Sintering Processes of the System Urania-Thoria

Abstract

Relating to the results of sinterings in the system urania-thoria in the author's preceding report, their processes were investigated by means of the thermogravimetric balance method and the dilatometry, and by the quenching experiments and the X-ray examinations. From their data the chemical and dimensional changes of their systems observed at the sintering at temperatures up to 1500°C were discussed. Specimens used in this study are the same as that described in the preceding paper.1) The sintering processes of the bar specimens (4×4×30mm) pressed at 3tons/cm2 were traced in air by means of the dilatometer (used silica or alumina as tube and rods materials) till 1400°C (Fig. 1). The slight shrinkages of 0.1-0.2% were observed till 300°C on the specimens with the compositions of thoria contents 20-30%. There were rapid shrinkages at about 800°C in all specimens and then followed by essential and large shrinkages in the range from 1100 to 1400°C, as shown in Fig. 1, whose temperature were higher with increasing thoria contents.2) A vaporisation of uranium component was observed in air at above 1100-1200°C on the specimen with larger uranium contents, but it was not occured in a nitrogen or argon atmosphere. This fact may be based on the presence of U3O8 in these specimens. The weight decreases at 1400°C in air are 0.18, 0.08 and 0.06%/hr in bodies of U3O8, 30% thoria, and 50% thoria specimens respectively (Fig. 2). Accordingly it is evident that the sintering in air is not so proper at the specimens containing larger content of uranium that the sintering in a neutral atmosphere is recommended.3) The dimensional changes of the bar specimens in the different atmospheres were determined at 800°C or a constant temperature up to 1400°C by means of the dilatometer (Figs. 4 and 5). It is noted that a small and slow contraction was observed in the change from air to nitrogen and a large and abrupt expansion or contraction in the change from nitrogen to hydrogen. In such a abrupt change at the reduction process, there was the contraction at lower temperature and the expansion at higher temperature. All of the specimens were disintegrated or cracked at the reduction process of 800°C, but these crackings were not observed at above 1200°C.4) From measurements of weight changes of sintered pellets containing 30% ThO2 in the change of atmosphere air-nitrogen-hydrogen at 1400°C, the weight change at the airnitrogen process was observed to be comparatively slow, whereas the abrupt increase and the abrupt decrease at oxidation and reduction process respectively (Figs. 6 and 7).5) The thermogravimetric curves of reduced or oxidized powder samples in the atmosphere of air or nitrogen showed the weight decrease at above 1000-1100°C (Fig. 8). The weight changes here were indicated by O/(U+Th) atomic ratio based upon the weights obtained when the samples were reduced at 800°C in hydrogen.6) The O/(U+Th) atomic ratio measurements and the X-ray examinations of the pressed specimens (12φ×1-2mm) were carried out after heated at each temperature between 800° and 1500C for 2 hrs in air or nitrogen and quenched into water or mercury. Results of the O/(U+Th) measurements are given in Fig. 9 which shows the decrease of these ratios at 1000°-1400°C.7) It was found from the X-ray examinations of the quenched samples that U3O8 phase only occured at above 2.6 of O/(U+Th) ratio, U3O8 phase plus solid solution phase of cubic fluorite type between 2.25 and 2.60, and cubic phase only between 2.0 and 2.25. The cell constants of cubic phases increased vigorously with lowering temperatures in the specimens contained U3O8 phase, since these cubic phases have a larger content of thoria than that in the starting specimens