Abstract:
A self-powered neutron detector having an emitter with a slightly negative bias voltage that assures that an increase in the electrons that enter the insulator are counted and decreases or eliminates the gamma induced prompt signal. Variations in the size of the bias is used as a diagnostic tool to estimate the gamma induced prompt signal.

Description:
BACKGROUND 
     1. Field 
     The present invention pertains generally to apparatus for monitoring radiation within the core of a nuclear reactor and, more particularly, to such apparatus that is self-powered. 
     2. Related Art 
     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 and radial locations. These sensors are used to measure the axial and radial 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 in modem nuclear reactors 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. 28, 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 within the fuel 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 that can be processed for determining core power at the sensor&#39;s location. The proportions of the signal from the full length emitter attributable to various axial regions of the core are determined from apportioning the signal generated by different lengths of primarily neutron 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. 
       FIG. 3  shows an example of a core monitoring system presently offered for sale by Westinghouse Electric Company LLC, Cranberry Township Pennsylvania, 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 assembly  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 work station  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 work station  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 environments surrounding the core region. 
     In previous nuclear plant designs, the in-core detectors enter the reactor vessel from the lower hemispherical end and enter the fuel assemblies&#39; instrumentation thimble from the bottom fuel assembly nozzle. In at least some of the current generation of nuclear plant designs, such as the AP 1000® nuclear plant, the in-core monitoring access is located at the top of the reactor vessel, which means that during refueling all the in-core instrument thimble assemblies will need to be removed from the core before accessing the fuel. In either arrangement, the long runs of signal cable are necessary to isolate the electronics from the harmful effects of radiation emanating from the core. 
     Self-powered detectors are generally coaxial in design with a center emitter wire, an annular alumina insulator and an outer metallic sheath. By some physical process the central wire emits electrons, some of which form the detector current. Some electrons slow down in the insulator leading to a space charge therein. The emitter and sheath are more or less at ground potential. The radius in the insulator where the minimum (most negative) potential occurs determines if charges that come to rest within the insulator are counted or not. For example, a Compton electron enters the insulator from the sheath with 300 keV of energy and comes to rest (due to collisions) in the insulator just inside the (probably less in magnitude than −1 volt) minimum potential radius. This particular electron is then directed to the emitter by the potential inside the insulator. As such, it creates a charge flow that subtracts from the total detector current. 
     Existing self-powered detectors have a reduced sensitivity due to the electrical potential trough that builds up in the insulating annulus. This is caused by a portion of the Compton electrons and beta particles coming to rest in the insulator due to kinetic interactions. The minimum potential formed by these particles is small, perhaps not even minus one volt, but is enough to direct the at rest electrons on the inside of the insulator back to the emitter. These charges are then not counted as they have finally not escaped the emitter. Although the minimum potential is small, it typically occurs at a depth in the insulator that takes 100&#39;s of keV of kinetic energy to reach. This then precludes low energy electrons or betas from contributing to the detector current. Similarly, the predominantly low energy photoelectric electrons are not able to penetrate the insulator to a depth on the outer side of the minimum potential. Consequently, they too are not counted. 
     It is an object of this invention to improve the sensitivity of self-powered neutron detectors. 
     Furthermore, it is an object of this invention to increase the sensitivity of the self-powered detectors without substantially altering the configuration of existing systems. 
     SUMMARY 
     These and other objects are achieved by employing a self-powered neutron detector having a neutron sensitive emitter element and an annular insulator substantially coaxially disposed around the emitter element. An outer electrically conductive sheath is disposed around the annular insulator and a negative bias voltage is applied across the neutron sensitive emitter. The negative bias voltage is approximately between 1 and 30 volts, and, more preferably, between 1 and 2 volts. In one embodiment, the self-powered detector includes a coating of selenium around at least a portion of the outside of the emitter. Preferably, the negative bias voltage is adjustable to identify the gamma induced part of the output signal as a fraction of the beta induced fraction. 
     The invention further contemplates a nuclear fuel assembly having an instrument thimble housing such as a self-powered neutron detector. The invention further contemplates a nuclear reactor system including a fuel assembly having an instrument thimble housing such a self-powered detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic representation of a self-powered irradiation detector; 
         FIG. 2A  is a plan view of an in-core instrument thimble assembly which can benefit from this invention; 
         FIG. 2B  is a schematic view of the interior of the forward sheath of the in-core instrument thimble assembly of  FIG. 2A ; 
         FIG. 2C  is a sectional view of the electrical connector at the rear end of the in-core instrument thimble assembly of  FIG. 2A ; 
         FIG. 3  is a schematic layout of an in-core monitoring system; 
         FIG. 4  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
         FIG. 5  is an elevational view, partially in section, of a nuclear reactor vessel and interior components to which this invention can be applied; 
         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; and 
         FIG. 7  is a schematic circuitry diagram illustrating the negative bias voltage applied to the emitter of the self-powered detector of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The primary side of many 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 of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. 
     For the purpose of illustration,  FIG. 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 the 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 . 
     An exemplary reactor design which can benefit from 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 function 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 in  FIG. 5  for simplicity), 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  59 , 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  in the upper core plate  66 . The coolant then flows upwardly and radially to one or more outlet nozzles  70 . 
     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 . 
     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 the upper internals  54  and into aligned fuel assemblies  80  by control rod guide tubes  79 . 
       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. 
     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 there between; 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. 
     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. 
     To control the fission process, a number of control rods  56  are reciprocally 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 a control rod  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 is coupled to the control rod hub  102 , all in a well known manner. 
     This invention provides an improvement to the in-core nuclear instrumentation thimble assemblies  16  that reside within the guide thimbles  86  within the fuel assemblies  80  during reactor operation. In accordance herewith, a negative bias voltage is applied to the detector emitter  12  to improve the sensitivity of the detector  10 . Just a few volts will be sufficient to overcome the magnitude of the potential in the insulator  38 , which is small, but sufficient to prevent some of the electrons leaving the emitter from reaching the outer sheath. This will then make the minimum potential occur at the surface of the emitter/inner surface of the insulator and any electron or beta that escapes the emitter will be propelled to the detector sheath  58  (shown in  FIGS. 1 and 2 ), where it would be counted. As typically above 25 percent of the beta particles are stopped in the insulator  38 , this would be a large increase in sensitivity. Also, photoelectrons and the associated Auger electrons would often be counted when created at the surface of the emitter. As the cross section for photoelectrons increases rapidly as the photon energy decreases, and as the number of photons in the core is high at low energies, this would be an additional substantial increase in the detector sensitivity beyond that involving the betas. 
     There would also be a benefit in simplifying the use and interpretation of the detector signals. At first the increase in detector sensitivity by counting more electrons formed from external gammas would tend to cancel the negative contribution they have to detector current. That is, one of the issues that arises in employing vanadium detectors is that part of the beta generated signal is canceled by Compton electrons generated by gammas created externally to the detector. In core measurement, this results in the sensitivity of the detectors varying from location to location in the core, notably as a function of enrichment. Any cancellation of the external gamma signal by other external gamma interactions, such as by photoelectrons, would tend to reduce this effect. Also, of concern is the fraction of the vanadium detector signal that is prompt. This makes the conversion of the signal to “instantaneous” power problematic as any signal processing algorithm has to deal with signal contributions on different time scales. Cancelling the prompt signal is then a benefit. A further benefit exists if the external gamma signal were “over cancelled”; counteracted to the degree that the net external gamma induced signal is now adding to the sensitivity. The concern over the prompt signal is exacerbated by the fact that it has the opposite sign to the beta signal; consequentially, in rapid transients the detector signal will initially change in the opposite direction to the power change. Consequently, forcing these contributions to have the same sign would also be beneficial. 
     The foregoing discussion addresses the effects of increasing the outgoing signal, the signal that adds to the beta decay signal, caused by external gammas. There would also be a reduction in the incoming external gamma induced current. Due to the negative bias  106 , the Compton electrons that are formed in the sheath  58  by the external gammas would have to penetrate through the entire thickness of the insulator  38  to the smaller radius emitter  12 . Thus, there would be a decrease in “external gamma induced signal cancelling,” due to both the requirement for higher energy needed to penetrate the entire insulation thickness and the smaller radius target area in the emitter. 
     For a cobalt detector, a bias voltage between approximately 1 and 30 volts, and preferably 1 and 2 volts, would allow the 58.6 keV internal transition electrons to be counted (although as these occur with a 10.47 minute half life, this would only be beneficial for steady state measurements). 
     Given that the photoelectrons would now be counted, coating the outside of the emitter  12  with a substance such as selenium that easily emits photoelectrons will be a benefit to sensitivity. Another benefit of applying the negative voltage bias  106 , is that varying the bias, one could get additional information on the origins of the detector signal. For example, removing the bias would affect the external gamma induced current more than it would affect the beta decay current (as the energy of the associated electrons would be less for the external gamma induced current). One could use this, to measure the variation of the external gamma induced part of the signal as a fraction of the beta induced fraction. This information could be used to correct the external gamma induced signal in an in-core flux map. 
     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.