Description of the Prior Art
This invention, in its preferred form, relates to welding fixture for welding sleeves to a fuel grid and to a method for assembling and locating the sleeves relative to the grid. More particularly, the invention relates to the apparatus, and the attendant method, for locating a grid on a fixture, and for locating sleeves on the grid, through use of the fixture. As herein used, the word "fixture" includes both a plate having abutments and locating pins, and independent pegs.
There have been provided nuclear fuel bundle assemblies which include a matrix of nuclear fuel rods which are arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids. These grids are formed of straps which are slotted so that the straps mate, one strap entering into the slot of the outer strap at each point of intersection. Control rods are included at selected positions within the array of nuclear fuel rods. These control rods are received in guide sleeves which are welded to the grid. More particularly, there has been provided in the past a construction in which the straps forming the grid had edges which were substantially in a common plane, and a plurality of guide sleeves were provided, each seated on four straps defining a common cell. The outside diameter of the cylindrical guide sleeve was such as to permit the base of the cylindrical guide sleeve to rest upon the edges of four straps forming a single cell, it being understood that guide sleeves were provided only at spaced cells, and not at adjacent cells.
To assemble the guide sleeve to the grid, the guide sleeves were butt welded onto the straps forming the particular cell at the upper edge of each of the four straps. This procedure required that the grid, after assembly, be machined to insure that the edges of the four straps forming the particular cell to which the cylindrical sleeve was to be welded lay as closely as possible in a common plane, and this required a fabrication step known as "spot facing". The "spot facing" step was time consuming and costly, requiring the use of a suitable machine and fixtures, as well as the expenditure of a workman's time. The requirement for relatively great precision in the location of the strap edges is based on the fact that if a gap greater than 0.005 inches remains between the sleeve and the edge of the grid strap, the weld which is effected may well be defective in that the fused metal will not extend entirely beneath that portion of the base of the cylindrical sleeve which is intended to engage the upper edge of the grip strap; the occurrence of a gap will result in either lack of welding or no welding, either of which is unsatisfactory.
Welding fixtures and methods have been heretofore provided, involving the provision of so called "chills" generally in the nature of pins which extended into cells and which positioned sleeves on the grid, for welding. These fixtures have not proven satisfactory, particularly in connection with the utilization of laser welding for welding the sleeves to the grid straps.