A boron carbide nuclear control rod with a gradient pore structure and a preparation method thereof

Abstract

The present invention belongs to the technical field of boron carbide ceramics, and relates to a boron carbide nuclear control rod with a gradient pore structure and a preparation method thereof. The boron carbide nuclear control rod includes an inner core, an intermediate layer, and an outer layer from the inside to the outside, and the porosity and average pore diameter of the boron carbide nuclear control rod increase sequentially from the inner core to the outer layer; the porosity and average pore diameter of the inner core of the boron carbide nuclear control rod are respectively regulated within the ranges of 6.1% - 11.6% and 1.5 - 5.6 μm, the porosity and average pore diameter of the intermediate layer are respectively regulated within the ranges of 13.2% - 18.6% and 4.7 - 12.4 μm, and the porosity and average pore diameter of the outer layer are respectively regulated within the ranges of 20% - 31.2% and 15.3 - 22.4 μm. The present invention prepares the boron carbide nuclear control rod by designing a gradient structure of boron carbide powders with multiple particle sizes, effectively solves the swelling stress problem caused by helium retention during the use of the boron carbide nuclear control rod, and has a simple preparation process.

Description

Technical Field

[0001] The present invention belongs to the technical field of ceramic preparation, and specifically relates to a boron carbide nuclear control rod with a gradient pore structure and a preparation method thereof. Background Art

[0002] Due to its excellent neutron absorption characteristics such as wide energy spectrum capture, high capture cross-section, no generation of radioactive isotopes, and low secondary ray energy release, boron carbide ceramics have become the preferred material for nuclear control rods in high-temperature gas-cooled reactors and have extensive and irreplaceable applications in the nuclear energy field. Usually, the operating temperature of nuclear control rods in high-temperature gas-cooled reactors is 900 - 1100 °C, with a large neutron radiation dose. The helium gas retained due to neutron absorption is likely to cause swelling failure. To avoid this problem, boron carbide nuclear control rods are mostly designed with a connected porous structure.

[0003] Patent CN201910042944.8 discloses "A preparation method of boron carbide porous ceramics for nuclear control rods in high-temperature gas-cooled reactors". This method uses ultra-fine boron carbide powder, boron carbide whiskers, boron nitride powder, and highly active carbon powder as raw materials, and is prepared through processes such as pulping, granulation, pressureless sintering, and machining. The prepared boron carbide ceramics have a porous structure and excellent comprehensive properties. However, although this patent uses boron nitride as a sintering aid to improve the sintering performance, it results in a reduction in product purity. In addition, the raw material costs of ultra-fine boron carbide powder and boron carbide whiskers are expensive, which is not conducive to industrial production.

[0004] Patent CN202411124474.7 discloses "A high-purity porous boron carbide ceramic for nuclear control rods and a preparation method thereof". This patent uses large-particle-size boron carbide powder as the raw material, and polyvinyl butyral and n-butyl acetate as binders. First, it is formed and oxidatively sintered to obtain a first semi-finished product with a certain porosity and strength; then phenolic resin is introduced as a carbon source into the first semi-finished product, and then carbon thermal reduction and secondary sintering are carried out to obtain boron carbide porous ceramics with excellent comprehensive properties. The disadvantages of this patent are: it requires multiple steps such as mixing, molding by die pressing, primary sintering, negative pressure impregnation, and secondary sintering, with a long production cycle and complex processes.

[0005] Patent CN202411346466.7 discloses "A preparation method of high-purity porous boron carbide ceramics based on high-frequency induction heating". This patent uses an induction heating process suitable for mass production, and uses the gas generated by foaming ceramic powder to pressurize boron carbide powder to achieve the effect of hot-pressing sintering. The prepared boron carbide ceramics have the advantages of pure phase, high porosity, uniform pore structure, good pore connectivity, and stable mechanical properties, and are suitable for nuclear control rods in high-temperature gas-cooled reactors.

[0006] In summary, the boron carbide nuclear control rods prepared at present have problems such as complex preparation processes, high preparation costs, and low product purity. In addition, the porous structure designed to avoid swelling failure reduces the effective concentration of boron elements per unit volume of the material, which to a certain extent damages its neutron absorption efficiency. Summary of the Invention

[0007] In view of the above-mentioned deficiencies in the prior art, the present invention provides a boron carbide nuclear control rod with a gradient pore structure and a preparation method therefor.

[0008] The specific technical solutions are as follows:

[0009] In the first aspect of the present invention, a boron carbide nuclear control rod with a gradient pore structure is provided, which sequentially includes an inner core, an intermediate layer, and an outer layer from the inside to the outside, and the porosity and average pore diameter of the boron carbide nuclear control rod increase sequentially from the inner core to the outer layer.

[0010] Specifically, the porosity and average pore diameter of the inner core of the boron carbide nuclear control rod are respectively regulated within the ranges of 6.1% - 11.6% and 1.5 - 5.6 μm, the porosity and average pore diameter of the intermediate layer are respectively regulated within the ranges of 13.2% - 18.6% and 4.7 - 12.4 μm, and the porosity and average pore diameter of the outer layer are respectively regulated within the ranges of 20% - 31.2% and 15.3 - 22.4 μm.

[0011] Preferably, the overall radius of the boron carbide nuclear control rod is 40 - 60 mm, the overall length is 100 - 150 mm, the radius of the inner core is 12 - 20 mm, the thickness of the intermediate layer is 15 - 20 mm, and the thickness of the outer layer is 13 - 20 mm.

[0012] In the second aspect of the present invention, a preparation method for a boron carbide nuclear control rod with a gradient pore structure is provided, including the following steps:

[0013] S1 Obtain three kinds of boron carbide powders with different particle sizes: the average particle size of the first boron carbide powder is 0.5 - 3 μm, the average particle size of the second boron carbide powder is 3 - 7 μm, and the average particle size of the third boron carbide powder is 7 - 15 μm, and the average particle size of the first boron carbide powder < the average particle size of the second boron carbide powder < the average particle size of the third boron carbide powder;

[0014] S2 Dissolve phenolic resin and boric acid in a solvent to obtain a mixed solution;

[0015] S3 Mix the first boron carbide powder, the second boron carbide powder, and the third boron carbide powder in step S1 with the mixed solution in step S2 respectively, and obtain slurries I, II, and III through ball milling respectively;

[0016] S4 Dry the slurry I, slurry II, and slurry III in step S3 respectively to obtain dry mixture I, dry mixture II, and dry mixture III;

[0017] S5 Grind, crush, and screen the dry mixture I, dry mixture II, and dry mixture III in step S4 to obtain sintered powder I, sintered powder II, and sintered powder III;

[0018] S6 Prepare a rod-shaped ceramic green body with sintered powder I as the inner core material, sintered powder II as the intermediate layer material, and sintered powder III as the outer layer material;

[0019] S7 Induction heat sinter the rod-shaped ceramic green body obtained in step S6 to obtain the boron carbide nuclear control rod with a gradient pore structure.

[0020] Among them, in step S1, dry the boron carbide powder. The drying method is preferably vacuum drying to remove the moisture and impurities therein; preferably, the vacuum degree is 0.01~0.1 MPa, the drying temperature is 50~100 °C, and the drying time is 12~48 h.

[0021] Preferably, in step S2, the addition amount of phenolic resin is 1 wt%~10 wt% of the solvent mass, and the molar ratio of the total amount of boron element in boric acid to the total amount of carbon element in phenolic resin is (2~6):1. The solvent is preferably anhydrous ethanol.

[0022] Preferably, in step S3: in slurry I, the mass ratio of the first boron carbide powder to phenolic resin is 1:(0.06~0.1); in slurry II, the mass ratio of the second boron carbide powder to phenolic resin is 1:(0.01~0.06); in slurry III, the mass ratio of the third boron carbide powder to phenolic resin is 1:(0.03~0.08).

[0023] Further, in step S3, the ball milling time is 4~12 h, the ball-to-material ratio is (3~12):1, and the rotation speed is 50~360 r / min.

[0024] Among them, in step S4: the drying method is atmospheric drying, preferably two-step drying; the first drying temperature is 40~60 °C, and the drying time is 1~4 h; the second drying temperature is 65~75 °C, and the drying time is 6~12 h.

[0025] Among them, in step S5: after grinding and crushing dry mixture I, screen it through a 2500-mesh sieve, and take the undersize to obtain sintered powder I; after grinding and crushing dry mixture II, screen it through a 1250-mesh sieve, and take the undersize to obtain sintered powder II; after grinding and crushing dry mixture III, screen it through a 625-mesh sieve, and take the undersize to obtain sintered powder III.

[0026] Preferably, in step S6: a rod-shaped ceramic green body is prepared by a molding method; the pressure for molding is 20-200 MPa, and the time is 5-300 s.

[0027] Among them, in step S7, the induction heating sintering is specifically as follows: in an inert atmosphere, first keep the rod-shaped ceramic green body at 600-900 °C for 0.5-2 h; then raise the temperature to 2050-2250 °C and keep it for 0.5-1.5 h for sintering; finally, cool it to 800-900 °C at a rate of 15-30 °C / min, and then naturally cool it to room temperature. During the induction heating sintering process, the rod-shaped ceramic green body first removes the binder at 600-900 °C, and at the same time decomposes phenolic resin and boric acid into carbon and boron oxide; then raise the temperature to 2050-2250 °C for sintering, and carbon and boron oxide undergo a carbothermal reaction to form boron carbide at this stage; cooling at a rate of 15-30 °C / min to 800-900 °C and then naturally cooling is to avoid cracking of the boron carbide ceramic.

[0028] The beneficial effects of the present invention are as follows:

[0029] (1) The present invention uses boron carbide powder as the raw material and replaces the pressureless sintering and hot pressing sintering processes with an induction heating process. It has a fast heating and cooling rate, a short production cycle, and a simple process, and can mass-produce boron carbide nuclear control rods for high-temperature gas-cooled reactors, solving the problems of high production cost and low efficiency.

[0030] (2) The phenolic resin added in the present invention has a multi-functional coupling effect. On the one hand, in the ceramic green body forming stage, the phenolic resin improves the forming ability of boron carbide powder through its three-dimensional cross-linked network characteristics, which is beneficial to the formation of a complete ceramic green body; on the other hand, the activated carbon skeleton formed by the carbonization of phenolic resin and boron oxide, the decomposition product of boric acid, can act as a sintering aid during the high-temperature sintering process, effectively promoting the sintering of the boron carbide ceramic green body and improving the mechanical properties of the boron carbide ceramic.

[0031] (3) The present invention precisely controls the addition amounts of phenolic resin and boric acid to regulate the stoichiometric ratio of pyrolytic carbon and boron oxide, so that carbon and boron oxide undergo a complete reaction during the high-temperature sintering process, achieving the effect of eliminating the residual carbon and boron oxide impurity phases in the traditional sintering process and improving the product purity.

[0032] (4) By controlling the ratio of sintering aids, namely phenolic resin and boric acid, and the particle size distribution of boron carbide powder, the present invention constructs a boron carbide ceramic with a radially gradient pore structure, realizing a synergistic distribution pattern in which the porosity decreases from the outside to the inside and the pore diameter decreases synchronously. This gradient pore structure has the following remarkable effects: First, on the premise of maintaining the structural integrity of the nuclear control rod, the gradient pore design significantly improves the effective content of boron element per unit volume of the material, increases the neutron absorption cross-section, and at the same time reduces the thermal neutron scattering loss through pore topology optimization; Second, the arrangement of multi-level pore diameters forms a three-dimensional connected gas path network, effectively improving the helium diffusion flux, eliminating the swelling stress caused by helium retention during the nuclear reaction process, and ensuring the safe use effect of the nuclear control rod.

[0033] (5) By designing the gradient structure of boron carbide powder with multiple particle sizes, the present invention can avoid the problem of inconsistent sinterability of the matrix caused by the high temperature outside and low temperature inside the temperature field during the sintering process. Boron carbide powder with three particle sizes of the first, second, and third is used to design a powder particle size increasing structure from the inner core to the outer layer. Compared with the second boron carbide powder (i.e., medium powder) in the middle layer, the first boron carbide powder (i.e., fine powder) in the inner core has higher sintering activity and can complete sintering at a slightly lower temperature; the third boron carbide powder (i.e., coarse powder) in the outer layer has the lowest sintering activity and must be sintered at an appropriately increased temperature. The sintering activity of the powder gradually increases from the outside to the inside, and the required sintering temperature gradually decreases, which exactly matches the actual situation of the high temperature outside and low temperature inside the temperature field during the sintering process, thus solving the problem of inconsistent sinterability inside and outside the sample. Description of the Drawings

[0034] Figure 1 It is a schematic structural diagram of the upper punch in the steel pressing die;

[0035] Figure 2 It is a main view cross-sectional view of the steel pressing die;

[0036] Figure 3 It is a top view of the steel pressing die;

[0037] Figure 4 It is a schematic diagram of the pressing process of the ceramic green body of the present invention;

[0038] Figure 5 It is a schematic diagram of the induction heating sintering process of the ceramic green body of the present invention;

[0039] Figure 6 It is an SEM image of the inner core of the boron carbide nuclear control rod in Example 1;

[0040] Figure 7 It is an SEM image of the middle layer of the boron carbide nuclear control rod in Example 1;

[0041] Figure 8 It is an SEM image of the outer layer of the boron carbide nuclear control rod in Example 1.

[0042] Wherein: 1 - Rubber tube I; 2 - Rubber tube II; 3 - Outer shell; 4 - Lower pressing head. Specific implementation manner

[0043] The principles and features of the present invention will be described below in conjunction with examples. The examples cited are only for explaining the present invention and are not used to limit the scope of the present invention. The experimental methods used in the following examples are all conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples can be obtained from commercial sources unless otherwise specified.

[0044] Figures 1-2 They are respectively the structural schematic diagram of the upper pressing head of the steel pressing mold used in the present invention and the main view cross-sectional view of the steel pressing mold; the steel pressing mold is cylindrical, including an upper pressing head, a lower pressing head 4, an outer shell 3, a rubber tube I 1, and a rubber tube II 2; Figure 3 It is the top view of the steel pressing mold, as Figure 3 shown, the outer shell 3, the rubber tube I 1, and the rubber tube II 2 are coaxial and sleeve structures. The first space for filling powder is between the outer shell 3 and the rubber tube I 1, the second space for filling powder is between the rubber tube I 1 and the rubber tube II 2, and the third space for filling powder is inside the rubber tube II 2.

[0045] A boron carbide nuclear control rod with a gradient pore structure, from the inside to the outside, is an inner core, an intermediate layer, and an outer layer in sequence, and the porosity and average pore diameter of the boron carbide nuclear control rod increase in sequence from the inner core to the outer layer. Specifically, the porosity and average pore diameter of the inner core of the boron carbide nuclear control rod are respectively regulated within the ranges of 6.1% - 11.6% and 1.5 - 5.6 μm, the porosity and average pore diameter of the intermediate layer are respectively regulated within the ranges of 13.2% - 18.6% and 4.7 - 12.4 μm, and the porosity and average pore diameter of the outer layer are respectively regulated within the ranges of 20% - 31.2% and 15.3 - 22.4 μm; the overall radius of the boron carbide nuclear control rod is 40 - 60 mm, the overall length is 100 - 150 mm, the radius of the inner core is 12 - 20 mm, the thickness of the intermediate layer is 15 - 20 mm, and the thickness of the outer layer is 13 - 20 mm.

[0046] The specific preparation method is as follows. Example 1

[0047] A preparation method of a boron carbide nuclear control rod with a gradient pore structure, comprising the following steps:

[0048] S1 Obtain boron carbide powders with three different particle sizes: Vacuum dry the boron carbide powders with three particle sizes at 0.01 MPa and 50 °C for 48 h; among them, the average particle size of the first boron carbide powder is 0.5 μm, the average particle size of the second boron carbide powder is 3 μm, and the average particle size of the third boron carbide powder is 7 μm;

[0049] S2 Dissolve phenolic resin in absolute ethanol, then add boric acid and stir evenly to obtain a mixed solution; among them, the addition amount of phenolic resin is 1 wt% of the mass of absolute ethanol, and the molar ratio of the total amount of boron elements in boric acid to the total amount of carbon elements in phenolic resin is 2:1;

[0050] S3 Mix the dried first boron carbide powder, second boron carbide powder, and third boron carbide powder in step S1 with the mixed solution in step S2 respectively, and obtain slurries I, II, and III through ball milling; among them, in slurry I, the mass ratio of the first boron carbide powder to phenolic resin is 1:0.06; in slurry II, the mass ratio of the second boron carbide powder to phenolic resin is 1:0.01; in slurry III, the mass ratio of the third boron carbide powder to phenolic resin is 1:0.03; the ball milling time is 4 h, the ball-to-material ratio is 12:1, and the rotation speed is 50 r / min; the materials of the ball milling tank and ball milling balls are polyurethane and agate balls respectively;

[0051] S4 First, dry the slurries I, II, and III in step S3 at normal pressure at 40 °C for 4 h, and then raise the temperature to 65 °C and dry at normal pressure for 12 h to obtain dry blends I, II, and III;

[0052] S5 Grind and crush the dry blends I, II, and III in step S4 and screen them to obtain sintered powders I, II, and III; specifically, after the dry blend I is ground and crushed, it is screened through a 2500-mesh sieve, and the material passing through the sieve is taken to obtain sintered powder I; after the dry blend II is ground and crushed, it is screened through a 1250-mesh sieve, and the material passing through the sieve is taken to obtain sintered powder II; after the dry blend III is ground and crushed, it is screened through a 625-mesh sieve, and the material passing through the sieve is taken to obtain sintered powder III;

[0053] S6 Use a steel pressing mold to prepare a rod-shaped ceramic green body by membrane pressing with sintered powder I as the inner core material, sintered powder II as the intermediate layer material, and sintered powder III as the outer layer material; among them, the pressure for die pressing is 20 MPa and the time is 300 s;

[0054] Specifically, as Figure 4This is a schematic diagram of the pressing process of the ceramic green body of the present invention. First, sintered powder I, sintered powder II, and sintered powder III are respectively placed into the third space, second space, and first space of a steel pressing mold. Then, rubber tube I 1 and rubber tube II 2 are taken out, and pressing is carried out using an upper punch. Subsequently, a ceramic green body is obtained through demolding.

[0055] S7 Induction heating sinter the rod-shaped ceramic green body obtained in step S6 to obtain the cylindrical boron carbide nuclear control rod with a gradient pore structure; the overall radius of the boron carbide nuclear control rod is 40 mm, the overall length is 100 mm, the inner core radius is 12 mm, the middle layer thickness is 15 mm, and the outer layer thickness is 13 mm.

[0056] Specifically, as Figure 5 This is a schematic diagram of the induction heating sintering process of the ceramic green body of the present invention; the rod-shaped ceramic green body obtained in step S6 is placed into a graphite sintering mold, and then placed into a zirconia ceramic tube for induction heating sintering. The induction heating sintering is specifically as follows: in an argon inert atmosphere, first keep the rod-shaped ceramic green body at 600 °C for 2 h for degumming, and at the same time decompose phenolic resin and boric acid into carbon and boron oxide; then raise the temperature to 2050 °C and keep it for 1.5 h for sintering, and at the same time carbon and boron oxide react to form boron carbide; finally, cool it to 800 °C at a rate of 15 °C / min to avoid cracking of the boron carbide ceramic, and then turn off the heating power supply and let it cool naturally to room temperature. After cooling and demolding, a cylindrical boron carbide nuclear control rod is obtained. Example 2

[0057] A preparation method of a boron carbide nuclear control rod with a gradient pore structure includes the following steps:

[0058] S1 Obtain three kinds of boron carbide powders with different particle sizes: vacuum dry the three kinds of boron carbide powders at 0.1 MPa and 100 °C for 12 h; among them, the average particle size of the first boron carbide powder is 3 μm, the average particle size of the second boron carbide powder is 7 μm, and the average particle size of the third boron carbide powder is 15 μm.

[0059] S2 Dissolve phenolic resin into absolute ethanol, then add boric acid and stir evenly to obtain a mixed solution; among them, the addition amount of phenolic resin is 10 wt% of the mass of absolute ethanol, and the molar ratio of the total amount of boron elements in boric acid to the total amount of carbon elements in phenolic resin is 6:1.

[0060] S3 Mix the dried first boron carbide powder, second boron carbide powder, and third boron carbide powder in Step S1 with the mixed solution in Step S2 respectively, and obtain Slurry I, Slurry II, and Slurry III through ball milling respectively; among them, in Slurry I, the mass ratio of the first boron carbide powder to phenolic resin is 1:0.1; in Slurry II, the mass ratio of the second boron carbide powder to phenolic resin is 1:0.06; in Slurry III, the mass ratio of the third boron carbide powder to phenolic resin is 1:0.08; the ball milling time is 12 h, the ball-to-material ratio is 3:1, and the rotation speed is 360 r / min; the materials of the ball milling tank and ball milling balls are polyurethane and agate balls respectively;

[0061] S4 First, dry Slurry I, Slurry II, and Slurry III in Step S3 at 60 °C under normal pressure for 1 h, and then raise the temperature to 75 °C and dry under normal pressure for 6 h to obtain Dry Mix I, Dry Mix II, and Dry Mix III;

[0062] S5 Grind and crush, and sieve Dry Mix I, Dry Mix II, and Dry Mix III in Step S4 to obtain Sintered Powder I, Sintered Powder II, and Sintered Powder III; specifically, after grinding and crushing Dry Mix I, sieve it through a 2500-mesh sieve, and take the material under the sieve to obtain Sintered Powder I; after grinding and crushing Dry Mix II, sieve it through a 1250-mesh sieve, and take the material under the sieve to obtain Sintered Powder II; after grinding and crushing Dry Mix III, sieve it through a 625-mesh sieve, and take the material under the sieve to obtain Sintered Powder III;

[0063] S6 Use a steel pressing mold to prepare a rod-shaped ceramic green body by membrane pressing with Sintered Powder I as the inner core material, Sintered Powder II as the intermediate layer material, and Sintered Powder III as the outer layer material; among them, the pressure for mold pressing is 200 MPa and the time is 5 s. The specific pressing process refers to Example 1;

[0064] S7 Induction heating sinter the rod-shaped ceramic green body obtained in Step S6 to obtain the cylindrical boron carbide nuclear control rod with a gradient pore structure; the overall radius of the boron carbide nuclear control rod is 60 mm, the overall length is 150 mm, the inner core radius is 20 mm, the intermediate layer thickness is 20 mm, and the outer layer thickness is 20 mm;

[0065] Among them, the induction heating sintering is specifically as follows: in an argon inert atmosphere, first keep the rod-shaped ceramic green body at 900 °C for 0.5 h to remove the binder, and at the same time decompose the phenolic resin and boric acid into carbon and boron oxide; then raise the temperature to 2250 °C and keep it at this temperature for 0.5 h for sintering, and at the same time carbon and boron oxide react to form boron carbide; finally, cool it to 900 °C at a rate of 25 °C / min to avoid cracking of the boron carbide ceramic, and then turn off the heating power supply and naturally cool it to room temperature. After cooling and demolding, a cylindrical boron carbide nuclear control rod is obtained. The specific sintering process refers to Example 1. Example 3

[0066] A preparation method of a boron carbide nuclear control rod with a gradient pore structure, comprising the following steps:

[0067] S1 Obtain three kinds of boron carbide powders with different particle sizes: Vacuum dry the three kinds of boron carbide powders at 0.05 MPa and 80 °C for 24 h; among them, the average particle size of the first boron carbide powder is 1 μm, the average particle size of the second boron carbide powder is 5 μm, and the average particle size of the third boron carbide powder is 10 μm;

[0068] S2 Dissolve phenolic resin in absolute ethanol, then add boric acid and stir evenly to obtain a mixed solution; among them, the addition amount of phenolic resin is 6 wt% of the mass of absolute ethanol, and the molar ratio of the total amount of boron elements in boric acid to the total amount of carbon elements in phenolic resin is 4:1;

[0069] S3 Mix the dried first boron carbide powder, second boron carbide powder, and third boron carbide powder in step S1 with the mixed solution in step S2 respectively, and obtain slurries I, II, and III by ball milling respectively; among them, in slurry I, the mass ratio of the first boron carbide powder to phenolic resin is 1:0.07; in slurry II, the mass ratio of the second boron carbide powder to phenolic resin is 1:0.05; in slurry III, the mass ratio of the third boron carbide powder to phenolic resin is 1:0.06; the ball milling time is 8 h, the ball-to-material ratio is 6:1, and the rotation speed is 200 r / min; the materials of the ball milling tank and ball milling balls are polyurethane and agate balls respectively;

[0070] S4 First dry the slurries I, II, and III in step S3 at 50 °C under normal pressure for 2 h, and then raise the temperature to 70 °C and dry under normal pressure for 8 h to obtain dry mixes I, II, and III;

[0071] S5 Grind and crush the dry mixes I, II, and III in step S4 and screen them to obtain sintered powders I, II, and III; specifically, after grinding and crushing the dry mix I, screen it through a 2500-mesh sieve, and take the material under the sieve to obtain sintered powder I; after grinding and crushing the dry mix II, screen it through a 1250-mesh sieve, and take the material under the sieve to obtain sintered powder II; after grinding and crushing the dry mix III, screen it through a 625-mesh sieve, and take the material under the sieve to obtain sintered powder III;

[0072] S6 Use the sintered powder I as the inner core material, the sintered powder II as the intermediate layer material, and the sintered powder III as the outer layer material to prepare a rod-shaped ceramic green body by membrane pressing using a steel pressing mold; among them, the pressure of the die pressing is 60 MPa and the time is 180 s. The specific pressing process refers to Example 1;

[0073] S7 Induction heating sintering is carried out on the rod-shaped ceramic green body obtained in step S6 to obtain the cylindrical boron carbide nuclear control rod with a gradient pore structure; the overall radius of the boron carbide nuclear control rod is 48 mm, the overall length is 120 mm, the inner core radius is 14 mm, the intermediate layer thickness is 17 mm, and the outer layer thickness is 17 mm;

[0074] Among them, the induction heating sintering is specifically as follows: under an argon inert atmosphere, first keep the rod-shaped ceramic green body at 800 °C for 1 h for degumming, and at the same time decompose phenolic resin and boric acid into carbon and boron oxide; then raise the temperature to 2100 °C and keep it for 1.2 h for sintering, and at the same time carbon and boron oxide react to form boron carbide; finally, cool it to 820 °C at a rate of 20 °C / min to avoid cracking of the boron carbide ceramic, and then turn off the heating power supply and naturally cool it to room temperature. After cooling and demolding, a cylindrical boron carbide nuclear control rod is obtained. The specific sintering process refers to Example 1. Example 4

[0075] A preparation method of a boron carbide nuclear control rod with a gradient pore structure includes the following steps:

[0076] S1 Obtain three kinds of boron carbide powders with different particle sizes: Vacuum dry the three kinds of boron carbide powders at 0.02 MPa and 80 °C for 30 h; among them, the average particle size of the first boron carbide powder is 2 μm, the average particle size of the second boron carbide powder is 3.5 μm, and the average particle size of the third boron carbide powder is 12 μm;

[0077] S2 After dissolving phenolic resin in absolute ethanol, add boric acid and stir evenly to obtain a mixed solution; among them, the addition amount of phenolic resin is 3 wt% of the mass of absolute ethanol, and the molar ratio of the total amount of boron element in boric acid to the total amount of carbon element in phenolic resin is 3.5:1;

[0078] S3 Mix the dried first boron carbide powder, second boron carbide powder, and third boron carbide powder in step S1 with the mixed solution in step S2 respectively, and obtain slurries I, II, and III through ball milling respectively; among them, in slurry I, the mass ratio of the first boron carbide powder to phenolic resin is 1:0.08; in slurry II, the mass ratio of the second boron carbide powder to phenolic resin is 1:0.03; in slurry III, the mass ratio of the third boron carbide powder to phenolic resin is 1:0.05; the ball milling time is 6 h, the ball-to-material ratio is 10:1, and the rotation speed is 150 r / min; the materials of the ball milling tank and ball milling balls are polyurethane and agate balls respectively;

[0079] S4 First, dry the slurries I, II, and III in step S3 at normal pressure at 55 °C for 3 h, and then raise the temperature to 65 °C and dry at normal pressure for 10 h to obtain dry mixes I, II, and III;

[0080] S5 Grind and crush the dry blend I, dry blend II and dry blend III in step S4, and sieve them to obtain sintered powders I, sintered powders II and sintered powders III; specifically, grind and crush the dry blend I, and pass it through a 2500-mesh sieve to obtain sintered powder I; grind and crush the dry blend II, and pass it through a 1250-mesh sieve to obtain sintered powder II; grind and crush the dry blend III, and pass it through a 625-mesh sieve to obtain sintered powder III;

[0081] S6: Using sintered powder I as the inner core material, sintered powder II as the middle layer material, and sintered powder III as the outer layer material, a rod-shaped ceramic green body is prepared by die pressing using a steel pressing mold; wherein the molding pressure is 100 MPa, the time is 150 s, and the specific pressing process is referred to Example 1;

[0082] S7: Induction heating and sintering the rod-shaped ceramic green body obtained in step S6 to obtain the cylindrical boron carbide nuclear control rod with a gradient pore structure; the boron carbide nuclear control rod has an overall radius of 53 mm, an overall length of 136 mm, an inner core radius of 19 mm, a middle layer thickness of 18 mm, and an outer layer thickness of 16 mm;

[0083] Among them, the induction heating sintering is specifically as follows: under an argon inert atmosphere, the rod-shaped ceramic green body is first kept at 700°C for 1.5 hours to remove the binder, and at the same time, the phenolic resin and boric acid are decomposed into carbon and boron oxide; then the temperature is increased to 2150°C and kept for 0.8 hours for sintering, and at the same time, carbon and boron oxide react to form boron carbide; finally, it is cooled to 860°C at 30°C / min to avoid cracking of the boron carbide ceramic, and then the heating power is turned off and naturally cooled to room temperature. After cooling and demolding, a cylindrical boron carbide nuclear control rod is obtained. The specific sintering process refers to Example 1.

[0084] Testing and analysis

[0085] The boron carbide nuclear control rods prepared in Examples 1 to 4 are high-purity boron carbide phases without the presence of carbon and boron oxide impurity phases. The boron carbide nuclear control rods prepared in Examples 1 to 4 have a purity of ≥99.9% as determined by combined inductively coupled plasma emission spectroscopy and high-frequency combustion infrared absorption.

[0086] The boron carbide nuclear control rod prepared in Example 1 was subjected to morphological analysis using a scanning electron microscope (SEM). Figures 6-8 The SEM images of the inner core, middle layer and outer layer of the boron carbide core control rod of Example 1 are shown in order. Figures 6-8It can be seen that the inner core of the boron carbide nuclear control rod has a low porosity and small pore diameter, the middle layer of the boron carbide nuclear control rod has a relatively high porosity and relatively large pore diameter, and the outer layer of the boron carbide nuclear control rod has the highest porosity and the largest pore diameter. This shows that the boron carbide nuclear control rod prepared by the present invention has a porous structure and a gradient distribution characteristic.

[0087] The physical and mechanical properties of the boron carbide nuclear control rods of Examples 1 to 4 were tested, and three examples in the patent with the publication number CN109704771A (a preparation method of boron carbide porous ceramics for nuclear control rods of high-temperature gas-cooled reactors) were used as Comparative Examples 1 to 3 for comparison. The specific results are shown in Table 1.

[0088] Among them, the compressive strength was measured by the compression test method in the national standard GB / T4740-1999; the density was measured by the Archimedes drainage method; the average pore diameter was measured by the mercury intrusion method in the national standard GB / T 21650.1-2008;

[0089] The porosity was calculated by the following formula:

[0090] (2.52 g / cm 3 - density of the nuclear control rod) / 2.52 g / cm 3 ×100%, where 2.52 g / cm 3 is the density of dense boron carbide ceramics, and the porosity of the inner core, middle layer, outer layer and the whole of the nuclear control rod was calculated based on the density of the inner core, middle layer, outer layer and the whole of the nuclear control rod respectively;

[0091] The through-hole rate was calculated by the following formula:

[0092] Open porosity of the nuclear control rod / overall porosity of the nuclear control rod × 100%, where the open porosity was measured by the bulk density method in the national standard GB / T25995-2010.

[0093] Table 1 Physical and mechanical properties of boron carbide nuclear control rods

[0094]

[0095] As can be seen from Table 1, for the boron carbide nuclear control rods prepared in Examples 1 to 4, the porosity of their inner cores, intermediate layers and outer layers are 6.1% - 11.6%, 13.2% - 18.6% and 20% - 31.2% respectively, and the corresponding average pore diameters are 1.5 - 5.6 μm, 4.7 - 12.4 μm and 15.3 - 22.4 μm respectively. This shows that the preparation method of the present invention can effectively adjust the pore structure of the boron carbide nuclear control rod by changing the proportion of sintering aids and the particle size distribution of boron carbide powder, and achieve its gradient distribution. The porosity and density of the whole boron carbide porous ceramics prepared in Comparative Examples 1 to 3 are 10.7% - 17.5% and 2.08 - 2.25 g / cm 3 , it can be seen that the inner cores of the boron carbide nuclear control rods prepared in Examples 1 to 4 have lower porosity (6.1% - 11.6%), thereby increasing the effective concentration of boron element per unit volume of the material and increasing the neutron absorption cross section. In addition, the gradient pore structure design makes the whole boron carbide nuclear control rods prepared in Examples 1 to 4 have higher porosity (13.9% - 25.4%) and through-hole rate (80.1% - 87.3%), which can ensure the timely discharge of helium and improve the safety of the nuclear control rod.

[0096] The compressive strength of the boron carbide nuclear control rods prepared in Examples 1 to 4 is 402 - 479 MPa. Comparing with Comparative Examples 1 to 3, it is found that the compressive strength of the boron carbide nuclear control rods prepared by the present invention is relatively low. However, since the nuclear control rods for high-temperature gas-cooled reactors do not need to bear external loads, the boron carbide nuclear control rods prepared in Examples 1 to 4 fully meet the service requirements.

[0097] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a boron carbide nuclear control rod with a gradient pore structure, characterized in that, The boron carbide nuclear control rod consists of an inner core, an intermediate layer and an outer layer from the inside to the outside, and the porosity and average pore diameter of the boron carbide nuclear control rod increase sequentially from the inner core to the outer layer; The preparation method includes the following steps: S1 Obtain three kinds of boron carbide powders with different particle sizes: the average particle size of the first boron carbide powder is 0.5 - 3 μm, the average particle size of the second boron carbide powder is 3 - 7 μm, and the average particle size of the third boron carbide powder is 7 - 15 μm, and the average particle size of the first boron carbide powder < the average particle size of the second boron carbide powder < the average particle size of the third boron carbide powder; S2 Dissolve phenolic resin and boric acid in a solvent to obtain a mixed solution; S3 Mix the first, second, and third boron carbide powders in step S1 with the mixed solution in step S2 respectively, and obtain slurries I, II, and III through ball milling respectively; S4 Dry the slurries I, II, and III in step S3 respectively to obtain dry blends I, II, and III; S5 Grind and crush, and sieve the dry blends I, II, and III in step S4 to obtain sintered powders I, II, and III; S6 Use the sintered powder I as the inner core material, the sintered powder II as the intermediate layer material, and the sintered powder III as the outer layer material to prepare a rod-shaped ceramic green body; S7 Induction heat sinter the rod-shaped ceramic green body obtained in step S6 to obtain the boron carbide nuclear control rod; The porosity and average pore diameter of the inner core of the boron carbide nuclear control rod are regulated within the ranges of 6.1% - 11.6% and 1.5 - 5.6 μm respectively, the porosity and average pore diameter of the intermediate layer are regulated within the ranges of 13.2% - 18.6% and 4.7 - 12.4 μm respectively, and the porosity and average pore diameter of the outer layer are regulated within the ranges of 20% - 31.2% and 15.3 - 22.4 μm respectively.

2. The preparation method according to claim 1, characterized in that, In step S2, the addition amount of phenolic resin is 1 wt% - 10 wt% of the mass of the solvent, and the molar ratio of the total amount of boron elements in boric acid to the total amount of carbon elements in phenolic resin is (2 - 6):

1.

3. The preparation method according to claim 1, characterized in that, In step S3: In slurry I, the mass ratio of the first boron carbide powder to phenolic resin is 1:(0.06 - 0.1); in slurry II, the mass ratio of the second boron carbide powder to phenolic resin is 1:(0.01 - 0.06); in slurry III, the mass ratio of the third boron carbide powder to phenolic resin is 1:(0.03 - 0.08).

4. The preparation method according to claim 1, wherein, In step S3, the ball milling time is 4 - 12 h, the ball-to-material ratio is (3 - 12):1, and the rotation speed is 50 - 360 r / min.

5. The preparation method according to claim 1, wherein In step S4: The drying method is two-step drying; the first drying temperature is 40 - 60 °C, and the drying time is 1 - 4 h; the second drying temperature is 65 - 75 °C, and the drying time is 6 - 12 h.

6. The preparation method according to claim 1, characterized in that, In step S5: After grinding and crushing the dry blend I, sieve it through a 2500-mesh sieve, and take the undersize to obtain the sintered powder I; after grinding and crushing the dry blend II, sieve it through a 1250-mesh sieve, and take the undersize to obtain the sintered powder II; after grinding and crushing the dry blend III, sieve it through a 625-mesh sieve, and take the undersize to obtain the sintered powder III.

7. The preparation method according to claim 1, characterized in that In step S6: A rod-shaped ceramic green body is prepared by a molding method; the pressure for molding is 20 to 200 MPa, and the time is 5 to 300 s.

8. The preparation method according to claim 1, characterized in that, The induction heating sintering is specifically as follows: In an inert atmosphere, first keep the rod-shaped ceramic green body at 600 to 900 °C for 0.5 to 2 h; then raise the temperature to 2050 to 2250 °C and keep it at this temperature for 0.5 to 1.5 h for sintering; finally, cool it to 800 to 900 °C at a rate of 15 to 30 °C / min, and then cool it naturally.

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