The phase study was carried out, in relation to the effective fabrication of the ceramic nuclear fuel of UO₂-ThO₂ system, for obtaining informations of phases present on its sintering or heat treatment in various atmospheres. In addition it is also very interesting that non-stoichionetric UO_2+x and ThO₂ make a non-stoichiometric solid solution U_yTh_1-yO_2+x in various composition.
Ammonia solution was added to the mixed soluton of uranyl nitrate and thorium nitrate to co-precipitate the ammonium diuranate plus thorium hydroxide. The intimate mixture of U₃O₈ and ThO₂ containing a solid solution as starting meterial was obtained by calcination of the co-precipitate in air at 500°-800°C. This powder was pressed to small round pellets which were suspended in the furnace tube to hold at certain constant temperature (700°-1500°C) in an atmosphere (oxygen, air or nitrogen), and then quenched. The specimens obtained were examined with their O/(U+Th) atomic ratio by means of the thermal balance hydrogen reduction, and the phases present by X-ray diffraction analysis.
Fig. 1 shows the variation of O(U+Th) ratio against various temperatures in several atmosheres. Each curve has no break that was seen in the case of UO₂ only. The lattice constants of cubic solutions containing excess oxygen were given in Fig. 2. On these curves the minimum points were obtained. This shows that the lattice parameters of stoichiometric UO₂-ThO₂ solid solution decreased with the O/(U+Th) ratio till the minimum points and then increased again. This fact explains that the non-stoichiometric dissolution of oxygen into lattice made decrease the lattice parameter, reached a saturation, and then the lattice constant of cubic solution increased owing to the increase in thorium content in it by a separation of U₅O₁₃(UO_2.60) phase. In Fig. 3a of X-ray diffraction line intensities of the cubic solution versus O/(U+Th) ratio, an appearance of U₅O₁₃ phase begins above 2.25 of O/(U+Th) and a disappearance of cubic solution at about 2.6 (Fig. 3b).
In order to confirm the limit of disappearance of the cubic solution, the UO₂-ThO₂ compositions rich in UO₂ were examined and the results are shown in Fig. 4. There is no break at 2.60 of O/(U+Th) on the isobaric curve in air for the specimens of 95-5 and 90-10wt.% UO₂-ThO₂, and the X-ray diffraction line intensities of the cubic solid solution almost disappear at 2.64-2.65 of O/(U+Th) passing 2.60. This shows that the terminal composition coexisting with pure thoria without solid solution is found to be UO_2.65.
In the compositions rich in ThO₂(60-90%), the isobaric curves are shown in Fig. 5 for oxygen, air, or nitrogen atmosphere, and the changes of the lattice parameters of cubic solution phase with O/(U+Th) are given in Fig. 6. From the minimum point of each curve, oxygen content of saturation in non stoichiometric solid solution decreases above the 59±1% ThO₂ with increasing in thoria content, and then tends to zero in pure ThO₂. This shows that the terminal composition of U₅O₁₃ phase in equilibrium with ThO₂ or the saturated solid solution in UO_2.65 (Table 1).
The lattice parameters of the non-stoichiometric solution do not satisfy the Vegard rule as shown in Fig. 7. Thus the phase diagram of the UO₂-ThO₂-O system was obtained in the temperature range less than 1500°C, as shown in Fig. 9, and the lattice parameters obtained by using this Figure were compared with the actual data and the agreement was found in the composition range rich in UO₂ (Table 3). The formation of