Patent Number: 053848136
Section: summary

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 "poison") 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 "poison") 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 "oil canning". 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.