Patent Number: 054229200
Section: claims

1. A method for estimating crystal grain sizes of nuclear fuel pellets comprising: (a) heating a plurality types of UO.sub.2 powders of a predetermined amount at a predetermined temperature raising speed in dry air of a constant flow amount, thereby measuring weight change ratios occurring due to the oxidation of each of the UO.sub.2 powders;  (b) determining for each kind of UO.sub.2 powders a temperature at which a composition of the powder arrives at from the UO.sub.2+x phase to the U.sub.3 O.sub.7 phase, on the basis of a change in the weight change ratios;  (c) producing UO.sub.2 sintered pellets from the plurality types of UO.sub.2 powders in which the arrival temperatures are known;  (d) measuring the crystal grain sizes of the plurality types of the sintered pellets produced in the (c);  (e) recognizing a correlation between the U.sub.3 O.sub.7 phase arrival temperature determined in the (b) and the crystal grain size of the sintered pellet measured in the (d);  (f) determining a U.sub.3 O.sub.7 phase arrival temperature of a UO.sub.2 powder of a test sample under the same conditions as those in the (a) and (b); and  (g) estimating a crystal grain size of the UO.sub.2 powder of the test sample upon production into a sintered pellet, according to the U.sub.3 O.sub.7 phase arrival temperature determined in the (f) and the correlation determined in the (e). 2. The method for estimating crystal grain sizes of nuclear fuel pellets according to claim 1 wherein the value at which the weight change ratio changes in said (b) is determined, after drawing an oxidation curve of the UO.sub.2 powder, from an inflection point of the curve. 3. The method for estimating crystal grain sizes of nuclear fuel pellets according to claim 1 wherein the measurement of the crystal grain size of the sintered pellet in said (d) is performed such that a cross section of the UO.sub.2 sintered pellet is polished, thereafter an etching treatment is performed to expose crystal grain boundaries, and image information obtained by an optical microscope or a scanning type electron microscope is used by means of a cross-sectional method.