Patent Number: 062051962
Section: claims

1. A boiling water type nuclear reactor core in which a plurality of fuel assemblies, each enclosed in a channel box, are loaded and a plurality of control rods, each having control blades with a neutron absorber, are arranged between said channel boxes comprising: square bundle regions, each of which is formed by a plurality of fuel assemblies;  short blade control rods, each of which has a latitudinal control rod blade length of about one half of the width of said square bundle region and is arranged between said channel boxes in the center of each of said square bundle regions; and  long blade control rods, each of which has a latitudinal control rod blade length of about twice as long as that of said short blade control rods and is arranged between said channel boxes on a diagonal line of each of said square bundle regions.  square bundle regions, each of which is formed by a plurality of fuel assemblies;  axially movable short blade control rods, each of which has a latitudinal control rod blade length of about one half of the width of one of said square bundle regions and is arranged in the center of each of said square bundle regions; and  axially movable long blade control rods, each of which has a latitudinal control rod blade length substantially the same as the width of said square bundle region and is arranged between said channel boxes on a diagonal line of each of said square bundle regions.  square bundle regions, each of which is formed by a plurality of fuel assemblies;  axially movable short blade control rods, each of which has a latitudinal control rod blade length of about one half of the width of one of said square bundle regions and is arranged in the center of each of said square bundle regions; and  axially movable long blade control rods, each of which has a latitudinal control rod blade length substantially the same as the width of one of said square bundle regions and is arranged at one of two diagonal corners of each of said square bundle regions. 2. A boiling water type nuclear reactor core according to claim 1, wherein said long blade control rods are arranged between channel boxes on the diagonal line of each square bundle region formed of 16 fuel assemblies each having a fuel rod lattice structure of 8.times.8, 9.times.9 or 10.times.10, each short blade control rod is arranged in the center of said square bundle region, the blade length of said long blade control rods is about 4 times as long as the width of said fuel assembly, and the blade length of said short blade control rods is about twice as long as the width of the fuel assemblies. 3. A boiling water type nuclear reactor core according to claim 1, wherein each of said fuel assemblies constituting said square bundle region is provided with a water rod, said water rod has therein an ascending flow path and a descending flow path, said flow paths are connected to an inflow hole and an outflow hole of said water rod, respectively, and said inflow hole is positioned at a position lower than said outflow hole. 4. A boiling water type nuclear reactor core according to claim 1, wherein as neutron absorber arranged in each of said control rod blades, a material which becomes higher in control rod worth is used in portion facing a central side portion of said fuel assembly. 5. A boiling water type nuclear reactor core in which a plurality of fuel assemblies enclosed in respective channel boxes are loaded and a plurality of control rods, each having control blades, are arranged between said fuel assemblies, comprising: 6. A boiling water type nuclear reactor core in which a plurality of fuel assemblies are loaded and a plurality of control rods, each having control blades, are arranged between said fuel assemblies, comprising: 7. A boiling water type nuclear reactor core according to claim 6, wherein each of said square bundle regions is formed of four fuel assemblies, and the latitudinal blade length of each of said long blade control rods is about twice as long as the width of one of said fuel assemblies. 8. A boiling water type nuclear reactor core according to claim 6, wherein said short blade control rods each have a neutron absorber used therein, which is high in reactivity effect, whereby control rod worth is raised. 9. A boiling water type nuclear reactor core according to claim 8, wherein said short blade control rods each have neutron absorber of a higher enrichment in an upper region thereof than in the other region thereof. 10. A boiling water type nuclear reactor core according to claim 6, wherein said fuel assemblies are enclosed in respective channel boxes, and each said long blade control rod is arranged between channel boxes on the diagonal line of a square bundle region formed of four fuel assemblies, each said fuel assembly having a fuel rod lattice structure of 8.times.8, 9.times.9 or 10.times.10. 11. A boiling water type nuclear reactor core according to claim 6, wherein each of said fuel assemblies constituting a square bundle region has a water rod arranged therein, said water rod having a cross-sectional area corresponding to a cross-sectional area of several fuel rods. 12. A boiling water type nuclear reactor core according to claim 6, wherein a hydraulic driving mechanism is provided for hydraulically driving said long blade control rods.