Patent Number: 
Section: claims

1. A natural-circulation type boiling water reactor comprising:a plurality of divided chimneys provided above a reactor core and charged with a number of fuel assemblies in the reactor core, each of the plurality of divided chimneys includes plural rectangular-columnar lattice plates,wherein a pressure equalization structure is provided on at least one of the rectangular-columnar lattice plates below an intermediate section in an axial height direction of the divided chimneys below a top end of the divided chimneys, and pressures of the divided chimneys are equalized by the pressure equalization structure. 2. The natural-circulation type boiling water reactor according to claim 1, wherein the pressure equalization structure is configured to form one or more connection holes on surfaces of the lattice plates of the divided chimneys so as to equalize the pressures of the divided chimneys by the connection holes of the divided chimneys. 3. The natural-circulation type boiling water reactor according to claim 1, wherein the pressure equalization structure is configured to form a slit on surfaces of the lattice plates of the divided chimneys so as to equalize the pressures of the divided chimneys by the slit of the divided chimneys. 4. The natural-circulation type boiling water reactor according to claim 1, wherein the pressure equalization structure is configured to form a plurality of connection holes or slits on surfaces of the lattice plates of the divided chimneys so as to equalize the pressures of the divided chimneys. 5. The natural-circulation type boiling water reactor according to claim 1, wherein the pressure equalization structure is configured to form a clearance between the outlet of fuel channels and the lattice plates of the divided chimneys so as to equalize the pressures of the fuel assemblies by the clearance at the outlet of the fuel channels. 6. The natural-circulation type boiling water reactor according to claim 1, wherein in the plurality of the divided chimneys provided above the reactor core, one or more connection holes or slits are formed on each divided chimney arranged in an outermost circumferential region, one or more connection holes or slits are formed on each divided chimney arranged in the residual regions, and a connection hole or slit of each divided chimney arranged in the outermost circumferential region is formed larger in size than a connection hole or slit of each divided chimney arranged in the residual regions so as to equalize the pressures of the fuel assemblies arranged in the peripheral section. 7. A natural-circulation type boiling water reactor comprising:a plurality of divided chimneys provided above a reactor core and charged with a number of fuel assemblies in the reactor core,wherein a region of the divided chimneys is divided into a plurality of regions in a chimney height direction, a plurality of divided chimney regions separated by rectangular-columnar lattice plates are arranged in an upper group, a plurality of divided chimney regions separated by rectangular-columnar lattice plates are arranged in a lower group, the lower group being combined with the upper group at a portion below an intermediate section in an axial height direction of the divided chimneys, and a cross sectional area enclosed by the rectangular-columnar lattice plates of the upper group of the divided chimneys is configured to be larger than a cross sectional area enclosed by the rectangular-columnar lattice plates of the lower group of the divided chimneys so as to equalize pressures of the fuel assemblies arranged in the intermediate section in the chimney height direction. 8. The natural-circulation type boiling water reactor according to claim 7, wherein the cross-sectional area of the upper divided chimney is configured substantially equally to a total cross-sectional area of a plurality of lower divided chimneys. 9. A natural-circulation type boiling water reactor comprising:a plurality of divided chimneys provided above a reactor core and charged with a number of fuel assemblies in the reactor core,wherein the divided chimneys are divided into a plurality of groups in a chimney height direction, each of the divided chimneys in an upper divided chimney group is arranged entirely above each of the divided chimneys in a lower divided chimney group, anda central position of rectangular-columnar lattice plates of the upper divided chimney group is laterally shifted from that of rectangular-columnar lattice plates of the lower divided chimney group coupled with the upper divided chimney group and having a same flow area so as to equalize pressures of the fuel assemblies in an intermediate portion of the divided chimneys in the divided-chimney height direction. 10. The natural-circulation type boiling water reactor according to claim 9, wherein each divided chimney of the upper divided chimney group is horizontally shifted from each divided chimney of the lower divided chimney group by an amount that is greater than zero and less than a width size of a divided chimney of the lower divided chimney group.