Patent Number: 061480541
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, particularly to FIG. 1, there is illustrated a nuclear fuel bundle generally designated B having a channel C encompassing an upper tie plate UTP and a lower tie plate LTP. Within the channel C, there is provided a plurality of nuclear fuel rods R supported on the lower tie plate LTP and which rods extend upwardly toward and to the upper tie plate UTP. A plurality of spacers S are vertically spaced one from the other throughout the height of the fuel bundle B and defines discrete vertically aligned openings at lattice positions in a regular array of such openings to receive and confine the rods R within the bundle B against lateral movement relative to one another. Generally, six to nine spacers are provided, only two of which are illustrated in FIG. 1. Such spacers may be of a ferrule type, for example, as described and illustrated in U.S. Pat. No. 5,178,825 of common assignee herewith or which may be of the type described and illustrated in U.S. Pat. No. 5,209,899, the disclosures of which are incorporated herein by reference. It will be appreciated that other types of spacers than those set forth in these two patents may be used in the context of the present invention. The ferrule-type spacer illustrated in the drawing figures includes a plurality of cylinders 11 arranged in side-by-side parallel relation and through which fuel rods R extend. The fuel rods are maintained spaced from the cylindrical walls of the ferrules by radially inwardly directed protrusions 13 (FIG. 4) in engagement with the fuel rods R. It will also be appreciated from a review of FIG. 1 that a 9.times.9 array of rods R is illustrated and that other arrays may be utilized with the present invention, for example, 8.times.8 or 10.times.10 arrays. A handle H is also illustrated in FIG. 1 for purposes of lifting the fuel bundle relative to a nuclear fuel core, not shown. When utilizing the fuel bundle B in the core of a nuclear reactor, for example, a BWR, coolant/moderator, e.g. water, enters through the lower tie plate LTP for flow upwardly and about the rods R. During upward passage of this water, steam is generated and a vapor and liquid mixture passes upwardly through the upper tie plate UTP. During steam generation, the channel C confines the coolant/moderator flow within the nuclear fuel bundle and isolates that flow from a core bypass volume flowing outside the channel C and between similarly disposed fuel bundles, not shown. Not each lattice position of the lattice or array of openings across the spacers, however, is occupied by a full length fuel rod R. For example, one or more water rods or moderator rods may pass upwardly through the central portion of the bundle B and occupy a number of lattice positions. Additionally, one or more part-length rods PLR may be provided in selected lattice positions in the fuel bundle B. Thus, for example, each part-length rod may extend from the lower tie plate LTP upwardly in the fuel bundle through the first three or four spacers and terminate just above the fourth spacer. Part-length rods are typically terminated in or just above the spacer to provide support for the otherwise cantilevered ends of the part-length rod. Further, a full-length fuel rod may be replaced by a rod containing no nuclear fuel. In accordance with this invention, separation devices are provided on structures above part-length fuel rods (FIGS. 1A, 2 and 3) or structures replacing a fuel rod and extending the full length of the fuel bundle. Referring now to FIG. 2, the upper end of a part-length rod PLR extends just above a spacer, for example, spacer S.sub.4. As illustrated, the volume above the part-length rod is not occupied by a fuel rod. Such volume constitutes a vent volume 16 which passes through superposed, axially registering, openings in spacers higher in the fuel bundle, e.g., spacers S.sub.4, S.sub.5 and S.sub.6. Also, as illustrated in FIGS. 2 and 3, the ferrule of each spacer S in the vent volume above the part-length rod is omitted. In accordance with the present invention, a separation device generally indicated 10 is provided in the vent volume 16 above one or more of the spacers superposed over the part-length fuel rod PLR and particularly projecting over substantially the entire plan area of the vent volume defined in the plane of the spacer(s) by the ferrules surrounding the omitted ferrule. Particularly, the separation devices are provided in the bundle in a manner enabling the removal of the separation devices as well as the part-length rods from the bundle and reinsertion of the separation devices and part-length rods through the upper end of the fuel bundle. To accomplish this, the separation devices, for example, swirlers 12, are mounted on and carried by a support structure, e.g., a rod 14 receivable in the fuel bundle through the upper end thereof. The swirlers and support rod are sized relative to the openings through the spacers such that the rods and swirlers can pass through the spacers. In certain instances, and in using certain spacers, a rotation of the support rod and swirlers may be necessary to pass the swirlers through the spacers avoiding inwardly projecting protuberances on the spacers which typically hold the fuel rods in their designed lattice position. Referring to FIG. 2, the lower end of the support rod 14 may be secured to the upper end of an underlying part-length fuel rod PLR. As a representative example of a connection between the support rod 14 and a PLR, the support rod 14 may have a reduced diameter threaded male part for threaded connection with a threaded female opening at the upper end of the PLR. Further, it will be appreciated that the PLR itself is releasably secured at its lower end to the tie plate. Such securement may comprise a threaded type joint indicated at 15 in FIG. 2. Consequently, the support rod 14, attached swirlers 12 and the part-length rod PLR may constitute a unitary assembly for insertion into and removal from the fuel bundle. Thus, the fuel bundle assembly process can be facilitated by first attaching the support rod to the part-length fuel rod and then inserting the assembled support and part-length rods through the openings in the spacers and securing the lower end of the part-length rod to the lower tie plate. Conversely, this enables both the support rod and the part-length rod to be removed as a unit from the fuel bundle by disconnecting the part-length rod from the lower tie plate. Preferably, upon insertion of the support rod and part-length rod, the swirlers are designed to be located in the vent volumes just above the spacers as illustrated. Consequently, flow through the spacer opening not occupied by a fuel length fuel rod encounters the swirler and the liquid is conveyed laterally onto and into the interstices of the surrounding full length fuel rods while the vapor flows upwardly in the vent volume. In a preferred form of the invention, the area of the swirler projected onto the opening through the underlying spacer is maximized. For example, and in the case of a ferrule-type spacer, it will be appreciated that the ferrule underlying the swirler is removed from the spacer as illustrated in FIGS. 1A, 2 and 3, leaving a substantially square opening with outwardly projecting apices at the corners adjacent diagonally located ferrules. To maximize the projected plan area of the swirler onto that opening while simultaneously enabling the swirler and support structure therefor to pass through the spacers during assembly and repair of the fuel bundle, a swirler having four twisted blades 21 (FIG. 3) is provided. As illustrated in FIG. 3, the blades 21 project laterally such that, when projected onto the spacer opening, the blade tips extend to lie closely adjacent the diagonally arranged ferrules 23 and between the middle ferrules 25 forming the sides of the opening. Thus, the support structure with attached swirler and PLR can be inserted from the top of the bundle through the aligned openings in the spacers at the lattice positions where the ferrules are absent with the PLR passing through ferrules in the lower spacers whereby the PLR can be screwthreaded to the lower tie plate. Further, the thickness (height) of the spacer and the twist (pitch) of the blades 21 can be correlated such that the projected plan area of the swirlers with blades 21 can be increased whereby the swirlers overlie a greater proportion of the opening as well as portions of the spacer. Rotation of the support structure enables the swirlers to pass through the spacer openings during assembly and/or repair. It will be appreciated that other types of separation devices may be used in the present invention than the disclosed and preferred swirlers 12. Additionally, the separation devices may extend the full length of the support rod 14 to its connection with the upper end of the part-length rod. Further, a support structure, e.g., a support rod may extend the full length of the fuel bundle having one or more separation devices or a continuous separation device along its length (FIGS. 4 and 5 described below). In any event, it will be appreciated that the support rod and separation devices are not supported by the spacers as typical in prior fuel bundles. Rather, the support rod and separation devices are directly supported by the part-length rod secured to the lower tie plate (FIGS. 1A, 2 and 3) or by the lower tie plate (FIGS. 4 and 5). The upper end of the support rod is suitably secured to the upper tie plate. In FIGS. 4 and 5, the support structure, e.g., support rod 14a extends the full distance between the upper and lower tie plates. Additionally, the swirler 12a extends continuously along the rod 14a between the upper and lower tie plates, for example, connecting with the lower tie plate LTP in a threaded connection 15a. It will be appreciated that the swirler 12a may extend fully or partly between the spacers, and not within the spacers. Also, the swirlers and support structure may be inserted through the ferrules left behind in the event a full-length fuel rod is removed. That is, the pitch of the swirlers is such to enable the swirler to be rotated past the protrusions 13 in the ferrules as illustrated in FIG. 4. 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.