Preparation method of nitride nuclear fuel

Abstract

The invention discloses a nitride nuclear fuel preparation method, which comprises: adding a nuclear fuel fluoride and a nitrogen source into a plasma reactor, completely mixing in the plasma reactor, and carrying out a reaction to form a product, the product comprising a solid phase product; the solid-phase product is nitride powder and is used as a nitride fuel raw material. According to the preparation method of the nitride nuclear fuel, the high-activity nitride fuel powder is prepared in one step by taking the nuclear fuel fluoride as the raw material and cooperating with the plasma reaction mode, and compared with various technologies in the prior art which are short-process technologies, the preparation method is suitable for industrial production, and the problems that the technology in the prior art is complex, and batch production is difficult are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of nuclear fuel, and in particular to a method for preparing nitride nuclear fuel. Background Art

[0002] Nitride nuclear fuels are mainly mono-nitrides such as uranium nitride (UN) and uranium-plutonium nitride ((U, Pu)N). Compared with traditional oxide nuclear fuels, nitride fuels have the advantages of high fissile nuclide density, high melting point, high thermal conductivity, low thermal expansion coefficient, good irradiation stability, low fission gas release rate, good compatibility with liquid metal coolants, good thermal stability and irradiation stability, etc. Nitride fuels have broad application prospects in the field of large metal-cooled fast reactors, and the combination of high uranium density accident-tolerant fuels (ATF) has attracted great attention from various countries. Countries around the world are also exploring the application of nitride fuels in water-cooled reactors such as pressurized water reactors, boiling water reactors, and supercritical water reactors.

[0003] Taking uranium nitride (UN) as an example, the synthesis of uranium nitride powder is one of the key links in the preparation of uranium nitride fuel; uranium nitride powder includes uranium mono-nitride (UN), uranium sesqui-nitride (U2N3), and uranium di-nitride (UN2) powder. Among them, U2N3 and UN2 powders can only be used to prepare nuclear fuel by pressure sintering processes such as hot pressing sintering and spark plasma sintering, but the common nuclear fuel industrial production processes use pressureless sintering, and only uranium mono-nitride powder in uranium nitride powder can be applied to pressureless sintering.

[0004] Currently, there are mainly two UN material synthesis routes that have been put into application:

[0005] (1) Metal nitridation method: First, metal U powder needs to be prepared. The metal U is broken into small pieces, then hydrogen is introduced and heated under vacuum conditions, and this process is repeated many times to form metal U powder; N2 is introduced into the metal U powder under heating conditions, and vacuum pumping is still required when cooling after the nitridation reaction ends to obtain U2N3. This method has the following defects: ① It is necessary to go through the hydrogenation-dehydrogenation process many times to obtain metal U powder; ② The particle size of the obtained metal U powder is greatly affected by the process and is difficult to control; ③ The nitridation reaction requires nitrogen charging and heating; ④ The product obtained from the nitridation reaction is U2N3, and denitrification treatment is required; ⑤ Both the dehydrogenation and denitrification processes require vacuum pumping, the process is slow, time-consuming, the equipment is complex, and the energy consumption is high; ⑥ In industrial production, the product of uranium enrichment is UF6, and the post-treatment product of spent fuel fluorination volatilization is UF6 and PuF6. It is necessary to convert UF6 into a metal form, then carry out hydrogenation-dehydrogenation to prepare metal U powder, and then nitride-denitrification to prepare nitrides, which increases the process complexity and the possibility of being contaminated by impurities.

[0006] (2) Carbon thermal reduction nitridation method of uranium oxide (CTRN): Using UO2 as raw material, adding excessive carbon into it, first removing oxygen in nitrogen atmosphere, then introducing hydrogen to remove carbon in it, that is: UO2 + C + N2 + H2 → UN bulk → UN powder → UN pellet → UN fuel. In order to improve the phase structure purity and chemical composition purity of the synthesized nitride, making the residual carbon and residual oxygen content less than 1000 μg / g, currently mainly the two-step carbon thermal reduction method is adopted: The first step is that the mixture of oxide and carbon black undergoes carbon thermal reduction and nitridation reaction at 1500 °C to 1650 °C in N2 gas, reducing and nitriding UO2 to UN, achieving oxygen removal but possibly containing a small amount of free carbon impurities; the second step is to carry out decarbonization and purification treatment at 1400 - 1450 °C in N2-H2 atmosphere. The overall reaction formula of the two-step carbon thermal reduction method is as follows: 2UO2 + C + N2 + 2H2 → 2UN + 4CO↑ + CH4↑. Generally speaking, the carbon thermal reduction method can use UO2 powder as raw material, but its reaction temperature is very high (about 1600 °C) and the impurity content is also relatively high. Factors such as C / UO2 in the raw material, reaction temperature, protective atmosphere, reaction time, etc. will all affect the purity and phase structure of uranium nitride powder. This method has the following defects: ① In the early one-step carbon thermal reduction nitridation method and currently the two-step carbon thermal reduction nitridation method. The raw material contains O, and excessive C is added during the reaction process. Even if the two-step method is adopted, there are still problems with unstable control of carbon and oxygen impurities, resulting in high impurity content in the product; ② The reaction temperature is very high and the energy consumption is large; ③ The nitride material produced by the UO2 carbon thermal reduction nitridation method is in bulk form, and processes such as crushing and grinding are required to obtain powder, and these treatment processes will inevitably lead to an increase in impurity content; ④ The particle size of the UN powder produced by the carbon thermal reduction method is relatively coarse and uncontrollable, and the sintering activity is not good, resulting in an increase in the difficulty of achieving the required sintering density of the pellet, such as high temperature and long time in pressureless sintering, etc.

[0007] In order to ensure that the UN fuel pellet has a certain strength, stiffness and structural integrity, generally the pellet density ≥ 80% TD is required. However, nitrides are ceramics mainly composed of covalent bonds, and the atomic diffusion rate is very low. The sintering difficulty of nitride fuel pellets is much greater than that of UO2. In research, hot pressing sintering or even spark plasma sintering (SPS) is commonly used in the preparation of UN pellets, which is not suitable for industrial production. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide an improved method for preparing nitride nuclear fuel.

[0009] The technical solution adopted by the present invention to solve its technical problem is: providing a method for preparing nitride nuclear fuel, including the following steps:

[0010] Adding nuclear fuel fluoride and nitrogen source into a plasma reactor, fully mixing and reacting in the plasma reactor to form products, wherein the products include solid phase products;

[0011] The solid phase product is nitride powder, which is used as nitride fuel raw material.

[0012] In some embodiments, in the plasma reactor, the average temperature of the reaction zone is ≤1500°C; and the inner wall temperature of the plasma reactor is controlled below 550°C.

[0013] In some embodiments, in the plasma reactor, the plasma is generated by at least one of high voltage discharge, DC arc discharge, high frequency discharge, microwave discharge and laser ionization.

[0014] In some embodiments, the product further comprises a gas-solid two-phase flow product; and the method for preparing nitride nuclear fuel further comprises the following steps:

[0015] The gas-solid two-phase flow product is subjected to gas-solid separation treatment, and the separated solid product and gas phase product are collected respectively; the solid product includes nitride powder;

[0016] The gas phase product is subjected to absorption and purification treatment.

[0017] In some embodiments, the method for preparing nitride nuclear fuel further comprises the following steps: 4 K / s~10 9 Rapid cooling was carried out at a cooling rate of K / s.

[0018] In some embodiments, the nuclear fuel fluoride includes at least one of UF6, UF4, PuF6, ThF6, and ThF4.

[0019] In some embodiments, the nitrogen source includes at least one of a mixed gas of N2 and H2 and a nitrogen-hydrogen compound, and the nitrogen-hydrogen compound includes NH3, N2H4, HN3, and NH4N3.

[0020] In some embodiments, when the nuclear fuel fluoride is hexafluoride, it is heated and then introduced into the plasma reactor in a gaseous state.

[0021] In some embodiments, when the nuclear fuel fluoride is tetrafluoride, it enters the plasma reactor in powder form under the action of a carrier gas; the carrier gas is an inert gas or a process gas, and the process gas includes at least one of N2, NH3 and H2.

[0022] In some embodiments, when the nitrogen source is a nitrogen hydrogen compound, H2 needs to be added into the plasma reactor.

[0023] In some embodiments, the method for preparing the nitride nuclear fuel further comprises the following steps:

[0024] Using the nitride powder as a raw material, a nitride fuel is prepared.

[0025] In some embodiments, the nitride fuel includes at least one of nitride pellets, nitride dispersion fuel, and nitride matrix TRISO particles.

[0026] Advantages of the present invention: Using nuclear fuel fluoride as a raw material and combining with a plasma reaction method, highly active nitride fuel powder is prepared in one step. Compared with the numerous processes of the prior art, it is a short-process technology, suitable for industrial production, and solves the problems of complex processes and difficulty in batch production in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below in conjunction with the drawings and embodiments. In the drawings:

[0028] Figure 1 is the T-ΔG data corresponding curve graph of the UF6-NH3 and UF4-NH3 reactions;

[0029] Figure 2 is the flow chart of the method for preparing the nitride nuclear fuel in an embodiment of the present invention;

[0030] Figure 3 is the flow chart of the method for preparing the nitride nuclear fuel in another embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] In order to have a clearer understanding of the technical features, objectives, and effects of the present invention, the specific embodiments of the present invention will now be described in detail with reference to the drawings.

[0032] The method for preparing the nitride nuclear fuel of the present invention may include the following steps:

[0033] S1. Adding the nuclear fuel fluoride and the nitrogen source into a plasma reactor, fully mixing and reacting in the plasma reactor to form a product;

[0034] S2. Rapidly cooling the product at a cooling rate of 10 4 K / s to 10 9 K / s. This rapid cooling can be carried out in the rapid cooling zone at the lower part of the plasma reactor.

[0035] The above reaction in the plasma reactor is based on the principle of plasma chemistry and can obtain the product in one step. The product obtained above includes a solid-phase product, which can be collected from the bottom of the plasma reactor. The solid-phase product is a nitride powder, which is used as a raw material for the preparation of nitride fuel, eliminating the crushing and grinding steps.

[0036] Among them, nuclear fuel fluoride as a raw material comes from uranium enrichment product UF6 or its conversion product UF4, reprocessing products (UF6, PuF6), and fluorides of other nuclear fuels (such as ThF6, ThF4), etc., and is combined with a nitrogen source to prepare nitride or mixed nitride powders. Thus, the nuclear fuel fluoride can include at least one of UF6, UF4, PuF6, ThF6, ThF4, and highly reactive nitride powders such as uranium nitride or thorium nitride can be prepared from the above raw materials. The nitride powder is used as a raw material and further prepared into nitride fuel through the fuel preparation process of the existing technology (such as sintering, etc.), such as nitride pellets, nitride dispersion fuels, nitride matrix TRISO particles, etc.

[0037] The nitrogen source includes at least one of a mixed gas of N2 and H2 and nitrogen-hydrogen compounds. The nitrogen-hydrogen compounds include NH3 (ammonia), N2H4 (hydrazine, colorless oily liquid), HN3 (hydrazoic acid, colorless pungent liquid), and NH4N3 (ammonium azide, colorless flaky crystal, melting point 160 °C). Considering the operation safety, the nitrogen source can preferably be NH3, a mixed gas of N2 and H2, or a mixed gas of NH3 + H2. Both N2 and H2 are high-purity gases.

[0038] When the nuclear fuel fluoride is a hexafluoride, it is heated and introduced into the plasma reactor in a gaseous state.

[0039] When the nuclear fuel fluoride is a tetrafluoride, it is transported in powder form through a nozzle or a screw conveyor, etc. under the action of a carrier gas into the plasma reactor; the carrier gas is an inert gas or a process gas, and the process gas includes at least one of N2, NH3, and H2.

[0040] In order to obtain the target product, when the nitrogen source is a nitrogen-hydrogen compound, H2 also needs to be added to the plasma reactor, which can recover fluorine resources in the form of HF and achieve different conversion rates.

[0041] Reference Figure 1 , according to the thermodynamic calculation results, the temperature required for UF4, UF6 to react with NH3 or a mixed gas of N2 + H2 to generate mononitride (UN) is > 3000 °C. In the present invention, based on the reaction activity of the plasma state substance, the average temperature of the reaction zone of the plasma reactor can be reduced to 1500 °C or below. In addition, the inner wall temperature of the plasma reactor is controlled below 550 °C to reduce the risk of corrosion.

[0042] In a plasma reactor, the plasma generation methods include at least one of high-voltage discharge, DC arc discharge, high-frequency discharge, microwave discharge, and laser ionization.

[0043] Furthermore, after the raw materials are fully mixed with a nitrogen source in the plasma reactor and react, the formed product also includes a gas-solid two-phase flow product. In this regard, the method for preparing a nitride nuclear fuel further includes the following steps:

[0044] S3. Perform gas-solid separation on the gas-solid two-phase flow product, and separately collect the separated solid product and gas product.

[0045] Among them, the gas-solid separation can be achieved by filtration using a filter. After the gas-solid two-phase flow product is discharged from the side outlet of the plasma reactor and enters the filter, the solid product in the gas-solid two-phase flow product can be sedimented and separated, and the gas product can be discharged from the upper outlet of the filter and further enter a gas treatment device for absorption and purification.

[0046] The solid product includes a nitride powder, and this nitride powder can be collected together with the solid phase product in the previous step and used as a raw material for preparing a nitride fuel. When the nitride powder contains unreacted raw materials, it can be returned to the plasma reactor for re-reaction if necessary according to the raw material content.

[0047] For the gas product, perform absorption and purification on it. According to the raw material being a nuclear fuel fluoride, HF is contained in this gas product. By absorbing it, hydrofluoric acid can be obtained and recycled, and the purified gas can be discharged.

[0048] As Figure 2 shown, in an embodiment of the present invention, when the nuclear fuel fluoride is hexafluoride (UF6), it is placed in a container, heated, and then introduced into the plasma reactor in a gaseous state (UF6 gas). At the same time, an N2 + H2 mixed gas, NH3, or other nitrogen-hydrogen compounds are added to the plasma reactor as a nitrogen source. After the hexafluoride and the nitrogen source are fully mixed and react in the plasma reactor, a solid phase product and a gas-solid two-phase flow product are formed. The solid phase product is collected to obtain a nitride powder (such as UN powder); the gas-solid two-phase flow product is filtered, the solid obtained by filtration is collected, the gas obtained by filtration is collected, and hydrofluoric acid can be obtained through absorption. The nitride powder obtained from the above solid phase product can obtain a nitride fuel (such as UN fuel) through a fuel preparation process.

[0049] As Figure 3As shown, in another embodiment of the present invention, when the nuclear fuel fluoride is tetrafluoride (UF4), it is fed into the plasma reactor in powder form (such as UF4 powder), and at the same time, a N2 + H2 mixed gas, NH3 or other nitrogen-hydrogen compounds are added into the plasma reactor as the nitrogen source. After the tetrafluoride and the nitrogen source are fully mixed and reacted in the plasma reactor, solid-phase products and gas-solid two-phase flow products are formed. The solid-phase products are collected to obtain nitride powder (such as UN powder); the gas-solid two-phase flow products are filtered, the solid phase obtained by filtration is collected, and the gas phase obtained by filtration is collected, and hydrofluoric acid can be obtained through absorption. The nitride powder obtained from the above solid-phase products can obtain nitride fuel (such as UN fuel) through the fuel preparation process.

[0050] For the nitride powder obtained in the above two embodiments, when it contains unreacted raw materials, the nitride powder can be selectively returned to the plasma reactor for re-reaction according to the raw material content.

[0051] Taking the preparation of a nitride powder from fluoride (UF6, UF4) and NH3 or N2 + H2 mixed gas or nitrogen-hydrogen compound as an example:

[0052] The fluoride UF6, UF4 and NH3 or N2 + H2 mixed gas or nitrogen-hydrogen compound are converted into plasma state substances to form active components including un-ionized molecules, excited atoms, ionized ions, etc. The reaction occurs based on the reactant ratio, and the reaction process is as follows:

[0053] (1) When using NH3 as the nitrogen source:

[0054] UF6(p) + NH3(p) = UN(s) + 3HF(g) + 3 / 2F2(g);

[0055] UF6(p) + NH3(p) + 3 / 2H2(p) = UN(s) + 6HF(g);

[0056] UF4(s) + NH3(p) = UN(s) + 3HF(g) + 1 / 2F2(g);

[0057] UF4(s) + NH3(p) + 3 / 2H2(p) = UN(s) + 3HF(g).

[0058] (2) When using N2 + H2 mixed gas as the nitrogen source:

[0059] 2UF6(p) + N2(p) + 6H2(p) = 2UN(s) + 12HF(g);

[0060] 2UF4(s) + N2(p) + 4H2(p) = UN(s) + 8HF(g).

[0061] In the above reaction formula, "(s)" and "(g)" represent solid state and gaseous state respectively, and "(p)" represents plasma state, where p: plasma.

[0062] It can be understood that when the fluoride is replaced by at least one of PuF6, ThF6, and ThF4, the reaction formula is the same as that of UF6 and UF4 above.

[0063] As can be seen from the process of the method for preparing nitride nuclear fuel of the present invention, the present invention directly prepares uranium nitride from uranium enrichment products / post-treatment products as raw materials, greatly simplifies the process flow, significantly reduces the process complexity and equipment used, reduces costs, and improves economic efficiency; directly prepares nitride from fluoride, reduces the probability of ingesting impurities in the process flow, thereby improving the product purity, with fewer impurities, improves the preparation efficiency of nitride fuel, and has good economic efficiency; directly obtains nitride powder, avoids the processes of crushing and grinding, and ensures the purity of the raw materials; the nitride powder has high sintering activity, reducing the difficulty of later sintering; the entire process flow operates under atmospheric pressure, reducing energy consumption and system complexity; according to the preparation process of the present invention, the equipment can continuously operate to produce nitride powder materials, and the materials do not need to be transported, which is suitable for industrial production.

[0064] The above are only the embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structural or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. A method for preparing nitride nuclear fuel, characterized in that: The following steps are involved: Adding nuclear fuel fluoride and nitrogen source into a plasma reactor, fully mixing and reacting in the plasma reactor to form products, wherein the products include solid phase products; The solid phase product is nitride powder, which is used as nitride fuel raw material.

2. The method for preparing nitride nuclear fuel according to claim 1, characterized in that: In the plasma reactor, the average temperature of the reaction zone is ≤1500°C; the inner wall temperature of the plasma reactor is controlled below 550°C.

3. The method for preparing nitride nuclear fuel according to claim 1, characterized in that: In the plasma reactor, the plasma is generated by at least one of high voltage discharge, direct current arc discharge, high frequency discharge, microwave discharge and laser ionization.

4. The method for preparing nitride nuclear fuel according to claim 1, characterized in that: The product also includes a gas-solid two-phase flow product; the nitride nuclear fuel preparation method also includes the following steps: The gas-solid two-phase flow product is subjected to gas-solid separation treatment, and the separated solid product and gas phase product are collected respectively; the solid product includes nitride powder; The gas phase product is subjected to absorption and purification treatment.

5. The method for preparing nitride nuclear fuel according to claim 1, characterized in that: The method for preparing nitride nuclear fuel further comprises the following steps: 4 K / s~10 9 Rapid cooling was carried out at a cooling rate of K / s.

6. The method for preparing nitride nuclear fuel according to claim 1, characterized in that: The nuclear fuel fluoride includes at least one of UF6, UF4, PuF6, ThF6, and ThF4; The nitrogen source includes at least one of a mixed gas of N2 and H2 and a nitrogen-hydrogen compound, and the nitrogen-hydrogen compound includes NH3, N2H4, HN3, and NH4N3.

7. The method for preparing nitride nuclear fuel according to claim 6, characterized in that: When the nuclear fuel fluoride is hexafluoride, it is heated and then introduced into the plasma reactor in a gaseous state; When the nuclear fuel fluoride is tetrafluoride, it enters the plasma reactor in powder form under the action of a carrier gas; the carrier gas is an inert gas or a process gas, and the process gas includes at least one of N2, NH3 and H2.

8. The method for preparing nitride nuclear fuel according to claim 6, characterized in that: When the nitrogen source is a nitrogen hydrogen compound, H2 also needs to be added into the plasma reactor.

9. The method for preparing nitride nuclear fuel according to any one of claims 1 to 8, characterized in that: The method for preparing nitride nuclear fuel further comprises the following steps: The nitride fuel is prepared by using nitride powder as a raw material.

10. The method for preparing nitride nuclear fuel according to claim 9, characterized in that: The nitride fuel includes at least one of nitride pellets, nitride dispersed fuel, and nitride core TRISO particles.

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