Abstract:
An electrical discharge machining (EDM) tool and method for removing tack welds on a component in a reactor pressure vessel are disclosed. The tool may include a motor, a housing which engages the component having tack welds, an electrode, positioned in the housing, a first plate, and a second plate attached to the electrode. The second plate actuates to align the electrode to the tack weld, and the electrode is discharged to remove the tack welds from the component.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method and apparatus for removing tack welds from reactor vessel components via electro-discharge machining (EDM). 
     2. Description of Related Art 
     Electrical Discharge Machining (EDM) is a common technique used for machining hard metals or those that would be impossible to machine with traditional techniques. EDM may be well-suited for cutting intricate contours or delicate cavities that would be difficult to produce with a grinder, an end mill or other cutting tools. EDM removes metal by producing a rapid series of repetitive electrical discharges. These electrical discharges are passes between an electrode and the piece of metal being machined. The repetitive discharges may remove the unwanted metal or create a set of successively deeper craters in the metal until a final shape is produced. Because EDM works with materials that are electrically conductive, metals that can be machined may include, for example, but not limited to, hastalloy, hardened tool-steel, titanium, carbide, inconel and kovar. 
     EDM is used to machine metals in a reactor pressure vessel (RPV) of a boiling water reactor (BWR). A typical reactor pressure vessel at a BWR has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud. 
       FIG. 1  is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV)  20  for a boiling water reactor. The RPV  20  has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate  22  within RPV  20 . A shroud  24  surrounds core plate  22  and is supported by a shroud support structure  26 . A downcomer annulus  28  is formed between the shroud  24  and a sidewall  30  of RPV  20 . 
     An annulet nozzle  32  extends through sidewall  30  of the RPV  20  and is coupled to a jet pump assembly  34 . The jet pump assembly  34  includes a thermal sleeve  36  which extends through nozzle  32 , a lower elbow (only partially visible in  FIG. 1 ), an inlet riser pipe  38  coupled to a pair of jet pumps  35 , and a jet pump riser brace assembly  40 . Thermal sleeve  36  is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve  36  is welded to the second end of the lower elbow. A first end of the lower elbow is similarly secured or welded to one end of the riser pipe  38 . The riser pipe  38  extends between and substantially parallel to the shroud  24  and sidewall  30 . The jet pumps  35  are circumferentially distributed around the core shroud  24 . The riser pipe  38  is coupled to the two jet pumps  35  by a transition assembly  39 . 
       FIG. 2  is a perspective view of a jet pump assembly  34 . Each jet pump  35  includes a jet pump nozzle  64 , a suction inlet  66 , an inlet mixer  41 , and a diffuser  42 . The jet pump nozzle  64  may be positioned in the suction inlet  66  which may be located at a first end (not shown) of inlet mixer  41 . The transition assembly  39  may include a base piece  70  and two elbows. Each elbow is coupled to a jet pump nozzle  64 . Locking arms  72 ,  74 ,  76 , and  78  extend from the transition assembly base piece  70 . Jet pump beams  86  are connected between the locking arms  72 ,  74 ,  76  and  78 . One jet pump beam  86  is engaged to a first pair of locking arms  72  and  76 , and another jet pump beam  86  is engaged to a second pair of locking arms  74  and  78 , as shown in  FIG. 2 . Each jet pump beam  86  includes a tongue member  81  at an end thereof which engages notches  82  in the locking arms  72 ,  74 ,  76  and  78  for preventing and/or reducing movement (e.g., rotational) of a corresponding beam bolt  94 . The jet pump beams  86  engage locking arms  72 ,  74 ,  76  and  78  by sliding the tongue member  81  into the notches  82 . 
     The jet pump beams  86  are generally attached to the inlet mixer  41  by a retainer plate (not shown) which is locked to the inlet mixer  41  with a retainer bolt  90 . To lock the retainer bolt  90 , tack welds are applied to prevent the retainer bolt  90  from vibrating loose. However, jet pump beams  86  occasionally need to be repaired or replaced. Thus, in order to repair/replace the jet pump beams  86 , the entire inlet mixer  41  is removed and moved into an open area to allow better access to separate the jet pump beams  86  from the inlet mixer  41  (e.g., by breaking the tack welds  91 , removing the retainer bolt  90  and removing the retainer plate). However, this procedure is time consuming, labor intensive, and requires additional equipment on-site. 
     Other techniques of breaking the tack welds  91  and removing the retainer bolt  90  use a torque multiplier to remove the tack welds  91 . However, torque multipliers are bulky and large, and thus cannot fit in the area of the jet pump beams  86 . 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the present invention is directed to an electrical discharge machining (EDM) tool for removing tack welds from surfaces of reactor pressure vessel components. The tool may include a motor, a housing which engages a retainer bolt having tack welds, a moveable electrode plate positioned in the housing, a first plate, and a second plate attached to the electrode. 
     In a further exemplary embodiment, the second plate actuates to align the electrode plate to the tack weld. 
     In yet a further exemplary embodiment, the electrode plate electrically discharges electrodes to remove tack welds from the components. 
     Another exemplary embodiment of the present invention is directed to a method of removing tack welds by electrical discharge machining (EDM). In the method, an EDM tool is installed over a component. The tool includes a housing, a first plate, a second plate and an electrode plate. The second plate is actuated to move the electrode in the housing. The, electrode plate electrically discharges electrodes to remove the tack welds. The EDM tool is then removed from the component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limitative of the present invention. 
         FIG. 1  is a schematic, partial cross-sectional view, with parts cut away, of a reactor pressure vessel (RPV) of a boiling water nuclear reactor. 
         FIG. 2  illustrates a riser brace assembly in accordance with an exemplary embodiment of the invention. 
         FIG. 3  is a schematic view of an EDM tool operable on a jet pump beam bolt in accordance with an exemplary embodiment of the invention. 
         FIG. 4  is a detailed schematic view of the EDM tool as shown in  FIG. 3 . 
         FIGS. 5A and 5B  are detailed schematic views of a head of the EDM tool in accordance with an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     These figures are intended to illustrate the general characteristics of method and apparatus of exemplary embodiments of this invention, for the purpose of the description of such exemplary embodiments herein. These drawings are not, however, to scale and may not precisely reflect the characteristics of any given embodiment, and should not be interpreted as defining or limiting the range of values or properties of exemplary embodiments within the scope of this invention. The relative dimensions and size of the jet pump assembly may be reduced or exaggerated for clarity. Like numerals are used for like and corresponding parts of the various drawings. 
       FIG. 3  is a schematic view of an EDM tool  100  operable on a jet pump beam bolt in accordance with an exemplary embodiment of the invention. It is appreciated that the portion of the jet pump assembly  34  shown in  FIG. 3  is similar to  FIG. 2 , and will not be described in detail. 
     Referring to  FIG. 3 , the jet pump beams  86  are attached to the inlet mixer  41 . A retainer plate  52  for supporting the jet pump beams  86  is attached below the jet pump beams  86 . The retainer plate  52  includes an opening which aligns with an opening on a block  49 . Once aligned, a retainer bolt  90  may be inserted for fastening the retainer plate  52  to the inlet mixer  41 . The retainer bolt  90  may include tack welds  91  for reinforcing the retainer bolt  90 . The tack welds  91  are secured to the retainer plate  52  in a conventional manner using known welding processes. 
     The EDM tool  100  engages the retainer bolt  90 . The EDM tool  100  is generally elongated (or linear) in shape so as to engage the retainer bolt  90 . Due to the design and operability of the EDM tool  100 , the EDM tool  100  may easily maneuver around the area of the jet pump assembly  34 . 
     The EDM tool  100  includes a bolt housing  110  for engaging (or indexing off) a surface of the head of retainer bolt  90 . In an example, the surface of the head of retainer bolt  90  may be hexagonal and thus the housing may assume a complimentary hexagonal shape. However, it should be appreciated that other shapes of the head of retainer bolt  90  may be used, so long as the bolt housing  110  corresponds to the shape of the head of retainer bolt  90 . 
     The bolt housing  110  is attached to a support frame plate  120  extending from the bolt housing  110  as shown in  FIG. 3 . The design of the support frame plate  120  may be construed as a generally narrow, extended linear slide shape. The support frame plate  120  is generally planar extending from the bolt housing  110 , and includes a gap  121  at an opposite end thereof. The gap  121  is designed to encompass ball screw nut  135 . A platform  125  is formed on top of gap  121  for holding an EDM motor  140 . The EDM motor  140  may be attached to the platform  125  with fasteners  128  (see  FIG. 4 ), i.e., screws or any other equivalent attachment means. The EDM motor  140  can be a conventional DC stepper that is typically used in EDM applications. The EDM motor  140  includes an extending wire  161  thereto for supplying power. As shown in  FIG. 3 , the extending wire  161  includes a connector  165  for connecting to a power supply (not shown). 
     As an example, the support frame plate  120  and the platform  125  may be made from stainless steel. However, it should be appreciated that other materials such as aluminum may be employed. It should further be appreciated that the support frame plate  120  and platform  125  may be formed as a unitary piece or as separate components fastened together. 
     An actuating plate  130  (which parallels support frame plate  120 ) extends into the bolt housing  110 . As will be described in detail later, the actuating plate  130  is used to engage an electrode which will be energized to release tack welds  91 . 
     The actuating plate  130  is attached to an electrode plate  150  (shown in  FIG. 5 ) in the bolt housing  110 . The actuating plate  130  provides the movement for engaging the electrode plate  150  to the head of retainer bolt  90  and electrically discharging the electrodes for removal of the tack welds  91 . The electrode plate  150  may be made from graphite or silver tungsten, and/or materials having equivalent conductive properties. 
       FIG. 4  is a detailed schematic view of an EDM tool  100  of  FIG. 3  in further detail.  FIG. 4  shows the EDM tool  100  without the surrounding jet pump assembly components. The EDM tool  100  may include a bolt housing  110 , a support frame plate  120 , an actuating plate  130 , a ball screw nut  135 , an EDM motor  140 , and an electrode plate  150  (see  FIGS. 5A and 5B ). 
     The bolt housing  110  engages the retainer bolt  90  for removing the tack welds  91 . When the bolt housing  110  is installed on the retainer bolt  90 , the EDM motor  140  operates via the ball screw nut  135  to move (retract and extend) the electrode plate  150  located in the bolt housing  110 . The ball screw nut  135  (moving in gap  121 , as discussed above) rotates a ball screw  160  to move the actuating plate  130  which then ultimately moves the electrode plate  150  a distance of approximately one inch. An extension piece  170  extending from the bottom of the bolt housing  110  prevents the actuating plate  130  from moving out of the ball screw  160  (e.g., acts as a stop piece). The extension piece  170  may be made from the same material as the support frame plate  120  and/or supports  139 . 
     The actuating plate  130  may include at least three holes  131 ,  132  and  133 . Hole  131  receives a ball screw  160  extending from the ball screw nut  135 ; and holes  132  and  133  receive supports  139  extending from the support frame plate  120 . The supports  139  may provide structural support for reinforcement, and may also be used for linear guides (e.g., when the actuating plate  130  is moving). The supports  139  may be made from the same material as the support frame plate  120 . The actuating plate  130  may be made from the same material as the support frame plate  120 , for example. 
     The bolt housing  110  includes index members  112  (shown in  FIGS. 5A and 5B ) which correspond to the shape of the head of retainer bolt  90 . The index members  112  allow alignment to, for example, a hexagonal head of retainer bolt  90 . This assures proper alignment to the welds. As an exemplary embodiment, two index members  112  extend inwardly in the bolt housing  110  to receive the retainer bolt  90 . It should be appreciated that more than two index members  112  may be used in the bolt housing  110 . It should further be appreciated that other design of the index member  112  may be implemented. The bolt housing  110  includes an indent  117  at a top end thereof to facilitate the insertion of the retainer bolt  90  into the bolt housing  110 . 
     As discussed above, the electrode plate  150  may retract and extend in the bolt housing  110 . During the placing of the bolt housing  110  onto the retainer bolt  90 , the electrode plate  150  is in the retracted position (shown in  FIG. 5A ). During the removal of the tack welds  91 , the electrode plate  150  is in the extended position (shown in  FIG. 5B ). In the extended position, the electrodes are discharged to remove the tack weld  91 .  FIG. 5B  illustrates the retainer bolt  90  with the tack welds  91  removed (e.g., reference character  91   a  indicates where tack welds  91  previously were located). Upon the removal of the EDM tool  100 , the electrode plate  150  retracts to disengage from the retainer bolt  90 . 
     Once the tack welds  91  are consumed by the discharging of the electrode plate  150  and the EDM tool  100  is removed from the work area, the retainer bolt  90  is removed by a standard low profile right angle nut runner. Since the tack welds  91  are no longer present, the bolt removal torque is low. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.