Method for sintering UO2 fuel pellet by taking direct ammonia decomposition gas as raw material

Abstract

The invention belongs to the technical field of nuclear power, and particularly relates to a sintering method of a UO2 fuel pellet taking direct ammonia decomposition gas as a raw material. Preparing nitrogen and hydrogen required for producing the UO2 fuel pellet by taking ammonia gas as a raw material; ammonia gas is subjected to a thermocatalytic ammonia decomposition reaction to generate ammonia decomposition gas, and the ammonia decomposition gas comprises ammonia gas, hydrogen and a small amount of ammonia gas which is not completely decomposed; ammonia gas which is not completely decomposed in the ammonia decomposition gas is removed to form mixed gas of hydrogen and nitrogen; and after being humidified, mixed gas of nitrogen and hydrogen is introduced into the sintering furnace to participate in preparation of the UO2 fuel pellet. According to the UO2 pellet sintering method using the direct ammonia decomposition gas as the raw material, the reaction gas required by pellet sintering can be obtained only by carrying out one-time thermal decomposition reaction and one-time purification operation on the gas, and then an original whole sintering system is simplified.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power technology, and specifically relates to a sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material. Background Technology

[0002] Uranium dioxide (UO2) is a non-stoichiometric compound with the actual molecular formula UO. 2+x Here, x represents the excess oxygen content. This deviation x, or more commonly expressed as the O / U atomic ratio, has a decisive impact on almost all key properties of UO2 fuel. An increased O / U ratio significantly reduces the thermal conductivity of UO2. This is the most fatal drawback. Reduced thermal conductivity means higher center temperatures in the fuel pellets during operation, increasing the risk of fuel melting and leading to a series of negative effects such as grain growth and increased fission gas release. The manufacturing goal is to obtain UO2 pellets that are as close as possible to stoichiometry (O / U = 2.000) and homogeneous. Hydrogen plays a dual role as a reducing agent and atmosphere conditioner during sintering. Its core chemical reaction is:

[0003] UO 2+x +xH2→UO2+xH2O

[0004] This reaction ensures that excess oxygen molecules in the UO2 core are fully reduced by hydrogen and discharged as water vapor, ultimately reducing the UO2 content. 2+x The hydrogen sintering process precisely reduces the fuel to stoichiometric UO2. Hydrogen sintering is a necessary and crucial core technology developed to address the specific chemical and physical performance requirements of UO2 fuel pellets, and is one of the cornerstones of the modern nuclear fuel industry. Modern UO2 pellet hydrogen sintering is typically carried out in a continuous pusher-type sintering furnace, with a nitrogen-hydrogen mixture in the sintering atmosphere and a reaction temperature of approximately 1750°C. During sintering, nitrogen is commonly used as a pressure-holding gas to maintain the pressure within the sintering furnace within a certain range. Currently, in industrial practice, the hydrogen and nitrogen used for UO2 fuel pellet sintering are mainly obtained through water electrolysis and air separation.

[0005] Ammonia (NH3) is considered as an important energy medium for achieving carbon neutralization goal due to its high hydrogen content (17.6wt%), easy liquefaction (liquefied at 0.8 MPa), mature storage and transportation infrastructure, and carbon-free characteristics. The ammonia decomposition hydrogen production technology mainly relies on thermal catalytic decomposition, which requires Ru-based catalysts above 500℃ or Ni-based catalysts above 600℃ to achieve high conversion rate, which is suitable for UO2 sintering environment. The technology of directly using ammonia decomposition gas, which is H2 / N2 mixed gas without separation and purification, as raw material is mainly applied in the fields of solid oxide fuel cells (SOFC) and ammonia-hydrogen mixed combustion. At present, the research on the combination of ammonia decomposition function and fuel pellets is still in the blank. Directly using ammonia decomposition gas as gas source input into the sintering furnace can simplify the preparation process of hydrogen and nitrogen to some extent, and further simplify the overall system structure. In addition, the energy temperature of thermal catalytic ammonia decomposition reaction is suitable for the high temperature environment of high temperature sintering furnace, and the ammonia gas is preheated by using a heat exchanger, which saves the energy consumption required for heating to a certain extent. SUMMARY

[0006] The purpose of the present application is to provide a sintering method of UO2 fuel pellets using direct ammonia decomposition gas as raw material, which is suitable for the preparation of UO2 pellets. The mixed gas of nitrogen, hydrogen and a small amount of ammonia obtained by thermal catalytic ammonia decomposition reaction of ammonia is called ammonia decomposition gas. After removing the unreacted ammonia in the ammonia absorber, the mixed gas of nitrogen and ammonia is obtained. Finally, the mixed gas is introduced into the sintering furnace to participate in the preparation of UO2 pellets. Compared with the existing method of preparing hydrogen and nitrogen by electrolysis of potassium hydroxide solution and separation of air, the preparation step of gas in the preparation process of UO2 pellets is simplified, and the preparation cost of reaction gas is further saved.

[0007] To achieve the above purpose, the technical scheme adopted by the present application is:

[0008] A sintering method of UO2 fuel pellets using direct ammonia decomposition gas as raw material: ammonia is used as raw material to prepare nitrogen and hydrogen required for the production of UO2 fuel pellets; ammonia is converted into ammonia decomposition gas by thermal catalytic ammonia decomposition reaction, which includes ammonia, hydrogen and a small amount of uncompleted ammonia; the uncompleted ammonia in the ammonia decomposition gas is removed to form a mixed gas of hydrogen and nitrogen; the mixed gas of nitrogen and hydrogen is humidified and introduced into the sintering furnace to participate in the preparation of UO2 fuel pellets.

[0009] The ammonia decomposition gas is removed by the ammonia absorber to form a mixed gas of hydrogen and nitrogen.

[0010] The mixed gas of nitrogen and hydrogen is humidified by a dew point instrument.

[0011] The ammonia gas is preheated by a pipeline into the fuel heat exchanger, and after preheating, the ammonia gas is introduced into the ammonia decomposer through the pipeline to generate ammonia decomposition gas through the thermal catalytic ammonia decomposition reaction.

[0012] The high-temperature tail gas generated after the preparation of the core block is used to heat the ammonia gas through the heat exchanger to reduce the heat difference in the thermal catalytic ammonia decomposition reaction process.

[0013] Hydrogen is used as the reaction gas.

[0014] Nitrogen is used as the protective gas.

[0015] The beneficial effects obtained by the present application are:

[0016] In industrial practice, water electrolysis or potassium hydroxide solution electrolysis is often used to obtain hydrogen gas through separation and purification technology, and nitrogen gas is prepared by separating air. After obtaining the required hydrogen and nitrogen gas, they are input into the high-temperature sintering furnace through the pipeline in proportion to perform UO2 sintering work. The gas preparation process needs to use separation and purification methods to treat the required gas to remove impurities in the gas.

[0017] Using ammonia gas to prepare nitrogen and hydrogen gas can obtain a nitrogen and hydrogen gas mixture with a volume ratio of 1:3, which is called ammonia decomposition gas. In actual production, although there is a small amount of unreacted ammonia gas in the ammonia decomposition gas, ammonia gas can be easily removed by negative pressure adsorption, molecular sieve, etc. After ammonia adsorption, the gas contains only a mixture of nitrogen and hydrogen. At this point, the mixture gas does not need to be separated, but is directly transported into the core block sintering furnace through the pipeline to participate in the core block sintering work.

[0018] Through comparison, the UO2 core block sintering method using direct ammonia decomposition gas as raw material only needs one thermal decomposition reaction and one purification operation of the gas to obtain the reaction gas required for core block sintering, thereby simplifying the original sintering system as a whole. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 It is a sintering method for UO2 fuel core block using direct ammonia decomposition gas as raw material.

[0020] In the figure: 1. Heat exchanger; 2. Ammonia decomposer; 3. Ammonia absorber; 4. Dew point instrument; 5. Sintering furnace. DETAILED DESCRIPTION

[0021] The present application will be described in detail below in combination with the drawings and specific embodiments.

[0022] A sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material: Nitrogen and hydrogen are prepared from NH3 as raw material to produce UO2 fuel pellets; ammonia is decomposed into ammonia decomposition gas via a thermocatalytic ammonia decomposition reaction, comprising ammonia, hydrogen, and a small amount of incompletely decomposed ammonia; the ammonia decomposition gas is passed through an ammonia absorber to remove the incompletely decomposed ammonia, forming a mixed gas of hydrogen and nitrogen; the nitrogen and hydrogen mixture is humidified by a dew point meter and then introduced into a sintering furnace to participate in the preparation of UO2 fuel pellets.

[0023] In the preparation of UO2 fuel pellets, nitrogen is used as a protective gas and hydrogen as a reactant gas, which are then fed into a high-temperature sintering furnace after preparation. This invention eliminates the need for separate preparation of hydrogen and nitrogen; the gas generated from the decomposition of ammonia is directly fed into the sintering furnace, eliminating the need for additional gas separation operations and simplifying the preparation process of the reactant gas for fuel pellets.

[0024] Example 1: As Figure 1 As shown, ammonia gas enters fuel heat exchanger 1 through a pipeline for preheating. After preheating, the ammonia gas is then passed through a pipeline to ammonia decomposer 2 for thermocatalytic ammonia decomposition, yielding ammonia decomposition gas, which includes hydrogen, nitrogen, and a small amount of unreacted ammonia. It is worth noting that theoretically, the decomposition products of ammonia are nitrogen and hydrogen. In actual experiments and production processes, a small amount of unreacted ammonia will inevitably be present in the ammonia decomposition gas.

[0025] The ammonia decomposition gas from actual production is passed into ammonia absorber 3, where ammonia is adsorbed, ultimately forming a mixture containing only nitrogen and hydrogen. This mixture is then piped into dew point meter 4 for humidification. The humidified mixture is then passed into sintering furnace 5 for UO2 block sintering. Finally, the high-temperature tail gas generated after block preparation is passed through a heat exchanger to raise the temperature of the ammonia, thereby reducing the heat difference during the thermocatalytic ammonia decomposition reaction.

Claims

1. A sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material, characterized in that: Nitrogen and hydrogen required for the production of UO2 fuel pellets using ammonia as a raw material; Ammonia gas undergoes a thermocatalytic decomposition reaction to produce ammonia decomposition gas, which includes ammonia, hydrogen, and a small amount of incompletely decomposed ammonia. The undecomposed ammonia gas in the ammonia decomposition gas is removed to form a mixture of hydrogen and nitrogen gas; the mixture of nitrogen and hydrogen gas is humidified and then introduced into the sintering furnace to participate in the preparation of UO2 fuel pellets.

2. The sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material according to claim 1, characterized in that: The ammonia decomposition gas is passed through an ammonia absorber to remove any undecomposed ammonia, forming a mixture of hydrogen and nitrogen.

3. The sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material according to claim 1, characterized in that: The mixture of nitrogen and hydrogen is humidified by a dew point meter.

4. The sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material according to claim 1, characterized in that: Ammonia gas enters the fuel heat exchanger through pipelines for preheating. After preheating, the ammonia gas is then passed through pipelines into the ammonia decomposer for thermocatalytic ammonia decomposition reaction to generate ammonia decomposition gas.

5. The sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material according to claim 1, characterized in that: The high-temperature exhaust gas generated after the pellet preparation is heated by a heat exchanger to reduce the heat difference in the thermocatalytic ammonia decomposition reaction process.

6. The sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material according to claim 1, characterized in that: Hydrogen is used as the reactant.

7. The sintering method for UO2 fuel pellets using direct ammonia decomposition gas as raw material according to claim 1, characterized in that: Nitrogen is used as a protective gas.

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