Abstract:
An in-core nuclear detector for detecting the neutron population surrounding the detector. The detector is an ion chamber having a cylindrical outer electrode that is insulated from a central electrode and capped to contain an Argon gas. An electron radiator that produces prompt neutron capture gamma radiation that is substantially, directly proportional to the local neutron population is disposed between the outer tubular electrode and the central electrode.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The present invention pertains generally to apparatus for monitoring the power distribution within the core of a nuclear reactor, and more particularly, to an ion chamber detector having an enhanced fission gamma radiation response. 
         [0003]    2. Related Art 
         [0004]    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 proof of the core design and some calibration measurements. 
         [0005]    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 power distributions. Test data showed that the split section out-of-core detectors, presently in use, responded to axial tilt, but the accuracy 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 have been developed for these affects, but there was some question as to whether out-of-core detectors will in all cases provide an adequate alarm of an adverse power distribution. 
         [0006]    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. The measurement system that performs the periodic core power distribution measurements typically controls the simultaneous insertion and withdrawal of as many as six moveable detectors until measurements are obtained from all of the prescribed core radial locations. Each detector used is inserted through a common radial location to ensure that the detector sensitivities can be normalized to allow the production of an accurate “relative” core power distribution from detectors having different absolute sensitivities. 
         [0007]    Preferably, a plurality of moveable detectors are arranged in electrically redundant groupings and 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 vessels&#39; 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 reactor operator. 
         [0008]    Moveable in-core detectors are now used by both boiling water reactors and pressurized water reactors to perform periodic detailed measurements of the core power distribution. The moveable detectors used are either primarily sensitive to neutron or gamma radiation. The type of detector most commonly used in both pressurized water reactors and boiling water reactors is a fission chamber style of detector. In this design, the signal output from the detector is directly proportional to the thermal neutron population surrounding the detector. The thermal neutron population is directly proportional to the local fission rate and local core power level. This response is generated by the use of significant amounts of highly enriched U 235  in the construction of the detector. Since U 235  is a special nuclear material, the cost to purchase and operate the moveable fission chambers is quite high. The moveable fission chambers are also quite delicate, so they are subject to frequent mechanical failure. The major technical benefit associated with their use is the direct relationship between the output signal and the local thermal neutron population and the direct relationship that the thermal neutron population has with the local core power production rate.  FIG. 1  provides a layout schematic of a miniature fission chamber  10 . The miniature fission chamber has a stainless steel tubular casing  12  that is capped at both ends and forms an outer electrode. Al 2 O 3  ceramic insulators  16  support a central mineral filled coaxial output electrode  18 , which is insulated from the outer electrode  12 . The stainless steel casing  12  surrounds a central chamber  14  that is filled with an Argon filler gas  22  with the walls of the chamber  14  coated with 90% enriched U 235  and U 3 O 3 . A detector bias voltage  32  of between 20 and 150 volts DC is maintained between the two electrodes. In operation, an incident thermal neutron  28  causes a fission event  30  within the enriched U 235  resulting in high energy ionizing fission fragments  26  which create ionized gas molecules  24  within the Argon gas. The voltage bias on the central electrode  18  collects the ionized gas particles  24  resulting in a detector output  34  which is proportional to the fission events  30  resulting from the incident thermal neutrons  28 . 
         [0009]    A moveable detector design using a miniature ion chamber  10  has been recently introduced for use in a boiling water reactor. This type of detector also produces a signal proportional to the local core power, but the signal is stimulated by gamma radiation interactions in the ionization chamber region of the detector. This type of detector does not require U 235  as the stimulation for the output signal, so the cost and upkeep of this type of detector is significantly less than for a fission chamber style. The detectors also tend to be more rugged than fission chamber moveable detectors. The most limiting issue associated with the use of an ion chamber detector is the much lower signal output corresponding to a given local core power level. The use of this style of detector requires the use of very sensitive and expensive signal processing electronics. There is also an uncertainty that needs to be included in the core power distribution measurement uncertainty to account for the non-single valued relationship between output signal and local fission rate or core power. This power distribution measurement uncertainty increase potential may actually reduce the maximum power output that can be achieved by the reactor operator. 
         [0010]      FIG. 2  provides a longitudinal cross section of a layout schematic of a miniature ion chamber  36  with  FIG. 2B  showing a cross section taken orthogonal to the sectional view shown in  FIG. 2A . The miniature ion chamber  36  has an outer metal casing  38  that forms the outer electrode and insulated end caps  40  and  42  that support a central electrode  44 . Similar in construction to the miniature fission chamber  10 , the casing  38  surrounds a central chamber  50  that is filled with a fill gas  48 . 
         [0011]    Each style of moveable detector has suboptimal performance characteristics that significantly increase the cost of operation of the detector system. Accordingly, an improved detector is desired that is more rugged and less expensive to operate than those currently employed. 
         [0012]    Additionally, such a detector design is desired that will minimize the uncertainty that has to be factored in to the core power measurements. 
       SUMMARY 
       [0013]    These and other objects are achieved by an ion chamber radiation detector with an enhanced fission gamma radiation response. The ion chamber has an outer tubular electrode having an upper and lower end closed off by insulated end caps that electrically insulate the outer tubular electrode from a central electrode that substantially extends from the lower end cap up and through the upper end cap. An electron radiator that produces prompt neutron capture gamma radiation that is substantially, directly proportional to the local neutron population is disposed between the outer tubular electrode and the central electrode. Desirably, the electron radiator is constructed from a material having a high Z value with which prompt neutron capture gamma radiation interacts through photoelectric and Compton scattering mechanisms. Preferably, the electron radiator is constructed from one or more materials selected from a group of metals comprising platinum, gold, manganese, tungsten and cadmium and, more preferably, mainly from a group of metals comprising manganese, tungsten and cadmium. 
         [0014]    In one embodiment, the electron radiator comprises a plurality of electron radiators that are spaced around the central electrode and preferably extends substantially between the lower end cap and the upper end cap. 
         [0015]    The invention also contemplates a nuclear reactor power distribution monitoring system having a plurality of in-core detectors that monitor power at different axial and radial locations within a core of the nuclear reactor. At least some of the in-core detectors comprise the foregoing ion chamber design. In one such embodiment, the ion chambers are moveable in-core detectors that substantially traverse an axial length of the core at different core radial locations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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: 
           [0017]      FIG. 1  is a cross section of a fission chamber employed as a moveable neutron detector; 
           [0018]      FIG. 2A  is a schematic representation of a longitudinal cross section of a miniature ion chamber; 
           [0019]      FIG. 2B  is a schematic cross section of a miniature ion chamber taken along a plane orthogonal to the plane of the cross section of  FIG. 2A ; 
           [0020]      FIG. 3  is a simplified schematic of a nuclear reactor system having moveable in-core detectors that can benefit from this invention; 
           [0021]      FIG. 4A  is a longitudinal cross sectional view of an ion chamber constructed in accordance with one embodiment of this invention; and 
           [0022]      FIG. 4B  is a cross section of the ion chamber shown in  FIG. 4A  taken along the plane orthogonal to the cross section shown in  FIG. 4A . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    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 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. 
         [0024]    For the purpose of illustration,  FIG. 3  shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  52  having a closure head  68  enclosing a nuclear core  54 . A liquid reactor coolant, such as water, is pumped into the reactor vessel  52  by pump  56  through the core  54  where heat energy is absorbed and is discharged to a heat exchanger  58 , typically referred to as a steam generator, in which is 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  56  completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor pressure vessel  52  by reactor coolant piping  60 . Though not shown, at least one such loop includes a pressurizer for maintaining the pressure in the system. 
         [0025]    As previously mentioned, moveable in-core detectors  62  are used by both boiling water reactors and pressurized water reactor style reactors to perform periodic detailed measurements of the core power distribution. The detectors  62  are inserted into the reactor core  54  during normal power operation according to a predetermined, intermittent, time 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. The outputs of the detectors are fed through a seal table  64  to a control and monitoring system  66  responsible for implementing the time program. The moveable detectors  62  used are either primarily sensitive to neutron or gamma radiation. Each style of detector has suboptimal performance characteristics that significantly increase the cost of operation of the detector system. The gamma radiation detection device claimed hereafter utilizes a combination of the advantages of fission chamber style detectors and ion chamber style detectors to provide a detector capable of measuring the relative core power distribution in both pressurized water reactor and boiling water reactor style reactors that allows a significant reduction in the cost to own and operate the core power distribution measurement system. 
         [0026]      FIGS. 4A and 4B  provide a schematic of the preferred embodiment of the invention claimed hereafter. The fundamental difference between  FIGS. 2A and 2B  and  4 A and  4 B is the inclusion of an additional structure  72  in the ionization volume of the device, made of a metal such as natural manganese, tungsten or cadmium, which serves to produce high energy prompt neutron capture gamma radiation that is directly proportional to the local neutron population. This additional prompt neutron capture gamma radiation interacts through photoelectric and Compton scattering mechanisms with a high Z material, such as platinum or gold, surrounding the gamma radiator element to produce high energy electrons that also enhance the ionization in the gas fill  48  of the detector  70 . This additional ionization greatly enhances the ionization current output  46  from the detector  70 . This approach serves to enhance the signal level output from the device relative to the standard ion chamber  36  design without use of special nuclear material. The “Z” score, in statistics, indicates how far and in what direction, that item deviates from its standard distribution mean. Expressed in units of its distribution standard deviation Z scores are sometimes called “standard scores.” The Z score transformation is especially useful when seeking to compare the relative standings of items from distributions with different means and/or different standard deviations. 
         [0027]    The addition of a number of electron radiators  72 , such as the four shown in  FIG. 4B , spaced around the center electrode  44  between the center electrode  44  and the outer electrode  38 , serves to enhance the signal level output from the device  70  relative to the standard ion chamber design  36  without the use of special nuclear material. The use of a relatively inexpensive prompt gamma radiator such as manganese, tungsten or cadmium allows the sensitive portion of the detector to be easily contained in a very robust physical package. Since the prompt sensitivity to the local neutron population is enhanced relative to the standard ion chamber  36 , the uncertainty in the relationship between the signal and the true local core power level is reduced. The increase in the output signal from the stimulation of the local power distribution reduces or eliminates the need for extremely sensitive and expensive signal measurement electronics. The optimized design of the central core of this device will allow the use of the existing moveable detector signal measurement electronics so that the cost of implementation of this detector design will be minimized. This device may also be used as a stationary detector in other ion chamber applications where enhanced neutron sensitivity is a benefit, such as in reactor power level monitoring applications. 
         [0028]    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.