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

Abstract

The present invention belongs to the technical field of ceramic preparation, and relates to a preparation method and device for a high-purity porous boron carbide nuclear control rod. The present invention uses boron trichloride gas as the boron source, methane as the carbon source, argon as the carrier gas and protective gas, and hydrogen as the catalytic gas and reducing gas to prepare a high-purity porous boron carbide nuclear control rod. The method of increasing the deposition temperature of boron carbide step by step and region by region from the back to the front is adopted, which can not only improve 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. By changing the particle size of the boron carbide powder for preparing the boron carbide blank and coordinating the adjustment of temperature, flow rate, pressure and deposition time, the porosity and pore size of the prepared high-purity porous boron carbide can be regulated and a high total boron and carbon content can be obtained, so as to meet various actual requirements.

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 of the above-mentioned existing technologies, 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 and carbon content, high sintering temperature, and raw material waste caused by machining during the preparation of the high-purity porous boron carbide nuclear control rod.

[0005] The specific technical solutions are as follows:

[0006] A method for preparing a high-purity porous boron carbide nuclear control rod includes the following steps:

[0007] S1 Obtain a boron carbide blank, and the boron carbide blank is sequentially divided into five regions: a front section, a front-middle section, a middle section, a middle-back section, and a rear section;

[0008] S2 Under an argon atmosphere, heat the entire boron carbide blank to a first temperature;

[0009] S3 Keep the temperatures of the front section and the middle section of the boron carbide blank at the first temperature, heat the rear section region to a second temperature, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank for reaction;

[0010] S4 Keep the temperatures of the front section and the rear section regions of the boron carbide blank at the first temperature, heat the middle section region to a third temperature, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank for reaction;

[0011] S5 Keep the temperatures of the middle section and the rear section regions of the boron carbide blank at the first temperature, heat the front section region to a fourth temperature, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank for reaction;

[0012] S6 After the reaction, a high-purity porous boron carbide nuclear control rod is obtained;

[0013] Among them, the second temperature, the third temperature, and the fourth temperature are all higher than the first temperature.

[0014] The reaction mechanism of the present invention is as follows:

[0015] The present invention uses boron trichloride gas as the boron source, methane as the carbon source, argon as the carrier gas and protective gas, and hydrogen as the catalytic gas and reducing gas to prepare a high-purity porous boron carbide nuclear control rod. Each gas is fully mixed by means of diffusion and convection. When the mixed gas flows through the pores inside the boron carbide green body, due to the action of high temperature, boron trichloride will decompose to produce boron atoms and chlorine atoms, and methane will decompose to produce carbon atoms and hydrogen atoms. Immediately afterwards, the chlorine atoms and hydrogen atoms react to form hydrogen chloride, which is discharged as tail gas, and the boron atoms and carbon atoms react to form boron carbide, which is deposited on the surface of the boron carbide particles of the boron carbide green body. With the continuous deposition of boron carbide, the originally loosely packed boron carbide green body particles are firmly bonded together.

[0016] Preferably, in step S1, the boron carbide green body 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 pressing at 3-5 MPa.

[0017] Preferably, in step S1, the length ratios of the front section, the front-middle section, the middle section, the middle-back section and the back section of the boron carbide green body are 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, overall increasing the temperature will cause significant differences in the porosity, pore size and mechanical properties of each region of the product. Therefore, the present invention uses zone temperature control to achieve the deposition of boron carbide when preparing the high-purity porous boron carbide nuclear control rod.

[0018] Preferably, in step S2, the first temperature is 1000-1050 °C.

[0019] Preferably, in steps S3-S5, the molar ratio of boron trichloride, methane, argon and hydrogen in the mixed gas is 1:(1-2):(3-6):(4-6).

[0020] In steps S3-S5, the front-middle section and the middle-back section of the boron carbide green body are temperature transition regions and do not require temperature control.

[0021] 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 flow rate of the mixed gas is 130-160 mL / min.

[0022] 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 flow rate of the mixed gas is 120-150 mL / min.

[0023] 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 flow rate of the mixed gas is 110 - 140 mL / min.

[0024] Preferably, the second temperature is higher than the third temperature, and the third temperature is higher than the fourth temperature.

[0025] In step S6, after the reaction ends, stop introducing the mixed gas of boron trichloride, methane, argon, and hydrogen, introduce argon and control the pressure to be 8000 - 9000 Pa, stop heating to let the graphite sleeve cool down naturally. When the temperature drops below 400 °C, stop introducing argon and the pressure returns to normal pressure; when the temperature drops below 100 °C, a high-purity porous boron carbide nuclear control rod is obtained.

[0026] Preferably, in step S6, the flow rate of argon is 60 - 90 mL / min.

[0027] The present invention also provides a preparation device for a high-purity porous boron carbide nuclear control rod, including a temperature control module, a flow rate adjustment module, a reaction chamber, and a pressure adjustment module connected in sequence; wherein, the flow rate adjustment module is used to regulate the flow rates of four gases, namely boron trichloride, methane, argon, and hydrogen; the pressure adjustment module is used to adjust the gas pressure in the reaction chamber.

[0028] The reaction chamber is composed 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 in close contact 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. 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 to load the boron carbide blank.

[0029] 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, the middle section, and the rear section of the graphite sleeve respectively. The temperature controller is used to regulate the temperature of the heating elements. By heating the positions of the quartz tube corresponding to the front section, the middle section, and the rear section of the graphite sleeve, the temperature of the graphite sleeve can be adjusted; the heating elements adjust the temperature by changing the output power, and the temperature controller is used to adjust the output power of the heating elements to further regulate the temperature. Since the graphite sleeve is in close contact with the quartz tube and both the graphite sleeve and the quartz tube are high thermal conductors, when using the temperature control module to adjust the temperature of the quartz tube, the temperature of the graphite sleeve can be adjusted by heat transfer.

[0030] Preferably, the temperature control module may further include heating elements corresponding to the front middle section and the middle rear section regions of the graphite sleeve.

[0031] Preferably, the temperature control module further includes a thermometer for detecting the temperature of the graphite sleeve; the thermometer can be a thermocouple.

[0032] The beneficial effects of the present invention are as follows:

[0033] (1) The present invention uses a step-by-step and region-by-region 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. By changing the particle size of the boron carbide powder for preparing the boron carbide green body and coordinating the regulation of temperature, flow rate, pressure, and deposition time, the porosity and pore size of the prepared high-purity porous boron carbide can be regulated and a high total boron and carbon content can be obtained, so as to meet various actual requirements.

[0034] (2) During the preparation of the high-purity porous boron carbide nuclear control rod of the present invention, high-temperature sintering is avoided, 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 the porous boron carbide.

[0035] (3) The present invention connects the flow rate adjustment module, the reaction chamber, and the pressure adjustment module in sequence, which can force the reaction mixed gas to flow through the pores of the boron carbide green body, and the temperature control module can be used to achieve precise zone temperature control and adjust the inner diameter and length of the graphite sleeve according to the actual size requirements of the required nuclear control rod, realizing the net size one-time forming of the high-purity porous boron carbide nuclear control rod, thus avoiding the problem of raw material waste caused by machining subsequently. Brief Description of the Drawings

[0036] Figure 1 It is a schematic diagram of the preparation device of the high-purity porous boron carbide nuclear control rod of the present invention;

[0037] Figure 2 It is a schematic diagram of the reaction chamber structure of the present invention;

[0038] Figure 3 It is an SEM image of the high-purity porous boron carbide nuclear control rod prepared in Example 2.

[0039] In the figure: 1 - flow rate adjustment module; 2 - temperature control module; 3 - pressure adjustment module; 4 - reaction chamber; 5 - graphite sleeve; 6 - quartz tube; 7 - valve. Detailed Embodiments

[0040] The principles and features of the present invention are described below in conjunction with examples. The examples given 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 all conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples can all be obtained from commercial channels unless otherwise specified. Example 1

[0041] As Figure 1 It is a preparation device for a high-purity porous boron carbide nuclear control rod, including a temperature control module 2 and a flow rate adjustment module 1, a reaction chamber 4, and a pressure adjustment module 3 that are connected in sequence. A valve 7 is provided between the flow rate adjustment module 1 and the reaction chamber 4, and a valve 7 is provided between the reaction chamber 4 and the pressure adjustment module 3; the flow rate adjustment module 1 is used to control the flow rates of four gases, namely boron trichloride, methane, argon, and hydrogen; the pressure adjustment module 3 is used to adjust the gas pressure in the reaction chamber 4.

[0042] As Figure 2 It is a schematic structural diagram of the reaction chamber 4. 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 in close contact with the inner wall of the quartz tube 6; the graphite sleeve 5 is used to fill the boron carbide blank, and is evenly divided into five regions, namely the front section, the front middle section, the middle section, the middle rear section, and the rear section in sequence starting from the air inlet end. 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.

[0043] 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 elements adjust 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 elements to 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 the temperature data of the thermocouple.

[0044] In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, the front middle section, the middle section, the middle rear section, and the rear section of the graphite sleeve 5 respectively. The heating elements adjust 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 elements to 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 the temperature data of the thermocouple. Example 2

[0045] A preparation method for a high-purity porous boron carbide nuclear control rod includes the following steps:

[0046] S1 Fill the graphite sleeve 5 with an inner diameter and length of 40 mm and 100 mm respectively with boron carbide powder with an average particle size of 7 μm, and compact the boron carbide powder in the graphite sleeve 5 with a pressure of 5 MPa to obtain a boron carbide blank; among them, the boron carbide blank is evenly divided into five regions, namely the front section, the front middle section, the middle section, the middle rear section, and the rear section in sequence starting from the air inlet end;

[0047] S2 Insert the graphite sleeve 5 into the quartz tube 6 to make them fit tightly, then introduce argon into the quartz tube 6, and heat the entire graphite sleeve 5 to 1050 °C;

[0048] S3 Use the temperature control module 2 to maintain the temperature of the front and middle sections of the graphite sleeve 5 at 1050 °C, then heat the rear section area of the graphite sleeve 5 to raise the temperature to 1200 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank into the reaction chamber 4 for reaction through the flow rate adjustment module 1; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. Use the pressure adjustment module 3 to adjust the pressure of the reaction chamber 4 to 2000 Pa. 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 130 mL / min, and the reaction time is 17 hours;

[0049] S4 Use the temperature control module 2 to maintain the temperature of the front and rear sections of the graphite sleeve 5 at 1050 °C, then heat the middle section area of the graphite sleeve 5 to raise the temperature to 1190 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank into the reaction chamber 4 for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. The pressure of the reaction chamber 4 is 1000 Pa. 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 120 mL / min, and the reaction time is 15 hours;

[0050] S5 Use the temperature control module 2 to maintain the temperature of the middle and rear sections of the graphite sleeve 5 at 1050 °C, then heat the front section area of the graphite sleeve 5 to raise the temperature to 1180 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank into the reaction chamber 4 for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. The pressure of the reaction chamber 4 is 500 Pa. 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 110 mL / min, and the reaction time is 13 hours;

[0051] S6 Stop introducing the three gases of boron trichloride, methane, and hydrogen, and only introduce argon with 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 and allows the graphite sleeve 5 to cool 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 out the graphite sleeve 5 from the quartz tube 6, and a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm can be obtained.

[0052] As Figure 3 Shown in the SEM image of the high-purity porous boron carbide nuclear control rod prepared in this example, it can be found that the boron carbide nuclear control rod has a uniform porous structure, with the pores communicating with each other and evenly distributed. This pore structure is beneficial for the discharge of helium gas, thus ensuring that the nuclear control rod will not bulge or rupture during actual use. In addition, the boron carbide particles in the boron carbide nuclear control rod do not show point contact, but rather a smooth transition with a large contact area. This structure helps to improve the mechanical properties of the boron carbide ceramic. Example 3

[0053] A preparation method of a high-purity porous boron carbide nuclear control rod includes the following steps:

[0054] S1 Fill the graphite sleeve 5 with an inner diameter and length of 40 mm and 100 mm respectively with boron carbide powder with an average particle size of 5 μm, and compact the boron carbide powder in the graphite sleeve 5 using a pressure of 4 MPa to obtain a boron carbide green body. Among them, the boron carbide green body is evenly divided into five regions: the front section, the front middle section, the middle section, the middle rear section, and the rear section starting from the gas inlet end.

[0055] S2 Insert the graphite sleeve 5 into the quartz tube 6 to make them fit tightly, then introduce argon into the quartz tube 6, and heat the entire graphite sleeve 5 to 1040 °C.

[0056] S3 Use the temperature control module 2 to maintain the temperature of the front section and the middle section of the graphite sleeve 5 at 1040 °C, then heat the rear section area of the graphite sleeve 5 to raise the temperature to 1195 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide green body into the reaction chamber 4 for reaction. Among them, the front middle section and the middle rear section are temperature transition regions and do not require temperature control. Use the pressure regulation module 3 to adjust the pressure in the reaction chamber 4 to 2300 Pa. 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 140 mL / min, and the reaction time is 15 hours.

[0057] S4 Use the temperature control module 2 to maintain the temperature of the front section and the rear section of the graphite sleeve 5 at 1040 °C, then heat the middle section area of the graphite sleeve 5 to raise the temperature to 1185 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide green body into the reaction chamber 4 for reaction. Among them, the front middle section and the middle rear section are temperature transition regions and do not require temperature control. The pressure in the reaction chamber 4 is 1400 Pa, 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 130 mL / min, and the reaction time is 13 hours.

[0058] S5 Use the temperature control module 2 to maintain the temperature of the middle and rear sections of the graphite sleeve 5 at 1040 °C, then heat the front section area of the graphite sleeve 5 to raise the temperature to 1175 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen into the reaction chamber 4 from the front section of the boron carbide green body for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. The pressure in the reaction chamber 4 is 700 Pa, 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 120 mL / min, and the reaction time is 11 hours;

[0059] S6 Stop introducing the three gases of boron trichloride, methane, and hydrogen, only introduce argon with a flow rate of 80 mL / min into the quartz tube 6, control the pressure in the reaction chamber 4 to 8700 Pa, and the temperature control module 2 stops heating to let the graphite sleeve 5 cool 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 atmospheric pressure; when the temperature of the graphite sleeve 5 drops below 100 °C, take out the graphite sleeve 5 from the quartz tube 6, and a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm respectively can be obtained. Example 4

[0060] A method for preparing a high-purity porous boron carbide nuclear control rod includes the following steps:

[0061] S1 Fill the graphite sleeve 5 with an inner diameter and length of 40 mm and 100 mm respectively with boron carbide powder with an average particle size of 3 μm, and compact the boron carbide powder in the graphite sleeve 5 with a pressure of 3 MPa to obtain a boron carbide green body; among them, the boron carbide green body is evenly divided into five regions: the front section, the front-middle section, the middle section, the middle-rear section, and the rear section in sequence from the intake end.

[0062] S2 Insert the graphite sleeve 5 into the quartz tube 6 so that they are closely fitted, then introduce argon into the quartz tube 6, and heat the whole graphite sleeve 5 to 1020 °C.

[0063] S3 Use the temperature control module 2 to maintain the temperature of the front and middle sections of the graphite sleeve 5 at 1020 °C, then heat the rear section area of the graphite sleeve 5 to raise the temperature to 1185 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen into the reaction chamber 4 from the front section of the boron carbide green body through the flow rate adjustment module 1 for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. Use the pressure adjustment module 3 to adjust the pressure in the reaction chamber 4 to 2600 Pa, 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 150 mL / min, and the reaction time is 13 hours.

[0064] S4 Use the temperature control module 2 to maintain the temperatures of the front and rear sections of the graphite sleeve 5 at 1020 °C, then heat the middle section area of the graphite sleeve 5 to raise the temperature to 1180 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank into the reaction chamber 4 for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. The pressure in the reaction chamber 4 is 1700 Pa, 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 140 mL / min, and the reaction time is 11 hours;

[0065] S5 Use the temperature control module 2 to maintain the temperatures of the middle and rear sections of the graphite sleeve 5 at 1020 °C, then heat the front section area of the graphite sleeve 5 to raise the temperature to 1170 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank into the reaction chamber 4 for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions and do not require temperature control. The pressure in the reaction chamber 4 is 800 Pa, 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 130 mL / min, and the reaction time is 9 hours;

[0066] S6 Stop introducing the three gases of boron trichloride, methane, and hydrogen, only introduce argon with a flow rate of 70 mL / min into the quartz tube 6, control the pressure in the reaction chamber 4 to 8400 Pa, and the temperature control module 2 stops heating to allow the graphite sleeve 5 to cool 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 out the graphite sleeve 5 from the quartz tube 6, and a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm respectively can be obtained. Example 5

[0067] A method for preparing a high-purity porous boron carbide nuclear control rod, comprising the following steps:

[0068] S1 Fill the graphite sleeve 5 with an inner diameter and length of 40 mm and 100 mm respectively with boron carbide powder with an average particle size of 1 μm, and use a pressure of 3 MPa to compact the boron carbide powder in the graphite sleeve 5 to obtain a boron carbide blank; among them, the boron carbide blank is evenly divided into five regions: the front section, the front-middle section, the middle section, the middle-rear section, and the rear section in sequence from the intake end;

[0069] S2 Insert the graphite sleeve 5 into the quartz tube 6 so that the two are in close contact, then introduce argon into the quartz tube 6, and heat the entire graphite sleeve 5 to 1000 °C;

[0070] S3 Use the temperature control module 2 to maintain the temperature of the front and middle sections of the graphite sleeve 5 at 1000 °C, then heat the rear section area of the graphite sleeve 5 to raise the temperature to 1180 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen into the reaction chamber 4 from the front section of the boron carbide blank through the flow rate adjustment module 1 for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions, and no temperature control is required. Use the pressure adjustment module 3 to adjust the pressure of the reaction chamber 4 to 3000 Pa. 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 160 mL / min, and the reaction time is 11 hours;

[0071] S4 Use the temperature control module 2 to maintain the temperature of the front and rear sections of the graphite sleeve 5 at 1000 °C, then heat the middle section area of the graphite sleeve 5 to raise the temperature to 1170 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen into the reaction chamber 4 from the front section of the boron carbide blank for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions, and no temperature control is required. The pressure of the reaction chamber 4 is 2000 Pa. 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;

[0072] S5 Use the temperature control module 2 to maintain the temperature of the middle and rear sections of the graphite sleeve 5 at 1000 °C, then heat the front section area of the graphite sleeve 5 to raise the temperature to 1160 °C, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen into the reaction chamber 4 from the front section of the boron carbide blank for reaction; among them, the front-middle section and the middle-rear section are temperature transition regions, and no temperature control is required. The pressure of the reaction chamber 4 is 1000 Pa. 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 140 mL / min, and the reaction time is 7 hours;

[0073] S6 Stop introducing the three gases of boron trichloride, methane, and hydrogen, and only introduce argon with a flow rate of 60 mL / min into the quartz tube 6. 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 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 atmospheric pressure; when the temperature of the graphite sleeve 5 drops below 100 °C, take out the graphite sleeve 5 from the quartz tube 6, and a high-purity porous boron carbide nuclear control rod with a diameter and length of 40 mm and 100 mm respectively can be obtained. Comparative Example 1

[0074] Referring to Example 2, the differences are as follows: in step S3, the entire graphite sleeve 5 is heated to 1200 °C; in step S4, the entire graphite sleeve 5 is heated to 1190 °C; in step S5, the entire graphite sleeve 5 is heated to 1180 °C; the remaining parameter steps remain unchanged.

[0075] Test

[0076] 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, and the specific results are shown in Table 1.

[0077] Among them, the porosity and density were measured by the bulk density method and the Archimedes drainage method in GB / T25995-2010 respectively; the compressive strength was measured by the compression test method in GB / T4740-1999; the fracture toughness was measured by the single-edge V-notch beam method in GB / T44547-2024; the total boron and carbon content was jointly measured by inductively coupled plasma emission spectrometry and high-frequency combustion infrared absorption method; the pore size was measured by the mercury intrusion method in GB / T 21650.1-2008.

[0078] Table 1 Physical and mechanical properties of high-purity porous boron carbide nuclear control rods

[0079]

[0080] Compared with the existing preparation method, the sintering temperature of the present invention is low, the preparation cost is low, 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%, the density is maintained in the range of 1.96-1.97 g / cm 3 range, the compressive strength is maintained in the range of 587-592 MPa, the fracture toughness is maintained in the range of 3.8-3.9 MPa·m 1 / 2 range, and the pore size is maintained in the range of 2.1-2.3 μm; on the contrary, in Comparative Example 1, since the method of increasing the deposition temperature of boron carbide step by step and region by region from the back to the front was not adopted, but the deposition temperature of boron carbide was increased as a whole, resulting in a large amount of boron carbide deposition in the front section of the nuclear control rod and a small amount of boron carbide deposition in the back section, thus leading to significant differences in the physical and mechanical properties of each section of the nuclear control rod: from the front section to the back section, the porosity of the nuclear control rod increases significantly from 15.6% to 32.2%, the density decreases significantly from 2.11 g / cm 3 to 1.70 g / cm 3 , the compressive strength decreases significantly from 776 MPa to 387 MPa, and the fracture toughness decreases significantly from 4.5 MPa·m 1 / 2 to 3.1 MPa·m 1 / 2, the pore size significantly increases from 1.3 μm to 2.8 μm. The above results show that: the preparation method of the present invention can ensure the consistency of the physical and mechanical properties of each part of the high-purity porous boron carbide nuclear control rod.

[0081] 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% - 36.6% and 1.58 - 1.97 g / cm 3 , respectively. Compared with the boron carbide materials prepared in Patent Application CN202411124474.7, the porosity and density ranges of the products prepared by the present invention are wider, and can meet the physical property requirements of more specifications of high-temperature gas-cooled reactors for boron carbide nuclear control rods; 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 - 592 MPa and 2.8 - 3.9 MPa·m 1 / 2 , respectively. Compared with the boron carbide materials prepared in Patent Application CN202411124474.7, the boron carbide materials of the present invention have a wider range of compressive strength and fracture toughness, and the present invention has a greater selection space for mechanical properties; in terms of purity, the total boron and 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 materials prepared in Patent Application CN202411124474.7, the boron carbide materials of the present invention have higher purity and higher neutron shielding efficiency.

[0082] The present invention can affect its porosity to a certain extent by changing the particle size of boron carbide powder and can effectively adjust the pore size of the green body, thereby affecting the resistance of the mixed gas passing through the green body; on this basis, by adjusting the reaction temperature, flow rate, pressure and deposition time, the deposition thickness of boron carbide in the green body can be changed, so as to achieve 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 boron carbide powder decreases from 7 μm to 1 μm, the porosity of the boron carbide nuclear control rod gradually increases from 21.5% - 21.8% to 36.3% - 36.6%, and the density gradually decreases from 1.96 - 1.97 g / cm 3 to 1.58 - 1.60 g / cm 3 , the compressive strength gradually decreases from 587 - 592 MPa to 344 - 351 MPa, and the fracture toughness gradually decreases from 3.8 - 3.9 MPa·m 1 / 2 to 2.8 - 2.9 MPa·m 1 / 2 , and 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. By coordinating the particle size of boron carbide powder and the deposition process parameters, the physical and mechanical properties of the porous boron carbide nuclear control rod can be effectively adjusted.

[0083] 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 principles of the present invention shall be included within 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, It includes the following steps: S1 Obtain a boron carbide blank, which is sequentially divided into five regions: a front section, a front-middle section, a middle section, a middle-back section, and a back section; S2 Under an argon atmosphere, heat the entire boron carbide blank to a first temperature; S3 Keep the temperatures of the front section and the middle section regions of the boron carbide blank at the first temperature, heat the back section region to a second temperature, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank for reaction; S4 Keep the temperatures of the front section and the back section regions of the boron carbide blank at the first temperature, heat the middle section region to a third temperature, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank for reaction; S5 Keep the temperatures of the middle section and the back section regions of the boron carbide blank at the first temperature, heat the front section region to a fourth temperature, and continuously introduce a mixed gas of boron trichloride, methane, argon, and hydrogen from the front section of the boron carbide blank for reaction; S6 After the reaction, obtain a high-purity porous boron carbide nuclear control rod; Wherein, the second temperature, the third temperature, and the fourth temperature are all higher than the first temperature. The first temperature is 1000 - 1050 °C, the second temperature is 1180 - 1200 °C, the third temperature is 1170 - 1190 °C, and the fourth temperature is 1160 - 1180 °C; the length ratio of the front section, the front-middle section, the middle section, the middle-back section, and the back section of the boron carbide blank is 1:(0.8 - 1.2):(0.8 - 1.2):(0.8 - 1.2):(0.8 - 1.2).

2. The preparation method according to claim 1, characterized in that, In step S3, the reaction pressure is 2000 - 3000 Pa, the reaction time is 11 - 17 h; the flow rate of the mixed gas is 130 - 160 mL / min.

3. The preparation method according to claim 1, characterized in that, In step S4, the reaction pressure is 1000 - 2000 Pa, the reaction time is 9 - 15 h, and the flow rate of the mixed gas is 120 - 150 mL / min.

4. The preparation method according to claim 1, wherein In step S5, the reaction pressure is 500 - 1000 Pa, the reaction time is 7 - 13 h, and the flow rate of the mixed gas is 110 - 140 mL / min.

5. 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.

6. An apparatus for preparing a high-purity porous boron carbide nuclear control rod prepared by the preparation method according to any one of claims 1-5, characterized in that, It includes a temperature control module (2) and a flow rate adjustment module (1), a reaction chamber (4), and a pressure adjustment module (3) 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 closely attached to the inner wall of the quartz tube (6); The graphite sleeve (5) is sequentially divided into a front section, a front-middle section, a middle section, a middle-back section, and a back section, and the length ratio of the front section, the front-middle section, the middle section, the middle-back section, and the back section is 1:(0.8 - 1.2):(0.8 - 1.2):(0.8 - 1.2):(0.8 - 1.2); The temperature control module (2) includes a temperature controller and heating elements corresponding to the front section, the middle section, and the back section of the graphite sleeve (5) respectively, and the temperature controller is used to control the temperature of the heating elements.

7. The device according to claim 6, characterized in that The length ratio of the front section, the front-middle section, the middle section, the middle-back section, and the back section of the graphite sleeve (5) is 1:1:1:1:

1.

8. The device according to claim 6, characterized in that, The temperature control module (2) further includes a thermometer for detecting the temperature of the graphite sleeve (5).

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