Patent Number: 048636800
Section: summary

The present application claims priority of Japanese Patent Application No. 62-82124 filed on Apr. 2, and No. 62-187295, No. 62-187296 and No. 62-187297 filed on July 27, 1987, respectively. FIELD OF THE INVENTION AND RELATED ART STATEMENT This invention relates to a fuel assembly for use in a light-water nuclear rector. In recent years, the trend of light-water nuclear reactors toward increase in power generation capacity has been urging an exacting demand for improvement of the fuel cycle cost of power generation. Various improvements, therefore, have been given to fuel assemblies. Since an extension of fuel burnup is an effective approach to the improvement of the fuel cycle cost of power generation, the desirability of improving the fuel for the purpose of alleviating a possible effect of an elevated fuel burnup upon the core operation characteristic has been finding approval. The fuel assembly heretofore used in the boiling-water reactor (BWR) is constructed by arranging cylindrical fuel rods each containing fuel pellets in a sealed state in the pattern of an 8-row 8-column tetragonal lattice within a channel box and disposing two water rods in the central part of the horizontal cross section of the interior of the channel box. In the core of the BWR, the adjacent channel boxes are spaced with a water gap of a width approximately in the range of 10 to 20 mm and cruciform control rods are inserted therein. In the BWR, the light water which flows inside the channel boxes boiled and forms a two-phase flow containing an average of about 40% by volume of steam while the reactor is in operation. In contrast, the light water flowing through the water-gap region outside the channel boxes does not boil even while the reactor is in operation. Owing to the effect manifested in moderating neutrons by the light water present in the water-gap region, the thermal neutron flux distribution in the horizontal direction in the fuel assembly tends to increase towards the periphery and decrease towards the center. The fuel assembly, therefore, is provided near the center thereof with two water rods adapted to pass non-boiling water through the interior thereof. The water rods, owing to the effect manifested in moderating neutrons by the water passed therethrough, serve the role of enhancing the thermal neutron utilization factor of thermal by moderating the depress of the thermal neutron flux in the central part, diminishing the output peaking, and heightening the thermal neutron flux inside the bundle. The effective multiplication factor of the core of a thermal reactor can be expressed with the four-factor formula as follows: EQU K.sub.eff =.epsilon..times.f.times.p.times.p.sub.L wherein K.sub.eff =effective multiplication factor, .epsilon.=fast fission factor, .eta.=regeneration factor, f=thermal utilization factor. p=resonance escape probability, and p.sub.L =ratio of neutrons leaking from core. The water rods mentioned above are intended to increase the effective multiplication factor by heightening the thermal neutron utilization factor, f. An effort to heighten the burnup for the purpose of improving the reduction of the fuel cycle cost, however, entails aggravation of the power mismatch among fuel assemblies and consequent rigidification of such thermal restrictions as the maximum linear power density and the minimum critical power ratio. The feasibility of a fuel assembly using an increased number, 9 (row).times.9 (column), or fuel rods as a countermeasure is being considered. The increase in the number of fuel rods, however, entails a decrease in the outside diameter of component fuel rods an increase in the resonance escape probability and cancels the effect brought about in the enhancement of thermal utilization factor by the aforementioned incorporation of water rods. For successful production of a fuel which permits extension of burnup and, at the same time, excels in economy, the enhance of thermal neutron utilization factor and the increase of resonance escape probability must be simultaneously satisfied. The attainment of an extension of the burnup requires an increase of the initial concentration of fissionable isotopes (enrichment of uranium-235 and fissionable plutonium isotopes) in the fuel and, therefore, brings about various effects on the core characteristic of a reactor. In all the effects, the decrease of the subcriticality (shut down margin) during the period of cold state and the increase of the change of core reactivity (void reactivity coefficient and moderator temperature reactivity coefficient) due to the change in the density of moderator constitute the hardest problems from the standpoint of design. One possible way of overcoming these problems may reside in increasing the water-to-fuel. An increase in the volume of the light-water region, however, entails an addition to the core volume and an increase in the construction cost of the reactor. A decrease in the amount of fuel material results in an increase in the number of fuel assemblies to be replaced per cycle and a decline of the economy of fuel. For successful production of a fuel assembly meeting the requirement for extension of burnup and excelling in core characteristic, therefore, it is further necessary to increase the shut down margin and lower the moderator density reactivity coefficient without entailing an increase in the volume of light water or a decrease in the amount of fuel material. Some of the boiling-water reactor have a core of the construction called D-lattice core. In this core, wide gaps of large width permitting insertion of control rods and narrow gaps of small width not permitting insertion of any control rod are formed outside a channel box. The width of the wide gaps is roughly twice that of the narrow gaps. In the D-lattice core, therefore, the power issues more readily from the wide gap side corners than from the narrow gap side corners. The power also issues more readily from the fuel rods facing on these gaps than from those not bordering on the gaps. Adjustment of the power, therefore, is accomplished by disposing a plurality of types of uranium rods differing in enrichment. For the increase of the average enrichment in the fuel assembly of this nature, it is necessary not only to add water rods and gadolinia rods but also to increase the number of types of uranium rods differing in enrichment (hereinafter referred to as "split number"). The addition of gadolinia rods and the increase in the number of splits, however, are nothing desirable from the standpoint of lowering the fuel cycle cost. OBJECT AND SUMMARY OF THE INVENTION As object of this invention, therefore, is to provide a fuel assembly which excels in reduction of the fuel cycle cost because of an increased multiplication factor during the course of operation as compared with the conventional fuel assembly, possesses a shut down margin enough to meet the requirement for extension of burnup and permits an effective improvement of the moderator density reactivity coefficient, and ensures a generous thermal margin during the operation. Another object of this invention is to provide a fuel assembly which minimizes the addition of gadolinia rods and obviates the necessity for increasing the number of splits in the improvement of the average enrichment of fuel assembly and, as compared with the conventional fuel assembly of equal average degree of concentration and equal water-to-fuel volumetric ratio, exhibits a high reactivity during the output operation, a small local power peaking, and a small difference of reactivity during the power operation and during the period of cold state. To be specific, the fuel assembly of this invention is constructed by preparing small units each having a small number of fuel rods bundled as spaced with a fixed intercentral distance, arranging a plurality of such small units in such a manner that the intercentral distance between the component fuel rods forming mutually juxtaposed sides of the adjacent small units is larger than the intercentral distance between the adjacent fuel rods within the small units, and disposing a water rod near the center of a cluster of the plurality of small units. Owing to this construction, the fuel assembly enjoys outstanding fuel economy, ensures an ample shut down margin even when the fuel to be used has a high enrichment, and permits a decrease in the moderator density reactivity coefficient.