Abstract:
A method to load a nuclear fuel rod, comprising, providing nuclear fuel pellets in a fuel plate transfer unit; transferring the nuclear fuel pellets from the fuel plate transfer unit to a fuel pellet column through the use of a belt, indexing the nuclear fuel pellets in the fuel pellet column to a nuclear fuel pellet loading machine; and pushing the fuel pellet column into a fuel rod cladding.

Description:
FIELD OF THE INVENTION 
   The present invention relates to nuclear fuel assemblies. More specifically, the present invention provides a method and device to load a nuclear fuel rod for a nuclear fuel assembly. 
   BACKGROUND INFORMATION 
   Production for nuclear fuel assemblies requires significant care during fabrication. The fabrication steps taken for such fuel assemblies is often costly and complicated due to the amount of precautionary steps that are required. Nuclear fuel rods are designed with several different components, wherein each of the components having a specific design purpose. The fissionable component of each nuclear fuel rod is generally a uranium enriched ceramic material (a uranium oxide) that is shaped in the form of a pellet. Individual pellets are placed end to end to form a fuel column. The fuel column is then inserted into an elongated rod made of corrosion resistant metal, such as a zirconium alloy, called a fuel clad. The fuel column is protected from mechanical and chemical wear by the fuel clad. The fuel clad protects the fuel column during operation of the reactor as well as handling of the fuel assembly. As an additional precaution, springs and/or other devices are also included inside the volume encapsulated by the fuel clad to allow the uranium fuel elements to swell and shift within prescribed limits in the fuel clad. This allows the fuel column to withstand several different loading scenarios without detrimental effects to the fuel column. The completed fuel rods are then stored. Completed fuel rods are then placed in a parallel arrangement, called a fuel assembly, to prevent the fuel rods from contacting each other during use. 
   In current automated loading systems, nuclear fuel pellets are taken from a fuel pellet elevator and transferred by a conveyor in a tray to a segment make-up table. The fuel pellets are removed from the fuel pellet tray by a worker and placed on the table. The fuel pellets are placed in a parallel orientation and then compacted by a pusher device to form columns of uranium containing ceramic material. The pushing device is connected to a linear variable displacement transducer which is configured to provide an electrical output signal. The electrical output signal is then read by a computer and an overall length of the individual fuel element column is determined. A computer then compares an overall design specification for the fuel rod with the overall length determined from the output signal. If the difference between the expected design value of the nuclear fuel element column length and the measured value meets a predetermined threshold value, the fuel rod cladding is then loaded with the nuclear pellet column. If the overall length of the fuel pellet column is outside of the threshold value, the fuel pellets are then rejected from the segment make-up table. A top end cap is then welded on the existing open side of the fuel rod cladding thereby completing the nuclear fuel rod. 
   There is a need to provide an apparatus and method which will enable an operator to perform additional quality assurance checks of the nuclear fuel elements during the manufacturing process of a nuclear fuel rod. 
   There is also a need to provide a method and device to load nuclear fuel pellets into a nuclear fuel rod in a safe, economical and non-damaging manner. 
   There is a further need to provide a method and device which will load cylindrical fuel pellets into an open fuel rod clad, i.e. a fuel rod clad without a lower plug welded to the fuel clad. 
   There is a further need to provide a method and device which will allow cylindrical fuel pellets to be loaded into an open fuel rod clad to eliminate slow insertion speeds for pellet placement found in existing methods and systems. 
   SUMMARY 
   It is therefore an objective of the present invention to provide a method and device to load pellets into a nuclear fuel rod for a nuclear fuel assembly. 
   It is also an objective of the present invention to provide a method and device to load nuclear fuel pellets into a nuclear fuel rod in a safe, economical and non-damaging manner. 
   It is also an objective of the present invention to provide a method and device which will load cylindrical fuel pellets into an open fuel rod clad. 
   It is a further objective of the present invention to provide a method and device which will allow cylindrical fuel pellets to be loaded into an open fuel rod clad to eliminate slow insertion speeds for pellet placement found in existing methods and systems. 
   The objectives of the present invention are achieved as illustrated and described. The present invention provides a method to load a nuclear fuel rod, comprising the steps of providing nuclear fuel pellets in a fuel plate transfer unit, transferring the nuclear fuel pellets from the fuel plate transfer unit to a fuel pellet column through the use of a belt; indexing the nuclear fuel pellets in the fuel pellet column to a nuclear fuel pellet loading machine, and pushing the fuel pellet column into a fuel rod cladding. The indexing the nuclear fuel pellets in the fuel pellet column to a nuclear fuel pellet loading machine can be performed through a v-trough station. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a handling tray supporting fuel elements to be incorporated into a nuclear fuel rod. 
       FIG. 2  is top perspective view of a pushing blade system for offloading nuclear fuel elements into the fuel plate transfer unit. 
       FIG. 3  is a top view of a fuel plate transfer unit and associated drive unit. 
       FIG. 4  is a side view of the fuel plate transfer unit and associated drive unit. 
       FIG. 5  is a side perspective view of a transfer belt for unloading a fuel plate transfer unit. 
       FIG. 6  is a top view of a juxtaposition of a transfer belt and a fuel plate transfer unit for use in transferring fuel elements to a loading area. 
       FIG. 7  is a top view of a rod loading device. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a handling tray  10  is illustrated. The handling tray  10  provides a housing by which nuclear fuel elements, in the illustrated embodiment fuel pellets  12 , are transported for ultimate incorporation into open end nuclear fuel clad. The handling tray  10  is made of a hardened corrosion resistant material, such as stainless steel compatible with nuclear materials. 
   Referring to  FIG. 2 , a handling tray  10  enters an unloading area  38  where nuclear fuel pellets  12  are to be inserted into the fuel plate transfer unit  30 . A hydraulic actuator  22  provides a motive force which causes supports  20  to extend in direction  24 . The hydraulic actuator  22  is activated through the instruction of a computer  25 , which has sensors  27  that indicate the presence of a handling tray  10 . At a first end of the supports  20 , a pushing blade  18  is positioned at a height which contacts the fuel pellets  12  in the handling tray  10  along the individual rows  40 . The blade  18 , in the example embodiment illustrated, is a flat bottom device. In an alternate embodiment of the present invention, the blade  18  has a scalloped edge. The blade  18  and the associated supports  20  move in direction  24  to push the fuel pellets  12  off of the tray  10 . At the completion of the pushing of the fuel pellets  12  off of the tray  10 , the blade  18  is then lowered by the hydraulic actuator  22  such that the edge contacts the tray  10 . The actuator  22  may be any unit that provides movement of the supports  20  and the associated blade  18 . As such, the actuator  22  may be an electromechanical device, a geared device or other similar arrangement. The actuator  22  may also be configured with a failsafe design to limit the imposition of force upon the fuel elements during pushing. To this end, the actuator  22  may be configured with a trip circuit that disconnects actuation of the power to the actuator if force is measured by the actuator to be above a predefined amount. The blade  18  is then moved in a tray removal direction  26 . The impact of the blade  18  on the tray  10  removes the tray  10  from the conveyor belt  29 . The tray  10  may then be stacked for further usage at another time. The conveyor belt  29  may then be indexed by the computer  25  to provide another handling tray  10  into the loading area The process of unloading fuel elements may then be repeated as often as desired. In the illustrated embodiment provided, a handling tray may be loaded every approximately 15 seconds. 
   A new tray  10  may then be moved into place along the roller system such that the pushing blade  18  is in a renewed position to push additional fuel pellets into the fuel plate transfer unit  30 . 
     FIG. 3  is a top view of a fuel plate transfer unit  30 . The fuel plate transfer unit  30  accepts fuel pellets  12  pushed by the combination of the hydraulic actuator  22  and the pushing blade  18 . The fuel plate transfer unit  30  has fuel element rows  28  which correspond to the fuel elements placed upon the handling tray  10 . Once the individual fuel elements are loaded into the fuel plate transfer unit rows  28 , the individual fuel elements are then removed from the element rows  28  through indexing of the fuel plate transfer unit  30 . The indexing of the fuel elements from the bottom of the fuel plate transfer unit  30  occurs along the indexing direction  32 . The indexing occurs through the use of a motor  36  in conjunction with a rail  34 . A slot  41  placed below the fuel plate transfer unit  30  allows the individual fuel elements rows to fall from the side of the fuel plate transfer unit  30  into the slot  41 . The pellets are then transferred down the slot  41  through the use of a belt  42 . The number of rows in the fuel plate transfer unit  30  and the tray  10  may be augmented such that larger or smaller batches of fuel elements  12  may be processed through the pellet loading apparatus. The fuel plate transfer unit  30  is indexed along direction  32  by the motor  36  through a ball screw connection. The ball screw is driven through motors controlled by a computer  37 . Although illustrated as a chain driven motor arrangement, other methods of operation such as hydraulic movement of the fuel plate transfer unit  30  are possible. The length of the rail  34  is such that the fuel plate transfer unit  30  can index all rows of fuel pellets into the single slot  41 . 
   Referring to  FIG. 4 , a side elevational view of the fuel plate transfer unit  30  is illustrated. The fuel plate transfer unit  30  indexes along direction  32  such that individual fuel elements  46  fall from the underside of the fuel plate transfer unit  30 . The rotational belt  42  accepts fuel pellets  46  falling from the fuel plate transfer unit from the slot  41 . A fuel pellet block  48  protects the alternate side of the slot  41  such that the individual fuel elements  46  directly drop into the slot  41  and onto the rotational belt  42 . The individual fuel elements  46  are housed in openings  31  placed in the fuel plate transfer unit  30 . The openings  31  are sized to allow the fuel elements  46  to roll along an interfacing surface  33 , but also keep individual fuel pellets from interacting with other fuel pellets during the indexing. Both the surface of the openings  31  and the interfacing surface  33  are configured as smooth surfaces to limit damage to the fuel elements  46 . The width of the slot  41  is minimized to restrict the amount of movement of the fuel element  46  along the indexing direction  32 . The depth of the slot  41  is also chosen such that fuel elements which transfer to the belt surface do not interfere with the indexing of the fuel plate transfer unit  30  during further indexing operations. Additionally, the speed of the rotational belt is maintained at levels that do not cause the fuel element  46  which transfers to the belt surface to jump do to sudden impartation of force. Although illustrated as a rectangular slot configuration, the slot  41  may also have an hourglass design to limit the possibility of fuel elements from being ejected from the surface of the belt  42  and back to the fuel plate transfer unit  30 . The illustrated embodiment is intended for use on a level floor. If the pellet loading apparatus is to be used on a non-level surface, the belt  42  can be provided with protection for fuel element slide back by increasing the coefficient of friction between the fuel element and the belt. An example of this is placing a rubber coating on the belt  42 . The belt  42  can be configured to assess material weight placed on the belt  42 . Additionally, the belt  42  may have a belt speed sensor and a microprocessor-based integrator to continuously compute the rate of materials transferred along the entire length of the conveyor per unit time. The belt  42  surface can be maintained in a taught configuration through a screw take-up system  51  which maintains a desired spacing between the individual rollers  52 . Although the loads handled by the belt  42  are anticipated to be light, impact idlers may be added in the impact area of the slot  41  to minimize belt deflection during loading. To additionally keep the belt surface taught, carrying idlers  53  can be accommodated along the entire space between the rollers  52 . A take up weight  55  is also located along the belt  42  to keep the top surface of the belt  42  in a taught condition under increasing and decreasing speeds as well as changes in temperature and humidity. 
   Referring to  FIG. 5 , the rotational belt  42  is illustrated in more detail. The rotational belt  42  is an endless loop of material  50  which is driven by motor driven rollers  52 . The motor driven rollers  52  are controlled through a controller, such as a computer. The speed of the motor driven rollers  52  and the rotational belt  42  may be a constant speed or may be variable according to the needs of the process. The endless loop of material  50  is provided to safely transfer ceramic tile fuel elements along the length of the rotational belt  42  without damage. In the example embodiment illustrated, the rotational belt may be a PVC matrix belt, a rubber track belt, or other arrangement which provides a non-maring surface to contact the individual fuel elements. 
   Referring to  FIGS. 6 and 7 , individual fuel elements are stacked to make up the full fuel rod pellet column by the rotational belt  42  and are then loaded into a fuel element transfer apparatus  70 . The fuel element transfer apparatus  70  accepts the individual fuel elements in a line progression to form a fuel column. The fuel column is then transported from the fuel element transfer apparatus  70  into an open ended fuel clad  72  placed in the fuel element transfer apparatus  70 . A blade  74  operated by a motor  76  pushes the fuel column down the fuel element transfer apparatus  70  and into the fuel clad  72 . The blade  74  and the motor  76  are controlled by a computer  80  which has positional sensors  77  indicating the amount of fuel elements within the fuel element transfer apparatus  70 . Instructed by the computer  80 , the motor  76  is activated and the blade  74  transfers the fuel column into the open ended fuel clad  72 . The fuel element transfer apparatus  70  is constructed such that the fuel column can be pushed into the fuel clad  72  without damage or need for lifting of the fuel elements. The blade  74  travels along guides  82  thereby allowing the blade  74  to maintain a perpendicular orientation to the fuel column placed within the fuel element transfer apparatus  70 . 
   The method and apparatus of the present invention provide significant advantages over conventional methods and apparatus to load nuclear fuel pellets into fuel rod cladding. The present method and apparatus of the present invention allow an individual fuel rod to be loaded with nuclear fuel material with a typical cycle time of 30 seconds. Conventional cycle times for pellet loaders have a typical cycle time of approximately 45 seconds. 
   The method and apparatus of the present invention provide for loading of nuclear fuel rods such the elements making up the fuel column are not damaged by hard loading. Furthermore, the present invention reduces radiation exposure for workers constructing nuclear fuel rods due to the increased loading speed. Airborne contamination is reduced since open rod loading eliminates pressure buildup as the pellet column is inserted (gas escape over fuel column.) 
   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 in an illustrative rather than in a restrictive sense.