Abstract:
A lifting fixture includes a housing; a plurality of jaws movable radially in and out of the housing to secure the fixture to a component to be lifted; at least one lifting bail extending from on end of the housing; and a screw mechanism operatively connected to the plurality of jaws for moving the jaws radially between an inward retracted position and an outward locking position. A related lifting and flipping process is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to nuclear reactor vessels, and specifically to a unique lifting fixture and related process for the core support plate. 
     In a nuclear reactor pressure vessel, the core support plate provides lateral support and guidance to the control rod guide tubes, peripheral support pieces, in-core flux monitor guide tubes, and start-up neutron sources. The core plate itself is formed with an array of holes through which the control rod guide tubes pass. The core plate assembly is bolted to a support ledge between the central and lower portions of the core shroud. During core shroud replacement procedures, the core plate is typically removed and transported underwater to another location where it is cut or sliced into multiple pieces for disposal. After removal and during transport, the core plate is typically supported in an upright orientation on a support stand provided with an annular array of vertical supports, each of which has a horizontal support ledge upon which the core plate rests. Because of the geometry of the core plate, as well as the configuration of certain cutting tools, it is necessary to flip the core plate to an upside down orientation prior to slicing to allow access to the support structure of the core plate on the underside. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention provides a core plate lifting fixture particularly useful for combined lifting and flipping of the core plate prior to slicing in an underwater environment. Despite inherent space limitations, the fixture is designed to withstand heavy loads, particularly moment loads encountered during the flipping operation. In a typical situation, four fixtures are attached at selective locations about the core plate. Each fixture can be installed within, or removed from a selected core plate hole via radially movable jaws actuated by a vertically oriented screw mechanism. The fixture is configured to have attachment or lifting eyes or bails projecting from both the upper and lower sides of the core plate, and the screw mechanism is accessible from both sides of the fixture (and hence, both sides of the core plate). Each of the lifting bails is also pivotable about a horizontal axis, within an approximate 180° range to enable easy access to the screw mechanism and to permit some swiveling motion for the hooks. 
     The invention also relates to a method of flipping or inverting the core plate, generally described as follows. The core plate is supported on a stand which is, in turn, supported on a turntable. Each of the four fixtures is inserted into a selected core plate hole, and the radial jaws are moved outwardly by turning the screw device from a remote location. After the fixtures are attached to the core plate, a crane is employed to lift the core plate by means of at least two of the fixtures, turned sideways and flipped over, and then set back down. The jaws are then radially retracted and the fixtures removed from the core plate. A more detailed description of the method is disclosed further herein. 
     In its broader aspects, therefore, the present invention relates to a lifting fixture comprising a housing; a plurality of jaws movable radially in and out of the housing to secure the fixture to a component to be lifted; at least one lifting bail extending from one end of the housing; and a screw mechanism operatively connected to the plurality of jaws for moving the jaws between a radial inward retracted position and a radial outward locked position. 
     In another aspect, the invention relates to a method of inverting a nuclear reactor core plate supported on a stand comprising a) attaching at least first and second lifting fixtures to the core plate, each fixture having an upper lifting bail and a lower lifting bail projecting, respectively, from upper and lower sides of the core plate; b) operatively attaching first and second grapples connected to a crane for selective movement to the upper lifting bails, respectively, of the first and second lifting fixtures; c) lifting the core plate off the stand; d) selectively manipulating the grapples to tilt the core plate on end and resting it on the stand, with the first grapple and first lifting fixture closest to the stand; e) releasing the first grapple, rotating the core plate and attaching the first grapple to the lower lifting bail of the second lifting fixture; f) releasing the second grapple from the upper lifting fixture of the second lifting fixture, rotating the core plate and attaching the second grapple to the lower lifting bail of the first lifting fixture; g) selectively manipulating the grapples to lift and level the core plate; and h) lowering the core plate onto the stand. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a nuclear reactor core plate supported on a support stand; 
     FIG. 2 is a top plan view of a lifting fixture in accordance with an exemplary embodiment of the invention; 
     FIG. 3 is a section view taken along the line  3 — 3  of FIG. 2; 
     FIG. 4 is a side elevation showing the core plate lifted and turned on edge during a flipping operation (grapples not shown for sake of clarity); 
     FIGS. 5A through 5H illustrate, sequentially, a lifting and flipping operation for a core plate, using fixtures as shown in FIGS. 2 and 3; and 
     FIG. 6 is an enlarged detail taken from FIG.  5 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIGS. 1 and 4, a core plate assembly  10  includes a flat, circular plate  12  with a downwardly projecting annular skirt  14  located slightly radially inwardly of a peripheral edge  16  of the plate, leaving a horizontal marginal edge  18  exposed outwardly of the skirt, on the underside of the plate. A plurality of vertically oriented flow guides  20  extend across the lower side of the plate  12  within, but projecting below, the skirt  14 . An array of through holes  22  are formed in the plate  12 , providing support for the reactor control rod guide tubes (not shown). As illustrated in FIG. 1, the assembly  10  is supported in an upright orientation on a support stand  24 . The stand  24  includes a plurality of upright brackets  26  fixed to an upper ring  28  of the stand, each bracket  26  having a horizontally oriented ledge  30 . The marginal edge  18  of the plate  12  is supported on the plurality of ledges  30 , and confined horizontally by vertically oriented stops  32  extending upwardly from respective ledges  30 . The stand  24  may be supported for rotation on a turntable described further herein in connection with FIGS. 5A-5H. This is the orientation of the core plate assembly  10  after removal from a nuclear reactor core shroud (not shown), and as situated under water, prior to a core plate slicing operation. 
     Four core plate lifting fixtures  34  are shown secured to the core plate  12  at equally spaced locations about the marginal edge of the plate. Each fixture  34  is mounted in a selected one of the holes  22  as described below. Since the fixtures are identical, only one need be described in detail. 
     With further reference to FIGS. 2 and 3, the core plate lifting fixture  34  includes a top or upper cap  36  and a lower cap  38  with a housing assembly therebetween, the latter including an upper housing  40  and a lower housing  42 . A lower plate  44  of the upper housing  40  is welded to the lower housing  42 , and the lower cap  38  is welded to the lower end of the lower housing  42 . 
     Mounted on the top cap  36  are a pair of clevis blocks or yokes  46 ,  48 , in which are pivotally secured respective T-shaped rod ends  50 ,  52  which are, in turn, welded to a generally inverted U-shaped attachment “eye” or lifting bail  54 . Clevis pins  56 ,  58  are employed to pivotally secure the rod ends within the yokes. As best seen in FIGS. 3 and 4, a second attachment eye or lifting bail  60  is secured to the lower cap  38  in the same manner as described above, but projecting in an opposite direction. With this arrangement, the bails  54  and  60  can be pivoted substantially 90° in either direction from the upright position shown in FIGS. 1 and 3. 
     Located within radial openings in the upper housing  40  is a jaw assembly including four jaw elements  62 ,  64 ,  66  and  68  located at 90° intervals about the upper housing  40 . Each jaw has a slot or recess (two shown at  70 ,  72 ) approximately mid-way from top to bottom of the respective jaw, the slot or recess adapted to engage the upper and lower plate surfaces about the edge of a hole  22  in the core plate when the fixture  34  is located within the hole as shown in FIG.  1 . The jaws are radially movable into locking engagement with the plate  12  by means of an actuator assembly which includes a sleeve  74  threadably engaged on a partially threaded rod or shaft  76  which is held in the fixture by nuts or bolt heads  78 ,  80  externally of the upper and lower caps  36 ,  38 . Rotation of the shaft or rod  76  will thus cause the sleeve  74  to move up or down on the shaft, depending on the direction of rotation of the latter. 
     Each jaw  62 ,  64 ,  66  and  68  is connected to the sleeve  74  by means of a link  82 , pivotally secured at opposite ends by dowel pins  84 ,  86 , thus permitting the link  82  to pivot relative to both the sleeve  74  and respective jaw. Vertical movement of the sleeve thus translates into horizontal radial movement of the four jaws  62 ,  64 ,  66  and  68  within the openings in the upper housing. As shown in FIG. 3, when the links  82  are horizontal, the jaws are in their radially outermost, locking position. There is also a stop or spacer  75  for limiting the downward travel of the sleeve  74 . 
     The components of the fixture as described above are made of high strength anti-galling materials which can carry the high moment loads and which enable relative movement of the various components. It is also to be noted that the screw mechanism is generally protected from the application of heavy shear (radial) and moment loads by the upper housing of the fixture. 
     In use, each of the four fixtures  34  is first inserted into a core plate hole  22 . The fixture can be swiveled to any desired position. The jaws  62 ,  64 ,  66  and  68  are then moved radially outwardly by turning the rod  76  from a remote location by means of a suitable tool engaging the nut or bolt head  78 . Access to both bolt heads  78 ,  80  is facilitated by the ability to pivot the attachment eyes or lifting bails  54 ,  60  to a substantially horizontal orientation. With reference now to FIGS. 5A-5H as well as FIG. 6, the sequence of steps necessary to lift and flip the core plate from its normal upright orientation to an inverted orientation to facilitate slicing will now be described. A conventional crane  88  is employed, the crane having a large grapple  90  and a smaller grapple  92 . The large grapple  90  has the capability to support the entire core plate while the smaller grapple  92  in some cases is capable of supporting half the core plate weight. Cables  94 ,  96  extend from the respective grapples  90 ,  92  with hooks  98 ,  100  (see also FIG. 6) at the free ends of the cables adapted to engage the attachment bails or eyes  54 ,  60  of the fixtures  34 . With reference to FIGS. 5A and 6, the hooks  98 ,  100  are secured to a selected pair of the four fixtures  34 . That will allow the cross plates attached to the bottom of the core plate to be in the vertical position when the core plate is supported by one side. (Typically, only two fixtures are required and the hooks  98 ,  100  can be attached to whichever two opposite fixtures  34  are most convenient). After the hooks have been attached to a pair of opposed bails or eyes  54 , the crane lifts the core plate vertically upwardly in a level orientation as shown in FIGS. 5A and 6. The larger grapple  90  is then lowered, thus tilting the core plate on end, allowing it to rest on the support stand  24 . As indicated earlier, the support stand  24  is supported on a rotatable turntable  102 . With the core plate in the position shown in FIG. 5B, the hook  98  of the large grapple  90  is released from the core plate as shown in FIG. 5C, and the core plate is rotated approximately 180° to the position shown in FIG.  5 D. The hook  98  is then attached to the lifting bail  60  of the same fixture  34  to which the smaller grapple  92  is attached by means of hook  100 . Then, the hook  100  of the small grapple  92  is released from the core plate (FIG.  5 E), and the core plate is again rotated 180° to the position shown in FIG.  5 F. The hook  100  of the small grapple  92  is then attached to the lower lifting bail  60  of a fixture  34  opposite that to which the large grapple hook  98  is secured as shown in FIG. 5G, using the small grapple  92 , the core plate is lifted and made level as shown in FIG.  5 H. Both grapples are then lowered to deposit the core plate on the support stand  24  in an inverted position, ready for slicing. In order to remove the fixtures  34 , the bails  60  are pivoted to a horizontal position and the remote access tool is engaged with the bolt head or nuts  80  and the rod  76  rotated to pull the jaws  62 ,  64 ,  66  and  68  radially inwardly, thereby enabling the fixtures  34  to be lifted out of the core plate holes  22  by the grapples  90 ,  92 . 
     It will be understood that the crane  88  as well as the grapples  90  and  92 , along with associated cables  94 ,  96  and hooks  98 ,  100  are conventional and themselves form no part of the invention. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.