阅读说明：本技术 采用氧化石墨烯掺杂制备二氧化铀复合燃料芯块的方法 (Method for preparing uranium dioxide composite fuel pellet by doping graphene oxide ) 是由 朱礼洋 张燕 杨志红 杨素亮 田国新 杜江平 冯海宁 于 2020-12-17 设计创作，主要内容包括：本发明属于核燃料制备技术领域,涉及采用氧化石墨烯掺杂制备二氧化铀复合燃料芯块的方法。所述的制备方法包括如下步骤：(1)向硝酸铀酰溶液中加入氧化石墨烯,混合均匀后加入氨水,获得含有氧化石墨烯的重铀酸铵沉淀；(2)过滤沉淀,滤饼经烘干、粉碎、筛分,得到重铀酸铵-氧化石墨烯复合物；(3)将重铀酸铵-氧化石墨烯复合物煅烧后再在氢气还原气氛下进一步进行加热处理,获得二氧化铀粉末；(4)将二氧化铀粉末装入模具冷压成型,脱模,得到生坯；(5)将生坯在还原性气氛下进行烧结,得到二氧化铀复合燃料芯块。利用本发明的制备方法,能够制备得到高导热性的新型复合燃料芯块,从而提高核反应堆运行时的安全性。(The invention belongs to the technical field of nuclear fuel preparation, and relates to a method for preparing a uranium dioxide composite fuel pellet by doping graphene oxide. The preparation method comprises the following steps: (1) adding graphene oxide into uranyl nitrate solution, uniformly mixing, and adding ammonia water to obtain ammonium diuranate precipitate containing graphene oxide; (2) filtering the precipitate, drying, crushing and screening a filter cake to obtain an ammonium diuranate-graphene oxide compound; (3) calcining the ammonium diuranate-graphene oxide compound, and further heating in a hydrogen reducing atmosphere to obtain uranium dioxide powder; (4) putting the uranium dioxide powder into a mould for cold press molding, and demoulding to obtain a green body; (5) and sintering the green body in a reducing atmosphere to obtain the uranium dioxide composite fuel pellet. By utilizing the preparation method, the novel composite fuel pellet with high thermal conductivity can be prepared, so that the safety of a nuclear reactor in operation is improved.)

1. The method for preparing the uranium dioxide composite fuel pellet by doping graphene oxide is characterized by comprising the following steps:

(1) adding graphene oxide into uranyl nitrate solution, uniformly mixing, and adding ammonia water to obtain ammonium diuranate precipitate containing graphene oxide;

(2) filtering the precipitate obtained in the step (1), and drying, crushing and screening a filter cake to obtain an ammonium diuranate-graphene oxide compound;

(3) calcining the ammonium diuranate-graphene oxide compound obtained in the step (2), and further heating in a hydrogen reduction atmosphere to obtain uranium dioxide powder;

(4) putting the uranium dioxide powder obtained in the step (3) into a mould for cold press molding, and demoulding to obtain a green body;

(5) and (4) sintering the green body obtained in the step (4) in a reducing atmosphere to obtain the uranium dioxide composite fuel pellet.

2. The method of claim 1, wherein: in the step (1), the concentration of the uranyl nitrate solution is 300-500g/L, the concentration of the ammonia water is 10-14mol/L, and the pH of the solution is 10-12 by adding excessive ammonia water.

3. The method of claim 1, wherein: in the step (1), the graphene oxide is a solution, a powder or a suspension.

4. The method of claim 1, wherein: in the step (2), the drying is drying in a vacuum drying oven.

5. The method of claim 1, wherein: in the step (2), the average particle size of the ammonium diuranate-graphene oxide compound is 0.1-10 μm.

6. The method of claim 1, wherein: in the step (3), the calcining temperature is 300-550 ℃, and the time is 1-4 h.

7. The method of claim 1, wherein: in the step (3), the temperature of the heating treatment is 600-700 ℃, and the time is 20-30 min.

8. The method of claim 1, wherein: in the step (4), the pressure of the cold press molding is 10-17kN, and the dwell time is 10-30 s.

9. The method of claim 1, wherein: in the step (5), the volume ratio of the hydrogen to the argon in the reducing atmosphere is 0.1-6: 94.

10. The method of claim 1, wherein: in the step (5), the sintering temperature is 1600-1800 ℃, and the time is 1-5 h.

Technical Field

The invention belongs to the technical field of nuclear fuel preparation, and relates to a method for preparing a uranium dioxide composite fuel pellet by doping graphene oxide.

Background

At present, ceramic uranium dioxide pellets are generally adopted internationally as reactor fuel. The ceramic uranium dioxide has many advantages, such as good nuclear physical properties, good irradiation stability, only one crystalline form within the melting point up to 2800 ℃, and isotropy. However, the thermal conductivity of the ceramic uranium dioxide is only a tenth of that of metallic uranium, and the thermal conductivity decreases with increasing temperature. At core temperature, if the heat transfer from the pellets to the coolant is not smooth, the pellets are easily overheated locally, and the cladding is swollen and cracked.

In order to improve the heat conductivity of the fuel pellet and improve the safety and economy of the nuclear reactor, new advanced fuels, such as UC fuel and U fuel, are researched and developed in various countries₃N₂Non-oxide fuels such as fuel; metal Mo-UO₂diamond-UO₂And the like; or UO doped with beryllium oxide or acetylene black₂Fuel, etc. in the existing UO₂Doping modification is carried out on the basis of fuel.

The graphene is a monoatomic layer graphite formed by a carbon atom SP2 hybridization mode, and has very good heat conduction performance due to delocalized pi bonds, and the heat conduction coefficient is as high as 5300W/mK. Thus in the UO₂The graphite doped in the core block is expected to improve the heat-conducting property of the core block. But graphene and UO₂Have large differences in density and other physical properties, resulting in grapheneDifficult to uniformly dope into UO₂In the pellet (the problems of low uniformity, low production efficiency and the like exist in the related research at abroad mainly by adopting a physical mixing and discharge plasma sintering process; the adoption of graphene doped UO is reported at home₂And (4) core block, wherein ultrasonic dispersion is required during material mixing. However, the graphene is easy to have segregation and agglomeration problems, and the uniformity of powder is affected, so that the uniformly doped UO is prepared by directly adopting the graphene₂Core block is difficult).

Disclosure of Invention

The invention aims to provide a method for preparing a uranium dioxide composite fuel pellet by doping graphene oxide, so that a novel composite fuel pellet with high thermal conductivity can be prepared, and the safety of a nuclear reactor in operation is improved.

To achieve the object, in a basic embodiment, the present invention provides a method for preparing a uranium dioxide composite fuel pellet by doping graphene oxide, wherein the preparation method comprises the following steps:

(1) adding graphene oxide into uranyl nitrate solution, uniformly mixing, and adding ammonia water to obtain ammonium diuranate precipitate containing graphene oxide;

(2) filtering the precipitate obtained in the step (1), and drying, crushing and screening a filter cake to obtain an ammonium diuranate-graphene oxide compound;

(3) calcining the ammonium diuranate-graphene oxide compound obtained in the step (2), and further heating in a hydrogen reduction atmosphere to obtain uranium dioxide powder;

(4) putting the uranium dioxide powder obtained in the step (3) into a mould for cold press molding, and demoulding to obtain a green body;

(5) and (4) sintering the green body obtained in the step (4) in a reducing atmosphere to obtain the uranium dioxide composite fuel pellet.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (1), the concentration of the uranyl nitrate solution is 300-500g/L, the concentration of ammonia water is 10-14mol/L, and the ammonia water is added in excess to ensure that the pH of the solution is 10-12.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (1), the graphene oxide is a solution, a powder or a suspension.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (2), the drying is drying in a vacuum drying oven.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (2), the average particle size of the ammonium diuranate-graphene oxide composite is 0.1-10 μm.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (3), the calcining temperature is 300-550 ℃, and the time is 1-4 h.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (3), the temperature of the heating treatment is 600-700 ℃, and the time is 20-30 min.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (4), the pressure of cold press molding is 10-17kN, and the dwell time is 10-30 s.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (5), the volume ratio of hydrogen to argon in the reducing atmosphere is 0.1-6: 94.

In a preferred embodiment, the invention provides a method for preparing uranium dioxide composite fuel pellets by doping graphene oxide, wherein in the step (5), the sintering temperature is 1600-1800 ℃ and the time is 1-5 h.

The method for preparing the uranium dioxide composite fuel pellet by doping the graphene oxide has the beneficial effects that the novel composite fuel pellet with high thermal conductivity can be prepared by using the method for preparing the uranium dioxide composite fuel pellet by doping the graphene oxide, so that the safety of a nuclear reactor in operation is improved.

The graphene oxide surface contains a large number of hydrophilic oxygen-containing functional groups (such as hydroxyl, carboxyl, epoxy and other functional groups), so that the hydrophilicity of the graphene is improved, good wettability and surface activity are obtained, and the graphene oxide can be uniformly dispersed in an aqueous solution. In the process of adding ammonia water into uranyl nitrate to form ammonium diuranate, graphene oxide is uniformly suspended in a solution and is separated out from the solution as crystal nuclei and the ammonium diuranate in a coprecipitation mode to obtain uniformly-dispersed ammonium diuranate-graphene oxide powder, and single-layer reduced graphene oxide can be generated through high-temperature reduction after reduction-pressing-sintering, so that UO (ultra-unsaturated) uniformly doped with graphene is obtained₂And the thermal conductivity of the composite fuel pellet is improved.

The beneficial effects of the invention are embodied in that:

(1) the invention adopts graphene oxide instead of graphene for doping. Commercial graphene production widely employs a redox method, in which graphite is oxidized and dispersed into graphene oxide, and then further reduced into graphene. Therefore, compared with graphene, the graphene oxide can greatly reduce the cost of raw materials; meanwhile, the surface of the graphene oxide is rich in oxygen functional groups, so that the graphene oxide is easy to uniformly disperse in an aqueous solution and is easy to prepare a uniformly mixed precursor.

(2) According to the method, graphene oxide and an aqueous solution of uranyl nitrate are mixed, ammonia water is added to enable the graphene oxide and uranium to be coprecipitated in the form of ammonium diuranate, and graphene oxide and UO are not mixed₂Or U₃O₈The powder is mixed, so that uniform mixing of molecular scale can be achieved.

(3) The uranium dioxide core block doped with the graphene oxide with the volume ratio of 2-5% has the same formability and appearance as the conventional uranium dioxide core block, and the density reaches 95% of theoretical density; at 1000 ℃, the thermal conductivity coefficient of the pellet is obviously improved compared with that of the conventional uranium dioxide pellet.

Detailed Description

The following examples further illustrate specific embodiments of the present invention.

Example 1: preparation of uranium dioxide composite fuel pellet

(1) Taking 180mL uranyl nitrate solution with the concentration of 500g/L and the acidity of 1.5mol/L HNO₃Adding 40g of graphene oxide suspension with the content of 1% (m/m), uniformly mixing, slowly adding 400mL of concentrated ammonia water to ensure that the pH value of the solution is 10-12, and obtaining Ammonium Diuranate (ADU) sediment containing graphene oxide.

(2) And (2) filtering the precipitate obtained in the step (1), and drying, crushing and screening a filter cake in a vacuum drying oven to obtain the ammonium diuranate-graphene oxide compound with the average particle size of 0.1-10 mu m.

(3) Transferring the ammonium diuranate-graphene oxide compound obtained in the step (2) into a quartz boat, feeding the quartz boat into a muffle furnace, introducing nitrogen, heating to 350 ℃, heating to 550 ℃, calcining for 2 hours, and firing into UO₃. Then the temperature is raised to 680 ℃ and the nitrogen is switched to hydrogen to reduce for 15min so as to reduce the UO₃Reduction to UO₂. And (4) cutting off the power and reducing the temperature, cutting the gas back into nitrogen, discharging after the furnace temperature is reduced to room temperature, and sieving the material by a 50-mesh sieve to obtain uranium dioxide powder.

(4) And (4) filling the uranium dioxide powder obtained in the step (3) into a mould for cold press molding (17kN, the pressure maintaining time is 10s), and demoulding to obtain a green body.

(5) And (3) placing the green body obtained in the step (4) in a molybdenum crucible, placing the molybdenum crucible in a high-temperature sintering furnace, and carrying out curing sintering (the sintering temperature is 1600-1800 ℃ and the sintering time is 2-4h) in a reducing atmosphere (the volume ratio of hydrogen to argon in the reducing atmosphere is 6:94) to obtain the uranium dioxide composite fuel pellet with the theoretical density of 95%, smooth and complete surface and no defects such as cracks, cap falling and the like.

Example 2: detection of prepared intermediate product and finally obtained uranium dioxide composite fuel pellet

The intermediate product prepared in example 1 and the uranium dioxide composite fuel pellet finally obtained were examined as follows.

(1) Uranium dioxide composite powder detection

According to GB/T118472008 BET volumetric method for determining specific surface area of uranium dioxide powder, the specific surface area of uranium dioxide composite powder prepared by doping graphene oxide is detected, and the composite specific surface area is 10.05m²(g) UO without addition of graphene oxide₂The specific surface area of the control sample was 3.01m²/g。

The uranium-oxygen ratio is determined according to GB/T11842-89 thermogravimetry for determining uranium-oxygen atomic ratio of uranium dioxide powder and core block, and the result shows that the uranium-oxygen ratio of the uranium dioxide composite powder prepared by doping graphene oxide is 2.11, while the uranium-oxygen ratio of the control sample UO not prepared by doping graphene oxide is 2.11₂The powder oxygen-uranium ratio is 2.12.

(2) Uranium dioxide composite fuel pellet detection

According to GB/T11927-.

And detecting the uranium content (mass percent) of the uranium dioxide composite fuel pellet according to GB/T11842-89 thermogravimetry for determining the atomic ratio of uranium oxide to uranium dioxide of uranium dioxide powder and pellet.

The results are shown in table 1 below.

TABLE 1 detection results of pellet density, open porosity and uranium content of uranium dioxide composite fuel

Sample (I)	Density (g/cm)3)	Open porosity (%)	U(％)
				UO2Core block	9.53	9.37	88.08
Composite fuel pellet	10.49	0.39	87.62

According to the detection results of the density and the open porosity, the following characteristics can be seen: the density of the uranium dioxide composite fuel pellet is larger than that of pure UO₂Density of the pellets; the porosity of the opening of the uranium dioxide composite fuel pellet is small mainly because of the graphene to UO₂Defects such as air holes and the like generated in the core block matrix are filled, so that the physical properties of the core block are improved to a certain degree.

The uranium content detection result shows that the uranium content of the doped pellets is not obviously reduced.

(3) Detection of heat conductivity coefficient of uranium dioxide composite fuel pellet

The thermal conductivity of the uranium dioxide composite fuel pellet is detected according to a laser flash method specified in GB/T5598-2015 beryllium oxide ceramic thermal conductivity determination method. As a result, at 1000 ℃, the thermal conductivity of the uranium dioxide fuel pellet not prepared by doping graphene oxide was 0.9W/m × K, the thermal conductivity of the uranium dioxide composite fuel pellet prepared in example 1 was 3.0W/m × K, and the thermal conductivity improvement effect of the uranium dioxide composite fuel pellet was significant.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is intended to include such modifications and variations. The foregoing examples or embodiments are merely illustrative of the present invention, which may be embodied in other specific forms or in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the invention should be indicated by the appended claims, and any changes that are equivalent to the intent and scope of the claims should be construed to be included therein.