Abstract:
A fuel rod or control rod for a nuclear reactor that has a spacer interposed between an upper end plug and a plenum spring which extends between the spacer and the fissile or absorber material. Preferably, the spacer is a relatively thin sleeve with a radially extending lip that sits above the coil spring wound at least in part around the sleeve.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention pertains generally to a pressurized water nuclear reactor fuel assembly and, more particularly, to hermetically sealed rods housing a reactive material that are employed with such a fuel assembly. 
         [0003]    2. Description of Related Art 
         [0004]    The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from and in heat exchange relationship with a secondary circuit for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. 
         [0005]    For the purpose of illustration,  FIG. 1  shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  10  having a closure head  12  enclosing a nuclear core  14 . A liquid reactor coolant, such as water, or borated water, is pumped into the vessel  10  by pump  16  through the core  14  where heat energy is absorbed and is discharged to a heat exchanger  18 , typically referred to, as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump  16 , completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . 
         [0006]    An exemplary reactor design is shown in more detail in  FIG. 2 . In addition to the core  14  comprised of a plurality of parallel, vertical, co-extending fuel assemblies  22 , for purpose of this description, the other vessel internal structures can be divided into the lower internals  24  and the upper internals  26 . In conventional designs, the lower internals&#39; function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assembly  22  (only two of which are shown for simplicity in  FIG. 2 ), and support and guide instrumentation and components, such as control rods  28 . In the exemplary reactor shown in  FIG. 2 , coolant enters the reactor vessel through one or more inlet nozzles  30 , flows down through an annulus between the reactor vessel and the core barrel, is turned 180° in a lower plenum  34 , passes upwardly to a lower support plate  37  and lower core plate  36  upon which the fuel assemblies are seated and through and about the fuel assemblies  22 . In some designs, the lower support plate  37  and the lower core plate  36  are replaced by a single structure, a lower core support plate having the same elevation as  37 . The coolant flow through the core and surrounding area  38  is typically large on the order of 400,000 gallons per minute at a velocity of approximately  20  feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate  40 . Coolant exiting the core  14  flows along the underside of the upper core plate  40  and upwardly through a plurality of perforations  42 . The coolant then flows upwardly and radially to one or more outlet nozzles  44 . 
         [0007]    The upper internals  26  can be supported from the vessel or the vessel head and include an upper support assembly  46 . Loads are transmitted between the upper support assembly  46  and the upper core plate  40 , primarily by a plurality of support columns  48 . A support column is aligned above a selected fuel assembly  22  and perforations  42  in the upper core plate  40 . 
         [0008]    Rectilinearly moveable control rods  28 , which typically include a drive shaft  50  and a spider assembly  52  of neutron poison rods, are guided through the upper internals  26  and into aligned fuel assemblies  22  by control rod guide tubes  54 . The guide tubes are fixedly joined to the upper support assembly  46  and the top of the upper core plate  40 . The support column  48  arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability. 
         [0009]      FIG. 3  is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character  22 . The fuel assembly  22  is the type used in a pressurized water reactor and has a structural skeleton which, at its lower end, includes a bottom nozzle  58 . The bottom nozzle  58  supports the fuel assembly  22  on the lower core plate  36  in the core region of the nuclear reactor. In addition to the bottom nozzle  58 , the structural skeleton of the fuel assembly  22  also includes a top nozzle  62  at its upper end and number of guide tubes or thimbles  84  which align with the guide tubes  54  in the upper internals. The guide tubes or thimbles  84  extend longitudinally between the bottom and top nozzles  58  and  62  and at opposite ends are rigidly attached thereto. 
         [0010]    The fuel assembly  22  further includes a plurality of transverse grids  64  axially spaced along and mounted to the guide thimbles  84  and an organized array of elongated fuel rods  66  transversely spaced and supported by the grids  64 . Also, the fuel assembly  22 , as shown in  FIG. 3 , has an instrumentation tube  68  located in the center thereof that extends between and is captured by the bottom and top nozzles  58  and  62 . With such an arrangement of parts, fuel assembly  22  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0011]    As mentioned above, the fuel rods  66  in the array thereof in the assembly  22  are held in spaced relationship with one another by the grids  64  spaced along the fuel assembly length. Each fuel rod  66  includes a plurality of nuclear fuel pellets  70  and is closed at its opposite ends by upper and lower end plugs  72  and  74 . The pellets  70  are maintained in a stack by a plenum spring  76  disposed between the upper end plug  72  and the top of the pellet stack. The fuel pellets  70 , composed of fissile material, are responsible for creating the reactive power of the nuclear reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system. 
         [0012]    To control the fission process, a number of control rods  78  are reciprocably moveable in the guide thimbles  84  located at predetermined positions in the fuel assembly  22 . Specifically, a rod cluster control mechanism  80 , positioned above the top nozzle  62  in selected fuel assemblies, supports a plurality of control rods  78 . The control mechanism has an internally threaded cylindrical hub member  82  with a plurality of radially extending flukes or arms  52  that form the spider previously noted with regard to  FIG. 2 . Each arm  52  is interconnected to a control rod  78  such that the control rod mechanism  80  is operable to move the control rods vertically in the guide thimbles  84  to thereby control the fission process in the fuel assembly  22 , under the motive power of a control rod guide shaft  50  which is coupled to the control rod hub  80  all in a well known manner. Like the fuel rod  66 , the control rods  78  are formed from an elongated, hollow, tubular cladding that is capped at either end by end plugs that are welded to the cladding. However, instead of fissile material, the reactive elements in a control rod is a neutron absorbing material such as silver-indium-cadmium, which occupies the lower interior region as in the case of fuel rods. During the manufacture of these rods some melting of the springs has been experienced during the tungsten inert gas girth welding process that seals the end plugs to the cladding. Spring melting with the hot end plug and formation of a eutectic can cause rod failures which can release fission by-products into the coolant which would further contaminate the coolant. 
         [0013]    Accordingly, a new rod design is desired that will overcome this manufacturing difficulty. 
       SUMMARY 
       [0014]    The foregoing object is achieved employing a new rod design formed from an elongated tubular cladding having a hollow interior with an axial dimension. A lower end plug closes off one end of the tubular cladding and an upper end plug closes off another end of the tubular cladding. A column of reactive elements occupies a lower portion of the hollow interior of the tubular cladding above the lower end plug and the upper portion of the hollow interior of the tubular cladding forms a gas plenum. A spring extends substantially between a top of the column of reactive elements and the upper end plug and a spacer is situated between the spring and the upper end plug. Preferably, an upper end of the spring that rests against a lower portion of a surface of the spacer is beveled and the thickness of the spacer above the lower portion of the surface of the spacer is greater than the thickness of the beveled upper end of the spring. In one embodiment, the spacer is a sleeve that is encircled by at least a portion of the spring with the sleeve having a radially outwardly extending lip just above an upper end of the spring. Desirably, the lip is approximately between 0.015 inch (0.038 cm) and 0.030 inch (0.076 cm) thick. In still another embodiment, a lower portion of the sleeve is bulged out against the spring and preferably the spring is a coil spring having approximately four closed coils encircling the sleeve. In one embodiment, the rod is a nuclear fuel element and the reactive element is a fissile material. In another embodiment, the rod is a nuclear control rod and the reactive element is a neutron absorbing material. In each case, the spacer insulates the spring against at least a portion of heat applied to the upper end plug. Preferably, the spacer is constructed from a metal selected from the group of materials consisting of zirconium, zirconium alloys, titanium alloys, niobium alloys and chromium alloys and more preferably from one or more of the latter three alloys. The invention also contemplates a nuclear fuel assembly having such a rod. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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: 
           [0016]      FIG. 1  is a simplified schematic of a nuclear reactor system to which the embodiments described hereafter can be applied; 
           [0017]      FIG. 2  is an elevation view, partially in section, of a nuclear reactor vessel and internal components to which the embodiments described hereafter can be applied; 
           [0018]      FIG. 3  is an elevation view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
           [0019]      FIG. 4  is a schematic view of a coil spring and spacer that can be employed in the gas plenum of a fuel rod or control rod of the embodiments described hereafter; and 
           [0020]      FIG. 5  is a schematic view of the spacer shown in  FIG. 4  with a bulging tool inserted within a central opening of the spacer. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The inventions claimed hereafter provide a new core component for a nuclear reactor and, more particularly, an improved fuel rod or control rod. As previously mentioned with respect to  FIG. 3 , the fuel rods  66  are basically formed from an elongated hollow tubular cladding  60  that is enclosed at its upper and lower ends respectively by upper end plug  72  and lower end plug  74 . The lower portion of the interior of the cladding has a number of fuel pellets  70  which are stacked in tandem and open up to a void region below the upper end plug  72 . The void region forms a gas plenum that has a plenum spring  76  extending between the upper plug  72  and the top of the stack of pellets  70 . The plenum  56  serves to contain fission gases which are generated during irradiation of the fuel pellets. The construction of a control rod  78  is much the same as that just described for the fuel rod except that the fissile material, i.e., the fuel pellets  70  are exchanged for a neutron absorbing material such as silver-indium-cadmium. 
         [0022]    The fuel rod cladding and end plugs are constructed from zirconium alloys and the plenum spring is typically made from 300 stainless steel. The end plugs are sealed to the cladding with a tungsten inert gas girth weld. Experience has shown that if the welding parameters are not set to a low enough temperature in welding the upper end plug some melting of the end of the spring that rests against the upper end plug will occur. If the spring starts to melt it can form a zirconium/iron and zirconium/nickel eutectics that have lower melting points and an affinity for hydrogen. The presence of eutectic and hydriding can adversely affect clad integrity which has to be avoided so that the fission gases are not released into the surrounding environment. 
         [0023]    In accordance with one embodiment of the inventions claimed hereafter an upper surface  86  of the plenum spring  76  is feathered or beveled to spread the contact area on the underside of a radially extending lip  90  of a spacer sleeve  88  around which an upper portion  92  of the plenum spring is closely wound. By being closely wound it&#39;s meant that the spacing between the coils of the spring around the spacer sleeve  88  is more closely packed than the spacing of the spring coils below the spacer sleeve  88 . The spacer sleeve  88  is preferably constructed from a material preferably from the group of metals comprising zirconium, zirconium alloys, titanium alloys, niobium alloys and chromium alloys and most preferably from one or more of the latter three alloys. The spacer sleeve  88  is also preferably thin walled, e.g., between 0.015 inch (0.038 cm) and 0.030 inch (0.076 cm) thick and more preferably between 0.015 inch (0.038 cm) and 0.020 inch (0.051 cm) thick. The sleeve is thin walled to maximize the volume available for the collection of fission by-products and makes it easier to bulge the spacer sleeve into the coils of the plenum spring to secure the coils in close proximity on the spacer sleeve  88 . The spacer sleeve  88  presents a heat transfer barrier and reduces the risk of spring melting with the hot end plug and formation of eutectic which can give rise to fuel rod failures. The radially extending lip  90  is inserted between the top of the spring and upper end plug  72  and prevents contact between the top end plug and the spring and the tubular portion of the spacer sleeve helps to dissipate the heat transmitted by the end plug. The lip  90  is thicker than the spring coil feathered end and will resist melting better than the spring, in that the feathered end of the coil tapers down to between approximately 0.002 inches (0.005 cm) and 0.005 inches (0.013 cm) and melts very easily. Without the spacer sleeve insert  88 , the weld heat input and temperatures have to be limited which reduces the weld parameter range which does not allow the weld process to be as robust as otherwise possible. If the spacer sleeve  88  is made from a zirconium alloy, some protection against eutectic formation is provided. If the spacer sleeve is made of a titanium alloy, niobium alloy or chromium alloy, no eutectic formation should occur even at high weld heat. 
         [0024]      FIG. 5  shows a bulge tool inserted within the spacer sleeve insert  88 . The tool  94  can be spread at its lower end by pulling up on a wedge extending axially through the tool and spreading the leaves  96 . When the lower end of the spacer sleeve insert  88  is bulged out into the spring, it anchors the sleeve to the spring and captures the spring coils above the bulge. 
         [0025]    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.