RT1: Molten Salt Structure, Dynamics, and Properties
Thrust 1 establishes the molecular-scale picture of how molten salts are structured, how ions move through them, and how these features determine bulk properties in reactor-relevant mixtures.
Scientific Focus
Thrust 1 examines neat molten salts and salt mixtures across broad ranges of composition and temperature, with particular emphasis on chloride salts of lanthanides and actinides and their mixtures with other metal ions. These systems are central to molten salt reactor (MSR) chemistry because local structure, ion motion, and composition together govern the properties needed for reliable fuel-salt and coolant performance.
A major insight from MSEE is that molten salts should not be treated as static networks of fixed coordination environments. At temperatures where these salts are liquid, metal ions can rapidly exchange counterions and sample multiple coordination states, often within extended ionic networks. Thrust 1 tests how general this behavior is and determines how dynamically exchanging structures should be represented when interpreting Raman, UV-vis and other optical spectra, X-ray absorption, and scattering measurements.
The thrust also investigates thermal and transport properties and the atomic mechanisms that give rise to them. Rather than treating diffusion, viscosity, conductivity, and related quantities only as measured or calculated coefficients, Thrust 1 seeks to understand how short-range structural correlations among ions form, persist, and relax. This rate-based view links fast local rearrangements in the melt to the macroscopic properties that govern salt behavior.
Approach and Impact
This work provides the microscopic foundation for predictive models used in nuclear energy applications. Coordination states that are short-lived or locally variable can still influence the thermodynamic and transport behavior captured by coarser models, so incorporating those states is essential for connecting fundamental measurements and simulations to practical descriptions of molten salt mixtures.
Mixtures containing cations with different formal charges introduce an additional level of complexity. MSEE studies indicate that such mixtures can develop nanoscale structural heterogeneity, creating local environments with distinct Lewis acidity or basicity. Thrust 1 explores how those environments affect speciation and solvation dynamics, especially for trivalent and higher-valent lanthanide and actinide ions dissolved in alkali chloride salts.
A further priority is the behavior of minority components in bulk molten salts. These species may arise from radiation chemistry, corrosion, reactions within the melt, or processes occurring at interfaces. Because they are often present at low concentrations and can be difficult to observe with scattering methods, their coordination structures and dynamics pose a substantial measurement challenge. Understanding them is nevertheless critical for building a complete picture of molten salt chemistry under realistic reactor-relevant conditions.
Thrust 1 Aims
Structural Dynamics and Transport Properties
Measure and compute thermal and transport coefficients in the bulk salt and close to interfaces. Use a rate theory perspective to derive the energetics of barriers to transport and the mechanisms of ion exchange that give rise to transport. Develop phase-transferable force fields based on machine learning.
Bulk Structure of Neat Salts and Salt Mixtures
Provide a complete picture of heterogeneous coordination states, ionic networks, and structural heterogeneity from a multimodal (e.g., X-ray scattering and X-ray absorption, Raman spectra) and computational perspective for chloride salts that include lanthanides, actinides, and other divalent and monovalent metal ions.
Dissolved Species in the Bulk
Determine the structure and dynamics of minority salt constituents and radiation and corrosion products that present challenges for traditional X-ray scattering structural determination. Measure and computationally predict how solutes, meaning components other than alkali and alkaline earth chlorides, affect the structural properties of molten salts.
