Abstract:
A method and apparatus for monitoring a parameter in an irradiated environment and communicating a signal representative of the monitored parameter to a less caustic environment that employs a wireless transmitter that is powered by the irradiated environment. The power for the wireless transmitter is derived from a self-powered radiation detector disposed within the radioactive environment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent application Ser. No. ______ (Attorney Docket NSD2010-009), entitled SELF-POWERED WIRELESS IN-CORE DETECTOR, filed concurrently herewith. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention pertains generally to methods and apparatus for monitoring the radiation within the core of a nuclear reactor and more particularly to such methods and systems that can economically replace movable in-core detectors in aged operating plants. 
         [0004]    2. Related Art 
         [0005]    In many of the aged reactors presently in operation, reliance for nuclear flux measurements, as used in the reactor control and protection systems, has been placed on out-of-core detectors supplemented by an in-core flux mapping system, which has been in use since as early as 1967; with the latter originally intended to provide proofs of core design and some calibration measurements. 
         [0006]    With a growing trend toward larger reactor cores, there has been some concern as to the possible adverse affects of core power distributions and whether out-of-core detectors could adequately detect such possible adverse distributions. Test data showed that the split section out-of-core detectors, presently in use, responded to axial flux tilt, but the accuracies of the tilt measurements were affected by the geometry and construction materials employed at the detector wells and by the spacing between the vessel and the detectors. Correction factors were developed for these affects, but there was still some question as to whether out-of-core detectors will in all cases provide an adequate alarm of an adverse power distribution. 
         [0007]    To obviate the foregoing concern, a method was developed for automatically monitoring the power distribution employing the moveable in-core detectors by providing a more accurate, detailed, automatic, frequently updated, data readout of the reactor core power distribution. The method, taught in U.S. Pat. No. 3,932,211, issued Jan. 13, 1976 and assigned to the Assignee of this invention, inserts the moveable detectors into the reactor core region during normal power operation according to a predetermined, intermittent, timed program. Upon insertion, the detectors are automatically driven through the core region along fixed predetermined paths. The outputs of the detectors are recorded as a function of core location to provide a graphical representation of the reactor power distribution. 
         [0008]    Preferably, a plurality of the moveable detectors are arranged in electrically redundant groupings are normally stored within the reactor thermal environment outside of the core reactivity region to minimize thermal cycling. In operation, the detectors are driven into the reactor through the reactor vessel&#39;s lower head, through the core support plate and through prescribed fuel assembly bottom nozzles to the fuel assembly instrumentation tubes through which the detector is extended to the desired core elevation. As dictated by the predetermined time program, alternate groupings of detectors are driven along corresponding linear paths within the instrumentation thimbles within the core at staggered time intervals governed by the reactor core physics. The programmed detector drive sequence is automatically reinitiated upon a given controlled reactivity change to provide the most meaningful data input to the plant operator. 
         [0009]    The movable detector drives and routing systems are complicated and, due to the increased use and age are subject to failure. An option for the replacement of an aging moveable in-core detector system in an operating plant, to allow continued or extended plant operation, is a system that uses nuclear sensors that remain inserted at fixed axial and radial positions in the reactor core throughout the entire operating cycle. This type of sensor is called a fixed in-core detector and has the ability to measure the radioactivity within the core at a number of axial elevations. These sensors are used to measure the radial and axial distribution of power inside the reactor core. This power distribution measurement information is used to determine whether the reactor is operating within nuclear design power distribution limits and, thus, avoids the complexity of the moveable in-core detectors. The typical in-core sensor used to perform this measurement function 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. 28, 1998, and assigned to the Assignee of this invention.  FIG. 1  provides a diagram of the mechanisms that produced 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 in-core 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 portions of the full length signal generated by the single neutron sensitive emitter element attributable to various axial regions of the core are determined from a 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 affects of the delayed gamma radiation due to fission products. The in-core instrumentation thimble assemblies also include a thermocouple  18  for measuring the coolant temperature 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 is 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. 
         [0010]      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  and communicates the thermocouple (TIC) and self-powered detector (SPD) signals to a single 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 (RJU)  32  which transmits the signals to an inadequate core cooling monitor (ICCM)  34  which communicates with the plant computer  36  which is also connected to the work station  30 . 
         [0011]    In order to replace an existing moveable in-core detector system at an operating plant, it is necessary to install the cables  22  routing the self-powered detectors  14  and thermocouples  18  signals. It is also necessary to install and power hardware  26  and  32  that can digitize and multiplex the self-powered detector signals for transmission out of the containment  28 . This equipment is very expensive, and represents approximately 60% of the costs of a WINCISE™ system. 
         [0012]    Accordingly, it is desirable to provide an improved method and apparatus for replacing moveable in-core detector systems in operating plants. 
         [0013]    Additionally, it is desirable to provide such an improved system that does away with much of the cable routing and the electronics that need to be placed inside the containment. 
         [0014]    Furthermore, such an improved system should preferably be self-powered and substantially maintenance free. 
       SUMMARY OF THE INVENTION 
       [0015]    This invention provides apparatus and a methodology that avoids most of the cost associated with the in-containment cables, power equipment, and self-powered detector signal processing electronic hardware currently associated with replacing moveable in-core detectors with fixed in-core detectors. The method of this invention that achieves the foregoing objectives monitors the parameters indicative of a condition of an irradiated environment and generates a signal representative of the monitored parameter. In monitoring the parameter, the method generates an electrical current from the irradiated environment and uses that electrical current to power a wireless transmitter. The wireless transmitter then communicates the signal indicative of the monitored parameter to a less caustic environment, preferably outside the containment. 
         [0016]    In one embodiment, the method includes the steps of powering the wireless transmitter from a rechargeable battery and recharging the rechargeable battery from a charger that derives its power from the irradiated environment. Desirably, the charger performing the recharging step is a self-powered radiation detector. In one application, the irradiated environment is a nuclear reactor and the wireless transmitter and charger are a part of an in-core instrument thimble assembly. 
         [0017]    The invention also contemplates an in-core instrument thimble assembly having a self-powered radiation detector assembly responsive to a radiation environment in which it is placed to generate an electric current representative of the strength of the radiation environment. A wireless transmitter is connected to the electric current generated by the self-powered radiation detector, for transmitting at least a portion of the electric current wirelessly to a less caustic environment. The in-core instrument thimble assembly further includes a power source for powering the wireless transmitter that derives its power from the radiation environment. 
         [0018]    Preferably, the self-powered radiation detector assembly includes a first self-powered radiation detector that supplies power to the power source. In one embodiment, the first self-powered radiation detector that suppliers power to the power source also generates the electric current representative of the strength of the radiation environment. In another embodiment, a second self-powered radiation detector generates the electric current representative of the strength of the radiation environment. 
         [0019]    Preferably, the power source is derived from a rechargeable battery for powering the wireless transmitter and a charger is connected to the rechargeable battery that derives its power from the irradiated environment. Desirably, the charger is a self-powered radiation detector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    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: 
           [0021]      FIG. 1  is a schematic representation of a self-powered detector; 
           [0022]      FIG. 2A  is a plan view of an in-core instrument thimble assembly; 
           [0023]      FIG. 2B  is a schematic view of the interior of the forward sheath of the in-core instrument thimble assembly of  FIG. 2A ; 
           [0024]      FIG. 2C  is a sectional view of the electrical connector at the rear end of the in-core instrument thimble assembly of  FIG. 2A ; 
           [0025]      FIG. 3  is a schematic layout of an in-core monitoring system; 
           [0026]      FIG. 4  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0027]      FIG. 5  is an elevational view partially in section of a nuclear reactor vessel and interior components to which this invention can be applied; 
           [0028]      FIG. 6  is an elevational view, partially in section, of a nuclear fuel assembly; 
           [0029]      FIG. 7  is a perspective view of a basic in-core moveable detector mapping system employed in many operating plants; 
           [0030]      FIG. 8  is a schematic circuitry diagram of a power supply that can be employed by this invention; and 
           [0031]      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 
       [0032]    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 a system of pipes which are connected to the vessel form a loop of the primary side. 
         [0033]    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 as 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 . 
         [0034]    An exemplary reactor design 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 and 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  58 , 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  62  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 . The coolant then flows upwardly and radially to one or more outlet nozzles  70 . 
         [0035]    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 perforations  68  in the upper core plate  66 . 
         [0036]    The rectilinearly moveable control rods  56  typically include a drive shaft  76  and a spider assembly  78  of neutron poison rods that are guided through upper internals  54  and into aligned fuel assemblies  80  by control rod guide tubes  79 . 
         [0037]      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. 
         [0038]    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 and integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0039]    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 by-products from entering the coolant and further contaminating the reactor system. 
         [0040]    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 a 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 rod 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 motive power of control rod drive shafts  76  (shown in  FIG. 5 ) which are coupled to the control rod hubs  102 , all in a well-known manner. 
         [0041]    As previously mentioned, to map the axial power distribution, to assure plant design limits are not exceeded, many conventional plants employ moveable in-core detectors such as are illustrated in  FIG. 7 . The in-core mapping system generally comprises four to six detector/drive assemblies, depending upon the size of the plant, which are interconnected in such a fashion that they can access various combinations of in-core flux thimbles that extend within the instrumentation tubes  92  within the fuel assemblies  80 . To obtain the thimble interconnection capability, each detector has associated with it a five-path and ten-path rotary mechanical transfer device. A core map is made by selecting, by way of the transfer devices, particular thimbles through which the detectors are driven. 
         [0042]      FIG. 7  shows the basic system for the insertion of the moveable in-core radiation detectors. Retractable thimbles  110  into which the moveable radiation detectors  112  are driven, take the routes approximately as shown. The thimbles  110  are inserted into the reactor core  44  through conduits  118  extending from the bottom of the reactor vessel  40  through the concrete shield area  114  and then up to a thimble seal table  24 . Since the moveable detector in-core flux thimbles are closed at the leading (reactor) end, they are dry inside. The in-core flux thimbles thus serve as a pressure barrier between the reactor water pressure (2500 psig design) and the atmosphere. Mechanical seals between the retractable thimbles  110  and the conduits  118  are provided at the sealed table  116 . The conduits  118  are essentially extensions of the reactor vessel  40 , with the thimbles  110  allowing the insertion of the in-core instrumentation moveable detectors  112 . During operation, the thimbles  110  are stationary and will be retracted only under depressurized conditions during fueling or maintenance operations. 
         [0043]    The drive system for insertion of the moveable in-core detectors includes drive units  120 , limit switch assemblies  122 , five-path rotary transfer devices  124 , ten-path rotary transfer devices  126  and isolation valves  128 , as shown. 
         [0044]    Each drive unit pushes a hollow, helical-wrap drive cable into the core with a miniature moveable detector  112  attached to the leading end of the cable and a small diameter coaxial cable, which communicates the detector output, threaded through the hollow center of the helical-wrap drive, back to the trailing end of the drive cable. The electronics for transferring the detector signals to the control room is similar to the schematic layout shown in  FIG. 3  and previously described for a fixed in-core detector system. The preferred embodiment of the methodology described herein for this invention allows elimination of most of the cost associated with in-containment cables, power equipment, and detector signal processing electronics hardware that would otherwise be necessary if the moveable flux mapping detector systems were replaced with the fixed in-core detector system previously described. 
         [0045]    The preferred embodiment relies on the use of small radio frequency transmitters that are attached to the electrical connector outputs of the in-core instrument thimble assemblies, like the one shown in  FIGS. 2A ,  2 B and  2 C, described above. However, it should be appreciated that other wireless transmitters, such as infrared transmitters may be used as well, with compatible receivers, provided the receivers can be situated within a line of sight. The electrical hardware used in accordance with this invention to measure and transmit the individual self-powered detector signals (either analog or digital) is contained in a small environmentally qualified container that is either integral with the in-core instrument thimble assembly  16  design, or may be attached as a stand-alone device. The primary electrical power source for the signal transmitting electrical hardware is a rechargeable battery  132  shown as part of the exemplary power supply illustrated in  FIG. 8 . In accordance with one embodiment of this invention, the charge on the battery  132  is maintained by the use of the electrical power produced by a dedicated power supply self-powered 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 a signal transmitter circuit  138  that transmits the signal received from the fixed in-core detector monitoring the core such as was described with respect to  FIGS. 2A ,  2 B and  2 C. Alternately, a certain percentage of the power being generated by the monitoring detectors could be diverted to the signal conditioning circuit  136  to charge the battery  132  to power the signal transmitter circuit  138 . In this latter embodiment, the remainder of the power from the monitoring circuits could be then communicated to the transmitter circuit  138  for communication wirelessly to a receiving circuit outside the nuclear containment. The monitored signal thus received would then have to be calibrated to compensate for the strength of the signal that was diverted to power the signal transmitting circuit  138 . The specific design of the power supply shown in  FIG. 8  will solely depend on the amount of power required to transmit the self-powered detector signal data to the receiver. The amount of power needed depends on the signal transmission distance and the radio frequency signal to noise characteristics of the particular operating environment. In general, the power supply is designed to produce power based on the amount of gamma radiation around it to provide the longest lifetime and power generation capabilities at low reactor power levels. There are a number of existing platinum-based self-powered detector designs, such as is described in U.S. Pat. No. 5,745,538, that will provide suitable performance for common applications. 
         [0046]      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 cabling within the containment above the seal table  24  has been replaced by the wireless transmitter  138  and wireless receivers  116  and 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. Thus, this invention greatly simplifies the replacement of the conventional in-core moveable detectors illustrated in  FIG. 7  with the fixed in-core detectors illustrated in  FIGS. 2A ,  2 B and  2 C. 
         [0047]    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.