JPH06138243A - Neutron detector - Google Patents

Abstract

PURPOSE:To decrease the quantity of radiation toward a detecting portion by varying the positions of a radiation shielding member and a neutron detecting portion relative to a reactor core using a drive. CONSTITUTION:The neutron detecting portion 5 of a radiation source region system and the neutron detecting portion 7 of an intermediate region system are incorporated into the same detecting portion assembly 6 and each include a cable 8 and a connector 9. A radiation shielding member 10 is rotated on the detecting portion 5 and is mounted to a drive 11 in such manner that its portion 10a with a low shielding ability and its portion 10b with a high shielding ability can be switched around between the detecting portion 5 and a reactor core. The shielding member 10 is made from an element such as lead in order to efficiently attenuate gamma-rays or made from an element such as cadmium in order to shield neutrons. Most of gamma-rays and radiation such as neutrons that the detector 5 receives come from one direction from the reactor ore. The intensity of radiation impinging on the detecting portion 5 can then be varied and the quantity of radiation can be reduced up to the low intensity at which characteristic deterioration does not occur over a long period of time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron detector used in a nuclear reactor.

[0002]

2. Description of the Related Art In order to safely operate a nuclear reactor, it is necessary to constantly measure neutrons in the core. This is to ensure the operation and safety of the reactor by measuring the neutron flux because the reactor output and the neutron number or neutron flux are in a proportional relationship. For this reason, the neutron detector is one of the most important measuring instruments used in nuclear reactors. An example of a conventional neutron detector in a nuclear reactor will be described with reference to FIG.
The nuclear reactor will be described as an example of a light water reactor, especially a pressurized water reactor in Japan. FIG. 4 is a schematic cross-sectional view showing the installation position of the neutron detector of the pressurized water reactor. The core fuel assembly 2 is installed inside the reactor pressure vessel 1, and although not shown in the figure, the upper part of the core is shown. A control rod for controlling the nuclear reaction of the reactor core is installed, and a partition wall for controlling the flow of cooling water and piping for measuring instruments in the reactor are provided in the reactor pressure vessel. By the way, since a part of neutrons generated in the core leaks to the outside of the reactor vessel, the outer periphery of the reactor pressure vessel 1 is generally surrounded by these neutrons and other radiation shielding materials leaking from the reactor vessel. Therefore, as shown in FIG. 4, the neutron detector guide tube 3 is arranged on the side surface of the reactor pressure vessel 1, and the neutron detector 4 is inserted thereinto to detect leaked neutrons from the core.
Since the neutron flux of leaked neutrons from the core changes by about 8 to 10 digits from startup to rated output, one type of neutron detector cannot cover all of this region. Therefore, a wide range of neutron flux is measured with a combination of two or three neutron detectors having different measurement ranges. Most domestic pressurized water reactors use this combination as a source area system,
Different types of neutron detectors are used for the intermediate region system and the output region system. Further, as shown in FIG. 5, in the neutron detector of the source region system, the neutron detector 5 and the neutron detector 7 of the intermediate region system are incorporated in the same neutron detector assembly 6, and a cable 8 and A measurement signal of the neutron flux is sent by the connector 9. This is called a source / intermediate region neutron detector assembly.

Since a neutron source region system having a role of monitoring core neutron flux during startup, shutdown, or shutdown is required to have high neutron sensitivity, BF ₃ with high neutron flux sensitivity is required.
A proportional counter is used as the neutron detector. In other intermediate region and output region neutron detectors, solid boron with an increased ratio of ¹⁰ B is applied to the electrodes in the neutron detector,
While operating as an ionization chamber filled with an inert gas such as nitrogen or argon, a BF ₃ proportional counter is filled with BF ₃ gas in which the ratio of ¹⁰ B having a high reaction cross section for neutrons is increased in the tube. It also serves as a neutron detection material and an ionizing gas. Further, the BF ₃ proportional counter has a characteristic that a pulsed electric signal proportional to the neutron flux can be obtained with higher efficiency by applying a higher electric field to the detectors of the intermediate region system and the output region system and operating in the proportional count region. .

[0004]

By the way, since BF ₃ gas is chemically active as compared with an inert gas such as nitrogen or argon, a part of activation or decomposition is caused by irradiation of neutrons or γ rays. Occurs, and is easily adsorbed by a structural material inside the neutron detector or causes a chemical reaction. Therefore, due to this, the BF ₃ proportional counter used as the neutron detector of the neutron detector in the radiation source / intermediate region system has a problem that its characteristics are easily deteriorated when exposed to strong radiation for a long period of time. . In addition, the neutron flux leaking from the core operating at the rated output to the neutron detector exceeds the measurable range of the highly sensitive BF ₃ proportional counter, and actually counts such a high neutron flux. It is known that deterioration of characteristics such as deterioration of sensitivity and collapse of pulse waveform occurs, and the life of the detection unit is shortened at an accelerated rate. Therefore, conventionally, the operation was stopped by cutting off the supply voltage while the neutron flux exceeded the measurable range of the BF ₃ proportional counter, but the neutron detector installed in the guide tube was always placed under high radiation. There is no change in being treated. Even when the supply voltage is cut off, the BF ₃ gas in the neutron detector has an interaction with radiation, and it is unavoidable that the characteristics change even slowly. Therefore, the source region neutron detector has a shorter life than the intermediate region neutron detector and the output region neutron detector, and thus the exchange period was short. Further, while the neutron flux is high, the operation cannot be confirmed for the above reason, and thus there is a problem that it is not possible to confirm the operation of the neutron detector during this period. In particular, the example of a pressurized water reactor in Japan has been described above, but even in other types of reactors, in the neutron flux measurement of a reactor that generally requires a very wide measurement range, a region with a low neutron flux is used. Take charge,
The same problem was caused by the fact that a neutron detector with high sensitivity was exposed to a high radiation field for a long period of time.

The present invention has been made to solve the above problems, and it is an object of the present invention to obtain a neutron detector capable of preventing characteristic deterioration and capable of confirming operation even in a high radiation field exceeding the operable range. Has an aim.

[0006]

A neutron detector according to claim 1 of the present invention comprises a neutron detector for detecting neutrons,
A radiation shield for shielding the radiation emitted from the core to the neutron detector, and a drive device for changing the relative position of the radiation shield and the neutron detector with respect to the core.

A neutron detector according to a second aspect of the present invention is a neutron detector in which a gas is enclosed in a tube equipped with an electrode and which detects neutrons radiated from the core, and a neutron detector in the vicinity of the neutron detector. An airtight chamber provided and communicating with the pipe,
A volume changing device for containing the gas sealed in the tube by changing the volume of the hermetic chamber, and a radiation shield for shielding the radiation emitted from the core to the hermetic chamber are provided.

Further, a neutron detector according to claim 3 of the present invention includes: a neutron detector for detecting neutrons, a radiation shield for shielding radiation emitted from the core to the neutron detector, and a core for the core. A drive device for changing the relative position of the radiation shield and the neutron detector, and a heat dissipation means for radiating the heat generated in the radiation shield to the outside.

A neutron detector according to a fourth aspect of the present invention is a neutron detector in which a gas is enclosed in a tube equipped with an electrode and which detects neutrons radiated from the core, and a neutron detector in the vicinity of the neutron detector. An airtight chamber provided and communicating with the pipe,
A volume changing device that accommodates the gas sealed in the tube by changing the volume of the airtight chamber, a radiation shield that shields radiation emitted from the core to the airtight chamber, and a radiation shield generated in the radiation shield. And a heat dissipation means for releasing heat to the outside.

[0010]

In the neutron detector according to claim 1 of the present invention,
The radiation shield can reduce the amount of radiation emitted to the neutron detector.

Further, in the neutron detector according to the second aspect of the present invention, when the radiation becomes strong, the influence of the radiation on the gas is reduced by containing the gas in the hermetic chamber by the volume changing device. can do. Further, the characteristic of the detection unit can be adjusted by changing the gas pressure in the tube by the volume changing device.

Further, in the neutron detector according to claim 3 of the present invention, since the radiation shield is provided with the heat radiating means, the amount of radiation radiated to the neutron detector can be reduced, and the radiation shield and the same It is possible to reduce the heat influence on the surroundings.

Further, in the neutron detector according to the fourth aspect of the present invention, since the radiation shield is provided with the heat radiation means, it is possible to reduce the influence of the radiation on the gas, and
The thermal effect on the radiation shield and its surroundings can be reduced.

[0014]

【Example】

Example 1. Embodiment 1 of the present invention will be described below with reference to the drawings as an application example of a neutron detector in a radiation source / intermediate region used in a pressurized water reactor. 1 is a schematic vertical sectional view of a neutron detector showing Example 1, and FIG. 2 is a horizontal schematic sectional view of the same neutron detector. In these figures,
The same reference numerals will be given to the same or corresponding portions as in the above description. Figure 1
In the same way as in the conventional example of FIG. 5, the neutron detector 5 of the source region system and the neutron detector 7 of the intermediate region system are incorporated in the same neutron detector assembly 6, and each is equipped with a cable 8 and a connector 9. There is. The first embodiment of the present invention shown in FIG. 1 is different from the conventional example shown in FIG. 5 in that the radiation shield 10 is provided at a position surrounding the outer periphery of the neutron detector 5 in the radiation source region system.
Is provided, and a drive device 11 having a function of changing the relative position of the shield 10 with respect to the neutron detection unit 5 is provided.

The radiation shield 10 placed at a position surrounding the outer periphery of the neutron detector 5 in the radiation source region system does not have a shield in the circumferential direction as shown in FIG. The portion 10a having a reduced radiation shielding ability and the shield 10
A portion 10b having a thickness that attenuates the radiation reaching the neutron detection section 5 placed inside the chamber to, for example, about two digits, and has a high radiation shielding capability. This radiation shield 10 rotates around the neutron detector 5 as an axis of rotation, and is attached to the drive unit 11 so that the portion 10a having low shielding ability and the portion 10b having high shielding ability can be exchanged between the neutron detecting portion 5 and the core. Has been. The drive unit 11 fixes the power unit 11a including a motor and the radiation shield 10 and
Driving force transmission portion 11b rotated by the power generator 11a
It is composed of As described above, this type of neutron detector is placed on the side surface of the reactor vessel at the position shown in FIG. 4 for the purpose of measuring the leaked neutrons from the core of the reactor. Most of the radiation such as γ rays and γ rays are incident from one direction from the core and a small amount from the other direction. Therefore, the neutron detector having the above-described structure has a built-in mechanism for neutron detector 5
It is possible to change the intensity of the radiation radiated to. Therefore, the neutron detector according to the present invention, while operating at the rated output in a high radiation field, the radiation dose reaching the neutron detector to a low intensity at which its characteristic deterioration does not occur for a long period of time, or to an intensity at which operation can be confirmed Can be reduced. The material and structure of the radiation shield 10 to be used may be determined according to its purpose. For example, in order to efficiently attenuate γ-rays, an element such as lead whose atomic number is as high as possible may be used to shield neutrons. As the material, a substance such as cadmium having an isotope with a large neutron absorption cross section may be used. Therefore, both materials can be combined to obtain arbitrary shielding properties.

Example 2. Although the radiation shield 10 rotates in the neutron detector assembly 6 about the neutron detector 5 as an axis in the first embodiment described above, the attenuation amount of the radiation amount reaching the neutron detector is variable. ,
The same effect can be obtained by moving the ionized gas in the neutron detector into an airtight container provided in the shield if necessary. FIG. 3 shows a schematic sectional view of the neutron detector of the embodiment. The same or corresponding parts as those in FIGS. 1, 2 and 5 are designated by the same reference numerals. The neutron detector 5 is in communication with the airtight container 12 through the pipe 13. The airtight container 12 is surrounded by the radiation shield 10A to shield the radiation. In addition, the internal volume is changed by the volume changing device 14. As described above, the neutron detector 5 normally uses a proportional counter containing BF ₃ gas.
In the neutron detector of the present invention, the volume of the airtight container 12 is maximized during the constant power operation in which the radiation from the core greatly exceeds the measurement range of the neutron detector. Since most of the BF ₃ gas in which the inner volume of the airtight container 12 is sufficiently larger than the inner volume of the neutron detection section 5 is in the airtight container 12, the radiation shield 10
Radiation from the core is shielded by A, and the BF ₃ gas can escape from the influence. When the neutron detector 5 is used, the volume of the airtight container 12 may be reduced by the volume changing device 14 and a required amount of BF ₃ gas may be sent around the neutron detector 5. If the amount of gas sent to the neutron detector 5 is reduced, the operation can be confirmed by applying the supply voltage to the neutron detector 5 without deteriorating the characteristics of the neutron detector even with a high neutron flux.

Embodiment 3. Embodiment 1 of the present invention. Is characterized in that a conventional neutron detector is provided with a radiation shield 10, but when the dose rate of radiation, especially γ rays, is high, the energy of γ rays shielded by the radiation shield 10 is converted into heat. Therefore, the radiation shield 10 generates heat. Therefore, the influence on surrounding components may be a problem. One of the simplest and most reliable means for releasing this heat generation is the radiation shield 10, the neutron detector assembly outer cylinder 6a, and the neutron detector guide tube 3 (see FIG. 4).
(See) to improve heat conduction and escape heat generation. Even if a so-called heat pipe is combined with the neutron detector described in Embodiments 1 and 2, the efficiency can be increased without lowering the reliability.
For more efficient heat release, a general cooling system that combines a compressor and a thermal catalyst or a cooling system using the Peltier effect is also suitable. These heat exchange devices are suitably incorporated in the drive device 11 of FIG.

Example 4. Embodiment 2 of the present invention Is to store most of the built-in gas originally held inside the neutron detector in an airtight container 12 shielded from radiation and save it, and prevent deterioration due to radiation during that period. Example 3. In addition to the problem of the influence of heat generated in the radiation shield 10, the characteristic of the neutron detector is susceptible to the influence of the gas pressure, so that the characteristic of the detector changes due to the temperature change of the gas pressure. Therefore, if a mechanism for controlling the temperature is provided inside the neutron detector assembly, the reliability of the neutron detector is further improved. As a mechanism for controlling the temperature, a heater may be used in addition to the combination of the heat conduction and the cooling system as in the third embodiment. These devices are preferably incorporated in the volume changing device 14 of FIG. 3, and the heater is preferably incorporated in the airtight container 12.

Example 5. In the above-described embodiments, the neutron detector of the radiation source / intermediate region system used in the pressurized water reactor has been described, but the present invention is also applicable to the neutron detector used for neutron flux measurement. Therefore, even if the radiation source is another type of nuclear reactor, for example, a fast breeder reactor or a gas cooling reactor, or an accelerator, a subcritical assembly, a fusion reactor, etc. It may be a radiation source. Further, the neutron detectors in Examples 1 and ₃ may be, for example, a semiconductor neutron detector, a scintillation neutron detector, or a thermoluminescence neutron detector other than the BF ₃ detector. Further, the method of shielding the neutron detector and adjusting the pressure of the ionized gas in the neutron detector according to the present invention can be used more positively for the measurement of radiation from the mere operation confirmation for knowing the soundness of the neutron detector. Therefore, it may be used for the purpose of expanding the conventional measurable range.

[0020]

As described above in detail, according to the neutron detector according to the first aspect of the present invention, the neutron detector for detecting neutrons and the radiation emitted from the core to the neutron detector are shielded. Since the radiation shield, and the drive device for changing the relative position of the radiation shield and the neutron detection unit with respect to the core, it is possible to reduce the amount of radiation emitted to the neutron detection unit, radiation It is possible to prevent the deterioration of the neutron detector due to and to extend its life. Further, it is possible to confirm the operation of the neutron detector even in a high neutron flux exceeding the operable range.

Further, according to the neutron detector of the second aspect of the present invention, a neutron detector for detecting neutrons radiated from the core in which a gas is enclosed in a tube provided with an electrode, and the neutron detector An airtight chamber that is provided in the vicinity and communicates with the tube, a volume changing device that stores the gas sealed in the tube by changing the volume of the airtight chamber, and radiation emitted from the core to the airtight chamber Since it has a radiation shield to shield, when the radiation becomes strong, the effect on radiation gas can be reduced by accommodating the gas in the airtight chamber by the volume changing device. The detector can be prevented from deteriorating and its life can be extended. Further, there is an effect that the characteristic of the detection unit can be adjusted by changing the gas pressure in the tube by the volume changing device. Further, it is possible to confirm the operation of the neutron detector even in a high neutron flux exceeding the operable range.

Further, according to the neutron detector of the third aspect of the present invention, the neutron detector for detecting neutrons, the radiation shield for shielding the radiation emitted from the core to the neutron detector, and the core With respect to the radiation shield and the drive device for changing the relative position of the neutron detection unit, and because the heat radiation unit for radiating the heat generated in the radiation shield to the outside, the radiation emitted to the neutron detection unit. The effect of being able to reduce the amount, thereby preventing the deterioration of the neutron detector due to radiation, extending its life, and reducing the thermal effect on the radiation shield and its surroundings. Play. Also,
It is possible to confirm the operation of the neutron detector even in a high neutron flux exceeding the operable range.

Further, in the neutron detector according to claim 4 of the present invention, a gas is enclosed in a tube provided with an electrode, and a neutron detector for detecting neutrons radiated from the core, and a neutron detector near the neutron detector. An airtight chamber that is provided and communicates with the pipe, a volume changing device that stores the gas sealed in the pipe by changing the volume of the airtight chamber, and shields radiation emitted from the core to the airtight chamber. Since the radiation shield and the heat radiation means for radiating the heat generated in the radiation shield to the outside are provided, it is possible to reduce the influence of the radiation on the gas, and thus prevent the deterioration of the neutron detector due to the radiation. It is possible to extend the life of the radiation shield and adjust the characteristics of the detector by changing the gas pressure inside the tube, and further to prevent the thermal effect on the radiation shield and its surroundings. An effect that can be reduced. Further, it is possible to confirm the operation of the neutron detector even in a high neutron flux exceeding the operable range.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing a first embodiment of the present invention.

FIG. 2 is a horizontal sectional view showing the first embodiment of the present invention.

FIG. 3 is a vertical sectional view showing a second embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view showing an installation position of a neutron detector in a pressurized water reactor as a conventional technique.

FIG. 5 is a vertical sectional view showing an assembly structure of a conventional neutron detector.

[Explanation of symbols]

 4 Neutron Detector 5 Radiation Source Region Neutron Detector 6 Neutron Detector Assembly 10 Radiation Shield 11 Drive Device

Claims (4)

[Claims] 1. A neutron detector that detects neutrons, a radiation shield that shields radiation emitted from a core to the neutron detector, and a relative position of the radiation shield and the neutron detector with respect to the core. A neutron detector, comprising: 2. A neutron detector for detecting neutrons radiated from a core, in which a gas is sealed in a tube provided with an electrode, an airtight chamber provided near the neutron detector and communicating with the tube, A volume changing device for containing the gas sealed in the tube by changing the volume of the airtight chamber; and a radiation shield for shielding the radiation emitted from the core to the airtight chamber. Neutron detector. 3. A neutron detector that detects neutrons, a radiation shield that shields radiation emitted from a core to the neutron detector, and a relative position of the radiation shield and the neutron detector with respect to the core. A neutron detector, comprising: a drive device for changing the temperature of the radiation shield; and a heat radiating unit for radiating heat generated in the radiation shield to the outside. 4. A neutron detector for detecting neutrons radiated from a core, in which a gas is sealed in a tube provided with an electrode, an airtight chamber provided near the neutron detector and communicating with the tube, A volume changing device that stores the gas sealed in the tube by changing the volume of the airtight chamber, a radiation shield that shields radiation emitted from the core to the airtight chamber, and heat generated in the radiation shield. A neutron detector, characterized by comprising: