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.

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.