Abstract:
A method and device to repair nuclear fuel assembly structural defects in the top nozzle to guide thimble connection, wherein the device includes a repair sleeve comprising tendons configured to deflect to allow establishment of a structural connection between a projection on each tendon and a dimple area of a guide thimble.

Description:
FIELD OF THE INVENTION 
   The present invention relates to handling of fuel assemblies for nuclear reactors. More specifically, the present invention relates to a method and device for installing a top nozzle repair sleeve in a nuclear fuel assembly to assist in structural transfer of forces when the fuel assembly is lifted. 
   BACKGROUND INFORMATION 
   Present-day nuclear power reactors use fuel in the form of fuel assemblies. The fuel assemblies usually are comprised of a uranium dioxide matrix, generally in the shape of pellets, stacked end-to-end. The stacked pellets are clad on their respective exterior by a zirconium metal alloy which prevents direct contact between the uranium dioxide pellets and reactor coolant. The fuel rods formed by the pellets cladded by the zirconium metal alloy are placed in a structural skeleton in a side by side arrangement to allow a dense packing of nuclear material. At the top and bottom of the fuel assembly, respective top and bottom nozzles are positioned to allow coolant to be channeled through the fuel assembly to remove heat generated by the nuclear fuel. 
   Industry experience has discovered that fuel assemblies can degrade over time near the top nozzle section of the fuel assembly. The top nozzle is connected to an interior shaft, (i.e. a guide thimble), to establish a structural connection between the top nozzle and a remainder of the fuel assembly. Industry experience has identified that a connection established between the top nozzle and the guide thimble, often in the form of a guide thimble sleeve, can be prone to stress corrosion cracking through continued fuel assembly use. When the nuclear reactor is refueled and depleted fuel assemblies removed from or repositioned in the reactor core, each fuel assembly is lifted and/or removed from the reactor using a fuel handling or polar crane. Degradation of the structural support established between the top nozzle and the guide thimbles prevents movement of the degraded fuel assembly as the fuel assembly may not be structurally adequate to lift its own weight. During inspections of the fuel assembly prior to lifting, if it is determined that the fuel assembly exhibits stress corrosion cracking or some other defect in the guide thimble structural load path, either the fuel assembly is moved in a piece-wise fashion, a special lifting device is fabricated to lift the assembly, or an extensive and lengthy structural repair is performed to allow the fuel assembly to be lifted. Current repair alternatives to correct stress corrosion cracking often delay nuclear reactor restart, thereby increasing the economic costs for the reactor owner. 
   There is a need to provide a method and device to repair damaged fuel assemblies such that a damaged fuel assembly may be lifted in a safe condition. 
   There is a further need to provide a method and device to provide a permanent repair for fuel assemblies which have stress corrosion cracking in areas such as the top nozzle, the guide thimble sleeves and the guide thimble. 
   There is a further need to provide an economically efficient method and device for repair of degraded fuel assemblies such that the repair may be accomplished quickly and efficiently. There is a still further need to provide a device and method to repair damaged fuel assemblies that are radioactive. 
   There is a still further need to provide a device and method to repair damaged fuel assemblies that are radioactive. 
   SUMMARY 
   It is an objective of the present invention to provide a method and device to repair damaged nuclear fuel assemblies such that a damaged fuel assembly may be lifted safely. 
   It is also an objective of the present invention to provide a method and device to provide a permanent repair for fuel assemblies which exhibit structural defects, such as stress corrosion cracking, between the top nozzle, the guide thimble sleeves and the guide thimble. 
   It is furthermore an objective of the present invention to provide an economically efficient method and device for repair of degraded fuel assemblies such that the repair may be accomplished quickly and efficiently. 
   It is also an objective of the present invention to provide a device and method to repair damaged fuel assemblies that are radioactive. 
   These and other objectives of the present invention will be achieved as illustrated and described. The present invention provides a repair sleeve for a nuclear fuel assembly. The present invention provides a shaft with a first end, a second end and a diameter, the diameter configured to fit into a guide thimble opening of a top nozzle of the nuclear fuel assembly, wherein the diameter of the shaft is dimensioned such that an exterior of the shaft snugly fits into the guide thimble opening, wherein the shaft has at least two openings. The present invention also provides for at least two tendons extending through the openings, wherein the tendons are configured to deflect in an instance of a horizontal load on the tendon, the tendons having a dimple configured to be inserted into a dimple of a guide thimble sleeve, wherein the shaft is configured to internally accept a control component of the fuel assembly and wherein at least two tendons do not deflect under a load when a thimble insert assembly is installed. 
   The invention furthermore provides a method to repair a fuel assembly. The method comprises providing a repair sleeve, the repair sleeve having a shaft with a first end, a second end and a diameter, the diameter configured to fit into a guide thimble opening of a top nozzle of the fuel assembly. The method also provides that the diameter of the shaft is dimensioned such that an exterior of the shaft snugly fits into the guide thimble opening, wherein the shaft has at least two openings. The method also provides that at least two tendons extend through the openings, the tendons configured to deflect in an instance of a load on the tendon, the tendons having a dimple configured to be inserted into a dimple of a guide thimble sleeve. The method furthermore provides for inserting the repair sleeve in the guide thimble opening in the top nozzle of the nuclear fuel assembly such that the dimples of the tendons project into the dimples of the guide thimble sleeve, and inserting a control component into an interior of the repair sleeve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side cross sectional view of a repair sleeve in conformance with the present invention. 
       FIG. 2  is a side cross sectional view of a second embodiment of a repair sleeve. 
       FIG. 3  is a graph illustrating structural load carrying capacity of the repair sleeve. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a repair sleeve  10  is illustrated which is used to aid in structural load transfer in a fuel assembly. Structural defects arising from stress corrosion cracking, for example, limit the capability of operators to move fuel assemblies as these structural defects may negatively impact safe lifting of the fuel assembly. Structural defects are often found in a swaged area  18  which connects the top nozzle  12  to the guide thimble  22 . 
   The repair sleeve  10  has a shaft  14  which is configured to allow a structural load transfer of force from the body of the fuel assembly through the top nozzle  12  of the fuel assembly. The repair sleeve  10  is configured with a first end  24  and a second end  26 . The second end  26  is configured to be inserted into a nozzle opening  30  in the top nozzle  12  of the fuel assembly. The first end  24  may protrude from a top surface  38  of the top nozzle to allow for connection of lifting components as required. The first end  24  may be configured such that an installer may easily differentiate the first end  24  from the second end  26 . Differentiation may be through geometric variation, such as an end which flairs outwardly, through the incorporation of a hole, or marking the exterior of the first end  24 . Other configurations are possible and as such, the configuration presented in  FIG. 1  is but one exemplary embodiment. 
   The shaft  14  of the repair sleeve  10  may be made of stainless steel, for example, to allow for rigidity during a fuel assembly lift. Other materials, such as Inconel, stainless steel, and zirconium alloys, may also be used as well as materials which will eliminate or lessen potential galvanic reaction between fuel assembly structural materials and the repair sleeve  10 . The material chosen for the repair sleeve  10  shall not exhibit permanent plastic deformation of the material upon insertion of the repair sleeve  10  into the top nozzle  12 , associated guide thimble sleeve  32  and guide thimble  22 . 
   The shaft  14  may be configured with a plurality of sleeve openings  28 . The number of sleeve openings  28  may be varied for the shaft  14 . Tendons  20  may be positioned through the sleeve openings  28 . The tendon width  40  may be varied such that the overall physical dimensions of the tendons  20  may be altered. The tendons  20  may be configured with a dimple  16  which corresponds to openings in a dimple area  34  in the guide thimble sleeve  32 . The embodiment provided in  FIG. 1  illustrates a connection between a first dimple area  34  and the projection  16  of the tendon  20 . The repair sleeve  10 , however, may have an overall length chosen such that the projection  16  of the tendons  20  extend to a second or third dimple area further inside the fuel assembly guide thimble. The diameter of the sleeve  10 , except for the projection  16 , may be a constant value. 
   The length of the projection  16  which projects into the dimple area  34  may be configured to closely fit into the overall shape defined by the dimple area  34 . The projections  16  may be configured in a trapezoidal shape, a hemispherical shape or other appropriate geometry. The number of dimples  16  in contact with dimple areas  34  may also be varied such that more or less structural support is established. The number of projections  16  per tendon  20  may also be varied. Tendon length may be chosen such that the tendon  20  may extend to and between multiple dimple areas, providing additional structural support connection. 
   Referring to  FIG. 2 , a second embodiment of a repair sleeve  70  is illustrated. The repair sleeve  70  has a shaft  72 . The repair sleeve  70  has a first end  56  and a second end  58 . The second end  58  is configured to be inserted into a guide thimble opening of the top nozzle  52 . The first end  56  of the repair sleeve  70  may be configured with a lapped edge  54  which extends beyond an external top surface  74  of the top nozzle  52 . The lapped edge  54  may have an external diameter which is greater in circumference than the external diameter of the opening of the top nozzle  52  for the guide thimble  50 . Although illustrated as a circular lapped edge  54 , other configurations are possible, such as square, hexagonal, or octagonal for example. The lapped edge  54  may be finally configured while the repair sleeve  70  is installed in the top nozzle  52 . Alternatively, the lapped edge  54  may be preformed prior to installation of the repair sleeve  70 . 
   A projection  60  may be formed on a tendon  66  of the repair sleeve  70 . The length of the tendon  66  may be chosen such that the projection  60  is placed in a dimple area  76  formed from the swaged area  78  of the guide thimble  50  and the guide thimble sleeve  68 . The contact established between the projection  60  and the dimple area  76  may be configured to allow a transfer of a specified amount of force. Similar to the first embodiment, the repair sleeve  70  may be modified such that the overall length of the sleeve  70  may reach multiple dimple areas in the guide thimble  50 . The projection  60  may be formed in any geometric configuration such as a hemispherical, trapezoidal or other arrangement. 
   Referring to  FIG. 3 , a graph of the structural capacity of the repair sleeve  10  is illustrated. The vertical axis of the graph represents load carrying capacity of the repair sleeve  10 . The horizontal axis of the graph represents overall position of the repair sleeve. As illustrated, the repair sleeve provides for an increase in load with a corresponding increase in displacement. Load carrying capacity then decreases after a maximum load carrying capacity is reached. Load carrying capacity is related to the amount of penetration of the projections into the dimple area. Greater penetration of the projections into the dimple area allows greater load carrying capacity. 
   The present invention provides a structural support that engages existing features of fuel assemblies to secure the upper or top nozzle of the fuel assembly to internal guide thimbles. Multiple repair sleeves may be used to transfer only a partial load of the fuel assembly if required. 
   This connection is used during lifting of the fuel assembly to allow the individual fuel rods to be lifted in unison with a desired factor of safety. The shaft  14  is split into separate sections (the tendons  20 ) by the sleeve openings  28  in the shaft material in the location of the dimple area  34  to deflect into the dimple area ( 34 ). 
   Operationally, a repair sleeve  10  is provided. The repair sleeve  10  is inserted into a guide thimble opening in the top nozzle  12  of the nuclear fuel assembly such that the projections  16  of the tendons  20  project into the dimple area  34  of the guide thimble sleeve. The insertion may be performed through a robotic device, or remote delivery tooling to install the sleeve in an irradiated environment or through use of a crane. The tooling can be configured to deliver singular or multiple sleeves at a time and install the sleeves to the engaged position. The insertion of the sleeve  10  in the top nozzle  12  causes the tendons  20  of the sleeve  10  to flex inward toward a centerline of the sleeve  10 . The sleeve insertion is then continued until the dimples  16  of the sleeve  10  intersect a dimple area  34  of the swaged area  18 . The tendons  20  of the sleeve  10  then extend away from a centerline of the sleeve  10  to allow the projections  16  to project into the dimple area  34 . A thimble insert assembly  1  (control component, BPRA, WABA, plug) of the fuel assembly is then inserted into an interior of the repair sleeve  10 . The installation of the thimble insert assembly into the repair sleeve  10  prevents the dimples  16  from exiting the dimple area  34  through plastic deformation of the sleeve  10 . A structural connection is thereby established between the projections  16  and the dimple area  34 . The first end  24  may be configured to extend from the top nozzle  12  or may be manipulated such that a desired geometric configuration is established. 
   The repair sleeve  10  may be removed from the guide thimble  22  by removing the control component from the interior of the repair sleeve  10 . The first end  14  of the repair sleeve  10  may then be pulled such that the tendons  20  of the sleeve  10  deflect allowing the dimple  16  to be removed from the dimple area  34 . 
   The present invention provides several advantages to other methods and devices for lifting fuel assemblies. The installation of the repair sleeve can be accomplished in a quick and efficient manner limiting economic expense. Moreover, the repair sleeve can be installed from a remote location, thereby limiting radiation exposure for workers installing the sleeves. The installation of the repair sleeve is performed with tools that are not complex, thereby allowing ordinary skilled craftspeople to install the sleeves with minimal special training. The present invention has a minimum of moving parts, thereby limiting potential failure over the life of the repair sleeve. The present invention also uses materials which are known to be successful in harsh environments, thereby limiting further degradation of the fuel assembly over time or affecting nuclear components in the facility. The present invention furthermore can be adapted to achieve different configurations to allow for differing attachment configurations and structural capacities. The present invention furthermore provides a configuration that will allow inspection of the sleeve through visual examination in an installed condition. The repair sleeve will not negatively affect overall fuel assembly pressure drop due to its relatively small size. The repair sleeve small size additionally limits the ultimate amount of disposed radiation waste for the fuel assembly. Additional sleeves  10  may be installed in a fuel assembly to provide further support. 
   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 claims. The specification and drawings are accordingly to be regarded in an illustrative rather than a restrictive sense.