Patent Number: 052572986
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear fuel pellets including UO.sub.2 and UO.sub.2 -Gd.sub.2 O.sub.3, and more particularly to nuclear fuel pellets having a satisfactory solid-solution state, greater grain diameters and a second phase precipitated in grain boundaries. This invention also relates to a method of manufacturing the above-described nuclear fuel pellets. 2. Description of the Prior Art As for nuclear fuel to be loaded into a light water reactor or a fast breeder reactor, intactness of fuel has been confirmed at a high burnup level ever experienced in a reactor. However, at present, extension of burnup to still higher levels has been planned. This plan inevitably involves the following disadvantages. Specifically, so-called bubble swelling occurs due to fission gas deposited in grain boundaries, i.e., an apparent volume of pellets increases due to bubbles produced in pellets because of gaseous fission products. Thus, PCI (pellet-cladding interaction), which is a mechanical interaction between pellets and a cladding, increases. Further, an inner pressure of a fuel rod increases because of fission gas release from fuel pellets. These phenomena may cause intactness of fuel to be deteriorated. To avoid these disadvantages, the following techniques have been attempted. Specifically, a fission gas release fraction (a ratio of the released to the produced of fission gas) is suppressed by increasing diameters of pellets grains. This is based on that a fission gas release from pellets is rate-controlled by diffusion of fission gas in pellet grains. However, when the diameters of pellet grains are increased, a creep rate of the pellets is decreased. This provides an adverse effect on PCI. To increase a creep rate of pellets there have been disclosed two technique (Japanese Patent Applications No. 1-193691 and No. 2-242195) in which a sintering agent consisting of aluminum oxide and silicon oxide is added to uranium dioxide powder so that a second soft phase can be precititated in the grain boundaries of the pellets. In these techniques, the total amounts of the sintering agents to be added are about 0.1 wt % through about 0.8 wt %, and about 0.05 wt % through about 0.4 wt %, respectively. In general, it has been known that sinterability of a mixed oxide of UO.sub.2 and Gd.sub.2 O.sub.3 is lower than that of pure UO.sub.2. Further, when sintering is performed under a given condition, a sintered density and grain diameters of the mixed oxide of UO.sub.2 and Gd.sub.2 O.sub.3 become smaller than those in the case of pure UO.sub.2. Further, when sintering is performed in a flowing dry hydrogen, a large number of micro-cracks occur in pellets. To avoid the above-described disadvantages, in the case of UO.sub.2 having Gd.sub.2 O.sub.3 added thereto, sintering is generally performed in a humid hydrogen atmosphere or in a mixed gas atmosphere of carbon dioxide and carbon monoxide. Further, the sintering is performed at a relatively high temperature (1700.degree. C. or higher). However, assume that sintering is performed in such atmospheres and a sintering agent, which consists of aluminum oxide and silicon oxide in the above-described conventional proportion, is added to the mixed oxide of UO.sub.2 and Gd.sub.2 O.sub.3. In this case, pores are generated in pellets, probably due to evaporation of silicon oxides, so that a density of pellets becomes decreased. As the pellet density becomes lower, a thermal conductivity of the pellets degrades. As a result, a fuel center temperature increases in service, so that both bubble swelling and a fission gas release rate are enhanced. This is disadvantageous to the performance of nuclear fuel pellets. Further, it has also been experimentally confirmed that grain diameters of pellets and a solid-solution state thereof can no longer be improved even when a sintering agent of 500 ppm or more is added. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide nuclear fuel pellets including fission substance, the fission substance being UO.sub.2 alone or UO.sub.2 having Gd.sub.2 O.sub.3 added thereto, the pellets comprising a satisfactory solid-solution state (homogeneous state), large grain diameters, and a second phase precipitated in grain boundaries, and still having a sufficiently high density. Another object of the present invention is to provide a method of manufacturing the above-described nuclear fuel pellets. Briefly, in accordance with one aspect of the present invention, there are provided nuclear fuel pellets including fission substance, the fission substance being UO.sub.2 alone or UO.sub.2 having Gd.sub.2 O.sub.3 added thereto. The nuclear fuel pellets comprises UO.sub.2 or (U, G) O.sub.2 grains and aluminosilicate deposition phases of glassy and/or crystalline state. An average diameter of the grains is in the range from about 20 .mu.m through about 60 .mu.m. The aluminosilicate deposition phases have a composition consisting of SiO.sub.2 of 40 wt % through 80 wt % and Al.sub.2 O.sub.3 of residual on average. The amount of the alumina plus silica is about 10 through 500 ppm with respect to the total weight of the nuclear fuel pellets. Further, the total volume of as-fabricated pores in the pellets is 5 vol % at a maxiumum. In accordance with another aspect of the present invention, there is provided a method of manufacturing the above-described nuclear fuel pellets. The method comprises the steps of compacting an oxide powder of UO.sub.2 or UO.sub.2 having Gd.sub.2 O.sub.3 added thereto, and sintering the oxide powder compacts. More specifically, the method comprises the steps of mixing a sintering agent (including the precursor thereof) consisting of SiO.sub.2 of about 40 wt % through about 80 wt % and Al.sub.2 O.sub.3 of the residual with the above-described oxide powder, the mixing proportion thereof being about 10 ppm through about 500 ppm with respect to the total amount of the oxide powder and the sintering agent; pressing the mixed oxide powder so as to obtain green pellets; and sintering the green pellets at a temperature in a range of about 1500.degree. C. through 1800.degree. C. so as to obtain sintered pellets.