[SY-B7] Thermal properties of fluorite-type metal dioxides: CeO2, ThO2, UO2, NpO2, PuO2 and AmO2
Invited
Actinide dioxide having a fluorite structure is one of prospective candidates as a fuel of advanced reactors. Thermal property of nuclear fuels is essential data to evaluate fuel performance. Therefore, great efforts to measure properties and understand their mechanism have been made so far. It is well-known that thermal properties of actinide oxides changes significantly depending on temperature, and their mechanism is complicated. Author’s research group has studied on thermal properties of CeO2, UO2, PuO2 and their solid solution. In this work, basic properties of various fluorite-type dioxides were summarized, and temperature dependence of thermal properties were evaluated.
In fluorite-type oxides of CeO2, ThO2, UO2, NpO2, PuO2 and AmO2, mechanical property, Debye temperature, Grunisen constant, thermal expansion, oxygen potential, heat capacity Cp and thermal conductivity were reviewed, and the data were compared. Heat capacity at volume constant Cv and thermal expansion term Cd were considered from their data in heat capacity evaluation. The calculation result of (Cv+Cd) was in good agreement with experimental data in CeO2 and ThO2, but the calculated data of other actinide dioxides underestimated the experimental data by 10-20 J/molK. The extra Cp from (Cv+Cd) was considered as a Schottky term Csch which related to 5f electrons. In fact, 5 f electrons do not exist in CeO2 and ThO2. Experimental data of UO2, NpO2, PuO2 and AmO2 were described with the calculated (Cv+Cd+Csch). The extra Cp was described assuming two level energy model.
Thermal conductivity of the dioxides was evaluated by phonon conduction mechanism using Slack’s equation. Phonon conduction of pure oxide can be represented by 1/(BT). Thermal conductivity of CeO2 and ThO2 was represented by Slack’s equation very well. But, other conduction mechanism is needed to evaluate thermal conductivity of UO2, NpO2, PuO2 and AmO2.
It is reported that excited term at high temperatures which is contributed with Frenckel defect, electron and so on exists in heat capacity and thermal conductivity. The experimental data at high temperatures are lacked to evaluate the excited terms. In addition of experimental data in high temperature region, computational approach is expected to understand deeply mechanism in property change.
In fluorite-type oxides of CeO2, ThO2, UO2, NpO2, PuO2 and AmO2, mechanical property, Debye temperature, Grunisen constant, thermal expansion, oxygen potential, heat capacity Cp and thermal conductivity were reviewed, and the data were compared. Heat capacity at volume constant Cv and thermal expansion term Cd were considered from their data in heat capacity evaluation. The calculation result of (Cv+Cd) was in good agreement with experimental data in CeO2 and ThO2, but the calculated data of other actinide dioxides underestimated the experimental data by 10-20 J/molK. The extra Cp from (Cv+Cd) was considered as a Schottky term Csch which related to 5f electrons. In fact, 5 f electrons do not exist in CeO2 and ThO2. Experimental data of UO2, NpO2, PuO2 and AmO2 were described with the calculated (Cv+Cd+Csch). The extra Cp was described assuming two level energy model.
Thermal conductivity of the dioxides was evaluated by phonon conduction mechanism using Slack’s equation. Phonon conduction of pure oxide can be represented by 1/(BT). Thermal conductivity of CeO2 and ThO2 was represented by Slack’s equation very well. But, other conduction mechanism is needed to evaluate thermal conductivity of UO2, NpO2, PuO2 and AmO2.
It is reported that excited term at high temperatures which is contributed with Frenckel defect, electron and so on exists in heat capacity and thermal conductivity. The experimental data at high temperatures are lacked to evaluate the excited terms. In addition of experimental data in high temperature region, computational approach is expected to understand deeply mechanism in property change.