Abstract:
A spacer grid design for a nuclear fuel assemblies that exhibits increased crush strength. The walls of the grid straps that surround the fuel elements have a number of dimples and/or springs with the flat surfaces of those walls formed with a plurality of emboss geometries that are formed in a symmetrical pattern with the pattern covering substantially an entire area of the wall except for the contact surfaces of the dimples and springs that interface with the fuel rods.

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 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 the lower internals  24  and the 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 threes 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 a 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 tithe deformation under accident conditions which could detrimentally affect control rod insertion capability. 
         [0009]      FIG. 3  is an elevation view, represented in vertically shortened form, of a fuel assembly being generally designed 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 atop 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 prevention 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 movable 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 fur the center location which is occupied by the instrumentation tubes  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 that promote the transfer of heat from the fuel rod cladding to the coolant. The significant rate of flow of coolant and the turbulence exerts 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 have received more attention, resulting in a tightening of the seismic requirements that fuel assemblies have to satisfy. Typically, 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 welt as added costs to the manufacturing process. Furthermore, adding additional metal to the grid increases the neutron capture cross section of the grid which detracts from the efficiency of the nuclear process within the core to produce heat for useful work. 
         [0014]    Accordingly, a new fuel assembly grid design is desired that will increase the crush strength of the grid without significantly increasing the manufacturing costs or pressure drop across the grid or detract from the efficiency of the nuclear reaction within the core. 
       SUMMARY 
       [0015]    These and other objects are achieved by a nuclear fuel assembly having a parallel array of elongated fuel elements and a support grid for supporting the elongated fuel elements along their longitudinal dimension. The grid has 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 a guide tube for a control rod with each of the cells having a plurality of wails which intersect at corners and surround the corresponding fuel element or a guide tube at the support locations. Each of the walls that supports the fuel elements has a number of dimples and/or springs and the walls that support the fuel elements are embossed with a plurality of emboss geometries that are formed in a staggered pattern with the pattern covering substantially an entire area of the wall except a contact surface of the dimples and springs that interface with the fuel rods. In one preferred embodiment, the geometry is generally circular in cross section. The geometry has a wall thickness, a wall pitch (i.e., the distance between corresponding points on the geometries), a height and a diameter, with the ratio of height to wall thickness greater than or equal to one-quarter and less than or equal to four; and a ratio of diameter to wall pitch greater than or equal to one-eighth and less than or equal to one. 
         [0016]    In another embodiment, the geometry is generally hexagonal in cross section. The geometry has a height and width, with the ratio of the height to width greater than or equal to one-quarter and less than or equal to four; and a ratio of width to wall pitch greater than or equal to one-eighth and less than or equal to one. 
         [0017]    In a third embodiment, the geometry is generally rectangular in cross section with rounded corners. The geometry has a height, width and length, with the ratio of the height to wall thickness greater than or equal to one-quarter and less than or equal to four; a ratio of width to length greater than or equal to one-tenth and less than or equal to one; and a ration of length to wall pitch greater or equal to one-eighth and less than or equal to one. For all embodiments, the geometries may extend on opposite sides of the wall or adjacent geometries may extend on the same side of the wall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    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: 
           [0019]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0020]      FIG. 2  is an elevation view, partially in section, of a nuclear reactor vessel and internal components to which this invention can be applied; 
           [0021]      FIG. 3  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
           [0022]      FIG. 4  is a plan view of an egg-crate support grid; 
           [0023]      FIG. 5  is a front view of one wall of a fuel element support cell having one embodiment of the embossed pattern of this invention; 
           [0024]      FIG. 6  is a perspective view of the fuel support cell wall illustrated in  FIG. 5 ; 
           [0025]      FIG. 7  is a bottom view of the fuel cells support wall illustrated in  FIG. 6 ; 
           [0026]      FIG. 8  is a schematic front view of the embossed wall pattern illustrated in  FIGS. 5-7 ; 
           [0027]      FIG. 9  is a laterally cross sectional view of the geometrical embossed pattern illustrated in  FIG. 8 ; 
           [0028]      FIG. 10  is a schematic front view of a second embodiment of the geometric embossed pattern of this invention; and 
           [0029]      FIG. 11  is a third embodiment of the embossed geometric pattern of this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    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  and  88  that incorporate the features of the embodiments described hereafter are not readily apparent from this view, but can better be appreciated from the view shown in  FIGS. 5-11 . 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 grid  64 . Though orthogonal straps  86  and  88  forming substantially square fuel rods 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&#39; elongated dimension between the intersections of four adjacent straps forms a wall  100  of the fuel rod support cells  94 . 
         [0031]    As previously mentioned, among the various functions, a spacer grid provides lateral support for a fuel assembly to assure the insertion of control rods is not impeded under any normal or accident conditions. However, postulated accident loads are always locally intense on the structural grids. These loads can, under certain circumstances, exceed the grid crush strength, which requires reevaluation of the loading conditions, or coolant geometry and control rod insertion analysis, or even a redesign of the spacer grid. This invention adds a three-dimensional embossed geometry to the walls of the cells that support fuel rods. One embodiment of the embossed geometry shown on a single wall of a support cell  100  is illustrated in  FIGS. 5-7 . Though one wall of a fuel element support cell with an embossed geometry is illustrated it should be appreciated that the embossed geometry may extend over two or more of the wails of each fuel element support cell. The embossed geometry in this embodiment is formed from rows  104  with alternate rows  106  offset in a staggered manner so that the alternate rows  106  are nested between the geometric shapes  102  of the adjacent rows  104 . Preferably, the geometric shapes  102  are not stamped into the contact surfaces of the dimples  92  or the springs  90  to avoid fretting of the fuel rods. 
         [0032]      FIGS. 8 and 9  are schematic views of the cell wall illustrated in  FIGS. 5-7  and show the height h and diameter d of the geometric pattern that can be stamped in one or in alternated directions into the grid strap wall. Computational results have shown that the optimal ranges for the height hand diameter d of the geometrical shape  102  are between the ratio of height to wall thickness greater than or equal to one-quarter and less than or equal to four, and the ratio of the diameter to wall pitch greater than or equal to one-eighth and less than or equal to one. 
         [0033]      FIG. 10  shows a second embodiment that employs hexagonal geometric shapes and  FIG. 11  shows a third embodiment that includes generally rectangular geometric shapes with rounded corners. Like reference characters are used among the several embodiments to identify corresponding features. For the rounded rectangular staggered patterns such as one illustrated in  FIG. 11 , the optimal ranges for the width w and the length l are between the ratio of width to length greater than or equal to one-tenth and less than or equal to one; and the ratio of length to wall pitch greater than or equal to one-eighth and less than or equal to one. It is expected that the hexagonal and rounded rectangular staggered patterns provide higher mechanical properties compared to the circular geometry. 
         [0034]    As previously mentioned, the three-dimensional embossed geometries are formed only on the flat surfaces of the straps, in one or in alternating directions. The fuel rod supports (i.e., the springs and dimples) preferably formed with smooth surfaces to minimize fretting wear. 
         [0035]    Thus, this invention provides improved grid strength with minimal increase in manufacturing costs and optionally enables the thickness of the straps to be slightly reduced which will contribute to a reduction in pressure drop across the grid. 
         [0036]    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.