High temperature heat capacities and electrical conductivities of UO 2 doped with Ti, Nb and Sc

Abstract

Heat capacities and electrical conductivities of (U -yM y)O 2 (M=Ti, y=0.007; M=Nb, y=0.010M=Sc, y=0.090) were measured by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. The heat capacity values of (U 0.993Ti 0.007)O 2 and (U 0.990Nb 0.010)O 2 were nearly equal to that of undoped UO 2 and no anomaly was seen in both heat capacity and the electrical conductivity curves up to 1500 K. On the other hand an anomalous increase in the heat capacity curve of (U 0.910Sc 0.090)O 2 was observed above 1300 K, similarly to the cases of (U - yGd y )O 2 and (U 1− 0 yLa y)O 2 y=0.044-0.142) found by the present authors. Assuming the anomalous increase is due to the formation of Frenkel defect of oxygen the enthalpy (δ H f) and the entropy of this process (Δ S f) were calculated from the excess heat capacity. The comparison with the previous values for (U 0.910La 0.090)O 2 and (U 0.899Gd 0.101)O 2 indicates the presence of the similar origin for the heat capacity anomaly found in th ese doped samples. No anomaly was seen in the electrical conductivity curve of (U 0.910Sc 0.090)O 2 around the onset temperature of the anomalous increase in the heat capacity curve. The difference in the onset temperatures of UO 2 doped with various cations such as Sc, La and Gd was discussed in relation to the change of the lattice constant calculated from the difference in ionic radius and the valency between host uranium ion and the dopant. The onset temperatures of UO 2 doped with Ti and Nb were also estimated from the change of the lattice constant.