Patent Number: 056404352
Section: claims

1. A fuel assembly including: a lower tie plate having a fuel supporting portion at the upper end thereof;  a plurality of fuel rods supported at the lower end thereof by said fuel supporting portion;  a water rod disposed between said fuel rods, and supported at the lower end thereof by said fuel supporting portion; and  a plurality of fuel spacers disposed in an axial direction of said fuel rods, and keeping spaces between said fuel rods; wherein  said fuel supporting portion has a plurality of second coolant paths for supplying a coolant from below said fuel supporting portion into a first coolant path defined above said fuel supporting portion, outside said water rod and between said fuel rods;  the total cross-sectional area of said second coolant paths is smaller than the cross-sectional area of said first coolant path at least at a position immediately above said fuel supporting portion;  said water rod includes an ascending tube path having therein an ascending path having a coolant inlet port opening to a region below said fuel supporting portion, for guiding upward a coolant supplied through said coolant inlet port, and a descending tube path having therein a coolant descending path having a coolant delivery port opening to said first coolant path, and connected to said ascending tube path, for guiding downward said coolant guided by said coolant ascending path and discharging said coolant through said coolant delivery port to said first coolant path; and  said descending tube path is so positioned outside said ascending tube path that the space through which said coolant in said first coolant path passes is defined between said descending tube path and said ascending tube path;  either one of said ascending tube path and said descending tube path is inserted into a coupling member, the upper end of said one of said ascending tube path and said descending tube path is welded to one part of said coupling member, the other one of said ascending tube path and said descending tube path is welded to a lower part of said coupling member at a position lower than said one part; and  a cover member is attached to said coupling member, said cover member and said coupling member delimiting a communication path for communication of said coolant ascending path with said coolant descending path.  a lower tie plate having a fuel supporting portion at the upper end thereof;  a plurality of fuel rods supported at the lower end thereof by said fuel supporting portion;  a water rod disposed between said fuel rods, and supported at the lower end thereof by said fuel supporting portion; and  a plurality of fuel spacers disposed in an axial direction of said fuel rods, and keeping spaces between said fuel rods; wherein  said fuel supporting portion has a plurality of second coolant paths for supplying a coolant from below said fuel supporting portion into a first coolant path defined above said fuel supporting portion, outside said water rod and between said fuel rods;  the total cross-sectional area of said second coolant path is smaller than the cross-sectional area of said first coolant path at least at a position immediately above said fuel supporting portion;  said water rod includes an ascending tube path having therein a coolant ascending tube path having a coolant inlet port opening to a region below said fuel supporting portion, for guiding upward a coolant supplied through said coolant inlet port, and a descending tube path having therein a coolant descending path having a coolant delivery port opening to said first coolant path, and connected to said ascending tube path, for guiding downward said coolant guided by said coolant ascending path and discharging said coolant through said coolant delivery port to said first coolant path;  said descending tube path is so positioned outside said ascending tube path that the space through which said coolant in said first coolant path passes is defined between said descending tube path and said ascending tube path;  each of said fuel spacers includes therein a plurality of cylindrical members into which said fuel rods are inserted;  the cross-sectional area of said coolant descending path is smaller than that of said coolant ascending path at least at a position about a middle of said coolant ascending path;  said ascending tube path is disposed in a region in which a plurality of said fuel rods can be disposed; and  said descending tube path is disposed between a plurality of said cylindrical members facing said ascending tube path and adjacent to one another;  either one of said ascending tube path and said descending tube path is inserted into a coupling member, the upper end of said one of said ascending tube path and said descending tube path is welded to one part of said coupling member, the other one of said ascending tube path and said descending tube path is welded to a lower part of said coupling member at a position lower than said one part; and  a cover member is attached to said coupling member, said cover member and said coupling member delimiting a communication path for communication of said coolant ascending path with said coolant descending path. 2. A fuel assembly according to claim 1, wherein said ascending tube path includes a large diameter tube portion and a small diameter tube portion positioned below said large diameter tube portion and connected to said large diameter tube portion, said large and small diameter tube portions having different outside diameters and the boundary between said large diameter tube portion and said small diameter tube portion is positioned between said fuel spacer positioned in a lowermost position and said fuel supporting portion. 3. A fuel assembly according to claim 1, wherein said coolant inlet port is disposed on the side wall of said ascending tube path in a position below said fuel supporting portion. 4. A fuel assembly according to claim 1, wherein the cross-sectional area of said coolant descending path is smaller than that of said coolant ascending path at least at a position about a middle of said coolant ascending path. 5. A fuel assembly according to claim 4, wherein said ascending tube path includes a large diameter tube portion and a small diameter tube portion positioned below said large diameter tube portion and connected to said large diameter tube portion, said large and small diameter tube portions having different diameters, and the cross-sectional area of said coolant descending path is smaller than 1/25 of that of said coolant ascending path at least at the position about the middle of said coolant ascending path. 6. A fuel assembly according to claim 1, wherein a tubular portion protruding upward, for blocking said coolant ascending path is disposed in said coolant ascending path in said ascending tube path, and said coolant ascending path below said tubular portion communicates with said coolant ascending path above said tubular portion through an opening bored in the side wall of said tubular portion. 7. A fuel assembly including: 8. A fuel assembly according to claim 7, wherein said ascending tube path includes a large diameter tube portion and a small diameter tube portion positioned below said large diameter tube portion and connected said large diameter tube portion, said large and small diameter portions different diameters, and the cross-sectional area of said coolant descending path is smaller than 1/25 of the cross-sectional area of said coolant ascending path at least at a position about a middle of said coolant ascending path. 9. A fuel assembly according to claim 7, wherein said coolant inlet port is disposed on the side wall of said ascending tube path in a position below said fuel supporting portion. 10. A fuel assembly according to claim 7, wherein a tubular member protruding upward, for blocking said coolant ascending path is disposed in said coolant ascending path in said ascending tube path, and said coolant ascending path below said tubular portion communicates with said coolant ascending path above said tubular portion through an opening bored in the side wall of said tubular portion. 11. A fuel assembly according to claim 1, wherein said coupling member has first and second openings extending therethrough, said first opening being spaced from and larger than said second opening, the upper end of said descending tube path being inserted into the second opening of said one part of said coupling member and being welded to said coupling member, said ascending tube path having a portion of said coupling member inserted therein so that said first opening of said coupling member communicates with the interior of said ascending tube path, said descending tube path being welded to said lower part of said coupling member, and said cover member being attached to an upper part of said coupling member. 12. A fuel assembly according to claim 1, wherein said coupling member has first and second openings extending therethrough, said first opening being spaced from and larger than said second opening, the upper end of said ascending tube path being inserted into the first opening of said coupling member and being welded to said one part of said coupling member, said descending tube path having a portion of said coupling member inserted therein so that said second opening of said coupling member communicates with the interior of said descending tube path, said descending tube path being welded to said lower part of said coupling member in the region of said second openings, said cover member being attached to an upper part of said coupling member. 13. A fuel assembly according to claim 7, wherein said coupling member has first and second openings extending therethrough, said first opening being spaced from and larger than said second opening, the upper end of said descending tube path being inserted into the second opening of said coupling member and being welded to said one part of said coupling member, said ascending tube path having a portion of said coupling member inserted therein so that said first opening of said coupling member communicates with the interior of said ascending tube path, said descending tube path being welded to said lower part of said coupling member, and said cover member being attached to an upper part of said coupling member. 14. A fuel assembly according to claim 7, wherein said coupling member has first and second openings extending therethrough, said first opening being spaced from and larger than said second opening, the upper end of said ascending tube path being inserted into the first opening of said coupling member and being welded to said one part of said coupling member, said descending tube path having a portion of said coupling member inserted therein so that said second opening of said coupling member communicates with the interior of said descending tube path, said descending tube path being welded to said lower part of said coupling member in the region of said second openings, said cover member being attached to an upper part of said coupling member.