Bakker et a/.59 performed a critical review of the phase diagram and of the thermodynamic proper­ties of the UO2-ThO2 system. Solid UO2 and ThO2 form an ideal continuous solid solution. The phase diagram proposed by Bakker et a/.59 on the basis of the available experimental data is presented in Figure 9(a). The authors report the large uncer­tainties on the phase diagram because of the experi­mental difficulties. The thermodynamic properties of (Th1 _JU3,)O2 solid solutions have been recently investigated by Dash et a/.60 using a differential scanning calorimeter and a high-temperature drop calorimeter. The ternary compound ThUO5 was synthesized and characterized by X-ray diffraction. The thermodynamic data on this compound were estimated.

Like UO2, PuO2 forms a continuous solid solution with ThO2 in the whole composition range. Limited melting point data were measured by Freshley and Mattys.61 The results indicate a nearly constant melt­ing point up to 25wt.% ThO2. In view of the

instability of PuO2 at high temperature (high pO2 over PuO2), this behavior could be due to a change of the stoichiometry of the samples. The available liquidus temperature measurements do not repro­duce the recommended value for the melting point of PuO2. The full lines in Figure 9 give the solidus and liquidus curves considering an ideal behavior of the PuO2-ThO2 system.