JP7211729B2 - Hydrogen gas vent for radioactive material storage container and radioactive material storage container using the same - Google Patents

Description

The present invention relates to a hydrogen gas vent for a radioactive substance storage container and a radioactive substance storage container using the same.

When radioactive materials are placed in a container during transport, storage/storage, burial disposal, etc., the water contained in the container is radiolyzed to generate hydrogen gas, oxygen gas, etc. There is a risk of damage or deformation of the container due to pressure rise, or hydrogen gas explosion.

When transporting, storing, storing, or disposing of radioactive materials that emit radiation, it is usually necessary to place them in a sealed container to prevent leakage of radioactive materials, prevent rainwater from entering from the outside, or shield radiation. placed in a burial chamber or managed within the facility. At this time, if water or the like exists, gases such as H ₂ , O ₂ , N ₂ and CO ₂ are generated by radiolysis of water. The generation of these gases increases the pressure inside the container, which may cause damage or deformation of the container, which may cause leakage of radioactive materials, or deformation that affects various shields that cover the container. there is a possibility.

Radioactive substances can be in the form of fine granules, in powder form such as incineration fly ash, or in liquid form. should avoid the danger of In order to stabilize these radioactive substances, it is effective to solidify them with cement-based materials, but most cement-based materials contain water. In addition, even in the case of solidification with a vitrified material, water may be included due to fixation using a cement-based material in a container, and the generation of gas due to radiation poses a problem.

In particular, H ₂ (hydrogen gas) derived from water has a concentration that reaches the explosion limit or combustion limit, and there is a risk of flame propagation due to the generation of sparks due to static electricity. It is also required to appropriately control the concentration of hydrogen gas, etc., below the limit value in facilities.

According to Patent Document 1, a ceramic porous membrane is formed on the surface of a porous substrate, and an organosilicon compound containing fluorine atoms is further applied thereon, and hydrogen gas is separated by a water-impermeable ceramic separation membrane. Separable. However, this structure is complicated, heat treatment is required in the manufacturing process, and it has been difficult to obtain a ceramic porous membrane with a pore size corresponding to the type of gas for the purpose of safe storage. In addition, the permeability of hydrogen gas is low, and the durability of the separation membrane, particularly the mechanical strength, is insufficient, making it unsuitable for shielding radioactive substances.

In addition, even if a leak valve that detects the pressure of the generated gas is installed in the sealed container, the explosion limit of hydrogen gas is 4% by volume, which is extremely low. It is difficult to operate the leak valve at the correspondingly low pressure, and the leak valve cannot prevent hydrogen gas explosion in the container.

WO2012/141033

Therefore, in the disposal of radioactive materials such as transportation, storage, storage, burial, etc., we prevent pressure deformation due to gas generated by radiation in the closed space inside the container and explosion due to hydrogen gas. An object of the present invention is to provide a gas vent that prevents leakage of radioactive substances from the closed space inside and intrusion of rainwater and the like from the outside, and efficiently releases hydrogen gas and other gases produced by radiolysis.

The present invention has been made in view of the above problems, and provides a hydrogen gas vent and a radioactive material container using the same. The present invention provides a gas vent that sequentially releases generated hydrogen gas and oxygen gas from the container.

In order to solve the above problems, the present invention
[1] A gas vent that is installed in a container for storing radioactive substances and discharges gas generated by radiolysis of water to the outside of the container, and has a gas permeable membrane that is impermeable to liquid but permeable to gas. , a hydrogen gas vent having an air permeable porous support to hold it.

[2] The hydrogen gas vent of [1], wherein the gas permeable membrane is silicone rubber.

[3] The method of [1] to [2], wherein the gas permeable porous carrier is a porous ceramic or a porous metal, and the thickness of the gas permeable membrane is 0.1 to 5 mm. Provide a hydrogen gas vent.

[4] The hydrogen vents of [1] to [3] hold the gas permeable membrane and the gas permeable porous support in a box having an air permeable port and an exhaust port, and the air permeable port is open to the inside of the container. , a radioactive substance storage container characterized in that the exhaust port is connected to an external exhaust port using a tube.

[5] The hydrogen vents of [1] to [3] are characterized by being installed in a cylindrical box 15 so that the gas permeable membrane and the gas permeable porous support form part of the outer wall of the container. to provide a radioactive material storage container.

FIG. 1 shows the concept of gas generation by radiation in a radioactive substance container 20 and the function of the gas vent 10 . The radioactive material 30 is stored in the container together with the solidified material 40 . In addition to preventing pressure deformation and explosion due to hydrogen gas (H ₂ in Fig. 1) due to gas generated by radiation in the closed space in the container, radioactive materials from the closed space in the container for radioactive material storage To provide a gas vent 10 that prevents the leakage of hydrogen gas 30 and the infiltration of rainwater from the outside and releases hydrogen gas safely and efficiently.

Gas-permeable membrane that is impermeable to liquid but permeable to gas Silicone rubber is preferable from the viewpoint of durability, such as heat resistance and cold resistance, and economical efficiency. Dimethyl-based, methylvinyl-based, and methylfluoroalkyl-based silicone rubbers are preferred. Furthermore, it is preferable to use SiO ₂ aerosil, quartz powder, diatomaceous earth, or the like as a filler (mixture material) to obtain a composite material with improved strength characteristics. Silicone rubber is excellent in permeability to hydrogen gas and also to other gases produced by radiation decomposition, so that it is suitable for preventing deformation and breakage of the sealed container.

Furthermore, when the molecular structure of the silicone rubber contains 3 to 20 mol% of phenyl groups, not only is the resistance to radiation improved, but also the container is sometimes stored in a low-temperature environment, so cold resistance is imparted. can be done. A methylphenyl vinyl silicone rubber having a phenyl group is exemplified.

The gas permeable membrane has a modulus of rigidity of 0.5 to 10 MPa, thereby increasing its ability to follow changes in the temperature of the external environment and preventing the contents of the container such as liquid water from leaking out. Moreover, the hydrogen gas permeability of the gas permeable membrane is desirably 0.1 cc·cm/(cm ² ·atm·day) or more.

Air-permeable porous support holding gas-permeable membrane Air-permeable porous support used for gas venting includes metal porous materials such as stainless steel, Al ₂ O ₃ , SiO ₂ , ZrO ₂ , MgO, TiO ₂ and Y ₂ . A porous ceramic containing O as a main component and having a pore size of 1 nm to 1 mm, a hardened cement material, or a hardened gypsum material can be used.

The gas permeability of these gas-permeable porous supports is preferably 100 cc·cm/(cm ² ·atm·day or more). Furthermore, the ratio of the gas permeation amounts of the air permeable porous support and the gas permeable membrane, (gas permeation amount of the air permeable porous support)/(gas permeation amount of the body permeable membrane) is hydrogen From the diffusivity of the gas permeable porous carrier, it is desirable to have a ratio of 10 or more.The value of this ratio is also important in designing a hydrogen gas vent. Since the gas permeation amount is determined by the permeability and the thickness, the gas permeation amount of the gas permeable porous support can be calculated by using this ratio of 10. The upper limit of the thickness of the permeable porous support can be determined from the measured gas permeability of the porous support and catalog values.

Further, if the breaking strength is 5 MPa to 200 MPa in a four-point bending test, the support will not be damaged even if subjected to external mechanical force, and the gas permeation performance as a support will be maintained. Desirably, when the pressure is 20 to 100 MPa, the gas pressure in the container can withstand a rise, and processing is easy. The thickness of the air-permeable porous carrier is 1 mm to 2 cm, and thin carriers having a thickness of 0.1 to 5 mm are laminated to form an air-permeable porous carrier, or thin carriers are laminated. A gas permeable membrane may be placed between them.

Configuration of Container Using Gas Vent FIG. 2 shows an example of installation of a gas vent on a container. The gas vent 10 has the gas permeable membrane 11 and the gas permeable porous carrier 12 and has a structure for holding them. Although the holding method is not particularly limited, the gas permeable membrane 11 and the gas permeable porous carrier 12 are held in a box 15 having an air permeable port 13 and an exhaust port 14, and the air permeable port 13 is open to the inside of the container. Further, the exhaust port 14 is configured to be connected to an external exhaust port 21 arranged in the container 20 using a tube 16 or the like. The gas vent 10 should be installed in the container 20 in which the radioactive material is stored so as not to form a protrusion, and to prevent liquid water from entering from the outside, the gas vent 10 should be installed above the position of the exhaust port outside the container. is desirable.

^The size of the hole provided in the container exterior exhaust port 21 is not particularly limited, but since gas ^diffusion is rapid, the opening area does not need to be large. good. For the purpose of increasing the resistance to mechanical action from the outside and for the purpose of preventing dust and the like from flying into the container, the container exterior exhaust port 21 is provided with a metal mesh or punching metal that does not hinder the gas permeability, or a gap. It is preferable to install a filter 22 using a metal porous material, porous ceramic, non-woven fabric, or the like having a rate of about 5 to 50%. If the exhaust port 14 of the gas vent is structured so as to be in direct contact with the external exhaust port 21, a connection tube becomes unnecessary.

Installation of the gas-permeable membrane 11 on the air-permeable porous support 12 can be carried out by, for example, setting cured silicone rubber on the support mechanically or using an adhesive or the like. The silicone rubber of the mold may be applied to the support and cured by the respective curing method. The gas permeable membrane 11 can be formed on one side or both sides of the gas permeable porous support 12, or can be laminated in the gas permeable porous support. At this time, the permeable porous carrier 12 can be installed on both sides, and liquid water and the like can be effectively blocked from inside and outside the container. Moreover, it is preferable to provide a drain port 17 for removing condensation water of steam.
The total film thickness of the gas permeable film 11 is preferably 0.1 to 5 mm, more preferably 0.1 to 2 mm in total. If the thickness is 5 mm or more, gas permeation may be hindered.

Construction of Container Using Gas Vent FIG. 3 shows another example of installation of a gas vent on a container. The gas vent 10 has a configuration in which the gas permeable membrane 11 and the gas permeable porous carrier 12 are installed in a cylindrical box 15 so as to form part of the outer wall of the container. The filter 22 and the mesh cap 21 were integrated and adhered to the cylindrical box 15 and part of the outer wall.

Hydrogen gas and oxygen generated by gas vents that efficiently release hydrogen gas and other radiolysis-generated gases by preventing leakage of radioactive materials from the closed space in the container for storing radioactive materials and infiltration of rainwater, etc. from the outside. Gas, etc., is released from the container one by one, and during the disposal of radioactive materials such as transportation, storage, storage, and burial, pressure deformation due to gas generated by radiation occurring in the closed space inside the container, explosion due to hydrogen gas, etc. can be prevented.

FIG. 2 is a conceptual diagram showing functions of gas generation by radiation and gas vent in a container. It is a figure which shows an example of installation to the container of a gas vent. It is a figure which shows another example of installation to the container of a gas vent. FIG. 4 is a diagram showing changes over time in the amount of hydrogen gas discharged from a gas vent.

10: Gas vent 11: Gas permeable membrane 12: Air permeable porous carrier 13: Air permeable port 14: Exhaust port 15: Box 16: Tube 17: Drain port 20: Container 21: Exhaust port outside container 22: Filter

As an air-permeable porous support, an aluminumite mixed sintered body (1300° C.) having a bending strength of 80 MPa was processed to a thickness of 0.98 cm and a size of 3.82 mm×3.82 mm. Moisture-curable methyl vinyl silicone containing 5 to 10% amorphous silica was coated on both sides of the support so as to have a thickness of 0.43 mm/side, and cured in a room to form a gas-permeable membrane. This was attached to the upper part of a metal box with an epoxy resin around the permeable porous carrier, and one side of the hydrogen gas vent was open to the atmosphere.

The internal volume of the metal box, excluding the air-permeable porous carrier, was 36.7 cc. In addition, when the air vent surface of the gas vent was immersed in water, no mass increase due to permeation of liquid water was observed, and the permeable membrane was in a state of repelling water. confirmed that it does not pass through

A hydrogen gas impermeable aluminum laminate sample bag containing 384 cc of hydrogen gas with a purity of 99.9% or more is connected to the gas vent attachment port, and the amount of hydrogen gas vented is the amount of decrease in the hydrogen gas volume in the sample bag. Measurements were taken over time and shown in FIG. The gas permeability was calculated from the amount of gas venting, the thickness of the silicone, the area of the permeable membrane in contact with the atmosphere via the carrier, and the hydrogen partial pressure difference obtained from the hydrogen in the aluminum bag and the volume of the metal container filled with air. As a result, the hydrogen gas permeability was 0.895 cm ³ /cm ² ·day·atm.

Here, when this gas vent is attached to a radioactive material containing Cs137 in a 200-liter closed container (radiation intensity 10 ¹⁰ Bq / kg, volume 180 liters, moisture content 20%, specific gravity 2.0), in the storage container is estimated as follows.

Assuming that the hydrogen gas generation G value (the number of products generated per 100 eV of absorbed energy) is G = 0.45, the hydrogen gas generation amount V per day under standard conditions is
V=10 ¹⁰ ×(24×60×60)×0.2×G×10 ⁴ ×22400/(6.02×10 ²³ )=10 cm ³ .

Since the volume of the container is 200 L and the sample filling rate is 90%, the partial pressure of the generated gas in the container cavity is 5×10 ⁻⁴ atm. Therefore, if there is no hydrogen gas vent, the hydrogen gas partial pressure will reach the flash point of 4 vol% = 0.04 atm in 80 days.

In order for the partial pressure to be equal to or lower than the flash point, the amount of hydrogen gas leaking from the hydrogen gas vent at a partial pressure of 0.04 atm should be 10 cm ³ or more per day under standard conditions. Since the hydrogen gas permeability of the hydrogen gas vent used is 0.895 cm ³ /(cm ² ·atm · day), the area S of the hydrogen gas permeable membrane is S = 10/(0.895 x 0.04) = 279 cm ² . Therefore, if a gas vent with a permeable membrane with a radius of 10 cm is installed, radioactive waste with a radiation intensity of 10 ¹⁰ Bq / kg, a moisture content of 20% by weight, and a specific gravity of 2 (specific gravity 2 is the standard for cement-solidified waste Physical properties) Even if 180 liters is enclosed in a 200 liter closed container (volume of a standard drum can) and stored, the hydrogen gas concentration is below the explosion limit, and it was confirmed that it functions effectively as a gas vent.

Claims (4)

A gas vent that is installed in a container for storing radioactive substances and allows gas generated by radiolysis of water to escape outside the container, and is a gas permeable membrane made of silicone rubber that is impermeable to liquids but permeable to gases; A hydrogen gas vent, comprising a stack of air-permeable porous supports that hold a hydrogen gas vent. 2. The hydrogen gas vent according to claim 1, wherein said permeable porous carrier is porous ceramics or porous metal, and said gas permeable membrane has a thickness of 0.1 to 5 mm. The hydrogen vent according to claim 1 or claim 2 holds the gas permeable membrane and the gas permeable porous support in a box having an air permeable port and an exhaust port, and the air permeable port is open to the inside of the container. A radioactive substance storage container, wherein the exhaust port is connected to the external exhaust port using a tube having a diameter smaller than the size of the box. The hydrogen vent according to claim 1 or claim 2 is characterized in that the gas-permeable membrane and the air-permeable porous carrier are installed in a cylindrical box so as to form part of the outer wall of the container. radioactive material storage container.

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