Abstract:
A neutron monitoring system for monitoring a state of a fuel used in a nuclear power plant includes the following: neutron detector; a structural material to which the neutron detector is mounted; a water detection sensor mounted at a position corresponding to the same height as is the neutron detector, the water detection sensor including a thermocouple and a heating element; an amplification circuit configured to amplify a neutron monitoring signal sent from the neutron detector; a temperature measuring circuit connected to a thermocouple of the water detection sensor via strands; a heater power supply connected to a heating element of the water detection sensor via heater lead wires; a water existence determining control section configured to determine whether the water detection sensor is underwater; and a correction circuit configured to correct an output signal value of the amplification circuit in accordance with a determination signal from the water existence determining control section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to neutron monitoring systems, and more particularly, to a neutron monitoring system that measures a neutron flux using neutron detectors placed inside a nuclear reactor or under the reactor vessel. 
         [0003]    2. Description of the Related Art 
         [0004]    Generally, neutron detectors are placed inside a nuclear reactor to monitor nuclear power of the reactor. For example, a boiling-water reactor includes an in-core instrumentation tube inserted in a gap of the fuel assemblies installed in a grid-like form inside the reactor. Inside the in-core instrumentation tube placed are neutron detectors such as fission chambers for monitoring of nuclear power. Upon entry of a neutron, the neutron detectors generate an electric signal and send a measured neutron flux signal to a power monitoring device installed outside the reactor pressure vessel, through independent cables connected to the detectors. The neutron detectors here detect the neutrons generated from the core fuel immersed in coolant water. 
         [0005]    A water level gauge for measuring the coolant water level in the reactor is provided to confirm whether appropriate cooling is performed on the core fuel. An instrument with a differential pressure gauge is commonly used as the water level gauge. Further, a method of detecting the reactor coolant water level directly inside the reactor has come to be considered to ensure diversity of detection methods. 
         [0006]    For example, Japanese Patent Application No. JP-1984-112290-A discloses a reactor core monitoring device that senses a coolant water level with a plurality of sheathed thermocouples placed in an in-core instrumentation tube of a boiling-water reactor. The sheathed thermocouples are placed at different heights from a lower section of the reactor, and differential output between upper and lower sheathed thermocouples are detected, thereby sensing the coolant water level. 
       SUMMARY OF THE INVENTION 
       [0007]    Analysis of a signal which a neutron detector has detected is effective for confirming core/fuel state associated with an accident, as well as for monitoring output power of the core during normal operation. The analysis is particularly crucial for obtaining information on the power and position of the core/fuel suffering a change in shape due to a severe accident such as meltdown of the fuel. 
         [0008]    For this reason, it is conceivable that in case of a core melting accident, for example, the fuel would be monitored using neutron detectors placed in the core positioned inside the reactor pressure vessel, and neutron detectors placed outside the reactor pressure vessel, in addition to neutron detectors used for monitoring during normal operation. In this case, it is necessary, from a viewpoint of neutron detector sensitivity, to detect whether the neutron detectors are immersed in coolant water or exposed above the water level. If the neutron detectors are immersed in the coolant water, a nuclear reaction during which the neutrons are sensed will be accelerated since neutrons are moderated in the water. Consequently, the neutron detector sensitivity will be increased. 
         [0009]    Under such a severe accident state that causes the melting of the core, however, it is very difficult to judge whether the neutron detectors are fully immersed in the coolant water or exposed. 
         [0010]    The present invention has been made on the basis of the above, and an object of the invention is to provide a neutron monitoring system that reliably senses whether neutron detectors are fully immersed in coolant water or exposed above the water level, and thus accurately monitors neutrons. 
         [0011]    The present invention includes a plurality of means to attain the above object. To cite an example, a neutron monitoring system for monitoring a state of a fuel used in a nuclear power plant includes the following: neutron detector; a structural material to which the neutron detector is mounted; a water detection sensor mounted at a position corresponding to the same height as is the neutron detector, the water detection sensor including a thermocouple and a heating element; an amplification circuit configured to amplify a neutron monitoring signal sent from the neutron detector; a temperature measuring circuit connected to a thermocouple of the water detection sensor via strands; a heater power supply connected to a heating element of the water detection sensor via heater lead wires; a water existence determining control section configured to determine whether the water detection sensor is underwater; and a correction circuit configured to correct an output signal value of the amplification circuit in accordance with a determination signal from the water existence determining control section. 
         [0012]    In the present embodiment, whether the each neutron detector is fully immersed in coolant water or exposed above the water level can be sensed, and the sensed result can correct the measured data by the neutron detectors. This in turn enables accurate detection of neutrons, thus contributing to estimating power and a fuel position more accurately even in case of such a severe accident that causes the fuel to melt. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a system block diagram showing an embodiment of a neutron monitoring system of the present invention; 
           [0014]      FIG. 2  is a conceptual diagram showing an example of water detection sensors which constitute part of the embodiment of the neutron monitoring system according to the present invention; 
           [0015]      FIG. 3  is a conceptual diagram showing another example of water detection sensors which constitute part of the embodiment of the neutron monitoring system according to the present invention; 
           [0016]      FIG. 4  is a partial, longitudinal cutaway view showing a fission chamber used as a neutron detector to constitute another part of the embodiment of the neutron monitoring system according to the present invention; 
           [0017]      FIG. 5  is a partial, longitudinal cutaway view showing a self-powered neutron detector used as a neutron detector to constitute yet another part of the embodiment of the neutron monitoring system according to the present invention; 
           [0018]      FIG. 6  is a characteristics diagram showing an example of a thermocouple output response in the embodiment of the neutron monitoring system according to the present invention; and 
           [0019]      FIG. 7  is a conceptual diagram showing an example of setting sensitivity for correction in the embodiment of the neutron monitoring system according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Hereunder, an embodiment of a neutron monitoring system of the present invention will be described using the accompanying drawings. 
         [0021]      FIG. 1  is a system block diagram showing the embodiment of the neutron monitoring system of the present invention. 
         [0022]    Referring to  FIG. 1 , a reactor pressure vessel  1  contains a shroud  2 . A fuel  27  is retained between a core support plate  3  and a core top guide  4 , inside the shroud  2 . A steam separator  5  and a steam dryer  6  are placed near an upper section of the shroud  2 . The steam separator  5  and the steam dryer  6  work together to separate core-generated steam from coolant water  26  and then supply the separated steam to a turbine not shown. The reactor pressure vessel  1  is fixed to a pedestal  8  via a support skirt  7 . 
         [0023]    An in-core instrumentation tube  10  is disposed in a gap of a large number of fuel assemblies  27  arrayed in a grid-like form, has a lower section inserted in an in-core instrumentation housing  12  and an in-core instrumentation guide tube  13 , and is fixed at an upper end of the tube  10  to the core top guide  4 . 
         [0024]    Neutron detectors  9   a  to  9   c  are housed in the in-core instrumentation tube  10 . Some of the detectors are placed inside the core, and some of them are placed under the core, inside the pressure vessel. Water detection sensors  11   a  to  11   c  are mounted at positions corresponding to the same heights as are the corresponding neutron detectors  9   a - 9   c  inside the in-core instrumentation tube  10  (e.g., the water detection sensor  11   a  is mounted at a position corresponding to the same height as is the neutron detector  9   a ). 
         [0025]    In addition to the neutron detectors  9   a - 9   c  and the water detection sensors  11   a - 11   c,  a guide tube of the traversing in-core probe  14  is stored within the in-core instrumentation tube  10 . An upper water hole  15   a  for cooling is provided at an upper side of the in-core instrumentation tube  10 , a lower water hole  15   b  is provided at a lower side of the in-core instrumentation tube  10 , and a seal  16  is disposed at an even lower portion relative to the water hole  15   b.  This core construction allows the coolant water to flow through the in-core instrumentation tube  10  and not to leak from the core. 
         [0026]    A first support structural material  17   a  and a second support structural material  17   b  are each connected at one end thereof to the pedestal  8 . On the first support structural material  17   a,  a neutron detector  9   d  and a water detection sensor  11   d  are placed at positions of the same height from a bottom section of the pedestal  8 . On the second support structural material  17   b,  a neutron detector  9   e  and a water detection sensor  11   e  are likewise placed at other positions of the same height from the bottom section of the pedestal  8 . In the present embodiment, the second support structural material  17   b  is mounted at a position corresponding to substantially the same height of the bottom of the pedestal  8 , and the first support structural material  17   a  is mounted at a position higher than that of the second support structural material. 
         [0027]    The neutron detectors  9   a - 9   e  are each connected to an amplification circuit  20  through a neutron detector cable  18 . 
         [0028]    In addition, the water detection sensors  11   a - 11   c  are each connected to a temperature measuring device  21  and a heater control device  22  via a water detection sensor cable  19   a  and a connector  28   a.  Similarly, the water detection sensors  11   d - 11   e  are each connected to a temperature measuring device  21  and a heater control device  22  via a water detection sensor cable  19   b  and a connector  28   b.    
         [0029]    The temperature measuring device  21  and the heater control device  22  are connected to a water existence determining control device  23 . The water existence determining control device  23  includes a storage device  23   a  within which a threshold data table is stored, and outputs a water existence determination signal to a correction circuit  24 . An output signal from the amplification circuit  20  is corrected in the correction circuit  24  and reaches a measured-data conversion device  25 . 
         [0030]    Next, the water detection sensors are described below using the accompanying drawings.  FIG. 2  is a conceptual diagram showing an example of water detection sensors which constitute part of the embodiment of the neutron monitoring system according to the present invention. In  FIG. 2 , the same reference numbers as those shown in  FIG. 1  denote the same elements, detailed description of which is omitted hereinafter. 
         [0031]      FIG. 2  shows construction of three water detection sensors,  11   a - 11   c.  These sensors are substantially of the same construction, so only the water detection sensor  11   a  is described below. 
         [0032]    The water detection sensor  11   a  contains a thermocouple  31   a,  a heating element  32   a,  and heater lead wires  33   a,    34   a.  The thermocouple  31   a  uses a plus-side strand  29   a  and a minus-side strand  30   a  bonded together. The heating element  32   a  heats a neighborhood of the thermocouple  31   a.    
         [0033]    The thermocouple  31   a  can be a commonly used K-type or N-type of thermocouple. A high-resistance wire of a nickel-chromium alloy, for example, is preferred as the heating element  32   a.  Use of copper wires, nickel wires, or other wires of relatively low resistance, as the heater lead wires  33   a,    34   a,  allows suppression of a voltage necessary for a heater power supply. 
         [0034]    The thermocouple  31   a  and the heating element  32   a  are electrically insulated from each other via an insulating material  35   a  such as alumina, and housed in a sheath  36   a  made of stainless steel, for example. The plus-side strand  29   a,  the minus-side strand  30   a,  and the heater lead wires  33   a,    34   a  are connected to respective conductors of the water detection sensor cable  19   a  via the connector  28   a.  Through these conductors, the plus-side strand  29   a  and the minus-side strand  30   a  are connected to the temperature measuring device  21 , and the heater lead wires  33   a,    34   a  are connected to the heater control device  22 . While the water detection sensors  11   a - 11   c  are shown by way of example in  FIG. 2 , substantially the same construction is also applied to the water detection sensors  11   d  and  11   e.    
         [0035]    Next, another example of water detection sensors is described below using  FIG. 3 .  FIG. 3  is a conceptual diagram showing another example of water detection sensors which constitute part of the embodiment of the neutron monitoring system according to the present invention. In  FIG. 3 , the same reference numbers as those shown in  FIGS. 1 and 2  denote the same elements, detailed description of which is omitted hereinafter. 
         [0036]    In each of the water detection sensors  11   a - 11   c  shown in  FIG. 3 , three thermocouples,  31   a  to  31   c,  are arranged at positions of different heights inside a sheath  36  made of stainless steel, for example. One heating element  32  is housed to heat in common a neighborhood of the thermocouples  31   a - 31   c.  The heating element  32  and heater lead wires  33 ,  34  have substantially the same construction as that of the heating element  32   a  and heater lead wires  33   a,    34   a  described above. Plus-side strand  29   a - 29   c  and minus-side strands  30   a - 30   c,  which constitute part of each of the thermocouples  31   a - 31   c,  are each connected to the temperature measuring device  21  via a connector  28   a  and conductors of a water detection sensor cable  19   a.  The heater lead wires  33 ,  34  are each connected to the heater control device  22 . While the water detection sensors  11   a - 11   c  are shown by way of example in  FIG. 3 , substantially the same construction is also applied to water detection sensors  11   d  and  11   e.    
         [0037]    Next, a neutron detector is described below using the accompanying drawings.  FIG. 4  is a partial, longitudinal cutaway view showing a fission chamber used as a neutron detector to constitute another part of the embodiment of the neutron monitoring system according to the present invention. In  FIG. 4 , the same reference numbers as those shown in  FIGS. 1 to 3  denote the same elements, detailed description of which is omitted hereinafter. 
         [0038]      FIG. 4  shows a structural example of a fission chamber  37  used as a neutron detector  9   a - 9   e.  The fission chamber  37  includes a metallic anode  38 , which is a positive electrode, and a metallic cathode  39 , which is a negative electrode. The cathode  39  is coated with a fission material such as U-235. The anode  38  and the cathode  39  are insulated from each other via an insulating body  40 , and are supported to maintain a fixed clearance between the electrodes. The clearance between the anode  38  and the cathode  39  is filled in with an inert gas such as argon. In addition, the cathode  39  has its outer region shrouded with a casing  41  electrically connecting to the cathode  39 . 
         [0039]    Connected to the fission chamber  37  is a mineral-insulated cable  42  which can withstand an internal environment of the reactor. The mineral-insulated cable  42  includes a conductor  43  formed from copper or stainless steel, a metal sheath  44  covering the conductor  43 , and a mineral insulator  45  such as alumina or magnesia. The mineral insulator  45  is disposed between the conductor  43  and the metal sheath  44  to electrically insulate them from each other. The conductor  43  is electrically connected to the anode  38 , and the metal sheath  44  to the housing  41 . 
         [0040]    In the fission chamber  37 , a voltage from about several tens of volts to a high voltage of nearly thousand volts is applied between the anode  38  and the cathode  39  to generate electric fields. Upon entry of a neutron into the fission chamber  37  under this state, part of the fission material within the cathode  39  undergoes fission, with a resulting fission product ionizing the inert gas and causing a current between the anode  38  and the cathode  39 . Additionally, a current is detected to measure a neutron flux (intensity of radiation). 
         [0041]    Next, another example of a neutron detector is described below using  FIG. 5 .  FIG. 5  is a partial, longitudinal cutaway view showing a self-powered neutron detector used as a neutron detector to constitute yet another part of the embodiment of the neutron monitoring system according to the present invention. In  FIG. 5 , the same reference numbers as those shown in  FIGS. 1 to 4  denote the same elements, detailed description of which is omitted hereinafter. 
         [0042]      FIG. 5  shows a structural example of a self-powered neutron detector  46  used as a neutron detector  9   a - 9   e  to constitute yet another part of the embodiment of the neutron monitoring system according to the present invention. 
         [0043]    The self-powered neutron detector  46  includes an emitter  47  and a collector  48 . The self-powered neutron detector  46  is constructed so that entry of a neutron into the emitter  48  causes a current between the emitter  47  and the collector  48 . The collector  48  covers the emitter  47 , and a mineral insulator  49  such as alumina is filled between the collector  48  and the emitter  47  to electrically insulate them from each other. An end cap  50  for protection is fitted at an end of the self-powered neutron detector  46 . A mineral-insulated cable  51  is connected to the other end of the self-powered neutron detector  46  and fixed at welds  52 . 
         [0044]    The mineral-insulated cable  51  includes a conductor  53  formed from copper or stainless steel, a metal sheath  54  covering the conductor  53 , and a mineral insulator  55  such as alumina or magnesia, disposed between the conductor  53  and the metal sheath  54  to electrically insulate them from each other. The conductor  53  is electrically connected to the emitter  47 , and the metal sheath  54  to the collector  48 . 
         [0045]    In the self-powered neutron detector  46 , the emitter  47  is comprised of either a substance that generates beta radiation, or a substance that generates Compton electrons or photo-electrons after generating gamma radiation, upon the occurrence of a nuclear reaction with entry of a neutron. For this reason, a current that corresponds to the entry of the neutron occurs without a power supply. A current is detected to measure a neutron flux (intensity of radiation). 
         [0046]    Next, operation of the embodiment of the neutron monitoring system according to the present invention is described below using  FIGS. 1 ,  6 , and  7 . 
         [0047]      FIG. 6  is a characteristics diagram showing an example of a thermocouple output response in the embodiment of the neutron monitoring system according to the present invention, and  FIG. 7  is a conceptual diagram showing an example of setting sensitivity for correction in the embodiment of the neutron monitoring system according to the present invention. 
         [0048]    First referring back to  FIG. 1 , a command signal is output from the water existence determining control section  23  to the heater control device  22  and the temperature measuring device  21 , to sense whether each of the neutron detectors  9   a - 9   e  monitoring the position and output power of the fuel  27  inside and outside the reactor pressure vessel  1  is fully immersed in the coolant water or exposed above the water level. Thus a current is applied for a predetermined time from the heater control device  22  to the heater leads  33   a - 33   e,    34   a - 34   e  (see  FIG. 2 ) of the water detection sensors  11   a - 11   e  placed at positions corresponding to the same heights as are the neutron detectors  9   a - 9   e.  When current flows into the heating elements  32   a - 32   e  via the heater leads  33   a - 33   e,    34   a - 34   e,  temperatures of the thermocouples  31   a - 31   e  increase since the thermocouples  31   a - 31   e  are arranged in close proximity to the heating elements  32   a - 32   e.  Thus, output voltages of the thermocouples  31   a - 31   e  increase. 
         [0049]    Under the command from the water existence determining control section  23 , the temperature measuring device  21  converts into temperature data a voltage that the thermocouples  31   a - 31   e  each output before the heater is electrically energized, and a voltage that the thermocouples  31   a - 31   e  each output while the heater is electrically energized. Then the temperature measuring device  21  transmits converted results to the water existence determining control device  23 . 
         [0050]    The water existence determining control device  23  calculates the increases in the temperatures of the thermocouples after a predetermined time of energizing on the basis of the temperature data obtained before and during the energizing of the heater. The water existence determining control device  23  then determines whether the water detection sensors  11   a - 11   e  are fully immersed in the coolant water or exposed above the water level. 
         [0051]    In the example of a thermocouple output response that is shown in  FIG. 6 , the response depends upon whether each water detection sensor  11   a - 11   e  is fully immersed in the coolant water or exposed above the water level. This example shows an increase in the temperature of the corresponding thermocouple, developed when current is supplied to one heating element  32   a  for five seconds. 
         [0052]    When the water detection sensor  11   a - 11   e  is fully immersed in the coolant water, the temperature increases by about 2° C. after five seconds from a start of energizing, and after energizing, decreases to a before-energizing temperature in about 20 seconds. On the contrary, when the water detection sensor  11   a - 11   e  is exposed above the water level, since the atmosphere whose heat transfer coefficient is small relative to that of water covers the water detection sensor  11   a,  energizing increases the temperature to about 12° C. Additionally a longer time is needed for an after-energizing temperature to decrease. 
         [0053]    The increase in the temperature, and response characteristics of heating/cooling, however, depend upon a design of the sensor and a magnitude of the current supplied, and the response characteristics in  FIG. 6  are only shown by way of example. 
         [0054]    As described above, in the water existence determining control device  23  that has calculated the increase in the temperature, for example if a value of 5° C. is set as a determination criterion (threshold value) with respect to the temperature increase developed after the five seconds of energizing, then it can be determined whether the water detection sensor  11   a - 11   e  is fully immersed in the coolant water or exposed above the water level. Because the temperature increase depends upon parameters such as temperatures and pressures of the coolant water and the atmosphere, the determination criterion with respect to the increase in the temperature of the thermocouple is taken as a function of temperature and pressure, and stored into a threshold-data table of the storage device  23   a.    
         [0055]    The water existence determining control device  23  acquires the temperature data obtained before the energizing of the heater, pressure data from a pressure gauge not shown, and other data, then reads the temperature/pressure-dependent determination criterion from the threshold-data table of the storage device  23   a,  and compares the determination criterion with the temperature increase. The water existence determining control device  23  thus determines whether water exists on the water detection sensor  11   a - 11   e  (i.e., the water detection sensor is fully immersed in the coolant water) or the sensor is exposed above the water level. This determination result is transmitted to the correction circuit  24 . 
         [0056]    The amplification circuit  20  amplifies a neutron detection signal sent from the relevant neutron detector  9   a - 9   e,  and then outputs the amplified signal to the correction circuit  24 . As described above, the water existence determination result by the water detection sensor  11   a - 11   e , placed at positions corresponding to the same heights as are the neutron detector  9   a - 9   e,  is also input to the correction circuit  24 . 
         [0057]    On the basis of the water existence determination result, the correction circuit  24  sets a sensitivity coefficient by which the amplified neutron detection signal is to be multiplied, and then outputs to the measured-data conversion device  25  the neutron detection signal that has been multiplied by the set sensitivity coefficient. 
         [0058]      FIG. 7  shows an example of setting sensitivity for correction. In this example, the sensitivity coefficient to be used when the water detection sensor is fully immersed in the coolant water is set to be “a”, or the sensitivity coefficient to be used when the sensor is exposed above the water level is set to be “b”, where “a” is smaller than “b”. 
         [0059]    For example, when the fission chamber  37  shown in  FIG. 4  is fully immersed in the coolant water, a moderation effect against the neutron is significant. Thus, sensitivity of the fission chamber  37  becomes increased compared with that obtained when the chamber is exposed above the water level. Accordingly, when the water detection sensor, disposed at a position corresponding to the same height as is the fission chamber  37 , is fully immersed in the coolant water, the neutron detection signal can be corrected by setting the sensitivity coefficient to be “a”, the smaller value. Conversely when the water detection sensor is exposed above the water level, the neutron detection signal can be corrected by setting the sensitivity coefficient to be “b”, the larger value. 
         [0060]    The neutron detection signal that the correction circuit  24  has thus multiplied by the set sensitivity coefficient, depending upon whether an enough amount of coolant water exists, is converted into a neutron count rate (counts/s) or a neutron flux (n/cm2/s) by the measured-data conversion device  25 . 
         [0061]    In the above embodiment of the neutron monitoring system according to the present invention, whether the individual neutron detectors are fully immersed in the coolant water or exposed above the water level can be sensed, so that the data measurements by the neutron detectors can be corrected accurately from the sensing results. This in turn enables accurate detection of neutrons, thus contributing to estimating power and the fuel position more accurately even in case of such a severe accident that causes the fuel to melt. 
         [0062]    It is to be noted that the present invention is not limited to the aforementioned embodiments, but covers various modifications. While, for illustrative purposes, those embodiments have been described specifically, the present invention is not necessarily limited to the specific forms disclosed. Thus, partial replacement is possible between the components of a certain embodiment and the components of another. Likewise, certain components can be added to or removed from the embodiments disclosed. 
         [0063]    Note also that some or all of the aforementioned components, functions, processors, and the like can be implemented by hardware such as an integrated circuit or the like. Alternatively, those components, functions, and the like can be implemented by software as well. In the latter case, a processor can interpret and execute the programs designed to serve those functions. 
         [0064]    Further note that the control lines and information lines shown above represent only those lines necessary to illustrate the present invention, not necessarily representing all the lines required in terms of products.