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 produc
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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 ΔfH°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.
