Patent Number: 041558081
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS For a more complete appreciation of the invention, attention is invited to FIG. 3 which shows a portion of a typical fuel element 10. The fuel element has a stud 11 to which control rods 12 through 18 are ganged by means of a cellular sheet metal orifice rod assembly 20, that characterizes the invention. It will be recalled in this respect that all of the control rods 12 thorugh 18, as well as those control rods in the fuel element 10 that are out of the plane of the FIG. 3 drawing are received within guide tubes 22 that ensheath the control rods, while the control rods within the tubes 22 are free to move into or out of the fuel element 10 in directions that are parallel to the longitudinal axes of fuel rods 21 as a unit in response to the motion of the stud 11 that is initiated by means of a control drive mechanism (not shown in the drawing). Thus, although not shown in FIG. 3 of the drawing, the stud 11 is welded, or otherwise suitably fastened to the cellular framework that comprises the orifice rod assembly 20. The ends of the control rods 12 through 18, inclusive, also are rigidly engaged in respective cells in the assembly as described subsequently in more complete detail. In these circumstances, the motion of the stud 11 in directions parallel to the longitudinal axes of the fuel rods 21 necessarily compels the control rods 12 through 18 to undergo the same motion in order to regulate the power level of the reactor core. As shown in FIG. 3, the orifice rod assembly 20, when the control rods 12 through 18 are fully inserted into the guide tubes 22 in the fuel element 10, rests upon a plate 23 which is sustained by means of an array of coil springs 24. In this manner, the plate 23 is capable of movement in the same direction as the control rod stud 11, the movement distance for the plate being limited only by the compression characteristics of the coil springs 24 and a set of stops (not shown in the drawing). These stops protrude from the corners of the upper end fitting 25 inwardly toward the orifice rod assembly 20 over a sufficient portion of the top surface of the plate 23 to retain the plate within the upper end fitting 25. In the reactor core, the fuel element 10 is kept in its proper relative position through a transversely disposed upper grid plate 26 which extends across the top of the reactor core (not shown in the drawing). A group of upper grid pads 27, 30 and 31 are secured to and protrude below the upper grid plate 26. The pads 27, 30, 31 (and a fourth pad that is not shown in the figure of the drawing) all bear against the flat upper surface of the plate 23. In this manner, the fuel element 10 is retained in its proper relative position in a resilient manner, the upper grid plate 26 bearing, in effect, against the coil springs 24. The control rods 12 through 18, inclusive, the orifice rod assembly to which they are attached, and the stud 11 that activates the control rod movement, moreover, all are free to enjoy the joint longitudinal movement that has been described above. For a more complete understanding of the cellular structure that characterizes the orifice rod assembly, attention is invited to FIG. 1 of the drawing. In FIG. 1 (and in companion view FIG. 2) the orifice rod assembly has a provision to stop reactor coolant flow through empty control rod guide tubes for the reasons hereinbefore considered. The structure that characterizes FIGS. 1 and 2, moreover, does not accommodate a group of stationary "burnable poison" rods, or the moveable control rods that were described in specific detail with respect to FIG. 1. Before undertaking a more complete description of the invention as it is illustrated in FIGS. 1 and 2 it is also important to note that the fuel element 10 shown in FIG. 3 is a "17.times.17" array, that is, an assembly that accommodates 205 fuel rods and a specific companion set of "burnable poison", empty guide tubes or guide tubes and control rods, as the case may be. The structure shown in FIGS. 1 and 2, however, is applied to a "17.times.17" array, which sustains a combination of 289 fuel rods in appropriate combination with "burnable poison" rods, empty guide tubes or guide tubes and control rods. Necessarily, these differences are reflected in relatively minor structural variations between the orifice rod assembly 20 for the longitudinally moveable control rod mechanism that is shown in FIG. 3 and the stationary empty control rod guide tube configuration that is shown in FIGS. 1 and 2. More specifically, a generally parallel first array of plates 32 through 37, inclusive, mesh and interlock with an essentially perpendicular second group of parallel plates 40 through 46. The individual plates in the arrays of the plates 33 through 37, 40 through 46 each have appropriately spaced mating slots formed in opposite plate halves, the individual slots being approximately half of the depth of the plate in which it is formed. By pressing the two mutually perpendicular groups of plates together at the respective mating slots, the entire structure is joined to form a rigid cellular grid, much in the way cardboard dividers are formed to protect the individual eggs in an egg-crate. To further enhance the integrity and strength of orifice rod assembly 47, junctions at the mutually slotted plate intersections, of which junctions 50, 51 are typical can be welded. Particular attention is invited to the plates 36, 37, 40, 41. As best seen in FIG. 2, the representative plate 37 is provided, near its ends with a somewhat greater depth that appears as a broad portion 52 which nests or is received on the upper surface of a recess 53 formed in the longitudinally moveable plate 23A that bears against the system of coil springs 24A. As shown in the drawing, the transverse extremity of the plate 37 has a member 55 that protrudes beyond the perimeter of the plate 23A and nests in a groove (FIG. 1) that is formed in the portion of the upper end fitting 25A (FIG. 2) which is pressed into contract with the lower bearing surface of the upper grid pad 27A. In a similar manner, the plates 40, 41 and 36 (FIG. 1) also have protruding members 56, 57 and 54, respectively, that are locked into associated grooves in the upper end fitting 25A by means of the upper grid pad 27A or 30A. Although not shown in the illustrated portion of the drawing, the opposite transverse ends of the plates 40, 41, 36 and 37 have protruding members which also are locked to the upper end fitting 25A through the action of the upper grid pads that are immediately above the protruding members in question. The orifice rod assembly 47 is, in this fashion, prevented from engaging in any longitudinal or transverse movement. The transverse plate 23A, however, can move longitudinally through the distance that is permitted by the stops in the upper end fitting 25A and against the force of the coil springs 24A. This rigid retention of the orifice rod assembly 47 is, of course, entirely contrary to the free longitudinal movement that the orifice rod assembly 20 in FIG. 3 transmits to the control rods 12 through 18. This feature of the invention is illustrative of the flexibility that the orifice rod assembly grid structure imparts to the problems of partially used fuel rod relocation within the reactor core. Continuing on with the description of the embodiment of the invention that is shown in FIGS. 1 and 2, it will be recalled that this specific illustration is used with empty control rod guide tubes. To overcome the thermal inefficiency that is caused by relatively cold water flowing from the inlet to the reactor core, through the empty guide tubes and out at the reactor discharge to mix with and reduce the temperature of the heated coolant that flows from the balance of the reactor core, control rod guide tube plugs 61, 62 and 63 are provided in the orifice rod assembly 47. As best shown in FIG. 2, each of the plugs 61, 62 and 63 is a generally tubular structure that has an outside diameter which will enable each plug to fit snugly within the interior of a control rod guide tube. To block flow through the guide tube, one extreme end of each of the plugs 61, 62 and 63 is swaged on otherwise suitably closed. The opposite end of each of the respective plugs is, however, expanded in a transverse direction to provide stops 65, 66 and 67 that limit the movement in a longitudinal direction of each of the plugs. In a manner similar to that which is described in connection with the control rods 12 through 18 (FIG. 3) "burnable poison" rods (not shown in the drawing) are mounted in a cellular grid of the type hereinbefore described. In this specific instance, however, it will be recalled that the "burnable poison" rods are not moved during reactor operation. Accordingly, to accommodate immobile "burnable poison" rods, the cellular grid structure that sustains these rods should be of the type described with respect to FIGS. 1 and 2, in which hooks or protruding members on the cellular grid or orifice rod assembly are rigidly engaged between the lower surfaces of the upper grid pads and respective grooves formed in the upper end fitting. This prevents the orifice rod assembly and the "burnable poison" rods from moving relative to the respective fuel element. In operation, as the need arises to reposition a partially used fuel element within a reactor core in order to obtain the maximum efficient use of the nuclear fuel within the fuel element, the fuel element is removed from the reactor core in a suitably shielded manner. The orifice rod assembly is changed, as appropriate, to provide control rod, empty control rod guide tube plugs or "burnable poison" rod functions as the new relative position of the partially used fuel element within the core dictates. In this way, the repositioned fuel element enjoys an optimum location within the core without imposing a need to remotely disassemble and rebuild the fuel element to satisfy special fitting requirements, the critical point being the longitudinal alignment between the cells and the associated control rod guide tubes, regardless of the particular use of the orifice rod assembly for control, "burnable poison" or guide tube plug application.