Abstract:
An in-core neutron monitor that employs vacuum microelectronic devices to configure an in-core instrument thimble assembly that monitors and wirelessly transmits a number of reactor parameters directly from the core of a nuclear reactor without the use of external cabling. The in-core instrument thimble assembly is substantially wholly contained within an instrument guide tube within a nuclear fuel assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent application Ser. No. ______ (Attorney docket NSD2010-006), entitled SELF-POWERED WIRELESS IN-CORE DETECTOR, filed concurrently herewith. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention pertains generally to apparatus for monitoring the radiation within the core of a nuclear reactor and, more particularly, to such apparatus that will not obstruct refueling of the reactor. 
         [0004]    2. Related Art 
         [0005]    In many state-of-the-art nuclear reactor systems in-core sensors are employed for measuring the radioactivity within the core at a number of axial elevations. These sensors are used to measure the radial and axial distribution of the power inside the reactor core. This power distribution measurement information is used to determine whether the reactor is operating within nuclear power distribution limits. The typical in-core sensor used to perform this function is a self-powered detector that produces an electric current that is proportional to the amount of fission occurring around it. This type of sensor does not require an outside source of electrical power to produce the current and is commonly referred to as a self-powered detector and is more fully described in U.S. Pat. No. 5,745,538, issued Apr. 20, 1998, and assigned to the Assignee of this invention.  FIG. 1  provides a diagram of the mechanisms that produce the current I(t) in a self-powered detector element  10 . A neutron sensitive material such as vanadium is employed for the emitter element  12  and emits electrons in response to neutron irradiation. Typically, the self-powered detectors are grouped within instrumentation thimble assemblies. A representative in-core instrumentation thimble assembly is shown in  FIG. 2 . The signal level generated by the essentially non-depleting neutron sensitive emitter element  12  shown in  FIG. 1 , is low, however, a single, full core length neutron sensitive emitter element provides an adequate signal without complex and expensive signal processors. The proportions of the full length signal generated by the single neutron sensitive emitter element attributable to various axial regions of the core are determined from apportioning the signal generated by different lengths of gamma sensitive elements  14  which define the axial regions of the core and are shown in  FIG. 2 . The apportioning signals are ratioed which eliminates much of the effects of the delayed gamma radiation due to fission products. The in-core instrumentation thimble assemblies also include a thermocouple  18  for measuring the temperature of the coolant exiting the fuel assemblies. The electrical signal output from the self-powered detector elements and the thermocouple in each in-core instrumentation thimble assembly in the reactor core are collected at the electrical connector  20  and sent to a location well away from the reactor for final processing and use in producing the measured core power distribution. 
         [0006]      FIG. 3  shows an example of a core monitoring system presently offered for sale by Westinghouse Electric Company LLC with the product name WINCISE™ that employs fixed in-core instrumentation thimble assemblies  16  within the instrument thimbles of fuel assemblies within the core to measure the core&#39;s power distribution. Cabling  22  extends from the instrument thimble assemblies  16  through the containment seal table  24  to a signal processing cabinet  26  where the outputs are conditioned, digitized and multiplexed and transmitted through the containment walls  28  to a computer workstation  30  where they can be further processed and displayed. The thermocouple signals from the in-core instrumentation thimble assemblies are also sent to a reference junction unit  32  which transmits the signals to an inadequate core cooling monitor  34  which communicates with the plant computer  36  which is also connected to the workstation  30 . Because of the hostile environment, the signal processing cabinet  26  has to be located a significant distance away from the core and the signal has to be sent from the detector  16  to the signal processing cabinet  26  through specially constructed cables that are extremely expensive and the long runs reduce the signal to noise ratio. Unfortunately, these long runs of cable have proved necessary because the electronics for signal processing has to be shielded from the highly radioactive environment surrounding the core region. 
         [0007]    In previous nuclear plant designs, the in-core detectors entered the reactor vessel from the lower hemispherical end and entered the fuel assemblies instrumentation thimble from the bottom fuel assembly nozzle. In at least some of the current generation of nuclear plant designs, such as the AP1000 nuclear plant, the in-core monitoring access is located at the top of the reactor vessel, which means that during refueling all in-core monitoring cabling will need to be removed before accessing the fuel. A wireless in-core monitor that is self-contained within the fuel assemblies and wirelessly transmits the monitored signals to a location remote from the reactor vessel would allow immediate access to the fuel without the time-consuming and expensive process of disconnecting, withdrawing and storing the in-core monitoring cables before the fuel assemblies could be accessed, and restoring those connections after the refueling process is complete. A wireless alternative would thus save days in the critical path of a refueling outage. A wireless system also allows every fuel assembly to be monitored, which significantly increases the amount of core power distribution information that is available. 
         [0008]    However, a wireless system requires that electronic components be located at or very near the reactor core where gamma and neutron radiation and high temperatures would render semiconductor electronics inoperable within a very short time. Vacuum tubes are known to be radiation insensitive, but their size and current demands have made their use impractical until recently. Recent developments in micro-electromechanical devices have allowed vacuum tubes to shrink to microscopic sizes and significantly reduced power draw demands. 
         [0009]    Accordingly, it is an object of this invention to improve the critical path for refueling a reactor by significantly reducing the number of cables attached to the reactor head that would have to be removed and reconnected in the course of the refueling process. 
         [0010]    It is a further object of this invention to provide a fuel assembly with a self-contained instrument thimble assembly that can be inserted into the core of a nuclear reactor and placed in operation without the necessity of routing cabling and connectors through the reactor vessel to activate the instrumentation. 
         [0011]    It is an additional object of this invention to increase the amount of in-core power distribution data that is communicated to the plant operator. 
       SUMMARY OF THE INVENTION 
       [0012]    These and other objectives are achieved by the apparatus of this invention which avoids the necessity of running expensive electrical cables through the reactor head and reactor internals to connect with and energize the in-core instrumentation. In accordance with this invention, a nuclear reactor in-core detector system is provided, including an in-core nuclear instrumentation thimble assembly that is substantially wholly contained within an instrument thimble within a nuclear fuel assembly. The instrument thimble assembly includes a self-powered, fixed, in-core detector for monitoring a reactor core parameter indicative of a state of the reactor core and providing an electric output representative of the monitored parameter. The instrument thimble assembly also includes a wireless transmitter that is connected to receive the electrical output from the self-powered fixed in-core detector and wirelessly transmit that signal to a location outside the reactor. Desirably, the wireless transmitter comprises a number of electronic components at least one of which is a vacuum microelectronic device and, preferably, a vacuum diode placed in a grid circuit of an amplifier which is connected to the electrical output of the self-powered, fixed, in-core detector and responds substantially logarithmically, thus enabling the electronic components to follow the monitored neutron flux from start-up to full power of a nuclear reactor in which the in-core detector system is disposed. 
         [0013]    In another embodiment, in addition to the amplifier, the electronics components include a current-to-voltage converter and a voltage controlled oscillator with an output of the amplifier connected to an input of the current-to-voltage converter whose output is connected to an input of the voltage controlled oscillator that provides a frequency output proportional to a voltage on the input of the voltage controlled oscillator. In that way, the current which is the electrical output representative of the monitored parameter, which is connected to the amplifier, is converted to a corresponding frequency signal that can be transmitted by a wireless transmitter. In still another embodiment, the voltage controlled oscillator comprises a micro-electronic reactance tube. 
         [0014]    Preferably, the electronic components comprise—an input of a first amplifier connected to the electrical output of the self-powered, fixed, in-core detector; the input of the current-to-voltage converter connected to an output of the amplifier; the input of the voltage controlled oscillator connected to the output of the current-to-voltage converter; an input of a second amplifier connected to the output of the voltage controlled oscillator; and a wireless transmission circuit connected to an output of the second amplifier for wirelessly transmitting the output of the second amplifier. Desirably, the nuclear reactor in-core detector system includes a wireless receiver circuit and signal conditioning component designed to be situated outside the highly radioactive environment of the nuclear reactor containment, and preferably, including conventional solid state components. 
         [0015]    In still another embodiment, the nuclear reactor in-core detector system includes a wireless receiver positioned outside and within the vicinity of the reactor vessel for receiving signals from the wireless transmitter and a retransmitter for transmitting outside the containment the signals received from the wireless transmitter. Desirably, the retransmitter is a second wireless transmission circuit that transmits the signals received from the wireless transmitter to a second wireless receiver that communicates the signals received from the wireless transmitter, by way of the wireless receiver and the retransmitter, to processing circuitry outside the containment. Desirably, the second wireless receiver is positioned within the vicinity of a containment wall that shields the primary circuit of a nuclear power generation facility in which the in-core detector system is placed. 
         [0016]    In a further embodiment, the invention comprises a nuclear fuel assembly having a top nozzle and a bottom nozzle and a plurality of thimble tubes extending between and substantially connected to the top nozzle and the bottom nozzle. At least one of the thimble tubes comprises an instrumentation thimble that houses and substantially completely contains the fixed in-core monitoring component of the detector system of this invention. 
     
    
     
       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 schematic representation of a self-powered radiation detector; 
           [0019]      FIG. 2A  is a plan view of an in-core instrument thimble assembly; 
           [0020]      FIG. 2B  is a schematic view of the interior of the forward sheath of the in-core instrument thimble assembly of  FIG. 2A ; 
           [0021]      FIG. 2C  is a sectional view of the electrical connector at the rear end of the in-core instrument thimble assembly of  FIG. 2A ; 
           [0022]      FIG. 3  is a schematic layout of an in-core monitoring system; 
           [0023]      FIG. 4  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0024]      FIG. 5  is an elevational view, partially in section, of a nuclear reactor vessel and interior components to which this invention can be applied; 
           [0025]      FIG. 6  is an elevational view, partially in section of a nuclear fuel assembly that contains the in-core nuclear instrument thimble assembly of this invention; 
           [0026]      FIG. 7  is a block diagram of the electronics of this invention; 
           [0027]      FIG. 8  is a schematic circuitry diagram of a power supply that can be employed by this invention to energize the electronic circuitry illustrated in  FIG. 7 ; and 
           [0028]      FIG. 9  is a schematic layout of a self-powered wireless in-core instrumentation core power distribution measurement system in accordance with this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    The primary side of nuclear power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from 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 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. 
         [0030]    For the purpose of illustration,  FIG. 4  shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  40  having a closure head  42  enclosing a nuclear core  44 . A liquid reactor coolant, such as water, is pumped into the vessel  40  by pump  46  through the core  44  where heat energy is absorbed and is discharged to a heat exchanger  48 , typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such a steam driven turbine generator. The reactor coolant is then returned to the pump  46  completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel  40  by reactor coolant piping  50 . 
         [0031]    An exemplary reactor design incorporating this invention is shown in  FIG. 5 . In addition to the core  44  comprised of a plurality of parallel, vertical, co-extending fuel assemblies  80 , for purposes of this description, the other vessel internal structures can be divided into the lower internals  52  and the upper internals  54 . 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  54  restrain or provide a secondary restraint for the fuel assemblies  80  (only two of which are shown for simplicity in this figure), and support guide instrumentation and components, such as control rods  56 . In the exemplary reactor shown in  FIG. 5 , coolant enters the reactor vessel  40  through one or more inlet nozzles, flows down through an annulus between the vessel  40  and the core barrel  60 , is turned 180° in a lower reactor vessel plenum  61 , passes upwardly through a lower support plate and a lower core plate  64 , upon which the fuel assemblies  80  are seated, and through and about the assemblies. In some designs, the lower support plate  62  and the lower core plate  64  are replaced by a single structure, the lower core support plate that has the same elevation as  62 . Coolant exiting the core  44  flows along the underside of the upper core plate  66  and upwardly and through a plurality of perforations  68  in the upper core plate  66 . The coolant then flows upwardly and radially to one or more outlet nozzles  70 . 
         [0032]    The upper internals  54  can be supported from the vessel  40  or the vessel head  42  and includes an upper support assembly  72 . Loads are transmitted between the upper support assembly  72  and the upper core plate  66  primarily by a plurality of support columns  74 . Each support column is aligned above a selected fuel assembly  80  and perforation  68  in the upper core plate  66 . 
         [0033]    The rectilinearly movable control rods  56  typically include a drive shaft  76  and a spider assembly  78  of neutron poison rods that are guided through the upper internals  54  and into aligned fuel assemblies  80  by control rod guide tubes  79 . 
         [0034]      FIG. 6  is an elevational view represented in vertically shortened form, of a fuel assembly being generally designated by reference character  80 . The fuel assembly  80  is the type used in a pressurized water reactor and has a structural skeleton which at its lower end includes a bottom nozzle  82 . The bottom nozzle  82  supports the fuel assembly  80  on the lower core support plate  64  in the core region of the nuclear reactor. In addition to the bottom nozzle  82 , the structural skeleton of the fuel assembly  80  also includes a top nozzle  84  at its upper end and a number of guide tubes or thimbles  86 , which extend longitudinally between the bottom and top nozzles  82  and  84  and at opposite ends are rigidly attached thereto. 
         [0035]    The fuel assembly  80  further includes a plurality of transverse grids  88  axially spaced along and mounted to the guide thimbles  86  (also referred to as guide tubes) and an organized array of elongated fuel rods  90  transversely spaced and supported by the grids  88 . Although it cannot be seen in  FIG. 6 , the grids  88  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  90  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  80  has an instrumentation tube  92  located in the center thereof that extends between and is mounted to the bottom and top nozzles  82  and  84 . With such an arrangement of parts, the fuel assembly  80  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0036]    As mentioned above, the fuel rods  90  in the array thereof in the assembly  80  are held in spaced relationship with one another by the grids  88  spaced along the fuel assembly length. Each fuel rod  90  includes a plurality of nuclear fuel pellets  94  and is closed at its opposite ends by upper and lower end plugs  96  and  98 . The fuel pellets  94  are maintained in a stack by a plenum spring  100  disposed between the upper end plug  96  and the top of the pellet stack. The fuel pellets  94 , 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 systems. 
         [0037]    To control the fission process, a number of control rods  56  are reciprocably movable in the guide thimbles  86  located at predetermined positions in the fuel assembly  80 . Specifically, a rod cluster control mechanism (also referred to as the spider assembly)  78  positioned above the top nozzle  84  supports the control rods  56 . The control mechanism has an internally threaded cylindrical hub member  102  with a plurality of radially extending flukes or arms  104  that with the control rods  56  form the spider assembly  78  that was previously mentioned with respect to  FIG. 5 . Each arm  104  is interconnected to the control rods  56  such that the control mechanism  78  is operable to move the control rods vertically in the guide thimbles  86  to thereby control the fission process in the fuel assembly  80 , under the motor power of control rod drive shaft  76  (shown in  FIG. 5 ) which are coupled to the control rod hubs  102 , all in a well known manner. 
         [0038]    As mentioned above, in the AP1000 nuclear plant design, the in-core monitoring access is through the top of the reactor vessel, which is a significant departure from previous designs which fed the fixed in-core detector cables through the bottom of the vessel and into the fuel assembly instrument thimbles through the lower fuel assembly nozzle. The change in design means that during refueling all conventional in-core monitoring cabling will need to be removed before accessing the fuel. This invention provides a wireless in-core monitor that is wholly contained within the instrument thimble within the fuel assemblies without any tether that extends outside the core and would permit access to the fuel without going through the costly and time-consuming steps of removing and reconnecting the cabling. In accordance with this invention, the in-core instrument thimble assembly is illustrated as a block diagram in  FIG. 7  and includes, in addition to the fixed in-core neutron detector, a self-contained power source and a wireless transmission circuit. Within the transmission circuit, the neutron detector output current is fed directly into an amplifier  112 , thus eliminating cabling concerns. One or more stages of amplification are provided within the amplifier  112 , using vacuum micro-electronic devices. A vacuum diode is preferably placed in the grid circuit of the amplifier to make the amplifier respond logarithmically, thus enabling the electronics to follow the neutron flux from start-up through full power. The amplified signal is then fed to a current-to-voltage converter  114 . The output voltage of the current-to-voltage converter  114  is used as the input to a voltage controlled oscillator  118  which converts the voltage input to a frequency output. As the neutron flux changes, so will the voltage input to the voltage controlled oscillator, which will vary the output frequency. A vacuum micro-electronic reactance tube can be used for the voltage controlled oscillator  118 . Such an arrangement provides a precise correlation between the neutron flux monitored by the neutron detector  10  and the output frequency of the voltage controlled oscillator  118 . That output is then amplified by amplifier  120  whose output is communicated to a wireless transmitter  122  within the in-core instrument thimble assembly  16 . The in-core instrument thimble assembly  16  can be made up of a single unit housing the neutron detector, power supply and transmission circuit or it can be made up of modular units, e.g., the self-contained power supply, neutron detector and transmission circuit, respectively. 
         [0039]    The primary electrical power source for the signal transmitting electrical hardware is the rechargeable battery  132  shown as part of the exemplary power supply illustrated in  FIG. 8 . The charge on the battery  132  is maintained by the use of the electrical power produced by a dedicated power supply self-power detector element  134  that is contained within the power supply  130 , so that the nuclear radiation in the reactor is the ultimate power source for the device, keeping the battery  132  charged. The power supply self-powered detector element  134  is connected to the battery  132  through a conditioning circuit  136  and the battery is in turn connected to the signal transmitter circuit  138  that transmits the signal received from the fixed in-core detector and the thermocouple monitoring the core such as was described with respect to  FIGS. 2A ,  2 B and  2 C. The self-contained power supply is more fully described in U.S. patent application Ser. No. ______ (Attorney Docket NSD2010-006). 
         [0040]      FIG. 9  shows a schematic layout of a self-powered wireless in-core detector instrumentation core power distribution measurement system constructed in accordance with this invention. The schematic layout illustrated in  FIG. 9  is identical to the schematic layout illustrated in  FIG. 3  for a conventional in-core monitoring system, except that the in-core instrument thimble assembly has been rotated 180° so that the electrical connectors for the detector element are closer to a receiver of the wireless transmitted signal and the cabling has been replaced by the wireless transmitters and receivers  122 ,  124 ,  138  and  116 , the in containment electronics  26  and  32  have been respectively replaced by the SPD signal processing system  108  and the core exit thermocouple signal processing system  106 , located outside the containment  28 . In all other respects, the systems are the same. 
         [0041]    As can also be appreciated from  FIG. 9 , the signal from the in-core instrument thimble assembly  16  wireless transmitter  122  is received by an antenna  124  on the underside of the reactor vessel head  42  which communicates with a combination wireless receiver and retransmitter  138  on the reactor head  42 . In that way, the reactor head  42  can be removed and the fuel assemblies accessed without the in-core instrumentation being an obstacle. Placement of the transmitting antenna on the reactor vessel will depend on the reactor design but the intent is to transmit from a close proximity to the reactor vessel at a location that would not be an impediment to accessing the fuel assemblies. The neutron signal is then retransmitted by the retransmission circuit  138  to a receiver  116  proximate the containment outer wall. The combination receiver and retransmitter  138  should similarly be constructed from vacuum microelectronic devices because of their close proximity to the reactor vessel; however, the receivers  116  and the processing circuitry  106  and  108  can be constructed from conventional solid state components and may be located within the containment remote from the reactor vessel or outside the containment. Thus, this invention greatly simplifies the transmission of the in-core detector signals and the refueling operation. 
         [0042]    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.