酸化ウラン-酸化トリウム系の焼結酸化ウラン-酸化トリウム系焼結体の研究 (第1報)

It is known that stoichiometric compositions of the urania-thoria system form a series of continuous solid solutions having some better properties as a nuclear ceramic fuel than urania only in oxidation resistivity, refractoriness, and usefulness of thoria as breeding materials. The sintered bodies of thoria-rich portion in the system have been studied since 1956-7 in Argonne National Laboratory, and subjected to the reactor tests, and going to be applied to fuel materials of power reactor.The authors have already published the results on the oxidation resistance and the sintering of the urania-rich portion of this system. This paper is concerned with their sinterings in air and reductions to stoichiometric compositions.The starting materials were the products of firing in air the coprecipitated mixtures of ammonium diuranate and thorium hydroxide obtained from mixed nitrate solutions of uranium and thorium by adding ammonia solution. The variation of pH by adding ammonium hydroxide to the mixed nitrate solution is shown in Fig. 1, and the calcination processes of the coprecipitates in air were traced by means of the thermogravimetric balance, with the results as given in Fig. 2. The tapped densities and Blain values of the calcinated powders in air at 800°C for 2 hrs. were measured, and also subjected to X-ray diffraction test. The results shown in Fig. 3 indicate that the grain size of he powders of containing 10-30% ThO2 are very small because of the hindrance of the crystal growth in the mixed phase of U3O8 and solid solution. The content of U3O8 phase has decreased and that of solid solution of fluorite type has increased as thorias were added to urania. U3O8 phase has disappered in the compositions containing thoria more than 60 weight%, forming only the solid solutions.The powders calcinated in air at 800°C were pressed at 3tons/cm2 to pellets of about 10mm in diameter, and sintered in air and reduced by CO gas. The results are given in Fig. 4 which show that the compacts containing 10, 20, and 30 wt.% of thoria have perfectly sintered at 1400°C, and those with 50% of thoria at 1500°C. The reduction of sintered pellets to stoichiometric composition by CO gas could be made more perfectly at higher sintering temperature, but the specimens containing less than 10% of ThO2 have cracked, as given in Table 1.The calcinated powder containing ThO2 30% was too fine to be pressed, and therefore the raising of calcination temperature up to 1000°C was tried. The sintering of these specimens showed a good results with the powder calcinated at 1000°C (Table 2). On the contrary the powders containing ThO2 50% was difficult to sinter at 1400°C, but this was possible by using the powder calcinated at 600°C (Table 3).The sintering in air of the uranium-rich specimens showed some faults of the volatilization of uranium component accompanying a large dimengional change at the reducing step. Therefore, the sinterings in neutral atomosphere, followed by hydrogen reduction were made at 1300°-1400°C. These sintering properties showed no particular differences between the atomosphere of nitrogen and carbon dioxide, and it was confirmed that good results may be obtained when the samples of 70-30 and 50-50 stoichiometric composition are sintered at 1400°C. The sintering processes will be reported in a following paper.