Patent Number: 046577262
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views. Also, in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. IN GENERAL Referring now to the drawings, and particularly to FIG. 1, there is shown an elevational view of a conventional fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. Fuel assembly 10 is the type used in a PWR (Pressurized Water Reactor) and basically comprises a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown); a number of longitudinally extending guide tubes or thimbles 14 projecting upwardly from the bottom nozzle 12; a plurality of transverse grids 16 axially spaced along the guide thimbles 14; an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16; an instrumentation tube 20 located in the center of the assembly; and an upper end structure or top nozzle, generally designated by the numeral 22, attached to the upper ends of the guide thimbles 14 to form an integral assembly capable of being conventionally handled without damaging the assembly components. The top nozzle 22 includes a transversely extending adapter plate 24 having upstanding sidewalls 26 (the front wall being partially broken away) secured to the perpherial edges thereof in defining an enclosure or housing. An annular flange 28 is secured to the top of the sidewalls 26. Suitably clamped to the annular flange 28 are leaf springs (not shown) which cooperate with the upper core plate (not shown) in a conventional manner to prevent hydraulic lifting of the fuel assembly caused by upward coolant flow, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Disposed within the opening defined by the annular flange 28 is the moderator control apparatus of the present invention, being designated generally by the numeral 30, which will be described in detail shortly hereafter. In that fuel assembly 10 does not form a part of the present invention, but is merely for illustrational purposes in representing the operative environment for use of the moderator control apparatus 30, a further description thereof will not be given. For a more detailed description of fuel assembly 10, reference should be made to the pending patent application of John M. Shallenberger et al, entitled "Nuclear Reactor Fuel Assembly With A Removable Top Nozzle"; filed Aug. 27, 1984; and assigned U.S. Ser. No. 644,758, a continuation-in-part of Ser. No. 537,775, filed Sept. 30, 1983 and now abandoned. MODERATOR CONTROL APPARATUS The moderator control apparatus 30 will now be discussed in further detail with particular reference to FIGS. 2, 3, and 4. As best seen in FIG. 2, the apparatus 30 includes a plurality of hollow elongated displacer rods 32 adapted to be inserted into respective ones of the guide thimbles 14 of the fuel assembly 10 for displacement of a predetermined volume of the moderator/coolant associated with the fuel rods 18. The displaced volume of the moderator/coolant decreases the H/U (hydrogen/uranium) ratio from a given normal level. The displacer rods 32 are interconnected by a manifold, generally designated by the numeral 34, located on the top of fuel assembly 10 and being disposed within the top nozzle and resting on the adapter plate 24 (see FIG. 1). In the preferred embodiment, the manifold 34 is in the form of a central hub 36 defining a central opening 38 (see FIGS. 3 and 4) and includes a plurality of, radially extending, hollow tube-like vanes 40 interconnecting the upper ends of the displacer rods 32 to the central hub 36. The hub 36 is provided with a number of radial bores or inlet ports 42 corresponding to the number of vanes 40. The inward end of the vanes are suitably secured to the hub such that the inlet ports 42 serve as unitary channel extensions of the vanes (best seen in FIG. 2), the arrangement being such that the inlet ports are circumferentially spaced about and adjacent the central opening 38 defined by the hub 36 (see FIGS. 3 and 4). It is preferred that each inlet port be disposed diametrically opposite another inlet port (the purpose for which being clearly understood from below). Each of the vanes 40 have at least one exit port 44 defined therein, some of the vanes have one such exit port whereas adjacent vanes have two exit ports. The number of exit ports 44 corresponds to the number and strategic location of the displacer rods 32. Although not specifically shown, it is preferred that the rods be threadably connected with the vanes 40 to facilitate assembly and dismantling, however, other suitable connections could equally be used. As can be appreciated, the connections are such that the inlet ports 42 are in fluid flow communication with the exit ports 44 of a respective vane 40, whereas, the exit ports 44 are in fluid flow communication with the respective displacer rods 32. Before continuing, it should be pointed out that the specific above-described manifold structure with its central hub and radially extending vanes is only illustrative of one possible type configuration and construction used, it being understood that other smaller structures and arrangements are equally applicale in keeping within the principles of the present invention. Again referring to FIG. 2, the control apparatus 30 further includes valves means operably associated with the manifold inlet ports 42 for controlling the flow and non-flow of the coolant into the displacer rods 32. More particularly, when the inlet ports are in closed position the flow of coolant therethrough is prohibited, whereas, in an opened position, coolant flows through the inlet ports 42, along through the vanes 40, and then out through the exit ports 44 and into the displacer rods 32. Thus, by opening the inlet ports, the original displacement of the coolant is removed as the rods are filled with coolant in thereby increasing or returning the H/U ratio back to its normal given level (shifting of the energy spectrum). In the preferred embodiment, the valve means takes on the form of a rotatable hollow stem 46 which is operable to open and close all of the inlet ports 42 in a predetermined sequential manner, however, as can be appreciated, a separate valve may be associated with each of the inlet ports. The valve stem 46 is circular with its lower section being provided with at least one, and preferably two diametrically opposite, flow aperatures or orifices 48. Still referring to FIG. 2, the valve stem 46 is mounted such that the lower end of the stem rests on an integral annular lip 50 of hub 36, whereas, a circumferential groove, on the stem at an axial location above the orifices 48, engages an annular flange 52, integrally formed on the hub 36 above the lower lip 50. The mounting arrangement permits rotation of the stem on the manifold, and more specifically, the rotation of the lower section of the stem 46 within the central opening 38. The dimension of the lower section of stem 46 is such that its exterior wall surface snugly abuts the inlet ports 42 and manifold 34 in sealing off the inlet ports so as to prevent the flow of coolant therethrough. Flow of coolant through the inlet ports 42, and thus into the displacer rods 32, only occurs when the valve stem 46 has been rotated to a point such that the orifices 48 are aligned with the inlet ports 42, as illustrated in FIG. 4. The valving is such that only two inlet ports (and the maximum of four displacer rods) are open to the flow of coolant therethrough at any one time, thereby insuring safety against accidental release or large change of reactivity during any single occurrence or transient. Further, such valving system provides an operator with increased flexibility to relieve unexpected power tilts during an operating cycle. Slidably mounted on the valve stem 48 is a perforated hold down plate 54 which compresses spring 56 as the upper core plate (not shown) is lowered down unto the assembly. Spring 56 is coiled about the valve stem 46 and is interposed between the hold down plate 54 and the central hub 36. This conventional arrangement prevents the control apparatus 30 from being ejected off the fuel assembly 10 by the forces of the upwardly flowing coolant. As best seen in FIG. 2, supported on plate 54 is a motor 58, battery 60, and transmitter/receiver 62 which diagramatically represent conventional means for rotating the valve stem 46 to cause the inlet ports 42 to open and close as described above. The motor 58 drives sprocket 64 which meshes with circular ring gear 66 that is attached to valve stem 46. From an external location, an operator sends a signal (electromagnetic, radio, or microwave) to the transmitter/receiver 60 which in turn actuates the motor 58, causing the valve stem to rotate. In the alternative, the motor 58 can be actuated by an incore buttom 68 which is accessible via the instrumentation tube 20 that is located within the center of fuel assembly 10. In that the displacement of the coolant in the guide thimbles 14 may not be sufficient to insure a negative moderator temperature coefficient and that selective displacement of the coolant may be insufficient for power shaping control, the invention further contemplates the use of a burnable poison. More specifically, each of the displacer rods 32 are filled with a burnable poison gas, preferably the gas is He.sup.3. The gas is released as the inlet ports 42 are opened to permit the flow of the coolant into the displacer rods 32. Due to the specific valving arrangement discussed above, the gas can only be released from two inlet ports (maximum of four displacer rods) at any one time, thus alleviating many safety problems. In order that the control apparatus 30 may be reused after completion of a cycle, in the preferred embodiment, each of the displacer rods 32 are provided with a refill valve 70, located at the lower end of the rod. In addition to the inherent cost savings in being able to reuse the rods 32 rather than discarding them, the refill valves 70 provide for last minute power distribution adjustment. Since the rods can be refilled on-site, the specific poison loading can be delayed until the time when the apparatus is placed on a fuel assembly. It is thought that the moderator control apparatus of the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore being merely a preferred or exemplary embodiment thereof.