Patent Number: 048184780
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 3, there is shown a nuclear feel assembly, generally designated 10 for a boiling water nuclear power reactor (BWR), in which the improvement of the present invention is incorporated. 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 serves 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 (also see FIGS. 4 and 5) 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, as seen in FIGS. 1, 2, 4 and 5 and 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 channels 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. Each mini-bundle incorporates the improved feature of the present invention to be described below. 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 relationships. 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. The above-described basic components of the BWR fuel assembly 10 are known in the prior art, except for modification of some of the fuel rods 42 so as to comprise the improved feature of the present invention, as described hereinafter. The BWR fuel assembly 10, disclosed in greater detail in the patent to Barry et al cited above, has been discussed in sufficient detail herein to enable one skilled in the art to understand the improved 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. Interior Fuel Rods Having Reduced Diameters The present invention provides an improved feature in each of the mini-bundles of fuel rods 42 in the form of two rod diameter sizes for interior and peripheral arrays 54,56. The fuel rods 42A in the interior array 54 thereof within each mini-bundle are of a first predetermined diameter size, whereas the fuel rods 42B in the peripheral array 56 thereof in each mini-bundle are of a second predetermined diameter size greater than the first diameter size. Preferably, the fuel rods 42A of the interior array 54 are ten to twelve percent less in diameter size compared to the fuel rods 42B of the peripheral array 56. Furthermore, the interior and peripheral arrays 54,56 of fuel rods 42 compose an overall squared array of fuel rods. The fuel rods 42A of the interior array 54 form an inner, centrally-located, generally squared pattern, and in the 4.times.4 pattern of each mini-bundle amount to four in number. The only fuel rods present in the inner squared pattern are the fuel rods 42A of the first smaller diameter size. The fuel rods 42B of the peripheral array 56 form an outer, peripherally-located, generally squared annular pattern which surrounds the interior array 54, and in the 4.times.4 pattern of each minibundle amount to twelve in number. The only fuel rods present in the outer squared pattern are the fuel rods 42B of the second larger diameter size. Thus, the fuel rods 42A,42B of the respective interior and peripheral arrays 54,56 which together comprise an overall squared array are aligned with one another in columns and rows. In summary, the improved feature of the present invention uses a demonstrated thermal-hydraulic effect to arrive at an unique BWR water cross-containing fuel assembly design with two rod diameter sizes, for interior and peripheral rod locations, thereby alleviating the inherent limitation on CHF performance due to additional "cold wall" surfaces. The relative rod diameter of the four interior fuel rods 42A in each mini-bundle would be set at a value of preferably about ten percent lower than the diameters of the peripheral fuel rods 42B. Experimental evidence for the W-QUAD+ critical heat flux (CHF) performance has demonstrated that the corner and side rods display vastly superior performance (more than 10-15% improvement in critical power) relative to the four interior rods. The present invention increases the flow area per rod for the interior rods thereby improving heat transfer and simultaneously reducing the power rating through smaller rods (and therefore fuel pellet) sizes. Standard nuclear design approaches would be employed to increase the enrichment of the side/corner rods so that the total fuel assembly power generated remains the same. This unique combination of factors leads to a fuel assembly with superior (approximately 10%) CHF performance, by improving the CHF performance of the interior rods to levels of the side/corner rods. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various 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.