Molten Salt Reactor Chemistry

Abstract

This thesis investigates the thermochemical behaviour and safety of chloride-based molten salt reactor (MSR) fuels, with a particular focus on the role of fission products during reactor operation. Although molten salt technology was first explored in the 1950s at Oak Ridge National Laboratory through the Aircraft Reactor Experiment and Molten Salt Reactor Experiment, interest declined in favour of other reactor types. Today, however, renewed attention has emerged due to the potential safety and efficiency advantages of molten salt systems, especially their low vapor pressure, high heat capacity, and favourable transport properties.

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.