Patent Number: 041359712
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and in particular to an apparatus for holding down fuel assemblies within the reactor core. In pressurized water reactors the coolant flow rate and fuel assembly flow resistance are such that the hydraulic uplift force is of sufficient magnitude to cause the assemblies to jitter and even lift off the core support structure. Various approaches have been used to eliminate this detrimental movement. One suggested solution involves the use of a lock down device which attaches the lower end of the fuel assemblies to the core support structure. While this device will function properly, it does introduce mechanical complexity since the device must not only lock and unlock remotely, but it must release reliably after a year of operation in the reactor environment. Another approach has been to use springs located above each fuel assembly which bear against an upper alignment plate, thereby urging the fuel assemblies down. As reactors have been designed with increasingly large hydraulic uplift forces the spring force has become very large. The springs themselves have become so large that an excessively large plenum is required between the alignment plate and the upper end fitting of the fuel assembly. The springs at this location produce an undesirable flow pattern and an excessively high pressure drop through the plenum. They are also potentially subject to flow induced vibration since the total reactor coolant passes over these springs. SUMMARY OF THE INVENTION It is an object of the invention to hold-down fuel assemblies in a simple uncomplicated manner which will eliminate or reduce the need for spring hold-down forces. It is a further object to introduce these forces in a manner which will compensate for variations in primary flow through the reactor. These and other objects are achieved in the invention by attaching to the fuel assembly, at either end, a pressure plate. The pressure plate is horizontally coextensive with the fuel assembly and has the side of the plate adjacent the fuel assembly exposed to the reactor fluid pressure existing in that area. The other side of the pressure plate is exposed to a pressure from another location in the reactor vessel. The pressure are selected so that the higher pressure is always above the pressure plate, thereby resulting in a downward force on the fuel assembly. Where the pressure plate is located above the fuel assembly the high pressure is preferably obtained from a location where the coolant enters the reactor vessel. Alternately this pressure may be obtained from a location at the bottom of the core. Where the pressure plate is located below the fuel assembly the low pressure is obtained by connecting this area to a portion of the fluid flow path near the outlet from the reactor. A pressurizable plenum is formed on the side of the sealing plate away from the fuel assemblies by providing a sealing arrangement between extensions on the sealing plate and a support plate or alignment plate which is adjacent thereto. Spring means to supplement the hold-down force may be used since they are compatible with the hydraulic structure and they may be of lesser magnitude than the prior art structure where the spring means supplied the entire hold-down force. Various other objects and advantages will appear from the following description of the embodiments of the invention, and the novel features will be particularly pointed out in connection with the appended claims.