Patent Number: 047284900
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIGS. 1 to 7, there is shown a nuclear fuel assembly, generally designated 10, for a BWR to which the improved features of the present invention can be advantageously applied. The fuel assembly 10 includes an elongated outer tubular flow channel 12 that extends along substantially the entire length of the fuel assembly 10 and interconnects an upper support fixture or top nozzle 14 with a lower base or bottom nozzle 16. The bottom nozzle 16 which seves as an inlet for coolant flow into the outer channel 12 of the fuel assembly 10 includes a plurality of legs 18 for guiding the bottom nozzle 16 and the fuel assembly 10 into a reactor core support plate (not shown) or into fuel storage racks, for example in a spent fuel pool. The outer flow channel 12 generally of rectangular cross-section is made up of four interconnected vertical walls 20 each being displaced about ninety degrees one from the next. Formed in a spaced apart relationship in, and extending in a vertical row at a central location along, the inner surface of each wall 20 of the outer flow channel 12, is a plurality of structural ribs 22. The outer flow channel 12, and thus the ribs 22 formed therein, are preferably formed from a metal material, such as an alloy of zirconium, commonly referred to as Zircaloy. Above the upper ends of the structural ribs 22, a plurality of upwardly-extending attachment studs 24 fixed on the walls 20 of the outer flow channel 12 are used to interconnect the top nozzle 14 to the channel 12. For improving neutron moderation and economy, a hollow water cross, generally designated 26, extends axially through the outer channel 12 so as to provide an open inner channel 28 for subcooled moderator flow through the fuel assembly 10 and to divide the fuel assembly into four, separate, elongated compartments 30. The water cross 26 has a plurality of four radial panels 32 composed by a plurality of four, elongated, generally L-shaped, metal angles or sheet members 34 that extend generally along the entire length of the channel 12 and are interconnected and spaced apart by a series of elements in the form of dimples 36 formed in the sheet members 34 of each panel 32 and extending therebetween. The dimples 36 are provided in opposing pairs that contact each other along the lengths of the sheet members 34 to maintain the facing portions of the members in a proper spaced-apart relationship. The pairs of contacting dimples 36 are connected together such as by welding to ensure that the spacing between the sheet members 34 forming the panels 32 of the central water cross 26 is accurately maintained. The hollow water cross 26 is mounted to the angularly-displaced walls 20 of the outer channel 12. Preferably, the outer, elongated longitudinal edges 38 of the panels 32 of the water cross 26 are connected such as by welding to the structural ribs 22 along the lengths thereof in order to securely retain the water cross 26 in its desired central position within the fuel assembly 10. Further, the inner ends of the panels together with the outer ends thereof define the inner central cruciform channel 28 which extends the axial length of the hollow water cross 26. Also, the water cross 26 has a lower flow inlet end 39 and an opposite upper flow outlet end 40 which each communicate with the inner channel 28 for providing subcoolant flow therethrough. Disposed within the channel 12 is a bundle of fuel rods 42 which, in the illustrated embodiment, number sixty-four and form an 8.times.8 array. The fuel rod bundle is, in turn, separated into four mini-bundles thereof by the water cross 26. The fuel rods 42 of each mini-bundle, such being sixteen in number in a 4.times.4 array, extend in laterally spaced apart relationship between an upper tie plate 44 and a lower tie plate 46 and connected together with the tie plates comprise a separate fuel rod subassembly 48 within each of the compartments 30 of the channel 12. A plurality of grids or spacers 50 axially spaced along the fuel rods 42 of each fuel rod subassembly 48 maintain the fuel rods in their laterally spaced relationship. Coolant flow paths and cross-flow communication are provided between the fuel rod subassemblies 48 in the respective separate compartments 30 of the fuel assembly 10 by a plurality of openings 52 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 52 serves to equalize the hydraulic pressure between the four separate compartments 30, thereby minimizing the possibility of thermal hydrodynamic instability between the separate fuel rod subassemblies 48. As seen generally in FIG. 4 and in greater detail in FIGS. 6-8, each spacer 50 includes a plurality of interleaved inner straps 54 having opposite terminal end portions 56 and being arranged in an egg-crate configuration to define a plurality of inner cell openings 58. Dimples 60 and springs 62 are formed in the straps 54 so as to project into the inner cell openings 58 and hold respective ones of the fuel rods 42 received therethrough in spaced apart and generally parallel extending relation to one another. The spacer 50 also includes an outer peripheral strap 64 attached to the respective terminal end portions 56 of the inner straps 54 so as to define a number of perimeter cell openings 66 into which others of the dimples 60 and springs 62, also being formed in the outer strap, extend to hold other ones of the fuel rods 42 in the spaced apart parallel relation. The perimeter cell openings 66 are arranged in the form of a ring which encompasses the inner cell openings 58 as a group. Portions of the inner straps 54 define a border, generally indicated as 68, which surrounds the group of inner cell openings 58 and separates them from the perimeter cell openings 66. The above-described basic components of the BWR fuel assembly 10 are known in the prior art, such as in the fuel assembly disclosed in the patent to Barry et al cited above, and have been discussed in sufficient detail herein to enable one skilled in the art to understand the feature of the present invention presented hereinafter. For a more detailed description of the construction of the BWR fuel assembly, attention is directed to the above-mentioned Barry et al patent. Improved Spacer for Avoiding CHF Performance Degradation The present invention provides improved features at least in the limiting CHF ones of the spacers 50 (i.e., the uppermost three spacers) of the fuel assembly 10. Referring to FIGS. 4 and 6-9, these features comprise a plurality of coolant flow diverting scoops 70 mounted on the outer peripheral strap 64. Basically, each of the scoops 70 is composed of a mounting portion 72 and a coolant flow deflecting portion 74 connected at its outer end 76 to the lower end of the mounting portion. The mounting portions 72 of the scoops 70 are in the form of flat extensions of the respective spacer outer strap 64 which extend downwardly therefrom, for instance from three to six inches, and generally parallel to the fuel rods 42 and are disposed in laterally spaced apart relation from one another along the upstream side of the spacer 50. The flow deflecting portions 74, having longitudinally tapered and arcuate-shaped configurations, extend inwardly from respective lower ends of the mounting portions 72 within the respective spaces between the ones of the fuel rods 42 received through the perimeter cell openings 66 of the spacer 50. Also, the flow deflecting portions 74 of the scoops 70 extend along and in spaced relation downwardly from the upstream side of the spacer 50 and particularly from the respective ones of the terminal end portions 56 of the inner straps 54. The flow deflecting portions 74 terminate at inner ends 78 being disposed generally below the border 68, as seen in FIG. 6, defined by portions of the inner straps 54. As depicted in FIGS. 7 and 9, the outer end 76 of the arcuate-shaped flow deflecting portion 74 of each scoop 70 is spaced farther from its respective one of the inner strap terminal end portions 56 than its inner end 78 such that it extends in an inclined relation thereto across the path of a portion of the coolant flowing upwardly toward the perimeter cell openings 66. Such inclined relation of the scoop flow deflecting portion 74 achieves scooping of liquid coolant from along the channel 12 and water cross panels 32 and redistributes it to the fuel rods 42 extending through the inner cell openings 58 of the spacer 50 just upstream of the same. Preferably, at least the three uppermost ones of the spacers 50 have the scoops 70 mounted as just described on their upstream sides. It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that varrious changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.