Layered Neutron Shielding and Tokamak Nuclear Fusion Reactor

Abstract

LAYERED NEUTRON SHIELDING. The present invention relates to neutron shielding. The neutron shielding comprises a plurality of absorption layers (201, 203), and at least one moderating layer (202). The plurality of absorption layers each comprises tungsten boride or tungsten carbide. The at least one moderating layer comprises a metal hydride. Each moderating layer is situated between at least two absorption layers.

Description

1 / 13 Layered Neutron Shielding and Tokamak Nuclear Fusion Reactor Field of Invention

[001] The present invention relates to neutron shielding, particularly, though not exclusively, for use in tokamak fusion reactors. Background

[002] The challenge of producing fusion energy is extremely 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, releasing energetic neutrons and fusion products (e.g., 4He for DT). To date, the most promising way to achieve this is to use a tokamak device; in the conventional tokamak approach to fusion (as realized by ITER), the plasma needs to have a high confinement time, high temperature, and high density to optimize this process.

[003] A tokamak features a combination of a strong toroidal magnetic field Bt, high plasma current Ip, and generally a large plasma volume and significant auxiliary heating, to provide a stable hot plasma so that fusion can occur. Auxiliary heating (e.g., through tens of megawatts of high-energy neutral beam injection H, D, or T) is necessary to raise the temperature to sufficiently high values ​​required for nuclear fusion to occur, and / or to maintain the plasma current.

[004] In order to ensure that the reactor is as compact as possible (which allows for greater efficiency, particularly for a spherical plasma tokamak configuration), the thickness of the radiation shielding must be reduced as much as possible, while maintaining Petition 870260072378, dated 07 / 21 / 2026, page 6 / 47 2 / 13 at the same time adequate protection for the other components. Minimizing the distance between the plasma and the field coils allows for a larger magnetic field in the plasma with a smaller current in the coils.

[005] Figure 1 shows a section of the central column and illustrates the problems that the shielding material must overcome. The central column comprises a central core of High Temperature Superconductor (HTS) coils 11 and an outer shielding layer 12. Depending on the material used for the shielding, there may be a layer of shielding material 13 on the outer surface. There are three main causes of damage originating from the plasma 14. First, the high-energy neutrons 15 generated by the fusion reaction can essentially knock atoms out of the shielding structure, creating cascades of damage 16 that propagate through the material and degrade the material's properties (such as mechanical, thermal, or superconducting properties).Secondly, the heat flux 17 from the fusion reaction is significant, and can damage the shield due to thermal stresses induced by uneven heating and the HTS core, as higher temperatures reduce the current that can be carried while maintaining superconductivity, and can cause the coil to suddenly gain resistance, causing the magnet to turn off. Lastly, 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 can also contaminate the plasma if the shield is directly exposed to it. It is desirable to have a shielding material that can withstand these effects, as well as prevent neutrons from reaching the superconducting coils.

[006] Current shielding designs also frequently make use of water channels both to cool the shielding and to moderate neutrons (which increases the effectiveness of the shielding). Petition 870260072378, dated 07 / 21 / 2026, page 7 / 47 3 / 13 However, this presents problems, as water is difficult to handle during disposal or maintenance of the application – due to the risks of pressurized systems, contamination, activation and vaporization of the water, and the possibility of reactor water entering the environment if mishandled.

[007] There is therefore a need for effective neutron shielding that does not require water for moderation. Summary

[008] According to a first aspect, neutron shielding is provided. The neutron shielding comprises a plurality of absorption layers, and at least one moderating layer. The plurality of absorption layers each comprises tungsten boride or tungsten carbide. The at least one moderating layer comprises a metal hydride. Each moderating layer is between at least two absorption layers.

[009] According to a second aspect, neutron shielding is provided. The neutron shielding comprises a metal hydride. The metal hydride comprises a neutron-absorbing element that: It has an average neutron absorption cross-section that is greater than 0.1 barns in a neutron energy range between 0.02 and 0.03 eV; and it has a solid solubility of at least 1 mol% with all other metals in the alloy.

[0010] The average neutron absorption cross-section is an average of the neutron absorption cross-sections in the specified range of all isotopes of that element present (weighted by their abundance). The distribution of the neutron absorption element isotopes may be their natural abundance, or it may be Petition 870260072378, dated 07 / 21 / 2026, page 8 / 47 4 / 13 some other distribution with the required average neutron absorption cross-section.

[0011] According to a third aspect, a tokamak nuclear fusion reactor is provided. The reactor comprises a toroidal plasma chamber, a plasma confinement system, and neutron shielding. The plasma confinement system is arranged to generate a magnetic field to confine a plasma within the plasma chamber. The neutron shielding is the neutron shielding according to the first or second aspect, and is arranged between the interior of the toroidal plasma chamber and the plasma confinement system.

[0012] Further embodiments are presented in claim 2 et seq. Brief Description of the Drawings

[0013] Figure 1 illustrates the neutron shielding of a tokamak center column, and the design challenges of such;

[0014] Figure 2 is a schematic illustration of an exemplary neutron shield;

[0015] Figure 3 is a schematic illustration of another exemplary neutron shield;

[0016] Figure 4 is a schematic illustration of a tokamak plasma chamber. Detailed Description

[0017] Effective and compact radiation shielding to reduce the absorbed dose in sensitive materials originating from a high-energy neutron source (such as in the central column of a spherical tokamak) requires a combination of high Z (atomic mass) and low Z elements in bulk materials. High-energy neutrons with kinetic energies greater than a few MeV are effectively moderated by two main mechanisms: first, through nuclear reactions. Petition 870260072378, dated 07 / 21 / 2026, page 9 / 47 5 / 13 inelastic reactions with high Z nuclei, and secondly through elastic scattering with low Z nuclei. After inelastic scattering, the energies of secondary neutrons are typically below the reaction threshold for subsequent inelastic nuclear reactions with high Z elements and therefore can only be effectively moderated by low Z elements. A combination of high Z and low Z elements can therefore be used to reduce the flux of high-energy neutrons incident on sensitive materials beyond the shielding.

[0018] Effective high-energy neutron shielding requires four main sections, in order from the neutron source to the objects being shielded: • A material containing elements with high atomic mass, to reduce the neutron energy of the plasma (especially a material with a high cross-section for inelastic scattering reactions or neutron multiplication); • A neutron moderator (i.e., a material containing elements with low atomic mass), to further moderate the neutrons to ideal cross-sectional energies; • A neutron absorber, to absorb moderated neutrons; • A gamma shield, to absorb the gamma rays produced by neutron interactions in earlier stages.

[0019] Where a single material or composite material can perform multiple functions, for example, as a neutron absorber and a gamma shield, then two or more sections can be combined.

[0020] In conventional shielding, the neutron moderator is often water - which requires careful handling for disposal and safety, as described above.

[0021] Tungsten is the ideal choice as a constituent element. Petition 870260072378, dated 07 / 21 / 2026, page 10 / 47 6 / 13 high Z for the first stage, due to both its high Z number (74) and its typically high mass and number density in stable compounds, compared to other high Z elements such as lead. Tungsten borides are especially favorable for shielding applications as they add boron as a constituent element of neutron shielding; boron is an effective absorber of low-energy neutrons, preventing low-energy neutrons from penetrating the shielding. Furthermore, tungsten is an effective absorber of gamma radiation. As such, tungsten borides can act as all the above sections except for the neutron moderator (since boron does not have a significant moderating effect compared to, for example, hydrogen).Tungsten carbides are also favorable for shielding applications because carbon provides neutron energy moderation, although they are not as effective as, for example, hydrogen – as such, they can be used for all the above sections, but will generally outperform moderators made from hydrogen-containing materials, or as neutron absorbers made from tungsten boride.

[0022] Hydrogen is the ideal neutron moderator below a few MeV, and is available as a constituent element of many potential materials. However, the most common, water and hydrocarbons, are problematic when integrated into a fusion power plant. Metal hydrides are comparable in terms of hydrogen density to water and hydrocarbons, but remain solid at substantially higher temperatures than typical water or hydrocarbons at ambient pressure. This allows for easier design of neutron shielding structures (since solid components are easier to integrate than liquid components), and results in easier maintenance and decommissioning (since there is less risk). Petition 870260072378, dated 07 / 21 / 2026, page 11 / 47 7 / 13 risk of leakage of solid components).

[0023] As such, a composite shield as shown in Figure 2, which is a sandwich of tungsten boride and metal hydride, with two layers 201,203 of tungsten boride and a layer 202 of metal hydride between them, is very effective as a neutron shield, to protect the sensitive components 204 from neutron irradiation 205. In fact, a multilayer shield with alternating layers of tungsten boride (or tungsten carbide) and metal hydride, with tungsten boride (or tungsten carbide) as the radially outer layers, will generally be effective.

[0024] Tungsten boride can be supplied in various forms, for example, as a cemented tungsten boride (tungsten boride particles in a metallic matrix), sintered tungsten boride, or as an alloy with metallic tungsten and / or other elements, etc. Any of the tungsten boride compounds can be used, as required by the particular structural considerations of the shielding application. A promising approach is a two-phase tungsten structure with W2B, which can be formed in a known process involving vacuum pressing of pure tungsten with boron nitride, producing a material with useful thermal and mechanical properties.

[0025] Tungsten carbide can also be supplied in various forms, for example, as cemented tungsten carbide (tungsten carbide particles in a metallic matrix), or as ceramic-metal tungsten carbide (cermet), reactive sintered tungsten carbide / boride, or monolithic tungsten carbide.

[0026] Potential metal hydrides for use as moderators include lithium hydride (LiHx), hafnium hydride (HfHx), yttrium hydride (YHx), and zirconium hydride (ZrHx), or combinations thereof. In each case, the hydride has been identified with the chemical formula of the compound. Petition 870260072378, dated 07 / 21 / 2026, page 12 / 47 8 / 13 is most common – but other hydrides (or combinations thereof, or alloys with the pure metal) can be used. The exact ratio of hydrogen to metal x can be chosen based on the degree of moderation required, the structural properties demanded, and will generally be between 0.1 and 4, or between 1 and 2. A value of 1.33 was used in neutron shielding simulation tests.

[0027] Tungsten boride generally comprises the majority of the shielding, with the total thickness of the tungsten boride layers being between 75% and 99% of the total thickness of the tungsten boride and metal hydride, more particularly, between 80% and 95% of the total thickness of the tungsten boride and metal hydride. The tungsten bromide layer 201 facing the neutron source (i.e., one of the outer tungsten bromide layers) may be between 30% and 90% of the total thickness of the tungsten bromide and metal hydride, more particularly, between 40% and 80% of the total thickness. Although layer 201 is represented in Figure 2 as a single layer, it may be constructed from several individual layers.One of the other tungsten boride layers (i.e., one in which a metal hydride layer lies between this layer and the neutron-facing layer, usually the final 203 layer) may be at least 10% of the total thickness of tungsten boride and metal hydride. It will be appreciated 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.

[0028] In any of the above discussions, tungsten boride (WB) can be replaced by tungsten carbide (WC), or by a combination of tungsten carbide and tungsten carbide. Petition 870260072378, dated 07 / 21 / 2026, page 13 / 47 9 / 13

[0029] Structures with additional layers of HfHx and WB can also work well - for example, with multiple layers of HfHx each separated by WB or WC layers, and having the WB or WC layers as radially outer layers. Similar compositions would perform well for the other metal hydrides.

[0030] Simulations of WB-HfHx-WB layered shielding (with x=1.33) provided shielding that was up to 5 times more effective (i.e., resulted in 5 times less energy deposition to the underlying components) than a reference standard of tungsten carbide and water shielding with the same total thickness. Adjusting to practically obtainable tungsten boride and hafnium hydride materials (instead of pure HfHx or the particular tungsten boride composition used in the simulation) gave similar results.

[0031] Tungsten boride or carbide layers can be formed from composite materials containing tungsten boride or carbide – for example, a cermet with tungsten boride or carbide particles in a metallic matrix. Furthermore, as discussed earlier, the tungsten boride / carbide layer can contain a mixture of tungsten boride or carbide, which may be a composite material with some other material (for example, a ceramate having both tungsten boride and tungsten carbide as aggregates).

[0032] Similarly, the metal hydride layer or layers can be formed from composite materials that make up the metal hydride, for example, a metal-clad metal hydride layer. Metal hydride layers can comprise hydrides of multiple metals in an alloy. This provides an advantage insofar as the thermal decomposition curve of the alloy hydride (i.e., the amount of hydrogen released from the material) Petition 870260072378, dated 07 / 21 / 2026, page 14 / 47 10 / 13 at temperature) will have a broader temperature profile, allowing any systems configured to absorb or remove hydrogen produced as a result of thermal decomposition to more easily accommodate hydrogen. As an example, metal hydride layers can contain any combination of metals known to form body-centered cubic solid solutions with each other, for example, those from 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.

[0033] One possibility is to use a structure similar to a highly centralized alloy, which has several (e.g., at least 5) different metals, with atomic proportions (i.e., the number of atoms of that metal divided by the total number of metal atoms in the hydride) of 5% to 50% (or 5%-30%) for each metal, so that no metal dominates the properties of the material. As in the case of 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.

[0034] When considering the possibility of composite materials, tungsten carbide and / or boride materials may be referred to as absorption layers, and metal hydride layers may be referred to as moderating layers. While these labels should not be taken as limiting the function of either layer – since, depending on the composition, some moderation may occur in the absorption layer and vice versa – the labels primarily serve to avoid any confusion that might imply that, for example, the tungsten boride layer must be purely tungsten boride.

[0035] The shielding can be continuous (as shown in Figure 2), that is, with the tungsten boride and metal hydride layers. Petition 870260072378, dated 07 / 21 / 2026, page 15 / 47 11 / 13 such layers are placed against each other. Alternatively, as shown in Figure 3, there may be coolant channels 301, 302 or other elements (e.g., temperature, radiation and / or voltage sensors) between the tungsten boride 310 and the metal hydride 320 layers, or within one or more of the layers. The coolant channels may be configured to carry liquid or gaseous coolant (water may be used as a coolant, and the shielding would still provide a useful alternative to existing designs, even if it did not achieve the elimination of water from the shielding).

[0036] The maximum operating temperature of the shield will generally be defined by the thermal decay 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 higher than the thermal decay temperature, as some hydrogen release may be permitted by the reactor's safety case. Encapsulating the metal hydride with stainless steel or similar could be used to prevent hydrogen release into the reactor system. For applications with low or transient neutron charge, such as fusion reactors with pulse durations of less than 10 seconds, direct cooling of the shield will generally not be necessary (although it may be provided as a precaution).In applications with sustained high neutron load, cooling will be necessary to keep the shielding below the thermal decay temperatures of the metal hydride used. In general, 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.

[0037] Cooling can be provided by refrigeration channels Petition 870260072378, dated 07 / 21 / 2026, page 16 / 47 12 / 13 te inside the shielding (as briefly described above), or by thermal conduction to refrigerant channels outside the shielding layers.

[0038] Figure 4 shows a tokamak plasma chamber, for use as a fusion reactor, comprising a plasma vessel 41 and a magnetic plasma confinement system 42, for confining a plasma 43. The tokamak also comprises a neutron shield 44, 45 located between the plasma vessel and the magnetic plasma confinement system, both in a central neutron shield column section 44 and in an outer shield section 45.

[0039] Although the above has been primarily directed toward multilayer shielding, similar principles can be applied to provide single-layer neutron shielding comprising metal hydrides. For example, a neutron shield can be provided comprising a neutron-absorbing and moderating material that is a hydride of a metal alloy comprising at least one neutron-absorbing element. In terms of this disclosure, a neutron-absorbing element has an average neutron-absorbing cross-section greater than 0.1 barns in a neutron energy range between 0.02 and 0.03 eV. The average neutron-absorbing cross-section is an average of the neutron-absorbing cross-sections in the specified range of all isotopes of that element present (weighted by their abundance).The distribution of neutron-absorbing element isotopes can be their natural abundance, or it can be some other distribution with the required average neutron-absorbing cross-section. The suitable elements for the alloy are the same as those discussed above for the metal hydride in multilayer solution. The suitable neutron-absorbing elements in... Petition 870260072378, dated 07 / 21 / 2026, page 17 / 47 13 / 13 include hafnium, tungsten, boron, dysprosium, and gadolinium (all suitable at their natural isotopic abundance, but can be used with a different isotopic abundance).

[0040] The proportion of the neutron-absorbing element 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 besides hydrogen (at least one of which is a strongly neutron-absorbing element), wherein each of the non-hydrogen elements is between 5 mol% and <50 mol% of the alloy (by atomic proportion, excluding hydrogen). The material may be a hydride of the neutron-absorbing element.

[0041] Such material may also be used as part of a multilayer shield, for example, a shield comprising the material that absorbs and moderates neutrons, in addition to other materials that absorb or moderate neutrons, or a shield comprising the material that absorbs and moderates neutrons and a protective coating layer, or similar.

[0042] The material can be classified in such a way that its composition varies across its thickness. For example, there may be a higher proportion of neutron-absorbing material toward the radially outer surfaces of the neutron shield, and a higher proportion of hydrogen toward the radially inner surfaces of the neutron shield. Petition 870260072378, dated 07 / 21 / 2026, page 18 / 47

Claims

1 / 3 CLAIMS 1. Neutron shielding (44, 45), characterized in that it comprises: a plurality of absorption layers (201, 203; 310), each comprising a tungsten boride or a tungsten carbide; and at least one moderating layer (202; 320) comprising a metal hydride; wherein each moderating layer (202; 320) is between at least two absorption layers; configured in such a way that, in use, the outermost absorption layer lies between the adjacent moderating layer and the neutron source.

2. Neutron shielding (44, 45), according to claim 1, characterized in that the metal hydride comprises one or more of the metals from groups 4, 5 and 6 of the periodic table and / or yttrium, beryllium, gadolinium or uranium.

3. Neutron shielding (44, 45), according to claim 2, characterized in that the metal hydride comprises one or more of hafnium, niobium, tantalum, titanium, yttrium and zirconium.

4. Neutron shielding (44, 45), according to any one of the preceding claims, characterized in that the metal hydride comprises at least 5 metals, each metal having an atomic proportion of 5 mol% to 50 mol% compared to the total number of metal atoms in the metal hydride.

5. Neutron shielding (44, 45), according to any one of the preceding claims, characterized in that the metal hydride has a ratio of hydrogen atoms to metal atoms between 0.1 and 4.

6. Neutron shielding (44, 45), according to any Petition 870260072378, of 07 / 21 / 2026, page 19 / 47 2 / 3 one of the preceding claims, characterized in that each absorption layer comprises a tungsten metal alloy and tungsten boride and / or carbide.

7. Neutron shielding (44, 45), according to any of the preceding claims, characterized in that the total thickness of the absorption layers (201, 203; 310) is at least 75% of the combined total thickness of the absorption layers (201, 203; 310) and the moderating layers (202; 320).

8. Neutron shielding (44, 45), according to any of the preceding claims, characterized in that the outermost absorption layer has a thickness that is between 30% and 90% of the total combined thickness of the absorption layers (201, 203; 310) and the moderating layers (202; 320).

9. Neutron shielding (44, 45), according to claim 8, characterized in that an additional absorption layer that is not the outermost absorption layer has a thickness that is at least 10% of the total combined thickness of the absorption layers (201, 203; 310) and the moderating layers (202; 320).

10. Neutron shielding (44, 45), according to any of the preceding claims, characterized in that it comprises coolant channels (301, 302) located between the absorption layers (201, 203; 310) and the moderation layers (202; 320).

11. Neutron shielding (44, 45), according to any of the preceding claims, characterized in that it comprises coolant channels (301, 302) integrated within one or more absorption layers (201, 203; 310) and / or moderation layers (202; 320).

12. Neutron shielding (44, 45), according to claim 10, as set out in Petition 870260072378, dated 21 / 07 / 2026, page 20 / 47 3 / 3, characterized in that it comprises a coolant source connected to the coolant channels, wherein the coolant source is configured to maintain the plurality of absorption layers and at least one moderation layer of the neutron shielding (44, 45) at a temperature below the metal hydride decay temperature.

13. Tokamak nuclear fusion reactor, characterized in that it comprises: a toroidal plasma chamber (41); a plasma confinement system (42) arranged to generate a magnetic field to confine a plasma within the plasma chamber; neutron shielding (44, 45), as defined in any one of claims 1 to 14-11, arranged between the interior of the toroidal plasma chamber and the plasma confinement system, such that the outermost layer faces the interior of the toroidal plasma chamber.

14. Tokamak nuclear fusion reactor, according to claim 13, characterized in that the neutron shield (44, 45) is the neutron shield (44, 45) as defined in claim 10 or 11, and comprising a coolant source connected to the coolant channels (301, 302), wherein the coolant source is configured to maintain the plurality of absorption layers and at least one moderating layer of the neutron shield (44, 45) at a temperature below a metal hydride decay temperature. Petition 870260072378, dated 21 / 07 / 2026, p. 21 / 47

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