Abstract:
A nuclear reactor having an upper internals control rod assembly guide tube formed from upper and lower sections that are connected along a central axial region of the guide tube at an intermediate coupling. An extended control rod axial support is provided for at least some of the control rods over a finite distance within at least one of the interiors of the lower guide tube section or the upper guide tube section.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention relates generally to nuclear reactors, and more particularly, to nuclear reactors that employ top mounted control rods. 
         [0003]    2. Related Art 
         [0004]    The primary side of nuclear 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 the system of pipes which are connected to the vessel form a loop of the primary side. 
         [0005]    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 . 
         [0006]    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  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 the 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  in 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 the core  14  flows along the underside of the upper core plate and upwardly through a plurality of perforations  42 . The coolant then flows upwardly and radially to one or more outlet nozzles  44 . 
         [0007]    The upper internals  26  can be supported from the vessel  10  or the vessel head  12  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 . 
         [0008]    As will be explained in more detail hereafter, the reactor internals also include rectilinearly moveable control rods  28  for controlling the nuclear reaction within the core. The control rod assemblies, commonly known as rod cluster control mechanisms, 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 to the upper support assembly  46  and are connected by a split pin  56  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 arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability. 
         [0009]      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 the 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  54 , which extend longitudinally between the bottom and top nozzles  58  and  62  and at opposite ends are rigidly attached thereto. 
         [0010]    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 extends 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. 
         [0011]    To control the fission process, a number of control rods  28  are reciprocally movable in the guide thimbles  55  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  28 . The control mechanism  80  has an internally threaded cylindrical hub member  82  with a plurality of radially extending flukes or arms  52 . Each arm  52  is interconnected to one or more control rods  28  (the arrangement of the central hub and radially extending flukes is also referred to as a spider mechanism), such that the control rod mechanism  80  is operable to move the control rods vertically in the guide thimbles  55  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 In the withdrawn position, the control rods are guided up into the control rod guide tubes  55  above the upper core plate  40  and in the fully inserted position the control rods occupy substantially the entire length of the guide thimbles  54  within the fuel assemblies as shown in  FIG. 3 . Alignment of the control rods through the upper internals  26  with the guide thimbles  55  in the fuel assemblies is maintained by guide cards  70  supported in a spaced tandem arrangement along the length of the control rod guide tubes  54 . 
         [0012]      FIG. 4  shows an enlarged view of the control rod assembly guide tube  54  shown between the upper support assembly  46  and the upper core plate  40  in  FIG. 2 . The guide tube  54  is made up of two sections, a lower guide tube section  78  and an upper guide tube section  84 . The lower guide tube section  78  has a generally square cross section while the upper guide tube section  84  has a generally rounded cross section. The lower guide tube section  78  is joined to the upper guide tube section  84  at an intermediate coupling  86 . The upper and lower guide tube sections  84  and  78  have a plurality of guide cards  70  supported in tandem in spaced relationship to each other along the length of the guide tube  54  with a continuous guided section  88  extending up from the bottom of the guide tube  54  a distance approximately equal to the spacing between the guide cards  70 . 
         [0013]      FIG. 5  is representative of the pattern of the openings in the continuous guided section  88 , the guide cards  70  and the guide plate at the intermediate coupling  86 , through which the control rod assembly  80  passes as it travels through the upper internals  26 . The three-quarter round openings  72  guide the individual control rods  28  with the flukes  52  passing through the straight portions  74  connecting the circular openings  72  to the central opening  76  through which the hub  82  passes. The guide card illustrated in  FIG. 5  is from the upper section  84  of the guide tube  54 , but the pattern of the openings are representative of the opening pattern in the other guides as well; the difference being that the shape of the outer circumference changes from circular to generally square as one transitions from the upper section  84  to the lower section  78  of the guide tube  54 . 
         [0014]    Aggressive guide card wear has been observed at some operating nuclear plants. When the special guide plate at the intermediate coupling  86  is located within the series of allowable worn guide cards  70 , the guide plate can be replaced during an outage to extend the life of the guide tube, in lieu of replacing the lower guide tube assembly  78 , if heavily worn. This mitigation technique reduces schedule, costs and radioactive waste generated while enabling continued safe plant operation, albeit for a limited portion of the remaining life of the plant. 
         [0015]    Accordingly, a more permanent fix for guide card wear is desired that can be achieved on a similar schedule to that required to replace the guide plate at the intermediate coupling  86 . 
         [0016]    Additionally, such a repair is desired that would not require the generation of additional radioactive waste and is substantially comparable in cost to replacement of the guide plate. 
       SUMMARY 
       [0017]    These and other objects are achieved for a nuclear reactor having a pressure vessel housing a core of fissile material and an upper core plate substantially covering the core. The nuclear reactor has a control rod guide tube with an extended axial length, for guiding a control rod assembly into and out of the core, which extends between the upper core plate and an upper support assembly supported above the upper core plate. The control rod guide tube has a lower guide section connected at a first end to the upper core plate and terminating at a second end at an intermediate coupling. The control rod guide tube also has an upper guide tube section connected at a first end portion to the upper support assembly and terminating at a second end at the intermediate coupling. The improvement comprises an extended control rod guide assembly that is supported substantially at the intermediate coupling and extends axially, a finite distance into at least one of an interior of the lower guide tube section or the upper guide tube section. 
         [0018]    The control rod assembly comprises a plurality of control rods and, in at least one embodiment, at least some of the control rods are continuously guided over substantially an entire axial length of the control rod guide assembly. Preferably, some of the control rods are guided over discrete, spaced axial elevations along the axial length of the control rod guide assembly and, desirably, the control rods extending along the cardinal axis of the control rod guide assembly are continuously guided over substantially the entire axial length of the control rod guide assembly. In an additional embodiment all of the control rods are continuously guided over substantially an entire axial length of the control rod guide assembly. 
         [0019]    In one embodiment, the control rod guide assembly has a first axially extending segment and a second axially extending segment with the first segment extending into the lower guide tube section and the second segment extending into the upper guide tube section. Preferably, in this latter embodiment, the first segment terminates at an upper end in a first guide plate, the second segment terminates at a lower end in a second guide plate and the first and second guide plates are joined at the intermediate coupling. Preferably, the first and second guide plates have openings through which all of the control rods pass and the first and second guide plates have either an alignment hole or an alignment pin for aligning the openings in the first guide plate with the corresponding openings in the second guide plate. Desirably, the first guide plate and the second guide plate respectively have a peripheral radially extending flange with each flange extending into a radial recess on a radial interior of the intermediate coupling, which clamps the flanges together. 
         [0020]    In another embodiment, the control rod guide assembly extends approximately between 0.9 and 23 inches (2.3 and 58.4 cm). More preferably, the control rod guide assembly extends axially approximately between 0.9 and 7 inches (2.3 and 17.8 cm). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    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: 
           [0022]      FIG. 1  is a simplified schematic of a nuclear reactor to which this invention may be applied; 
           [0023]      FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internal components to which this invention may be applied; 
           [0024]      FIG. 3  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form with parts broken away for clarity; 
           [0025]      FIG. 4  is an enlarged isometric view of the control rod guide tube  54  illustrated in  FIG. 2 ; 
           [0026]      FIG. 5  is a plan view of one of the guide cards in the upper guide tube  84  illustrated in  FIG. 4 ; 
           [0027]      FIG. 6  is a perspective view of an extended control rod guide assembly which is inserted within the intermediate coupling in place of a worn guide plate, in accordance with one embodiment of this invention; 
           [0028]      FIG. 7  is a perspective view of the upper segment of the extended control rod guide assembly shown in  FIG. 6 ; 
           [0029]      FIG. 8  is a perspective view of the lower segment of the extended control rod guide assembly shown in  FIG. 6 ; 
           [0030]      FIG. 9  is a cross sectional view of the upper internals guide tube  54  taken at the intermediate coupling location with the extended control rod guide assembly of one embodiment of this invention incorporated therein; 
           [0031]      FIG. 10  is a perspective view of the extended control rod guide assembly of one embodiment of this invention installed in the lower guide tube  78 ; 
           [0032]      FIG. 11  is a plan view of an upper section of a second embodiment of the control rod guide assembly of this invention; 
           [0033]      FIG. 12  is a plan view of the lower section of the second embodiment of the control rod guide assembly illustrated in  FIG. 11 ; 
           [0034]      FIG. 13  is a cross sectional view of another embodiment of the control rod guide assembly in which the upper guide tube section and the lower guide tube section are constructed as a solid member; and 
           [0035]      FIG. 14  is a perspective view of the embodiment shown in  FIG. 13 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    The useful life of a guide tube  54  can be substantially extended by replacing the guide plate at the intermediate coupling  86  during a refueling outage with the extended control rod guide assembly of this invention, one embodiment of which is illustrated in  FIG. 6 . This invention provides an extended control rod guide assembly  90  that is supported substantially at the intermediate coupling  86  and extends axially, a finite distance into at least one of the interior of the lower guide tube section  78  or the upper guide tube section  84 . The embodiment illustrated in  FIG. 6  extends into both the upper guide tube section  84  and the lower guide tube section  78  and is formed from a lower segment  94  and an upper segment  92 . The upper segment  92  has a guide plate  114  at its lower end with openings  72  that substantially match the openings in the guide card  70  and terminates at its upper end  106  with a guide ring with peripheral openings  72  that support a portion of the circumference of the outer row of control rods  28 . In addition, continuous support between the lower support plate  114  and the upper support ring  106  is provided by the support channels  110  for the control rods at the cardinal axes of the control rod assembly. Thus, discrete axially spaced support for some of the control rods  28  are provided while continuous support is provided for other of the control rods  28  over the upper segment  92  of the extended control rod guide assembly  90 . 
         [0037]    The lower segment  94  of the extended control rod guide assembly of this embodiment terminates at its upper end in a guide plate  116  with openings that correspond to the openings in the guide plate  114  on the upper segment  92 . The lower segment  94  terminates at its lower end in a guide ring  108  which is connected to the guide plate  116  by the continuous support channels  110  as shown in  FIG. 8 . The upper guide plate  114  and the lower guide plate  116  are joined by the bolts  104  and aligned through an alignment pin in one or the other of the guide plates, that fits through an alignment hole  100  in the other of the guide plates to assure the openings  72  in the guide plates  114  and  116  are lined up. 
         [0038]      FIG. 9  shows the extended control rod guide assembly  90  with its flanges  96  and  98  installed in a recess in the intermediate coupling  86  which is locked in place by the bolts  112 .  FIG. 10  shows a perspective view of the upper segment  92  of the extended control rod guide assembly  90  installed within the lower guide tube section  78  with the flange  96  resting on the lower flange of the intermediate coupling  86 . In this embodiment where not all of the control rods receive continuous support over the length of the extended control rod guide assembly, the control rod guide assembly may extend approximately between 0.9 and 23 inches (2.3 and 58.4 cm). More preferably, this type of control rod guide assembly extends axially approximately between 0.9 and 7 inches (2.3 and 17.8 cm). 
         [0039]    Like reference characters are employed for corresponding components among the several views. In another embodiment shown in  FIGS. 11-14  the upper segment  92  and the lower segment  94  of the extended control rod guide assembly  90  are respectively made of a solid continuous length of material such as stainless steel with the openings  72 ,  74  and  76  providing continuous guidance for all of the control rods over substantially the entire length of the extended control rod guide assembly. As in the above case the upper segment  92  and the lower segment  94  may be joined at the intermediate coupling  86  which captures the respective flanges  96  and  98 . Alternately, the upper segment  92  and the lower segment  94  can be constructed as one piece with the flange  96 / 98  radially extending from an intermediate elevation for capture within the intermediate coupling. In this latter embodiment, the control rod guide assembly may extend approximately between 1.6 and 23 inches (4.1 and 58.4 cm). More preferably, the control rod guide assembly extends axially approximately between 1.6 and 7 inches (4.1 and 17.8 cm). 
         [0040]    Thus, the extended control rod guide assembly of this invention, when installed, provides extended support for the control rod assembly that compensates for some of the wear in the guide cards  70 , while adding a minimum of additional friction to the control rod travel path and substantially extends the life of a control rod guide tube  54  without requiring replacement of either of the guide tube sections. 
         [0041]    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.