Neptunium-uranium mixed target for heavy water reactor irradiation production of plutonium-238 and preparation method of neptunium-uranium mixed target

Abstract

The invention provides a neptunium-uranium mixed target for producing plutonium 238 through irradiation of a heavy water reactor. The neptunium-uranium mixed target comprises at least one fuel rod, the fuel rod comprises at least one fuel rod; the fuel rod comprises a first pellet, the first pellet comprises neptun-237 and enriched uranium, and R * mU1 = 0.72% * (mU1 + mNp) is met, mU1 represents the mass of the enriched uranium in the first pellet, and mNp represents the mass of the enriched uranium in the first pellet. MNp represents the mass of the neptun-237 in the first pellet; r represents the content of uranium-235 in the enriched uranium.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear fuel, and in particular relates to a neptunium-uranium mixed target for producing plutonium-238 isotopes by heavy water reactor irradiation and a preparation method thereof. Background Art

[0002] Pu-238 is an important radioactive isotope that is widely used in fields such as spacecraft power supply and medical radioisotope batteries. The production of Pu-238 usually includes four steps: extracting neptunium raw materials in the spent fuel reprocessing plant, preparing neptunium targets, irradiating neptunium targets in the reactor, and extracting Pu-238. At present, Pu-238 is mainly produced by irradiating neptunium-237 targets in reactors. Traditional methods usually use research reactors (such as the High Flux Isotope Reactor HFIR) to produce Pu-238, but due to the high operating costs and limited production capacity of research reactors, the production cost of Pu-238 remains high.

[0003] In recent years, the production of plutonium-238 by irradiating neptunium targets in nuclear power plant reactors (such as heavy water reactors) has gained increasing attention. Compared to research reactors, nuclear power plant reactors offer advantages such as lower operating costs, larger capacity, and continuous operation. They can simultaneously produce plutonium-238 while generating electricity, significantly reducing production costs. However, when pure neptunium-237 targets are irradiated in heavy water reactors, their high thermal neutron absorption cross-section can lead to localized reactivity deficiencies in the reactor, compromising safe and stable operation. Furthermore, using low-enriched uranium as a compensating material results in excessive amounts of total uranium (uranium-235 + uranium-238), resulting in a high concentration of isotopes other than plutonium-238, making it difficult to meet the plutonium-238 purity requirement of greater than 80%.

[0004] Therefore, improvements in the production of Pu-238 are still needed. Summary of the Invention

[0005] The present invention aims to provide a neptunium-uranium mixed target for heavy water reactor irradiation production of plutonium-238 and its preparation method. This solves the problem of insufficient local reactivity in heavy water reactors caused by pure neptunium targets and effectively reduces the content of isotopes other than plutonium-238, meeting the requirement for a plutonium-238 purity greater than 80%. Furthermore, the present invention utilizes neptunium targets irradiated in nuclear power plant heavy water reactors to produce plutonium-238, enabling simultaneous power generation and plutonium-238 production, offering significant cost advantages.

[0006] In order to achieve the above object, the present invention provides a neptunium-uranium mixed target for heavy water reactor irradiation production of plutonium-238, comprising at least one fuel rod; the fuel rod comprises a first pellet, the first pellet comprises neptunium-237 and enriched uranium, and satisfies R×m U1 =0.72%×(m U1 +m Np );

[0007] wherein m U1 represents the mass of the enriched uranium in the first pellet; m Np represents the mass of the neptunium-237 in the first pellet; R represents the content of uranium-235 in the enriched uranium.

[0008] Thus, the overall reactivity of the uranium-neptunium mixture fuel rod of the present application is equivalent to that of the original natural uranium fuel rod, thereby effectively compensating for the insufficient reactivity caused by the low reactivity of neptunium-237.

[0009] In some embodiments, the enriched uranium comprises one or more of low enriched uranium with a uranium-235 content of 3%-20%, medium enriched uranium with a uranium-235 content of 20%-80%, and high enriched uranium with a uranium-235 content greater than 80%.

[0010] In some embodiments, the density of the pellet is 10.0 g / cm 3 to 11.0 g / cm 3 .

[0011] In some embodiments, the mass ratio of the neptunium-237 to the enriched uranium is 1:4 to 99:1.

[0012] In some embodiments, the remaining material other than the pellet in each of the fuel rods respectively adopts a material with a thermal neutron macroscopic absorption cross section less than 10 barns.

[0013] In some embodiments, the fuel rod comprises a second pellet, and the second pellet comprises natural uranium.

[0014] In some embodiments, the length of the fuel rod is 500 mm to 1000 mm, and the diameter is 10 mm to 20 mm.

[0015] The present application also provides a preparation method of a neptunium-uranium mixed target for irradiation production of plutonium-238 in a heavy water reactor, wherein the neptunium-uranium mixed target comprises at least one fuel rod; and the fuel rod comprises a first pellet. The method comprises:

[0016] ball-milling neptunium-237 oxide powder and enriched uranium oxide powder for 2 hours to 10 hours to obtain a mixed powder;

[0017] extruding the mixed powder under a pressure of 100 MPa to 500 MPa to obtain a green body; and

[0018] sintering the green body at 1600 ℃ to 1800 ℃ for 2 hours to 8 hours to obtain the first pellet.

[0019] Therefore, the neptunium-uranium mixed target provided by the application has a simple preparation process and is easy to realize industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A structure schematic diagram of a neptunium-uranium mixed target provided by an embodiment of the application is shown.

[0021] Figure 2 An end view of a neptunium-uranium mixed target provided by an embodiment of the application is shown. DETAILED DESCRIPTION

[0022] The following detailed description discusses example embodiments. The detailed description contained herein should not be interpreted as a limitation on the generality of the present application. Further, although specific language can be used in describing the embodiments herein, the claims are not limited to the specific features, acts, or structures described.

[0023] Throughout this specification, unless specifically stated otherwise, the terms used herein are to be interpreted as generally used in the art. Thus, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0024] Heavy water reactors use natural uranium as fuel, which has the advantage of good neutron economy. However, due to the low content of uranium-235 (0.72%) in natural uranium, the burnup depth of the reactor is relatively low. A typical heavy water reactor fuel element is usually composed of a natural uranium dioxide (UO2) fuel pellet and a zirconium alloy cladding. For example, a CANDU reactor has a fuel rod length of 500 mm and a diameter of 10 mm to 20 mm, and the mass of natural uranium in a single fuel rod is about 500 g to 1000 g. The fuel rods are usually bundled in 37 or 28 as a fuel assembly.

[0025] In the present application, part of the natural uranium is replaced by a mixture of neptunium-237 and a certain enrichment of enriched uranium, wherein the content of uranium-235 in the enriched uranium is equivalent to that in the natural uranium, and the total amount of the enriched uranium and neptunium is equivalent to the total amount of the natural uranium before replacement. In this way, after the replacement, the overall reactivity of the uranium-neptunium mixture fuel rod is equivalent to that of the original natural uranium fuel rod, which can effectively make up for the insufficient reactivity caused by the low reactivity of neptunium-237, and effectively reduce the content of isotopes other than plutonium-238, meeting the requirement that the purity of plutonium-238 is greater than 80%. At the same time, the present application uses the heavy water reactor of a nuclear power plant to irradiate the neptunium target to produce plutonium-238, which can produce plutonium-238 while generating electricity, having a certain cost advantage.

[0026] Specifically, in a conventional heavy water reactor fuel rod, it includes a core block loaded with natural uranium m u grams, wherein the amount of uranium-235 is m 235 = 0.72% x m U , and the amount of uranium-238 is m 238 = 99.28% x m U . When replacing the core block of the present application, wherein the amount of enriched uranium is m U1 , the uranium-235 enrichment is R, and the amount of neptunium-237 is m Np , the following relationships must be met:

[0027] m U = m U1 + m Np (total mass remains unchanged)

[0028] R x m U1 = 0.72% x m U (u-235 reactivity compensation).

[0029] It should be understood that in some embodiments, the present application can involve replacement in the same fuel rod. For example, a fuel rod includes a plurality of core blocks, possibly some or all of which are core blocks of the present application, while the rest still use natural uranium core blocks.

[0030] In other embodiments, the present application can involve replacement in different fuel rods. For example, taking a rod bundle consisting of 37 fuel rods as an example, there can be some or all of the core blocks in the 37 fuel rods that are core blocks of the present application, while the rest still use natural uranium core blocks.

[0031] The present application will be further described in detail below with reference to the accompanying drawings.

[0032] Figure 1 and Figure 2 respectively show the schematic structural view and end view of the neptunium-uranium mixed target. Referring to Figure 1 and Figure 2 , the neptunium-uranium mixed target can include a bearing pad 1, a cladding tube 2, an end plug 3, an end plate 4, a core block 5, a coating 6, a spacer block 7, and a pressure tube 8.

[0033] The bearing pad 1 is an important support component, used to reduce friction between moving parts or surfaces, help reduce energy loss and prolong the service life of the component, and absorb impact and vibration to protect the structural integrity of the fuel target; and uniformly distribute the load to prevent local overload and ensure the stability and safety of the fuel target.

[0034] In the present application, there are no special requirements for the material and size of the bearing pad, and those conventionally used in the art can be used, such as zirconium alloy bearing pads.

[0035] The cladding tube 2 can be composed of an outer cladding tube and an inner cladding tube, which serves to protect and contain the fuel. The pellets 5 are loaded between the inner and outer cladding tubes, and the inner and outer cladding tubes are closed into a whole by the end plugs 3 at both ends.

[0036] In the present application, the material and size of the cladding tube are not particularly limited, and those conventionally used in the art can be used, such as any one of the following nuclear-grade materials: zirconium alloy, niobium alloy, molybdenum alloy, stainless steel, aluminum alloy, nickel-based alloy, silicon carbide. In an embodiment, the cladding tube is a zirconium alloy cladding tube.

[0037] The end plugs 3 are fixed (e.g. welded) at both ends of the cladding tube 2, which serves to close the cladding tube and ensure the sealing and structural integrity of the fuel element.

[0038] The end plate 4 is mainly used to fix and support the fuel element, which ensures the overall structural stability and operational safety of the fuel target.

[0039] Referring to Figure 2 , the spacer 7 fixes and supports the fuel element, which ensures that the fuel element maintains the correct geometric position and spacing during the operation of the reactor, and the pressure tube 8 is used to contain and protect the fuel element while bearing the internal pressure generated during operation.

[0040] The fuel rod in the neptunium-uranium mixed target provided by the present application has a length of 500 mm to 1000 mm and a diameter of 10 mm to 20 mm, and the total mass of neptunium-uranium in a single fuel rod is 500 g to 1000 g.

[0041] The size of the pellets 5 of the present application is not particularly limited, and the size conventionally used in the art can be used, for example, φ12.15x17.7mm, but is not limited thereto.

[0042] The neptunium-237 and enriched uranium mixture fuel pellet is prepared by the following steps:

[0043] The neptunium-237 oxide powder and the enriched uranium oxide powder are mixed in a mass ratio and ball milled for 4 hours.

[0044] The mixed powder is extruded at a pressure of 100 MPa to 500 MPa.

[0045] The formed blank is sintered at 1600°C to 1800°C for 2 hours to 8 hours to produce a fuel pellet with a density of 10.0 g / cm 3 to 11.0 g / cm 3 .

[0046] The prepared neptunium-237 and enriched uranium mixture fuel pellet is used to replace part of the natural uranium pellets in the heavy water reactor fuel rod, so that the overall reactivity of the replaced fuel rod is equivalent to that of the original natural uranium fuel rod.

[0047] In this application, the uranium-neptunium mixture fuel is loaded into a heavy water reactor and irradiated for 70 to 100 days. After the irradiation is completed, the fuel rods are removed and the plutonium-238 is separated and extracted, and then the purity of the plutonium-238 is tested.

[0048] The present invention is described in more detail below by way of examples. It should be understood that the examples described below are illustrative and are intended only to explain the present application and are not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in the art or in accordance with the product instructions were used. Reagents or instruments used without manufacturer indication are conventional products that can be obtained commercially. The raw materials used in the examples are nuclear-grade raw materials.

[0049] Embodiment one:

[0050] 14.4 kg of low-enriched uranium with a 5% uranium-235 content was mixed with 85.6 kg of neptunium-237 and ball-milled for 4 hours.

[0051] The mixed powder was then extruded at a pressure of 300 MPa.

[0052] The formed green body was sintered at 1700℃ for 4 hours to obtain a density of 10.5g / cm 3 of fuel pellets.

[0053] The resulting fuel pellets were loaded into a heavy water reactor and irradiated for 70 days. After irradiation, the fuel rods were removed and the plutonium-238 was separated and extracted. Testing showed that the plutonium-238 had a purity greater than 80%.

[0054] Example 2

[0055] 4 kg of low-enriched uranium with an 18% uranium-235 content was mixed with 96 kg of neptunium-237 and ball-milled for 4 hours.

[0056] The mixed powder was then extruded at a pressure of 300 MPa.

[0057] The formed green body was sintered at 1700℃ for 4 hours to obtain a density of 10.5g / cm 3 of fuel pellets.

[0058] The resulting fuel pellets were loaded into a heavy water reactor and irradiated for 71 days. After irradiation, the fuel rods were removed and the plutonium-238 was separated and extracted. Testing showed that the plutonium-238 had a purity greater than 80%.

[0059] Embodiment three:

[0060] The 1.4 kg of medium enriched uranium with 50% of uranium-235 is mixed with 98.6 kg of neptunium-237, and the mixture is ball-milled for 4 hours.

[0061] The mixed powder is then extruded at a pressure of 300 MPa.

[0062] The extruded body is sintered at 1700 °C for 4 hours to obtain a fuel pellet with a density of 10.5 g / cm 3 .

[0063] The obtained fuel pellet is loaded into a heavy water reactor for irradiation for 75 days. After the irradiation, the fuel rod is taken out, and plutonium-238 is separated and extracted. It is detected that the purity of the plutonium-238 is greater than 80%.

[0064] Example Four:

[0065] The 0.9 kg of high enriched uranium with 80% of uranium-235 is mixed with 99.1 kg of neptunium-237, and the mixture is ball-milled for 4 hours.

[0066] The mixed powder is then extruded at a pressure of 300 MPa.

[0067] The extruded body is sintered at 1700 °C for 4 hours to obtain a fuel pellet with a density of 10.5 g / cm 3 .

[0068] The obtained fuel pellet is loaded into a heavy water reactor for irradiation for 71 days. After the irradiation, the fuel rod is taken out, and plutonium-238 is separated and extracted. It is detected that the purity of the plutonium-238 is greater than 80%.

[0069] The neptunium-uranium mixed target provided by the present application can effectively solve the problem of insufficient local reactivity of the pure neptunium target in the heavy water reactor, improve the safety and stability of the reactor, and at the same time has a high plutonium-238 yield, which can meet the production demand of plutonium-238. In addition, the present application uses enriched uranium as a compensation material, which can effectively reduce the content of isotopes other than plutonium-238, and meet the requirement that the purity of plutonium-238 is greater than 80%. At the same time, the present application uses the heavy water reactor of the nuclear power plant to irradiate the neptunium target to produce plutonium-238, which can produce plutonium-238 while generating electricity, and has a certain cost advantage.

[0070] Although the present disclosure has been described with reference to its specific exemplary embodiments, many different alterations, modifications and the like will become apparent to those skilled in the art.

[0071] Variations of the disclosed embodiments can become apparent to those skilled in the art from the disclosure and the accompanying claims and are intended to be within the scope of the present disclosure.

Claims

1. A neptunium-uranium mixed target for heavy water reactor irradiation production of plutonium-238, comprising at least one fuel rod; the fuel rod comprises a first pellet, the first pellet comprises neptunium-237 and enriched uranium, and satisfies R×m U1 =0.72%×(m U1 +m Np ), in, m U1 represents the mass of the enriched uranium in the first pellet; m Np represents the mass of the neptunium-237 in the first pellet; and R represents the content of uranium-235 in the enriched uranium.

2. The neptunium-uranium mixed target according to claim 1, wherein: The enriched uranium includes one or more of low enriched uranium with a uranium-235 content of 3%-20%, medium enriched uranium with a uranium-235 content of 20%-80%, and highly enriched uranium with a uranium-235 content greater than 80%.

3. The neptunium-uranium mixed target according to claim 1 or 2, wherein: The density of the core block is 10.0 g / cm 3 Up to 11.0g / cm 3 .

4. The neptunium-uranium mixed target according to any one of claims 1 to 3, wherein: The mass ratio of the neptunium-237 to the enriched uranium is 1:4 to 99:

1.

5. The neptunium-uranium mixed target according to any one of claims 1 to 4, wherein: The remaining materials in each fuel rod except the pellets are made of materials with a thermal neutron macroscopic absorption cross section of less than 10 barns.

6. The neptunium-uranium mixed target according to any one of claims 1 to 5, wherein: The fuel rod includes a second pellet including natural uranium.

7. The neptunium-uranium mixed target according to any one of claims 1 to 6, wherein: The fuel rod has a length of 500 mm to 1000 mm and a diameter of 10 mm to 20 mm.

8. A method for preparing a neptunium-uranium mixed target for producing plutonium-238 by heavy water reactor irradiation, wherein the neptunium-uranium mixed target comprises at least one fuel rod; the fuel rod comprises a first pellet; the method comprises: ball milling neptunium-237 oxide powder and enriched uranium oxide powder for 2 to 10 hours to obtain a mixed powder; Extruding the mixed powder at a pressure of 100 MPa to 500 MPa to obtain a green body; The green body is sintered at 1600° C. to 1800° C. for 2 to 8 hours to obtain the first core block.

Images