Patent Number: 048760633
Section: description

Referring to FIG. 1, a fuel bundle assembly B is illustrated. Assembly B includes a surrounding channel C. Channel C extends from an upper tie-plate 14 at the upper end to a lower tie plate 16 at the lower end. As is common in the prior art, the lower tie-plate is tapered, supported on a so-called elephant's foot casting (not shown) and makes provisions for receiving water and passing the water upwardly through the fuel bundle. In the disclosed fuel bundle, a 9 by 9 array of fuel rods is shown. In FIG. 1, many of these fuel rods are omitted so that the double "D" water rods 30 and 32 may be seen. The fuel rods include full length rods 18. Full length rods 18 extend between the lower tie plate 16 and the upper tie plate 14. Also shown are partial length rods 20. Partial length rods 20 extend from the lower tie plate to a spacer 24. The partial length rods terminate above spacer 24. A spacer 26 is provided with enlarged apertures overlying each of the partial length rods. Likewise the upper tie-plate includes enlarged apertures overlying each partial length rod. The partial length rods are set forth in both function and purpose in that patent application entitled Two-Phase Pressure Drop Reduction, BWR Assembly Design filed Apr. 4, 1988, No. 176,975, owned by the common Assignee herein. Generally, the partial length rods 20 serve to form steam voids overlying their terminated ends. These voids, during operation of the fuel bundle, form an escape path for the steam. The present invention is directed to the water rods 30 and 32. In order to set forth this invention, the presence of moderator required for burning of the fuel rods will be set forth with respect to FIG. 2. Thereafter, the placement of the double "D" water rods back-to-back will be set forth with respect to FIG. 3. Referring to FIG. 4, the locking of the "D" water rods together for the support of spacers will be discussed. Finally, and with respect to FIGS. 5 and 6, the upper and lower transitions from the double "D" water rods to smaller diameter cylindrical rods will be illustrated. Referring to FIG. 2, channel C is illustrated having a 9 by 9 row and column array of rods. These rods are shown with respective columns numbered 01-09 and rows numbered 11-19. Partial length rods 20 are illustrated. Full length rods 18 are illustrated. It will be observed that the central portion of the fuel bundle has the rods removed. This removal is for the purpose of providing sufficient water moderator in the center of the bundle to optimize burning of fuel placed within the rods. The configuration of the region without fuel rods can be easily understood. Realizing that row 05 has sufficient interval to accommodate nine rods, it will be seen that the three center rods are removed. In the direction of the corners 40 and 42, it is required that more void space be placed. Accordingly, rods in column 04 are removed from rows 15, 16. Likewise, rods in column 06 are removed from rows 14, 15. Thus, the water region comprises two overlying square sections. These overlying square sections have in common that spatial interval that would be occupied by a fuel rod placed at column 05, row 15. It has been found that this configuration provides a suitable increase of water moderator throughout most of the length of the fuel bundle. What has been required is to find the optimum shape of water rod for placement within this interval. The advantages of the present invention become apparent when alternate cross-sectional shapes of water rods are reviewed for placement to the 9 by 9 array of fuel rods herein set forth. Presuming that a polygon cross-section is utilized, tracking the borders of the removed rods, efficient space utilization will be present. However, that kind of construction is expensive, and would not provide for any rotation of the members during insertion. Hence, the locking feature of this disclosure would not be available. Similarly, two side by side cylindrical members would not use the available space efficiently. Such cylindrical members contact one another at their respective tangent points. In such contact at tangent points the contacted cylinders will define wasted spatial intervals that will not include the desirable water containing volumes. In short, it is believed that the following double "D" configuration has optimal volume usage for the containment of the desired moderating water. Referring to FIG. 3, the double "D" water rod can now be described in cross-sectional configuration. Referring to FIG. 3, the central twenty five spatial intervals for rods are shown. All these rods are full length rods 18. Insertion of the double "D" water rods is shown at 30, 32. Each double "D" water rod includes a generally cylindrical, vertically extending wall 50. Wall 50 spans approximately 270.degree. of arc of a cylinder. In the case of double "D" water rod 30, wall 50 spans from the approximate 3:00 o'clock position to the 12:00 o'clock position. In the case of rod 32, wall 50 spans from the 9:00 o'clock position to the 6:00 o'clock position. Each of the walls is truncated by a flat, vertically extending chord wall 52. Cord wall 52 serves two purposes. First, cord wall 52 provides a flat surface. Utilizing this flat surface, the two water rods can be placed interior of the fuel bundle in back to back relation. As placed in back to back relation, the rods utilized most of the volume of the removed seven rods. Confinement of water to an efficient moderating volume occurs. Secondly, the backs 52 serve to lock the respective water rods from rotation. As will hereinafter be seen with respect to FIG. 4, by the expedient of placing supporting tabs on one water rod and allowing that water rod to be locked in place by the remaining water rod, a sure support of spacers at their desired elevation can occur. The ferrule spacer shown in FIG. 3 is preferred. For each spacer, 2 tabs are welded to the water rod one above and one below the spacer (spacer must be restrained in both directions, up and down). Referring to FIG. 4, rod 30 has spacer support tabs 61 placed thereon. The support tabs are here shown supporting a square grid spacer S. The particular supported square grid spacer S is that one shown at 26 (see FIG. 1). The reader will realize that if "D" shaped water rod 30 is inserted before water rod 32, it will be possible to insert tabs 61-64 without interference with the spacer S. Simply stated, "D" shaped water rod 30 will be rotated 45.degree.. In this alignment, each of the tabs 61-64 will pass in non-interfering alignment to the corners of the spacers. In such passage, full insertion of the water rod will occur. Once rod 30 is fully inserted, it will be rotated. Upon rotation, the respective tabs 61-64 will rotate into positions underlying the structure of spacer S. Presuming that tabs are immediately under the spacer, it will be seen that the spacer will be supported at its appropriate elevational interval inside a fuel bundle B. Once rod 30 is inserted and rotated to its final position, rod 32 is placed. Rod 32 is placed with its back 52 confronting the back 52 of rod 30. Thus, it will be appreciated that rod 30 is locked from further location. Thus, the tabs 61-64 are likewise locked. Support of the spacers at their desired elevational within the fuel bundle will occur. Referring to FIG. 5, a section at the bottom of the assembly is taken along lines 5--5 of FIG. 3. Specifically, "D" section water rods 30, 32 terminate each at ends 70. At ends 70 they are fitted to conical fittings 72. Conical fittings 72 are, in fact, truncated at three planes. The first plane of truncation is at 73. This plane produces on conical fitting 72 the same outside diameter as each of the water rods 30, 32. As each is common, the rods are each provided with an annulus 74 for receiving the lower inside diameter of the respective "D" shaped rods 70. The second plane of truncation is along plane 75. This provides a truncated lower portion to the water rod support. This truncation occurs at a diameter equal to a tube 76 which is supported from the lower tie-plate. A tube 76 is fitted over the reduced diameter 77, and the tube 76 is welded to the transition piece 72. The final plane of truncation is at 78. The truncation of 78 is a vertical plane which cuts off the side edge of what would otherwise be a cone. This defines on both fittings 72 a flat planar area wherein the fitting 72 can fit together in back to back relationship.