Patent Publication Number: US-2023137322-A1

Title: Control rod remote disconnect mechanism

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. provisional patent application No. 63/273,687 filed Oct. 29, 2021, the disclosure of which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The presently disclosed invention relates generally to systems and methods of use thereof for controlling reactor power levels in nuclear reactors and, more specifically, to systems and methods of use thereof for controlling the operation of control rods for nuclear thermal reactors. 
     BACKGROUND 
     In thermal nuclear power plants, a nuclear reactor core comprises a fissile material having size and composition selected to support a desired nuclear fission chain reaction. The core is disposed in a pressure vessel immersed in primary coolant water. It is further known to control or stop the reaction by inserting “control rods” comprising a neutron-absorbing material into guide tubes passing through the reactor core. When inserted, the control rods absorb neutrons so as to slow or stop the chain reaction. 
     The control rods are operated by control rod drive mechanisms (CRDMs). With “regulating” control rods, the insertion of the control rods is continuously adjustable so as to provide continuously adjustable reaction rate control. For “shutdown” control rods, the insertion is either fully in or fully out. During normal operation the shutdown rods are fully retracted from the reactor core, whereas during a SCRAM, the shutdown rods are fully inserted so as to rapidly stop the chain reaction. Control rods can also be designed to perform both regulating and shutdown rod functions. In some such dual function control rods, the control rod is configured to be detachable from the CRDM in the event of a SCRAM, such that the detached control rod falls into the reactor core under the influence of gravity. In some systems, such as naval systems, a hydraulic pressure or other positive force (other than gravity) is also provided to drive the detached control rods into the core. 
     To complete the control system, a control rod/CRDM coupling is provided. A known coupling includes a connecting rod having a lower end at which a spider is secured. The upper portion of the connecting rod operatively connects with the CRDM. In regulating rods, this connection includes a lead screw or other incremental adjustment element. Conventionally, the lead screw scrams with the connecting rod, spider, and control rods as a translating assembly (also known as the “control rod assembly”). In some known approaches, however, the lead screw may be retained in the CRDM and the remainder of the control rod assembly scrams. To reduce cost and overall system complexity, a single CRDM is typically connected with a plurality of control rods via a spider. In this arrangement, all the control rods coupled with a single spider together as a translating control rod assembly (CRA). In practice a number of CRDM units are provided, each of which is coupled with a plurality of control rods via a spider, so as to provide some redundancy. The spider extends laterally away from the lower end of the connecting rod to provide attachment points for multiple control rods. 
     During certain operations, for example, extended shutdown for maintenance, etc., it may be required that the translating control rods of the CRAs be fully inserted into the reactor core for extended periods of time. As such, it is desirable to have the ability to remotely engage and disengage the translating control rods from the CRDMs at a fixed location, such as between the connecting rods and the spiders, by vertical motion of the connecting rods. 
     SUMMARY OF INVENTION 
     One embodiment of the present disclosure provides a control rod drive mechanism having a torque tube with an inner surface defining a central bore, a control rod assembly including a connecting rod disposed within the central bore of the torque tube and a spider, the connecting rod being releasably securable to the spider, a lock cam assembly rotatably secured to a bottom end of the connecting rod, the lock cam assembly including a body portion and at least one locking cam extending radially-outwardly therefrom, and a locking collar disposed non-rotatably within the spider, the locking collar including an inner surface defining a central bore and at least one locking recess therein, the locking recess including an entry slot extending downwardly from a top edge of the locking collar, wherein the connecting rod is axially-movable with respect to the torque tube between a first position in which the lock cam assembly is rotatable with respect to the torque tube, and a second position in which the lock cam assembly is non-rotatable with respect to the torque tube. 
     Another embodiment of the present disclosure provides a disconnect mechanism for use with a control rod drive mechanism having a torque tube, including a connecting rod that is non-rotatably disposed within the torque tube, a lock cam assembly rotatably secured to a bottom end of the connecting rod, the lock cam assembly including a body portion and at least one locking cam extending radially-outwardly therefrom, and a locking collar disposed non-rotatably within the torque tube, the locking collar including an inner surface defining a central bore and at least one locking recess therein, the locking recess including an entry slot extending downwardly from a top edge of the locking collar, wherein the connecting rod is axially-movable with respect to the control rod drive mechanism between a first position in which the lock cam assembly is rotatable with respect to the connecting rod, and a second position in which the lock cam assembly is non-rotatable with respect to the connecting rod. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not, all embodiments of the invention are shown. Indeed, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. 
         FIG.  1    is a partial perspective, cross-sectional view of a lower portion of a nuclear reactor pressure vessel including an illustrative control rod assembly; 
         FIG.  2    is a side view of the control rod assembly shown in  FIG.  1   ; 
         FIG.  3    is a perspective view of the control rods and the connecting rod of the control rod assembly shown in  FIG.  2   ; 
         FIGS.  4 A and  4 B  are a perspective view and a side view, respectively, of the bottom end of a control rod assembly including a disconnect mechanism in accordance with an embodiment of the present disclosure; 
         FIGS.  5 A and  5 B  are a perspective view and a side view, respectively, of the bottom end of the connecting rod of the control rod assembly shown in  FIGS.  4 A and  4 B ; 
         FIGS.  6 A through  6 C  are side, bottom, and cross-sectional views of the locking collar of the disconnect mechanism of the control rod assembly shown in  FIGS.  4 A and  4 B ; and 
         FIGS.  7 A and  7 B  are partial side views of the disconnect mechanism of the control rod assembly shown in  FIGS.  4 A and  4 B , in the engaged and disengaged states, respectively. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention according to the disclosure. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope and spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     As used herein, terms referring to a direction or a position relative to the orientation of the control rod assembly with a remote disconnect mechanism, such as but not limited to “vertical,” “horizontal,” “upper,” “lower,” “above,” or “below,” refer to directions and relative positions with respect to the disconnect mechanism&#39;s orientation in its normal intended operation, as indicated in the Figures herein. Thus, for instance, the terms “vertical” and “upper” refer to the vertical direction and relative upper position in the perspectives of the Figures and should be understood in that context, even with respect to a reactor that may be disposed in a different orientation. 
     Further, the term “or” as used in this disclosure and the appended claims is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise, or clear from the context, the phrase “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, the phrase “X employs A or B” is satisfied by any of the following instances: X employs A; X employs B; or X employs both A and B. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from the context to be directed to a singular form. Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provided illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     With reference to  FIG.  1   , a relevant portion of an illustrative nuclear reactor pressure vessel  10  includes a reactor core  12  located proximate to a bottom of the pressure vessel  10 . The core  12  includes or contains radioactive material such as, by way of illustrative example, enriched uranium oxide (that is, UO 2  processed to have an elevated  235 U/ 238 U ratio). A control rod drive mechanism (CRDM)  14  assembly is diagrammatically illustrated. The illustrative CRDM  14  is an internal CRDM that is disposed within the pressure vessel  10 . In alternate embodiments, an external CRDM may be employed. Typically, there are multiple CRDM units each coupled with a plurality of control rods, although these additional CRDM units are not shown in  FIG.  1   . The pressure vessel  10  is drawn showing the space for such additional CRDM units. 
     Below the CRDM  14  is a control rod guide frame  16 , which in the perspective view of  FIG.  1    blocks from view the control rod/CRDM coupling assembly (i.e., the spider  32  and connecting rod  30 , both shown in  FIG.  3   ). Extending below the guide frame  16  is a plurality of control rods  18 .  FIG.  1    shows the control rods  18  in their fully inserted position in which the control rods  18  are maximally inserted into the core  12 . In the fully inserted position, the spider  32  ( FIG.  3   ) is located at a lower location  20  within the control rod guide frame  16 . In the illustrative embodiment of  FIG.  1   , the CRDM  14  and the control rod guide frame  16  are spaced apart by a standoff  22  comprising a hollow tube having opposite ends coupled with the CRDM  14  and the guide frame  16 , respectively, and through which the connecting rod  30  ( FIG.  3   ) passes. 
       FIG.  1    shows only a lower portion of the illustrative pressure vessel  10 . In an operating nuclear reactor, an open upper end  24  of the illustration is connected with one or more upper pressure vessel portions (not shown) that together with the illustrated lower portion of the pressure vessel  10  forms an enclosed pressure volume containing the reactor core  12 , the control rods  18 , the guide frame  16 , and the internal CRDM  14 . In an alternative embodiment, the CRDM  14  is external, located above the reactor pressure vessel. In such embodiments, the external CRDM is connected with the control rods  18  by a control rod/CRDM coupling assembly in which the connecting rod  30  extends through a portal in the upper portion of the pressure vessel. With reference to  FIG.  2   , the control assembly including the CRDM  14 , the control rod guide frame  16 , the intervening standoff  22 , and the control rods  18  is illustrated isolated from the reactor pressure vessel. With reference to  FIG.  3   , the control rods  18  and the connecting rod  30  of the control rod assembly  40  are shown without any of the occluding components (e.g., without the guide frame, standoff, or CRDM). The spider  32  provides connection of the plurality of control rods  18  with the lower end of the corresponding connecting rod  30 . 
     Referring now to  FIGS.  4 A and  4 B , a disconnect mechanism  50  in accordance with the present disclosure is shown. The disconnect mechanism  50  includes a locking collar  52  that is non-rotatably secured within a central bore  28  of the spider  32  of the control rod drive mechanism  14 . As shown, the spider  32  includes a mounting tube  26  that extends upwardly from a body portion  21  thereof and defines the central bore  28 . The mounting tube  26  is configured to selectively receive the bottom end of the connecting rod  30  therein. As discussed in greater detail below, the locking collar  52  ( FIGS.  6 A through  6 C ) includes at least one locking recess  54  that is configured to selectively receive a cam  38  that is formed by a projection that extends radially-outwardly from a lock cam assembly  33  that is rotatably received in the bottom end  33  of the connecting rod  30  of the control rod assembly  40 . Although embodiments of the disconnect mechanism  50  may include as few as one locking recess  54  and one corresponding cam  38 , it is preferable that the disconnect mechanism  50  include at least a pair of opposed locking recesses  54  and a pair of corresponding opposed locking cams  38 , as is shown in the present embodiment. 
     As is known in the art, friction forces between the lead screw (not shown) of a control rod assembly  40  and the roller nuts of a control rod drive mechanism  14  may cause the connecting rod  30  to rotate with respect to the torque tube (not shown) of the CRDM  14 . As discussed in greater detail below, non-rotation of the connecting rod  30  is desirable as it maintains proper alignment of the connecting rod  30  with the locking collar  52 . As shown, as is known in the art, in the present embodiment, rotation of the connecting rod  30  with respect to the torque tube is prevented by way of a key (not shown) which is non-rotatably fixed to the inner surface of the torque tube, and key slot (not shown) arrangement, the key slot being formed in an outer surface of the connecting rod  30 , as is known. 
     As best seen in  FIGS.  5 A and  5 B , the bottom edge  27  of the connecting rod  30  includes a plurality of projections  25  formed by a series of alternating peaks  45  and valleys  47 . The adjacent peaks  45  and valleys  47  are connected by a plurality of angled camming surfaces  49  that are configured to slidably engage the locking cams  38  of the lock cam assembly  33 . Referring additionally to  FIG.  4 B , the bottom end of the connecting rod  30  also defines a cylindrical bore  31  that extends between an upper internal ledge  41  and a lower internal ledge  43 . The cylindrical bore  31  is configured to slidably receive a portion of the body  35  of the lock cam assembly  33  therein so that the lock cam assembly  33  is limitedly slidable both into and out of the connecting rod  30 . An outwardly-depending flange  37  on the top of the lock cam assembly  33  is slidably received between the upper and lower ledges  41  and  43 , thereby limiting axial motion of the lock cam assembly  33  with respect to the connecting rod  30 . An optional coil spring (not shown) may be disposed between the upper ledge  41  of the connecting rod  30  and the top flange  37  of the lock cam assembly  33 . The coil spring may be used to reduce vibration of the lock cam assembly  33  during normal reactor operations by urging the assembly downwardly until the top flange  37  of the assembly abuts the lower ledge  43  of the connecting rod  30 . 
     As shown in  FIGS.  6 A through  6 C , the locking collar  52  is formed by concentric outer and inner surfaces  64  and  66 , respectively. The embodiment shown includes two locking recesses  54 , each locking recess  54  being defined by a projection  55  that extends radially-inwardly from the inner surface  66  of the locking collar  52 . Each locking recess  54  includes a first angled camming surface  60 , a stop surface  61 , and a second angled camming surface  62 . As best seen in  FIGS.  6 C , the first camming surface  60  extends upwardly from a bottommost edge  67  of the locking collar  52  to a top end of the stop surface  61 , which extends upwardly from the top of the stop surface  61  and is parallel to a longitudinal center axis of the locking collar  52 . The second camming surface  62  also slopes upwardly from the bottommost edge  67  of the locking collar  52  until intersecting a corresponding one of the entry slots  56 . A pair of entry slots  56  separate the projections  55 , with each entry slot  56  extending downwardly from a top edge  68  of the locking collar  52  and being configured to slidably receive a corresponding locking cam  38  therein. Preferably, the top end of each entry slot  56  is flared to facilitate slidably receiving both a corresponding locking cam  38  and key  51  of the lock cam assembly  33 . Note, interaction of the keys  51  of the connecting rod  30  with the entry slots  56  and pass-through slots  53  of the locking collar  52  further prevents rotation of the connecting rod  30  with respect to the locking collar  52 . The locking collar  52  is non-rotatably fixed at the top end of the spider  32 , as shown in  FIGS.  4 A and  4 B . 
     Referring again to  FIGS.  5 A and  5 B , the lock cam assembly  33  of the present embodiment preferably includes a pair of opposed cam projections  38 , one for each locking recess  54 . As shown, each locking cam  38  includes an angled bottom surface  51 , an angled camming surface  53 , and a stop surface  57  that extends upwardly from the bottom surface  51  to the camming surface  53  and is parallel to the longitudinal center axis of the lock cam assembly  33 . As shown, the slope of the angled camming surface  53  of each cam  38  is the same as the slopes of the first camming surfaces  60  and second camming surfaces  62  of the lock recesses  54 , as well as the camming surfaces  49  of the bottom edge  27  of the connecting rod  30 . 
     Referring to  FIGS.  4 A,  4 B,  7 A, and  7 B , the operation of the disconnect mechanism  50  is now discussed. As shown in  FIG.  7 A , during extended shutdown periods for the reactor, the locking cams  38  of the lock cam assembly  33  are aligned with the entry slots  56  of the locking collar  52  so that motion of the connecting rod  30  does not alter the position of the spider  32  and, therefore, control rods  18  ( FIG.  3   ). As such, the locking cams  38  of the lock cam assembly  33  may be disposed above the locking collar  52  so that the locking collar  52  is not engaged with the connecting rod  30  of the control rod assembly  40 . As previously noted, the connecting rod  30  is non-rotatably fixed to the torque tube  26 . 
     When an operator desires to engage the spider  32  of the control rod assembly  40  with the connecting rod  30 , the control rod drive mechanism  14  is utilized to move the connecting rod  30  downwardly within the torque tube  26 . As shown in  FIG.  7 B , the top end of each entry slot  56  is flared to facilitate entry of the corresponding locking cam  38 , which includes the angled bottom end  51  to facilitate entry into the slot  56 . When the locking cams  38  are not engaged with the locking collar  52 , the coil spring  67  of the spider  32  is fully extended. Continued downward motion of the connecting rod  30  causes the end face  29  of the lock cam assembly  33  to come into contact with the spring  67 , at which point the lock cam assembly  33  moves upwardly with respect to the connecting rod  30  until the flange  37  comes into contact with the upper ledge  41  of the bore  31 . At this point, the coil spring  67  begins to become compressed. 
     Continued downward movement of the connecting rod  30  with respect to the torque tube  26  causes the locking cams  38  to exit the bottom ends of the entry slots  56 , thereby clearing the bottom most edges  67  of the projections  55 , at which point the lock cam assembly  33  is free to rotate with respect to the torque tube  26  and, therefore, the locking collar  52 . Referring additionally to  FIG.  7 A , once the top of the stop surface  57  of each locking cam  38  clears the bottom end of the corresponding entry slot  56 , upward force exerted by the coil spring  67  on the lock cam assembly  33  forces the lock cam assembly  33  to move upwardly with respect to both the connecting rod  30  and locking collar  52 . As shown, interaction of the camming surface  49  of the bottom edge  27  of the connecting rod  30  causes the cam lock assembly  33  to rotate in the clockwise direction when viewing the connecting rod  30  from above. As well, the lock cam assembly  33  rotates with respect to the locking collar  52  and connecting rod  30  as the camming surfaces  53  of the locking cams  38  slide upwardly along the first camming surfaces  60  of the locking recesses  54 . Upward motion and clockwise rotation of the lock cam assembly  33  continues until the stop surface  57  of each cam  38  comes into abutment with the lock surface  61  of the corresponding locking recess  54 . At this point, the locking cams  38  are firmly seated within the locking recesses  54  of the locking collar  52  so that the corresponding control rod assembly  40  ( FIG.  3   ) is fully supported by the locking collar  52 , and the control rods  18  may be manipulated as desired with the CRDM  14 . 
     To disconnect the connecting rod  30  and, therefore, the CRDM  14 , from the spider  32 , the control rod drive mechanism  14  is energized and a lead screw of the control rod assembly  40  ( FIG.  3   ) is engaged to move the connecting rod  30  in the downward direction. Eventually, a camming surface  49  of the bottom edge  27  of the connecting rod  30  comes into contact with the camming surface  53  of a corresponding locking cam  38 . Continued downward motion of the connecting rod  30  once again causes the end face  29  of the lock cam assembly  33  to come into contact with the coil spring  67 , thereby causing it to be compressed. Once the bottom end of the stop surface  57  of each locking cam  38  clears the bottommost end of the lock surface  61  of each locking recess  54 , the lock cam assembly  33  is once again free to rotate in the clockwise direction with respect to both the locking collar  52  and the connecting rod  30 . As best seen in  FIG.  7 B , the upward force exerted by the coil spring  67  and interaction of camming surfaces  49  of the connecting rod  30  causes the camming surfaces  53  of the locking cams  38  to slide upwardly along the second camming surfaces  62  of the locking recesses  54  until each locking cam  38  enters the corresponding entry slot  56 . As shown in  FIG.  7 B , the connecting rod  30  is now free to move upwardly independently of the control rods  18  so that they may remain fully inserted in the reactor. 
     While one or more preferred embodiments of the invention are described above, it should be appreciated by those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope and spirit thereof. It is intended that the present invention cover such modifications and variations as come within the scope and spirit of the appended claims and their equivalents.