Abstract:
A quick disconnect for a control rod drive mechanism seismic support tie rod system that is remotely operable from a nuclear power plant&#39;s operating deck. A wall mounted anchor in the reactor cavity contains one half of a disconnect coupling that interfaces with the other half of the disconnect coupling on the ends of the tie rods employing a remote winching system that is actuated from the top of the reactor head assembly. A latching mechanism is then actuated from the refueling cavity operating deck to lock the tie rod in place and prevent displacement during a seismic or pipe break event. The tie rod may similarly be unlocked from the wall anchor and raised above the reactor head assembly as part of a reactor head disassembly operation to gain access to the core of the reactor vessel for refueling.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to control rod drive mechanism seismic supports for nuclear power plants and more particularly to a quick disconnect seismic support tie rod system. 
     2. Description of the Related Art 
     In conventional reactors, the head package includes the pressure vessel head which seals the reactor vessel, control rod drive mechanisms which are used to raise and lower control rods in the core of the reactor, a seismic platform adjacent the upper ends of the control rod drive mechanisms, which laterally restrains the drive mechanisms, and various cables for operation of the control rod drive mechanisms. A missile shield, which conventionally was formed of a concrete slab, is positioned above the head package to protect the containment housing and associated equipment from penetration by any of the control rod drive mechanisms in the event of a major pipe break. The problems associated with such conventional head packages are more fully described in U.S. Pat. No. 4,678,623, issued Jul. 7, 1987, and assigned to the assignee of this invention. In such conventional plants, the large concrete slabs installed above the reactor vessel to act as a missile shield must be removed and stored prior to head disassembly and refueling of the reactor, and then must be replaced after the refueling and head reassembly. Such operations affect overall refueling time and radiation exposure and require space in the containment area for placement of the missile shield slabs when removed from the position above the reactor vessel. 
     In order to reduce the refueling time, personal exposure and space requirements, an improved system, designated as an integrated head package was developed which incorporates an integral missile shield and head lift rig. The missile shield is in the form of a perforated circular plate which is directly attached to a head lift rig. Such an integral head package system is described in U.S. Pat. No. 4,830,814, issued May 16, 1989 and assigned to the assignee of this invention. 
     As described therein, and illustrated in  FIG. 1  of the present drawings, an integral head package  10  includes a three-legged head lifting rig  12  that is pin connected at  14  by lift lugs  16 , to a missile shield assembly  18 . The perforated circular plate  20  that forms the missile shield  18  acts as a spreader for the head lift load, and as a seismic support for the tops of the control rod drive mechanisms  22 , with rod travel housings extensions  24  of the control rod drive mechanisms protruding through apertures  26  in the circular plate  20 . The missile shield  18  interfaces with the tops of the control rod travel housings  22  which limits the overall vertical travel (and impact force) of a missile before it impacts the shield. The impact load of the missile against the underside of the perforated plate  20  is transmitted to head lift rods  28 , through vessel head lift lugs  30  secured to the vessel head  32 , and closure studs  34  to the vessel head  32 , and ultimately to the vessel supports. A cooling shroud  36  surrounds the control rod drive mechanisms  22 , while electric cabling  38  is routed from the top of the control rod drive mechanisms  22  to a connector plate  40  and then along a cable tray  42  to respective cable terminations. Cooling fans  44  circulate air within the shroud  36  to transfer waste heat from the control rod drive mechanisms  22 . Hoist supports  46 , and trolleys  48  on hoist assemblies  50  are used to position stud tensioner tools and stud removal tools during refueling operations. 
     The integrated head package and variants of the design which have since evolved, were a marked improvement over conventional head package designs, and are adaptable for retrofitting existing reactors or for incorporation into new reactor designs as will be described hereafter. However, there is still room for improvement in reducing the number of steps that have to be performed in the critical path of a refueling outage. For example, many nuclear power plants have control rod drive mechanism seismic support tie rods. Typically, there are five to six tie rods which are pinned at the reactor head assembly attachment, and pinned at a refueling cavity wall mounted anchor. During plant refueling, the tie rods need to be removed in order to move the vessel head assembly to the head storage stand. The ends of the tie rods that are attached to the wall mounted anchors (and in most cases the head assembly mounted ends also) are disconnected and reconnected by operators in a man basket supported by the overhead polar crane. Because these activities utilize the polar crane, which is also required for numerous refueling activities, they are considered to be in the critical path of the refueling outage. Any reduction in critical path time results in significant savings in the form of the refueling schedule and electric utility dollars. 
     Accordingly, a new tie rod support system is desired that can reduce the number of steps required to disconnect the tie rod wall anchors from the vessel head so that the vessel head can be removed. 
     Furthermore, a new tie rod support system is desired that can remove the disconnection of the tie rods from the refueling outage critical path. 
     Additionally, such a system is desired that can enable the vessel head assembly to be removed from the vessel with the tie rods attached to the head assembly. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by the control rod drive mechanism seismic support tie rod system of this invention that has the tie rod ends that engage anchors on the vessel cavity walls and are connected to the anchors with a locking mechanism that is directly operable from a location remote from the anchor to lock or unlock the tie rod from engagement with the anchor. In one preferred form, the locking mechanism is latchable in a locked position to lock the tie rod into engagement with the anchor. In a second position, the locking mechanism is preferably latchable in an unlocked position to maintain the locking member in an open state so that the tie rod can be removed. In one embodiment, the end of the tie rod that engages the anchor includes a lateral extension that extends from the tie rod end in a first direction and is engaged by a pivotable hook on the locking mechanism when the locking mechanism is in a locked position. In the foregoing embodiment, the pivotable hook has a distal end that is spaced from a pivot coupling on the locking mechanism. The distal end is pivotably connected to an actuation arm which is operable from the location remote from the anchor to lock or unlock the end of the tie rod. Preferably, in the foregoing embodiment, a stationary arm, over which the actuation arm rides, has a plurality of holes along a length thereof, at least one of which mates with a corresponding hole in the actuation arm when the actuation arm moves the locking mechanism into the locked position and into the unlocked position. In still another embodiment, the end of the tie rod that engages the wall anchor includes a second lateral extension that extends from the tie rod end in a second direction that is opposite the first direction and is engaged by a clevis on the locking mechanism. 
     Preferably, the nuclear containment facility employing this invention includes an operating deck within the vicinity of the vessel cavity. A generally vertical oriented wall extends down from the operating deck into the cavity, opposed from at least a portion of the reactor vessel, on which the tie rod anchors are mounted. The anchors are preferably secured at or between a foot to two feet (30.5-61 cm.) below the operating deck. 
     Preferably, one end of the tie rods are pivotably connected to the reactor vessel so that the tie rods can pivot up into a generally vertical position when they are released from the anchors. Desirably, a winching system is located on the reactor vessel head assembly for raising and lowering the tie rods. 
     In still another embodiment wherein the anchor includes a plate that extends generally toward the reactor vessel, substantially in line with the corresponding tie rod that it connects with, the anchor has a first dowel portion extending laterally from one side of the plate and a second dowel portion extending from an opposite side of the plate. The adjacent end of the tie rod is formed to slip over and engage the first and second dowel portions from above when the tie rods are pivoted down at the reactor vessel head assembly. The plate has a locking bar that is operable from a remote location on an operating deck of the containment to move over the tie rod end when the tie rod end fully engages the first and second dowel portions to lock the tie rod end to the anchor. Preferably, the tie rod end is configured as a fork with two tines spaced to receive the plate therebetween. Each of the tines has a downward facing clevis opening that receives a corresponding dowel portion as the tie rod is pivoted downward over the plate. Desirably, a locking bar is operable from the remote location on the operating deck to rotate between an open position wherein the locking bar lies over the top of the plate, clear of the tines of the tie rod end and a closed position, approximately 90 degrees from the open position, where the locking bar lies over each of the tines. The locking bar is preferably rotated by a long-handled tool that has a forked lower end that grips an upwardly extending wall on the locking bar. Desirably, the locking bar can be latched in either or both the open or closed position. Preferably, the clevis opening in the tie rod end has a taper at a lower end of at least one wall of the clevis opening slot to facilitate alignment over the dowel pin portions. 
     In still another embodiment, the anchor includes an engagement interface which is angularly adjustable to align with the corresponding tie rod. Preferably, the angular adjustment is in the vertical orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with accompanying drawings in which: 
         FIG. 1  is a perspective view of a prior art reactor vessel head assembly; 
         FIG. 2  is a perspective view of a portion of the reactor vessel head assembly anchored to the reactor vessel cavity sidewalls by tie rods, which shows a portion of the operation deck; 
         FIG. 3  is a planned view of the reactor vessel cavity showing the reactor vessel head assembly anchored to the vessel cavity sidewalls by the tie rods; 
         FIG. 4  is a perspective view of one embodiment of the tie rod system of this invention extending between the vessel cavity wall and an upper portion of the reactor vessel head assembly; 
         FIG. 5  is a perspective view of a prior art coupling between a tie rod end and the wall anchor; 
         FIG. 6  is a side view of a wall anchor clevis of the embodiment of this invention illustrated in  FIG. 4 ; 
         FIG. 7  is a perspective view of the coupling between the tie rod end and the wall anchor of the embodiment of this invention illustrated in  FIGS. 4 and 6 ; 
         FIG. 8  is a side view of the coupling, illustrated in  FIG. 7 , between the tie rod end and the wall anchor from the side of the pivotable hook with the hook in a locked position; 
         FIG. 9  shows a side view of  FIG. 8  with the pivotable hook in an open position; 
         FIG. 10  is a perspective view of a tie rod illustrating another embodiment of this invention that connects with the anchor on the reactor cavity wall; 
         FIG. 11  is a perspective view of the connection interface between the tie rod embodiment of  FIG. 10  and the anchor; 
         FIG. 12  is a perspective view of the angle adjustment assembly of  FIG. 11  that extends between the rigid wall mounted portion of the anchor and the open ended clevis of the tie rod; 
         FIG. 13  is a cross-sectional view of the anchor extension plate at the locking bar assembly illustrated in  FIG. 12 ; and 
         FIG. 14  is a perspective view of the long-handled tool that can be used for actuating the locking bar assembly shown in  FIGS. 11-13 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  shows a perspective view of the portion of the operating deck  52  which surrounds the reactor vessel cavity  54  and a portion of the integrated head package  10  showing a missile shield  18  seated over the control rod travel housing extensions  24  and coupled to the lifting rig  12  that was previously described with respect to  FIG. 1 . The integrated head package also includes a seismic ring  56  that is secured around the control rod travel housing extensions  24  and is secured against lateral movement by the tie rods  58  and  60  which are connected between the seismic ring  56  and the vessel cavity wall  62 . As can be seen from plan view in  FIG. 3  of this prior art arrangement, six tie rods, four radial tie rods  60  and two tangential tie rods  58  secure the control rod travel housing extensions  24  from lateral movement. The tie rods are connected at one end  72  to the seismic ring  56  on the integrated head package  10  through a pinned coupling  70  and are connected at another end  74  to the anchor  64  secured to the vessel cavity wall  62  through a similar connection  66 . As previously mentioned, during plant refueling, these tie rods  58  and  60  have to be removed in order to move the vessel head assembly  10  to the head storage stand. The end of the tie rods  74  that is attached to the wall mounted anchor  64  (and in some cases the head assembly mounted end  72  also) is disconnected and reconnected by operators in a man basket supported by the overhead polar crane. The quick disconnect control rod drive mechanism seismic support tie rod system of this invention eliminates the need to use the overhead polar crane for this purpose and thus takes this task outside of the critical path. 
     One embodiment of the quick disconnect control rod drive mechanism seismic support tie rod system of this invention is illustrated in  FIG. 4  and includes a winching system  76  attached to the head assembly  10  which is used to raise and lower the tie rods  58  and  60  without the use of a man basket. The tie rods  58  and  60  are secured to the head assembly structure through the pivot connection  70  and remain with the head assembly  10  for the move to the head storage stand. In this embodiment, a wall anchor attachment  64  includes a slotted clevis  90  which is designed to receive a laterally extending dowel  92  on the forward end  74  of the tie rod  58 ,  60 . The dowel  92  spans between two spaced circular brackets  94  that are supported at the end of the tie rod  58 ,  60  with the circular brackets  94  fitting on either side of the clevis  90  when the dowel  92  is seated in the slot of the clevis. A locking mechanism (more clearly shown in  FIGS. 7, 8 and 9 ), is actuated from the operating deck  52  and is designed to prevent the tie rods  58 ,  60  from becoming displaced from the clevis  90  during a seismic or pipe break activity. 
     Referring more specifically to  FIG. 4 , it can be seen that the winch system  76  is supported on a post  86  that is mounted on the seismic ring  56 . A winch crank  88  is mounted on the post  86  and has a cable  78  that extends from the post  86  to a pulley system  80  which is connected to the forward end  70  of the tie rod  58 ,  60  through a forward linkage  82 . The winch cable  78  extends from the post  86  to the pulley system  80  and back and around guide wheels  84  on the post  86  with the end of the cable connected to the crank  88  so that when the crank is turned in a direction to draw in the cable  78  the tie rod  58 ,  60  is raised toward the vertical axis lifting the dowel out of the clevis, assuming the locking mechanism  96  is in an open position. Conversely, when the crank  88  is turned in a direction to let out the winch cable  78 , the tie rods  58 ,  60  is lowered to be received in the slotted clevis  90  when the plant is in a cold start-up condition. A winching system  76  is provided for each tie rod  58 ,  60 . Preferably, the winch system  76  includes a lock, such as on the crank  88  that will lock the winch cable  78  in position when the tie rods  58 ,  60  are in their fully withdrawn position so the tie rods can be removed with the reactor vessel head to the head stand. 
     The design of the locking mechanism  96  of this invention is more fully illustrated in  FIGS. 6-9 . To appreciate the improvement of this invention, it is helpful to first understand the prior art coupling between the end of the tie rods  74  and the anchor  64  illustrated in  FIG. 5 . The anchor plate  64  of the prior art has two spaced, parallel plates that extend orthogonally from a base plate which is affixed to the reactor cavity wall  62 . The spaced parallel plates  100  have aligned holes through which a dowel  92  passes. The design of the end  74  of the tie rod is very similar to that of the current invention shown in  FIGS. 6 and 7  in which the end is formed from a split yoke that is designed to receive at least one of the parallel plates therebetween. The split yoke  102  of the prior art, shown in  FIG. 5 , has an enlarged circular rounded end  94  on each of the fork tine terminations of the split yoke. The rounded ends  94  have a central opening through which the dowel  92  passes and secures the split yoke  102  to the spaced parallel plate  100 . The dowel is affixed on one side of the parallel plates  100  with an enlarged end and on the other side with a cotter pin  98 . 
     One side of the anchor plate  64  of the embodiment of this invention described above, is illustrated in the side view shown in  FIG. 6 . The anchor plate  64  which is attached to the reactor cavity wall  62  has a slotted clevis  90  that extends orthogonally into the reactor well. The slotted opening  104  in the clevis  90  is designed to receive the dowel  92  on the tie rod end  74  between the two rounded ends  94  of the split yoke  102  of the tie rod end  74 . 
     A better view of the locking mechanism  96  of the anchor assembly  64  of the foregoing embodiment is shown in  FIG. 7 . As previously stated, the tie rod end  74  is substantially similar to the prior art tie rod end except the dowel  92  extends laterally from the rounded end  94  of the split yoke  102  in a direction away from the slotted opening  104  in the clevis  90  and is captured by the locking mechanism  96  in the closed position as will be more fully explained hereafter. The dowel  92  in this embodiment can be permanently affixed to both rounded ends  94  of the split yoke tie rod end  102 . Alternately, one of the tines of the split fork could be removed and the dowel  92  could extend out of one or both sides of the rounded end  94  so long as the dowel  92  was captured within the clevis slot  104  and the locking mechanism  96  as explained hereafter. 
     The locking mechanism  96  includes a pivotable hook  106  that is attached to a base spacer member  108  at a pivot point  110 . The base spacer member  108  is desirably connected to both the base anchor plate  64  and the slotted clevis  90  and is sized to capture the rounded end  94  of at least one tine of the split yoke  102  of the tie rod end  74  between the pivotable hook  106  and the clevis  90 . The distal end  112  of the pivotable hook  106  is connected to the end of an actuation arm  114  through a second pivot point  116 . The actuation arm  114  extends from the pivot point  116  vertically to a height above the operating deck  52  where the actuation arm  114  terminates in a horizontal handle  118  that extends over the operating deck  52 . The operating deck is typically one and one-half feet to two feet (45.72-60.96 cm) above the wall mounted anchor  64 . The actuation arm  114  rides over a stationary arm  120  that extends along the anchor wall plate  64 , along side the slotted clevis  90 . The stationary arm has a locking pin hole  122  which mates with corresponding holes  122  in the actuation arm  114  to receive a locking pin to lock the actuation arm  114  in position when in either the open or, closed orientation. Alternately, the stationary arm can have two holes that will mate with a single hole in the actuation arm. Thus, when the actuation arm  114  is pulled up in the vertical direction, the pivotable hook  106  rotates around the pivot  116  to an open position as shown in  FIG. 9 . Similarly, when the actuation arm  118  is pushed down in the vertical direction the pivoted hook  106  rotates about the pivot  110  and second pivot point  116  to the closed position illustrated in  FIG. 8 , locking the dowel  92  in the slot  104  of the clevis  90 . Thus, the locking mechanism  96  pins the tie rod end  74  to the wall mounted anchor  64  and can be locked in place in either the open or closed position from the operating deck elevation  52  using locking pins inserted through the holes  122 . The position locking pins utilize a lanyard to prevent them from being dropped into the reactor cavity  54  or misplaced. 
       FIG. 10  is a perspective view of another embodiment  124  of the tie rod  58 ,  60  employed by this invention. The coupling  70  to the reactor head is the same as that previously illustrated in  FIG. 2 . The short length of threaded piping  126  and the long threaded rod  130  on either side of the turn buckle  128  that are employed to adjust the length of the tie rod has been reversed from the embodiments illustrated in  FIGS. 2 and 4 . The hex jam nuts  132  are provided to lock in the adjusted length of the pipe. A set screw  134  on the open end clevis  136  on the distal end of the tie rod prevents the clevis from rotating. The open end clevis  136  has two fork tines  138  and  140  are separated by a distance that will accommodate the anchor plate fitting there between, as will be appreciated from the description to follow. The open end clevis  136  includes a downwardly facing slot  142  that is sized to accommodate a dowel pin that it will fit over. The lower portions  144  and  146  of the vertical walls of the slot  142  are angled to guide the open end clevis  136  over the dowel pins. 
       FIG. 11  is a perspective view of the coupling between the anchor  64  and the distal ends  74  of the tie rod  58 ,  60 , in accordance with the embodiment  124 . The hex jam nut  132  and the set screw  134  has been omitted for convenience. Like reference characters are used for the corresponding components among the several figures. The anchor  64  includes an embedment plate  148  and a laterally extending lug  150  which is reinforced by the gussets  152 . The lug  150  is connected to the open end clevis  136  through an extension plate  154  that is pivotably connected to the lug  150  by way of the extension plate pin  156 . The extension plate pin  156  enables the extension plate  154  to rotate in a vertical plane to align the clevis pin dowel  158  with the downwardly facing clevis pin slot  142  in the clevis tines  138  and  140 . A U-shaped bracket of an angle adjustment arm assembly  160  is connected to each side of the extension plate  154  and spans over the lug  150 . Set screw  162  and lock nut  164  adjusts the height of the U-shaped bracket of the angle adjustment arm assembly  160  over the lug  150  and thus raises or lowers the angle of the extension plate  154  until it is in alignment with the tie rod  58 ,  60 . A tie rod locking plate assembly  168  is rotationally connected on top of the extension plate  154 . The tie rod locking plate assembly  168  has a locking bar  170 , shown in  FIG. 11  in the locked position where it sits over the tines  138  and  140 , preventing the open end clevis  136  from being dislodged from the clevis pin  158 . The locking bar  170  can be rotated 90 degrees over the top  172  of the extension plate  154  to uncover the tines  138  and  140  for removal of the tie rod end  74  from the clevis pin  158 . 
       FIG. 12  shows the extension plate  154  in more detail. The clevis pin  158  passes through an opening in the extension plate  154  and extends on both sides of the extension plate. As can better be appreciated from  FIG. 13 , a keeper plate  174  sits within a notch in the clevis pin  158  and is secured by screws to the extension plate to firmly hold the clevis pin  158  in place. The hold down bar  170  is connected to the top of the extension plate  172  by a shoulder screw  176  which is loosely tightened to enable the hold down bar  170  to rotate about the screw  176 . As can be seen from  FIG. 13 , spring plungers  178  which fit in indentations in the underside of a hold down bar  170  engage the hold down bar in the closed position to resist rotation. A similar set of plungers can be provided to engage the underside of the hold down bar  170  when it is rotated 90 degrees to the open position to avoid interference with removal of the tines  138  and  140  from the clevis pin  158  as the tie rod  58 ,  60  is lifted. 
     Rotation of the hold down bar  170  between closed and open positions can be achieved through the use of the long-handled hold down tool  182  illustrated in  FIG. 14 . The hold down tool has a forked end  184  for engaging the recess  180  in the locking bar  170 . The forked end  184  is connected through an elongated shaft  188  to a handle  186  which can be turned from the operating deck  52  previously illustrated in  FIG. 2 . 
     Accordingly, in addition to eliminating the need for use of a polar crane for this refueling activity, the quick disconnect control rod drive mechanism seismic support tie rod system of this invention eliminates the need for unsafe ladders; eliminates the need for laydown space for the tie rods; and eliminates the potential for the dropping of loose parts. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, other attachment configurations between the anchor plates and the tie rod ends that can be engaged and disengaged remotely can be employed without departing from the scope of this invention. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breath of the appended claims and any and all equivalents thereof.