A moderator element for a nuclear reactor and a method of making the same

Abstract

The present application relates to a kind of nuclear reactor moderator elements, it includes shell and core block, wherein, core block is metal hydride moderator and is encapsulated in shell, shell is ceramic shell or metal shell and is made of sealingly connected shell and cover body.The present application also relates to the preparation method of the above-mentioned nuclear reactor moderator elements.According to the nuclear reactor moderator elements of the present application, the shell is prepared by additive manufacturing technology, and the densification of the moderator is realized by combining with heat treatment process, which solves the problems of easy oxidation and easy corrosion in the use of moderator, and meets the requirements of advanced nuclear reactor type for moderator.

Description

Technical Field

[0001] This invention relates to nuclear reactors, and more specifically to a moderator element for nuclear reactors and a method for preparing the same. Background Technology

[0002] Nuclear energy, as a clean and efficient energy source, has attracted much attention. Compared with traditional large nuclear reactors, small modular reactors (SMRs) are characterized by their small size, light weight, flexible deployment, and diverse applications, showing broad application prospects and development potential in specific scenarios such as deep space exploration, ship propulsion, and remote areas. Therefore, the development of small modular reactors has become a global research hotspot. One of the key components of a small modular reactor is the moderator, which slows down fast neutrons in the reactor into thermal neutrons.

[0003] Moderator elements are core components of reactor design. Their design and fabrication determine the reactor's sophistication and economic efficiency.

[0004] Traditional graphite moderator element manufacturing processes suffer from problems such as long production cycles, high costs, and low raw material utilization. Furthermore, graphite and other moderators are large in size when used in small reactors, which cannot achieve reactor miniaturization and flexibility.

[0005] Metal hydrides (zirconium hydride, yttrium hydride) have advantages such as high hydrogen content, good thermal conductivity, and low neutron absorption cross section, making them ideal materials for moderators in small nuclear reactors. Among them, yttrium hydride has the advantages of low hydrogen partial pressure and good high-temperature stability.

[0006] The moderator comes into contact with the coolant inside the reactor and is subjected to high temperatures and strong neutron radiation. Under long-term service conditions, the moderator material will suffer from high temperature, corrosion and radiation damage. In the service environment, it will provide a diffusion path for hydrogen and accelerate hydrogen evolution, thereby reducing the moderation performance of yttrium hydride and seriously affecting the service life of the material and the safety of the nuclear reactor. Summary of the Invention

[0007] To address the problems of susceptibility to high temperatures, corrosion, and radiation damage in the use of moderators in the prior art, this invention provides a moderator element for nuclear reactors and its preparation method.

[0008] The nuclear reactor moderator element according to the present invention includes a shell and a core, wherein the core is a metal hydride moderator and is encapsulated within the shell, and the shell is a ceramic shell or a metal shell and consists of a sealed shell and a cover.

[0009] In a preferred embodiment, the outer shell is a silicon carbide shell, an alumina shell, a zirconium carbide shell, a tungsten shell, a molybdenum shell, or a tungsten-molybdenum alloy shell.

[0010] In a preferred embodiment, the chip is a yttrium hydride moderator or a zirconium hydride moderator.

[0011] In a preferred embodiment, the fill factor of the chip is between 10% and 80%.

[0012] The method for preparing a moderator element for a nuclear reactor according to the present invention includes the following steps: S1, providing a cover and a shell with a pre-reserved opening through an additive manufacturing process, and performing a first heat treatment on the cover and the shell, the first heat treatment including a first densification treatment on the cover and the shell through a chemical vapor infiltration process or a silicon infiltration process; S2, providing a core block, placing the core block into the cavity of the shell through the opening, filling the gap between the shell and the core block with filler powder, and connecting the cover to the shell to obtain a preform; S3, performing a second heat treatment on the preform, the second heat treatment including a sintering treatment and a second densification treatment, the filler powder being sintered through the sintering treatment, and the connection between the shell and the cover being sealed through the second densification treatment to ensure a seal.

[0013] In a preferred embodiment, in step S1, the additive manufacturing process is 3D printing.

[0014] In a preferred embodiment, in step S1, a first thread or a first slot is printed at the opening of the housing, and a second thread or a second slot is printed on the cover. The second thread is connected to the first thread to achieve a threaded connection between the cover and the housing, or the second slot is connected to the first slot to achieve a slotted connection between the cover and the housing.

[0015] In a preferred embodiment, in step S2, the core block is formed by hot pressing sintering, plasma sintering, or direct hydrogenation.

[0016] In a preferred embodiment, in step S2, the filling powder is silicon carbide powder, alumina powder, zirconium carbide powder, tungsten powder, molybdenum powder, or tungsten-molybdenum alloy powder.

[0017] In a preferred embodiment, in steps S1 and S3, the first and second densification treatment temperatures are 900°C and 1200°C, respectively.

[0018] The moderator element for nuclear reactors according to the present invention utilizes additive manufacturing technology to prepare the outer shell, combined with heat treatment processes, to achieve dense encapsulation of the moderator. This solves problems such as easy oxidation and corrosion during moderator use, meeting the requirements of advanced nuclear reactors for moderators. Furthermore, the moderator element for nuclear reactors according to the present invention not only offers flexible design, simple manufacturing process, and rapid molding speed, but also improves the service life of the moderator and the safety of reactor operation, significantly reducing the manufacturing cost of the moderator element for nuclear reactors. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a moderator element for a nuclear reactor according to the present invention.

[0020] Figure 2 yes Figure 1 A schematic diagram of the preparation method of moderator elements for nuclear reactors.

[0021] Figure 3 The cracking phenomenon of the yttrium hydrogenation moderator block according to Comparative Example 1 of the present invention is shown. Detailed Implementation

[0022] The preferred embodiments of the present invention are given below with reference to the accompanying drawings and described in detail.

[0023] like Figure 1 As shown, the nuclear reactor moderator element 1 according to the present invention includes a shell 11 and a core 12, wherein the shell 11 is a ceramic shell or a metal shell and is composed of a sealed shell 111 and a cover 112, and the core 12 is a metal hydride moderator and is encapsulated within the shell 11.

[0024] Metal hydrides, used as moderator materials in nuclear reactors, experience hydrogen evolution due to high temperatures, radiation, and corrosion under actual operating conditions. This occurs when the hydrogen partial pressure in the operating environment equals YH. 2-x At equilibrium hydrogen pressure (where hydrogen release leads to a decrease in the H / Y ratio), the hydrogen in the metal hydride is in dynamic equilibrium with the hydrogen in the environment. When the metal hydride is in an open space, YH... 2-x Hydrogen will continue to be released until all hydrogen is released, leaving only pure metallic yttrium. This invention provides a hydrogen barrier or controls the hydrogen content in the environment through the outer shell 11, thereby enabling the metal hydride moderator to remain effective in the nuclear reactor for a long period. It should be understood that when applied in a nuclear reactor, neutrons pass through the outer shell 11 and are moderated by the core 12. That is, the encapsulation material does not affect the use of the metal hydride moderator, and by preventing the metal hydride moderator from reacting with reactor materials and losing hydrogen, it avoids structural changes and degradation of the moderator's moderating performance, ensuring the stability of the moderator element 1 in extreme service environments, thereby improving the safety and economy of the nuclear reactor.

[0025] In a preferred embodiment, the outer shell 11 is a silicon carbide shell because silicon carbide has advantages such as high thermal conductivity, high melting point, corrosion resistance, radiation resistance, and good compatibility with moderators, making it suitable for the high-temperature, high-radiation environment of reactors. It should be understood that the outer shell 11 can also be an alumina shell, zirconium carbide shell, tungsten shell, molybdenum shell, or tungsten-molybdenum alloy shell, etc.

[0026] In a preferred embodiment, the core 12 is a yttrium hydride moderator or a zirconium hydride moderator, etc. Compared to graphite moderators, which require mixing, drying, crushing, sieving, pressing, carbonizing, and graphitizing processes, the metal hydride moderator of this invention has a simpler preparation process, solving the problems of long cycle time and high cost. Moreover, for the same moderation efficiency, graphite requires 1 cubic meter, while metal hydride moderator only requires 0.2 cubic meters, thus reducing the core size and achieving reactor miniaturization and operational flexibility (vehicle-mounted, ship-mounted, and vessel-mounted).

[0027] In a preferred embodiment, the housing 111 and the cover 112 are connected by a threaded connection or a slot connection.

[0028] In a preferred embodiment, the outer shell 11 is spherical, cylindrical, hexagonal prism, cube, cuboid, or other irregularly shaped. Taking a cylindrical shape as an example, the inner diameter of the shell 111 is slightly larger than the outer diameter of the cover 112, so that the cover 112 can be placed on top of the core block 12 for connection.

[0029] In a preferred embodiment, the thickness of the outer shell 11 is between 1 mm and 3 mm.

[0030] The volume fraction of the reactor moderator element 12 relative to the total volume of the reactor pellet 1 is referred to as the fill factor or loading. In a preferred embodiment, the fill factor is between 10% and 80%, preferably between 11% and 60%, and more preferably between 30% and 45%. It should be understood that the fill factor can be adjusted according to the actual application, for example, by increasing or decreasing the thickness of the outer shell 11.

[0031] In a preferred embodiment, the inner diameter of the outer shell 11 is 1 mm larger than the diameter of the core block 12. It should be understood that the inner diameter of the outer shell 11 is only slightly larger than the diameter of the core block 12, so that the core block 12 can be placed inside the cavity of the outer shell 11.

[0032] like Figure 2As shown, the method for preparing the moderator element 1 for a nuclear reactor according to the present invention first includes providing a shell 111 and a cover 112 with pre-reserved openings respectively through an additive manufacturing process (e.g., 3D printing). Specifically, three-dimensional models of the shell 111 and the cover 112 are designed using computer-aided software (such as Pro / Engineering, Unigraphics, CATIA, Solidworks, etc.), then sliced ​​and layered (using Magics, Mimics, etc.), and the contours are imported into a 3D printing device. The 3D printing device is then used to print the silicon carbide shell 111 and the cover 112 respectively using silicon carbide powder. It should be understood that the silicon carbide powder here can also be replaced with other ceramic powders or metal powders suitable for 3D printing, such as alumina, zirconium carbide ceramic powder, and tungsten, molybdenum, tungsten-molybdenum alloy powder, etc. It should be understood that a 3D printing device is a printing device that utilizes at least one of the following technologies: photopolymerization, three-dimensional printing, inkjet printing, selective laser sintering, selective laser melting, direct metal laser sintering, and electron beam fused deposition modeling. During the 3D printing process, a first thread is printed at the opening of the shell 111, and a second thread is printed on the cover 112, which connects with the first thread to achieve a threaded connection between the cover 112 and the shell 111. It should be understood that the first thread can also be replaced by a first slot, and the second thread can also be replaced by a second slot, which connects with the first slot to achieve a slotted connection between the cover 112 and the shell 111.

[0033] The method for preparing the moderator element 1 for a nuclear reactor according to the present invention further includes densifying the shell 111 and the cover 112 by a chemical vapor infiltration process or a silicon infiltration process. It should be understood that the densification temperature is 900°C-1200°C.

[0034] The method for preparing the moderator element 1 for a nuclear reactor according to the present invention further includes providing a pellet 12. In a preferred embodiment, the pellet 12 is formed by processes such as hot pressing sintering, plasma sintering, or direct hydrogenation. In a preferred embodiment, the pellet 12 is obtained by spark plasma sintering. Specifically, a certain mass of yttrium hydride powder and yttrium oxide powder as a sintering aid are weighed and uniformly mixed, and the pellet 12 is obtained by spark plasma sintering.

[0035] The method for preparing the moderator element 1 for nuclear reactors according to the present invention further includes placing a core block 12 into the cavity of a shell 111 through an opening, filling the gap between the shell 111 and the core block 12 with silicon carbide powder, and connecting the cover 112 to the shell 111 to obtain a preform.

[0036] The method for preparing the moderator element 1 for a nuclear reactor according to the present invention further includes sintering and densifying a preform, sintering silicon carbide powder filling the space between the shell 111 and the core 12, and sealing the connection between the shell 111 and the cover 112 through densification to ensure a tight seal. It should be understood that the sintering temperature is set according to the material properties of the powder filling the space between the shell 111 and the core 12, and the densification temperature is 900°C-1200°C. Densification can be achieved through in-situ reaction. It should be understood that sintering and densification can be a single step (e.g., densification occurs during the sintering process for some materials) or two steps (e.g., some materials require the addition of additives for in-situ reaction to achieve densification during heat treatment), ultimately providing the thermodynamic properties of the moderator element 1 for the nuclear reactor.

[0037] Example 1

[0038] The printing equipment was the EXOne S-MAX Pro adhesive jet printing system, which used EXOne's BA005 adhesive.

[0039] A cylindrical shell 111 is printed, with an outer diameter of 12.7 mm, a wall thickness of 1.5 mm, a bottom thickness of 2 mm, and a height of 30 mm. Threads are also printed to facilitate connection with the cover 112. After printing, the powder bed is cured at 180°C for 8 hours. The sample blank is then removed, and the remaining green blank and powder are recovered from the powder bed.

[0040] A silicon carbide disc (slightly smaller in diameter than the inner diameter of the cylindrical barrel) is printed as the cover 112, with a thickness of 1.5 mm. Threads or slots are printed to facilitate connection with the housing 111. After printing, the powder bed is cured at 180°C for 8 hours. The sample preform is then removed, and the remaining green preform and powder are recovered from the powder bed.

[0041] At a temperature of 950°C, methyltrichlorosilane is used as a precursor and is permeated for 100 hours to densify the shell 111 and the cover 112.

[0042] Yttrium hydride powder (98 wt.%) and yttrium oxide sintering aid (2 wt.%) were weighed according to their nominal composition, uniformly mixed, and cold-pressed in an argon glove box before being placed in a vacuum graphite crucible. The sintering temperature was 1000℃, and the sample was held at 1000℃ for 10 minutes. After the sample was placed in the furnace, a vacuum was applied. A pressure of 65 MPa was applied to the punch before the heating process, and the pressure was removed after the holding period. After holding, the furnace was allowed to cool naturally, yielding core block 12.

[0043] The core block 12 is placed inside the cavity of the housing 111, and then silicon carbide powder is used to fill the gap between the housing 111 and the core block 12, and then the cover 112 is used to seal it to obtain a preform.

[0044] The preformed blank is subjected to sintering and densification treatment. The total pressure of the system is 3 kPa, the molar ratio of H2, Ar and MTS is 6:8:1, the temperature is set at 995℃, the reaction time is 50 hours, and finally the seal is achieved to obtain the final moderator element 1 for nuclear reactor. The moderator accounts for about 45% of the volume of the element.

[0045] Comparative Example 1

[0046] A certain mass of yttrium hydride powder and yttrium oxide powder (a sintering aid) were weighed and uniformly mixed. Yttrium hydride moderator blocks were then prepared using a spark plasma sintering process. The pressure was 0.3t, and the sintering temperature was 1800℃. Figure 3 As shown, the prepared yttrium hydrogenation moderator blocks, when stored in an air environment, exhibit a certain degree of cracking due to their inability to withstand water and oxygen corrosion in the air.

[0047] Obviously, unencapsulated yttrium hydride moderator blocks are susceptible to corrosion by water and oxygen, thus failing to achieve neutron moderation throughout their lifespan. Compared to the unencapsulated use of moderator elements in traditional gas-cooled reactors, this invention uses an outer shell 11 to prevent the core 12 from directly contacting the coolant, thereby reducing the thermodynamic properties and moderation capability of the moderator. The outer shell 11, formed by layer-by-layer stacking of materials, facilitates the sealing of the core 12, making the nuclear reactor moderator element 1 according to this invention more advantageous. In particular, direct compression encapsulation is prone to cracking due to the mismatch in thermal expansion coefficients between the outer shell material and the moderator material. Machining-based subtractive processing of the outer shell is not conducive to raw material recycling and makes it difficult to process complex structural components. This invention achieves flexible design of the encapsulated outer shell through additive manufacturing, which not only simplifies the process and increases molding speed but also improves raw material utilization, significantly reducing the manufacturing cost of nuclear reactor moderator elements. Thus, by using additive manufacturing technology to prepare the outer shell 11, specific non-moderator areas (i.e., the outer shell 11) and moderator areas (i.e., the core block 12) can be realized as needed, thereby achieving an orderly distribution of the moderator, increasing molding speed, reducing costs, improving material utilization, and enhancing the thermodynamic properties and moderation capability of the nuclear reactor moderator element 1. Simultaneously, it allows for integrated molding of the component's structural design and manufacturing, enabling repeated digital mold modification and printed component verification, thereby accelerating the development cycle and saving development costs. This is an effective technical solution for low-cost rapid prototyping manufacturing of the nuclear reactor moderator element 1. Practice has shown that the encapsulated nuclear reactor moderator element 1 according to the present invention has advantages such as radiation resistance, corrosion resistance, and a longer service life.

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims of this patent. All aspects not described in detail in this invention are conventional technical content.

Claims

1. A method for preparing a moderator element for a nuclear reactor, characterized in that, The preparation method includes the following steps: S1, a silicon carbide cover and a silicon carbide shell with a pre-drilled opening are provided by 3D printing process. A first thread or a first slot is printed at the opening of the silicon carbide shell, and a second thread or a second slot is printed on the silicon carbide cover. The second thread is connected to the first thread to realize the threaded connection between the silicon carbide cover and the silicon carbide shell, or the second slot is connected to the first slot to realize the slot connection between the silicon carbide cover and the silicon carbide shell. The silicon carbide cover and the silicon carbide shell are subjected to a first heat treatment, which includes a first densification treatment of the silicon carbide cover and the silicon carbide shell by chemical vapor infiltration process. S2, providing a metal hydride moderator core, the core being yttrium hydride moderator, the core being placed into the cavity of the silicon carbide shell through the opening, the gap between the silicon carbide shell and the core being filled with silicon carbide powder, the silicon carbide cover being connected to the silicon carbide shell to obtain a preformed blank; S3, the preform is subjected to a second heat treatment, which includes a sintering treatment and a second densification treatment. The silicon carbide powder is sintered by the sintering treatment, and the connection between the silicon carbide shell and the silicon carbide cap is sealed by the second densification treatment to ensure a seal, thereby obtaining a moderator element for a nuclear reactor.

2. The preparation method according to claim 1, characterized in that, The fill factor of the core is between 10% and 80%.

3. The preparation method according to claim 1, characterized in that, In step S2, the core block is formed by hot pressing sintering, plasma sintering, or direct hydrogenation.

4. The preparation method according to claim 1, characterized in that, In steps S1 and S3, the first and second densification treatment temperatures are 900 ℃-1200 ℃, respectively.

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