Patent Number: 054229200
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION An example of the present invention will be explained in detail on the basis of the drawings. 16 types of UO.sub.2 powders having different average particle sizes were weighed by about 10 mg respectively using a chemical balance. The weighed UO.sub.2 powder was individually placed into a quartz boat of a thermogravimetric analysis apparatus (TBA-50 model, made by Shimadzu Corporation), and after inputting the weighed value into this apparatus, heating was performed under the following heating condition, and the weight change ratio with respect to the heating temperature was determined. The heating and measurement were performed systematically under automatic control, and 16 types of oxidation curves were obtained. One example thereof is shown in FIG. 2. Air flow amount at the inlet of the apparatus: 10 mL/minute Moisture of dry air (dew point): not more than -70.degree. C. Temperature of initiation of temperature raising: room temperature Temperature raising speed: 2.degree. C./minute According to inflection points P initially appearing in the oxidation curves, temperatures of the arrival of the powders to the U.sub.3 O.sub.7 phase were determined respectively. In this example, the arrival temperatures of the 16 types of the UO.sub.2 powders to the U.sub.3 O.sub.7 phase were distributed in a wide range from about 170.degree. C. to about 250.degree. C. From the above-mentioned 16 types of the UO.sub.2 powders were produced 16 individuals of UO.sub.2 sintered pellets by means of the following method. At first, 0.2% of zinc stearate was added to and mixed with the UO.sub.2 powder as a lubricant, and this mixture was placed in a mold to form a green pellet having a diameter of 10 mm and a height of 15 mm under a pressure of about 2 t/cm.sup.2. Next, this formed article was sintered in a hydrogen gas flow at 1750.degree. C. for 5 hours to obtain a UO.sub.2 sintered pellet. Next, the obtained UO.sub.2 sintered pellet was placed in a cylindrical container, an acrylic ester resin was poured around it to embed the pellet in the resin, and thereafter bubbles in the resin were removed using a reduced-pressure pump. Subsequently the pellet embedded in the resin was cut. Its cut face was polished. and then the polished face is etched with a hydrofluoric acid solution to expose crystal grain boundaries. The exposed crystal grain boundaries were observed with an optical microscope, and a negative film of the crystal grain boundaries was manufactured. Using this negative film, according to the cross-sectional method in accordance with ASTM E-112, crystal grain sizes were measured from 16 individuals of the UO.sub.2 sintered pellets respectively. A relation between the arrival temperatures to the U.sub.3 O.sub.7 phase of the above-mentioned 16 types of the UO.sub.2 powders and the crystal grain sizes of the UO.sub.2 sintered pellets manufactured from each of the UO.sub.2 powders was plotted, and a curve R showing the correlation between the U.sub.3 O.sub.7 phase arrival temperature and the crystal grain size shown in FIG. 3 was obtained. Subsequently, about 10 mg of a UO.sub.2 powder of a test sample was weighed, and the thermogravimetric analysis was performed by means of the same operation as described above to obtain an oxidation curve. As shown in FIG. 2, the temperature of arrival of this powder to the U.sub.3 O.sub.7 phase was 210.degree. C. according to an inflection point P initially appeared on the oxidation curve. This temperature of 210.degree. C. was allowed to correspond to the axis of abscissa in FIG. 3, and according to an intersection (a) with the curve R, it was estimated that a UO.sub.2 sintered pellet to be manufactured from the UO.sub.2 powder of the test sample was estimated to have a crystal grain size of about 18 .mu.m. In order to confirm reliability of the present invention, when a UO.sub.2 sintered pellet was manufactured in the same manner as described above using the same UO.sub.2 powder as the above-mentioned test sample, and the crystal grain size was directly measured by means of the above-mentioned cross-sectional method, then the crystal grain size was about 19 .mu.m. Thereby it was found that the crystal grain size of the UO.sub.2 sintered pellet estimated by the present invention is approximately coincident with the crystal grain size measured directly, and the present invention is a reasonable estimating method. As described above, according to the present invention, when the crystal grain size of the UO.sub.2 sintered pellet manufactured from the UO.sub.2 powder is determined, the crystal grain size can be estimated from the oxidation behavior of the UO.sub.2 powder without actually manufacturing a sintered pellet. Thereby the measurement cost for the crystal grain size can be reduced. Especially, when a UO.sub.2 sintered pellet having a large crystal grain size is produced in order to realize a high degree of combustion of nuclear fuel, the judgment of suitability of a raw material powder for the sintered pellet can be performed rapidly and economically, and the ratio of occurrence of an unexpected sintered pellet having a small crystal grain size can be suppressed to be low. INDUSTRIAL APPLICABILITY The method of the present invention is useful for rapidly and economically performing the judgment of suitability of raw material powders for sintered pellets having large crystal grain sizes for realizing a high degree of combustion of nuclear fuel.