JPS6252273B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inspection method and apparatus for a neutron detector in a nuclear reactor.

The sensitivity of in-core neutron detectors installed in operating power reactors gradually deteriorates due to reactions with neutrons. Conventionally, the sensitivity has been corrected by adjusting the gain of the neutron flux amplifier, but as the sensitivity to neutron flux decreases, the proportion of current caused by gamma rays in the detection output increases, resulting in the above-mentioned problem. Depending on the gain adjustment, sufficient sensitivity correction may not be made.

The above-mentioned current due to gamma rays is an unnecessary component from the viewpoint of core monitoring, and if it becomes a significant proportion, it will adversely affect the accuracy of core performance calculations.

Currently, the replacement period for neutron detectors is determined as follows. In other words, when a current is applied to the input side of the neutron flux amplifier and the input current required for the amplified output to reach a predetermined value falls below a predetermined value, the detector is replaced as a limit for deterioration. I have to. This becomes a problem when the ratio of the neutron flux component to the gamma ray component in the detection output exceeds a certain value with the gain of the amplifier, if the detector has deteriorated normally. It is based on the estimation of

This judgment of replacement time is not made by directly calculating the ratio of the component due to neutron flux to other components, but is based on the above estimation. The S/N ratio of is completely unknown. In addition, even if the insulation of the cables, connectors, etc. between the detector and amplifier deteriorates, dark current increases, and the S/N ratio of the detection system deteriorates.
This cannot be detected using the methods described above.

The present invention has been made in view of the above circumstances, and is an in-core neutron detector capable of directly determining sensitivity deterioration of an in-core neutron detector and detecting an increase in dark current due to a decrease in insulation of the detector wiring system. We provide testing methods and testing equipment.

The invention will be explained in detail below with reference to the drawings. 1st
The figure is a conceptual diagram of the arrangement of in-core neutron detectors in the nuclear reactor 1. A plurality of fixed in-core neutron detectors 2 ₁ , 2 _{2 .} . . 2 _o are provided in the axial direction of the reactor core, and a movable neutron detector 3 that scans the vicinity of these in the axial direction is provided. A large number of sets of detectors arranged as described above are provided in the reactor core, distributed within the cross section.

The movable neutron detector 3 is used to calibrate the fixed neutron detectors 2 ₁ , 2 ₂ . . . 2 _o , and is placed outside the reactor core except when necessary. Sensitivity deterioration due to neutron irradiation can be ignored throughout the lifetime.

In the above configuration, the power spectral density of the output signals of the two coaxial fixed neutron detectors and the coherence between them will be considered.
Figure 2 shows the power spectral density. In this figure, the power spectrum density is standardized by dividing the variation by the DC value. From this figure, the coherence between the two neutron detectors was calculated and illustrated in Figure 3. In Figure 3, the coherence below 1 Hz is extremely high, and in this frequency range, there is almost no coherence due to the reactivity relationship. It can be seen that the behavior is the same.

On the other hand, in the frequency range above 1 Hz, the coherence decreases rapidly, but this is due to local void fluctuations near the neutron detector.

FIG. 4 shows the calculation of the coherence of the output signals of the two fixed neutron detectors by passing the movable neutron detector 3 near one of the fixed neutron detectors.
It can be seen from this figure that both output signals exhibit high coherence even in the frequency range of 1 Hz or higher.
This means that for two neutron detectors placed at almost the same location, local effects such as void fluctuations are
By having almost the same effect.

Therefore, the mutual positional relationship between the two detectors can be determined by the coherence between the output signals of the fixed neutron detector and the movable detector in a frequency range of 1 Hz or more. Figure 5 shows the power spectrum density of a normal stationary neutron detector when a movable neutron detector is passed near the detector, and it can be seen from this figure that both the pattern and the level agree well. .

On the other hand, FIG. 6 shows the power spectrum densities of both when a movable neutron detector is passed near a fixed neutron detector whose sensitivity is significantly degraded.
This figure shows that although the pattern is the same, the level of power spectral density of the fixed neutron detector is lower.

This level drop is due to the flow of current due to gamma rays and dark current due to insulation deterioration in the wiring system, in addition to the neutron flux signal. Gamma rays from fissile materials often have a large time constant, and fluctuations in gamma ray intensity are much smaller than those of neutron flux. Further, the dark current has a constant value depending on the degree of deterioration of the insulation. Therefore, the input to the amplifier has a bias corresponding to the gamma ray current and dark current. Therefore, when the sensitivity of the neutron detector to neutrons deteriorates, the ratio of the bias to the total input becomes large, resulting in a decrease in the level of power spectral density.

Therefore, by passing a movable neutron detector near a fixed neutron detector, comparing the power spectral densities of the two, and detecting a decrease in the level as described above, the sensitivity of the fixed neutron detector can be improved. Deterioration can be detected.

The present invention provides an inspection method and an inspection apparatus for detecting deterioration of a neutron detector based on the above principle.

The details of the present invention will be explained below with reference to FIG. In FIG. 7, the outputs of the fixed neutron detectors ₂₁ to _2o , shown as only one in the figure, and the output of the movable neutron detector 3, which scans in the axial direction, are transmitted through current amplifiers 4 and 5. Each route is divided into two routes. Each of the divided outputs passes through the bandpass filters 6 and 7, and is sent to the correlation coefficient calculating device 8, which calculates the correlation coefficient of only the components in a fixed frequency band among the divided outputs. The wave band of the wave device is set so as to pass only the local variation component due to voids among the variation components of the neutron flux signal. Specifically speaking, a band of 1Hz to 10Hz is appropriate.

The correlation coefficient γ is expressed by the following equation.

Here, Xi and Yi are the sampling values of the band wave generators 5 and 4 at the same time, i=1, 2,
...n indicates that n pieces of data are collected at appropriate sampling intervals.

The output of the correlation coefficient calculation device 8 is sent to the correlation coefficient maximum value detection device 9, and when this device 9 detects the maximum correlation coefficient value, the output of the other divided output of the detectors ₂₁ to _2o , 3 is sent to the correlation coefficient maximum value detection device 9. A signal is given to the variation ratio calculation devices 12 and 13 via the low frequency wave generators 10 and 11.

The low frequency amplifiers 10 and 11 convert the output of the current amplifiers 4 and 5 into DC components having appropriate fluctuation components, and the corner frequency is appropriately selected between 1 Hz and 10 Hz. .

The outputs of the fluctuation ratio calculators 12 and 13 are divided by the divider 1
4 and its output is given to an output device 15.

In the above configuration, when the maximum correlation coefficient is detected, the detectors 2 ₁ to 2 _o and the detector 3 are in the closest state. At this time, the correlation coefficient maximum value detection device 9 gives a signal to the variation ratio calculation devices 12 and 13. The fluctuation ratio calculation devices 12 and 13 perform N samplings at appropriate intervals before and after the time point at which the signal is given, and calculate an average value.

is the average value of the signal of the movable neutron detector 3,
is the average value of the fixed signal.

N is selected based on the scanning speed and sampling interval of the movable neutron detector 3 within a range where the two detectors are not far apart. Further, the fluctuation ratio calculation device calculates deviations ΔAi and ΔBi from the average value from the average value and the input at that time.

△Ai=Ai− ... △Bi=Bi− ... Ratio α 1 of the fluctuation component of the signal from the movable neutron detector 3 to the average value ₁ is the fluctuation ratio calculation device 13
In, It is required as.

On the other hand, as described above, the signal from the fixed neutron detector is the sum of the neutron flux signal In and the sum Iγ of the γ-ray current and the dark current, and is expressed as follows. Here, as is clear from the above, the variation in I can be ignored. Therefore, the ratio α ₂ of the fluctuation components of the signals from the fixed-position neutron detectors 2 ₁ to 2 _o to the average value is determined by the fluctuation ratio calculation device 12
In, It is calculated as

The divider 14 calculates B=α ₁ /α ₂ .

In the equation, if the fixed-position neutron detector has not deteriorated, I is very small, I/In0, which is β1, and if the deterioration progresses, I can no longer be ignored, and β>1.

The output device 15 has a function of displaying β obtained by the above formula and generating an alarm when β exceeds a predetermined set value _β0 . The set value β ₀ is variable and is determined depending on how much of the dark current and γ-ray current to the current due to the neutral flux is allowed.

As described above, according to the present invention, it is possible to determine the deterioration of the neutron detector with high accuracy while the reactor is operating without causing any disturbance to the reactor, and it is possible to accurately determine when to replace the in-reactor neutron detector. Since it can be grasped, there are great advantages both in terms of core performance evaluation and economically.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the arrangement of a neutron detector in a reactor, FIGS. 2 to 6 are graphs for explaining the principle of the present invention, and FIG. 7 is a block diagram of an embodiment of the present invention. 1... Nuclear reactor, 2 ₁ ~ 2 _o ... Fixed position neutron detector, 3... Movable neutron detector, 6, 7...
...bandwidth wave device, 8...correlation coefficient calculation device, 9...
Correlation coefficient maximum value detection device, 12, 13... Fluctuation ratio calculation device, 14... Divider, 15... Output device.

Claims (1)

[Claims] 1. Detection output signals of a fixed neutron detector fixed in a nuclear reactor core and a movable neutron detector that scans the vicinity of the fixed neutron detector in the axial direction of the reactor core, Calculate the correlation coefficient of those fluctuation components in a specific frequency band, calculate the ratio of the fluctuation component to the DC value of the signal from both detectors before and after the time when the calculated correlation coefficient shows the maximum value, A nuclear reactor in which it is determined that the fixed neutron detector has deteriorated when the calculated ratio of the movable neutron detector exceeds a preset value when compared with the calculated ratio of the fixed neutron detector. Inspection method for in-reactor neutron detector. 2 a first current amplifier that amplifies the output of the movable neutron detector; a second current amplifier that amplifies the output of the fixed-position neutron detector; A first and a second channel that respectively pass those in a specific frequency band among the fluctuating components of the output signal of the current amplifier.
a correlation coefficient calculating device for calculating a correlation coefficient between fluctuation components of the outputs of the first and second band wave generators; and a correlation coefficient maximum value for calculating the time point at which the maximum value of the correlation coefficient appears. a detection device and a first and second low frequency filter that passes a specific frequency or lower in the output signal of the first and second current amplifier; first and second fluctuation ratio calculation devices that calculate the average value of the output of each low-frequency device before and after and the deviation from the average value of the output and calculate the ratio of the deviation to the average value; and the first and second fluctuation ratio. A nuclear reactor characterized by having a division circuit for determining the ratio of outputs of a calculation device, and an output device that displays the output of the division circuit and issues an alarm when the output exceeds a predetermined value. Inspection equipment for internal neutron detectors.