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. @en

The low-temperature heat capacity of (U_1−yTh_y)O₂ and ²³⁸Pu-doped UO₂ samples were determined using hybrid adiabatic relaxation calorimetry. Results of the investigated systems revealed the presence of the magnetic transition specific for UO₂ in all three intermediate compositions of the uranium-thorium dioxide (y = 0.05, 0.09 and 0.12) and in the ²³⁸Pu-doped UO₂ around 25 K. The magnetic behaviour of UO₂ exposed to the high alpha dose from the ²³⁸Pu isotope was studied over time and it was found that 1.6% ²³⁸Pu affects the magnetic transition substantially, even after short period of time after annealing. In both systems the antiferromagnetic transition changes intensity, shape and Néel temperature with increasing Th-content and radiation dose, respectively, related to the increasing disorder on the crystal lattice resulting from substitution and defect creation.

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The low-temperature heat capacity of (U _1-y,Am_y)O _2−x solid solution with y = 0.0811 and 0.2005 and x = 0.01–0.03 was determined from a minimum of 12.52 K up to 297.1 K and from 9.77 K up to 302.3 K, respectively, using hybrid adiabatic relaxation calorimtry. The low temperature heat capacity results of the investigated system revealed the absence of the magnetic transition specific for UO₂ in the temperature region of 30 K. Since there are no experimental data available for AmO₂ in this temperature region, the results obtained for the intermediate compositions are validated based on the experimental data of UO₂ end-member and the low-temperature heat capacity computation of AmO₂. In the measured temperature interval, excess heat capacity was observed for the two investigated intermediate compositions, which is concluded to be dominated by self-radiation effects at very low temperature.

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The enthalpy increment data for the (Th,U)O₂ and (U,Pu)O₂ solid solutions are reviewed and complemented with new experimental data (400–1773 K) and many-body potential model simulations. The results of the review show that from room temperature up to about 2000 K the enthalpy data are in agreement with the additivity rule (Neumann-Kopp) in the whole composition range. Above 2000 K the effect of Oxygen Frenkel Pair (OFP) formation leads to an excess enthalpy (heat capacity) that is modeled using the enthalpy and entropy of OFP formation from the end-members. A good agreement with existing experimental work is observed, and a reasonable agreement with the results of the many-body potential model, which indicate the presence of the diffuse Bredig (superionic) transition that is not found in the experimental enthalpy increment data.

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