Patent Number: 
Section: description

Hereinafter, embodiments of the present invention will be described with reference to the drawings. A first embodiment of the present invention will be described with reference to FIGS. 1 to 16. FIG. 2 is a horizontally transverse sectional view showing a partial schematic arrangement (symmetric quarter portion) of a boiling water reactor core according to this embodiment, and FIG. 3 is a partial enlarged view of FIG. 2. Referring to FIGS. 2 and 3, a number of fuel assemblies 1 are arranged in a reactor pressure vessel (not shown), to constitute a core 2. In the core 2, each control rod 3 is inserted among the four adjacent fuel assemblies 1 placed in a square array. The core 2 is configured as a so-called D-lattice core, in which a gap between the fuel assemblies 1 on the control rod 3 side is larger than that on the anti-control rod 3 side. FIG. 3 shows fuel assemblies 1A, 1B, 1C and 1D as one example of the four adjacent fuel assemblies placed in the square array. The control rod 3 is formed into an approximately cruciform in transverse cross-section. The fuel assemblies 1A to 1D are identical to each other in terms of structure but are different from each other in arrangement orientation. That is to say, the fuel assemblies 1A to 1D are point-symmetrically arranged around the axis of the cruciate control rod 3. The control rod 3 is inserted in such a manner as to be close to two sides of the square shape in transverse cross-section of each fuel assembly 1. FIG. 1 is a partial enlarged view of FIG. 3, showing a detailed structure of the fuel assembly 1A, and FIG. 4 is a vertical sectional view showing a detailed structure of the fuel assembly 1A shown in FIG. 1. Referring to FIGS. 1 and 4, the fuel assembly 1A includes a fuel bundle 6 composed of a number of fuel rods 4 and two water rods 5 (these rods are not shown in FIG. 4), fuel spacers 8, a upper tie plate 9, a lower tie plate 10, and a channel box 11. The fuel bundle 6 includes, as shown in FIG. 1, the fuel rods 4 placed in a square lattice array of 9-rows/9-columns. The square lattice array of the fuel rods 4 is offset as a whole in the upward, leftward direction in FIG. 1 (toward a channel fastener to be described later, or toward the control rod 3). The xe2x80x9cixe2x80x9d which is the center of the square lattice array of the fuel rods 4 is offset from the xe2x80x9cjxe2x80x9d which is the center in a cross section of the channel box 11, which is equal to the center in a cross section of the upper tie plate 9 and the lower tie plate 10, toward the channel fastener side or the control rod 3 side. The offset amount Y is set at 2xe2x88x921/2 mm in the upward, leftward direction in FIG. 1. In other words, as shown in FIG. 1, the offset amount Y has an offset component of 0.5 mm in the upward direction and an offset component of 0.5 mm in the leftward direction. The means for offsetting and holding the square lattice array of the fuel rods 4 is configured by an insertion hole 14a of a channel fastener 14, fuel rod insertion holes 10b and water rod insertion holes 10c of the lower tie plate 10, and tabs 8b1 and 8b2 of each fuel spacer 8. These will be described in detail later one by one. The fuel rod 4 contains fuel pellets. The fuel pellet is formed of a sintered body of uranium as a fissile material. The outside diameter d of each fuel rod is set at 11.2 mm. The fuel rod pitch p is set at 14.4 mm. In this embodiment, 74 pieces of the fuel rods 4 are placed in a square lattice array of 9-rows/9-columns. The 74 pieces of the fuel rods 4 are composed of usual fuel rods (long-length fuel rods) 4a and partial length fuel rods (short-length fuel rods, not shown in FIG. 4) 4b. The fuel active length (charging length of nuclear fuel) of the short-length fuel rod 4b is shorter than that of the long-length fuel rod 4a.  While not particularly shown and described, there are used a plurality of kinds of the fuel rods 4a which are different in enrichment distribution of uranium contained in pellets. The local peaking factor is flattened by suitably adjusting the fuel rod pitch 4a and 4b. Further, a suitable enrichment distribution in the axial direction is given for each the fuel rods 4a and 4b in order to flatten the power peaking in the axial direction (axial peaking factor). These configurations may be the same as those for the known fuel assembly of this type. The two water rods 5 are arranged in the fuel assembly 1A at an approximately central portion having an area in which 7 pieces of the fuel rods 4 can be placed in a square lattice array of 3-rows/3-columns. While not particularly shown in detail, the water rods 5 are formed by hollow tubes having the known structure for forming a coolant flow passage so as to flatten a thermal neutron flux at the central area of the fuel assembly 1A. The upper tie plate 9 is used for supporting the upper end portion of the fuel bundle 6. Guide posts 9a and 9b are integrally formed on the upper tie plate 9 on the control rod 3 side and the anti-control rod 3 side, respectively. The upper tie plate 9 is supported by an upper lattice plate 12 (see the later figure, FIG. 5) via the guide post 9a in a state in which the lateral movement of the upper tie plate 9 is restricted by the upper lattice plate 12. FIG. 5 is a top view showing a supporting structure in which the upper portion of the fuel assembly 1A is supported by the upper lattice plate 12. For a clear understanding of the structure, the structure of supporting the four fuel assemblies 1A to 1D (see FIG. 3) with the control rod 4 put therebetween is shown in FIG. 5. The upper lattice plate 12 has a lattice structure corresponding to the positions of the fuel assemblies 1 and the fuel rods 3 arranged in the core. To be more specific, as shown in FIG. 5, the upper lattice plate 12 has a number of lattices 13 each having a size surrounding one of the four fuel assemblies 1A, 1B, 1C and 1D. The four fuel assemblies 1A to 1D are provided under respective lattices 13. One control rod 3 is positioned among the four fuel assemblies 1A to 1D. The leading end of the guide post 9a of each of the fuel assemblies 1A to 1D is inserted in one of the insertion holes 14a provided in the channel fastener 14, and is fixed to the channel fastener 14 with a fixture, typically, a bolt (not shown). At this time, each insertion hole 14a of the channel fastener 14 is formed at a position slightly offset from that of the prior art 9-rows/9-columns fuel assembly toward the corner side. With this offset of the insertion holes 14a of the channel fastener 14, as described above, the center in a cross section of the fuel bundle 6 is offset from the center in a cross section of the upper tie plate (center in a cross section of the channel box) toward the control rod side. The channel fastener 14 is connected to the channel box 11, whereby the channel box 11 is fixed to the fuel bundle 6 surrounded by the channel box 11. The channel fastener 14 is provided for keeping constant gaps each being formed between the channel boxes 11 of the adjacent two of the fuel assemblies 1, thereby ensuring spaces in which the control rod 3 is to be inserted. Each of the guide posts 9b on the anti-control rod side is taken as a dummy for taking a balance in weight between the guide post 9a on the control rod 3 side and the same. In addition, a guard 15 is provided between the channel fastener 14 and each channel box 11 for preventing excessive deformation of the channel fastener 14. The side surface, on which the channel fastener 14 is not provided, of the channel box 11 of each of the fuel assemblies 1A to 1D is supported by the upper lattice plate 12 (see FIG. 5). To be more specific, the above side surface of the channel box 11 is simply pressed to the upper lattice plate 12 by the elastic force of the channel fastener 14, to be thus supported by the upper lattice plate 12. As shown in FIG. 4, the lower tie plate 10 supports the lower end of the fuel bundle 6. FIG. 6 is a top view showing a detailed structure of the lower tie plate 10. As shown in FIGS. 4 and 6, an upper surface 10a of the lower tie plate 10 has the fuel rod insertion holes 10b, the water rod insertion holes 10c, and coolant introduction holes 10d, 10e and 10f. In this embodiment, 74 pieces of the fuel rod insertion holes 10b are provided, and the lower end portions of the fuel rods 4 are inserted in and supported by the fuel rod insertion holes 10b; and two pieces of the water rod insertion holes 10c are provided, and the lower end portions of the water rods 5 are inserted in and supported by the water rod insertion holes 10c.  The fuel rod insertion holes 10b and the water rod insertion holes 10c are provided at the positions corresponding to those of the fuel rods 4 and the water rods 5 of the fuel bundle 6 shown in FIG. 1. To be more specific, the fuel rod insertion holes 10b and the water rod insertion holes 10c are also placed in the square lattice array of 9-rows/9-columns. The square lattice array of the fuel rod insertion holes 10b and the water rod insertion holes 10c is offset as a whole in the upward, leftward direction in FIG. 6, that is, toward the channel fastener 14 side or the control rod 3 side. More specifically, the center i in a cross section, which is equivalent to the center position of the coolant introduction hole 10f0 between the two water rod insertion holes 10c, of the square lattice array of the fuel rod insertion holes 10b and the water rod insertion holes 10c is offset from the center j in a cross section of the lower tie plate 10 in the upward, leftward direction in FIG. 6. The offset amount Y is, as described above, set at 2xe2x88x921/2 mm in the upward, leftward direction in FIG. 6. This means that the offset component of the offset amount Y in the leftward direction in FIG. 6 is 0.5 mm and the offset component thereof in the upward direction in FIG. 6 is 0.5 mm. As shown in FIGS. 1 and 4, the channel box 11 surrounds the outer periphery of the fuel bundle 6 to form an outer wall of the fuel assembly 1A. The inner width W of the channel box 11 is set at 134.1 mm. The fuel spacers 8 are provided at a plurality of the axial positions of the fuel bundle 6. At each axial position of the fuel bundle 6, the fuel spacer 8 bundles the fuel rods 4 and the water rods 5 in such a manner that they are spaced at specific gaps. Accordingly, the center in a cross of the fuel spacers 8 is equal to the center i in a cross section of the fuel bundle 6. The fuel spacer 8 includes a band 8a and a plurality (eight in this embodiment) of tabs 8b projecting outwardly from the outer periphery of the band 8a. The fuel spacer 8 also includes the known cylindrical members and spring members (not shown in FIGS. 1 and 3). To be more specific, the cylindrical members of the number corresponding to that of the fuel rods 4 are provided. The fuel rods 4 are inserted in the cylindrical members and pressed to the side opposed to the cylindrical members by the spring members provided on the cylindrical members. The fuel rods 4 are thus supported in the cylindrical members while being restricted in their lateral movements by the spring members. FIG. 7A is a side view showing a detailed structure of one of the four sides of the square-shaped band 8a and the tabs 8b provided thereon, and FIG. 7B is a sectional view taken on line Axe2x80x94A of FIG. 7A. The band 8a is a band-like member having a uniform thickness which is formed into a square shape. The tabs 8b are formed, typically, by extruding portions of the band 8a. The height of the tab 8b is designated by character X in FIG. 7B. The tabs 8b are provided on the tab 8a at eight positions. Of these tabs 8b, the four tabs 8b1 are provided on the control rod side and the four tabs 8b2 are provided on the anti-control side with respect to a diagonal line of the square shape of the tab 8a (see FIG. 1). The height X2 of the tab 8b2 is different from the height X1 of the tab 8b1. The difference X2xe2x88x92X1 therebetween is set at 1 mm. By setting the difference X2xe2x88x92X1 at 1 mm, there can be realized the structure in which the center i in a cross section of the fuel bundle 6 is offset a value Y=2xe2x88x921/2 mm from the center j in a cross section of the channel box 11 on the control side, and more concretely, offset by 0.5 mm in the leftward direction and by 0.5 mm in the upward direction in FIG. 1. This will be more fully described with reference to FIGS. 1 and 8. In the structure shown in FIG. 1, as described above, the thickness t of the band 8a is equalized over the entire peripheral length, and the center in a cross section of the fuel spacers 8 is the same as the center i in a cross section of the fuel bundle 6. Accordingly, a distance u between each fuel rod 4 positioned at the outermost periphery of the square lattice array and the band 8a is equalized over the entire periphery of the square lattice array. Letting the distance between the leading end of the tab 8b2 and each fuel rod 4 positioned at the outermost periphery of the square lattice array on the anti-control rod side be L2, and the distance between the leading end of the tab 8b1 and each fuel rod 4 positioned at the outermost periphery of the square lattice array on the control rod side be L1, the heights X2 and X1 of the tabs 8b2 and 8b1 are given by X2=L2xe2x88x92(t+u) and X1=L1xe2x88x92(t+u), and accordingly, the difference in height between the tabs 8b2 and 8b1 is expressed by X2xe2x88x92X1=L2xe2x88x92L1. In the prior art non-offset structure, L2 is equal to L1, each of which is taken as L (L2=L1=L, see FIG. 8A). On the other hand, according to this embodiment, the fuel bundle 6 is offset up to the center i in a cross section. At this time, letting each of the leftward offset amount and the upward offset amount in FIG. 1 be H, the offset amount Y and the above distances L1 and L2 are given by Y=Hxc3x972xe2x88x921/2 (H=Yxc3x972xe2x88x921/2), L1=Lxe2x88x92H, and L2=L+H. Accordingly, referring to FIG. 8A, the difference L2xe2x88x92L1 in distance between the tabs 8b2 and 8b1 and the fuel rods 4 positioned at the outer periphery of the square lattice array becomes L2xe2x88x92L1=2H=Yxc3x972xe2x88x921/2. Here, since X2xe2x88x92X1=L2xe2x88x92L1, the following equation is given. X2xe2x88x92X1=Yxc3x972xe2x88x921/2xe2x80x83xe2x80x83(1) Accordingly, by setting the difference X2xe2x88x92X1 at 1 mm, the offset amount Y can be set at Y=2xe2x88x921/2 mm. The function of this embodiment will be described below. (1) Reduction in Local Peaking Factor Due to Offset Structure In the D-lattice core 2, when the fuel assemblies 1 are arranged, a gap between the adjacent two of the fuel assemblies 1 on the control rod side (channel fastener side) is wider than that on the anti-control rod side (anti-fastener side). The continuous water region on the channel fastener side on which the gap between the fuel assemblies 1 is thus wide is larger than that on the anti-fastener side on which the gap between the fuel assemblies 1 is narrow, and accordingly, the effect of moderating neutrons on the channel fastener side becomes larger than that on the anti-fastener side. As a result, on the channel fastener side, the power obtained from the fuel rod 4 becomes relatively larger and thereby the local peaking factor tends to become larger. In the fuel assembly 1A according to this embodiment, the channel box 11 is left as it is and the center in a cross section of the fuel bundle 6 is offset toward the channel fastener 14 side. To be more specific, it is possible to get almost the same effect as follows, the narrow gap between the fuel assemblies on the anti-channel fastener side is made wide and simultaneously the wide gap between the fuel assemblies on the channel fastener side is made narrow. With this configuration, it is possible to reduce the difference between the above two gaps on both the channel fastener side and the anti-channel fastener side, and hence to relieve the difference between the continuous water regions on both the channel fastener side and the anti-channel fastener side. This makes it possible to lower the difference in power of fuel rods between the channel fastener side and the anti-channel fastener side, and hence to reduce the local peaking factor. In this way, according to the D-lattice core using the fuel assembly of the present invention, it is possible to obtain a core characteristic comparable to that of a C-lattice core and hence to achieve the fuel economy comparable to that of the C-lattice core. Since the local peaking factor can be reduced, the maximum value of the powers of the fuel rods 4 can be reduced. For example, when the local peaking factor is reduced by 5%, the maximum linear heat generation rate of the fuel rods 4 can be reduced by 5%. Accordingly, it is possible to increase the critical power obtainable from one fuel assembly 1A, and hence to increase the thermal margin of the fuel assembly 1A if the power of the core 2 is fixed at a certain value and increase the power of the core 2 if the thermal margin is fixed at a certain value. The above-described function of the fuel assembly in this embodiment will be more fully described below. During usual operation, water in the fuel assembly 1 (excluding water in the water rods 5) is in a mixed state of steam and liquid. At this time, the volume ratio of steam in the fuel assembly 1 is about 40% in average. Meanwhile, the gap between the fuel assemblies 1 is basically filled with only water in the liquid state. That is to say, the volume ratio of steam in the gap between the fuel assemblies 1 is 0%. Under each of the above conditions (in the gap between the fuel assemblies 1 and in the fuel assembly 1), the volume density of hydrogen atoms in water mainly contributing to moderation of neutrons is calculated at 70 atm (about 7 MPa) used for usual operation of the boiling water reactor, and the calculated values under both the conditions are compared with each other. Consequently, assuming that the calculated volume density of hydrogen atoms in the gap between the fuel assemblies 1 (volume ratio of steam: 0%) is taken as 1, the calculated volume density of hydrogen atoms in the fuel assembly 1 (volume ratio of steam: 40%) becomes about 0.6. Accordingly, the effect of moderating neutrons obtained by offsetting the fuel bundle by H=1 mm in the leftward direction (or upward direction) in FIG. 1 is substantially equal to the effect of moderating neutrons obtained by reducing the gap between the fuel assemblies by 0.6 mm. It may be considered that the critical power be reduced by offsetting the fuel bundle. To be more specific, since the gap between the inner side of the channel box 11 and the fuel rod 4 positioned at the outermost periphery becomes wider on the side opposed to the offset side, a larger amount of water flows in such a wider gap, causing a possibility that the amount of water directly contributing to cooling of the fuel rods 4 is reduced. However, if the gap is in the order of several mm, the area of the gap is sufficiently small with respect to the area of the total flow passage of water in the fuel assembly 1A. Also, while the critical power generally has an approximately one-to-one relationship with local peaking factor, the critical power has no one-to-one relationship with the flow rate of water contributing to cooling. Accordingly, even if the amount of water is reduced by 5%, the critical power is changed only by about 2-3%. As a result, in actual, it may be considered that the critical power is not reduced by offsetting the fuel bundle. To solve the non-uniformity of flow of water, the following measure may be adopted. Namely, the flow passage between the channel box 11 and the fuel rod positioned at the outermost periphery, which is widened by offsetting the fuel bundle, may be made narrow by providing a number of tabs on the inner side of the channel box 11 facing to the wide flow passage. Tabs may be additionally provided on the lower end or upper end of the band 8a of the fuel spacer 8 facing to the wide flow passage. In this case, the heights of the tabs must be larger than the heights of the tabs 8b1 and 8b2. The thickness of the portion, facing to the wide flow passage, of the channel box 11, may be increased; or a structural member may be additionally provided in the wide flow passage. With these configurations, it is possible to reduce the amount of water flowing in the wide flow passage. (2) Certain Reduction in Local Peaking Factor Due to Setting of Offset Amount The present inventors have examined, by numerical analysis, a relationship between the offset amount Y and the effect of reducing the local peaking factor, and obtained a result shown in FIG. 9. FIG. 9 is a graph showing a change in local peaking factor depending on a change in offset amount Y for the fuel assembly 1A in this embodiment. In this figure, the ordinate designates a relative value of the local peaking factor, which value represents a reduction ratio x% on the basis of the local peaking factor upon the offset amount Y=0 in which the center i in a cross section of the fuel bundle 6 is overlapped to the center j in a cross section of the channel box 11; and the abscissa designates a difference between the heights of the spacer tabs 8b1 and 8b2 directly related to the offset amount Y (X2xe2x88x92X1=Yxc3x972xe2x88x921/2, see the above-described equation 1). Referring to FIG. 9, as the difference X2xe2x88x92X1 starts to be increased from zero, the reduction ratio x% of the local peaking factor is rapidly increased, and when the difference X2xe2x88x92X1 becomes 0.4 mm, the reduction ratio x% becomes 1%. In a region in which the difference X2xe2x88x92X1 is more than 0.5 mm, the incremental rate of the reduction ratio x% of the local peaking factor is lowered. After that, the reduction ratio x% of the local peaking factor is gradually saturated even if the difference X2xe2x88x92X1 is further increased, and when the difference X2xe2x88x92X1 becomes 4 mm, the reduction ratio x% becomes 5%. On the basis of the above result, the present inventors have decided that in order to ensure the effect of reducing the local peaking factor and hence to improve the fuel economy, the difference X2xe2x88x92X1 may be preferably set at a value of 0.5 mm or more (X2xe2x88x92X1xe2x89xa70.5 mm), which is equivalent to Yxe2x89xa72xe2x88x923/2 mm in the above-described equation (1). The present inventors have further examined, by numerical analysis, a relationship between the offset amount Y and the average neutron infinite multiplication factor of the fuel assembly, and obtained a result shown in FIG. 10. The neutron infinite multiplication factor is an indicator for deciding the effect of neutrons generated by fission exerted on the next fission. FIG. 10 is a graph showing a change in incremental ratio of the neutron infinite multiplication factor depending on a change in offset amount Y. In this figure, the ordinate designates a relative value of the neutron infinite multiplication factor, which represents an incremental ratio y% on the basis of the neutron infinite multiplication factor upon the offset amount Y=0; and the abscissa designates the difference (X2xe2x88x92X1) mm between the heights of the spacer tabs 8b1 and 8b2. Referring to FIG. 10, as the value on the ordinate is increased, the neutron infinite multiplication factor is increased. To be more specific, as the value on the ordinate is increased, the neutrons come to be effectively used, so that the fuel is correspondingly saved and the fuel economy is improved. In FIG. 10, as the difference X2xe2x88x92X1 starts to be increased from zero, the incremental ratio y%. is rapidly increased. However, in a region in which the difference X2xe2x88x92X1 is 0.5 mm or more, the increased rate of the incremental ratio y% is reduced. After that, the incremental ratio y% is gradually saturated even if the difference X2xe2x88x92X1 is further increased, and when the difference X2xe2x88x92X1 becomes 4 mm, the incremental ratio Y% becomes 0.2%. The increase in neutron infinite multiplication factor by 0.2% corresponds to the reduction in the enrichment of U-235 by about 0.03%. As a result, to improve the fuel economy, the difference X2xe2x88x92X1 may be preferably set at a value of 0.5 mm or more (X2xe2x88x92X1xe2x89xa70.5 mm) In this embodiment, since the difference X2xe2x88x92X1 is set at 1 mm (Y=2xe2x88x921/2 mm) which satisfies the above relationship of X2xe2x88x92X1xe2x89xa70.5 mm, it is possible to ensure the effect of reducing the local peaking factor and hence to improve the fuel economy. However, in actual, the value X2xe2x88x92X1 has an upper limit. This will be described below. In the case of the fuel assembly including fuel rods placed in a square lattice array of 9-rows/9-columns, as described in xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, vol. 43, No. 530 (September, 1988; Wilmington Business Publication) p12-31, the diameter of a fuel rod is generally about 11.0 mm. To ensure the thermal margin, it is required to set a gap between the adjacent fuel rods at about 3 mm. In this case, a distance between both ends of the nine pieces of the fuel rods becomes 123 mm (11.0xc3x979+3xc3x97(9xe2x88x921)=123), and the inner width W of a channel box which surrounds the fuel bundle is about 134 mm as described in the above-described xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d and xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d (February, 1998, Hitachi, Ltd.). Accordingly, the remaining distance between a fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box on both sides is 11 mm at maximum (134xe2x88x92123=11). Meanwhile, a gap of 2 mm or more is generally required between the fuel rod positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box because a band of each fuel spacer must be inserted in the gap. It is generally required to give the same gap from the viewpoint of the thermal margin. As a result, the actually usable remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box for offsetting the fuel bundle becomes 7 mm (11xe2x88x922xc3x972=7). To be more specific, when the fuel bundle is offset on the channel fastener side, the actually movable maximum distance in the row or column direction of the square lattice array becomes 7 mm. In this embodiment, the difference X2xe2x88x92X1 is set at 1 mm which is less than the maximum value, that is, 7 mm. (3) Prevention of Reduction in Thermal Margin In this embodiment, unlike the structure in the above-described U.S. Pat. No. 2,791,132, the fuel bundle 6 is offset, that is, the fuel rod pitch is not changed. Accordingly, the thermal margin is not reduced as compared with the fuel assembly of the prior art D-lattice core. This will be described with reference to FIG. 11. The present inventors have examined, by numerical analysis, a relationship between the arrangement pitch of fuel rods and the critical power of a fuel assembly, and obtained a result shown in FIG. 11. FIG. 11 shows a change in critical power depending on a change in arrangement pitch of fuel rods. In this figure, the abscissa designates the fuel rod pitch, and the ordinate designates a relative value of the critical power of the fuel assembly. The fuel rod pitch of 9-rows/9-columns array in the prior art D-lattice fuel assembly is 14.4 mm, as described in the above-described xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d. Such a value (existing pitch) is equivalent to the right end of the abscissa and the critical power corresponding to the existing pitch is expressed by 1. As shown in FIG. 11, there is a linearly increasing relationship between the fuel rod pitch and the critical power. To be more specific, as the fuel rod pitch is reduced from the existing pitch, the critical power is linearly reduced. As a result, according to the configuration of the prior art fuel assembly, the local peaking factor can be improved; however, the thermal margin is reduced due to lowering of the critical power. To prevent the reduction in thermal margin, the fuel rod pitch must be substantially equal to that of the prior art D-lattice fuel assembly. As described above, the fuel rod pitch of the prior art D-lattice fuel assembly is 14.4 mm. In consideration of manufacturing errors, the present inventors have considered that it may be desirable to set the fuel rod pitch in a range of 14.15 mm to 14.65 mm. In this embodiment, since the fuel rod pitch is 14.4 mm which is within the above range, it is possible to prevent the reduction in thermal margin, which has appeared in the prior art fuel assembly. (4) Usability of Existing Fuel Spacer In general, a fuel spacer includes holding members (for example, cylindrical members) for holding fuel rods and water rods such that they are spaced from each other at specific gaps. If the fuel rod pitch is changed as in the prior art structure, the pitch of the holding members must be correspondingly changed. As a result, in the prior art structure, the existing fuel spacers cannot be used and new fuel spacers must be prepared. On the contrary, in this embodiment, since the fuel rod pitch is not changed, the existing fuel spacers 6 can be used as they are. (5) Function of Ensuring Reactor Shutdown Margin In general, for a fuel assembly of a boiling water reactor, as the size of a channel box becomes smaller, the reactor shutdown margin becomes smaller. This is shown, for example, in FIG. 6 of Japanese Patent No. 2791132. In this figure, the channel box is made small to increase the narrow water gap width relative to the wide water gap width, a difference in reactivity between upon power operation and upon cold shutdown becomes small. The reason for this will be described below. The width of a region in which water is continuously present exerts the largest effect on the reactor shutdown margin. In particular, in the gap between the fuel assemblies, water is continuously present over a wide region. The width of the water region in the gap between the fuel assemblies is specified by the channel box. To be more specific, by increasing the size of the channel box, the amount of water in the gap between the fuel assemblies is correspondingly reduced, so that the reactor shutdown margin is made small. Accordingly, to ensure the reactor shutdown margin substantially comparable to that of the prior art fuel assembly, the size of the channel box may be equal to that of the channel box of the prior art fuel assembly. As described in the above-documents xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d and xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, the inner width of the channel box of the prior art D-lattice fuel assembly is about 134 mm. In consideration of manufacturing errors, the present inventors have considered that it may be desirable to set the inner width of the channel box in a range of 133.5 mm to 134.5 mm. In this embodiment, since the inner width of the channel box 11 is 134.1 mm which is within the above range, it is possible to ensure the reactor shutdown margin comparable to that of the prior art D-lattice fuel assembly. As described above, according to the fuel assembly 1A in this embodiment, it is possible to achieve the fuel economy comparable to that of a C-lattice core without reducing the thermal margin, and to utilize the existing fuel spacers as they are. It should be noted that many variations of the above-described embodiment may be made without departing from the scope of the present invention. Some of such variations may be described below. Variation (a)xe2x80x94Structure of Fixing Tabs of Fuel Spacer by Welding FIG. 12 is a sectional view showing a detailed structure of a fuel spacer 8 according to this variation, in which cylindrical members and spring members are not shown like FIG. 1. Referring to FIG. 12, tabs 8b2 having a large height and tabs 8b1 having a small height are fixed on a band 8a by welding. In this case, it is possible to eliminate the necessity of forming the tabs 8b1 and 8b2 by extruding associated portions of the band 8a as in the above embodiment, and hence to make the degree of freedom in design larger than that in the above embodiment. Additionally, only the tabs 8b2 may be fixed by welding and the tabs 8b1 may be formed by extrusion like the above embodiment. This leads to an advantage in reducing the working cost. Variation (b)xe2x80x94Structure of Providing Tabs Even on Channel Box FIG. 13 shows a transverse sectional structure of a fuel spacer 8 and a channel box 11 according to this variation, in which cylindrical members and spring members are not shown like FIG. 12. Referring to FIG. 13, the tabs 8b1 having the same relatively small height are provided on the fuel spacer 8, and inwardly projecting tabs 11a1 and 11a2 are provided on the inner peripheral surface of the channel box 11 in such a manner that the tabs 11a1 and 11a2 are in contact with the tabs 8b1. To be more specific, the tabs 11a2 having a large height Z2 are disposed on one side (lower right side, anti-control rod side in FIG. 13) with respect to a diagonal line m of the transverse cross-section, and the tabs 11a1 having a small height Z1 are disposed on the other side (upper left side, control rod side in FIG. 13) with respect to the diagonal line m. These tabs 11a1 and 11a2 also function as the above-described offsetting/holding means. The thickness of the channel box 11 is generally larger than that of the band 8a of the fuel spacer 8, and accordingly, the degree of freedom in design becomes larger than that in the above embodiment. In addition, the tabs than having the small height may be omitted and only the tabs 11a2 may be provided. This is advantageous in reducing the working cost. It may be considered to omit the tab 8b1 on the fuel spacer 8 side and provide only the tabs 11a1 and 11a2 on the channel box 11 side. Further, as shown in FIG. 14, tabs 8b1 having a small height and tabs 8b2 having a large height may be disposed on the fuel spacer 8. This is particularly suitable for a structure in which the offset amount of the fuel bundle is made larger. Variation (c)xe2x80x94Application to Structure Including Square Type Water Rod In the above embodiment, the present invention is applied to the structure in which the two water rods 5 are disposed in the square lattice array of 9-rows/9-columns; however, the present invention is not limited thereto but may be applied to a structure shown in FIG. 15 in which one square water rod is disposed in a square lattice array of 9-rows/9-columns. In this case, the same effect can be obtained. Further, the present invention can be applied to a structure shown in FIG. 16 in which a square water rod 5A is disposed in such a manner as to be offset from the center in a cross section of the square lattice array. Variation (d)xe2x80x94Application to Array of 10-rows/10-columns In the above embodiment, the present invention is applied to the fuel assembly of the square lattice array of 9-rows/9-columns; however, the present invention is not limited thereto and may be applied to a fuel assembly of a square lattice array of 10-rows/10-columns. FIG. 17 is a horizontally transverse sectional view of a fuel assembly 201 in this variation, which is equivalent to FIG. 1 showing the fuel assembly 1 in the above embodiment. In FIG. 17, parts corresponding to those of the fuel assembly 1 in the above embodiment are designated by reference numerals obtained by adding 200 to the reference numerals of the parts shown in FIG. 1, and the overlapped explanation thereof is omitted. Referring to FIG. 17, a fuel bundle 206 includes fuel rods 204 placed in a square lattice array of 10-rows/10-columns. Like the structure shown in FIG. 1, the position of the square lattice array is offset as a whole in the upward, leftward direction in FIG. 17. To be more specific, the center in a cross section of the array is offset from the center in a cross section of a channel box 211 toward the control rod side. The offset amount Y is 2xe2x88x921/2 mm in the upward, leftward direction in FIG. 17. In other words, the offset amount Y includes an offset component of 0.5 mm in the upward direction and an offset component of 0.5 mm in the leftward direction in FIG. 17. In this variation, 92 pieces of the fuel rods 204 are disposed; the outside diameter d of each fuel rod 204 is 10.05 mm; and the fuel rod pitch p of the fuel rods 204 is 12.9 mm. These fuel rods 204 may include short-length fuel rods like the fuel rods 4b shown in FIG. 1. Two water rods 205 are disposed at an approximately central portion of the fuel assembly 201. In this case, four pieces of the fuel rods 204 of 2-rows/2-columns are replaced with each water rod 205. Like the structure shown in FIG. 1, the inner width W of the channel box 211 is 134.1 mm. The center in a cross section of a fuel spacer 208 is equal to the center in a cross section of the fuel bundle 206. Like the fuel spacer 8 shown in FIG. 1, the fuel spacer 208 includes a band 208a and tabs 208b. To be more specific, four pieces of the tabs 208b1 are disposed on the control rod side, and four pieces of the tabs 208b2 are disposed on the anti-control rod side. Like the structure shown in FIG. 1, a difference (X2xe2x88x92X1) between the height X2 of the tab 208b2 and the height X1 of the tab 208b1 is 1 mm. The other configuration in this variation is substantially the same as that of the fuel assembly 1 in the above embodiment. As is apparent from the above description, even in this variation, there can be obtained the same functions as those of the above embodiment, that is, the functions of (1) reducing the local peaking factor due to the offset structure, (2) ensuring the reduction in the local peaking factor due to setting of the offset amount, (3) preventing lowering of the thermal margin, (4) realizing usability of the existing fuel spacers, and (5) ensuring the reactor shutdown margin. With respect to the function (3), a relationship between the fuel rod pitch placed in the prior art square lattice array of 10-rows/10-columns and the fuel rod pitch in this variation will be described in detail below. According to the document described in the above embodiment, xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d (February, 1998, Hitachi, Ltd.xe2x80x9d, the inner width W of the channel box surrounding the fuel bundle of the square lattice array of 9-rows/9-columns is the same as that in the square lattice array of 8-rows/8-colmns, that is, about 134 mm. Further, even for the square lattice array of 10-rows/10-columns, as described in the above document xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, the inner width W of the channel box is about 134 mm. As described in the above document xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d, the distance g between the fuel rod positioned at the outer periphery of the square lattice array and the inner peripheral surface of the channel box in the case of the fuel assembly of 8-rows/8-columns is the same as that in the case of the fuel assembly of 9-rows/9-columns. To be more specific, for the fuel assembly of 8-rows/8-columns, since the fuel rod pitch is 16.3 mm and the diameter of the fuel rod is 12.3 mm, the distance g becomes 3.85 mm [(134xe2x88x9216.3xc3x977xe2x88x9212.3)/2=2.85]; and for the fuel assembly of 9-rows/9-columns, since the fuel rod pitch is 14.4 mm and the diameter of the fuel rod is 11.2 mm, the distance g becomes 3.85 mm [(134.1xe2x88x9214.4xc3x978xe2x88x9211.2)/2=3.85)]. Even for the fuel assembly of 10-rows/10-columns, the distance g similarly becomes 3.85 mm. On the other hand, for the fuel assembly of 10-rows/10-columns, as described in the above document xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, the diameter d of the fuel rod is generally 10.05 mm. From the above inner width W=134 mm, the distance g=3.85 mm, and the diameter d=10.05 mm, the fuel rod pitch p becomes 12.9 mm [(134xe2x88x922xc3x973.85xe2x88x9210.05)/9]. In this way, the arrangement of the fuel rods in the prior art fuel assembly of 10-rows/10-columns becomes 12.9 mm. In consideration of manufacturing errors, the present inventors have considered that it may be desirable to set the fuel rod pitch in a range of 12.65 mm to 13.15 mm. In this variation, since the fuel rod pitch is 12.9 mm which is within the above range, it is possible to prevent the reduction in thermal margin. With respect to the function (2), even for the fuel assembly of 10-rows/10-columns, like the fuel assembly of 9-rows/9-columns, the difference X2xe2x88x92X1 has an upper limitation. This will be described below. As described, for the fuel assembly of 10-rows/10-columns, the diameter d of the fuel rod is generally 10.05 mm, and to ensure the thermal margin, it is required to set the gap between the adjacent fuel assemblies at about 2.5 mm. In this case, the distance between both ends of 10 pieces of the fuel rods becomes 123.0 mm (10.05xc3x9710+2.5xc3x97(10xe2x88x921)=123.0). The inner width W of the channel box which surrounds the fuel bundle is, as described above, 134 mm. Accordingly, the remaining distance between the fuel rods positioned at the outer periphery of the square lattice array and the inner peripheral surface of the channel box on both sides is 11 mm at maximum (134xe2x88x92123=11). Meanwhile, a gap of 2 mm or more is generally required between the fuel rod positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box. As a result, the actually usable remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box for offsetting the fuel bundle becomes 7 mm (11xe2x88x922xc3x972=7). In this variation, the difference X2xe2x88x92X1 is set at 1 mm which is less than the maximum value, that is, 7 mm. As described above, even in this variation, it is possible to achieve the fuel economy comparable to that of a C-lattice core without reducing the thermal margin, and to utilize the existing fuel spacers as they are. In addition, as shown in FIG. 18, the present invention can be applied to a structure in which a square water rod 205A is disposed in such a manner as to be offset from the center in a cross section of the square lattice array. Variation (e)xe2x80x94Application to Fuel Assembly of 11-rows/11-columns The present invention can be also applied to a fuel assembly of a square lattice array of 11-rows/11-columns. FIG. 19 is a horizontally transverse sectional view of a fuel assembly 301 in this variation, which is equivalent to FIG. 1 showing the fuel assembly 1 in the above embodiment. In FIG. 19, parts corresponding to those of the fuel assembly 1 in the above embodiment are designated by reference numerals obtained by adding 300 to the reference numerals of the parts shown in FIG. 1, and the overlapped explanation thereof is omitted. Referring to FIG. 19, a fuel bundle 306 includes fuel rods 304 placed in a square lattice array of 11-rows/11-columns. Like the structure shown in FIG. 1, the position of the square lattice array is offset as a whole in the upward, leftward direction in FIG. 19. To be more specific, the center in a cross section of the array is offset from the center in a cross section of a channel box 311 toward the control rod side. The offset amount Y is 2xe2x88x921/2 mm in the upward, leftward direction in FIG. 19. In other words, the offset amount Y includes an offset component of 0.5 mm in the upward direction and an offset component of 0.5 mm in the leftward direction in FIG. 19. In this variation, 112 pieces of the fuel rods 304 are disposed, and the outside diameter d of each fuel rod 304 is 9.2 mm. These fuel rods 304 may include short-length fuel rods like the fuel rods 4b shown in FIG. 1. The fuel rod pitch p of the fuel rods 304 is 11.7 mm. One square water rod 305 is disposed at an approximately central portion of the fuel assembly 301. In this case, nine pieces of the fuel rods 304 of 3-rows/3-columns are replaced with the water rod 305. Like the structure shown in FIG. 1, the inner width W of the channel box 311 is 134.1 mm. The center in a cross section of a fuel spacer 308 is equal to the center in a cross section of the fuel bundle 306. Like the fuel spacer 8 shown in FIG. 1, the fuel spacer 308 includes a band 308a and tabs 308b. A difference (X2xe2x88x92X1) between the height X2 of the tab 308b2 on the control rod side and the height X1 of the tab 308b1 on the anti-control rod side is 1 mm. The other configuration in this variation is substantially the same as that of the fuel assembly 1 in the above embodiment. Even in this variation, there can be obtained the same functions as those of the above embodiment, that is, the functions of (1) reducing the local peaking factor due to the offset structure, (2) ensuring the reduction in the local peaking factor due to setting of the offset amount, (3) preventing lowering of the thermal margin, (4) realizing usability of the existing fuel spacers, and (5) ensuring the reactor shutdown margin. With respect to the function (3), a relationship between the fuel rod pitch placed in the prior art square lattice array of 11-rows/11-columns and the fuel rod pitch in this variation will be described in detail below. As described in the above document xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, the number of the fuel rods in the prior art fuel assembly of the square lattice array of 10-rows/10-columns is 91. The number of the long-length fuel rods having a relatively long fuel effective length (usual fuel rods, overall length fuel rods) is 83, and the number of the short-length fuel rods having a relatively short fuel effective length is 8. The fuel effective length of the short-length fuel rod is not particularly described. Here, like the prior art square lattice array of 9-rows/9-columns (for example, described in the above document xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d), the length of the short-length fuel rod is set at about 15/24 of the length of the long-length fuel rod. In this case, 91 pieces of the fuel rods are equivalent to 88 pieces of the long-length fuel rods (83+8xc3x97(15/24)=88) It is assumed that the number of the prior art fuel assembly of a square lattice array of 11-rows/11-columns is 112 as in this variation. The ratio of the number of the short-length fuel rods to the total number of the fuel rods is assumed to be the same as that in the array of 10-rows/10-columns. In this case, the number of the short-length fuel rods becomes 10 (112xc3x97(8/91)=10). That is to say, in the prior art array of 11-rows/11-columns, the number of the long-length fuel rods is 102 and the number of the short-length fuel rods is 10. Assuming that the fuel effective length of the short-length fuel rod is taken as 15/24 of the length of the long-length fuel rod, 112 pieces of the fuel rods are equivalent to 108 pieces of the long-length fuel rods (102+10xc3x97(15/24)=108). In the prior art fuel assembly of the array of 11-rows/11-columns, the value of the fuel effective length is not particularly described. However, since the height of the core of the existing boiling water reactor is generally within a certain range, the fuel effective length of the array of 11-rows/11-columns may be considered to be substantially the same as that of the array of 10-rows/10-columns. The same is substantially true for the fuel inventory. On the assumption of the same fuel effective length, the condition with the same fuel inventory is expressed by (diameter of fuel pellet)xc3x97(conversion number of fuel rods based on long-length fuel rod)=(constant) As described in the above document xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, the diameter of the fuel pellet in the array of 10-rows/10-columns is generally 8.67 mm. The conversion number of the fuel rods based on the long-length fuel rods in the array of 10-rows/10-columns is, as described above, 88. On the other hand, the conversion number of the fuel rods based on the long-length fuel rods in the array of 11-rows/11-columns is, as described above, 108. Letting the diameter of the fuel pellet in the array of 11-rows/11-columns be D, an equation of 8.67xc3x9788=Dxc3x97108 is established. That is to say, the diameter of the fuel pellet D becomes 7.83 mm. Next, a relationship between the diameter d of the fuel rod and the diameter D of the fuel pellet will be examined. As described in the above document xe2x80x9c9xc3x979 Fuel in Boiling Water Type Nuclear Power Stationxe2x80x9d (February, 1998, Hitachi, Ltd.), in the square lattice array of 9-rows/9-columns, D/d becomes 0.86 (9.6/11.2=0.86). As described in the above document xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, in the square lattice array of 10-rows/10-columns, D/d becomes 0.86 (8.67/10.05=0.86). That is to say, D/d becomes substantially a specific value irrespective of the array of n-rows/n-columns. Accordingly, for the array of 11-rows/11-columns, it can be assumed that D/d is 0.86. As a result, since the diameter D of the fuel pellet is 7.83 mm, the diameter d of the fuel rod becomes 9.2 mm (7.83/0.86=9.2). As described in the variation (d), the inner width W of the channel box which surrounds the fuel bundle is 134.0 mm irrespective of the array of n-rows/n-columns. The fuel rod pitch p of the square lattice array of 11-rows/11-columns thus becomes 11.7 mm [(134xe2x88x929.2xe2x88x923.85xc3x972)/10=11.7]. In this way, the arrangement of the fuel rods in the prior art fuel assembly of 11-rows/11-columns becomes 11.7 mm. In consideration of manufacturing errors, the present inventors have considered that it may be desirable to set the fuel rod pitch in a range of 11.45 mm to 11.95 mm. In this variation, since the fuel rod pitch is 11.7 mm which is within the above range, it is possible to prevent the reduction in thermal margin. With respect to the function (2), even for the fuel assembly of 11-rows/11-columns, like the fuel assembly of 9-rows/9-columns, the difference X2xe2x88x92X1 has an upper limitation. This will be described below. As described, for the fuel assembly of 11-rows/11-columns, the diameter d of the fuel rod is generally 9.2 mm, and to ensure the thermal margin, it is required to set the gap between the adjacent fuel assemblies at about 2.0 mm. In this case, the distance between both ends of 11 pieces of the fuel rods becomes 121.2 mm (9.2xc3x9711+2.0xc3x97(11xe2x88x921)=121.2). The inner width W of the channel box which surrounds the fuel bundle is, as described above, 134 mm. Accordingly, the remaining distance between the fuel rods positioned at the outer periphery of the square lattice array and the inner peripheral surface of the channel box on both sides is 12.8 mm at maximum (134xe2x88x92121.2=12.8). Meanwhile, a gap of 2 mm or more is generally required between the fuel rod positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box. As a result, the actually usable remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box for offsetting the fuel bundle becomes 8.8 mm (12.8xe2x88x922xc3x972=8.8). In this variation, the difference X2xe2x88x92X1 is set at 1 mm which is less than the maximum value, that is, 8.8 mm. As described above, even in this variation, it is possible to achieve the fuel economy comparable to that of a C-lattice core without reducing the thermal margin, and to utilize the existing fuel spacers as they are. In addition, as shown in FIG. 20, the present invention can be applied to a structure in which a square water rod 305A is disposed in such a manner as to be offset from the center in a cross section of the square lattice array.