Patent Number: 
Section: description

Referring now to the drawing figures, particularly to FIG. 1, there is illustrated a nuclear fuel bundle, generally designated 10. Bundle 10 includes an outer channel 12 surrounding an upper tie plate 14 and a lower tie plate 16. A plurality of fuel rods 18 are disposed in a matrix within the fuel bundle 10 and pass through a plurality of spacers 20 vertically spaced one from the other maintaining the fuel rods in the predetermined matrix thereof. In the illustrated form of FIG. 1, the matrix is a 10xc3x9710 array. A pair of water rods 22 and 24 are disposed in the fuel rods between the lower tie plate 16 and the upper tie plate 14. The water rods serve to transfer moderator fluid from the lower regions of the nuclear fuel bundle to the upper regions, where the water is dispersed through openings in the water rod for flow into the bundle in and about the fuel rods as illustrated in FIG. 1A, the lower tie plate 16 receives end plugs 26 formed on the lower ends of the water rods. The lower ends of the water rods have openings 28 for receiving moderator fluid for flow upwardly within the water rod through passages 30 for flow into passageways 32 formed by the tubular portions of the water rod 22. As illustrated in FIG. 1A, the spacer 20 is bounded by a pair of radially directed flanges 34 and 36 which lie on opposite sides of the spacer 20, maintaining the spacer at the desired elevation along the water rod. Referring now to FIG. 1B, at least one and preferably a pair of water rods 40 are illustrated passing through vertically adjacent spacers 42 and 44. The upper ends of the water rods 40 terminate in end guides which are received in the upper tie plate 45 of the fuel bundle. The spacers 42 and 44 comprise two of a plurality of spacers, typically between five and seven spacers, spaced along the entire length of the fuel bundle for maintaining the fuel rods in the desired array thereof. The spacers 42 and 44 useful with the present invention may comprise any type of spacers, for example, ferrule-type spacers or spacers of the type described and illustrated in U.S. Pat. No. 5,209,899 In the present invention, however, and instead of forming a unitary water rod throughout its entire height between the upper and lower tie plates, the water rod may be formed of two or more water rod segments. For example, and referring to FIG. 2, the water rods 40 may comprise a series of water rod segments interconnected one to the other at adjoining ends. For example, as illustrated in FIG. 2, a water rod segment 46 may have an upper end joined to the lower end of the next-adjacent upper water rod segment 48. The upper end of segment 48 may be joined to the lower end of an additional water rod segment. The process may be repeated until eventually the upper end of the final water rod segment is secured to the upper tie plate. The joints between the adjoining segments are disposed within the openings provided in the spacer through which the water rod extends. For example, the upper end of the segment 46 terminates in a female threaded nipple 50 which may be formed integrally with the segment 46 or formed separately and secured thereto as by threading or welding. The nipple 50 includes an axial passage therethrough in communication with the interior passage 51 of the underlying water rod segment 46. The adjoining upper water rod segment 48 has a male threaded plug 52 on its lower end for threaded engagement with the female threaded nipple 50. Like the nipple 50, the male plug has a central passage 54 through which moderator may flow from the lower segment to the upper segment. In the illustrated form of the present invention, the water rod segments 46 and 48 capture the spacer, for example, spacer 44, therebetween. To accomplish this capture, the segments 46 and 48 have elements, e.g., capture flanges, which project laterally beyond the peripheral confines of the opening in the spacer through which the water rod extends. Thus, for example, the diameter of the overlying and underlying water rod segments 48 and 46, respectively, may be larger than the diameter of the opening through the spacer. By employing a threaded connection between the adjoining two segments, the spacer can be assembled onto the water rods by inserting the nipple 51 and male plug 52 through the opening and threading the segments one to the other. The enlarged diameter portions of the upper and lower segments thus lie in registry with portions of the spacer laterally of the spacer opening and capture the spacer therebetween. Preferably, a small clearance 56 is provided between the enlarged diameter portions and the spacers to accommodate thermal expansion and contraction of the spacer relative to the water rod. In a preferred form of the present invention, the capture flanges may comprise terminal portions of swirler vanes 60 extending externally about the water rod segments 46 and 48. As illustrated in FIG. 2, the swirler vanes form a double helix about the water rod segments, although a single helix or helices in excess of two helices may be used. Ends of the swirler vanes extend beyond the lateral confines of the opening of the spacer, e.g., spacer 44, and on opposite sides thereof to capture the spacer. With a double helix, the ends of the vanes are diametrically opposite and form diametrically opposite stops or flanges 55 engageable by the captured spacer. Where a single helix is employed, only one stop is used, although an additional tab or stop can be used, preferably diametrically opposite the end of the swirler vane. It will be appreciated that the swirler vanes about the water rods serve to deflect moderator onto the adjacent fuel rods and into interstices thereof. In a preferred embodiment of the present invention, a separate segment is provided for each axial spacer-to-spacer region above a lower segment, which may be of a standard water rod configuration and extend through the first two or more spacers in the lower region of the bundle. For example, a third segment 70 above segment 48 is illustrated at FIG. 2. Segment 70 preferably includes a swirler vane(s) 72 terminating at its lower end in stops engageable with the spacer 74. Openings 76 are also provided in the uppermost water rod segment, e.g., segment 70, to flow water from the lower region of the bundle via an interior passage 78 to the upper region of the bundle against and into the interstices between the fuel rods. By using the segmented water rod of the present invention, the water rod tabs previously used to capture the spacer are entirely eliminated. Moreover, the present invention provides an installation whereby damage, marring or scratching of the spacers and water rods is minimized or eliminated. It will be appreciated that with elongated water rods, the spacers are typically displaced along the water rods to their desired location in the bundle. In that displacement, there is potential for damaging the contacting parts of the spacer and water rod. However, in the present invention, the threaded couplings between the water rod segments within the openings of the spacers afford a construction which virtually eliminates any sliding action of the water rod and spacers relative to one another, hence eliminating the potential for damage to one or the other, or both, of the spacers and water rods. In short, the spacer cannot be dislodged from its location along the water rod. 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.