Patent Number: 048204781
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 nuclear reactor fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. Basically, the fuel assembly 10 includes 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), and a number of longitudinally extending guide tubes or thimbles 14 which project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the guide thimbles 14 and an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16. Also, the assembly 10 has an instrumentation tube 20 located in the center thereof and an upper end structure or top nozzle 22 removably attached to the upper ends of the guide thimbles 14 to form an integral assembly capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 18 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 16 spaced along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets 24 and the opposite ends of the rod are closed by upper and lower end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the pellets 24 to maintain the pellets in a tight, stacked relationship within the rod 18. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the nuclear reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work. Control Rod With Uniformly Changeable Axial Worth Turning now to FIGS. 2 to 12, there is shown the preferred embodiment of the control rod of the present invention, being generally designated 32, which can be used in some of the fuel assemblies 10 of the reactor core to compensate for xenon transients, such as occur at load follow when the reactivity of the core is reduced. As will become clear, the control rod 32 constitutes means which compensates for the reduction in reactivity in a manner which matches it generally uniformly in a transverse direction and symmetric in the axial direction. Basically, the control rod 32 includes an elongated inner cylindrical member 34 and an elongated outer cylindrical member 36 surrounding the inner member, with each of the members 34,36 being composed respectively of alternating poison and nonpoison regions 38,40 and 42,44 (in FIGS. 4 and 6, the poison regions have an "X" on them) and with one of the inner and outer members 34,36 being axially movable relative to the other to adjust the degree to which the poison regions 38,42 of the members 34,36 overlap with the nonpoison regions 40,44 thereof and thereby change the overall worth of the rod 32. More particularly, as seen in FIG. 5, the inner cylindrical member 34 from end to end has a solid cross-sectional configuration, whereas, as seen in FIG. 7, the outer cylindrical member 36 from end to end has an annular cross-sectional configuration and concentrically surrounds and is generally coextensive with the inner member 34, as seen in FIGS. 8 and 9. The poison regions 38,42 of the respective members 34,36 are composed exclusively of black poison material, such as unclad hafnium, and extend axially in an alternating arrangement with the nonpoison regions 40,44 of the members which are composed exclusively of a nonpoison material, such as Zirc-4. The regions of the members can be in the form of poison and nonpoison annular and solid pellets and thus the members themselves are formed by connecting the respective pellets together in any suitable manner, such as by welding them. It will be observed that there is no cladding material needed, and no plenum is needed since no oxide material is used and thus no reaction gas is given off. As will be observed in FIG. 8, each of the black poison regions 38,42 of the inner and outer members 34,36 has substantially the same axial height. Also, each of the nonpoison regions 40,44 of the members 34,36 has substantially the same axial height. Additionally, preferably the axial heights of the poison regions 38,42 are about the same as the nonpoison regions 40,44. A height of one foot for the regions was arbitrarily chosen. Such height was considered to be short enough to not cause significant axial offset changes with rod movement. The inner member 34 has a conical shaped lower head 46 which defines an outwardly projecting annular ledge 48 at the lower end of the member 34 upon which rests on an inner portion of a lower edge 50 of the outer member 36, as seen in FIGS. 8 and 10. In uch manner, the inner cylindrical member 34 supports the outer cylindrical member 36 prior to insertion of the control rod 32 into one of the guide thimbles 14 of the fuel assembly 10. The control rod 32 is placed in one guide thimble 14 of the fuel assembly 10 at beginning of core life (BOL). The inner or central member 34 is connected at an upper threaded end 51 to the control rod drive line, in the same manner as a conventional control rod, through a radially extending fluke or arm 52 of a conventional spider assembly 54, as seen in FIG. 1. As the inner cylindrical member 34 is lowered into the guide thimble 14, the outer cylindrical member 36 rides along with the inner member 34 by sitting on the lower ledge 48 of the inner member. When the two members 34,36 are almost completely inserted into the core, i.e., the guide thimble 14 of the fuel assembly 10, an outer portion of the lower edge 50 of the outer cylindrical member 36 encounters an annular stop 56 in the form of a sleeve in a lower portion of the guide thimble 14, for instance, fixed to the thimble 14 just above a dashpot 58 defined in its lower portion. The sleeve 56 is sized to support the outer portion of the lower edge 50 of the outer member 36 and has a central hole 60 sized to allow passage of the inner member 34 therethrough. While the outer member 36 encounters the stop 56, the inner member 34 can still continue down relative to the outer member 36 an additional foot to full insertion, as seen in FIG. 12. In such offset position of the poison and nonpoison regions 38,40 and 42,44 of the members 34,36, the control rod 32 has its maximum worth. At full reactor power, the members 34,36 are in the offset position so that they have maximum worth. To adjust the degree to which the poison regions 38,42 of the cylindrical members 34,36 overlap with the nonpoison regions 40,44 thereof and thereby change the overall worth of the rod 32, the inner member 34 is withdrawn upwardly relative to the outer member 36. At the aligned position of the poison and nonpoison regions 38,40 and 42,44 of the members 34,36, as seen in FIG. 11, the control rod 32 has its minimum worth, since the poison regions 42 in the outer member 36 shields the poison regions 38 in the inner member 34 from neutrons. In summary, therefore, the inner member 34 is axially movable relative to the outer member 36 between an upper axially displaced position (FIG. 12), in which the black poison regions 38,42 of the members 34,36 are disposed in side-by-side alignment and the nonpoison regions 40,44 thereof are also disposed in side-by-side alignment, and a lower axially displaced position (FIG. 11), in which the black poison regions 38,42 of the members 34,36 are disposed in side-by-side alignment with the nonpoison regions 40,44 of the members. In such manner, the overall worth of the control rod 32 can be changed in a substantially axially uniform manner. While the annular sleeve 56 fixed in the guide thimble 14 is the means illustrated herein for supporting and retaining the outer member 36 in a stationary position in the guide thimble 14 while the inner member 34 is movable axially thereto between the aligned and offset positions of the poison and nonpoison regions 38,42 and 40,44, respectively shown in FIGS. 11 and 12, it will be readily understood that other means could be used to accomplish this same purpose. For example, the outer cylindrical member 36 could engage a stop associated with the top nozzle to hold it stationary in the guide thimble 14. The xenon compensating control rods 32 are not used for power reduction because they do not control the change in axial offset which results as power is decreased. However, the rods might be used to also compensate for the Doppler reactivity which is changed during load follow since these changes are also symmetric in the axial direction. If Doppler control is to be performed, then the rods would not be in their most absorbing condition at full power so that their worth could be increased at reduced power to cancel the Doppler reactivity increase. For use just in xenon control, the rods are at their highest worth position at full power, as mentioned above. At reduced power, their worth would be decreased to compensate for the xenon concentration buildup. Since the regular control rods inserted to reduce power have not had to be withdrawn to compensate for xenon, there is enough worth in the core so that it can be returned to full power as fast as permissible. The xenon compensating rods have, therefore, preserved return to power capability and have removed the requirement of changing dissolved boron concentration in the core during load follow. Also, the rods are removed before EOL so that there is no fuel cycle penalty associated with their being in the core. They may not all be withdrawn at once, but in stages. It is thought that 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 the 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 described being merely a preferred or exemplary embodiment thereof.