Patent Number: 048266540
Section: claims

1. A fuel assembly comprising a plurality of fuel rods, moderator rods disposed among said fuel rods, upper and lower tie plates for holding both end portions of each of said fuel rods and said moderator rods and fuel spacers for keeping spaces between said fuel rods and said moderator rods as wide as a predetermined width, the improvement wherein there is provided a space region substantially void of any solid, extending in an axial direction from said lower tie plate to said upper tie plate and having a space sufficient to disposed at least one of said fuel rods therein, at least one of said moderator rods being disposed immediately adjacent to said space region for delimiting a boundary of said space region in a direction transverse to the axial direction, said upper and lower tie plates delimiting a boundary of said space region in the axial direction, and said fuel rods, said moderator rods and said space region being disposed in a grid form, said space region having substantially the same cross-sectional area from an upper end portion thereof proximate to said upper tie plate to a lower end portion thereof proximate to said lower tie plate. 2. A fuel assembly as defined in claim 1, wherein said moderator rod is a water rod. 3. A fue1 assembly as defined in claim 1, wherein said space region is disposed in a central portion of a bundle of said fuel rods and said water rods. 4. A fuel assembly as defined in claim 3, wherein wherein said space region corresponds to a unit cell which which has a space as wide as one of said fuel rods and said moderator rods being positioned to surround said space region. 5. A fuel assembly as defined in claim 4, wherein said space region is encompassed by eight of said water rods so as to face directly said eight water rods. 6. A fuel assembly as defined in claim 4, wherein said space region is encompassed by four pairs of said water rods so as to face directly one of each said pair of water rods, and each of said pairs of water rods are arranged radially. 7. A fuel assembly as defined in claim 3, wherein said space region corresponds to five unit cells each of which has a space as wide as one of said fuel rods, said moderator rods being disposed to surround said space region. 8. A fuel assembly comprising: a channel box axially elongated;  a plurality of fuel rods disposed in said channel box;  a plurality of moderator rods disposed among said fuel rods;  upper and lower tie plates holding both end portions of said fuel rods and said moderator rods, respectively;  fuel spacers for keeping spaces between said fuel rods and said moderator rods as wide as a predetermined width;  a space region substantially void of any solid positioned at a central portion of said channel box, said space region extending in the axial direction from said lower tie plate to said upper tie plate so as to be delimited in the axial direction by said lower and upper tie plates, and a space size of said space region being such that at least one of said fuel rods can be disposed therein;  wherein at least four of said moderator rods are disposed immediately adjacent to said space region so that each of said four moderator rods faces said space region and for a boundary of said space region in a direction transverse to the axial direction; and  wherein said space region has substantially the same cross-sectional area along the axial direction from an upper end portion thereof to a lower end portion thereof.  a channel box axially elongated;  a plurality of fuel rods disposed in said channel box;  a plurality of moderator rods disposed among said fuel rods;  upper and lower tie plates holding both end portions of said fuel rods and said moderator rods, respectively;  fuel spacers for keeping spaces between said fuel rods and said moderator rods as wide as a predetermined width;  means for delimiting a space region substantially void of any solid at a predetermined portion of said channel box and a size sufficient to enable at least one of said fuel rods to be disposed therein, said means delimiting said space region in the axial direction of said channel box including said upper and lower tie plates and in a direction transverse to the axial direction including at least one moderator rod disposed immediately adjacent to said space region so that said at least one moderator rod faces said space region and forms a part of a boundary of said space region; and  wherein said space region has substantially the same cross-sectional area from an upper end portion thereof to a lower end portion thereof in the axial direction of said channel box. 9. A fuel assembly as defined in claim 8, wherein each of said moderator rods is a water rod and said water rods are arranged symmetrically with respect to said space region. 10. A fuel assembly comprising: 11. A fuel assembly as defined in claim 10, wherein said predetermined portion of said channel box is a central portion of said channel box in the transverse direction, and at least four of said moderator rods are disposed immediately adjacent to said space region so that each of said four moderator rods faces said space region and forms a part of a boundary thereof. 12. A fuel assembly as defined in claim 11, wherein each of said moderator rods is a water rod and said water rods are arranged symmetrically with respect to said space region. 13. A fuel assembly as defined in claim 10, wherein said space region region is adapted to be filled with water when said fuel assembly is loaded in a core of a nuclear reactor.