Abstract:
A nuclear fuel assembly having a plurality of multi-leaf hold down spring sets extending from a top nozzle. Each spring set consists of a multiple number of springs leafs in order to provide a large working range of spring deflection. Each spring leaf has a straight, flat base section followed by a straight, flat tapered beam with a secondary spring set having a curvature at its peripheral end.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to a nuclear reactor fuel assembly and more particularly to an improved hold down spring on the top nozzle of the fuel assembly. 
     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 the 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  (also shown in  FIG. 2 ), 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 . 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 structure 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 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 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 plates  40 . 
     The rectilinearly movable control rods  28  typically include a drive shaft  50  and a spider assembly  52  of neutron poison rods  28  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 to the upper support assembly  46  and connected to the top of the upper core plate  40 . 
       FIG. 3  is an elevational view, represented in vertically shortened form, of a typical fuel assembly being generally designated by reference character  22 . The fuel assembly  22  is of the type used in a pressured water reactor and has a structural skeleton which, at its lower end, includes a bottom nozzle  58  sometimes referred to as the lower end fitting. The bottom nozzle  58  supports the fuel assembly  22  on a lower core support plate  60  in the core region of the nuclear reactor (the lower core support plate  60  is represented by reference character  36  in  FIG. 2 ). In addition to the bottom nozzle  58 , the structural skeleton of the fuel assembly  22  also includes a top nozzle  62  (sometimes referred to as the upper end fitting or top end fitting) at its upper end and a number of guide tubes or thimbles  54  (also referred to as guide tubes), 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  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 a 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 therebetween; exerting pressure on the fuel rods 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 by-products from entering the coolant and further contaminating the reactor system. 
     To control the fission process, a number of control rods  78  are reciprocably movable 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. These forces are countered by a combination of the weight of the fuel assemblies  22  and a plurality of hold down spring assemblies  56  on the top nozzles  62  which push against the upper core plate  40  ( FIG. 2 ) of the reactor. The hold down spring assemblies  56  thereby prevent the force of the upward coolant flow from lifting the fuel assemblies into damaging contact with the upper core plate, while allowing for changes in fuel assembly length due to core-induced thermal expansion and radiation growth. Operating experience has shown that these hold down springs can be subject to stress corrosion cracking which can reduce their effectiveness. 
     Accordingly, a new hold down arrangement is desired that will maintain its resiliency over extended fuel cycles. Furthermore, a new hold down assembly is desired that will be more resistant to stress corrosion cracking. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by an improved fuel assembly having a top end fitting and a bottom end fitting connected together by a structural assembly having an axial dimension that extends from the bottom end fitting to the top end fitting, with the top end fitting having a hold down spring assembly projecting above an upper surface of the top end fitting. The hold down spring has a primary spring member extending above the top end fitting, that includes a first straight leg portion having one end attached to a frame of the top end fitting at an acute angle to a plane orthogonal to the axial dimension of the fuel assembly, with the acute angle being greater than 0°. An arcuate transition portion extends at the other end of the first straight leg with a straight second leg portion extending from the transition portion toward the frame at an acute included angle with the first leg. The primary spring member is oriented on the top end fitting so that the transition portion is at the vertical highest elevation, whereby movement of the end fitting and an upper plate of the reactor that the nuclear fuel assembly is designed to operate in, relatively towards each other, primarily loads the transition portion and deflects the first leg portion about the attachment to the end fitting frame. The hold down spring assembly also includes at least one secondary spring that has a first and second end. The first end is attached to the top end fitting adjacent the first end of the primary spring first leg. The second end terminates adjacent the transition portion and includes means for interacting with the transition portion to resist downward movement of the transition portion as the primary spring member deflects in a cantilever fashion. 
     Preferably, the end of the primary spring that is attached to the frame of the top end fitting is supported in a slot in the frame that extends substantially at the acute angle. Desirably, the spring is clamped on a first portion of the surface on the frame that extends substantially at the acute angle where a periphery of the first portion of the surface of the frame under the primary spring is radiused to transition to a second portion of the surface of the frame under the primary spring that extends substantially parallel to the plane orthogonal to the axial dimension. 
     In still another embodiment, the at least one secondary spring above has a substantially flat leg that extends from the first end to an intermediate portion near the second end where the flat leg is radiused in the direction of the frame. Preferably, the radiused intermediate portion is curved at between 10° and 70°. 
    
    
     
       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 internals 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 prior art top fuel assembly nozzle showing four cantilevered leaf spring assemblies supported from diametrically opposite corners; 
         FIG. 5  is a schematic view of the fuel assembly top nozzle partially cut away to show the support of the cantilevered leaf spring assembly of this invention; 
         FIG. 6  is a perspective view of the spring assembly of this invention illustrated in  FIG. 5 , captured in a section of the top nozzle; 
         FIG. 7  is graphical representation of the strain distribution of the prior art leaf spring design; and 
         FIG. 8  is a graphical representation of the strain distribution of the leaf spring design of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As previously indicated, the hold down spring assemblies  56  shown in  FIG. 3  are important structural members for a nuclear fuel assembly. Several leafs are assembled together to form a spring set in order to provide the needed hold down force to the fuel assembly to counteract the upward lift forces due to the hydraulic flow and to permit fuel assembly growth due to differential thermal expansion and irradiation dosage during normal plant operation. 
     Conventional hold down springs  56  are mounted on the top fuel nozzles  62  and are retained by a pin  60  located at diametrically opposite corners of the top plate  20  as shown in  FIG. 3 . Typically, the top nozzle  62  supports four spring sets  56  as illustrated in  FIG. 4 . Each spring set has a primary spring  84  and at least one secondary spring  86  with two secondary springs being shown in  FIG. 3  and three being shown in  FIG. 4 . In accordance with the prior art, spring leafs  84  and  86  have a flat, horizontal base  88  that is secured against the top plate  20  of the top nozzle  62  by a pin  60 . The leafs then curve upward away from the top nozzle  20  with the primary spring member  84  having a first flat leg  90  that extends at an acute angle, greater than 0°, with the top plate  20  to an arcuate transition portion  92  at the other end of the first leg  90 . A straight second leg portion  94  extends from the transition portion  92  toward the frame of the top nozzle  62  at an acute included angle with the first leg  90 . The secondary spring leafs  86  of this prior art embodiment have a short flat section that corresponds to the flat spring base  88  of the primary spring and then curve upward under the primary spring, extending over a straight portion under the primary spring and terminating adjacent the transition portion  92  at a second end, with the second ends of the secondary leafs  86  interacting with the transition portion  92  of the primary spring  84  to resist downward movement of the transition portion  92  as the primary spring  84  deflects downward in a cantilevered fashion. The second leg  94  of the primary spring extends through an opening in the secondary spring leafs  86  to the top nozzle frame  62  where it interacts with a stop that is not shown. 
     Fuel assemblies  22  are installed vertically in the reactor core  14  and stood upright on the lower core plate  60  ( 36 ). As can be appreciated from  FIG. 2 , after the fuel assemblies are set in place, the upper support structure  26  is installed. The upper core plate  40  then bears down against the hold down springs  56  on the top nozzle  62  of each fuel assembly  22  to hold the fuel assemblies in place. The springs are generally made of nickel-chromium-iron alloy  718 . The retaining pin  60 , which holds the spring set in place, can be either threaded into the top nozzle or welded to prevent loosening while in service. 
     The improvement of this invention is illustrated in  FIG. 5 . Like reference characters are used for corresponding components of the spring  56  and top nozzle  62 , though it should be appreciated that the design of the individual components will deviate from the corresponding components of the prior art illustrated in  FIGS. 3 and 4 , as hereafter described. In accordance with this invention, the spring base  88  of each leaf, i.e., the primary spring members and secondary spring members, are formed from a short section of straight, flat beam followed by a long straight, flat beam  90  whose thickness is tapered, extending in a direction along the leafs away from the base  88 . The beams  88  and  90  thus form one continuous flat leg. Other than the top primary spring  84 , there is a slight bend  96  at the end portion of the straight secondary beams  86 . Since the spring set  56  is a cantilevered structural system, the maximum bending moment and stretch occur at the support end  98 . From the flexure loading of a straight beam, the absolute magnitude of strain or stress on the inner and outer fibers are equal. However, as for the curved base of a conventional spring design, the absolute magnitude of strain or stress on the inner and outer fibers are not equal due to a curvature effect which can be appreciated from the graphical representation of the strain distribution for the prior art leaf spring design shown in  FIG. 7  and the strain distribution of the leaf spring design of this invention illustrated in  FIG. 8 . This analysis assumes an elastic-plastic deflection up to the operation condition. Based on the same loading analysis, the maximum absolute strain for the straight (flat) end spring design is equally distributed on the inner and outer fibers. The maximum strains are 0.014247 and 0.010104, respectively for the curved base of the prior art and the straight base design of this invention. This means the maximum strain is reduced by approximately 29% for the straight base design. 
       FIG. 6  provides another view of the spring design illustrated in  FIG. 5  taken from another angle. The sections  88  and  90  shown in  FIGS. 5 and 6  provide a straight, flat beam leaf spring set that extends from a slanted slot  100  in the top nozzle. The straight beams extend until the transition portion  92  in the primary spring leaf and the slightly curved sections  96  in the second end of the secondary spring leafs. The slanted slot  100  extends at an acute angle, greater than 0°, to a plane orthogonal to the longitudinal axis of the fuel assembly. The bends  96  are radiused at between 10° and 70°. Similarly, the lower lip  102  of the slot  100  is similarly radiused between 10° and 70°. In other respects, the top nozzle  62  is similar to that shown in  FIG. 4 . 
     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.