Patent Number: 047160110
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 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 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, 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 and 2 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 channel 12. The sheet members 34 of each panel 32 are interconnected and spaced apart by a series of elements in the form of dimples (not shown) formed therein and extending therebetween. The dimples are provided in opposing pairs that contact each other along the lengths of the sheet members to maintain the facing portions of the member in a proper spaced-apart relationship. The pairs of contacting dimples 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 maintainedd. The hollow water cross 26 is mounted to the angularly-displaced walls 20 of the outer channel 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, 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 36 and an opposite upper flow outlet end 38 which each communicate with the inner channel 28 for providing subcoolant flow therethrough. Disposed within the channel 12 is a bundle of fuel rods 40 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 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. The fuel rods 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 channel 12. A plurality of grids 48 axially spaced along the fuel rods 40 of each fuel rod subassembly 46 maintain the fuel rods in their laterally spaced relationships. The lower and upper tie plates 42,44 of the respective fuel rod subassemblies 46 have flow openings 50 defined therethrough for allowing the flow of the coolant/moderator fluid into and from the separate fuel rod subassemblies. Also, coolant flow paths provide flow communication between the fuel rod subassemblies 46 in the respective separate compartments 30 of the fuel assembly 10 through 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 46. The above-described basic components of the BWR fuel assembly 10 are known in the prior art, being disclosed particularly in the Doshi application cross-referenced above, and have 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 both of the above cross-referenced Barry et al and Doshsi patent applications. Coolant Flow Direction Control Device Referring now to FIG. 1, and more specifically to FIGS. 3 to 5, there is seen the feature incorporated in the BWR fuel assembly 10 which constitutes the present invention, namely a coolant flow direction control device, generally indicated by the numeral 54. The flow direction control device 54 is mounted in the bottom nozzle 16 of the fuel assembly 10 on an annular ledge or surface 56 formed thereon at an inner end of an inlet 58 of the bottom nozzle so as to surround the inlet. In such location, the flow direction control device 54 is operable basically to open the inlet 58 to flow of coolant fluid in an inflow direction into the flow channel 12 through the bottom nozzle inlet 58 but close the inlet to flow of coolant fluid from the channel 12 through the bottom nozzle inlet upon reversal of coolant liquid flow from the inflow direction. More particularly, the coolant flow direction control device 54 is preferably in the form of a one-way or unidirectional flow check valve positioned across the inlet 58 of the fuel assembly bottom nozzle 16. The coolant flow check valve 54 is operable to sense the direction of coolant flow through the inlet 58 and automatically, by action of the fluid on the valve, open when the flow direction sensed is into the bottom nozzle 16 (in the direction of arrow A in FIG. 4) and close when the flow direction sensed is out of the bottom nozzle (in the direction of arrow B in FIG. 4). In an exemplary embodiment, the flow check valve 54 is composed of four parts 54a,54b,54c,54d. Each part of the check valve 54 is quarter pie-shaped and has an outer portion 60 mounted to the bottom nozzle 16 on its annular surface 56 and adjacent to its inlet 58 and an inner portion 62 being connected to the respective outer portion 60 by a middle hinge portion 64 for pivotal movement relative to one another. The inner portions 62 of the valve 54 together pivot about the respective outer portions 60 between lowered positions, as seen in solid line form in FIGS. 4 and 5, and raised positions, as seen in broken line form in FIG. 4. The inner valve portions 62 are configured to extend in close fitting relationship adjacent to one another and coplanarly across the inlet 58, as seen in FIGS. 4 and 5, so as to close it when disposed in their respective lowered positions. Then, when disposed in their respective raised positions, as seen in FIG. 4, the inner portions 62 extend in generally parallel relationship to the direction of flow A an are disposed remote from one another so as to open the inlet 58. The inner valve portions 62 are arranged in first and second pairs which, as depicted in FIG. 5, are angularly displaced about ninety degrees from one another. In such arrangement, the inner valve portions 62 of each pair are placed in opposing relation to one another such that one is a mirror image of the other. Additionally, the inner valve portions 62 located directly opposite to one another in the respective pairs thereof extend generally parallel to one another when in their raised positions. The outer valve portions 60, in being mounted to the annular surface 56 of the bottom nozzle 16 which concentrically surrounds its inlet 58, are configured for attachment on respective circumferentially spaced sectors 66 of the surface 56, as seen in FIG. 5. The inner valve portions 62, when in their respective lowered positions as seen in FIG. 5, are configured for seating on respective circumferentially spaced segments 68 of the annular surface 56 which alternate with the spaced sectors 66 of the surface 56 and constitute the remainder thereof. Whereas in their raised positions the inner valve portions 62 extend toward the bottom nozzle 16 in the direction of coolant flow (arrow A) into the bottom nozzle, in their lowered positions their seating on the annular surface 56 stops them from pivoting past the lowered position so as to prevent them from extending away from the bottom nozzle 16 in a direction (arrow B) opposite to that of their raised positions which would allow reverse flow of coolant therefrom. Thus, the primary characteristic of the flow direction control device 54, whatever specific form it might take, should be that it provides unrestricted flow through the bottom nozzle inlet 58 into the fuel assembly during normal operation. Then, upon occurrence of LOCA flow reversal (initiation of core coolant inventory depletion), the flow reversal should cause the device to shut automatically, preventing any further depletion via the bottom nozzle. A principle requirement of this device should be reliability in operation and freedom from corrosion. The material of the device 54 might be a titanium alloy. The advantage of having multiple closure members or portions making up the device 54 is that in case one portion gets stuck and fails to close upon flow reversal, the others would then introduce a partial closure, which would still be beneficial. However, it is possible to use only one closure member. 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.