability to offer exceptionally safe and carbon-free energy, we can be glad that this is a technology you don’t have to take with a grain of salt.
