SiC composite material, cladding tube and application thereof

Abstract

The invention provides a SiC composite material, a cladding tube and application thereof, and belongs to the technical field of nuclear reactors. The SiC composite material comprises at least one SiC / SiC composite material layer and at least one elementary substance SiC layer, the rigidity of the SiC / SiC composite material layer is progressively increased in a gradient mode in the direction of the elementary substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the rigidity of the elementary substance SiC layer is progressively decreased in a gradient mode in the direction of the SiC / SiC composite material layer adjacent to the elementary substance SiC layer. The SiC composite material has a rigidity gradient distribution structure, so that the stress concentration and stress difference between layers and in the layers can be greatly reduced, the failure probability of the device or the expansion of cracks of the SiC / SiC composite material layer to the elementary SiC layer can be reduced, and the air tightness of the device can be remarkably improved; the SiC composite material can be used for preparing SiC composite material cladding pipe fittings of which the mechanical strength, the heat conduction capability and the air tightness all meet in-reactor application requirements.

Description

Technical Field The invention belongs to the technical field of nuclear reactors, and in particular relates to a SiC composite material, a cladding tube and applications thereof. Background Art Nuclear fuel cladding tubes are structures used to contain nuclear fuel pellets and prevent the released radioactive fission products from entering the external environment through the coolant. The cladding tube material originally used in the reactor was zirconium (Zr)-based alloy, which has high strength at high temperatures, good corrosion resistance, and a small neutron absorption interface. However, in the Fukushima nuclear accident in 2011, the zirconium-based alloy cladding tubes reacted with steam at high temperatures to produce a large amount of hydrogen, which eventually led to a hydrogen explosion. In order to prevent such accidents from happening, researchers at home and abroad are committed to developing accident-tolerant nuclear fuel cladding tube materials. Silicon carbide (SiC) materials have natural advantages in high-temperature mechanical strength and accident resistance. They can replace the existing zirconium (Zr)-based alloys with good service performance and commercial value but dangerous upper temperature limits as nuclear fuel cladding tube materials. They are considered to be ideal candidates for fission reactor cladding or fusion reactor first wall materials. SiC materials include single-element SiC (m-SiC) materials and SiC-reinforced SiC composites (SiC / SiC composites), among which SiC / SiC composites include continuous silicon carbide fiber-reinforced SiC composites (SiC f / SiC composites) and SiC whisker-toughened SiC composites (SiC w / SiC composite materials), etc., among which SiC f / SiC composite materials also include winding-molded SiC f / SiC composite materials and braided SiC f / SiC composite materials. However, although m-SiC material has many advantages such as high high temperature mechanical strength, high thermal conductivity, high density, good air tightness, and small neutron absorption cross section, [1-2] However, due to its mechanical brittleness, the material cannot withstand the complex mechanical environment of the reactor and is difficult to apply directly. Although SiC / SiC composite materials have certain pseudoplasticity and can provide additional fracture toughness to the material, single SiC / SiC composite materials have a certain porosity and there is a theoretical upper limit to their density improvement, making it difficult to meet the technical requirements for air tightness and thermal conductivity. [3-5] . In order to combine the advantages of both, the current cladding tube is mainly designed as a multi-layer structure with various SiC forms. For example, patent application document CN112876257A discloses a SiC f / SiC composite material two-layer composite cladding tube and preparation method thereof, the SiC f / SiC composite material two-layer composite cladding tube, including SiCf / SiC composite layer and SiC ceramic layer. Patent application document US200610039524 provides a method for preparing a multi-layer cladding tube, wherein the cladding tube has inner and outer layers of m-SiC and an intermediate layer of SiC f The SiC composite cladding tubes obtained by these structures or preparation methods can obtain certain mechanical properties, but cannot fully meet the requirements of reactor application for material airtightness.

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[0004] Gyanender Singh, Steve Gonczy, Christian Deck, Edgar Lara-Curzio, Yutai Katoh, "Interlaboratory round robin study on axial tensil properties of SiC-SiC CMC tubular test specimens", Int J Applied Ceramic Thechnology 2018;15:1334-1349.

[0005] CPDeck, GM Jacobsen, J. Sheeder, O. Gutierrez, J, Zhang, J. Stone, HE Khalifa, CA Back, "Characterization of SiC-SiC composites for accidenttolerant fuel cladding", J. Nucl. Mater. 466 (2015) 667-681. Summary of the invention The present invention aims to solve one of the technical problems in the related art at least to a certain extent. To this end, an embodiment of the present invention provides a SiC composite material and an application thereof. The SiC composite material of the embodiment of the present invention comprises at least one SiC / SiC composite material layer and at least one single-substance SiC layer, wherein the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer. The advantages and technical effects brought by the SiC composite material of the embodiment of the present invention are as follows: The SiC composite material of the embodiment of the present invention combines a SiC / SiC composite material layer and a single SiC layer together. The stiffness of the SiC / SiC composite material layer is relatively small, while the stiffness of the single SiC layer is relatively large, so that the layers have the characteristic of stiffness gradient distribution. At the same time, the SiC composite material of the embodiment of the present invention constructs a suitable stiffness gradient distribution structure in the SiC / SiC composite material layer and / or the single SiC layer serving as the load-bearing unit of the device, so that the layer also has the characteristic of stiffness gradient distribution. Therefore, the stress concentration and stress difference between and within the layers can be greatly reduced, thereby reducing the failure probability of the device or the extension of the cracks of the SiC / SiC composite material layer to the single SiC layer, and further significantly improving the airtightness of the device. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least two layers of the SiC / SiC composite material layers and at least one layer of the single-substance SiC layer. When one layer of the single-substance SiC layer is arranged between the two layers of the SiC / SiC composite material layers, the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually from the middle position of the single-substance SiC layer toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least one layer of the SiC / SiC composite material layer and at least two layers of the single-substance SiC layers. When one layer of the SiC / SiC composite material layer is arranged between the two layers of the single-substance SiC layers, the stiffness of the SiC / SiC composite material layer increases gradually from the middle position of the SiC / SiC composite material layer toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least 2 layers of the SiC / SiC composite material layers and at least 2 layers of the single-substance SiC layers. When the SiC / SiC composite material layers and the single-substance SiC layers are arranged alternately, the stiffness of the SiC / SiC composite material layers increases gradually from the middle position of the SiC / SiC composite material layers toward the single-substance SiC layers adjacent to the SiC / SiC composite material layers, and / or the stiffness of the single-substance SiC layers decreases gradually from the middle position of the single-substance SiC layers toward the single-substance SiC layers adjacent to the single-substance SiC layers. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least 2 layers of the SiC / SiC composite material layers and at least 2 layers of the single-substance SiC layers. When 2 layers of the single-substance SiC layers in contact with each other are arranged between the 2 layers of the SiC / SiC composite material layers, the stiffness of the SiC / SiC composite material layer increases gradually toward the direction of the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the direction of the SiC / SiC composite material layer adjacent to the single-substance SiC layer, and the stiffness of the 2 layers of the single-substance SiC layers increases gradually toward the position where the 2 layers of the single-substance SiC layers are in contact with each other. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least 2 layers of the SiC / SiC composite material layers and at least 2 layers of the single-substance SiC layers. When 2 layers of the single-substance SiC layers are provided with 2 layers of the SiC / SiC composite material layers in contact with each other, the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer, and the stiffness of the 2 layers of the SiC / SiC composite material layers decreases toward the position where the 2 layers of the SiC / SiC composite material layers are in contact with each other. In some embodiments, when the stiffness of the SiC / SiC composite material layer increases gradually toward the single-element SiC layer adjacent to the SiC / SiC composite material layer, the SiC / SiC composite material layer includes 2-8 different stiffnesses. In some embodiments, when the stiffness of the SiC / SiC composite material layer increases gradually toward the single SiC layer adjacent to the SiC / SiC composite material layer, the SiC / SiC composite material layer includes a wound SiC f / SiC composite materials, braided SiC f / SiC composites and SiC w At least two of the / SiC composite material layers. In some embodiments, when the stiffness of the single SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single SiC layer, the single SiC layer includes 2-20 different stiffnesses. In some embodiments, when the stiffness of the single SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single SiC layer, the stiffness of the single SiC layer changes continuously. In some embodiments, when the stiffness of the elemental SiC layer decreases gradually toward the direction of the SiC / SiC composite material layer adjacent to the elemental SiC layer, the elemental SiC layer is prepared by chemical vapor deposition, and the chemical vapor deposition temperature at different stiffness positions of the elemental SiC layer increases gradually toward the direction of the SiC / SiC composite material layer adjacent to the elemental SiC layer. In some embodiments, the chemical vapor deposition temperature of the single-element SiC layer at a position adjacent to the SiC / SiC composite material layer is 1100-1400°C; the chemical vapor deposition temperature of the single-element SiC layer at a position away from the SiC / SiC composite material layer is 800-1100°C; and / or the chemical vapor deposition temperatures of two adjacent positions with different stiffness in the single-element SiC layer differ by 30-150°C. In some embodiments, the chemical vapor deposition temperature of the single-element SiC layer at different stiffness positions changes continuously; and / or the chemical vapor deposition temperature of the single-element SiC layer at a position adjacent to the SiC / SiC composite material layer is 1100-1400°C, and the chemical vapor deposition temperature of the single-element SiC layer at a position away from the SiC / SiC composite material layer is 800-1100°C; and / or the rate of change of the chemical vapor deposition temperature of the single-element SiC layer is 0.2-2°C / min. In addition, an embodiment of the present invention further provides a cladding tube, which is prepared from the SiC composite material of an embodiment of the present invention. The advantages and technical effects brought by the cladding tube of the embodiment of the present invention are as follows: The cladding tube of the embodiment of the present invention can achieve good airtightness of maintaining pressure for 100 hours without obvious leakage under 15MPa positive pressure conditions, and can also achieve a thermal conductivity higher than 19W / m·k, as well as a bending strength higher than 180MPa and a burst strength higher than 170MPa, meeting the application requirements of nuclear reactors, especially meeting the requirements of the pressurized water reactor service environment, and can effectively reduce catastrophic failure problems such as micro-damage and crack propagation or reduced airtightness caused by stress mismatch problems in different functional gradient regions during the service of the pressurized water reactor, meet the mechanical, heat transfer and airtightness requirements of long-term high fuel consumption applications in the reactor, extend the service life, improve fuel consumption and provide higher accident fault tolerance. The embodiments of the present invention further provide the use of the SiC composite material according to the embodiments of the present invention in a nuclear reactor, or the use of the cladding tube according to the embodiments of the present invention in a nuclear reactor. The advantages and technical effects brought by the application of the embodiments of the present invention are as follows: In addition to being used as a cladding material for a nuclear reactor, the SiC composite material of the embodiment of the present invention can also be applied to a heat exchanger of a nuclear reactor, a first wall of a fusion reactor, an engine nozzle, a nose cone, a burner, a flow channel, and the like. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 It is a schematic structural diagram of a SiC composite material of the present invention; Figure 2 is a schematic diagram showing the change in stiffness of a SiC composite material according to the present invention with respect to position; Figure 3 is a schematic diagram showing the change in stiffness of another SiC composite material of the present invention with respect to position; Figure 4 It is a structural schematic diagram of another SiC composite material of the present invention; Figure 5 is a schematic diagram showing the change in stiffness of another SiC composite material of the present invention with respect to position; Figure 6 is a schematic diagram showing the change in stiffness of another SiC composite material of the present invention with respect to position; Figure 7 is a SEM image of the SiC composite cladding tube of Example 1 of the present invention; Figure 8 is the internal pressure curve of the SiC composite cladding tube of Example 1 of the present invention maintained at an internal pressure of 15 MPa for 100 hours; Fig. 9 This is a schematic diagram of the structure of the SiC composite cladding tube of Comparative Example 1; Fig.10 It is a schematic diagram showing the change of the stiffness of the SiC composite cladding tube of Example 1 with position. DETAILED DESCRIPTION Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention. An embodiment of the present invention provides a SiC composite material, comprising at least one SiC / SiC composite material layer and at least one single-substance SiC layer, wherein the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer. Working principle: The SiC composite material of the embodiment of the present invention combines a SiC / SiC composite material layer and a single SiC layer together. The stiffness of the SiC / SiC composite material layer is relatively small, while the stiffness of the single SiC layer is relatively large, so that the layers have the characteristic of stiffness gradient distribution; at the same time, the SiC composite material of the embodiment of the present invention constructs a suitable stiffness gradient distribution structure in the SiC / SiC composite material layer and / or the single SiC layer serving as the load-bearing unit of the device, so that the layer also has the characteristic of stiffness gradient distribution; therefore, the stress concentration and stress difference between and within the layers can be greatly reduced, thereby reducing the failure probability of the device or the extension of cracks in the SiC / SiC composite material layer to the single SiC layer, and further significantly improving the airtightness of the device. It should be noted that if Figure 7 As shown, in the SiC composite material of the embodiment of the present invention, the SiC / SiC composite material layer and the single SiC layer are tightly combined without obvious interface. In addition, when a stiffness gradient is provided in the SiC / SiC composite material layer and / or the single SiC layer, there is no obvious interface in the layer. The SiC composite material of the embodiment of the present invention comprises at least one layer of the SiC / SiC composite material layer and at least one layer of the single substance SiC layer. For example, the SiC composite material is a two-layer structure, such as Figure 1 and Figure 2 As shown, the inner layer is a SiC / SiC composite material layer, the outer layer is a single-substance SiC layer, the stiffness of the inner SiC / SiC composite material layer increases gradually toward the single-substance SiC layer of the outer layer, and / or the stiffness of the single-substance SiC layer of the outer layer decreases gradually toward the inner SiC / SiC composite material layer, wherein Figure 2 The solid line in the middle represents that a stiffness gradient structure is constructed in both the inner SiC / SiC composite layer and the outer single SiC layer. Figure 2 The long dashed line represents that the stiffness gradient structure is constructed only in the single SiC layer. Figure 2 The midpoint dashed line represents that the stiffness gradient structure is constructed only in the SiC / SiC composite layer. Alternatively, the SiC composite material is a 2-layer structure, such as Figure 3 As shown, the inner layer is a single-substance SiC layer, the outer layer is a SiC / SiC composite material layer, the stiffness of the outer SiC / SiC composite material layer increases gradually toward the inner single-substance SiC layer, and / or the stiffness of the inner single-substance SiC layer decreases gradually toward the outer SiC / SiC composite material layer, wherein Figure 3 The solid line in the middle represents that a stiffness gradient structure is constructed in both the inner SiC / SiC composite layer and the outer single SiC layer. Figure 3 The long dashed line represents that the stiffness gradient structure is constructed only in the single SiC layer. Figure 4 The midpoint dashed line represents that the stiffness gradient structure is constructed only in the SiC / SiC composite layer. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least two layers of the SiC / SiC composite material layers and at least one layer of the single-substance SiC layer. When one layer of the single-substance SiC layer is arranged between the two layers of the SiC / SiC composite material layers, the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually from the middle position of the single-substance SiC layer toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer. For example, the SiC composite material has a three-layer structure, such as Figure 4 and Figure 5 As shown, the inner layer is a SiC / SiC composite material layer, the middle layer is a single-substance SiC layer, and the outer layer is a SiC / SiC composite material layer, and the stiffness of the inner and outer SiC / SiC composite materials increases gradually toward the middle single-substance SiC layer, and / or the stiffness of the middle single-substance SiC layer decreases gradually from the middle position of the single-substance SiC layer toward the inner and outer SiC / SiC composite materials. Figure 5 The solid line in the middle represents that a stiffness gradient structure is constructed in both the inner SiC / SiC composite layer and the outer single SiC layer. Figure 5 The long dashed line represents that the stiffness gradient structure is constructed only in the single SiC layer. Figure 5 The midpoint dashed line represents that the stiffness gradient structure is constructed only in the SiC / SiC composite layer. Alternatively, the SiC composite material is a three-layer structure, such as Figure 6As shown, the inner layer is a SiC layer, the middle layer is a SiC / SiC composite material layer, the outer layer is a SiC layer, and the stiffness of the SiC / SiC composite material layer in the middle increases gradually from the middle position of the SiC / SiC composite material layer toward the inner and outer SiC layers, and / or the stiffness of the inner and outer SiC layers decreases gradually toward the SiC / SiC composite material layer in the middle. Figure 6 The solid line in the middle represents that a stiffness gradient structure is constructed in both the inner SiC / SiC composite layer and the outer single SiC layer. Figure 6 The long dashed line represents that the stiffness gradient structure is constructed only in the single SiC layer. Figure 6 The midpoint dashed line represents that the stiffness gradient structure is constructed only in the SiC / SiC composite layer. Alternatively, the SiC composite material has a 6-layer structure, the 1st layer (innermost layer) is a SiC / SiC composite material layer, the 2nd layer is a single-substance SiC layer, the 3rd layer is a SiC / SiC composite material layer, and the 4th to 6th layers repeat the 1st to 3rd layers. The stiffness of the SiC / SiC composite material layers of the 1st and 3rd layers increases gradually toward the single-substance SiC layer of the 2nd layer, and / or the stiffness of the single-substance SiC layer of the 2nd layer decreases gradually from the middle position of the single-substance SiC layer toward the direction of the SiC / SiC composite material layers of the 1st and 3rd layers. Similarly, the 4th to 6th layers also satisfy the above rule. Alternatively, the SiC composite material has a four-layer structure, wherein the first layer (innermost layer) is a SiC / SiC composite material layer, the second layer is a single-element SiC layer, the third layer is a SiC / SiC composite material layer, and the fourth layer is also a SiC / SiC composite material layer. The stiffness of the first and third SiC / SiC composite material layers increases gradually toward the second single-element SiC layer, and / or the stiffness of the second single-element SiC layer decreases gradually from the middle position of the single-element SiC layer toward the first and third SiC / SiC composite material layers. There is no special requirement for the stiffness of the fourth layer of the SiC / SiC composite material layer, which can be the same as that of the third layer, or higher or lower than that of the third layer. Alternatively, the first layer (the innermost layer) is a C / SiC composite layer, the second layer is a SiC / SiC composite layer, the third layer is a single-substance SiC layer, and the fourth layer is a SiC / SiC composite layer, and the stiffness of the first and third SiC / SiC composite layers increases gradually toward the second single-substance SiC layer, and / or, the stiffness of the second single-substance SiC layer decreases gradually from the middle position of the single-substance SiC layer toward the first and third SiC / SiC composite layers, and the stiffness of the C / SiC composite layer is generally lower than that of the SiC / SiC composite layer. Since there are too many possible arrangements, they are not listed here one by one. It should be noted that the middle position here can be the position in the middle of the elemental SiC layer, or a position close to the middle of the elemental SiC layer, that is, the position with the maximum stiffness in the elemental SiC layer is not necessarily the position in the middle of the elemental SiC layer, as long as the stiffness of the elemental SiC layer gradually decreases toward the SiC / SiC composite material layers on both sides adjacent to the elemental SiC layer. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least one layer of the SiC / SiC composite material layer and at least two layers of the single-substance SiC layers. When one layer of the SiC / SiC composite material layer is arranged between the two layers of the single-substance SiC layers, the stiffness of the SiC / SiC composite material layer increases gradually from the middle position of the SiC / SiC composite material layer toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer. For example, the SiC composite material has a three-layer structure, an inner layer is a single-substance SiC layer, a middle layer is a SiC / SiC composite material layer, and an outer layer is a single-substance SiC layer. The stiffness of the middle SiC / SiC composite material layer increases gradually from the middle position of the SiC / SiC composite material layer toward the inner and outer single-substance SiC layers, and / or the stiffness of the inner and outer single-substance SiC layers decreases toward the inner SiC / SiC composite material layer. Alternatively, the SiC composite material is a 6-layer structure, the first layer (the innermost layer) is a single SiC layer, the second layer is a SiC / SiC composite layer, the third layer is a single SiC, and the fourth to sixth layers repeat the first to third layers. Alternatively, the multi-layer cladding tube material is a 4-layer structure, the first layer (the innermost layer) is a single SiC layer, the second layer is a SiC / SiC composite layer, the third layer is a single SiC layer, and the fourth layer is also a single SiC layer. The structures of the first to third layers are the same as above, and there is no special requirement for the rigidity of the fourth layer of the single SiC layer, which can be the same as that of the third layer, or higher or lower than that of the third layer. Since there are too many possible arrangements, they are not listed here one by one. It should be noted that the middle position here can be the position in the middle of the SiC / SiC composite material layer, or it can be a position in the SiC / SiC composite material layer away from both sides and close to the middle position, that is, the position with the minimum stiffness in the SiC / SiC composite material layer is not necessarily the position in the middle of the SiC / SiC composite material layer, as long as the stiffness of the SiC / SiC composite material layer increases gradually toward the direction of the single SiC layer on both sides adjacent to the SiC / SiC composite material layer. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least 2 layers of the SiC / SiC composite material layers and at least 2 layers of the single-substance SiC layers. When the SiC / SiC composite material layers and the single-substance SiC layers are arranged alternately, the stiffness of the SiC / SiC composite material layers increases gradually from the middle position of the SiC / SiC composite material layers toward the single-substance SiC layers adjacent to the SiC / SiC composite material layers, and / or the stiffness of the single-substance SiC layers decreases gradually from the middle position of the single-substance SiC layers toward the single-substance SiC layers adjacent to the single-substance SiC layers. For example, the SiC composite material includes a 4-layer structure, the first layer (innermost layer) is a SiC / SiC composite material layer, the second layer is a single-substance SiC layer, the third layer is a SiC / SiC composite material layer, and the fourth layer is a single-substance SiC layer. The stiffness of the SiC / SiC composite material layer of the first layer increases gradually toward the single-substance SiC layer of the second layer, and / or the stiffness of the single-substance SiC layer of the second layer decreases from the middle position of the single-substance SiC layer toward the SiC / SiC composite materials of the first and third layers, and / or the stiffness of the SiC / SiC composite material layer of the third layer increases gradually from the middle position of the SiC / SiC composite material layer toward the single-substance SiC layers of the second and fourth layers, and / or the stiffness of the single-substance SiC layer of the fourth layer decreases toward the SiC / SiC composite material layer of the third layer. Alternatively, the multilayer cladding tube material includes an 8-layer structure, the 1st layer (innermost layer) is a SiC / SiC composite material layer, the 2nd layer is a single-element SiC layer, the 3rd layer is a SiC / SiC composite material layer, the 4th layer is a single-element SiC layer, and the 5th to 8th layers repeat the structure of the 1st to 4th layers. Alternatively, the multilayer cladding tube material includes a 5-layer structure, the first layer (innermost layer) is a C / SiC composite material layer, the second layer is a SiC / SiC composite material layer, the third layer is a single-substance SiC layer, the fourth layer is a SiC / SiC composite material layer, and the fifth layer is a single-substance SiC layer. The stiffness of the second layer of the SiC / SiC composite material layer increases gradually toward the third layer of the single-substance SiC layer, and / or the stiffness of the third layer of the single-substance SiC layer decreases from the middle position of the single-substance SiC layer toward the second and fourth layers of the SiC / SiC composite material layer, and / or the stiffness of the fourth layer of the SiC / SiC composite material layer increases from the middle position of the SiC / SiC composite material layer toward the second and fifth layers of the single-substance SiC layer. The stiffness of the C / SiC composite layer is generally lower than that of the SiC / SiC composite layer. Alternatively, the SiC composite material includes a 5-layer structure, the 1st layer (innermost layer) is a SiC / SiC composite material layer, the 2nd layer is a single-element SiC layer, the 3rd layer is a SiC / SiC composite material layer, the 4th layer is a single-element SiC layer, and the 5th layer is also a single-element SiC layer. The structures of the 1st to 4th layers are the same as above. There is no special requirement for the stiffness of the single-element SiC layer of the 5th layer, which can be the same as that of the 4th layer, or lower or higher than that of the 4th layer. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least 2 layers of the SiC / SiC composite material layers and at least 2 layers of the single-substance SiC layers. When 2 layers of the single-substance SiC layers in contact with each other are arranged between the 2 layers of the SiC / SiC composite material layers, the stiffness of the SiC / SiC composite material layer increases gradually toward the direction of the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the direction of the SiC / SiC composite material layer adjacent to the single-substance SiC layer, and the stiffness of the 2 layers of the single-substance SiC layers increases gradually toward the position where the 2 layers of the single-substance SiC layers are in contact with each other. For example, the SiC composite material includes a 4-layer structure, the first layer (innermost layer) is a SiC / SiC composite material layer, the second layer is a single-substance SiC layer, the third layer is a single-substance SiC layer, and the fourth layer is a SiC / SiC composite material layer. The stiffness of the SiC / SiC composite material layer of the first layer increases gradually toward the single-substance SiC layer of the second layer, and / or the stiffness of the single-substance SiC layer of the second layer decreases gradually toward the SiC / SiC composite material layer of the first layer, the stiffness of the SiC / SiC composite material layer of the fourth layer increases gradually toward the single-substance SiC layer of the third layer, and / or the stiffness of the single-substance SiC layer of the third layer decreases gradually toward the SiC / SiC composite material layer of the fourth layer, and the stiffness of the single-substance SiC layers of the third and fourth layers increases gradually toward the position where they are in contact. In some embodiments, the SiC composite material of the embodiments of the present invention includes at least 2 layers of the SiC / SiC composite material layers and at least 2 layers of the single-substance SiC layers. When 2 layers of the single-substance SiC layers are provided with 2 layers of the SiC / SiC composite material layers in contact with each other, the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer, and the stiffness of the 2 layers of the SiC / SiC composite material layers decreases toward the position where the 2 layers of the SiC / SiC composite material layers are in contact with each other. For example, the SiC composite material includes a 4-layer structure, the 1st layer (innermost layer) is a single-substance SiC layer, the 2nd layer is a SiC / SiC composite material layer, the 3rd layer is a SiC / SiC composite material layer, and the 4th layer is a single-substance SiC layer. The stiffness of the SiC / SiC composite material layer of the 2nd layer gradually increases toward the direction of the single-substance SiC layer of the 1st layer, and / or the stiffness of the single-substance SiC layer of the 1st layer gradually decreases toward the direction of the SiC / SiC composite material layer of the 2nd layer, the stiffness of the SiC / SiC composite material layer of the 3rd layer gradually increases toward the direction of the single-substance SiC layer of the 4th layer, and / or the stiffness of the single-substance SiC layer of the 4th layer gradually decreases toward the direction of the SiC / SiC composite material layer of the 3rd layer, and the stiffness of the SiC / SiC composite material layers of the 3rd and 4th layers gradually decreases toward the position where they are in contact. In some embodiments, when the stiffness of the SiC / SiC composite material layer increases gradually toward the single-element SiC layer adjacent to the SiC / SiC composite material layer, the SiC / SiC composite material layer includes 2-8 different stiffnesses. Preferably, the SiC / SiC composite material layer includes 3-8 different stiffnesses. By optimizing the stiffness gradient of the SiC / SiC composite material layer, it is beneficial to further reduce the stress concentration and stress difference between layers and within layers, thereby further reducing the failure probability of the cladding tube or the expansion of the cracks of the SiC / SiC composite material layer to the single-element SiC layer, and then significantly improving the airtightness of the cladding tube. When the stiffness gradient in the SiC / SiC composite material layer is too small, the process is complicated, which is not conducive to cost reduction and efficiency improvement. When the stiffness gradient in the SiC / SiC composite material layer is too large, the stress difference between various positions in the layer is large, and the stiffness of the position of the SiC / SiC composite material layer in contact with the single-element SiC layer is also too different from the stiffness of the single-element SiC layer, which is not conducive to reducing the failure probability of the device or the extension of the cracks in the SiC / SiC composite layer to the single-element SiC layer, and is further not conducive to improving the airtightness of the device. In some embodiments, when the stiffness of the SiC / SiC composite material layer increases gradually toward the single SiC layer adjacent to the SiC / SiC composite material layer, the stiffness gradient change in the same SiC / SiC composite material layer in the SiC composite material of the embodiment of the present invention can be achieved by arranging different types of materials in the layer. For example, SiC wound forming can be used at each position. f / SiC composite materials, braided SiC f / SiC composites and SiC w / SiC composite materials, etc. Generally speaking, the stiffness of the three materials listed above is ranked as follows: winding-molded SiC f / SiC composite material is smaller than woven SiC f / SiC composite materials, braided SiC f / SiC composite materials are smaller than SiC w Therefore, by selecting different types of SiC / SiC composite materials, a structure with a gradient change in stiffness in the SiC / SiC composite material layer can be achieved. In some embodiments, when the stiffness of the single SiC layer decreases gradually in the direction of the SiC / SiC composite material layer adjacent to the single SiC layer, the single SiC layer includes 2-20 different stiffnesses. Preferably, the single SiC layer includes 3-15 different stiffnesses. By optimizing the stiffness gradient of the single SiC layer, it is beneficial to further reduce the stress concentration and stress difference between layers and within layers, thereby further reducing the failure probability of the device or the expansion of the cracks of the SiC / SiC composite material layer to the single SiC layer, and then significantly improving the airtightness of the device. When the stiffness gradient in the single SiC layer is too small, the process is complicated, which is not conducive to cost reduction and efficiency improvement. When the stiffness gradient in the single-element SiC layer is too large, the stress difference between various positions in the layer is large, and the stiffness of the position of the single-element SiC layer in contact with the SiC / SiC composite material layer is also too different from the stiffness of the SiC / SiC composite material layer, which is not conducive to reducing the failure probability of the device or the extension of the cracks in the SiC / SiC composite layer to the single-element SiC layer, and is further not conducive to improving the airtightness of the device. In the related art, chemical vapor deposition is usually used to prepare a single-element SiC layer. In an embodiment of the present invention, when the stiffness of the single-element SiC layer decreases gradually in the direction of the SiC / SiC composite material layer adjacent to the single-element SiC layer, the stiffness gradient change in the same layer of the single-element SiC layer in the SiC composite material of the embodiment of the present invention can be controlled by adjusting the parameters of the chemical vapor deposition method, for example, by controlling the chemical vapor deposition temperature. Specifically, the chemical vapor deposition temperature at different positions of the single-element SiC layer is set to increase gradually in the direction of the SiC / SiC composite material layer adjacent to the single-element SiC layer. It can be understood that if the method used to prepare the single-element SiC layer is other methods, the stiffness gradient change at different positions of the single-element SiC layer can also be controlled by adjusting the preparation parameters of the method accordingly. In some embodiments, the chemical vapor deposition temperature of the single-element SiC layer at a position adjacent to the SiC / SiC composite material layer is 1100-1400°C, for example, 1100°C, 1130°C, 1150°C, 1180°C, 1200°C, 1230°C, 1250°C, 1280°C, 1300°C, 1330°C, 1350°C, 1380°C, 1400°C, etc.; and / or the chemical vapor deposition temperature of the single-element SiC layer at a position away from the SiC / SiC composite material layer is 800-1100°C, for example, 800°C, 830°C, 880°C, 900°C, 930°C, 950°C, 980°C, 1000°C, 1030°C, 1050°C, 1080°C, 1100°C, etc. By preferably adjusting the deposition temperature of the single-element SiC layer in contact with the SiC / SiC composite material layer, and / or preferably adjusting the deposition temperature of the position in the single-element SiC layer farthest from the SiC / SiC composite material layer, the stress difference between the position and the SiC / SiC composite material layer can be further reduced, and the interlayer stress concentration can be reduced, which is beneficial to further reduce the failure probability of the device or the extension of the cracks in the SiC / SiC composite material layer to the single-element SiC layer, and further beneficial to improve the airtightness of the device. In some embodiments, the chemical vapor deposition temperature of two adjacent different stiffness positions in the single-substance SiC layer differs by 30-150°C, such as 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, etc., preferably 30-70°C. By optimizing the difference in the chemical vapor deposition temperature at each position, a stiffness gradient distribution within the layer can be effectively generated, and the layer is tightly bonded, which is beneficial to further improve the airtightness of the cladding tube. When the difference in the chemical vapor deposition temperature of different stiffness positions of the single-substance SiC layer is too small, in order to reduce the interlayer stress difference with the SiC / SiC composite material layer, the stiffness gradient within the single-substance SiC layer is too small, the process is complicated, and it is not conducive to cost reduction and efficiency improvement. When the difference in chemical vapor deposition temperature at different stiffness positions of the single-element SiC layer is too large, the stiffness gradient within the single-element SiC layer is too large, which is not conducive to reducing the stress difference and stress concentration within the layer, and is not conducive to reducing the failure probability of the device or the extension of cracks in the SiC / SiC composite material layer to the single-element SiC layer, and is further not conducive to improving the airtightness of the device. In some embodiments, the chemical vapor deposition time of a single pass of the single-element SiC layer is 1-3 hours. In some embodiments, when the stiffness of the single-substance SiC layer decreases gradually in the direction of the SiC / SiC composite material layer adjacent to the single-substance SiC layer, the stiffness of the single-substance SiC layer changes continuously. The continuous change of stiffness here means that the change of stiffness at different positions in the layer is small enough, and the change is basically uninterrupted. The intra-layer stiffness gradient distribution of the single-substance SiC layer has the characteristics of continuous change, which can significantly reduce the stress concentration and stress difference in the layer, thereby reducing the failure probability of the device or the expansion of the cracks of the SiC / SiC composite material layer to the single-substance SiC layer, and then significantly improving the airtightness of the device. In some embodiments, the chemical vapor deposition temperature of the elemental SiC layer at different stiffness positions changes continuously, so that the stiffness gradient distribution within the elemental SiC layer also has the characteristic of continuous change, which can significantly reduce the stress concentration and stress difference within the layer, thereby reducing the failure probability of the device or the extension of cracks in the SiC / SiC composite material layer to the elemental SiC layer, and further significantly improving the airtightness of the device. In some embodiments, the chemical vapor deposition temperature of the single-element SiC layer at a position adjacent to the SiC / SiC composite material layer is 1100-1400°C, and / or the chemical vapor deposition temperature of the single-element SiC layer at a position away from the SiC / SiC composite material layer is 800-1100°C, for the same reasons as above. In some embodiments, the rate of change of the chemical vapor deposition temperature of the single-substance SiC layer is 0.2-2°C / min, such as 0.2°C / min, 0.5°C / min, 0.8°C / min, 1°C / min, 1.2°C / min, 1.5°C / min, 1.8°C / min, 2°C / min, etc., which is conducive to improving the continuity of the stiffness gradient distribution within the layer of the single-substance SiC layer, and further reducing the stress concentration and stress difference within the layer, thereby reducing the failure probability of the device or the extension of the crack of the SiC / SiC composite material layer to the single-substance SiC layer, and thus significantly improving the airtightness of the device. When the rate of change of the chemical vapor deposition temperature of the single-substance SiC layer is too small, the deposition time is too long, which is not conducive to cost reduction and efficiency improvement; when the rate of change of the chemical vapor deposition temperature of the single-substance SiC layer is too large, although the stiffness change also has the characteristics of gradient change, it is not conducive to improving the continuity of the stiffness change, and thus is not conducive to improving the airtightness of the device. In addition, an embodiment of the present invention further provides a cladding tube, which is prepared from the SiC composite material of an embodiment of the present invention. The cladding tube of the embodiment of the present invention can achieve good airtightness of maintaining pressure for 100 hours without obvious leakage under 15MPa positive pressure conditions, and can also achieve a thermal conductivity higher than 19W / m·k, as well as a bending strength higher than 180MPa and a burst strength higher than 170MPa, meeting the application requirements of nuclear reactors, especially meeting the requirements of the pressurized water reactor service environment, and can effectively reduce catastrophic failure problems such as micro-damage and crack propagation or reduced airtightness caused by stress mismatch problems in different functional gradient regions during the service of the pressurized water reactor, meet the mechanical, heat transfer and airtightness requirements of long-term high fuel consumption applications in the reactor, extend the service life, improve fuel consumption and provide higher accident fault tolerance. The embodiments of the present invention further provide the use of the SiC composite material according to the embodiments of the present invention in a nuclear reactor, or the use of the cladding tube according to the embodiments of the present invention in a nuclear reactor. It is understandable that the SiC composite material of the embodiment of the present invention can be used not only as the cladding material of the nuclear reactor, but also in the heat exchanger of the nuclear reactor, the first wall of the fusion reactor, the engine nozzle, the nose cone, the burner and the flow channel, etc. In some embodiments, the nuclear reactor includes a pressurized water reactor, a high temperature gas-cooled reactor, a lead-cooled fast reactor, a sodium-cooled fast reactor, or a molten salt reactor. The present invention is described in detail below with reference to the embodiments and the accompanying drawings. Example 1 A SiC composite cladding tube, comprising an inner layer of braided SiC f / SiC composite material layer, an outer single SiC layer, and the stiffness of the single SiC layer decreases gradually from the outside to the inside. The method for preparing the SiC composite material cladding tube comprises the following steps: (1) The domestic third-generation SiC fiber braided tube densified by the CVI process is further densified by a multi-pass PIP process, with each pass taking 1-2 hours to obtain a braided SiC f / SiC composite tube; (2) After the device of step (1) is lightly polished, CVI treatment is performed at 1000° C. for 5 hours; (3) placing the device processed in step (2) into a CVD furnace and performing CVD epitaxial growth at 1350° C. for 2 hours; (4) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1300°C for 2 hours; (5) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1250°C for 2 hours; (6) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1200°C for 2 hours; (7) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1150°C for 2 hours; (8) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1100°C for 2 hours; (9) After the CVD furnace is cooled, the sample is taken out, the surface of the material is lightly polished to remove surface particles and protrusions to make the surface as smooth as possible, and the dust remaining on the surface during the polishing process is removed. Then, the sample is annealed at 1000°C for 3 hours to prepare a SiC composite cladding tube. Example 2 A SiC composite cladding tube comprises an inner SiC / SiC composite layer and an outer single SiC layer, wherein the inner SiC / SiC composite layer is formed by winding the innermost SiC f / SiC composite material layer and the braided SiC f / SiC composite material layer composition, winding SiC f The stiffness of the SiC / SiC composite layer is less than that of the braided SiC f / SiC composite material layer, and the stiffness of the single SiC layer decreases gradually from the outside to the inside. The method for preparing the SiC composite material cladding tube comprises the following steps: (1) High-temperature CVI densification winding of domestic third-generation SiC fiber outside the graphite core rod; (2) Weaving SiC fibers based on step (1), performing CVI densification, and then performing three PIP processes for further densification; (3) placing the device processed in step (2) into a CVD furnace and performing CVD epitaxial growth at 1350° C. for 2 hours; (4) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1300°C for 2 hours; (5) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1250°C for 2 hours; (6) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1200°C for 2 hours; (7) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1150°C for 2 hours; (8) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1100°C for 2 hours; (9) After the CVD furnace is cooled, the sample is taken out, the surface of the material is lightly polished to remove surface particles and protrusions to make the surface as smooth as possible, and the dust remaining on the surface during the polishing process is removed. Then, the sample is annealed at 1000°C for 3 hours to prepare a SiC composite cladding tube. Example 3 A SiC composite cladding tube, comprising an inner layer of braided SiC f / SiC composite layer, the middle layer of single SiC layer, and the outer layer of SiC w / SiC composite material layer, the stiffness of the single SiC layer in the middle layer increases from the middle position to the inner layer of the woven SiC f The SiC composite material layer decreases gradually, and the stiffness of the single SiC layer in the middle layer increases from the middle position to the outer SiC layer. w / SiC composite material layer also decreases gradually. The method for preparing the SiC composite material cladding tube comprises the following steps: (1) The domestic third-generation SiC fiber braided tube densified by the CVI process is further densified by a multi-pass PIP process, with each pass taking 1-2 hours to obtain a braided SiC f / SiC composite tube; (2) After the device of step (1) is lightly polished, CVI treatment is performed at 1000° C. for 5 hours; (3) placing the device processed in step (2) into a CVD furnace and performing CVD epitaxial growth at 1350° C. for 2 hours; (4) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1300°C for 2 hours; (5) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1250°C for 2 hours; (6) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1200°C for 2 hours; (7) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1150°C for 2 hours; (8) After reducing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1100°C for 2 hours; (9) After increasing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1150°C for about 2 hours; (10) After increasing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1200°C for about 2 hours; (11) After increasing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1250°C for about 2 hours; (12) After increasing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1300°C for about 2 hours; (13) After increasing the CVD furnace temperature by 50°C, the CVD treatment was continued at 1350°C for about 2 hours; (14) After the CVD furnace has cooled, take out the sample, lightly grind the surface of the material to remove surface particles and protrusions to make the surface as smooth as possible, remove the dust remaining on the surface during the grinding process, and then anneal at 1000°C for 3 hours; (15) After coating the periphery of the device obtained in step (14) with SiC whisker slurry, the slurry is then pyrolyzed at 1200° C. to prepare a SiC composite cladding tube. Example 4 A SiC composite cladding tube, comprising an inner layer of braided SiC f / SiC composite material layer, an outer single SiC layer, and the stiffness of the single SiC layer decreases continuously from the outside to the inside. The method for preparing the SiC composite material cladding tube comprises the following steps: (1) The domestic third-generation SiC fiber braided tube densified by the CVI process is further densified by a multi-pass PIP process, with each pass taking 1-2 hours to obtain a braided SiC f / SiC composite tube; (2) After the device of step (1) is lightly polished, CVI treatment is performed at 1000° C. for 5 hours; (3) placing the device processed in step (2) into a CVD furnace, starting CVD epitaxial growth at 1350° C., and then continuing CVD epitaxial growth while cooling at a rate of 0.5° C. / min until the temperature drops to 1100° C.; (4) After the CVD furnace is cooled, the sample is taken out, the surface of the material is lightly polished to remove surface particles and protrusions to make the surface as smooth as possible, and the dust remaining on the surface during the polishing process is removed. Then, the sample is annealed at 1000°C for 3 hours to prepare a SiC composite cladding tube. Comparative Example 1 A method for preparing a SiC composite cladding tube comprises the following steps: (1) The domestic third-generation SiC fiber braided tube densified by the CVI process is further densified by a multi-pass PIP process, with each pass taking 1-2 hours to obtain a braided SiC f / SiC composite tube; (2) After the device of step (1) is lightly polished, CVI treatment is performed at 1000° C. for 5 hours; (3) The device treated in step (2) was placed in a CVD furnace and subjected to CVD epitaxial growth at 1350° C. for 12 hours to obtain the following: Fig. 9 and Fig.10 SiC composite cladding tube with the structure shown. The air tightness, thermal conductivity and mechanical properties of the SiC composite cladding tubes of Examples 1-4 and Comparative Example 1 were tested, and the results are shown in Table 1. Table 1. Air tightness, thermal conductivity and mechanical properties of SiC composite cladding tubes of Examples 1-4 and Comparative Example 1 <![CDATA[Air tightness (Pa·m 3 / s)]]> Thermal conductivity (W / m·K) Bending strength(MPa) Burst strength(MPa) Example 1 <![CDATA[2.74×10 -11 ]]> 22.4 183.5 176.7 Example 2 <![CDATA[3.16×10 -11 ]]> 21.7 186.7 182.1 Example 3 <![CDATA[5.61×10 -10 ]]> 20.1 197.6 191.3 Example 4 <![CDATA[7.82×10 -11 ]]> 19.7 185.4 188.4 Comparative Example 1 <![CDATA[1.77×10 -6 ]]> 15.4 171.6 154.2 It can be seen from Table 1 that the SiC composite cladding tubes of Examples 1-4 of the present invention have a variable stiffness functional gradient distribution design, especially the SiC composite cladding tube of Example 4 has a continuous variable stiffness distribution design, which can effectively reduce the catastrophic failure problems such as micro-damage and crack propagation or reduced airtightness caused by stress mismatch problems in different functional gradient areas during the service of the pressurized water reactor, meet the mechanical, heat transfer and airtightness requirements of long-term high fuel consumption applications in SiC composite reactors, extend the service life, improve fuel consumption and provide higher accident fault tolerance. In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction. Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A SiC composite material, characterized in that: The invention comprises at least one SiC / SiC composite material layer and at least one single-substance SiC layer, wherein the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer.

2. The SiC composite material according to claim 1, characterized in that: The method comprises at least two layers of the SiC / SiC composite material layers and at least one layer of the single-substance SiC layer. When one layer of the single-substance SiC layer is arranged between the two layers of the SiC / SiC composite material layers, the stiffness of the SiC / SiC composite material layer increases gradually toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually from the middle position of the single-substance SiC layer toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer.

3. The SiC composite material according to claim 1, characterized in that: The method comprises at least one SiC / SiC composite material layer and at least two single-substance SiC layers. When one SiC / SiC composite material layer is arranged between the two single-substance SiC layers, the stiffness of the SiC / SiC composite material layer increases gradually from the middle position of the SiC / SiC composite material layer toward the single-substance SiC layer adjacent to the SiC / SiC composite material layer, and / or the stiffness of the single-substance SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-substance SiC layer.

4. The SiC composite material according to claim 1, characterized in that: The method comprises at least two layers of the SiC / SiC composite material layers and at least two layers of the single-substance SiC layers. When the SiC / SiC composite material layers and the single-substance SiC layers are arranged alternately, the stiffness of the SiC / SiC composite material layers increases gradually from the middle position of the SiC / SiC composite material layers toward the single-substance SiC layers adjacent to the SiC / SiC composite material layers, and / or the stiffness of the single-substance SiC layers decreases gradually from the middle position of the single-substance SiC layers toward the SiC / SiC composite material layers adjacent to the single-substance SiC layers.

5. The SiC composite material according to claim 1, characterized in that: The method comprises at least two layers of the SiC / SiC composite material layers and at least two layers of the single-substance SiC layers. When two layers of the single-substance SiC layers in contact with each other are arranged between the two layers of the SiC / SiC composite material layers, the stiffness of the SiC / SiC composite material layers increases gradually toward the direction of the single-substance SiC layers adjacent to the SiC / SiC composite material layers, and / or the stiffness of the single-substance SiC layers decreases gradually toward the direction of the SiC / SiC composite material layers adjacent to the single-substance SiC layers, and the stiffness of the two layers of the single-substance SiC layers increases gradually toward the position where the two layers of the single-substance SiC layers are in contact with each other.

6. The SiC composite material according to claim 1, characterized in that: The method comprises at least two layers of the SiC / SiC composite material layers and at least two layers of the single-substance SiC layers. When two layers of the SiC / SiC composite material layers in contact with each other are arranged between the two layers of the single-substance SiC layers, the stiffness of the SiC / SiC composite material layers increases gradually in a direction toward the single-substance SiC layers adjacent to the SiC / SiC composite material layers, and / or the stiffness of the single-substance SiC layers decreases gradually in a direction toward the SiC / SiC composite material layers adjacent to the single-substance SiC layers, and the stiffness of the two layers of the SiC / SiC composite material layers decreases toward the position where the two layers of the SiC / SiC composite material layers are in contact with each other.

7. The SiC composite material according to claim 1, characterized in that: When the stiffness of the SiC / SiC composite material layer increases gradually toward the single-element SiC layer adjacent to the SiC / SiC composite material layer, the SiC / SiC composite material layer includes 2-8 different stiffnesses.

8. The SiC composite material according to claim 1 or 7, characterized in that: When the stiffness of the SiC / SiC composite material layer increases gradually toward the single SiC layer adjacent to the SiC / SiC composite material layer, the SiC / SiC composite material layer includes a wound SiC f / SiC composite materials, braided SiC f / SiC composites and SiC w At least two of the above-mentioned materials.

9. The SiC composite material according to claim 1, characterized in that: When the stiffness of the single SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single SiC layer, the single SiC layer includes 2-20 different stiffnesses.

10. The SiC composite material according to claim 1, characterized in that: When the stiffness of the single-element SiC layer decreases gradually toward the SiC / SiC composite material layer adjacent to the single-element SiC layer, the stiffness of the single-element SiC layer changes continuously.

11. The SiC composite material according to claim 1, 9 or 10, characterized in that: When the stiffness of the elemental SiC layer decreases gradually toward the direction of the SiC / SiC composite material layer adjacent to the elemental SiC layer, the elemental SiC layer is prepared by chemical vapor deposition, and the chemical vapor deposition temperature at different stiffness positions of the elemental SiC layer increases gradually toward the direction of the SiC / SiC composite material layer adjacent to the elemental SiC layer.

12. The SiC composite material according to claim 9, characterized in that: The chemical vapor deposition temperature of the single-element SiC layer at a position adjacent to the SiC / SiC composite material layer is 1100-1400°C; and / or the chemical vapor deposition temperature of the single-element SiC layer at a position away from the SiC / SiC composite material layer is 800-1100°C; and / or the chemical vapor deposition temperatures of two adjacent positions with different stiffness in the single-element SiC layer differ by 30-150°C.

13. The SiC composite material according to claim 10, characterized in that: The chemical vapor deposition temperature of the single-substance SiC layer at different stiffness positions changes continuously; and / or the chemical vapor deposition temperature of the single-substance SiC layer at a position adjacent to the SiC / SiC composite material layer is 1100-1400°C, and / or the chemical vapor deposition temperature of the single-substance SiC layer at a position away from the SiC / SiC composite material layer is 800-1100°C; and / or the rate of change of the chemical vapor deposition temperature of the single-substance SiC layer is 0.2-2°C / min.

14. A cladding tube, characterized in that: Prepared from the SiC composite material according to any one of claims 1 to 13.

15. Use of the SiC composite material according to any one of claims 1 to 13 in a nuclear reactor, or use of the cladding tube according to claim 14 in a nuclear reactor.

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