Abstract:
A storage rack for storing nuclear fuel rod assemblies is provided with an array of cell housings having damping elements which are preloaded against the outer walls of the individual cells. The damping elements are slabs which when preloaded against the cell walls provide a coulomb damping function which is highly effective in absorbing vibration from rough handling or seismic events. The cell housings may be located in alternating positions in the array and cells may be formed from the outer walls of the surrounding cell housings. The cell housings are held together in the array by support bars which are affixed to the top and bottom ends of the cell housings. The support bars may include recesses to align the cell housings.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to storage racks for storing nuclear fuel assemblies both during transport and during stationary storage. Preferably, the racks are highly overdamped, enabling them to best withstand vibrations caused by seismic events or rough handling. 
     Fuel for nuclear reactors is typically configured in the form of elongated fuel rods, which may be separate, stand-alone elements, or may be positioned within canisters. Hereinafter, the fuel rods and rod/canister combinations are referred to as fuel assemblies. Both before and following use, the fuel assemblies must be stored and/or transported with great care. To assure that such care is achieved, storage racks are often used to support a plurality of fuel assemblies in a generally parallel, spaced-apart configuration, while maintaining the fuel assemblies in a subcritical array environment. During storage, the racks and the fuel assemblies contained therein, may be completely submerged in a pool of water. The water provides cooling and additional shielding from nuclear radiation. 
     The fuel storage racks of the prior art typically consist of an assembly of hollow cells, each defined by an array of elongated rectangular cross-section boxes or compartments. The boxes are typically made by forming sheets of stainless steel into elongated rectangular cross-section tubes and welding the corners of the elongated tubes together to form a matrix of elongated hollow cells, each adapted the receive a single fuel assembly. Exemplary storage racks are disclosed in U.S. Pat. Nos. 4,695,424, 4,857,263, 4,948,553, and 4,366,115. A neutron absorbing (or &#34;poison&#34;) material, such as borated stainless steel, is typically welded or otherwise rigidly affixed to each of the walls of boxes to absorb neutron flux from the fuel assemblies which may be positioned within the boxes, thereby avoiding an undesirable concentration of neutrons. 
     These prior art storage racks suffer from several disadvantages. For example, neutron absorbing elements, and particularly those made borated stainless steel, are expensive and difficult to form and weld to the walls of the boxes. Further, the individual cells are known to be weak along the top edge and have little torsional or crush strength. In addition, storage racks constructed in this way have little resistance to vibration, such as may be caused by seismic events. Due to the reactive nature of the nuclear fuel assemblies, such damage to the storage racks can be disastrous. 
     Accordingly, it is an object of the present invention to provide an improved storage and/or transport rack for nuclear fuel assemblies. 
     Another object of the present invention is to provide an improved storage rack for nuclear fuel assemblies which is highly overdamped to enable the rack to withstand the vibration of seismic events or rough handling such as may be encountered during transportation of the rack. 
     It is another object to provide a storage rack for nuclear fuel assemblies which has improved torsional and crush strength. 
     A further object is to provide an improved storage rack for nuclear fuel assemblies which may De easily and inexpensively manufactured. 
     Other objects of the invention will in part be obvious and will in part appear hereinafter. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a rack structure is provided for long term storage and/or transport of nuclear fuel assemblies. The storage rack includes an array of individual storage cells. The cells of the array are defined by a plurality of substantially polygonal cross-section, elongated cell housings, each extending along an elongated central axis, wherein the central axes are substantially parallel to each other. In accord with an important aspect of the invention, a slab of neutron absorbing (or &#34;poison&#34;) material is biased against the outer surfaces of the cell housings. Preferably, the cell housings are positioned in alternate points of a rectangular grid configuration, so that each cell housing defines one cell in its interior and so that the outer walls of three or more adjacent cell housings define one cell. A stiffener wall may be welded to the adjacent cell housings along the perimeter of the rack to enclose the open cells along the perimeter. The cell housings and the stiffener walls are held in parallel alignment by support bars affixed thereto, for example by welding, at both the top and bottom ends of the array of cell housings. Preferably, the support bars are positioned at the top and bottom ends between each row of cell housings and along the outer perimeter of the rack. The support bars may be recessed on one side or on alternating sides to provide positioning of the cell housings prior to affixation. A base plate is affixed to bottom of the array to define the lower boundary of the respective cells and to support the fuel assemblies therein. To facilitate water flow for cooling of the nuclear fuel assemblies, the base plate may include holes at positions within each cell. Pedestals extending from the base plate may be used to raise the rack above a floor. 
     With this configuration, a new or spent fuel assembly may be placed in each of the cells. When in place, neutron flux from the fuel assemblies is absorbed by the poison material on the cell housing walls. Retaining devices, or clamp assemblies, hold the poison slabs in position, while preloading (i.e. forcing) the slabs against the walls, and permit easy assembly of the rack without requiring welding of the poison material to the housings. 
     The retaining devices press the slabs firmly against the walls of the cell housings. The resulting friction between the slab and cell walls results in a coulomb damping function that has proven extremely effective in deadening vibration. The slabs also serve to strengthen the cell walls making them more resistant to deformation or &#34;oil canning&#34;. Thus, the slabs, as held in place by the preload forces, preferably establish an overdamped characteristic for the cell housings, resulting in a substantially stronger, vibration resistant configuration as compared with the prior art. In the preferred form of the invention the slabs, which are preloaded against the cell walls, are made of a neutron absorbing material. Alternatively, the invention may be configured with other materials that are preloaded against the cell walls, which merely provide the coulomb damping function. In the latter configurations, other forms of neutron absorption may or may not be used. 
     Alternatively, the retaining device may include a single cover plate which extends over and protects the entire poison slab. The cover plate is provided with flanges along the perimeter which are fixed to the housing to preload or force the slab against the outer surface. Additionally, the cover plate may be provided with raised bumps or ridges which bear on the poison slab and further preload it against the housing in a substantially uniform manner. 
     In view of the potential danger inherent in handling and storing nuclear fuel assemblies, it is critical that the storage racks effectively isolate and support the nuclear fuel assemblies under adverse conditions. In use, the racks are subject to stresses resulting from normal insertion and removal of the nuclear fuel assemblies, rough handling during transportation of the rack and possibly natural phenomena such as an earthquake. By providing the rack with a highly overdamped characteristic, it is better able to withstand these stresses and insure long term stability, as compared to prior art racks. 
     Typically, in prior art storage racks, individual cells are welded together at the corners of their respective cell housings. This method has proven to be undesirable not only because is it difficult and therefore expensive to manufacture but also because it has proven to be structurally inferior. In contrast, by welding the cell housings to support bars at the top and bottom of the rack, and with little or no other cell-to-cell fixation, the present invention avoids both the difficulty and expense of the prior art process and provides improved torsion and crush strength. The support bars also reinforce the upper edges of the cell housing to protect them from damage during normal insertion and removal of the nuclear fuel assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: 
     FIG. 1 is a plan view of a storage rack in accordance with the invention, along lines 1--1 of FIG. 2, showing the cell locations with the support bars removed. 
     FIG. 2 is an elevation view of the storage rack of FIG. 1. 
     FIG. 3 is a detailed section of the plan view of FIG. 1. 
     FIG. 4 is a detailed view of an perimeter cell of the rack of FIG. 1, showing the stiffener wall and retainer clips. 
     FIG. 5 is a plan view of the storage rack of FIG. 1, showing the cell locations and the support bars. 
     FIG. 6 is a plan view of an alternative embodiment of the invention showing the cover plate retaining device. 
     FIG. 7 is an elevation view of an alternative embodiment of the invention showing the cover plate retaining device. 
     FIG. 8 is a plan view of an alternate embodiment of the invention showing cell housings having a pentagonal cross-section. 
     FIG. 9 is a plan view of an alternate embodiment of the invention showing cell housings having a triangular cross-section. 
     FIG. 10 is a plan view of an alternate embodiment of the invention showing cell housings having a hexagonal cross-section. 
     FIG. 11 is a plan view of the alternate embodiment of the invention shown in FIG. 10 wherein specific cell positions are formed by adjacent cells. The spacing between the cells has been exaggerated for clarity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-5 show a storage rack 10 of the invention, which forms a close packed array 10, 3 rows by 5 columns of elongated cells C1-C15 in this embodiment, the odd numbered or primary cells C1, C3, C5, C7, C9, C11, C13, C15 are formed from rectangular cell housings 3 which extend along housing axes 3&#39;. The even numbered or secondary cells C2, C4, C6, C8, C10, C12, C14 are formed by the walls of the surrounding cells. It is noted that the secondary cells along the perimeter of the array C2, C4, C6, C10, C12, C14 are not completely surrounded by primary cells. If it is necessary that one of these secondary cells is to be utilized, a stiffener wall 3A is inserted substantially flush with exterior walls of the adjacent primary cell housings to completely enclose the secondary cell as shown in FIG. 3. 
     The individual cell housings 3 and the stiffener walls 3A are held in parallel alignment by upper and lower support bars 1, 7, and 8 which extend transverse to the longitudinal axis of the cell housings 3. The support bars 1, 7 and 8 are located between each of the rows of cells and along the perimeter of the array as shown in FIG. 5. Support bars 1, 7 and 8 are provided at both the top and bottom end portions of the rack. The cell housings are held in parallel alignment by welding the individual cell housings 3 to the upper and lower support bars 1, 7 and 8. The support bars also add strength to the upper and lower ends of the cell housings to resist damage during inserting and removal of the nuclear fuel rod assemblies. 
     As shown in FIG. 2, a base plate 2 is welded to the bottom of the rack to close the bottom of the cells and support the nuclear fuel assemblies. The base plate 2 may also be provided with holes 12 at locations within each cell and pedestal feet 9 to facilitate the flow of water for enhanced cooling. 
     Each cell housing 3 is an elongated tube having a rectangular cross section. The housing is constructed from suitable material, for example, .050 inch thick stainless steel tubing. Typically the tubes are square, approximately 81/2 inches along each side and 14 feet long. Each outer surface of the housing is planar, to which is applied, with a preload force, an elongated slab 4 constructed of a damping material. The damping material may also be a neutron absorbing material, such as borated stainless steel, borated aluminum, boral (such as manufactured by Brooks &amp; Perkins, Minneapolis, Minnesota), or other neutron absorbing materials may be used. The damping material is preloaded against the outer surface by retainer clips 5, 5A, 5B which are welded to the outside of the housings along the perimeter of each surface. The retainer clips 5, 5A, 5B are composed from a flat plate of steel by forming an S-shaped bend which causes the plane of the raised edge portion 5C to be offset relative to the fixed edge portion 5D of the same surface. Preferably, the offset is the same size as the thickness of the slab 4. When retainer clips 5, 5A, 5B are plug or spot welded the housing 3, the weld shrinks and the clips are pulled tight against the housing thus preloading the elongated slab against the housing. The fixed edge portion 5D may be provided with holes to facilitate plug welding. Alternatively, the offset 5E may be less than the thickness of the damping material and therefore, when the fixed edge portion 5D is fixed to the housing 3, the raised edge portion 5C forces the damping material 4 against the outer surface of the housing. In this embodiment, the retainer clips may be fixed by welding as well as by rivets or threaded fasteners. 
     As shown in FIG. 2, there are upper and lower horizontal retainer clips 5B attached at the upper and lower ends of each lateral surface of the cell housings. Also shown are vertical retainer clips 5 which are attached to the housings 3 along the vertical edges of the outer surfaces. As shown in FIG. 4, the vertical retainer clip 5 has a right angle bend 5&#39; along its center and raised edge portions 5C on each side of the bend. This permits the vertical retainer clip 5 to preload the damping material on two adjacent surfaces of the housings 3. 
     As shown in FIG. 4, the slabs 4 may be provided with recesses 4A along each edge. The depth of the recess 4A is preferably equal to the thickness of the retainer clip 5. The recess 4A permits the retainer clips 5 to be flush with the outer surface of the slab 4. This avoids damaging the retainer clips 5 during insertion and removal of the fuel assemblies. 
     As shown in FIGS. 3 and 4, a stiffener wall 3A may be utilized to enclose the secondary cells (e.g., cell C4) along the perimeter of the array 10. The stiffener wall 3A is a U-shaped channel of the same material, length and thickness as the cell housing. It is of sufficient width to fill the space between two primary cells (e.g., cells C3 and C5). The outer surface of the stiffener wall 3A may have a damping slab 4 preloaded against it made of a material of a similar size and type of material preloaded against the walls of the cell housings 3. The slab 4 of damping material is preferably preloaded against the outer surface of the stiffener wall by horizontal retainer clips 5A along the upper and lower ends of the stiffener wall 3A and by vertical retainer clips 5B along the longitudinal edges of the stiffener wall 3A. 
     In an alternate embodiment as shown in FIGS. 6 and 7, the retaining device is a cover plate 15 which extends over the entire housing surface. The cover plate 15 has a flange extending from each edge which is fixed to the housing 4. Each flange has a S-shaped bend which includes a sloped portion 15A and fixed portion 15B. After the fixed portion 15B is welded to the housing, the resulting weld shrinkage causes the sloped portion 15A to pull the cover plate 15 against the housing thus preloading the poison slab 4 against the outer surface. 
     As shown in FIGS. 8-11, the invention may be embodied utilizing cells having different geometric cross-sections. By utilizing different cell geometries, the packing or spacing of nuclear fuel assemblies can be controlled to accommodate different system requirements. FIG. 8 shows an embodiment of the present invention utilizing cells of pentagonal cross-section. While the cells are shown in a circular array configuration, the cells may be arranged in other geometric or non-geometric configurations. FIGS. 9-11 show exemplary close packed arrays. FIG. 9 shows an embodiment of the present invention utilizing cells of triangular cross-section. In this embodiment, stiffener walls to enclose the outer cells 3A may also be provided. FIG. 10 shows an embodiment of the present invention utilizing cells of hexagonal cross-section. This configuration provides increased packing of the nuclear fuel assemblies and increased cell strength over rectangular configurations. FIG. 11 shows a modification of the embodiment of FIG. 10 wherein central cells A are formed by the walls of the adjacent surrounding cells B. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.