Abstract:
A skeleton rack for storing nuclear fuel rods, the rack having a rectangular array of vertically extending cells, the cells being formed by a plurality of elongated, relatively narrow rigid metal shafts, each disposed at a corner of a cell, rigid metal bridge members fixed to adjacent shafts proximal to upper ends of the shafts, apertured rigid metal end walls proximal to lower ends of the shafts and fixed to four shafts at corners of a respective cell.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to improvements in storage racks for nuclear fuel rod assemblies. 
       PRIOR ART 
       [0002]    Spent nuclear fuel rod bundles or assemblies are commonly stored in vertically oriented racks submerged in a pool of water. The racks typically have vertically extending walls that form an array of square columnar cells. Sheets of neutron absorbing material are attached to the cell walls. A spent fuel rod bundle is lowered into a cell for storage for some period and is ultimately retrieved by raising it from the cell. Water is allowed to circulate by convection upwardly through a cell to carry heat from the fuel rod bundle. 
         [0003]    U.S. Pat. No. 6,741,669 discloses a novel neutron absorber in the form of a two plane sheet of chevron cross-section. The neutron absorber is configured to be held in a storage rack cell by an interference fit with the cell walls. 
       SUMMARY OF THE INVENTION 
       [0004]    The invention provides a spent nuclear fuel storage rack having a skeleton frame forming an array of square, vertically extending cells. Each cell is arranged to receive a closely fitting neutron absorber sheet of chevron cross-section. The neutron absorbers are arranged in the cells in a regular pattern that allows the two planes of an absorber to block four cell faces. 
         [0005]    In the disclosed embodiment, the framing shafts of the rack are of three cross-sectional shapes. The shapes at the periphery of the rack are angles at the corners, and tees at the sides; the shafts of the rack interior are of cruciform or cross shape. Adjacent shafts are fixed together at their ends with elements that allow vertical flow of coolant water through each cell. At their upper ends, adjacent shafts are rigidly joined by bridge plates at the periphery of associated cells. At their lower ends, adjacent shafts are joined by an apertured end wall extending across an associated cell. 
         [0006]    The shafts, bridge plates and end walls are preferably joined with mechanical fasteners to allow a rack to be reliably and efficiently constructed with limited skill and equipment. This feature makes the rack especially suited for on-site erection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a simplified perspective view of a skeleton frame fuel storage rack embodying the invention; 
           [0008]      FIG. 2  is a fragmentary partially exploded view of an upper portion of the rack of  FIG. 1 ; 
           [0009]      FIG. 3  is a fragmentary perspective view of a bottom of the rack of  FIG. 1 ; 
           [0010]      FIG. 4  is a fragmentary cross-sectional view in a vertical plane of a bottom portion of the rack of  FIG. 1 ; 
           [0011]      FIG. 5  is a perspective view of a typical bottom end wall of a cell of the rack of  FIG. 1 ; 
           [0012]      FIG. 6  is a perspective view of a typical leg assembly of the rack of  FIG. 1 ; 
           [0013]      FIG. 7  is a somewhat schematic horizontal cross-sectional view of a corner portion of the rack and associated neutron absorber sheets; 
           [0014]      FIG. 8  is a perspective view of a typical neutron absorber sheet; and 
           [0015]      FIG. 9  is a fragmentary cross-sectional view of the upper end of an absorber sheet taken in the plane  9 - 9  indicated in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]      FIG. 1  illustrates an example of a skeleton frame rack  10  for spent nuclear fuel rods. The rack  10  is ordinarily submerged in a pool of water used to cool the spent nuclear fuel. The rack  10  has an array of vertical cells  11  that individually receive and store a bundle of fuel rods sometimes referred to as a fuel assembly (not shown). By way of example, but not limitation, the cell array may be a 10 by 10 matrix, each cell can be, for example, nominally 9 or 10 inches square, and 9½ feet in length. 
         [0017]    The rack  10  is primarily constructed of vertical shafts  12 ,  13  and  14 , bridge plates  15 , and lower end walls  16  ( FIG. 5 ). Typically, these parts  12 - 16  are all aluminum or all stainless steel to minimize electrolytic corrosion. With reference to  FIG. 2 , for example, the shafts  12  at the corners of the rack  10  are right angles in cross-section. At the side of the rack  10 , the shafts  13  are T-shaped in cross-section and at the interior of the rack, the shafts  14  have a cruciform or cross cross-section. By way of example, legs  21  of the shafts  12 - 14  an be between 4 inches and 6 inches in width and between ¼ inch to ⅜ inch in thickness. Preferably each shaft  12 - 14  is a monolithic element. In general, the shafts  12 - 14  are of equal length and are joined at their upper and lower ends by the bridge plates  15  and end walls  16 , respectively. Preferably, the shaft, plate and end wall elements  12 - 16  are all pre-drilled or pre-punched with holes  22  that align with one another for reception of mechanical fasteners  23 . The fasteners  23 , for example, can be rivets, bolts and nuts, and/or self-drilling and/or self-tapping screws or any combination of some or all of the same. 
         [0018]    The rack  10  can be erected at a factory, job shop, or at a site of use. The rack  10  is assembled using the selected form of fasteners, typically by technicians, starting at one side, one row of cells  11  at a time. If desired or necessary, additional bridge plates (not shown) can be provided at mid-length of the shafts  12 - 14 . 
         [0019]    Referring to  FIG. 5 , the end walls  16  are fabricated of a selected metal sheet stock. An end wall  16  has a large circular central aperture  24  and integral or monolithic depending flanges  25 . The aperture  24  assures that water can freely circulate through the end wall  16  and respective cell  11  by convection. The bridge plates  15 , residing in vertical planes, also assure free circulation of water upwardly through the cells  11 . Upper ends of the shafts  12 - 14  are milled or otherwise machined with shallow pockets  26  to receive the thickness of a bridge plate  15  and to enable the plates to be quickly registered with the shafts. 
         [0020]    The rack  10  sits on the floor of the pool with a plurality of legs  28  provided on a lower face of the rack represented by the end walls  16 . With reference to  FIG. 6 , each leg  28  is an assembly of a lower foot  29  providing a socket for a ball  30  depending from an upper spider-like portion  31  of the leg. Ideally, the legs  28  are formed of the same metal as the elements  12 - 16 . In the illustrated arrangement, each leg  28  is substituted for an end wall  16  at selected locations in the lower face of the rack  10 . 
         [0021]    After the rack  10  is assembled, neutron absorber sheets  36 , illustrated in  FIG. 8 , are lowered into respective cells  11 . This can be accomplished, for example, with the general method and type of apparatus disclosed in aforementioned U.S. Pat. No. 6,741,669. The absorber sheets  36  can be, for example, an aluminum boron composite or a stainless steel boron alloy with a wall thickness of, for example, 0.070 inch. The sheets  36  have a length generally equal to the height of the rack  10  above the end walls  16 . These and other suitable neutron-absorbing materials are disclosed in the just-cited U.S. Pat. No. 6,741,669. The neutron absorber sheet  36  has a pair of panels  37  that lie in planes that preferably diverge from one another at an angle exceeding 90 degrees, for example, 93 to 95 degrees. A lower end of each panel  37  is tapered at locations  38 ,  39  to facilitate insertion of these lower ends into a respective cell  11 . Upper ends of the neutron absorber sheet panels  37  have holes  41  for gripping by a robotic device (not shown) that inserts a neutron absorber sheet  36  into a cell  11  or retrieves the same from a cell. As shown in  FIG. 9 , fixed to upper edges of the neutron absorber sheet panels  37  are guide bars  42 , typically made of the material used for the main parts of the rack  10 . The guide bars have the cross-section of an inverted U so as to provide a slot  43  into which is received the upper edge of a panel  37 . The groove  43  is wide enough to receive the thickness of a panel  37 , the thickness of a bridge plate  15  and the local thickness of the end of a shaft  12 - 14 . The guide bars  42  are fixed to respective neutron absorber sheet panels  37  with suitable fasteners. It will be understood from the discussion below, that two outside faces of the rack  10  may not have an associated neutron absorber sheet panel associated therewith. In such cases, a guide bar  42  may be attached to the upper ends of adjacent perimeter shafts  12 ,  13 . 
         [0022]    As shown in  FIG. 7 , a neutron absorber sheet  36  is proportioned so that when the distal vertical edges of the panels  37  are elastically drawn towards one another by the installation device and/or by forces developed by the shafts  12 - 14  when the neutron absorber sheet  36  is lowered into a cell  11 , an elastic friction fit is developed between the absorber sheet and abutting surfaces of the shafts  12 - 14 . The guide bars  42  are beveled at their upper surfaces to assist in guiding fuel assemblies into associated cells  11 . Additionally, the guide bars serve to protect the upper edges of the neutron absorber sheet panels  37  when fuel assemblies are being manipulated into or from a cell. 
         [0023]    Consideration of  FIG. 7  will reveal that two side faces of a rack  10  will be devoid of a neutron absorber sheet panel  37 . Where a plurality of racks  10  exist, adjacent ones of the racks can be used to provide the absorber function from an adjacent rack. It will be seen from  FIG. 7 , that each cell, apart from two lines of peripheral cells, has an associated neutron absorber sheet blocking two faces of a cell and has its remaining two faces blocked by the neutron absorber sheet panels of the respective adjacent cells. 
         [0024]    The disclosed rack construction reduces manufacturing costs by reducing material content and inventory requirements. The same shafts can be used to produce any common cell size. The construction can be provided as a kit for on-site erection thereby greatly reducing shipping costs. Assembly with mechanical fasteners reduces labor costs and the level of required skill. 
         [0025]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.