Nuclear fuel and preparation method thereof

Abstract

The invention belongs to the field of nuclear fuels, and particularly relates to a nuclear fuel and a preparation method thereof.The nuclear fuel comprises a uranium-containing fuel, a U2CrN3 coating layer and a CrN coating layer which are sequentially arranged from inside to outside; the thickness of the U2CrN3 coating layer ranges from 10 nm to 20 nm. The thickness of the CrN coating layer ranges from 20 nm to 40 nm. The particle size of the uranium-containing fuel is 5-20 [mu] m. The U2CrN3 coating layer and the CrN coating layer are arranged on the surface of the uranium-containing fuel, so that double protection on the uranium-containing fuel is realized, the oxidation resistance and the chemical stability of the uranium-containing fuel are effectively improved, and the weight gain rate of the nuclear fuel can be lower than 4% after the nuclear fuel is oxidized by steam at the high temperature of 400 DEG C for 4 hours.

Description

Technical Field

[0001] This invention belongs to the field of nuclear fuel, specifically relating to a nuclear fuel and its preparation method. Background Technology

[0002] Nitride nuclear fuel is an important direction for future nuclear fuel development. It possesses advantages in uranium density, stability, mechanical properties, thermal properties, and accident resistance, showing great research potential and development prospects. However, nitride nuclear fuel has poor oxidation and corrosion resistance. Since the fuel core in the most widely used pressurized water reactor (PWR) operates under high-temperature, high-pressure water conditions in the primary loop, its application in PWRs is limited. Currently, the main modification techniques for improving the oxidation resistance of nitride nuclear fuel include second-phase composite and doping. Second-phase composite involves adding an appropriate proportion of uranium oxide (UO2) or other non-fuel materials (such as chromium nitride CrN, aluminum nitride AlN, zirconium nitride ZrN, etc.) with good oxidation resistance to uranium nitride (UN) fuel pellets for modification. Doping involves adding appropriate proportions of Th, Cr, Zr, Al, etc., to UN fuel pellets for modification. However, existing technologies mostly modify fuel from a macroscopic perspective, which has very limited effect on improving the oxidation resistance of UN nuclear fuel. Furthermore, the second-phase composite method significantly reduces the uranium density of the fuel. Summary of the Invention

[0003] In order to overcome at least one of the technical problems existing in the prior art, one of the objectives of the present invention is to provide a nuclear fuel.

[0004] The second objective of this invention is to provide a method for preparing nuclear fuel.

[0005] The third objective of this invention is to provide a nuclear fuel pellet.

[0006] The fourth objective of this invention is to provide a fuel rod.

[0007] The fifth objective of this invention is to provide a nuclear reactor.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] The first aspect of the present invention provides a nuclear fuel comprising, from the inside out, uranium-containing fuel, a U2CrN3 coating layer, and a CrN coating layer.

[0010] In some embodiments of the present invention, the thickness of the U2CrN3 coating layer is 10-20 nm.

[0011] In some embodiments of the present invention, the thickness of the CrN coating layer is 20–40 nm.

[0012] In some embodiments of the present invention, the particle size of the uranium-containing fuel is 5 to 20 μm.

[0013] In some embodiments of the present invention, the uranium-containing fuel includes at least one of uranium nitride, uranium oxide, and uranium silicide.

[0014] A second aspect of the present invention provides a method for preparing the nuclear fuel described in the first aspect of the present invention, comprising the following steps:

[0015] Raw materials including chromium salts, organic acids, and solvents are mixed to obtain a sol; then the sol is mixed with carbon black, and then heated and mixed with uranium-containing fuel to obtain gel-coated uranium-containing fuel;

[0016] The uranium-containing fuel coated with the gel was heat-treated and cooled in a nitrogen-containing reducing atmosphere to obtain the nuclear fuel.

[0017] In some embodiments of the present invention, the temperature of the heat treatment is 1200–1400°C.

[0018] In some embodiments of the present invention, the heat treatment time is 2 to 5 hours.

[0019] In some embodiments of the present invention, the heating rate of the heat treatment is 10-15°C / min.

[0020] In some embodiments of the present invention, the heat treatment is carried out under a mixture of hydrogen and nitrogen.

[0021] In some embodiments of the present invention, the gas flow rate during the heat treatment is 100 to 300 mL / min.

[0022] In some embodiments of the present invention, the cooling step is as follows: cooling to 580-620°C at a cooling rate of 5-10°C / min, and then allowing it to cool naturally.

[0023] In some embodiments of the present invention, the molar ratio of the chromium salt to the organic acid is 1:(2 to 2.5).

[0024] In some embodiments of the present invention, the molar ratio of the chromium salt to the carbon black is 1:(2-5).

[0025] In some embodiments of the present invention, the molar ratio of the chromium salt to the uranium-containing fuel is 1:(5-8).

[0026] A third aspect of the present invention provides a nuclear fuel pellet, the material for which the nuclear fuel pellet is prepared includes the nuclear fuel described in the first aspect of the present invention or the nuclear fuel prepared by the method described in the second aspect of the present invention.

[0027] A fourth aspect of the invention provides a fuel rod comprising the nuclear fuel pellets described in the third aspect of the invention.

[0028] A fifth aspect of the invention provides a reactor comprising the fuel rods described in the fourth aspect of the invention.

[0029] The beneficial effects of this invention are: by setting a double coating layer of U2CrN3 and CrN on the surface of uranium fuel, this invention achieves double protection for uranium fuel, thereby effectively improving the oxidation resistance and chemical stability of uranium fuel, and enabling the nuclear fuel to have a weight gain rate of less than 4% after being oxidized by high-temperature steam at 400°C for 4 hours.

[0030] Furthermore, this invention employs a nanoscale double-layer coating method to modify uranium-containing fuel. Without affecting the uranium density of the uranium-containing fuel, it can significantly improve the oxidation resistance and chemical stability of the uranium-containing fuel, thus enabling the pellets and fuel rods made from nuclear fuel as raw materials to have a longer service life.

[0031] The preparation method of this invention first forms a gel coating layer on the surface of uranium-containing fuel, and then heat-treats it in a nitrogen-containing atmosphere, thereby forming a uniform and completely coated double layer on the surface of uranium-containing fuel. The preparation method is simple and easy to operate, the preparation process is short, and it can be mass-produced industrially. Attached Figure Description

[0032] Figure 1 This is a flowchart of the preparation process of the nitride nuclear fuel in Example 1.

[0033] Figure 2 This is a schematic diagram of the structure of the nitride nuclear fuel in Example 1. Detailed Implementation

[0034] The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings and examples, but the implementation and protection of the present invention are not limited thereto. It should be noted that any processes not specifically described in detail below are those that can be implemented or understood by those skilled in the art by referring to the prior art. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0035] In some embodiments of the present invention, the present invention provides a nuclear fuel comprising, from the inside out, uranium-containing fuel, a U2CrN3 coating layer, and a CrN coating layer.

[0036] This invention provides dual protection for uranium-containing fuel by setting a double coating layer (i.e., a U2CrN3 coating layer and a CrN coating layer) on the surface of the uranium-containing fuel, thereby effectively improving the oxidation resistance and chemical stability of the uranium-containing fuel, and enabling the nuclear fuel to have a weight gain rate of less than 4% after being oxidized by high-temperature steam at 400°C for 4 hours.

[0037] In some embodiments of the present invention, the nuclear fuel comprises the following raw materials: uranium-containing fuel, chromium salt, organic acid, and carbon black.

[0038] In this invention, chromium salts and organic acids react to form a gel that coats the surface of uranium-containing fuel, and carbon black acts as a reducing agent to participate in the nitriding reaction, promoting the formation of U2CrN3 coating layer and CrN coating layer.

[0039] In some embodiments of the present invention, the molar ratio of the chromium salt to the organic acid is 1:(2-2.5); in some embodiments of the present invention, the molar ratio of the chromium salt to the organic acid is any value or a range formed by any two of 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, and 1:2.5. When the chromium salt and organic acid are within the range of the present invention, a uniform and complete coating layer can be formed on the surface of the uranium-containing fuel, and the thickness of the coating layer is appropriate, which can give the nuclear fuel both good oxidation resistance and maintain a high uranium content.

[0040] In some embodiments of the present invention, the molar ratio of chromium salt to carbon black is 1:(2-5); in some embodiments of the present invention, the molar ratio of chromium salt to carbon black is any value of 1:2, 1:3, 1:4, 1:5 or a range formed by any two of these values. The role of carbon black in the present invention is to participate in the nitriding reaction as a reducing agent, promoting the formation of the U2CrN3 coating layer and the CrN coating layer.

[0041] In some embodiments of the present invention, the molar ratio of chromium salt to uranium-containing fuel is 1:(5-8); in some embodiments of the present invention, the molar ratio of chromium salt to uranium-containing fuel is any value of 1:5, 1:6, 1:7, 1:8, or a range formed by any two of these values. When the chromium salt and uranium-containing fuel are within the ranges of the present invention, a coating layer of suitable thickness can be formed on the surface of the uranium-containing fuel.

[0042] In some embodiments of the present invention, the thickness of the U2CrN3 coating layer is 10-20 nm; in some embodiments of the present invention, the thickness of the U2CrN3 coating layer is any value of 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm or a range formed by any two of these values.

[0043] In some embodiments of the present invention, the thickness of the CrN coating layer is 20–40 nm; in some embodiments of the present invention, the thickness of the CrN coating layer is any value or a range formed by any two of 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, 30 nm, 32 nm, 34 nm, 36 nm, 38 nm, and 40 nm.

[0044] When the thicknesses of the U2CrN3 coating and the CrN coating are within the ranges defined in this invention, the nuclear fuel can possess both good oxidation resistance and a high uranium content.

[0045] In some embodiments of the present invention, the particle size of the uranium-containing fuel is 5–20 μm; in other embodiments, the particle size of the uranium-containing fuel is any value or a range formed by any combination of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, and 20 μm. When the particle size of the uranium-containing fuel is within the range defined by the present invention, the nuclear fuel can have higher thermal conductivity, higher mechanical strength, and better structural stability.

[0046] In some embodiments of the present invention, the uranium-containing fuel includes at least one of uranium nitride, uranium oxide, and uranium silicide.

[0047] In some embodiments of the present invention, the present invention also provides a method for preparing the above-mentioned nuclear fuel, comprising the following steps:

[0048] Raw materials including chromium salts, organic acids, and solvents are mixed to obtain a sol; then the sol is mixed with carbon black, then heated and mixed with uranium-containing fuel, and dried to obtain gel-coated uranium-containing fuel.

[0049] The uranium-containing fuel coated with the gel was heat-treated and cooled in a nitrogen-containing reducing atmosphere to obtain the nuclear fuel.

[0050] The drying process allows the dry gel to coat the surface of the uranium-containing fuel, facilitating subsequent thermal treatment steps.

[0051] In some embodiments of the present invention, the chromium salt includes at least one of chromium nitrate, chromium chloride, chromium hypochlorite, and chromium sulfate.

[0052] In some embodiments of the present invention, the chromium salt is chromium nitrate nonahydrate.

[0053] In some embodiments of the present invention, the organic acid includes at least one selected from citric acid, acetic acid, oxalic acid, lactic acid, L-tartaric acid, and L-malic acid.

[0054] In some embodiments of the present invention, the solvent includes water; in some embodiments of the present invention, the solvent is deionized water.

[0055] Using the above-mentioned raw materials can reduce raw material costs and has high environmental performance.

[0056] In some embodiments of the present invention, the step of mixing raw materials including chromium salt, organic acid and solvent specifically involves mixing the raw materials including chromium salt, organic acid and solvent, and then stirring for 30 to 60 minutes to form a sol.

[0057] In some embodiments of the present invention, the stirring time is any value or a range formed by any two of 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, and 60 min. When the stirring time is 30 to 60 min, the chromium salt and organic acid can react fully to form a sol.

[0058] In some embodiments of the present invention, the step of mixing the sol with carbon black is as follows: adding carbon black to the sol and sonicating for 60-90 minutes to make the carbon black uniformly dispersed.

[0059] In some embodiments of the present invention, the ultrasonic time is any value or a range formed by any two of 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, and 90 min. When the ultrasonic time is 60 to 90 min, the carbon black can be fully dispersed in the sol, avoiding the aggregation of carbon black particles, which would lead to inconsistent reduction and nitriding reactions during subsequent heat treatment, resulting in poor compositional uniformity of the coating layer on the surface of the uranium-containing fuel.

[0060] In some embodiments of the present invention, the drying temperature is 60–80°C; in other embodiments, the drying temperature is any value of 60°C, 65°C, 70°C, 75°C, 80°C, or 85°C, or a range formed by any two of these values. At this drying temperature, the solvent can be removed, resulting in lower energy consumption and no impact on subsequent heat treatment.

[0061] In some embodiments of the present invention, the drying time is 12–24 hours; in other embodiments, the drying time is any value or a range formed by any two of the following: 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, and 24 hours. At this drying time, the drying effect is good and the energy consumption is low.

[0062] In some embodiments of the present invention, the heating and mixing temperature is 90–95°C; in other embodiments, the heating and mixing temperature is any value of 90°C, 91°C, 92°C, 93°C, 94°C, or 95°C, or a range formed by any two of these values. At this temperature, the gel can fully coat the surface of the uranium-containing fuel, forming a uniform coating layer, which can then be dried to form a dry gel-coated uranium-containing fuel.

[0063] In some embodiments of the present invention, the heating and mixing time is 6 to 12 hours; in other embodiments, the heating and mixing time is any value of 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours, or a range formed by any two of these values. When the heating and mixing time is within the range defined by the present invention, the chromium salt and organic acid can fully react to form a gel coating on the surface of the uranium-containing fuel, while also avoiding increased energy consumption caused by excessively long heating and mixing times.

[0064] In some embodiments of the present invention, the carbon black particle size is 100–200 nm; in other embodiments, the carbon black particle size is any value or a range formed by any combination of 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, and 200 nm. When the carbon black particle size is 100–200 nm, it can be fully dispersed in the sol during ultrasonic dispersion to avoid the aggregation of carbon black particles, and carbon black with this particle size has a high specific surface area, which is beneficial to the subsequent nitriding and formation of the coating layer.

[0065] In some embodiments of the present invention, the heat treatment temperature is 1200–1400°C; in other embodiments, the heat treatment temperature is any value or a range formed by any combination of 1200°C, 1220°C, 1240°C, 1260°C, 1280°C, 1300°C, 1320°C, 1340°C, 1360°C, 1380°C, and 1400°C. At this heat treatment temperature, the formation of the U2CrN3 coating layer and the CrN coating layer can be ensured. If the heat treatment temperature is too low, sufficient nitriding cannot be achieved; if the heat treatment temperature is too high, energy will be wasted and byproducts will easily be generated.

[0066] In some embodiments of the present invention, the heat treatment time is 2 to 5 hours; in other embodiments, the heat treatment time is any value of 2 hours, 3 hours, 4 hours, or 5 hours, or a range formed by any two of these values. When the heat treatment temperature is within the range defined by the present invention, the gel can be fully nitrided to form a U2CrN3 coating layer and a CrN coating layer. If the heat treatment time is too short, the nitriding reaction will be incomplete; if the heat treatment time is too long, the energy consumption will be high, and the cost will be high.

[0067] In some embodiments of the present invention, the heating rate of the heat treatment is 10–15 °C / min; in other embodiments, the heating rate of the heat treatment is any value or a range formed by any combination of 10 °C / min, 11 °C / min, 12 °C / min, 13 °C / min, 14 °C / min, 15 °C / min, 16 °C / min, 17 °C / min, 18 °C / min, 19 °C / min, and 20 °C / min. At this heating rate, the U₂CrN₃ coating layer and the CrN coating layer can be slowly and stably formed and coated on the surface of the uranium-containing fuel.

[0068] In some embodiments of the present invention, the heat treatment is carried out under a mixture of hydrogen and nitrogen.

[0069] In some embodiments of the present invention, the nitrogen-containing reducing atmosphere consists of 4-10% hydrogen and 90-96% nitrogen by volume. The role of hydrogen in this invention is to remove excess carbon from the nuclear fuel. This specific atmosphere ratio ensures sufficient nitrogen to guarantee the amount of nitrogen required for the formation of the U₂CrN₃ and CrN coatings.

[0070] In some embodiments of the present invention, the gas flow rate during heat treatment is 100–300 mL / min; in other embodiments, the gas flow rate during heat treatment is any value or a range formed by any combination of 100 mL / min, 120 mL / min, 140 mL / min, 160 mL / min, 180 mL / min, 200 mL / min, 220 mL / min, 240 mL / min, 260 mL / min, 280 mL / min, and 300 mL / min. Within the gas flow rate range defined by the present invention, the formation of the U2CrN3 coating layer and the CrN coating layer can be ensured, and gas waste can be avoided.

[0071] In some embodiments of the present invention, the heat treatment is carried out in a graphite crucible. Reacting in a graphite crucible avoids the participation of components from the reaction vessel in the reaction, thus preventing side reactions.

[0072] In some embodiments of the present invention, the heat treatment is performed using at least one heating method selected from resistance heating, microwave heating, and plasma heating.

[0073] In some embodiments of the present invention, the cooling step is as follows: cooling to 580-620°C at a cooling rate of 5-10°C / min, and then allowing it to cool naturally.

[0074] In some embodiments of the present invention, the cooling rate is any value of 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or a range formed by any two of these values. At this cooling rate, the U2CrN3 coating layer and the CrN coating layer can remain stable, avoiding side reactions.

[0075] In some embodiments of the present invention, the temperature is reduced at a rate of 5 to 10 °C / min to any value of 580 °C, 590 °C, 600 °C, 610 °C, 620 °C, or any range formed by both.

[0076] In some embodiments of the present invention, the molar ratio of the chromium salt to the organic acid is 1:(2-2.5); in some embodiments of the present invention, the molar ratio of the chromium salt to the organic acid is any value or a range formed by any two of 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, and 1:2.5. When the chromium salt and organic acid are within the range of the present invention, a uniform and complete coating layer can be formed on the surface of the uranium-containing fuel, and the thickness of the coating layer is appropriate, which can give the nuclear fuel both good oxidation resistance and maintain a high uranium content.

[0077] In some embodiments of the present invention, the molar ratio of chromium salt to carbon black is 1:(2-5); in some embodiments of the present invention, the molar ratio of chromium salt to carbon black is any value of 1:2, 1:3, 1:4, 1:5 or a range formed by any two of these values. The role of carbon black in the present invention is to participate in the nitriding reaction as a reducing agent, promoting the formation of the U2CrN3 coating layer and the CrN coating layer.

[0078] In some embodiments of the present invention, the molar ratio of chromium salt to uranium-containing fuel is 1:(5-8); in some embodiments of the present invention, the molar ratio of chromium salt to uranium-containing fuel is any value of 1:5, 1:6, 1:7, 1:8, or a range formed by any two of these values. Carbon black in the present invention acts as a reducing agent in the nitriding reaction, promoting the formation of the U2CrN3 coating layer and the CrN coating layer.

[0079] In some embodiments of the present invention, the present invention also provides a nuclear fuel pellet, the material for which the nuclear fuel pellet is prepared includes the nuclear fuel described above or the nuclear fuel prepared by the above method.

[0080] In some embodiments of the present invention, the present invention also provides a fuel rod comprising the aforementioned nuclear fuel pellets.

[0081] In some embodiments of the present invention, the present invention also provides a reactor comprising the above-described fuel rods.

[0082] The specific implementation of the present invention will be further described in detail below with reference to specific embodiments:

[0083] Example 1

[0084] This example provides a method for preparing nitride nuclear fuel, the process flow diagram of which is shown below. Figure 1 As shown, the specific steps include:

[0085] 5.0g of chromium nitrate and 4.8g of citric acid were dissolved in 80mL of deionized water and magnetically stirred for 60min to form a sol; 0.3g of nano carbon black particles were added and ultrasonicated for 60min to disperse them evenly; 20g of UN powder was added, heated to 90℃ and magnetically stirred for 6h to obtain gel-coated UN particles;

[0086] The gel-coated UN particles were dried at 60℃ for 12 hours, placed in a graphite crucible, and heated to 1200℃ in a tube furnace at a rate of 10℃ / min, holding for 2 hours to complete the heat treatment. During the cooling process, a cooling rate of 5℃ / min was used to control the cooling above 600℃, and furnace cooling was used below 600℃. A mixed gas of N2 and H2 was introduced throughout the nitriding heat treatment process, with H2 accounting for 4% by volume and N2 accounting for 96% by volume. The flow rate of the mixed gas was 300 mL / min, resulting in a double-layer coated nitride nuclear fuel powder (denoted as UN@U2CrN3@CrN). The outer coating layer of the double-layer coated nitride nuclear fuel powder is a 20 nm thick CrN coating layer, and the inner coating layer is a 10 nm thick U2CrN3 coating layer. Its structural schematic diagram is shown below. Figure 2 As shown.

[0087] Example 2

[0088] This example provides a method for preparing nitride nuclear fuel, including the following steps:

[0089] Dissolve 5.0g chromium nitrate and 4.8g citric acid in 100mL deionized water and stir magnetically for 60min to form a sol; add 0.4g nano carbon black particles and sonicate for 60min to disperse them evenly; add 20g UN powder, heat to 90℃ and stir magnetically for 12h to obtain gel-coated UN particles;

[0090] The gel-coated UN particles were dried at 60℃ for 12 hours, placed in a graphite crucible, and heated to 1400℃ in a tube furnace at a rate of 10℃ / min and held for 3 hours to complete the heat treatment. During the cooling process, the cooling rate was controlled at 5℃ / min above 600℃, and furnace cooling was used below 600℃. A mixture of N2 and H2 gas was introduced throughout the nitriding heat treatment process, with H2 accounting for 5% by volume and N2 accounting for 95% by volume. The flow rate of the mixed gas was 200 mL / min. A double-layer coated nitride nuclear fuel powder was obtained, with an outer coating layer of 30 nm thick CrN coating layer and an inner coating layer of 15 nm thick U2CrN3 coating layer.

[0091] Example 3

[0092] This example provides a method for preparing nitride nuclear fuel, including the following steps:

[0093] Dissolve 5.0g of chromium nitrate and 6g of citric acid in 150mL of deionized water and stir magnetically for 90min to form a sol; add 0.7g of nano carbon black particles and sonicate for 120min to disperse them evenly; add 20g of UN powder, heat to 90℃ and stir magnetically for 12h to obtain gel-coated UN particles.

[0094] The gel-coated UN particles were dried at 60℃ for 12 hours, placed in a graphite crucible, and heated to 1400℃ at a rate of 10℃ / min in a microwave furnace for 5 hours to complete the heat treatment. During the cooling process, the cooling rate was controlled at 5℃ / min above 600℃, and furnace cooling was used below 600℃. A mixture of N2 and H2 gas was introduced throughout the nitriding heat treatment process, with H2 accounting for 5% by volume and N2 accounting for 95% by volume. The flow rate of the mixed gas was 300 mL / min. A double-layer coated nitride nuclear fuel powder was obtained, with an outer coating layer of 40 nm thick CrN coating layer and an inner coating layer of 20 nm thick U2CrN3 coating layer.

[0095] The present invention uses transmission electron microscopy (TEM) to test the thickness of the CrN coating layer and the U2CrN3 coating layer, and uses electron diffraction (SAED) to test the composition of the CrN coating layer and the U2CrN3 coating layer.

[0096] Comparative Example 1

[0097] This example provides a method for preparing nitride nuclear fuel, including the following steps:

[0098] Dissolve 2.5g chromium nitrate and 4.8g citric acid in 80mL of deionized water and stir magnetically for 60min to form a sol; add 0.15g nano carbon black particles and sonicate for 60min to disperse them evenly; add 20g UN powder, heat to 90℃ and stir magnetically for 6h to obtain gel-coated UN particles;

[0099] The gel-coated UN particles were dried at 60℃ for 12 hours, placed in a graphite crucible, and heated to 1200℃ in a tube furnace at a rate of 10℃ / min and held for 2 hours to complete the heat treatment. During the cooling process, the cooling rate was controlled at 5℃ / min above 600℃, and furnace cooling was used below 600℃. A mixture of N2 and H2 gas was introduced throughout the nitriding heat treatment process, with H2 accounting for 4% by volume and N2 accounting for 96% by volume. The flow rate of the mixed gas was 300 mL / min. A single-layer coated nitride nuclear fuel powder (denoted as UN@CrN) was obtained, with a CrN coating layer of 20 nm thickness on the surface of the UN particles.

[0100] Comparative Example 2

[0101] This example provides a method for preparing nitride nuclear fuel, including the following steps:

[0102] 1.3g of chromium nitrate and 4.8g of citric acid were dissolved in 80mL of deionized water and magnetically stirred for 60min to form a sol; 0.08g of nano carbon black particles were added and sonicated for 60min to disperse them evenly; 20g of UN powder was added, heated to 90℃ and magnetically stirred for 6h to obtain gel-coated UN particles;

[0103] The gel-coated UN particles were dried at 60℃ for 12 hours, placed in a graphite crucible, and heated to 1200℃ at a rate of 10℃ / min in a microwave furnace for 2 hours to complete the heat treatment. During the cooling process, the cooling rate was controlled at 5℃ / min above 600℃, and furnace cooling was used below 600℃. A mixture of N2 and H2 gas was introduced throughout the nitriding heat treatment process, with H2 accounting for 4% by volume and N2 accounting for 96% by volume. The flow rate of the mixed gas was 100 mL / min. A single-layer coated nitride nuclear fuel powder was obtained, and the surface of the UN particles was coated with a CrN coating layer with a thickness of 10 nm.

[0104] Comparative Example 3

[0105] This example uses uncoated pure UN particles.

[0106] Performance testing:

[0107] The oxidation resistance of the nuclear fuels in Examples 1-3 and Comparative Examples 1-3 was tested using a high-temperature steam oxidation experiment. The specific testing method was as follows: First, the obtained nitride nuclear fuel powder was pressed into blocks. The samples were placed in a quartz boat and pushed into the constant-temperature zone of the furnace. The temperature was increased to 400°C at a rate of 5°C / min, while an inert gas was introduced. After the temperature stabilized, the steam generator was turned on, and 80% partial pressure water steam was introduced. The temperature was maintained for 4 hours. The mass change of the samples after high-temperature steam oxidation was measured, and then the weight gain rate of the samples was tested. Weight gain rate = (m... 测试后 -m 测试前 ) / m 测试后 *100%; The test results obtained according to the above test method are shown in Table 1 below.

[0108] Table 1. Results of nuclear fuel weight gain rate test

[0109] Test group Weight gain rate Example 1 3.1％ Example 2 2.4％ Example 3 1.5％ Comparative Example 1 5.9％ Comparative Example 2 10.6％ Comparative Example 3 43.2％

[0110] Therefore, compared with the use of a single-layer coating in Comparative Examples 1-2, Examples 1-3 can significantly improve the oxidation resistance of nuclear fuel by setting a U2CrN3 coating between the uranium nitride and chromium nitride coating layers, so that its weight gain in a high-humidity environment at 400°C is less than 4%.

[0111] In summary, this invention proposes a solution to improve the oxidation resistance of UN at the micro-nano scale. A CrN layer is used as the first protective layer to protect the UN grains, while a U2CrN3 layer is formed between the two as a second protective layer, achieving dual protection for UN and effectively improving its oxidation resistance and chemical stability. The double-layered nitride nuclear fuel powder formed by this preparation method can be directly used for subsequent sintering to obtain fuel pellets. Each UN grain is coated inside the pellet, avoiding the problems of incomplete and uneven macroscopic protective layers and reducing the complexity of subsequent operations. Weight gain after oxidation at 400℃ for 4 hours: Double-layered nitride nuclear fuel of this invention < Single-layered nitride nuclear fuel < Pure UN nuclear fuel, i.e., oxidation resistance: Double-layered nitride nuclear fuel of this invention > Single-layered nitride nuclear fuel > Pure UN nuclear fuel.

[0112] The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A nuclear fuel, characterized in that: It includes, from the inside out, uranium-containing fuel, a U2CrN3 coating layer, and a CrN coating layer.

2. The nuclear fuel according to claim 1, characterized in that: The thickness of the U2CrN3 coating layer is 10–20 nm; And / or, the thickness of the CrN coating layer is 20–40 nm; And / or, the particle size of the uranium-containing fuel is 5 to 20 μm.

3. The nuclear fuel according to claim 1, characterized in that: The uranium-containing fuel includes at least one of uranium nitride, uranium oxide, and uranium silicide.

4. The method for preparing nuclear fuel according to any one of claims 1 to 3, characterized in that: Includes the following steps: Raw materials including chromium salts, organic acids, and solvents are mixed to obtain a sol; then the sol is mixed with carbon black, then heated and mixed with uranium-containing fuel, and dried to obtain gel-coated uranium-containing fuel. The uranium-containing fuel coated with the gel was heat-treated and cooled in a nitrogen-containing reducing atmosphere to obtain the nuclear fuel.

5. The method for preparing nuclear fuel according to claim 4, characterized in that: The heat treatment has at least one of the following characteristics: (a1) The heat treatment temperature is 1200–1400 °C; (a2) The heat treatment time is 2 to 5 hours; (a3) The heating rate of the heat treatment is 10-15℃ / min; (a4) The heat treatment is carried out under a mixture of hydrogen and nitrogen; (a5) The gas flow rate during the heat treatment is 100-300 mL / min.

6. The method for preparing nuclear fuel according to claim 4, characterized in that: The cooling process involves cooling the temperature to 580-620°C at a rate of 5-10°C / min, followed by natural cooling.

7. The method for preparing nuclear fuel according to claim 4, characterized in that: The molar ratio of the chromium salt to the organic acid is 1:(2-2.5); And / or, the molar ratio of the chromium salt to the carbon black is 1:(2-5); And / or, the molar ratio of the chromium salt to the uranium-containing fuel is 1:(5-8).

8. A nuclear fuel pellet, characterized in that: The materials used to prepare the nuclear fuel pellets include the nuclear fuel described in any one of claims 1 to 3 or the nuclear fuel prepared by the method described in any one of claims 4 to 7.

9. A fuel rod, characterized in that: Includes the nuclear fuel pellets as described in claim 8.

10. A reactor, characterized in that: Includes the fuel rod as described in claim 9.

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