WO2019037688A1

WO2019037688A1 - Uranium carbide pellet, preparation method therefor, and fuel rod

Info

Publication number: WO2019037688A1
Application number: PCT/CN2018/101376
Authority: WO
Inventors: 薛佳祥; 张显生; 刘彤; 李锐; 严岩; 李思功; 黄华伟; 龚星; 任啟森; 严俊; 卢志威
Original assignee: 中广核研究院有限公司; 中国广核集团有限公司; 中国广核电力股份有限公司
Priority date: 2017-08-21
Filing date: 2018-08-20
Publication date: 2019-02-28
Also published as: GB2581903B; CN107500767A; GB202007032D0; CN107500767B; GB2581903A; RO134863A2

Abstract

A uranium carbide pellet, a preparation method therefor, and a fuel rod. The preparation method for the uranium carbide pellet comprises the following steps: S1, weighing a uranium nitride powder and a carbon source according to a molar ratio of 1:0.8-1.5 and adding them to a solvent, mixing the mixture uniformly to form a slurry; S2, drying the slurry to obtain a mixed powder; S3, pressing the mixed powder into green bodies with a density of 50% or more; and S4, carrying out high-temperature pressureless sintering, to give uranium carbide pellets with a density of ≥95%.

Description

Uranium carbide pellet and preparation method thereof, fuel rod

Technical field

The invention relates to the technical field of nuclear reactors, in particular to a uranium carbide pellet, a preparation method thereof and a fuel rod.

Background technique

The existing commercial pressurized water reactor nuclear fuel is mainly uranium dioxide (UO ₂ ) pellets, but the UO ₂ pellets have low thermal conductivity and high centerline temperature. Even in the event of an accident, even if it has been safely shut down, the pellets are still stored. A large amount of decay heat, in the case of loss of coolant, the residual heat is difficult to export, so that the temperature of the fuel rod cladding rapidly rises to a dangerous level. Due to its own chemical properties, the existing zirconium alloy material begins to undergo a significant zirconium-water reaction above 650 °C. This reaction is an exothermic reaction and releases a large amount of hydrogen, which seriously deteriorates the safety of the fuel assembly, which can lead to core melting and severe hydrogen. Explosive and other catastrophic consequences.

If uranium carbide (UC) pellets are used, the thermal conductivity is higher and the uranium density is also improved. From the perspective of economy and safety, there are obvious advantages.

The preparation of the existing uranium carbide pellets is mainly divided into two steps. One is the synthesis of uranium carbide powder, and the uranium carbide powder is usually synthesized by the method of carbothermal reduction of uranium dioxide at high temperature; the second is the compaction and sintering of the pellets. Pressureless sintering with a sintering aid or external field assisted hot press sintering is usually employed.

The following problems exist in the preparation of existing uranium carbide pellets:

1. The uranium carbide powder is easily dissolved into oxygen atoms, and the dissolution of oxygen atoms stabilizes UC ₂ and decomposes U ₂ C ₃ . The concentration of solid dissolved oxygen in uranium carbide can reach 12.5at%, which will cause the thermal conductivity of uranium carbide to decrease. Uranium carbide is easily oxidized to UO ₂ or other uranium oxides at temperatures above 200 ° C or oxygen partial pressures exceeding 20 kPa. Therefore, the milling and mixing of uranium carbide must be carried out in an argon-filled environment, and the purity of argon is also high.

2. When the sintering aid is added to sinter the uranium carbide pellet, although it is advantageous for densification, the melting point of the pellet is significantly reduced and the safety is lowered.

3. It is easy to realize the densification of the uranium carbide pellet by external field-assisted hot pressing sintering. However, this type of method can only sinter one core block per furnace, which has extremely low efficiency, high energy consumption and high equipment requirements, and is not suitable for industrial mass production.

technical problem

The technical problem to be solved by the present invention is to provide a method for preparing a uranium carbide pellet which is one-step reaction synthesis and sintering densification and can be industrially mass-produced, and a prepared uranium carbide pellet and a fuel having the uranium carbide pellet. Baton.

Technical solution

The technical solution adopted by the present invention to solve the technical problem thereof is to provide a preparation method of a uranium carbide pellet, which comprises the following steps:

S1, the uranium nitride powder and the carbon source are weighed according to a molar ratio of 1:0.8-1.5 and added to the solvent, and uniformly mixed to form a slurry;

S2, drying the slurry to obtain a mixed powder;

S3, pressing the mixed powder into a blank having a density of 50% or more;

S4, high temperature pressureless sintering, obtaining a uranium carbide pellet with a density of ≥95%.

In the preparation method of the present invention, in step S1, the uranium nitride powder has a purity greater than 95% and a particle size of 0.1-50 μm;

The carbon source is carbon black and/or graphite, the purity is greater than 95%, and the particle size is 0.1-50 μm.

In the production method of the present invention, in the step S1, the solvent is ethanol or acetone.

In the preparation method of the present invention, step S3 comprises the following steps:

S3.1, pre-compressing the mixed powder under a pressure of 5-30 MPa;

S3.2, vacuum-sealing the pre-formed blank, pressing the sealed blank at a pressure of 150-300 MPa, and obtaining a green body having a density of 50% or more after the pressure is maintained.

In the preparation method of the present invention, in step S3.1, the mixed powder is placed in a pellet steel mold for pre-compression molding;

In step S3.2, the pre-formed green body is vacuum-sealed with oil paper, and the sealed green body is subjected to high pressure pressing using a cold isostatic press.

In the preparation method of the present invention, the step S4 comprises the following steps:

S4.1, the blank obtained in step S3 is placed in a high temperature pressureless furnace, in a vacuum environment, the temperature is raised to 1300 ° C -1600 ° C at a rate of 5-30 ° C / min and held for 0.5-4 h;

S4.2. Under an inert atmosphere, the temperature is raised to 1700 ° C - 2000 ° C at a rate of 5-30 ° C / min and held for 1-14 h to obtain a uranium carbide pellet.

In the preparation method of the present invention, in the step S4.2, argon gas is introduced into the high-temperature pressureless furnace and maintained at an atmospheric pressure to form an inert atmosphere.

In the preparation method of the present invention, the ratio of uranium, carbon and nitrogen atoms in the obtained uranium carbide pellet is 1: (0.8-1.5): (0-0.2).

The present invention also provides a uranium carbide pellet, which is produced by the preparation method described in any of the above.

The present invention also provides a fuel rod comprising the above-described uranium carbide pellet.

Beneficial effect

The invention has the beneficial effects of using the uranium nitride and carbon source as raw materials to realize the two processes of carbothermal reduction reaction and densification sintering under high temperature pressureless sintering, without using a hot pressing sintering process with low production efficiency, without introducing Sintering aids, avoiding the problem of lowering the melting point of the pellets, can achieve batch sintering, low energy consumption, and is suitable for industrial production of fuel pellets.

Embodiments of the invention

The preparation method of the uranium carbide pellet of the invention comprises the following steps:

S1, the uranium nitride powder and the carbon source are weighed according to a molar ratio of 1:0.8-1.5 and added to the solvent, and uniformly mixed to form a slurry.

Among them, the uranium nitride (UN) powder has a purity of more than 95% and a particle diameter of 0.1-50 μm. The carbon source (C) is carbon black and/or graphite having a purity of more than 95% and a particle diameter of 0.1 to 50 μm.

The solvent can be ethanol or acetone. The uranium nitride powder and the carbon source are added to the solvent in order to sufficiently mix and evenly distribute the two.

S2, drying the slurry to obtain a mixed powder.

The resulting slurry can be dried by rotary evaporation to obtain a dried mixed powder. The evaporation drying time is adjusted according to the actual situation.

S3. The mixed powder is pressed into a green body having a density of 50% or more.

The step S3 may further include the following steps:

S3.1, pre-compressing the mixed powder under a pressure of 5-30 MPa.

The holding time can be determined according to the situation, for example, 5 min, and the main purpose is to stabilize the shape of the blank after pressing.

Specifically, in step S3.1, the mixed powder is placed in a pellet steel mold, and a pressure of 10 MPa may be applied for pre-compression molding. In step S3.2, the pre-formed green body is vacuum-sealed with oil paper, and the sealed green body is subjected to high-pressure pressing such as 200 MPa using a cold isostatic press.

Step S4 may further include the following steps:

S4.1, the blank obtained in step S3 is placed in a high-temperature pressureless furnace, and heated to a temperature of 1300 ° C - 1600 ° C at a rate of 5-30 ° C / min in a vacuum environment and maintained for 0.5-4 h; vacuum.

This step mainly realizes the carbothermal reduction reaction of the green body, and the reaction is sufficiently carried out by heat preservation to complete the formation of uranium carbide.

After the end of the heat preservation, the uranium carbide pellets can be taken out after cooling to room temperature.

This step mainly achieves densification of the green body to form a pellet having a predetermined density.

Alternatively, in step S4.2, argon gas may be introduced into the high temperature pressureless furnace and maintained at an atmospheric pressure to form an inert atmosphere.

In the preparation method of the present invention, the in-situ reaction of uranium nitride and a carbon source is pressureless sintering, and densification is achieved by solid solution and carbon-nitrogen vacancy migration mass transfer.

In the uranium carbide pellet obtained by the invention, the ratio of uranium, carbon and nitrogen atoms is 1: (0.8-1.5): (0-0.2), and the above ratio is adjustable. The regulation of uranium carbon atom ratio and solid solution nitrogen includes changing the particle size and group distribution ratio of the raw materials, changing the reaction temperature and reaction holding time, changing the sintering temperature and sintering holding time, and changing the sintering atmosphere.

The uranium carbide pellet of the present invention is obtained by the above preparation method.

The fuel rod of the present invention includes the above-described uranium carbide pellet.

The invention is further illustrated by the following specific examples.

Example 1

A 98% by weight uranium carbide pellet was prepared and weighed according to a UN:C molar ratio of 1:1.2: 249 g of UN powder and 14.4 g of carbon black.

The mixture was mixed with Si ₃ N ₄ balls at a speed of 120 rpm for 24 hours using ethanol as a solvent, and the obtained slurry was dried by rotary evaporation to obtain a uniformly mixed powder. The mixed powder is placed in a pellet steel mold, pre-compressed by applying a pressure of 10 MPa, and then the preformed blank is vacuum-sealed with oil paper, and then a cold isostatic press is used to apply a vacuum of 200 MPa to the vacuum-sealed blank of the oil-paper. The isostatic load is held at 200 MPa for 5 minutes to achieve an initial density of the core block of 50% or more.

The green body was taken out and placed in a high temperature pressureless furnace for reaction sintering. With pressureless sintering, in the first reaction stage, the heating rate is 10 ° C / min, the temperature is raised to 1400 ° C and held for 1 hour, which must maintain a vacuum environment. After the end of the heat preservation, the vacuum was stopped, high-purity argon gas was introduced into the furnace and maintained at an atmospheric pressure, and then heated to 1800 ° C at a temperature rising rate of 10 ° C / min and kept for 4 hours. After the end of the heat preservation, cool to room temperature and take out the pellet.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peaks were not significantly shifted. It was found that the uranium carbide pellets obtained by thermal sintering at 1800 ° C for 4 hours had a density of 98%, a porosity of 2%, a pore diameter of 100 nm, and were open pores.

Example 2

A 95% density uranium carbide pellet was prepared. The carbon source was changed from carbon black in Example 1 to graphite. Weighed according to the UN:C molar ratio of 1:0.8: 249 g of UN powder and 9.6 g of graphite.

The green body was prepared in the same manner as in Example 1, and the in-situ reaction temperature was maintained at 1500 ° C for 2 hours, and the sintering temperature was maintained at 1900 ° C for 1 hour.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peak shifted slightly to the left, indicating the presence of nitrogen solid solution. It was found that the uranium carbide pellet obtained by sintering at 1900 ° C for 1 hour had a density of 95%, a porosity of 5%, a pore diameter of 500 nm, and an open pore.

Example 3

A uranium carbide pellet having a density of 99% was prepared. Weighed according to the UN:C molar ratio of 1:1: 249 grams of UN powder, 12 grams of graphite.

The green body was prepared in the same manner as in Example 1, and the in-situ reaction was carried out, and the pellet was prepared by pressureless sintering at 2000 ° C for 3 hours.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peaks were not significantly shifted. It was found that the density of uranium carbide pellets produced by heat-sintering at 2000 ° C for 14 hours was 99%, and no open pores were present.

Example 4

A 95% density uranium carbide pellet was prepared. The carbon source was changed from carbon black in Example 1 to graphite. Weighed according to the UN:C molar ratio of 1:1.5: 249 grams of UN powder and 18 grams of graphite.

The green body was prepared in the same manner as in Example 1, and the in-situ reaction temperature was maintained at 1600 ° C for 1.5 hours, and the sintering temperature was maintained at 1700 ° C for 10 hours.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peak shifted slightly to the right, indicating the presence of carbon. It was found that the uranium carbide pellet obtained by holding and sintering at 1700 ° C for 10 hours has a density of 95%, a porosity of 5%, a pore diameter of 200 nm, and an open pore.

Example 5

A 97% dense uranium carbide pellet was prepared. Carbon black is used as a raw material. The molar ratio of UN:C was 1:0.9: 249 g of UN powder and 10.8 g of carbon black.

The mixture was mixed with SiC balls at a speed of 120 rpm for 12 hours using acetone as a solvent, and the obtained slurry was dried by rotary evaporation to obtain a uniformly mixed powder. The mixed powder is placed in a pellet steel mold, and a pressure of 20 MPa is first applied to pre-press, and then the preformed blank is vacuum-sealed with oil paper, and then 250 MPa is applied to the vacuum-sealed blank of the oil paper using a cold isostatic press. The isostatic load is held at 250 MPa for 5 minutes to achieve an initial density of the core block of 50% or more.

The green body was taken out and placed in a high temperature pressureless furnace for reaction sintering. Using pressureless sintering, in the first reaction stage, the heating rate is 20 ° C / min, the temperature is raised to 1550 ° C and held for 0.5 hours, which must maintain a vacuum environment. After the end of the heat preservation, the vacuum was stopped, the high-purity argon gas was introduced into the furnace and maintained at an atmospheric pressure, and then heated to 1900 ° C at a temperature increase rate of 20 ° C / min and held for 8 hours. After the end of the heat preservation, cool to room temperature and take out the pellet.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peaks were not significantly shifted. It was found that the uranium carbide pellet obtained by sintering at 1900 ° C for 8 hours had a density of 97%, a porosity of 3%, a pore diameter of 300 nm, and an open pore.

Example 6

A 100% density uranium carbide pellet was prepared. Carbon black is used as a raw material. Weighed according to the UN:C molar ratio of 1:1.1: 249 grams of UN powder and 13.2 grams of carbon black.

The body was prepared in the same manner as in Example 5, and the in-situ reaction temperature was maintained at 1600 ° C for 2 hours, and the sintering temperature was maintained at 1950 ° C for 12 hours.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peaks were not significantly shifted. It was found that the density of uranium carbide pellets obtained by holding and sintering at 1950 ° C for 12 hours was 100%, and the porosity was 0%.

Example 7

A 95.5% density uranium carbide pellet was prepared. Carbon black is used as a raw material. Weighed according to the UN:C molar ratio of 1:1.5: 249 grams of UN powder and 18 grams of carbon black.

The mixture was mixed with SiC balls at a speed of 120 rpm for 24 hours using acetone as a solvent, and the obtained slurry was dried by rotary evaporation to obtain a uniformly mixed powder. The mixed powder is placed in a pellet steel mold, and a pressure of 15 MPa is applied first to pre-press, and then the preformed blank is vacuum-sealed with oil paper, and then 150 MPa is applied to the vacuum-sealed blank of the oil paper using a cold isostatic press. The isostatic load is held at 150 MPa for 5 minutes to achieve an initial density of the core block of 50% or more.

The green body was taken out and placed in a high temperature pressureless furnace for reaction sintering. Using pressureless sintering, in the first reaction stage, the heating rate is 30 ° C / min, the temperature is raised to 1600 ° C and held for 2 hours, which must maintain a vacuum environment. After the end of the heat preservation, the vacuum was stopped, the high-purity argon gas was introduced into the furnace and maintained at an atmospheric pressure, and then heated to 1850 ° C at a temperature increase rate of 30 ° C / min and held for 6 hours. After the end of the heat preservation, cool to room temperature and take out the pellet.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peak shifted slightly to the right, indicating the presence of carbon. It was found that the density of the uranium carbide pellets obtained by holding and sintering at 1850 ° C for 6 hours was 95.5%, and the porosity was 4.5%.

Example 8

A 96.5% density uranium carbide pellet was prepared. Carbon black is used as a raw material. Weighed according to the UN:C molar ratio of 1:1.3: 249 grams of UN powder and 15.6 grams of carbon black.

The mixture was mixed with Si ₃ N ₄ balls at a speed of 120 rpm for 18 hours using acetone as a solvent, and the obtained slurry was dried by rotary evaporation to obtain a uniformly mixed powder. The mixed powder is placed in a pellet steel mold, pre-compressed by applying a pressure of 30 MPa, and then the preformed blank is vacuum-sealed with oil paper, and then 300 MPa is applied to the vacuum-sealed blank of the oil paper using a cold isostatic press. The isostatic load is held at 300 MPa for 5 minutes to achieve an initial density of the core block of 50% or more.

The green body was taken out and placed in a high temperature pressureless furnace for reaction sintering. Using pressureless sintering, in the first reaction stage, the heating rate is 15 ° C / min, the temperature is raised to 1450 ° C and held for 2.5 hours, which must maintain a vacuum environment. After the end of the heat preservation, the vacuum was stopped, the furnace was purged with high-purity argon gas and maintained at an atmospheric pressure, and then heated to 1950 ° C at a temperature increase rate of 15 ° C / min and held for 2 hours. After the end of the heat preservation, cool to room temperature and take out the pellet.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peak shifted slightly to the right, indicating the presence of carbon. It was found that the density of the uranium carbide pellets obtained by holding and sintering at 1950 ° C for 2 hours was 96.5%, and the porosity was 3.5%.

Example 9

A 96% density uranium carbide pellet was prepared. Carbon black is used as a raw material. Weighed according to the UN:C molar ratio of 1:0.8: 249 g of UN powder and 9.6 g of carbon black.

The mixture was mixed with a Si ₃ N ₄ ball roll at a speed of 120 rpm for 20 hours using acetone as a solvent, and the resulting slurry was dried by rotary evaporation to obtain a uniformly mixed powder. The mixed powder is placed in a pellet steel mold, pre-compressed by applying a pressure of 5 MPa, and then the preformed blank is vacuum-sealed with oil paper, and then 150 MPa is applied to the vacuum-sealed blank of the oil paper using a cold isostatic press. The isostatic load is held at 150 MPa for 5 minutes to achieve an initial density of the core block of 50% or more.

The green body was taken out and placed in a high temperature pressureless furnace for reaction sintering. Using pressureless sintering, in the first reaction stage, the heating rate is 5 ° C / min, the temperature is raised to 1300 ° C and held for 4 hours, which must maintain a vacuum environment. After the end of the heat preservation, the vacuum was stopped, the high-purity argon gas was introduced into the furnace and maintained at an atmospheric pressure, and then heated to 1850 ° C at a temperature increase rate of 5 ° C / min and held for 5 hours. After the end of the heat preservation, cool to room temperature and take out the pellet.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peak shifted slightly to the left, indicating the presence of solid solution nitrogen. It was found that the density of uranium carbide pellets obtained by holding and sintering at 1850 ° C for 5 hours was 96% and the porosity was 4%.

Example 10

A 98.5% density uranium carbide pellet was prepared. Carbon black is used as a raw material. Weighed according to the UN:C molar ratio of 1:1.1: 249 grams of UN powder and 13.2 grams of carbon black.

The mixture was mixed with Si ₃ N ₄ balls at a speed of 240 rpm for 24 hours using acetone as a solvent, and the resulting slurry was dried by rotary evaporation to obtain a uniformly mixed powder. The mixed powder is placed in a pellet steel mold, pre-compressed by applying a pressure of 10 MPa, and then the preformed blank is vacuum-sealed with oil paper, and then a cold isostatic press is used to apply a vacuum of 200 MPa to the vacuum-sealed blank of the oil-paper. The isostatic load is held at 200 MPa for 5 minutes to achieve an initial density of the core block of 50% or more.

The green body was taken out and placed in a high temperature pressureless furnace for reaction sintering. Using pressureless sintering, in the first reaction stage, the heating rate is 10 ° C / min, the temperature is raised to 1350 ° C and held for 4 hours, which must maintain a vacuum environment. After the end of the heat preservation, the vacuum was stopped, the high-purity argon gas was introduced into the furnace and maintained at an atmospheric pressure, and then heated to 1750 ° C at a temperature increase rate of 10 ° C / min and held for 13 hours. After the end of the heat preservation, cool to room temperature and take out the pellet.

After X-ray diffraction analysis, the sintered core phase was uranium carbide, and the diffraction peaks were not significantly shifted. It was found that the density of uranium carbide pellets obtained by holding and sintering at 1750 ° C for 13 hours was 98.5%, and the porosity was 1.5%.

The above is only the embodiment of the present invention, and is not intended to limit the scope of the invention, and the equivalent structure or equivalent process transformation made by using the content of the specification of the present invention, or directly or indirectly applied in other related technical fields, The same is included in the scope of patent protection of the present invention.

Claims

A method for preparing a uranium carbide pellet, comprising the steps of:

S1, the uranium nitride powder and the carbon source are weighed according to a molar ratio of 1:0.8-1.5 and added to the solvent, and uniformly mixed to form a slurry;

S2, drying the slurry to obtain a mixed powder;

S3, pressing the mixed powder into a blank having a density of 50% or more;

S4, high temperature pressureless sintering, obtaining a uranium carbide pellet with a density of ≥95%.
The method for preparing a uranium carbide pellet according to claim 1, wherein in step S1, the uranium nitride powder has a purity greater than 95% and a particle diameter of 0.1-50 μm;

The carbon source is carbon black and/or graphite, the purity is greater than 95%, and the particle size is 0.1-50 μm.
The method for preparing a uranium carbide pellet according to claim 1, wherein in the step S1, the solvent is ethanol or acetone.
The method for preparing a uranium carbide pellet according to claim 1, wherein the step S3 comprises the following steps:

S3.1, pre-compressing the mixed powder under a pressure of 5-30 MPa;

S3.2, vacuum-sealing the pre-formed blank, pressing the sealed blank at a pressure of 150-300 MPa, and obtaining a green body having a density of 50% or more after the pressure is maintained.
The method for preparing a uranium carbide pellet according to claim 4, wherein in step S3.1, the mixed powder is placed in a pellet steel mold for pre-compression molding;

In step S3.2, the pre-formed green body is vacuum-sealed with oil paper, and the sealed green body is subjected to high pressure pressing using a cold isostatic press.
The method of preparing a uranium carbide pellet according to claim 1, wherein the step S4 comprises the following steps:

S4.1, the blank obtained in step S3 is placed in a high temperature pressureless furnace, in a vacuum environment, the temperature is raised to 1300 ° C -1600 ° C at a rate of 5-30 ° C / min and held for 0.5-4 h;

S4.2. Under an inert atmosphere, the temperature is raised to 1700 ° C - 2000 ° C at a rate of 5-30 ° C / min and held for 1-14 h to obtain a uranium carbide pellet.
The method for preparing a uranium carbide pellet according to claim 6, wherein in the step S4.2, argon gas is introduced into the high temperature pressureless furnace and maintained at an atmospheric pressure to form an inert atmosphere.
The method for preparing a uranium carbide pellet according to any one of claims 1 to 7, wherein the ratio of uranium, carbon and nitrogen atoms in the obtained uranium carbide pellet is 1: (0.8-1.5): (0) -0.2).
A uranium carbide pellet, which is produced by the production method according to any one of claims 1-8.
A fuel rod comprising the uranium carbide pellet of claim 9.