Patent Number: 047388196
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 prior art 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 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 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, an 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 formed in and disposed in a vertical column (FIG. 6) along the axial length of the sheet members 34. Preferably, the dimples 36 in each of the sheet members 34 are laterally and vertically aligned with corresponding dimples 36 in adjacent sheet members 34 in order to provide pairs of opposed dimples that contact each other along the lengths of the sheet members 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. Disposed within the channel 12 is a bundle of fuel rods 40 which, in the illustrated embodiment, number sixty-four and form a 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 40 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 42 and a lower tie plate 44 and connected together with the tie plates comprise a separate fuel rod subassembly 46 within each of the compartments 30 of the channel 12. A plurality of grids or spacers 48 axially spaced along the fuel rods 40 of each fuel rod subassembly 46 maintain the fuel rods in their laterally spaced relationships. Coolant flow paths and cross-flow communication are provided between the fuel rod subassemblies 46 in the respective separate compartments 30 of the fuel assembly 10 by a plurality of openings 50 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 50 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 46. The above-described basic components of the BWR fuel assembly 10 are known in the prior art, being disclosed particularly in the patent applications cross-referenced above, and have been discussed in sufficient detail herein to enable one skilled in the art to understand the improvements of the present invention presented hereinafter. For a more detailed description of the construction of the BWR fuel assembly, attention is directed to both of the above cross-referenced Barry et al and Doshi patent applications. Features for Avoiding CHF Performance Degradation While the openings 50 allow improved flow stability and pressure equalization, analysis shows that at the spacer locations sharp changes in pressure gradients raise the cross-flow levels between the mini-bundle subassemblies 46 by more than 100 times the nominal cross-flows, becoming comparable to axial mass flow rates in the separate mini-bundle subchannels or compartments 30. This is a large number which can affect and degrade the predicted CHF margin by five to ten percent depending on the operating conditions. Data shows that such undesirable cross-flow behavior around the locations of the spacers 48 washes off beyond plus or minus approximately three inches of the spacer location. Also, in a BWR, CHF occurs predominantly at the top three spacer locations. Thus, the solution of the present invention is to provide means for blocking lateral flow around the top three spacer locations, at least up to three inches in either direction. More particularly, two different embodiments of the features of the present invention for accomplishing lateral cross-flow blockage and thereby eliminating or minimizing CHF performance degradation are shown in FIGS. 8 and 9 incorporated in respective BWR fuel assemblies 10A and 10B. The same parts of the fuel assemblies 10A,10B as described previously with respect to the prior art fuel assembly 10 are identified with the same reference numerals but with the addition of either an A or B suffix depending upon whether the part identified is in the fuel assembly 10A or in the fuel assembly 10B. As seen in FIGS. 8 and 9, each of the respective blocking means which interconnects each water cross radial panel 32A,32B and channel 12A, 12B for closing predetermined ones of the openings 50A,50B at upper ones of the spacers 48A,48B is in the form of a solid continuous structure impervious to cross-flow of moderator/coolant fluid. Specifically, in FIG. 8 the blocking means is a continuous rib 52 formed in the channel 12A and connected to a respective one of the water cross radial panels 32A. Each rib 52 extends along the channel 12A through a distance which encompasses the levels or regions of the adjacent subassemblies 46A occupied by the upper ones of the spacers 48A, preferably the upper three of the spacers 48A. In FIG. 9, the blocking means is a continuous weld or bar 54 interconnecting the channel 12B with a respective one of the water cross radial panels 32B, which covers the same distance as the ribs 52 of FIG. 8. Due to the continuous connection provided between the water cross panels and the channel at the top three spacer locations or the BWR fuel assembly, it is believed that the water cross and channel will be better coupled and hence improvement in structural integrity can be expected. Also, elimination of large cross-flows at upper spacer locations would lead to an improvement in the area of flow induced structural vibrations. As discussed earlier, the impact on the disclosed BWR fuel assembly thermal hydraulic design would be vastly improved due to elimination of cross-flow related penalties and uncertainties and, as a consequence, allow for enormous reduction in analytical complexity by permitting one-dimensional thermal hydraulic analysis, rather than multi-dimensional analysis (which would require development, qualification and licensing of a code to handle it). 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 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.