A preparation method of a non-coated neutron absorption sphere and the neutron absorption sphere

Abstract

The present invention relates to a method for preparing a non-coated neutron absorption sphere, which comprises the following technological steps carried out in sequence: a pulping process, a forming process, and a firing process. It is characterized in that it further comprises an impregnation and baking process, that is, the sintered spherical blank is placed in a pressure vessel filled with a liquid impregnating agent for impregnation, and after being taken out, it is transferred to a heating furnace for baking, and the above process is repeated at least once. At the same time, the present invention also provides a neutron absorption sphere prepared according to the above-mentioned method for preparing a neutron absorption sphere. Such a method for preparing a non-coated neutron absorption sphere and the neutron absorption sphere have better electrical conductivity and neutron absorption efficiency while having good wear resistance by improving the microstructure of the absorption sphere.

Description

Technical Field

[0001] The present invention relates to the technical field of nuclear reactors, and particularly to a method for preparing a non-coated neutron absorber ball and the neutron absorber ball. Background Art

[0002] The high-temperature gas-cooled reactor is an advanced nuclear reactor. A key component of the high-temperature gas-cooled reactor is the absorber ball shutdown system, whose working principle is to use the negative pressure conveying process to transport small balls containing high-absorption cross-section materials (such as B4C) into the reflector channels of the reactor core. In case of an emergency, these small balls will enter the core and reduce the power of the reactor by absorbing neutrons, thus achieving the purpose of shutting down the reactor; in non-emergency situations, these small balls can be recovered and stored in the storage ball tank at the top of the reactor. These small balls are usually called neutron absorber balls (Neutron Absorber Balls, abbreviated as NABs).

[0003] According to the usage scenario, the neutron absorber ball needs to have good wear resistance and conductivity at the same time: Since the neutron absorber ball needs to move in the reflector channels of the reactor core, to prevent the channels from being blocked and causing the shutdown system to fail, it is necessary to require the neutron absorber ball to have good wear resistance; the operation of the nuclear reactor may involve the use of electromagnetic fields. Conductive materials can provide electromagnetic compatibility, reduce electromagnetic interference, and ensure the stability of the reactor control system. In some designs, the neutron absorber ball may be connected to the drive mechanism of the control rod, and conductivity helps the precise movement and positioning of the control rod.

[0004] However, the conductivity of the neutron absorber ball usually needs to be achieved by adding graphite, and the crystal structure of graphite is a hexagonal planar network structure formed by layered carbon atoms with sp 2 hybrid orbitals, and the layers are connected to each other by van der Waals forces. This structure makes it easy for the graphite layers to slide and peel off, resulting in poor wear resistance.

[0005] To solve the above problems, in the published materials, there is a technical solution that uses acetylene gas to perform a surface coating treatment on the absorber ball blank by chemical vapor deposition (CVD) process to form a surface-coated pyrolytic carbon layer. The principle is to form a wear-resistant material on the surface of the neutron absorber ball to improve wear resistance. Although this solution can improve the wear resistance of the absorber ball, it affects the conductivity and neutron absorption efficiency to a certain extent. Summary of the Invention

[0006] Aiming at the defects existing in the prior art, the technical problem to be solved by the present invention is to propose a method for preparing a non-coated neutron absorber ball and the neutron absorber ball, which can make the neutron absorber ball have better conductivity and neutron absorption efficiency while having good wear resistance by improving the microstructure of the absorber ball.

[0007] To solve the above technical problems, the present invention provides a method for preparing a non-coated neutron absorption sphere, comprising the following technological steps:

[0008] A method for preparing a non-coated neutron absorption sphere, comprising the following technological steps carried out in sequence:

[0009] Pulp-making process: Take boron carbide powder and silicon carbide powder, and add graphite powder, and mix them evenly through a wet ball-milling process to form a paste-like slurry;

[0010] Forming process: Inject the paste-like slurry into a forming mold to make a spherical blank;

[0011] Firing process: High-temperature sinter the spherical blank;

[0012] The method for preparing a non-coated neutron absorption sphere of the present invention further comprises an impregnation and baking process: Place the sintered spherical blank in a pressure vessel filled with a liquid impregnating agent for impregnation, take it out and transfer it to a heating furnace for baking, and repeat the above process at least once.

[0013] The main components of the neutron absorption sphere are boron carbide and silicon carbide, both of which are ceramic materials and have good wear resistance. After analysis, the decrease in the wear resistance of the neutron absorption sphere is mainly due to the addition of graphite material in order to have good electrical conductivity. In view of this, the present invention changes the thinking, uses graphite as the main component to improve wear resistance, and draws on the impregnation and baking process of graphite electrode manufacturing to improve the graphite crystal structure, thereby improving the wear resistance of the neutron absorption sphere. At the same time, since the liquid impregnating agent will enter the pores of the spherical blank during the impregnation and baking process, and carbonize and solidify in the spherical blank after baking, it will not form a covering outer film on the surface of the neutron absorption sphere, reducing the impact on electrical conductivity and neutron absorption efficiency.

[0014] Specifically, the impregnation and baking operations in the impregnation and baking process are carried out a total of four times, that is, the four-impregnation and four-baking process is adopted. Through multiple impregnations and bakings, the volume density and mechanical strength of the neutron absorption sphere can be gradually improved. Such a repeated process helps to obtain a denser structure, but excessive impregnation and baking may lead to an increase in production costs. Graphite electrodes usually adopt the "three-impregnation and three-baking" process, but the unit price of neutron absorption spheres is high and the wear resistance requirements are higher. More detailed step-by-step impregnation and baking can more precisely control the product quality and reduce the rejection rate, thereby controlling the production cost as a whole.

[0015] Among them, the liquid impregnating agent used in the first two impregnations in the impregnation and baking process is asphalt, and the liquid impregnating agent used in the last two impregnations is high-residual-carbon resin, and the residual carbon content of the high-residual-carbon resin is greater than 30%. Asphalt is the most common impregnating agent, which has low cost and good fluidity at appropriate temperatures, so it is used as the impregnating agent for the first two impregnations. Resin has better permeability and can penetrate more deeply into the micropores of porous materials, improving the impregnation effect. At the same time, it can improve the interlaminar shear strength and fatigue resistance of the composite material, so it is used as the impregnating agent for the last two impregnations.

[0016] Preferably, before impregnation in the impregnation and baking process, the spherical blank is preheated to make the temperature of the spherical blank reach 500 - 550 °C. During the manufacturing process of graphite electrodes, the blank is also preheated, and the preheating temperature is usually controlled between 200 - 400 °C. Since the higher the preheating temperature, the impregnating agent can maintain more appropriate fluidity and temperature during the penetration process. More importantly, it can reduce the thermal shock caused by the temperature difference during impregnation or baking of the material, and avoid cracks or damage to the material. However, too high a temperature may cause thermal degradation or damage to the material structure, and also cause an increase in thermal stress resulting in material cracking. As a product with a ceramic matrix, the neutron absorption ball has good thermal stability and at the same time has higher requirements for sufficient impregnation. Therefore, the preheating temperature is selected to be 500 - 550 °C.

[0017] Furthermore, after the spherical blank is placed in the pressure vessel in the impregnation and baking process, the pressure vessel is evacuated, and then the liquid impregnating agent is injected into the pressure vessel by a pressure device to make the pressure in the pressure vessel reach 2 - 2.5 MPa. Similarly, the impregnation pressure adopted in the present invention is higher than that in the processing of graphite electrodes because the neutron absorption ball has higher material strength, and the pores of the blank also have higher strength. Higher pressure can better push the impregnating agent into the neutron absorption ball on the premise of ensuring the safety of the product.

[0018] A further preferred solution is that after the paste-like slurry is obtained in the pulping process, boron carbide particles are added to the paste-like slurry. The boron carbide particles are obtained by crushing a boron carbide ceramic body sintered by hot pressing, and the density ρ of the boron carbide particles is 2.48 - 2.52 g / cm 3Since silicon carbide is easier to sinter and densify than boron carbide, when the silicon carbide reaches a dense state during the firing of neutron-absorbing balls, the structure of boron carbide often fails to meet the density requirements, which to a certain extent affects the wear resistance of the neutron-absorbing balls. However, in fact, pure boron carbide can be sintered and densified by hot pressing. Therefore, in addition to the boron carbide contained in the original balls, another portion of boron carbide is taken, sintered and densified by hot pressing, and then crushed. The obtained boron carbide particles have a dense structure and thus have good wear resistance. After adding the boron carbide particles to the paste-like slurry, when the formed spherical green body shrinks during roasting and densification, the boron carbide particles distributed on the surface of the spherical green body protrude, forming a structure similar to that of a sphere densely covered with numerous protrusions. When the fired neutron-absorbing ball rolls, the boron carbide particles with high wear resistance come into contact with other objects, reducing the possibility of wear of the graphite in the neutron-absorbing ball. At the same time, the protruding boron carbide particles can also improve the neutron absorption efficiency.

[0019] Preferably, the particle size distribution of the boron carbide particles is between the 60-mesh sieve and the 80-mesh sieve, and the D50 value is 70 mesh. Large boron carbide particles may cause the surface of the neutron-absorbing ball to be too rough, and at the same time may also cause damage to the structure of the neutron-absorbing ball, making it prone to cracking. If the boron carbide particles are too small, it will be difficult for the boron carbide particles to protrude from the surface of the neutron-absorbing ball, affecting the design goal.

[0020] The method for preparing the non-coated neutron-absorbing ball of the present invention further includes a ball grinding process, which is carried out after the dipping and roasting process. In the ball grinding process, the bilateral grinding allowance of the spherical green body does not exceed 0.15 mm. Since the boron carbide particles may have some sharp edges and corners, which may cause damage to the contacted objects, they are ground to remove the sharp edges and corners. However, the grinding allowance should not be too large, as an excessive grinding allowance may also cause insufficient protrusion of the boron carbide particles.

[0021] The total mass percentage of the boron carbide powder and the boron carbide particles is 20%-35%, the mass percentage of the silicon carbide powder is 45%-60%, and the mass percentage of the graphite powder is 5%-10%. Neutron-absorbing balls are required to have comprehensive properties such as appropriate strength, hardness, wear resistance, and impact resistance. Therefore, it is necessary to select appropriate matrix materials and ratios of neutron absorbers.

[0022] The present invention also provides a neutron-absorbing ball prepared according to the above-described method for preparing a neutron-absorbing ball.

[0023] In summary, such a method for preparing a neutron-absorbing ball and the neutron-absorbing ball, by improving the microstructure of the absorbing ball, enable the neutron-absorbing ball to have good wear resistance while having better electrical conductivity and neutron absorption efficiency. Detailed Embodiments

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with specific embodiments.

[0025] A method for preparing a non-coated neutron absorption sphere includes the following technological steps carried out in sequence:

[0026] (1) Pulp making process: Take boron carbide powder and silicon carbide powder, and add graphite powder, and mix them evenly through a wet ball milling process to form a paste-like slurry.

[0027] After obtaining the paste-like slurry in the pulp making process, boron carbide particles are added to the paste-like slurry. The boron carbide particles are obtained by crushing a boron carbide ceramic body obtained by hot pressing and sintering. The density ρ of the boron carbide particles is 2.48 - 2.52 g / cm 3 .

[0028] The component ratio selected in this embodiment is as follows: The total mass percentage of boron carbide powder and boron carbide particles is 20% - 35%, the mass percentage of silicon carbide powder is 45% - 60%, and the mass percentage of graphite powder is 5% - 10%. The neutron absorption sphere is required to have comprehensive properties such as appropriate strength, hardness, wear resistance, and impact resistance. Therefore, it is necessary to select a suitable matrix material and the ratio of neutron absorbers. It should be noted that in the pulp making process, auxiliary materials such as a dispersant with a dispersing effect, a phenolic resin to promote sintering, a release agent to facilitate molding and demolding, and an adhesive to facilitate molding are also added. These related auxiliary materials are common knowledge, and the present invention does not specifically limit and describe them.

[0029] Since silicon carbide is easier to sinter and densify than boron carbide, when silicon carbide reaches a dense state during the sintering process of the neutron absorption sphere, the structure of boron carbide often does not meet the dense requirements, which to a certain extent affects the wear resistance of the neutron absorption sphere. However, in fact, pure boron carbide can be sintered and densified by hot pressing. Therefore, in addition to the boron carbide contained in the original sphere, another portion of boron carbide is taken, hot pressed and sintered, and then crushed. The obtained boron carbide particles have a dense structure and thus have good wear resistance. After adding the boron carbide particles to the paste-like slurry, the formed spherical green body shrinks densely during roasting, and the boron carbide particles distributed on the surface of the spherical green body protrude, forming a structure similar to numerous protrusions densely distributed on the sphere. When the sintered neutron absorption sphere rolls, the boron carbide particles with high wear resistance come into contact with other objects, reducing the possibility of graphite wear in the neutron absorption sphere. At the same time, the protruding boron carbide particles can also improve the neutron absorption efficiency.

[0030] Preferably, the particle size distribution of the boron carbide particles is between 60-mesh and 80-mesh sieves, and the D50 value is 70-mesh. Large boron carbide particles may cause the surface of the neutron-absorbing sphere to be too rough, and at the same time, they may also cause damage to the structure of the neutron-absorbing sphere and make it prone to cracking. If the boron carbide particles are too small, it will be difficult for the boron carbide particles to protrude from the surface of the neutron-absorbing sphere, affecting the design goal.

[0031] (2) Forming process: Inject the paste-like slurry into a forming mold to make a spherical green body;

[0032] (3) Firing process: High-temperature sinter the spherical green body;

[0033] (4) Impregnation and baking process: Place the sintered spherical green body in a pressure vessel filled with a liquid impregnating agent for impregnation, take it out and transfer it to a heating furnace for baking, and repeat the above process at least once.

[0034] The main components of the neutron-absorbing sphere are boron carbide and silicon carbide, both of which are ceramic materials with good wear resistance. After analysis, the decrease in the wear resistance of the neutron-absorbing sphere is mainly due to the addition of graphite material in order to have good electrical conductivity. In view of this, the present invention changes the idea, uses graphite as the main component to improve wear resistance, and draws on the impregnation and baking process of graphite electrode manufacturing to improve the graphite crystal structure, thereby improving the wear resistance of the neutron-absorbing sphere. At the same time, since the liquid impregnating agent will enter the pores of the spherical green body during the impregnation and baking process, and carbonizes and solidifies in the spherical green body after baking, it will not form a coating outer film on the surface of the neutron-absorbing sphere, reducing the impact on electrical conductivity and neutron absorption efficiency.

[0035] As a preference, before impregnation in the impregnation and baking process, the spherical green body is preheated to make the temperature of the spherical green body reach 500 - 550 °C. During the manufacturing process of graphite electrodes, the green body is also preheated, and the preheating temperature is usually controlled between 200 - 400 °C. Since the higher the preheating temperature, the impregnating agent can maintain more appropriate fluidity and temperature during the penetration process. More importantly, it can reduce the thermal shock caused by the temperature difference during impregnation or baking, and avoid cracking or damage of the material. However, too high a temperature may cause thermal degradation or damage to the material structure, and will also cause an increase in thermal stress and lead to material cracking. The neutron-absorbing sphere is a product based on ceramics, has good thermal stability, and at the same time has higher requirements for sufficient impregnation. Therefore, the preheating temperature is selected to be 500 - 550 °C.

[0036] Specifically, in this embodiment, the impregnation and baking processes in the impregnation-baking process are carried out a total of four times, that is, a four-impregnation and four-baking process is adopted. Through multiple impregnations and bakings, the bulk density and mechanical strength of the neutron absorption balls can be gradually increased. Such a repeated process helps to obtain a denser structure. However, excessive impregnation and baking may lead to an increase in production costs. Graphite electrodes usually adopt a "three-impregnation and three-baking" process. However, the unit price of neutron absorption balls is high and the wear resistance requirement is higher. More meticulous step-by-step impregnation and baking can more precisely control the product quality and reduce the rejection rate, thereby controlling the production cost as a whole.

[0037] Among them, the liquid impregnating agent used in the first two impregnations in the impregnation-baking process is pitch, and the liquid impregnating agent used in the last two impregnations is a high residual carbon resin, and the residual carbon content of the high residual carbon resin is greater than 30%. Pitch is the most common impregnating agent, which has low cost and good fluidity at an appropriate temperature, so it is used as the impregnating agent for the first two impregnations. Resin has better permeability and can penetrate more deeply into the micropores of porous materials, improving the impregnation effect. At the same time, it can improve the interlaminar shear strength and fatigue resistance of the composite material, so it is used as the impregnating agent for the last two impregnations.

[0038] Furthermore, after the spherical blank is placed in the pressure vessel in the impregnation-baking process, the pressure vessel is evacuated, and then the liquid impregnating agent is injected into the pressure vessel by a pressure device to make the pressure in the pressure vessel reach 2 - 2.5 MPa. Similarly, the impregnation pressure adopted in the present invention is higher than that in the processing of graphite electrodes. The reason is that the neutron absorption balls have higher material strength, and the pores of the blank also have higher strength. Higher pressure can better push the impregnating agent into the neutron absorption balls on the premise of ensuring product safety.

[0039] The method for preparing non-coated neutron absorption balls of the present invention further includes a ball grinding process. The ball grinding process is carried out after the impregnation-baking process. In the ball grinding process, the bilateral grinding allowance of the spherical blank does not exceed 0.15 mm. Since the boron carbide particles may have some sharp corners and may cause damage to the contacting objects, they are ground to remove the sharp corners. However, the grinding allowance should not be too large, as too large a grinding allowance may also cause insufficient protrusion of the boron carbide particles.

[0040] The present invention also provides a neutron absorption ball prepared according to the above-mentioned method for preparing neutron absorption balls.

[0041] In summary, such a method for preparing neutron absorption balls and the neutron absorption balls, by improving the microstructure of the absorption balls, enable the neutron absorption balls to have good wear resistance while having better electrical conductivity and neutron absorption efficiency.

[0042] In summary, 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 non-coated neutron absorbing sphere, comprising the following process steps in sequence: Slurry preparation process: Take boron carbide powder and silicon carbide powder, add graphite powder, mix them evenly through wet ball milling process to form a paste slurry; Forming process: injecting the paste slurry into a forming mold to form a spherical body; Firing process: spherical green body is sintered at high temperature; Features: The process also includes an impregnation and baking step: placing the sintered spherical green body in a pressure vessel containing a liquid impregnating agent for impregnation, taking it out and transferring it to a heating furnace for baking; In the impregnation and roasting process, the impregnation and roasting work is carried out four times in total, that is, a four-impregnation and four-roasting process is adopted; The liquid impregnating agent used in the first two impregnations in the impregnation and baking process is asphalt, and the liquid impregnating agent used in the last two impregnations is a high carbon residue resin, and the carbon residue of the high carbon residue resin is greater than 30%; After the paste slurry is prepared in the slurry making process, boron carbide particles are added to the paste slurry. The boron carbide particles are obtained by crushing the hot-pressed boron carbide ceramic body. The density of the boron carbide particles is ρ=2.48-2.52 g / cm 3 After the spherical green body is sintered and densely shrunk, the boron carbide particles distributed on the surface of the spherical green body protrude, forming a structure similar to that of a sphere with numerous protrusions.

2. The method for preparing a non-coated neutron absorbing sphere according to claim 1, characterized in that: In the soaking and roasting process, the spherical green body is preheated before soaking, so that the temperature of the spherical green body reaches 500-550°C.

3. The method for preparing a non-coated neutron absorbing sphere according to claim 1, characterized in that: In the impregnation and roasting process, after the spherical green body is placed in a pressure container, the pressure container is evacuated, and then the liquid impregnating agent is injected into the pressure container by a pressure device, so that the pressure in the pressure container reaches 2-2.5MPa.

4. The method for preparing a non-coated neutron absorbing sphere according to claim 1, characterized in that: The particle size distribution of the boron carbide particles is between 60 mesh and 80 mesh, and the D50 value is 70 mesh.

5. The method for preparing a non-coated neutron absorbing sphere according to claim 4, characterized in that: The method further comprises a ball grinding step, which is performed after the soaking and roasting step, and the grinding bilateral allowance of the spherical blank in the ball grinding step does not exceed 0.15 mm.

6. The method for preparing a non-coated neutron absorbing sphere according to claim 5, characterized in that: The total mass percentage of the boron carbide powder and the boron carbide particles is 20%-35%, the mass percentage of the silicon carbide powder is 45%-60%, and the mass percentage of the graphite powder is 5%-10%.

7. Neutron absorbing sphere, characterized in that: The neutron absorbing sphere is prepared according to the method for preparing the neutron absorbing sphere according to any one of claims 1 to 6.