1. Field of the Invention
The present invention relates to a nuclear reactor start-up monitoring apparatus installed in, for example, a boiling-water reactor, particularly relates to the contrivance of the constitution of a digital apparatus for signal-processing of an output pulse from a radiation sensor disposed inside or outside of a reactor pressure vessel as data for measuring and monitoring reactor power.
2. Description of the Related Art
In a recent boiling-water reactor, there are provided with radiation sensors such as six to ten SRNM (Start up Ranged Neutron Monitor) detectors and 100 to 200 LPRM (Local Power Ranged Monitor) sensors installed in a reactor pressure vessel of the reactor, and a reactor power monitoring apparatus such as, for example, a start-up ranged monitor and power ranged monitor, for measuring and monitoring reactor power based on the detected signals by these radiation sensors.
Among them, the start up ranged neutron monitor (to be referred to as "reactor start-up monitoring apparatus " hereinafter) monitors reactor power at the time of reactor start-up or the like based on output pulses in accordance with neutron fluxes detected by the SRNM detectors. This apparatus performs processing for counting the number of output pulses of the SRNM detectors (to be referred to as "pulse measurement" hereinafter) in low reactor power ranges (10.sup.-9 % to 10.sup.-4 %) and performs processing for measuring the power of a fluctuabon component generated by the overlapping output pulses of the SRNM detectors, i.e., processing based on the principle of Campbell Law (to be referred to as "Campbell measurement" hereinafter) in high reactor power ranges (10.sup.-5 % to 10%).
An example of a reactor start-up monitoring apparatus conducting the pulse measurement and Campbell measurement as stated above will be described based on FIGS. 5 and 6.
A reactor start-up monitoring apparatus shown in FIG. 5 consists of an SRNM detector 100 detecting, as data for measuring reactor power, neutron fluxes in the reactor, an analog amplifier 101 amplifying and rectifying the detector output pulses, two signal processing systems connected in parallel to the signal output side, i.e., a pulse measurement system 102 and a Campbell measurement system 103, and reactor power evaluating (monitoring) means 104 for continuously monitoring/evaluating reactor power at least at time of starting up the reactor based on the processing results of the measurement systems 102 and 103.
Among those elements, the pulse measurement system 102 consists of a pulse wave height comparator 105 comparing the wave height value of the detector output pulse amplified by the analog amplifier 101 with a preset wave height value and counting the number of the detector output pulses having wave heights higher than the preset wave height value, and pulse measurement evaluating means 106 evaluating reactor power at the time of low power output by converting the counted pulse number to the output level of the reactor power.
Also, the Campbell measurement system 103 consists of a plurality of amplifiers (a small gain amplifier 107, a medium gain amplifier 108 and a large gain amplifier 109) amplifying and attenuating the detector output pulses amplified by the analog amplifier 101 under different conditions of a plurality of amplification factors and thereby limiting frequency bands to a specified band, a plurality of MS (Mean Square) operators 110, 111, 112 calculating mean square roots of the outputs of the amplifiers 107 to 109 and Campbell output evaluating means 113 for selecting an optimum value from the outputs of the operators 110 to 112 and for evaluating reactor power at the time of high power output.
In the reactor start-up monitoring apparatus stated above, if reactor power is low, pulse measurement is executed to adjust an output pulse to a signal level optimal to the pulse wave height comparator 105 and to accurately count the number of output pulses of the SRNM detector 100 by the analog amplifier 101. If reactor power is high, Campbell measurement is executed. When the Campbell measurement is executed, about five figures which is the measurement range thereof, cannot be covered by a single MS (Mean Square) arithmetic element 110, 111 or 112. Due to this, as shown in this example, a plurality of amplifiers 107 to 109 amplify the output of the SRNM detector 100 at different amplification factors to thereby cover the overall measurement range. In that case, an amplifier can be formed of a single logarithmic amplifier. Recently, however, there are many cases where divided amplifiers are employed as described above in light of temperature characteristics and the like, which constitution makes it possible to continuously monitor the wide measurement range of the reactor.
Nevertheless, since the above-stated reactor start-up monitoring apparatus is constituted to signal-process detector output pulses with an analog circuit, there is a possibility that counting only based on the magnitude of pulses may cause error measurement by the influence of discharge pulses generated as a result of discharge within the SRNM detector and noise erroneously generated by electromagnetic induction in the vicinity of the monitoring apparatus. As a result, this apparatus has a disadvantage in that information contained in the waveforms of the detector output pulses cannot be always utilized efficiently.
To deal with the above disadvantage, it is necessary to sufficiently shield signal cables and the like to prevent the induction of external noise at the time of execution. Additonally, as measures to prevent erroneous measurement due to the external noise during execution, there is a method of using not only pulse wave heights but also other waveform information since the waveforms of the erroneous pulses are, in most cases, different from those of signal pulses. Further, since a detector output pulse is changed by the leakage of gas sealed in the detector or by the abnormal distance between electrodes, it is possible to detect these abnormal states during measurement by monitoring waveforms. In the present system, however, it is necessary to, for example, diagnose the detector while precluding the system from monitoring targets. According to the present system, it is necessary to provide circuits dedicated to Campbell measurement and pulse measurement, respectively to simultaneously conduct the measurements, which results in larger-sized and complicated circuits. Due to this, the measurements are conducted by different hardware and, therefore, the apparatus shown in FIG. 5 has a problem that maintenance and inspection operations are carried out for the measurements, respectively. To solve this problem, it is desired that hardware parts are integrated, particularly, signal processing parts are integrated.
Considering the above, as means for solving the erroneous measurement problems with the analog circuit of that type, there is proposed a digital type reactor start-up monitoring apparatus for converting a detector output pulse to a digital signal and for conducting signal processing (e.g., in Japanese Patent Application Laid-Open No. 9-274095). Description will now be given to an example of this apparatus based on FIG. 6.
A digital type reactor start-up monitoring apparatus shown in FIG. 6 consists of an SRNM detector 100, an analog amplifier 101, as in the case of the above apparatus, a pulse measurement system 102 and a Campbell measurement system 103 connected to the signal output side in parallel, and reactor power evaluating (monitoring) means 104, such as a monitor, for continuously monitoring reactor power at the time of reactor start-up based on the measurement results of the measurement systems 102 and 103.
Among the elements, the pulse measurement system 102 consists of the first A/D converter 120 sampling the detector output pulse amplified by the analog amplifier 101 at intervals shorter than the pulse width, pulse counting means 121 for counting the number of detector output pulses from the obtained sampling data and pulse measurement evaluating means 106 for converting the counted pulse number to a reactor power level and evaluating reactor power at the time of low power output. The pulse counting means 121 performs arithmetic processing to not only pulse wave height values but also other features using sampling data and recognizes output pulses. Thus, it is possible to count only the output pulses and to remove different signals such as noise.
In addition, the Campbell measurement system 103 consists of a plurality of analog amplifiers (a small gain amplifier 107, a medium gain amplifier 108 and a large gain amplifier 109) amplifying and attenuating the detector output pulse amplified by the analog amplifier 101 under different conditions of a plurality of amplification factors, at least one, the second A/D converter 122 A/D converting the outputs of the respective amplifiers 107 to 109 and Campbell measurement evaluating means 113 for calculating a mean square value corresponding to the power of a specific frequency band using the obtained sampling data and then evaluating reactor power at the time of high power output. Among them, the Campbell measurement evaluating means 113 limits a frequency band after A/D conversion and operates the mean square value. Since the operation of the means 113 can be processed by software-based calculation, the measurement frequency band can be advantageously easily changed.
The above-stated conventional digital type reactor start-up monitoring apparatus has, however, the following problems.
1): The pulse measurement and Campbell measurement differ in necessary sampling cycle (interval) and A/D conversion accuracy and require individual A/D converters, respectively. PA0 2): Since Campbell measurement has a sampling cycle of about 2 MH.sub.Z as stated above, it is necessary to remove frequency components of 1 MH.sub.Z or more before sampling according to the sampling theorem. For that reason, an antialiasing analog filter is normally provided. PA0 3): If a digital filter is used as a band limiting filter for Campbell measurement, it is necessary to execute a simple method capable of real-time processing operation using the digital filter. This is because the present Campbell measurement elements do not have enough operation speeds. PA0 4): In regard to an algorithm for digitally sampling pulse waveforms and selecting a detector output pulse from the sampling values, it is difficult to make waveform selection in a real-time manner at a cycle of 100 nsec or less as in the case of the digital filter for Campbell measurement. To deal with this, a method for simplifying a waveform selection algorithm is desired.
That is, in pulse measurement, it is required that a sampling cycle is shorter than the pulse width of a detector output pulse. Since the width of the output pulse is about 100 nsec, an N/D converter capable of digitizing a pulse with 20 MH.sub.Z is required for the pulse measurement. For example, a commercially available A/D converter having accuracy of about eight to ten bits is employed.
In Campbell measurement, by contrast, the measurement band is normally 1 MH.sub.Z or less and an A/D converter having a sampling cycle of about 2 MH.sub.Z is, therefore, required. Besides, to broaden the measurement range, the accuracy of the converter needs to be higher than that of the above-stated N/D converter for pulse measurement. For example, a commercially available AND converter having accuracy of about 16 bits is employed.
Meanwhile, Campbell measurement, as in the case of the conventional analog type apparatus described above, covers a measurement range as wide as five figures and a plurality of analog amplifiers are, therefore, required.
As stated above, due to the need to prepare a plurality of A/D converters and analog amplifiers, the overall constitution of the conventional digital type reactor start-up monitoring apparatus becomes disadvantageously complicated.
If such an antialiasing analog filter is provided, however, analog filtering characteristics play a major role and digital response expected by the digital type reactor start-up monitoring apparatus cannot be disadvantageously realized. That is to say, since power operation is performed by the Campbell measurement system in the digital type reactor start-up monitoring apparatus in a frequency band of about 100 kH.sub.Z to 400 kH.sub.Z, a digital filter instead of an analog filter is expected to be adopted as the band limiting filter to enhance time response and performance against external noise.