Abstract:
A solid state semiconductor neutron detector that automatically varies its sensitivity to provide a pulsed output over the entire range of operation of a nuclear reactor. The sensitivity is varied by changing the thickness or makeup of a converter layer that emits charged particles to the active region of the semiconductor surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C. §119(e) to provisional application Serial No. 60/241,340, filed Oct. 18, 2000 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to neutron detectors, and more particularly relates to an array of solid state neutron sensors capable of measuring a wide range of neutron fluxes generated by nuclear power reactors and the like.  
         BACKGROUND OF THE INVENTION  
         [0003]    Neutron detection is conventionally accomplished by using a detector of ionizing radiation and employing a conversion nuclear reaction whereby the neutron produces a charge particle product. The first neutron detectors were gas proportional counters which detected ionization produced by the highly charged fission fragments produced in neutron induced fission of  235 U.  
           [0004]    In pressurized water reactors, three types of gas proportional counters are typically used to provide coverage over the full range of neutron fluxes that are normally experienced during reactor operation. For the low neutron fluxes at reactor startup, a BF 3  proportional counter is used to record pulses from the  10 B(n,α) reaction. This detector is operated in the pulse-counting mode until the neutron flux reaches the 10 4  to 10 5  cm −2 -second −1  range, corresponding to 10 5  to 10 6  counts per second in the detector. At these count rates, linearity in count rate as a function of neutron flux begins to deteriorate due to the pulse pile-up, and at about 10 6  counts per second, individual pulses cannot be distinguished. The neutron flux range from startup to about 10 4 cm −2 -second −1  is referred to as the Source Range. A second type of detector, a boron-lined ionization chamber, is used to monitor neutron flux above 10 3 cm −2 -second −1 . Because of the inherently large count rate, this counter is operated in the current mode, i.e., individual pulses are summed to form a current which is monitored. However, although the current generated by neutron counts is proportional to neutron flux and to reactor power, current is also generated by gamma ray interactions with the detector. Therefore, gamma compensation must be used, because gamma ray intensity is not proportional to reactor power in the low power range. A second detector, without a boron liner, which was used in the first detector to generate neutron-induced particles for counting, is used to determine current generated by gammas only and this current is subtracted from the observed current for the boron-lined first detector. This pair of ionization chambers is referred to as a compensated ion chamber (CIC). CICs are used to monitor reactor power from neutron fluxes of about 10 3 cm −2 -second −1  to above 10 10 cm −2 -second −1 , which is close to full reactor power. This range of neutron fluxes is referred to as the Intermediate Range, and the BF 3  and CIC power monitors are referred to as the Source Range and Intermediate Range detectors, respectively. For neutron fluxes higher than about 10 7 cm −2 -second −1 , gamma ray intensity becomes proportional to power, and gamma compensation is no longer necessary. Boron-lined ion chambers, without gamma compensation, are used from 10 7 cm −2 -second −1  to 2.5×10 10 cm −2 -second −1 , the range of neutron fluxes referred to as the Power Range. The Source, Intermediate and Power Ranges are indicated in FIG. 1, which shows the thermal neutron flux in neutrons/cm 2 /second at the detector location over the Source Range  10 , Intermediate Range  12  and Power Range  14 . The Source Range is shown in counts per second and the Intermediate and Power Ranges are shown in amperes.  
           [0005]    In order to monitor power from startup to full power, data from all three types of detectors are needed. During the transition from the high end of the Source Range into the lower end of the Intermediate Range, the responses from the Source Range and Intermediate Range detectors must be matched. This matching can be particularly difficult because two inherently different types of detectors are being used. A pulse mode Source Range detector must be matched to a current mode, gamma compensated Intermediate Range detector. Difficulties in matching these responses when changing power can result in operating delays and, in extreme cases, in reactor trips. To further complicate the matter, due to the extreme sensitivity of the Source Range detector, to avoid early failure, the Source Range is turned off during Power Range operation. It has not been uncommon for the Source Range detector to fail when an attempt is made to reactivate the detector upon power down of the reactor.  
           [0006]    An alternative to using a gas-filled detector is to use a semiconductor or solid state detector. Conventional semiconductor neutron detectors consist of a silicon surface barrier detector with a layer of boron, lithium or fissionable material adjacent to the active volume of the detector. One such solid state neutron detector using silicon semiconductor is disclosed in U.S. Pat. No. 3,227,876.  
           [0007]    A problem with prior art neutron detectors is sensitivity of the detector to non-neutronic components of the radiation field, particularly gamma ray sensitivity. Gas-filled detectors are favored in nuclear reactor applications because low density gases are inherently inefficient detectors for gamma rays which deposit their energy over large volumes. Solid state detectors, on the other hand, are more sensitive to gamma rays because of their higher electron density.  
           [0008]    Solid state semiconductor detectors are candidates for replacement of conventional gas filled detectors, but they have not found widespread use in the nuclear industry because of problems associated with background signal and deterioration of detector performance during operation in intense, hostile radiation environments.  
           [0009]    U.S. Pat. Nos. 5,726,453, issued Mar. 10, 1998 and 5,940,460, issued Aug. 17, 1999, describe a radiation resistant solid state neutron detector that can discriminate between neutron and gamma responses on the basis of pulse height. Based on these two properties displayed by, but not necessarily limited to, silicon carbide (SiC) neutron detectors, an improved solid state neutron detector is desired that can operate over the wide range of reactor operation. Furthermore, it is an objective of this invention to provide such a detector that can operate over the entire range of operation of a reactor in the pulse mode. Additionally, it is an object of this invention to provide a detector that can function over the entire range of operation of the nuclear reactor employing a single type of electronics train to process the detector output.  
         SUMMARY OF THE INVENTION  
         [0010]    These and other objects are achieved by employing a detector having at least one array of sensors, each sensor having a semiconductor active region for generating an electronic output signal in response to charged particles emitted from a converter layer in response to the neutron emissions from a neutron source such as a reactor core. The electronic output is processed by a single electronics train for each detector for processing the electronics output signals of each of the arrays of sensors associated with that detector to provide an output indicative of the power of the source. Means are also provided for changing the sensitivity to neutron emissions of the detector when the electronic output pulse count exceeds or drops below pre-selected rates, e.g., the count rate exceeds the rate the electronics train can discriminate counts from or the count rate is too slow to provide meaningful information.  
           [0011]    In one embodiment, the detector includes at least two arrays of sensors with each array having a different sensitivity. The electronics train then switches between arrays when the pre-selected rates of count are detected.  
           [0012]    In another preferred embodiment, the means for changing the sensitivity of the detectors changes the converter layer composition either by changing the depth or makeup of the converter material.  
           [0013]    An additional feature of the invention is that during full power operation, it alters the converter material associated with sensors whose outputs are not being monitored so the corresponding semiconductor active region is not bombarded with charged particles. Additionally, the detector of this invention can be employed to provide a more accurate display of the axial power distribution of a reactor core than current ex-core detectors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    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:  
         [0015]    [0015]FIG. 1 is a graphical illustration of the prior art Source, Intermediate and Power Ranges of reactor operations; and  
         [0016]    [0016]FIG. 2 is a schematic of the detector and electronic signal processing train of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Recently, semiconductor neutron detectors, and particularly silicon carbide (SiC) neutron detectors in embodiments described in U.S. Pat. Nos. 5,726,453 and 5,940,460, have been shown to be reliable devices for monitoring neutron flux over a wide range of fluxes. Furthermore, the gamma ray and neutron responses of these detectors have been shown to be separable on the basis of pulse height. Based on these two properties displayed, but not necessarily limited to, SiC neutron detectors, this invention provides a wide range neutron detector that will overcome many of the reactor power monitoring difficulties noted previously. This is accomplished by providing a solid state semiconductor power monitor that is designed to cover the entire power range from startup to full power in the pulse mode of operation without any gamma compensation.  
         [0018]    A typical SiC neutron detector consists of a SiC diode, either a Schottky diode or P-N junction, or an array of diodes juxtaposed with a layer of  6 LiF or other neutron-sensitive material which will interact with neutrons to produce charged particles which will produce electronic pulses when they pass through the SiC diodes. In this case, the  6 Li(n,α) 3 H reaction is used. Both the alpha particle and the triton ( 3 H) can interact with the active volume of an adjacent diode to produce an electrical pulse. Three factors can be used to adjust the sensitivity of a SiC neutron detector, the area of the SiC diode, the number of diodes and the thickness of the  6 LiF layer. In accordance with this invention, a single detector is constructed that contains combinations of diodes and converter layers that permit it to operate in the Source, Intermediate and Power Ranges.  
         [0019]    A single SiC semiconductor reactor power monitor detector is designed in accordance with this invention to have multiple sensitivities. For example, a set of ten 6-mm diameter SiC diodes at each of twelve axial locations juxtaposed with 24.2 μm 6 LiF layers would have a sensitivity, S- 1 , equal to 0.614 cps/nv (where nv refers to neutron flux in units of cm 2 -second −1 ). A second array of sets of ten 500 μm diameter diodes at each of twelve axial locations and juxtaposed with a 24.2 μm 6 LiF layers would have a sensitivity, S- 2 , equal to 4.264×10 −3  cps/nv. A third array of sets of ten 500 μm diameter diodes at each of twelve axial locations and juxtaposed with 0.05 μm 6 LiF layers would have the sensitivity, S- 3 , equal to 1.319×10 −5  cps/nv. The count rates for each of these diode arrays as a function of thermal; neutron flux are shown below in Table 1. 
                                                   TABLE 1                           Detector count rates as a function of thermal neutron flux            Neutron Flux                   (nv)   S-1 (cps)   S-2 (cps)   S-3 (cps)                    0.1   0.0614   0.0004264           1   0.614   0.004264       10   6.14   0.04264       100   61.4   0.4264       1000   614   4.264       10000   6140   42.64   0.1319       100000   61400   426.4   1.319       1000000   614000   4264   13.19       10000000   6140000   42640   131.9       100000000   61400000   426400   1319       1000000000   614000000   4264000   13190       10000000000   6140000000   42640000   131900       50000000000   30700000000   213200000   659500                  
 
         [0020]    It can be seen from the data of Table 1 that useable count rates are obtained in the sensitivity S- 1  range from startup to about 10 5  nv, at which point very fast electronics would be required to measure count rates above 6×10 4  cps. However, the diode arrays corresponding to sensitivity S- 2  range have a count rate of approximately 400 cps at this point and could be used to record data with the same pulse processing electronics used for the first range, up to a neutron flux of approximately 10 7  nv. At that flux level, the diodes from array  3  have a count rate of greater than 100 cps and can record data with the same electronics as used for the other two sensitivity ranges, at neutron fluxes up to full power, which is approximately 2.5×10 10  nv. The sensitivity of diode array  3  can be further reduced by selecting counts from one diode rather than ten at each axial location, resulting in a count rate of 3.30×10 4  cps at full reactor power. This count rate is well within the pulse processing capabilities of typical electronic systems. The term “diode” comprises the active region of the semiconductor and is used interchangeably with the term “sensor”.  
         [0021]    The detector of this invention, with multiple built in sensitivity ranges, can be connected to a single electronic pulse-processing train. Transitions between ranges can be accomplished automatically based on detector count rate. A computer can be used to control the detector internal sensitivity changes and to correct the output data to the proper sensitivity to allow the reactor power level to be inferred.  
         [0022]    The above combination of internal sensitivities is provided as an example. The sensitivity ranges chosen will depend upon the speed of the electronics components that are employed and will result from an optimization of the detector response characteristics to the nuclear plant measurement environment in which it is deployed. If faster electronics become available, a wider dynamic range of output count rates can be used, and internal sensitivities can be adjusted accordingly. While three ranges of sensitivities are used in this example, it should be appreciated that as few as two or as many as four or more may be used as well.  
         [0023]    In order to provide longer service lifetimes, the detector arrays with higher sensitivities can be protected when not in use from bombardment by charged particles produced in the neutron-converter layer. This can be accomplished by either removing the  6 LiF layer from the vicinity of the SiC diodes or by inserting a shutter between the  6 LiF foil and the active region of the diodes to absorb the charged particles. An example of several materials which can be used for such a shutter are stainless steel, aluminum, zirconium, zircalloy or other material of at least 50 micrometers thickness that is capable of withstanding the nuclear environment.  
         [0024]    The response of SiC neutron detectors reflects a much more localized neutron fluence rate than that measured by the much larger gas filled ex-core detectors. Therefore, detectors must be deployed at sufficient locations to give an accurate representation of the axial power profile. Whereas the integral count rate over all of the axial locations can be used to provide a measure of total core power, the individual count rates can be used to evaluate axial power profiles. Additionally, the multisensor detector arrays positioned at twelve axial locations can provide data at each of those locations independently to enable the plant operator to use the data to evaluate core flux tilt and axial offset.  
         [0025]    The detector of this invention can also replace the functions of the so called wide range fission chamber used in boiling water reactors and some pressurized water reactors. The detector of this invention has been shown to have a more linear response to reactor power than a fission chamber at close to full power. Furthermore, the detector of this invention contains no uranium or other materials that will result in radioactivation and exposures of plant personnel to high levels of radiation during detector repair or replacement. Furthermore, radioactive waste disposal costs are much less than for conventional detectors.  
         [0026]    [0026]FIG. 2 illustrates two alternative embodiments for carrying out the principles of this invention. Arrays of SiC sensors  46  are positioned along the length of the detector system. The arrays  46  are more fully described in U.S. Pat. No. 5,940,460, issued to the assignee of this invention. These groupings  46  can be used to monitor the axial profile of the reactor core when positioned between the reactor vessel and the biological shield. While only two groupings are shown for illustrative purposes, it should be appreciated that as many as twelve or more groupings may be used for a more precise profile. The two groupings  46  each include a more sensitive sensor array  16  and a less sensitive sensor array  18 . The sensitivity of the array is established by the number of individual diodes that are connected in parallel. The more diodes connected in parallel, the more sensitive the sensor. The counting circuits  44  can be switched between the sensor arrays  16  and  18  having different active areas. Separate electronic trains for each detector array  46  can allow simultaneous data acquisitions for each detector, higher total throughput, and reduced noise due to a lower input capacitance. The detector array output cables  52  and  54  for the arrays  18  and  16  are routed to a switch  24 , which can alternately connect the cables  54  and  52  to the electronics train  44 . The output signals are routed through a preamplifier  26  and for conditioning the signal to an amplifier  28 . Then the amplified signal is sent to a single channel analyzer  30  that discriminates between the lower amplitude pulses, representative of the gamma response, from the higher amplitude pulses indicative of the neutron response. The counts representative of the neutron response are then sent to a counter, which provides an output in counts per second. This output is then modified by a diode factor  34 , which compensates for the sensitivity of the sensor employed. The signal is then further modified by a converter layer factor  36  to account for the sensitivity of the converter layer. The counts per second are monitored by the logic circuit  38 . If the counts per second are above or below pre-selected values, then the logic circuit signals the switch drive  40  to switch the sensitivity of the detector by, for example, transferring switch  24  from output  54  to output  52 . The pre-selected values are chosen so that if the counts per second are so high that the electronics cannot distinguish between pulses, the logic circuit directs the switch drive  40  to transfer the switch  24  from the higher sensitive array  16  to the lower sensitive array  18  by transferring connections from the output cable  54  to the output cable  52 . Conversely, if the electronics train  44  is connected to the output cable  54  and the logic train detects that the counts per second are so low that meaningful information is not being transferred, then the logic circuit directs the switch drive  40  to transfer the connection from the output cable  52  to the more sensitive detector output  54 . A return signal is sent by the switch drive  40  to the logic circuit to verify the position of the switch  24 . A shutter system (not shown in FIG. 2) can be built into each array whereby a material such as stainless steel, aluminum, zirconium, zircalloy, or other material of at least 50 μm thickness that is capable of withstanding the nuclear environment can be interposed between the converter and semiconductor active region, e.g., by a solenoid, to absorb the charged particles and preserve the semiconductor active region when it is not in use. Two groupings of converter foils are illustrated in FIG. 2 represented by the rotating drum  22 , converter foils  48  and  50  and drum drive  42 . In this embodiment, spaced converter foils  48  and  50  are supported on the surface of the drum  22  in the vicinity of the detector arrays  16  and  18 , preferably coextensive with and spaced from the active area of the semiconductor region. Two such foils are shown,  48  and  50 , for each such array  16  and  18 . Each group of converter foils,  48  and  50 , corresponds to a converter layer of different thickness to provide a different sensitivity. Converter foils  48  provide high sensitivity whereas converter foils  50  provide low sensitivity. The logic circuit activates the drum drive  42  to rotate the drum to have the appropriate foil placed in front of the active region of the semiconductor array  16  or  18 . The converter foil can also be rotated away from the sensor arrays to locations that do not emit charged particles in response to incident neutrons and function similar to the shutter shade previously described. Instead of employing a separate foil, the shutter material can be the drum surface with the drum drive  42  programmed to position the drum to a fourth position outside the sensor arrays view of the converter foils.  
         [0027]    Alternatively, the array outputs for each detector grouping  46  can be processed through the same electronics train by multiplexing the outputs. That would further reduce the cost of the system, but result in a loss of redundancy.  
         [0028]    Another alternative for varying the sensitivity of the detectors is to multiplex the sensor outputs. The electronics train would then select which and how many sensors within an array it monitored based upon the rate of pulses received.  
         [0029]    Thus, the detector system of this invention covers the neutron flux range of operation of a nuclear reactor from startup to full power without any requirement for gamma ray compensation. A single electronic pulse processing train can be used to acquire data from the detector at all reactor power levels. The sensitivity of the detector can be adjusted automatically to optimize the signal output and transitions between ranges can be accomplished smoothly and rapidly. More accurate axial profile information can be obtained than is currently available from ex-core detectors. It is anticipated that this solid state design will be more reliable and have a longer service lifetime than either boron gas-filled or fission chamber neutron detectors. Additionally, the detector requires no materials that undergo radioactivation, thereby reducing occupational radiation exposures and radioactive waste disposal costs.  
         [0030]    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 all equivalents thereof.