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Mixed uranium and americium dioxides (U, Am)O 2±x are candidates as possible transmutation targets for generation IV reactors. In this work, the enthalpy increments of this solid solution were measured in the 470–1750 K temperature range by drop calorimetry for Am/(Am + U) ratios equal to 0.32, 0.39, 0.49, 0.58 and 0.68. Then, the heat capacity functions were obtained by derivation of the enthalpy data. The results of this work were compared to the heat capacity and enthalpy functions reported in the literature for the UO 2 [1] and AmO 2 [2] binary oxides and for the U 0.9 Am 0.1 O 2±x , U 0.8 Am 0.2 O 2±x mixed oxides [3]. From the obtained trend, it was found out that an excess contribution to the enthalpy increment appears for T > 1100 K in the compositions with Am/(Am + U)≥0.4 and a possible explanation attributing this effect to oxygen hypostoichiometry is provided. Finally, to verify the hypothesis, thermodynamic computations based on the CALPHAD method were performed for AmO 2-x under air and the results confirmed that the source of the excess contribution is the formation of oxygen vacancies.

The low-temperature heat capacity of the (Th1−y,Puy)O2 solid solution with y=0.03, 0.08, 0.30, 0.54 and 0.85 was determined from 4 K up to about 300 K with a PPMS instrument using the hybrid adiabatic relaxation method. In addition, the heat capacity of 239PuO2 end member was also measured and compared with available literature data for 242PuO2, observing an indistinguishable match. The obtained heat capacity values of the intermediate compositions are greater than the data obtained from Neumann–Kopp's molar additivity rule based on the ThO2 and PuO2 end-members, and a substantial excess capacity is observed for all intermediate compositions, particularly at the lowest temperatures.

The enthalpy increments of ( U 1 - y , Am y ) O 2 - x mixed oxides with y = 0.0877 and 0.1895 and x = 0.01–0.03 were measured using drop calorimetry in the temperature range 425–1790 K and the heat capacity was obtained as differential of the obtained enthalpy increments with respect to temperature. The thermal diffusivity was measured using the laser flash technique from 500 to 1550 K. The thermal conductivity was calculated from the measured thermal diffusivity, density and heat capacity. Measured enthalpy increments of the ( U 1 - y , Am y ) O 2 - x solid solutions are very close to the end members, indicating no excess contribution. The derived heat capacities for the two intermediate compositions are slightly higher than that of UO2 and in a good agreement with literature data of AmO2 up to 1100 K. For the thermal conductivity of (U, Am)O2−x mixed oxides a correlation using the classical phonon transport model in crystal structures is proposed.

The enthalpy increments of the (Th1-y, Puy)O2 solid solution with y= 0.03, 0.08, 0.30, 0.54 and 0.85 and the PuO2 and ThO2 end-members were measured using drop calorimetry in the temperature range 476-1790 K. The heat capacity was obtained by derivation of the obtained enthalpy data with respect to temperature. The presented results for PuO2 and ThO2 are compared with the available literature data whereas the results obtained for (Th1-y, Puy)O2 solid solutions were compared with the Neumann-Kopp's molar additivity rule to search for possible non-ideal behaviour. This paper presents the first heat capacity data obtained for this system., JRC.E.3-Materials research