Developing Practical Models of Complex Salts for Molten Salt Reactors

Abstract

Molten salt reactors (MSRs) utilize salts as coolant or as the fuel and coolant together with fissile isotopes dissolved in the salt. It is necessary to therefore understand the behavior of the salts to effectively design, operate, and regulate such reactors, and thus there is a need for thermodynamic models for the salt systems. Molten salts, however, are difficult to represent as they exhibit short-range order that is dependent on both composition and temperature. A widely useful approach is the modified quasichemical model in the quadruplet approximation that provides for consideration of first- and second-nearest-neighbor coordination and interactions. Its use in the CALPHAD approach to system modeling requires fitting parameters using standard thermodynamic data such as phase equilibria, heat capacity, and others. A shortcoming of the model is its inability to directly vary coordination numbers with composition or temperature. Another issue is the difficulty in fitting model parameters using regression methods without already having very good initial values. The proposed paper will discuss these issues and note some practical methods for the effective generation of useful models.

Complete Description of the LaCl3–NaCl Melt Structure and the Concept of a Spacer Salt That Causes Structural Heterogeneity

“…Although the literature on molten salts is significant and spans over a century, the prospect of using them for cleaner and safer energy harvesting and processing to generate electricity has resulted in a massive spike in interest recently. − The coordination of ions influences reactivity, including for corrosion and the complex cascade of radiation-driven processes occurring in nuclear reactors. − Consequently, a highly desirable objective is to gain complete and consistent understanding of (1) the speciation of ions, (2) the heterogeneity of speciation, and (3) intermediate-range structural correlations that go beyond the arrangement of nearest neighbors in complex multi-ion melts. An added bonus is that speciation information, and its heterogeneity will serve as precious input to simpler thermodynamic models that compute complex salt phase diagrams. − …”

Heterogeneous Structure, Mechanisms of Counterion Exchange, and the Spacer Salt Effect in Complex Molten Salt Mixtures Including LaCl3

“…Molten salt mixtures of the actinides or lanthanides are becoming quite useful for industry and in the energy sector. For example, UCl 3 –NaCl–MgCl 2 and PuCl 3 –NaCl–MgCl 2 are promising molten salt fast reactor fuel candidates, , LaCl 3 –KCl–MgCl 2 mixtures are used to synthesize various LaMg alloys used as tunable mirrors, and various Cl – -based mixture melts have been studied in the context of corrosion. − It is also important to have fast database access to thermodynamic properties of complex binary and ternary salt combinations. − There is a direct link between structural and thermodynamic studies such as ours and the quasi-thermodynamic models that are at the core of such databases since free-energetic properties in those are modeled based on the coordination number of the ions. − Of course, in reality it was found − that, at the temperatures at which salts are in the molten state, cations have an ensemble of interconverting coordination structures that are temperature dependent. For multivalent cations in combination with chloride, a significantly polarizable anion, there are further complexities.…”

Deep neural network based quantum simulations and quasichemical theory for accurate modeling of molten salt thermodynamics

“…The CALPHAD method is a principal method for phase diagram and molten salt database development. [32][33][34][35][36][37] Experimental thermodynamic data (from phase equilibria measurements and activities of solution species) and/or AIMD simulations are used to empirically t models and then predict stable phases at different temperatures or compositions. As mentioned above it would be difficult, expensive, and timeconsuming to obtain data for high-dimensional salt systems through either experiments or AIMD simulations.…”