Fuel element containing moderator and preparation method thereof

Abstract

The invention relates to a moderator-containing fuel element which comprises a shell and a core block, the core block is packaged in the shell and comprises fuel particles distributed in a moderator in a dispersed mode, and the shell is a ceramic shell or a metal shell and is composed of a shell body and a cover body which are connected in a sealed mode. The invention further relates to a preparation method of the fuel element containing the moderator. According to the fuel element containing the moderator, the shell is prepared through the hot pressing sintering or additive manufacturing forming technology, then the heat treatment technology is combined, densified packaging of the moderator and the fuel is achieved, and the problems that the moderator and the fuel are prone to oxidation and corrosion in the using process are solved; and the design requirements of advanced nuclear reactor types on moderator-containing fuel elements are met.

Description

Technical Field

[0001] The present invention relates to a nuclear reactor, and more particularly to a fuel element containing a moderator and a preparation method thereof. Background Art

[0002] Small modular reactors (SMRs) have the characteristics of small size, light weight, flexible deployment, and multiple uses. They have broad application prospects and development expectations in specific scenarios such as deep space exploration, ship propulsion, and remote areas. Therefore, the development of small modular reactors has become a hot topic in global research. The design and preparation of fuel elements of small modular reactors determine the advancement and economy of the reactor.

[0003] The traditional graphite-based fuel element preparation process has problems such as long production cycle, high cost, and low raw material utilization rate. In addition, moderators such as graphite are large in size when used in small reactors, which is not conducive to achieving the miniaturization and flexibility of the reactor.

[0004] Metal hydrides (zirconium hydride, yttrium hydride) have the advantages of high hydrogen content, good thermal conductivity and low neutron absorption cross section, and are one of the ideal materials for small nuclear reactor moderators. Among them, yttrium hydride has the advantages of low hydrogen partial pressure and good high temperature stability.

[0005] Generally, moderators and fuel elements are made into components and arranged in the core. In order to optimize the thermal hydraulic performance and prevent the moderators and fuel from being damaged by high temperature, corrosion and radiation in the core, the moderators and fuel must be packaged to ensure their stability in extreme service environments, thereby improving the safety and economy of nuclear reactors. Summary of the invention

[0006] In order to solve the problems of the moderator and fuel in the prior art being susceptible to high temperature, corrosion and radiation damage when used, the present invention provides a fuel element containing a moderator and a preparation method thereof.

[0007] The moderator-containing fuel element according to the present invention comprises an outer shell and a pellet, wherein the pellet is encapsulated in the outer shell and comprises fuel particles dispersed in the moderator, and the outer shell is a ceramic shell or a metal shell and consists of a shell body and a cover body which are sealed and connected.

[0008] In a preferred embodiment, the shell is a silicon carbide shell, an aluminum oxide 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 filling factor of the pellets is between 10% and 50%.

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

[0012] According to the preparation method of the moderator-containing fuel element of the present invention, it includes the following steps: S1, providing a cover body and a shell with an opening reserved therein respectively by hot pressing sintering or additive manufacturing process, and performing a first heat treatment on the cover body and the shell respectively, and the first heat treatment includes performing a first densification treatment on the cover body and the shell respectively by a chemical vapor infiltration process or a siliconizing 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 filling powder, connecting the cover body to the shell, and obtaining a preformed body; S3, performing a second heat treatment on the preformed body, and the second heat treatment includes a sintering treatment and a second densification treatment, the filling powder is sintered by the sintering treatment, and the connection between the shell and the cover body is sealed by the second densification treatment to ensure sealing.

[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, a second thread or a second slot is printed on the cover body, the second thread is connected to the first thread to achieve a threaded connection between the cover body and the shell, or the second slot is connected to the first slot to achieve a slot connection between the cover body and the shell.

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

[0015] In a preferred embodiment, in step S2, the filling powder is silicon carbide powder, aluminum oxide powder, zirconium carbide powder, tungsten powder, molybdenum powder or tungsten-molybdenum alloy powder.

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

[0017] According to the moderator fuel element of the present invention, the shell is prepared by hot pressing sintering or additive manufacturing molding technology, and then combined with the heat treatment process, the dense packaging of the moderator and the fuel is realized, the problems of easy oxidation and corrosion of the moderator and the fuel in use are solved, and the design requirements of the advanced nuclear reactor type for the moderator fuel element are met. Moreover, the moderator fuel element according to the present invention is not only flexible in design, simple in process, and fast in molding speed, but also improves the service life of the moderator and the fuel and the safety of the reactor operation, and greatly reduces the preparation cost of the moderator element. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It 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 Schematic diagram of the microstructure of a fuel element containing a moderator.

[0020] Figure 3 yes Figure 1 Schematic diagram of a method for preparing a fuel element containing a moderator.

[0021] Figure 4 The cracking phenomenon of the yttrium hydride moderator block according to Comparative Example 1 of the present invention is shown. DETAILED DESCRIPTION

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

[0023] like Figure 1-Figure 2 As shown, the moderator-containing fuel element 1 according to the present invention includes an outer shell 11 and a pellet 12, wherein the outer shell 11 is a ceramic shell or a metal shell and is composed of a shell body 111 and a cover body 112 that are sealed and connected, and the pellet 12 is encapsulated in the outer shell 11 and includes fuel particles 122 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-containing fuel element 1 in an extreme service environment, thereby improving the safety and economy of the nuclear reactor.

[0025] In a preferred embodiment, the shell 11 is a silicon carbide shell, because silicon carbide has the advantages of high thermal conductivity, high melting point, corrosion resistance, radiation resistance, good compatibility with the moderator, etc., and is suitable for the high temperature and high radiation environment of the reactor. It should be understood that the shell 11 can also be an aluminum oxide shell, a zirconium carbide shell, a tungsten shell, a molybdenum shell or a tungsten-molybdenum alloy shell, etc. In particular, the shell 11 has good compatibility with the moderator 121.

[0026] In a preferred embodiment, the mass ratio of the moderator 121 to the fuel particles 122 is 10%-45%: 55%-90%. It should be understood that the fuel element 1 containing the moderator according to the present invention includes a pellet 12 composed of the moderator 121 and the fuel particles 122, and may also include a pellet 12 provided only by the moderator 121 (that is, only the moderator 121 is encapsulated in the shell 11), so as to adjust the reactivity of the reactor.

[0027] In a preferred embodiment, the moderator 121 is a yttrium hydride moderator or a zirconium hydride moderator. Compared with the graphite moderator, which needs to be prepared by mixing, mixing, drying, crushing, screening, pressing, carbonization, graphitization and other processes, the preparation steps of the metal hydride moderator of the present invention are simple, which can solve the problems of long cycle and high cost. Moreover, for the same moderation efficiency, graphite requires 1 cubic meter, while the metal hydride moderator only requires 0.2 cubic meters, so the core size can be reduced, and the miniaturization and flexibility of use of the reactor (car owner, ship, shipboard) can be achieved.

[0028] In a preferred embodiment, the fuel particles 122 are coated fuel particles with a four-layer structure, which are fuel core, loose pyrolytic carbon, dense pyrolytic carbon, silicon carbide and dense pyrolytic carbon layer from the inside to the outside. It should be understood that the fuel particles 122 do not chemically react with the moderator 121 and have good compatibility. The fuel particles 122 are selected according to the requirements of the reactor type. For example, metal tungsten fuel particles, metal niobium fuel particles and silicon carbide fuel particles all have good compatibility with the yttrium hydride moderator.

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

[0030] In a preferred embodiment, the shell 11 is spherical, cylindrical, hexagonal, cube-shaped, cuboid-shaped or other special-shaped structures. Taking the 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 in the shell 111 above the pellet 12 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 microns to 1000 microns uniformly distributed in the moderator 121. It should be understood that the specific size of the pellet 12 can be adjusted according to the design of the reactor, and multiple pellets 12 can also be stacked into assemblies and arranged in the reactor.

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

[0032] The total volume fraction of the pellets 12 in the moderator-containing fuel element 1 is called the filling factor or loading. In a preferred embodiment, the filling factor is between 10% and 50%, preferably between 15% and 45%, and more preferably between 30% and 40%. It should be understood that the filling factor can be adjusted according to actual applications, for example, by increasing or decreasing the thickness of the shell 11.

[0033] In a preferred embodiment, the inner diameter of the shell 11 is 1 mm larger than the diameter of the core block 12. It should be understood that the inner diameter of the shell 11 only needs to be slightly larger than the diameter of the core block 12, so that the core block 12 can fit into the cavity of the shell 11.

[0034] like Figure 3 As shown, the preparation method of the moderator fuel element 1 according to the present invention first includes providing a shell 111 and a cover 112 with openings reserved by hot pressing sintering or additive manufacturing process (such as 3D printing). Specifically, the three-dimensional model of the shell 111 and the cover 112 is designed by computer-aided software (such as Pro / Engineering, Unigraphics, CATIA, Solidworks, etc.), and then sliced ​​and layered (using Magics, Mimics, etc.), and the outline is imported into a 3D printing device, and the silicon carbide shell 111 and the cover 112 are printed by silicon carbide powder using the 3D printing device. It should be understood that the silicon carbide powder here can also be replaced by other ceramic powders or metal powders suitable for 3D printing, such as aluminum oxide, zirconium carbide ceramic powders and tungsten, molybdenum, tungsten-molybdenum alloy powders, etc. It should be understood that the 3D printing device is a printing device that uses at least one of the following technologies: photocuring molding technology, three-dimensional printing molding technology, inkjet printing molding technology, laser selective sintering molding technology, laser selective melting molding technology, direct metal laser sintering molding technology and electron beam fuse deposition molding technology. During the 3D printing process, the first thread is printed at the opening of the shell 111, and the second thread is printed on the cover 112, which is connected to the first thread to achieve the threaded connection between the cover 112 and the shell 111. It should be understood that the first thread here can also be replaced by a first slot, and the second thread can also be replaced by a second slot, which is connected to the first slot to achieve the slot 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 then includes densifying the shell 111 and the cover 112 by chemical vapor infiltration or siliconization. It should be understood that the densification temperature is 900°C-1200°C.

[0036] The preparation method of the fuel element 1 containing a moderator according to the present invention then includes uniformly mixing the moderator 121 and the fuel particles 122 to provide a pellet 12. In a preferred embodiment, the pellet 12 is formed by a process such as hot pressing sintering, plasma sintering, or direct hydrogenation to achieve 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 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.

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

[0038] The preparation method of the fuel element 1 containing a moderator according to the present invention then includes sintering and densification of the preformed body, the silicon carbide powder filled between the shell 111 and the pellet 12 is sintered by sintering, and the connection between the shell 111 and the cover 112 is sealed by densification to ensure sealing. It should be understood that the temperature of the sintering treatment is set according to the material properties of the filling powder between the shell 111 and the pellet 12, and the densification treatment temperature is 900°C-1200°C. The densification treatment can be achieved by in-situ reaction. It should be understood that the sintering treatment and densification treatment can be one step (for example, some materials will be densified during the sintering process), or two steps (for example, some materials need to add additives for in-situ reaction to achieve densification during the heat treatment process), which is used to finally provide the thermodynamic properties of the fuel element 1 containing a moderator.

[0039] Example 1

[0040] The printing equipment is the S-MAX Pro adhesive jetting printing system produced by EXOne, which uses the BA005 adhesive produced by the company.

[0041] The cylindrical barrel-shaped 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. The threads are printed to facilitate connection with the cover 112. After printing, the powder bed is cured at 180°C for 8 hours, and then the sample green body is taken out, and the remaining green body and powder are recovered from the powder bed.

[0042] A silicon carbide disc with the same diameter as the inner diameter of the cylindrical barrel (the diameter of the disc is slightly smaller than the inner diameter of the cylindrical barrel) is printed as the cover 112, with a thickness of 1.5 mm, and a thread or a slot is printed to facilitate connection with the shell 111. After printing, the powder bed is cured at 180°C for 8 hours, and then the sample green body is taken out, and the remaining green body and powder are recovered from the powder bed.

[0043] The shell 111 and the cover 112 are densified by using methyltrichlorosilane as the precursor at a temperature of 950° C. for 100 hours of infiltration.

[0044] According to the nominal composition of yttrium hydride powder (40wt.%) and metal tungsten fuel particles (60wt.%), they were weighed, mixed evenly, cold pressed in an argon glove box, and then placed in a vacuum graphite crucible. The sintering temperature was 1000°C, and the temperature was kept at 1000°C for 10 minutes. After the sample was placed in the furnace, it was evacuated. Before the heating process, a pressure of 65MPa was applied to the punch, and the pressure was removed after the insulation process. After insulation, the furnace was cooled naturally to obtain a pellet 12.

[0045] The core block 12 is placed in the cavity of the shell 111 , and then the gap between the shell 111 and the core block 12 is filled with silicon carbide powder, and sealed with a cover 112 to obtain a preformed body.

[0046] The preformed body is sintered and densified, 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 1000°C, the reaction time is 50 hours, and finally sealing is achieved to obtain the final moderator-containing fuel element 1, and the particle filling factor of the moderator-containing fuel element 1 is about 40 vol%.

[0047] Comparative Example 1

[0048] Weigh a certain mass of yttrium hydride powder and sintering aid yttrium oxide powder and mix them evenly, and use spark plasma sintering process to prepare yttrium hydride moderator block. The pressure is 0.3t and the sintering temperature is 1000℃. Figure 4 As shown, the prepared yttrium hydride moderator block is stored in an air environment. Since it is not resistant to corrosion by water and oxygen in the air, the yttrium hydride moderator block has a certain degree of cracking.

[0049] Obviously, the unpackaged yttrium hydride moderator block is easily corroded by water and oxygen, and thus cannot achieve the moderation capacity of neutrons within the life of the moderator. Compared with the unpackaged use of moderators and fuels for conventional gas-cooled reactors, the present invention uses the shell 11 to avoid direct contact between the pellet 12 and the coolant, thereby reducing the thermodynamic properties and moderation capacity of the moderator and fuel. In a preferred embodiment, the shell 11 provided by the layer-by-layer stacking molding of the material is conducive to the sealing of the pellet 12, and the fuel element 1 containing the moderator according to the present invention is more advantageous. In particular, direct pressing and packaging is prone to cracking due to the mismatch between the thermal expansion coefficients of the shell material and the moderator material. The shell processed by the machining subtractive method is not conducive to the recovery of raw materials and it is difficult to realize the processing of complex-shaped structural parts. The present invention realizes the flexible design of the package shell through the additive manufacturing process, which is not only simple in process and fast in molding speed, but also improves the utilization rate of raw materials and greatly reduces the preparation cost of the moderator element. In this way, by using additive manufacturing technology to prepare the shell 11, a specific moderator-free area (i.e., the shell 11) and a moderator area (i.e., the pellet 12) can be realized as needed, thereby realizing the orderly distribution of the moderator and the fuel, improving the molding speed, reducing the cost, improving the material utilization rate, and enhancing the thermodynamic properties and moderation capacity of the fuel element 1 containing the moderator. At the same time, the structural design and manufacturing of the parts can be integrated, and the digital mold can be repeatedly repaired and the printed parts can be verified, thereby speeding up the development cycle and saving development costs. It is an effective technical solution for low-cost rapid prototyping and manufacturing of the fuel element 1 containing the moderator. Practice shows that the encapsulated fuel element 1 containing the moderator according to the present invention has the advantages of radiation resistance, corrosion resistance, and longer service life.

[0050] The above is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. The above embodiment of the present invention can also be modified in various ways. That is, all simple, equivalent changes and modifications made according to the claims and the description of the present invention fall within the scope of protection of the claims of the present invention. The contents not described in detail in the present invention are all conventional technical contents.

Claims

1. A fuel element containing a moderator, characterized in that: The moderator-containing fuel element comprises an outer shell and a pellet, wherein the pellet is encapsulated in the outer shell and comprises fuel particles dispersedly distributed in the moderator, and the outer shell is a ceramic shell or a metal shell and consists of a shell body and a cover body which are sealed and connected.

2. The moderator-containing fuel element according to claim 1, characterized in that: The shell is a silicon carbide shell, an aluminum oxide shell, a zirconium carbide shell, a tungsten shell, a molybdenum shell or a tungsten-molybdenum alloy shell.

3. The moderator-containing fuel element according to claim 1, characterized in that: The moderator is a yttrium hydride moderator or a zirconium hydride moderator, and the fuel particles are coated fuel particles.

4. The moderator-containing fuel element according to claim 1, characterized in that: The filling factor of the pellets is between 10% and 50%.

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

6. The method for preparing a fuel element containing a moderator according to any one of claims 1 to 5, characterized in that: The preparation method comprises the following steps: S1, providing a cover body and a shell with an opening reserved therein by hot pressing sintering or additive manufacturing process, respectively, and performing a first heat treatment on the cover body and the shell respectively, wherein the first heat treatment includes performing a first densification treatment on the cover body and the shell respectively by a chemical vapor infiltration process or a siliconizing 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 filling powder, connecting the cover body to the shell, and obtaining a preformed body; S3, performing a second heat treatment on the preformed body, the second heat treatment including a sintering treatment and a second densification treatment, the filling powder is sintered by the sintering treatment, and the connection between the shell and the cover is sealed by the second densification treatment to ensure sealing.

7. The preparation method according to claim 6, characterized in that: 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, 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 slot connection between the cover and the shell.

8. The preparation method according to claim 6, characterized in that: In step S2, the moderator and the fuel particles are mixed, and pellets are formed by hot pressing, plasma sintering or direct hydrogenation process to achieve uniform distribution of the fuel particles in the moderator.

9. The preparation method according to claim 6, characterized in that: In step S2, the filling powder is silicon carbide powder, aluminum oxide powder, zirconium carbide powder, tungsten powder, molybdenum powder or tungsten-molybdenum alloy powder.

10. The preparation method according to claim 6, characterized in that: In step S1 and step S3, the first and second densification treatment temperatures are 900°C-1200°C, respectively.

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