Patent Number: 
Section: description

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 place UTP and a lower tie plate LTP. Within the channel C there is provided a plurality of nuclear fuel rods and moderator 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 define 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 or seven spacers are provided only three of which are illustrated at S4, S5 and S6 positions. Such spacers may be of the type disclosed in U.S. Pat. No. 5,209,899, of common assignee herewith, the disclosure of which is incorporated herein by reference. It will be appreciated from a review of FIG. 1 that a 9xc3x979 array of rods R is illustrated and that other arrays may be utilized with the present invention, e.g., 8xc3x978 or 10xc3x9710 arrays. A handle H is also illustrated for purposes of lifting the fuel bundle relative to a nuclear fuel core, not shown. In 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 flow bundle and isolates that flow from a core bypass volume flowing outside the channel C and between similarly disposed fuel bundles, not shown. As those of skill in the art will recognize, not each lattice position of the lattice or array of openings across the spacer 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 a spacer, for example spacer S4, and terminate just above spacer S4. 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. As best seen for example in FIG. 2, the termination of a part-length rod PLR for example above the spacer S4 in a certain lattice position of the 9xc3x979 array, leaves a vent volume 10 above the upper end of the part-length rod including the superposed opening(s) of the overlying spacer(s). By employing part-length rods, the associated flow blockage effects at each lattice position above the part-length rod, which would otherwise have been occupied by a full-length rod, is eliminated. That is, the opening through the spacer above the part-length rod has a flow area therethrough as large as each flow area through the openings of the spacer without a fuel rod received therethrough, e.g., a flow area equal to the combined first area through the spacer opening with a fuel rod received therein and the flow area otherwise occupied by the fuel rod in the spacer opening. Consequently, additional flow area is provided through the vent volume 10 including through the opening(s) in the overlying spacers at the lattice position of the underlying part-length rod, thereby providing additional flow area and a reduction in pressure drop across the spacers. This reduction in pressure drop, however, diverts flow from the surrounding full-length fuel rods into the vent volume 10 which can cause reduction in critical power performance. However, the reduction in pressure drop is highly advantageous and, according to the present invention, separation devices are used to divert the flow of liquid in the vent volume laterally outwardly onto the surfaces and into the interstitial regions about the full-length fuel rods. Thus, the present invention advantageously maximizes flow diversion at locations just above the spacers while simultaneously minimizing pressure loss of the flow passing through those spacers. Consequently, according to the present invention and in a preferred embodiment, the separation devices are advantageously placed within the vent volume just above the spacers and in the lattice position which would have been occupied by a full-length rod but for the creation of a vent volume, e.g., by the installation of the part-length rod at that lattice position in underlying spacers. Accordingly, referring to FIG. 2, there is illustrated a separation device 20 which, in the specific illustrated form, comprises a swirler. The purpose of the separation device is to deflect or divert flow laterally outwardly onto the surfaces and into the interstices of the full-length fuel rods with minimum pressure loss across the spacer. Thus, the separation device 20 is disposed just above the opening 22 in the spacer which would otherwise have been occupied by a full-length rod but for the installation of a part-length rod. Further, the separation device 20 extends in an axial direction in the vent volume 10 sufficiently only to achieve the flow diversion effect recognizing that the greater the axial length of the separation device the greater the pressure drop across the device. Therefore, the separation device 20 preferably has a very short axial length. Placing the swirler just above a spacer is preferable because the higher fluid velocities that result from the spacer flow diversion improves separation efficiency and the helical flow pattern caused by the swirler persists for a substantial distance downstream from the swirler allowing a shorter axial length of separation devices to be used. As illustrated, the separation device 20 may be repeated for each overlying spacer at each lattice position forming part of a vent volume, for example, the vent volume 10 above a part-length rod. In FIG. 3, the swirler 20a occupies a vent volume above several clustered part-length rods. It has also been extended toward the next adjacent spacer. This advantageously provides for the helical flow pattern to persist with substantial centrifugal forces as far as possible toward the next overlying spacer and thus the swirler continues to aggressively feed liquid onto the laterally adjacent fuel rod surfaces. While this extension of the separation device toward the next spacer advantageously enhances liquid/vapor separation, it also increases the pressure drop. The latter effect can be mitigated, however, by employing non-uniform separation devices such as the non-uniform diameter swirler illustrated in FIG. 9 discussed below. Referring to FIG. 4, an alternate separation device may comprise an auger mounted on a vertical shaft 24. The helical blade 26 of the auger is thus essentially wound on edge about the shaft 24, the edge of the helical blade 26 being secured to the shaft 24. Multiple flights may be used on edge about the central shaft 24. While the extension of an auger shaft through the upper tie plate and through the openings of the spacers increases the pressure drop when coolant/moderator flows through the openings of the spacers, the cross-sectional dimensions of the auger shaft can be minimized to minimize that pressure drop with the concurrent advantage that the auger can be removed from the fuel bundle through the upper tie plate. Further, the blade(s) of the auger may, but preferably do not, extend over the entire length of the shaft 24. Auger blade segments may be disposed on the shaft located for disposition just above the spacers in the vent volume 10. Also, the auger blade segments may extend only a short distance axially above the spacers, similar to the distance swirlers 20 extending above the spacers as illustrated in FIG. 2. In FIGS. 5A-5C there is illustrated a preferred form of separation device comprising a swirler 20. In this simplest form of swirler, it will be appreciated that its minimum axial length for effective separation is that which results in a horizontal projected area covering a full 360xc2x0. Consequently, swirler 20 may comprise a single strip 27 of material twisted 180xc2x0 between its opposite ends to form a helical vane and hence provide a helical flow pattern in the vent volume. In FIG. 5C, the periphery of the swirler defines a circular projected plan and hence a majority of the area of the vent volume occupied by the swirler is subjected to the helical flow pattern. More complex configurations of separation devices, for example, two or more twisted strips to form more complex swirlers may be provided. Thus, in FIGS. 6A-6E, two strips of material 29 and 31 are slotted at their opposite ends and interleaved along their axes. The strips 29 and 31 are maintained perpendicular along their length and are twisted 90xc2x0 to complete the full 360xc2x0 horizontal projected area necessary to provide effective separation. In FIGS. 7A-7F three strips 33, 35 and 37 of sheet material are slotted adjacent their ends as illustrated and joined along their axes to initially provide strips 60xc2x0 apart. By rotating or twisting the strips 60xc2x0, a full 360xc2x0 projected area is provided as illustrated in FIG. 7F. In the case of three strips the length of unslotted material in each strip is one-third the height of the strips. The unspotted material is at the top, middle and bottom among the three strips, to permit interlocking assembly. The same design technique is used as the number of strips increases. To improve the efficiency of the swirl device, it will be recognized that in the generally rectilinear array of fuel rods, the vent volume 10 has a generally rectilinear configuration, i.e., square or rectangular. With the typical projected circular plan area of the swirler, for example, the swirler of FIG. 2, the regions between the corners of the square vent volume area and the circular projected plan area; of the swirler constitute flow bypass regions. Thus, flow upwardly into the vent volume may bypass the swirler. To provide for more efficient swirl flow patterns without flow bypass or with only minimum flow bypass, the perimeter of the separation device can be shaped to generally conform to the perimeter of the vent volume defined by the adjacent fuel rods. Thus, the generally rectilinear vent volume can be substantially covered in plan area by the separation device. To accomplish this, and as illustrated in FIG. 8A, a swirler, e.g., of the type illustrated in FIGS. 7A-7F, having a diameter corresponding to the longest diagonal of the area of the vent volume is formed. The circular edges 41 of the strips forming the swirler may be removed to form a linear swirler having a generally rectilinear projected plan view. Thus, the perimeter of the separation device is shaped so that the resultant projected area in plan closely conforms with the vent volume flow passage whereby bypass flow around the edges of the separation device is substantially eliminated. It is recognized that the circular cross-section of the adjacent fuel rods causes the edges of vent volumes to have non-linear shapes. For maximum swirler effectiveness, the projected area of the swirlers can match those shapes, as illustrated in FIG. 8B. Thus, the edges of the separation device can be rounded in plan view as illustrated at 49. In FIG. 9, there is illustrated a separation device, e.g., in the form of a swirler 50, which extends a substantial distance above the spacer on which the swirler is mounted. Adverse pressure drop created by an axially extended swirl device can be ameliorated by using a non-uniform separation device. Thus, in this form, the swirler 50 may decrease in horizontal dimension, i.e., diameter, with the distance above the spacer or the helical pitch may vary. Specifically, the swirler illustrated in FIG. 9 has a progressively decreasing diameter with increasing distance from the spacer on which the swirler is mounted. Step-wise progression of decreasing lateral extent with increasing distance above the spacer may likewise be provided. It will be appreciated from the foregoing that the separation devices are preferably mounted directly to the spacers for high performance and reliability. However, this prevents ready removal of the underlying part-length rod. Thus, as an alternative, the separation devices may be removably attached to the spacer or may be attached in groups to a removable central shaft or other structural support. The structural support may have the separation devices, e.g., in the form of swirlers at axially spaced positions along the support which, when inserted into the fuel bundle, align with the spacers at a location just above the upper surface of each spacer. As a further alternative, where more than one separation device is utilized, different flow patterns can be achieved. For example, the swirlers may be arranged to rotate the flow in a common direction or in opposite directions. Alternatively, various patterns of flows in opposite directions may be provided. Also, in a general sense, the necessary characteristic of a separation device according to this invention is the requirement that the device impart a lateral or horizontal component to the flow. Thus, in addition to swirlers and augers formed of one or more vanes which are twisted to form a helical pattern and a consequent helical flow pattern, the separation devices hereof may comprise discrete vanes with laterally outwardly flared edges such as, for example, the flaring bell-shaped cones or outwardly directed deflecting tabs described and illustrated in U.S. Pat. No. 5,416,812 of common assignee herewith, the disclosure of which is incorporated herein by reference. 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.