Patent Number: 055263879
Section: summary

TECHNICAL FIELD This invention relates to spacers used in a boiling water nuclear reactor (BWR) fuel bundle for maintaining the fuel rods within the fuel bundle in their designed spaced apart relationship. More specifically, the spacer construction in accordance with this invention incorporates improved flow tabs on the peripheral band of the spacer to direct more of the liquid coolant flow toward the outer fuel rods in the bundle rather than merely into the spaces between the fuel rods. BACKGROUND Fuel bundles in boiling water nuclear reactors (BWR's) include an array of upstanding side-by-side fuel rods supported between upper and lower tie plates. Each bundle requires multiple spacers (e.g., seven, axially spaced along the bundle) for the maintenance of the fuel rods in designed spaced apart relationship. The bundle is surrounded between the tie plates by a fuel bundle channel. The lower tie plate is configured to permit the inflow of moderating water coolant while the upper tie plate permits the outflow of both water coolant and generated steam. The surrounding channel confines the flow of coolant to a path around the steam generating fuel rods separate from a water flooded core bypass region surrounding each fuel bundle. Flow tabs are used on the peripheral bands of each spacer to deflect liquid coolant flow from the band to the outer row of fuel rods. This flow redirection deposits water droplets on the fuel rods, increasing the water film thickness and improving the critical power performance of the outer fuel rods. Currently utilized flow tabs consist of simple, planar projections which extend upwardly from the band and which are bent inward at their outer ends. With reference to FIGS. 1 to 3, a conventional fuel assembly 10 comprises a plurality of fuel elements or rods 12 supported between an upper tie plate 14 and a lower tie plate 16. The fuel rods 12 pass through a plurality of fuel rod spacers 18 which provide intermediate support and retain the elongated rods in spaced relation. Each spacer contains a matrix of ferrules F, each adapted to receive and surround a corresponding fuel rod 12. In a typical arrangement, seven such spacers 18 may be located along the approximate 13 foot length of the fuel bundle. Each of the fuel rods 12 includes an elongated tube containing fissile fuel and other materials, such as fertile fuel, burnable poison, inert material or the like, sealed in the tube by upper and lower end plugs 20, 22, respectively. Lower end plugs 22 are formed with extensions for registration and support within openings formed in the lower tie plate 16. At the same time, the upper end plugs 20 are formed with extensions which fit into support openings in the upper tie plate 14. The fuel assembly 10 also includes a thin walled tubular flow channel 24 of substantially square cross section, sized to form a sliding fit over the upper and lower tie plates 14 and 16 as well as spacers 18 so that the channel may be easily mounted to or removed from the fuel bundle. The lower tie plate 16 is formed with a nosepiece 26 adapted to support the fuel assembly 10 in a socket in a core support plate (not shown) in the reactor pressure vessel. The end of this nosepiece is formed with an opening 28 which receives pressurized coolant in an upward flow direction. FIGS. 2 and 3 of the drawings show top and side views, respectively, of the corner region of a conventional spacer 18. Flow tabs 20 are shown extending upwardly from an upper edge 30 of the peripheral band 32 of the spacer, and between adjacent fuel rods 12. Each tab comprises a lower, substantially vertical portion 34 and an upper inwardly bent portion 36. Side edges of each tab are generally inwardly tapered in the upward direction, and both portions 34 and 36 are substantially planar. The bent upper portions 36 of the flow tabs 20 project into the region between each pair of fuel rods 12. The two phase (water and steam) coolant flow is upward, and the steam water mixture flows around the flow tabs while some of the flow is deflected in the direction normal to the flow tab, toward the interior of the spacer. This results in some water being deposited on the adjacent fuel rods 12 and increasing the thickness of the water film. Other representative examples of flow tabs incorporated into fuel bundle spacers may be found in U.S. Pat. Nos. 5,180,548; 5,080,858; 4,879,090; 4,692,302; 4,698,204; 4,683,115 and 4,039,379. SUMMARY OF THE INVENTION The present invention relates to an improved flow tab design which redirects more of the liquid component of the steam water mixture toward the outer fuel rods rather than into the space between the fuel rods, and which offers less resistance generally to the flow of coolant through the channel. In an exemplary embodiment, both the lower and upper portions of the flow tab are provided with secondary bends about vertical and inclined center lines. More specifically, the upper inwardly bent portion has a secondary bend along its inclined center line such that this center line is located downward relative to the outer or side edges of the tab. This shape imparts velocity components to the flow which are parallel to the spacer band and away from the flow tab. As a result, more water droplets are channeled directly to the surfaces of the outer fuel rods. The lower, substantially vertical portion of the flow tab is also formed with a secondary bend about its vertical center line such that the latter is located outwardly of the side edges of the tab, and outwardly of the spacer band itself. This improved lower tab configuration increases the stiffness of the flow tab, and makes it easier to form. In addition, the overall size of the tab relative to conventional tabs as shown in FIGS. 1-3, has been increased to deflect greater amounts of coolant flow. In its broader aspects, the invention relates to a spacer for use with a fuel bundle in a nuclear reactor, the spacer comprising a matrix of ferrules for surrounding individual fuel rods within a bundle; a band surrounding the matrix and defining a peripheral wall of the spacer, the band having an upper edge; a plurality of laterally spaced flow tabs extending upwardly from the upper edge, each flow tab having a substantially vertical portion and an inclined portion; and first means on the inclined portion for imparting velocity components to a flow of coolant along the fuel rods, which components are substantially parallel to the band. In another aspect, the invention relates to a spacer for use with a fuel bundle in a nuclear reactor, the spacer comprising a matrix of ferrules for surrounding individual fuel rods within a bundle; a band surrounding the matrix and defining a peripheral wall of the spacer, the band having an upper edge; and a plurality of laterally spaced flow tabs extending upwardly from the upper edge, each flow tab having a substantially vertical portion joined to the band and an inclined portion joined to the vertical portion and extending upwardly and inwardly relative to the vertical portion, the inclined portion having a first center line located downwardly and inwardly relative to a pair of lateral free edges of the inclined portion. In still another aspect, the invention relates to a spacer for use with a fuel bundle in a nuclear reactor comprising a matrix of ferrules for surrounding individual fuel rods within a bundle; a band surrounding the matrix and defining a peripheral wall of the spacer, the band having an upper edge; and a plurality of laterally spaced flow tabs extending upwardly from the upper edge, each flow tab having a lower substantially vertical portion and an upper inclined portion extending away from the vertical portion, the vertical portion having a vertical crease and the inclined portion having an inclined crease, each located centrally of the tab. It will be appreciated that the improved flow tab configuration in accordance with this invention directs more of the liquid flow directly onto the outer fuel rods and offers less resistance to coolant flow, thereby enhancing overall fuel bundle performance. Other objects and advantages of the invention will become apparent from the detailed description which follows.