Patent Number: 041860489
Section: description

Very generally, the system of the invention is employed in a nuclear reactor wherein a plurality of neutron flux detector assemblies 11 provide on respective output channels 12, pulsed output signals at pulse rates corresponding to detected neutron flux. Signal processing means 24 provide for each channel an indication of neutron flux in the intermediate and power ranges of the reactor. The system of the invention includes summing means 13 responsive to pulsed input signals to provide a pulsed output signal corresponding to the sum of the input pulses thereof. A plurality of coupling means 14 each connect a respective one of the output channels to the summing means for providing a pulsed signal thereto substantially corresponding to the pulsed output signals provided by the corresponding detector assembly. The coupling means each include isolation means 15 for preventing spurious communication between the summing means and the cables. Indicating means 17 are connected to the output of the summing means to represent the pulse rate of the output pulses of the summing means. Referring now more particularly to FIG. 1, the invention is shown in a preferred embodiment for use in a power reactor wherein approximately eleven decades of neutron flux measurement are required for monitoring neutron flux over the full range of the reactor. The monitoring system employs a plurality of detector assemblies, each of which utilizes one or more fission chambers 21, two being shown in FIG. 1. Only one detector assembly is shown in the drawing but a typical reactor may employ four of these detector assemblies located at four different positions around the reactor core. Typically, two or three fission chambers are used per assembly for the purpose of monitoring vertical flux tilts and the four separate assemblies are used in a two out of four logic redundant system for safety purposes. The typical length of the detector assembly is approximately 31/2 meters. In the drawings, the power supplies for the fission chambers are not illustrated for the purpose of simplification. The range of flux typically covered by a complete neutron flux monitoring system extends from about 0.1 nv to greater than 10.sup.10 nv (approximately 111/2 decades). A single fission chamber will cover the range of about 1 nv to 2.times.10.sup.10 nv. The system sensitivity when pulses from all twelve detectors are summed will extend below 0.1 nv. The output of each of the detector assemblies 11 is applied to an output channel 12. In the illustrated embodiment, each channel comprises the two output lines from the fission chambers 21 and a safety drawer 24. A separate safety drawer 24 is provided for each detector assembly 11. The outputs of the fission chambers are fed to the safety drawer through a suitable junction box (not shown) containing line and decoupling filters (not shown) for the chamber high voltage (not shown). The secondary containment is indicated schematically by the line 25 and 23 represents the penetration for chamber leads. In the illustrated embodiment, the safety drawer 24 contains capacitors 31 and 33 through which the a-c component of each of the signals from the outputs of the fission chambers 21 are applied to pulse amplifiers 35 and 37. The output of the amplifier 35 is applied through a resistor 38 to a ten decade log circuit 39 (such as shown in U.S. Pat. No. 3,579,127) in the safety drawer 24. The circuit 39 is suitably constructed to provide a log output signal of flux for the upper ten decades of the flux range of the reactor. The circuit 39 provides its output signal through a buffer amplifier 41 to provide an indication of the log of the output of the fission chamber 21 in the detector assembly. To provide a rate output, the output of the ten decade log circuitry 39 is also applied to a rate circuitry 43, the output of which is applied to an indicator, not shown, through a suitable buffer amplifier 45. In the event the rate of change of flux exceeds a preset value, a bistable trip 47 is coupled to the output of the rate circuit 43 to produce a rate trip signal. The d.c. components of the outputs of the fission chambers 21 from each detector assembly 11 are also applied to d.c. amplifiers 51 and 53 in the safety drawer 24. These signals are proportional to flux level in the upper decades and are amplified by buffer amplifiers 57 and 59 to indicate the level of the particular fission chamber, thus providing an indication of flux tilts. In addition, the outputs of the amplifiers 51 and 53 are summed in a summing amplifier 69 and are applied through a buffer amplifier 71 to indicate the power sum or average power in that particular detector assembly. The power sum signal is also applied to a bistable trip 73 which is set to produce a power level trip signal for use in the reactor safety logic system. The foregoing described circuitry within the safety drawer provides, for each detector assembly, a suitable indication of the neutron flux for both intermediate and power range operation. In accordance with the invention, the same detector assemblies 11 are utilized to provide an indication of flux levels during the shutdown condition and in the source range. This is done without the need for an additional costly and sometimes unreliable monitoring instrument array of the necessary sensitivity. More particularly, in the illustrated embodiment, the output channel 12 of the detector assembly 11 is summed in a pulse summer 72 and coupling means 14 to a summing means 13. The other detector outputs are similarly coupled to the summing means. To provide electrical isolation between the summing means 13 and the detector outputs 12 and between the signal outputs 12 of the four redundant and separate safety assemblies and thus prevent spurious signals from being communicated therebetween, the isolation means 15 are provided. In the embodiment of FIG. 1, the coupling means 14 include a light emitting diode 75 connected to the pulse summer 72. An optical fiber cable 15 serves as the isolation means and couples the diode 75 to the summing means 13. The summing means 13 preferably includes an array of photo diodes 77 one for each channel. The electrical pulses generated by two of the photo diodes 77 are combined in a log count rate circuit 78, and the other two by a log count rate circuit 79. By dividing the channels into groups and summing separately, two independent count rate outputs are provided. Thus, if one channel should develop an oscillation or source of noise that makes its count rate output read higher, the other independent output is not affected. The independent outputs are indicated by meters 81 and 83. To provide an output indicating the total count rate of all channels, buffer amplifiers 85 and 87 connect the separate rate outputs to a single meter 89. Referring now to FIG. 2, another embodiment of the invention is shown. In FIG. 2, the detector assemblies 111, two of which are shown, each contain three fission chambers 121. The output of each of the detector assemblies 111 is applied to an output channel 112. In FIG. 2, each channel comprises three output lines from the fission chambers 121, a discriminating pulse summer 122, and a safety drawer 124, described below. A separate safety drawer 124 is provided for each detector assembly 111. The outputs of the fission chambers are fed to the summer 122 through a suitable junction box (not shown) containing line and decoupling filters (not shown) for the chamber high voltage (not shown). The secondary containment is indicated schematically by the line 125, the penetration by 123, and the line connections on the interior and exterior of the pressure vessel are indicated at 127 and 129, respectively. In FIG. 2, the discriminating pulse summer 122 comprises capacitors 131, 133 and 135 through which the a-c component of each of the signals from the outputs of the fission chambers 121 are applied to a summing amplifier 137. The output of the summing amplifier 137 is applied to a ten decade log circuit 139 (such as shown in U.S. Pat. No. 3,579,127) in the safety drawer 124. The circuit 139 is suitably constructed to provide a log output signal of flux for the upper ten decades of the flux range of the reactor. The circuit 139 provides its output signal through a buffer amplifier 141 to provide an indication of the log of the output of the sum of the three fission chambers in the detector assembly. To provide a rate output, the output of the ten decade log circuitry 139 is also applied to a rate circuit 143, the output of which is applied to an indicator, not shown, through a suitable buffer amplifier 145. In the event the rate of change of flux exceeds a preset value, a bistable trip 147 is coupled to the output of the rate circuit 143 to produce a rate trip signal. The outputs of the fission chambers 121 from each detector assembly 111 are also applied through resistors 151, 153 and 155, in the discriminating pulse summer 122 to the safety drawer 124. These signals, which comprise the d.c. component of fission chamber signal and are proportional to flux level in the upper decades, are amplified by amplifiers 157, 159 and 161, respectively, and are applied through buffer amplifiers 163, 165 and 167, to indicate the level of the particular fission chamber, thus providing an indication of flux tilts. In addition, the outputs of the amplifiers 157, 159 and 161 are summed in a summing amplifier 169 and are applied through a buffer amplifier 171 to indicate the power sum in that particular detector assembly. The power sum signal is also applied to a bistable trip 173 which is set to produce a power level trip signal for use in the reactor safety logic system. The foregoing described circuitry within the safety drawer provides, for each detector assembly, a suitable indication of the neutron flux for both intermediate, start-up and power range operation. In this embodiment as well as the first, the same detector assemblies 111 are utilized to also provide an indication of flux levels during the shutdown condition or source range. More particularly, in the illustrated embodiment the output channel 112 of the detector 111 is coupled by coupling means 114, from the ten decade log circuit 139 to a summing means 113. The other detector outputs are similarly coupled to the summing means. To provide electrical isolation between the summing means 113 and the detector outputs 112 and between the signal outputs 112 of the four redundant and separate safety assemblies and thus prevent spurious signals from being communicated therebetween, the isolation means 115 are provided. In the embodiment of FIG. 2, the coupling means 114 include a light emitting diode 175 connected to the log circuit 139. An optical fiber cable 115 serves as the isolation means and couples the diode 175 to the summing means 113 which preferably comprises an array of photo detectors such as photo diodes. The outputs of all of the channels of the detector assemblies 111 are so coupled so that, as illustrated, four light inputs to the photo diode summer 113 result. The photo diode summer 113 operates in response to light pulses applied thereto to produce a pulsed electrical output signal having a pulse rate corresponding to the sum of the applied light pulses. By applying the output of the summer 113 to the count rate circuit 116, an output indicating the total count rate of all pulses produced in all of the fission chambers 121 in all four of the detector assemblies 111 results. For the purpose of providing redundancy so that failure of one portion of the system will not result in an inability to monitor flux, an additional light emitting diode 179 is provided corresponding to the light emitting diode 175 in each of the safety drawers. The output of the light emitting diode 179, which is a series of pulses, corresponding to the electrical pulses applied thereto, is passed by means of a fiber optic cable 181 to a second photo diode 183. The photo diode 183 operates in the same manner as the photo diode 113 to provide an electrical output signal representing the sum of the light pulses applied thereto. The electric output signal of the photo diode 183 is then applied to a count rate circuit 185 and a redundant output indicative of the total neutron flux in the lowermost range or shutdown range of the reactor is provided. Redundancy and separation for the two shutdown and source range signals required can also be obtained by combining pulse signals from only two of the four detector assemblies for each separate shutdown monitoring channel. This will result in a sensitivity decrease of each channel by a factor of two. Although light emitting diodes, fiber optic cables and photo diodes are shown as separate elements in FIG. 1, it is possible to provide a single unit which functions identically. Such devices are available commercially as optical isolators, for example from Texas Instruments under the designation 3N219 Opto-coupler. Referring now to FIG, 3, an alternative to the coupling means 114 of FIG. 2 is shown. Instead of utilizing a fiber optic cable as the isolation means, the embodiment of FIG. 3 employs a radio frequency transformer 215. A radio frequency oscillator 275 is gated on and off by summed pulses in the log circuit 139. The oscillator 275 is responsive to applied pulses to produce an output signal comprised of short bursts of radio frequency signals, each burst corresponding to one of the applied pulses. After passage through the RF transformer 215 (having adequate electrical insulation and isolation), the RF bursts are converted back into electrical pulses. To this end a detector 276 of suitable construction is employed. A summing amplifier 213, which is connected to the detectors 276 for all channels, provides a pulsed output corresponding to the sum of the input pulses. A suitable count rate circuit 216 connected thereto provides the desired output. By utilizing a second similar coupling means, a redundant output may be provided. It may be seen therefore that the invention provides an improved flux monitoring system for a nuclear reactor which eliminates the need for proportional counters or similar sensing means for the lowermost range to be monitored. The lower decade of sensitivity for monitoring the shutdown condition is achieved directly from the same equipment required for the upper ten decades of reactor operation. The invention operates successfully with neutron flux monitoring systems which use fission chambers for the power range and intermediate range for the detectors. The system has a sensitivity to average neutron flux of many times that of a single detector (in the illustrated embodiment of FIG. 2, twelve times) and, with readily available fission chamber neutron sensitivity, can achieve the equivalent sensitivity to that of a proportional counter and adds at least an additional decade of neutron flux measurement in the source range of the reactor. Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.