D.C. Alders
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1
Thermodynamic Studies of Cesium and Iodine in Molten Salt Systems
CALPHAD Modeling of the (Na,Cs,Mg,Nd,Pu)(Cl,I) System
A thermodynamic model of the molten salt system (Na,Cs,Mg,Pu,Nd)(Cl,I) has been developed in this work to assess the effect of CsI on the melting and vaporization behavior of the nuclear fuel in molten salt reactors. Investigation using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) of the binary systems NdCl3–NdI3 and MgI2–NdI3, as simulant systems for the analogous Pu system, is presented for the first time. Both systems were found to be binary eutectic systems, with solid solutions NdCl3–3xI3x (hexagonal in space group P63/m) and NdCl3yI3–3y (orthorhombic in space group Cmcm) in the NdCl3–NdI3 system, and Mg1–xNdxI2+x (hexagonal in space group P 3 m1) in the MgI2–NdI3 system. Additionally, the system CsI–MgI2 was scrutinized using DSC, confirming the available experimental data in the literature. Furthermore, the investigation of the reciprocal diagonals in the systems (Na,Nd)(Cl,I), (Cs,Mg)(Cl,I), (Mg,Nd)(Cl,I), and (Cs,Nd)(Cl,I) is presented, allowing the characterization of the quaternary behavior of these salts. Based on the experimental data obtained in this work, a CALPHAD model is presented using the quasi-chemical formalism in the quadruplet approximation for the liquid solution. With the aim of modeling the complete (Na,Cs,Mg,Pu)(Cl,I) system, the binary systems NaCl–CsCl, CsCl–MgCl2, CsCl–NdCl3, NaI–CsI, and CsI–NdI3 were reassessed based on data from the literature. Furthermore, a CALPHAD model of the PuCl3 and PuI3 systems is also presented using Nd as a simulant for Pu in molten halide salts. With the developed thermodynamic models, calculations were finally performed to assess the fission product retention of Cs and I in a molten chloride environment. As opposed to their behavior in molten fluorides, the fission products are well retained in the fuel matrix up to concentrations of at least 5 mol%.
Fission products Sr and Ba in actinide (U, Th) chloride systems
Thermodynamic modelling, experimental investigation and application calculations
In this study, new insights into the solid state chemistry and melting behaviour of the Image 1001 system are presented, building on results in the simulant system Image 1002. Our studies have revealed the solubility of U in the high-temperature β[jls-end-space/]-phase of BaCl2 (i.e Image 1003 ) and an intermediate compound, Ba3U2Cl12, which has led us to revisit or present for the first time the phase diagrams of this system accordingly. Furthermore, we present revised thermodynamic models for the systems Image 1004, Image 1005, Image 1006 and Image 1007 (AE = Sr, Ba) based on existing literature data. With the constructed multi-component database Image 1008, the effect of fission products on the melting behaviour of molten chloride salts containing uranium and thorium is investigated through higher order phase equilibria calculations.
Molten Salt Reactor Chemistry
Fission Products and Simulants
Modern MSR designs increasingly consider chloride-based fuels instead of traditional fluoride systems, as chlorides allow higher actinide solubility and compatibility with existing reprocessing technologies. However, a major challenge in assessing their safety lies in understanding the behaviour of fission products within these complex, multi-component molten salt systems. These fission products can exist as dissolved species, volatile compounds, or solid precipitates, each posing different operational risks such as precipitation or vaporisation.
The research focuses on both salt-soluble fission products (e.g. barium, strontium, rare earth elements) and volatile species such as cesium and iodine. Their interactions with molten salt fuels are studied experimentally using techniques including Differential Scanning Calorimetry, X-ray diffraction, and neutron diffraction. These experiments are complemented by thermodynamic modelling based on the CALPHAD method, which enables prediction of phase stability and system behaviour under various temperatures and compositions. Central to this modelling is the Gibbs free energy, which determines the most stable phase configurations.
Due to the hazardous nature and limited availability of plutonium chloride (PuCl₃), the study employs simulant materials such as neodymium chloride (NdCl₃) and cerium chloride (CeCl₃), which closely mimic the thermochemical behaviour of plutonium and uranium chlorides. These simulants allow safe experimental investigation while maintaining scientific relevance. The work establishes a thermodynamic description of base fuel systems such as NaCl–MgCl₂–PuCl₃ and validates the use of simulants in representing real reactor conditions.
Significant findings include the identification of previously unknown solid solutions and intermediate compounds in systems containing barium and strontium, improving the understanding of precipitation risks. The research also demonstrates that while individual fission products can increase the likelihood of solid formation, realistic mixtures of fission products—reflecting actual reactor conditions—do not significantly alter the melting behaviour due to their lower concentrations.
The study further explores volatile fission products, particularly iodine and cesium, which may contribute to vaporisation risks. Modelling of complex mixed cation–anion systems shows that chloride-based fuels exhibit stronger retention of these volatile species compared to fluoride-based systems, indicating a safety advantage.
Finally, the developed thermodynamic database is validated against experimental data and applied to simulate real reactor conditions. Results confirm its reliability in predicting phase equilibria and assessing risks related to precipitation and vaporisation. Overall, the thesis provides a comprehensive thermodynamic framework for evaluating the safety of molten salt fuels under irradiation, while also identifying remaining knowledge gaps, such as the need for further experimental validation of certain compounds and interactions.
In conclusion, this work significantly advances the understanding of fission product behaviour in chloride-based molten salt reactors and supports their development as a safe and promising nuclear energy technology. ...
Modern MSR designs increasingly consider chloride-based fuels instead of traditional fluoride systems, as chlorides allow higher actinide solubility and compatibility with existing reprocessing technologies. However, a major challenge in assessing their safety lies in understanding the behaviour of fission products within these complex, multi-component molten salt systems. These fission products can exist as dissolved species, volatile compounds, or solid precipitates, each posing different operational risks such as precipitation or vaporisation.
The research focuses on both salt-soluble fission products (e.g. barium, strontium, rare earth elements) and volatile species such as cesium and iodine. Their interactions with molten salt fuels are studied experimentally using techniques including Differential Scanning Calorimetry, X-ray diffraction, and neutron diffraction. These experiments are complemented by thermodynamic modelling based on the CALPHAD method, which enables prediction of phase stability and system behaviour under various temperatures and compositions. Central to this modelling is the Gibbs free energy, which determines the most stable phase configurations.
Due to the hazardous nature and limited availability of plutonium chloride (PuCl₃), the study employs simulant materials such as neodymium chloride (NdCl₃) and cerium chloride (CeCl₃), which closely mimic the thermochemical behaviour of plutonium and uranium chlorides. These simulants allow safe experimental investigation while maintaining scientific relevance. The work establishes a thermodynamic description of base fuel systems such as NaCl–MgCl₂–PuCl₃ and validates the use of simulants in representing real reactor conditions.
Significant findings include the identification of previously unknown solid solutions and intermediate compounds in systems containing barium and strontium, improving the understanding of precipitation risks. The research also demonstrates that while individual fission products can increase the likelihood of solid formation, realistic mixtures of fission products—reflecting actual reactor conditions—do not significantly alter the melting behaviour due to their lower concentrations.
The study further explores volatile fission products, particularly iodine and cesium, which may contribute to vaporisation risks. Modelling of complex mixed cation–anion systems shows that chloride-based fuels exhibit stronger retention of these volatile species compared to fluoride-based systems, indicating a safety advantage.
Finally, the developed thermodynamic database is validated against experimental data and applied to simulate real reactor conditions. Results confirm its reliability in predicting phase equilibria and assessing risks related to precipitation and vaporisation. Overall, the thesis provides a comprehensive thermodynamic framework for evaluating the safety of molten salt fuels under irradiation, while also identifying remaining knowledge gaps, such as the need for further experimental validation of certain compounds and interactions.
In conclusion, this work significantly advances the understanding of fission product behaviour in chloride-based molten salt reactors and supports their development as a safe and promising nuclear energy technology.
The reference list in the originating article was incorrect. The correct list of references is shown here.
The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. ...
The reference list in the originating article was incorrect. The correct list of references is shown here.
The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.
Graphical abstract: Simulant chemistry for uranium and plutonium molten fuel salts: crystallographic investigation and thermodynamic modelling assessment of the NaCl–RECl3 and NaCl-MgCl2-RECl3 (RE = Ce, Nd) systems ...
Graphical abstract: Simulant chemistry for uranium and plutonium molten fuel salts: crystallographic investigation and thermodynamic modelling assessment of the NaCl–RECl3 and NaCl-MgCl2-RECl3 (RE = Ce, Nd) systems
The thermochemistry of the quaternary molten salt system NaCl-NaI-MgCl2-MgI2 has been studied using an experimental and thermodynamic modeling approach. The binary subsystems NaCl-NaI and NaCl-MgCl2 were reassessed based on existing data in the literature. The binary subsystem NaI-MgI2 was subjected to a renewed experimental investigation, to complement and revisit the data in the literature. The subsystem MgCl2-MgI2 was investigated for the first time in this work using Differential Scanning Calorimetry (DSC). Furthermore, the phase equilibria in the pseudobinary phase diagrams of NaCl-MgI2 and NaI-MgCl2 in the quaternary system were investigated by DSC, while the condensed phases in the quaternary system were investigated using X-ray diffraction (XRD). A thermodynamic model of the quaternary system was developed using the CALPHAD (CALculation of PHase Diagrams) method with the quadruplet approximation in the modified quasichemical model for the liquid phase, and two-sublattice polynomial models for the solid solution phases. With this model, the liquidus surface of the NaCl-NaI-MgCl2-MgI2 quaternary system has been described for the first time.
The thermodynamic and thermo-physical properties of the molten salt system [Formula presented] have been investigated using an experimental and modelling approach. This molten salt system includes a single intermediate compound [Formula presented], whose structure has been investigated using X-ray and neutron diffraction. Furthermore, this system exhibits solubility of [Formula presented] in [Formula presented] at high temperatures up to a concentration of around 25% [Formula presented] at 1060 K. Additionally, our measurements show solubility of [Formula presented] in [Formula presented] up to about 5% [Formula presented] at 973 K. The investigation of these solid solutions has been performed using quenching experiments and subsequent post-characterisation by X-ray diffraction (XRD). Phase diagram equilibria have also been investigated using differential scanning calorimetry (DSC). Using the aforementioned information on phase transitions, intermediate compound formation, and mutual solid solubility, a thermodynamic assessment of the system has been performed using the CALPHAD method. The model for the Gibbs energy of the liquid solution is the quasi-chemical formalism in the quadruplet approximation, while the model for the Gibbs energy of the solid solutions is a two-sublattice polynomial model.
The thermochemistry of the ternary system CsI-PbI2-BiI3, of interest for applications in photovoltaics, memory devices, and nuclear applications, among other things, is investigated in this work. The binary phase diagrams CsI-PbI2 and CsI-BiI3 were subjected to renewed experimental investigation, and the compounds CsPbI3, Cs4PbI6, and Cs3Bi2I9 were found to be the only stable phases in the investigated temperature window. The liquidus lines and invariant equilibria were determined. The phase equilibria in the BiI3-PbI2 system were measured for the first time by using Differential Scanning Calorimetry (DSC). The end-members form a solid solution over the entire composition range. The pseudobinary section CsPbI3-Cs3Bi2I9 of the CsI-PbI2-BiI3 ternary system was moreover measured by DSC, as well as the ternary eutectic points. A thermodynamic model of the complete CsI-PbI2-BiI3 system was developed by using the Compound Energy Formalism (CEF) for the solid phases and the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) for the liquid phase. The binary systems were modeled first, and no ternary interaction parameters were found necessary to reproduce accurately the phase equilibria in the ternary system. With our model, the whole liquidus surface of the field CsI-PbI2-BiI3 is described for the first time.