Layered neutron shield

Abstract

Provided is a neutron shield. The neutron shield includes a plurality of absorbing layers, and at least one decelerating layer. Each of the plurality of absorber layers includes tungsten boride or tungsten carbide. The at least one deceleration layer includes a metal hydride. Each decelerating layer is between at least two absorbing layers.

Description

[0001] This application is a divisional application with application number 202080088997.4, application date June 20, 2022, and subject matter “Layered Neutron Shielding”. Technical Field

[0002] The present invention relates to neutron shielding, and in particular, but not exclusively, to neutron shielding for a tokamak fusion reactor. Background Art

[0003] The challenges of producing fusion power are very complex. Fusion neutrons are produced when a deuterium-tritium (DT) or deuterium-deuterium (DD) plasma is heated so that the nuclei have enough energy to overcome the Coulomb electrostatic repulsion to fuse together, releasing high-energy neutrons and fusion products (e.g., in the case of DT, 4 The most promising way to achieve this so far is to use a tokamak device; in the traditional tokamak fusion approach (such as that implemented in ITER), the plasma needs to have high confinement time, high temperature and high density to optimize the process.

[0004] The tokamak is characterized by a strong toroidal magnetic field B T , high plasma current I p A combination of a typically large plasma volume and significant auxiliary heating provides a thermally stable plasma so that fusion can occur. Auxiliary heating (e.g., by performing tens of megawatts of high energy neutral beam injection of H, D, or T) is necessary to increase the temperature to a sufficiently high value for nuclear fusion to occur and / or to maintain the plasma current.

[0005] To ensure that the reactor is as compact as possible (which allows for higher efficiency, especially for a "spherical tokamak" plasma configuration), the thickness of the radiation shield should be reduced as much as possible while still maintaining adequate protection for the other components. Minimizing the distance between the plasma and the field coils allows for higher magnetic fields in the plasma and less current in the coils.

[0006] Figure 1A portion of a central column is shown, and the problems that shield materials must overcome are shown. 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, there may be a layer of oxidized shield material 13 on the outer surface. There are three main causes of damage due to plasma 14. First, the high energy neutrons 15 produced by the fusion reaction can essentially knock atoms out of the structure of the shield, thereby creating a damage cascade 16 that propagates through the material and degrades the material properties (such as mechanical properties, thermal properties, or superconducting properties). Secondly, the heat flux 17 from the fusion reaction is significant and can damage the shield due to thermal stress caused by uneven heating, and damage the HTS core due to the high temperature reducing the current that can be carried while maintaining superconductivity, and may cause the coil to suddenly gain resistance, resulting in a magnet quench. Finally, the energetic particles of the plasma will ablate 18 the outer surface of the shield. This not only causes damage to the shield itself, but also may 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 coils.

[0007] Current shield designs also often use water channels to cool the shield, and for moderating neutrons (which increases the effectiveness of the shield). However, this is problematic because during disposal or maintenance of an application, the water is difficult to handle – due to the risk of pressurizing the system, contaminating the water, activating the water, and evaporating the water, as well as the potential for water from the reactor to enter the environment if not handled properly.

[0008] Therefore, there is a need for an effective neutron shield that does not require water for moderation. Summary of the invention

[0009] According to a first aspect, a neutron shield is provided. The neutron shield comprises a plurality of absorbing layers and at least one moderation layer. Each of the plurality of absorbing layers comprises tungsten boride or tungsten carbide. At least one moderation layer comprises a metal hydride. Each moderation layer is between at least two absorbing layers.

[0010] According to a second aspect, a neutron shield is provided. The neutron shield comprises a metal hydride. The metal hydride comprises a neutron absorbing element, wherein:

[0011] having an average neutron absorption cross section greater than 0.1 barn over the neutron energy range between 0.02 eV and 0.03 eV; and

[0012] It has a solid solubility of at least 1 mol% with all other metals in the alloy.

[0013] The "mean neutron absorption cross section" is the average of the neutron absorption cross sections over a specified range for all isotopes of the element that exist (weighted by their abundance). The isotopic distribution of a neutron absorbing element may be its natural abundance, or it may be some other distribution that has the desired mean neutron absorption cross section.

[0014] According to a third aspect, a tokamak nuclear fusion reactor is provided. The reactor comprises 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 plasma in the interior of the plasma chamber. The neutron shield is a neutron shield according to the first aspect or the second aspect, and is arranged between the interior of the toroidal plasma chamber and the plasma confinement system.

[0015] Further embodiments are set forth in claim 2 et seq. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 shows the neutron shielding of the central column of a tokamak and its design challenges;

[0017] Figure 2 schematically illustrates an exemplary neutron shield;

[0018] Figure 3 Other exemplary neutron shields are schematically shown;

[0019] Figure 4 A tokamak plasma chamber is shown schematically. DETAILED DESCRIPTION

[0020] An effective and compact radiation shield for reducing the absorbed dose in sensitive materials originating from high-energy neutron sources, such as the central column of a spherical tokamak, requires a combination of high-Z (atomic mass) and low-Z elements in the bulk material. High-energy neutrons with kinetic energies greater than a few MeV are effectively slowed down by two main mechanisms: first, by inelastic nuclear reactions with high-Z nuclei; and second, by 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. Therefore, a 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.

[0021] An effective high energy neutron shield requires four main parts, which in order from the neutron source to the object being protected are:

[0022] Materials containing elements with high atomic mass for reducing the energy of neutrons from the plasma (especially materials with high cross-sections for inelastic scattering or neutron multiplication reactions)

[0023] Neutron moderators (i.e., materials containing elements with low atomic mass) used to further slow down the neutrons to an energy of optimal cross section

[0024] Neutron absorbers for absorbing the slowed-down neutrons

[0025] • Gamma shielding to absorb gamma rays produced by neutron interactions in the early stages.

[0026] Where a single material or composite material can perform multiple functions (e.g., such as being a neutron absorber and a gamma shield), then two or more parts may be combined.

[0027] In conventional shielding, the neutron moderator is typically water - which requires careful handling for disposal and safety, as described above.

[0028] Tungsten is an ideal choice as a high-Z constituent element for the first stage due to its high Z value (74) and its generally high mass and number density in stable compounds compared to other high-Z elements (e.g., lead). Tungsten boride is particularly advantageous for shielding applications due to the addition of boron as a constituent element of neutron shielding; boron is an effective neutron absorber at low energies used to prevent low-energy neutrons from penetrating shielding. In addition, tungsten is an effective absorber of gamma radiation. Thus, tungsten boride can be used as all of the above components except as a neutron moderator (since boron does not have a significant moderation effect compared to, e.g., hydrogen). Tungsten carbide is also advantageous for shielding applications because carbon provides neutron energy moderation, although not as effectively as, e.g., hydrogen - thus, they can be used for all of the above components, but will generally be preferred over moderators of hydrogen-containing materials or neutron absorbers of tungsten boride.

[0029] Hydrogen is an ideal neutron moderator below a few MeV and can be used as a constituent element of many potential materials. However, the most common water and hydrocarbons are problematic when integrated into fusion power plants. Metal hydrides are comparable to water and hydrocarbons in terms of hydrogen density, but remain solid at room temperature at temperatures significantly higher than water or typical hydrocarbons. These two aspects make it easier to design neutron shield structures (because solid components are easier to integrate than liquid components) and make it easier to maintain and decommission (because solid components have a lower risk of leakage).

[0030] Therefore, if Figure 2The composite shield shown as a "sandwich structure" of tungsten boride and metal hydride (having two tungsten boride layers 201, 203 and a metal hydride layer 202 between them) is very effective as a neutron shield, protecting sensitive components 204 from neutron radiation 205. In practice, a multi-layer shield having alternating layers of tungsten boride (or tungsten carbide) and metal hydride, with the tungsten boride (or tungsten carbide) being the radially outer layer, will generally be effective.

[0031] 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 as required by 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 involving vacuum hot pressing of pure tungsten with boron nitride, thereby producing a material with useful thermal and mechanical properties.

[0032] Tungsten carbide can also be provided in various forms, such as as cemented tungsten carbide (tungsten carbide particles in a metal matrix), or as ceramic-metal (cermet) tungsten carbide, reaction-cemented tungsten carbide / boride, or monomeric tungsten carbide.

[0033] Potential metal hydrides for use as moderators include lithium hydride (LiH x ), Hafnium Hydride (HfH x ), yttrium hydride (YH x ) and zirconium hydride (ZrH x ), or a combination thereof. In each case, it has been determined that hydrides with the chemical formula of the most common compounds are used - but other hydrides (or combinations thereof, or alloys with pure metals) may be used. The specific hydrogen-to-metal ratio x may be selected based on the desired degree of moderation, the desired structural properties, and will typically be between 0.1 and 4, or between 1 and 2. A value of 1.33 was used in simulation tests of neutron shields.

[0034] The tungsten boride will typically constitute the majority of the shield, with the total thickness of the tungsten boride layer being 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) may 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 Figure 2203. It is depicted as a single layer in FIG. 204, but it may be composed of multiple separate layers. One of the other tungsten boride layers (i.e., the tungsten boride layer in which the metal hydride layer is located between the layer and the neutron-facing layer, typically the final layer 203) may 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.

[0035] In any of the above discussions, tungsten boride (WB) may be replaced by tungsten carbide (WC) or a combination of tungsten boride and tungsten carbide.

[0036] With additional HfH x and WB layers can also perform well – for example, with multiple HfH x Layer, each HfH x The layers are separated by WB or WC layers and have WB or WC layers as radially outer layers. It is expected that similar compositions will also perform well for other metal hydrides.

[0037] For layered WB-HfH X - Simulations of WB shields (where x = 1.33) provide up to 5 times more effective shielding (i.e., 5 times less energy deposition to underlying components) than the baseline of tungsten carbide and water shields of the same total thickness. For practically available tungsten boride and hafnium hydride materials (rather than pure HfH x or the specific tungsten boride composition used in the simulations) yielded similar results.

[0038] The tungsten boride or carbide layer may be formed of a composite material including the tungsten boride or carbide - for example, a cermet having tungsten boride or carbide particles in a metal matrix. In addition, as previously mentioned, the tungsten boride / carbide layer may contain a mixture of tungsten boride or carbide, which may be a composite material with some other material (for example, a cermet having both tungsten boride and tungsten carbide as aggregates).

[0039] Similarly, one or more metal hydride layers may be formed of a composite material including a metal hydride (e.g., a metal-clad metal hydride layer). The metal hydride layer may include hydrides of multiple metals in an alloy. This provides the advantage that because the thermal decomposition curve of the alloy hydride (i.e., the amount of hydrogen released from the material at a temperature) will have a wider temperature distribution, any system configured to absorb or remove hydrogen produced by thermal decomposition can be more easily accommodated by hydrogen. As an example, the metal hydride layer may contain any combination of metals known to form a bulk-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.

[0040] One possibility is to use a "high entropy alloy" class structure with 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 5% to 50% (or 5%-30%), so that no one 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.

[0041] When considering the possibility of composite materials, the "tungsten carbide and / or boride" material may be referred to as an "absorber layer" and the "metal hydride layer" may be referred to as a "decelerator layer." Although it will be understood that these designations should not be considered limiting of the function of either layer - since some deceleration may occur in the absorber layer and vice versa depending on the composition. The designations are primarily used to avoid any confusion that might suggest, for example, that a tungsten boride layer should be pure tungsten boride.

[0042] The shield may be continuous (e.g. Figure 2 As shown), that is, the tungsten boride layer and the metal hydride layer are adjacent to each other. Figure 3 As shown, there may be coolant channels 301, 302 or other elements (e.g., temperature sensors, radiation sensors, and / or stress sensors) between the tungsten boride 310 and metal hydride 320 layers, or within one or more layers. The coolant channels may be configured to carry a liquid or gaseous coolant (water may be used as a coolant, and the shield would still provide a useful alternative to existing designs, even if the shield would not achieve the elimination of water from the shield).

[0043] The maximum operating temperature of the shield is generally limited 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 may be slightly above the thermal decomposition temperature because the reactor containment can allow some hydrogen release. Encapsulation of the metal hydride with stainless steel or the like may be used to prevent hydrogen from being released into the reactor system. For applications with low or transient neutron loads (such as fusion reactors with pulse lengths of less than 10 seconds), direct cooling of the shield is generally not required (although this may be provided as a precaution). In applications with sustained high neutron loads, cooling will be required to keep the shield below the thermal decomposition temperature of the metal hydride being used. Typically, for hafnium hydride and yttrium hydride, the temperature should be maintained below about 600 °C. º C. For zirconium hydride, the temperature should be kept below about 300 º C, and for lithium hydride, the temperature should be kept below about 200 º C.

[0044] Cooling may be provided by coolant passages within the shield (as briefly described above), or by thermal conduction to coolant passages external to the shield.

[0045] Figure 4 A tokamak plasma chamber for use as a fusion reactor is shown, comprising a plasma vessel 41 and a magnetic plasma confinement system 42 for confining plasma 43. The tokamak also comprises neutron shields 44, 45 in a central column neutron shield section 44 and an outer shield section 45 located between the plasma vessel and the magnetic plasma confinement system.

[0046] Although the above is mainly directed to multilayer shields, similar principles can be applied to provide a single-layer neutron shield including metal hydrides. For example, a neutron shield including a neutron absorbing and decelerating material can be provided, which is a hydride of a metal alloy including at least one neutron absorbing element. For the purposes of this disclosure, a "neutron absorbing 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 of the neutron absorption cross sections within a specified range of all isotopes of the element (weighted by their abundance). The isotopic distribution of the neutron absorbing element can be their natural abundance, or it can be some other distribution with a desired average neutron absorption cross section. Suitable elements for alloys are the same as those discussed above for metal hydrides in multilayer solutions. Suitable neutron absorbing elements include hafnium, tungsten, boron, dysprosium and gadolinium (all of which are suitable for their natural isotopic abundances, but can be used with different isotopic abundances).

[0047] The proportion of neutron absorbing elements may be at least 5 mol%. The ratio of hydrogen to other elements within the material is between 0.1 and 4, more preferably between 1 and 2. The material may be a high entropy alloy comprising at least 5 elements other than hydrogen (at least one element being a strong neutron absorbing element), wherein each non-hydrogen element is between 5 mol.% and <50 mol.% of the alloy (in atomic proportions, excluding hydrogen). The material may be a hydride of a neutron absorbing element.

[0048] Such materials may also be used as part of a multi-layer shield, such as a shield that includes neutron absorbing and moderating material in addition to other materials that absorb or moderate neutrons, or a shield that includes neutron absorbing and moderating material and a blanket or similar protective layer.

[0049] The material may be graded so that the composition of the material varies over its thickness. For example, there may be a greater proportion of neutron absorbing material towards the radially outer surface of the neutron shield, and a greater proportion of hydrogen towards the radially inner surface of the neutron shield.

Claims

1. A neutron shield, comprising a metal hydride, wherein the metal hydride comprises a plurality of metals, wherein the plurality of metals comprises a neutron absorbing element, wherein the neutron absorbing element is a metal having: An average neutron absorption cross section greater than 0.1 barn over the neutron energy range between 0.02 eV and 0.03 eV; and It has a solid solubility of at least 1 mol % with all other metals in the metal hydride.

2. The neutron shield according to claim 1, wherein: The metal hydride is a hydride of a high entropy alloy.

3. The neutron shield according to claim 1, wherein: The metal hydride includes at least 5 elements other than hydrogen, and each element other than hydrogen has an atomic ratio of 5 mol.% to 50 mol.% relative to the total number of atoms of the elements other than hydrogen in the metal hydride.

4. The neutron shield according to claim 3, wherein: Every element except hydrogen is a metal.

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

6. The neutron shield according to claim 1, wherein: The proportion of the neutron absorbing element is at least 5 mol.% of the metal hydride.

7. The neutron shield according to claim 1, wherein: The composition of the metal hydride varies with the thickness of the neutron shield.

8. The neutron shield according to claim 7, wherein: The proportion of hydrogen in the neutron shield is greater toward the first surface of the neutron shield than toward the second surface of the neutron shield, and the proportion of the neutron absorbing element is greater toward the second surface of the neutron shield than toward the first surface of the neutron shield.

9. The neutron shield according to claim 8, wherein: The first surface is a radially outer surface and the second surface is a radially inner surface.

10. The neutron shield of claim 1, the metal hydride comprising a metal from Groups 4, 5, and 6 of the periodic table and / or one or more of yttrium, beryllium, gadolinium, or uranium.

11. The neutron shield according to claim 10, wherein: The neutron absorbing element is one of the following: hafnium; Tungsten; boron; Dysprosium; or gadolinium.

12. The neutron shield according to claim 11, wherein: The isotopic abundance of the neutron absorbing element is different from the abundance of a natural isotope of the neutron absorbing element.

13. The neutron shield of claim 1, comprising multiple layers, and further comprising a coolant channel, wherein: The coolant channels are provided between the layers, in each of the one or more layers, or a combination thereof.

14. The neutron shield of claim 1 including coolant channels in the metal hydride.

15. An assembly comprising a neutron shield according to claim 13 or 14, and a coolant source connected to a coolant channel, wherein the coolant source is configured to maintain the neutron shield at a temperature below the decomposition temperature of the metal hydride.

16. A tokamak nuclear fusion reactor, comprising: Toroidal plasma chamber; a plasma confinement system arranged to generate a magnetic field for confining plasma in an interior of the plasma chamber; The neutron shield according to any one of claims 1 to 14, arranged between the interior of the toroidal plasma chamber and the plasma confinement system.

17. The tokamak nuclear fusion reactor according to claim 16, wherein: The neutron shield is a neutron shield according to claim 13 or 14, and the tokamak nuclear fusion reactor comprises a coolant source connected to a coolant channel, 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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