Neutron shielding multilayer structure

Abstract

Provided is a structure having excellent neutron shielding performance. When designing a neutron shielding structure, this structure is made into a multilayer structure (1) obtained by laminating: at least two first layers (2) comprising a resin or elastomer; and at least two second layers (3) formed from a resin or elastomer and an inorganic fiber bundle containing neutron adsorbing element. This multilayer structure (1) has superior neutron shielding performance compared to previous particle dispersion structures, even those structures having the same neutron adsorbing element content.

Description

Neutron shielding multilayer structure

[0001] The present invention relates to a multilayer structure having excellent neutron shielding properties.

[0002] Advances in nuclear and aerospace technologies have created a greater need for neutron-shielding materials than ever before. To protect humans from neutron radiation, there is a continuing need to improve the performance of neutron-shielding materials. Furthermore, the increasingly high integration of semiconductors used in electronic devices, essential for controlling nuclear facilities and space-related equipment, has led to an increase in "soft errors" in communication devices. Soft errors occur when neutrons from cosmic rays penetrate semiconductor elements, causing bit data to flip, resulting in malfunctions in electronic devices. Therefore, materials that can effectively shield against neutrons are needed to protect not only humans but also electronic devices. While concrete has long been known as a representative neutron-shielding material, it is not suitable for precision-machined components such as electronic device housings. As an alternative, composites of neutron-shielding substances and resins have been proposed. For example, Japanese Patent Laid-Open Publication No. 6-180388 (Patent Document 1) proposes a heat-resistant neutron shielding material in which a phenolic resin is blended with an inorganic boron compound, gadolinium oxide, or the like as a thermal neutron shielding substance. Furthermore, Japanese Patent Laid-Open Publication No. 2020-30088 (Patent Document 2) proposes a resin composition in which a curable resin is blended with boron carbide, boric acid, gadolinium, or a mixture thereof, each of which has a specific particle size range. The inventions described in Patent Documents 1 and 2 both involve adding a neutron-absorbing "powder" raw material to a resin material. Meanwhile, the present inventor has developed an inorganic fiber with excellent neutron shielding properties that contains a high content of gadolinium, etc. (WO2022 / 145401 A1, Patent Document 3).

[0003] Japanese Patent Application Laid-Open No. 6-180388 Japanese Patent Application Laid-Open No. 2020-30088 International Publication WO2022 / 145401 A1

[0004] The inventions of Patent Documents 1 to 3 all use rare earth elements such as gadolinium and samarium as neutron absorbing elements. Therefore, from the perspective of effectively utilizing scarce resources, it is desirable to have materials that maximize the neutron shielding function of rare earth elements while minimizing the amount of rare earth elements used. Therefore, the present inventors have conducted research aimed at creating a neutron shielding structure that maximizes neutron shielding performance while minimizing the amount of neutron absorbing elements used.

[0005] As a result, the present inventors discovered that in a composite material in which a matrix material such as a resin is filled with a material containing a neutron-absorbing element, the neutron-absorbing element-containing material is filled in the form of fiber bundles rather than particles, resulting in superior neutron shielding properties, and completed the present invention. That is, the present invention is a neutron-shielding multilayer structure in which at least one first layer made of a resin or elastomer and at least one second layer made of neutron-shielding inorganic fiber bundles and a resin or elastomer are laminated together. In the multilayer structure of the present invention, the first layer attenuates the energy of neutrons (slows down the velocity of neutrons), and the second layer absorbs the slowed-down neutrons.

[0006] The multilayer structure of the present invention has superior neutron shielding properties because the material containing the neutron absorbing element filled in the base material is in the form of inorganic fiber bundles rather than powder. Furthermore, because the filler material is inorganic fiber bundles, it is possible to effectively arrange and form neutron attenuating layers and neutron absorbing layers in the structure.

[0007] FIG. 3 is a diagram schematically showing the layer structure of a multilayer structure of the present invention. FIG. 4 is a diagram schematically showing a conventional neutron shielding structure in which the material containing a neutron absorbing element is a powder. FIG. 5 is a graph showing the results of a PHITS calculation. FIG. 6 is a partially enlarged view of the graph of FIG. 3. FIG. 7 is a schematic diagram of two examples of multilayer structure models used in the PHITS calculation. FIG. 8 is another graph showing the results of a PHITS calculation. FIG. 9 is a diagram showing a preferred embodiment of the multilayer structure of the present invention. FIG. 10 is a diagram showing another preferred embodiment of the multilayer structure of the present invention. FIG. 11 is a diagram showing yet another preferred embodiment of the multilayer structure of the present invention.

[0008] The multilayer structure of the present invention will be described below with reference to the appropriate drawings. In the present invention, neutron-absorbing inorganic fibers are used as the neutron-absorbing component, rather than the neutron-absorbing inorganic powder that has been conventionally used. The inorganic fibers can be bundled into various fiber bundles and combined with a resin or elastomer to form a composite material in which the fiber bundles are arranged in layers in a resin or elastomer matrix. The multilayer structure of the present invention is formed by making the most of the above-mentioned characteristics of the fiber bundles.

[0009] FIG. 1 shows a basic embodiment of the present invention. In the figure, a multilayer structure (1) is disposed between and protects an object to be protected (O) from neutron exposure. The multilayer structure (1) is composed of two layers (i.e., a first layer (2) and a second layer (3)). At least one of the first layer (2) and the second layer (3) each contributes to the construction of the multilayer structure (1). However, by combining two or more of the first layer (2) and the second layer (3), the ratio of neutron-shielding inorganic fiber bundles present in the multilayer structure (1) is adjusted in the direction from the neutron source (S) to the object to be protected (O). The first layer (2) is made of a resin (or elastomer). The second layer (3) is made of a neutron-shielding inorganic fiber bundle and a resin (or elastomer). Here, the resin (or elastomer) constituting the first layer (2) and the resin (or elastomer) constituting the second layer (3) do not need to be the same, but it is preferable to use the same resin (or elastomer) constituting the first layer (2) and the same resin (or elastomer) constituting the second layer (3), since this simplifies the manufacturing process of the multilayer structure (1) as the final product.

[0010] A resin (or elastomer) is used as a common constituent component of the first layer (2) and the second layer (3). The resin (or elastomer) involved in the formation of the second layer (3) may be referred to as a base material. The resin or elastomer used in the present invention will be described below. Depending on the molding method of the multilayer structure (1), either a thermosetting or thermoplastic resin (or elastomer) can be used. While a thermosetting resin is preferred from the viewpoint of emphasizing the heat resistance and strength of the multilayer structure (1), a thermoplastic resin can also be used in consideration of productivity. Furthermore, resins and elastomers with a large amount of hydrogen in their molecular skeletons are more preferred because they have an increased ability to moderate neutrons.

[0011] When the multilayer structure (1) is required to have a suitable degree of flexibility, an elastomer is used instead of a resin.

[0012] Examples of the curable resin include, but are not limited to, epoxy resin, unsaturated polyester resin, vinyl ester resin, bismaleimide resin, and phenolic resin. From the viewpoint of neutron shielding, among these, those containing a large amount of hydrogen in the skeleton are preferred. Examples of such curable resins include bisphenol A epoxy resin and hydrogenated bisphenol A epoxy resin. The above-mentioned thermosetting resin is finally converted into a thermoset product by the addition of a curing agent, and becomes a component of the first layer (1) and the second layer (3).

[0013] Examples of thermoplastic resins include, but are not limited to, polypropylene, polyethylene, polystyrene, ABS resin, polyphenylene ether, polycarbonate, polyamide, etc. Among these, polypropylene and polyethylene are more preferred because they are not only inexpensive but also contain a large amount of hydrogen in their molecular skeletons.

[0014] Both thermosetting elastomers and thermoplastic elastomers can be used. Examples of thermosetting elastomers (including their cured products) include ethylene propylene rubber (EPDM), nitrile rubber (NBR), acrylic rubber (ACM), urethane rubber (PUR), silicone rubber, ethylene vinyl acetate rubber (EVA), and epichlorohydrin rubber (ECO). All of these have excellent weather resistance, but ethylene propylene rubber (EPDM) and silicone rubber are more preferred because they contain a lot of hydrogen in their molecular skeletons.

[0015] Examples of thermoplastic elastomers include polystyrene thermoplastic rubber, polyolefin thermoplastic rubber, polyurethane thermoplastic rubber, polyester thermoplastic rubber, polyamide thermoplastic rubber, and polyvinyl chloride thermoplastic rubber.

[0016] Examples of polystyrene-based thermoplastic rubbers include styrene-ethylene-butylene-styrene block copolymer (SEBS) and styrene-ethylene-propylene-styrene block copolymer (SEPS). Of the above thermoplastic elastomers, ethylene-propylene rubber (EPDM), polyolefin-based thermoplastic rubber, styrene-ethylene-butylene-styrene block copolymer (SEBS), and styrene-ethylene-propylene-styrene block copolymer (SEPS) are more preferred because they have excellent weather resistance and contain a large amount of hydrogen in their molecular skeletons.

[0017] The second layer (3) constituting the multilayer structure (1) of the present invention is composed of the above-mentioned resin or elastomer and inorganic fiber bundles. In the present invention, inorganic fiber bundles refer to fiber cloth, roving, roving cloth, chopped strand mat, nonwoven fabric, filament mat, and other secondary processed products of inorganic fibers. These can be appropriately selected depending on the molding method.

[0018] In the present invention, the inorganic fibers that form the base of the inorganic fiber bundles contain a high content of any one of gadolinium, samarium, and cadmium, which are excellent in neutron absorption. More specifically, the inorganic fibers preferably contain 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and most preferably 40% by mass or more of an element selected from gadolinium, samarium, and cadmium, calculated as oxide. While two or more of gadolinium, samarium, and cadmium may be contained, gadolinium has overwhelmingly superior neutron absorption properties. Therefore, in order to increase the neutron absorption per unit mass of the inorganic fibers, the inorganic fibers preferably contain 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and most preferably 40% by mass or more of gadolinium, calculated as oxide. When producing the inorganic fibers, these three elements can be used as raw materials either as simple elements (metals) or as oxides. In addition to the above three elements as essential components, the inorganic fibers contain known oxides with excellent glass-forming properties, namely silicon oxide and aluminum oxide, as main components, and may contain calcium oxide as a secondary component.

[0019] The second layer (3) is composed of inorganic fiber bundles and a resin (or elastomer), and more specifically, the voids in the inorganic fiber bundles are filled with the same base material (resin or elastomer) that forms the first layer (2). The mass ratio of the inorganic fiber bundles to the total mass of the second layer (3) is 10 to 95 mass%. From the viewpoint of increasing the neutron absorption efficiency of the second layer (3), it is preferably 25 mass% or more, more preferably 50 mass% or more, and even more preferably 70 mass% or more. The mass ratio of the inorganic fiber bundles to the total mass of the second layer (3) largely depends on the volume porosity (void fraction) of the inorganic fiber bundles, but can also be adjusted by the specific molding method for combining the inorganic fiber bundles with the resin (or elastomer). Therefore, when increasing the mass ratio of inorganic fiber bundles to the total mass of the second layer (3), it can be adjusted by using inorganic fiber bundles with a small volume void ratio and, if necessary, increasing the pressure when combining the inorganic fiber bundles with a matrix (resin or elastomer) and molding them. With the above in mind, when preparing the second layer (3), if a cured resin or cured elastomer is used as the matrix, the second layer (3) can be obtained by impregnating the inorganic fiber bundles with a prepolymer of the resin or elastomer, followed by a pressurized and heated operation. Specific molding methods that can be suitably used include known methods such as autoclave molding, infusion molding, and hand lay-up molding. On the other hand, if a thermoplastic resin or thermoplastic elastomer is used as the matrix, the second layer (3) can be obtained, for example, by preparing a sheet made of the thermoplastic resin or thermoplastic elastomer in advance, layering the sheet on a fiber cloth, and molding it under heat and pressure. Calender molding is an example of heat and pressure molding.

[0020] In the present invention, since the resin or elastomer used to form the first layer (2) and the second layer (3) is the same, the first layer (2) and the second layer (3) can be produced in separate steps and then laminated to form the desired multilayer structure, but when the base material is a thermosetting type, the following molding method can be adopted to simplify the process: That is, by first stacking inorganic fiber bundles (e.g., fiber cloth) into multiple layers and then impregnating them with a base material prepolymer, and adjusting the pressure, heating, and time, it is possible to form, in a single step, a first layer (2) consisting of only the base material and a second layer (3) consisting of inorganic fiber bundles impregnated with the base material between the inorganic fiber bundles.

[0021] There are no particular restrictions on the ratio of the first layer (2) to the second layer (3) in the multilayer structure (1), but preferably, the ratio of the total thickness of the first layer (2) to the total thickness of the second layer (3) is in the range of 10 / 90 to 90 / 10.

[0022] The multilayer structure (1) of the present invention is configured by laminating at least one or more first layers (2) and at least one or more second layers (3). In a multilayer structure consisting of one first layer (2) and one second layer (3), the first layer (2) is naturally disposed on the neutron source side and the second layer (3) is disposed on the protected object side. However, simulations performed by the present inventors using a Monte Carlo particle calculation code unexpectedly showed that in a multilayer structure (1) having two or more first layers (2) and two or more second layers (3), if the first layer (2) is the outermost layer on the protected object side, the neutron shielding properties are deteriorated. Therefore, it is preferable that the layer closest to the protected object is the second layer (3). From this finding, it was found that in a multilayer structure (1) in which the layer closest to the object to be protected is the first layer (i.e., the neutron moderating layer), even if the first layer is thin, the neutron shielding properties of the multilayer structure (1) are impaired due to the presence of the first layer (2). Therefore, it is preferable that the layer closest to the object to be protected is the second layer (3).

[0023] The simulation results will be described below. While several Monte Carlo particle transport calculation codes are known and in use, the present inventors used a Monte Carlo particle transport calculation code known as PHITS (Particle and Heavy Ion Transport code System). PHITS is available on the Japan Atomic Energy Agency's website (https: / / phits.jaea.go.jp / indexj.html), and its usage instructions are publicly available. Figure 3 shows the energy spectrum of transmitted neutrons obtained using the above calculation code when a multilayer structure (1) is irradiated with neutrons of a predetermined energy spectrum. Here, the thermal reactor neutron spectrum was used as the energy spectrum of the incident neutrons. The thickness of the multilayer structure (1) was set to 30 mm, the first layer (2) was assumed to be a cured bisphenol A epoxy resin, and the second layer (3) was assumed to be an inorganic fiber cloth containing gadolinium. For the sake of convenience in the calculations, the second layer (3) was assumed to consist only of inorganic fiber cloth, and the volume void ratio of the inorganic fiber cloth was assumed to be 0.5. The assumptions for the multilayer structure model used in the following calculations are as follows: - Ratio of total thickness of first layer (2) / total thickness of second layer (3): 90 / 10 - Mass ratio of first layer (2): bisphenol A / hardener: 80 / 20 - Composition of second layer (3) (composition of inorganic fiber cloth): gadolinium oxide (Gd2O3) 10 mass%, silicon dioxide (SiO2) 48 mass%, aluminum oxide (Al2O3) 15 mass%, iron oxide (Fe2O3) 9 mass%, calcium oxide (CaO) 10 mass%, boron oxide (B2O3) 5 mass%, potassium oxide (K2O) 2 mass% - Number of first layers (2): 20 (each having the same thickness) - Number of second layers (3): 19 (each having the same thickness) As a comparative structure for the multilayer structure model, calculations were also carried out on a particle dispersion structure (Fig. 2) in which the same amount of gadolinium oxide as contained in the second layer of the multilayer structure model was dispersed in the same resin as that constituting the first layer of the multilayer structure model. The particle diameter was set to 0.50 mm. The calculation results are shown in Fig. 3. From Fig. 3, it can be seen that the neutron energy was 10-7 It is clear that the multilayer structure of the present invention is superior in neutron shielding to the conventional particle dispersion structure in the region of neutron energy of 10 MeV or less. -7 This is an enlarged view of Figure 3, focusing on the sub-MeV region. The above calculation results suggest the advantages of a layered distribution of neutron-absorbing elements within the base resin. In the above calculations, the first layer (2R) was the outermost layer on the protected object side (Figure 5(a)). However, when compared with a model in which the first layer was moved to the front row on the neutron source side (Figure 5(b)), the neutron shielding performance was unexpectedly further improved (Figure 6). Similar calculation results were also supported by the simplest model. Specifically, when a two-layer structure model consisting of one first layer (2) (neutron source side) and one second layer (3) (protected object side) was compared with a model in which an additional first layer of the same thickness was added and stacked behind the second layer, the neutron shielding performance of the latter multilayer structure was found to be worse. Therefore, in the multilayer structure (1) of the present invention, it is preferable that the outermost layer on the side of the object to be protected is formed of the second layer (3) (neutron absorbing layer).

[0024] In FRP molding, which combines fiber bundles and thermosetting resin, the surface of the molded product is usually covered with a layer consisting only of the base material, although the thickness varies. Therefore, when producing the multilayer structure (1) of the present invention, the above calculation results should be kept in mind, and care should be taken to ensure that the first layer (2) (a layer consisting only of resin or elastomer) is not formed as much as possible on the layer closest to the object to be protected (the outermost layer), or that if it is formed, its thickness is as thin as possible. From this perspective, when using a curable resin (or elastomer) to produce the multilayer structure (1) of the present invention, infusion molding and filament winding molding are more preferred molding methods than other thermosetting molding methods. In particular, infusion molding is a molding method in which a fiber bundle such as fiber cloth or nonwoven fabric is sandwiched between an upper mold film and a lower mold, and resin is sucked in by vacuum pressure to fill and impregnate the material while maintaining the airtightness of the film. This has the advantage that there is little seepage of resin (or elastomer) onto the surface of the molded product, and it is also easy to apply to molded products with uneven surfaces.

[0025] When the multilayer structure (1) of the present invention comprises two or more first layers (2), the first layers (2) may be configured such that the thickness of the layers increases in order from the layer (2R) closest to the object to be protected to the layer (2F) closest to the neutron source (FIG. 7).

[0026] Furthermore, when the multilayer structure (1) of the present invention includes two or more second layers (3), the following variations can be employed for these second layers (3). That is, for each of the multiple second layers (3), from the layer (3F) closest to the neutron source to the layer (3R) closest to the object to be protected, the following variations are possible: i) a configuration in which the thickness of each layer increases sequentially [thickness increase pattern of each layer] ( FIG. 8 ), ii) a configuration in which the content of inorganic fiber bundles per unit volume of each layer increases sequentially [pattern of increasing inorganic fiber bundle content (or decreasing volume porosity of inorganic fiber bundles) of each layer] ( FIG. 9 ), or iii) a configuration in which the content of neutron-absorbing elements (gadolinium, samarium, cadmium) in the inorganic fiber bundles constituting each layer increases sequentially [neutron-absorbing element concentration increase pattern of each layer] ( FIG. 10 ). These variations can be combined, such as a combination of variations i) and ii), a combination of variations ii) and iii), a combination of variations i) and iii), and all combinations of variations i), ii), and iii).

[0027] As a variation of the multilayer structure of the present invention, a multilayer structure may be formed by laminating two or more layers, essentially consisting of the second layer described above, i.e., layers composed of neutron-shielding inorganic fiber bundles and a resin (or elastomer). More specifically, examples of such multilayer structures include a multilayer structure formed by laminating two or more layers composed of a resin (or elastomer) and inorganic fiber bundles, in which, from the layer closest to the neutron source to the layer closest to the object to be protected, a) the inorganic fiber bundle content per unit volume of each layer increases sequentially, or b) the neutron-absorbing element (gadolinium, samarium, cadmium) content of the inorganic fiber bundles constituting each layer increases sequentially. For multilayer structures having the above configurations, known methods such as autoclave molding, infusion molding, and hand lay-up molding can be suitably employed.

[0028] The multilayer structure of the present invention has excellent neutron shielding properties. Furthermore, because it is composed of an inorganic fiber bundle filler material with a resin or elastomer matrix, it has a higher degree of freedom in shape than concrete-based materials. Therefore, it is suitable for use as equipment, devices, and components in the fields of atomic energy, aerospace, and medicine. Nuclear power equipment, devices, and components include: Nuclear power generation equipment, devices, and components for preventing critical reactions during debris (molten nuclear fuel) removal and storage, uranium ore mining and processing equipment, secondary processing of nuclear fuel (including conversion, enrichment, reconversion, fabrication, and MOX production), spent nuclear fuel storage, processing, and reprocessing equipment, and components for neutron-exposed waste storage, processing, and disposal, transport equipment and components for uranium ore, secondary nuclear fuel products, spent nuclear fuel, or neutron-exposed waste, and other nuclear-related equipment, devices, and components. More specific examples of nuclear power generation equipment include reactor buildings (including research and test reactors), reactor containment vessels, piping within nuclear reactor facilities, and decommissioning robots. Examples of facilities, equipment, and components in the aerospace field include: Space base buildings, space stations, artificial satellites, planetary exploration satellites, etc. Examples of facilities, equipment, and components in the medical field include: Medical devices that use particle beams. The above use examples are presented as examples to demonstrate the usefulness of the composite material of the present invention, and are not intended to limit the scope of the present invention.

[0029] REFERENCE SIGNS LIST 1 Multilayer structure 2 First layer 2F First layer closest to neutron source 2R First layer closest to object to be protected 3 Second layer 3F Second layer closest to neutron source 3R Second layer closest to object to be protected 4 Conventional neutron shielding structure 5 Dispersed particles 6 Resin S Neutron source O Object to be protected

Claims

1. A neutron-shielding multilayer structure that separates a neutron source from an object to be protected, said multilayer structure comprising at least one first layer made of a resin or elastomer and at least one second layer formed of neutron-absorbing inorganic fiber bundles and said resin or elastomer laminated together.

2. The multilayer structure according to claim 1, wherein the total mass of the one or more elements selected from gadolinium, samarium and cadmium in the inorganic fiber bundles is 10 mass % or more.

3. The multilayer structure according to claim 2, wherein the inorganic fiber bundles are any one of fiber cloth, roving, roving cloth, chopped strand mat, milled fiber, nonwoven fabric, and filament mat.

4. The multilayer structure according to claim 3, wherein the resin or elastomer of said first layer and the resin or elastomer of said second layer are the same.

5. The multi-layer structure according to claim 4, wherein the layer closest to the object to be protected is formed by the second layer.

6. The multi-layer structure of claim 4, wherein the layer closest to the neutron source is formed by the first layer.

7. The multilayer structure according to claim 5 or 6, wherein each of the first layer and the second layer is two or more.

8. The multi-layer structure of claim 7, wherein the thickness of each of the first layers increases in sequence from the side closest to the object to be protected to the side closest to the neutron source.

9. The multi-layer structure of claim 7, wherein the thickness of each of the second layers increases in sequence from the side closest to the neutron source to the side closest to the object to be protected.

10. A multilayer structure as claimed in claim 7, in which, for each layer of the second layer, in the order from the side closest to the neutron source to the side closest to the object to be protected, i) the inorganic fiber bundle content per unit volume of each layer increases, or ii) the volumetric porosity of the inorganic fiber bundles constituting each layer decreases, or iii) the content of neutron absorbing elements (gadolinium, samarium, cadmium) in the inorganic fiber bundles constituting each layer increases.

11. A neutron shielding multi-layer structure that separates a neutron source from an object to be protected, the multi-layer structure being a laminate of at least two layers formed of neutron absorbing inorganic fiber bundles and a resin or elastomer, the multi-layer structure being configured such that, from the side closest to the neutron source to the side closest to the object to be protected, i) the inorganic fiber bundle content per unit volume of each layer increases, or ii) the volume porosity of the inorganic fiber bundles constituting each layer decreases, or iii) the neutron absorbing element (gadolinium, samarium, cadmium) content in the inorganic fiber bundles constituting each layer increases.

12. The multilayer structure according to claim 10, wherein the resin is a cured product of a bisphenol A type epoxy resin or a hydrogenated bisphenol A type epoxy resin.

13. The multilayer structure according to claim 11, wherein the resin is a cured product of a bisphenol A type epoxy resin or a hydrogenated bisphenol A type epoxy resin.

14. A method for producing the multilayer structure of claim 10 by infusion molding.

15. A method for producing the multi-layer structure of claim 11 by infusion molding.

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