Patent Number: 048184730
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a fuel assembly 21 including a plurality of fuel bundles 23. Each bundle 23 includes a plurality of conventional fuel rods 25 and one or more tie rods 26 in accordance with this invention. The fuel rods 25 and tie rods 26 are arrayed in a square array typically of 16 rods (4.times.4). The bundles 23 are typically arrayed in a square array (2.times.2) each defining a quadrant of the assembly. The bundle array is wrapped in a plate 27 of generally square cross-section defining a channel for coolant. The bundles 23 are spaced from each other defining a cruciform gap 31 through which coolant flows. A coolant conductor 33 extends into the gap 31. The conductor 33 is in the form of a cross whose arms are hollow shells of generally elongated oval cross section defining channels 35 through which coolant is conducted. Coolant also flows outside of the conductor 33 through the gap 31. The channels are open only at the bottom and the top and coolant is typically conducted through the channels between the bottom and the top. The plate 27 is secured to the coolant conductor 33 by welding at dimple 37 distributed axially along the plate. There is communication of coolant (not shown) between the quadrants of the square array of fuel bundles 23. Each conventional fuel rod 25 includes a cylindrical cladding tube 39 within which there is nuclear fuel. The tube 39 has a cap 41 from which a pin 43 extends (FIGS. 1, 2). A tapered pin 44 (FIG. 2) also extends from the lower end of the cladding 39. Each tie rod 26 (FIG. 6) also includes a cylindrical cladding tube 45 containing nuclear fuel. Alternatively, the rod 26 may not contain fuel. At the lower end the tie rod 26 tapers into a stem 47 from which a flat key 49 extends. The key tapers from the stem 47 into a generally rectangular portion. At the upper end the tube 45 has a plug 51. The plug 51 is flattened along the upper portion 53 of its length having in this portion a transverse cross section having a boundary which resembles a race track with rounded ends 57 and flat sides. The upper portion 53 is threaded along the rounded ends. From the end of the rounded portion 53 a flat tip 59 extends. Each fuel bundle 23 has a lower end plate 61 and an upper end plate plate 63 (FIGS. 1, 2). The lower plate 61 is a generally rectangular block having holes 65 (FIG. 4) for receiving the tapered pins 44 extending from the conventional fuel rods 25 and holes 66 for passage of coolant. The plate 61 also has a plurlaity of holes 67 in each of which a slot 69 is interposed. The holes 67 are countersunk from the top and bottom as shown in FIG. 10. The tie rod 26 is inserted in each of the holes 67 oriented so that the key 49 passes through the slot 69. When the tie rod is rotated about its axis through an angle such as 45.degree. or 90.degree., the key 49 locks the tie rod in the lower plate against removal (FIG. 9). The upper plate 63 (FIG. 5) is a block in the form of a grid having openings 71, whose boundaries are generally octagonal for upward coolant flow. The openings 71 are defined by intersecting members 73 and 75 structured to form regions 77 of generally circular section at their intersections. Holes 79 pass concentrically through regions 77. Certain (most) of these holes 79 are coaxial with the holes 65 in the lower plate 61. Each conventional fuel rod 25 is supported between a hole 65 in the lower plate 61, in which its pin 44 (FIG. 2) is engaged, and the corresponding coaxial hole 79 in upper plate 63. The rod or pin 43 (FIGS. 1, 2) which extends from the cap 41 engages the hole 79 slideably penetrating through the hole. Each rod 43 is encircled by a coil spring 81 which engages the corresponding cap 41 at one end and the upper plate 63 at the opposite end positioning the upper plate 63 with reference to the fuel rods 25. The upper plate 63 has diagonally disposed posts 83 (FIGS. 1 and 5) which serve to align the fuel bundle in the upper nozzle (not shown) associated with the fuel bundle. The upper plate 63 also has one or more slots 85 (FIG. 11) each of which is coaxial with a slotted hole 67 in the lower plate 61. Each of these slots has the race track contour of the outer surface of the upper portion 53 of the plug 51. Each slot 85 is so oriented that with a tie rod 26 turned so that the key 49 is in the locked position, the upper portion 53 passes through the slot 85 and is prevented from turning. In assembling the apparatus, the fuel rods are positioned with the pins 44 in the holes 65 and the tie rods 26 are locked in the holes 67. The springs 81 are mounted on the rods 43. Similar springs may also be mounted on the lower portion of plug 51. Then the upper plate 63 is positioned with pins 43 of each fuel rod 25 passing through the hole 79 in the upper plate which is coaxial with the hole 65 in the lower plate 61, in which the fuel rod is seated on its pin, and the portion 53 of each time rod 26 engaging and passing through the hole 85 in the upper plate 63 which is coaxial with the hole 67 in the lower plate 61 in which the tie rod is locked. A nut 87 (FIGS. 2, 3, 13, 14) is threaded on the thread of portion 53 of the plug 51 of the tie rod 26. The nut 87 engages the upper surface of the upper plate 63. A crimping lip or shell 89 (FIG. 14) of circularly cylindrical transverse cross-section extends integrally from the locking lip of the nut. After the nut 87 is threaded onto portion 53 and seated on the surface of upper plate 63, the crimping lip 89 (FIGS. 13, 14) is crimped to the portion 53 above the nut forming a crimped surface 91 (FIGS. 2, 3) of generally frusto conical shape whose transverse cross-section is generally oval. The tie rods are now secured firmly against turning about their axes. The fuel rods 25 of the fuel bundle 23 are secured together by the grids 93 (FIG. 2) spaced along the length of the bundle. Each grid includes dimples 95 and leaf springs 97 in each cell (not shown) which springs exert a resilient force on one side of each cell urging the dimples on the opposite side into engagement with each fuel rod 25 passing through the cell. To prevent the grids 93 from sliding along the bundle 23 under the force exerted by the coolant, which usually flows in and out of the reactor at a velocity typically of 50 ft. per second, tabs 99 (FIG. 6) are provided along the outer surface of each tie rod 26. Since the coolant flows upwardly in the normal operation of a reactor, it may be adequate to provide tabs 99 only above each grid 92 (six sets in the core of bundle shown in FIG. 2). However, typically tabs 99 are provided both above and below each grid 93. The tie rod 26 thus serves the additional purpose of the spacer-capture rods of the prior art and the spacer-capture rods may be dispensed with. To disassemble a fuel bundle 23, the tip 59 of each tie rod 26 is held by a wrench which is usually remotely operated. The nut 87 is then turned by a second remotely operated wrench. The crimp 91 is broken and the nut 87 is removed. After the nuts 87 are removed from all tie rods 26, the upper plate 63 is removed. The tie rods 26 are turned to the unlocked position and the fuel bundle can be reconstituted as required. While a preferred embodiment of this invention has been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.