Heat Capacity Measurements of U_<1-y>Gd_yO_2 (y=0-0.27) from 325 to 1,673K

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Molar heat capacities were measured for U1−yGdyO2 (y=0–0.27) by differential scanning calorimetry (DSC) in the temperature region from 325 to 1,673 K. The molar heat capacities of U1−yGdyO2 (0<y<0.27) were close to, or slightly lower than, the values calculated by using the summation rule, and anomalous increases of heat capacities were not clearly observed at temperatures above 1,000 K. The heat capacity analysis for (U, Gd)O2 showed that the excess heat capacities of (U, Gd)O2 tended to decrease with increasing cation concentration excluding U4+ ions in crystals. This suggested that the Schottky term contribution to the heat capacity, which was mainly caused by the excitation of f electrons in the 5f2 configuration in U4+ ions, decreased in (U, Gd)O2, compared to that in undoped UO2 due to the formation of U5+ and Gd3+ ions in (U, Gd)O2 by addition of Gd2O3. Thermal functions of (U, Gd)O2 were also evaluated.
2004-02-25