JP2005134306A - Neutron detection unit and life diagnostic method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate an influence of precision of a calculation function in a process computer, and an influence of a gamma ray or the like to enhance precision of life diagnosis, in a life diagnostic method for a local output area monitor detector in a neutron detection unit. <P>SOLUTION: A value of an average square voltage is detected in the local output area monitor detector (S11), a ratio of the detected average square voltage in the local output area monitor detector to an initial average square voltage measured preliminarily in an initial time in attaching the local output area monitor detector is calculated (S13), detection sensitivity is measured, an integrated neutron irradiation amount is calculated over from the time point when the local output area monitor detector is equipped in a reactor up to the time point when the detection sensitivity is measured (S14), a detection sensitivity change transition curve of the local output area monitor detector with respect to the integrated neutron irradiation amount is obtained (S15), the detection sensitivity found by measuring a direct current of the local output area monitor detector is compared with the detection sensitivity measured by the average square voltage (S16) to calculate a gamma ray component based on a difference therebetween, and the life of the local output area monitor detector is determined based on a ratio of a neutron component and the gamma ray component (S17). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fission ionization chamber type neutron detector used in a nuclear reactor and a life diagnosis method thereof.

In nuclear power plants, it is important to constantly monitor the power distribution in the reactor for ensuring the safety of the reactor core.
Conventionally, a neutron detector called a local power range monitor (LPRM = Local Power Range Monitor) has been used as a means for monitoring the core power of a boiling water reactor.

  This local power region monitor measures the local neutron flux level in the horizontal and axial directions in the reactor by a plurality of LPRM detectors arranged around the reactor control rod, and monitors the core power. I do.

The LPRM detector is a nuclear fission ionization chamber that is fixed in the reactor. The inner surface of the detector is coated with uranium and reacts with neutrons to generate a current. For example, in a 1,100 MWe class boiling water reactor, A LPRM detector is loaded in the furnace.
When this LPRM detector is irradiated with neutrons in the furnace for a long period of time and reacted, the output characteristics deteriorate. Therefore, it is necessary to perform timely life diagnosis, and to perform periodic correction and replacement.

In general, the life of an LPRM detector, which is a guideline for replacement, is diagnosed from the core life that the detection sensitivity deteriorates when irradiated with thermal neutrons and the service life of the structural materials that make up the LPRM detector assembly. Yes.
Regarding the nuclear lifetime of the LPRM detector, the actual change in detection sensitivity is calculated from the change in the detector signal and used for the lifetime diagnosis.

The LPRM detector is a fission ionization chamber as described above, and uranium is applied to the cathode surface of the detector. As the uranium to be applied, in addition to uranium 235, which is a fissile material, uranium, which is a fission conversion material. 234 is mixed and applied (for example, refer to Patent Document 1).
In this case, the mixing ratio of uranium is set to be about uranium 235: uranium 234 = 1: 4.

FIG. 6 shows a change in detection sensitivity due to deterioration of the LPRM detector.
In FIG. 6, the vertical axis represents the sensitivity ratio S / S ₀ which is the ratio of the current sensitivity S to the initial sensitivity S ₀ of the LPRM detector, and the horizontal axis represents the neutron irradiation amount for the LPRM detector.

  Due to the effect of mixing uranium 234, the sensitivity of the LPRM detector first increases once with an increase in the neutron irradiation amount (T1) and then gradually decreases (T2), as shown in FIG. At a certain time (T3), the sensitivity ratio L indicating the life limit sensitivity is reached.

In FIG. 6, the sensitivity ratio S / S ₀ , which is the ratio of the current sensitivity S to the initial sensitivity S ₀ of the LPRM detector shown on the vertical axis, is a calibration current (LPRM detector) with respect to the initial calibration current value I ₀ of the LPRM detector. May be regarded as substantially equivalent to the ratio I / I _{0 of the} value I). That is,
S / S ₀ ≒ I / I ₀
Holds.

  From this, as a life diagnosis of the LPRM detector in the conventional local output region monitor, instead of the change in the detection sensitivity S of the LPRM detector, the change in the calibration current value I is used to grasp the change in the sensitivity, Life diagnosis is performed.

  The detection current component of the LPRM detector includes a current caused by neutrons and a current caused by other than neutrons. However, the current caused by other than neutrons is usually dominated by gamma rays, and for the sake of simplicity, it will be described as a current caused by gamma rays in the following description.

In the case of the LPRM detector, the detection sensitivity of neutrons is high at the initial stage of the reactor interior load, and there are many current components due to neutrons. Can no longer be ignored.
When the ratio of the current component due to gamma rays in the detected current component increases, a response delay of the neutron detector occurs due to the influence of delayed gamma rays.

Therefore, the time point at which this response delay is not negligible is set as the nuclear life limit of the LPRM detector. That is, the ratio between the current caused by neutrons and the current caused by gamma rays is a factor that determines the nuclear lifetime limit of the LPRM detector.
As described above, conventionally, as means for diagnosing the core lifetime of the LPRM detector, means for diagnosing from the degree of decrease in the calibration current value of the detector is used.

The flow of this life diagnosis method is shown in the flowchart of FIG.
In general, the lifetime is determined by the ratio of the calibration current value of the LPRM detector obtained by the core performance calculation of the process computer to the initial value.

As shown in FIG. 7, first, the calibration current value I is obtained by the core performance calculation function of the process computer (S21).
Next, using the initial calibration current value _{I 0} obtained in advance as well (S22), calculating the ratio I / _{I 0} of the calibration current value I with respect to the initial calibration current value _{I 0} (S23).

This result and the calculated neutron irradiation amount (S24) obtained by the core performance calculation are plotted on a graph to obtain an LPRM sensitivity change transition curve as shown in FIG. 6 (S25).
It is predicted that the neutron irradiation amount at which the transition curve reaches the sensitivity ratio L indicating the life limit sensitivity becomes the life of the LPRM detector (S26).
Japanese Patent Laid-Open No. 2003-232862

However, since the conventional LPRM detector life diagnosis method evaluates the detection sensitivity from the calibration current value obtained in the core performance calculation, it is affected by the accuracy of the core performance calculation function of the process computer, so accurate life prediction is possible. Difficult to do.
Further, since the measurement of the calibration current value is a measurement method based on direct current measurement, the measurement value is a value including the influence of gamma rays and the like, and there is a problem that life diagnosis cannot be performed based on highly accurate evaluation.

  The present invention has been made to solve the above problems, and a neutron detection apparatus capable of appropriately and more accurately performing a life diagnosis based on a change in detection sensitivity of an LPRM detector of a local output region monitor, and a life diagnosis thereof. It aims to provide a method.

  In order to achieve the above object, the invention according to claim 1 is directed to a local output area monitor detector, a power source for applying a voltage to the local output area monitor detector, and an average square voltage of the local output area monitor detector. A mean square voltage measuring circuit for detecting a value and a data sampling device for measuring a neutron amount from the mean square voltage of the local output region monitor detector.

  According to a second aspect of the present invention, there is provided means for detecting the value of the mean square voltage of the local output area monitor detector, the detected mean square voltage of the local output area monitor detector, and the initial output of the local output area monitor detector. The ratio between the initial mean square voltage measured in advance and the means for measuring the detection sensitivity and the time when the detection sensitivity measurement is carried out after loading the local output region monitor detector inside the furnace Means for calculating the integrated neutron dose, means for obtaining a change curve of detection sensitivity of the local output region monitor detector with respect to the integrated neutron dose, detection sensitivity and mean square obtained by DC measurement of the local output region monitor detector The gamma ray component is calculated from the difference in detection sensitivity measured by the voltage, and the life is determined from the ratio between the neutron component and the gamma ray component.

  According to the neutron detection apparatus and the lifetime diagnosis method of the present invention, the lifetime diagnosis of the LPRM detector can be appropriately performed, and the lifetime diagnosis of the LPRM detector with higher accuracy can be performed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description of the embodiments, detailed description of the same components as those in the prior art will be omitted.

FIG. 1 is a system configuration diagram showing a first embodiment of the present invention, and FIG. 2 is a flowchart for explaining a life diagnosis method.
In FIG. 1, 1 is an LPRM detector of a local output region monitor, 2 is a signal measuring device connected to the LPRM detector 1 through a high voltage conductor 3, and this signal measuring device 2 is an average square voltage (hereinafter referred to as MSV). A power supply (HV) 5 for applying a high voltage to the measurement circuit 4 and the LPRM detector 1 is provided.

Here, in order to reduce the quantity of the signal measuring device 2, the MSV measuring circuit 4 and the power source 5 are built in the same signal measuring device 2, but the MSV measuring circuit 4 and the power source 5 are individually installed. It doesn't matter.
A data collection device 6 is connected to the signal measuring device 2 and includes a storage medium for transmitting and storing a detector signal caused by the neutrons measured by the signal measuring device 2.

Here, when the measurement data from the signal measuring device 2 is stored in the data sampling device 6, the measurement date and time, the LPRM detector identification number (loading position coordinates, serial number, etc.), etc. are stored at the same time. This is convenient when calculating the neutron sensitivity described later.
In addition, as a storage medium of the data collection device 6, it is possible to use a generally used medium such as a hard disk or an MO disk.

Next, a method for calculating the neutron detection sensitivity from the measurement data of the MSV measurement circuit 4 will be described with reference to the flowchart of FIG.
First, the MSV value of the LPRM detector 1 is detected by the MSV measuring circuit 4 of the signal measuring device 2, and the neutron quantity is measured based on this value to monitor the normal core power (S11).

Next, it was detected using the MSV signal data of the LPRM detector 1 obtained in the step of S11 and MSV ₀ (S12) which is an initial MSV value measured in advance at the initial stage of LPRM detector loading. The ratio MSV / MSV ₀ between the MSV value and the initial MSV ₀ value is calculated (S13), and the sensitivity is measured.

  Similarly, the integrated neutron irradiation at the position of the LPRM detector obtained by the core performance calculation from when the LPRM detector is loaded inside the reactor until the sensitivity measurement is performed is calculated (S14). The LPRM sensitivity change transition with respect to the amount is plotted on a graph to obtain an LPRM sensitivity change transition curve A as shown in FIG. 3 (S15).

Here, when plotted on the same graph as the LPRM sensitivity change transition curve B of I / I ₀ obtained by the conventional DC measurement method, the DC measurement method is affected by gamma rays in addition to neutrons, so as shown in FIG. When both LPRM sensitivity change transition curves A and B are compared (S16), a difference D appears.

  As shown in the figure, the neutron component and the gamma ray component have a relationship in which the ratio of the gamma ray component to the neutron component gradually increases as the amount of neutron irradiation increases. It is determined that the neutron irradiation time point P, which is a non-negligible ratio, is the lifetime of the LPRM detector (S17).

  As described above, according to the embodiment of the present invention, since the mean square voltage is detected and the signal processing is performed in the life diagnosis of the LPRM detector, it is not affected by the accuracy of the calculation function of the process computer as in the past. In addition, the lifetime of the neutron detector can be accurately diagnosed by eliminating the influence of the gamma ray component.

In the above description of the embodiment, it has been described that the neutron flux at the position of the neutron detection device hardly changes during the evaluation period, but in reality, the neutron flux measurement level changes. When the change cannot be ignored, the same error-free evaluation as described above can be performed by correcting the following equation (1) using the calculated value of the neutron flux at the detector position.
MSV ′ = (φ ₀ / φ) MSV (1)

Here, φ ₀ is the neutron flux calculated by the three-dimensional simulation calculation of the core when measuring MSV ₀ , φ is the neutron flux at the time of MSV measurement similarly calculated, MSV ′ is the measured value MSV by the mean square voltage measurement Represents a correction value.

  By using this corrected MSV ′, it becomes possible to evaluate with higher accuracy and to use this lifetime diagnostic method even when there is a change in neutron flux that cannot be ignored.

Next, a second embodiment of the present invention will be described with reference to FIG.
In FIG. 4, reference numerals 1 to 1n denote a plurality of LPRM detectors arranged appropriately in the nuclear reactor. The detection signals of the LPRM detectors 1 to 1n are the same LPRMs installed in a central control room (not shown). Input to the monitoring device 7.

However, the LPRM detection signal as a local output is individually signal-processed in the same LPRM monitoring device 7, and sent to the monitoring display devices 9 to 9n via the amplifiers 8 to 8n, respectively, for instruction.
The detection signals of the LPRM detectors 1 to 1n are transmitted to the process computer 10 via a signal transmission device (not shown) and used for core performance calculation.

Further, in the LPRM monitoring device 7, detection signals of the LPRM detectors 1 to 1n are switched and connected to the MSV measuring circuit 4 and the display devices 9 to 9n via switching switches 11 to 11n, The normal monitoring by the display devices 9 to 9n and the data collection for life diagnosis by the MSV measurement can be appropriately switched and executed.
According to this method, it is possible to reduce the burden on the measurer by carrying an individual MSV measurement device for each MSV measurement, and connecting and disconnecting the LPRM detector.

In this embodiment, the data collection device 3 for storing the MSV measurement data is individually installed. However, it can be stored in the process computer 5 in the same manner as the normal signal of the LPRM detector. It is.
In addition, this method can be automatically performed by software processing, and the switching cycle can be arbitrarily set.

Next, a third embodiment of the present invention will be described with reference to FIG.
FIG. 5 is a conceptual diagram showing the relationship between the measured value of the sensitivity change and the fitting curve C for applying it to the sensitivity curve. Black dots in the figure represent sensitivity ratios obtained by actual measurement.

As described above, in both DC (direct current) measurement and MSV measurement, it is necessary to extrapolate the sensitivity change and predict the neutron irradiation amount that will become the lifetime.
In this case, fitting with the uranium atom number density change equation represented by the following equation in consideration of the characteristics of the fission ionization chamber of the LPRM detector is effective for accurate prediction.
S / S ₀ = Aexp (−aE) + (1−A) exp (−bE) (2)
A = n (a / ba)
Here, E represents the neutron irradiation amount, and n represents the ratio of uranium 234 to uranium 235.

Therefore, the sensitivity change curve can be extrapolated by determining a and b, which are the coefficients of the above equation, by the least square method based on the measurement data.
The same effect can be obtained by using an n-order (n is a natural number) polynomial for the fitting curve C instead of the equation (1).

  Since the sensitivity change of the LPRM detector is asymmetric, the order of the polynomial of the fitting curve C is preferably a third or higher order polynomial that can take into account the asymmetry, but for a simpler evaluation, It is also possible to fit the data in a region where the sensitivity rises in FIG.

  (1) Although highly accurate evaluation can be performed by using an expression combining exponential functions such as the expression (1), extrapolation by fitting can be performed by a simple polynomial as a simpler method.

The block block diagram which shows the neutron detection apparatus by the 1st Embodiment of this invention. The flowchart which shows the detection sensitivity diagnostic method of the neutron detection apparatus by the 1st Embodiment of this invention. The graph which shows the evaluation concept of the detection sensitivity change of the neutron detection apparatus in the 1st Embodiment of this invention. The block block diagram which shows the neutron detection apparatus by the 2nd Embodiment of this invention. The graph showing a part of detection sensitivity change evaluation method of the neutron detection apparatus in the 3rd Embodiment of this invention. The graph which shows the example of the detection sensitivity change of a conventional general neutron detection apparatus, and a lifetime limit. The flowchart which shows the procedure of the detection sensitivity diagnosis of the conventional neutron detection apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1-1n ... LPRM detector, 2 ... Signal measuring device, 3 ... High voltage conductor, 4 ... Mean square voltage measuring circuit, 5 ... Power supply, 6 ... Data sampling device, 7 ... LPRM monitoring device, 8 ... Amplifier, 9 ... Display Device: 10 ... Process computer, 11-11n: Switch, A: LPRM sensitivity change transition curve determined by mean square voltage, B: LPRM sensitivity change transition curve determined by DC measurement, C: Fitting curve, L: Life limit sensitivity.

Claims (5)

  Local output area monitor detector, power supply for applying voltage to the local output area monitor detector, average square voltage measurement circuit for detecting the value of the mean square voltage of the local output area monitor detector, and the local output area A neutron detector comprising a data sampling device for measuring the amount of neutrons from the mean square voltage of a monitor detector.   Means for detecting the value of the mean square voltage of the local output area monitor detector, the detected mean square voltage of the local output area monitor detector and the initial average previously measured at the initial stage of loading of the local output area monitor detector Means for calculating the ratio to the square voltage and measuring the detection sensitivity; means for calculating the integrated neutron irradiation amount from when the local output region monitor detector is loaded into the furnace until the detection sensitivity measurement is performed; , A means for obtaining a change curve of detection sensitivity of the local output region monitor detector with respect to the integrated neutron irradiation amount, and a difference between the detection sensitivity obtained by DC measurement of the local output region monitor detector and the detection sensitivity measured by the mean square voltage A lifetime diagnostic method for a neutron detector comprising a means for calculating a gamma ray component and determining a lifetime from a ratio of a neutron component and a gamma ray component.   3. The lifetime of a neutron detector according to claim 2, wherein when measuring the detection sensitivity, the detection sensitivity is corrected by a neutron flux calculation value at a detector position calculated by calculation such as three-dimensional simulation calculation of the core. Diagnosis method.   3. The neutron detector according to claim 2, wherein the uranium atom number density change equation of the neutron detector is used for the change curve of the detection sensitivity, and the detector lifetime is estimated from the change equation fitted to the measured value. Life diagnosis method. The method of diagnosing the lifetime of a neutron detector according to claim 2, wherein an n-order (n is a natural number) polynomial is used for the change curve of the detection sensitivity.

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