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. ... Read more

Fast-neutron spectrum nuclear reactors allow generating carbon-free energy from fissile uranium and plutonium isotopes with increased fuel utilisation compared to currently used light-water reactors (LWRs), whereby they contribute to closing the nuclear fuel cycle. The increased fuel utilisation influences the chemistry of the fuel pin, leading among others to the formation of the so-called JOG-layer (joint oxyde-gaine, French for the connecting layer between the oxide fuel and cladding material). This JOG-phase is chemically approximated as Cs2MoO4. Unlike conventional LWRs, liquid metals are used as coolant to accommodate the fast-neutron spectrum. In this work, liquid lead (Pb) and lead-bismuth eutectic (LBE), a liquid mixture of lead and bismuth (Bi) are considered. The class of reactors using these coolants and a fast neutron spectrum is known as Lead-cooled Fast Reactors (LFRs). This dissertation studies chemical interactions that can occur between coolant and fission products following cladding failure in LFRs, first focussing on the interaction between coolant and JOG-layer. Other important fission products to assess are cesium (Cs) and iodine (I), so-called volatile fission products and barium (Ba), present in the so-called grey phase. It uses the chemical thermodynamics approach, as complementary to post-irradiation examinations and kinetic and release studies.

Chemical thermodynamics centres around a proper description of the Gibbs energy of the possible phases. This Gibbs energy description is informed by available experimental data. Important compounds are typically synthesised using solid state synthesis. Their characterisation involves X-ray and neutron diffraction at ambient and nonambient temperatures, along with X-ray absorption spectroscopy. After the characterisation, thermodynamic properties like the enthalpy of formation and standard entropy are determined. Phase diagrams are measured using differential scanning calorimetry to study phase transition points and know the aggregation state of mixtures in (composition, temperature)-space. The acquired thermodynamic data are used to perform thermochemical calculations or to develop thermodynamic models using the so-called CALPHAD approach.

In this work, the possibility of chemical interaction between Pb-coolant and JOG-layer was studied. Thermodynamic properties of the compounds PbMoO4, Pb2MoO5 and Cs2Pb(MoO4)2, such as standard entropy, enthalpy of formation and melting enthalpy were determined experimentally. Based on this, a complete thermodynamic model of the Pb-Mo-O system, including PbMoO4, Pb2MoO5 and Pb5MoO8 was developed using computational thermochemical software (ThermoCalc). Finally, thermodynamic calculations show that Cs2Pb(MoO4)2, PbMoO4, Pb2MoO5 and Pb5MoO8 can form in LFR operating conditions i.e. with typical oxygen concentrations present in the coolant. Next to this, thermal expansion and Mo-oxidation state ofPbMoO4, Pb2MoO5 and Cs2Pb(MoO4)2 were measured, in order to for example assess the mechanical interaction of these phases after formation.

The compound CsBi(MoO4)2 was studied as a possible formation product between LBE and JOG-phase. A long-standing issue in the understanding of the crystal structure of this compound has been solved using neutron diffraction. The thermal expansion of CsBi(MoO4)2 was determined.

To assess the interaction between coolant and volatile fission products, the system CsI-PbI2-BiI3 was studied experimentally. The low-temperature heat capacity of the three compounds in the system(CsPbI3, Cs4PbI6 and Cs3Bi2I9) were determined and the standard entropy was calculated. The phase diagrams CsI-PbI2, CsI-BiI3 and PbI2-BiI3 were measured using differential scanning calorimetry. A thermodynamic model was developed to predict the liquidus surface of the CsI-PbI2-BiI3 system. The accuracy of the model was confirmed by selective measurements of the ternary eutectics and the pseudo-binary CsPbI3-Cs3Bi2I9.

Study of the interaction between the grey-phase element Ba, fuel and coolant was initiated. During this work, a BaO-deficient plutonium-based perovskite with a composition close to Ba3PuO6 was synthesised. Its crystal structure was studied, as well as the phase transitions at high temperature. The standard entropy of the compound and magnetic susceptibility were determined experimentally. This work, valuable in itself as a contribution to the understanding of the irradiated nuclear fuel pin, is needed as a building block to study coolant-grey phase interaction.

Overall, this thesis describes potential chemical interaction products in the scenario of cladding failure in LFRs. In the concluding chapter, it is shown that the oxygen concentration present in operating conditions allows for the formation of several complex oxide compounds in case of Pb-JOG interaction. In general, this work provides new and necessary data to assess the stability of iodide and oxide compounds. The results present in this thesis should be combined with post-irradiation examination and kinetic studies to assess the scenario of cladding failure from different perspectives.
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The severe accident at the Fukushima-Daiichi Nuclear Power Station in 2011 has shown the necessity to study the impact of the release of hazardous fission products. This work investigates the Ba-Cs-Sr-Mo-O system, which contains some of the most abundantly produced fission products, as well as fission products that carry a great health risk on release. The study of this system is broken up into four subsystems: Ba-Sr-O, Ba-Mo-O, Sr-Mo-O and Ba-Cs-Mo-O. A literature study into the ternary Ba-Sr-O system, including existing thermodynamic models, showed the formation of no stoichiometric ternary compounds due to the mutual miscibility of Ba and Sr. Despite this mutual solubility, a miscibility gap is shown to be present in the solid region of the binary BaO-SrO phase diagram below a certain temperature. Thermogravimetric differential scanning calorimetry (TGDSC) investigations of the BaMoO₄ – MoO₃, SrMoO₄ – MoO₃ and BaMoO₄ – Cs₂MoO₄ pseudo-binary systems revealed likely compositions for the eutectic equilibria at 0.792 ≤ x(MoO₃) ≤ 0.80, 0.806 ≤ x(MoO₃) ≤ 0.82 and 0.909 ≤ x(Cs₂MoO₄) ≤ 0.976, respectively. These measurements also allowed for the development and optimisation of a new thermodynamic model of the BaMoO₄ – Cs₂MoO₄ system using the CALPHAD (Calculation of Phase Diagram) method. Syntheses of BaMoO₄, BaMo₃O₁₀, Ba₂MoO₅ and BaCs₂(MoO₄)₂ were successfully completed. A partially successful synthesis method was developed for Ba₃MoO₆ that needs further optimisation. The novel synthesis of Ba₂MoO₅ allowed for solution calorimetry measurements to be performed, leading to the determination of its standard enthalpy of formation Δ_fH°_m(298.15K, Ba₂MoO₅) = -(2169.0 ± 14.7) kJ/mol. Vapour pressure studies of BaMoO₄ by means of Knudsen EffusionMass Spectrometry (KEMS) gave insight into the composition of the vapour formed above BaMoO₄ after vaporisation. The results showed extensive influence of fragmentation reactions and only a small amount of congruent evaporation, indicated by the high partial pressure of BaO(g) and other binary molecules. A partial reduction of the BaMoO₄ sample to BaMoO₃ could have occurred, but this cannot be confirmed due to the full evaporation of the KEMS sample. Further studies are required to investigate a potential reduction. ... Read more

In the recent years, there has been a growing interest in molten salt reactors as a source of energy. To ensure molten salt reactor safety, it is vital to know the thermodynamic properties of the systems involved. An investigation into the uncertainty of the mixing enthalpy, excess heat capacity and Gibbs energy parameters of the LiF-KF system is presented in this study. The program FactSage 7.2 [19] is used, which takes optimized Gibbs energy parameters as an input and uses these to calculate phase diagram data and the values of different thermodynamic properties. The uncertainty is quantified using the polynomial chaos expansion, which analyzes the relationship between the input Gibbs energy parameters and the output; which is the phase diagram data, mixing enthalpy and excess heat capacity of the system. Firstly, an investigation into the accuracy of the polynomial chaos expansion, when applying different settings, is given. Once the most accurate settings are found, this expansion is used to generate many different samples of phase diagrams and the corresponding mixing enthalpy and excess heat capacity values. A margin of 10 Kelvin is then introduced as a maximum deviation from the experimentally determined phase diagram. The input Gibbs energy parameters and the mixing enthalpy and excess heat capacity values, that correlate with the phase diagrams within the margin of the experimentally determined phase diagram, can then be extracted. Once these values are known, the maximum uncertainty half width of the mixing enthalpy and excess heat capacity can be given that is still consistent with sensible phase diagrams. The found values for the maximum uncertainty half range are 1.65 kJ/mol for the mixing enthalpy which is a 35.6% deviation from the mixing enthalpy value computed with the original Gibbs energy parameters. For the excess heat capacity, 1.676 J/K/mol was found as a maximum uncertainty half range which is a 44.7% deviation from the excess heat capacity value computed with the original Gibbs energy parameters. The one-dimensional uncertainty half range (the uncertainty half range of one parameter assuming all other parameters possess zero uncertainty) of Gibbs energy parameters were calculated. Additionally, scattering plots are generated to illustrate the two- and three-dimensional uncertainty of the different Gibbs energy parameters. ... Read more