Patent Number: 054065992
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is generally related to nuclear fuel assemblies and particularly to the fabrication of spacer grids used in nuclear fuel assemblies. 2. General Background Fuel assemblies for nuclear reactors are formed from a number of fuel rods held in position radially by spacer grids located along the length of the fuel rods. The spacer grids are formed from slotted metal strips containing stamped features, which are crisscrossed on edge to define individual cells for each fuel rod. The crossing metal strips are aligned at approximate right angles to each other and then welded together. The alignment method currently used is generally known as the pinning and framing process. After the metal strips are placed in the crisscross pattern, the next step is the insertion of pins into the individual cells of the egg crate. The "egg crate" is used to refer to the unwelded spacer grid inner structure comprised of intersecting, slotted strips that form a checkerboard-like array of cell locations. A tooling pin is then inserted into each cell. The pin interfaces with the base strip areas, as opposed to the contact points in each cell, to position and hold the strips in the proper orientation. The pinned egg crate is then placed into a welding frame that contains pressure pads which secure the egg crate to the frame. Pressure applied directly to the outer tooling pins is transferred to the inner locations through each strip and the adjacent pins. The load is repeatedly applied and released to act as a settling process to remove any tiny gaps or misalignments that may be present. The grid is then ready to be welded. This leaves a need for an improved fixturing system that can be used to align the egg crate with a minimum of steps and equipment, and provide greater dimensional control. SUMMARY OF THE INVENTION The present invention addresses the above need for a more straightforward manner in which to prepare the spacer grid egg crate for intersection welding. What is provided is a weld fixture for receiving and aligning the fuel rod contact points of a nuclear fuel bundle spacer grid, which eliminates the individual pin placement and settlement process in the previous equipment. Two plates each have a series of intermittent intersecting slots on one side used to align and position the fuel rod contact points referred to as outboard hardstops. The intermittent slots are not equally spaced, and the slot pitch is different in each intersecting direction to account for shrinkage during the welding process. One plate has a bore therethrough at each corner and a counterbore in a selected number of the polygons defined by the intermittent slots. A second plate has a bore therethrough at each corner and a counterbore in each of the polygons formed by the intermittent slots. Both plates have bores therethrough substantially at the intersection of the slots. A guide rod is received through each corner bore such that the intersecting slots on each plate are facing and aligned with each other. The guide rod in the orientation indicator corner is larger than the others to assure that the two fixture plates are properly oriented during assembly. Guide cell pins are positioned on the first plate in the recessed milled areas referred to as guide tube cell locations. Short alignment pins, located in counterbores in the polygons formed by the slots, are used to align and position fuel rod contact points referred to as inboard hardstops in the required locations on the first plate. The second plate contains tapered alignment pins, located in counterbores in the polygons formed by the slots, which are used to align and position fuel rod contact points referred to as softstops, with the exception of locations that also require positioning of an inboard hardstop, in which a combination pin is used to align and position both contact points simultaneously. The slots and assorted pins properly align the contact points of the spacer grid and the two plates are bolted together to secure the spacer grid in place. Laser welding of the interior intersections of the strips is accomplished by directing a laser beam through the bores at the slot intersections.