Abstract:
A nuclear fuel assembly grid having a torpedo-shaped mixing vane assembly supported at each intersection of the grid straps that surrounds a fuel rod support location. The torpedo-shaped stem supports mixing vanes that extend over each of the fuel rod support locations.

Description:
BACKGROUND 
     1. Field 
     This invention pertains generally to a nuclear reactor fuel assembly and more particularly to such a fuel assembly with spacer grids that enhance heat transfer and reduce pressure drop. 
     2. Related Art 
     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. 
     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 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 . 
     An exemplary reactor design is shown in more detail in  FIG. 2 . It should be appreciated that like reference characters are employed to designate corresponding components in the several figures. 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 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 degrees in a lower plenum  34 , passes upward through a lower support plate  37  and a lower core plate  36  upon which the fuel assemblies 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, 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 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 . 
     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 and perforations  42  in the upper core plate  40 . 
     Rectilinearly movable 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. 
       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 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 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. 
     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 grid  64 . A plan view of a grid  64  without the guide thimbles  84  and fuel rods  66  is shown in  FIG. 4 . The guide thimbles  84  pass through the cells labeled  96  and the fuel rods occupy the remaining cells  94 . As can be seen from  FIG. 4 , the grids  64  are conventionally formed from an array of orthogonal straps  86  and  88  that are interleaved in an egg-crate pattern with the adjacent interface of four straps forming approximately square support cells through which the fuel rods  66  are supported in the cells  94  in transverse, spaced relationship with each other. In many designs, springs  90  and dimples  92  are stamped into opposite walls of the straps that form the support cells  94 . The springs and dimples extend radially into the support cells and capture the fuel rods  66  therebetween; exerting pressure on the fuel rod cladding to hold the rods in position. The orthogonal array of straps  86  and  88  is welded at each strap end to a bordering strap  98  to complete the grid structure  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. 
     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 stack of a plurality of nuclear fuel pellet s  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 plugs  72  and the top of the pellet stack. The 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 movable in the guide thimbles  84  located at predetermined positions in the fuel assembly  22 . The guide thimble locations can be specifically seen in  FIG. 4  represented by reference character  96 , except for the center location which is occupied by the instrumentation tube  68 . Specifically, a rod cluster control mechanism  80 , positioned above the top nozzle  62 , 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 drive shaft  50  which is coupled to the control rod hub  80 , all in a well-known manner. 
     As mentioned above, 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 assemblies. In addition, there is significant turbulence in the coolant in the core caused by mixing vanes  56  upon the upper surfaces of the straps of many grids which promote the transfer of heat from the fuel rod cladding to the coolant. Mixing of the coolant is very important because the power output of the core is limited by the hottest fuel rod to a temperature that will avoid compromising the fuel rod cladding. The more uniform the temperature across the core, the higher the power output of the reactor that can safely be achieved. Mixing of the coolant not only draws more heat off the fuel element cladding, but also distributes that heat across the fuel assembly. The turbulence also causes a pressure drop which has to be balanced across the core to maintain the uniformity of flow of the coolant. To the extent that the pressure drop can be reduced, it enables the fuel assembly designers to add other features that will enhance the contribution of the fuel assembly. 
     Accordingly, it is an object of this invention to provide an improved fuel assembly design that promotes enhanced mixing, enhanced heat transfer from the fuel rods to the coolant, enhanced critical heat flux performance and has a reduced pressure drop. 
     SUMMARY 
     These and other objects are achieved by a new fuel assembly design having a parallel, spaced array of a plurality of elongated nuclear fuel rods supported between a lower nozzle and an upper nozzle and having an axial length along the elongated dimension of the nuclear fuel rods. A plurality of spaced support grids are arranged in tandem along the axial length of the fuel rods, between the upper nozzle and the lower nozzle, at least partially enclosing an axial portion of the circumference of each fuel rod within a corresponding support cell of the support grids to maintain a lateral spacing between fuel rods. At least one of the support grids comprises a plurality of elongated, intersecting straps that define the support cells at the intersection of each four adjacent straps that surround the nuclear fuel rods. A length of each strap along its elongated dimension, between the intersections of the four adjacent straps forms a wall of the corresponding support cell. An intersection of each wall that surrounds a part of the circumference of the fuel rods, with an adjoining wall that surrounds a part of the circumference of the fuel rods, supports a mixing vane that extends over the corresponding support cell. In one embodiment, the mixing vanes that extend on opposite sides of the walls that support the fuel rods are tilted in an opposite direction to form a counter-rotating effect on reactor coolant. In another embodiment, the mixing vanes that extend on opposite sides of the walls that support the fuel rods are tilted in the same direction to reinforce a flow pattern on reactor coolant. 
     Preferably, the mixing vanes are supported at the intersection from a stem that has a rounded cross section with the stem extending along the intersection into the support cells with the radius of curvature of the rounded cross section of the stem decreasing as the stem extends into the support cell. Desirably, the stem extends and is tapered above an attachment of the vanes to the stem and the stem is rounded at its upper-most and lower-most extensions. In such an arrangement, the stem and vane assembly generally resemble a torpedo. In still another embodiment, the stem has an elongated body and a lower end of at least some of the stems have diametrically extending slits that fit over the walls extending from the intersection. Preferably, the stems are welded to the walls at their slits. 
    
    
     
       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 plan view of an egg-crate support grid of the prior art; 
         FIG. 5  is a perspective view of a support grid constructed in accordance with one embodiment of this invention; 
         FIG. 6  is a perspective view of the mixing vane and stem assembly shown in  FIG. 5 ; 
         FIG. 7  is a plan view of a five-by-five matrix of the support cells shown in  FIG. 5 , with the springs and dimples omitted for simplicity, that shows the mixing vanes tilted in the same direction in accordance with one embodiment of this invention; and 
         FIG. 8  is another example of the five-by-five matrix shown in  FIG. 7  with the springs and dimples in place to support the fuel rods and adjoining vanes around the circumference of each stem and vane assembly titled in a counter-rotating direction. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention provides a new fuel assembly design for a nuclear reactor and more particularly an improved spacer grid design for a nuclear fuel assembly. The improved grid is generally formed from a matrix of approximately square (or hexagonal) cells, some of which  94  support fuel rods  66  while others of which  96  are connected to guide thimbles and a central instrumentation tube. The perspective view shown in  FIG. 5  looks very much like the prior art grid shown in  FIG. 4  since the improvement is mainly focused on the mixing vanes  56 . As shown in  FIGS. 5-8 , the grid of this embodiment is also formed from two orthogonally positioned sets of parallel, spaced straps  86  and  88  that are interleaved in a conventional manner and surrounded by an outer strap  98  to form the structural make-up of the grid  64 . Though orthogonal straps  86  and  88 , forming substantially square fuel rod support cells, are shown in this embodiment, it should be appreciated that this invention can be applied equally as well to other grid configurations, e.g., hexagonal and circular grids. The orthogonal straps  86  and  88  and in the case of the outer rows, the outer strap  98  define the support cells  94  at the intersection of each four adjacent straps that surround the nuclear fuel rods  66 . A length of each strap along the straps&#39; elongated dimension, between the intersections of four adjacent straps, forms a wall  100  of the fuel support cells  94 . 
     As previously mentioned, the critical heat flux performance of the fuel assembly is the key factor to determine the operating range of a pressurized water reactor. The critical heat flux performance of a fuel assembly can be enhanced and the pressure drop across the fuel assembly reduced by employing the mixing vane assembly illustrated in  FIGS. 5 and 6 .  FIG. 5  shows a perspective view of a five-by-five array of fuel support cells that has a novel vane assembly  102  supported at each of the intersections  60  of the straps  86  and  88  of the cells  94  that support fuel rods. The vane assembly  102  combines the streamlined, elongated body stem  104  with separate mixing vanes that respectively extend over the adjoining fuel support cells  94 . Though not shown in  FIGS. 5-8 , no vanes  56  are provided over the cells through which the guide thimbles extend as shown in  FIG. 4 . Accordingly, the intersections with the outer straps  98  and those adjoining the guide thimble cell  96  will only have zero, one or two vanes  56  extending from the stem  104  over the adjoining fuel rod support cells  94  while all the other fuel rod support cells  94  will have four vanes supported at substantially equally spaced circumferential positions respectively extending outwardly from the stem  104  over each of the adjoining fuel rod support cell  94  as shown in  FIGS. 5, 7 and 8 . The streamlined body stem  104  and mixing vanes  56  shown in this embodiment provides enhanced heat convection by mixing the cold coolant in the middle of the sub-channel with the hot coolant near the fuel rod surface. The streamlined body pushes the cooler coolant from the center of the channel to the surface of the fuel rod and minimizes irrecoverable pressure losses. The two or four mixing vanes create a swirl that displaces the hot coolant near the rods&#39; surface with the cooler coolant from the channel center. A two-phase computational fluid dynamics model has predicted that the critical heat flux performance of this grid spacer is improved relative to the conventional split vane spacer grid design illustrated in  FIG. 4 . In addition, the computational fluid dynamics model has predicted that the single phase pressure drop produced by this invention is significantly reduced relative to the conventional split vane spacer grid design illustrated in  FIG. 4 . The streamlined torpedo body  104  has two diametrically extending slots in its lower end that are circumferentially offset by ninety degrees to fit over the grid straps  86  and  88  at the intersection  60  to which it is welded. The streamlined stem  104  enhances the grid crush strength. The mixing vanes are connected to the strap and the streamlined torpedo body  104 , such as by welding. Extra support from the torpedo body  104  will enhance the structural integrity of the grid cell. Grids manufactured in accordance with this invention are still constructed in the conventional way with the addition of the vane assembly  102  which is slotted onto the intersection  60  at each corner of a fuel element support cell  94  and welded. The addition in manufacturing costs is estimated to be low in impact. 
       FIG. 7  is a plan view illustrating one embodiment of this invention (with the springs and dimples removed to more easily focus on the vane structure) with the vanes all tilted in the same direction.  FIG. 8  illustrates a second embodiment where adjoining vanes are tilted in opposite directions to establish a counter-rotating effect. 
     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.