Neutron shielding material

Abstract

A neutron shielding composite is described comprising a magnesium borate matrix and a tungsten boride filler, optionally comprising other fillers such as tungsten. Methods of making the composite material are also described.

Description

TECHNICAL FIELD

[0001] The present invention relates to materials suitable for use in nuclear reactor components, methods of manufacture thereof and uses thereof. BACKGROUND

[0002] Nuclear energy, for example civilian fusion reactors, require effective neutron shielding for safe operation. Tungsten boride (WB) is a very effective neutron shielding material, but is difficult to manufacture. They require a binder interface phase (typically cobalt based) to consolidate into a sintered block of material. These binders are problematic as they become radioactive in a fusion environment. While monolithic tungsten boride can be manufactured, the technology is complex and costly.

[0003] Other neutron shielding materials are bulkier and less effective. It is therefore desirable to be able to manufacture a neutron shielding material containing tungsten boride, avoiding the disadvantages of existing tungsten boride shielding materials. SUMMARY

[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 can comprise 1 to 60 volume % of the composite material, for example 1 to 55 volume %, or 1 to 50 volume %, for example 1 to 40 volume % (approximately 9-86 wt %) based on the total amount of starting materials. The filler can be particulate. Suitable particle sizes (Feret diameter) range from 0.05 to 50 microns, for example 0.05 to 40 microns, for example 0.25 to 30 microns. The filler can comprise one or more of tungsten, tungsten carbide, tungsten oxide, graphite and other suitable materials having neutron attenuation properties in addition to tungsten boride.

[0006] The composite material of the present invention can suitably take the form of a block. Such blocks are typically ceramic. The blocks of the present invention can have a compressive strength of 11 to 13 GigaPascals and / or a density of 5 to 15 g / cm3. 3 The block form of the composite material of the present invention can be particularly suitable for use in modular construction. The blocks of the present invention can be manufactured using off-the-shelf equipment and methods and can be formed into a variety of shapes to meet construction requirements.

[0007] In bulk form, the composite material can be manufactured in a way that results in an uneven distribution of the filler. For example, the concentration of the filler can exhibit a gradient in the thickness direction of the bulk. The filler can exhibit a composition gradient in the thickness direction of the bulk, i.e. the filler comprises a secondary filler component in addition to tungsten boride, for example, 100% tungsten boride on one surface of the bulk composite and 100% secondary filler component on the opposite surface of the bulk (the ratio of tungsten boride to secondary filler component is 100:0 to 0:100 in the thickness direction of the bulk). The secondary filler component can be one or more of tungsten, tungsten carbide, tungsten oxide, graphite, or any other suitable material having neutron-attenuating properties.

[0008] The composite material of the present invention can be in particulate form, for example ceramic or metal particles. For applications requiring a lower degree of neutron attenuation, the particulate form of the material of the present invention can be incorporated into other materials. For example, concrete incorporating the particulate form of the material of the present invention can be used for neutron attenuation in a nuclear fission reactor device in a thinner amount than concrete not incorporating the material of the present invention, thereby making the reactor assembly and / or the entire energy device more compact.

[0009] The present invention provides a neutron shielding assembly. The neutron shielding assembly comprises a layer of the composite material of the present invention disposed against a layer of tungsten. The two layers can be placed in either order in sequence between a source of radiation and an object or organism to be shielded from the radiation. Preferably, the tungsten metal is placed closest to the source of neutrons to slow down the fast neutrons, followed by the composite material of the present invention which is capable of capturing thermal neutrons. In a tokamak-type (magnetically confined) fusion reactor, the assembly can comprise the composite material of the present invention interposed between the tungsten layer and a coolant layer. In this arrangement, the central solenoid is shielded from the neutron radiation originating from the plasma.

[0010] The present invention provides a method of manufacturing the composite material of the present invention. The method comprises the steps of:

[0011] a. providing particulate MgO and particulate filler, wherein the filler comprises tungsten boride;

[0012] b. mixing the particulate MgO and particulate filler to form a first mixture;

[0013] c. mixing the first mixture with boric acid to form a second mixture;

[0014] d. sintering the second mixture to form the composite material.

[0015] In the method of the invention, the sintering step can be performed at a temperature of 200 to 300 °C. The sintering step can be performed at a pressure of 25 to 35 megaPascal. These conditions are relatively mild for the manufacture of ceramics and materials suitable for nuclear applications, making the composite material of the invention more attractive compared to materials that require very harsh production conditions. These sintering temperatures and pressures can be achieved by standard ceramic hot-pressing and sintering equipment, making it easy to scale up production. Using these standard equipment, custom shapes can be manufactured. For example, during sintering, the composite material can take the shape of a custom mold. For example, modular, stackable blocks can be produced to enable simple custom nuclear reactor designs. The sintering step can also be performed at a pressure of 25 to 200 megaPascal, for example 30 to 175 megaPascal, for example 35 to 150 megaPascal, for example 35 to 100 megaPascal. The sintering step can comprise a series of alternating heating and pressurizing treatments at any of the temperatures or pressures mentioned. In this series, the pressurizing treatment is only started after the temperature of the sintering equipment has dropped below 100 °C, for example below 75 °C, for example below 50 °C, for example about 25-30 °C. In other such series, the sintering can comprise a heating step at a temperature between 200 °C and 300 °C, stopping the heating, and a pressurizing step while the system cools. The sintering step can comprise a heating and pressurizing treatment that is performed simultaneously at any of the temperatures and pressures mentioned. In one example, the sintering step is performed as a single step at a temperature of 200 to 300 °C and a pressure of 25 to 200 megaPascal, for example 25 to 50 megaPascal, i.e. a sintering step that comprises a heating and pressurizing treatment that is performed simultaneously.

[0016] The invention also provides the use of the composite material of the invention as a neutron shield in a nuclear fusion or fission reactor, as a radiation shield in a nuclear medicine application, or as a radiation shield in an aerospace application, preferably as a neutron shield for a nuclear fusion reactor. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 : SEM micrograph of magnesium borate matrix with tungsten and tungsten boride fillers;

[0018] Figure 2 Figure 4: XRD pattern of magnesium borate with tungsten boride fillers after sintering at 1000 °C;

[0019] Figure 3 Figure 5: Schematic representation of a neutron shield assembly comprising the material of the invention;

[0020] Figure 4 Figure 6: Cross-sectional schematic representation of a solenoid of a neutron shield assembly with Figure 3

[0021] Figure 5 Figure 7: Graph of the relative toroidal field coil lifetime versus the ratio of total inboard shield thickness for the material of the invention; ​

[0022] Figure 6 : Histogram of tungsten boride particle size distribution. DETAILED DESCRIPTION

[0023] Matrix

[0024] The matrix material comprises, consists essentially of, or consists of magnesium borate. The matrix acts as a binder for the filler particles, such that the filler material can be more easily used as, for example, a neutron attenuating material. The stoichiometry of the magnesium borate used in the present invention can be Mg3B2O6. Other stoichiometries of magnesium borate can be present in the composite material of the present invention.

[0025] Magnesium borate and methods of making it have been described in GB 1035811, inventor Donald Albert Gebbett. Magnesium borate is also known as jeanite.

[0026] Unlike traditional tungsten boride binders, magnesium borate does not produce significant radioactivity when used in nuclear reactor components. Magnesium borate can be considered a low-activation material, aimed at safe disposal within 100 years of shutdown of a nuclear power plant, and without production of long-lived radionuclides. This is a large advantage of the material as a binder, as it simplifies and makes safer the disposal of decommissioned reactor components. In contrast, commonly used binders such as cobalt become highly radioactive during use, requiring difficult and expensive safe disposal processes.

[0027] Another large advantage of magnesium borate as a tungsten boride binder is ease of manufacture. It can be manufactured and sintered at temperatures accessible in a household oven, and pressures accessible in a hand press. Ease of manufacture means that the final size and shape of the sintered block is not limited, so the size and shape of the mould can be varied, enabling the manufacture of customised geometric components. For example, the blocks can be made in unit form so as to be stackable.

[0028] Filler

[0029] The filler comprises tungsten boride. The tungsten boride used in the present invention preferably has the stoichiometry WB. WB has a tetragonal structure below 1200°C. Other stoichiometries of tungsten boride can be used in the present invention, for example W2B5. Unless otherwise stated, "tungsten boride" as used herein refers to tungsten and boron in any stoichiometry.

[0030] The simultaneous presence of tungsten and boron in the filler can bring a dual advantage in neutron attenuation. Tungsten and boron are currently the most effective neutron shielding materials when used together.

[0031] The filler is preferably in particulate form. The particle size and morphology is not particularly limited, but the particle size is preferably in the range of 0.05 to 50 microns, for example 0.05 to 40 microns, for example 0.05 to 30 microns, for example 0.25 to 30 microns. The filler particle size can be in the range of 0.1 to 10 microns. Other filler morphologies, such as fibers, can also be used in the present invention. Metal fillers in fiber form can be advantageous to increase part strength, for example to provide pseudo-ductility under tensile loading. The filler morphology and method of mixing the filler with the matrix can be selected to obtain a uniform product and to minimize product porosity.

[0032] Tungsten metal is commercially available in powder form, with a particle size distribution d 10 of at least 15 microns, for example about 21 microns. The tungsten boride can be milled to the desired size for the composite material. The particle size can be measured by SEM imaging in combination with visual inspection and measurement.

[0033] Figure 6 The results of the analysis of the WB particle size distribution are shown. The particle size analysis was performed using FIJI-ImageJ (V2.1.0 / 1.53C) image analysis software. The SEM secondary electron images of the powders were processed for image thresholding at 2000x magnification. The automated powder sieve program was applied in FIJI-ImageJ to measure 1400 powder particles. The Feret diameter of the powder was found to range from 0.429-2.923 microns, with an average powder Feret diameter of 1.03 microns. A fine WB filler that conforms to the results of this particle size distribution analysis is shown in FIG. 1. Figure 1C

[0034] In addition to tungsten boride, the filler can also include one or more of tungsten carbide, tungsten, tungsten oxide, graphite, and other fillers with neutron attenuation properties to provide neutron shielding over a wider energy range. The filler is preferably a mixture of tungsten boride and tungsten. The use of a gradient of different fillers can take advantage of the neutron attenuation capabilities of each element, for example to first slow down the neutrons and then capture the neutrons within the same material, and to adjust the gradient of fillers according to the incident direction of the fast neutrons.

[0035] The composite material can have a bulk form

[0036] The composite material can have a substantially granular structure, and can include some microporosity. FIG. 1 shows a micrograph of magnesium borate including WB and W filler particles. Each SEM micrograph is of a composite material including 9:21 volume percent W:WB filler particles. In Figure 1A the coarse W particles are clearly visible. In Figure 1B the W particles are still visible; the mottled background visible in this micrograph is composed of fine mixed WB. In Figure 1C ​At the highest magnification, the fine WB powder structure is more clearly visible. Neither the WB nor the W particles are dissolved into the matrix during fabrication. Figure 1A A large number of unreacted MgO inclusions are also shown distributed throughout the matrix. The porosity and amount of MgO inclusions depend on the fabrication method.

[0037] The concentration of the filler can present a gradient in the direction of the thickness of the block. For example, a combination of tungsten and tungsten boride can be used as the filler, the volume % of the filler remaining constant throughout the matrix, but the concentration ratio of tungsten to tungsten boride presenting a gradient from 1 :0 (all tungsten) to 0:1 (all tungsten boride) in the direction of the thickness of the composite. In a neutron shielding assembly, the side of the block with the highest concentration of tungsten filler particles should be located closest to the source of radiation, since pure tungsten is better at slowing down fast neutrons, while tungsten boride is better at capturing slow neutrons.

[0038] Preliminary studies have found that the compressive strength of the composite block is 11 to 13 megapascals (measured using the Brazilian disc test at room temperature), approximately twice the strength of concrete. Concrete is widely used to construct traditional nuclear fission reactors. The composite of the present invention can be used as a replacement for concrete, for example, where a more compact design is required or where higher strength materials are required for special structural performance requirements.

[0039] Preliminary studies have shown that, when using a 9:21 volume % mixture of tungsten and tungsten boride respectively in a magnesium borate matrix, the density reaches 8.17 g cm -3 The density of a magnesium borate matrix without any filler is approximately 2.5 g cm -3 .

[0040] Composite in pellet form

[0041] Some applications do not require the high level of neutron absorption as in a nuclear fusion reactor. For such applications, the amount of composite can be reduced while achieving the required neutron absorption performance, thereby saving costs.

[0042] The composite can be crushed from block form into pellet form for incorporation into another material. For example, it can be mixed with concrete for use in neutron attenuation in a traditional nuclear fission reactor. This enables thinner concrete components to be used, thereby enabling more compact reactor assemblies. The composite can be fabricated using a mould that produces small pieces directly, thereby reducing or eliminating the need for a crushing step.

[0043] Neutron shielding assembly

[0044] The composite of the present invention is particularly suitable for use in any neutron shielding assembly. Figure 3 and Figure 4A schematic of one such assembly is shown. A central solenoid a is typically present in a tokamak fusion reactor for containing a plasma. The central solenoid must be protected from the neutron radiation emitted by the plasma. A coolant layer b provides thermal cooling and can provide a good level of thermal neutron capture for any neutrons that are not captured by a composite or metal shielding layer. The composite of the present invention is Figure 3 and Figure 4 layer c in the assembly shown. Magnesium borate with tungsten filler can capture 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 shown schematically at e. Other arrangements of the neutron shielding assembly containing the composite of the present invention are possible. For example, the coolant layer can be omitted depending on the particular application. However, it is to be understood that the composite can also be deployed in other areas where neutron shielding is required, and the example of shielding a tokamak fusion reactor central solenoid is merely one example.

[0045] Manufacture

[0046] When preparing the composite in bulk form, the basic procedure is as follows: first, mix the particulate magnesium oxide with the filler to form a first mixture. Then, mix the boric acid with the first mixture to form a second mixture. The boric acid can be replaced with other suitable boron compounds, such as boron oxide or alkali borate. Finally, sinter the second mixture. In the sintering step, the assumed reactions are as follows:

[0047] MgO + H3BO3 + WB -> Mg3B2O6 + WB + H2O

[0048] The sintering temperature is preferably 200 to 300 °C. The sintering pressure is preferably 25 to 35 megaPascals. In one example, the sintering is performed simultaneously at the high temperature and high pressure as described above. That is, the sintering can include simultaneously applying heat and pressure at the temperature and pressure as described above. In other examples, the sintering can include sequentially applying heat and pressure within the ranges described herein.

[0049] Use of the material

[0050] The design of the composite material of the present invention makes it particularly suitable for use as a neutron shield in nuclear fusion applications. For example, the composite material can be used as part of a neutron shield assembly in a tokamak-type reactor. Due to the design of the tokamak reactor, it is desirable to make the neutron shield material thinner. The composite material of the present invention can produce a thinner neutron shield than some known materials, while having a scalable manufacturing process and a mechanically strong and stable bulk product. Fusion reactions produce more neutrons than fission reactions for the production of civilian energy. The addition of tungsten and boron in the same material allows the fast neutrons produced by fusion reactions to be slowed down and captured in the same material. In addition, the magnesium borate matrix is not 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 designs.

[0051] Conventional fission reactors can also benefit from the composite material of the present invention as a neutron shielding material. Although the level of neutron radiation is much lower than in fusion reactors, the composite material of the present invention can be used to produce a much thinner neutron shield, allowing for a compact reactor design. The composite material of the present invention can be produced in granular or particulate form and incorporated into another material such as concrete. This tertiary composite material can strike a balance between the low cost of concrete and the excellent neutron attenuation properties of the composite material of the present invention.

[0052] The composite material can also be used as a radiation shield in medical applications. Radiation sources in medicine are a beneficial treatment tool, but often there is not enough space to provide radiation shielding. Given the efficiency of the neutron shielding provided by the composite material of the present invention, it can be suitable for radiation shielding in medical applications that must contend with neutron radiation. In addition, due to the high density of tungsten, it is a known X-ray and gamma radiation shielding material. In the present invention, the inclusion of tungsten metal as a filler material in addition to tungsten boride as a filler material can make the manufacture of medical radiation shielding components more affordable and convenient than pure tungsten metal components.

[0053] Another area of application for the composite material of the present invention is aerospace. Both the human body and components must be shielded from environmental radiation, and the intensity of radiation is higher in aerospace applications than at sea level. The composite material can be used as a neutron shield in aerospace applications, particularly because it is thinner, stronger, and lighter than some known radiation shielding materials. The composite material can be used to attenuate radiation from power sources or cosmic radiation. The ability to manufacture the composite material in custom shapes is particularly useful for these applications.

[0054] Examples

[0055] Example 1

[0056] Tungsten boride is considered one of the most effective neutron shielding materials. However, this material has been difficult to manufacture previously, requiring adhesive interfacial phases such as cobalt to consolidate into a material bulk. The problem is that these adhesives are prone to high activation in a fusion environment. Thus, the MgOB-WB composite ceramic according to the present invention is able to utilize tungsten and boron in a low activation MgOB matrix.

[0057] A bulk composite material was prepared according to the present invention. Tungsten boride filler particles were mixed with ground MgO, where the tungsten boride filler particles comprised up to 30 volume percent. This mixture was reacted with boric acid (H3BO3) in a volume ratio of MgO: WB: boric acid of 4.7: 1: 10 (this ratio is applicable where tungsten boride is present in an amount of 30 volume percent). The boric acid was used in a concentrated solid form with a purity of >99.5%. The reaction was carried out under sintering conditions of 200-300 °C and 30.9 MPa pressure for 20 minutes to produce a solid material bulk. Without being bound by theory, it is hypothesized that the reaction proceeds as follows:

[0058] MgO + H3BO3 + WB → Mg3B2O6 + WB + H2O

[0059] MgO will only be present in the product if the reaction is not complete. In some embodiments, the unreacted MgO content can be zero.

[0060] The density of the product was found to be in the range of 7.1 to 8 g cm -3 .

[0061] Figure 2 XRD pattern of a bulk sample of the composite material of the present invention. This sample contained 10 volume percent WB. The volume ratio of WB:MgO:H3BO3 was 1.3:1:10.

[0062] Example 2

[0063] A neutron shielding assembly was modelled where the amount of the composite material of the present invention used as the inner side shielding was varied as a proportion of the total shielding thickness. In each model, the composite material of the present invention as the inner side shielding was placed behind a pure tungsten outer side shielding, i.e. the tungsten was closest to the plasma and the present material was closest to the central solenoid. The shielding in the model was water cooled and the total shielding thickness was 45.5 centimetres. The specific composite material of the present invention used in the model was a magnesium borate matrix with 21 volume percent WB filler and 9 volume percent W filler. The purpose of the modelling was to investigate the relationship between the toroidal field coil lifetime and the neutron shielding material.

[0064] Figure 5The results are shown in the graph. The horizontal line indicates the relative toroidal field coil lifetime when the shield is pure tungsten, pure tungsten carbide and pure tungsten boride. The relative toroidal field coil lifetime indicates the shield effectiveness. When 15-25% of the shield thickness is the inventive material and the remaining 75-85% of the shield thickness is tungsten, the performance of the combined shield is superior to a pure tungsten boride shield.

[0065] Measurement method

[0066] Particle size analysis can be performed using the FIJI ImageJ software, using the particle analysis program to automatically assess the size of 1400 particles from SEM images.

[0067] Beneficial effects

[0068] The present invention has the following advantages in addition to the above-mentioned advantages.

[0069] An effective method of capturing thermal neutrons is provided, particularly in the absence of moderator materials such as water.

[0070] The process requires very low temperatures (<300°C) and can be produced using a manual press.

[0071] The use of reaction sintering (powder-based consolidation) allows the use of a variety of mold shapes and permits the use of useful bulk geometry in the future (e.g., stacked bricks in shield regions).

[0072] The composition employs low-activation elements (instead of the traditional cobalt binder used for tungsten carbide and tungsten boride).

[0073] The use of different fillers in the material allows the shielding performance of the composite to be adapted to a range of neutron energies.

[0074] The material maintains its integrity at 1000°C for up to 48 hours.

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 accounts for 1 to 60% by volume of the composite material, wherein the volume percentage is given relative to the total volume of the starting material.

3. The composite material according to claim 1 or claim 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 with a particle size in the range of 5 to 50 micrometers.

5. A ceramic block comprising the composite material of any one of the preceding claims.

6. The ceramic block according to claim 5, wherein the compressive strength of the block is 11 to 13 MPa.

7. The ceramic bulk according to claim 5 or claim 6, wherein the density of the bulk is 3 to 15 g / cm³. 3 Preferably 5 to 15 g / cm³ 3 .

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

9. The ceramic bulk according to claim 8, wherein the filler comprises tungsten boride particles and tungsten particles, wherein the concentration ratio of tungsten boride to tungsten ranges from 1:0 to 0:1 over the thickness of the bulk.

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

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

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

13. A method for manufacturing the composite material according to any one of claims 1 to 9, comprising the following steps: a. Provide granular MgO and granular filler, wherein the filler contains tungsten boride; b. Mixing granular MgO and granular filler to form a first mixture; c. Mix the first mixture with boric acid to form a second mixture; d. Sinter the second mixture to form a composite material.

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

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

16. Use of the composite material according to any one of claims 1 to 10, for use as neutron shielding in nuclear fusion or fission reactors, for use as radiation shielding in medical nuclear applications, or for use as radiation shielding in aerospace applications, preferably for use as nuclear fusion neutron shielding.

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