Patent Number: 047088460
Section: claims

1. A fuel assembly for a boiling water reactor comprising a group of vertical, parallel, elongated fueld tubes containing a fissionable material, said assembly having a critical heat transfer zone in its upper portion; at least one water tube parallel to said fuel tubes and located among them, said water tube having an inlet adjacent its lower end; means closing the upper end of said water tube; an inner tube within the upper portion of said water tube having a lower end in sealing relationship with the inner surface of said water tube, forming an annulus with said water tube and having at least one opening adjacent its upper end; at least one upper exit opening in said water tube above the lower end of said inner tube and located within the critical heat transfer zone of said assembly, whereby water will flow upwardly through said water tube, upwardly through said inner tube, then downwardly through said water tube annulus and out through said upper exit opening, into the critical heat transfer zone of said assembly. 2. An assembly as defined in claim 1, and further including an intermediate exit opening in said water tube adjacent the lower end of said critical heat transfer zone. 3. An assembly as defined in claim 2, wherein said intermediate exit opening is located in the range of about 50% to about 65% of the height of the fuel assembly measured from the bottom. 4. An assembly as defined in claim 3, wherein said upper exit opening is located in the range of about 65% to about 75% of the height of the assembly measured from the bottom, said intermediate and upper exit openings being spaced a substantial distance apart. 5. An assembly as defined in claim 1, wherein said water tube extends through a lower tie plate of said assembly, and is open at its lower end. 6. An assembly as defined in claim 2, wherein said water tube extends through a lower tie plate in said assembly and is open at its lower end. 7. An assembly as defined in claim 3, wherein said water tube extends through a lower tie plate of said assembly and is open at its lower end. 8. An assembly as defined in claim 4, wherein said water tube extends through a lower tie plate of said assembly and is open at its lower end. 9. An assembly as defined in claim 1, and comprising an upper end plug, closing said water tube and said inner tube, both said tubes being joined to said end plug, and said inner tube being provided with openings near its upper end. 10. An assembly as defined in claim 9, wherein said inner tube is flared at its lower end into sealing engagement with said water tube. 11. An assembly as defined in claim 9, wherein said inner tube is secured at its lower end to an inner plug which seals said water tube.