Patent Number: 052020851
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly, a reactor core, and a method for loading of the fuel assembly, especially, relates to the preferable fuel assembly being loaded in boiling water type nuclear reactors (hereinafter called BWR) for increment of reactor shut down margin, improvement of fuel economy, and maintenance of reactivity control, and relates to the preferable reactor core loaded with the fuel assemblies and the preferable method for loading of the fuel assembly. A conventional fuel assembly which is used in a BWR is generally composed of a plurality of fuel rods and one or a plurality of water rods which are arranged in a channel box by being supported at an upper end and a lower end of the fuel rod and the water rod with an upper tie plate and a lower tie plate. During operation of the reactor, slightly unsaturated cooling light water enters from a hole of the lower tie plate into an interval among the fuel rods in the fuel assembly, and flows out from a hole of the upper tie plate as vapor-liquid two phase flow after being heated by the fuel rods and boiled during flow from lower portion to upper portion of the fuel rod interval. As the result, void fraction of the coolant increases from 0% at the lower portion to about 70% at the upper portion of the fuel assembly. Consequently, the ratio of hydrogen atoms to heavy metal atoms; that is, the ratio of moderator to fuel (H/U ratio), which is an important factor for determining of nuclear characteristics of the fuel assembly alters remarkably depending on a position in an axial direction. On the other hand, it is necessary to install control rods and instrument tubes for neutron detectors exterior of the channel box in the BWR and, therefore, such intervals (hereinafter called water gap) as to enable the above mentioned rods and tubes be inserted are provided between the fuel assemblies. The water gap is filled with saturated water and, consequently, effects of the saturated water existing in the water gap to the fuel rods in the fuel assembly are different depending on whether the fuel rods locate at periphery of the fuel assembly (a region near the water gap) or central region of the fuel assembly. That is, the peripheral region of the fuel assembly near the water gap has larger H/U ratio than the central region. Accordingly, such H/U ratio as an important factor for determining the nuclear characteristics of the fuel assembly differs depending on the radial position in the fuel assembly. The H/U ratio is a parameter to determine an average energy of a neutron. As the ratio becomes larger, the average energy of the neutron becomes smaller (softer neutron spectrum), and the nuclear fission reaction with nuclear fissile material is enhanced. Concurrently, the softening of neutron spectrum increases the neutron absorbing reaction by the moderator (light water as coolant) as well as the nuclear fission reaction. Accordingly, there is an optimum H/U ratio in view of fuel economy. Moreover, fuel rod power generation which depends on the reactivity of nuclear fission is determined by the H/U ratio. That is, in view of thermal margin and controlability of excess reactivity of the fuel assembly, it is necessary to consider the H/U ratio. On the other hand, with related to conventional nuclear reactors, extension of an operation cycle of the reactor and high burn up of fuel are considered for increasing of a plant utilization factor and effective utilization of uranium resources. For increasing of discharged burn up of the fuel assembly, it is necessary to increase enrichment of the fuel assembly. The increment of the fuel enrichment influences the optimum H/U ratio. Further, the extension of loading period of the fuel assembly in the reactor means that the fuel is effected under different H/U ratios for a long period in the reactor, and the above mentioned influence of the H/U ratio is enhanced. In regard to improvement of distribution of the H/U ratio in a radial direction and an axial direction of the fuel assembly, there are such methods as enlarging of a saturated water region at a necessary portion and regulation of distribution of the nuclear fissile material. The former is a method for improving the H/U ratio by enlarging the saturated water region at the central and the upper region of the fuel assembly, wherein moderating effect of the neutron is deteriorated. And the latter is a method for improving the H/U ratio in the axial direction by regulation of loading quantity of the fuel. For example, in JP-A-62-211584 (1987), a method to increase horizontal cross sectional area at the upper region in the axial direction of the fuel assembly and to arrange a water rod having a horizontal cross section of cruciform at the upper region in the axial direction is proposed. Short length fuel rods are loaded beneath the cruciform protruded region of the cruciform water rod. And, in JP-A-52-50498 (1977), a method to arrange fuel rods having different length in order to form a flow channel of coolant having reversely tapered shape toward the upper region in the axial direction of the center of the fuel assembly is disclosed. In USP-4,968,479, a fuel assembly for achieving high burn up by increasing of fuel enrichment is disclosed. The fuel assembly is composed of a water rod having larger horizontal cross sectional area at the upper region in the axial direction than the area at the lower region and of fuel rods having three kinds of different length in order to reduce an increment of local power peaking accompanying with using of the highly enriched fuel with a burnable poison at the beginning of operation and to optimize a reactivity distribution at the upper and the lower region of the fuel assembly during a designated operation period. The shortest fuel rod is arranged at the position adjacent to the lower small diameter region of the water rod, and contains fuel having equal to or lower enrichment than the fuel assembly average enrichment, the medium length fuel rod contains fuel having equal to the fuel assembly average enrichment, and a part of the longest fuel rods contain fuel having the burnable poison (column 15, line 25-60, FIG. 22, 30B-30D). Further, in JP-A-63-311195 (1988), on a fuel assembly for achieving high burn up by increment of fuel enrichment, an improving method for increasing the reactor shut down margin in considering that the increasing of the enrichment at the upper region of the fuel assembly increases the reactivity of the upper region at the reactor shut down margin is disclosed. The fuel assembly improved by the above described method has two water rods each of which have a large diameter and uniform horizontal cross section in the axial direction and fuel rods, which are arranged adjacent to the two large diameter water rods, containing lower enriched fuel at least at the upper region of the fuel rod than the fuel in other next fuel rods. Other prior techniques relating to the increment of burn up are disclosed in USP-4,229,258, JP-A-63-21589 (1988), and JP-A-64-28587 (1989). In USP-4,229,258, a fuel assembly having higher enriched fuel at the upper region than at the lower region is disclosed. In JP-A-63-21589 (1988), a fuel assembly in which high enriched fuel rods are arranged at the outermost periphery in the horizontal cross section and the enrichment at the lower region in the axial direction of the fuel rods is higher than the enrichment at the upper region is disclosed. In JP-A-6428587 (1989), a fuel assembly in which enrichment of fuel pellets in fuel rods containing enriched uranium and gadolinium is the highest in the fuel assembly and the effective fuel length of the fuel rod is shorter than the length of fuel rods containing enriched uranium but not gadolinium is described. Further, in JP-A-53-43193 (1978), a conventional method in which the saturated water region at the upper region of the fuel assembly is increased by making the thickness of the channel box wall thin at the upper region of the fuel assembly is disclosed. Among above described prior techniques, the conventional method disclosed in JP-A-63-311195 (1988), wherein large water rods having uniform horizontal cross section in the axial direction are used, improves the distribution of the moderator to fuel ratio (H/U ratio) at the upper region of the fuel assembly. Nevertheless, the improvement of the distribution of the H/U ratio at the lower region of the fuel assembly is not considered in the conventional method. In accordance with the prior art wherein the improvement of the H/U ratio distribution in the axial direction of the fuel assembly is aimed at, the characteristics at the lower region of the fuel assembly is sacrificed for the improvement of the H/U ratio distribution in the axial direction and, consequently, the improvement of the H/U ratio distribution in the radial direction at the lower region of the fuel assembly is not sufficient. And the distribution of the moderator and fuel materials (fissile materials and parent materials) in the axial and the radial direction is not considered sufficiently in the prior art. That is, in the methods disclosed in JP-A-62-211584 (1987), JP-A-52-50498 (1977), and USP-4,968,479, the horizontal cross sectional area of water rod or moderator flow channel at the upper region in the axial direction of the fuel assembly is made larger than the area at the lower region in order to increase the H/U ratio at the upper region of the fuel assembly. But the methods have such problems that the cross sectional area of the water rod at the lower region is not sufficient, and flattening of thermal neutron flux distribution is not achieved sufficiently. The problems cause lowering of the fuel economy. Moreover, in the methods disclosed in JP-A-62-211584 (1987) and JP-A-52-50498 (1977), when the enrichment of the short fuel rods arranged in a region which is yielded by decreasing of H/U ratio at the lower region of the fuel assembly is excessively high, fissile materials are generated more at the lower region than at the upper region of the fuel assembly and, consequently, a large peak in power distribution is caused at the lower region of the fuel assembly. Accordingly, there are such problems that stability becomes insufficient and fuel economy is lowered by increasing of average void fraction in the axial direction of the fuel assembly. SUMMARY OF THE INVENTION One of the objects of the present invention is to provide a fuel assembly for improving fuel economy by making the moderator to fuel ratio, which alters depending on the position in the fuel assembly, close to the optimum value as possible everywhere including the lower portion of the fuel assembly, a reactor core using the fuel assembly, and a method of usage of the fuel assembly thereof. Another object of the present invention is to provide a fuel assembly for improving fuel economy and controlability of excess reactivity by optimizing the distribution of fuel materials and moderators in the axial and radial direction of the fuel assembly, a reactor core using the fuel assembly, and a method of usage of the fuel assembly thereof. The feature of the present invention is to provide a fuel assembly comprising a plurality of first fuel rods, a means for moderating which is surrounded with the first fuel rods and have larger horizontal cross sectional area at upper region in the axial direction than the area at lower region, and second fuel rods which are arranged at adjacent to the lower region of the means for moderating and have lower enriched fuel than the horizontal cross sectional average enrichment of the fuel assembly, characterized in that the horizontal cross sectional area at the lower region of the means for moderating is so determined that both of the minimum values of thermal neutron flux distribution and resonance neutron flux distribution in the vertical direction to the longitudinal axis of the fuel assembly are located at an exterior region to the second fuel rod in the vertical direction to the longitudinal axis. The horizontal cross sectional area of the means for moderating at the lower region is preferably larger than sum of the two first fuel rods. The enrichment of the fuel contained in the second fuel rod is preferably lower than 0.7 of the average enrichment at horizontal cross section of the fuel assembly, and more preferably, lower than 0.5 of the average enrichment at horizontal cross section of the fuel assembly. The second fuel rod contains, for example, natural uranium. The second fuel rod is a short length fuel rod which is preferably arranged adjacent to the lower region of the means for moderating, and the length of the fuel rod is preferably less than a half of the effective fuel length of the first fuel rod. Further, the means for moderating is preferably a water rod having wider horizontal cross sectional area at the upper region than the area at the lower region. And the means for moderating is able to be composed of a water rod having an uniform horizontal cross sectional area along the axial direction and a coolant flow channel which surrounds the upper region of the water rod, and is able to be composed of a water rod having an uniform horizontal cross sectional area along the axial direction and a plurality of solid moderating rods which surround the upper region of the water rod. In order to achieve the objects, the present invention provides a reactor core loaded with the above described fuel assemblies. The reactor core preferably has at least a central region and a peripheral region, and the fuel assemblies are arranged more in the central region than in the peripheral region. Further, for achieving the objects, the present invention provides a method of usage of the fuel assemblies characterized in that the fuel assemblies are loaded more in the central region than in the peripheral region at fuel exchange. The distribution of the moderator to fuel ratio in the axial direction of the fuel assembly and in the vertical direction to the axis at the upper region of the fuel assembly are improved respectively by arranging a means for moderating having larger horizontal cross sectional area at the upper region in the axial direction than at the lower region. And, increment of resonance neutron flux absorbing effect and flattening of thermal neutron flux in the radial direction are achieved by arranging the second fuel rods containing lower enriched fuel than the average enrichment in horizontal cross section of the fuel assembly adjacent to the means for moderating, and determining of the horizontal cross sectional area of the means for moderating at the lower region in the axial direction of the fuel assembly so that the minimum value of both the thermal neutron flux distribution and the resonance neutron flux distribution in the vertical direction to the axis are located at an exterior region to the second fuel rod in the vertical direction to the axis and, accordingly, the fuel economy and the controlability of excess reactivity are improved. By the present invention, as the moderator, the fissile material, and the fertile material are optimally arranged in the axial direction and vertical direction to the axis of the fuel assembly, the moderator to fuel ratio comes close to the optimum value at everywhere of the fuel assembly including the lower region and, consequently, the increment of resonance neutron flux absorbing effect and thermal neutron flux flattening in the vertical direction to the axis are able to be utilized, and the effects of improving the fuel economy, the controlability of the excess reactivity, and the thermal margin are realized.