Phase diagram studies of the ThO2-UO2 system have been reported by several authors. Lambertson et al. [21] used a quench technique, Christensen [27, 37] a tungsten filament technique, whereas Latta et al. [34] applied a thermal arrest method. The phase diagram of ThO2-UO2 can be constructed with the help of the melting points and enthalpies of fusion of the end members, assuming ideal solid solution behavior in both liquid and solid state. The results of the phase diagram measurements are given in Fig. 1 showing UO2 and ThO2 form a continuous series of solid solutions. The measurements of Christensen [27] and Latta et al. [34] show a shallow minimum at 2 and 5 mol% ThO2, respectively. The melting points calculated for some intermediary composition are shown below in Table 3 [38-49].

There is no direct experimental measurement on the heat of fusion of ThO2 or ThO2-UO2 solid solution. The most probable value is that of Fink for UO2 as 74.8 ± 1 kJ/mol [50]. The recommended value for ThO2 is 90.8 kJ/mol [40].