FUEL BUNDLE FOR USE IN A NUCLEAR REACTOR

Abstract: The invention relates to a fuel bundle with fuel rods extending between and supported by a top plate and a bottom plate, and with tie rods holding the top and bottom plates connected together against the action of hold-down springs interposed between the fuel rods and one of the plates. Each tie rod (26) has on its lower end a key (49) which is inserted through a slot (69) in the bottom plate (61) and, with the tie rod turned to a locking position, cooperates with the bottom plate to prevent separation of the tie rod therefrom, and the tie rod has further an upper end plug with a flat upward extension extending through a generally rectangular hole in the top plate (63) to hold the tie rod against rotation from its locking position with the bottom plate. A nut (87) is threaded onto and crimped to an upper end portion of the flat extension to hold the top plate (63) in position against the action of the hold-down springs (81).

Description: This invention relates to nuclear reactors and has particular relationship to fuel bundles or fuel assemblies. A fuel bundle typically includes a top plate and a bottom plate between which there are mounted fuel rods held together by axially spaced straps or spacers. In accordance with teachings of the prior art, each bundle is provided with tie rods which are screwed into, or bolted to, the bottom plate, and it has spacer-capture rods which are secured to the bottom plate by pins to suppress rota­ tion about their longitudinal axes and which have axially spaced tabs to prevent axial displacement of the spacers. Removal of the tie rods during reconstitution, particularly complete reconstruction, of a fuel bundle presents formidable difficulties because it demands access to the bottom plate, usually in a radioactive environment, to unscrew or unbolt the tie rods and to disengage the space-capture rods. It is the principal object of the invention to overcome this problem by providing a fuel bundle which can be completely reconstituted without access to the bottom plate. Accordingly, there is provided a fuel bundle having one or more tie rods each secured to the bottom plate by a key by means of which the tie rod can be locked to, and unlocked from, the bottom plate without direct access to the bottom plate. Each tie rod is also secured to the top plate by means for preventing rotation of the tie rod. Specifically, the tie rod has at the bottom a flat key which passes through a slot in the bottom plate. The tie rod can be locked to the bottom plate by turning the rod so that the key is at an angle to the slot under the bottom plate. At the top the tie rod has a flattened plug with a transverse cross-section resembling a race track. The plug has a thread on its rounded sides. A nut is threaded onto the thread on the plug and engages the top plate. The plug passes through a slot in the top plate. The nut has a crimping lip which is crimped to the plug. To disassemble the bundle the crimp for each tie rod is broken by turning the nut. The nut and top plate can then be removed and the tie rod turned to unlock the key and removed by access to the top of the bundle. The plug has a flat tip which may be engaged to prevent rotation of a tie rod when the crimp is broken and the nut unscrewed by turning the nut. Since the tie rods of this invention are not rotatable, they may carry the tabs which prevent displacement of the straps, and the spacer-capture rods may be dispensed with. A preferred emtodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a fragmental isometric view of a fuel assembly including fuel bundles embodying the invention; Fig. 2 is a view in side elevation, of some of the fuel bundles; Fig. 3 is a fragmental enlarged view of the part of the fuel bundle of Fig. 2 in circle III; Fig. 4 is a plan view of the bottom plate of the fuel bundle shown in Fig. 2; Fig. 5 is a plan view of the top plate of the fuel bundle shown in Fig. 2; Fig. 6 is a view in side elevation of a tie rod embodying the invention; Fig. 7 is a bottom view of this tie rod taken in the direction VII of Fig. 6; Fig. 8 is a plan view of the top of the tie rod taken in the direction VIII of Fig. 6; Fig. 9 is a fragmental plan view of the portion of a bottom plate enlarged through which the key at the lower end of the tie rod penetrates; Fig. 10 is a view in section taken along line X-X of Fig. 9; Fig. 11 is a fragmental plan view showing the part of the top plate which is adapted to receive the upper end of a tie rod according to the invention; Fig. 12 is a fragmental view in side elevation and partly in section taken in the direction XII of Fig. 11; Fig. 13 is a plan view of the nut for locking the tie rod against rotation; and Fig. 14 is a view in side elevation taken in the direction XIV of Fig. 13. Fig. 1 shows a fuel assembly 21 including several fuel bundles 23 each including a plurality of conventional fuel rods 25 and one or more tie rods 26 embodying the invention. The fuel rods 25 and tie rods 26 are arranged in a square array typically of 16 rods (4x4). The bundles 23 are typically arranged in a square array (2x2) 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 flow there­ through and consequently through the fuel rod bundles. The bundles 23 are spaced from each other, defining a cruciform gap 31 through which extends a coolant conductor 33 in the form of a cross with arms which are hollow and define channels 35 for coolant flow therethrough. The channels 35 are open only at the bottom and the top and coolant typi­ cally is conducted through the channels from the bottom toward the top. The plate 27 forming the outer or inner flow channel is secured to the coolant conductor 33 by being welded thereto at dimples 37 distributed axially along the plate and leaving spaces for fluid flow to exist between the quadrants of the square array of fuel bundles 23. Each conventional fuel rod 25 includes a cylin­ drical cladding tube 39 containing nuclear fuel and includ­ ing an end cap 41 from which a pin 43 extends (Figs. 1, 2). A tapered pin 44 also extends from the lower end of the cladding 39. Each tie rod 26 (Fig. 6) likewise includes a cylindrical cladding tube 45 which may or may not contain nuclear fuel. At its 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 outwardly into a generally rectangular portion. At its upper end, the tube 45 has a plug 51. The plug 51 has a flat upper portion 53 with a transverse cross section having a boundary which resembles a race track with flat sides and rounded ends 57. The upper portion 53 is threaded along the rounded ends. From the end of the portion 53 there extends a flat tip 59. 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 plurality 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. Each tie rod 26 is inserted in one of the holes 67 so that the key 49 passes through the slot 69, after which the tie rod is rotated about its longitudinal axis through an angle, such as 45° or 90°, causing the key 49 in coopera­ tion with the lower surface of the end plate 61 to lock the tie rod to the lower plate against removal therefrom. The upper plate 63 is a block in the form of a grid having generally octagonal openings 71 for upward coolant flow therethrough. The openings 71 are defined by intersecting members 73 and 75 structured to form, at their intersections, generally circular regions 77 with holes 79 extending concentrically therethrough. Certain (most) of these holes 79 are axially aligned with the holes 65 in the lower plate 61. Each conventional fuel rod 25 has its lower pin 44 (Fig. 2) engaged and seated in one of the holes 65 in the lower plate 61, and has the pin 43 in its upper cap 41 slideably extending through the aligned hole 79 in the upper plate 63. Each pin 43 is encircled by a coil spring 81 held under compression between the end cap 41 of the fuel rod and the upper plate 63. The upper plate 63 has diagonally disposed corner 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 holes 85 each of which is axially aligned with one of the slotted holes 67 in the lower plate 61 and has the form of a slot having the same race-track-like contour as the outer surface of the flat upper portion 53 of the plug 51 and thus forming a receptacle for the flat upper portion of the plug. Each hole 85 is so oriented that, with the associat­ ed tie rod 26 turned to have its key 49 in the locked position thereof, the upper portion 53 of the tie rod extends through the hole 85 and, hence, prevents turning of the tie rod. In assembling the apparatus, the fuel rods are positioned with their pins 44 seated in the holes 65 of the lower plate 61, and the tie rods are inserted into the holes 67 and locked to the lower plate 61. 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 lowered into position, with the pins 43 of the various fuel rods 25 passing through the associated holes 79 in the upper plate, and with the portions 53 of the respective tie rods 26 passing through the associated holes 85 in the upper plate 63. A nut 87 (Figs. 2, 3, 13, 14) is threaded onto the threaded flat portion 53 of the plug 51 of each tie rod 26 to limit upward movement of the upper plate 63 under the action of the springs 81. The nut 87 has a cylindrical crimp lip or sleeve 89 (Fig. 14) adapted, after the nut 87 is threaded onto portion 53 and is seated on the surface of upper plate 63, to be crimped onto the flat portion 53 as shown at 91 in Figs. 2, 3, thereby to positively secure the nut 87 against rotation thereof on and relative to the tie rods. The fuel rods 25 of the fuel bundle 23 are secured together by 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 15 m/sec., 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 93 (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 addi­ tional 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.

Claims: 1. A fuel bundle for use in a nuclear reactor, including a top plate, a bottom plate, and a plurality of nuclear fuel elements and at least one tie rod extending between the top and bottom plates, characterized in that said tie rod (26) has a key (49) extending from the lower end thereof, and said bottom plate (61) has a receptacle (67, 69) for receiving the key (49) and enabling the tie rod to be rotated to a locking position in which the key locks the tie rod to the bottom plate against separation therefrom, and further characterized in that said tie rod (26) has on its upper end an end plug (51) including a retaining portion (53), and said upper plate (63) has a receptacle (85) for receiving said retaining portion (53), when the tie rod is in the locking position thereof, and for cooperating with the retaining portion to prevent rotation of the tie rod from its locking position. 2. A fuel bundle according to claim 1, charac­ terized in that said tie rod (26) is hollow and contains nuclear fuel. 3. A fuel bundle according to claim l or 2, characterized in that said key (49) at the lower end of the tie rod (26) comprises a flat plate, and said receptacle (67, 69) in the bottom plate (61) comprises a slot (69) shaped substantially as said flat plate in cross-section and permitting the flat plate to be inserted through the slot and to a position below the bottom plate. 4. A fuel bundle according to claim 1, 2 or 3 characterized in that said retaining portion (53) of the end plug (51) is substantially rectangular in cross-section, and said receptacle (85) in the upper plate (63) comprises a hole substantially conformed to said retaining portion (53) with respect to its cross-sectional size and configuration. 5. A fuel bundle according to claim 4, charac­ terized in that said retaining portion (53) extends slideably through said hole (85) and projects from the upper plate (63), said retaining portion (53) having an external threading formed at least on the projecting part thereof and having a nut (87) disposed thereon and thread­ edly engaged therewith. 6. A fuel bundle according to claim 5, charac­ terized in that said nut (87) includes a locking lip (89) which is crimped against the flat sides of the projecting part of said retaining portion (53) to secure the nut against rotation thereof relative to the tie rod (26). 7. A fuel bundle according to any one of the preceding claims, including rod-spacing grids spaced from each other in the longitudinal direction of the fuel and tie rods, characterized in that said tie rod (26) has tabs (99) disposed thereon and cooperating with the grids (93) to secure the latter against displacement thereof relative to and longitudinally of the tie rod.

Inventors: LUI, CHUN KEE
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G21C3
IPC codes: G21C3