Abstract:
A fuel assembly handling tool that can be lowered onto the top nozzle of a fuel assembly, positively latch the top nozzle, unlatch from the top nozzle, and be raised off the top nozzle of the fuel assembly. The tool head, that interfaces with the top nozzle has load bearing grippers that latch onto the fuel assembly, that are located in a storage position up within the tool while the tool is lowered onto the fuel assembly. The gripper fingers are then lowered into position during the latching process, and are raised back to the storage position during the unlatching process. In the storage position, the gripping fingers are spaced above the fuel assembly top nozzle when the tool head is resting on the nozzle.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119(e) from Provisional Application Ser. No. 61/756,136, entitled “Retracting Finger Spent Fuel Assembly Handling Tool,” filed Jan. 24, 2013. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The invention relates generally to equipment used to transport nuclear fuel assemblies within a nuclear power generating facility, and more particularly to such a piece of equipment that will not get tangled in the components of a top nozzle of a fuel assembly as the equipment is being aligned to grip the top nozzle. 
         [0004]    2. Related Art 
         [0005]    In a nuclear reactor power plant, one design of a fuel assembly is comprised of a plurality of fuel elements or rods oriented in a square-shaped array. For a typical pressurized water reactor, there are on the order of approximately 200 to 300 of these elongated fuel rods in each fuel assembly. At either end of the fuel assembly is a top and bottom nozzle which direct the flow of coolant, typically water, through the fuel assembly. Interspersed among the fuel rods are hollow tubes, or thimbles, into which control rods are inserted. The control rods contain neutron absorbent material and are moved into and out of the plurality of guide thimbles to help control the nuclear reaction. These fuel assemblies also contain a centrally located instrumentation tube which allows the insertion of in-core instrumentation during reactor operation. The thimbles and instrumentation tube project between the top and bottom nozzles. Between the top and bottom nozzles, a plurality of spacer grids are positioned at intervals to provide lateral support for the fuel rods. The top nozzle is positioned at the upper end of the fuel assembly and connects to one end of the guide thimbles to allow the load of the entire fuel assembly to be carried from the bottom nozzle, which is connected to the other end of the guide thimbles, with the fuel assembly weight transferred up the guide thimbles to the top nozzle. The fuel assembly top nozzle and bottom nozzle are configured to aid in channeling coolant through the assembly during operation. In this configuration, the weight of the fuel rods is born by the guide thimbles and not by the fuel rods when the fuel assembly is lifted by the top nozzle. 
         [0006]    In an equilibrium core a typical fuel assembly will see three operating cycles before it is removed from the reactor and transported under water through a refueling canal to a spent fuel pool in a separate spent fuel building outside the reactor containment. 
         [0007]    When handling the fuel assembly, a fixture such as a refueling mast or other overhead crane is positioned over the reactor after the reactor head and upper internals are removed and connected to the top nozzle of the fuel assembly. The fuel assembly is then lifted from the core by the refueling machine which transports the fuel assembly under water through a flooded area in the containment above the reactor vessel, to a fuel assembly transport cart. The transport cart translates the fuel assembly to a horizontal position so it can pass through a refueling canal which connects to the spent fuel pool. A separate fuel handling machine in the spent fuel building uprights the fuel assembly and transports it to an appropriate location within the racks within the spent fuel pool. 
         [0008]    Existing designs of spent fuel assembly handling tools built for certain styles of fuel assemblies, such as that described above, include gripper fingers at a fixed elevation below a tool head of the handling tool. These gripper fingers pivot between a latched and unlatched position by raising and lowering an actuator. This design requires an operator to lower the tool onto a fuel assembly until the tool is resting on the top nozzle of the fuel assembly. The existing tools incorporate two alignment “S-pins” that must be inserted in two alignment “S-holes” on the top nozzle by a skilled technician. If the alignment of these pins to the holes is incorrect, the tool can be lowered in an orientation in which the gripper fingers contact or interfere with the top nozzle hold down springs. Such interference can cause the gripper fingers to become locked under the hold down springs requiring non-normal recovery efforts. In the Spring of 2012, a refueling machine gripper was lowered onto a fuel assembly and became stuck due to gripper finger to top nozzle interaction, which caused a seven-day delay in the refueling outage. During the Fall of 2012, a spent fuel tool lowered onto a fuel assembly became stuck due to finger to top nozzle interaction resulting in an 18-hour delay. 
         [0009]    The refueling operation usually determines the critical path for an outage during which replacement power has to be purchased at a relatively high cost. Anything that delays the refueling process is to be avoided wherever possible. 
         [0010]    Accordingly, it is an object of this invention to provide a fuel handling tool design that will not adversely get caught up in the components of the top nozzle. 
         [0011]    Additionally, it is a further object of this invention to provide such a tool design that is simple to operate. 
       SUMMARY 
       [0012]    These and other objects are achieved by a fuel assembly handling tool having a bail configured to be connected to an overhead crane or other hoist. A bail plate is connected to and is freely supported by the bail. A tool body is freely supported at an upper end from the bail plate and extends between the bail plate and a lower end with the length between the upper and lower ends being gauged to access a top nozzle of a fuel assembly. A tool head is connected to the lower end of the tool body and is sized to house a gripper assembly in a withdrawn position so that the gripper assembly is out of contact with the top nozzle of the fuel assembly when the tool head contacts or otherwise rests on the top nozzle. The gripper assembly is operable to extend below its withdrawn position to an extended position to grip a portion of the top nozzle of the fuel assembly to support the fuel assembly as the crane or other hoist lifts the bail. Preferably, an actuator arm is accessible from the bail plate and is operable to extend or withdraw the gripper assembly to the extended or withdrawn position. 
         [0013]    In one embodiment, the gripper assembly is moved to the withdrawn position or the extended position by respectively raising or lowering the actuator arm in a linear motion. In one such embodiment, the gripper assembly fully grips the fuel assembly as the actuator arm is lowered. Desirably, the gripper assembly positively locks in the fully withdrawn and fully extended positions. In another embodiment, the fuel assembly handling tool includes guide pins extending down from the tool head for aligning the tool head with the fuel assembly top nozzle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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: 
           [0015]      FIG. 1  is an elevational view, partially in section, of a fuel assembly that can benefit from the handling tool of this invention, the fuel assembly being illustrated in vertically shortened form, with parts cut away for clarity; 
           [0016]      FIG. 2  is a perspective view of one embodiment of the fuel assembly handling tool of this invention showing the bail, actuator handle, body and tool head, with the body shown in vertically shortened form; 
           [0017]      FIG. 3  is a cross sectional view of the fuel assembly handling tool shown in  FIG. 2 ; 
           [0018]      FIG. 4  is an enlarged cross sectional view of the tool head shown in  FIG. 3 ; 
           [0019]      FIG. 5  is a perspective view of the tool head with the gripper fingers shown retracted; and 
           [0020]      FIG. 6  is a perspective view of the tool head with the gripper fingers shown in the extended position. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    Referring to the drawings, in particular  FIG. 1 , there is shown an elevational view of a nuclear fuel assembly, of the type employed in pressurized water reactors, represented in vertically shortened form and generally designated by reference character  10 . The fuel assembly  10  has a structural skeleton which, at its lower end, includes a bottom nozzle  12 . During the operating life of the fuel assembly  10 , the bottom nozzle  12  supports the fuel assembly  10  on a lower core support plate  14  in the core region of the nuclear reactor (not shown). In addition to the bottom nozzle  12 , the structural skeleton of the fuel assembly  10  also includes a top nozzle  16  at its upper end and a number of guide tubes or thimbles  18  which extend longitudinally between the bottom and top nozzles  12  and  16  and at opposite ends are rigidly attached thereto. 
         [0022]    The fuel assembly  10  further includes a plurality of transverse grids  20  axially spaced along and mounted to the guide thimbles  18  and an organized array of elongated fuel rods  22  transversely spaced and supported by the grids  20 . Also, the assembly  10  has an instrumentation tube  24  located in the center thereof and extending between and mounted to the bottom and top nozzles  12  and  16 . With such an arrangement of parts, fuel assembly  10  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0023]    As mentioned above, the fuel rods  22  in the array thereof in the fuel assembly  10  are held in spaced relationship with one another by the grids  20  spaced along the fuel assembly length. Each fuel rod  22  includes nuclear fuel pellets  26  and is closed at its opposite ends by upper and lower end plugs  28  and  30 . The pellets  26  are maintained in a stack by a plenum spring  32  disposed between the upper end plug  28  and the top of the pellet stack. The pellets  26 , composed of fissile material, are responsible for creating the reactive power of the reactor. A liquid moderator/coolant such as water or water containing boron is pumped upwardly through apertures  42  in the lower core support plate  14  to the fuel assembly  10 . The bottom nozzle  12  of the fuel assembly  10  passes the coolant upward along the fuel rods  22  of the assembly in order to extract heat generated therein for the production of useful work. The coolant exits the core through apertures in an upper core plate (not shown) that sits over the fuel assembly. Hold down springs  40  that extend up from the top nozzle  16  seat against the underside of the upper core plate and serve to hold down the fuel assembly, counteracting the upward force exerted by the flowing coolant. 
         [0024]    To control the fission process, a number of control rods  34  are reciprocally movable in the guide thimbles  18  located at predetermined positions in the fuel assembly  10 . Specifically, a rod cluster control mechanism  36  positioned above the top nozzle  16  supports the control rods  34 . The control mechanism has an internally threaded cylindrical member  37  which is coupled to a drive rod (not shown) and a plurality of radially extending flukes or arms  38 . Each arm  38  is interconnected to a control rod  34  such that the control rod mechanism  36  is operable to move the control rods vertically in the guide thimbles  18  to thereby control the fission process in the fuel assembly  10 , all in a well known manner. 
         [0025]    To refuel such a reactor, the refueling area above the reactor is flooded, the reactor head and upper internals are removed, including the upper core plate, to expose the fuel assemblies. A refueling machine is then lowered and actuated to couple with the top nozzle  16  of the fuel assembly, gripping on an upper lip  44 .  FIG. 2  shows one embodiment of a refueling machine which incorporates the principles of this invention for coupling to and lifting a fuel assembly without the risk of entangling the gripping fingers of the lifting device in the hold down springs  40  or other component parts of the top nozzle. The fuel assembly handling machine  46 , shown in  FIG. 2 , basically includes a bail  48  that is connected to a bail plate  60  that supports the remainder of the tool. The bail  48  is designed to be connected to a hook on an overhead crane that will raise and lower the tool  46 . In addition to the bail  48  and bail plate  60 , the tool includes a handle  52  that raises and lowers to operate the gripping feature of the tool, a mechanism  62  employed to separately lock the handle in an “engaged” and “disengaged” position, a long and slender tubular tool body  54  and a tool head  56 . Alignment pins  58  extend from the lower part of the tool head and are designed to be inserted into corresponding openings in the upper surface of the top nozzle  16  of the fuel assembly  10 . The tool head  56  in this embodiment has four gripper fingers  64 , three of which can be observed in  FIGS. 3 and 4  with all four gripper fingers shown in the perspective view shown in  FIG. 6 . Though it should be appreciated that any number of gripping fingers may be used. In accordance with this invention, the gripper fingers  64  are retracted in a storage position within the tool head  56  when in a “disengaged” or “unlatched” position as shown in  FIG. 5 . The gripper fingers  64  are lowered and rotated into place when actuated to the “engaged” or “latched” position. Actuation of the tool is achieved by a single linear motion of the tool handle  52 . 
         [0026]    As can be seen from the cross sectional view shown in  FIG. 3 , the tool bail  48 , tubular housing body  54  and tool head frame  56  are all fixed and welded or bolted together. The handle  52  is pinned to an actuating rod which is slidably movable within the tool body  54  and extends substantially the whole length of the tool. The actuating rod  50  is attached at its lower end to an actuator  66 , which can better be seen in  FIG. 4 . The actuator  66  is connected to four “carriers”  68 . The fingers  64  are respectively pinned at  70  to the corresponding carrier  68 , but are able to rotate. Each finger  64  has a lobe  72  that rides in a cam slot  74 . As the handle  52  is raised, the actuator rod  50  to which the handle is coupled, raises the actuator  66 , the four carriers  68  and the four fingers  64 . As the fingers  64  are raised by raising the handle  52 , the lobes  72  on each finger ride in the cam slots which rotate the fingers in a specific way determined by the contour of the cam slot. In the image shown in  FIG. 4 , the tool is shown in an “engaged” position. As the handle  52  is raised, the fingers  64  would raise slightly while rotating approximately 10° in the first half-inch or so of travel. Then, as the handle continues to rise, the gripping fingers rise vertically because the cam slots are vertical from that point up. As previously stated,  FIG. 5  shows the tool head  56  in a completely “disengaged” condition with the fingers  64  completely retracted within the tool head housing  76 .  FIG. 6  shows the fingers  64  in a fully extended and “engaged” condition. 
         [0027]    Though, the invention was shown in an embodiment in which four fingers are employed and the disengaged and engaged positions are achieved by way of a single linear motion of the tool handle, to either fully retract or fully extend the gripping fingers relative to the tool head housing, other arrangements are contemplated in which those steps can be separately carried out with any number of grippers. 
         [0028]    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.