A moderated fuel element and method of making same

Abstract

The present application relates to a kind of moderator-containing fuel elements, it includes shell and pellet, wherein, pellet is encapsulated in shell and includes fuel particles of dispersion type distribution in moderator, 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 moderator-containing fuel element.According to the moderator-containing fuel element of the present application, the shell is prepared by hot-pressing sintering or additive manufacturing technology, and then combined with heat treatment process, the densification encapsulation of moderator and fuel is realized, the problems of easy oxidation and easy corrosion in the use of moderator and fuel are solved, and the design requirements of advanced nuclear reactor type on moderator-containing fuel element are met.

Description

Technical Field

[0001] This invention relates to nuclear reactors, and more specifically to a moderator-containing fuel element and its preparation method. Background Technology

[0002] 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, marine propulsion, and remote areas. Therefore, the development of SMRs has become a global research hotspot. The design and fabrication of fuel elements in SMRs determine the reactor's advancement and economic efficiency.

[0003] Traditional graphite-based fuel element manufacturing processes suffer from problems such as long production cycles, high costs, and low raw material utilization. Furthermore, the large volume of moderators such as graphite in small reactors hinders the miniaturization and flexibility of the reactor.

[0004] 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.

[0005] Typically, the moderator and fuel elements are assembled into separate modules and arranged within the reactor core. To optimize thermal-hydraulic performance and prevent damage to the moderator and fuel from high temperatures, corrosion, and radiation within the reactor core, the moderator and fuel must be encapsulated to ensure their stability in extreme service environments, thereby improving the safety and economy of the nuclear reactor. Summary of the Invention

[0006] In order to solve the problems of susceptibility to high temperature, corrosion and radiation damage when using moderators and fuels in the prior art, the present invention provides a fuel element containing a moderator and a method for preparing the same.

[0007] According to the present invention, a fuel element containing a moderator includes a housing and a pellet, wherein the pellet is encapsulated within the housing and includes fuel particles dispersed in the moderator, and the housing is a ceramic or metal housing and consists of a sealed housing and a cover.

[0008] 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.

[0009] In a preferred embodiment, the moderator is a yttrium hydride moderator or a zirconium hydride moderator, and the fuel particles are coated fuel particles.

[0010] In a preferred embodiment, the fill factor of the chip is between 10% and 50%.

[0011] In a preferred embodiment, the mass ratio of moderator to fuel particles in the pellet is 10%-45%:55%-90%.

[0012] According to the method for preparing a moderator-containing fuel element of the present invention, the method comprises the following steps: S1, providing a cover and a shell with a pre-reserved opening by hot pressing sintering or additive manufacturing processes, and performing a first heat treatment on the cover and the shell, the first heat treatment comprising performing a first densification treatment on the cover and the shell by chemical vapor infiltration or silicon infiltration processes; 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 comprising sintering and a second densification treatment, the filler powder being sintered by the sintering treatment, and the connection between the shell and the cover being sealed by the second densification treatment to ensure a seal.

[0013] In a preferred embodiment, in step S1, the additive manufacturing process is 3D printing. A first thread or a first slot is printed at the opening of the shell, 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 shell, or the second slot is connected to the first slot to achieve a slotted connection between the cover and the shell.

[0014] In a preferred embodiment, in step S2, the moderator and fuel particles are mixed and formed into a pellet by hot pressing sintering, plasma sintering, or direct hydrogenation processes to achieve uniform distribution of fuel particles in the moderator.

[0015] 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.

[0016] In a preferred embodiment, in steps S1 and S3, the first and second densification treatment temperatures are 900°C-1200°C, respectively.

[0017] The moderator-containing fuel element of the present invention utilizes hot-pressing sintering or additive manufacturing techniques to prepare the outer shell, followed by heat treatment to achieve dense encapsulation of the moderator and fuel. This addresses the problems of easy oxidation and corrosion of the moderator and fuel during use, meeting the design requirements of advanced nuclear reactors for moderator-containing fuel elements. Furthermore, the moderator-containing fuel element of the present invention not only offers flexible design and simple manufacturing processes with rapid molding speeds, but also improves the service life of the moderator and fuel and the safety of reactor operation, significantly reducing the manufacturing cost of the moderator element. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of a fuel element containing a moderator according to the present invention.

[0019] Figure 2 yes Figure 1 A schematic diagram of the microstructure of a fuel element containing a moderator.

[0020] Figure 3 yes Figure 1 A schematic diagram of the preparation method of a fuel element containing a moderator.

[0021] Figure 4 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 Figures 1-2 As shown, the fuel element 1 containing a moderator according to the present invention includes a housing 11 and a pellet 12, wherein the housing 11 is a ceramic shell or a metal shell and is composed of a housing 111 and a cover 112 that are sealed together, and the pellet 12 is encapsulated in the housing 11 and includes fuel particles 122 that are dispersedly distributed in the moderator 121.

[0024] The present invention provides a barrier through the outer shell 11 to ensure the stability of the moderator fuel 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. In particular, the outer shell 11 has good compatibility with the moderator 121.

[0026] In a preferred embodiment, the mass ratio of moderator 121 to fuel pellets 122 is 10%-45%:55%-90%. It should be understood that the moderator-containing fuel element 1 according to the present invention includes a pellet 12 composed of moderator 121 and fuel pellets 122, and may also include a pellet 12 provided solely by moderator 121 (i.e., only moderator 121 is encapsulated within the outer casing 11), thereby modulating the reactor's reactivity.

[0027] In a preferred embodiment, the moderator 121 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).

[0028] In a preferred embodiment, the fuel particle 122 is a four-layer coated fuel particle, consisting of, from the inside out, a fuel core, loose pyrolytic carbon, dense pyrolytic carbon, silicon carbide, and a dense pyrolytic carbon layer. It should be understood that the fuel particle 122 does not chemically react with the moderator 121 and has good compatibility. The selection of the fuel particle 122 is based on the reactor type and its requirements. For example, tungsten metal fuel particles, niobium metal fuel particles, and silicon carbide fuel particles all have good compatibility with yttrium hydride moderators.

[0029] In a preferred embodiment, the housing 111 and the cover 112 are connected by a threaded connection or a slot connection.

[0030] 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 shell 111 for connection. In a preferred embodiment, the pellet 12 is cylindrical with a diameter of 10mm-12.7mm and a height of 10mm-25mm, wherein the fuel particles 122 are spherical with a diameter of 100 micrometers-1000 micrometers, uniformly distributed in the moderator 121. It should be understood that the specific size of the pellet 12 can be adjusted according to the reactor design, and multiple pellets 12 can also be stacked into a component and arranged in the reactor.

[0031] In a preferred embodiment, the thickness of the outer shell 11 is between 1 mm and 3 mm.

[0032] The volume fraction of the fuel element 12 containing the moderator in the total volume of the pellet 12 is referred to as the fill factor or loading. In a preferred embodiment, the fill factor is between 10% and 50%, preferably between 15% and 45%, and more preferably between 30% and 40%. 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 housing 11.

[0033] 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.

[0034] like Figure 3 As shown, the method for preparing the moderator-containing fuel element 1 according to the present invention first includes providing a shell 111 and a cover 112 with pre-reserved openings by hot pressing sintering or additive manufacturing processes (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 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.

[0035] The method for preparing the moderator-containing fuel element 1 according to the present invention further includes densifying the housing 111 and the cover 112 by a chemical vapor infiltration process or a silicon infiltration process. It should be understood that the densification process temperature is 900°C-1200°C.

[0036] The method for preparing the moderator-containing fuel element 1 according to the present invention further includes uniformly mixing the moderator 121 and fuel particles 122 to provide a pellet 12. In a preferred embodiment, the pellet 12 is formed by processes such as hot pressing sintering, plasma sintering, or direct hydrogenation to achieve a uniform distribution of the fuel particles 122 in the moderator 121. In a preferred embodiment, the pellet 12 is obtained by spark plasma sintering. Specifically, a certain mass of coated fuel particles, yttrium hydrogenation powder, and yttrium oxide powder (a combustion aid) are weighed and uniformly mixed, and the pellet 12 is obtained by spark plasma sintering.

[0037] The method for preparing the moderator-containing fuel element 1 according to the present invention further includes placing the pellet 12 into the cavity of the housing 111 through an opening, filling the gap between the housing 111 and the pellet 12 with silicon carbide powder, and connecting the cover 112 to the housing 111 to obtain a preform.

[0038] The method for preparing the moderator-containing fuel element 1 according to the present invention further includes sintering and densifying a preform, sintering the 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-containing fuel element 1.

[0039] Example 1

[0040] The printing equipment was the EXOne S-MAX Pro adhesive jet printing system, which used EXOne's BA005 adhesive.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] Yttrium hydride powder (40 wt.%) and tungsten fuel particles (60 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 mixture 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.

[0045] 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.

[0046] The preformed blank was sintered and densified. The total pressure of the system was 3 kPa, the molar ratio of H2, Ar and MTS was 6:8:1, the temperature was set at 1000℃, and the reaction time was 50 hours. Finally, the blank was sealed to obtain the final fuel element 1 containing the moderator. The particle filling factor of the fuel element 1 containing the moderator was about 40 vol.

[0047] Comparative Example 1

[0048] 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 1000℃. Figure 4 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.

[0049] Obviously, unencapsulated yttrium hydride moderator blocks are susceptible to corrosion by water and oxygen, thus failing to achieve neutron moderation capabilities throughout their lifespan. Compared to the unencapsulated use of moderators and fuels in traditional gas-cooled reactors, this invention uses an outer shell 11 to prevent direct contact between the pellet 12 and the coolant, thereby avoiding a reduction in the thermodynamic properties and moderation capabilities of the moderator and fuel. In a preferred embodiment, the outer shell 11, formed by layer-by-layer stacking of materials, facilitates the sealing of the pellet 12, which is even more advantageous for the moderator-containing fuel element 1 according to this invention. 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 and significantly reduces the manufacturing cost of the moderator element. Thus, by using additive manufacturing technology to prepare the outer shell 11, specific regions without moderators (i.e., the outer shell 11) and regions with moderators (i.e., the fuel cell 12) can be created as needed, thereby achieving an orderly distribution of moderators and fuel. This increases molding speed, reduces costs, improves material utilization, and enhances the thermodynamic properties and moderation capability of the moderator-containing fuel 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 moderator-containing fuel element 1. Practice has shown that the encapsulated moderator-containing fuel element 1 according to the present invention has advantages such as radiation resistance, corrosion resistance, and a longer service life.

[0050] 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 fuel element containing a moderator, 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 pellet including fuel particles dispersed in a moderator, the moderator being yttrium hydride moderator, the fuel particles being coated fuel particles, the pellet being placed into the cavity of a silicon carbide shell through an opening, the gap between the silicon carbide shell and the pellet being filled with silicon carbide powder, and the silicon carbide cap being connected to the silicon carbide shell to obtain a preform. 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 fuel element containing a moderator.

2. The preparation method according to claim 1, characterized in that, The fill factor of the core is between 10% and 50%.

3. The preparation method according to claim 1, characterized in that, The mass ratio of moderator to fuel particles in the pellets is 10%-45%: 55%-90%.

4. The preparation method according to claim 1, characterized in that, In step S2, the moderator and fuel particles are mixed and formed into pellets by hot pressing, plasma sintering, or direct hydrogenation processes to achieve uniform distribution of fuel particles in the moderator.

5. 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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