Abstract:
A nuclear fuel assembly having lateral support provided by a bimetallic spring that extends from a side of the fuel assembly under certain core conditions to pressure against an adjacent component and withdraws under other core conditions, such as shutdown, to enable the nuclear fuel assembly to be aligned or withdrawn from the core and repositioned.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/096,017, filed Dec. 23, 2014, entitled A NUCLEAR FUEL ASSEMBLY SUPPORT FEATURE. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This invention relates in general to nuclear fuel assemblies and more particularly to a support feature for laterally supporting a Nuclear Fuel Assembly. 
         [0004]    2. Related Art 
         [0005]    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 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. 
         [0006]    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 . 
         [0007]    The core  14  comprises a large number of fuel assemblies.  FIG. 2  shows 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  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 atop nozzle  62  at its upper end and a number of guide tubes or thimbles  54 , which 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  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 interleafed 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 therebetween 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. 
         [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 the fission by-products from entering the coolant and further contaminating the reactor system. 
         [0010]    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  (shown in phantom) which are coupled to the control rod hubs  80 , all in a web-known manner. 
         [0011]    The pressurized water reactor fuel assemblies  22  are thus long elongated structures that are supported within a core  14  of a nuclear reactor at their lower ends by a bottom nozzle  58  that has holes that sit on pins that extend from the upper surface of a bottom core plate  60  and are supported at their upper ends by alignment pins that fit in holes in the surface of the underside of an upper core support plate. Some space exists between fuel assemblies for the passage of coolant. There have been instances of upper core plate alignment pins being bent during reactor internals reassembly after the reactor core has been accessed for maintenance. These pins have to be removed before the upper core plate can be resealed over the fuel assemblies. Reseating of the upper core plate after the alignment pins have been removed can result in the fuel assembly for that location being misaligned. The plant then typically has to impose a power penalty for that misalignment, which can be significant. The top nozzle pop out spring  48  provides some alignment to that core location during operation because the fuel assembly is supported. by the adjacent assemblies. Also, Fuel assembly to fuel assembly gaps allow for impact between fuel assemblies, resulting in increased impact loads on spacer grids during Seismic/LOCA events. However, some gap between fuel assemblies during outages is desirable to facilitate fuel handling. A need exists to reduce or eliminate the gap only during reactor operation while enabling some clearance between fuel assemblies during outages in which the assemblies need to be moved. 
       SUMMARY 
       [0012]    This invention achieves the foregoing objectives by providing a nuclear fuel assembly having an elongated dimension and comprising a plurality of interconnected components. At least some of the interconnected components comprise: a top nozzle; a bottom nozzle; a plurality of guide thimbles extending between the top nozzle and the bottom nozzle; and a plurality of grids arranged in a tandem spaced relationship that extends between the top nozzle and the bottom nozzle along the elongated dimension, with each of the grids having a plurality of cells some of which support fuel rods and others through which the guide thimbles respectively pass and attach to the grid. At least some of the interconnected components have a peripheral surface area that extends in a plane a distance along the elongated dimension. At least some of the interconnected components have a bimetallic spring that moves between a first and second position relative to the plane as the fuel assembly transitions in a reactor core from a shutdown temperature to an operating temperature, with one of the first and second positions placing the bimetallic spring in contact with an adjoining component of the reactor core. In one embodiment, the bimetallic spring does not extend substantially out of the plane at temperatures substantially below the nuclear reactor operating temperature and protrudes outwardly from the nuclear fuel assembly at operating temperatures of the nuclear reactor to an extent to contact the adjoining component of the core of the nuclear reactor. 
         [0013]    In another embodiment, the bimetallic spring is placed on a border grid strap at a mid-grid location and in another embodiment, the bimetallic spring is supported on an upper or lower border grid strap or both the upper and lower border grid strap. In still another embodiment, the bimetallic spring is supported on the top nozzle. The bimetallic spring may be a circular disc shape, optionally with relief holes or it may have an elongated rectangular shape. The rectangular shape may be oriented horizontally or vertically. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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: 
           [0015]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0016]      FIG. 2  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
           [0017]      FIG. 3  is a plan view of a corner portion of a border grid strap with one embodiment of the bimetallic spring of this invention shown protruding outward away from the interior of the fuel assembly as the spring would protrude as the reactor is brought up to power; 
           [0018]      FIG. 4  is a side view of the outside of the portion of the border grid strap shown in  FIG. 3 ; 
           [0019]      FIG. 5  is a side view of the inside of the portion of the border grid strap shown in  FIG. 3 ; 
           [0020]      FIG. 6  is a perspective view of  FIG. 4 ; 
           [0021]      FIG. 7  is a perspective view of a second embodiment of the bimetallic spring of this invention formed in a circular disc configuration; 
           [0022]      FIG. 8  is a side view of  FIG. 7 ; 
           [0023]      FIG. 9  is a plan view of four adjacent fuel assemblies at operating temperatures with their opposing springs in contact; 
           [0024]      FIG. 10  is a perspective view of a portion of a top fuel assembly nozzle with a disc metallic spring shown in one corner of each side with a recess in the other corner to receive the forward portion of an opposing spring extending from an adjacent nozzle; 
           [0025]      FIG. 11  is a perspective view of a disc bimetallic spring with optional relief openings; 
           [0026]      FIG. 12  is a perspective view of a portion of a top nozzle employing the embodiment of the bimetallic spring shown in  FIG. 11 ; 
           [0027]      FIG. 13  is a plan view of three adjacent top nozzles; and 
           [0028]      FIG. 14  is an enlarged plan view of  FIG. 13 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    The attached figures in  FIGS. 3-14  show varying configurations of the bimetallic concept to attain different benefits. One embodiment of this invention is a fuel assembly  22  with a mid-grid with a bimetal protrusion spring  26  arrangement formed in an outer grid strap  24  shown in  FIGS. 3-6 . Any arrangement of these springs  26  could be present on each outer strap  24  (such as one in each corner as shown in the  FIG. 9 ). A rectangular shaped spring/protrusion  28  will likely be oriented vertically to minimize concerns for fuel handling should the spring not return fully to the flat original shape or return beyond the outer strap envelope, However, the rectangular shaped spring may also be mounted horizontally and fall within this concept. The spring may also be of a circular shape  30  such as a “pop-out” disc or other configuration that result in the desired deflection and load capability as shown in  FIGS. 7 and 8 . The bimetal area may be a lamination attached mechanically or a coating on the base strap material. This grid spring  26  will provide the benefit of reducing or eliminating the fuel assembly gaps during operation to reduce seismic/LOCA (Loss Of Coolant Accident) impact loads, and to provide energy absorption during such accident conditions to prevent grid damage. 
         [0030]    Another embodiment disclosed herein is a bimetallic protrusion spring attached to a fuel assembly top or bottom nozzle  62  and  58 , or Inconel top or bottom grid. This feature would provide alignment benefits for conditions such as damaged upper core plate fuel assembly alignment pins that have been removed. The top nozzle  62  or other host component would support itself upon all adjacent nozzles  62  (or other like-adjacent components) with this feature, ensuring alignment with the intent of reducing or eliminating penalties for removed pins. This second feature may be on all the fuel assemblies or just the ones with damaged alignment. However, preferably it is on the fuel assemblies with damaged pins and the adjacent fuel assemblies have recesses  32  in which the springs can seat. Sample calculations for many of the arrangements are also available showing significant load capability can be attained depending on the feature geometry. Thus, this invention employs bimetallic features to provide either fuel assembly alignment benefits or improved fuel assembly response during seismic/LOCA accident conditions. 
         [0031]    More specifically, one such feature is the top nozzle alignment spring shown in  FIGS. 10-14 . This bimetallic spring attached to a fuel assembly top nozzle would provide alignment benefits for conditions such as upper core plate fuel assembly alignment pins that have been damaged and removed. The top nozzle would support itself upon all adjacent nozzles with this feature, ensuring alignment with the intent of reducing or eliminating cut pin penalties that such plants must impose. This spring may be of varying shapes such as a rectangular beam  28  or circular disc  30  as shown in the figures. It may be attached with varying methods such as brazing, riveting, or welding. The spring may be fabricated from various materials or alloys, but will most likely be an INVAR™/Stainless combination to attain the desired deflection and load capability while facilitating attachment to the nozzle or other host component. INVAR™ is generally known as FeNi36 or 64FeNi. 
         [0032]    Another concept feature is a typical outer grid strap  24  containing a bimetal material spring feature  26  as shown in  FIGS. 3-11 . The outer strap bimetal features would extend outside the nominal grid envelope at operating temperature due to the high expansion side of the material to reduce or eliminate the gap between fuel assemblies, allowing for support between adjacent assemblies. This support between adjacent fuel assemblies during operation would result in lower fuel assembly loads during Seismic/LOCA events. The features may be of long rectangular beam designs similar to grid springs, “pop out” disc shaped designs shown in  FIGS. 7 and 8 , or some other unique design shape needed to attain the desired deflection and load capability. The bimetallic spring features may only be needed at one or two mid-grid locations near the axial center of the fuel assembly. This would reduce any neutronic penalty due to the material used. The bimetal combination could consist of many materials, but for the purposes of doing sample calculations to determine the feasibility of the concepts, a bimetal laminate of INVAR™ as the low expansion material and a stainless steel variant as the high expansion material was considered. The bimetal feature may also be obtained by coating the base strap material with a low-to negative coefficient of thermal expansion material, The basic feature could also be joined to the grid similar to the top nozzle feature described above. 
         [0033]    Significant load capability can be designed into the features for Seismic/LOCA load absorption. This feature can also be used in reverse, i.e., wherein the spring retracts at operating temperature and is in an expanded state below operating temperature to facilitate alignment of the fuel assemblies when the upper core plate is being installed. 
         [0034]    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.