Abstract:
A device and method to move a fuel assembly of a nuclear reactor, wherein the device eliminates the potential for dropping the fuel assembly due to stress corrosion cracking of the upper guide thimble sleeves that attach the top nozzle to guide thimbles.

Description:
FIELD OF THE INVENTION 
       [0001]    The current invention relates to handling of fuel assemblies for nuclear reactors. More specifically, the invention relates to a tool and method for movement of nuclear fuel assemblies without over stressing the top nozzle of the fuel assembly. 
       BACKGROUND INFORMATION 
       [0002]    Present-day light water nuclear reactors, both pressurized water reactors and boiling water reactors, require fuel to be installed in a reactor pressure vessel prior to operation. The fuel is often in the form of a fuel assembly, where individual fuel rods, which contain a series of parallel arranged rods each containing stacked pellets, are held together by a structural skeleton arrangement. 
         [0003]    After the reactor pressure vessel is operated and the nuclear fuel in the fuel assemblies is depleted, the depleted fuel assemblies must be removed and replaced with new fuel assemblies. Additionally, non-depleted fuel assemblies must be moved to other areas of the reactor core and inspected for defects prior to restarting the reactor. The moving of the fuel assemblies, both depleted and non-depleted, must be performed in a safe and controlled manner as dropping of a fuel assembly could lead to damage of nuclear fuel and potential release of radioactive material. 
         [0004]    Detailed visual and radiographic inspections of fuel assemblies and special lifting devices (devices that move the fuel assembly) help prevent load lifting accidents due to degraded structural components. It has been found that after exposure to radiation, some fuel assemblies exhibit stress corrosion cracking of the upper guide thimble sleeves which attach the top nozzle of the fuel assembly to the guide thimbles. 
         [0005]    Industry experience has also found that current devices and methods to lift structurally damaged fuel assemblies are costly for plant operators, while not eliminating the potential for load lifting accidents from problem prone areas such as failure of the top nozzle to guide thimble connection from stress corrosion. 
         [0006]    If a fuel assembly exhibits stress corrosion cracking or some other structural defect, current practices require underwater disassembly of the fuel assembly and removal of the individual pieces or establishment of additional structural corrections to the damaged fuel assembly to allow an adequate transfer of the structural load of the assembly. Both of these repair alternatives are expensive and require significant structural analysis of the damaged fuel assembly to ensure a safe load lift. 
         [0007]    There is a need for a fuel assembly lifting device and method which can be used with existing fuel assembly configurations in nuclear reactors, both pressurized and boiling water reactors. 
         [0008]    There is a further need to provide a fuel assembly lifting device and method which will allow for safe movement of a fuel assembly when the fuel assembly has stress corrosion cracking of the upper guide thimble sleeves that attach the top nozzle to the guide thimbles. 
         [0009]    There is still further need for a device which will properly interface with existing fuel handling equipment and storage racks for cost efficient effective movement of fuel assemblies. 
       SUMMARY 
       [0010]    It is an objective of the present invention to provide a fuel assembly nozzleless handling tool and method which can be used with existing configurations of fuel assemblies used in nuclear reactors. 
         [0011]    It is a further objective of the present invention to provide a fuel assembly nozzleless handling tool which will allow movement of a fuel assembly in a safe manner when the fuel assembly exhibits stress corrosion cracking of the upper guide thimble sleeves that attach the top nozzle to the guide thimbles. 
         [0012]    It is a further objective of the present invention to provide a tool and method which will properly interface with existing fuel handling equipment and storage racks for cost efficient and effective movement of fuel assemblies. 
         [0013]    The invention provides a method of handling a fuel assembly. The method provides for supporting a tool configured to handle the fuel assembly, and positioning the tool over a top of the fuel assembly. The method further provides for lowering the tool onto the top of the fuel assembly such that alignment pins engage a top nozzle of the fuel assembly and actuating a shaft to lower lock fingers into guide thimbles of the fuel assembly. The method also provides for positioning the lock fingers to a position below divots in the guide thimble in the fuel assembly to be engaged, engaging the lock fingers into the divots to an extended position, and lifting the fuel assembly and the tool. 
         [0014]    The invention also provides a fuel assembly nozzleless handling tool configured to handle a fuel assembly. The tool comprises a mandrel shaft configured to travel a length of the tool. The tool also provides a mandrel plate connected to the mandrel shaft, the mandrel plate configured to move through actuation of the mandrel shaft and a finger shaft configured to travel from an unengaged position to an engaged position. The device further provides a finger plate configured to insert and retract the lock fingers into the guide thimbles, the finger plate moved by actuation of the finger shaft and a load plate. The device still further provides at least two lock fingers configured to interface with a fuel assembly guide thimble, and a mandrel for each of the two lock fingers, wherein the mandrel is configured to secure the lock fingers and retract the lock fingers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a side elevation view of a fuel handling nozzleless lifting tool in accordance with the present invention. 
           [0016]      FIG. 2  is a side elevation view of the fuel handling lifting tool of  FIG. 1  wherein lock fingers are in an unextended position. 
           [0017]      FIG. 3  is a side view of the fuel handling lifting tool of  FIG. 1 , wherein the mandrel shaft is lowered to extend the lock fingers. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  illustrates a fuel assembly nozzleless handling tool  10  in conformance with the present invention. The fuel assembly nozzleless handling tool  10  is comprised of a mandrel  14 , a mandrel shaft  12 , a mandrel plate  16 , a finger shaft  18 , a lock finger  22 , a finger plate  26  and a load plate  24 . 
         [0019]    The fuel assembly nozzleless handling tool  10  is a device which allows for lifting of fuel assemblies which exhibit structural defects which may affect overall lifting safety. The handling tool  10 , therefore, prevents the need for permanent structural repair from being accomplished on structurally degraded fuel assemblies. 
         [0020]    The tool  10  interfaces with the existing structures in the fuel assembly  20  in order to accomplish a load lift. To accomplish this, lock fingers  22  are used to interface with fuel assembly guide thimbles. The external diameters of the lock fingers  22  are configured to allow insertion of the lock fingers  22  into the openings of the guide thimbles in the top nozzle. In the extended position, the ends or tabs of the lock fingers  22  are configured to engage divots within the guide thimble where the guide thimble is swaged to the upper sleeve. The lock fingers  22  may be fabricated from a variety of materials, including stainless steel or Inconel. The lock finger ends or tabs may be in any configuration such that contact established between the lock fingers  22  and the dimples of guide thimble is sufficient to allow a vertical lift of the fuel assembly  20 . In an unextended position, the lock fingers  22  are configured to be inserted into the guide thimbles with sufficient clearance such that the lock fingers  22  do not contact the inside contact edges of the guide thimbles. The ends of the lock fingers  22  may be made of a material which is softer than the material of the guide thimbles, thereby preventing damage to the fuel assembly  20  in the extended position. 
         [0021]    The mandrel  14  is used to secure the lock fingers  22  into place in a fuel assembly  20  prior to lifting a fuel assembly  20 . The mandrels  14  also retract the lock fingers  22  to a smaller profile in the unengaged position for insertion into the guide thimbles. The mandrels  14  are individually spring loaded to their respective lock fingers  22  to allow independent locking and unlocking of the lock fingers  22 . The number of mandrels  14  and lock fingers  22  may be varied according to the number of connections desired to be established to the fuel assembly  20  to be lifted. In an exemplary embodiment illustrated, 20 lock finger/mandrels are used to lift the fuel assembly  20 . Other numbers of lock finger/mandrels may be used according to the configuration of the fuel assembly  20  and the desired factor of safety for lifting. 
         [0022]    The load plate  24  is configured to lift the weight of the fuel assembly  20  transferred through the lock fingers  22 . The lock fingers  22  are supported with a spring interface by the load plate  24  to allow for variation in fuel assembly growth and divot position. The load plate  24  is attached to the mast  34  of the tool  10  via support rods  32  at each corner of the tool. Alignment pins  28  attach the load plate  24  to provide accurate positioning onto the fuel assembly  20 . The alignment pins  28  may allow for fine adjustment of the tool  10  so that precise positioning occurs. 
         [0023]    The finger plate  26  is used to insert and retract the lock fingers  22  into the guide thimbles of the fuel assembly  20 . The finger plate  26  is a non-load bearing component and is connected to members  38  which connect to the mandrel plate  15  of the tool  10 . The finger plate  26  may have an attachment  36  which allows/assists in the insertion and retraction of the lock fingers  22  during actuation. 
         [0024]    The finger shaft  18  is configured to travel the full length of the full assembly nozzleless handling tool  10  to allow operator movement of the finger plate  26  via a screw drive  44 . As illustrated, the finger shaft  18  is a screw  40  which may be controlled in a precision manner. The screw  40  may be of such a configuration such that the threads of the screw  40  allow for accurate positioning. The screw  40  may also be configured such that any configuration of threads per unit length may be used. Other types of actuation are possible, such as hydraulic actuation, for example, and as such the exemplary embodiment illustrated should not be considered limiting. 
         [0025]    The mandrel plate  16  is used to allow insertion and retraction of the mandrels  14  into the lock fingers  22 . The mandrel plate  16  is used to provide a structural connection which may be actuated for movement through movement of the mandrel shaft  12 . The mandrel plate  16  is also configured to allow visual verification that the mandrels  14  are fully inserted through visual identification of position. 
         [0026]    The mandrel shaft  12  travels the full length of the tool  10  to allow operator movement of the mandrel plate  16  via rotation and elevation change. The mandrel shaft  12  may be configured to rotate as illustrated, or may be moved through hydraulic actuation. 
         [0027]    Operationally, control components for the reactor core and fuel assemblies are removed to provide access to the individual fuel assembly  20  to be moved. The fuel assembly nozzleless handling tool  10  is positioned over the fuel assembly  20  to be moved as illustrated in  FIG. 2 . The fuel assembly nozzleless handling tool  10  is lowered onto the fuel assembly  20  and alignment pins  28  engage the top nozzle  30  of the fuel assembly  20 . The finger shaft  18  is rotated by the tool operator to lower the lock fingers  22  into the guide thimbles of the fuel assembly  20 . The mandrel shaft  12  is then lowered by the operator to allow spring tension of the mandrels  14  to be applied to the lock fingers  22 . The finger shaft  18  is then rotated to perform a slow withdrawal of the lock fingers  22 . As the individual lock fingers  22  reach divots that they are to engage, the lock fingers  22  spring into place, and the mandrel  14  is therefore positioned inside the guide thimble of the fuel assembly  20 , locking a finger tab of the lock finger  22  into a divot of the guide thimble. The lock fingers  22  are withdrawn from the guide thimbles by actuation of the tool  10  until all of the mandrels  14  used are locked into position. Next, the mandrel shaft  12  is rotated to verify if all of the mandrels  14  are in the locked position and illustrated in  FIG. 3 . Finally, the fuel assembly nozzleless handling tool  10  is lifted by the auxiliary hoist of a fuel handling crane, as an example, and the assembly  20  is lifted and transferred to a new location. 
         [0028]    The fuel assembly nozzleless handling tool  10  may be configured of any material, such as stainless steel, that is corrosion resistant and provides sufficient strength to lift the fuel assembly  20  with a desired factor of safety. The handling tool  10  is configured to engage guide thimble dimples below the upper end grid of the full assembly  20 . Other dimple areas in the guide thimble may be used, however, and the illustrated embodiment is but one alternative configuration. Although illustrated as a configuration that will attach to a 17×17 pressurized water reactor fuel assembly, the fuel assembly nozzleless handling tool  10  could be reconfigured to fit any fuel assembly design that uses a swaged top nozzle sleeve. 
         [0029]    The fuel assembly nozzleless handling tool  10  provides an alternative to expensive and lengthy permanent fixes to damaged fuel assemblies. The handling tool  10  may be configured to be single failure proof wherein failure of a single component on the handling tool  10  will not result in the dropping of the assembly  20  during a load lift. 
         [0030]    In the foregoing specification, the invention has been described with reference to specific exemplary embodiments, thereof. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded as an illustrative rather than a restrictive sense.