JPH0377474B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear instrumentation device for measuring neutron flux levels in a nuclear reactor.

A conventional device of this type is shown in FIG. In the figure, 1 is a neutron detector assembly, 2 is a housing of the neutron detector assembly,
3 is a neutron proportional counter housed in the housing,
4 is a gamma-ray compensated neutron ionization chamber housed in a housing; 11 is a measurement circuit for a neutron proportional counter; 12 is a measurement circuit for a gamma-ray compensation neutron ionization chamber; 13 is a DC voltage power source A constituting the measurement circuit 12. and B, and the polarities of DC voltage power supplies A and B are opposite to each other. 21 is a coaxial cable connecting the neutron proportional counter tube to its measurement circuit 11;
Coaxial cables 2, 23 and 24 connect the signal electrode, excitation electrode and compensation electrode of the gamma ray compensated neutron ionization chamber to the current measuring device A and the DC high voltage power supplies A and B, respectively.

Next, the functions will be explained. Nuclear instrumentation equipment needs to measure neutron flux over approximately 10 orders of magnitude from reactor shutdown to power operation. 5~ from the bottom of the measurement range
Six digits are measured with a neutron proportional counter, and the upper seven to eight digits are measured with a gamma-ray compensated neutron ionization chamber and its measurement circuit. Therefore, there is an overlap of more than two orders of magnitude in the measurement ranges of the two. The nuclear instrumentation system thus measures the reactor's neutron flux and, with equipment not shown in FIG. 1, provides an indication of its level and the signals necessary to protect the safety of the reactor. Furthermore, in order to measure and monitor only the neutron flux level during power operation of a nuclear reactor, a separate gamma-ray non-compensated neutron ionization chamber and its measurement circuit are often used.

Now, in a nuclear reactor, gamma rays are present in addition to the neutron flux that should be measured. However, neutron detectors are sensitive to gamma rays, albeit to a small extent. Gamma rays from a nuclear reactor are not necessarily proportional to the reactor's output, so gamma ray signals become noise for nuclear instrumentation equipment that is trying to measure neutron flux levels in order to extract a signal proportional to the reactor's output. . The following measures are taken to reduce the contribution of noise caused by gamma rays.
In other words, in the case of a neutron proportional counter, the electric pulses generated by neutrons are larger than the electric pulses generated by gamma rays, so pulse height discrimination technology is used to count only electric pulses larger than a certain wave height as signals. However, if the counting rate per unit time is output, this becomes a signal proportional to the neutron flux.
A gamma-ray compensated ionization chamber attempts to cancel out the interference signal caused by gamma rays in a neutron detector. The principle of operation will be explained using FIG. In FIG. 2, 31 is a signal electrode, 32 is an excitation electrode, 33 is a compensation electrode, and 34 is a case that houses these electrodes. The opposing surfaces of the signal electrode and the excitation electrode are coated with a neutron-sensitive material. The case is also filled with ionizing gas. Therefore, the signal electrode and the excitation electrode form a neutron ion chamber, and this part is sensitive to neutrons and at the same time sensitive to gamma rays.
Therefore, when a positive DC high voltage is applied to the excitation electrode, a signal proportional to the neutron flux and a signal proportional to the gamma ray flux flow into the signal electrode. On the other hand, the compensation electrode and the signal electrode also form an ionization chamber, but since no neutron-sensitive substance is introduced here, this part is sensitive only to gamma rays. Therefore, when a negative high DC voltage is applied to the compensation electrode, a current proportional to the gamma ray flux flows from the signal electrode. A gamma-ray compensated neutron ionization chamber is designed so that the gamma-ray sensitivity of the two ionization chamber parts is equal, so that a current proportional to the gamma-ray flux flowing from the excitation electrode into the signal electrode and a gamma-ray flux flowing from the signal electrode to the compensation electrode are generated. The currents proportional to the flux become equal, and eventually only the current proportional to the neutron flux flows from the excitation electrode into the signal electrode. Note that the polarities of the high voltages applied to the excitation electrode and the compensation electrode need only be opposite to each other, and either positive or negative may be used.

As described above, a signal proportional to the neutron flux is obtained over a wide measurement range in a field where gamma rays are mixed.

However, such conventional devices have the following drawbacks. A neutron proportional counter creates a strong electric field near a linear anode and causes gas amplification to increase the electrical pulses and count them. However, when the surrounding gamma ray level increases, a large amount of charge generated by the gamma rays increases. weakens the strong electric field near the linear anode, resulting in a decrease in the gas amplification factor, that is, a decrease in the wave height of the signal pulse. Then, even if the neutron flux level is constant, when a pulse signal exceeding a certain wave height is measured, the counting rate decreases and the neutron proportional counter appears to behave as if its sensitivity has decreased. This situation is explained in Figure 3, which shows how the counting rate changes when only the gamma ray level is changed under a constant neutron flux. The horizontal axis is the level of the wave height to be discriminated, and the vertical axis is the counting rate of pulses exceeding that wave height. The sharply rising part on the left side of the curve is due to circuit noise and minute pulses of gamma rays.In order to distinguish between these and measure only pulses caused by neutrons, in the case of the figure, the discrimination level must be set to L ₁ or higher. Must be set to . Then, even though the surrounding neutron flux remains unchanged, as the gamma ray level increases, the counting rate decreases, making it appear as if the neutron sensitivity of the neutron proportional counter has decreased. As described above, conventional nuclear instrumentation devices have the drawback that the output counting rate of the neutron proportional counter is not necessarily proportional to the surrounding neutron flux.

This invention was made to eliminate the drawbacks of the conventional ones as described above, and by providing a current measuring device to measure the current flowing through the compensation electrode of a gamma-ray compensated neutron chamber, it is possible to know the ambient gamma-ray flux level. , by correcting the output count rate of the neutron proportional counter according to the gamma ray flux level,
It is an object of the present invention to provide a nuclear reactor nuclear instrumentation device that makes the output counting rate of a neutron proportional counter tube proportional to the surrounding neutron flux.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 4, 1 is a neutron detector assembly, 2 is a housing of the neutron detector assembly,
3 is a neutron proportional counter housed in a housing, 4 is a gamma ray compensated neutron ion chamber housed in the housing, 11 is a measurement circuit for the neutron proportional counter, and 12 is a measurement circuit for a gamma ray compensated neutron ion chamber. , 13 is a current measuring device A which constitutes the measuring circuit 12, 14 and 15 are DC high voltage power supplies A and B which constitute the measuring circuit 12, and 16 is a current measuring device A which constitutes the measuring circuit 12.
This is current measuring device B that constitutes the. DC high voltage power supply A
The polarities of and B are opposite to each other. 21 is a coaxial cable connecting a neutron proportional counter and its measurement circuit 11; 22, 23 and 24 are a signal electrode, an excitation electrode and a compensation electrode of a gamma ray compensated neutron ionization chamber; respectively, a current measuring device A, a DC high voltage power supply A and a B
It is a coaxial cable that connects to the Current measuring device B is connected between DC high voltage power source B and ground.

Here, FIG. 5 will be used to explain the characteristics of the current flowing through the current measuring device connected to the compensation electrode.
FIG. 5 is a diagram showing the gamma-ray compensated neutron ion chamber and its measurement circuit from FIG. 4, and schematically shows the electrodes inside the gamma-ray compensated neutron ion chamber for explanation. The compensation electrode 33 is the signal electrode 3
1 and case 34, respectively, to form an ionization chamber. Therefore, the current flowing into the compensation electrode is
There are two signals, Iγ' from the signal electrode and Iγ'' from the case, but both are proportional to gamma rays only, so the current flowing to current measuring device B becomes a signal proportional to the gamma ray flux. Once the gamma ray sensitivity has been determined, the surrounding gamma ray flux can be determined by measuring the current flowing through the compensation electrode with current measuring device B. In a nuclear reactor, this gamma ray flux and the gamma ray flux around the neutron proportional counter tube are approximately equal to each other. By the way, the output counting rate of a neutron proportional counter changes depending on the surrounding gamma ray flux, but by measuring this dependence in advance, it is possible to The ambient gamma ray flux level can be determined as follows. Therefore, by correcting the output counting rate of the neutron proportional counter tube with a correction means (not shown) according to the measured output of the current measuring device, it is possible to detect the influence of gamma rays. An output count rate that is properly proportional to the previous ambient neutron flux can be obtained.

In the above embodiment, the output count rate of the neutron proportional counter is corrected separately, but the correction coefficient can be generated by using the signal from the current measuring device B of the gamma ray compensated neutron ionization chamber and using this signal as input. By providing a circuit and a multiplication circuit that multiplies the output of this circuit by the output counting rate of the neutron proportional counter tube, it is possible to remove the influence of the ambient gamma ray flux and automatically obtain a signal that is correctly proportional to the ambient neutron flux.

In the above embodiment, the current measuring device B was inserted between the DC high voltage power source B and the ground, but the positions of the two were swapped and the current measuring device B was directly connected to the compensation electrode with a cable, and the DC high voltage power source B was connected to the compensation electrode. Current measuring device B
It may be placed between the ground and ground.

Furthermore, the neutron proportional counter and gamma-compensated neutron ionization chamber need not be housed within the same neutron detector assembly housing, but may be housed in separate containers, and no special enclosure is necessarily required.

As described above, according to the present invention, since the current measuring device flowing through the compensation electrode of the gamma ray compensated neutron ion chamber is provided, the surrounding gamma ray flux level can be measured without using a separate gamma ray detector. Since the output counting rate of the neutron proportional counter, which depends on the gamma ray level, can be corrected using

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing a conventional nuclear instrumentation device, Figure 2
The figure is a diagram explaining the operating principle of a gamma-ray compensated neutron ionization chamber, Figure 3 is a diagram showing the gamma-ray flux level dependence of the output count rate of a neutron proportional counter, and Figure 4 is a schematic diagram showing an embodiment of the present invention. , FIG. 5 is an explanatory diagram of the operating principle of the embodiment of the invention shown in FIG. DESCRIPTION OF SYMBOLS 1... Neutron detector assembly, 2... Housing, 3... Neutron proportional counter, 4... Gamma ray compensated neutron ionization chamber, 11... Measurement circuit for neutron proportional counter, 12... Measurement circuit for gamma ray compensated ionization chamber, 1
3...Current measuring device A, 14...DC high voltage power supply A, 15
...DC high-voltage power supply B, 20, 22, 23, 24... Coaxial cable, 16... Current measuring device B, 31... Signal electrode, 32... Excitation electrode, 33... Compensation electrode, 34... Case. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. In a nuclear reactor nuclear instrumentation system that uses at least a neutron proportional counter and a gamma-ray compensated neutron ionization chamber as a neutron detector, a current that measures the current flowing through the compensation electrode of the gamma-ray compensated neutron ionization chamber. A nuclear reactor nuclear instrumentation device comprising: a measuring device; and a correction means for correcting the output counting rate of the proportional counter according to the measured output of the current measuring device. 2. In a nuclear reactor nuclear instrumentation system that uses at least a neutron proportional counter and a gamma-ray compensated neutron ionization chamber as a neutron detector, a current measuring device that measures the current flowing through the compensation electrode of the gamma-ray compensated neutron ionization chamber; A correction coefficient generation circuit that generates a predetermined correction coefficient according to the measurement output of the measuring instrument, and a multiplication circuit that multiplies the output of the correction coefficient generation circuit by the output counting rate of the neutron proportional counter tube. Features of nuclear reactor instrumentation equipment.