Abstract:
An unirradiated nuclear fuel assembly component transport system that includes a clamshell-type inner liner that opens either along its axial dimension or from the top to load and unload the fuel assembly being transported. The exterior dimensions of the liner conform to a generic overpack tubular container that protects the liner from impact loads and fires.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a shipping container for nuclear fuel components and, in particular, to such a container for unirradiated nuclear fuel assemblies and nuclear fuel rods. 
         [0003]    2. Related Art 
         [0004]    In the shipping and storage of unirradiated nuclear fuel elements and assemblies which contain large quantities and/or enrichments of fissile material, U 235 , it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, nuclear reactor fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments; i.e., weights and weight-percent U 235 , for each fuel assembly design. In order for a new shipping container design to receive licensing approval, it must be demonstrated to the satisfaction of the NRC that the new container design will meet the requirements of the NRC rules and regulations, including those defined in 10 CFR  71. These requirements define the maximum credible accident (MCA) that the shipping container and its internal support structures must endure in order to maintain the sub-criticality of the fuel assembly housed therein. 
         [0005]    U.S. Pat. No. 4,780,268, which is assigned to the assignee of the present invention, discloses a shipping container for transporting two conventional nuclear fuel assemblies having a square top nozzle, a square array of fuel rods and a square bottom nozzle. The container includes a support frame having a vertically extending section between the two fuel assemblies which sit side by side. Each fuel assembly is clamped to the support frame by clamping frames which each have two pressure pads. This entire assembly is connected to the container by a shock mounting frame and a plurality of shock mountings. Sealed within the vertical section are at least two neutron absorber elements. A layer of rubber cork cushioning material separates the support frame and the vertical section from the fuel assemblies. 
         [0006]    The top nozzle of each of the conventional fuel assemblies is held along the longitudinal axis thereof by jack posts with pressure pads that are tightened down to the square top nozzle at four places. The bottom nozzle of some of these conventional fuel assemblies has a chamfered end. These fuel assemblies are held along the longitudinal access thereof by a bottom nozzle spacer which holds the chamfered end of the bottom nozzle. 
         [0007]    These, and other shipping containers, e.g., RCC-4, for generally square cross-sectional geometry pressurized water reactor (PWR) fuel assemblies used by the assignee of the present invention, are described in Certificate of Compliance Number 5454, Docket Number 71-5450, U.S. Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of the Nuclear Material Safety and Safeguards, Washington, D.C. 20555. 
         [0008]    U.S. Pat. No. 5,490,186, assigned to the assignee of the present invention, describes a completely different nuclear fuel shipping container designed for hexagonal fuel, and more particularly, for a fuel assembly design for a Soviet-style VVER reactor. Still, other shipping container configurations are required for boiling water reactor fuel. 
         [0009]    There is a need, therefore, for an improved shipping container for a nuclear fuel assembly that can be employed interchangeably with a number of nuclear reactor fuel assembly designs. 
         [0010]    There is a further need for such a fuel assembly shipping container that can accommodate a single assembly in a lightweight, durable and licensable design. 
         [0011]    These and other needs have been partially resolved by U.S. Pat. No. 6,683,931, issued Jan. 27, 2004 and assigned to the assignee of the instant invention. The shipping container described in this latter patent includes an elongated inner tubular liner having an axial dimension at least as long as a fuel assembly. The liner is preferably split in half along its axial dimension so that it can be separated like a clamshell for placement of the two halves of the liner around the fuel assembly. The external circumference of the liner is designed to be closely received within the interior of an overpack formed from an elongated tubular container having an axial dimension at least as long as the liner. Preferably, the walls of the tubular container are constructed from relatively thin shells of stainless steel and the liner is coaxially positioned within the tubular container with close-cell polyurethane disposed in between. Desirably, the inner shell includes boron impregnated stainless steel. The tubular liner enclosing the fuel assembly is slidably mounted within the overpack and the overpack is sealed at each end with end caps. The overpack preferably includes circumferential ribs that extend around the circumference of the tubular container at spaced axial locations that enhance the circumferential rigidity of the overpack and form an attachment point for peripheral shock-absorbing members. An elongated frame, preferably of a birdcage design, is sized to receive the overpack within the external frame in spaced relationship with the frame. The frame is formed from axially spaced circumferential straps that are connected to circumferentially-spaced, axially-oriented support ribs that fixedly connect the straps to form the frame design. A plurality of shock absorbers are connected between certain of the straps and at least two of the circumferential ribs extending around the overpack, to isolate the tubular container from a substantial amount of any impact energy experienced by the frame, should the frame be impacted. 
         [0012]    Although the shipping container described in the aforementioned &#39;931 patent is a substantial improvement in that it can accommodate different fuel assembly designs through the use of complementary liners while employing the same overpack and birdcage frame, that improvement has been taken one step further by U.S. Pat. No. 6,748,042, assigned to the assignee of the instant invention. The &#39;042 patent describes a transport system that provides a liner and overpack system that will achieve the same objectives as the &#39;931 patent while further improving the protective characteristics of the transport system and the ease of loading and unloading the nuclear fuel components transported therein. The shipping container includes an elongated tubular container, shell or liner designed to receive and support a nuclear fuel product such as a fuel assembly therein. The interior of the tubular liner preferably conforms to the external envelope of the fuel assembly. The exterior of the tubular container has at least two substantially abutting flat walls which extend axially. In the preferred embodiment, the cross-section of the tubular member is rectangular or hexagonal to match the outer envelope of the fuel assembly and three of the corner seams are hinged so that removal of all the kingpins along a seam will enable two of the sidewalls to swing open and provide access to the interior of the tubular container. The tubular container or liner is designed to seat within an overpack for transport. The overpack is a tubular package having an axial dimension and cross-section larger than the tubular liner. The overpack is split into a plurality of circumferential sections (for example, two sections, a lower support section and an upper cover, or three sections, a lower support section and two upper cover sections) that are respectively hinged to either circumferential side of the lower support section and joined together when the overpack is closed. The lower support section includes an internal central V-shaped groove that extends substantially over the axial length of the overpack a distance at least equal to the axial length of the tubular liner. Shock mounts extend from both radial walls of the V-shaped groove to an elevation that will support the tubular liner in spaced relationship to the groove. The axial location, number, size and type of shock mount employed is changeable to accommodate different loadings. The tubular liner is seated on the shock mounts, preferably with a corner of the liner aligned above the bottom of the V-shaped groove. The top cover section (sections) of the overpack has a complementary inverted V-shaped channel that is sized to accommodate the remainder of the tubular liner with some nominal clearance approximately equal to the spacing between the lower corner of the tubular liner and the bottom of the V-shaped groove. The ends of the overpack are capped and the overpack sections are latched. 
         [0013]    Though the transport system of the &#39;402 patent provides a substantial improvement in the protective characteristics and ease of loading and unloading of the nuclear fuel components being transported, further improvement in the ease of loading and unloading the liner is desired. 
       SUMMARY OF THE INVENTION 
       [0014]    This invention provides an improved liner that facilitates the loading and unloading of nuclear components, especially components having hexagonal contour such as the VVER nuclear fuel assemblies. The liner comprises an elongated tubular container designed to receive and support the nuclear fuel product or components therein. An exterior of the tubular container has at least two substantially flat walls with at least one circumferential end of at least one of the walls having a hinged interface with a stationary wall of the container to provide access to the interior thereof. The hinged wall extends axially in the direction of one end of the container and terminates a pre-selected distance short of the corresponding end of the stationary wall. The stationary wall has a lateral groove on an interior surface thereof at an elevations starting substantially at the elevation of the one end of the hinged wall. An access cover is slidable in the groove in the stationary wall to close off the one end of the container so that the interior of the container may be accessed either through the one end by sliding out the access cover, or from the side by rotating the hinged wall. The elongated tubular container has the other end opposite the one end capped and sealed and is sized to fit within the overpack of the &#39;042 patent. 
         [0015]    Preferably, a mechanism is provided for locking the access cover in a closed position when the container is prepared for transport. Desirably, the locking mechanism is a pair of radially extending arms that pivot proximate one end on each of the radially extending arms that faces towards the center of the access cover. The pivot enables the radially extending arms to rotate from a position orthogonal to the axis of the elongated tubular container toward the axis. Each of the radially extending arms extends at a distal end into a slot in the stationary wall that extends axially to the one end of the stationary wall so that when the radially extending arms are rotated into a horizontal position and engage the slot in the stationary wall, the access cover cannot slide in the groove. In this preferred embodiment, the radially extending arms are laterally restrained in a slot in an outwardly projecting face of the access cover. Preferably, the outwardly-projecting face of the access cover is formed from a raised fork having two spaced prongs of a given width that form the walls of the slot in the outwardly-projecting face of the access cover. A hole is formed in the width of the wall of each prong that is aligned with a hole in the corresponding radially extending arm when the radially extending arm is rotated in the horizontal position to engage the slot in the outwardly-projecting face of the access cover. Thus, when a pin is inserted through the holes when the radially extending arm is in the horizontal position, the radially extending arm is locked in engagement with the slot in the stationary wall. 
         [0016]    Preferably, the liner has at least two hinged walls that interface at their non-hinged circumferential ends in a closed position. One of the non-hinged circumferential ends of the hinged wall has an axially extending tongue and the other of the non-hinged circumferential ends of the hinged wall has an axially extending groove that mates with the tongue when the two hinged walls are in the closed position. Preferably, the stationary and hinged walls of the liner are constructed from three extruded sections. 
         [0017]    In another embodiment, the access cover has an axially-extending lip extending in the direction of the hinged door. The lip of the access cover extends over an outer surface of the hinged door at the one end when the access cover is fully seated in the groove. Thus, when the access cover is fully seated to close off the one end of the tubular liner, it prevents the hinged door from rotating toward an open position. 
         [0018]    In still another embodiment, the access cover includes a hold-down plate supported on an underside of the cover. The hold-down plate is adjustable in the axial direction to bring pressure on the nuclear product being transport to secure the nuclear product against a bottom member of the elongated tubular liner. Preferably, in the withdrawn position, the hold-down plate is secured within a recess in the access cover. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    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: 
           [0020]      FIG. 1  is a side view of a nuclear fuel assembly having a top nozzle, a hexagonal array of fuel rods, and a bottom nozzle; 
           [0021]      FIG. 2  is a front view of the shipping container system of this invention, showing neutron moderated material lining the inner channel of the overpack; 
           [0022]      FIG. 3  is a front view of the shipping container system of this invention with thermal insulation lining the interior of the stainless steel shell and neutron-absorbing material lining the exterior of the tubular container surrounding a fuel assembly; 
           [0023]      FIG. 4  is a perspective view of the latch mechanism used to anchor the overpack segments together; 
           [0024]      FIG. 5  is a perspective view of the tubular container housing a nuclear fuel assembly with two sides of the tubular container swung open; 
           [0025]      FIG. 6  is a top view of the tubular container having two hinged walls with all six sidewalls closed; 
           [0026]      FIG. 7  is a perspective view of the top end of the tubular liner of this invention showing the access cover removed; 
           [0027]      FIG. 8  is a perspective view showing the underside of the access cover to the tubular liner; and 
           [0028]      FIG. 9  is a perspective view of the top end of the tubular liner of this invention showing the access cover seated in a closed position with the locking mechanism shown open. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    In the preferred embodiment, this invention provides a transport system for transporting nuclear fuel assemblies and particularly, nuclear fuel assemblies having a hexagonal profile such as those employed in the VVER nuclear reactors. An exemplary VVER 1000 nuclear fuel assembly  2  manufactured by Westinghouse Electric Company LLC, which is the assignee of the present invention, is shown in  FIG. 1 . The fuel assembly  2  includes a top nozzle  4 , a hexagonal array of a plurality of fuel rods  6  and a bottom nozzle  8 . The top nozzle  4 , the fuel rods  6  and the bottom nozzle  8  are positioned about a central longitudinal axis  9  of the fuel assembly  2 . The top nozzle  4  includes a cylindrical outer barrel  10  having a top end  11  and two lifting lugs  13  (only one is shown), a cylindrical inner barrel  12  which telescopes into the outer barrel  10 , and a shoulder  14  between the outer barrel  10  and the inner barrel  12 . The fuel rods  6  are held in the hexagonal array by a plurality of hexagonal grids  16  spaced longitudinally along the fuel rods  6 . The exemplary fuel assembly  2  includes 9 hexagonal grids  16 . Each of the grids  16  has six sides. 
         [0030]    The bottom nozzle  8  includes a longitudinally-extending recess  18  formed by a hexagonal barrel  20 , a spherical taper  22 , and a cylindrical barrel  24  which has a diameter smaller than the hexagonal barrel  20 . Disposed on the cylindrical barrel  24  are two alignment pins  25  (only one is shown). The spherical taper  22  interconnects the hexagonal barrel  20  and the cylindrical barrel  24  which forms a bottom end  26  of the fuel assembly  2 . The longitudinally-extending recess  18  tapers towards the bottom end  26  and also forms an internal shoulder between the hexagonal barrel  20  and the bottom end  26 . The fuel assembly  2  will be secured within a liner  28  which will be described hereafter with respect to  FIGS. 3 ,  5 ,  6 ,  7 ,  8  and  9 . The liner  28  will, in turn, be secured within an overpack  30  which is intended to protect the fuel assembly  2  from impacts and fires. The overpack  30  and the internal components of the nuclear fuel product containment and transport system of this invention is illustrated in  FIG. 2 . A tubular liner, sometimes referred to as container or shell  28 , constructed from a material such as aluminum, houses the nuclear fuel assembly  2 . The tubular liner  28  is suspended over a V-shaped groove  32  in the overpack  30  and supported on shock mounts  32  that are affixed in a recess  34  in an upper wall section of the groove  32  and spaced along the axial length of the lower overpack support section  36 . The shock mounts can be those identified by part number J-3424-21, which can be purchased from Lord Corporation, having offices in Cambridge Springs, Pa. Angle irons  24  can be used at the corners of the tubular liner  28  to spread the load on the liner walls. The number and resiliency of the shock mounts are chosen to match the weight of the liner, which depends upon the nuclear product being transported within the liner  28 . The orientation of the lower section  36  of the overpack  30  is fixed by the legs  40  so that the weight of the liner  28  holds the liner centered in the groove  32 . One capped end  42  of the overpack  30  forms part of the lower overpack support section  36 , while a second capped end  44  is formed as an integral part of the top cover  46 . The end  44  of the upper overpack segment  46  seals against the lip  48  in the lower support section  36 . Keys  50  on each side of the upper section  46  of the overpack  30  fit in complementary keyways in the lower overpack support section  36 , as can be better appreciated from the frontal view shown in  FIG. 3 . 
         [0031]      FIG. 3  shows a frontal view of the shipping container system  27  of this invention with the end plate  44  removed. Both the top segment  46  and the bottom segment  36  of the overpack  30  are formed from hollow stainless steel sheet  52 . For example, an 11 gauge stainless steel shell filled with polyurethane can be employed. Preferably, in this embodiment, the polyurethane has a minimum 3″ (7.62 cm) thickness. In the preferred embodiment, the hollow channel in the overpack  54  is shaped to substantially conform to the outer profile of the tubular liner  28  and the walls of the hollow channel  54  can be lined with a neutron-absorbing material, such as a half-inch (1.27 cm) of borosilicate. Alternately, the outer surface of the tubular liner  28  can be lined with a neutron-absorbable material, such as a ⅛″ (0.318 cm) thick layer of borosilicate, or a combination of neutron-absorbing material on the walls of the tubular liner  28  and the walls of the hollow channel  54  can be employed.  FIG. 3  provides a better view of the recess  34  that the shock mounts  32  are mounted in than can be derived from  FIG. 2 . Similarly, the keys  50  and keyways  56  that aid in positioning the top section  46  on the lower support section  36  of the overpack  30  are shown more clearly in  FIG. 3 . The top and bottom overpack sections  46  and  36 , respectively, are formed from a stainless steel shell  58  that is filled with polyurethane  60 . Thermal insulation  62  can be incorporated to line the interior of the stainless steel sheet overpack shell  52 . 
         [0032]    The top segment  46  of the overpack is latched to the bottom support segment  36  in the preferred embodiment using the latch assembly shown in  FIG. 4 . Both the lip  53  on the upper overpack section  46  and the lip  55  on the lower overpack section  36  include a plurality of axially-spaced slots. A latchbar  66  is affixed to either the upper lip  53  or the lower lip  55  in a manner to permit the clamp arm  64  to slide within a corresponding slot in the lip. For example, with the latchbar  66  coupled to the lower lip  55 , the clamp arm  64  would protrude through the corresponding slot in a downward direction and have a large protruding end to anchor the latchbar  66  to the lower lip  55 . The upper clamp arm  64  can have an L-shape, as shown in  FIG. 4 , so that when the lip  53  is seated over its corresponding clamp arm  64 , the latchbar  66  can be moved in a direction into the Figure to lock the upper section  46  to the lower section  36  of the overpack  30 . The clamp arm  64  can then be secured in that locked position and an external lever can be used to slide the latchbar  66  to an open and closed position with an approximate 4″ stroke desirable. To facilitate the locking and unlocking action, a low-friction coating can be applied to the sliding surfaces. 
         [0033]      FIG. 5  illustrates a perspective view of an open tubular liner  28  with a fuel assembly  2  positioned therein. As previously mentioned with respect to  FIG. 1 , the fuel assembly  2  is made up of a parallel spaced array of fuel elements  6  that are maintained in spaced relationship and in position by grid straps  16 , bottom nozzle  8  and a top nozzle which is not shown. The grid straps are constructed in an egg crate design to maintain the spacing between the fuel elements  6  that form flow channels for the reactor coolant to flow through during reactor operation. The fuel assembly  2  is seated on a neoprene or cork rubber bottom pad  72  which is affixed to the bottom  68  of the tubular liner  28 . The neoprene or cork rubber pad  72  supports and cushions the fuel assembly  2 . A similar arrangement is provided above the fuel assembly  2  by a neoprene or cork rubber hold down plate that is supported by a top access cover to the tubular liner  28  as will be more fully described with regard to  FIG. 8 . In this embodiment, the tubular container has four stationary sides,  74 ,  76 ,  78  and  80  (shown in  FIG. 6 ) which are affixed to the bottom  68  of the tubular liner  28 . The tubular liner  28  has two movable sides  70  and  71  which are hinged to the adjacent edges of the stationary sides  74  and  78  through hinges  82  that rotate around a kingpin  84 . The two movable sides are in turn connected, when latched, by similar hinges  82 , with the insertion of the kingpin in the hinge forming the latch. In this way, the movable sides  70  and  71  can be opened from any of the hinged seams to provide access to the interior of the tubular liner  26  from a number of different directions to facilitate loading and unloading in different environments that may present obstructions. For quick access, the hinges connecting a given side may be connected by a single kingpin that extends through the lower hinge and up through each of the individual hinges  82  extending up the hinged seam. The tubular liner  28  is preferably constructed out of aluminum of a thickness, for example, of 0.375″ (0.9525 cm). 
         [0034]    The interior walls of the sides  70 ,  71 ,  74 ,  76 ,  78  and  80  are covered with an iron ferrite composite sheet  86  and neoprene or cork rubber pads with magnetic backing  88  attached and affixed by the magnetic force at the grid elevations to seat the neoprene or cork rubber side of the pads against the outside straps of the grids  16 . The magnetic coupling on the pads make them adjustable to accommodate different nuclear fuel component designs. The neoprene or cork rubber pads are not as hard as the material that the grids are constructed of and secures the grids in position when the movable sides  70  and  71  are in the closed position, without damaging the grids, and cushions the fuel assembly  2  during transport. The inside of the tubular liner  28  can be used to transport other fuel components, such as fuel rods, separately by employing inserts within the tubular container  28  that will hold those components securely. Alternatively, clips on the backs of the neoprene or cork rubber pads can be supported in slots at multiple elevations on the interior walls of the sides  70 ,  71 ,  74 ,  76 ,  78  and  80 . Axial adjustment of the pads can be made by moving the pads from slot to slot.  FIG. 6  provides a better view of the iron ferrite composite sheet  86  and hinged locations.  FIG. 6  shows the bottom  68  of the tubular line  28  supported on the shock mounts  32  within the overpack  30 . From  FIG. 6 , it can be appreciated that one of the opening edges of the movable walls  70  and  71  has a groove that extends axially down its entire length while the other of the edges of the movable walls  70  and  71  has an axially extending tongue that mates with the groove when the movable walls  70  and  71  are in the closed position, as shown in  FIG. 6 . Though the preferred embodiment is shown with a hexagonal liner compatible with VVER 1000 fuel, it should be appreciated that the novel features of this invention can be applied equally as well to a square reactor fuel assembly such as those employed in Westinghouse Electric Company LLC designed reactors. This invention has particular benefit for handling hexagonal fuel because it provides additional choices for access to the interior of the liner for loading the hexagonal fuel which can present handling difficulties that are not encountered with square fuel configurations. 
         [0035]      FIG. 7  shows the top  90  of the liner  28  with an access cover  92  in the open position. With the access cover  92  removed from the top of the tubular liner  28 , as shown in  FIG. 7 , the fuel assembly  2  may be loaded into the liner from the top of the liner as an alternative to being loaded from the side through the movable sides  70  and  71 . To close the liner  28 , the access cover  92  slides within a circumferential groove  94  in the stationary walls  74 ,  76 ,  78  and  80 . The access cover  92 , on its upper surface  103 , has diametrically opposed raised forks  104  that are connected by a central hub  112 . The tines  114  of the forks  104  define a groove  113  within which radially extending arms  98  are laterally restrained and pivot about pivot points  96 . When the access cover  92  is in the closed position seated within the grooves  94 , the radially extending arms  98  can be rotated about the pivots  96  to the horizontal position in which they engage the slots  108  in the upper end  90  of the stationary walls  74  and  80 , thus locking the access cover  92  in the closed position. A retaining pin or lock can then be inserted through aligned holes  100  in the fork tines and  102  in the radially extending arms  98  to restrain the radially extending arms in the locked position. A downwardly projecting lip  110  on the access cover  92  seats up against the outer upper surface of the movable sides  70  and  71  to lock the movable sides in the closed position when the access cover  92  is in place fully seated in the groove  94 . 
         [0036]      FIG. 8  shows another perspective view of the upper portion of the liner  28  with the access cover  92  in an open position showing the underside of the access cover. The underside of the access cover has a recess  116  in which the hold down plate  118  can be withdrawn as the access cover  92  is inserted into the annular groove  94  to close off the top of the tubular liner  28 . A hole in the top of the access cover  106  (shown in  FIG. 7 ) provides access to an adjustment screw that adjust the axial elevation of the hold down plate  118  so that it brings pressure against the top nozzle  4  of the fuel assembly  2  to restrain the fuel assembly in a secure position within the tubular liner  28 . 
         [0037]      FIG. 9  shows the access cover  92  in the fully seated closed position locking the movable sides  70  and  71  in the closed position. 
         [0038]    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.