Patent Number: 041750047
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a fuel assembly 10, oriented with its longitudinal axis in the vertical plane, including a lower end fitting assembly 11, an upper end fitting assembly 12, a plurality of elongated fuel elements 13, hollow guide tubes 14 and spacer grid assemblies 15. The fuel elements 13 and guide tubes 14 are laterally spaced and supported in a square array parallel to each other at intervals along their length by the spacer grid assemblies 15. Threaded sleeves 16, 17 welded to each end of the guide tubes 14 respectively secure the guide tubes to the lower and upper end fitting assemblies 11, 12 by lock welded nuts 21, 22 in a manner such as is described in U.S. Pat. No. 3,828,868 by F. S. Jabsen and assigned to The Babcock & Wilcox Company. Each one of the spacer grid assemblies 15, as is best shown in FIG. 3, is composed of a multiplicity of grid plates 23 which are slotted and fitted together in an "egg-crate" fashion to form the cells 24 through which the fuel elements 13 (shown in FIG. 1) and guide tubes 14 extend. The spacer grid plates 23 are generally of the type described in U.S. Pat. No. 3,665,586 by F. S. Jabsen and assigned to The Babcock & Wilcox Company and, as is shown in FIG. 2, have indentations 26 laterally extending into those cells 24 that contain fuel elements 13 for engagement and support of the fuel elements. The lengthwise edges of the grid plates 23 are provided with arcuate saddles 25 in the wall portions of the plates that form those cells 24 through which the guide tubes 14 extend. The guide tubes 14 generally have larger diameters than the fuel elements 13. The arcuate saddles 25 are generally shaped to conform to the cylindrical shape of the guide tubes. As is best shown in FIG. 2, the guide tubes 14 are formed with protuberancies 30 that radially extend the diameter of the guide tubes, and which are circumferentially spaced about the perimeter of the guide tube 14. The protuberancies 30, moreover, are formed at longitudinally aligned and spaced intervals along the guide tube length. In the guide tube 14 shown in FIG. 2, longitudinally separated groups of protuberancies are spaced apart so as to span the width of a spacer grid plate 23. Some longitudinal clearance is generally maintained between the protuberancies 30 and the grid plate 23. As is shown in FIGS. 2 and 3, each spacer grid assembly 15 (shown in FIG. 1) is restricted from longitudinal movement by the protuberancies 30 which are longitudinally aligned with the saddles 25 of the grid plates. Thus, the protuberancies 30 bear against and restrain the plates at the saddles 25 if the grid plates longitudinally shift position. The guide tube 14 shown in FIGS. 2 and 3 is formed with groups of four equidistantly circumferential or perimetrical spaced protuberancies located at intervals along the length of the guide tube. An alternative method of forming protuberancies is shown in FIGS. 4 and 5 wherein a generally cylindrical guide tube 40 is provided with a plurality of protuberancies 41 that radially extend the diameter of the guide tube 40, and are circumferentially spaced about the guide tube. The protuberancies 41 are longitudinally aligned and spaced apart along the guide tube length. The protuberancies 41, as is shown in FIG. 4, are located between the saddles 25 that are formed in the grid plate to be restrained by the protuberancies. The protuberancies 41 are spaced at ninety degree intervals about the circumference of the cylindrical guide tube 40 as is shown in FIG. 5. The guide tube protuberancies may take various shapes. FIG. 6, for example, shows the general arrangement of FIG. 2. However, cuts have been made through the protuberancy 43 edge, adjacent to the spacer grid plate 23 edges or saddles, of the guide tube 44 in order to form a lip 42 for more positively engaging the grid plate saddles 25. FIGS. 7 and 8 illustrate, in detail, the formation of the protuberancies 30 and 41 of guide tubes 14 and 40, respectively. The protuberancies, as shown, are an integral part of the guide tube walls. A guide tube is inserted through the spacer grid assemblies by orienting the protuberancies so that each projects into a corner of the cell 23, as is illustratively shown in FIGS. 4 and 5. When the guide tube has been inserted to the desired length, it is rotated approximately forty-five degrees, to bring the guide tube protuberancies into longitudinal alignment with the spacer grid plate saddles, as is best shown in FIGS. 2 and 3. As shown in FIGS. 2 and 3, the cross-section of the guide tube 14 at each protuberancy 30 is greater than the distance between the saddles 25 of parallel grid plates forming the cell 24 through which the guide tube extends. In this manner, a fuel assembly can be assembled without the need for rigidly attaching the guide tube to grid plate by welding or brazing; and, the fuel assembly can be disassembled without the need for cutting. Moreover, no additional components such as sleeves, clips or tabs, which introduce additional parasitic absorbing materials are required. As explained hereinbefore, a suitable hoist and grapple mechanism is generally utilized to transfer a fuel assembly to and from the reactor core. The handling device usually engages the upper end fitting assembly for lifting and movement of the fuel assembly. During handling of the fuel assembly, the assembly load, which bears on the lower end fitting assembly, is transferred to the guide tubes. Should a failure of all the guide tubes occur, the fuel elements will begin to slip through the spacer grid assemblies 15. The upper end of each fuel element is plugged with an upper end cap 44 (FIG. 1). The upper end cap 44, moreover, has a diametrical portion exceeding the width of the cells 24. Thus, the fuel elements 13 slip until the end cap 44 engages the uppermost spacer grid assembly. The upper spacer grid, in turn, will longitudinally slip until it is restricted by the guide tube protuberancies, thereby precluding separation of the fuel assembly into its component parts. It will be evident to those skilled in the art that changes may be made, e.g., the use of a straight edge in lieu of a saddle for engaging the protuberancies and grid plates, or the use of different multiples of protuberancies, or protuberancies having different shapes, or non-cylindrical guide tubes, without departing from the spirit of the invention covered in the claims.