1. Field of the Invention
The present invention relates to grid assemblies for fuel assemblies within a nuclear power plant. More specifically, the invention provides a lowermost grid assembly having a grid insert secured therein for removably securing the lowermost grid between the guide tubes and lower nozzle of the fuel assembly.
2. Description of the Related Art
A typical fuel assembly for a nuclear reactor has a plurality of fuel rods held in place by a plurality of grids. The grids are distributed along the length of the fuel assembly, and are held in place by guide tubes extending lengthwise through the fuel assembly. The guide tubes are structured to receive devices such as control rods or instrumentation. The top and bottom of the fuel assembly each include a nozzle.
A typical grid includes a first plurality of grid straps substantially parallel to each other, and a second plurality of grid straps substantially parallel to each other and substantially perpendicular to the first grid straps. A perimeter strap surrounds the grid. A plurality of substantially square cells are thereby defined within each grid. Each cell includes one or more springs and one or more dimples which engage the fuel rod, thereby holding it in place within the cell. The grid assembly may also include mixing vanes for the coolant which flows from the bottom to the top of the fuel assembly. The lowermost grid assembly further includes debris-trapping features.
The lowermost grid assembly within the fuel assembly is typically secured to the lower nozzle by a continuous perimeter weld of the perimeter strap of the grid to the lower nozzle. In addition to being time consuming, this method of attachment of the lower nozzle requires that fuel rods be installed after completion of the welding, because there is no access to the fuel rods from the bottom of the fuel assembly once the lower nozzle is in place. Therefore, the fuel rods must be inserted into the fuel assembly by pushing them through the top of the fuel assembly, instead of the preferred method of pulling the fuel rods through from the bottom of the fuel assembly.
Accordingly, there is a need for an apparatus and method of securing the lowermost grid assembly to the lower nozzle of the fuel cell that is simpler, less time consuming and less costly to accomplish. There is a further need for a method of attaching the lower nozzle to the lowermost grid of a fuel assembly that permits installation of the fuel rods into the fuel assembly by pulling them through from the bottom instead of pushing them through from the top.
The present invention provides a lowermost grid for a fuel assembly having a grid insert secured therein. The grid insert is structured so that a guide tube may be mechanically secured therein, with a lower nozzle secured to the lower grid assembly and guide tube.
The grid assembly includes a first and second plurality of metal straps, with the first plurality arranged substantially parallel to each other, and the second plurality arranged substantially parallel to each other and substantially perpendicular to the first plurality of metal straps, thereby forming an egg crate pattern of substantially square cells within the grid assembly. Each of the first and second plurality of metal straps have spring and/or dimple elements projecting therefrom into the cells reserved for fuel rods within the matrix, so that a fuel rod may be secured within each of these cells. A plurality of larger cells are provided within the matrix for receiving the guide tubes and an instrument tube.
A grid insert is secured within these larger cells. The grid insert comprises a tube having a retention lip projecting inwardly from its lower end. The grid insert is dimensioned and configured to receive a guide tube therein, with the end of the guide tube abutting the grid insert retention lip. The guide tube itself includes a fashioned lower end for receiving a mechanical fastener therethrough. Fashioned is defined herein as being dimensioned and configured to mate with a mechanical fastener in a secure fashion. An example of a fashioned lower end includes one having a threaded bore for receiving a bolt or screw, or having an end fitting that is so threaded.
The lower nozzle includes an opening corresponding to each of the grid inserts, with one preferred embodiment including an opening corresponding to each corner, and a fifth opening in the center. A mechanical fastener such as a bolt or screw may be passed through the opening, being threadedly secured to the end of the guide tube, and may possibly have a locking disk placed over the screw slot.
By assembling the fuel assembly in this manner, the prior art step of continuous perimeter welding the perimeter strap to the nozzle is avoided. Additionally, the prior continuous perimeter weld required fuel rods to be pushed through the top of the fuel assembly, rather than pulling the fuel rods from the bottom. Once the lower nozzle is welded in place, there is no access to the fuel rods from the bottom of the fuel assembly.
The present invention therefore provides a precise, secure alignment of the lowermost grid to the lower nozzle. The invention further provides a grid insert structured to facilitate securing the grid assembly and lower nozzle together by sandwiching the retention lip of the grid insert between the guide tube end fitting and the lower nozzle when the guide tube is bolted or screwed into place. The invention provides a method of securing the lowermost grid assembly to the lower nozzle that is less expensive, less time-consuming, and simpler than previous methods. The present invention additionally provides a method of assembling a fuel assembly that permits rods to be pulled from the bottom.
Accordingly, it is an object of the present invention to provide a lowermost grid assembly having a grid insert for receiving a guide tube.
It is another object of the present invention to provide a lowermost grid having a grid insert secured within the grid, and having a retention lip that is sandwiched between a guide tube end fitting and the lower nozzle, thereby securing the lowermost grid assembly and lower nozzle together.
It is a further object of the present invention to provide an improved method of securing a lowermost grid to a lower nozzle.
It is another object of the present invention provide a method of assembling a fuel assembly that permits fuel rods to be installed by pulling them from the bottom instead of pushing them in from the top.
It is a further object of the present invention to provide a more efficient, less expensive means for making a fuel assembly.
These and other objects of the invention will become more apparent through the following description and drawings.
Like reference numbers denote like elements throughout the drawings.
The present invention provides a lowermost grid for a fuel cell having a grid insert to facilitate securing the lowermost grid to the fuel assembly's lower nozzle.
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While a specific embodiment of the invention has been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.