Abstract:
A nuclear fuel assembly having a bottom nozzle with protrusions that extend from the upstream (lower or fluid entry) and downstream (upper or fluid exit) side of a horizontally supported perforated flow plate. The protrusions have a funnel-like shape that gradually decreases the lateral flow area on the upstream side of the perforated flow plate and gradually increases the lateral flow area on the downstream side of the perforated plate. The protrusions on the downstream side are preferably recessed to accommodate the ends of the fuel rods.

Description:
BACKGROUND 
     1. Field 
       [0001]    The present invention relates generally to nuclear reactors and, more particularly, is concerned with reducing the pressure drop across the bottom nozzle of a nuclear fuel assembly. 
       2. Related Art 
       [0002]    The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated 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. 
         [0003]    For the purpose of illustration,  FIG. 1  shows a simplified nuclear 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 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 . 
         [0004]    An exemplary conventional 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 the 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  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  10  through one or more inlet nozzles  30 , flows down through an annulus between the reactor vessel and the core barrel  32 , is turned 180° in a lower plenum  34 , passes upwardly through lower support plate  37  and lower core plate  36  upon which the fuel assemblies are seated and through and about the fuel assemblies. 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 cause an upward force on the fuel assembly whose 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 . 
         [0005]    The upper internals  26  are 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 . 
         [0006]    Rectilinearly moveable control rods  28 , which typically include a drive shaft  50  and a spider assembly  52  of neutron poison rods (shown and described more fully with regard to  FIG. 3 ), 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 effect control rod insertion capability. 
         [0007]      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 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 a number of guide tubes or thimbles  84  which align with 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. 
         [0008]    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 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, the fuel assembly  22  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0009]    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 fission byproducts from entering the coolant and contaminating the reactor system. 
         [0010]    To control the fission process, a number of control rods  78  are reciprocally 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 nozzles  62  of selected fuel assemblies, supports a plurality of 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  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 corresponding fuel assembly  22 , under the motive power of a control rod drive shaft  50  which is coupled to the control rod hub  80 , all in a well-known manner. 
         [0011]    It is desirable to have a balanced flow across the reactor core, i.e., substantially the same pressure drop across each of the fuel assemblies, so that some of the fuel assemblies do not operate at higher temperatures than other fuel assemblies. Power output is limited by the hottest operating fuel element. Reducing pressure drop without comprising other beneficial features provides the fuel assembly designer the opportunity to add other features to make up for the reduced pressure drop, that can, for example, promote mixing which will enhance heat transfer that can translate into increased power output of the core. The bottom nozzles of the fuel assemblies, which include a horizontal top plate with a large number of flow through holes, is a significant contributor to that pressure drop. That is especially true for debris filter bottom nozzles which require that the holes be small enough to prevent the passage of debris that could damage the fuel rod cladding such as is described in U.S. Pat. No. 7,822,165, assigned to the Assignee of this application. Any modification that reduces the pressure drop across the fuel assemblies without otherwise adversely impacting the operation of the reactor core is desirable. 
         [0012]    Therefore, it is an object of this invention to reduce the pressure drop across the fuel assemblies by modifying the design of the bottom nozzles to alter the shape of the flow through holes. 
         [0013]    It is a further object of this invention to achieve that reduction in pressure drop by gradually changing the lateral flow area on either or both the upstream and downstream sides of the perforated flow plate of the bottom nozzle. 
       SUMMARY 
       [0014]    These and other objects are achieved by a nuclear fuel assembly having a plurality of elongated nuclear fuel rods with an extended axial length. At least a lower most grid supports the fuel rods in an organized array having unoccupied spaces defined therein adapted to allow flow of fluid coolant therethrough and past the fuel rods when the fuel assembly is installed in a nuclear reactor. A plurality of guide thimbles extend along the fuel rods through and supporting the grid. A bottom nozzle is disposed below the lower most grid, below lower ends of the fuel rods and supports the guide thimbles. The bottom nozzle has openings therethrough to allow the flow of fluid coolant into the fuel assembly. The bottom nozzle includes a substantially horizontal plate supported orthogonal to the axis of the fuel rods. The horizontal plate has an upper face directed substantially toward the lower most grid and a lower face on an underside of the horizontal plate with the openings extending therethrough for the flow of coolant. At least some of the openings in the lower face have a funnel-like first appendage respectively extending below the lower face, around at least some of the openings in the lower face with an opening at the first appendage&#39;s substantially lowest extent having a larger diameter than a diameter of the opening in the lower face. An internal wall of the first appendage substantially gradually decreases in diameter from the opening at the first appendages substantially lowest extent until the wall of the first appendage transitions to the opening in the lower face. In one preferred embodiment, a lip in the opening in at least some of the first appendage&#39;s substantially lowest extent has a scalloped contour and preferably, the scalloped lip has a plurality of spaced depressions, resembling the contour of an egg receptacle in an egg carton and more preferably all of the lip of the opening at the first appendage&#39;s substantially lowest extent has such a scalloped contour. 
         [0015]    In one embodiment, the nuclear fuel assembly includes a funnel-like second appendage that extends up from at least some of the openings in the upper face with an opening at the second appendage&#39;s substantially highest extent having a larger diameter than a diameter of the opening in the upper face. An internal wall of the second appendage substantially, gradually increases in diameter from the transition at the opening in the upper face to the second appendage&#39;s substantially highest extent. In this latter embodiment, a lip of the opening at the second appendage&#39;s substantially highest extent has a scalloped contour. Desirably, the second appendage is at least partially recessed within the opening in the upper face. In one embodiment the highest extent of the second appendage terminates below the lower ends of the fuel rods and desirably, the highest extent of the second appendage is smaller than the lowest extent of the first appendage. At least some of the openings in the bottom nozzle substantially align with the unoccupied spaces in the lower most grid. 
         [0016]    In general, the internal wall of the first appendage gradually decreases the lateral flow area axially through the first appendage as the first appendage transitions to the opening in the lower face. The internal wall of the second appendage gradually increases the lateral flow area axially through the second appendage as the second appendage transitions from the opening in the upper face to the unoccupied flow spaces defined within the organized array of fuel rods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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: 
           [0018]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0019]      FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internal components to which this invention can be applied; 
           [0020]      FIG. 3  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
           [0021]      FIG. 4  is an isometric view of a portion of one embodiment of the bottom nozzle top plate and flow through holes of this invention showing recesses in the upper face that interface with the fuel rod end plugs; 
           [0022]      FIG. 5  is an isometric view of the embodiment shown in  FIG. 4  with the fuel rod end plugs in position; 
           [0023]      FIG. 6  is a partial side view of the embodiment illustrated in  FIGS. 4 and 5 ; 
           [0024]      FIG. 7  is a bottom plan view of the embodiment illustrated in  FIGS. 4 and 5 ; 
           [0025]      FIG. 8  is a top plan view of the embodiment illustrated in  FIGS. 4 and 5  (with portions of the end plugs removed); 
           [0026]      FIG. 9  is a sectioned isometric view of another embodiment of the bottom nozzle top plate and flow through holes of this invention; 
           [0027]      FIG. 10  is the sectioned isometric view shown in  FIG. 9  with the lower portion of a number of fuel element end plugs shown interfacing with the appendages of this invention; 
           [0028]      FIG. 11  is a top view of the horizontal lower nozzle plate of  FIGS. 9 and 10 , showing the positioning of the flow through holes; 
           [0029]      FIG. 12  is a bottom view of the hole pattern shown in  FIG. 11 ; and 
           [0030]      FIG. 13  is a sectioned side view of the flow holes (shown with the lower end of the fuel rod end plugs). 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]    The present invention relates to a bottom nozzle  58  for a fuel assembly which, in addition to supporting the fuel assembly  22  on the lower core plate  36 , also contains features which function to reduce the pressure drop across the nozzle. This can be appreciated from  FIG. 3 . The bottom nozzle includes a support means, for example, the skirt  56  shown in  FIG. 3 . The support means, skirt  56  in this embodiment, includes a plurality of corner legs  60  for supporting the fuel assembly  22  on the lower core plate  36 . A generally rectangular, planar plate  86  is suitably attached to the upper surface of the support skirt  56 . In the nozzle plate  86  of this embodiment, a large number of relatively small holes are provided to accommodate the passage of coolant from below the plate  86  to and through the lower most grid  88 . These holes may be small enough to trap debris to shield the fuel element cladding from damage as described in U.S. Pat. No. 7,822,165, though it should be appreciated that this invention can provide a benefit to most any type of flow through hole in a fuel assembly seeking to minimize pressure drop. 
         [0032]    This invention recognizes that a significant portion of the pressure drop associated with the bottom nozzle flow plate  86  is due to abrupt changes in flow area. This advanced bottom nozzle concept incorporates “egg-crate” type features on either or both the upstream and downstream sides of the bottom nozzle flow plate  86  to gradually change the lateral flow area in the flow through path through the flow plate  86 . 
         [0033]      FIGS. 4 through 8  show a portion of one embodiment of a flow plate  86  incorporating the features claimed hereafter.  FIG. 4  shows an isometric view of a portion of the flow plate  86  with parts of the flow through holes  90  broken away to observe the interior of the flow through holes.  FIG. 5  is the isometric view illustrated in  FIG. 4  with the fuel rod end plugs  74  shown in position above the flow plate  86 .  FIG. 6  is a side view of a portion of the flow plate shown in  FIG. 5 .  FIG. 7  is a bottom plan view of the flow plate shown in  FIG. 5 ; and  FIG. 8  is a top plan view of the portion of the flow plate shown in  FIG. 4 . On the upstream side (i.e., from the underside of the flow plate  86 ), which can best be appreciated from  FIG. 7 , streamlined “egg-crate” protrusions  92  gradually reduce the lateral flow area to minimize form losses associated with the rapid contraction that the coolant flow must undergo as it enters the perforated flow plate at the entrance to the flow holes  90 . These “egg-crate” protrusions  92  also eliminate high pressure pockets of recirculating flow below each fuel rod location. The protrusions  92  are funnel-like extensions of the openings of the flow through holes  90  with a lip  98  that surrounds an opening on the lower most extent of the protrusions  92  having depressions  94  that in one embodiment are approximately equally spaced around its circumference; though it should be appreciated that the depressions need not be equally spaced to obtain some reduction in pressure drop. The depressions in the lip  94  form a scalloped contour. Additionally, though the protrusions  92  extending on either side of the flow plate  86  are shown to be approximately the same height, the height may vary over the surface of the plate and still obtain a reduction in pressure drop. 
         [0034]    On the downstream side (i.e., between the flow plate  86  and the fuel rods), streamlined “egg-crate” protrusions  96  gradually increase the lateral flow area to minimize form losses associated with the rapid expansion and contraction that the coolant experiences in the transition from the flow plate  86  to the fuel rod bundle. Due to the close proximity of the fuel rod bottom end plugs  74 , the downstream “egg-crate” protrusions are recessed in the upper face of the plate  86  to interface with the fuel rods  66 . There are no changes to the axial elevations of the fuel rods. 
         [0035]    The development of advanced fabrication techniques such as additive manufacturing makes the manufacture of this design more convenient, though it should be appreciated that traditional manufacturing techniques can also be employed. Though the egg-crate protrusion design has been applied to the upper and lower surfaces of the flow through plate  86  it should be appreciated that either of these designs may be employed alone to obtain some reduction in pressure drop or together to minimize the pressure drop for maximum benefit. 
         [0036]    Furthermore, an additional reduction in pressure drop can be achieved employing the embodiment illustrated in  FIGS. 9-13 . This embodiment retains the streamlined flow passages unique to the foregoing embodiment, which has the flow through holes substantially aligned with the unoccupied spaces between the lowermost grid and the fuel rod, but adds an additional flow path substantially in-line with the fuel rods. The additional flow holes  100  are of a similar design to the other flow holes  90 , but are positioned directly under the fuel rods, are preferably smaller in diameter and have a set of standoffs  102  supporting the fuel rods and allowing the coolant flow to exit the bottom nozzle. The standoff may be the peaks of the scalloped lips of the appendages and ensure that the fuel rods don&#39;t block the flow holes during operation. Because the additional flow holes  100  are directly under the fuel rods they provide a “no-line-of-sight” path for the flow which helps minimize debris from passing thru the bottom nozzle yet help reduce the overall loss coefficient of the bottom nozzle by providing an additional flow path. Testing of this added feature showed a significant improvement over the embodiment employing the appendages without the additional flow holes in-line with the fuel rods. 
         [0037]    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.