Patent Number: 048030449
Section: description

DETAILED DESCRIPTION In describing my invention, reference will be made to the above application, Ser. No. 850,104, now U.S. Pat. No. 4,708,846. A fuel assembly for a boiling water reactor of the type to which this inention applies is shown in FIG. 1 of that patent. Like the assembly shown in that figure, the embodiment of this invention illustrated in FIG. 1 and 2 includes an outer flow channel 2, and a plurality of parallel vertical fuel rods 4. Within this outer flow channel is an inner flow channel 6 which, as best shown in FIG. 1, is cruciform in section, with its ends at approximately the mid points of the sides of outer flow channel 2. Like the water tubes of the above mentioned patent, inner flow channel 6 will either be provided with openings for inlet of water slightly above the lower tieplate of the assembly, or it may extend below that tieplate, as shown in FIGS. 2 and 4 respectively of that patent. In the upper portion of the inner flow channel 6 is a closure 8, through which extends an inner tube 10. The upper end of the inner flow channel 6 is closed at 12 while the upper end of the inner tube 10 is open as shown at 14. Alternatively, the inner tube 10 may be closed at its upper end, and provided with egress openings, such as shown in FIGS. 2 through 5 of the above patent. The inner channel 6 is provided with intermediate egress openings 16, positioned below the closure 8, near the lower end of the critical heat flux zone of the assembly. It is also provided with upper egress openings 18, positioned above the closure 8 within the critical heat flux zone. The intermediate openings 16 will normally be positioned in the range of about 50 percent to 65 percent of the height of the assembly measured from the bottom, and the upper openings 18 will be distributed in the range of about 65 percent to 75 percent of the height. The two sets of openings may be spaced a substantial distance apart, e.g. 10 percent of the height of the assembly, although openings uniformly distributed along the upper half of the assembly could also yield satisfactory results. In operation, cooling water will enter the inner channel 6, at or near the bottom of the assembly, and flow upwardly therethrough. The greater proportion of the water, e.g. about two-thirds, will exit through openings 16. The remainder, e.g. one-third, will enter the inner tube 10 and flow upwardly through it, downwardly within inner channel 6, and out through openings 18, where it will enter the critical heat flux zone. The openings 16 and 18 are so sized that the entire inner channel 6 is maintained full of water at all times, thereby increasing the moderation of the assembly at the same time the flow outwardly through openings 16 and 18 supplies coolant to the critical heat flux zone, where it is most needed. This has been found to materially increase the power that can be generated by an assembly containing a given amount of fuel. While I have shown only one inner tube 10, it is obvious that there be others, of the same form, positioned in the arms of cruciform inner channel 6. Moreover, the centraltube could be omitted and all the inner tubes located in the arms. In FIGS. 3 and 4 I have shown an alternate embodiment of my invention. In this embodiment the assembly is provided with a centrally positioned inner channel 17 of square cross section. This inner channel is provided with a closure 19, through which extends an inner tube 20. The inner channel 17 is closed at its upper end 22 while the inner tube 20 is open at its upper end 24. Once again, the inner tube may be closed at its upper end, e.g., by being joined to the closed upper end 22 of channel 17, and provided with egress openings near or at the top. The inner channel 17 is provided with openings 26, and the inner tube 20 with openings 28, which correspond in their position and function to openings 16 and 18, respectively, in the embodiment of FIGS. 1 and 2. As shown in FIGS. 1 and 3, the openings 16, 18, 26, and 28 are so positioned that they do not impinge directly on a fuel rod, but rather direct the jet streams into the spaces between the rods. This avoids inducing vibration in the fuel rods, which may result in fretting at their points of support. While I have shown and described two embodiments in detail, it will be obvious to those skilled in the art that various changes can be made. I therefore wish my invention to be limited solely by the scope of the appended claims.