JPS6331061B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a wide area instrumentation system in which multiple signals of neutron flux are simultaneously and continuously measured by a single neutron detector.

As is well known, the neutron flux level of a nuclear reactor has a wide measurement range. For example, a boiling water reactor has an 11-digit measurement range. Therefore, it is technically difficult to measure with a single measuring means. Therefore, measurements are generally performed using a measurement system that combines the following three measurement means.
One of them is a measurement method in the low neutron flux range (startup system region). In this case, since the reactor output is proportional to the counting rate, the neutron flux level is measured by determining the counting rate by pulse counting using a low range six digits. The second method is a measurement method using the intermediate neutron flux range (intermediate system region). This measuring means focuses on the fact that when the furnace output is in the intermediate range, it is proportional to the root mean square value, and measures using the Campbell method. In other words, measurement is performed according to Campbell's second theory using the effective value or root mean square value of the alternating current component of the output signal of the detector. The third is a measurement means using a high neutron flux range (output system area). This measuring means focuses on the fact that when the furnace output is in the power system region, it is proportional to direct current, and measures the direct current from the detector.

As shown in Figure 1, the above pulse measurement system connects the pulse measurement system detector 1 and pulse preamplifier 2 with a coaxial cable 3, but since this cable 3 cannot be shortened, the cable capacity is Very large, 2000~5000PF. Therefore, when performing high count rate measurements, pulses are received by the input resistor R1 of the amplifier 2 with a low input impedance. Further, if the coaxial cable 3 is long, signal reflection occurs, so it is necessary to match the coaxial cable 3 and the preamplifier 2 in the input circuit 4.

Furthermore, in the case of the above-mentioned intermediate measurement system, the effective value of the AC component of the input current is measured instead of counting pulses, and since the signal level is very small, it is necessary to perform low-noise measurement. As shown in FIG. 2, the measurement system connects a coaxial cable 3 to the output side of the intermediate measurement system detector 5, and connects the other end of the cable 3 via an input circuit 6 to a high input impedance type low noise Campbell preamplifier. (voltage amplifier) 7
It is connected to the Note that the above amplifier 7
is a low noise voltage type amplifier having an input impedance of, for example, about 5 to 10 kΩ.

As is clear from the above-mentioned differences in characteristics, it is extremely difficult to measure the pulse signal and the Campbell signal using the same measuring device.
In other words, it is technically very difficult to separate the pulse signal from the canvas signal while using the same detector for both systems and satisfying the measurement requirements of each system. It is also difficult to get enough.

Thus, each detector is typically used separately. Looking at the number of detectors installed in a nuclear reactor, conventional boiling water reactors have four startup system detectors, eight intermediate system detectors, and 100 to 200 power system detectors each in the core. It is installed.

In this way, conventionally, neutron flux is measured using different detectors for each system. Therefore, not only a large number of detectors but also a large number of detector guide tubes, cables, etc. are required. As a means to improve this point, there is a so-called wide area instrumentation system.
There are two signal separation methods in conventional wide-area instrumentation systems. One of them is, as shown in FIG. 3, in which a single wide-area amplifier 10 is connected to a neutron detector 8 via an input circuit 9 consisting of a coaxial cable 3, a resistor RH, and a capacitor _C3 . The preamplifier 10 is of a low noise type with low input impedance. The other one is an input circuit 11 connected to a neutron detector 8 via a coaxial cable 3, as shown in FIG.
A low input impedance type high frequency band amplification pulse preamplifier 12 is connected via a low input impedance circuit consisting of a resistor RH and a capacitor _C4 of this input circuit 11, and a high input impedance circuit consisting of an input resistor RH and a coupling capacitor _C5 is connected. A high input impedance type low noise intermediate frequency band amplifying canvas amplifier 13 is connected via a circuit.

In the case shown in FIG. 3, assuming that the input resistance of the preamplifier 10 is Rin, the amplifier input conversion noise cannot be reduced due to the thermal noise of this input resistance Rin. Now, assuming that the input conversion noise is ln (rms), it is expressed as ln (rms) = √...(1). K is the Boltzmann constant
1.3804×10 ^-23 (Joul/〓), T is the absolute temperature of the resistor (〓), B is the amplifier bandwidth (Hz), and Rin is the input (signal source) resistance (Ω). The input equivalent noise current in (rms) at this time is becomes. When considering this method of pulse measurement, the input impedance must be low.
Rin becomes smaller. Therefore, the input conversion noise becomes large. Therefore, when the signal current from the detector 8 is equal to or less than equation (2), S/N separation cannot be performed and measurement becomes impossible.

On the other hand, in the case shown in FIG. 4, when the detector 8 is a fission counter for neutron detection, its output current is generated by electrons and ionic components. The electron and ionic components are due to the ionization of the gas in the fission counter, such as argon gas.
Both have the same amount of charge, and the moving speed in the gas is, for example, 10 atmospheres of argon gas, 0.5 mm between the electrodes,
If the applied voltage is 300V, then in the case of electrons it is 3.7×10 ⁵
cm/sec, and for ions it is 9.78×10 ² cm/sec. Therefore, the charge collection time for each is 135 nS for electrons and 51.1 μS for ions,
This means that the difference is only about 379 times. It is extremely difficult to frequency-separate the input from such a detector using the coupling capacitors C ₄ and C ₅ of the input circuit 11, and causes loss of signal current and generation of reflected waves due to mismatching. Therefore, this method is not currently being adopted at all.

The present invention was developed in consideration of these circumstances, and its purpose is to enable continuous measurement of neutron flux using a single detector, and to significantly reduce the number of in-core detectors. We provide a neutron wide-area instrumentation system that not only greatly reduces equipment costs but also facilitates maintenance by reducing detection guide tubes, cables, cable penetration, etc. It's about doing.

The present invention will be explained in detail below with reference to embodiments shown in the drawings. In FIG. 5, reference numeral 21 denotes a wide area instrumentation detector, and an input circuit 23 is connected to the output end of this detector 21 via a coaxial cable 22. This input circuit 23 has a low input impedance circuit consisting of an input resistance RH and a coupling capacitor _C6 , and a high input impedance circuit consisting of an input resistance RH and a coupling capacitor _C7 . A low input impedance type high frequency band amplifying pulse amplifier 24 having an input resistance R6 is connected to the low input impedance circuit side of the input circuit, and a high input impedance type low noise intermediate having an input resistance R7 is connected to the other high input impedance circuit side. A frequency band amplifying Campbell amplifier 25 is connected.

The wide-area instrumentation detector 21 is, for example, a nuclear fission counter, with an electrode gap of 0.5 mm, a sealed gas pressure of 10 atm,
The applied voltage was 300 V, and a mixed gas of 95% argon and 5% nitrogen was used as the filler gas, that is, the ionized gas. As a result, the speed of electron movement increases significantly. When pure argon gas is used, the electron transfer rate is
The speed is 3.7×10 ⁵ cm/sec, but when 5% nitrogen is mixed in, the speed increases to 2.1×10 ⁶ cm/sec, about 6 times faster, and the electron charge collection time becomes 23.8 nS. On the other hand, the ion charge collection time is 50.4 μS, which is almost the same as the ion charge collection time of pure argon gas, which is 51.1 μS. By mixing a small amount of nitrogen gas into the ionized gas in this way, the charge collection times for electrons and ions differ greatly, resulting in values that differ by a factor of 2100.
Here, the component ratio of argon gas and nitrogen gas will be explained in more detail. In this example, as already mentioned, argon gas is 95% and nitrogen gas is 5%, but the present invention is not limited to this. Argon gas is 99.9% to 80% and nitrogen gas is 0.1 to 80%. 20
% range is sufficient.

By the way, the reason for this numerical limitation will be explained with reference to FIG. FIG. 7 is a diagram showing the change in the argon gas/nitrogen gas mixture ratio on the horizontal axis and the electron transfer speed on the vertical axis. As is clear from Figure 7, when nitrogen gas is less than 0.1%, the electron transfer rate does not increase much compared to pure argon gas, but when nitrogen gas is mixed with 0.1% or more, When using pure argon gas, the amount will be approximately twice that of pure argon gas. Furthermore, by increasing the ratio of nitrogen gas, the electron transfer rate is further improved, and when the mixture is 5%, the electron transfer rate increases to about 5.7 times. Up to about 20%, the electron transfer rate is about 5 to 6 times higher.
Therefore, we first set the lower limit of nitrogen gas to 0.1%.

Next, the reason why the upper limit of nitrogen gas was set at 20% will be explained. First, by the incidence of neutrons (n,
Nuclear reactions such as α) and (n, f) occur, and the gas is ionized by α rays and fission fragments passing through the ionized gas, but the average energy loss required for the ionization (creating a pair of ions) The amount (w value) varies depending on the type of gas. For example, w of argon gas is 26.4eV
, and the w value of nitrogen gas is 36.6 eV. The energy of alpha rays and nuclear fission fragments is a constant value, and the number of ion pairs generated by each is determined by the w value of the ionized gas used; the smaller the w value, the more ion pairs are produced. can be generated. In addition, the signal obtained from the neutron detector is obtained by collecting ionized ion pairs by electrolysis to obtain a pulse signal proportional to the collected charge, and by superimposing the pulse signals, a Campbell signal can be obtained. .

From this consideration, in order to perform pulse measurement and Campbell measurement with good S/N characteristics, it is better to have a larger pulse signal, and to increase the pulse signal, it is necessary to use a gas with a small w value. Therefore, it is not preferable to use a large amount of nitrogen gas, which has a relatively large w value. Therefore, in the present invention, the nitrogen gas is The upper limit is set at 20%. As described above, by using a nuclear fission counter with different charge collection times for electrons and ions as a wide-area instrumentation detector, frequency separation becomes possible, and signal current loss and reflected waves are also eliminated.

Further, specifically, it is assumed that the amplifiers 24 and 25 have characteristics as shown in FIG. In the same figure
_CO is the center recuperation frequency of the Campbell signal, _CL and _CH are the lower and upper limits of the Campbell signal, _PO are the center frequencies of the pulse signal, and _PL and _PH are the lower and upper limits of the pulse signal. Therefore, for example, if the value of the coupling capacitor C ₆ of the input circuit 23 is determined according to the band of the pulse signal, 1/2π _PL ·C ₆ =R6, the pulse signal can be sufficiently passed. At this time, since the Campbell signal (ion current) component in the intermediate frequency band is sufficiently small, it can be sufficiently removed by pulse height discrimination.

Further, the Campbell amplifier 25 is a high input impedance type intermediate frequency band amplifier suitable for the Campbell signal (ion current component), and if the value of the input resistance R7 is determined by R7 = 1/2π _CO・C ₇ , the Campbell signal is With high impedance input, the detector component (ion current component) can be separated based on the S/N ratio. The influence of noise from the pulse amplifier side can be sufficiently removed by the frequency characteristics of the coupling capacitor C6 and the Campbell amplifier 25. In a device with such a configuration, the wide area instrumentation detector 2
The neutron flux signal output from 1 is sent to the coaxial cable 22
Input circuit 23 installed at a predetermined location via
The frequency bands are separated by this circuit 23. In the pulse measurement system, only the high frequency signal component is received and amplified by the pulse amplifier 24 with low impedance, and in the Campbell measurement system, only the intermediate frequency band component is amplified by the Campbell 25 with high input impedance. In this way, neutron flux can be measured with a high S/N ratio.

Furthermore, by using the wide-area instrumentation detector of the present invention, the output linearity in the high count region of the pulse system can be improved.
It has been improved from 3.7×10 ⁵ CPS to 2.1×10 ⁶ CPS, and a large amount of overlap between the pulse system and the canvas signal system can be obtained.

In the above example, the case where 5% nitrogen was mixed with argon was described, but substantially the same effect can be expected with a mixture of 0.1 to 20% nitrogen.

As explained above, according to the present invention, the output signal of the detector can be separated into the pulse signal and the canvas signal by the S/N ratio, so the neutron flux can be continuously measured with a single detector, and the neutron flux inside the reactor core can be measured continuously. Not only can the number of detectors be significantly reduced, but also the number of detector guide tubes, cables, and cable penetrations can be reduced, which can significantly reduce equipment costs and improve maintenance. It is possible to provide a neutron wide-area instrumentation system that is easy to use.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional pulse measurement system, Fig. 2 is a block diagram of a conventional Campbell measurement system, Figs. 3 and 4 are block diagrams of a conventional wide-area instrumentation system, and Fig. 5 is a block diagram of a conventional wide-area instrumentation system. A configuration diagram showing an embodiment of the wide-area instrumentation system of the present invention, FIG. 6 is a diagram showing the amplifier characteristics of the same embodiment, and FIG. 7 shows changes in electron transfer speed with respect to the mixing ratio of argon gas and nitrogen gas. It is a characteristic diagram. 21... Neutron detector for wide area instrumentation, 22... Coaxial cable, 23... Input circuit, 24... Pulse amplifier, 25... Campbell amplifier, HV... High voltage power supply, R6, R7... Amplifier input resistance, C6,
C7...Coupling capacitor.

Claims (1)

[Claims] 1. In a device that measures neutron flux over a wide area with a single neutron detector using measurement means based on pulse measurement and the Campbell method, 99.9% to 80% argon gas is used as the ionized gas of the detector; 0.1%~20
A neutron wide-area instrumentation system characterized by using a mixed gas with % nitrogen gas. 2. The neutron wide-area instrumentation system according to claim 1, wherein the neutron detector is a fission counter.