Freshley and Mattys [41, 42] have shown that ThO2 and PuO2 form a complete solid solution (Fig. 2) in the whole composition range like ThO2 and UO2. A continuous series of solid solution has also been reported by Mulford and Ellinger [44]. They found only a single fluorite structure by X-ray diffraction (XRD) and also showed that the lattice parameter varied linearly with composition.

Dawson [45] has made magnetic susceptibility measurements on PuO2 and ThO2 mixtures and implied that PuO2 and ThO2 form solid solutions which follow the Vegard’s law. The lattice parameter of fluorite type cubic phase was found to decrease regularly from 0.5597 nm for ThO2 to 0.5396 nm for pure PuO2 [43]. Due to the limited amount of experimental data, a more accurate assessment of the phase diagram is not yet possible. The melting point of the ThO2-PuO2 solid solutions, containing various amounts of PuO2, was measured in helium. The melting point of the specimens containing less than 25 % ThO2 was found to be unchanged as shown in Fig. 3 [42]. The melting point of a PuO2 sample (whose purity was not specified) used in their study is 2,533 K, which is 130 K lower than the assessed value of Table 2.

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