Heat capacity measurement of U1−yGdyO2 (0.00 ≦ y ≦ 0.142) from 310 to 1500 K

Abstract

Heat capacity measurement of UO 2.004, U 0.956Gd 0.044O 2.000 U 0.927Gd 0.073O 1.998,U 0.101Gd 0.101O 2.001 and U 0.858Gd 0.142O 1.998 was conducted from 310 to 1500 K by means of direct heating pulse calorimetry. An increase in the heat capacity of each sample was observed at high temperatures and the excess heat capacity increased as the gadolinium content of U 1−y Gd y O 2 increased. The enthalpy of activation, ΔH f , of the thermally activated process which causes the excess heat capacity was obtained as 2.25, 1.88, 1.75,1.10 eV for U 0.956Gd 0.044O 0.073. U 0.927Gd 0.073O 1.998, U 0.899Gd 0.101O 2.001 and U 0.858Gd 0.142O 1.998, respectively. The enthalpy of activation for UO 2 was estimated to be about 3.1 eV by extrapolation to UO 2, which is in good agreement with that obtained by Szwarc for the excess heat capacity of UO 2. This value is higher than that of the thermally activated process for the formation of an electron-hole pair obtained by the reported theoretical calculation, but lower than that for the formation of Frenkel pairs of oxygen. It may be concluded that the excess heat capacity is originated from the predominant contribution of the formation of oxygen clusters and from the small contribution of the formation of electron-hole pairs.