Abstract:
A fixture for performing visual inspections of the underside of the top guide of a boiling water reactor. The inspections are performed in the reactor vessel, under water, and includes a framed structure that rests on top of the top guide and supports a wheel track within a fuel assembly opening in the top guide, that follows the contour of the opening. A camera support, suspended from the frame, is then remotely, manually rotated to follow the contour of the wheel track as the fixture maintains the camera at a fixed angle and known constant distance from the underside of the top guide.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention pertains in general to the inspection of boiling water reactor internals and more particularly to the inspection of the underside of a top guide for a boiling water reactor. 
         [0003]    2. Related Art 
         [0004]      FIG. 1  is a sectional view, with parts cut away, of a typical reactor pressure vessel  10  for a boiling water nuclear reactor. During operation of the boiling water reactor, coolant water circulating in the reactor pressure vessel  10  is heated by nuclear fission produced in the core  12 . Feedwater is admitted into the reactor pressure vessel  10  by a feedwater inlet  14  and feedwater sparger  16 . The sparger  16  is a ring-shaped pipe that includes apertures for circumferentially distributing the feedwater inside the reactor pressure vessel  10 . The feedwater from the feedwater sparger  16  flows downwardly through downcomer annulus  18 , which is an annular region between the reactor pressure vessel  10  and the core shroud  20 . 
         [0005]    The core shroud  20  is a stainless steel cylinder that surrounds the core  12 . Core  12  includes a multiplicity of fuel bundle assemblies  22 ; two arrays of which are shown in  FIG. 1 . Each array of fuel bundle assemblies  22  is supported at its top by top guide  24  and at the bottom by core plate  26 . Top guide  24  provides lateral support for the top of the fuel bundle assemblies  22  and maintains correct fuel channel spacing to permit control rod insertion. 
         [0006]    The coolant water flows downward through downcomer annulus  18  and into core lower plenum  28 . The coolant water in the core lower plenum  28  in turn flows upward through the core  12 . The coolant water enters fuel bundle assemblies  22  wherein a boiling boundary layer is established. A mixture of water and steam exits core  12  and enters core upper plenum  30  under shroud head  32 . Core upper plenum  30  provides a standoff between the steam-water mixture exiting the core  12  and entering standpipes  34 . Standpipes  34  are disposed atop shroud head  32  and in fluid communication with core upper plenum  30 . 
         [0007]    The steam-water mixture flows through standpipes  34  and enters steam separators  36 , which may be, for example, of the axial-flow centrifugal type. Steam separators  36  substantially separate the steam-water mixture into liquid water and steam. The separated liquid water mixes with feedwater in mixing plenum  38 . This mixture then returns to the core  12  via downcomer annulus  18 . The separated steam passes through steam dryers  40  and enters the steam dome  42 . The dried steam is withdrawn from the reactor pressure vessel  10  via steam outlet  44  for use in turbines and other equipment (not shown). 
         [0008]    The boiling water reactor also includes a coolant recirculation system that provides the forced convection flow through the core  12  necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus  18  via recirculation water outlet  46  and forced by a centrifugal recirculation pump (not shown) into a plurality of jet pump assemblies  48  (only one of which is shown) via recirculation water inlets  50 . The jet pump assemblies  48  are circumferentially distributed around the core shroud  110  and provide the required reactor core flow. 
         [0009]    The United States Nuclear Regulatory Commission requires that for nuclear plant license extensions the reactor internals components subject to age degradation be inspected for deterioration through mechanism such as intergranular stress corrosion cracking. The previous method of inspecting the bottom side of the top guide was conducted with a single camera secured back on itself via a piece of tape such that the camera was pointed directly up. Inaccuracies in the motion and inspection angle were common using this method. Visual inspection criteria set forth in the EPRI Boiling Water Reactor Vessel and Internals Project (BWRVIP) Report-03 (Revision 12), requires the camera angle to be placed 30° or less from the perpendicular with a known distance from the inspection piece. BWRVIP-26A and BWRVIP-183 are both applicable to Top Guide examinations. Employing the previous method it was difficult to verify that the inspection was within the inspection criteria. 
         [0010]    Accordingly, a new method is desired that can verify that the inspection criteria has been followed. More particularly, a new apparatus is desired that can carry out such a method and maintain a known camera angle and distance from the inspection piece as well as provide rigidity to keep flow induced impact on the inspection process at a minimum if not entirely eliminated. 
       SUMMARY OF THE INVENTION 
       [0011]    The foregoing objectives are achieved by the apparatus of this invention which provides an inspection fixture for a top guide of a boiling water reactor. The inspection fixture includes a frame sized to rest and be supported on a top edge of a fuel assembly opening within the top guide. A wheel track is supported from the frame in a manner to extend around a periphery of the fuel assembly opening, substantially proximate a border thereof, when the frame is supported on the top edge of the fuel assembly opening. A central shaft is rotably supported from the frame and extends down below the fuel assembly opening, substantially along an axis of a fuel assembly to be positioned below the opening, when the frame is supported on the top edge of the fuel assembly opening. A linkage bracket is fixedly connected to the central shaft and extends laterally therefrom with a hinged distal link having a hinge line substantially parallel to and spaced from the central shaft. A cam follower rides on the wheel track and is supported from the distal link. An inspection sensor bracket is connected between the central shaft and the distal link for supporting a camera at a fixed angle to survey the underside of the top guide. 
         [0012]    In one embodiment, the fixture includes a hard stop ring for preventing the central shaft from rotating more than approximately 380°. Preferably, the hinged distal link is spring-biased in a preselected orientation that is preferably at an angle of 0° with another portion of the linkage bracket between the hinge line and the central shaft. 
         [0013]    In a preferred embodiment, the wheel track is substantially rectangular and preferably substantially square. Desirably, the wheel track is suspended from the frame within the fuel assembly opening in the top guide when the frame rests on the top edge of the fuel assembly opening and in one embodiment the wheel track sits just below the top surface of the top guide. 
         [0014]    In another embodiment, the inspection fixture of this invention includes a coupling at an upper end of the central shaft for coupling with a drive means for rotating the central shaft. Preferably, the drive means includes a pole that connects to the coupling for positioning the inspection fixture on the top edge of the fuel assembly opening and rotating the central shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0016]      FIG. 1  is a sectional view, with parts cut away, of a typical reactor pressure vessel for a boiling water reactor; 
           [0017]      FIG. 2  is an exploded view of the inspection fixture of this invention; 
           [0018]      FIG. 3  is an exploded view of the frame and wheel track of the inspection fixture shown in  FIG. 2 ; 
           [0019]      FIG. 4  is a perspective view of a portion of the central shaft and the link assembly that is connected directly to the central shaft of the inspection fixture of this invention; 
           [0020]      FIG. 5  is a perspective view of the distal end of the link assembly which is hingedly attached to the portion of the link assembly shown in  FIG. 4 ; 
           [0021]      FIG. 6  is a perspective view of the underside of the fixture of this invention in place within the fuel assembly opening of a top guide, with the inspection sensor bracket in a first orientation; and 
           [0022]      FIG. 7  is a perspective view of the inspection fixture of this invention shown in  FIG. 6 , in a second orientation. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    The top guide inspection fixture of this invention is shown in the exploded view illustrated in  FIG. 2  and is supplied from a reactor work platform by way of a small handling pole  52  that has a dimple  54  that fits in a J-slot coupling  56  in a distal end of a sleeve  58  which is the upper part of the central shaft  60  of the inspection fixture  62  of this invention. The small handling pole  52  sets the fixture  62  into the cell of the top guide, i.e., the fuel assembly opening. The fixture then allows for manual mechanical manipulation and rotation of a camera or other inspection sensor through the small handling poles to visually inspect the underside surface of the top guide to meet visual inspection requirements. The inspection can be performed in either a clockwise or counterclockwise direction, as desired. 
         [0024]    The inspection fixture  62  features a cross-member frame  64  and wheel track  66  that is shown in more detail in the exploded view shown in  FIG. 3 . The frame has a central hub  68  with four flukes or struts  70  that are spaced 90° apart around the hub  68  and extend out radially. The struts  70  have support ledges  72  that are designed to sit on the top surface of the walls of the fuel assembly openings in the top guide. The central hub  68  has a lower sleeve bearing  74  and a modified upper sleeve bearing  76  that accommodates rotation of the central shaft  60  that passes therethrough. A hard stop floating ring  78  is connected to the central shaft  60  and includes a raised land  80  that cooperates with a notch  82  in the flange of the upper bearing  76  to limit rotation of the central shaft to a little more than one full revolution, i.e., approximately 380°. The approximately 380° rotation from hard stop to hard stop assures an overlap of the inspection area for full coverage. The hard stops are desirable to prevent the device, e.g., camera cable, from becoming entangled should the user continue performing all remaining inspections in the same direction and not reset the fixture after a full rotation, to a “home” position. Two cable restraint hooks  84  are located on the hub  68  of the frame to properly secure a video (or other sensor) cable and prevent the cable from having excessive or insufficient slack. A home position visual indicator  86  is located on the top side of the frame to provide feedback of the relative camera location during operation. A wheel track  66  is welded to the frame and sits within the fuel assembly opening just below the top surface of the top guide when the frame is in position. Preferably, the wheel track  66  is a rectangle and more desirably a square, to follow the inside surface of the fuel assembly opening so as to guide a camera  88  in a camera support bracket  90 , around the underside of the walls of the fuel assembly opening in the top guide. 
         [0025]    The central shaft  60  comprises an upper J-slot sleeve  58 , an upper central shaft member  92 , a linkage assembly sleeve  94  and a lower shaft member  97  each of which are fixedly coupled together end to end, such as by welding, with the pins  98  serving to assure that there is no rotation of slippage. The weight of the camera support bracket  90  is carried by a shaft collar assembly  100  which is secured to the lower central shaft member  96 . The height of the shaft collar assembly  100  on the lower central shaft member  96  is adjustable so that the camera support bracket  90  can be raised or lowered to adjust for different top guide configurations. The markings  102  on the lower central shaft  96  are provide to identify the height adjustments for specific top guide designs. A slotted camera bracket  104  connects the camera support bracket  90  with the lower central shaft member  96  and permits lateral movement of the camera support bracket as a cam follower  124  connected to the camera support bracket  90  through a distal linkage block  110  follows the wheel track  66  as will be explained hereafter. 
         [0026]    A linkage assembly  106  generally comprises the sleeve  94 , first linkage block  108  and second, distal linkage block  110 . First linkage block  108  is more fully shown in  FIG. 4  and is welded to the linkage assembly sleeve  94  to form an integral member. Second or distal linkage block  110  is better shown in  FIG. 5 . First linkage block  108  is connected to second linkage block  110  through a hinged coupling  112  which is pivotably coupled by linkage pin  114 . The hinged coupling  112  includes a torsion spring  116  that returns the hinge orientation to a neutral position, i.e., 0° offset between the first linkage block  108  and the second linkage block  110 . The bottom side of the second linkage block has a hole  118  for one end of a slip fit rod  120  that allows for rotational movement. The other end of the slip fit rod  120  is fixedly connected to the top of the camera bracket  90  to maintain the fixed orientation of the camera bracket. A hole  122  in the upper end of the second linkage block  110  seats the shaft of the cam follower  124 . The cam  124  rides on the inside surface of the wheel track  66 . 
         [0027]    As the central shaft  60  is manually rotated with the small handling pole  52 , the configuration of the fixture  62  ensures that the camera position is always in the same position as the cam roller  124 . It should be appreciated that while the fixture is stated as supporting a camera  88 , other nondestructive sensors can also be supported by this fixture to examine the state of the top guide. As previously mentioned, a hard stop ring  78  is positioned on the central shaft  60  to limit the fixture to one full revolution, which prevents the camera cable from binding in the fixture, as well as gives mechanical feedback for beginning and ending the inspection. 
         [0028]    The fixture positions the camera such that inspections performed through the fixture of this invention are within BWRVIP requirements for a VT-1 examination. This includes the relation of the camera viewing angle to the inspection surface (less than 30° from the perpendicular angle between the surface and the camera field of view) as well as inspection distance (15.24 cm from the inspection surface). For an inspection to be performed, the top guide cell (i.e., the fuel assembly opening) designated for inspection must be fully evacuated of all fuel assemblies, control rod blades, double blade guides and/or single blade guides. Fuel assemblies, double blade guides, and/or single blade guides located in immediately adjacent cells do not require evacuation to perform inspections as there will be no contact or interference with any of these objects. The foregoing fixture of this invention will also not interfere or make contact with any instrumentation, such as local power range monitors. 
         [0029]    To accommodate five different set-ups respectively required for all BWR/2-6 top guide designs, three different length rods  120  may be used. Each rod is designed to maintain significant contact in the slip fit hole  118  on the distal block  110  of the linkage while allowing vertical adjustment depending on the position of the shaft collar  100 . The lower central shaft member  96  that holds the shaft collar  100  has small grooves  102  to indicate where to properly position the shaft collar depending on the top guide being inspected. Once the fixture is configured for a particular set-up, all intended inspections can be performed without any further alteration, modification, or manipulation of the set configuration. The main construction materials employed for the various components in this embodiment are formed from 300 series stainless steel and 6061 aluminum for corrosion resistance. Thus, the fixture of this invention provides for a smooth inspection with a known camera angle and distance from the inspection piece, as well as provides rigidity to keep flow induced impact to the inspection at a minimum, if not entirely eliminated. 
         [0030]      FIG. 6  provides a perspective view of the fixture of this invention installed on the top guide with the cam  124  at a mid location along a straight rail of the wheel track  66  as the camera support  90  surveys the underside of the top guide  24 .  FIG. 7  is a perspective view of the fixture shown in  FIG. 6  with the cam wheel  124  in a corner location of the wheel track  66 .  FIGS. 6 and 7  give a better appreciation of the functioning of the linkage assembly  106  as the hinge coupling  112  bends to accommodate the straight portions of the wheel track  66 . 
         [0031]    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. 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 breadth of the appended claims and any and all equivalents thereof.