Patent Publication Number: US-2023133343-A1

Title: Control rod remote holdout mechanism

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Provisional Pat. Application No. 63/273,694 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, shutdown core removal, etc., it may be required that the translating CRAs be fully withdrawn from the reactor core for extended periods of time. As such, it is desirable to have the ability to remotely engage and disengage the translating CRAs at a fixed location, such as by vertical motion of those CRAs. 
     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, the connecting rod including at least one cam extending radially-outwardly from an outer surface thereof, and an annular collar defining a key slot, an elongated key that is slidably receivable within the key slot, the elongated key being non-rotatably fixed to the inner surface of the torque tube, and a holdout collar disposed non-rotatably within the torque tube, the holdout collar including an inner surface defining a central bore and at least one locking recess therein, the locking recess including an entry slot extending upwardly from a bottom edge of the holdout collar, wherein the connecting rod is axially-movable with respect to the torque tube between a first position in which the elongated key is disposed within the key slot so that the connecting rod is non-rotatable with respect to the torque tube, and a second position in which the elongated key is removed from the key slot and the connecting rod is rotatable with respect to the torque tube. 
     Another embodiment of the present disclosure provides a holdout mechanism for use with a control rod drive mechanism having a torque tube, including a connecting rod with at least one cam extending radially-outwardly from an outer surface of the connecting rod, and an annular collar defining a key slot, the annular collar extending radially-outwardly from the outer surface of the connecting rod, an elongated key that is slidably receivable within the key slot, the elongated key being non-rotatably fixed to an inner surface of the torque tube, and a holdout collar disposed non-rotatably within the torque tube, the holdout collar including an inner surface defining a central bore and at least one locking recess therein, the locking recess extending upwardly from a bottom edge of the holdout collar, wherein the connecting rod is axially-movable with respect to the control rod drive mechanism between a first position in which the elongated key is disposed within the key slot so that the connecting rod is non-rotatable with respect to the control rod drive mechanism, and a second position in which the elongated key is removed from the key slot and the connecting rod is rotatable with respect to the control rod drive mechanism. 
     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   ; 
         FIG.  4    is a partial cut-away side view of a control rod drive mechanism including a holdout mechanism in accordance with an embodiment of the present disclosure; 
         FIGS.  5 A and  5 B  are a side view and a perspective view, respectively, of the top end of the connecting rod of the control rod drive mechanism shown in  FIG.  4   ; 
         FIGS.  6 A and  6 B  are a perspective view and a cross-sectional view, respectively, of the holdout collar of the control rod drive mechanism shown in  FIG.  4   ; and 
         FIGS.  7 A and  7 B  are partial cutaway side views of the holdout mechanism of the control rod drive mechanism shown in  FIG.  4   , 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 including a remote holdout 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’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 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 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  FIG.  4   , a holdout assembly  50  in accordance with the present disclosure is shown. The holdout assembly  50  includes a holdout collar  52  that is non-rotatably secured within a central bore  28  of the torque tube  26  of the control rod drive mechanism  14 . Further, the torque tube  26  is non-rotatably secured within a central bore  22  of a motor tube  20  of the control rod drive mechanism  14 . As discussed in greater detail below, the holdout collar  52  ( FIGS.  6 A and  6 B ) 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 top end  33  of the connecting rod  30  of the control rod assembly  40  ( FIG.  3   ). Although embodiments of the holdout assembly  50  may include as few as one locking recess  54  and one corresponding cam  38 , it is preferable that the holdout assembly  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. 
     Referring now to  FIGS.  5 A and  5 B , in addition to the pair of cams  38 , the top end  33  of the connecting rod  30  also includes an annular collar  44  that defines an axially extending key slot  46  therein. The annular collar  44  extends radially-outwardly from the outer surface of the connecting rod  30  and is disposed below the pair of locking cams  38 . As discussed in greater detail below, the key slot  46  of the annular collar  44  is configured to slidably receive an elongated key  34  therein. As shown in  FIG.  4   , the elongated key  34  is non-rotatably affixed to the inner surface of the central bore  28  of the torque tube  26 . As such, when the elongated key  34  is received within the key slot  46  of the annular collar  44 , the connecting rod  30  is non-rotatably fixed to the torque tube  26 . However, when the elongated key  34  is not received within the key slot  46 , the connecting rod  30  is free to rotate with respect to the torque tube  26  when the control rod assembly  40  is being moved vertically by the control rod drive mechanism  14 . 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  cause the connecting rod  30  to rotate with respect to the torque tube  26 . The direction of rotation of the connecting rod  30  with respect to the torque tube  26  is dependent upon the direction of axial travel of the connecting rod  30  with respect to the torque tube  26 . For example, in the instant case, when the control rod drive mechanism  14  is viewed from above and the connecting rod  30  is moved upwardly, the connecting rod  30  will rotate in the clockwise direction with respect to the torque tube  26 . Conversely, when the connecting rod  30  is moved downwardly with respect to the torque tube  26 , the connecting rod  30  will rotate in the counter-clockwise direction with respect to the torque tube  26 . 
     As shown in  FIGS.  6 A and  6 B , the holdout collar  52  is formed by concentric outer and inner surfaces  64  and  66 , respectively. In the embodiment shown, each locking recess  54  is defined in the inner surface  66  of the holdout collar  52  and includes an entry slot  56 , a lock surface  60 , and a camming surface  58  that extends therebetween. As shown, the entry slot  56   extends upwardly from a bottom edge  68  of the holdout collar  52  and is configured to slidably receive a corresponding locking cam  38  therein. The holdout collar  52  is non-rotatably fixed at the top end of the central bore  28  of the torque tube  26 , as shown in  FIG.  4   . 
     Referring to  FIGS.  4 ,  7 A, and  7 B , operation of the holdout assembly  50  is now discussed. As shown in  FIG.  7 A , during normal reactor operations, the locking cams  38  of the holdout assembly  50  are disposed below the holdout collar  52  so that the holdout collar  52  is not engaged with the connecting rod  30  of the control rod assembly  40 . Referring additionally to  FIG.  4   , when the locking cams  38  are disposed below the holdout collar  52 , the elongated key  34  is disposed within the key slot  46  of the connecting rod  30  such that the locking cams  38  are aligned with the entry slots  56  of the corresponding locking recesses  54 . When an operator desires to engage the control rod assembly  40  ( FIG.  3   ) with the holdout assembly  50 , the control rod drive mechanism  14  is utilized to move the control rod assembly  40  within the torque tube  26 . As noted, interaction of the elongated key  34  with the key slot  46  maintains alignment of the locking cams  38  with the corresponding entry slots  56  of the locking recesses  54 . As shown in  FIG.  4   , once the locking cams  38  have entered the corresponding entry slots  56 , upward motion of the connecting rod  30  causes the annular collar  44  to move upwardly beyond a top end  36  of the elongated key  34 , meaning that interaction between the elongated key  34  and the key slot  46  no longer prevent rotation of the connecting rod  30  with respect to the torque tube  26  of the control rod drive mechanism  14 . Note, however, as shown in  FIG.  4   , rotation of the connecting rod  30  with respect to the torque tube  26  is still prevented by interaction of the locking cams  38  with the side walls of the corresponding entry slots  56  of the locking recesses  54 . 
     Continued upward movement of the connecting rod  30  with respect to the torque tube  26  causes the locking cams  38  to exit the top ends of the entry slots  56 , thereby allowing the connecting rod  30  to rotate with respect to the torque tube  26  and, therefore, holdout collar  52 . As such, the locking cams  38  ride along the corresponding camming surfaces  58  of the locking recesses  54  until reaching the upper end thereof, at which point the locking cams  38  ride along the lock surfaces  60  until abutting the stop surfaces  61  of the locking recesses  54 . Note, prior to the locking cams  37  reaching the upper end of the corresponding camming surfaces  58 , the top end of the connecting rod  30  comes into contact with coil spring  67 , which is seated in end cap  69 . Coil spring  67  maintains a downward force on connecting rod  40  as it is compressed. When locking cams  38  come to rest on lock surfaces  60 , the control rod drive mechanism  14  may be de-energized, with the weight of the control rod assembly  40  being supported by the locking cams  38  resting upon the locking surfaces  60  of the holdout collar  52 , as shown in  FIG.  7 B . 
     To return to normal operation, the control rod drive mechanism  14  is energized and the lead screw of the control rod assembly  40  is engaged to move the assembly in a downward direction. As previously noted, friction between the roller nuts (not shown) of the control rod drive mechanism  14  and the lead screw (not shown) of the control rod assembly  40  will cause the connecting rod  30  to rotate the counter-clockwise direction when viewed from above. As such, the locking cams  38  slide along the lock surfaces  60  and then downwardly along the camming surfaces  58  until the locking cams  38  enter the corresponding entry slots  56  of the locking recesses, as shown in  FIG.  4   . When the locking cams  38  are disposed within the entry slots  56 , the key slot  46  of the connecting rod collar  44  is axially aligned with the elongated key  34  such that further downward motion of the control rod assembly causes the elongated key  34  to be slidably received in the key slot. As such, rotation of the connecting rod  30  with respect the torque tube  26  of the control rod drive mechanism is prevented. An operator is now free to manipulate the control rod assembly  40  as required by desired reactor conditions. 
     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.