Patent Number: 047088460
Section: description

DETAILED DESCRIPTION Referring to the drawing, FIG. 1 shows a vertical section of a boiling water fuel assembly incorporating one of our improved water rods. The assembly comprises an outer flow channel 2. At the bottom is an inlet nozzle 4 which supports a lower tie plate 6, and which has, at its lower end, an opening 8 for the reception of cooling water. Within the flow channel 2 are a large number of fuel rods 10, which are supported at their lower ends on lower tie plate 6. The upper ends extend into an upper tie plate 12. Springs 14 are compressed between the fuel rods and the upper tie plate. The fuel rods 10 are held spaced from one another by grid spacers 16. The structure described thus far is typical of boiling water reactors in general. Our assembly includes one, and often several, water rods 18, one form of which is shown in more detail in FIGS. 2 and 3. In this embodiment, the lower end of the tube 18 is closed by an end plug 20, which rests on lower tie plate 6. Immediately above the tie plate are a plurality of inlet openings 22, through which assembly cooling water enters the water rod. A plurality of intermediate exit holes 24 are provided preferably somewhat above the midpoint of the tube and near the lower end of the critical heat transfer zone 32. The upper end of the water rod is closed by an end plug 26. Within the upper portion of the water tube is an inner tube 28, which is open at or near its top. Near the lower end of tube 28 are a plurality of exit openings 30 in tube 18. These are located at or near the midpoint of the critical heat transfer zone referred to above, and shown at 32. The upper portion of tube 18 is shown in more detail in FIG. 3. As shown in that figure, the lower end of inner tube 28 is flared outwardly at 34. This flared portion is originally made with a diameter the same as the internal diameter of tube 18. At several points in its periphery, it is deformed outwardly to a diameter which gives an interference fit with the inner surface of 18. The upper end is welded at 36 to end plug 26, and the inner end 34 is then forced downwardly to the point where end plug 26 fits within tube 18. The latter is then welded at 38 to the end plug. As shown, inner tube 28 is provided with several holes 40 near its upper end. In operation, cooling water from the assembly flows inward through openings 22. Some of it exits through intermediate outlets 24. The remainder flows upwardly, passing through inner tube 28 and out through holes 40, then downwardly and out through exit openings 30. As described above, the openings 30 are positioned so that the water flowing outwardly through them enters the critical heat transfer zone of the assembly. FIG. 4 shows an alternate form of "water rod," which is the one we prefer at present. In this embodiment, the tube 18 extends through the lower tie plate 6, and has an open lower end through which water enters directly from the coolant supply zone of the reactor, as shown by arrow 42. The remainder of the structure is the same as in FIG. 2 and either FIG. 4 or FIG. 5. This arrangement gives a greater water flow through a tube of a given size than that of FIG. 2. FIG. 5 shows an alternate upper end structure, appliable to either the embodiment of FIG. 2 or that of FIG. 4. In assembling it, the tube 28 is dimensioned so as to have a tight fit on reduced portion 43 of end plug 26. It is welded into inner plug 44 at 46. Plug 44 has a peripheral groove 48. After tube 28 has been pushed onto end cap 26, the assemblage is pushed into tube 18. The latter is then deformed into groove 48 and welded at 38 to end plug 26. While we have shown the intermediate openings 24 at only a single level, in practice there will be multiple holes distributed over several inches of the tube 18. The same is true of the upper openings 30. The intermediate openings should be positioned in the range of about 50% to about 65%, and the upper openings in the range of about 65% to about 75% of the height of the assembly, measured from the bottom. The two sets of openings should be spaced a substantial distance apart, e.g., 10% of the height of the assembly. While we have described various details of our structure, it will be apparent to those skilled in the art that various changes can be made. We therefore wish our invention to be limited solely by the scope of the appended claims.