Thermodynamic properties of the actinide oxides solid solutions: A calorimetric study

The work presented in this thesis provides new data on the behaviour of the binary mixed actinide oxides of the following systems: (Th,Pu)O2, (U,Pu)O2, (Th,U)O2, (U,Am)O2. The results were obtained by performing measurements in a large temperature interval, for some compounds the thermodynamic properties were studied in the range from about 2 to 2400 K, from cryogenic to sub-melting state. The goal of the thesis was extending the knowledge of the thermophysical behaviour of the oxides containing different mixture of actinides in various ratios by performing analysis and use the results for providing and improving the foundational knowledge of such materials. Enthalpy, heat capacity, thermal diffusivity and thermal conductivity of the investigated mixed oxides were determined. In addition, the effect of dilution and radiation damage on the magnetic transition present in uranium dioxide-based solid solutions was studied. Since most of the studied samples are highly radioactive, they were prepared in limited quantities in glove boxes, and techniques suitable for measuring thermal properties of small samples (< 100 mg) were used. Experiments have been performed in the low- and high-temperature regimes. Heat capacity was either measured directly or indirectly by measuring the enthalpy increments using different calorimeter types. Thermal diffusivity was measured in the temperature range from 500 to 1550 K, with a laser flash instrument, and thermal conductivity was determined from this.