JPH0815497A - Method and facility for storing and monitoring spent fuel - Google Patents

Abstract

PURPOSE:To highly accurately detect leakage gas without sampling by enclosing helium gas labeled by a small quantity of labeled gas in a sealing vessel and optically detecting the gas leaked into a housing tube from the sealing vessel by a sensor attached to the housing tube. CONSTITUTION:For instance, a sail quantity of carbon monoxide is enclosed in a sealing vessel 2 enclosing used fuel 1 as labeled gas together with helium gas of good heat conduction. In the case where a hole is opened in the sealing vessel 2 by corrosion or the like, helium gas containing carbon monoxide is leaked in the inside of a housing tube 3. Since carbon monoxide absorbs an infrared ray in the neighborhood of wavelength of 5.5mum and an absorption coefficient is very large, the infrared ray not exceeding the wavelength of 5.5mum is transmitted from a detection part 8 to a sensor 6. If a quantity of attenuation is measured in the inside of the housing tube 3, leakage can be detected in high accuracy. Thereby radiation leakage can be monitored in high accuracy without performing troublesome sampling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a monitoring method and storage equipment for dry storage of spent fuel generated from a nuclear power plant, and in particular, radioactive substances in the spent fuel may leak to the outside. The present invention relates to a storage monitoring method and a storage facility suitable for reducing the amount as much as possible.

[0002]

2. Description of the Related Art In recent years, dry storage has been studied as a method of storing spent fuel generated from a nuclear power plant. For example, Japanese Examined Patent Publication No. 5-11598 shows an example of a dry storage method. In one embodiment of this system, the spent fuel is sealed in a sealed container, and the sealed container is stored inside a storage pipe. There is air outside the storage pipe, and the heat generated from the spent fuel can be removed by convection of the air.
Since the storage of spent fuel by this method may take several decades or longer, it is necessary to monitor that radioactive substances in the spent fuel have not leaked to the outside of the sealed container.

As a method of monitoring leakage, the Japanese Patent Publication No.
No. 98 discloses that the air inside the storage pipe is periodically sampled and the radioactivity concentration in the sampling air is measured. However, this method not only increases the cost of the entire storage facility because it requires sampling equipment, but it also complicates the disposal work because the sampling air must be handled as radioactive gaseous waste.

In order to solve such a problem, Japanese Patent Publication No. 3-7
No. 8960 discloses a method of detecting a pressure fluctuation inside a sealed container to monitor leakage. This method is excellent in that it does not require sampling, but it is difficult to distinguish it from leakage because the pressure inside the sealed container changes due to decay of decay heat of spent fuel and fluctuation of ambient temperature. There was a lack of certainty.

[0005]

The above-mentioned conventional techniques have not always been sufficiently considered with respect to prevention of complication of work associated with sampling or improvement of monitoring accuracy. An object of the present invention is to monitor substances leaking from the inside of a sealed container into the storage pipe without sampling and with high accuracy.

[0006]

The above-mentioned object can be achieved by detecting the helium gas leaking from the inside of the sealed container to the storage tube by optical means such as absorption of light. Further, the above object can be achieved by detecting that the labeling gas previously sealed in the sealed container leaks to the storage tube. As a method of detecting the labeling gas, an optical method, a radiation measuring method or the like can be applied.

[0007]

In order to efficiently remove the heat generated from the spent fuel, it is conceivable to fill the inside of the hermetically sealed container with helium gas having excellent thermal conductivity together with the spent fuel. Further, in consideration of handleability, it may be considered to enclose air inside the storage tube. Helium gas leaks into the storage pipe before the radioactive substance in the spent fuel leaks if the sealed container is pierced due to corrosion etc. during the dry storage of the spent fuel for a long time. To do. By monitoring and detecting this helium gas by an optical method such as light absorption or light emission, it is possible to quickly find a defect and take appropriate measures such as replacement of the sealed container.

However, since the detection accuracy of helium gas is generally poor, in order to improve the accuracy, a labeling gas such as carbon monoxide, carbon dioxide, or argon is made to coexist with the helium gas in a sealed container and stored. It is desirable to detect the marker gas that leaks into the pipe. When carbon monoxide or carbon dioxide is used as the labeling gas, it is possible to monitor with extremely high accuracy by examining the infrared absorption spectrum. Further, when argon is used as the labeling gas, it is possible to monitor with extremely high accuracy by measuring the radiation of Ar-41 which has been activated by the spent fuel.

[0009]

Embodiment 1 Hereinafter, one embodiment of the present invention will be described with reference to FIGS. The basic structure of the spent fuel storage facility is shown, for example, in FIGS. 1 and 3 of Japanese Examined Patent Publication No. 5-11598, so that only the part directly related to the present invention will be described here. As shown in FIG. 1, the spent fuel 1 is sealed in a sealed container 2 in which helium gas is sealed, and the sealed container 2 is further stored in a storage pipe 3. The storage tube 3
The upper part of is sealed by a sealing plug 4, and the space between the storage tube 3 and the sealed container 2 is filled with air. Cooling air 5 is flowing to the outside of the storage pipe so that the decay heat generated from the spent fuel 1 can be removed.
A sensor unit 6 for detecting helium is attached to the sealed plug 4, and the sensor unit 6 is connected to a helium detection unit 8 via an optical fiber 7.

In FIG. 2, the sensor section 6 and the detection section 8 are shown.
Shows the details of. The sensor unit 6 is provided with a cell 9 so that the air filled in the storage tube 3 can freely move in and out of the cell. The cell 9 has a light transmitting section 1
0 and a light receiving unit 11 are provided. Light (e.g. 388.9 nm) corresponding to the absorption wavelength of helium is emitted from the dye laser 12 provided in the detection section 8 and transmitted to the light transmission section 10 through the optical fiber 7. This light is in cell 9
Although it passes through the inside air portion to reach the light receiving portion 11, the received light is sent to the photodetector 13 again using the optical fiber 7 and the light intensity is measured. The measured light intensity is compared with the light intensity emitted from the dye laser 12 in the calculator 14 to obtain the amount of light attenuation in the cell 9.

When the hermetic container 2 is sound and the hermeticity is maintained, the light is hardly attenuated because only the air is present inside the housing tube 3. On the other hand, when the sealed container 2 is corroded and a pinhole or the like is generated, the helium gas inside the sealed container 2 is stored in the storage pipe 3.
Leak to. Therefore, the light from the dye laser 12 is absorbed and attenuated by the helium gas in the cell 9.
Moreover, since the attenuation of light is proportional to the leakage of helium gas, the degree of corrosion of the sealed container 2 can be known. By monitoring the amount of light attenuation in this way, the soundness of the sealed container 2 can be monitored without sampling the air inside the storage tube. In measuring the amount of light attenuation due to helium gas, it is natural that a high precision method adopted in a general measurement field can be used in order to reduce an error caused by a baseline shift or the like. Specifically, the drift component correction using the reference light,
There is a method in which white light is used instead of monochromatic light as a light source to obtain an absorption spectrum to improve accuracy.

Considering that the spent fuel may be stored for a long period of several tens of years or longer depending on the case, and the corrosion of the sealed container may occur gradually, the soundness monitoring is not always continuous. There is no need to carry out. Specifically, it is considered that once a day to several months is sufficient.
Further, although one monitoring device is used to monitor one storage pipe in FIG. 1, it is also possible to monitor a plurality of storage pipes with one device. Specifically, the sensor unit 6 is provided in all of the plurality of storage tubes, but only one detection unit 8 is provided, and it is possible to switch the light transmitted and received using the switching circuit and sequentially monitor the storage tubes. is there. These methods are not limited to this embodiment, and are basically the same in other embodiments described below.

Further, although the light absorption phenomenon of helium is used in the above embodiment, the light emission phenomenon may be used.
Specifically, instead of the dye laser 12, an ultraviolet laser having a short wavelength or a xenon lamp is used to irradiate the inside of the cell 9. When helium gas leaks, the specific emission associated with the excitation of helium (wavelength 388.9, 5
(87.6, 706.5 nm, etc.) is observed, and the soundness can be monitored by detecting this. Also, monitoring is possible by means other than the optical method. For example, it is possible to monitor the soundness of the hermetically sealed container 2 by utilizing the fact that helium gas has better thermal conductivity than air and the like, and measuring the thermal conductivity of the filling gas in the storage tube 3. .

Example 2 In Example 1, helium gas leaking from the sealed container was detected, but the lower limit of detection of helium gas may not be sufficient when sampling analysis is not performed. For this reason,
Leakage can be detected only after 0.1% or more of helium leaks from the sealed container. This embodiment makes it possible to detect a slight amount of leakage by making helium coexist with a labeling gas.

Since the structure of the apparatus is basically the same as that of FIG. 1, description will be given using this. In the present embodiment, a small amount of carbon monoxide (concentration of about 0.1 to 10%) is enclosed as a marker gas together with helium gas in a sealed container 2 for sealing the spent fuel 1. When the sealed container 2 has a hole due to corrosion or the like, carbon monoxide leaks into the storage tube 3 together with helium. It is known that carbon monoxide absorbs infrared rays having a wavelength of around 5.5 μm and its absorption coefficient is extremely large. Therefore, the leak can be detected by sending infrared rays having a wavelength of about 5.5 μm from the detection section 8 to the sensor section 6 and measuring the amount of attenuation inside the storage tube 3. Moreover, since it is possible to detect even if the carbon monoxide concentration is about 1 ppm, it is possible to monitor and detect even a leak that is two orders of magnitude less than when directly detecting helium. As an infrared light source having a wavelength of about 5.5 μm, there are an infrared lamp, an iodine laser, and the like.

Further, carbon monoxide is not limited to the use as a labeling gas, and carbon dioxide and sulfur dioxide have strong absorption peaks in the infrared region, and thus they can be used. Further, it is natural that a gas having a strong absorption peak in the visible region or the ultraviolet region can also be used as the labeling gas, and, for example, fluorine gas can be considered.

Third Embodiment In the second embodiment, an optical method is used to detect the labeling gas, but in this embodiment, a case of using radiation measurement will be described.

A small amount of non-radioactive argon (Ar-40) is sealed as a marker gas together with helium gas in a sealed container 2 for sealing the spent fuel 1. Part of Ar-40 is changed to radioactive Ar-41 by neutrons emitted from the spent fuel. If there is a hole in the sealed container 2 due to corrosion or the like, this Ar together with helium
-41 leaks into the storage tube 3 and reaches the sensor unit 6. In this embodiment, the leak from the hermetically sealed container 2 is detected by detecting the radiation of Ar-41 that has reached the sensor unit 6, but the structure of the sensor unit 6 used in this embodiment is illustrated. 3 shows.

Although the entire sensor portion 3 is covered with lead 15 so as to shield the radiation from the spent fuel 1, the air in the storage pipe 3 can freely enter and leave the center of the sensor portion. It is considered to be possible. A NaI (Tl) crystal 16 is provided in the central portion, and when Ar-41 leaked from the sealed container 3 reaches the sensor, A
The NaI (Tl) crystal 16 emits light by the γ-ray emitted from r-41. This light is collected by a condenser 17 and taken out of the storage tube 3 by an optical fiber 7.
The extracted light is measured by a photoelectric tube or the like to detect Ar-41. This type of radiation measurement method is based on a general principle.
Naturally, it is effective to carry out the energy discrimination of the lines, in which case it is easy to distinguish it from the radiation emitted from the spent fuel 1.

In the present embodiment, the non-radioactive gas activated by neutrons from the spent fuel was used as the labeling gas, but the radioactive gas was used from the beginning and the radiation of this should be detected when leaking from the sealed container. Is also possible. However, the use of non-radioactive gas as in this embodiment has the following advantages. The first advantage is that it is not necessary to handle radioactive gas when the spent fuel is sealed in the sealed container. However, even when the radioactive gas is sealed, the radioactive gas is once sealed in an ampoule such as glass and put in a sealed container together with the spent fuel, and the ampoule is sealed after the sealing work of the sealed container is completed. If it is broken from the outside of the container and the radioactive gas is diffused, handling becomes extremely easy. Next, the second merit of using the non-radioactive gas will be described. When radioactive gas is used as the labeling gas, it is unavoidable to use radioactive gas with a considerably long half-life considering that the monitoring period is several decades, and in the unlikely event a leak occurred from the sealed container. In some cases complicated gas waste treatment is required. On the other hand, when non-radioactive Ar-40 is used, neutrons generated from the spent fuel constantly radiate the radioactive Ar
-41 is produced and its half-life is short, about 2 hours. Therefore, even if a leak from the sealed container occurs, it can be easily treated. Other examples of such a non-radioactive labeling gas include He-3 and Kr-84. Neutrons produce radioactive H-3 in the former and Kr-85m in the latter. In particular, in the case of He-3, since the specific gravity of the gas is close to that of normal helium gas (about 100% He-4) used when sealing the spent fuel 1 in the sealed container 2, It has an advantage that it can be accurately detected even if the leakage from the sealed container occurs anywhere in the container.

In the above first to third embodiments, the explanation has been made on the premise that the helium gas is sealed in the sealed container, but the same monitoring can be performed when other gas is sealed. For example, even if hydrogen gas with good thermal conductivity is filled, the leak can be detected by optical means or the like. Further, even when air is enclosed, leakage can be detected by coexisting an appropriate labeling gas such as carbon monoxide or argon.

Further, in the above embodiment, the detection of the gas leaking from the sealed container to the storage pipe has been described, but it can also be used for the detection of the gas leaking from the inside of the storage pipe to the outside cooling air. The sensor unit 6 may be installed outside the storage pipe on the side of the cooling air 5. Such leakage monitoring is particularly effective when the spent fuel 1 is directly stored in the storage pipe 3 without once sealing the spent fuel 1 in the sealed container 2.

[0023]

As described above, according to the present invention, the soundness of the sealed container can be accurately monitored without sampling the air or the like inside the storage tube. Further, although there is a relatively high radiation field inside the storage tube, it is not necessary to use electronic parts such as semiconductor devices inside the storage tube, so that reliable monitoring can be performed for a long period of time.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a device configuration according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a structure of a sensor unit and a detection unit for gas detection used in an embodiment of the present invention.

FIG. 3 is a diagram illustrating the structure of a sensor unit for radiation detection used in another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Spent fuel, 2 ... Sealed container, 3 ... Storage tube, 5 ... Cooling air, 6 ... Sensor part, 7 ... Optical fiber, 8 ... Detection part, 9
... cell, 12 ... dye laser, 13 ... photodetector, 16 ... N
aI (Tl) crystal.

Front Page Continuation (72) Inventor Hidetoshi Kanai 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd., Hitachi Works

Claims (12)

[Claims] 1. A spent fuel dry storage facility, in which a spent fuel is sealed in a sealed container together with an enclosed gas, the sealed container is housed in a storage pipe, and the outside of the storage pipe is cooled. A method for monitoring storage of spent fuel, wherein the enclosed gas leaking from a container into the storage pipe is monitored by a sensor provided in the storage pipe. 2. The spent fuel storage monitoring method according to claim 1, wherein in monitoring the enclosed gas leaking from the sealed container, the absorption or emission characteristics of the enclosed gas are detected by the sensor. 3. The spent fuel storage monitoring method according to claim 1, wherein in the monitoring of the enclosed gas leaked from the sealed container, the thermal conductivity characteristic of the enclosed gas is detected by the sensor. 4. The spent fuel storage monitoring method according to claim 1, wherein the enclosed gas is helium. 5. A spent fuel dry storage facility in which a spent fuel is hermetically sealed in a hermetically sealed container together with a sealed gas, and the hermetically sealed container is housed in a storage pipe to cool the outside of the storage pipe. A method for monitoring storage of spent fuel, characterized in that a labeling gas is allowed to coexist in the gas, and the labeling gas leaking from the sealed container into the storage pipe is monitored by a sensor provided in the storage pipe. 6. The labeling gas is carbon monoxide, carbon dioxide,
The spent fuel storage monitor according to claim 5, wherein the sensor detects the absorption or emission characteristics of the enclosed gas in the monitoring of the labeling gas that is one of sulfur dioxide and leaks from the sealed container. Method. 7. The labeling gas is non-radioactive Ar-40, K.
It is either r-84 or He-3, and in the monitoring of the labeling gas leaked from the hermetically sealed container, the sensor detects a radioactive component in the labeling gas. For monitoring storage of spent fuel in Japan. 8. The spent fuel according to claim 5, wherein the marker gas is a radioactive gas, and the radiation of the marker gas is detected by the sensor in monitoring the marker gas leaked from the sealed container. Storage monitoring method. 9. A spent fuel dry storage facility, in which a spent fuel is sealed in a sealed container together with a sealed gas, the sealed container is housed in a storage pipe, and the outside of the storage pipe is cooled. A spent fuel storage facility, wherein a sensor unit for detecting the enclosed gas leaking from the container into the storage pipe is provided in the storage pipe. 10. A spent fuel dry storage facility for sealing a spent fuel together with an enclosed gas in a hermetically sealed container, accommodating the hermetically sealed container in a storage pipe, and cooling the outside of the storage pipe.
A storage facility for spent fuel, characterized in that a sensor portion for allowing the labeling gas to coexist in the enclosed gas and detecting the labeling gas leaking from the sealed container into the storage pipe is provided in the storage pipe. 11. A detection unit for detecting a signal from the sensor unit is provided outside the housing pipe, and a plurality of sensor units are connected to the detection unit, using a switching circuit. 11. The spent fuel storage facility according to claim 9, wherein signals from a plurality of the sensor units are detected by one detection unit. 12. The sensor section is provided inside the storage tube, and a detection section for detecting a signal from the sensor section is provided outside the storage tube, and the sensor and the detection section are optical fibers. The spent fuel storage facility according to claim 9 or 10, characterized in that the spent fuel storage facility is connected by means of.