HEAT-CAPACITY MEASUREMENT OF U1-YGDYO2 (0.00-LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-0.142) FROM 310-K TO 1500-K

Heat capacity measurement of UO//2//. //0//0//4, U//0//. //9//5//6Gd//0//. //0//4//4O//2//. //0//0//0, U//0//. //9//2//7Gd//0//. //0//7//3O//1//. //9//9//8, U//0//. //8//9//9Gd//0//. //1//0//1O//2//. //0//0//1 and U//0//. //8//5//8Gd//0//. //1//4//2O//1//. //9//9//8 was conducted from 310 to 1500 K by direct heating pulse calorimetry. An increase in heat capacity of each sample was observed at high temperatures and the excess heat capacity increased as the gadolinium content of U//1// minus //yGd//yO//2 increased. The enthalpy of activation, DELTA H//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//. //9//5//6Gd//0//. //0//4//4O//2//. //0//0//0, U//0//. //9//2//7Gd//0//. //0//7//3O//1//. //9//9//8, U//0//. //8//9//9Gd//0//. //1//0//1O//2//. //0//0//1 and U//0//. //8//5//8Gd//0//. //1//4//2O//1//. //9//9//8, respectively. The enthalpy of activation for UO//2 is 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. The excess heat capacity originated from the predominant contribution of the formation of oxygen clusters and the small contribution of the formation of electron-hole pairs.