Patent Number: 052572986
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, manufacturing processes according to one embodiment of the present invention will be described with reference to FIG. 1. First, a sintering agent was manufactured in the following manner. Specifically, Al.sub.2 O.sub.3 of about 40 wt % and SiO.sub.2 of about 60 wt % were coarsely mixed. The mixture thereof was exposed to a mixed gas flow of 8%-H.sub.2 /N.sub.2, and was heated up to about 2100.degree. C., and then melted. Thereafter, the melt was cooled, and homogeneous aluminum silicate was obtained. The thus obtained aluminum silicate was then ground. As a result, uniform powder was obtained as a sintering agent. By use of this sintering agent, nuclear fuel pellets were manufactured in accordance with processes shown in FIG. 1. Specifically, the sintering agent was added to UO.sub.2 powder, and then Gd.sub.2 O.sub.3 powder was mixed. Further, a lubricant (stearic acid, polyethylene glycol and the like) was added, as shown in FIG. 1. This mixture was compacted by an uniaxial press and then green pellets were obtained. The adding amount of the sintering agent was about 30 through about 500 ppm, and the adding amount of the Gd.sub.2 O.sub.3 powder was about 10 wt %, both with respect to the total amount of UO.sub.2, Gd.sub.2 O.sub.3 and the above-described sintering agent. Next, the thus obtained green pellets were processed in a degreasing process. Thereafter, the green pellets were sintered in a humid hydrogen atmosphere at about 1760.degree. C. for about 5.6 hours. In some cases, the lubricant-mixing process and the degreasing process may be omitted. The sintering density and average grain diameter of the thus manufactured nuclear fuel pellets were compared with those of the nuclear fuel pellets obtained by use of the sintering agent of adding amount of 0.25 wt %. The results of comparison are as follows: ______________________________________ sintered average grain adding amount of density diameter sintering agent (g/cm.sup.3) (.mu.m) ______________________________________ 30 ppm 10.40 27.2 70 ppm 10.34 25.6 130 ppm 10.39 25.7 250 ppm 10.37 32.7 500 ppm 10.33 33.3 0.25 wt % 10.20 24.7 ______________________________________ As can be seen from the comparison results, the sintered densities of the pellets manufactured in accordance with this embodiment are significantly higher than that of the pellet manufactured with addition of the sintering agent by 0.25 wt %. Further, the grain diameters of the pellets of this invention are also increased. For the sake of comparison, FIG. 2 and FIG. 3 respectively show the microstructure of each nuclear fuel pellet after polished and chemically etched, when observed by a microscope. Specifically, FIG. 2 shows a UO.sub.2 -10 wt % Gd.sub.2 O.sub.3 nuclear fuel pellet including the sintering agent of 30 ppm according to this embodiment. FIG. 3 shows conventional a UO.sub.2 -10 wt % Gd.sub.2 O.sub.3 without a sintering agent. As can be seen from FIG. 2 and FIG. 3, in the pellet of this embodiment (FIG. 2), the portions of free UO.sub.2 phase, which are indicated by the shaded portions (blue portions in the actual microphotograph), are much smaller (2 vol % at a maximum) than those shown in FIG. 3. In this embodiment according to the present invention, a humid hydrogen gas was used as a sintering atmosphere. However, a mixed gas of carbon monoxide and carbon dioxide may also be used as a sintering atmosphere. Further, in this embodiment, aluminum oxide and silicon oxide were mixed and melted, and then aluminum silicate was obtained as a sintering agent. However, besides this, the mere mixed powder of aluminum oxide and silicon oxide may also be used as a sintering agent. Further, the mixture of alkoxides such as aluminum isopropoxide, tetraethyl orthosilicate, etc. and precursors such as aluminum hydroxide, aluminum stearate and the like may also be used as a sintering agent. As described above, when nuclear fuel pellets are manufactured in accordance with the manufacturing method of this invention, a series of phenomena occur in the following manner. Specifically, a sintering agent becomes a single liquid phase during the sintering, and through this liquid phase, Gd.sub.2 O.sub.3 is dispersed into the entire pellets. Thus, the effective inter-diffusion distances between UO.sub.2 and Gd.sub.2 O.sub.3 become smaller, so that the generation of solid-solution phase is promoted. Further, a liquid phase-sintering mechanism promoters reaction between particles, so that the growth of grains is promoted. This growth of grains increases the diffusion distance between FP gas and grain boundaries. Thus, a FP gas release rate from the pellets decreases. In the present invention, the mixing proportion of a sintering agent (consisting of aluminum oxide and silicon oxide) is determined to be about 10 ppm through about 500 ppm with respect to the total amount of nuclear fuel pellets. This is based on that the following facts have been confirmed. Specifically, the adding amount of the sintering agent must be 10 ppm at a minimum in order to improve the solid-solution state (i.e., to obtain homogeneous pellets) while a free UO.sub.2 phase is maintained to be 5% at a maximum. Further, the grain-growth-promoting effect reaches a maximum when the adding amount of the sintering agent is about 250 ppm. Further, when the adding amount of the sintering agent exceeds 500 ppm, this is not only insignificant but also decreases the pellet density. Further, if the sintering agent includes Al.sub.2 O.sub.3 of more than 60 wt %, the grain-growth-promoting effect decreases. As described above, according to the present invention, the solid-solution state (i.e., homogeneous state) of nuclear fuel pellets can be improved. Further, the creep characteristics of nuclear fuel pellets can also be improved by grain boundaries softened by glassy phase in spite of large grain diameter. Therefore the nuclear fuel pellets of this invention can decrease a FP gas release rate, and can also improve the PCI resistance, whereby burnup extensin toward higher levels of the nuclear fuel can be achieved. Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.