JPH11311677A - Radioactive gas monitor and fuel rod surveillance device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To exclude the effect of<13> N gas produced by activation during monitoring offgas taken out of reactor and existing in a large quantity and determine abnormality of fuel rod. SOLUTION: On both sides of a gas chamber 12 in which offgas is introduced, a first detector 16 and a second detector 18 are provided. For each annihilation of a positron emitted from<13> N, two annihilation γ-rays are emitted in directions opposite to each other, which are simultaneously detected by both the first detector 16 and the second detector 18. Such a specific annihilation γ-rays are eliminated by anticoincidence count, so that the γ-rays from rare gas are measured with good sensitivity. In an operation part 54, each of species concentration is operated based on the spectrum. In a determining part 56, abnormality of fuel rod is determined and the degree of leakage is determined, based on the concentration ratio of at least two species with different half lives.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a radioactive gas monitor and a fuel rod monitoring device used in a nuclear power plant or the like.

[0002]

2. Description of the Related Art As the demand for energy increases year by year, a nuclear power plant that extracts energy by nuclear fission has become indispensable. There is a need for safer equipment to promote the peaceful use of nuclear energy.

[0003] A large number of fuel rods are inserted into a nuclear reactor, and nuclear fission in the fuel rods transfers heat to cooling water surrounding the fuel rods to form a thermal cycle. The fuel rods are the primary key to containment of fission products and are well designed to prevent them from breaking. Double and triple safety systems are in place to prevent any possible damage to the fuel rods from affecting the outside world.

[0004] As described above, the fuel rods perform important functions, and it is desirable to constantly monitor the health of the fuel rods.
For this reason, monitoring of the gas exhausted from inside the reactor (hereinafter referred to as off-gas) is being performed. A conventional gas monitor is provided near a chamber into which a gas is introduced, and a gamma ray emitted from the gas is detected as a current output of the ionization chamber.

[0005] However, oxygen atoms emitted by the action of neutrons in a nuclear reactor, ¹³ N gas having radioactive are mass produced. The annihilation of the positron emitted from the ¹³ N generates a 511-keV gamma ray, but the detection by the gas monitor becomes dominant. Furthermore, Compton scattering of the γ-rays makes observation more difficult. Even if a fuel rod leaks and noble gas as a fission product is generated, it is difficult to detect the γ-ray if the amount is small, so it is difficult to detect the leak with the current gas monitor alone. difficult.

The noble gas as a fission product includes, for example, Xe-138, Kr-87, Kr-85,
Kr-88, Kr-85m, Xe-135, Xe-13
3, Xe-135m, Xe-137, Kr-89, Ar
-41 and the like. The noble gas components in the off-gas are generally about 3%, and the remaining 97% is N-13.
It is.

[0007] The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to remove a rare gas generated by fission.
It is to measure in the presence of ¹³ N gas.

It is another object of the present invention to monitor a rare gas and determine an abnormality of a fuel rod.

[0009]

(1) In order to achieve the above object, the present invention provides a radioactive gas monitor provided near a gas chamber into which a gas generated inside a nuclear reactor is introduced. A pair of radiation detectors that are disposed opposite to each other and that detect γ-rays from the gas chamber; and, when simultaneous detection of γ-rays is performed by the pair of radiation detectors, the positrons emitted from ¹³ N are detected. It is characterized in that the signal processing circuit is regarded as 511 keV γ-rays generated by the annihilation and is excluded from the detection result, and the sensitivity of γ-rays emitted from the noble gas which is a fission product is improved.

According to the above configuration, the nuclear reactor (particularly the light water reactor)
Even if a large amount of ¹³ N is generated inside by the activation of oxygen atoms, the detection of off-gas as a radioactive gas causes the positrons emitted from ¹³ N to be extinguished.
1 keV γ-rays can be discriminated and detected and excluded from the detection results. That is, when the positron is annihilated, the two annihilated γ-rays (511 keV) fly out in opposite directions.
Γ-rays are specified and removed. Such annihilation gamma rays are basically not observed for rare gases, and as a result, the detection sensitivity of rare gases can be improved. In addition,
The off-gas taken out of the reactor is generally re-introduced into the reactor.

(2) In order to achieve the above object, the present invention provides a radioactive gas monitor provided near a gas chamber into which gas generated inside a nuclear reactor is introduced, and an output signal from the radioactive gas monitor. An abnormality monitoring unit that monitors an abnormality of a fuel rod inside the nuclear reactor, wherein the radioactive gas monitor is disposed to face the gas chamber across the gas chamber, and detects γ-rays from the gas chamber. When the simultaneous detection of γ-rays is performed by a pair of radiation detectors and the pair of radiation detectors, the detection is regarded as 511 keV γ-rays generated by annihilation of positrons emitted from ¹³ N and is excluded from the detection result. A signal processing circuit, wherein the abnormality monitoring unit calculates the concentration of a noble gas that is a fission product based on an output signal from the radioactive gas monitor, Characterized in that it comprises an abnormality determination means for determining abnormality of the fuel rods, the based on the concentration of the rare gas.

According to the above configuration, the concentration of the rare gas or the concentration of one or more nuclides constituting the rare gas is measured, and the abnormality of the fuel rod is determined based on the measured concentration. That is,
When the fuel rod container is damaged, the noble gas trapped in it is released into the reactor, which is determined by the increase in the concentration of the noble gas, and the abnormality of the fuel rod is determined indirectly. is there.

Preferably, the fuel rod monitoring device includes a nuclide analyzing means for performing a nuclide analysis of a rare gas based on an output signal from the radioactive gas monitor, and at least two nuclides having different half-lives according to the nuclide analysis result. And a leakage degree determining means for determining the degree of leakage from the density. If the hole formed in the fuel rod is a pinhole, the abundance ratio of nuclides with a long half-life in the rare gas component increases, while if the hole formed in the fuel rod is relatively large, the rare gas component The abundance ratio of nuclides with short half-life increases in the medium. Therefore, the size of the hole (the degree of leakage) formed in the fuel rod can be estimated from the concentration ratio of at least two nuclides having different half-lives.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a preferred embodiment of a fuel rod monitoring device according to the present invention, and FIG. 1 is a conceptual diagram showing the entire configuration. This fuel rod monitoring device is a device that monitors off-gas emitted from a nuclear reactor, particularly a light water reactor, and detects an abnormality of the fuel rod.

In FIG. 1, off-gas taken out of a nuclear reactor is introduced into a gas chamber 12 through a gas pipe 10. The gas chamber 12 has a certain volume in order to increase the efficiency of detecting off-gas. Gas chamber 1
The off-gas discharged from 2 is re-introduced into the reactor.

The detection unit 14 detects radiation emitted from the off-gas,
In particular, it is means for detecting γ-rays, and in this embodiment,
The detection unit 14 includes a first detector 16 and a second detector 18 which are arranged to face each other with the gas chamber 12 therebetween. The first detector 16 detects γ from each nuclide in the off-gas.
The second detector 18 is a detector mainly detecting a line.
^This device mainly detects gamma rays generated by the annihilation of positrons emitted from 13N. The first detector 16 is roughly composed of a lead collimator 20, a scintillator 22, and a photomultiplier tube 24. One of the second detectors 18 includes a lead collimator 26, a scintillator 28, and the photomultiplier tube 3.
0.

As described above, ¹³ N in the off-gas
Since about 97% is present, if the first detector 16 simply detects γ-rays, the detection of 511-keV γ-rays caused by annihilation of positrons emitted from ¹³ N becomes dominant. Therefore, it is desirable that the counting efficiency of the 511 keV γ-rays in the γ-ray detection of the first detector 16 be set to about 1/100 by the removal by anticoincidence with the opposing detector. Therefore, the second detector 18
As much as possible due to the annihilation of positrons from ¹³ N
It is desirable to design so as to have a larger counting efficiency than the first detector 16 for detecting 1 keV γ-rays.

In the first detector 16, the lead collimator 2
Numeral 0 has a cylindrical shape, and a scintillator 22 and a photomultiplier tube 24 are arranged in an inner cavity thereof. As the scintillator 22, a NaI scintillator having a size of about 2 inches × 2 inches can be used.
When gamma rays enter the scintillator 22, photons are generated there, and the photons are detected by the photomultiplier tube 24.

In the second detector 18, a scintillator 28 and a photomultiplier tube 30 are arranged in an internal cavity of a cylindrical lead collimator 26. The scintillator 28
For example, an inorganic scintillator such as NaI, CsI, or BGO is used, or a plastic scintillator is used. In this case, as the inorganic scintillator, 10
It is desirable to have a depth of about 15 cm, and when using a plastic scintillator, it is desirable to have a depth of about 50 cm. For example, it is desirable to use a CsI scintillator having a size of 4 inches × 3 inches as the scintillator 28.

In any case, the first detector 16 needs to be designed to detect γ-rays over a wide energy range in consideration of the pile-up by 511 keV γ-rays described above. On the other hand, the second detector 1
In 8, it is preferable to configure the detector to detect the 511 keV γ-rays as much as possible.

The high voltage power supply 32 is a device for supplying a high voltage power to the photomultiplier tube 24. The detection pulse output from the photomultiplier tube 24 is amplified by the amplifier 34 and then input to the pulse height discriminator 36, and only the detection pulse having a certain peak value or more is extracted. The extracted detection pulse is input to the inverse coincidence unit 40. On the other hand, the high voltage power supply 42 supplies a high voltage to the photomultiplier tube 30. The detection pulse output from the photomultiplier tube 30 is amplified by an amplifier 44, and then sent to a pulse height discriminator 46, where a detection pulse having a certain peak value or more is extracted.

The inverse coincidence unit 40 is a circuit for executing anti-coincidence. When the detection pulse output from the first detector 16 and the detection pulse output from the second detector 18 are input simultaneously, Is excluded from the counting. As described above, when the positron emitted from ¹³ N is annihilated, two annihilation γ-rays (511 keV) are generated in opposite directions, and these are detected by the first detector 16 and the second detector 18, respectively. The inverse coincidence unit 40 specifically excludes the γ-rays.

Therefore, a detection pulse corresponding to a nuclide other than ¹³ N is output from the inverse coincidence unit 40, and the detection pulse is converted into a digital signal in the A / D converter 50. The spectrum measuring unit 52 measures an energy spectrum by counting the number of detection pulses for each energy.

The calculating section 54 is a circuit for calculating the concentration of each nuclide based on the measured spectrum. The calculation result is sent to the display unit 58, and the concentration of each nuclide is displayed on the display unit 58. For example, the rare gases Xe-138, Kr-87, Kr-88, and Ar-
41, Kr-85m, Xe-135, Xe-135m,
The concentrations of Xe-133, Xe-137, Kr-89, etc. are displayed.

The determination unit 56 has a function of determining an abnormality of the fuel rod based on the concentration of the rare gas contained in the off-gas or the concentration of one or more nuclides in the rare gas in the present embodiment, and two different halving methods. A function of estimating the size of a hole formed in the fuel rod based on the concentration ratio of at least two nuclides having a period. In other words, when a defect occurs in the fuel rod, and as a result, a rare gas as a fission product confined in the fuel rod appears in the offgas, the concentration of the rare gas increases, so that it is detected. As a result, the abnormality of the fuel rod is determined. Also, if the holes formed in the fuel rods are relatively small, the concentration of nuclides having a relatively long half-life will increase relatively, while if the holes formed in the fuel rods are relatively large, The concentration of nuclides with a relatively short half-life increases. Therefore, utilizing such properties, the degree of leakage is determined according to the concentration ratio based on the difference in half-life.

Accordingly, the concentration of each nuclide is displayed on the display section 58 as described above, and a numerical value representing the alarm, the degree of leak, and the like is displayed as necessary. In determining the degree of leakage, it is desirable to compare the concentrations of at least two nuclides having different half-lives as much as possible. When introducing the off-gas into the gas chamber 12, it is desirable to provide a dehumidifying agent or the like in order to remove vapor or the like in the off-gas.

According to the above configuration, the characteristic of ¹³ N annihilation γ-ray is utilized to perform the anti-coincidence.
It is possible to detect only the rare gas with high sensitivity by eliminating the influence of ¹³ N.

FIG. 2 shows a configuration example of the detection unit 14 shown in FIG. A first detector 16 and a second detector 18 are provided on both sides of the cylindrical gas chamber 12. A scintillator 22 and a photomultiplier tube 24 are arranged in the cavity of the lead collimator 20, while a scintillator 28 and a photomultiplier tube 30 are arranged inside the lead collimator 26. Here, as described above, the first detector 16 outputs ¹³ N
511 keV caused by the annihilation of positrons emitted from
In order to minimize the influence of the γ-ray, the size of the γ-ray entrance aperture of the lead collimator 20 is set to be small. on the other hand,
In the second detector 18, the size of the γ-ray entrance aperture of the lead collimator 26 is set to be large in order to increase the detection efficiency of the above-mentioned 511 keV γ-ray.

FIG. 3 is a flow chart showing the operation of the arithmetic unit 54 and its judging unit 56 shown in FIG. In S101, the concentration of each nuclide is calculated based on the measured spectrum. In S102, the determination unit 56
In the above, abnormality of the fuel rod is determined based on the rare gas concentration or the concentration of one or more nuclides. In S103, the determination unit 56 determines the degree of leakage based on the concentration ratio of at least two nuclides having different half-lives.

[0031]

As described above, according to the present invention,
The noble gas generated by fission can be measured with high sensitivity in the presence of ¹³ N gas. Therefore, according to the present invention, it is possible to accurately determine the abnormality of the fuel rod and the like.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a preferred embodiment of a fuel rod monitoring device according to the present invention.

FIG. 2 is a diagram illustrating a specific configuration example of a detection unit.

FIG. 3 is a flowchart illustrating operations of a calculation unit and a determination unit.

[Description of Signs] 12 gas chamber, 14 detector, 16 first detector, 18 second detector, 20 lead collimator, 22 scintillator, 24 photomultiplier tube, 26 lead collimator, 28 scintillator, 30 photomultiplier tube , 40
Inverse coincidence unit, 52 spectrum measurement unit, 54 operation unit,
56 Judgment unit.

Claims (3)

[Claims] 1. A radioactive gas monitor provided near a gas chamber into which a gas generated inside a nuclear reactor is introduced, wherein a pair of radiations which are opposed to each other with the gas chamber interposed therebetween and detect γ-rays from the gas chamber. When simultaneous detection of γ-rays is performed by the detector and the pair of radiation detectors, the detection is performed by annihilation of positrons emitted from ¹³ N.
A signal processing circuit which regards as 11 keV γ-rays and excludes the detection results from the detection results, and wherein the sensitivity of γ-rays emitted from a noble gas as a fission product is improved. 2. A radioactive gas monitor provided in the vicinity of a gas chamber into which gas generated inside the nuclear reactor is introduced, and an abnormality monitor for monitoring an abnormality of a fuel rod inside the nuclear reactor based on an output signal from the radioactive gas monitor. A fuel rod monitoring device comprising: a pair of radiation detectors, wherein the radioactive gas monitor is disposed opposite to the gas chamber, and detects a γ ray from the gas chamber; and the pair of radiation detection devices. When simultaneous detection of γ-rays is performed by the detector, it is caused by the annihilation of positrons emitted from ¹³ N.
A signal processing circuit that considers it as 11 keV γ-rays and excludes it from the detection result, wherein the abnormality monitoring unit calculates a concentration of a noble gas that is a fission product based on an output signal from the radioactive gas monitor. A fuel rod monitoring device, comprising: means; and abnormality determination means for determining an abnormality of the fuel rod based on the concentration of the rare gas. 3. The apparatus according to claim 2, wherein the fuel rod monitoring device is configured to perform a nuclide analysis of a noble gas based on an output signal from the radioactive gas monitor, and to reduce the nuclide in half according to the nuclide analysis result. A fuel rod monitoring device, comprising: a leak level determining means for determining a leak level from concentrations of at least two nuclides having different periods.