Patent Number: 
Section: description

In accordance with the present invention, an improved method for metallurgically bonding a complete leak-tight enclosure to a matrix-type fuel element penetrated longitudinally by a multiplicity of coolant channels is provided. Coolant tubes containing solid filler pins are disposed in the coolant channels penetrating the fuel matrix. Header plates, perforated to match the coolant channels in the fuel matrix, are disposed at each end of the fuel matrix to accommodate the coolant tube ends. Metal cladding is placed about the fuel matrix and welded to the header plates. Metal cover plates cover the perforated ends of the header plates and are welded to the header plates under vacuum conditions to complete a leak-tight evacuated enclosure for the fuel matrix and coolant tubes disposed therein. The completely enclosed and sealed fuel element assembly is then exposed to a high temperature and pressure gas environment so as to effect a metallurgical bond between the contacting surfaces of the fuel matrix, coolant tubes, header plates and cladding. After the operation is completed, the ends of the assembly are machined to expose the coolant tube ends and filler pins contained therein. Selective leaching is used to remove the filler pins from the coolant tubes so as to leave the coolant tubes with internal coolant passageways of the proper hydraulic diameter. Final machining of the header plates then prepares the fuel element for mounting within a reactor core. In order to facilitate an understanding of the invention, reference is made to the drawings wherein like reference characters designate like or corresponding parts throughout the several views. A fuel element assembly which has been prepared for a hot gas pressure-bonding operation according to this invention is illustrated in FIG. 1. A fuel matrix 1, with a refractory metal such as tungsten forming the matrix material and uranium dioxide as fuel, is penetrated longitudinally by a multiplicity of coolant channels 2. The fuel matrix is formed from a plurality of axially aligned hexagonal fuel compacts. Sacrificial metal pins 3 are fitted within the bores of refractory metal coolant tubes 4 which are, in turn, disposed within coolant channels 2. The hexagonal configuration of the fuel compacts and the manner in which the filler pins 3 and coolant tubes 4 are disposed therein will be more apparent from an examination of FIG. 2 where a sectional view of the fuel element assembly of FIG. 1 is shown. FIGS. 1, 2 and 3 also illustrate how a refractory metal cladding or can 5 is used to enclose the lateral surface of the fuel assembly so as to prevent the escape of fission products therefrom during operation in a neutronic reactor. The refractory metal can 5 is butt-welded to lips 6 and 7 extending about the lateral periphery of header plates 8 and 9, respectively. A relatively large area of contact is provided between the refractory metal can and header plates 8 and 9 to ensure a satisfactory diffusion bond therebetween during the subsequent high-temperature and pressure-bonding operation. Circular closure plates 10 and 11 are placed over header plates 8 and 9, respectively, and butt-welded thereto about their peripheries to complete a leak-tight enclosure for the fuel element assembly during the subsequent pressure bonding operation. Header plates 8 and 9, which are otherwise hexagonal in shape, are provided with circular flanges 12 and 13 to facilitate the welding operation. FIG. 3 provides an end view of closure plate 10 which is identical to closure plate 11. FIGS. 4 and 5 are a plan and an end view, respectively, of the fuel element assembly of FIG. 1 after it has been pressure bonded, its header plates 8 and 9 machined, and filler pins 3 chemically leached from coolant tubes 4 to provide a finished fuel element. As shown at end 14 of the finished fuel element, closure plate 10 has been removed and header plate 8 machined to provide a threaded central extension 15 suitable for engagement with a fuel element latching mechanism. Other suitable configurations for engaging fuel element latching mechanisms will be apparent to those skilled in the nuclear reactor art. At end 16 of the finished fuel element, closure plate 11 has been removed and a hexagonal depression encompassing all of the coolant tubes has been machined into header plate 9. The hexagonal depression facilitates making spacing nubs 17 which permit a multiplicity of fuel elements to be assembled closely together in a reactor core with only point contact therebetween. The close assembly minimizes neutron streaming and coolant flow induced vibration, while the slight spacing provided by the nubs permits thermal expansion of the fuel elements without excessive interference. The fuel element assembly illustrated in FIG. 1 must be leak-tight to the hot high-pressure gas used in a bonding operation in order that a complete metallurgical bond will be developed between the fuel matrix, header plates, coolant tubes and cladding. Any leakage through the enclosure during the bonding operation will tend to equalize the pressure within the enclosure with that outside, so that a pressure differential no longer exists to press the cladding against the fuel matrix. Since the development of a diffusion bond between the cladding, fuel matrix and coolant tubes depends in part upon their being pressed together, no diffusion bond will develop in the presence of an enclosure leak. In a typical fuel element fabrication done in accordance with the present invention, a fuel element assembly similar to that shown in FIG. 1 was assembled using electron beam welding in vacuum to join tantalum enclosure components as taught herein. Solid molybdenum pins were inserted in the coolant tube bores which were in turn placed within coolant channels penetrating the fuel matrix and header plates. Following the welding of closure plates to the header plates, the entire assembly was heated to 3000xc2x0-3200xc2x0 F. and subjected to helium gas at 10,000 psig for 1-1xc2xd hours. Because of the pressure differential between the interior and exterior of the assembly, all clearances within the fuel element assembly were eliminated and all corresponding parts were brought into intimate contact by contraction upon the molybdenum pins. During the pressure bonding operation, solid state diffusion occurs between all metal components which are in contact, thereby effecting a metallurgical bond between fuel segments and the coolant tubes, header plates and outer hexagonal cladding or can. The coolant tubes are also bonded to the header plates and to the molybdenum pins within their bores. Due to the isostatic nature of the pressure application, no deformation of the molybdenum pins occurs so that the pins fix the hydraulic diameter of the coolant tube channels. Following the bonding operation, the closure plates are machined away to expose the molybdenum pins within the bores of the coolant tubes. The fuel element assembly is then immersed in a heated dilute solution of nitric and sulfuric acid which preferentially dissolves the molybdenum pins without attacking the tantalum coolant tubes and other cladding. The significant advantages of this fuel element fabrication method include the simplicity of the final closure or sealing operation prior to the bonding operation and the control of the desired coolant tube hydraulic diameter through the use of sacrificial molybdenum pins within the coolant tube bores. Another advantage is that the quality level of the refractory metal tubing is not as critical to successful bonding and to the integrity of the finished fuel element as it would be if the bores of the coolant tube were exposed to the high temperature and pressure gas during the bonding operation. Exposure of the tube bores in such a manner tends to aggravate existing defects in the tubing as well as increasing the effect of any defects in the coolant channel wall when the tubing expands against the wall. The method of this invention, on the other hand, thickens the slightly oversize coolant tube wall by compressing the tube against the filler pin and thus tends to mitigate the effect of existing coolant tube defects rather than aggravating them. Although only tantalum closure components have been referred to in the above description, the closure design and bonding technique can be applied to other refractory metals such as tantalum-10 tungsten, tantalum-8 tungsten-2 hafnium, tungsten, tungsten-25 rhenium, molybdenum and molybdenum base alloys, columbium and others. When molybdenum coolant tubing is used, however, dissimilar metal sacrificial filler pins must be used in order that they may be selectively leached from the coolant tubes. Dissimilar refractory metals may be used for tubing and for the inner header plate without encountering serious welding problems inasmuch as the tubing-to-header plate joints are effected by solid state diffusion bonding. In an alternative embodiment, a fuel element assembly may be provided wherein filler pins plated with refractory metal are used in place of filler pins disposed in refractory metal tubes. Such an embodiment may be desirable where a supply of a particular refractory metal tubing is not available in the quantity and/or of the quality desired. The above description of the invention was offered for illustrative purposes only, and should not be interpreted in a limiting sense. It is intended rather that the invention be limited only by the claims appended hereto.