Patent Number: 047770163
Section: claims

1. A fuel assembly comprising: upper and lower tie plates disposed so as to be spaced from each other;  a plurality of fuel rods both end portions of each of which is held by said upper and lower tie plates;  a plurality of spacers disposed between said upper and lower tie plates for providing spacings between said fuel rods;  a water rod disposed among said fuel rods and held at its lower end portion by said lower tie plate, said water rod having holes for passing water therethrough at its side portion, and a closed upper end portion of said water rod being substantially as high as an upper end of an effective enriched fuel section, which is filled with enriched fuel, of said each fuel rod; and  a channel box enclosing said fuel rods and said water rod, said upper and lower tie plates being fitted in said channel box;  wherein said water rod has a larger diameter than that of each of said fuel rods, and is disposed among said fuel rods so as to be positioned at a substantially central portion of said fuel assembly.  a channel box elongated axially;  upper and lower tie plates mounted in upper and lower end portions of said channel box;  fuel rods disposed in said channel box and held by said upper and lower tie plates at upper and lower ends of said fuel rods, respectively;  a large diameter central water rod disposed among said fuel rods around an axis of said channel box, and having a larger diameter than said each fuel rod, said large diameter central water rod having a closed upper end portion, and having holes for passing water at upper and lower side portions, and having a lower end plug held by said lower tie plate, and the upper end portion of said large diameter central water rod being substantially as high as an upper end of an effective enriched fuel section of said each fuel rod; and  spacers, disposed between said upper and lower tie plates, for keeping said fuel rods and said large diameter central water rod spaced from one another.  upper and lower tie plates;  a plurality of fuel rods each held by said upper and lower tie plates at upper and lower ends of said fuel rods, respectively, each of said fuel rods including an effective enriched fuel section and a plenum section thereon;  a plurality of spacers arranged axially for keeping said fuel rods spaced from each other; and  a large diameter central water rod, disposed among said fuel rods so as to be positioned at a central portion of said fuel rods, and having a larger diameter than said each fuel rod;  said large diameter central water rod having a lower end plug held by said lower tie plate and an upper closed end spaced from said upper tie plate so as to not engage said upper tie plate, said large diameter central water rod having holes for passing water therethrough proximate to the upper and lower ends thereof, said large diameter central water rod having a height at the upper end substantially as high as an upper end of said effective enriched fuel section of said each fuel rod which is filled with enriched fuel pellets. 2. The fuel assembly as defined in claim 1, wherein each of said fuel rods has a natural uranium blanket on said effective fuel portion, said upper end of said water rod being set substantially as high as a lower end of said natural uranium blanket region. 3. The fuel assembly as defined in claim 1, wherein said closed upper end portion of said water rod is spaced from said upper tie plate so that said water rod does not engage said upper tie plate. 4. The fuel assembly as defined in claim 3, wherein each fuel rod includes an effective enriched fuel section and a plenum section at the upper end of said fuel rod, said closed upper end portion of said water rod having a height substantially as high as said upper end of said effective enriched fuel section and a height substantially less than an upper end of said plenum section of said fuel rod. 5. The fuel assembly as defined in claim 4, wherein each of said fuel rods has a natural uranium blanket region on said effective fuel portion, said plenum section being disposed above said natural uranium blanket region, said upper end of said water rod having a height substantially as high as a lower end of said natural uranium blanket region. 6. A fuel assembly comprising: 7. The fuel assembly as defined in claim 6, wherein the diameter of said large diameter central water rod is extended to the extent that said large diameter central water rod occupies a space corresponding to one occupied by four of said fuel rods. 8. The fuel assembly as defined in claim 7, wherein said large diameter central water rod is provided with tabs engaged with said spacers. 9. The fuel assembly as defined in claim 6, wherein said closed upper end portion of said water rod is spaced from said upper tie plate so that said water rod does not engage with upper tie plate. 10. The fuel assembly as defined in claim 9, wherein each fuel rod includes an effective enriched fuel section and a plenum section at the upper end of said fuel rod, said closed upper end portion of said water rod having a height substantially as high as said upper end of said effective enriched fuel section and a height substantially less than an upper end of said plenum section of said fuel rod. 11. The fuel assembly as defined in claim 10, wherein each of said fuel rods has a natural uranium blanket region on said effective fuel portion, said plenum section being disposed above said natural uranium blanket region, said upper end of said water rod having a height substantially as high as a lower end of said natural uranium blanket region. 12. A fuel assembly comprising: 13. The fuel assembly as defined in claim 12, wherein said large diameter central water rod occupies a space corresponding to four of said fuel rods, said large diameter central water rod having tabs for engaging said spacers. 14. The fuel assembly as defined in claim 13, wherein said upper closed end of said large diameter central water rod has a substantially flat shape and has a height so as to provide a space between said upper closed end of said large diameter central water rod and said upper tie plate.