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• Reassessed the thermodynamic functions for CsF(g) and Cs 2 F 2 (g) to improve fit to experimentally measured vapor pressure and monomer–dimer distributions over condensed phase CsF. • Original phase equilibria determinations for the LiF-CsF, KF-CsF, LiF-NaF-CsF and LiF-NaF-KF-CsF systems. • First-time representation of the LiF-CsF system using non-default SNN coordination numbers to account for short range ordering in the melt. • First time representation of the LiF-NaF-KF-CsF system using the modified quasichemical model. • Successfully reproduced the experimentally measured vapor pressure of cesium-containing species over a dilute mixture of CsF with FLiNaK. Gibbs energy models were developed to describe the thermochemical behavior of CsF in molten FLiNaK (46.5LiF-11.5NaF-42KF mol%), a proposed molten salt reactor (MSR) fuel solvent and coolant, as cesium is of concern due to its high radiotoxicity and volatility. Initially, it was necessary to obtain a more accurate Gibbs energy function for CsF which required fitting parameters to reported vapor pressures over condensed phase CsF. The pseudo-binary systems CsF-LiF, CsF-NaF and CsF-KF were then evaluated utilizing phase equilibria and enthalpy of mixing (Δ mix H) values, together with original differential scanning calorimetry (DSC) measurements performed for the CsF-LiF and CsF-KF systems. The CsF-LiF-NaF, CsF-LiF-KF and CsF-NaF-KF pseudo-ternary system representations were obtained by interpolation of the constituent pseudo-binary systems, with DSC measurements performed for the CsF-LiF-NaF system to corroborate the calculated liquidus temperature. Ultimately, the pseudo-ternary systems were interpolated to obtain Gibbs energy models for the pseudo-quaternary CsF-LiF-NaF-KF system, supported by DSC measurements at low CsF compositions (1–10 mol%), yielding computed equilibria and cesium-containing vapor pressures that compare favorably with reported values. The Molten Salt Thermal Properties Database – Thermochemical (MSTDB-TC) was subsequently expanded to include these Gibbs energy models allowing description of the thermochemical behavior of the CsF-LiF-NaF-KF system. [ABSTRACT FROM AUTHOR]

The extensively evaluated and consistent thermodynamic database, the Molten Salt Thermal Properties Database—Thermochemical (MSTDB-TC), was used along with additional thermodynamic values from other sources as examples of ways to examine molten salt reactor (MSR) fuel behavior. Relative stability with respect to halide potential and temperature for likely fuel and fission product components were mapped in Ellingham diagrams for the chloride and fluoride systems. The Ellingham diagrams provide a rich, visual means for identifying halide-forming components in proposed fuel/solvent salt systems. Thermochemical models and values from MSTDB-TC and ancillary sources were used in global equilibrium calculations to provide compositions for a close analysis of the behavior of a possible Molten Chloride Salt Fast Reactor and a Molten Salt Reactor Experiment-type system at high burnup (100 GWd/t). The results illustrated the oxidative nature of burnup in MSRs and provided information about redox behavior and possible control. [ABSTRACT FROM AUTHOR]

• Continued additions and improvements for upcoming MSTDB-TC versions. • 5 novel assessments and 25 reassessments of MSR-relevant systems. • Discussion of each system (re)assessment and comparison to available data. In conjunction with the development of the Molten Salt Thermal Properties Database – Thermochemical (MSTDB-TC), the thermodynamic assessment and reassessment of 30 systems are detailed. The NdF 3 –LiF, NdF 3 –NaF, CrCl 3 –KCl, CrCl 3 –MgCl 2 , and CrCl 3 –NaCl systems are newly assessed in this work. The remaining 25 systems have been reassessed from published models to ensure compatibility with the MSTDB-TC and reproduction of available data and models. Liquid solutions are modeled using the modified quasichemical model in the quadruplet approximation and solid solutions are modeled using either single sublattice polynomial models or the compound energy formalism. For each system, original and reassessed model parameters are provided and the calculated pseudo-binary phase diagram or pseudo-ternary liquidus projection is compared to available phase equilibria. [ABSTRACT FROM AUTHOR]

• Thermochemical properties of lithium halide pseudobinary systems are assessed based on reported experimental data. • LiF-LiI system is essential for understanding fission product iodine behavior, thus developing source terms for accident analysis of fluoride-based MSRs. • The calculated vapor pressures of LiI and LiF endmembers are verified using available experimental partial pressures. • For example, vapor pressures are computed by our assessed models for LiF-70 mol% LiI and compared to reported experimental vapor pressures. • Extrapolations to the pseudoternary and pseudoquaternary systems have been performed. Lithium halides are of interest because of their unique applications as fuel or coolants in molten salt reactors (MSRs) and electrolytes in thermally activated batteries. Because iodine is an important fission product for MSRs, the Molten Salt Thermal Properties Database-Thermochemical (MSTDB-TC) is being expanded to include relevant iodide reciprocal salt systems. In this work, we assess the thermochemical properties of lithium halide pseudobinary systems based on reported experimental phase diagrams, eutectic temperatures, and enthalpies of mixing. Within the framework of the modified quasi-chemical model in the quadruplet approximation (MQMQA), extrapolations to the related pseudoternary and pseudoquaternary systems have been performed. The pseudoquaternary representations of phase equilibria, enthalpy, entropy, and heat capacity are represented in the resultant models. Of interest for generating source terms for accident analysis is the ability to compute the vapor pressures of the dominant iodide species for the systems at any composition. An example of such calculations is provided for the LiF-LiI system at x LiI = 0.01 and x LiI = 0.70, in this case indicating that the LiI vapor species are predominant. [ABSTRACT FROM AUTHOR]

• Initial release of the MSTDB-TC for thermodynamic modeling of molten salt reactor coolant/fuel salt systems. • Available with no-cost license through Oak Ridge National Laboratory's code.ornl GitLab repository. • Additional systems and improvements continue to be added to future database versions. The Molten Salt Thermal Properties Database − Thermochemical (MSTDB−TC) has been developed to support thermodynamic modeling of fluoride- and chloride-based systems for molten salt reactors (MSRs). Utilizing data from available literature and original work, the current MSTDB−TC Ver. 1.2 contains models for 96 pseudo-binary systems, 37 pseudo-ternary systems, 42 solid solutions, 229 stoichiometric compounds, and 130 gaseous species. This includes 67 reassessed systems and 13 original system models. The reassessment of the LiCl−RbCl pseudo-binary system and the original assessment of the UF 3 −UF 4 pseudo-binary system are included as examples of those efforts and provide improved representations of those systems. The database continues to be expanded, adding systems of relevance to the wide variety of MSR concepts. To illustrate applications for MSTDB−TC , phase equilibria were computed to understand the effect of varying uranium content in the KCl−MgCl 2 −NaCl−UCl 3 system. In an example of how such calculations can support corrosion modeling, the control of redox potential by the UF 3 :UF 4 ratio in the LiF−BeF 2 −UF 3 −UF 4 system is demonstrated and used to assess corrosion potential for typical alloy constituents. [ABSTRACT FROM AUTHOR]