Abstract:
A spacer for a nuclear fuel assembly includes dividers orthogonally interdigitated relative to one another defining fuel and water rod cells. The dividers defining fuel rod cells have openings carrying an integrally formed spring which projects into the fuel cell to bear the fuel rod in the cell against an opposite stop carried by an opposing divider. The dividers are arranged in a 10×10 array with diagonally opposing quadrants being mirror images of one another. The dividers are surrounded by a peripheral band with integrally formed stops projecting into the peripheral cells enclosing the spacer.

Description:
[0001]     The present invention relates to a spacer for a nuclear fuel bundle assembly and particularly relates to a nuclear fuel bundle spacer having a limited number of piece parts and a low pressure drop and enabling high critical power.  
       BACKGROUND OF THE INVENTION  
       [0002]     Spacers in nuclear fuel bundle assemblies comprise structural elements located at axially spaced positions along the length of the fuel bundle to maintain fuel rod-to-rod spacing in normal operation and to accommodate shipping and handling movements of the bundles. The spacer also impacts thermal hydraulic performance of the fuel bundle and is designed to enable the bundle to achieve critical power objectives without substantial pressure drop.  
         [0003]     Nuclear fuel spacers are typically formed of a multiplicity of different parts which, upon assembly, define cell openings for receiving fuel rods as well as water rods. Spacer designs conventionally provide one or more springs in each round, square or octagonal fuel rod cell to positively locate the fuel rods in the center of the cell against stops that comprise separate elements or are incorporated into the cell defining the structures and bands surrounding the assembly. The production and assembly of these various and many parts is complicated, laborious and costly. Moreover, the assembled spacer affects thermal hydraulic performance in terms of critical power and pressure drop. Particularly, current spacer designs use separate springs captured and positioned by other parts of the spacer, making them susceptible to flow induced vibrations, the possibility of loose parts and rendering it difficult to control the spring characteristics such as stiffness and to reduce stresses.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     In accordance with a preferred aspect of the present invention, there is provided a nuclear fuel spacer formed of a plurality of dividers with integrated springs with the dividers being interdigitated one with the other to form a grid structure within an outer band. The dividers have common elements, and the number of discrete divider parts is maintained at a minimum, particularly by providing back-to-back dividers secured to one another along orthogonally related centerlines of the spacer, enabling quadrants of the spacer to be mirror images of orthogonally related quadrants. By using back-to-back centerline dividers, the divider springs of adjacent quadrants face in opposite directions and always face toward the peripheral band, avoiding placement of springs in the outer peripheral band. The springs per se also are formed integrally with the dividers to minimize the number of parts, simplify assembly, precisely position the springs and minimize the possibility of loose parts. The outer band is moved outward by over sizing the integrated stops and under sizing the bathtubs creating rectangular fuel rod cells on the periphery of the spacer with the fuel rod cells on the periphery of the spacer with the fuel rods not residing in the center of the cell. This allows the band to reside in a lower coolant velocity and thus lower pressure drop regime. It also provides for more coolant flow inside the band in conjunction with more room for over sized flow tabs on the band to direct more coolant back onto the edge fuel rods to improve critical power. Other features of the present invention will become apparent from the ensuing description.  
         [0005]     In a preferred embodiment according to the present invention, there is provided a spacer for a nuclear fuel bundle comprising an outer peripheral band, a plurality of orthogonally arranged interdigitated dividers within the band and defining with the band a grid having an array of cells for receiving fuel rods, each cell having orthogonally related sides, each divider including along a side of the cells a plurality of openings and at least one spring spanning the openings, the springs projecting out of a plane generally containing the divider and to one side of the divider into the cell and formed integrally with the divider, each spring having an intermediate portion for engaging a fuel rod, legs extending from opposite sides of the intermediate portion toward respective top and bottom edges of the spacer and pairs of laterally projecting flanges interconnecting opposite end portions of the legs and lateral margins of the divider defining the openings, the flanges adjacent the top and bottom edges of the spacer being angled from the leg end portions in directions toward the bottom and top edges, respectively, of the spacer.  
         [0006]     In a further preferred embodiment according to the present invention, there is provided a spacer for a nuclear bundle comprising an outer rectilinear peripheral band, a plurality of orthogonally arranged interdigitated dividers within the band and defining with the band a grid having an array of cells for receiving fuel rods, each cell having orthogonally related sides, each divider including along a side of the cells a plurality of openings and at least one spring spanning each opening, the springs projecting out of a plane generally containing the divider and to one side of the divider into the cell and formed integrally with the divider, a first set of the plurality of dividers extending orthogonally relative to one another for the full length and width of the spacer between opposite sides of the band, a second set of the plurality of dividers extending along orthogonally related centerlines of the spacer, pairs of discrete dividers of the second set thereof being secured to one another back-to-back and forming the dividers along the respective centerlines of the spacer, the springs of the discrete dividers of each back-to-back pair of dividers extending back-to-back to one another and in opposite directions toward opposite sides of the band. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a partial fragmentary schematic representation of a nuclear fuel bundle assembly illustrating a pair of spacers with fuel rods extending into the spacers;  
         [0008]      FIG. 2  is a top plan view of a spacer according to an aspect of the present invention;  
         [0009]      FIG. 3  is a side elevational view of the spacer of  FIG. 2 ;  
         [0010]      FIG. 4  is a top elevational view of one of four peripheral spacer bands;  
         [0011]      FIG. 5  is a side elevational view of a regular divider forming part of the grid within the spacer band;  
         [0012]      FIGS. 5   a  and  5   b  are top and end elevational views of the regular divider of  FIG. 5 ;  
         [0013]      FIG. 5   c  is a side elevational view of a portion of a divider defining a single cell;  
         [0014]      FIG. 6  is a view similar to  FIG. 5  illustrating an identical regular divider with oppositely directed slots affording interdigitation;  
         [0015]      FIGS. 7, 8 ,  9  and  10  are side elevational views of partial dividers for accommodating water rod openings within the spacer;  
         [0016]      FIG. 11  is a side elevational view of a center divider;  
         [0017]      FIG. 12  is a top plan view of end-to-end back-to-back dividers of  FIG. 11  forming a centerline divider assembly;  
         [0018]      FIG. 13  is a side elevational view of the dividers of  FIG. 11  arranged back-to-back;  
         [0019]      FIG. 14  is a side elevational view of another center divider;  
         [0020]      FIG. 15  is a top plan view of a pair of the dividers of  FIG. 14  arranged end-to-end back-to-back;  
         [0021]      FIG. 16  is a side elevational view of the dividers of  FIG. 14  arranged back-to-back;  
         [0022]      FIG. 17  is a side elevational view of a special divider;  
         [0023]      FIG. 18  is a top plan view thereof;  
         [0024]      FIG. 19  is a side elevational view of another special divider; and  
         [0025]      FIG. 20  is a top plan view thereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Referring now to the drawings, particularly to  FIG. 1 , there is illustrated a fuel bundle assembly generally designated  10  comprised of a fuel bundle channel  12  surrounding a plurality of fuel rods  14  and water rods  16  arranged in a generally rectilinear grid configuration. Spacers  18  are axially spaced one from the other as illustrated and maintain the fuel rods and water rods spaced one from the other in the grid configuration.  
         [0027]     Referring to  FIG. 2 , the spacer  18  is illustrated in a top plan view. As illustrated, the spacer comprises a grid structure defined by orthogonally related dividers  20  interdigitated with one another to form a grid. In this preferred embodiment, it will be appreciated that a 10×10 array of cells  22  are formed by the interdigitated dividers  20  bounded by the peripheral band  24 . The grid structure defined by the dividers and peripheral band also affords larger openings  26 . The cells  22  and openings  26  receive the fuel and water rods, respectively. Two water rod openings  26  are illustrated, each occupying space in the grid structure which would otherwise constitute four fuel cell openings  22 . By interdigitation, it is meant the dividers  20  have respective slots opening through top and bottom edges of the dividers so that orthogonally related dividers may be received in the slots of one another.  
         [0028]     As illustrated in  FIG. 2 , the dividers  20  are elongated in length and width directions of the spacer and extend between top and bottom faces of the spacer. The dividers  20  also include in each fuel cell a spring  28  which projects inwardly of the cell. The orthogonally related dividers  20  defining each fuel cell  22  have two orthogonally related springs  28 . With the exception of the corner fuel cells  30 , each fuel cell is provided with two orthogonally related stops  32  provided on orthogonally related dividers. Oversized stops  34  are provided adjacent top and bottom faces of the spacer and on the outer peripheral band  24  in conjunction with undersized bathtubs  44  to permit rectangular shaped peripheral cells. The corner fuel cells  30  have springs  28  provided by orthogonally related dividers and a single stop  36  formed in a diagonally related corner piece  38  forming an integral part of the outer band  24 . Consequently, with the exception of the corner cells  30 , each cell  22  includes orthogonally related intermediate springs  28  which lie opposite stops  32  of orthogonally related dividers or stops of one divider and the stop  34  of the band for the marginal fuel cell openings  22 .  
         [0029]     Referring to  FIGS. 3 and 4 , the outer peripheral band of the spacer is comprised of four identical parts, each part  40  as illustrated in  FIG. 4  spans the width of the spacer and includes a corner piece  38 . Additionally, the oversized stops  34 , illustrated in  FIG. 3 , lie adjacent the top and bottom edges of the bands. In conjunction with the oversized stops, the undersized bathtubs  44  move the band outward creating rectangular cells on the spacer periphery allowing more flow inside the band and space for oversized inwardly directed flow deflecting tabs  42  for directing flow of coolant within the band onto the fuel rods. The outwardly directed projecting bathtubs  44  engage the interior wall surfaces of the channel  12 . In assembly, the band portions  40  are seam welded one to the other about the periphery of the spacer.  
         [0030]     Dividers  20  are formed of sets of regular dividers  50 ,  52 , partial dividers  54 ,  56 ,  58  and  60 , center dividers  62  ( FIGS. 11-13 ) and  64  ( FIGS. 14-16 ) and special dividers  66  ( FIGS. 17 and 18 ) and  68  ( FIGS. 19 and 20 ). Each of the sets of dividers has a plurality of openings  70  containing the springs  28  and a description of one spring  28  and its interconnection with a divider, for example, either of the dividers  50  or  52  of  FIGS. 5 and 6 , respectively, will suffice as a description of the springs  28  for the remaining sets of dividers.  
         [0031]     Referring to  FIGS. 5 and 6 , each divider includes a plurality of openings  70  spaced one from the other along the length of the divider, each opening  70  containing a spring  28 . As best illustrated in  FIG. 5   b , each spring  28  projects outwardly to one side of the divider  50  and includes an intermediate or central projection  72  having legs  74  extending therefrom in opposite directions terminating in generally T-shaped ends  76 . The T-shaped ends  76  integrally include flanges  78  and angled elements  80  connecting the T-shaped portions  76  and margins of the openings  70 . The T-shaped portions  76  including flanges  78  lie spaced from and generally parallel to the plane containing the divider with the elements  80  extending between the flanges  78  and the divider.  
         [0032]     As best illustrated in  FIGS. 5   b  and  5   c , the flanges  78  and elements  80  interconnected with the flanges  78  and which are directed or angled inwardly relative to the opening  70  and toward the central spring  28 . That is, the elements  80  together with the flanges  78  form generally flattened shallow arrowheads at opposite ends of the spring  28  pointing in opposite directions away from one another. It will be appreciated from a review of  FIG. 5   c  that the T-shaped portions  76  and flanges  78  are spaced from the margins  84  and  86  of the spacer adjacent the respective top and bottom edges of the divider. Consequently, the flanges  78  and elements  80  form the sole connection between the springs  28  and the dividers. It will be appreciated that by making the T-shaped portions  76  and flanges  78  inclined toward the spring, the effective length of the spring  28  is increased and the stiffness of the spring is reduced. Note also the large radii  88  between the legs  74  of the spring  28  and the T-shaped portions  76  which accommodate the substantial stresses at those locations.  
         [0033]     As illustrated in  FIG. 5   a , flow directing tabs  90  extend at angles from the upper edge of the regular divider  50 . The tabs  90  alternately project to opposite sides of the divider. The tabs are bent or struck and reinforcing stiffeners  92  ( FIG. 5 ) are struck and projected to opposite sides of the tabs to reinforce the tabs and minimize or preclude deflection of the tabs in response to water pressure.  
         [0034]     Referring to  FIG. 6 , the regular divider  52  is identical to the divider  50  except that slots  94  open through the top edge of divider  52  while slots  96  open through the bottom edge of divider  50  to redirect coolant back onto the fuel rods. It will be appreciated that by arranging the dividers  50  and  52  orthogonally one to the other, each divider may be received in the slots of the other divider to form a grid type structure.  
         [0035]     Referring now to  FIGS. 7-10 , partial dividers  54 ,  56 ,  58  and  60  are illustrated and have corresponding structure to portions of the regular dividers  50  and  52  previously described. The spring and stops previously described with respect to  FIGS. 5 and 6  are applicable to the partial dividers of  FIGS. 7-10 . The difference between the partial dividers  54  and  60  resides in the slots  94  and  96  disposed through the top and bottom edges of the spacer such that the partial dividers  54  and  60  can be interdigitated with regular dividers  52  and  50 , respectively. Similarly, the partial dividers  56  and  58  are identical to one another except for the slots  96  and  94  opening through the bottom and top edges of the dividers which permit interdigitation of the dividers  56 ,  58  with the regular dividers  50 ,  52  and special dividers  66 ,  68  to be described.  
         [0036]     Referring now to  FIGS. 11-13 , there are illustrated center dividers  62 . The center dividers  62  and  64  are provided to accommodate the water rod openings in the grid structure and to avoid spring assemblies in the peripheral band of the spacer. The springs  28  of center dividers  62  and  64  are identical to the springs  28  previously described with respect to the regular dividers  50 ,  52 . However, springs  28  are omitted in a pair of adjacent openings  100  ( FIG. 11 ) of center divider  62 . Additionally, a strut  102  of divider  62  lies between the adjacent openings  100  and includes a stop  104  projecting to one side of the divider. Two center dividers  62  are arranged in end-to-end back-to-back relation and are welded to one another, for example, by top and bottom spot welds indicated  106  in  FIG. 13  to form one of the two center dividers of the spacer.  
         [0037]     Two center dividers  64  of  FIGS. 14-16  are provided identical to one another and similarly have adjacent opening  100  separated by a strut  102  mounting a stop  104 . The two center dividers  64  are arranged end-to-end and back-to-back and welded to one another, for example, by top and bottom spot welding indicated  110  in  FIG. 16 . Thus, the center dividers  62  may form a center divider assembly  120  ( FIGS. 12 and 13 ) while the center dividers  64  welded back-to-back form a center divider assembly  122  ( FIGS. 16 and 17 ). It will be appreciated that the slots  119  and  121  of assemblies  120  and  122 , respectively, may be interdigitated by the center dividers, and slots  94  and  96  may be interdigitated by the regular dividers  50  and  52 . It will also be appreciated that by disposing the center dividers  62  back-to-back to one another and dividers  64  back-to-back to one another, the springs  28  of each center divider assembly project in opposite directions except in those areas which in part bound or define the water rod openings  26 .  
         [0038]     Referring now to  FIGS. 17-20 , there are illustrated special dividers  66  and  68 . The springs  28  and flow tabs  90  of these special dividers are similar to the regular dividers. The deflector tabs  90  alternate to opposite sides of the divider. Referring to divider  66 , the two openings  70  which will lie adjacent a water rod opening  26  have a bridging strut  130  which forms a spring projecting into the water rod opening  26 . The deflecting tabs  90  adjacent those openings deflect to the opposite side of the divider from the spring projection, as can be seen in  FIG. 2 . Note also that stops  32  are omitted with respect to the openings adjacent the water rod openings. The special divider  68  is similarly configured with a water rod spring  130  projecting to a side of the divider  68  opposite from which the springs  28  project. The slots  96  and  94  for the special dividers  66  and  68 , respectively, lie adjacent the respective bottom and top edges of the spacer as illustrated to facilitate interdigitation.  
         [0039]     Consequently, it will be appreciated that only a limited number of generally corresponding parts are required. For example, four each of the regular dividers  50  and  52  are provided. Two each of the partial dividers  54 ,  56 ,  58  and  60  are provided. Two each of the center dividers  62  and  64  are provided, and two each of the special dividers  66  and  68  are provided. Ancillary parts include only corner plates  131  ( FIG. 2 ) for the water rod openings. Consequently, a total of 24 divider parts are necessary to form the grid structure together with four peripheral bands  4  and two corner plates  131 .  
         [0040]     For illustrative and representative purposes, the regular dividers  52 , the partial dividers  58  and  60 , the center dividers  62  and the special dividers  68  are illustrated in  FIG. 2  as extending in a vertical direction. Those dividers have slots  94  opening in an upward direction. In addition, center dividers  62  and special dividers  68  have slots  119  opening in an upward direction. The regular dividers  50 , partial dividers  54  and  56 , center dividers  64  and special dividers  66  extend in a horizontal direction in  FIG. 2  and have slots  96  opening in a downward direction. In addition, center dividers  64  and special dividers  66  have slots  121  opening in a downward direction. Consequently, the slots  94  and  96  of the illustrated vertically and horizontally extending dividers enable orthogonal interdigitation of the dividers to form the rectilinear grid structure.  
         [0041]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.