Patent Number: 054467730
Section: description

DETAILED DESCRIPTION OF THE EMBODIMENTS As mentioned above, the present invention relates to a heterogeneously loaded type of fast reactor core in which a reduced number of (e.g., 30 to 50) target fuel assemblies containing minor actinide nuclides are heterogeneously dispersed and loaded. The target fuel rods within each fuel assembly contain 20 to 50% of minor actinide nuclides, and are smaller in diameter than ordinary core fuel rods, so that more target fuel rods can be loaded in the target fuel assembly. Shown in FIG. 1 is the relation between the content of minor actinide nuclides and the melting point of fuel. As the content of minor actinide nuclides increases, the melting point of fuel decreases, and the thermal conductivity of fuel decreases as well. For these reasons, target fuel assemblies containing large amounts of minor actinide nuclides are lower than ordinary core fuel assemblies (i.e., those containing no minor actinide nuclides) in terms of the fuel permissible linear power, i.e., the uppermost linear power with which the fuel does not melt. Shown in FIG. 2 is the relation between the content of minor actinide nuclides and the permissible linear power. For instance, when the content of minor actinide nuclides is 50%, the permissible linear power is about 20% lower than that in the absence of minor actinide nuclides. To avoid a decrease in the permissible linear power, it is required to lower the operating power. It is thus expected that no sufficient annihilation of the minor actinide nuclides would occur in the target fuel assemblies. According to the present invention, this decrease in the permissible linear power can be avoided by reducing the diameter of target fuel rods so that more target fuel rods can be loaded in a wrapper cylinder or tube of the same hexagonal shape in section as that used with an ordinary core fuel assembly. Characterized by this is the present invention. Referring to FIG. 3, there is shown an embodiment of the fuel assembly used in the present invention. FIG. 3A represents a cross-section of an ordinary core fuel assembly 10, while FIG. 3B illustrates a cross-section of a target fuel assembly 12 containing minor actinide nuclides. The ordinary core fuel assembly 10 is of the structure the 271 core fuel rods 16 are regularly loaded in a hexagonal wrapper cylinder or tube 14, whereas the target fuel assembly 12 is of the structure that as many as 397 target fuel rods 18 are regularly loaded in a hexagonal wrapper tube 14 of the same geometry. Referring then to FIG. 4, there is schematically shown one embodiment of the core arrangement in which such core and target fuel assemblies are loaded. For power flattening, this core arrangement is of a two-zone type wherein plutonium (Pu) is more enriched in the outer zone than in the inner zone. Provided in the inner and outer zones are 154 and 180 core fuel assemblies, respectively, with 39 target fuel assemblies dispersed in these zones. On the outermost side there are provided blanket fuel assemblies. At suitable positions main and backup control rods are located. Referring to the case where the number of the target fuel rods (containing 50% of minor actinide nuclides) within the target fuel assembly is used as a parameter at such a core arrangement, a change in the maximum linear power is as shown in FIG. 5. The maximum linear power decreases with an increase in the number of the target fuel rods, and becomes lower than the permissible linear power when the number of the target fuel rods loaded within the target fuel assembly reaches 397. Also, the sodium void and Doppler coefficients are equivalent to those of a homogeneous core in which minor actinide nuclides are used at the same content; no problem arises during operation and control. In addition, the rate of annihilation of minor actinide nuclides in the core of the present invention is equivalent to that of a homogeneous core (about 11% per year). It is here to be noted that when fuel rods are regularly inserted in a hexagonal wrapper tube, it is inevitable that their number take discrete values of, e.g., 127, 217, 271, 331, 397. . . The current standard number of ordinary core fuel rods loaded per ordinary core fuel assembly is set at 271 in view of wrapper tube size and fuel pellet production. On the other hand, all wrapper tubes must be of the same shape, as can be seen from FIG. 4. According to the present invention wherein target fuel rods of reduced diameter can be used, it is thus possible to load as many as 331 or 397 target fuel rods in the same wrapper tube. However, it is to be understood that it is practically impossible to use more target fuel rods, because it is not only required to reduce their diameter further but also some problems arise in connection with mechanical strength. In the present invention, the basic material of the fuel containing minor actinide nuclides is usually a mixed oxide of uranium and plutonium. However, it is to be understood that the present invention is applicable to a mixed nitride of uranium and plutonium that is superior to the mixed-oxide fuel in terms of the rate of annihilation of minor actinide nuclides. While the above embodiment has been described as using uranium and plutonium-containing fuel for the target fuel assembly, it is also to be understood that any uranium and plutonium-free fuel can be used as the target assembly. In other words, for the target assembly it is possible to make use of aluminum oxide, magnesium oxide, aluminum magnesium oxide, cerium oxide, gadolinium oxide, aluminum nitride or zirconium nitride, to which minor actinide nuclides are added. This forecloses the possibility of formation of minor actinide nuclides from uranium and plutonium, and so enables minor actinide nuclides to be annihilated with high efficiency. While the present invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing form the spirit and scope of the invention.