Patent Number: 049869582
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly and, more particularly, to a fuel assembly suitable for use in a boiling water reactor. 2. Description of the Related Art The core of such a boiling water reactor is provided with a plurality of fuel assemblies which are spaced apart from one another at predetermined intervals and in which a plurality of control rods which are inserted between adjacent fuel assemblies. The fuel assembly has an upper tie plate, a lower tie plate and a plurality of fuel rods whose opposite ends are supported by the upper and lower tie plates. Each of the fuel rod includes a multiplicity of fuel pellets. A channel box is mounted on the upper tie plate to surround a fuel bundle. The maximum power in the core provided with such fuel assemblies is obtained by multiplying the product of the following three kinds of peaking and the average power of the fuel assemblies within the core. A first peaking from among the three kinds of peaking is a radial power peaking which is the proportion of the maximum power of the fuel assemblies within the reactor core to the average power of fuel assemblies. A second peaking is an axial power peaking which is the proportion of the maximum power to the average power of the reactor core in the vertical direction thereof. A third peaking is a local power peaking which is the proportion of the maximum power of the fuel rods in the fuel assembly to the average rods power in the fuel assembly. The power P of each fuel rod in the fuel assembly is given by EQU P=.phi..multidot..delta..sub.r .multidot.N where .phi. is the thermal neutron flux in the position of the fuel rod, .delta..sub.r is the fission cross section of a fissile material and N is the density of atoms in the fissile material in the fuel rod (hereinafter referred to as "fuel atoms"). In order to efficiently burn the fuel and to prolong its burn-up period, it is necessary to increase the so-called infinite multiplication factor of the fuel assembly. As is known, to increase the infinite multiplication factor, it is effective to increase the density of fuel atoms in a region in which thermal neutron flux level is high, as well as to decrease the density of fuel atoms in a region in which thermal neutron flux level is low. In the above-described boiling water reactor, the thermal neutron flux level is high in the periphery of the fuel assembly, but is low in the central portion due to the non-uniform distribution of a moderator for neutrons, the neutron absorption effect of the fuel rod itself and so forth. Accordingly, it is desirable that a fuel assembly of the type which is used in the boiling water reactor be formed such that the density of fuel atoms in the periphery of the fuel assembly is greater than that in the central portion. The fuel assembly disclosed in Japanese Patent Unexamined Publication No. 58-26292 is known as a fuel assembly capable of satisfying such a demand. The fuel assembly disclosed in Japanese Patent Unexamined Publication No. 58-26292 is constructed in the following manner. A plurality of fuel rods for use in a reactor which employs a fissile material as fuel are incorporated in the fuel assembly in parallel to one another and in an integral form. The average density of the fissile material in the fuel rods in the periphery of the fuel assembly is selected so as to be greater than the average density of the fissile material in the fuel rods in the central portion of the fuel assembly. In a manner similar to that disclosed in U.S. Pat. No. 4,229,258, the proportion of fissile material contained in each of the fuel rods is changed in the axial direction so that the infinite multiplication factor in the upper portion of the fuel assembly becomes greater than the infinite multiplication factor in the lower portion of the fuel assembly. Thus, the infinite multiplication factor of the overall fuel assembly is increased and the burn-up period of the fuel assembly is consequently prolonged. Recently, fuel with a high degree of burn-up has been developed and there has been a trend toward an increase in the enrichment of a fuel pellet of the type which is loaded in a fuel rod. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a fuel assembly whose axial power peaking can be suppressed and whose fuel economy is improved by the effective utilization of neutrons. It is another object of the present invention to provide a fuel assembly such that a decrease of its infinite multiplication factor can be suppressed. A first feature of the present invention resides in the fact that, the proportion of fuel rods whose enrichment in their lower regions is greater than the average enrichment in the lower region of the fuel assembly among the fuel rods located in the periphery of a fuel assembly, is smaller than the proportion of fuel rods whose enrichment in their upper regions is greater than the average enrichment in the upper region of the fuel assembly among the fuel rods located in the periphery of a fuel assembly. A second feature of the present invention resides in the fact that, in the upper region of a fuel assembly, the proportion of fuel rods whose enrichment in their upper regions is greater than the average enrichment in the upper region of the fuel assembly is not less than 50 percent of the total number of fuel rods located in the periphery of the fuel assembly, whereas, in the lower region of the fuel assembly, the proportion of fuel rods whose enrichment in their lower regions is greater than the average enrichment in the lower region of the fuel assembly is not greater than 20 percent of the total number of fuel rods located in the periphery of the fuel assembly. In accordance with the first feature of the present invention, at least the fuel rods whose fuel enrichment in the upper portions thereof is greater than the average enrichment in the upper region of the fuel assembly are present in the periphery of the upper region of the fuel assembly. Accordingly, since the neutrons in the periphery of the fuel assembly, in which neutron flux level is high, can be effectively utilized, good fuel economy can be achieved. Among the fuel rods located in the periphery of the fuel assembly, the proportion of fuel rods whose enrichment in their respective lower regions are greater than the average enrichment in the lower region of the fuel assembly is less than the proportion of fuel rods whose enrichment in their respective upper regions are greater than the average enrichment in the upper region of the fuel assembly. Accordingly, it is possible to decrease the local power peaking in the lower region of the fuel assembly which has a low void fraction, in particular, the local power peaking in the periphery of the lower region. In accordance with the second feature of the present invention, in the upper region of the fuel assembly, at least 50 percent of the fuel rods located in the periphery of the fuel assembly are fuel rods whose enrichment in their respective upper regions are greater than the average enrichment in the upper region of the fuel assembly. Accordingly, since neutrons in the periphery in which neutron flux level is high can be effectively utilized, good fuel economy can be achieved. With this arrangement at least 50 percent of the fuel rods located in the periphery of the fuel assembly and has enrichment in their respective upper regions greater than the average enrichment in the upper region of the fuel assembly. Accordingly, the increment of the infinite multiplication factor of the fuel assembly does not rapidly decrease. Further, the proportion of fuel rods whose enrichment in their respective lower regions are greater than the average enrichment of the lower region of the fuel assembly among the fuel rods located in the periphery of the fuel assembly does not exceed 20 percent. Accordingly, it is possible to diminish the local power peaking in the lower region of the fuel assembly having a low void fraction, in particular the local power peaking in the periphery of the fuel assembly, and hence the axial power peaking. In addition, owing to the above-noted functions, the quantity of fissile material in a new fuel assembly (the exposure of fuel assembly is zero) to be loaded in a reactor core can be increased, and therefore the exposure of fuel can be increased.