Neutron shielding materials

Abstract

A neutron shielding composite material comprising a magnesium borate matrix and a tungsten boride filler is described, optionally including additional fillers such as tungsten. A method for manufacturing the composite material is also described.

Description

Technical Field

[0001] The present invention relates to materials suitable for nuclear reactor components, methods for their manufacture, and their use.

Background Art

[0002] Nuclear energy, for example a civilian fusion reactor, requires effective neutron shielding for safe operation. Tungsten boride (WB) is a very effective neutron shielding material, but it is difficult to manufacture. They typically require a binder interface, which is typically cobalt-based, to solidify into a sintered material block. These binders are problematic because they become radioactive in a fusion environment. It is possible to manufacture monolithic tungsten boride, but this is technically complex and expensive.

[0003] Other neutron shielding materials are larger and less effective. Therefore, it is desirable to manufacture a neutron shielding material containing tungsten boride without the drawbacks of existing tungsten boride shielding materials.

Summary of the Invention

[0004] The present invention provides a composite material comprising a matrix and a filler, wherein the matrix is magnesium borate and the filler comprises tungsten boride.

[0005] The filler may constitute 1 to 60% by volume, for example 1 to 55% by volume, or 1 to 50% by volume, for example 1 to 40% by volume (about 9 to 86% by weight) of the composite material, based on the total amount of starting materials. The filler may be fine particles. The range of suitable particle sizes (Feret particle diameter) is 0.05 to 50 μm, for example 0.05 to 40 μm, for example 0.25 to 3 μm. In addition to tungsten boride, the filler may comprise one or more of tungsten, tungsten carbide, tungsten oxide, graphite, and other suitable materials having neutron attenuation properties.

[0006] The composite material of the present invention may suitably take the form of a block. Such a block is typically ceramic. The block according to the present invention has a compressive strength of 11-13 MPa and / or 5-15 g / cm². 3 It may have a density of . The block form of the composite material of the present invention may be particularly suitable for use in modular construction. The blocks of the composite material of the present invention can be manufactured using readily available apparatus and methods and can be formed into various shapes to satisfy construction constraints.

[0007] In block form, the composite material may be manufactured so that the filler is not uniformly distributed. For example, the concentration of the filler may exhibit a gradient across the thickness of the block. The filler may include a secondary filler component in addition to tungsten boride and may exhibit a compositional gradient across the thickness of the block, for example, from 100% tungsten boride on one surface of the block composite to 100% secondary filler component on the opposite surface of the block (a tungsten boride:secondary filler ratio of 100:0 to 0:100 across the thickness of the block). The secondary filler component may be tungsten, tungsten carbide, tungsten oxide, graphite, or one or more of any other suitable materials having neutron attenuation properties.

[0008] The composite material of the present invention may be in the form of fine particles, such as ceramic or metal fine particles. In applications where a lower degree of neutron attenuation is required, the material of the present invention in the form of fine particles may be incorporated into another material. For example, concrete incorporating the material of the present invention in the form of fine particles can be used for neutron attenuation in a nuclear fission reactor plant in a smaller quantity than concrete that does not incorporate the material of the present invention, thereby making the nuclear fission reactor assembly and / or the entire energy plant more compact.

[0009] The present invention provides a neutron shielding assembly. The neutron shielding assembly includes a layer of the composite material of the present invention, positioned between a layer of tungsten. These two layers may be positioned sequentially between a radiation source and an object or organism to be protected from radiation, in any order. Preferably, the tungsten metal is positioned closest to the neutron source to slow down fast neutrons, followed by the composite material of the present invention, which can trap thermal neutrons. In a tokamak (magnetic confinement) fusion reactor, the assembly may include the composite material of the present invention interposed between the tungsten layer and the coolant layer. In this configuration, the central solenoid is shielded from neutron radiation originating from the plasma.

[0010] The present invention provides a method for producing the composite material of the present invention. This method comprises the following steps: a. A step of providing fine particles of MgO and a fine particle filler, wherein the filler comprises tungsten boride, b. A step of mixing fine particles of MgO and fine particle filler to form a first mixture, c. The first mixture is combined with boric acid to form a second mixture, d. The step of sintering the second mixture to form a composite material. Includes.

[0011] In the method of the present invention, the sintering step may be carried out at a temperature of 200–300°C. The sintering step may be carried out at a pressure of 25–35 MPa. These are relatively mild conditions for producing ceramics and materials suitable for use in nuclear applications, making the composite materials of the present invention attractive compared to materials that require very harsh manufacturing conditions. These temperatures and pressures for sintering are achievable with standard ceramic hot presses and sintering equipment, enabling easy scale-up of manufacturing. Custom shapes can be produced using this standard equipment. For example, during sintering, the composite material can take the shape of a custom die form. For example, modular, stackable blocks can be produced to enable simple custom reactor designs. Alternatively, the sintering step may be carried out at a pressure of 25–200 MPa, e.g., 30–175 MPa, e.g., 35–150 MPa, e.g., 35–100 MPa. The sintering step may include a series of alternating heat and pressure treatments at any of the temperatures or pressures described. In such a sequence, pressurization can only be initiated when the temperature of the sintering apparatus has dropped to below 100°C, for example below 75°C, for example below 50°C, for example about 25-30°C. In other such sequences, sintering may include performing a heating step at a temperature of 200-300°C, terminating the heating, and performing a pressurization step while the system is cooling. The sintering step may include simultaneous heating and pressurization at any of the temperatures and pressures described. In one example, the sintering step is performed as a single step at a temperature of 200-300°C and a pressure of 25-200 MPa, for example 25-50 MPa, i.e., the sintering step includes simultaneous heating and pressurization.

[0012] The present invention also provides the use of the composite material of the present invention as a neutron shield in a fusion reactor or nuclear fission reactor, as a radiation shield in nuclear medicine applications, or as a radiation shield in aerospace applications, preferably as a neutron shield in a fusion reactor. [Brief explanation of the drawing]

[0013] [Figure 1A] This is a SEM micrograph of a magnesium borate matrix containing tungsten and tungsten boride packing materials. [Figure 1B] This is a SEM micrograph of a magnesium borate matrix containing tungsten and tungsten boride packing materials. [Figure 1C] This is a SEM micrograph of a magnesium borate matrix containing tungsten and tungsten boride packing materials. [Figure 2] This is an XRD trace of magnesium borate containing tungsten boride filler after sintering at 1000°C. [Figure 3] This is a schematic diagram of a neutron shielding assembly containing the materials of the present invention. [Figure 4] Figure 3 is a schematic cross-sectional view of a solenoid with a neutron shielding assembly. [Figure 5] This is a graph showing the ratio of the relative toroidal magnetic field coil lifetime to the total inner shielding thickness of the material of the present invention. [Figure 6] This is a histogram of the tungsten boride particle size distribution. [Modes for carrying out the invention]

[0014] [matrix] The matrix material contains, substantially consists of, or consists of magnesium borate. The matrix acts as a binder for the filler particles, and as a result, the filler material can be more readily used, for example, as a neutron attenuation material. The magnesium borate used in the present invention may have stoichiometric Mg3B2O6. Other stoichiometric forms of magnesium borate may be present in the composite material of the present invention.

[0015] Magnesium borate and its production are described in British Patent Application Publication No. 1035811, inventor Donald Albert Gebbett. Magnesium borate is also known as jeanite.

[0016] Unlike conventional binders for tungsten boride, magnesium borate does not become highly radioactive when used in reactor components. Magnesium borate can be considered a low-activation material aimed at being safely disposed of without long-lived radionuclides within 100 years after plant shutdown. This is the main advantage of the material as a binder, as it simplifies and makes the disposal of end-of-life reactor components safer. In contrast, commonly used binders such as cobalt become highly radioactive during use and require difficult and expensive processes for safe disposal.

[0017] Another main advantage of magnesium borate as a binder for tungsten boride is the ease of its production. It can be produced and sintered at temperatures achievable in a household oven and pressures achievable with a hand press. The ease of production means that the final size and shape of the sintered block are not limited, and the size and shape of the die can be changed to enable the production of custom component shapes. The blocks can be made, for example, in unit form for stacking.

[0018] [Filler] The filler contains tungsten boride. Preferably, the tungsten boride used in the present invention has a stoichiometry of WB. WB has a tetragonal structure at temperatures below 1200 °C. Other stoichiometric tungsten borides such as W2B5 may also be used in the present invention. As used herein, "tungsten boride" refers to any stoichiometric tungsten and boron, unless otherwise specified.

[0019] Including both tungsten and boron in the filler provides a twofold advantage for neutron attenuation. When used together, tungsten and boron are the most efficient neutron shielding materials available.

[0020] The filler is preferably in particulate form. The particle size and shape are not particularly limited, but the particle diameter can preferably range from 0.05 to 50 μm, such as 0.05 to 40 μm, such as 0.05 to 30 μm, such as 0.25 to 30 μm. The particle size of the filler may also range from 0.1 to 10 μm. Other filler forms such as fibers are also possible in the present invention. A fibrous form of metal filler can be beneficial for component strength, for example, by providing pseudo-ductility under tensile load. The form of the filler and the method of blending the filler with the matrix can be selected to achieve a homogeneous product and minimize the porosity in the product.

[0021] W metal is commercially available in powder form and can have a particle size distribution d of at least 15 μm, such as about 21 μm. 10 Tungsten boride can be milled to a desired particle size for inclusion in the composite material. The particle size can be measured using SEM imaging in combination with visual inspection and measurement.

[0022] Analysis of the WB particle size distribution is shown in FIG. 6. The particle size analysis was performed using image analysis with FIJI-ImageJ (V2.1.0 / 1.53C). Image thresholding was applied to SEM secondary electron images of the powder collected at a magnification of 2000 times. An automated powder size measurement routine was applied to FIJI-ImageJ, enabling the measurement of 1400 powder particles. It was found that the powder Feret diameter range was 0.429 to 2.923 μm and the average powder Feret diameter was 1.03 μm. A fine WB filler consistent with this particle size distribution analysis is shown in FIG. 1C.

[0023] In addition to tungsten boride, the filler may include one or more other filler materials having neutron attenuation properties, such as tungsten carbide, tungsten, tungsten oxide, graphite, and tungsten boride to provide neutron shielding over a wider energy range. Preferably, the filler is a mixture of tungsten boride and tungsten. By using different filler gradients, it is possible to efficiently utilize the neutron attenuation capabilities of each element, for example, by slowing and trapping neutrons within the same material and aligning the filler gradient with the direction of incidence of fast neutrons.

[0024] [Block-shaped composite materials] The composite material may have a substantially granular structure and may contain some micropores. Micrographs of magnesium borate containing WB and W-filler particles are shown in Figure 1. Each SEM micrograph is of a composite material containing 9:21 volume% W:WB-filler particles. In Figure 1A, coarse W particles are clearly visible. In Figure 1B, W particles are still visible, and the mottled background seen in this micrograph consists of blended fine WB. In Figure 1C, at the highest magnification, the fine WB powder structure is more clearly visible. Neither WB particles nor W particles dissolve in the matrix during manufacturing. Figure 1A also shows large unreacted MgO inclusions distributed throughout the matrix. Porosity and the amount of MgO inclusions depend on the manufacturing method.

[0025] The concentration of the filler can exhibit a gradient across the thickness of the block. For example, a combination of tungsten and tungsten boride can be used as a filler, where the volume % of the filler is constant across the entire matrix, but the tungsten to tungsten boride concentration ratio exhibits a gradient across the thickness of the composite material from 1:0 (all tungsten) to 0:1 (all tungsten boride). In a neutron shielding assembly, the side of the block with the highest concentration of tungsten filler particles is positioned closest to the radiation source because pure tungsten is better at slowing down fast neutrons, while tungsten boride is better at trapping slow neutrons.

[0026] Early studies have shown that composite material blocks have a compressive strength of 11–13 MPa, measured using the Brazilian disk test at room temperature, which is about twice the strength of concrete. Concrete is widely used in the construction of conventional nuclear fission reactors. The composite materials of the present invention can be used as an alternative to concrete, for example, when a compact design is required or when there are special structural performance requirements that necessitate a material with higher strength.

[0027] In early studies, when a 9:21 volume% mixture of tungsten and tungsten boride in a magnesium borate matrix was used, 8.17 g / cm³ was obtained. -3 The density was achieved. The magnesium borate matrix without fillers was approximately 2.5 g / cm³. -3 It has a density of .

[0028] [Composite materials in the form of microparticles] Some applications do not require the high levels of neutron absorption found in fusion reactors. In such applications, the required neutron absorption characteristics can be achieved while using fewer composite materials to save costs.

[0029] Composite materials can be ground from block form to particulate form for incorporation into other materials. For example, this can be combined with concrete and used for neutron decay in conventional fission reactors. This allows for the use of thinner concrete components, resulting in more compact fission reactor assemblies. Composite materials can be manufactured using die forms that directly create small blocks to reduce or eliminate the need for a grinding step.

[0030] [Neutron Shielding Assembly] The composite material of the present invention is particularly suitable for use in any neutron shielding assembly. A schematic diagram of one such assembly is shown in Figures 3 and 4. A central solenoid a is typically located within a tokamak fusion reactor to contain the plasma. The central solenoid must be protected from neutron radiation emitted by the plasma. A coolant layer b provides thermal cooling and can provide a good level of thermal neutron capture for neutrons not captured by the composite material or metal shielding layer. The composite material of the present invention is layer c in the assembly shown in Figures 3 and 4. Magnesium borate containing a tungsten boride filler captures thermal neutrons. A tungsten layer d acts as a first shield in the assembly and slows down fast neutrons. High-energy neutrons entering the neutron shielding assembly are schematically shown in e. Other arrangements of the neutron shielding assembly containing the composite material of the present invention are also possible. For example, depending on the application, the coolant layer may be omitted. However, it should be understood that composite materials may be deployed in other areas where neutron shielding is required, and the example of shielding the central solenoid of a tokamak fusion reactor is just one such example.

[0031] [Manufacturing] When fabricating composite materials in block form, the following basic procedure is followed. First, fine-particle magnesium oxide is mixed with a filler to form the first mixture. Next, boric acid is combined with the first mixture to form the second mixture. Boric acid may be substituted with another suitable boron compound, such as boron oxide or alkali metal borate. Finally, the second mixture is sintered. The expected reactions during the sintering step are as follows: MgO + H3BO3 + WB → Mg3B2O6 + WB + H2O

[0032] Preferably, the sintering temperature is 200 to 300°C. Preferably, the sintering pressure is 25 to 35 MPa. In one example, sintering is carried out at the high temperature and high pressure described above. That is, sintering may involve simultaneous heating and pressurization at the temperatures and pressures described above. In other examples, sintering may involve sequential application of heat and pressure within the ranges described herein.

[0033] [Usage of materials] The composite material design of the present invention is particularly well-suited for use as a neutron shield in fusion applications. For example, the composite material can be used as part of a neutron shielding assembly in a tokamak fusion reactor. For tokamak fusion reactor design, it is desirable to make the neutron shielding material thinner. The composite material of the present invention enables thinner neutron shielding than some known materials, while having a large-scale manufacturing process and a mechanically strong and stable bulk product. Fusion reactions produce a much larger amount of neutron radiation than fission reactions for civilian energy production. By including tungsten and boron in the same material, it becomes possible to slow down fast neutrons from fusion reactions and trap them in the same material. Furthermore, the magnesium borate matrix does not become highly activated, unlike previously used binders such as cobalt. This makes the composite material of the present invention an attractive new option for neutron shielding in fusion reactor design.

[0034] Conventional nuclear fission reactors can also benefit from the composite material of the present invention as a neutron shielding material. Even when the degree of neutron emission is much lower than that of a fusion reactor, the composite material of the present invention can be used to create a much thinner neutron shield, thereby enabling a more compact reactor design. The composite material of the present invention may be manufactured in the form of fine particles or granules and incorporated into another material such as concrete. Such a tertiary composite material can offer a balance between the low cost of concrete and the excellent neutron attenuation properties of the composite material of the present invention.

[0035] Composite materials can also be useful as radiation shielding in medical applications. While radiation sources in medicine are beneficial therapeutic tools, there is typically little space to provide radiation shielding. Given the efficiency of neutron shielding provided by the composite materials of the present invention, they can be appropriately implemented as radiation shielding in medical applications where neutron radiation must be addressed. Furthermore, tungsten is a known material for shielding both X-rays and gamma rays due to its high density. By including tungsten metal alongside tungsten boride as a filler material in the present invention, medical radiation shielding components can be manufactured in a cheaper and more convenient way than pure tungsten metal components.

[0036] Another application of the composite materials of the present invention is in aerospace. Both people and components must be shielded from ambient radiation experienced at levels higher than sea level in aerospace applications. The composite materials can be implemented as neutron shields in aerospace applications, particularly because they are thin, strong, and lighter than some known radiation shielding materials. The composite materials can also be used to attenuate radiation from power sources or from cosmic radiation. The ability to manufacture composite materials in custom shapes is particularly useful for these applications. [Examples]

[0037] [Example 1] Tungsten boride has been shown to be one of the most effective neutron shielding materials. However, it has been difficult to manufacture until now, requiring a binder interface such as cobalt to solidify the material into blocks. The problem is that these binders are highly activated in the fusion environment. Therefore, the MgOB-WB composite ceramic material according to the present invention enables the use of tungsten and boron in a low-activation MgOB matrix.

[0038] A block composite material was prepared according to the present invention. Tungsten boride filler particles were mixed with pulverized MgO, with a maximum of 30 volume% being tungsten boride filler particles. This combination was reacted with boric acid (H3BO3) in a volume ratio of MgO to WB to boric acid of 4.7:1:10 (this ratio applies when tungsten boride is present in an amount of 30 volume%). Boric acid was used in a concentrated solid form with a purity of over 99.5%. The reaction was carried out for 20 minutes using sintering conditions of 200-300°C and a pressure of 30.9 MPa to produce a solid block of the material. While we do not wish to be bound by theory, the assumed reaction is as follows: MgO + H3BO3 + WB → Mg3B2O6 + WB + H2O

[0039] MgO is present in the product only if it has not reacted completely. In some embodiments, the amount of unreacted MgO may be zero.

[0040] The product is 7.1-8 g / cm³. -3 It was found to have a density within the range of [this range]. Figure 2 shows the XRD trace of a sample composite material block of the present invention. The sample contained 10 volume% WB. The ratio of WB:MgO:H3BO3 was 1.3:1:10 by volume.

[0041] [Example 2] The neutron shielding assembly was modeled by varying the amount of the present invention composite material used as the inner shield as a percentage of the total shielding thickness. In each model, the present invention composite material as the inner shield was positioned behind a pure tungsten outer shield, i.e., the tungsten was closest to the plasma and the present invention material was closest to the central solenoid. The shields in the models were water-cooled, and the total shielding thickness was 45.5 cm. The specific present invention composite material used in the models was a magnesium borate matrix containing 21 vol% WB filler and 9 vol% W filler. The objective of the modeling was to investigate the toroidal magnetic field coil lifetime as a function of the neutron shielding material.

[0042] The results are shown in Figure 5. The horizontal line represents the relative toroidal magnetic field coil lifetime when the shield is pure tungsten, pure tungsten carbide, and pure tungsten boride. The relative toroidal magnetic field coil lifetime indicates the shielding effect. When 15-25% of the shield thickness was the material of the present invention and the remaining 75-85% was tungsten, the shield combination was superior to a pure tungsten boride shield.

[0043] [Measurement method] Using a particle size analysis routine, particle size analysis can be performed using FIJI-ImageJ software to generate an automated evaluation of the particle size of 1400 particles from SEM images.

[0044] [advantage] In addition to the advantages mentioned above, the present invention has the following advantages. ● This provides an effective method for capturing thermal neutrons, especially in the absence of mitigating materials such as water. ● It can be manufactured in solid block form using a very easy process (<300°C) and a hand press. ● By using reaction sintering (powder-based solidification), it is possible to use various die shapes, which will enable block shapes that will be useful in the future (for example, bricks that can be stacked in shielding areas). ● The composition utilizes low-activation elements (replacing the Co binder conventionally used in tungsten carbide and tungsten boride). ● By using different fillers within the material, it becomes possible to adapt the shielding properties of the composite material to various neutron energies. ● The integrity of the material was demonstrated for up to 48 hours at 1000°C.

Claims

1. A composite material comprising a matrix and a filler, wherein the matrix is ​​magnesium borate and the filler comprises tungsten boride.

2. The composite material according to claim 1, wherein the filler constitutes 1 to 60% by volume of the composite material, and the volume percentage is estimated with respect to the total volume of the starting materials.

3. The composite material according to claim 1 or 2, wherein the filler further comprises one or more of tungsten, tungsten carbide, tungsten oxide, and graphite.

4. The composite material according to any one of claims 1 to 3, wherein the filler consists of particles having a particle size in the range of 5 to 50 μm.

5. A ceramic block made of the composite material described in any of the preceding claims.

6. A ceramic block according to claim 5, having a compressive strength of 11 to 13 MPa.

7. 3-15 g / cm 3 Preferably 5 to 15 g / cm³ 3 A ceramic block according to claim 5 or 6, having a density of the specified density.

8. The ceramic block according to any one of claims 5 to 7, wherein the density of the filler exhibits a gradient over the thickness of the block.

9. The ceramic block according to claim 8, wherein the filler comprises tungsten boride particles and tungsten particles, and the concentration ratio of tungsten boride to tungsten is in the range of 1:0 to 0:1 over the thickness of the block.

10. A ceramic microparticle comprising the composite material according to any one of claims 1 to 4.

11. A neutron shielding assembly comprising a layer of tungsten disposed on a layer of composite material according to any one of claims 1 to 10.

12. The neutron shielding assembly according to claim 11, comprising the coolant layer such that the layer of composite material is interposed between the tungsten layer and the coolant layer.

13. A method for producing a composite material according to any one of claims 1 to 9, comprising the following steps: a. A step of providing fine particles of MgO and a fine particle filler, wherein the filler comprises tungsten boride, b. A step of mixing the fine particles MgO and the fine particle filler to form a first mixture, c. The first mixture is combined with boric acid to form a second mixture, d. The step of sintering the second mixture to form the composite material. Methods that include...

14. The method according to claim 13, wherein the sintering step is performed at a temperature of 200 to 300°C.

15. The method according to claim 13 or 14, wherein the sintering step is performed at a pressure of 25 to 200 MPa.

16. Use of the composite material according to any one of claims 1 to 10 as a neutron shield in a fusion reactor or nuclear fission reactor, as a radiation shield in medical nuclear applications, or as a radiation shield in aerospace applications, preferably as a fusion neutron shield.

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