JP6253930B2 - Radiation shielding material, radiation waste storage container, and method for manufacturing radiation waste storage container - Google Patents

Description

  The present invention relates to a radiation shielding material comprising a specific laminate, a radiation waste storage container provided with the radiation shielding material, and a method for producing the radiation waste storage container.

  In the Great East Japan Earthquake that occurred in 2011, a nuclear power plant accident occurred, and radioactive materials were scattered over a wide area inside and outside the power plant. After the accident, the removal of waste containing radioactive materials and the decontamination of radioactive materials targeting soil, houses, etc. have been started, and a large amount of radioactive waste materials have been recovered. The collected radioactive waste is packed in simple containers such as bags and boxes and collected at a point away from the living area. The amount of such radioactive waste will continue to increase as the nuclear power plant is repaired and decontaminated. The accumulated radioactive waste must be stored for a long time in a sealed state in a container that blocks the radiation until the radiation dose of the radioactive waste is below the reference value. In the future, it will be necessary to seal a large amount of radioactive waste in a suitable container and store it in a large-scale long-term storage facility. In this work, first, a radioactive waste storage container that has both high radiation shielding properties and durability to withstand weather and disasters during storage is required. A storage facility that fills a large number of storage containers as much as possible is required. In addition, the enormous cost required for the treatment of radioactive waste is an important matter affecting the national finance. Therefore, there is a need for a facility and method for safely storing long-term radioactive waste at the lowest possible cost.

JP 2013-145119 A JP 2010-8224 A JP 2000-304893 A

  Patent Document 1 describes a storage facility for bags containing radioactive waste. The storage facility described in Patent Document 1 is intended to safely store a bag containing radiation waste by devising the structure of the base, each layer, and the outline of the storage facility. This storage facility is suitable for temporary storage until a bag containing radiation waste is transferred to a long-term storage facility. However, there is concern about using this for long-term storage until radiation waste is rendered harmless. Considering that storage facilities may be exposed to harsh environments such as frequent temperature fluctuations and earthquake disasters during long storage periods, the bags containing radiation waste and the structures covering them are sufficiently robust. Nonetheless, the radiation shielding of the storage facility is never guaranteed.

  Patent Document 2 describes a cylindrical radiation shielding container having excellent durability and low manufacturing cost. However, this radiation shielding container is intended for temporary storage of radioactive waste generated when a nuclear power plant is operating normally. In a long-term storage facility for radiation waste collected through decontamination work, which is currently required, it is necessary to store tens of thousands of containers containing radiation waste. A radiation shielding container using stainless steel or various connecting parts as described in Patent Document 2 has a high manufacturing cost and is not realistic as a material used for a large amount of radiation waste. Moreover, when the cylindrical containers are arranged, a gap is formed, and the amount of radiation waste that can be accommodated in one storage facility is reduced. As a result, there is a problem that the storage cost increases.

  Patent Document 3 also describes a radiation shielding concrete container. However, the radiation shielding concrete container described in Patent Document 3 is also intended to store a relatively small amount of radiation waste generated during normal operation of a nuclear power plant. There's a problem. Moreover, the radiation shielding concrete container described in Patent Document 3 is also typically cylindrical, and is not suitable for storing and storing a large amount of radiation waste.

  If the cylindrical radioactive waste storage container described in Patent Document 2 and Patent Document 3 is changed to a square or a rectangular parallelepiped, there is no gap when the containers are arranged and it is easy to be efficient. Can understand. A rectangular container is suitable for containing large amounts of radioactive waste. However, in a rectangular container, the dispersibility of the impact force from the outside is poor and distortion is likely to occur. For this reason, the risk of breakage of the container is high, and there is a concern that the radiation shielding property is lowered. On the other hand, the amount of radioactive waste materials that are newly collected is increasing day by day, and it is necessary to move the radioactive waste that is temporarily stored to a safe long-term storage facility as soon as possible. Producing a rectangular container with excellent durability and radiation shielding properties at low cost is an urgent issue.

  Reinforced concrete and hard resin are suitable for materials for long-term storage containers because they have high strength and can be freely changed in size and shape. Moreover, these are low in manufacturing cost compared with metal containers such as stainless steel, and are suitable for storing a large amount of radiation waste. However, when a container made of reinforced concrete or a container made of hard resin is manufactured with realistic wall thickness and sealed with radioactive waste, the radiation shielding degree of the container is only about 30% and the radiation should be shielded sufficiently. I can't.

  The inventor has intensively studied for a container for radiation waste that can be manufactured at low cost and has excellent durability and radiation shielding properties. As a result, it was found that the specific laminate is effective as a radiation shielding material for various containers.

That is, the first present invention is as follows.

A radioactive waste storage container characterized in that each of the main body and the lid of the reinforced concrete container is covered with a radiation shielding material,
A radiation waste storage container, wherein the radiation shielding material comprises a laminate comprising the following layers (1), (2), and (3).
(1) Lead plate layer
(2) Layer containing thermosetting resin and lead compound
(3) Layer made of fiber reinforced plastic (FRP)
(However, the stacking order of the layers (1), (2), and (3) is not limited. Any one or more of the layers (1), (2), and (3) may be used.) )

The second aspect of the present invention is as follows.

  The radioactive waste storage container according to invention 1, wherein each of the main body and the lid of the reinforced concrete container is covered with the radiation shielding material via an adhesive layer (4).

The third aspect of the present invention is as follows.
A method for manufacturing a container for storage of radioactive waste, comprising a step of covering each of a main body and a lid of a reinforced concrete container with a radiation shielding material comprising a laminate comprising the following layers (1), (2), and (3): .
(1) Lead plate layer
(2) A layer containing a thermosetting resin and a lead compound.
(3) A layer made of fiber reinforced plastic (FRP).
(However, the stacking order of the layers (1), (2), and (3) is not limited. Any one or more of the layers (1), (2), and (3) may be used.) )

  The radiation shielding material of the present invention has excellent radiation shielding properties, is relatively lightweight, has a low production cost, and is excellent in durability. As a result, the container provided with the radiation shielding material of the present invention exhibits high radiation shielding properties and is excellent in durability.

The schematic which looked at the container main body manufactured in Example 1 before radiation shielding material coating | cover from the top. 1 is a schematic cross-sectional view of a container body manufactured in Example 1 before coating with a radiation shielding material. BRIEF DESCRIPTION OF THE DRAWINGS The schematic which looked at the container lid before the radiation shielding material coating manufactured in Example 1 from the top. 1 is a schematic cross-sectional view of a container lid manufactured in Example 1 before coating with a radiation shielding material. 1 is a schematic view of an upper end portion of a radiation waste storage container main body manufactured in Example 1. FIG. 1 is a schematic diagram of an end portion of a radioactive waste storage container lid manufactured in Example 1. FIG. 1 is a schematic diagram of a radioactive waste storage container manufactured in Example 1. FIG. 1 schematically illustrates a method for measuring radiation shielding properties of a ray waste storage container manufactured in Example 1. Schematic of the radioactive waste storage container manufactured in Reference Example 2. The schematic diagram which looked at the radiation waste storage container main body before the radiation shielding material coating | covering manufactured by the reference example 2 from the top. The figure which showed typically a mode that the state which arranged the radioactive waste storage container manufactured in the reference example 2 was seen from the top.

[Lead plate layer (1)]
The radiation shielding material of the present invention has a lead plate layer (1). The lead plate layer (1) functions as a radiation shielding layer. A thin lead plate is used as the lead plate layer (1). The thickness of the lead plate layer is 0.5-4 mm and can be adjusted according to the level of radiation to be shielded. Since wastes having various levels of radiation dose are mixed in decontamination waste during temporary storage, it is preferable to set the thickness of the lead plate layer so that even a relatively strong radiation equivalent can be shielded. Therefore, the thickness of the lead plate layer is preferably 1 to 3 mm.

[Layer containing thermosetting resin and lead compound (2)]
The radiation shielding material of the present invention has a layer (2) containing a thermosetting resin and a lead compound. The layer (2) is a layer obtained by curing a thermosetting resin composition containing a thermosetting resin, a curing agent, and a lead compound. Since the layer (2) contains a lead compound, the layer (2) also functions as a radiation shielding layer.
When the radiation waste storage container is covered with the radiation shielding material of the present invention, the layer (2) also has a function of improving the impact resistance and sealing performance of the radiation waste storage container.

  The thermosetting resin is a resin that becomes a base material of fiber reinforced plastic (FRP), and unsaturated polyester, epoxy resin, polyamide resin, and phenol resin are used. In the present invention, it is preferable to use unsaturated polyester from the viewpoint of cost and strength. As a hardening | curing agent, what is suitable for each thermosetting resin can be used. The compounding quantity of a hardening | curing agent is suitably determined according to a usual method. In the present invention, when an unsaturated polyester is used as the thermosetting resin, various organic peroxides are used as a curing agent. In order to adjust the curing time, a curing accelerator can be blended. The layer (2) has a thickness of 0.5 to 3 cm, preferably 1 to 2 cm.

A lead compound is mixed in the thermosetting resin. A lead compound is preferable because it is excellent in dispersibility. As such a lead compound, a lead oxide that is easy to handle is preferably used. Examples of the lead oxide include lead monoxide (PbO), lead dioxide (PbO ₂ ), trilead tetroxide (Pb (II) ₂ Pb (IV) O ₄ ), and dilead trioxide (Pb (II) Pb (IV) ) O ₃ ) and mixtures thereof are used. The compounding quantity of a lead compound can be changed suitably. When there is much compounding quantity of a lead compound, the radiation shielding property of a layer (2) will become high. When the lead oxide is used, the thermosetting resin and the lead compound are mixed at a ratio of 5: 1 to 1: 5 (weight ratio), preferably 3: 1 to 1: 3 (weight ratio). Thus, a sufficient radiation shielding property can be obtained for radiation waste temporarily stored without impairing the performance of the thermosetting resin.

[Layer made of fiber reinforced plastic (FRP) (3)]
The radiation shielding material of the present invention also has a layer (3) made of fiber reinforced plastic (FRP). The layer (3) improves the durability of the radiation shielding material of the present invention. As the FRP, a general one, that is, one obtained by impregnating a thermosetting resin into a high-strength fiber and curing it is used. The same thermosetting resin as that used in the layer (2) is used. In the present invention, it is preferable to use an unsaturated polyester from the viewpoint of cost and strength. The same curing agent as that used in the layer (2) is used. If necessary, a curing accelerator can be added to the thermosetting resin. The same curing accelerator as that used in the layer (2) is used. The high-strength fiber may be a general fiber for FRP, and specifically, glass fiber, carbon fiber, or resin fiber is used. In the present invention, it is preferable to use glass fibers in view of balance of handleability, cost, and strength, and a glass chopped strand mat is particularly preferable. The layer (3) has a thickness of 0.5 to 3 cm, preferably 1 to 2 cm.

[Laminate]
The radiation shielding material of the present invention comprises a laminate in which at least the above layer (1), layer (2), and layer (3) are laminated. The stacking order of the layers (1), (2), and (3) is not limited. Any one or more of the layers (1), (2), and (3) can be used. When the number of layers (1) and (2) containing lead is increased, radiation shielding properties are improved. In the present invention, layers (1), (2), and (3) are laminated one by one. But sufficient radiation shielding can be obtained. Further, if the number of layers (2) and (3) containing thermosetting resin as a main material is increased, the durability of the reinforced concrete member is improved. In the present invention, the layers (1), (2), (3 ) Sufficient durability can be obtained even when each layer is laminated. From the viewpoint of adhesion and cost of each layer, a laminate in which the above layer (1), layer (2), and layer (3) are laminated in this order is preferable. On the surface of the laminate on which at least the above layer (1), layer (2), and layer (3) are laminated, an outermost layer such as a coating layer made of various paints and / or a protective layer made of a transparent material is further provided. be able to.

[Radiation waste storage container and its manufacturing method]
The radiation shielding material of the present invention can be used as a coating material for a radiation waste storage container. By bringing the layer (1), the layer (2), and the layer (3) into close contact with a predetermined surface of the container, which is a substrate, a container coated with the radiation shielding material of the present invention is obtained. By cutting the lead plate into a predetermined size in advance, the layer (1) can be adhered to the surfaces of various sizes. The layer (1) can be adhered to a container made of any material such as metal, plastic, ceramic and concrete by using an adhesive. Moreover, a layer (2) and a layer (3) can be closely_contact | adhered by apply | coating the material of the layer (2) and (3) containing the heat | fever constituent resin before hardening with a roller or a brush. Thus, the radiation shielding material comprising a laminate in which at least the layer (1), the layer (2), and the layer (3) of the present invention are laminated adheres to both a curved surface and a flat surface. Therefore, the radiation shielding material of the present invention can be used for radiation waste storage containers having various shapes and materials. From the viewpoint of storage efficiency of radioactive waste, the capacity of the radioactive waste storage container is 0.5-5m ³ , its shape is a rectangular parallelepiped or hexagonal column that can be loaded without any gaps, and the material is mainly cheap concrete Those are preferred. Moreover, the radiation shielding material of this invention can be attached to a part with a weak radiation shielding property among containers, and the radiation shielding property of the whole container can also be improved. Generally, the radiation shielding material of the present invention is attached to each of the lid and the main body of the container to shield the radiation emitted from the container contents in all directions.

  When the layer (1), the layer (2), and the layer (3) are laminated in this order on the container serving as the base, an adhesive is applied on the base in advance, or the surface of the layer (1) that contacts the base It is preferable to apply an adhesive in advance. Adhesive is appropriately selected according to the material of the substrate. Generally, it is suitable for bonding of synthetic rubber, leather, metal plate, cloth, wood, ceramics, plastic, etc. to wood, slate, PC board, concrete. Adhesives for construction and construction are used. Among them, the rubber-based adhesive is preferable because the adhesive itself after drying has elasticity and has a function of improving the durability of the radiation waste storage container of the present invention.

  In terms of cost, handleability, and durability, a container provided with the radiation shielding material of the present invention on each of a reinforced concrete lid and a main body is preferable as the radiation waste storage container of the present invention. Reinforced concrete is obtained by reinforcing a cured product of a cement composition containing cement, aggregate, and water with reinforcing bars. The cement to be blended with the above cement composition may be a known one. Specifically, ordinary Portland cement, early-strength Portland cement, ultra-early strength Portland cement, medium heat Portland cement, low heat Portland cement, acid salt salt Portland cement, etc. Portland cement, blast furnace cement, fly ash cement, mixed cement such as silica cement, other cement such as eco-cement, and special cement such as white Portland cement. Aggregates used in the cement composition may be known ones, specifically, natural aggregates such as river sand, river gravel, sea sand, sea gravel, mountain sand, mountain gravel, half sand such as crushed sand and intention to crushed. Lightweight aggregates such as artificial aggregate, blast furnace slag aggregate and artificial lightweight aggregate. The cement composition may further contain a known cement additive. Examples of the additive include a cement dispersant, a water reducing agent, a quick hardener, a slow hardener, a low specific gravity agent, and a high specific gravity agent. Additives are appropriately blended according to the construction period and cost. Both the reinforced concrete lid and the main body are manufactured by a general manufacturing method of cast reinforced concrete. That is, components such as cement, aggregate, and water are kneaded to produce a cement composition containing cement, aggregate, and water, and this cement composition is poured into a mold in which reinforcing bars are fixed in advance. Next, the cement composition is cured in the mold. Finally, the hardened reinforced concrete structure is punched out of the mold, and the reinforced concrete container of the present invention is completed. The lid and the main body can be manufactured using different molds. The final thickness of the reinforced concrete container is preferably 10 to 30 cm for both the lid and the main body. The range of 12-20 cm is particularly preferable.

Further, the main body and the lid of the container may be made of hard resin selected such as phenol resin and melamine resin . A hard resin such as a phenol resin or a melamine resin is preferable because it is lighter than concrete and metal and has excellent durability.

In addition, since elastic materials such as rubber and thermoplastic elastomer are durable against impact and vibration, they are useful as materials for radioactive waste storage containers. However, since these are inferior to reinforced concrete, hard resin, and metal in strength when the container is loaded, they are used in combination with reinforced concrete, hard resin, metal, and the like .

By appropriately selecting the adhesive, any surface of any container such as concrete, hard resin, rubber, metal, etc. can be coated with the radiation shielding material of the present invention.

Example 1
The main body of the container made of reinforced concrete before being coated with the radiation shielding material was manufactured. That is, the materials shown in Table 1 were kneaded to prepare a cement composition. The cement composition satisfies the standards of density, setting property, stability, compressive strength, heat of hydration, and chemical composition defined in JIS R 5201, JIS R 5202, JIS R 5203, and JIS R 5204.

  A concrete mold with a wire mesh fixed by an embedded metal, this cement composition was poured and cured, and the cured product was punched out. The main body after punching has a box shape of 95 cm long × 105 cm wide × 108 cm high. The thickness of each surface is 15 cm. The schematic shape of the main body is shown in FIGS.

  Using the materials shown in Table 1, a container lid made of reinforced concrete before being coated with the radiation shielding material was manufactured in the same procedure as the above-mentioned main body. The lid has a plate shape having a projection of 95 cm in length and 105 cm in width, and the thickness is 15 cm at the thin part and 20 cm at the thick part. The schematic shape of the lid is shown in FIGS.

  The main body was coated with a lead plate layer (1) made of the material shown in Table 2, a layer (2) containing thermosetting resin and lead, and a layer (3) made of fiber reinforced plastic (FRP) in this order. .

  That is, an adhesive ("Bond G17Z" manufactured by Konishi Co., Ltd.) is applied to the outer side surface, outer bottom surface, and upper end surface of the main body with a roller, and the lead plate is cut into the shape of each surface as a layer (1). Adhered to the coated surface. Next, what knead | mixed the thermosetting resin, the hardening | curing agent, and PbO powder as a layer (2) was apply | coated to the surface of the lead plate with the roller. Immediately after coating, a glass chopped strand mat having a size of 450 g / m 2 was pressed and adhered on the coated surface in order to form the layer (3). And what knead | mixed the thermosetting resin and the hardening | curing agent was apply | coated to the glass chopped strand mat with the roller, and the kneaded material was impregnated into the glass chopped strand mat. Thereafter, it was allowed to stand until the thermosetting resin was cured. FIG. 5 shows an outline of the upper end portion of the container main body covered with the laminate composed of the layers (1), (2), and (3).

  Cover the lid with a lead plate layer (1) made of the materials shown in Table 2, a layer (2) containing a thermosetting resin and a lead compound, and a layer (3) made of fiber reinforced plastic (FRP) in this order. did. That is, an adhesive (“Bond G17Z” manufactured by Konishi Co., Ltd.) was applied to the lower surface of the lid with a roller, and the lead plate cut into the shape of each surface as a layer (1) was adhered to the application surface. Next, what knead | mixed thermosetting resin, the hardening | curing agent, and the lead compound as a layer (2) was apply | coated to the surface of the lead plate with the roller. Immediately after coating, a glass chopped strand mat was pressed and adhered on the coated surface to form a layer (3). And what knead | mixed the thermosetting resin and the hardening | curing agent was apply | coated to the glass chopped strand mat, and the kneaded material was impregnated to the glass chopped strand mat. It was applied with a roller to form a layer in which a glass chopped strand mat was impregnated with a thermosetting resin. Thereafter, it was allowed to stand until the thermosetting resin was cured. FIG. 6 shows an outline of the end of the lid covered with the laminate composed of the layers (1), (2), and (3).

  As described above, a reinforced concrete box-type container including the radiation shielding material composed of the layers (1), (2), and (3) was completed. FIG. 7 shows a side sectional view when the radioactive waste is accommodated in this container. As shown in FIG. 7, when radioactive waste is stored in the container and the lid is loaded on the main body, the heavyweight lid made of reinforced concrete adheres to the main body without a gap through a relatively flexible radiation shielding material. ,Stabilize. And the whole circumference | surroundings of waste can be shielded with the said radiation shielding material.

  Separately, when the nominal strength of the cured product of the cement composition used in the container was measured, it showed a sufficient strength of 24. Therefore, it can be said that this container has a strength suitable for a long-term storage container for radiation waste.

  The radiation shielding property of the obtained radioactive waste storage container was measured. As shown in FIG. 8, the radiation source (137Cs 3.7MBq) was fixed inside the container, and the container was sealed. Dose was measured at six points outside the container. The six measurement points are the center of each of the top surface (upper part) of the lid, the bottom surface (lower part) of the main body, and the four side surfaces (side surfaces 1, 2, 3, 4) of the main body, and a point 1 cm from each surface. Table 3 shows the measurement results of the dose at each measurement point. In Table 3, the reduction rate was calculated by subtracting the background (BG, 0.07 μSv / h) from the measurement results.

  From the results of Table 3, it can be seen that the radiation waste storage container of the present invention exhibits a high radiation shielding rate of 80 to 90%.

( Reference Example 2)
Using a member made of phenol resin and a member made of synthetic rubber, a hexagonal columnar container having a width of 130 cm, a height of 150 cm, a lid thickness of 15 cm at the thin part, and a thickness of 20 cm was produced. A summary of the overall shape is shown in Table 10. FIG. 11 is a schematic view of the main body before coating with the radiation shielding material as seen from above. Both the lid and the main body are formed of a plate material in which a rubber plate is covered with a phenol resin layer. The outer peripheral surface of the main body of the container and the lower surface of the lid were covered with a radiation shielding material composed of the adhesive layers (1), (2), and (3) using the same material as that used in Example 1. . When the radiation shielding rate of this container was measured in the same manner as in Example 1, it showed a high radiation shielding rate of 80 to 90% on any of the upper, lower, and six side surfaces. Since such a hexagonal column-shaped radioactive waste storage container can be loaded with a honeycomb structure as shown in FIG. 11, the filling efficiency is high and it is robust against external impacts.

The radiation shielding material of the present invention has high radiation shielding properties and can coat various substrates. Therefore, radiation waste storage containers of various shapes and materials can be manufactured using the radiation shielding material of the present invention, and these radiation waste storage containers have high radiation shielding properties. For example, even if the container itself is made of reinforced concrete, metal, hard resin, etc. and has insufficient radiation shielding property, it is suitable for long-term storage of radiation waste by covering the container with the radiation coating material of the present invention. Radiation shielding is obtained. For this reason, if the radiation shielding material of the present invention is used, a high-performance radiation waste storage container can be manufactured by diverting and applying existing containers, containers, water tanks, and tanks without manufacturing special containers. Can do. Such a radiation shielding material of the present invention greatly contributes to the long-term storage of a large amount of radiation waste, which is an urgent issue at present. Furthermore, since the radiation shielding material of the present invention is a laminate that adheres to surfaces of various shapes, it is a building material for radiation waste storage facilities, a member for preventing exposure of workers of nuclear power plants, a radiation waste storage facility, It can also be used as a repair material for radiation storage containers.

DESCRIPTION OF SYMBOLS 1 Concrete side surface 2 Reinforcing metal fixture 3 Reinforcing bar 4 Outer surface 5 Inner surface 6 Upper end part of main body before covering with radiation shielding material Bottom end part of main body before covering with radiation shielding material 8 (It is not actually visible at the boundary between the thin part and the thin part.)
9 Thin lid part (The dotted line is not actually visible at the boundary between thick part and thin part.)
DESCRIPTION OF SYMBOLS 10 Lid upper part 11 Lid and main part fitting part 12 Lid lower part 13 Main body upper end part 14 after radiation shielding material coating | cover Radiation shielding material 15 Lead board layer (1)
16 Layer containing thermosetting resin and lead compound (2)
17 Layer made of fiber reinforced plastic (FRP) (3)
18 Radiation waste storage container lid 19 Radiation waste storage container body 20 Radiation waste 21 Reinforced concrete 22 Radiation source 23 Measuring instrument 24 Phenolic resin member 25 Synthetic rubber member 26 Radiation waste storage container Actually not visible at the border between thick and thin parts)

Claims (3)

A radioactive waste storage container characterized in that each of the main body and the lid of the reinforced concrete container is covered with a radiation shielding material,
A radiation waste storage container, wherein the radiation shielding material comprises a laminate comprising the following layers (1), (2), and (3).
(1) Lead plate layer
(2) Layer containing thermosetting resin and lead compound
(3) Layer made of fiber reinforced plastic (FRP)
(However, the stacking order of the layers (1), (2), and (3) is not limited. Any one or more of the layers (1), (2), and (3) may be used.) ) The radioactive waste storage container according to claim 1, wherein each of the main body and the lid of the reinforced concrete container is covered with the radiation shielding material via an adhesive layer (4). A method for manufacturing a container for storage of radioactive waste, comprising a step of covering each of a main body and a lid of a reinforced concrete container with a radiation shielding material comprising a laminate comprising the following layers (1), (2), and (3): .
(1) Lead plate layer
(2) A layer containing a thermosetting resin and a lead compound.
(3) A layer made of fiber reinforced plastic (FRP).
(However, the stacking order of the layers (1), (2), and (3) is not limited. Any one or more of the layers (1), (2), and (3) may be used.) )

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