CN116705361A - Preparation method of anti-oxidation and wear-resistant spherical fuel element and matrix composite material powder - Google Patents

Abstract

The invention provides a preparation method of an antioxidant wear-resistant spherical fuel element and matrix composite powder. Wherein, the preparation method of the matrix composite powder for the antioxidant and wear-resistant spherical fuel element comprises the following steps: step S1, obtaining nuclear pure grade high-purity graphite powder; s2, obtaining a binder, wherein the binder is a silicon carbide precursor or an organic solution of the silicon carbide precursor; step S3, adding the binder into the nuclear pure grade high-purity graphite powder, and performing wet mixing to obtain a paste; and S4, granulating and drying the paste, and crushing to obtain the matrix composite powder. When the matrix composite powder prepared by the method provided by the embodiment of the invention is used for preparing the fuel element, a novel oxidation-resistant and wear-resistant fuel element can be obtained, the oxidation problem of the fuel element in a water vapor or air invasion accident is effectively solved, the generation of graphite dust is reduced, and the inherent safety of the high-temperature gas cooled reactor is enhanced.

Description

Preparation method of anti-oxidation and wear-resistant spherical fuel element and matrix composite material powder

technical field

The invention relates to the field of nuclear engineering and nuclear materials, in particular to a method for preparing a novel oxidation-resistant and wear-resistant spherical fuel element and matrix composite material powder for high-temperature gas-cooled reactors.

Background technique

The high temperature gas-cooled reactor is an internationally recognized fourth-generation nuclear energy system. According to the different fuel elements used, it can be divided into prismatic stacks and pebble bed stacks, the latter using spherical fuel elements. The existing spherical fuel element is a sphere with a diameter of 60mm, and the interior is composed of a fuel zone with a diameter of about 50mm and a fuel-free shell with a thickness of 5mm. The fuel zone contains a large number of TRISO fuel particles, which are uniformly distributed in the matrix graphite of the fuel element.

At present, the raw materials used in the manufacture of spherical fuel elements or fuel-free graphite balls are made of 64% natural graphite powder, 16% artificial graphite powder and 20% phenolic resin binder, also known as A3-3 matrix graphite pink. After carbonization and high-temperature purification, the phenolic resin is transformed into resinous carbon, so the fuel element substrate graphite exhibits the general characteristics of graphite-like materials.

Matrix graphite has good high-temperature mechanical properties, high thermal conductivity and low neutron absorption cross-section, and plays the role of moderator, heat conduction and structural material in fuel elements. However, the graphite-based spherical fuel element also has the problems of poor oxidation resistance and weak wear resistance.

On the one hand, improving the oxidation resistance of spherical fuel elements is helpful to the safe operation of high-temperature gas-cooled reactors. First, there are trace amounts of oxidizing impurities in the helium coolant of the high-temperature gas-cooled reactor, such as water vapor, oxygen, etc., which will oxidize the fuel elements during the long-term operation of the reactor; second, in the event of an accident, such as water Or air enters the core, and the matrix graphite in the fuel element will be oxidized. The Jülich Institute in Germany has carried out the SUPERNOVA experiment to systematically study the oxidation behavior of spherical fuel elements under the intrusion of water vapor and air.

On the other hand, improving the wear resistance of spherical fuel elements can help solve the problem of graphite dust in high-temperature gas-cooled reactors. As the spherical fuel elements circulate through the reactor through the fuel handling system, they experience surface wear, especially in the fuel start-up riser. After the graphite on the surface wears out, the fallen graphite fragments become graphite dust, which accumulates in the primary circuit of the reactor, which may absorb radioactive substances and form a potential safety hazard.

Therefore, it is urgent to provide a novel oxidation-resistant and wear-resistant high-temperature gas-cooled reactor fuel element.

Contents of the invention

Silicon carbide is a new type of nuclear material, which has the characteristics of high hardness, good oxidation resistance and radiation resistance. It is reported that a layer of gradient silicon carbide coating is formed on the surface of fuel elements by embedding method, so as to improve the oxidation resistance and wear resistance of fuel elements. However, this method still belongs to surface modification treatment, and the inside of the fuel element is still composed of graphite material.

The purpose of the present invention is to provide a novel anti-oxidation and wear-resistant spherical fuel element and a method for preparing matrix composite material powder, fundamentally improve the anti-oxidation and wear resistance of the fuel element itself, and keep the fuel element in an accident state. function and finish.

The inventors have found through repeated research that using high-purity graphite powder as a raw material, mixing it with a silicon carbide liquid precursor as a binder in a certain proportion, and then crushing it to make a matrix composite material powder. When it is directly applied to the preparation of spherical fuel elements and fuel-free matrix spheres, the obtained matrix material is no longer pure graphite, but a composite material formed of graphite and silicon carbide, which can improve the oxidation resistance and wear resistance of the fuel element matrix . On this basis, the present invention has been accomplished.

To achieve the above object, the present invention adopts the following technical solutions:

In the first aspect, the preparation method of the matrix composite material powder for anti-oxidation and wear-resistant spherical fuel elements according to the embodiment of the present invention includes the following steps:

Step S1, obtaining nuclear-grade high-purity graphite powder;

Step S2, obtaining a binder, which is a silicon carbide precursor or an organic solution of a silicon carbide precursor;

Step S3, adding the binder into the nuclear-grade high-purity graphite powder, and performing wet mixing to obtain a paste;

In step S4, the paste is granulated, dried, and then pulverized to obtain the matrix composite material powder.

Among them, the so-called nuclear-grade high-purity graphite powder has an ash content of less than 100 ppm and a total boron equivalent of less than 0.9 ppm. The raw materials for configuring the nuclear-grade high-purity graphite powder should meet the relevant requirements of nuclear reactor fuel elements, including purity, particle size distribution, packing density and tap density. Further, the nuclear-grade high-purity graphite powder includes natural flake graphite powder and artificial graphite powder, wherein the ratio of the natural flake graphite powder to the artificial graphite powder is (3-5):1.

The nuclear-grade high-purity graphite powder can be configured according to the graphite powder raw material for A3-3 matrix graphite, and other formulas can also be used. According to the A3-3 matrix graphite powder raw material configuration, obtain natural flake graphite powder and artificial graphite powder, and mix them at a ratio of 4:1.

In addition, the silicon carbide precursor includes a silicon carbide solid precursor or a silicon carbide liquid precursor. In the case of using a solid precursor, it is preferable to use an organic solution of the silicon carbide precursor so as to mix well with the graphite powder. In the case of using a liquid precursor, it may be used as it is, or it may be used after being dissolved in an organic solvent.

Further, the silicon carbide precursor includes polymethylsilane. In addition, the organic solution of the silicon carbide precursor includes a solution obtained by dissolving the silicon carbide precursor in toluene.

Further, the content of the silicon carbide precursor in the organic solution of the silicon carbide precursor is more than 50wt%.

Further, the step S3 is carried out under the protection of nitrogen atmosphere.

Further, in the step S3, the content of the nuclear-grade high-purity graphite powder is 30-80wt%, and the content of the silicon carbide liquid precursor is 20-70wt%.

In the second aspect, the method for preparing an oxidation-resistant and wear-resistant spherical fuel element according to an embodiment of the present invention includes the following steps:

Step S100, obtaining the matrix composite material powder prepared by the preparation method described in any one embodiment of the first aspect;

Step S200, using the matrix composite material powder to manufacture a spherical fuel element body;

Step S300, subjecting the spherical fuel element body to a low-temperature heat treatment so that the silicon carbide precursor is pyrolyzed to generate silicon carbide to obtain a primary product;

Step S400, performing high-temperature heat treatment on the primary product at 1600-2500° C. to obtain a spherical fuel element.

Further, the step S200 includes:

blending coated fuel particles into the matrix composite material powder, and pressing it into a central sphere of the fuel element with a rubber mold;

The shell layer of the matrix composite material powder prepared according to the preparation method described in any embodiment of the first aspect is pressed around the center sphere to form a fuel-free zone to obtain the shaped fuel element body.

Further, in the step S300, the low temperature heat treatment temperature is 800-1200°C.

Further, in the step S400, before the heat treatment, the primary product is also machined so that its size meets the predetermined requirements.

The technical solution of the present invention has at least the following beneficial effects:

When the matrix composite material powder prepared according to the method of the embodiment of the present invention is used to prepare fuel elements, a new type of oxidation-resistant and wear-resistant fuel element can be obtained, which can effectively solve the oxidation of fuel elements in water vapor or air intrusion accidents problems, reduce the generation of graphite dust, and enhance the inherent safety of high-temperature gas-cooled reactors.

Furthermore, since the silicon carbide precursor is used as a binder, the performance of the matrix material itself is improved, and even after the fuel element or the fuel-free matrix material ball is broken, it still has good oxidation resistance.

In addition, during the pyrolysis and sintering process of the silicon carbide precursor, the silicon element can diffuse into the graphite particles to a certain extent to form an atomic-level bond. The obtained composite matrix material has compact structure, low porosity, and good oxidation resistance and wear resistance.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Apparently, the described embodiments are some, not all, embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention belong to the protection scope of the present invention.

In the following, the preparation method of the matrix composite material powder for anti-oxidation and wear-resistant spherical fuel elements according to the embodiment of the present invention will be firstly introduced.

The preparation method of the matrix composite material powder for anti-oxidation and wear-resistant spherical fuel elements according to the embodiment of the present invention includes the following steps:

Step S1, obtaining nuclear-grade high-purity graphite powder.

Among them, the so-called nuclear-grade high-purity graphite powder has an ash content of less than 100 ppm and a total boron equivalent of less than 0.9 ppm. The raw materials for configuring the nuclear-grade high-purity graphite powder should meet the relevant requirements of nuclear reactor fuel elements, including purity, particle size distribution, packing density and tap density.

In addition, the nuclear-grade high-purity graphite powder may include natural flake graphite powder and artificial graphite powder, wherein the ratio of the natural flake graphite powder to the artificial graphite powder is (3-5):1.

Specifically, the nuclear-grade high-purity graphite powder can be configured according to the graphite powder raw material for A3-3 matrix graphite, and of course other formulations can also be used. According to the A3-3 matrix graphite powder raw material configuration, obtain natural flake graphite powder and artificial graphite powder, and mix them at a ratio of 4:1.

Step S2, obtaining a binder, which is a silicon carbide precursor or an organic solution of a silicon carbide precursor.

That is to say, in addition to using nuclear-grade high-purity graphite powder, according to the preparation method of the embodiment of the present invention, a binder is also used. As a binder, the preparation method of the present invention uses a silicon carbide precursor or an organic solution of the silicon carbide precursor. That is to say, in the preparation method of the present invention, the silicon carbide precursor (or the organic solution of the silicon carbide precursor) is used as the binder, and the silicon carbide precursor (or the organic solution of the silicon carbide precursor) after heat treatment It can be converted into silicon carbide, which is highly dispersed among graphite particles and forms a good interface with graphite particles, which can effectively improve the oxidation resistance of the matrix composite material powder as the target product, and can also improve the use of the matrix composite material powder. Wear resistance of prepared spherical fuel elements. In addition, by using an organic solvent to disperse the silicon carbide precursor, the dispersion uniformity of the silicon carbide liquid precursor can be improved when the silicon carbide liquid precursor is mixed with the graphite powder.

In some embodiments of the present invention, the silicon carbide precursor includes polymethylsilane (ie silicon carbide liquid precursor), and the organic solution of the silicon carbide precursor is obtained by dissolving the silicon carbide precursor in toluene The solution. Among them, polymethylsilane is preferred due to its high purity, liquid state at room temperature, high pyrolysis yield of silicon carbide, and close to stoichiometric ratio of pyrolysis products. In addition, the toluene raw material is easy to obtain and has strong solubility, so it is preferable.

Further, the content of the silicon carbide precursor in the organic solution of the silicon carbide precursor is more than 50wt%. In the organic solution of silicon carbide precursor, when the content of silicon carbide precursor is too low, although its viscosity is lower, in order to obtain enough silicon carbide, the amount of addition is too large, and it is easy to cause silicon carbide to not be fully covered after heat treatment The surface of the graphite particles, therefore, the content of the silicon carbide precursor in the organic solution of the silicon carbide precursor is preferably 50 wt % or more.

Step S3, adding the binder into the nuclear-grade high-purity graphite powder, and performing wet mixing to obtain a paste.

That is to say, after obtaining nuclear-grade high-purity graphite powder and binder, it only needs to be wet-mixed to obtain a paste as a mixture. Here, as means for wet mixing, for example, kneading, kneading, stirring, etc. can be performed, and the present invention is not specifically limited thereto.

Preferably, the step S3 is carried out under the protection of nitrogen atmosphere. Wet mixing under the protection of nitrogen atmosphere can avoid the oxidation of silicon carbide liquid precursor.

Further, in the step S3, the content of the nuclear-grade high-purity graphite powder is 30-80wt%, and the content of the silicon carbide liquid precursor is 20-70wt%.

Here, it should be noted that when the organic solution of the silicon carbide precursor is used, the content ratio is calculated based on the silicon carbide precursor in the organic solution of the silicon carbide precursor.

In step S4, the paste is granulated, dried, and then pulverized to obtain the matrix composite material powder.

After the paste is obtained, it can be granulated, dried, and finally pulverized to obtain matrix composite material powder. Wherein, in order to obtain matrix composite material powder with uniform particles and a predetermined particle size, sieving may also be performed after pulverization.

The matrix composite material powder prepared according to the preparation method of the present invention can be prepared by the following method to prepare anti-oxidation and wear-resistant spherical fuel elements.

Specifically, the method for preparing an oxidation-resistant and wear-resistant spherical fuel element according to an embodiment of the present invention includes the following steps:

Step S100, obtaining the matrix composite material powder prepared by the preparation method described in any embodiment of the first aspect above.

Step S200, using the matrix composite material powder to manufacture a spherical fuel element green body.

In some embodiments of the present invention, the step S200 includes:

blending coated fuel particles into the matrix composite material powder, and pressing it into a central sphere of the fuel element with a rubber mold;

The shell layer of the matrix composite material powder prepared by the preparation method described in any of the above-mentioned embodiments is pressed on the periphery of the central sphere to form a fuel-free zone to obtain the shaped fuel element body.

In step S300, the spherical fuel element body is subjected to low-temperature heat treatment so that the silicon carbide precursor is pyrolyzed to generate silicon carbide, and a primary product is obtained.

In some embodiments of the present invention, in the step S300, the low temperature heat treatment temperature is 800-1200°C.

Step S400, performing high-temperature heat treatment on the primary product at 1600-2500° C. to obtain a spherical fuel element.

In some embodiments of the present invention, in the step S400, before the heat treatment, the preliminary product is also machined so that its size meets predetermined requirements.

The preparation of embodiment 1 matrix composite material powder

(1) Configure nuclear-grade high-purity graphite powder

Use natural flake graphite and artificial graphite with an ash content less than 100ppm and a boron equivalent less than 0.9ppm as raw materials, and mix them in a cone mixer for 1-5 hours at a mass ratio of 4:1 for later use.

(2) kneading

Knead the above-mentioned mixed graphite powder and polymethylsilane at a mass ratio of 1:1 using a kneader, and knead for 2 hours under nitrogen protection to obtain a paste. The polymethylsilane used in the kneading process was dissolved in toluene with a mass fraction of 70%.

(3) Granulation and drying

The resulting paste was made into granules with a diameter of about 2 mm using a granulator. The obtained particles were dried in a vacuum drying oven with a vacuum degree of less than 0.01 MPa, a drying temperature of 150° C., and a drying time of 8 hours.

(4) Smash

The above-mentioned dried particles were pulverized by a small mechanical pulverizer, and passed through a 100-mesh sieve to obtain the matrix composite material powder.

Embodiment 2 Utilizes the matrix composite material powder of embodiment 1 to prepare spherical fuel element

(1) core ball pressing

The coated fuel particles (TRISO) sprayed with toluene solution on the surface were mixed with the matrix composite material powder obtained in Example 1 in a drum, so that the surface of the TRISO particles was covered with the matrix composite material powder with a thickness of about 0.2-0.6 mm. The TRISO particles whose surface is covered with the matrix composite material powder are placed in a silicone rubber mold (specially used for indenters), and preformed under a pressure of 5 MPa to obtain a ball core.

(2) Ball

The above-mentioned core ball was placed in another silicone rubber mold (specially used for making balls), filled with the matrix composite material powder obtained in Example 1, and molded under a pressure of 300 MPa by a cold quasi-isostatic pressing process to obtain a spherical fuel element product.

(3) Low temperature heat treatment

Using an atmosphere furnace, carbonize the above-mentioned spherical fuel element raw product to 800°C under a nitrogen atmosphere at a heating rate of about 1°C/min, and obtain a carbonized spherical fuel element after cooling in the furnace.

(4) Machining

The carbonized spherical fuel element is turned by using a special lathe to obtain a spherical fuel element with a diameter of about 60mm.

(5) High temperature heat treatment

The above-mentioned carbonized spherical fuel element was subjected to high-temperature heat treatment in a graphitization furnace, the heating rate was about 10°C/min, the purification temperature was 1950°C, and the temperature was maintained for 0.5h. During this period, the purified spherical fuel element was obtained by argon protection.

In Comparative Example 1, spherical fuel elements were prepared with the powder obtained in combination with A3-3

1) Configure high-purity graphite powder

Use natural flake graphite and artificial graphite with an ash content less than 100ppm and a collision equivalent less than 0.9ppm as raw materials, and mix them in a conical mixer at a mass ratio of 4:1 for 1 hour for later use.

(2) kneading

Use a kneader to knead the above mixed powder and phenolic resin at a mass ratio of 4:1 for 2 hours to obtain a paste. The phenolic resin used in the kneading process was dissolved in methanol with a mass fraction of 50%.

(3) Milling

The above paste was granulated, dried, crushed and sieved using the same preparation process as in Example 1 to obtain A3-3 base powder.

(4) Preparation of spherical fuel elements using A3-3 substrate

The coated fuel particles (TRISO) sprayed with methanol solution on the surface were mixed with the A3-3 base powder in a drum, so that the surface of the TRISO particles was covered with the A3-3 base powder with a thickness of about 0.5 mm. Put the above TRISO granules covered with A3-3 matrix powder in a silicone rubber mold (specially used for pressing cores), and preform under a pressure of 5 MPa to obtain core balls. Put the above-mentioned ball core in another silicone rubber mold (specially used for making balls), fill the surrounding with A3-3 matrix powder, adopt cold quasi-isostatic pressing process, and mold it under a pressure of 300 MPa to obtain a spherical fuel element product. The raw product was carbonized, purified and turned by the same preparation process as in Example 2 to obtain a finished spherical fuel element using an A3-3 substrate.

Performance tests such as oxidation resistance and wear resistance were performed on the fuel elements obtained in the above-mentioned Example 2 and Comparative Example 1. The results are shown in Table 1.

Table 1 Fuel element oxidation resistance, wear resistance test results

It can be known from the above Table 1 that the fuel element prepared by using the matrix composite material powder prepared by the preparation method of the present invention has higher density, better wear resistance, and higher oxidation resistance. It is expected to promote the further development of high temperature gas-cooled reactor technology.

The above description is a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications can also be made. It should be regarded as the protection scope of the present invention.

Claims (10)

1. A preparation method of matrix composite material powder for anti-oxidation and wear-resistant spherical fuel elements, characterized in that, comprising the steps: Step S1, obtaining nuclear-grade high-purity graphite powder; Step S2, obtaining a binder, which is a silicon carbide precursor or an organic solution of a silicon carbide precursor; Step S3, adding the binder into the nuclear-grade high-purity graphite powder, and performing wet mixing to obtain a paste; In step S4, the paste is granulated, dried, and then pulverized to obtain the matrix composite material powder. 2. preparation method according to claim 1, is characterized in that, described nuclei grade high-purity graphite powder comprises natural flake graphite powder and artificial graphite powder, and wherein, the ratio of described natural flake graphite powder and artificial graphite powder For (3-5):1. 3. The preparation method according to claim 1, wherein the silicon carbide precursor comprises polymethylsilane, and the organic solution of the silicon carbide precursor comprises a solution obtained by dissolving the silicon carbide precursor in toluene solution. 4. The preparation method according to claim 1, characterized in that the content of the silicon carbide precursor in the organic solution of the silicon carbide precursor is more than 50 wt%. 5. The preparation method according to claim 1, characterized in that, the step S3 is carried out under the protection of a nitrogen atmosphere. 6. The preparation method according to claim 5, characterized in that, in the step S3, the content of the high-purity graphite powder is 30-80wt%, and the content of the silicon carbide precursor is 20-70wt%. 7. A method for preparing an oxidation-resistant and wear-resistant spherical fuel element, comprising the steps of: Step S100, obtaining the matrix composite material powder prepared by the preparation method according to any one of claims 1 to 6; Step S200, using the matrix composite material powder to manufacture a spherical fuel element body; Step S300, subjecting the spherical fuel element body to a low-temperature heat treatment so that the silicon carbide precursor is pyrolyzed to generate silicon carbide to obtain a primary product; Step S400, performing high-temperature heat treatment on the primary product at 1600-2500° C. to obtain a spherical fuel element. 8. The preparation method according to claim 7, wherein the step S200 comprises: blending coated fuel particles into the matrix composite material powder, and pressing it into a central sphere of the fuel element with a rubber mold; A shell layer made of the matrix composite material powder is pressed around the center sphere to form a fuel-free area, and the shaped fuel element green body is obtained. 9. The preparation method according to claim 7, characterized in that, in the step S300, the low temperature heat treatment temperature is 800-1200°C. 10 . The preparation method according to claim 7 , characterized in that, in the step S400 , before heat treatment, the primary product is also machined so that its size meets predetermined requirements. 11 .