Neutron-shielding multilayer structure

Abstract

Provided is a structure having excellent neutron shielding properties. When designing a neutron-shielding structure, the structure is made into a multilayer structure (1) in which at least two or more first layers (2) comprising a resin or an elastomer and at least two or more second layers (3) comprising a resin or an elastomer and inorganic fiber bundles containing a neutron-absorbing element are laminated. The neutron shielding properties of the multilayer structure (1) are superior to those of conventional particle dispersed structures even with the same neutron absorbing element content.

Description

Technical Field

[0001] This invention relates to a multilayer structure with excellent neutron shielding properties. Background Technology

[0002] With the advancement of nuclear energy and aerospace technologies, the necessity of neutron shielding materials has increased compared to the past.

[0003] There is a growing demand for improved performance of neutron-shielding materials to protect humans from neutron radiation. Furthermore, when focusing on the electronic devices necessary for controlling nuclear facilities or space-related machinery, the high integration of semiconductors in these devices has led to a gradual increase in "soft errors" in communication equipment. Soft errors refer to the malfunctions of electronic devices caused by neutrons from space rays interfering with semiconductor elements and resulting in bit data reversal. Therefore, materials capable of effectively shielding against neutrons are needed to protect not only humans but also electronic devices.

[0004] Concrete has been known as a representative neutron shielding material, but it is not suitable for precision-machined parts such as the casings of electronic devices. As an alternative, a material combining neutron shielding material and resin has been proposed.

[0005] For example, Japanese Patent Application Publication No. 6-180388 (Patent Document 1) discloses a heat-resistant neutron shielding material made by incorporating inorganic boron compounds, gadolinium oxide, etc., into a phenolic resin as a thermal neutron shielding material. Furthermore, Japanese Patent Application Publication No. 2020-30088 (Patent Document 2) discloses a resin composition made by incorporating boron carbide, boric acid, gadolinium, or mixtures thereof within a specific particle size range into a curable resin. Both inventions described in Patent Documents 1 and 2 involve adding neutron-absorbing "powder" raw materials to the resin material.

[0006] On the other hand, the inventors have developed an inorganic fiber with excellent neutron shielding properties containing a high content of gadolinium, etc. (WO2022 / 145401 A1, Patent Document 3).

[0007] [Previous Technical Documents] [Patent Literature] [Patent Document 1] Japanese Patent Application Publication No. 6-180388 [Patent Document 2] Japanese Patent Application Publication No. 2020-30088 [Patent Document 3] International Publication WO2022 / 145401 A1 Summary of the Invention [The problem that the invention aims to solve] The inventions in Patent Documents 1-3 all use rare earth elements such as gadolinium and samarium as neutron-absorbing elements. Therefore, from the perspective of effectively utilizing rare resources, the ideal material is one that minimizes the use of rare earth elements and maximizes their neutron-shielding function.

[0008] Therefore, the inventors conducted research on the subject of creating a neutron shielding structure that suppresses the amount of neutron-absorbing elements used and maximizes neutron shielding performance.

[0009] [Technical means to solve the problem] As a result, it was found that in composite materials in which a material containing a neutron-absorbing element is filled into a base material such as resin, the neutron-absorbing element has excellent neutron shielding properties when it is filled in the form of fiber bundles rather than particles, thus completing the present invention.

[0010] That is, the present invention is a neutron-shielding multilayer structure, which is a multilayer structure formed by laminating at least one first layer made of resin or elastomer and at least one second layer formed of neutron-shielding inorganic fiber bundles and resin or elastomer. In the multilayer structure of the present invention, the first layer plays the role of attenuating the energy of neutrons (decelerating the velocity of neutrons), and the second layer plays the role of absorbing the decelerated neutrons.

[0011] [The effects of the invention] In the multilayer structure of the present invention, the material containing neutron-absorbing elements filling the parent material is in the form of inorganic fiber bundles rather than powder, thus providing superior neutron shielding. Furthermore, since the filling material is inorganic fiber bundles, layers that attenuate neutrons and layers that absorb neutrons can be effectively configured and formed within the structure. Attached Figure Description

[0012] Figure 1 This is a schematic diagram illustrating the layer structure of the multilayer structure of the present invention.

[0013] Figure 2 This is a schematic diagram illustrating a conventional neutron shielding structure made of powder containing neutron-absorbing elements.

[0014] Figure 3 This is a graph representing the results of the PHITS calculation.

[0015] Figure 4 yes Figure 3 A magnified view of a portion of the curve graph.

[0016] Figure 5 These are schematic diagrams of two examples of multi-layer structure models used in PHITS calculations.

[0017] Figure 6 This is another graph representing the results of the PHITS calculation.

[0018] Figure 7 This is a diagram illustrating a preferred configuration of the multilayer structure of the present invention.

[0019] Figure 8 This is a diagram illustrating another preferred embodiment of the multilayer structure of the present invention.

[0020] Figure 9 This is a diagram illustrating other preferred configurations of the multilayer structure of the present invention.

[0021] Figure 10 This is a diagram illustrating another preferred embodiment of the multilayer structure of the present invention. Detailed Implementation

[0022] The multilayer structure of the present invention will be described below with appropriate use of figures.

[0023] In this invention, neutron-absorbing inorganic fibers are used as the neutron-absorbing component, instead of the neutron-absorbing inorganic powders used previously. By forming fiber bundles of various shapes that bind them together, and combining them with resins or elastomers, composite materials can be formed in which the fiber bundles are arranged in layers within a resin or elastomer matrix. The multilayer structure of this invention is formed to maximize the aforementioned advantages of the fiber bundles.

[0024] Figure 1 The basic form of the present invention is shown. In the figure, a multilayer structure (1) is arranged in the middle to prevent the protected object (O) from being irradiated by neutrons.

[0025] The multilayer structure (1) is composed of two layers (i.e., the first layer (2) and the second layer (3)). The first layer (2) and the second layer (3) each participate in the construction of the multilayer structure (1) by combining the first layer (2) and the second layer (3) into two or more layers respectively. The ratio of the inorganic fiber bundles with neutron shielding properties present in the multilayer structure (1) increases from the neutron source (S) toward the protected object (O).

[0026] The first layer (2) is composed of resin (or elastomer). The second layer (3) is composed of neutron-shielding inorganic fiber bundles and resin (or elastomer).

[0027] Here, the resin (or elastomer) constituting the first layer (2) does not need to be the same as the resin (or elastomer) constituting the second layer (3). However, by making the resin (or elastomer) constituting the first layer (2) and the resin (or elastomer) constituting the second layer (3) the same, the manufacturing process of the multilayer structure (1) as the final product can be simplified, and therefore it is preferred.

[0028] Resin (or elastomer) is used as a common component of the first layer (2) and the second layer (3) described above. In addition, the term "base material" is sometimes used for the resin (or elastomer) involved in the formation of the second layer (3).

[0029] The resin or elastomer used in this invention will be described below.

[0030] According to the molding method of the multilayer structure (1), the resin (or elastomer) can be either thermosetting or thermoplastic. From the viewpoint of valuing the heat resistance or strength of the multilayer structure (1), the thermosetting type is preferred, but considering the productivity, the thermoplastic type can also be used. In addition, resins and elastomers containing a large amount of hydrogen in the molecular backbone are more preferred because they increase the performance of neutron slowing.

[0031] In cases where moderate flexibility is required in multilayer structures (1), elastomers are used instead of resins.

[0032] Examples of curable resins include epoxy resins, unsaturated polyester resins, vinyl ester resins, bismaleimide resins, and phenolic resins, but are not limited to these. From the viewpoint of neutron shielding, among these, those containing a large amount of hydrogen in the framework are preferred. Examples of such curable resins include bisphenol A type epoxy resins, and further examples include hydrogenated bisphenol A type epoxy resins. The above-mentioned thermosetting resins are ultimately thermosetting by using a curing agent, becoming components of the first layer (1) and the second layer (3).

[0033] Examples of thermoplastic resins include polypropylene, polyethylene, polystyrene, ABS resin, polyphenylene ether, polycarbonate, and polyamide, but these are not the only options. Among these, polypropylene and polyethylene are preferred because they are not only inexpensive but also contain a large amount of hydrogen in their molecular backbone.

[0034] As an elastomer, both thermosetting elastomers and thermoplastic elastomers can be used.

[0035] Examples of thermosetting elastomers (including their cured forms) 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). They all exhibit excellent weather resistance. EPDM and silicone rubber, in particular, contain a large amount of hydrogen in their molecular backbone and are therefore preferred.

[0036] Examples of thermoplastic elastomers include: polystyrene-based thermoplastic rubber, polyolefin-based thermoplastic rubber, polyurethane-based thermoplastic rubber, polyester-based thermoplastic rubber, polyamide-based thermoplastic rubber, and polyvinyl chloride-based thermoplastic rubber.

[0037] Examples of polystyrene-based thermoplastic rubbers include: styrene-ethylene-butene-styrene block copolymer (SEBS) and styrene-ethylene-propylene-styrene block copolymer (SEPS).

[0038] Among the above thermoplastic elastomers, ethylene propylene rubber (EPDM), polyolefin thermoplastic rubber, styrene-ethylene-butene-styrene block copolymer (SEBS), and styrene-ethylene-propylene-styrene block copolymer (SEPS) have excellent weather resistance and contain a large amount of hydrogen in their molecular backbone, and are therefore preferred.

[0039] The second layer (3) constituting the multilayer structure (1) of the present invention comprises the aforementioned resin or elastomer and inorganic fiber bundles. In the present invention, inorganic fiber bundles refer to secondary processed products of fiber cloth, roving, roving fabric, woven felt, nonwoven fabric, fiber felt, and other inorganic fibers. They can be appropriately selected according to the forming method.

[0040] In this invention, the inorganic fibers that form the basis of the inorganic fiber bundle contain a high content of any one of gadolinium, samarium, or cadmium, which are rich in neutron absorbers. More specifically, the inorganic fiber oxide preferably contains at least 10% by mass of any one of gadolinium, samarium, or cadmium, more preferably at least 20% by mass, even more preferably at least 30% by mass, and most preferably at least 40% by mass.

[0041] Furthermore, the fiber may contain two or more of gadolinium, samarium, and cadmium, with gadolinium exhibiting overwhelmingly superior neutron absorption. Therefore, to improve the neutron absorption per unit mass of the inorganic fiber, it is preferable to contain at least 10% by mass of gadolinium (calculated as oxides), more preferably at least 20% by mass, even more preferably at least 30% by mass, and most preferably at least 40% by mass.

[0042] In the manufacture of inorganic fibers, any of the three elements, either in their elemental form (metal) or in their oxide, can be used as a raw material.

[0043] In addition to the three elements mentioned above that are essential components, the inorganic fiber also contains silicon oxide and aluminum oxide, well-known oxides with excellent glass-forming properties, as main components. Calcium oxide may also be included as a secondary component.

[0044] The second layer (3) is composed of inorganic fiber bundles and resin (or elastomer), and more specifically, is a structure in which the voids of the aforementioned inorganic fiber bundles are filled by the same parent material (resin or elastomer) that forms the first layer (2) as described above.

[0045] The proportion of the inorganic fiber bundle in the total mass of the second layer (3) is 10 to 95% by mass. From the viewpoint of improving the neutron absorption efficiency of the second layer (3), it is preferably 25% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more.

[0046] The proportion of the inorganic fiber bundle in the total mass of the second layer (3) depends on the volume porosity (void fraction) of the inorganic fiber bundle, and can also be adjusted by the specific molding method of combining the inorganic fiber bundle with resin (or elastomer).

[0047] Therefore, when increasing the proportion of the inorganic fiber bundle in the total mass of the second layer (3), it can be adjusted by using inorganic fiber bundles with small volume porosity and increasing the pressure when the inorganic fiber bundle is combined with the parent material (resin or elastomer) as needed.

[0048] Considering the above, when making the second layer (3), if a hardened resin or hardened elastomer is used as the base material, it can be obtained by impregnating the prepolymer of the resin or elastomer into an inorganic fiber bundle and then by pressurizing and heating.

[0049] As a specific forming method, well-known methods such as autoclave forming, injection forming, and hand-forming can be appropriately adopted.

[0050] On the other hand, when using thermoplastic resin or thermoplastic elastomer as the base material, for example, it can be obtained by pre-preparing a sheet containing thermoplastic resin or thermoplastic elastomer, overlapping the sheet with a fiber cloth, and then heating and pressing it. Calendering is an example of heating and pressing.

[0051] In this invention, since the resin or elastomer forming the first layer (2) and the second layer (3) are the same, although the first layer (2) and the second layer (3) can be made in different processes and then laminated to form the target multilayer structure, the following molding method can be used to simplify the process when the base material is thermosetting.

[0052] That is, if inorganic fiber bundles (e.g., fiber cloth) are overlapped in multiple layers beforehand, and then impregnated with a prepolymer of the parent material, and the pressure, temperature and time are adjusted, a first layer (2) consisting only of the parent material and a second layer (3) consisting of inorganic fiber bundles impregnated in the parent material can be formed between the aforementioned inorganic fiber bundles in one process.

[0053] There is no particular limitation on the ratio of the first layer (2) and the second layer (3) in the multilayer structure (1), but it is preferred that 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.

[0054] The multilayer structure (1) of the present invention is formed by laminating at least one first layer (2) and at least one second layer (3).

[0055] In a multi-layer structure consisting of a first layer (2) and a second layer (3), the first layer (2) is disposed on the neutron source side and the second layer (3) is disposed on the object to be protected.

[0056] However, the inventors used Monte Carlo particle calculation codes for simulation and found that when both the first layer (2) and the second layer (3) are two or more multilayer structures (1), the neutron shielding performance unexpectedly deteriorates if the outermost layer on the protected object side is the first layer (2). Therefore, it is preferable that the layer closest to the protected object is the second layer (3). Based on this insight, it was found that in a multilayer structure (1) where the layer closest to the protected object is the first layer (i.e., the neutron deceleration layer), the presence of the first layer (2), even if its thickness is thin, will impair the neutron shielding performance of the multilayer structure (1). Therefore, in this multilayer structure, it is preferable that the layer closest to the protected object is the second layer (3).

[0057] The simulation results will be mentioned below. Although various Monte Carlo particle transport codes are known and used, the inventors used the Monte Carlo particle transport code known as PHITS (Particle and Heavy Ion Transport code System). PHITS is available from the website of the Japan Nuclear Energy Agency (https: / / phits.jaea.go.jp / indexj.html), and the method of using it has been disclosed.

[0058] Figure 3 The result is obtained by using the above calculation code to calculate the energy spectrum of the transmitted neutron rays when the multilayer structure (1) is irradiated with neutron rays of a specified energy spectrum.

[0059] Here, the energy spectrum of the incident neutrons is the thermal neutron furnace neutron spectrum.

[0060] In addition, the thickness of the multilayer structure (1) is set to 30 mm, the hardened body of bisphenol A epoxy resin is assumed to be the first layer (2), and the inorganic fiber cloth containing gadolinium is assumed to be the second layer (3). Furthermore, for ease of calculation, the second layer (3) is assumed to be composed only of inorganic fiber cloth, and the volume porosity of the inorganic fiber cloth is set to 0.5.

[0061] The following are the premises for the multi-layer structure model used in the calculation.

[0062] The ratio of the total thickness of the first layer (2) to the total thickness of the second layer (3) is 90 / 10. First layer (2): Bisphenol A / hardener mass ratio: 80 / 20 The composition of the second layer (3) (the composition of the inorganic fiber cloth): Gadolinium oxide (Gd2O3) 10% by mass, silicon dioxide (SiO2) 48% by mass, aluminum oxide (Al2O3) 15% by mass, iron oxide (Fe2O3) 9% by mass, calcium oxide (CaO) 10% by mass, boron oxide (B2O3) 5% by mass, potassium oxide (K2O) 2% by mass The number of layers (2) in the first layer: 20 (each with the same thickness) The number of layers (3) in the second layer: 19 (each has the same thickness) Furthermore, as a comparative structure relative to this multi-layered structure model, a particle dispersion structure was also considered ( Figure 2 Calculations were performed showing that the particle-dispersed structure contains gadolinium oxide in the same amount as the second layer of the multilayer structure model, dispersed in the same resin as the first layer. Furthermore, the particle size was set to 0.50 mm. The calculation results are shown below. Figure 3 .

[0063] according to Figure 3 It is known that the energy of a neutron is 10. -7 In regions below MeV, the multilayer structure of this invention exhibits superior neutron shielding compared to conventional particle dispersion structures.

[0064] Figure 4 It focuses on the neutron's energy of 10-7 Areas below MeV Figure 3 A magnified view of a portion of the image.

[0065] The above calculation results suggest the advantage of neutron-absorbing elements being distributed in a layered, localized manner in the parent resin.

[0066] In the above calculations, the configuration of using the outermost layer on the protected object side as the first layer (2R) is adopted. Figure 5 (a)), but compared to the model that moves the first layer to the forefront of the neutron source side ( Figure 5 (b) was compared, and the results were contrary to expectations, with neutron shielding further improved. Figure 6 ).

[0067] The same calculation results were confirmed even in the simplest model.

[0068] That is, when comparing a two-layer structure model consisting of one layer each of the first layer (2) (neutron source side) and the second layer (3) (protected object side) with a model in which another first layer of the same thickness is added and stacked on the back of the second layer of the two-layer structure, the result is that the neutron shielding of the latter multilayer structure is worse.

[0069] Therefore, the multilayer structure (1) of the present invention is preferably formed by the outermost layer (3) (neutron absorption layer) on the side of the object being protected.

[0070] However, in FRP molding where fiber bundles are combined with thermosetting resin, the surface of the molded article is usually covered by layers of varying thicknesses consisting only of the base material. Therefore, when manufacturing the multilayer structure (1) of the present invention, taking into account the above calculation results, care should be taken to avoid forming the first layer (2) (a layer consisting only of resin or elastomer) in the layer closest to the object being protected (the outermost layer), or if it is formed, to make it as thin as possible.

[0071] Based on this viewpoint, when using a curable resin (or elastomer) to manufacture the multilayer structure (1) of the present invention, among the many thermosetting molding methods, injection molding or filament winding molding is a more preferred molding method.

[0072] In particular, the injection molding method involves sandwiching fiber bundles such as fiber cloth or non-woven fabric between the upper and lower molds to maintain the airtightness of the film. At the same time, it uses vacuum pressure to draw out resin for filling and impregnation. Therefore, it has the advantages of less resin (or elastomer) seepage to the surface of the molded product and is also easy to apply to molded products with uneven surfaces.

[0073] In the case where the multilayer structure (1) of the present invention has two or more first layers (2), the configuration of these first layers (2) can also be such that the thickness of the layers increases sequentially from the layer (2R) closest to the object being protected to the layer (2F) closest to the neutron source. Figure 7 ).

[0074] In addition, when the multilayer structure (1) of the present invention has two or more second layers (3), the following variations can be adopted regarding these second layers (3).

[0075] That is, the following changes: 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 being protected. i) The structure of layers with progressively increasing thickness [the pattern of increasing thickness of each layer] Figure 8 ); ii) The composition of the inorganic fiber bundle content per unit volume of each layer increasing sequentially [the pattern of increasing inorganic fiber bundle content (or decreasing volumetric porosity of inorganic fiber bundles) in each layer] Figure 9 ); iii) The composition of the inorganic fiber bundles that make up each layer, with the content of neutron-absorbing elements (gadolinium, samarium, cadmium) increasing in sequence [the pattern of increasing concentration of neutron-absorbing elements in each layer] ( Figure 10 ).

[0076] The above variations can be combined. That is, variations i) and ii) can be combined, variations ii) and iii) can be combined, variations i) and iii) can be combined, and all variations i), ii) and iii) can be combined.

[0077] As an improvement to the multilayer structure of the present invention, a multilayer structure can also be formed by substantially making two or more laminated layers consisting of a second layer, namely, a neutron-shielding inorganic fiber bundle and a resin (or elastomer).

[0078] More specifically, the following multilayer structures can be listed, which are composed of two or more laminated layers of resin (or elastomer) and inorganic fiber bundles, arranged in a sequence from the layer closest to the neutron source to the layer closest to the object being protected. a) The composition of inorganic fiber bundle content per unit volume in each layer increases sequentially; or b) The composition of the inorganic fiber bundles that make up each layer has an increasing content of neutron-absorbing elements (gadolinium, samarium, cadmium).

[0079] Regarding the formation of the multi-layered structure described above, known methods such as autoclave forming, injection molding, and hand-forming can also be appropriately employed.

[0080] [Industry availability] The multilayer structure of this invention exhibits excellent neutron shielding. Furthermore, because it is constructed from inorganic fiber bundles filled with resin or elastomer as the matrix, it offers greater flexibility in shape compared to concrete-based materials. Therefore, it is suitable for use as equipment, machinery, and components in various fields such as nuclear energy, aerospace, and medicine.

[0081] As for equipment, machinery, and components in the nuclear energy field, the following can be listed: Equipment, machinery, and components used in nuclear power generation; Equipment, machinery, and components used to prevent critical reactions in operations related to the extraction and storage of molten nuclear fuel; Equipment, machinery, and components used for the mining and processing of uranium ore; Equipment, machinery, and components used for the secondary processing of nuclear fuel (including the conversion, enrichment, reconversion, shaping, and MOX manufacturing of the fuel); Equipment, machinery, and components used for the storage, processing, and reprocessing of used nuclear fuel; Equipment, machinery, and components used for the storage, treatment, and disposal of neutron radiation irradiation waste; Transport machinery and components for uranium ore, secondary processed nuclear fuel products, used nuclear fuel, or neutron radiation-exposed waste; Other nuclear-related equipment, machinery, and components.

[0082] More specific examples of the equipment, machinery, and components used in nuclear power generation include: nuclear reactor buildings (including research and experimental reactors), nuclear reactor storage containers, piping within nuclear reactor facilities, and robots for waste disposal.

[0083] As for equipment, machines, and components in the aerospace field, the following can be listed: Space base buildings, space stations, artificial satellites, planetary exploration satellites, etc.

[0084] As for equipment, machines, and components in the medical field, the following can be listed: Medical devices that utilize particle beams.

[0085] The above examples are provided to demonstrate the usefulness of the composite materials of the present invention and do not limit the scope of the invention.

[0086] [Symbol Explanation] 1: Multi-layer structure 2: First layer 2F: The first layer closest to the neutron source 2R: The first layer closest to the object being protected. 3: Second layer 3F: The second layer closest to the neutron source 3R: The second layer closest to the object being protected. 4: Previous neutron shielding structures 5: Dispersed particles 6: Resin S: Neutron Source O: Protected object

Claims

1. A multi-layered structure, which is a neutron-shielding multi-layered structure that separates a neutron source from the object to be protected. The multi-layer structure is Comprising at least one or more first layers, and It consists of at least one second layer formed by neutron-absorbing inorganic fiber bundles and the resin or elastomer.

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

3. The multilayer structure according to claim 2, wherein the inorganic fiber bundle is any one of fiber cloth, roving, roving cloth, diced felt, ground fiber, nonwoven fabric, or fiber felt.

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

5. The multilayer structure according to claim 4, wherein the layer closest to the protected object is formed by a second layer.

6. The multilayer structure according to 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 the first layer and the second layer are both two or more.

8. The multilayer structure according to claim 7, wherein the thickness of each first layer increases sequentially from the side closest to the protected object to the side closest to the neutron source.

9. The multilayer structure according to claim 7, wherein the thickness of each second layer increases sequentially from the side closest to the neutron source to the side closest to the object being protected.

10. The multilayer structure according to claim 7, wherein each layer of the second layer, from the side closest to the neutron source to the side closest to the object being protected, i) The content of inorganic fiber bundles per unit volume in each layer increases sequentially; or ii) The volumetric porosity of the inorganic fiber bundles constituting each layer decreases sequentially; or iii) The content of neutron-absorbing elements (gadolinium, samarium, cadmium) in the inorganic fiber bundles that make up each layer increases in sequence.

11. A multi-layered structure, which is a neutron-shielding multi-layered structure that separates a neutron source from the object to be protected. The multilayer structure is composed of at least two laminated layers formed by neutron-absorbing inorganic fiber bundles and resin or elastomer, with the following configuration: from the side closest to the neutron source to the side closest to the protected object. i) The content of inorganic fiber bundles per unit volume in each layer increases sequentially; or ii) The volumetric porosity of the inorganic fiber bundles constituting each layer decreases sequentially; or iii) The content of neutron-absorbing elements (gadolinium, samarium, cadmium) in the inorganic fiber bundles that make up each layer increases in sequence.

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

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

14. The method for manufacturing a multilayer structure according to claim 10, wherein the method utilizes an injection molding method.

15. The method for manufacturing a multilayer structure according to claim 11, wherein the method utilizes injection molding.

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