Patent Number: 054229200
Section: summary

TECHNICAL FIELD The present invention relates to a method for estimating crystal grain sizes of uranium dioxide (UO.sub.2) sintered pellets to be used as nuclear fuel. In particular, it relates to a method for estimating crystal grain sizes of UO.sub.2 sintered pellets according to oxidation behavior of UO.sub.2 powder. BACKGROUND ART Such UO.sub.2 sintered pellets are tightly enclosed in coating tubes made of zircaloy, and are used as nuclear fuel. Recently, in order to make the life of the nuclear fuel to be long to enable continuous operation for a long period of light water reactors or fast breeder reactors, it is advanced to realize a high degree of combustion of nuclear fuel. When the nuclear fuel is allowed to have a high degree of combustion, the amount of fission products (FP) generated from nuclear fuel pellets is increased. Among the products, gaseous one such as radon (Rn) scarcely make solid solutions in the matrix of the nuclear fuel pellet, which diffuse into the crystal grain boundary and generate bubbles there. Swelling occurs due to the bubble formation, and the volume of the pellet increases to give stress to the coating tube. This makes a cause to generate a mechanical interaction (PCI, Pellet Clad Interaction) between the pellet and the coating tube. In addition, the FP gas diffused into the grain boundary is released to the exterior of the pellet later, which increases the internal pressure of the fuel rod to make a cause to decrease the thermal conductivity of the gap between the pellet and the; coating tube. In order to prevent the increase in PCI and the decrease in the thermal conductivity, it has been attempted that the nuclear fuel pellet is allowed to have a large grain size so as to enclose the FP gas in the pellet. This is based on the fact that although the generation of the FP gas itself cannot be suppressed, when the pellet is allowed to have a large grain size, for example, when the crystal grain size is made to be two-fold, the arriving distance to the grain boundary of the FP gas generated in the crystal grain becomes two-fold, and consequently the release speed of the FP gas becomes half. Until now, methods for increasing the crystal grain size of the UO.sub.2 sintered pellet have been disclosed in Unexamined Published Japanese Patent Application No. 2-242195/1990; Unexamined Published Japanese Patent Application No. 3-287096/1991; Unexamined Published Japanese Patent Application No. 4-70594/1992 and the like. According to these methods, nuclear fuel pellets having crystals of a large grain size of 20-120 .mu.m are obtained. In the prior art, not only for the nuclear fuel pellets produced by these methods as a matter of course, but also for nuclear fuel pellets produced by other methods, the crystal grain size has been mainly measured by a cross-sectional method defined in accordance with ASTM E-112. In this cross-sectional method, at first a produced UO.sub.2 sintered pellet is embedded in a synthetic resin, the pellet embedded in the resin is cut, and then its cross section is polished. Next, a wet etching treatment is performed to expose crystal grain boundaries of the pellet, and then the crystal grain boundaries are photographed by an optical microscope or the like. Next, in a state in which a scale line having a predetermined length is projected on a screen using a slide type projector, a photographed negative film is projected on the same screen so as to overlay a grain boundary texture to the scale line. The number of grains intersecting the scale line on the screen is measured at a plurality of places by sliding the negative film, and an average value of crystal grain sizes is determined according to the grain number. However, in the above-mentioned cross-sectional method, there is such an advantage that the crystal grain size of the UO.sub.2 sintered pellet is directly observed by the photographing with the optical microscope or the like, and a value relatively having a high accuracy is obtained, but on the contrary, it is necessary that every time when the measurement is performed, the UO.sub.2 powder is placed in a mold frame to conduct formation and calcination to make the sintered pellet, as well as complicated works for preparation of the measurement are required, and fine operation and observation must be performed. For example, when UO.sub.2 sintered pellets having large crystal grain sizes are produced for a high degree of combustion of nuclear fuel, in order to decrease the ratio of deficiency of the sintered pellets, it is necessary to perform judgment of the suitability of raw material powders for the sintered pellets beforehand. However, in the case of the conventional measurement method, there has been such a problem that relatively much time is consumed for this judgment, and consequently management cost for the raw material powder is raised. An object of the present invention is to provide a method for estimating crystal grain sizes of UO.sub.2 sintered pellets without actually producing UO.sub.2 sintered pellets. Another object of the present invention is to provide a method for estimating crystal grain sizes of nuclear fuel pellets in which when UO.sub.2 sintered pellets having large crystal grain sizes are produced for realizing a high degree of combustion of nuclear fuel, the judgment of suitability of raw material powders for the sintered pellets can be performed rapidly and economically. DISCLOSURE OF THE INVENTION In order to achieve the above-mentioned objects, a method for estimating crystal grain sizes of nuclear fuel pellets according to the present invention includes the following procedures as shown in FIG. 1. (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 (c); (e) recognizing a correlation between the U.sub.3 O.sub.7 phase arrival temperature determined in (b) and the crystal grain size of the sintered pellet measured in (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 (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 0.sub.7 phase arrival temperature determined in (f) and the correlation determined in (e). The above-mentioned procedures of (a) to (e) of the present invention are basic procedures for determining the correlation between the U.sub.3 O.sub.7 phase arrival temperature of the UO.sub.2 powder and the crystal grain size of the sintered pellet produced with the powder, and the above-mentioned procedures of (f) and (g) are procedures for estimating the crystal grain size of the sintered pellet produced from the UO.sub.2 powder of the test sample. Once when the above-mentioned basic procedures of (a) to (e) are established only the above-mentioned procedures (f) and (g) for estimating the crystal grain size of the sintered pellet of the UO.sub.2 powder of the test sample are performed repeatedly. At first, a predetermined amount of the UO.sub.2 powder is heated at a predetermined temperature raising speed while allowing a constant amount of dry air to flow, thereby it is oxidized by oxygen in the dry air. As the UO.sub.2 powder to be oxidized, in order to provide different values of U.sub.3 O.sub.7 phase arrival temperature described hereinafter as far as possible, a plurality types of powders having various average grain sizes are prepared. It is known that when the UO.sub.2 powder is oxidized under the above-mentioned condition, the powder changes from the UO.sub.2+x phase to the U.sub.3 O.sub.7 phase, which ultimately becomes the U.sub.3 O.sub.8 phase to be stabilized. The phase-change of the powder can be known from the change ratio of the powder weight, so that the weight change ratio of the powder is measured. This measurement is performed by means of a commonly used thermogravimetric analysis apparatus (thermobalance). The weight change ratio is determined by measuring an increment in weight per unit time because the temperature raising speed is constant. According to the value at which the weight change ratio changes, the temperature of the arrival from the UO.sub.2+x phase to the U.sub.3 O.sub.7 phase is determined. It is preferable that the arrival temperature is determined, after drawing an oxidation curve of the UO.sub.2 powder as shown in FIG. 2, from a inflection point P of the curve. In FIG. 2, the axis of ordinate is the weight change ratio, and the axis of abscissa is the oxidation temperature. The inflection point Q indicates the U.sub.3 O.sub.8 phase arrival temperature. Using raw materials of a plurality types of UO.sub.2 powders having different U.sub.3 O.sub.7 phase arrival temperatures, sintered pellets are produced respectively under the same condition by means of a known method. Concretely, the production is performed such that a lubricant is added to the UO.sub.2 powder to perform powder-pressing formation to provide a green pellet, next the lubricant is removed, and thereafter sintering is performed in a hydrogen gas flow at a specified temperature in a range of 1400.degree.-1800.degree. C. The crystal grain sizes of produced plurality types of UO.sub.2 sintered pellets are measured by means of the following method. At first the produced UO.sub.2 sintered pellet is embedded in a synthetic resin, the pellet embedded in the resin is cut, and thereafter its cross section is polished. Next, after the polishing, an etching treatment is performed to expose crystal grain boundaries, and the measurement is performed by the above-mentioned cross-sectional method using a negative film photographed by an optical microscope or the like. As the synthetic resin in which the UO.sub.2 sintered pellet is embedded, acrylic type, silicone type, vinyl type and the like can be exemplified, however, the acrylic type is preferable. The U.sub.3 O.sub.7 phase arrival temperatures of the UO.sub.2 powder before the production of the sintered pellets and the values of the crystal grain sizes determined by the above-mentioned cross-sectional method are plotted on a chart shown in FIG. 3, and a curve R, which indicates a correlation between the U.sub.3 O.sub.7 arrival temperature and the crystal grain size of the sintered pellet, is drawn. On the basis of the obtained correlation, a UO.sub.2 powder for which the crystal grain size is intended to be estimated is prepared. The UO.sub.2 powder as a test sample is heated under the same condition as that of (a) as described above so as to measure its weight change ratio, and using the same procedure as (b) as described above its U.sub.3 O.sub.7 phase arrival temperature is determined from its oxidation curve. The determined U.sub.3 O.sub.7 phase arrival temperature is attributed to the curve R showing the above-mentioned correlation, and the crystal grain size upon production into a sintered pellet of the UO.sub.2 powder of the test sample is estimated. Incidentally, in the present invention, the weight change ratio is used as a parameter for obtaining the oxidation curve of the UO.sub.2 powder, however, according to an oxidation curve using a parameter of a calorific value corresponding to chemical energy required during change of crystals of uranium oxide, it is also possible to determine the temperature of arrival from the UO.sub.2 powder to the U.sub.3 O.sub.7 phase.