Reactivity control assembly for nuclear reactor
BACKGROUND OF THE INVENTION 
     This invention, which resulted from a contact with the United States Department of Energy, relates to a control mechanism for a nuclear reactor and, more particularly, to an assembly for selectively shifting different numbers of reactivity modifying rods into and out of the core of a nuclear reactor. 
    
    
     It has been proposed heretofore to control the reactivity of a breeder reactor by varying the depth of insertion of control rods (e.g., rods containing a fertile material such as ThO 2 ) in the core of the reactor, thereby varying the amount of neutron-thermalizing coolant and the amount of neutron-capturing material in the core. This invention relates to a mechanism which can advantageously be used in this type of reactor control system. 
     SUMMARY OF THE INVENTION 
     It is therefore a general object of this invention to provide an improved means for controlling reactivity of a nuclear reactor of the breeder type. 
     Another more specific object of the invention is to control reactivity of a breeder reactor by selectively shifting different groups of reactivity modifying rods into and out of the core of the reactor. 
     Still another object of the invention is to provide a compact, reliable latch mechanism for moving different numbers of reactivity control rods into and out of the core of a nuclear reactor. 
     These objects are attained by means of a preferred latch assembly comprising (1) at least two bodies each having an aperture extending between opposite ends thereof, each aperture including a first axially slotted bore located at one end thereof, a second middle bore having an increased diameter, and an unslotted third bore located at the other end thereof and having a smaller diameter than the second bore, and (2) at least one coupler including a cylindrical connector portion rotatably fitted in the third bore of one of said bodies and projecting outward therefrom, a flange portion projecting radially outward from one end of said connector portion and rotatably held in the second bore of the last-mentioned body, an axially slotted cavity formed in said flange portion and extending axially from the end thereof remote from said connector portion, and a lug projecting radially from said connector portion and spaced from said flange portion. Rotation of the aforesaid couplers to different positions relative to the hubs permits interconnection of different numbers of the hubs. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagrammatic view of components of a preferred embodiment of the invention, portions of a housing, drive elements, and support arms being broken away in the drawing for clarity and simplification. 
     FIG. 2 is an enlarged view of a representative one of the three control rod supports illustrated in FIG. 1, portions of components also being broken away in this drawing. 
     FIG. 3 is a pictorial view of a holding member illustrated in FIG. 1, a portion of the wall of this part being away from in the drawing so that the configuration of an aperture therein can be seen. 
     FIGS. 4 and 5 are detail plan views of components illustrated in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     In FIG. 1, reference numbers 10a,10b,10c respectively generally designate three identical support bodies, or hubs, that are positioned in vertically stacked relation above a nuclear reactor core (not shown) and centered on a vertical axis 12 that passes through the center of the core. The internal configuration of a typical hub 10c is illustrated in FIG. 2, wherein it can be seen that an aperture extends between the upper and lower end surfaces 12,14 of the hub, this aperture being centered on a vertical axis 16 that passes through the center of the reactor core. The aperture in each hub has (1) a first cylindrical bore 18 at its upper end, (2) two slots 20a, 20b located at diametrically opposed points on the wall of said first bore and extending axially thereof, only one slot 20b being illustrated in hub 10c in FIG. 2 but both slots being illustrated in hubs 10a and 10c in FIG. 1, (3) a second cylindrical bore 22 disposed next to said first bore and having a diameter larger than the diameter of said first bore, thereby forming a shoulder 24 at the upper end of the described aperture, and (4) a threaded counterbore 26 located at the lower end of the aperture. A washer-shaped retainer plate 28 is threadedly engaged within counterbore 26 and extends transversely of the lower end of each hub 10a-10c, a set screw 30 mounted in the side wall of the hub holding this retainer plate in position. The cylindrical aperture 32 of retainer plate 28 is centered on axis 16 and its diameter is equal to the diameter of first bore 18. In certain claims appended hereto, aperture 32 is referred to as the third bore of a hub. 
     Associated with each hub 10a-10c is a coupler 34 having the configuration illustrated in FIG. 2. More particularly, the coupler of each hub includes the following parts: (1) a cylindrical connector portion 36 which has a diameter slightly smaller than the diameter of aperture 32 in retainer plate 28 and which is rotatably fitted in that aperture; (2) a frustoconical tip 38 at the lower end of the connector portion; (3) two connecting lugs 40a,40b which project radially outward from the lower end of the connector portion at diametrically opposed points thereon and which, on the couplers of hubs 10a and 10b, are sized to slide through the slots 20a,20b of the subjacent hub and, on the coupler for hub 10c, are sized to slide through corresponding slots in a holding member 42 further described hereinafter: and (4) a cylindrical flange 44 (generically referred to as the detent portion of the coupler in certain claims appended hereto) which projects radially outward from the upper end of the connector portion, which has a diameter slightly smaller than the diameter of second bore 22 of the associated hub, and which is rotatably held in said bore between shoulder 24 and retainer plate 28. A cylindrical cavity 46 extends downwardly into flange 44 from its upper surface, this cavity being coaxial with connector portion 36 and terminating above the lower surface of the flange. Two slots 48a,48b extend axially of the wall of cavity 46 at diametrically opposed points thereon, these slots being sized, on the couplers for hubs 10b and 10c, to slidably receive the connecting lugs 40a,40b of the couplers of hubs 10a and 10b, respectively, and, on the coupler for hub 10a, to slidably receive connecting lugs 50a,50b which project radially outward from the lower end of a vertically disposed shaft 52 at diametrically opposed points thereon. 
     Two horizontal support arms 54a,54b are attached to the side wall of each hub 10a-10c at opposed points thereon, these arms (the ends of which are removed in FIG. 1 to simplify the drawing) extending radially outward from the associated hub to a point substantially in vertical alignment with the perimeter of the reactor core thereunder. The upper ends of a number of vertically disposed control rods 56 are attached to each support arm 54a,54b by means of connecting elements 58. Control rods 56 are spaced apart from one another in the reactor core in the usual manner and their lower ends are slidably held in guide tubes 60 that extend into the core. It will be understood that the number of support arms and control rods connected to hubs 10a-10c can be varied as required for controlling reactivity of the reactor core. 
     Coaxially positioned below the stack of hubs 10a-10c and fixedly centered relative to the reactor core is the aforementioned holding member 42. As illustrated in FIG. 3, the internal configuration of holding member 42 corresponds with that of each hub 10a-10c inasmuch as the holding member has an aperture therein which includes (1) a first cylindrical bore 62 at its upper end, (2) two slots 64a,64b extending axially across said first bore at diametrically opposed points thereon, and (3) a second cylindrical bore 66 which has a diameter larger than the diameter of the first bore and thereby forms a shoulder 68 at the upper end of the holding member. Holding member 42, like hubs 10a-10c, is centered on axis 16. 
     Projecting downwardly from a fixedly positioned housing 70 and also centered on axis 16 is a vertically disposed tube 72 which terminates above the point to which the uppermost hub 10a is raised when the control rods connected thereto are removed from the reactor core. A second tube 74 is concentrically and rotatably held in tube 72 by suitable means including a bearing 76. Splines 78 are spaced around and extend lengthwise of the inner surface of tube 74, and gear teeth 80 on the underside of the driven element 82 of a geneva movement mesh with these splines at the upper end of the tube. Gear teeth 84 on the driving element 86 of the geneva movement mesh with teeth of a gear 88 mounted at the lower end of the drive shaft 90 of a suitable drive means 92 located in the upper portion of housing 70. 
     Fixed to the upper end of shaft 52 is a gear 94, the teeth of which mesh with splines 78 on tube 74. As will be understood by persons skilled in the design of nuclear reactors, a rotatable nut can be engaged with a helical thread 96 formed on shaft 52 to translate the shaft in opposite directions along its longitudinal axis 16. A rotating fixed nut drive used for raising or lowering a nuclear reactor control rod is described on page 197, Vol. 2, of &#34;The Technology of Nuclear Reactor Safety&#34;, published by The M.I.T. Press in 1973. 
     Rotation of hubs 10a-10c about axis 16 is prevented by control rods 56 which extend into the fixed guide tubes 60. Thus the slots 20a,20b in each hub 10a-10c are maintained in the same angular position relative to axis 16 when they are raised or lowered by shaft 52 as will be presently described. It can be seen in FIG. 1 that the angular positions of slots 20a,20b in hubs 10a and 10c relative to axis 10 are different. Moreover, as illustrated in FIG. 4 (which is a plan view of portions of hubs 10a-10c and holding member 42), the angular position of a pair of slots 20a,20b in any one of the hubs 10a-10c, relative to axis 16, is different from that of the slots 20a,20b in the other two hubs and that of the slots 64a,64b in holding member 42. In the described preferred embodiment of the invention, there is an angle of 72° between the lines a and b in FIG. 4, which respectively pass through axis 16 and bisect slots 20a,20b in hubs 10a and 10b. Likewise, there is an angle of 72° between the aforesaid line b and the line c which passes through axis 16 and bisects the slots 20a,20b in hub 10c, and the angular relation between the last-mentioned slots and slots 64a,64b in holding member 42 follows the same pattern. Also, as illustrated in FIG. 5, there is an angle of 72° between lines e, f, g, which respectively pass through axis 16 and bisect the slots 98a,98b,98c in the driven element 82 of the above-described geneva movement for rotating shaft 52. 
     OPERATION OF PREFERRED EMBODIMENT OF THE INVENTION 
     The length of first bore 18 of each hub 10a-10c and the length of first bore 62 in holding member 42 (i.e., the vertical width of shoulder 24 in each hub and shoulder 68 in the holding member) is less than the spacing between flange 44 and connecting lugs 40a,40b on couplers 34. Hence when two hubs are in juxtaposition (as illustrated by hubs 10a and 10b in FIG. 1), the connecting lugs 40a,40b on the coupler 34 of the uppermost hub are disposed in slots 48a,48b formed in the wall of the cavity 46 in the coupler of the lowermost hub. Furthermore, when hub 10c is juxtaposed to holding member 42 as illustrated in FIG. 1, the connecting lugs 40a,40b on the coupler 34 associated with that hub are disposed in second bore 66 of the holding member. 
     By means of the components which have been described, groups of the control rods 56 connected to hubs 10a-10c can be successively lowered into or lifted out of the reactor core which lies under the hubs. For example, if all hubs are in lowered position (i.e., hub 10c resting on and connected to holding member 42, and the hubs stacked in contact with and connected to one another) and shaft 52 is raised above the stacked hubs, either partial or complete removal of the control rods 56 connected to hub 10a from the reactor core can be accomplished by rotating lugs 50a,50b on shaft 52 to a position wherein said lugs are vertically aligned with slots 20a,20b in hub 10a. It can be assumed that in this position of shaft 52, the components of the geneva movement are in the position illustrated in FIG. 5. Shaft 52 is then lowered to slide its connecting lugs 50a,50b through slots 20a,20b of hub 10a and into slots 48a,48b of the coupler 34 associated with said hub. Next shaft 52 is rotated 72° about axis 16 by the geneva movement to bring the connecting lugs 50a,50b on the shaft into vertical alignment with slots 20a,20b in hub 10b, which movement also rotates connecting lugs 40a,40b of the coupler 34 associated with hub 10a through the same angle and aligns said lugs 40a,40b of that coupler with slots 20a 20b, respectively, of hub 10b. Finally, shaft 52 is lifted, which engages its connecting lugs 50a,50b with the underside of shoulder 24 on hub 10a and thus lifts hub 10a and the control rods 56 connected thereto to either a partially removed or completely removed position relative to the reactor core, depending on the distance travelled by the shaft. When connecting lugs 50a,50b on shaft 52 and connecting lugs 40a,40b of the coupler 34 associated with hub 10a are positioned as just described, lugs 40a,40b of said coupler slide through slots 20a,20b of hub 10b, and hub 10b is thus not lifted. As indicated hereinbefore, when all three hubs 10a-10c are juxtaposed in their lowered position, lugs 40a,40b of the couplers 34 of hubs 10a and 10b are respectively positioned in slots 48a,48b of the couplers of hubs 10b and 10c, and therefore when shaft 52 is rotated its connecting lugs 50a,50b rotate all of the couplers and their lugs 40a,40b are maintained in vertical alignment with lugs 50a,50b. 
     If partial or complete removal of the control rods 56 connected to both hub 10a and hub 10b is required, after shaft 52 has been lowered to insert its connecting lugs 50a,50b into slots 48a,48b of the coupler 34 associated with hub 10a, drive means 92 is operated to rotate the shaft 144° about axis 16. Rotation of shaft 52 through that angle moves connecting lugs 50a,50b on shaft 52 and connecting lugs 40a,40b on the couplers 34 associated with hub 10a and hub 10b into vertical alignment with slots 20a,20b in hub 10c. Then when shaft 52 is lifted, its connecting lugs 50a,50b engage the underside of shoulder 24 on hub 10a and thus lift hub 10a. When hub 10a is raised, connecting lugs 40a,40b on its coupler 34 are now in a position out of alignment with slots 20a,20b in hub 10b, and therefore the connecting lugs of that coupler engage the underside of shoulder 24 on hub 10b and the hub 10b is lifted along with hub 10a. In the described position of shaft 52, connecting lugs 40a,40b of the coupler associated with hub 10b are vertically aligned with slots 20a,20b in hub 10c, and consequently hub 10c is not lifted. 
     It will be evident from the above description that all three of the hubs 10a-10c can be elevated by rotating shaft 52 through an angle of 216° about axis 16 after its connecting lugs 50a,50b have been inserted through slots 20a,20b in hub 10a and into slots 48a,48b of the coupler 34 of that hub. It will also be evident that all hubs not lifted by shaft 52 will be interconnected with each other and with holding member 42 by couplers 34. Insertion of the control rods 56 connected to one, two, or three of the hubs 10a-10b into the reactor core can be effected by reversing the above-described steps. The geneva movement illustrated in FIG. 5 provides the four angular position of shaft 52 required for lifting one, two, or three hubs 10a-10c or for releasing all of the hubs. 
     Various modifications can obviously be made in the disclosed latching assembly without departing from its principles of construction and operation. For example, in some applications only one connecting lug 50a on shaft 52 and one connecting lug 40a on each coupler 34 may suffice, and with this construction only one slot 20a in hubs 10a-10c and one slot 48a in the couplers would be required.