Abstract:
A spacer grid for a nuclear fuel assembly that exhibits increased crush strength. Each grid strap at the ligaments that support fuel rods has a spring or dimple to support the fuel rods under anticipated external loads during shipping and handling or in a seismic event. One or more elongated embossed ribs are provided on each of the fuel rod grid strap support ligaments to increase its moment of inertia by forming various shapes on the ligaments of the grid strap. Preferably, the ribs have a streamlined shape to prevent any excessive pressure drop. In this manner, the crush strength of a conventional short grid strap is increased without meaningful additional manufacturing costs or adverse effects to the neutron economy of the grid.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention pertains generally to a nuclear reactor fuel assembly and, more particularly, to a nuclear fuel assembly that employs a robust spacer grid. 
         [0003]    2. Description of the Related Art 
         [0004]    The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary circuit for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump, and a system of pipes which are connected to the vessel form a loop of the primary side. 
         [0005]    For the purpose of illustration,  FIG. 1  shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  10  having a closure head  12  enclosing a nuclear core  14 . A liquid reactor coolant, such as water, is pumped into the vessel  10  by pump  16  through the core  14  where heat energy is absorbed and is discharged to a heat exchanger  18 , typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump  16 , completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . 
         [0006]    An exemplary reactor design is shown in more detail in  FIG. 2 . In addition to the core  14  comprised of a plurality of parallel, vertical, co-extending fuel assemblies  22 , for purpose of this description, the other vessel internal structures can be divided into lower internals  24  and upper internals  26 . In conventional designs, the lower internals&#39; function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies  22  (only two of which are shown for simplicity in  FIG. 2 ), and support and guide instrumentation and components, such as control rods  28 . In the exemplary reactor shown in  FIG. 2 , coolant enters the reactor vessel  10  through one or more inlet nozzles  30 , flows down through an annulus between the vessel and the core barrel  32 , is turned 180° in a lower plenum  34 , passes upwardly through a lower support plate  37  and a lower core plate  36  upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate  37  and the lower core plate  36  are replaced by a single structure, a lower core support plate having the same elevation as  37 . The coolant flow through the core and surrounding area  38  is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate  40 . Coolant exiting the core  14  flows along the underside of the upper core plate  40  and upwardly through a plurality of perforations  42 . The coolant then flows upwardly and radially outward to one or more outlet nozzles  44 . 
         [0007]    The upper internals  26  can be supported from the vessel or the vessel head and include an upper support assembly  46 . Loads are transmitted between the upper support assembly  46  and the upper core plate  40 , primarily by a plurality of support columns  48 . Support columns are respectively aligned above selected fuel assemblies  22  and perforations  42  in the upper core plate  40 . 
         [0008]    Rectilinearly moveable control rods  28 , which typically include a drive shaft  50  and spider assembly  52  of neutron poison rods, are guided through the upper internals  26  and into aligned fuel assemblies  22  by control rod guide tubes  54 . The guide tubes are fixedly joined through the upper support assembly  46  and the top of the upper core plate  40 . The support column  48  arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally effect control rod insertion capability. 
         [0009]      FIG. 3  is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character  22 . The fuel assembly  22  is the type used in a pressurized water reactor and has a structural skeleton, which at its lower end includes a bottom nozzle  58 . The bottom nozzle  58  supports the fuel assembly  22  on the lower core plate  36  in the core region of the nuclear reactor. In addition to the bottom nozzle  58 , the structural skeleton of the fuel assembly  22  also includes a top nozzle  62  at its upper end and a number of guide tubes or thimbles  84  which align with the guide tubes  54  in the upper internals. The guide tubes or thimbles  84  extend longitudinally between the bottom and top nozzles  58  and  62  and at opposite ends are rigidly attached thereto. 
         [0010]    The fuel assembly  22  further includes a plurality of transverse grids  64  axially spaced along and mounted to the guide thimbles  84  and an organized array of elongated fuel rods  66  transversely spaced and supported by the grids  64 . A plan view of a grid  64  without the guide thimbles  84  and fuel rods  66  is shown in  FIG. 4 . The guide thimbles  84  pass through the cells labeled  96  and the fuel rods occupy the cells  94 . As can be seen from  FIG. 4 , the grids  64  are conventionally formed from an array of orthogonal straps  86  and  88  that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells through which the fuel rods  66  are supported in the cells  94  in transverse, spaced relationship with each other. In many designs, springs  90  and dimples  92  are stamped into the opposite walls of the straps that form the support cells  94 . The springs and dimples extend radially into the support cells and capture fuel rods  66  therebetween; exerting pressure on the fuel rod cladding to hold the rods in position. The orthogonal array of straps  86  and  88  is welded at each strap end to a bordering strap  98  to complete the grid structure  64 . Also, the assembly  22 , as shown in  FIG. 3 , has an instrumentation tube  68  located in the center thereof that extends between and is captured by the bottom and top nozzles  58  and  62 . With such an arrangement of parts, fuel assembly  22  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0011]    As mentioned above, the fuel rods  66  in the array thereof in the assembly  22  are held in spaced relationship with one another by the grids  64  spaced along the fuel assembly length. Each fuel rod  66  includes a plurality of nuclear fuel pellets  70  and is closed at its opposite ends by upper and lower end plugs  72  and  74 . The pellets  70  are maintained in a stack by a plenum spring  76  disposed between the upper end plug  72  and the top of the pellet stack. The fuel pellets  70 , composed of fissile material are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system. 
         [0012]    To control the fission process, a number of control rods  78  are reciprocally moveable in the guide thimbles  84  located at predetermined positions in the fuel assembly  22 . The guide thimble locations can be specifically seen in  FIG. 4  represented by reference character  96 , except for the center location which is occupied by the instrumentation tube  68 . Specifically, a rod cluster control mechanism  80  positioned above the top nozzle  62 , supports a plurality of control rods  78 . The control mechanism has an internally threaded cylindrical hub member  82  with a plurality of radially extending flukes or arms  52  that form the spider previously noted with regard to  FIG. 2 . Each arm  52  is interconnected to a control rod  78  such that the control rod mechanism  80  is operable to move the control rods vertically in the guide thimbles  84  to thereby control the fission process in the fuel assembly  22 , under the motive power of a control rod drive shaft  50  which is coupled to the control rod hub  80 , all in a well known manner. 
         [0013]    As mentioned above, the fuel assemblies are subject to hydraulic forces that exceed the weight of the fuel rods and thereby exert significant forces on the fuel rods and the assemblies. In addition, there is significant turbulence in the coolant in the core caused by mixing vanes on the upper surfaces of the straps of many grids which promote the transfer of heat from the fuel rod cladding to the coolant. The significant rate of flow of the coolant and the turbulence exert substantial forces on the grid straps. In addition, the grid straps have to withstand external loads incurred during shipping and handling or from all postulated accidents such as seismic and loss of coolant accidents. Recently, the concerns over seismic events at nuclear power plants has received more attention, resulting in a tightening of the seismic requirements that fuel assemblies have to satisfy. Typically, the fuel assembly grids have been strengthened by increasing the strap height, or the strap thickness or by adding additional welds. However, each of these design improvements results in an increased pressure drop of the coolant across the fuel assembly as well as added costs to the manufacturing process. For example, a high strength strap height of 2.25 inches (5.72 cm) that is 1.5 times taller than the standard height of 1.50 inches (3.81 cm) would increase the pressure drop across the grid assembly by approximately 10%. Additionally, adding a weld at the middle of the intersection of the grid straps to increase its crush strength, would add to the manufacturing costs. 
         [0014]    Accordingly, a new fuel assembly grid design is desired that will increase the strength of the grid without significantly increasing the manufacturing costs or pressure drop across the grid. 
       SUMMARY 
       [0015]    A new support grid for a nuclear fuel assembly is herein provided that will fulfill the foregoing objectives. The new support gird, for supporting elongated fuel elements along the longitudinal dimension, includes a lattice structure which defines a plurality of cells, some of through which the fuel elements are respectively supported. Others of the cells respectively support guide tubes for control rods. Each of the cells has a plurality of walls which intersect at the corners of the cells and surround the corresponding fuel element or guide tube at the support locations. At least one wall of each cell that supports the fuel elements has an elongated rib formed from an indentation within the wall, that is an integral part of the wall, without substantially any perforations along the periphery of the indentation. 
         [0016]    In one embodiment, the support grid has the elongated rib oriented substantially in the horizontal direction. Desirably, the elongated rib extends substantially the entire width between the corners of the walls. Preferably, the indentation is discontinued substantially at the corners. In the preferred embodiment, each of the cells that support fuel elements has an upstream end and a downstream end, wherein the upstream end first encounters a reactor coolant flow when the fuel assembly is situated in an operating reactor. Preferably, the surfaces of the indentation are rounded on the upstream side of the indentation and more desirably, all of the surfaces of the indentation are rounded to reduce pressure drop. 
         [0017]    In another embodiment, the at least one wall has a plurality of the elongated ribs and preferably they are located at an elevation on either side of a dimple or spring that is employed to restrain vertical movement of the fuel rod. 
         [0018]    In another embodiment, the lattice structure, in part, is made up of two parallel arrays of intersecting straps with the walls on a strap of adjacent cells that support fuel rods having an elongated rib formed in different directions. Preferably, all of the walls of each cell that supports the fuel elements has the elongated rib. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0020]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0021]      FIG. 2  is an elevational view, partially in section of a nuclear reactor vessel and internal components to which this invention can be applied; 
           [0022]      FIG. 3  is an elevational view, partially in section of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
           [0023]      FIG. 4  is a plan view of an egg-crate support grid of this invention; 
           [0024]      FIG. 5  is a perspective view of two cell portions of one grid strap of the grids shown in  FIG. 4 , that borders two fuel support cells with the cell strap section showing the ribs of this invention; 
           [0025]      FIG. 6  is a perspective rear view of the grid strap section shown in  FIG. 5 ; 
           [0026]      FIG. 7  is a front perspective view of the grid strap cell sections shown in  FIG. 5 , with the elongated ribs oriented on a diagonal; 
           [0027]      FIG. 8  is a perspective rear view of the grid strap cell sections shown in  FIG. 7 ; and 
           [0028]      FIGS. 9A-9G  are side cross sectional views, respectively, of different grid strap rib contours that can be applied in accordance with this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    This invention provides a new fuel assembly design for a nuclear reactor and more particularly an improved spacer grid design for a nuclear fuel assembly. The improved grid is generally formed from a matrix of approximately square (or hexagonal) cells, some of which  94  support fuel rods while others of which  96  are connected to guide thimbles and a central instrumentation tube. The plan view shown in  FIG. 4  looks very much like the prior art grids since contour of the individual grid straps  86  an  88  that incorporate the features of the embodiments described herein are not readily apparent from this view, but can be better appreciated from the view shown in  FIGS. 5-9 . The grid of this embodiment is formed from two orthogonally positioned sets of parallel, spaced straps  86  and  88 , that are interleaved in a conventional manner and surrounded by an outer strap  98  to form the structural make-up of the grids  64 . Though orthogonal straps  86  and  88  forming substantially square fuel rod support cells are shown in this embodiment, it should be appreciated that this invention can be applied equally as well to other grid configurations, e.g., hexagonal grids. The orthogonal straps  86  and  88  and in the case of the outer rows the outer straps  98 , define the support cells  94  at the intersection of each four adjacent straps that surround the nuclear fuel rods  66 . A length of each strap along the straps elongated dimension between the intersections of four adjacent straps forms a wall  100  of the fuel rod support cells  94 . 
         [0030]      FIGS. 5 and 6 , and  7  and  8 , each illustrate two walls  100  of adjacent cells  94  that support fuel rods that have many of the features of conventional grid straps  86  or  88  shown in  FIG. 4 . Though  FIG. 4  illustrates a 17×17 array of cells, it should be appreciated that the application of the principals of this invention are not affected by the number of fuel elements in an assembly. The lattice straps which form the orthogonal members  86  and  88  shown in  FIG. 4  are substantially identical in design. While the lattice straps  86  and  88  are substantially identical, it should be appreciated that the design of some of the lattice straps will vary from other lattice straps, to accommodate guide tube and instrument thimble locations identified by reference character  96 . As can be best appreciated by reference to  FIGS. 5-8 , most of the walls  100  of the cells  94  that accommodate fuel elements are provided with a number of stamped protruding segments that are tooled by appropriate dies as is known and used in the industry. The upper and lower stamped segments  92  bulge out in one direction and form dimples for supporting the fuel elements against juxtaposed diagonal springs  90  which protrude from the opposite cell wall. The remaining centrally located, stamped section  90  in the same wall  100  as the previously described dimples  92 , bulges in the opposite direction into the adjacent cells and forms a diagonal spring  90  for pressuring the fuel element against dimples  92  which protrude into that adjacent cell from its opposite wall. A preferred design of the diagonal spring can better be appreciated by reference to U.S. Pat. No. 6,144,716, issued Nov. 7, 2000. 
         [0031]    Mixing vanes  102  extend from the upper edges of the lattice straps at some of the segments which form the walls of the cells  94  through which the fuel elements pass. The cells  96  that support the guide tubes and an instrumentation thimble through which the control rods and the in-core instrumentation pass differ from the fuel element support cells  94  in that they have none of the support members  90  or  92  protruding into their interior or mixing vanes  102  extending from their walls. The cells  96  may further differ in that they may have a concave, embossed section at the center of the cell walls extending from the bottom to the top of the lattice strap as described in U.S. Pat. No. 6,526,116, issued Feb. 25, 2003. 
         [0032]    In accordance with the embodiments described herein, the crush strength of the spacer grid walls are increased by adding one or more embossed ribs  104  on one or more of the walls  100  as illustrated in  FIGS. 5 ,  6 ,  7  and  8 .  FIGS. 6 and 8  present a rear view, respectively, of  FIGS. 5 and 7 . Preferably, the embossed ribs  104  extend in a horizontal direction in between the intersection of the orthogonal straps that define the fuel support cells  94 . Desirably, the ribs  104  are on either side of the springs  90  between the dimples  92  and springs  90 . However, it should be appreciated that one or more of the ribs  104  may be provided on one or more of the walls  100  to add strength to the grid straps  86  or  88 . Furthermore, the ribs  104  may be provided at an orientation other than the horizontal orientation illustrated in  FIGS. 5 and 6 , as shown in  FIGS. 7 and 8 , in which the ribs extend on a diagonal. The shallow dome or cylinder type of ribs  104  illustrated in the figures can easily be formed during the strap stamping process without adding much additional cost to the manufacturing process. To prevent any excessive pressure drop increase, preferably the edges of the ribs  104  should be streamlined as illustrated in  FIGS. 5-8 , on the upstream side of the coolant and, desirably, all of the edges of the ribs should be streamlined. Also, the embossed ribs  104  can be oriented in alternate directions to minimize strap bowing or fanning, i.e., on alternate sides of the grid cell strap in adjacent cells. The ribs of this invention will prevent or minimize any undesirable deformation during the stamping process used to form the dimples and springs. Undesirable deformation of the thin plate straps that form the walls of the fuel rod support cells has been experienced in the past. The deformation makes it difficult to assemble the straps to be welded at the intersecting joints. In the past the straps were hammered to overcome this difficulty. The ribs of this invention obviate the need for the additional hammering step. Based on conventional Euler buckling theory, the buckling strength is a linear function of the moment of inertia. Therefore, the increase moment of inertia introduced by the embossed ribs  104  will enhance the crush strength of the space grid. 
         [0033]    Based on a strap height, the moment of inertia is a function of the geometry, location, direction, and number of ribs as shown in Table 1 below. 
         [0000]                                                        FIGS. 9   9A   9B   9C   9D   9E   9F   9G                   Type (inch)   Straight   Single Rib   Double Rib   Double Rib   Double Rib   Double Rib   Double Rib           (1.5 × 0.018)   Shape A   Shape A   Shape B   Shape B   Shape A   Shape B               Location A   Location A   Location A   Location B   Location A   Location B               Direction A   Direction A   Direction A   Direction A   Direction B   Direction B       Moment of Inertia   7.3   ~26.8   ~44.7   ~51.6   ~51.6   ~57.3   ~66.5       (×10 −7  inch 4 )       Projected Area   3.6   11.1   11.1   12.3   12.3   13.7   21.0       (×10 −3  inch 2                      
Table 1 corresponds to the rib configurations illustrated in  FIGS. 9A-9G , showing the approximate moment of inertia and projected area of the ribs for each of the configurations illustrated.  FIG. 9A  shows a straight strap without any ribs as a point of reference.  FIG. 9B  shows a single rib having a shape A, location A in the upper region of the strap and a direction A, i.e., protruding to the left side of the strap.  FIG. 9C  shows a double rib configuration having the shape A, at location A, albeit in the upper and lower regions of the strap, in direction A.  FIG. 9D  shows a double rib configuration of shape B, i.e., having a sharper angle than that of shape A, at location A, in direction A.  FIG. 9E  shows a double rib configuration of shape B, at location B, i.e., more to the center of the strap, in direction A.  FIG. 9F  shows a double rib configuration of shape A, location A and in direction B, i.e., protruding on either side of the strap.  FIG. 9G  shows a double rib configuration of shape B, at location B, in direction B. Thus, the parameters in Table 1 can be optimized by satisfying the pressure drop allowance limit since a higher moment of inertia grid strap design could lead to a higher pressure drop. Another consideration is the manufacturing concerns regarding cracking, bowing and fanning during strap stamping.
 
         [0034]    Thus, this invention will enhance the crush strength of a spacer grid without increasing the height of the strap and/or adding additional, meaningful, manufacturing expense. 
         [0035]    Accordingly, while specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.