JP7153532B2 - Self-powered detectors and nuclear instrumentation systems - Google Patents

Description

The present invention relates to a self-powered detector, which is a type of radiation detector, and a nuclear instrumentation system using the self-powered detector.

Self-powered detectors do not require the application of voltage, and have a simple structure that measures the current value generated by changes in the number of electrons in the emitter, so they can be installed in relatively harsh environments. is a detector. A self-powered gamma ray detector, which is a kind of self-powered detector, measures a current value generated as electrons in the emitter are reduced when electrons in the emitter are ejected by gamma ray irradiation. Similarly, the self-powered neutron detector measures the current value generated by the change in the number of electrons in the emitter caused by the emission of electrons by the nuclear reaction between the emitter material and neutrons. Decay phenomena such as beta decay are also used to increase or decrease electrons in the emitter.

Thus, the self-powered gamma ray detector and the self-powered neutron detector operate on similar principles and have substantially the same structure. These differences are mainly caused by differences in emitter materials. However, there is no substance that only interacts with gamma rays or nuclear reactions with neutrons. For this reason, self-powered gamma-ray detectors are used when a substance that easily emits electrons due to interaction with gamma rays is used as the emitter, and self-powered gamma-ray detectors are used when a substance that easily emits electrons due to a nuclear reaction with neutrons is used as the emitter. It works as a type neutron detector. The easiness of electron emission referred to here depends on the environment, and there is a wide selection of emitters for self-powered gamma-ray detectors to be applied to environments with few neutrons. There is a wide range of emitters to choose from.

Considering the application in a nuclear reactor, the dose rate of gamma rays is high in the nuclear reactor, and the neutron flux is also large. Furthermore, the temperature inside the nuclear reactor is high. Patent document 1 is a self-powered gamma ray detector applicable to such an environment. Patent Literature 1 discloses a self-powered gamma ray detector in which tungsten, which has a heat resistance of 600° C. or more and is highly sensitive to gamma rays, is applied to the emitter.

Japanese Patent No. 6095099

Emitters of self-powered gamma-ray detectors used in nuclear reactors have a high probability of interaction with gamma-rays, and the smaller the reaction cross-section with neutrons, the more suitable they are for monitoring gamma-ray intensity. The probability of interaction with gamma rays varies depending on the energy of the gamma rays. Elements with large ray attenuation coefficients include Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, and Pb with atomic numbers of 72 to 83. , Bi. These elements have linear attenuation coefficients that are several to one order of magnitude larger than many elements in a wide energy band.

On the other hand, when comparing the thermal neutron absorption cross-sections of these elements with atomic numbers of 72 to 83, the cross-sections of Pb and Bi are small, and the others are about 50 to about 2500 times that of Pb. Bi has a smaller thermal neutron cross section than Pb, which is about 0.2 times the thermal neutron cross section of Pb. For these reasons, Bi or Pb is suitable as a self-powered gamma ray detector that is applied to an environment with a high dose rate and a large neutron flux.

The inside of the nuclear reactor is a high temperature environment of about 280°C. On the other hand, Pb has a melting point of 330° C. and Bi has a melting point of about 270° C. When Pb and Bi are installed in a nuclear reactor, Pb breaks or deforms, and Bi melts. However, from the viewpoint of gamma ray sensitivity and neutron sensitivity, Pb and Bi are optimal as emitters for self-powered gamma ray detectors to be applied in nuclear reactors. , is a promising nuclear instrumentation system to monitor gamma rays.

The same is true for self-powered neutron detectors. If a material with a large cross section for neutrons, a low interaction probability with gamma rays, and a low melting point can be used for the emitter, it is promising as a self-powered neutron detector installed in a high-temperature environment such as a reactor.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and to provide a self-powered detector and a nuclear instrumentation system that can operate even if the emitter melts at high temperatures.

To achieve the above object, a self-powered detector according to the present invention is a self-powered detector comprising an internal emitter and an external collector, insulated between the emitter and the collector, wherein the emitter is enclosed. The emitter container is gas-tight (liquid-tight), and the emitter is lead-bismuth . Other aspects of the present invention are described in embodiments below.

The invention can operate even if the emitter melts at high temperatures.

It is a figure which shows the basic concept of the self-powered detector which concerns on 1st Embodiment. It is a figure which shows the arrangement position of the instrumentation pipe in the containment vessel which concerns on 1st Embodiment. FIG. 3 is a diagram showing a cross section near an instrumentation tube in the core according to the first embodiment; 1 is a diagram showing an example of a nuclear instrumentation system that monitors reactor internal power with a self-powered detector according to the first embodiment; FIG. FIG. 10 is a diagram showing a structure in which the emitter container of the self-powered detector according to the second embodiment also serves as an insulating material; FIG. 10 is a diagram showing a structure for mitigating the effect of an increase in the volume of the emitter in the emitter container of the self-powered detector according to the third embodiment;

Embodiments for carrying out the present invention will be described in detail with reference to the drawings as appropriate.
<<First Embodiment>>
FIG. 1 is a diagram showing the basic concept of the self-powered detector according to the first embodiment. FIG. 1 shows a cylindrical self-powered gamma-ray detector for nuclear instrumentation system applications. The self-powered detector 1 has an emitter 11 made of Bi in the center and a collector 14 made of SUS on the outside. The emitter 11 is covered with an emitter container 12 made of quartz glass. The emitter container 12 is airtight (liquid-tight), and even if the emitter 11 melts, it will not leak outside the emitter container 12 .

As a method of making the emitter container 12 airtight, a method of sealing a glass tube such as a light bulb can be applied. For example, the emitter 11 is melted and put into a glass tube molded with appropriate dimensions, and a metal core wire is inserted into the emitter 11 . Since only the core wire protrudes from the opening of the glass tube to the outside, it can be sealed by sandwiching the opening while heating it, or by gradually reducing the diameter. The volume of the emitter container 12 is controlled by the heating position. Also, the amount of emitters 11 put into the glass container is controlled by measuring the mass inside the container. This makes it possible to arbitrarily determine the volume of the emitter at room temperature and at assumed high temperature.

Between the emitter container 12 and the collector 14 there is an insulator 13 made of Al ₂ O ₃ . The emitter 11 is connected to a core wire 15 made of Ni, and the emitter 11 and the core wire 15 are electrically connected. At this time, the volume of the emitter 11 and the depth (insertion length) of the core wire inserted into the emitter container 12 are adjusted so that the core wire 15 and the emitter are electrically connected even when the emitter 11 is melted. ing. The emitter container 12 is secured by a holder 16 made of Al ₂ O ₃ to avoid movement within the self-powered gamma ray detector. A core wire 15 is a core wire of the coaxial cable 17 . A coaxial cable 17 is connected to a current measuring device. The circumference of the self-powered gamma ray detector is covered with a SUS housing 18 except for the portion where the coaxial cable 17 exists or the collector portion.

When the self-powered gamma ray detector of FIG. 1 is installed in the reactor nuclear instrumentation tube (see reference numeral 41 in FIG. 2), it is exposed to a high temperature of about 280° C., and the melting point of Bi in the emitter 11 is about 270° C. , the emitter 11 melts. At this time, the emitter container 12 is made of quartz glass and can maintain its shape even at 1000° C. or higher. The emitter container 12 is airtight, and the volume of the emitter 11, the volume of the emitter container 12, and the depth of the core wire 15 in the emitter container 12 are appropriately adjusted. are in contact and electrically connected.

Electrons are emitted from the emitter 11 when the gamma ray interacts with the emitter even when the emitter 11 is molten. At this time, a current flows through the electrically connected core wire 15 due to the change in the number of electrons in the emitter 11, similar to the operation of a general self-powered gamma ray detector. The gamma ray interacts with the emitter 11, and the more electrons emitted from the emitter 11, the greater the current value generated. Therefore, by measuring the current value with a current measuring device connected to the end of the coaxial cable 17, the gamma ray dose or dose rate irradiated to the self-output type gamma ray detector can be monitored.

In a nuclear reactor, at least during rated operation, the dose rate in the reactor is proportional to the reactor power. Therefore, the reactor output can be monitored by monitoring the current value obtained by the self-powered gamma ray detector of FIG. 1 installed in the reactor instrumentation tube.

Here, the emitter 11 is made of Bi, but other materials such as Pb can also fulfill the same function. Furthermore, when operating as a self-powered neutron detector, a substance with high neutron sensitivity and a low melting point can be used as the emitter 11 to satisfy the same function.

Further, quartz glass is used as the emitter container 12, but it is not necessarily made of quartz glass as long as it can be shaped as an airtight container and can hold the emitter 11 melted in a high-temperature environment.

One possible application of self-powered detectors to nuclear instrumentation systems is the calibration of local area monitor detectors and the monitoring of detailed axial distributions of power. At this time, it is necessary to install a plurality of self-powered detectors per instrumentation tube.

FIG. 2 is a diagram showing the arrangement positions of instrumentation tubes in the containment vessel according to the first embodiment. FIG. 3 is a diagram showing a cross section near an instrumentation tube in the core according to the first embodiment. FIG. 2 shows the position of the instrumentation tube 41 in which the self-powered detector 1 is installed. An instrumentation tube 41 is installed so as to penetrate a nuclear pressure vessel 42 , and the self-powered detector 1 is installed within the range of a core 44 within the instrumentation tube 41 .

FIG. 3 shows a cross-sectional view of the vicinity of the instrumentation tube 41 in the core 44. As shown in FIG. The channel box 45 houses fuel rods 48 and water rods 47 therein. A control rod 46 having a cross-shaped cross section is arranged in the center of the four channel boxes 45, and an instrumentation pipe 41 is installed around the place where the control rod 46 is arranged.

FIG. 4 is a diagram showing an example of a nuclear instrumentation system that monitors the power in the reactor with the self-powered detector according to the first embodiment. FIG. 4 shows an axial sectional view of multiple self-powered gamma-ray detectors installed in the same instrumentation tube. A plurality of self-powered gamma-ray detectors are installed in the instrumentation tube 41, and the self-powered gamma-ray detector 1a at the highest position and the self-powered gamma-ray detector 1b at the lowest position are respectively installed in the nuclear reactor. They are installed at the upper and lower ends of the output monitoring range. Each self-powered gamma ray detector is connected to a coaxial cable 17 to transmit current outside the nuclear pressure vessel 42 . With such a configuration, it is possible to monitor reactor power at different heights by the number of self-powered gamma-ray detectors per instrumentation tube.

Although a plurality of self-powered gamma ray detectors are installed in the instrumentation tube 41 in FIG.

<<Second Embodiment>>
FIG. 5 is a diagram showing a structure in which the emitter container of the self-powered detector according to the second embodiment also serves as an insulator. In the first embodiment, the insulator 13 was present between the emitter container 12 and the collector 14 . Here, an example in which the emitter container 12 also serves as an insulating material is shown. FIG. 5 shows the structure when Al ₂ O ₃ is adopted as the emitter container 12 of the self-powered detector 1A.

The emitter container 12 made of Al ₂ O ₃ is airtight, and even if the emitter 11 made of Bi melts, it does not leak out of the emitter container 12, and serves as an insulating material to prevent direct contact between the emitter 11 and the collector 14. is also working. Although Al ₂ O ₃ is also used for the holder 16 here, it does not have to be an insulating material as long as the emitter 11 and the collector 14 are not in electrical contact with each other. Since the melting point of Al ₂ O ₃ is about 2000° C., it is possible to maintain its shape even at about 280° C., which is the temperature during rated operation of the nuclear reactor.

With such a configuration, it is possible to operate in the same manner as when the emitter container 12 and the insulating material 13 are made of different materials, and the current value can be measured by a current measuring device connected to the tip of the coaxial cable 17 . By measuring , the dose or dose rate of gamma rays irradiated to the self-powered gamma ray detector can be monitored. Therefore, the reactor output can be monitored by monitoring the current value obtained by the self-powered gamma ray detector of FIG. 4 installed in the reactor instrumentation tube.

Although the emitter container 12 is made of Al ₂ O ₃ here, quartz glass, MgO, or the like also has an insulating function, so that similar effects can be obtained.

<<Third Embodiment>>
Depending on the choice of material for emitter 11, it may expand in high temperature environments. As a countermeasure, a structure is shown in which a space is provided in the emitter container 12 as shown in FIG.

FIG. 6 is a diagram showing a structure for mitigating the influence of an increase in the volume of the emitter in the emitter container of the self-powered detector according to the third embodiment. In the self-powered detector 1B shown in FIG. 6, the emitter container 12 is an airtight container containing the emitter 11 and a gas 31 (for example, air) inside. The pressure of the gas 31 is desirably at room temperature or at a negative pressure before the emitter 11 expands, such as by molding the airtight container in a low-pressure environment. Except for the gas 31, it is composed of the same substances as those in FIG.

As the temperature rises, the emitter 11, which has a low melting point, expands or melts to increase its volume. At this time, by providing the negative pressure gas 31 in the emitter container 12, a large pressure is not applied to the emitter container 12 from the inside, and it is possible to avoid breaking airtightness due to deformation or breakage.

The presence of the gas 31 may create a space in the emitter container 12. However, when the emitter 11 melts or expands, the emitter 11 and the core wire 15 come into contact with each other and can be electrically connected. The volume, the volume of the emitter container 12, and the depth of the core wire 15 in the emitter container 12 are appropriately adjusted.

With such a configuration, it is possible to operate as a self-powered gamma ray detector without breaking airtightness even when the volume of the emitter increases at a high temperature. Therefore, by measuring the current value with a current measuring device connected to the end of the coaxial cable 17, the gamma ray dose or dose rate irradiated to the self-output type gamma ray detector can be monitored. Therefore, the reactor output can be monitored by monitoring the current value obtained by the self-powered gamma ray detector of FIG. 4 installed in the reactor instrumentation tube.

Here, the gas 31 in the emitter container 12 is negative pressure air, but a rare gas such as He or Ar or a gas such as CO ₂ may be used. It is possible.

<<Fourth Embodiment>>
In the operation of the nuclear reactor, after reaching criticality, the output of criticality is increased step by step to reach the rated output. This process increases the frequency of fission and raises the temperature. The emitter 11 changes in volume and density before and after melting. Therefore, the correlation between the current value obtained before the emitter 11 is melted and the output of the current value obtained after the emitter 11 is melted may not be continuous. For this reason, in the case of a self-powered gamma ray detector that monitors the output when the emitter 11 is melted during reactor operation, by adopting a substance with a lower melting point as the emitter 11, linearity from low temperature to rated operation can be achieved. It may be possible to monitor the output with high sensitivity. Also, in such a state, it can be confirmed that the self-powered gamma ray detector is operating normally while the reactor power is increasing.

Up to the above-described embodiments, the description has focused on the case where Bi, which has a melting point of about 270° C., is used as an example of the emitter 11 . Pb—Bi (lead bismuth) is a substance with a lower melting point, a higher probability of interaction with gamma rays, and a lower neutron absorption cross section. The melting point of Pb—Bi is approximately 125° C., which is lower than that of Pb and Bi. Therefore, if Pb--Bi is used for the emitter 11, the reactor power can be monitored from a lower power stage.

The nuclear reactor reaches a critical state at about 80°C, the output is increased step by step, and eventually the temperature at the installation position of the self-powered gamma ray detector adopting Pb-Bi for the emitter 11 reaches 125°C. At this time, the emitter 11 melts, and the dose rate and the current value obtained from the self-powered gamma ray detector have a linear relationship. If the dose rate is proportional to the reactor output, the self-powered gamma ray detector using Pb—Bi as the emitter 11 can monitor the output from this point.

When the reactor power is low, there is little nuclear fission in the reactor, and the dose rate depends on the gamma rays emitted by the activation of the structures in the reactor and the decay of the nuclei that make up the fuel. There is no correlation between reactor current and reactor power. However, as the reactor approaches rated operation, the proportion of gamma rays caused by nuclear fission reactions increases, and eventually the reactor output and dose rate reach a proportional relationship.

The detectors that monitor the reactor power during rated operation and the start-up range monitor or intermediate range monitor that monitors the power up to rated power need to overlap each other for continuous power monitoring. By adopting a self-powered gamma ray detector that adopts Pb-Bi for the emitter 11, which has linearity in the dose rate and current value, from a dose rate lower than the dose rate during rated reactor operation, the start-up area monitor and It is possible to widen the output range in which the monitoring output of the intermediate range monitor and the monitoring output of the self-powered gamma ray detector overlap.

To summarize the above, conventionally, there is a self-powered gamma ray detector as one of the detectors for monitoring nuclear reactor output with gamma rays. Since the self-powered gamma ray detector cannot measure neutrons and gamma rays separately, it was necessary to adopt an element with high sensitivity to gamma rays and low sensitivity to neutrons for the emitter when monitoring gamma ray dose. Pb and Bi are the most suitable elements for this condition, but since these have a melting point near the reactor temperature during rated operation of the nuclear reactor, a self-powered gamma ray detector (self output type detector) could not be installed in the reactor during rated operation.

The self-detecting detector of this embodiment is a self-powering detector in which an emitter is provided inside and a collector is provided outside as conductors, and the emitter and collector are insulated, and the emitter is sealed in an airtight container.

By configuring the container covering the emitter with a material having a higher melting point than the emitter, the shape of the emitter can be maintained even when the emitter is melted or deformed. From the principle of operation of the self-powered detector, when electrons are emitted from the emitter by gamma rays or neutrons, and the emitter is electrically connected to a current measuring device, the detector operates as a radiation detector. Therefore, even if the emitter is melted, it is electrically insulated from the collector and can operate as a radiation measuring device as long as it is electrically connected to the current measuring device.

In addition, assuming that the emitter expands or melts at high temperatures, an emitter with a low melting point is applied by adjusting the mass of the emitter, the volume of the emitter container, and the length of the core wire from which the current is taken out inside the emitter container. Even with the self-powered detector, it is possible to monitor the reactor power by generating and measuring a current corresponding to the amount of gamma rays in the reactor during rated operation.

In the above description, the self-powered gamma ray detector has been mainly described, but the same applies to the self-powered neutron detector. If a material with a large cross-sectional area for neutrons, a low probability of interaction with gamma rays, and a low melting point can be used for the emitter, it can be used as a self-powered neutron detector installed in a high-temperature environment such as a reactor.

1, 1A, 1B self-powered detector 1a self-powered gamma ray detector (upper end)
1b Self-powered gamma ray detector (lower end)
11 Emitter 12 Emitter Container 13 Insulator 14 Collector 15 Core Wire 16 Holder 17 Coaxial Cable 18 Housing 31 Air 41 Instrumentation Tube 42 Reactor Pressure Vessel 43 Containment Vessel 44 Core 45 Channel Box 46 Control Rod 47 Water Rod 48 Fuel Rod

Claims (8)

In a self-powered detector comprising an internal emitter and an external collector, wherein insulation is provided between the emitter and the collector,
an emitter container enclosing the emitter, wherein the emitter container is airtight ;
A self-powered detector, wherein said emitter is lead-bismuth . The self-powered detector of claim 1, wherein
A self-powered detector, wherein gas is sealed in the emitter container. 3. The self-powered detector of claim 2, wherein
A self-powered detector, wherein the pressure of the gas in the emitter container is negative at a point before the volume of the emitter increases due to high temperature. The self-powered detector of claim 1, wherein
A self-powered detector, comprising a vacuum space within the emitter container. The self-powered detector of claim 1, wherein
A self-powered detector, wherein the emitter container is made of an insulator. In a self-powered detector comprising an internal emitter and an external collector, wherein insulation is provided between the emitter and the collector,
an emitter container enclosing the emitter, wherein the emitter container is airtight;
The emitter mass and the volume of the emitter container are adjusted so that the local emitter density in the emitter container is constant regardless of the orientation of the self-powered detector when the emitter melts due to high temperature. A self-powered detector characterized by: A nuclear instrumentation system, wherein the self-powered detector according to any one of claims 1 to 6 is arranged in an instrumentation tube of a nuclear reactor. 8. The nuclear instrumentation system of claim 7 , wherein a plurality of self-powered detectors are installed for each of said instrumentation tubes.

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