Patent Number: 054811171
Section: summary

CROSS REFERENCE TO RELATED APPLICATION The inventions taught herein are related to a commonly assigned copending application Ser. No. 08/298,503 entitled "Expandable Top Nozzle and Device for Securing Same to a Nuclear Fuel Assembly" by DeMario et at. (Attorney Docket No. 58,227). BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a shipping container for a nuclear fuel assembly, and in particular, to such a container for nuclear fuel assemblies which have a plurality of fuel rods held in a hexagonal array by a plurality of grids spaced longitudinally along the fuel rods. The invention also relates to a hold-down device for securing the bottom nozzle of the nuclear fuel assembly. 2. Background of Information In the shipping and storage of nuclear reactor fuel elements and assemblies, which contain large quantities and/or enrichments of the fissile material, U.sup.235, it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments (i.e., weights and weight percent U.sup.235) for each fuel assembly design. In order for a new shipping container design to receive licensing, 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 which is incorporated herein by reference. These requirements define the maximum credible PG,3 accident (MCA) that the shipping container and its internal support structures must endure in order to maintain the subcriticality of the fuel assemblies therein. U.S. Pat. No. 4,780,268, which is assigned to the assignee of the present invention and which is incorporated herein by reference, 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 plural 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. The top nozzle of each of the conventional fuel assemblies is held, along the longitudinal axis thereof, by four longitudinally attached bolts at the four corners of the square top nozzle. The bottom nozzle of some of these conventional fuel assemblies has a chamfered end. These fuel assemblies are held, along the longitudinal axis thereof, by a bottom nozzle spacer which holds the chamfered end of the bottom nozzle. This 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 No. 5450, Docket 71-5450, U.S. Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of Nuclear Material Safety and Safeguards, Washington, D.C. 20555, which is incorporated herein by reference. In nuclear reactors of the type originally designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies. Each of these fuel assemblies includes a plurality of fuel rods held in an organized hexagonal array by a plurality of hexagonal grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimble robes. Subsequently, the Soviet-style fuel assemblies were redesigned by the assignee of the present invention in order to provide, for example, more reliable operation. The guide thimble tubes of the redesigned fuel assemblies extend above and below the ends of the fuel rods and are attached to top and bottom nozzles, respectively. Such fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles. These fuel assemblies may contain U.sup.235 concentrations of up to about 4.80 to 5.00 weight percent U.sup.235. Under normal manufacturing conditions, the dimensions of the fuel assemblies may vary. For example, the dimensions of the six sides of the hexagonal array may differ by about .+-.2.0 mm between individual fuel assemblies. The top nozzle of the fuel assembly includes a cylindrical outer barrel, a cylindrical inner barrel and a hub. The outer barrel forms a first end of the top nozzle at the top of the fuel assembly. The inner barrel, which has a diameter smaller than the outer barrel, is attached to the hub, which forms a second end of the top nozzle opposite from the first end. The outer barrel has a shoulder facing the second end. The inner barrel telescopes into the outer barrel. The hub interfaces the plurality of fuel rods at the second end. The relatively heavy (e.g., 70 pounds) top nozzle is susceptible to transportation induced damage to the guide thimble tubes. For example, during normal transportation, vibration in the top nozzle inner barrel may be detrimental to the guide thimble tubes. Because of the unique design of the fuel assembly, which allows movement of the outer barrel along the longitudinal axis of the fuel assembly with respect to the relatively smaller inner barrel, it is not feasible to position adjustable hardware for securing the top nozzle in order to provide the necessary supporting restraint of the fuel assembly during shipment thereof. The bottom nozzle includes a longitudinally extending recess formed by a hexagonal barrel, a spherical taper, and a cylindrical barrel which has a diameter smaller than the hexagonal barrel. The spherical taper forms a tapered bore within the longitudinally extending recess tapering toward the bottom end. The spherical taper, also, forms an internal shoulder between the hexagonal barrel and the bottom end. There is a need, therefore, for an improved shipping container for a nuclear fuel assembly having a double-barrelled top nozzle. There is also a need for an improved shipping container for a nuclear fuel assembly having a double-barrelled bottom nozzle. More particularly, there is a need for such a container for a nuclear fuel assembly having a hexagonal geometry. There is an even more particular need for such a container which accommodates for manufacturing tolerances in the hexagonal geometry. There is another more particular need for such a container for a nuclear fuel assembly including a top nozzle having an outer barrel and an inner barrel of smaller diameter which telescopes into the outer barrel. There is yet another more particular need for such a container for a nuclear fuel assembly including a bottom nozzle having a longitudinally extending recess formed by a hexagonal barrel, a spherical taper, and a cylindrical barrel having a diameter smaller than the hexagonal barrel. There is still another more particular need for such a shipping container for transporting high enrichment fuel assemblies. SUMMARY OF THE INVENTION These and other needs are satisfied by the invention which is directed to a shipping container for a nuclear fuel assembly. The fuel assembly includes an array of a plurality of fuel rods; and a top nozzle having a top end, an outer barrel, an inner barrel, and a shoulder between the barrels. The shipping container may include a support mechanism for supporting the top nozzle and the plurality of fuel rods, a housing for the support mechanism and the fuel assembly, and a top nozzle holder secured to the support mechanism for holding the top nozzle. The top nozzle holder may include a shoulder holder for holding the shoulder. The top nozzle holder may also include an end holder for enclosing and holding the top end. The end holder may further include a spacer member, a resilient spacer and a support member. The spacer member may be secured to the support mechanism. The resilient spacer may be attached to the support member which forms a surface supported by the spacer member for holding the top end of the top nozzle therein. The resilient spacer may separate the support member from the top end of the top nozzle. The top nozzle holder may further include a shoulder clamp for clamping the shoulder holder to the support mechanism. The shoulder holder may include a resilient split ring having a first gap for positioning around the inner barrel, and a resilient split support for encasing the resilient split ring. The resilient split support may have a bore running therethrough, a second gap, and a counter-bore which encases the resilient split ring therein adjacent the shoulder. The shoulder clamp may clamp the resilient split support thereby closing the first gap of the resilient split ring, closing the second gap of the resilient split support, and securing the inner barrel to the support mechanism. The nuclear fuel assembly may also include a bottom nozzle and a plurality of grids supporting the array. The shipping container may further include a support mechanism for supporting the top nozzle, the plurality of grids, and the bottom nozzle; a housing for housing the support mechanism and the nuclear fuel assembly; a top nozzle holder secured to the support mechanism for holding the top nozzle; a plurality of grid supports for supporting the array; a plurality of clamping mechanisms for clamping the array; a plurality of guide plates for guiding the nuclear fuel assembly between adjacent ones of the plurality of grid supports; and a bottom nozzle holder secured to the support mechanism for holding the bottom nozzle. The support mechanism may have a first surface for abutting the array and a second surface which is perpendicular to the first surface. Each of the plurality of clamping mechanisms may clamp a corresponding one of the plurality of grids to a corresponding one of the plurality of grid supports. Each of the plurality of grid supports may support a corresponding one of the plurality of grids on the second surface. The nuclear fuel assembly array may be a hexagonal array having six sides. The first surface of the support mechanism may abut a first side of the array. Each of the guide plates may have two surfaces for guiding a second side and a third side of the hexagonal array. Each of the grid supports may include a first support for supporting the second side of the array, a second support for supporting the third side of the array, a base plate for fixedly supporting the first and second supports thereto, a bearing pad for slidably supporting the base plate, and a limiter for limiting a sliding motion of the base plate on the bearing pad which is fixedly mounted to the second surface of the support mechanism. Alternatively, each of the guide plates may have a guide side for guiding the nuclear fuel assembly, and an absorbing side having a coating of gadolinium oxide. The bottom nozzle of the nuclear fuel assembly may include a longitudinally extending recess. The bottom nozzle holder may be secured to the support mechanism for holding the bottom nozzle and may include a recess holder for holding the bottom nozzle within the longitudinally extending recess. The recess holder may include a wedge mechanism for wedging against the bottom nozzle within the longitudinally extending recess and a moving mechanism for moving the wedge mechanism within the longitudinally extending recess. The bottom nozzle may further include a bottom end and a tapered bore or shoulder within the longitudinally extending recess tapering toward the bottom end. The recess holder may include a gripper mechanism for gripping the tapered bore or shoulder within the bottom nozzle and a moving or engaging mechanism for moving the gripper mechanism against the tapered bore or shoulder. The gripper mechanism may include a plurality of grippers for gripping the shoulder within the bottom nozzle. Each of the grippers may have a gripping end and a pivot end. The engaging mechanism may include a base for pivotally mounting the pivot end of each of the grippers and a moving mechanism for moving the gripping end of each of the grippers. The moving mechanism may include an operating mechanism for moving the., moving mechanism which engages each of the gripping ends in order to move the gripping ends toward the shoulder within the bottom nozzle. The operating mechanism may also disengage the moving mechanism in order to move the gripping ends away from the shoulder within the bottom nozzle. The base may be inserted adjacent the support mechanism and within the bottom end of the bottom nozzle. The bottom nozzle may include a hexagonal barrel, a spherical taper, and a cylindrical barrel having a diameter smaller than the hexagonal barrel. The spherical taper may interconnect the hexagonal barrel and the cylindrical barrel which forms the bottom end of the nuclear fuel assembly. The bottom nozzle holder may further include a spacer having a hole for inserting the cylindrical barrel therein and a tapered surface for abutting the spherical taper in order to space the bottom end of the nuclear fuel assembly from the support mechanism. The moving mechanism may include a cam mechanism having a plurality of cam surfaces for camming a corresponding one of the gripping ends of the plurality of grippers. Adjacent ones of the plurality of grippers may include a spring mechanism for forcing each of the adjacent grippers against a corresponding one of the plurality of cam surfaces. The nuclear fuel assembly may have a central longitudinal axis. Each of the support mechanism, the base and the moving mechanism may have a hole which is positioned on the central longitudinal axis. The support mechanism may have a surface and the hole of the moving mechanism may be threaded. The operating mechanism may include a screw mechanism for rotating the moving mechanism, a collar, and a spring biased between the moving mechanism and the collar in order to provide a pre-load force for the screw. The screw may have a head and a shaft. The head may abut the surface of the support mechanism. The shaft may have a non-threaded portion and a threaded portion. The non-threaded portion may be adjacent the head and may pass through the holes of the support mechanism and the base. The threaded portion may be adjacent the non-threaded portion and may be threaded through the threads of the hole of the moving mechanism. The collar may be fixedly attached to the threaded portion and separated from the moving mechanism. The moving mechanism may further include a first blocking mechanism for blocking rotation of the moving mechanism. The first blocking mechanism may include a plurality of blocking surfaces which are between adjacent ones of the plurality of cam surfaces. Each of the blocking surfaces may abut the corresponding one of the gripping ends of the grippers whenever the moving mechanism is fully disengaged. The moving mechanism may further include a second blocking mechanism for blocking rotation of the moving mechanism. The second blocking mechanism may include a plurality of blocking tabs. Each of the blocking tabs may be attached to a corresponding one of the cam surfaces in order that each one of the blocking tabs abuts the corresponding one of the gripping ends of the grippers whenever the moving mechanism is fully engaged. Alternatively, a bottom nozzle holder may be provided for use with a shipping container for a nuclear fuel assembly. The nuclear fuel assembly may include a plurality of fuel rods; and a bottom nozzle having a longitudinally extending recess, a bottom end, and a shoulder within the longitudinally extending recess. The bottom nozzle holder may include a gripper mechanism for gripping the shoulder within the bottom nozzle, and an engaging mechanism for engaging the gripper mechanism against the shoulder.