Steam Sintering of Uranium Dioxide 179 . IV. Summary the primary-phase surface of silica. The ternary critical solution curve originates at the Critical point of the binary system BaO-B2Q3 and proceeds, approximately linearly, to 1405°C. and the composition 18 weight ' % BaO; 10.5% B203, 71.5% SiOz. By extrapolation of the ternary critical solution curve, the critical point of the postulated metastable immiscibility region weight ' % BaO. According to theory, the temperature difference of almost 250'C. between the liquidus and 1430" seems excessively large, considering the flat nature of the liquidus curve.The shape of the immiscibility volume in the system BaO-B203-Si02 was determined from data on seventeen composimaking small experimental glass melts. Dome temperatures were obtained mainly with a temperature-gradient apparatus.An upper critical solution temperature in the binary system tions. The method of sample preparation was that used for its On the silica primaq-phase surface, at about BaO-B203 was found to exist at 1225°C. and about 15 weight in the binary system BaO-SiOn is located at 1430°C. and 18.5 % BaO.One end of the immiscibility volume of the ternary system originates in the binary system BaO-B20a and rests on the primary-phase surface of BaO .4B203. The other end rests onThe following variables were considered in an investigation of the sintering of uranium dioxide: (1) sintering aids, i.e., small additions of substances that might aid the densification process, (2) soaking temperature, (3) soaking time, (4) furnace atmosphere, and (5) particle size of the uranium dioxide. The incipient sintering temperature, the effect of binders, and the effect of fabricating pressure also were determined. The data obtained in the investigation led to the following optimum conditions for obtaining dense UOz by cold pressing and sintering : (1) preparing the UOa as a submicron powder but not fine enough to be pyrophoric, (2) cold pressing at 20 to 30 tons per sq. in. without the use of organic binders, (3) using the following sintering schedule: (a) in Hz to 1400°C., (b) in steam or a wet inert gas from 1400' to 15OO0C., (c) in steam or a wet inert gas at 15OO0C. for a 1-hour soak, and (d) cooling in Hz from 1500°C. to room temperature. By following this procedure, ?ellets of UOz (% in. in diameter by l,/* in. lugh) were obtained that were 97 to 98% of theoretical density.
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