Patent Number: 041526023
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to restraint devices and more particularly provides a flexible lateral restraint device for an underwater nuclear fuel storage rack. 2. Description of the Prior Art Nuclear power generating plants are typically fueled by elongated fuel assemblies, such as those including a bundle of nuclear fuel rods. Subsequent to utilization of the fuel assemblies in a nuclear core they are stored within fuel racks positioned within an enclosure such as a spent fuel pit. A typical spent fuel pit or pool includes leak tight vertical walls and a floor made of concrete and other support materials lined with stainless steel. The fuel racks include a rectangular, closely spaced array of cells, each cell sized to receive a fuel assembly. Because the assemblies have been irradiated they must be shielded, and water is typically utilized for this purpose. The assemblies and fuel racks are therefore maintained submerged within water within the pit. It is of significant importance that the fuel racks be restrained laterally within the fuel pit under accident conditions, such as a large seismic occurrence. Excessive deflections of the racks and contained fuel assemblies could result in damage to the assemblies and the surrounding environs. It is also recognized that differential thermal expansions can occur in time between the fuel racks and the walls of the containing fuel pit. Accordingly, several types of lateral restraint devices have been proposed in the past, which respond to these differential expansions as well as seismic occurrences. One type has been proposed which is affixed to the side of the fuel rack and positions, through an arc-like motion between the fuel rack and the containing wall, a load pad surface against the fuel pit wall. A pre-load is imposed upon the pads by a series of Belleville-type springs between a portion of the restraining device and the load pads. With such arc-type devices the pre-load is very difficult to adjust and the device is difficult to properly and accurately position. An excessive amount of space between the fuel rack and the pit wall is also required to accommodate the arc-like motion. Further, the arc motion devices are difficult to properly seat against the pit wall due to the high frictional forces and slippage among the contact pad surface and the pit wall. A significant improvement upon the arc motion device utilizes the substantially horizontal motion of a device affixed to the side of the rack and remotely extendible into contact with the pit wall. It operates in a manner similar to a scissors jack. This device is described in detail in application Ser. No. 789,912, filed Apr. 22, 1977, in the name of June S. Knight, Personal representative of the estate of Charles B. Knight, deceased, and entitled "Nuclear Fuel Rack Lateral Support and Pre-Load Device". Among the prior art, the arc-motion devices imposed loads between the racks and walls resulting from differential expansions, as well as imposing loads for pre-load purposes. Thus, the pit walls, for example, are loaded needlessly high. The pre-loaded scissors jack device suffers the same deficiency. Although these prior art devices will perform the intended function, it is desirable to have an alternative and simple lateral support which avoids unnecessarily high loadings, particularly in new plant installations where remote positioning capability is not necessarily required. SUMMARY OF THE INVENTION This invention provides a lateral support for nuclear fuel racks positioned underwater which provides the capability of a substantially rigid support in the event of large accidental loadings, while also providing the capability to inherently absorb differential thermal expansions between a fuel rack and the adjacent enclosing wall. The device can be manufactured simply, and can be sized to require a small amount of space. The device includes, similar to prior art systems, one or more load pads with a flat surface that can be positioned into contact with a vertical surface such as the wall of a spent fuel pit. The structure utilized for extending and positioning the pads, however, is significantly different and more simple than prior art structures and can eliminate the use of Belleville-type springs in a preferred embodiment. The pads are joined directly or preferably through a leveling foot to a sliding piston. The piston is disposed partially or completely within a horizontally oriented cylinder affixed to the side of the spent fuel rack. At the opposite end of the piston, an elastic structure such as a compression spring is positioned so as to continuously apply a positioning force to the piston. The device also includes structure for maintaining the piston in a fixed position within the cylinder with the spring in a significant compressed orientation. This structure can include a simple pin inserted through an opening in the wall of the cylinder matingly engaged with a receiving aperture in the piston. Means are also provided for controlling the amount of fluid, such as the water in which the assemblies and racks are immersed, which can flow into or out of the cylinder portion behind the piston in a predetermined fashion. This can be accomplished by contouring the piston to the shape of the cylinder, with a preselected clearance between the two components, or by incorporating selectively sized flow relief openings. The properly sized lateral support device can therefore be installed onto a fuel rack with the spring compressed and the piston maintained in place by the retaining pin. Subsequently, the holding pin is removed, allowing the spring to laterally move the piston and affixed surface so that the surface contacts the pit wall. Differential thermal expansions between the pit wall and the side of the fuel rack will then be accommodated by movement of the piston and the respective expansion or contraction of the compression spring. The water in the pit will also flow into the area behind the piston within the cylinder containing the compression spring. Under a large lateral loading, such as a seismic accident condition, the substantially incompressible fluid within the cylinder will resist and damp the motion of the piston, thereby providing a substantially rigid restraint between the fuel rack and the wall so as to alleviate the potential for excessive loading or bending forces upon the contained fuel assemblies. In view of the prior art, it is readily apparent that forces imposed between the fuel racks and pit walls from differential thermal expansion are avoided by water bleeding out of the cylinder. However, for a seismic event the substantially incompressible liquid will act as a rigid restraint.