Patent Number: 056639937
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to the drawings, particularly FIG. 1, there is illustrated a nuclear fuel bundle assembly generally designated 10, incorporating the feature of the present invention. Briefly, the nuclear fuel bundle assembly 10 includes a lower tie plate 12, an upper tie plate 14, a plurality of fuel rods 16 extending between the lower tie plate 12 and upper tie plate 14. A plurality of spacers 18 (one shown) are disposed at vertically spaced locations along the height of the fuel bundle for maintaining the fuel rods and water rods in predetermined lateral positions relative to one another. The fuel bundle assembly 10 is encased within a casing or channel 20. In conventional fashion, coolant/moderator liquid flows into an opening through the lower tie plate 12 and into and about the fuel rods 16 within the channel 20 and upwardly through the spacers 18, whereby steam is generated for use in, for example, the production of electricity. In this fuel bundle assembly, a pair of water rods 22, 24 structurally interconnect the upper and lower tie plates 14, 12, and no other structural connections between the upper and lower tie plates are employed. While only a pair of water rods are disclosed and discussed herein, it will be appreciated that one or more additional water rods may be employed to structurally interconnect the upper and lower tie plates to one another. Alternatively, a single water rod may be employed for the same purpose. It will be appreciated however, that the invention disclosed herein is equally applicable to other fuel bundle assemblies where, for example, fuel rods configured as tie rods serve as the load bearing members of the bundle. The water rods or tubes 22 and 24 extend vertically through central regions of the fuel bundle, although other locations may be appropriate. The water rods in the exemplary embodiment are threadedly attached at their lower ends to the lower tie plate 12 (FIG. 2) and extend upwardly within the fuel bundle through the spacers 18 for coupling to the upper tie plate 14 in the manner described in commonly owned copending application Ser. No. 08/380,587 filed Jun. 30, 1995, and now allowed. That application is incorporated herein by reference. The invention here has to do with the lower end plugs 26, 28, respectively, of the water rods 22, 24 by which the latter are secured to the lower tie plate 12. With reference specifically to FIGS. 2 and 3, the end plug 26 (end plug 28 is identical, and both are made of Zircaloy), it may be seen from FIG. 3 that the end plug is secured to the lower end of the water rod 24, by welding for example, and is in threaded engagement within a cylindrical boss 30 provided in the lower tie plate. With the water rods so attached to the lower tie plate, and attached to the upper tie plate in the manner described in the '587 application, it may be appreciated that the water rods serve as the sole load bearing means within the fuel bundle assembly 10. In other words, the entire fuel bundle assembly may be lifted out of the reactor core via the handle 32 which is structurally connected to the water rods by means of the latch mechanism disclosed in the '587 application. For other structural arrangements, however, the water rod end plug may be received in the lower tie plate in various other ways, and this invention is not limited to any particular fastening technique. As already noted above, in this invention, the water rods 22, 24 and specifically the water rod end plugs 26 and/or 28 incorporate a flow restriction or metering mechanism best seen in FIG. 3. The end plug 26 includes a coolant entrance end at 34 and a coolant exit end at 36. At the coolant entrance end 34, an enlarged inlet counterbore 38 opens to the upstream coolant flow. This opening is reduced to a smaller diameter center bore 40 which extends through the plug to an exit where it opens to another larger counterbore 42. The center bore 40 has a greater axial length than either the inlet or outlet counterbores. The exterior of the metering device is formed with an annular locating shoulder 44 on which seats the lower end of the water rod, where the two are welded together by conventional welding techniques. The threaded cylindrical exterior portion of the end plug terminates at a slightly reduced neck portion 46 Which merges with a tapered transition 48, which in turn merges with a cylindrical portion 50 terminating at the shoulder 44. In the exemplary embodiment, the reduced diameter center bore 46 may have a diameter of about 0.297 inches, but it will be appreciated that the center bore 40 may be altered in accordance with the desired regulation of flow through the water rod. In an alternative arrangement, because the water rod inlet may in fact bypass a debris filter arrangement in the vicinity of the lower tie plate, a debris screen 52 (FIG. 4) may be incorporated into the metering device inlet counterbore 38, with flow openings sized and arranged to provide the desired pressure drop and flow characteristics. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.