Molten Salt for Molten Salt Reactors (MS4MSRs) : A thermodynamic exploration and characterisation of alternative nuclear molten fuel salt systems

Findlay, Joshua and Degueldre, Claude and Cheneler, David and Chartrand, Patrice (2026) Molten Salt for Molten Salt Reactors (MS4MSRs) : A thermodynamic exploration and characterisation of alternative nuclear molten fuel salt systems. PhD thesis, School of Engineering.

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Abstract

This thesis presents a comprehensive thermodynamic investigation of alternative molten salt systems for use in both thermal- and fast-spectrum molten salt reactors (MSRs). The work integrates new experimental data with advanced thermodynamic modelling to establish a consistent and validated database for both fluoride- and chloride-based fuel salts, many of which had not been previously examined. A series of Differential Scanning Calorimetry (DSC) experiments were conducted to determine phase equilibria, eutectic compositions, and enthalpies of mixing in key binary and ternary systems. A custom hermetically sealable, high-sensitivity crucible was designed and developed to enable accurate DSC measurements of volatile and corrosive salt systems – representing a novel technical contribution of this work. These data formed the foundation for thermodynamic optimizations using Gibbs energy functions, allowing reliable prediction of phase equilibria, melting points and respective compositions, redox potentials, chemical activities, vapor pressure and other excess thermodynamic properties relevant to reactor operation and safety. The thesis follows a hybrid structure comprising three peer-reviewed publications and supporting chapters. The RbCl-CeCl3 and RbF-CeF3 systems were experimentally investigated and modelled to establish reliable surrogates for actinide chlorides and fluorides (PuCl3 and PuF3, respectively). The RbCl-UCl4 system was measured for the first time using DSC, providing entirely novel experimental data, while the RbF-UF4 system was re-measured to validate historical data and generate additional thermodynamic property information. The CeF3-UF4 and CeCl3-UCl4 surrogate systems were measured for the first time using DSC, providing novel experimental data and enabling calculation of new thermodynamic properties. The systems were subsequently modelled as solid solutions incorporating lattice vacancies, which better reflect real crystal behaviour than interstitial substitution, improving both reliability and predictive accuracy. Building upon these binary descriptions, new ternary systems (RbF-PuF3-UF4 and RbCl-PuCl3-UCl4) were constructed and analysed to assess the thermodynamic stability and reactor suitability of Rb-based salts compared to traditional Oak Ridge National Laboratory (ORNL) LiF-BeF2 containing mixtures. Collectively, this work provides the novel integrated experimental–computational framework linking measured thermodynamic data with predictive modelling across multiple halide systems. The resulting database and modelled liquid solutions significantly enhance the understanding of MSR-relevant salts and supports the identification of optimized compositions that balance critical properties, laying the groundwork for Rb containing alternative fuel salts.