High temperature heat capacities and electrical conductivities of UO 2 doped with yttrium and simulated fission products

Abstract

Heat capacities and electrical conductivities of (U 0.190M 0.090)O 2 (M = Y and simulated fission products (FP) for 10% burnup) were measured simultaneously by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. Anomalous increases in the heat capacity curves of UO 2 doped with yttrium and simulated fission products were observed above about 1100 and 600 K, respectively. These anomalous increases were similar to those of ( U 1− y M y ) O 2 (M = Gd, La, Eu and Sc) found previously by the authors. The values for the enthalpy and entropy of defect formation in (U 0.910M 0.090)O 2 (M = Y and FP) were calculated from the excess heat capacity assuming the presence of Frenkel pairs of oxygen, and were found to be similar to that for UO 2 doped with Gd, La, Eu and Sc. On the other hand, no anomaly was seen in the electrical conductivity curve around the onset temperature of the anomalous increase in the heat capacity of doped UO 2. It was, therefore, concluded that the excess heat capacity originates from the predominant contribution of the formation of Frenkel pair-like defects of oxygen and from the small contribution of the formation of electron-hole pairs. The difference in the onset temperatures of UO 2 doped with various cations was thought to be originated from that of the elastic strain induced by the change of the lattice parameters of doped UO 2.