Preparation method and device of high-purity porous boron carbide nuclear control rod

Abstract

The invention belongs to the technical field of ceramic preparation, and relates to a preparation method and device of a high-purity porous boron carbide nuclear control rod. The high-purity porous boron carbide nuclear control rod is prepared by taking boron trichloride gas as a boron source, methane as a carbon source, argon as a carrier gas and a protective gas and hydrogen as a catalytic gas and a reducing gas, and the deposition temperature of boron carbide is increased by adopting a step-by-step and region-divided method from back to front, so that the deposition rate of boron carbide can be increased; the physical and mechanical properties of all parts of the prepared high-purity porous boron carbide nuclear control rod can be ensured to be consistent, the porosity and aperture of the prepared high-purity porous boron carbide can be regulated and controlled by changing the particle size of the boron carbide powder for preparing the boron carbide blank and cooperatively regulating the temperature, flow, pressure and deposition duration, and the high boron-carbon total content can be obtained; therefore, various actual requirements can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic preparation, and in particular relates to a method and a device for preparing a high-purity porous boron carbide nuclear control rod. Background Art

[0002] In the field of nuclear energy, boron carbide ceramics containing high-abundance B10 are key materials for preparing nuclear reactor control rods. Boron carbide has extremely strong covalent bonds and a very low atomic self-diffusion coefficient. Its defects can be effectively overcome by introducing sintering aids, increasing the sintering temperature, and applying pressure during the sintering process. In recent years, with the rapid development of high-temperature gas-cooled reactor technology, the demand for porous boron carbide ceramic nuclear control rods has increased day by day. Compared with dense boron carbide ceramics, porous boron carbide ceramics are more difficult to sinter. Due to the low surface energy of boron carbide particles, the sintering driving force between boron carbide particles is weak, which greatly hinders the agglomeration of boron carbide particles and the formation of sintering necks. To solve this problem, the conventional idea is to increase the molding pressure of the boron carbide ceramic body, but this approach is contrary to the process requirements of porous boron carbide ceramics. This is because, in order to achieve the goal of making boron carbide ceramics porous, the boron carbide ceramic body should adopt a lower molding pressure, and a pore-forming agent should be introduced into the body. In this way, the boron carbide particles cannot fully contact each other, resulting in poor bonding strength during the sintering process, causing the mechanical properties of porous boron carbide ceramics to be seriously reduced.

[0003] In order to improve the mechanical properties of porous boron carbide ceramics, patent CN201910042944.8 discloses "a method for preparing porous boron carbide ceramics for high-temperature gas-cooled reactor nuclear control rods". This method successfully makes the prepared boron carbide ceramics have both high porosity and mechanical properties by introducing boron carbide whiskers and boron nitride into the raw materials. However, the disadvantage of this method is that the introduction of boron nitride will reduce the total boron and carbon content of boron carbide ceramics, and the use of expensive boron carbide whiskers will increase the manufacturing cost of boron carbide ceramics. Patent CN202411124474.7 discloses "a high-purity porous boron carbide ceramic for nuclear control rods and its preparation method". This method can improve the porosity and total boron-carbon content of porous boron carbide ceramics while taking into account the mechanical properties of porous boron carbide ceramics. For example, for porous boron carbide ceramics prepared by this method, when its porosity is 35.3%, its compressive strength is as high as 379 MPa, and its total boron-carbon content is as high as 99.8%. However, the methods disclosed in the above two patents still have two common problems: first, the sintering temperature is higher than 2100°C, which will greatly increase the manufacturing cost; second, in order to meet the size requirements of nuclear control rods, the sintered porous boron carbide ceramics need to be machined. Due to the high hardness of boron carbide ceramics, it is very difficult to machine them, and the machining process will cause serious waste of boron carbide, especially for high-abundance porous boron carbide ceramics. This waste will greatly increase the manufacturing cost of nuclear control rods. Summary of the invention

[0004] In view of the deficiencies in the above-mentioned prior art, the present invention provides a method and device for preparing a high-purity porous boron carbide nuclear control rod, which solves the problems of low total boron-carbon content, high sintering temperature and waste of raw materials caused by machining in the process of preparing high-purity porous boron carbide nuclear control rods.

[0005] The specific technical solutions are as follows: A method for preparing a high-purity porous boron carbide nuclear control rod comprises the following steps: S1. Obtaining a boron carbide blank, wherein the boron carbide blank is divided into five regions: a front section, a front middle section, a middle section, a middle rear section, and a rear section; S2, heating the boron carbide blank as a whole to a first temperature in an argon atmosphere; S3: maintaining the temperature of the front section and the middle section of the boron carbide blank at a first temperature, heating the rear section to a second temperature, and continuously introducing a mixed gas of boron trichloride, methane, argon and hydrogen from the front section of the boron carbide blank to react; S4: maintaining the temperature of the front section and the rear section of the boron carbide blank at the first temperature, heating the middle section to the third temperature, and continuously introducing a mixed gas of boron trichloride, methane, argon and hydrogen from the front section of the boron carbide blank to react; S5: maintaining the temperature of the middle and rear sections of the boron carbide blank at the first temperature, heating the front section to a fourth temperature, and continuously introducing a mixed gas of boron trichloride, methane, argon and hydrogen from the front section of the boron carbide blank to react; After the S6 reaction, a high-purity porous boron carbide nuclear control rod is obtained; Among them, the second temperature, the third temperature and the fourth temperature are all higher than the first temperature.

[0006] The reaction mechanism of the present invention is as follows: The present invention uses boron trichloride gas as a boron source, methane as a carbon source, argon as a carrier gas and a protective gas, and hydrogen as a catalytic gas and a reducing gas to prepare a high-purity porous boron carbide nuclear control rod. The gases are fully mixed by diffusion and convection, and when the mixed gas flows through the pores inside the boron carbide blank, due to the effect of high temperature, the boron trichloride will decompose to produce boron atoms and chlorine atoms, and the methane will decompose to produce carbon atoms and hydrogen atoms. Then, the chlorine atoms and hydrogen atoms react to produce hydrogen chloride, which is discharged as tail gas, and the boron atoms and carbon atoms react to produce boron carbide, which is deposited on the surface of the boron carbide particles of the boron carbide blank. With the continuous deposition of boron carbide, the originally loosely stacked boron carbide blank particles are firmly bonded together.

[0007] Preferably, in step S1, the boron carbide blank is obtained by pressing boron carbide powder; the average particle size of the boron carbide powder is 1-7 μm, and the pressing method is preferably to pressurize to 3-5 MPa.

[0008] Preferably, in step S1, the length ratio of the front section, front middle section, middle section, middle rear section and rear section of the boron carbide blank is 1:(0.8~1.2):(0.8~1.2):(0.8~1.2):(0.8~1.2), preferably 1:1:1:1:1. Since the deposition rate of boron carbide is usually very low, although increasing the temperature can increase the deposition rate of boron carbide, increasing the temperature overall will lead to significant differences in porosity, pore size and mechanical properties of various regions of the product. Therefore, the present invention adopts regional temperature control to achieve the deposition of boron carbide when preparing high-purity porous boron carbide nuclear control rods.

[0009] Preferably, in step S2, the first temperature is 1000-1050°C.

[0010] Preferably, in steps S3 to S5, the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:(1-2):(3-6):(4-6).

[0011] In steps S3 to S5, the front middle section and the middle and rear section of the boron carbide blank are temperature transition areas and no temperature control is required.

[0012] Preferably, in step S3, the second temperature is 1180-1200° C., the reaction pressure is 2000-3000 Pa, the reaction time is 11-17 h, and the mixed gas flow rate is 130-160 mL / min.

[0013] Preferably, in step S4, the third temperature is 1170-1190° C., the reaction pressure is 1000-2000 Pa, the reaction time is 9-15 h, and the mixed gas flow rate is 120-150 mL / min.

[0014] Preferably, in step S5, the fourth temperature is 1160-1180° C., the reaction pressure is 500-1000 Pa, the reaction time is 7-13 h, and the mixed gas flow rate is 110-140 mL / min.

[0015] Preferably, the second temperature is higher than the third temperature, and the third temperature is higher than the fourth temperature.

[0016] In step S6, after the reaction is completed, the introduction of the mixed gas of boron trichloride, methane, argon and hydrogen is stopped, argon is introduced and the pressure is controlled to be 8000-9000Pa, heating is stopped to allow the graphite sleeve to cool naturally, and when the temperature drops below 400°C, the introduction of argon is stopped and the pressure is restored to normal pressure; when the temperature drops below 100°C, a high-purity porous boron carbide nuclear control rod is obtained.

[0017] Preferably, in step S6, the argon gas flow rate is 60-90 mL / min.

[0018] The present invention also provides a device for preparing high-purity porous boron carbide nuclear control rods, comprising a temperature control module and a flow regulating module, a reaction chamber and a pressure regulating module connected in sequence; wherein the flow regulating module is used to regulate the flow of four gases, namely boron trichloride, methane, argon and hydrogen; and the pressure regulating module is used to regulate the gas pressure in the reaction chamber.

[0019] The reaction chamber consists of a quartz tube and a graphite sleeve, the graphite sleeve is located in the quartz tube, and the outer wall of the graphite sleeve is tightly fitted with the inner wall of the quartz tube; the graphite sleeve is divided into a front section, a front middle section, a middle section, a middle rear section and a rear section in sequence, and the length ratio of the front section, the front middle section, the middle section, the middle rear section and the rear section is 1:(0.8-1.2):(0.8-1.2):(0.8-1.2):(0.8-1.2), preferably 1:1:1:1:1; the graphite sleeve is used for filling a boron carbide blank.

[0020] The temperature control module is connected to the reaction chamber. Specifically, the temperature control module includes a temperature controller and heating elements corresponding to the front section, middle section and rear section of the graphite sleeve, respectively, and the temperature controller is used to control the temperature of the heating element. The heating element can adjust the temperature of the graphite sleeve by heating the quartz tube corresponding to the front section, middle section and rear section of the graphite sleeve; the heating element adjusts the temperature by changing the output power, and the temperature controller is used to adjust the output power of the heating element and then control the temperature. Since the graphite sleeve and the quartz tube are tightly fitted and both the graphite sleeve and the quartz tube are high thermal conductors, when the temperature of the quartz tube is adjusted by the temperature control module, the temperature of the graphite sleeve can be adjusted by heat transfer.

[0021] Preferably, the temperature control module may further include heating elements corresponding to the front middle section and the middle and rear section areas of the graphite sleeve.

[0022] Preferably, the temperature control module further comprises a thermometer for detecting the temperature of the graphite sleeve; the thermometer may be a thermocouple.

[0023] The beneficial effects of the present invention are as follows: (1) The present invention adopts a step-by-step and regional method from back to front to increase the deposition temperature of boron carbide, which can not only increase the deposition rate of boron carbide, but also ensure that the physical and mechanical properties of each part of the prepared high-purity porous boron carbide nuclear control rod are consistent. The present invention can adjust the porosity and pore size of the prepared high-purity porous boron carbide and obtain a high total boron-carbon content by changing the particle size of the boron carbide powder used to prepare the boron carbide blank, and adjusting the temperature, flow rate, pressure and deposition time, so as to meet various practical needs.

[0024] (2) In the process of preparing high-purity porous boron carbide nuclear control rods, the present invention avoids high-temperature sintering, and no binder and sintering aid need to be added to the raw materials, which can significantly reduce the manufacturing cost and improve the purity of porous boron carbide.

[0025] (3) The present invention sequentially connects the flow regulating module, the reaction chamber and the pressure regulating module, which can force the reaction mixture to flow through the pores of the boron carbide blank, and utilizes the temperature control module to achieve precise zoned temperature control. The inner diameter and length of the graphite sleeve can be adjusted according to the actual size requirements of the required nuclear control rod, thereby achieving one-time molding of the net size of the high-purity porous boron carbide nuclear control rod, thereby avoiding the problem of subsequent raw material waste caused by machining. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of a preparation device for high-purity porous boron carbide nuclear control rods of the present invention; Figure 2 It is a schematic diagram of the reaction chamber structure of the present invention; Figure 3 This is the SEM image of the high-purity porous boron carbide nuclear control rod prepared in Example 2.

[0027] In the figure: 1-flow regulating module; 2-temperature control module; 3-pressure regulating module; 4-reaction chamber; 5-graphite sleeve; 6-quartz tube; 7-valve. DETAILED DESCRIPTION

[0028] The principles and features of the present invention are described below in conjunction with examples, which are only used to explain the present invention and are not used to limit the scope of the present invention. The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples, unless otherwise specified, can all be obtained from commercial sources. Example 1

[0029] like Figure 1 The invention relates to a preparation device for a high-purity porous boron carbide nuclear control rod, comprising a temperature control module 2 and a flow regulating module 1, a reaction chamber 4 and a pressure regulating module 3 which are connected in sequence. A valve 7 is provided between the flow regulating module 1 and the reaction chamber 4, and a valve 7 is provided between the reaction chamber 4 and the pressure regulating module 3. The flow regulating module 1 is used for regulating the flow of four gases, namely, boron trichloride, methane, argon and hydrogen. The pressure regulating module 3 is used for regulating the gas pressure in the reaction chamber 4.

[0030] like Figure 2: is a schematic diagram of the structure of the reaction chamber 4, which is composed of a quartz tube 6 and a graphite sleeve 5. The graphite sleeve 5 is located in the quartz tube 6, and the outer wall of the graphite sleeve 5 is tightly fitted with the inner wall of the quartz tube 6; the graphite sleeve 5 is used to load the boron carbide blank, and is divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section and a rear section, starting from the air inlet end, and the length ratio of the front section, the front middle section, the middle section, the middle rear section and the rear section is 1:1:1:1:1:1.

[0031] The temperature control module 2 is connected to the reaction chamber 4. In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, the middle section, and the rear section of the graphite sleeve 5, respectively. The heating element adjusts the temperature of the graphite sleeve 5 in the reaction chamber 4 by changing the output power; the temperature controller is used to adjust the output power of the heating element and thus control the temperature, the thermocouple is used to detect the temperature of the graphite sleeve 5, and the temperature controller can be used to collect temperature data of the thermocouple.

[0032] In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, front middle section, middle section, middle rear section, and rear section of the graphite sleeve 5, respectively. The heating element adjusts the temperature of the graphite sleeve 5 in the reaction chamber 4 by changing the output power; the temperature controller is used to adjust the output power of the heating element and thus control the temperature, the thermocouple is used to detect the temperature of the graphite sleeve 5, and the temperature controller can be used to collect temperature data of the thermocouple. Example 2

[0033] A method for preparing a high-purity porous boron carbide nuclear control rod comprises the following steps: S1. A graphite sleeve 5 having an inner diameter of 40 mm and a length of 100 mm is filled with boron carbide powder having an average particle size of 7 μm, and the boron carbide powder in the graphite sleeve 5 is compacted at a pressure of 5 MPa to obtain a boron carbide blank; wherein the boron carbide blank is evenly divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section, and a rear section, starting from the air inlet end; S2 Insert the graphite sleeve 5 into the quartz tube 6 so that the two are tightly fitted, then introduce argon gas into the quartz tube 6, and heat the entire graphite sleeve 5 to 1050° C.; S3: The temperature of the front section and the middle section of the graphite sleeve 5 is maintained at 1050°C by using the temperature control module 2, and then the rear section of the graphite sleeve 5 is heated to raise the temperature to 1200°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced from the front section of the boron carbide blank into the reaction chamber 4 through the flow regulating module 1 for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required, and the pressure of the reaction chamber 4 is adjusted to 2000Pa by using the pressure regulating module 3, the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:2:6:6, the flow rate of the mixed gas is 130mL / min, and the reaction time is 17 hours; S4: Using the temperature control module 2, the temperature of the front section and the rear section of the graphite sleeve 5 is maintained at 1050°C, and then the middle section of the graphite sleeve 5 is heated to raise the temperature to 1190°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced from the front section of the boron carbide blank into the reaction chamber 4 for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 1000Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:2:6:6, the flow rate of the mixed gas is 120mL / min, and the reaction time is 15 hours; S5: Using the temperature control module 2, the temperature of the middle section and the rear section of the graphite sleeve 5 is maintained at 1050°C, and then the front section of the graphite sleeve 5 is heated to raise the temperature to 1180°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced from the front section of the boron carbide blank into the reaction chamber 4 for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 500Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:2:6:6, the flow rate of the mixed gas is 110mL / min, and the reaction time is 13 hours; S6: Stop introducing the three gases of boron trichloride, methane and hydrogen, and only introduce argon at a flow rate of 90 mL / min into the quartz tube 6, and control the pressure of the reaction chamber 4 to 9000 Pa. The temperature control module 2 stops heating to allow the graphite sleeve 5 to cool down naturally. When the temperature of the graphite sleeve 5 drops below 400°C, stop introducing argon into the quartz tube 6, and restore the reaction chamber 4 to normal pressure. When the temperature of the graphite sleeve 5 drops below 100°C, take the graphite sleeve 5 out of the quartz tube 6, and you can get a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm, respectively.

[0034] like Figure 3The SEM image of the high-purity porous boron carbide nuclear control rod prepared in this embodiment shows that the boron carbide nuclear control rod has a uniform porous structure, and the pores are interconnected and evenly distributed. This pore structure is conducive to the discharge of helium, thereby ensuring that the nuclear control rod will not bulge or break during actual use. In addition, the boron carbide particles in the boron carbide nuclear control rod are not in point contact, but rather show a smooth transition with a large contact area. This structure helps to improve the mechanical properties of boron carbide ceramics. Example 3

[0035] A method for preparing a high-purity porous boron carbide nuclear control rod comprises the following steps: S1. A graphite sleeve 5 having an inner diameter of 40 mm and a length of 100 mm is filled with boron carbide powder having an average particle size of 5 μm, and the boron carbide powder in the graphite sleeve 5 is compacted at a pressure of 4 MPa to obtain a boron carbide blank; wherein the boron carbide blank is evenly divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section, and a rear section, starting from the air inlet end; S2 Insert the graphite sleeve 5 into the quartz tube 6 so that the two are tightly fitted, then introduce argon gas into the quartz tube 6, and heat the entire graphite sleeve 5 to 1040° C.; S3: The temperature of the front section and the middle section of the graphite sleeve 5 is maintained at 1040°C by using the temperature control module 2, and then the rear section of the graphite sleeve 5 is heated to raise the temperature to 1195°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced from the front section of the boron carbide blank into the reaction chamber 4 through the flow regulating module 1 for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required, and the pressure of the reaction chamber 4 is adjusted to 2300Pa by using the pressure regulating module 3, the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1.5:5:6, the flow rate of the mixed gas is 140mL / min, and the reaction time is 15 hours; S4: The temperature of the front section and the rear section of the graphite sleeve 5 is maintained at 1040°C by using the temperature control module 2, and then the middle section of the graphite sleeve 5 is heated to raise the temperature to 1185°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced into the reaction chamber 4 from the front section of the boron carbide blank for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 1400Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1.5:5:6, the flow rate of the mixed gas is 130mL / min, and the reaction time is 13 hours; S5: The temperature of the middle section and the rear section of the graphite sleeve 5 is maintained at 1040°C by using the temperature control module 2, and then the front section of the graphite sleeve 5 is heated to raise the temperature to 1175°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced into the reaction chamber 4 from the front section of the boron carbide blank for reaction; wherein the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 700Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1.5:5:6, the flow rate of the mixed gas is 120mL / min, and the reaction time is 11 hours; S6: Stop introducing the three gases of boron trichloride, methane and hydrogen, and only introduce argon at a flow rate of 80 mL / min into the quartz tube 6, and control the pressure of the reaction chamber 4 to 8700 Pa. The temperature control module 2 stops heating to allow the graphite sleeve 5 to cool down naturally. When the temperature of the graphite sleeve 5 drops below 400°C, stop introducing argon into the quartz tube 6, and restore the reaction chamber 4 to normal pressure. When the temperature of the graphite sleeve 5 drops below 100°C, take the graphite sleeve 5 out of the quartz tube 6, and you can get a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm, respectively. Example 4

[0036] A method for preparing a high-purity porous boron carbide nuclear control rod comprises the following steps: S1. A graphite sleeve 5 having an inner diameter of 40 mm and a length of 100 mm is filled with boron carbide powder having an average particle size of 3 μm, and the boron carbide powder in the graphite sleeve 5 is compacted at a pressure of 3 MPa to obtain a boron carbide blank; wherein the boron carbide blank is evenly divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section, and a rear section, starting from the air inlet end; S2 Insert the graphite sleeve 5 into the quartz tube 6 so that the two are tightly fitted, then introduce argon gas into the quartz tube 6, and heat the entire graphite sleeve 5 to 1020° C.; S3: The temperature of the front section and the middle section of the graphite sleeve 5 is maintained at 1020°C by using the temperature control module 2, and then the rear section of the graphite sleeve 5 is heated to raise the temperature to 1185°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced from the front section of the boron carbide blank into the reaction chamber 4 through the flow regulating module 1 for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required, and the pressure of the reaction chamber 4 is adjusted to 2600Pa by using the pressure regulating module 3, the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1.5:4:5, the flow rate of the mixed gas is 150mL / min, and the reaction time is 13 hours; S4: The temperature of the front section and the rear section of the graphite sleeve 5 is maintained at 1020°C by using the temperature control module 2, and then the middle section of the graphite sleeve 5 is heated to raise the temperature to 1180°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced into the reaction chamber 4 from the front section of the boron carbide blank for reaction; wherein the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 1700Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1.5:4:5, the flow rate of the mixed gas is 140mL / min, and the reaction time is 11 hours; S5: The temperature of the middle section and the rear section of the graphite sleeve 5 is maintained at 1020°C by using the temperature control module 2, and then the front section of the graphite sleeve 5 is heated to raise the temperature to 1170°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced into the reaction chamber 4 from the front section of the boron carbide blank for reaction; wherein the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 800Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1.5:4:5, the flow rate of the mixed gas is 130mL / min, and the reaction time is 9 hours; S6: Stop introducing the three gases of boron trichloride, methane and hydrogen, and only introduce argon at a flow rate of 70 mL / min into the quartz tube 6, and control the pressure of the reaction chamber 4 to 8400 Pa. The temperature control module 2 stops heating to allow the graphite sleeve 5 to cool down naturally. When the temperature of the graphite sleeve 5 drops below 400°C, stop introducing argon into the quartz tube 6, and restore the reaction chamber 4 to normal pressure. When the temperature of the graphite sleeve 5 drops below 100°C, take the graphite sleeve 5 out of the quartz tube 6, and you can get a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm, respectively. Example 5

[0037] A method for preparing a high-purity porous boron carbide nuclear control rod comprises the following steps: S1. A graphite sleeve 5 having an inner diameter of 40 mm and a length of 100 mm is filled with boron carbide powder having an average particle size of 1 μm, and the boron carbide powder in the graphite sleeve 5 is compacted at a pressure of 3 MPa to obtain a boron carbide blank; wherein the boron carbide blank is evenly divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section, and a rear section, starting from the air inlet end; S2 Insert the graphite sleeve 5 into the quartz tube 6 so that the two are tightly fitted, then introduce argon gas into the quartz tube 6, and heat the entire graphite sleeve 5 to 1000° C.; S3: The temperature of the front section and the middle section of the graphite sleeve 5 is maintained at 1000°C by using the temperature control module 2, and then the rear section of the graphite sleeve 5 is heated to raise the temperature to 1180°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced from the front section of the boron carbide blank into the reaction chamber 4 through the flow regulating module 1 for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required, and the pressure of the reaction chamber 4 is adjusted to 3000Pa by using the pressure regulating module 3, the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1:3:4, the flow rate of the mixed gas is 160mL / min, and the reaction time is 11 hours; S4: The temperature of the front section and the rear section of the graphite sleeve 5 is maintained at 1000° C. by using the temperature control module 2, and then the middle section of the graphite sleeve 5 is heated to raise the temperature to 1170° C., and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced into the reaction chamber 4 from the front section of the boron carbide blank for reaction; wherein the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 2000 Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1:3:4, the flow rate of the mixed gas is 150 mL / min, and the reaction time is 9 hours; S5: Using the temperature control module 2, the temperature of the middle section and the rear section of the graphite sleeve 5 is maintained at 1000°C, and then the front section of the graphite sleeve 5 is heated to raise the temperature to 1160°C, and a mixed gas of boron trichloride, methane, argon and hydrogen is continuously introduced into the reaction chamber 4 from the front section of the boron carbide blank for reaction; wherein, the front middle section and the middle and rear sections are temperature transition areas, and no temperature control is required. The pressure of the reaction chamber 4 is 1000Pa, and the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:1:3:4, the flow rate of the mixed gas is 140mL / min, and the reaction time is 7 hours; S6: Stop introducing the three gases of boron trichloride, methane and hydrogen, and only introduce argon at a flow rate of 60 mL / min into the quartz tube 6, and control the pressure of the reaction chamber 4 to 8000 Pa. The temperature control module 2 stops heating to allow the graphite sleeve 5 to cool down naturally. When the temperature of the graphite sleeve 5 drops below 400°C, stop introducing argon into the quartz tube 6, and restore the reaction chamber 4 to normal pressure. When the temperature of the graphite sleeve 5 drops below 100°C, take the graphite sleeve 5 out of the quartz tube 6, and you can get a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm, respectively. Comparative Example 1

[0038] Referring to Example 2, the difference is that in step S3, the graphite sleeve 5 is heated to 1200° C. as a whole; in step S4, the graphite sleeve 5 is heated to 1190° C. as a whole; in step S5, the graphite sleeve 5 is heated to 1180° C. as a whole; and the other parameter steps remain unchanged. test

[0039] The physical and mechanical properties of the high-purity porous boron carbide nuclear control rods prepared in Examples 2-5 and Comparative Example 1 were tested, as shown in Table 1.

[0040] Among them, the porosity and density are measured by the bulk density method and Archimedes drainage method in the national standard GB / T25995-2010 respectively; the compressive strength is measured by the compression test method in the national standard GB / T4740-1999; the fracture toughness is measured by the single-sided V-notch beam method in the national standard GB / T44547-2024; the total boron and carbon content is measured jointly by inductively coupled plasma emission spectroscopy and high-frequency combustion infrared absorption method; the pore size is measured by the mercury injection method in the national standard GB / T 21650.1-2008.

[0041] Table 1 Physical and mechanical properties of high-purity porous boron carbide nuclear control rods

[0042] Compared with the existing preparation method, the present invention has a lower sintering temperature and a lower preparation cost, and the physical and mechanical properties of each section of the boron carbide nuclear control rod prepared by the method of the present invention are basically the same. Taking Example 2 as an example, the porosity of all sections is maintained in the range of 21.5-21.8%, and the density is maintained in the range of 1.96-1.97 g / cm 3 The compressive strength is maintained in the range of 587~592MPa, and the fracture toughness is maintained in the range of 3.8~3.9MPa·m 1 / 2 In contrast, in Comparative Example 1, since the deposition temperature of boron carbide was not increased step by step from the back to the front, but the deposition temperature of boron carbide was increased as a whole, the amount of boron carbide deposited in the front section of the nuclear control rod was large, and the amount of boron carbide deposited in the rear section was small, resulting in significant differences in the physical and mechanical properties of each section of the nuclear control rod: from the front section to the rear section, the porosity of the nuclear control rod increased significantly from 15.6% to 32.2%, and the density increased from 2.11 g / cm 3 Significantly reduced to 1.70g / cm 3 The compressive strength dropped significantly from 776MPa to 387MPa, and the fracture toughness dropped from 4.5MPa·m 1 / 2 Significantly reduced to 3.1MPa·m 1 / 2 The pore size increased significantly from 1.3 μm to 2.8 μm. The above results show that the preparation method of the present invention can ensure that the physical and mechanical properties of each part of the high-purity porous boron carbide nuclear control rod are consistent.

[0043] According to Table 1, in terms of physical properties, the porosity and density of the high-purity porous boron carbide nuclear control rods prepared in Examples 2 to 5 are 21.5% to 36.6% and 1.58 to 1.97 g / cm, respectively. 3 Compared with the boron carbide material prepared in patent application CN202411124474.7, the porosity and density range of the product prepared in the present invention is wider, and it can adapt to the physical performance requirements of boron carbide nuclear control rods of more specifications of high-temperature gas-cooled reactors; in terms of mechanical properties, the compressive strength and fracture toughness of the high-purity porous boron carbide nuclear control rods prepared in Examples 2 to 5 are 344~592MPa and 2.8~3.9MPa·m 1 / 2 Compared with the boron carbide material prepared in patent application CN202411124474.7, the boron carbide material of the present invention has a wider range of compressive strength and fracture toughness, and the present invention has a larger selection space for mechanical properties; in terms of purity, the total boron-carbon content of the high-purity porous boron carbide nuclear control rods prepared in Examples 2 to 5 is ≥99.9%. Compared with the boron carbide material prepared in patent application CN202411124474.7, the boron carbide material of the present invention has higher purity and higher neutron shielding efficiency.

[0044] The present invention can affect the porosity to a certain extent by changing the particle size of the boron carbide powder and can effectively adjust the pore size of the blank, thereby affecting the resistance of the mixed gas passing through the blank; on this basis, by adjusting the reaction temperature, flow rate, pressure and deposition time, the deposition thickness of boron carbide in the blank can be changed, thereby achieving the purpose of adjusting the physical and mechanical properties of the boron carbide nuclear control rod. As shown in Table 1, from Example 2 to Example 5, as the particle size of the boron carbide powder decreases from 7μm to 1μm, the porosity of the boron carbide nuclear control rod gradually increases from 21.5~21.8% to 36.3~36.6%, and the density increases from 1.96~1.97g / cm 3 Gradually decreased to 1.58~1.60g / cm 3 The compressive strength gradually decreased from 587~592MPa to 344~351MPa, and the fracture toughness decreased from 3.8~3.9MPa·m 1 / 2 Gradually decreased to 2.8~2.9MPa·m 1 / 2 The pore size gradually decreases from 2.1-2.3 μm to 0.3-0.4 μm. The above results show that the preparation method of the present invention has the significant advantage of easy process control, and the physical and mechanical properties of the porous boron carbide nuclear control rod can be effectively adjusted by coordinating the particle size of the boron carbide powder and the deposition process parameters.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a high-purity porous boron carbide nuclear control rod, characterized in that: The following steps are involved: S1. Obtaining a boron carbide blank, wherein the boron carbide blank is sequentially divided into five regions: a front section, a front middle section, a middle section, a middle rear section, and a rear section; S2, heating the boron carbide blank as a whole to a first temperature in an argon atmosphere; S3: maintaining the temperature of the front section and the middle section of the boron carbide blank at a first temperature, heating the rear section to a second temperature, and continuously introducing a mixed gas of boron trichloride, methane, argon and hydrogen from the front section of the boron carbide blank to react; S4: maintaining the temperature of the front section and the rear section of the boron carbide blank at the first temperature, heating the middle section to the third temperature, and continuously introducing a mixed gas of boron trichloride, methane, argon and hydrogen from the front section of the boron carbide blank to react; S5: maintaining the temperature of the middle and rear sections of the boron carbide blank at the first temperature, heating the front section to a fourth temperature, and continuously introducing a mixed gas of boron trichloride, methane, argon and hydrogen from the front section of the boron carbide blank to react; After the S6 reaction, a high-purity porous boron carbide nuclear control rod is obtained; Among them, the second temperature, the third temperature and the fourth temperature are all higher than the first temperature.

2. The preparation method according to claim 1, characterized in that: In step S1, the length ratio of the front section, front middle section, middle section, middle rear section and rear section of the boron carbide blank is 1:(0.8~1.2):(0.8~1.2):(0.8~1.2):(0.8~1.2).

3. The preparation method according to claim 1, characterized in that: In step S2, the first temperature is 1000-1050°C.

4. The preparation method according to claim 1, characterized in that: In step S3, the second temperature is 1180-1200°C, the reaction pressure is 2000-3000 Pa, the reaction time is 11-17 h, and the mixed gas flow rate is 130-160 mL / min.

5. The preparation method according to claim 1, characterized in that: In step S4, the third temperature is 1170-1190° C., the reaction pressure is 1000-2000 Pa, the reaction time is 9-15 h, and the mixed gas flow rate is 120-150 mL / min.

6. The preparation method according to claim 1, characterized in that: In step S5, the fourth temperature is 1160-1180° C., the reaction pressure is 500-1000 Pa, the reaction time is 7-13 h, and the mixed gas flow rate is 110-140 mL / min.

7. The preparation method according to claim 1, characterized in that: In step S1, the boron carbide blank is obtained by pressing boron carbide powder, and the average particle size of the boron carbide powder is 1-7 μm.

8. A device for preparing a high-purity porous boron carbide nuclear control rod prepared by the preparation method according to any one of claims 1 to 7, characterized in that: It comprises a temperature control module (2) and a flow regulating module (1), a reaction chamber (4) and a pressure regulating module (3) which are connected in sequence; the reaction chamber (4) is composed of a quartz tube (6) and a graphite sleeve (5), the graphite sleeve (5) is located in the quartz tube (6), and the outer wall of the graphite sleeve (5) is tightly fitted with the inner wall of the quartz tube (6); The graphite sleeve (5) is divided into a front section, a front middle section, a middle section, a middle rear section and a rear section in sequence, and the length ratio of the front section, the front middle section, the middle section, the middle rear section and the rear section is 1:(0.8-1.2):(0.8-1.2):(0.8-1.2):(0.8-1.2); The temperature control module (2) comprises a temperature controller and heating elements corresponding to the front section, the middle section and the rear section of the graphite sleeve (5), respectively, and the temperature controller is used to control the temperature of the heating element.

9. The device according to claim 8, characterized in that The length ratio of the front section, the front middle section, the middle section, the middle rear section and the rear section of the graphite sleeve (5) is 1:1:1:1:

1.

10. The device according to claim 8, characterized in that The temperature control module (2) also includes a temperature measuring instrument for detecting the temperature of the graphite sleeve (5).

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