Abstract:
A nuclear fuel rod plenum spring assembly that has a spacer affixed to the lower end of the ground torsion spring. The spacer has a substantially flat surface on its underside that presses against the upper surface of the upper fuel pellets to spread the load of the spring over the top surface of the upper most fuel pellet.

Description:
BACKGROUND 
     1. Technical Field 
     This invention pertains generally to a nuclear reactor internals structure and more particularly to components such as fuel rods that employ an active ingredient within a cladding that is held in position by a plenum spring. 
     2. Related Art 
     The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprise a closed circuit which is isolated and in heat exchange relationship with a secondary side 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. 
     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, 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 the 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, the plurality of the above described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . 
     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 purposes 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 assemblies  22  (only two of which are shown for simplicity in this figure), 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  10  through one or more inlet nozzles  30 , flows down through an annulus between the vessel and the core barrel  32 , is turned 180° in a lower plenum  34 , passes upwardly through a lower support plate  37  and a lower core plate  36  upon which the fuel assemblies  22  are seated and through and about the assemblies. In some designs, the lower support plate  37  and the lower core plate  36  are replaced by a single structure, the lower core support plate, at 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 for a four loop plant (generally, the flow rate is approximately 100,000 gallons per minute per loop), 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 . 
     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 . 
     The rectilinearly moveable control rods  28  typically include a drive shaft  50  and a spider assembly  52  of neutron poison rods that 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 at one end to the upper support assembly  46  and connected at the other end to the top of the upper core plate  40  by a split pin force fit into the top of the upper core plate  40 . The pin configuration provides for ease of guide tube assembly and replacement if ever necessary and assures that the core loads, particularly under seismic or other high loading accident conditions are taken primarily by the support columns  48  and not the guide tubes  54 . This support column arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability. 
       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 a lower core support 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 a number of guide thimbles  54 , which extend longitudinally between the bottom and top nozzles  58  and  62  and at opposite ends are rigidly attached thereto. 
     The fuel assembly  22  further includes a plurality of transverse grids  64  axially spaced along and mounted to the guide thimbles  54  (also referred to as guide tubes) and an organized, array of elongated fuel rods  66  transversely spaced and supported by the grids  64 . Although it cannot be seen in  FIG. 3 , the grids  64  are conventionally formed from orthogonal straps that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells through which the fuel rods  66  are supported in transversely spaced relationship with each other. In many conventional designs, springs and dimples are stamped into the opposing walls of the straps that form the support cells. The springs and dimples extend radially into the support cells and capture the fuel rods there between; exerting pressure on the fuel rod cladding to hold the rods in position. Also, the assembly  22  has an instrumentation tube  68  located in the center thereof that extends between and is mounted to 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. 
     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 reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission byproducts from entering the coolant and further contaminating the reactor system. 
     To control the fission process, a number of control rods  78  are reciprocally moveable in the guide thimbles  54  located at predetermined positions in the fuel assembly  22 . Specifically, a rod cluster control mechanism  80 , positioned above the top nozzle  62 , supports the control rods  78 . The control mechanism has an internally threaded cylindrical hub member  82  with a plurality of radially extending flukes or arms  52 . Each arm  52  is interconnected to the control rods  78  such that the control rod mechanism  80  is operable to move the control rods vertically in the guide thimbles  54  to thereby control the fission process in the fuel assembly  22 , under the motive power of control rod drive shafts  50  which are coupled to the control rod hubs  80 , all in a well known manner. 
     As previously mentioned, the fuel assemblies are subject to hydraulic forces that exceed the weight of the fuel rods and thereby exert significant forces on the fuel rods and the fuel assemblies. In addition, there is significant turbulence in the coolant in the core caused by mixing vanes on the upper surfaces of the straps of many grids, which promote the transfer of heat from the fuel rod cladding to the coolant. The substantial flow forces and turbulence can result in severe vibration of the fuel rod cladding if motion of the fuel rods is not restrained. 
     Recently, a concern has been expressed about small pellet chips found in the fuel rod plenum in a fraction of fuel rods following back fill and sealing during manufacture. An investigation suggests that one mechanism responsible for top pellet chipping is non-uniform pressure distribution on the top surface of the fuel pellets. It was concluded that the end coil of the plenum spring does not make perfect contact with the top pellet. This leads to some part of the top pellet surface experiencing a significant axial load which could cause chipping. This affect was confirmed during pressure tests. It should be noted that the plenum spring design cannot provide uniform pressure distribution on the top surface of the pellet that it interfaces with due to limited contact area corresponding to the end coil spring geometry. A better view of the plenum spring  76  can be had by reference to  FIG. 4  which clearly shows the end coil geometry  84 . 
     Accordingly, an improved means of holding down the fuel pellets within the fuel element cladding is desired that will provide uniform pressure on the upper surface of the top pellet. 
     Furthermore, such an improved design is desired that will facilitate installation, limit consequences of unlikely installation mistakes and minimize potential performance issues. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by an improved elongated reactive member, such as a fuel element or control rod, for use in a nuclear core. The reactive member is formed from a tubular cladding substantially extending the elongated length of the reactive member with a top end plug sealing off a top end of a central hollow cavity of the tubular cladding and a bottom end plug sealing off a bottom end of the central hollow cavity of the tubular cladding. An active element substantially occupies a lower portion of the central hollow cavity and a spring substantially extends between the top end plug and an upper surface of the active element, pressuring the active element toward the lower end plug. A spacer is positioned between a lower end of the spring and the upper surface of the active element, spreading the force of the spring over a larger portion of the upper surface of the active element than would be applied by the spring directly. 
     In one embodiment, the spring is a ground torsion spring and preferably the spring is either mechanically or metallurgically attached to the spacer. Preferably, the spacer has a substantially flat head facing the upper surface of the active element and an opposite side that extends in an axial direction of the elongated dimension of the cladding with the opposite side being attached to the spring. Desirably, a distal portion of the opposite side has a width that is smaller than the width of the head and a fillet is formed between the width of the head and the width of the distal portion. 
     In another embodiment, an opening extends through the head from a side facing the upper surface of the active element through the spacer and out the distal end of the opposite side. Preferably, at least a portion of the opening has a hex contour. 
     In one embodiment where the spring is mechanically attached to the spacer, a spiral thread extends axially along a radial surface of the opposite end of the spacer and a lower portion of the spring is wound around the spiral thread. In another embodiment, where the spring is mechanically attached to the spacer, an upper portion of the opposite side of the spacer is a split tube with an outwardly, radially extending lip that mechanically attaches to the spring. 
     In one embodiment the reactive member is a nuclear fuel element and in still another embodiment, the reactive member is a nuclear control rod. 
     Preferably, the spacer is substantially round and spaced from the inner wall of the cladding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
         FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internal components to which this invention can be applied; 
         FIG. 3  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
         FIG. 4  is a perspective view of a plenum spring illustrating the end coil geometry; 
         FIG. 5  is a perspective view of a threaded spacer of one embodiment of this invention; 
         FIG. 6  is a sectional view of the embodiment shown in  FIG. 5 ; 
         FIG. 7  is a perspective view of a two-slot split tube embodiment of the spacer of this invention; 
         FIG. 8  is a perspective view of a four-slot split tube embodiment of the spacer of this invention; and 
         FIG. 9  is a side view of a welded washer embodiment of the spacer-spring assembly of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To achieve the foregoing objectives, this invention introduces an intermediate part between the plenum spring and top pellet of the fuel pellet stack to create a uniform contact distribution and reduce chip migration potential between the fuel rod plenum and the pellet stack. The new intermediate element between the top pellet and the spring end coil is designed to provide a uniform pressure distribution and reduce chip migration potential. Desirably, this element is attached to the existing plenum spring. In one embodiment, illustrated in  FIGS. 5 and 6 , the intermediate element is a threaded spacer  86  that forms an interface between the plenum spring and the top surface of the top pellet. The spacer  86  is designed to provide a uniform contact pressure over its substantially flat head  88 , on the top surface of the top pellet and reduce the potential for small pellet chip migration. The threaded spacer also has features to facilitate the spacer to spring assembly process. The threaded spacer  86  has a central hole  90  to facilitate proper fuel rod pressurization in an unlikely event where the plenum spring assembly is incorrectly installed and to prevent any related performance issues. If the fuel rod is not properly pressurized with He it could experience a reduction in diameter due to the high system external operating pressure that is not compensated by the proper fuel rod internal pressure, which could reduce the holding forces applied by the fuel assembly grid springs. Also, improper pressurization can lead to increased fuel element operating temperatures, due to lower thermal conductivity between the pellets and the cladding, possibly resulting in excessive clad corrosion and potential fuel melting. Any of these performance issues can lead to fuel rod failure resulting in an undesirable fission product release into the coolant. The central hole  90  has a hex contour  92  at its opening in the flat head  88  to facilitate coupling the rear tubular portion of the spacer  86  to the spring  76 . The rear tubular portion has a spiral thread  94  that extends from the opposite end  96  just short of the rear side of the head  88 . A hexed tool can be inserted in the hex opening  92  to wind the spacer  86  onto the plenum spring  76  until the end coil seats snuggly on the back of the head  88 . 
     Desirably, the threaded spacer  86  is one machined piece that basically comprises two functional regions in the fuel element axial direction: a pellet/clad interface region  88  and a spring interface region  98 . Preferably, the total length of the spacer  86  should prevent rotation of the spacer inside of the cladding. The pellet/clad interface  88  maximum diameter of the spacer should be less than the pellet minimum outside diameter under all conditions to ensure that the spacer does not compromise clad structural integrity. Preferably, the pellet/clad interface  88  maximum length, i.e., the dimension in the fuel element axial direction, should be as minimal as practically possible. The pellet/clad interface length minimum value is limited by the ability to uniformly distribute the spring force and distortion during manufacturing. The maximum value of the pellet/clad interface length is limited by the additional spring compression and rod internal pressure penalty. Generally, the plenum spring is compressed during fuel rod fabrication to a pre-determined force within a range of forces for each fuel rod type. The maximum force within the range is established to assure the structural integrity of the fuel rod welds and pellets. A force above the maximum set by the range could impair the ability to produce a proper end plug weld. The amount of compression of the plenum spring is controlled by the plenum length. The free volume within the fuel element cladding has to accommodate the fission gases released during reactor operation. Therefore, any reduction in plenum volume will result in increases in fuel rod internal pressure over its operating life, which may lead to an unpredicted fuel rod outer diameter increase resulting in a decrease in thermal conductivity between the cladding and the pellets. The pellet/clad interface region  88  will reduce the plenum length and plenum volume and increase the plenum spring deflection/force and rod internal pressure. It was confirmed that the length of the pellet/clad region of the spacer is acceptable so long as it is factored into the design of the spring. A fillet radius  100  should be present between the back side of the head  88  and the tubular section  102  to prevent pressure concentration at the pellet to spacer bearing surface. The thread dimensions and profile on the spring interface  98  depends upon the spring design to allow for proper fit between the spring wire and the thread profile. The thread vanish zone is the area between the thread  94  and the fillet  100  and the thread vanish zone diameter plus two times the spring interface fillet radius should not exceed the minimum spring inner diameter to ensure proper interface between the spring end coil and the spacer  86 . The central hole  90  diameter should be present to allow for fuel rod pressurization in case of “incorrect” assembly installation and the hex size should be sufficient to apply the required torque during assembly. The torque should be sufficient to prevent the spacer “from becoming” loose during shipping and handling and to mitigate spring damage during installation. 
     Pressure tests have demonstrated that the spacer is able to provide a uniform pressure distribution and confirm that the spacer design reduces small pellet chipping frequency. Additionally, the fuel rod plenum spring assembly design of this embodiment is capable of meeting the design objectives to provide a uniform pressure distribution in pellet-to-spacer contact and to reduce the potential for small pellet chip migration. The design also includes features to facilitate spacer installation, limit consequences of unlikely installation mistakes and minimize potential performance issues. 
       FIGS. 7 ,  8  and  9  show alternate embodiments to the threaded spacer illustrated in  FIGS. 5 and 6 . The embodiments illustrated in  FIGS. 7 ,  8  and  9  each have the same flat head  88  as was previously described with regard to the threaded spacer shown in  FIGS. 5 and 6 . In  FIGS. 7 and 8 , the rear side of the spacer is a slotted tubular member  106  with the embodiment shown in  FIG. 7  having the slots spaced 180° apart while the embodiment in  FIG. 8  has the slots spaced 90° apart. Each of the two embodiments has a lip  108  that fits over a rung of the spring  76  to secure the spacer to the spring. In the embodiment shown in  FIG. 9 , the head  88  is welded directly to the end coil of the plenum spring  76 . 
     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. For example, though the previous embodiments have described as being applied to a nuclear fuel element, the spring and spacer assembly taught herein can be applied to control rods as well, wherein the active element will be a neutron absorber rather than the fissile fuel pellets. 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.