Patent Number: 047537744
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 4, there is shown a nuclear fuel 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 tubular outer flow channel member 12 (hereinafter outer channel member 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 serves as an inlet for coolant flow into the outer channel member 12 of the fuel assembly 10, and 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 channel member 12 (FIG. 4) is generally of rectangular cross-section, is made up of four interconnected vertical walls 20 each being displaced about ninety degrees one from the next. A plurality of inwardly facing structural ribs 22 are formed in the walls 20 of the outer channel member 12. The ribs 22 are formed in a spaced-apart relationship in the inner surface of each wall 20 of the outer channel member 12. Above the upper ends of the structural ribs 22, a plurality of upwardly-extending attachment studs 24 (FIG. 2) are fixed on the walls 20 of the outer channel member 12 and are used to interconnect the top nozzle 14 to the outer channel member 12. Fuel rods 40 are supported in an exemplary 8.times.8 array in a bundle within the outer channel member 12. Upper and lower tie plates 42 and 44, secured in the outer channel member 12, support opposite ends of the fuel rods. A plurality of grids 48 secure the rods laterally in a known manner. For improving neutron moderation and economy, a hollow water cross 26, as seen in FIGS. 1, 2, 4 and 5, extends axially through the outer channel member 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 length of the outer channel member 12. The sheet members 34 of each panel 32 are interconnected and spaced apart by a series of elements in the form of dimples 36 (FIGS. 4 and 5) as shown. Opposed pairs of contacting dimples 36 are connected together such as by welding to insure that the spacing between confronting sheet members 34 forming the panels 32 of the central water cross 26 is accurately maintained. Upper and lower closures 54 and 52 (FIG. 5) seal or close the respective upper and lower ends 38 and 39 of the water cross 26. Outlets 66 are provided in the upper closure 54. The hollow water cross 26 is mounted to the angularly displaced walls 20 of the outer channel member 12. Preferably, the outer elongated, lateral ends 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, inner surfaces of the sheet members 34 together with the outer ends thereof define the inner central cruciform channel 28 which extends the axial length of the hollow water cross 26. The bundle of fuel rods 40 which, in the illustrated embodiment, number sixty-four in an 8.times.8 array, are separated by the water cross 26 into the four compartments 30, each housing a fuel mini-bundle or subassembly 46. 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 upper and lower tie plates 42 and 44. The fuel rods 40 in each mini-bundle are connected to the upper and lower tie plates 42, 44 and together therewith comprise a separate fuel rod subassembly 46 within each of the compartments 30 of the outer channel member 12. The grids 48, axially spaced along the fuel rods 40 of each fuel rod subassembly 46, maintain the fuel rods 40 in laterally spaced relationship. The lower and upper tie plates 44, 42 of the respective fuel rod subassemblies 46 have flow openings 50 defined therethrough for allowing the low of the coolant/moderator fluid into and from each separate fuel rod subassembly 46. Also, coolant flow paths provide flow communication between the fuel rods subassemblies 46 in the respective separate compartments 30 of the fuel assembly 10 through a plurality of openings 53 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 53 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. Openings 68 in the sheet members 34 may be provided to supply subcooled moderator fluid to the central portion of each fuel rod subassembly 46. Cross Flow Inlet Means at Fuel Bundle Entrance Referring now to FIGS. 1, 4 and 5, there is seen the feature incorporated in the BWR fuel assembly 10 which provides an inlet to the water cross 26 and allows cross flow of fluid near the lower ends of the respective fuel rod mini-bundles of the separate fuel rod subassemblies 46 and in such manner minimizes maldistribution of flow between the mini-bundles. Referring again to FIG. 5, the water cross 26 shown in perspective includes the aforementioned respective bottom and top closure means 52 and 54. The improvement comprises side entry cross flow inlet means 58 in the form of holes in the water cross sheet members 34 at a lower end 38 thereof. The top closure 54 at the upper end 39 of the water cross 26 has its outlet holes 66 sized so as to limit the outlet of the water cross 26 to an open area less than the open area of the cross flow inlet means 58. In such manner, a positive pressure gradient is maintained in the subcooled moderator flow through the water cross inner flow channel 58 relative to the coolant/moderator flow through the fuel rod subassemblies 46 in outer flow channel 12. Specifically, the outlet holes 66 in the top closure means 54 are each of a predetermined diameter size which is less than the predetermined diameter size of each of the holes forming cross flow inlet means 58 in the sheet members 34. Such relationship distributes the hydraulic losses such that greater loss is experienced at the outlet than at the inlet of the water cross 26. Also, the arrangement reduces the static pressure loads at the inlet and the risk of failure of dimple welds which interconnect the pairs of sheet members 34 of the water cross 26. The holes 58 are preferably located in opposition to each other in confronting sheet members 34 and are generally axially aligned as shown. The holes 58 are preferably within a few centimeters above the respective lower ends of the mini-bundles 46 and the lower tie plate 44. In the current conventional design, inlet pressure drives the coolant/moderator into the water cross 26 through openings below the lower tie plate 44, and coolant/moderator enters the mini-bundle through openings the lower tie plate 44. In the proposed design, the inlet pressure drives coolant/moderator into the mini-bundle through resized lower tie plate holes 50, and coolant/moderator is driven into the water cross 26 through the side entry cross flow inlet means 58 therein above lower tie plate 44. The driving force for coolant into the water cross 26 in the current design is the pressure gradient or difference in inlet pressure P1 less the resulting water cross pressure P3 (or P1-P3). In the invention, the driving force is the difference between the water cross pressure P3 and the mini-bundle pressure P2 (or P2-P3), which is less than (P1-P3). In order to achieve a certain selected flow W.sub.C to the water cross for a given pressure drop .DELTA.P the following expression is used: EQU .DELTA.P=KW.sub.C.sup.2 where K denotes the effective loss coefficient. If .DELTA.P is to be reduced for a selected W.sub.C, then K must necessarily be decreased. In the present invention, the pressure gradient P2-P3 driving the coolant/moderator into the water cross is less than the pressure gradient (P1-P3) in the current design. Therefore, the effective loss coefficient at the inlet to the water cross can be decreased. This is accomplished by increasing the flow area at the water cross inlet. Thus, in FIG. 6, the reduced loss coefficient allows the operation in the shallow part of the curve at S.sub.2. In the proposed design, the lower tie plate loss coefficient is also decreased in order to achieve the same pressure drop P1-P2 between the inlet and mini-bundle. This occurs because the total flow W.sub.T, although the same in both designs, is rerouted through the mini-bundles in the proposed design by closure of the inlets to the water cross below the lower tie plates. The low to the mini-bundles is thus increased by about 10%, by upwardly resizing the holes in the lower tie plates to produce a lower loss coefficient according to the following expression: EQU K'W.sub.T.sup.2 =K(W.sub.T -W.sub.C).sup.2 where K' is the new loss coefficient for the lower tie plate, W.sub.T is total flow through the tie plates, and K is the original loss coefficient for the lower tie plate. Hole size determination may be made by relatively straightforward and simple experiments based on known parametric data. The important feature of the present invention is to determine the apppropriate loss coefficients which achieve improved results. Thereafter, hole sizing is implemented to achieve the desired loss coefficients. The proposed improvement provides for better control and lesser uncertainty in the amount of flow entering the water cross 26. This has direct bearing on the amount of voids formed and consequent degradation in nuclear performance. Again, crud buildup effects are also subdued. The opposed axial alignment of holes 58 provides for direct communication between the minibundles and helps minimize flow maldistribution, consequently also minimizing any degradation in bundle CHF performance which might result from the flow maldistribution. About 4% to 7% improvement in bundle CHF margin can be expected from total elimination of inlet flow maldistribution. Reducing the possibility of generating voids in the water cross also improves the transient thermal hydraulic performance, especially the hydrodynamic stability of the assembly. It is felt that the proposed modification should prove largely beneficial from the standpoint of structural characteristics. For example, the proposed side entry cross flow inlet means reduces the pressure gradient across each of the four water cross panels. This pressure gradient reduction consequently improves the fatigue-related performance of the fuel bundles 10 and water cross structure 26. This disclosure has focused on an invention to minimize water cross flow rate uncertainties and related undesirable effects. The proposed design modification involves closing off the water cross inlet and diverting liquid into the water cross from above the minibundle lower tie plates via holes or slots machined into the water cross walls. This simple modification miminizes uncertainties in the amount of flow entering the water cross and the flow maldistribution between the minibundles. The modification also provides direct benefits by reducing any crud buildup-related penalties. The present invention also has some important safety implications in the context of a so-called loss of coolant accident (LOCA). The repositioning of the water cross entry inlet flow holes 58 to the side of the water cross panels 34 and above the lower tie plates 44 is expected to provide benefits during a loss of coolant accident. In the early stages of such an accident, coolant is sprayed into the top of each fuel assembly 10 to provide cooling heat transfer. The coolant spray may be unable to penetrate into the assembly, however, because it is resisted by the steam produced from the residual heat in the fuel. This condition is referred to as being counter-current flow limited (CCFL). Later in the accident, water is injected from the safety injection systems (not shown) at the bottom of the fuel assemblies in the same manner that coolant normally enters the fuel bundles 10. This is called the reflood portion of the accident. The holes 58 in the water cross 26 situated above the lower tie plates 44 provide benefits during both the CCFL and reflood portions of the accident. During the CCFL portion, spray coolant can travel down through the water cross 26, exit through the holes 58 and flow up through the fuel mini-bundles. This helps to more quickly quench the steam and allow the coolant spray, restricted to the top of the bundle, to penetrate throughout the bundle. With the entry holes 58 located above the lower tie plates 44, the downwardly directed water cross flow does not have to overcome the resistance of the lower tie plate 44 as in the current design where the watercross flow holes are located below the lower tie plate. Therefore, more flow is available from the water cross 26 to travel up the fuel mini-bundles and help quench the steam. The CCFL condition is therefore more quickly overcome. In the current design, with the flow holes located below the lower tie plate, during the reflood portion of the accident, a certain fraction of the injected water is diverted to the water cross 26. Consequently, that flow is not available to provide heat transfer cooling of the fuel rods. In the proposed invention, the holes 58 are moved instead to the side of the water cross sheet members 34 and above the lower tie plates 44. With this design, all of the injected water flows up through the lower tie plates 44 and most of that injected water continues to travel up through the fuel mini-bundles to provide cooling. Only after sufficient coolant pressure is re-established within the fuel mini-bundles does some of the injected water get diverted up through the water cross instead of up through the mini-bundles. Thus, the invention results in a design which has safety advantages not available with prior arrangements. The pressure drop across the tie plate of the improved fuel bundle without water cross inlets shall be adjusted to be less than the pressure drop across a conventional tie plate having inlets to the water cross because the tie plate orifice losses associated with the water cross inlets have been eliminated. Thus, in the present invention the loss coefficient shown in FIG. 6 is reduced from S.sub.1 to S.sub.2. As the slope of the curve in FIG. 6 becomes shallow in the S.sub.2 portion, uncertainties in the inlet area result in lower uncertainties in the loss coefficient. Therefore, the embodiment of the present invention is less sensitive to crud buildup and the flow to the water cross is less uncertain. 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.