Layered neutron shield

Abstract

A neutron shield. The neutron shield includes a plurality of absorption layers and at least one moderation layer. Each of the plurality of absorption layers includes tungsten boride or tungsten carbide. The at least one moderation layer includes a metal hydride. Each moderation layer is between at least two absorption layers.

Description

Technical Field

[0001] The present invention relates to neutron shields, and particularly but not exclusively, to neutron shields for tokamak fusion reactors. Background Art

[0002] The challenges of generating fusion power are very complex. When deuterium-tritium (D-T) or deuterium-deuterium (D-D) plasmas are heated, fusion neutrons are produced, enabling atomic nuclei to have sufficient energy to overcome the Coulomb electrostatic repulsion to fuse together, thereby releasing high-energy neutrons and fusion products (e.g., for the D-T case, releasing 4 He). To date, the most promising way to achieve this is to use a tokamak device; in the conventional tokamak fusion method (as implemented in ITER), the plasma needs to have a high confinement time, high temperature, and high density to optimize the process.

[0003] A tokamak is characterized by the combination of a strong toroidal magnetic field B T 、a high plasma current I p 、usually a large plasma volume, and significant auxiliary heating, thereby providing a thermally stable plasma such that fusion can occur. Auxiliary heating (e.g., by performing neutral beam injection of high-energy H, D, or T in the order of several tens of megawatts) is necessary to increase the temperature to a high enough value required for nuclear fusion to occur and / or to maintain the plasma current.

[0004] To ensure that the reactor is as compact as possible (which allows for higher efficiency, especially for the "spherical tokamak" plasma configuration), the thickness of the radiation shield should be minimized while still providing sufficient protection to other components. Minimizing the distance between the plasma and the field coils allows for a higher magnetic field in the plasma and less current in the coils.

[0005] Figure 1Shows a part of the central column and shows the problems that the shield material must overcome. The central column includes a high-temperature superconductor (HTS) coil 11 in the central core and a shield 12 in the outer layer. Depending on the material used for the shield, an oxidized shield material layer 13 may be present on the outer surface. There are three main causes of damage due to the plasma 14. First, high-energy neutrons 15 generated by the fusion reaction can basically knock atoms out of the structure of the shield, thereby generating a damage cascade 16 that propagates through the material and reduces the material properties (such as mechanical properties, thermal properties, or superconducting properties). Second, the heat flux 17 from the fusion reaction is significant and may damage the shield due to thermal stress caused by non-uniform heating, and damage the HTS core due to the reduction in the current that can be carried while maintaining superconductivity at high temperatures, and may cause the coil to suddenly acquire resistance, resulting in a quench of the magnet. Finally, the high-energy particles of the plasma will ablate 18 the outer surface of the shield. This not only causes damage to the shield itself, but may also contaminate the plasma if the shield is directly exposed to the plasma. It is desirable to have a shield material that can resist these effects and prevent neutrons from reaching the superconducting coil.

[0006] Current shield designs also often use water channels to cool the shield and to slow down neutrons (which increases the effectiveness of the shield). However, this has problems because during the disposal or maintenance of the application, it is difficult to handle water - due to the risks of pressurizing the system, contaminating the water, activating the water, and evaporating the water, and if handled improperly, there is a possibility of water from the reactor entering the environment.

[0007] Therefore, there is a need for an effective neutron shield that does not require water to slow down neutrons. Summary of the Invention

[0008] According to a first aspect, there is provided a neutron shield. The neutron shield includes a plurality of absorption layers and at least one moderation layer. Each of the plurality of absorption layers includes tungsten boride or tungsten carbide. At least one moderation layer includes a metal hydride. Each moderation layer is between at least two absorption layers.

[0009] According to a second aspect, there is provided a neutron shield. The neutron shield includes a metal hydride. The metal hydride includes a neutron-absorbing element that:

[0010] has an average neutron absorption cross-section greater than 0.1 barn in the neutron energy range between 0.02 eV and 0.03 eV; and

[0011] has a solid solubility of at least 1 mol% with all other metals in the alloy.

[0012] "Average neutron absorption cross section" is the average (weighted by their abundances) of the neutron absorption cross sections within a specified range of all the isotopes of the element. The isotopic distribution of the neutron absorbing element can be their natural abundances, or it can be some other distribution having a desired average neutron absorption cross section.

[0013] According to a third aspect, there is provided a tokamak fusion reactor. The reactor includes a toroidal plasma chamber, a plasma confinement system, and a neutron shield. The plasma confinement system is arranged to generate a magnetic field for confining the plasma inside the plasma chamber. The neutron shield is a neutron shield according to the first or second aspect and is arranged between the inside of the toroidal plasma chamber and the plasma confinement system.

[0014] Other embodiments are set forth in claim 2 and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Showing the neutron shield of the central column of a tokamak and its design challenges;

[0016] Figure 2 Schematically showing an exemplary neutron shield;

[0017] Figure 3 Schematically showing other exemplary neutron shields;

[0018] Figure 4 Schematically showing a tokamak plasma chamber. DETAILED DESCRIPTION

[0019] An effective and compact radiation shield for reducing the absorbed dose in sensitive materials (such as the central column of a spherical tokamak) from a high-energy neutron source requires the combination of high-Z (atomic mass) and low-Z elements in a bulk material. High-energy neutrons with kinetic energies greater than a few MeV are effectively slowed down by two main mechanisms: first, through inelastic nuclear reactions with high-Z nuclei; second, through elastic scattering with low-Z nuclei. After inelastic scattering, the secondary neutron energy is typically below the reaction threshold energy for subsequent inelastic nuclear reactions with high-Z elements and can therefore only be effectively slowed down by low-Z elements. Thus, the combination of high-Z and low-Z elements can be used to reduce the high-energy neutron flux incident on sensitive materials outside the shield.

[0020] An effective high-energy neutron shield requires four main parts, which in order from the neutron source to the object being protected are:

[0021] · A material containing elements with high atomic mass for reducing the energy of neutrons from the plasma (especially materials having a high cross section for inelastic scattering or neutron multiplication reactions)

[0022] · A neutron moderator for further slowing down neutrons to an energy with an optimal cross-section (i.e., a material containing elements with a low atomic mass)

[0023] · A neutron absorber for absorbing the slowed-down neutrons

[0024] · A gamma shield for absorbing gamma rays generated by neutron interactions in an earlier stage.

[0025] In cases where a single material or composite material can perform multiple functions (e.g., as a neutron absorber and a gamma shield), two or more parts can be combined.

[0026] In traditional shields, the neutron moderator is typically water – which requires careful handling for disposal and safety, as described above.

[0027] Due to the high Z value of tungsten (74), and the fact that tungsten generally has a high mass and number density in stable compounds compared to other high-Z elements (e.g., lead), tungsten is an ideal choice as a high-Z constituent element in the first stage. Tungsten boride is particularly advantageous for shield applications, as tungsten boride adds boron as a constituent element of the neutron shield; boron is an effective neutron absorber for preventing low-energy neutrons from penetrating the shield at low energies. In addition, tungsten is an effective absorber of gamma radiation. Thus, tungsten boride can be used for all of the above parts, except for the neutron moderator (since boron does not have a significant moderating effect compared to, for example, hydrogen). Tungsten carbide is also advantageous for shield applications, as carbon provides neutron energy moderation, although not as effectively as, for example, hydrogen – thus, they can be used for all of the above parts, but will generally be inferior to hydrogen-containing materials as moderators or tungsten boride as neutron absorbers.

[0028] Hydrogen is an ideal neutron moderator below a few MeV and can be used as a constituent element of many potential materials. However, when integrated into a fusion power plant, the most common water and hydrocarbons are problematic. Metal hydrides are comparable to water and hydrocarbons in terms of hydrogen density, but have a significantly higher temperature at which they remain solid at room temperature than water or typical hydrocarbons. These two aspects make it easier to design the neutron shield structure (since solid components are easier to integrate than liquid components), and make it easier to maintain and decommission (since solid components have a lower risk of leakage).

[0029] Therefore, as Figure 2The composite shield in the form of a "sandwich structure" of tungsten boride and metal hydride (with two tungsten boride layers 201, 203 and a metal hydride layer 202 therebetween) shown is very effective as a neutron shield, thereby protecting the sensitive component 204 from neutron radiation 205. In fact, a multi-layer shield having alternating layers of tungsten boride (or tungsten carbide) and metal hydride, and with tungsten boride (or tungsten carbide) as the outermost layer in the radial direction, will generally be effective.

[0030] Tungsten boride can be provided in various forms, such as sintered tungsten boride (tungsten boride particles in a metal matrix), sintered tungsten boride, or as an alloy with metallic tungsten and / or other elements, etc. Any tungsten boride compound can be used according to the requirements of the specific structural considerations of the shield application. A promising approach is a two-phase structure of tungsten with W2B, which can be formed in a known process including vacuum hot pressing pure tungsten with boron nitride, thereby producing a material with useful thermal and mechanical properties.

[0031] Tungsten carbide can also be provided in various forms, such as sintered tungsten carbide (tungsten carbide particles in a metal matrix), or as cermet (ceramic-metal) tungsten carbide, reaction-sintered tungsten carbide / boride, or monomeric tungsten carbide.

[0032] Potential metal hydrides used as moderators include lithium hydride (LiH x ), hafnium hydride (HfH x ), yttrium hydride (YH x ), and zirconium hydride (ZrH x ), or combinations thereof. In each case, hydrides with the chemical formula of the most common compounds have been determined to be used - but other hydrides (or combinations thereof, or alloys with pure metals) can be used. The specific hydrogen-metal ratio x can be selected based on the required moderation degree, the required structural properties, and will generally be between 0.1 and 4, or between 1 and 2. The value 1.33 was used in the simulation tests of the neutron shield.

[0033] Tungsten boride will generally constitute most of the shield, where the total thickness of the tungsten boride layer is between 75% and 99% of the total thickness of the tungsten boride and metal hydride, more specifically between 80% and 95% of the total thickness of the tungsten boride and metal hydride. The tungsten boride layer 201 facing the neutron source (i.e., one of the outer tungsten boride layers) can be between 30% and 90% of the total thickness of the tungsten boride and metal hydride, more specifically between 40% and 80% of the total thickness. Although layer 201 is in Figure 2is depicted as a single layer in the figure, but it can be composed of multiple individual layers. One of the other tungsten boride layers (i.e., the tungsten boride layer between this layer and the neutron-facing layer where the metal hydride layer is located, usually the final layer 203) can be at least 10% of the total thickness of the tungsten boride and the metal hydride. It will be understood that the upper limit of the thickness of the other tungsten boride layers is defined by the difference between the total thickness of the tungsten boride and the thickness of the tungsten boride in the neutron-facing layer.

[0034] In any of the above discussions, tungsten boride (WB) can be replaced by tungsten carbide (WC) or a combination of tungsten boride and tungsten carbide.

[0035] With additional HfH x and the structure of the WB layer can also perform well – for example, having multiple HfH x layers, each HfH x layer is separated by a WB or WC layer and has a WB or WC layer as the radial outer layer. It is expected that for other metal hydrides, similar compositions will also perform well.

[0036] For the stratified WB-HfH X -WB shield (where x = 1.33), the simulation provides a shielding effect (shield) that is up to 5 times more effective than the benchmark of tungsten carbide and water shield of the same total thickness (i.e., reducing the energy deposition on the component below by 5 times). For the actually available tungsten boride and hafnium hydride materials (instead of pure HfH x or the specific tungsten boride composition used in the simulation), making such adjustments gives similar results.

[0037] The tungsten boride or carbide layer can be formed from a composite material including tungsten boride or carbide – for example, a cermet having tungsten boride or carbide particles in a metal matrix. Additionally, as mentioned before, the tungsten boride / carbide layer can contain a mixture of tungsten boride or carbide, which can be a composite material with some other materials (e.g., a cermet having both tungsten boride and tungsten carbide as aggregates).

[0038] Similarly, one or more metal hydride layers can be formed from a composite material including a metal hydride (e.g., a metal-coated metal hydride layer). The metal hydride layer can include hydrides of multiple metals in an alloy. The advantage this provides is that since the thermal decomposition curve of the alloy hydride (i.e., the amount of hydrogen released from the material at a given temperature) will have a wider temperature distribution, allowing any system configured to absorb or remove the hydrogen generated due to thermal decomposition to more easily accommodate hydrogen. As an example, the metal hydride layer can contain any combination of metals known to form a body-centered cubic solid solution with each other, such as metals in Groups 4, 5, and 6 of the periodic table (such as hafnium, niobium, tantalum, titanium, tungsten, and zirconium) and / or yttrium, gadolinium, beryllium, and uranium.

[0039] One possibility is to use a structure of the "high-entropy alloy" type, which has several (e.g., at least 5) different metals, where the atomic proportion of each metal (i.e., the number of atoms of that metal divided by the total number of metal atoms in the hydride) is from 5% to 50% (or 5%-30%), such that no single metal dominates the material properties. As with single-metal hydrides, the ratio of hydrogen atoms to (total) metal atoms in the hydride can be between 0.1 and 4, more preferably between 1 and 2.

[0040] When considering the possibility of composite materials, a "tungsten carbide and / or boride" material can be referred to as an "absorbing layer", and a "metal hydride layer" can be referred to as a "moderating layer". Although it will be understood that these labels should not be taken as restricting the function of either layer - since depending on the composition, some moderation may occur in the absorbing layer and vice versa. The labels are mainly used to avoid any confusion that might imply, for example, that a tungsten boride layer should be pure tungsten boride.

[0041] The shield can be continuous (as Figure 2 shown), i.e., the tungsten boride layer and the metal hydride layer are adjacent to each other. Optionally, as Figure 3 shown, between the tungsten boride 310 and metal hydride 320 layers, or within one or more of the layers, there can be coolant channels 301, 302 or other elements (e.g., temperature sensors, radiation sensors, and / or stress sensors). The coolant channels can be configured to carry a liquid or gas coolant (water can be used as the coolant, and the shield will still provide a useful alternative to existing designs even if the shield does not achieve the elimination of water from the shield).

[0042] The maximum operating temperature of the shield is typically defined by the thermal decomposition temperature of the metal hydride (i.e., the temperature at which the metal hydride will partially decompose and release hydrogen). The actual operating temperature can be slightly higher than the thermal decomposition temperature since the reactor safety shell can allow some hydrogen release. Encapsulating the metal hydride with something like stainless steel can be used to prevent hydrogen from being released into the reactor system. For applications with low or transient neutron loads (such as a fusion reactor with a pulse length below 10 seconds), direct cooling of the shield is usually not required (although this can be provided as a precautionary measure). In applications with a continuously high neutron load, cooling will be required to keep the shield below the thermal decomposition temperature of the metal hydride used. Generally, for hafnium hydride and yttrium hydride, the temperature should be kept below about 600 °C, for zirconium hydride, the temperature should be kept below about 300 °C, and for lithium hydride, the temperature should be kept below about 200 °C.

[0043] Cooling can be provided either through the coolant channels within the shield (as briefly described above), or by heat conduction to coolant channels external to the shield layer.

[0044] Figure 4 Shows a tokamak plasma chamber used as a fusion reactor, which includes a plasma vessel 41 and a magnetic plasma confinement system 42 for confining plasma 43. The tokamak also includes neutron shields 44, 45 in the central column neutron shield section 44 and the outer shield section 45, which are located between the plasma vessel and the magnetic plasma confinement system.

[0045] Although the above mainly focuses on multi-layer shields, similar principles can be applied to provide a single-layer neutron shield including metal hydrides. For example, a neutron shield including neutron absorption and moderation materials can be provided, and the neutron absorption and moderation materials are hydrides of metal alloys including at least one neutron absorption element. For the purposes of this disclosure, a "neutron absorption element" has an average neutron absorption cross-section greater than 0.1 barn in the neutron energy range between 0.02 eV and 0.03 eV. The "average neutron absorption cross-section" is the average (weighted by their abundances) of the neutron absorption cross-sections within a specified range of all isotopes of the element. The isotope distribution of the neutron absorption element can be their natural abundances, or it can also be some other distribution with a desired average neutron absorption cross-section. The suitable elements for the alloy are the same as those discussed above for the metal hydrides in the multi-layer solution. Suitable neutron absorption elements include hafnium, tungsten, boron, dysprosium, and gadolinium (all of which are suitable for their natural isotope abundances, but can be used with different isotope abundances).

[0046] The proportion of the neutron absorption element can be at least 5 mol%. The ratio of hydrogen to other elements in the material is between 0.1 and 4, more preferably between 1 and 2. The material can be a high-entropy alloy including at least 5 elements (at least one element being a strong neutron absorption element) other than hydrogen, where each non-hydrogen element is between 5 mol.% and <50 mol.% of the alloy (by atomic ratio, excluding hydrogen). The material can be a hydride of a neutron absorption element.

[0047] Such a material can also be used as part of a multi-layer shield, such as a shield that includes neutron absorption and moderation materials in addition to other materials that absorb or moderate neutrons, or a shield that includes neutron absorption and moderation materials and a cladding or similar protective layer.

[0048] The material can be graded such that the composition of the material varies with its thickness. For example, there may be a larger proportion of neutron absorption material towards the radial outer surface of the neutron shield, and a larger proportion of hydrogen towards the radial inner surface of the neutron shield.

Claims

1. A neutron shield, the neutron shield comprising: a plurality of absorption layers, each absorption layer comprising tungsten boride or tungsten carbide; and at least one moderation layer, the at least one moderation layer comprising a metal hydride; wherein each moderation layer is between the at least two absorption layers; the neutron shield is configured such that, in use, the outermost absorption layer is between an adjacent moderation layer and a neutron source.

2. The neutron shield according to claim 1, wherein, The metal hydride comprises one or more of metals from Groups 4, 5 and 6 of the Periodic Table and / or yttrium, beryllium, gadolinium or uranium.

3. The neutron shielding member according to claim 2, wherein, The metal hydride comprises one or more of hafnium, niobium, tantalum, titanium, yttrium and zirconium.

4. The neutron shield according to claim 1, wherein, The metal hydride comprises at least 5 metals, each metal having an atomic proportion of 5 mol.% to 50 mol.% relative to the total number of metal atoms in the metal hydride.

5. The neutron shield according to claim 1, wherein, The ratio of hydrogen atoms to metal atoms in the metal hydride is between 0.1 and 4.

6. The neutron shielding member according to claim 1, wherein, Each absorption layer comprises an alloy of tungsten metal and tungsten boride and / or carbide.

7. The neutron shield according to claim 1, wherein, The total thickness of the absorption layers is at least 75% of the total combined thickness of the absorption layers and the moderation layers.

8. The neutron shielding member according to claim 1, wherein The thickness of the outermost absorption layer is between 30% and 90% of the total combined thickness of the absorption layers and the moderation layers.

9. The neutron shield according to claim 8, wherein, The thickness of another absorption layer other than the outermost absorption layer is at least 10% of the total combined thickness of the absorption layers and the moderation layers.

10. The neutron shield according to claim 1, comprising coolant channels located between the absorption layers and the moderation layers.

11. The neutron shield according to claim 1, comprising coolant channels integrated in one or more of the absorption layers and / or the moderation layers.

12. An assembly, the assembly comprising the neutron shield according to claim 10, and a coolant source connected to the coolant channels, wherein the coolant source is configured to maintain the neutron shield at a temperature below the decomposition temperature of the metal hydride.

13. A tokamak fusion reactor, the tokamak fusion reactor comprising: a toroidal plasma chamber; a plasma confinement system arranged to generate a magnetic field for confining a plasma inside the plasma chamber; the neutron shield according to claim 1, the neutron shield being arranged between the inside of the toroidal plasma chamber and the plasma confinement system such that the outermost layer faces the inside of the toroidal plasma chamber.

14. The tokamak fusion reactor according to claim 13, wherein, The neutron shield comprises coolant channels integrated in one or more of the absorption layers and / or the moderation layers, and the tokamak fusion reactor comprises a coolant source connected to the coolant channels, wherein the coolant source is configured to maintain the neutron shield at a temperature below the decomposition temperature of the metal hydride.

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