EA043044B1 - MULTILAYER NEUTRON SHIELD - Google Patents

Description

Field of invention

The present invention relates to neutron shielding, in particular, although not exclusively, for use in tokamak fusion reactors.

Background of the invention

The problem of fusion energy production is extremely complex. Fusion neutrons are produced by heating deuterium-tritium (DT) or deuterium-deuterium (DD) plasma so that the nuclei have enough energy to overcome the Coulomb electrostatic repulsion to fuse with each other, releasing energetic neutrons and fusion reaction products (e.g., ⁴ He for DT). To date, the most promising way to achieve this is through the use of a tokamak device; In the traditional approach to fusion in tokamaks (implemented by ITER), to optimize this process, the plasma must have a high confinement time, high temperature, and high density.

The tokamak is characterized by a combination of a strong toroidal magnetic field BT, a high plasma current Ip and a generally large plasma volume and significant concomitant heating to provide a hot stable plasma whereby nuclear fusion can occur. Co-heating (eg, by injection of tens of megawatts of H, D, or T high energy neutral particle beams) is needed to increase the temperature to high enough temperatures required for nuclear fusion to occur and/or to maintain plasma current.

In order to keep the reactor as compact as possible (which allows higher efficiency, in particular with a spherical tokamak plasma configuration), the shielding thickness should be reduced as much as possible while still providing adequate shielding for other components. Minimizing the distance between the plasma and the excitation coils allows for a stronger magnetic field in the plasma with a lower current in the coils.

Fig. 1 shows a section of the central column and illustrates the problems that the protective material must overcome. The central column contains a central core of high temperature superconductor (HTSC) coils 11 and an outer protection layer 12. Depending on the material used for protection, there may be a layer of oxidized protective material 13 on the outer surface. There are three main causes of damage that arise from for plasmas 14. First, the high-energy neutrons 15 produced by the fusion reaction can essentially knock atoms out of the shielding structure, creating damage cascades 16 that propagate through the material and degrade the properties of the materials (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 the thermal stresses introduced by uneven heating and the HTS core, since higher temperature reduces the current that can be carried during superconductivity maintenance and can cause the coil resistance to rise sharply. causing the loss of superconductivity of the magnet. Finally, the plasma energy particles will cause ablation 18 of 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 resist these effects as well as prevent neutrons from reaching the superconducting coils.

Existing shielding structures also often use water channels both to cool the shield and to moderate neutrons (which increases the effectiveness of the shield). However, this creates problems because water is difficult to handle during installation or maintenance of apparatus due to the risks of pressurized systems, contamination, activation and evaporation of water, and the possibility of water from the reactor being released into the environment if handled improperly.

Therefore, there is a need for effective neutron shielding that does not require water for moderation.

The essence of the invention

According to the first aspect, neutron shielding is proposed. The neutron shield contains a plurality of absorbing layers and at least one moderating layer. Each of the plurality of absorbent layers contains tungsten boride or tungsten carbide. Said at least one retardation layer contains a metal hydride. Each slowing layer is located between at least two absorbing layers.

According to the second aspect, neutron shielding is proposed. The neutron shield contains a metal hydride. The metal hydride contains a neutron absorbing element that:

has an average neutron absorption cross section that is greater than 0.1 B (barn) in the neutron energy range between 0.02 and 0.03 eV; and has a solid phase solubility of at least 1 mol% with all other metals in the alloy.

The mean neutron absorption cross section is the average of the neutron absorption cross sections in the specified range of all isotopes of that element present (weighted by their number).

- 1 043044 laziness). The isotope distribution of the neutron absorbing element may correspond to their abundance in nature, or it may be some other distribution with the required average neutron absorption cross section.

According to a third aspect, a tokamak fusion reactor is provided. The reactor contains a toroidal plasma chamber, a plasma confinement system and a neutron shield. The plasma confinement system is configured to create a magnetic field to ensure plasma confinement in the interior of the plasma chamber. The neutron shield is a neutron shield in the first or second aspect and is located between the interior of the toroidal plasma chamber and the plasma confinement system.

Additional options for implementation are presented in paragraph 2 and subsequent paragraphs of the claims.

Brief description of the figures

Fig. 1 illustrates the neutron shielding of the central column of the tokamak and its design problems.

Fig. 2 is a schematic illustration of an exemplary neutron shield.

Fig. 3 is a schematic illustration of another exemplary neutron shield.

Fig. 4 is a schematic illustration of a tokamak plasma chamber.

Detailed description of the invention

An efficient, compact radiation shield to reduce the radiation dose absorbed in sensitive materials from a source of high energy neutrons (such as in the central column of a spherical tokamak) requires a combination of elements with high atomic number Z (atomic mass) and elements with low Z in bulk materials. High-energy neutrons with kinetic energies greater than a few MeV are effectively slowed down by two main mechanisms: firstly, through nuclear inelastic scattering reactions with high-Z nuclei and secondly, through elastic scattering with low-Z nuclei. After inelastic scattering, secondary neutron energies are typically lower threshold reaction energies for subsequent nuclear inelastic scattering reactions with high-Z elements, and therefore they can 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 flux of high-energy neutrons incident on sensitive protective screen materials.

An effective high-energy neutron shield requires four main sections, in order from the neutron source to the objects to be protected:

a material containing high atomic mass elements to reduce the energy of neutrons from the plasma (especially a material with a large cross section for inelastic scattering or neutron multiplication reactions);

a neutron moderator (ie, a material containing low atomic mass elements) to further moderate neutrons to optimal cross-sectional energies;

a neutron absorber to absorb delayed neutrons;

gamma shield to absorb gamma rays produced by earlier neutron interactions.

If a single material or composite material can serve multiple functions, such as neutron absorber and gamma shield, then two or more sections can be combined.

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

Tungsten is an ideal choice as a high Z constituent element for the first stage due to its high Z atomic 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 particularly favorable for shielding applications as they add boron as a constituent element of the neutron shield; Boron is an effective low energy neutron absorber, preventing low energy neutrons from penetrating the shield. In addition, tungsten is an effective absorber of gamma radiation. Essentially, tungsten borides can act as all of the above sections, with the exception of the neutron moderator (because boron does not have a significant moderating effect compared to, for example, hydrogen). Tungsten carbides are also favorable for shielding applications, as carbon moderates energetic neutrons, although not as effectively as, for example, hydrogen - in fact, they can be used for all of the above sections, but will generally be superior to hydrogen-containing materials as moderators. or tungsten boride as neutron absorbers.

Hydrogen is an ideal neutron moderator below a few MeV and is available as a constituent of many potential materials. However, the most common ones, water and hydrocarbons, are problematic when incorporated into a fusion power plant.

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Metal hydrides are comparable in terms of hydrogen density to water and hydrocarbons, but remain solid at significantly higher temperatures than water or typical hydrocarbons at room pressure. This allows for easier design of neutron shield structures (since solid components are easier to combine than liquid components) and results in easier maintenance and decommissioning (since there is a lower risk of leakage of solid components).

As such, the composite shield as shown in FIG. 2, which is a sandwich of tungsten boride and metal hydride, with two tungsten boride layers 201, 203 and a metal hydride layer 202 in between, is very effective as a neutron shield to protect sensitive components 204 from neutron irradiation 205. In fact, multi-layer a 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.

The tungsten boride can be provided in various forms, such as cemented tungsten boride (particles of tungsten boride in a metal matrix), sintered tungsten boride, or as an alloy with metallic tungsten and/or other elements, etc. Any of the tungsten boride compounds may be used as required by the particular design considerations for the shield application. One promising approach is the two-phase structure of tungsten with W ₂ B, which can be formed in a known process involving vacuum hot pressing of pure tungsten with boron nitride, yielding a material with useful thermal and mechanical properties.

The tungsten carbide can also be provided in various forms, for example, as cemented tungsten carbide (particles of tungsten carbide in a metal matrix) or as sintered tungsten carbide (cermet), reaction sintered tungsten carbide/boride, or solid tungsten carbide.

Potential metal hydrides for use as a moderator include lithium hydride (LiH _x ), hafnium hydride (HfHx), yttrium hydride (YH _x ), and zirconium hydride (ZrH _x ), or combinations thereof. In each case, the hydride has been identified by the chemical formula of the most common compound, but other hydrides (or combinations thereof, or pure metal alloys) may be used. The exact ratio x of hydrogen to metal can be chosen based on the degree of retardation required, the structural properties required, and will generally be between 0.1 and 4, or between 1 and 2. A value of 1.33 was used in a simulated neutron shield test.

Tungsten boride will typically make up the majority of the protection, with the total thickness of the tungsten boride layers being between 75% and 99% of the total thickness of tungsten boride and metal hydride, more specifically between 80% and 95% of the total thickness of tungsten boride and metal hydride. The tungsten boride layer 201 facing the neutron source (i.e., one of the outer layers of tungsten boride) may be between 30% and 90% of the total thickness of the tungsten boride and metal hydride, more specifically between 40% and 80% of their total thickness. Although layer 201 is depicted in FIG. 2 as a single layer, it can be built from many individual layers. One of the other tungsten boride layers (i.e., the layer where the metal hydride layer is between this layer and the neutron-facing layer, generally end layer 203) may be at least 10% of the combined thickness of the tungsten boride and metal hydride. It will be understood that the upper limit of the thickness of the other tungsten boride layers is determined by the difference between the total thickness of the tungsten boride and the thickness of the tungsten boride in the neutron facing layer.

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

Designs with additional HfH _x and WB layers may also work well, such as with multiple HfH _x layers each separated by WB or WC layers and having WB or WC layers as radially outer layers. Similar formulations are expected to work well for other metal hydrides.

Simulations of layered WB-HfH _x -WB protection (with x=1.33) provided protection that was up to 5 times more effective (i.e. resulted in 5 times less energy impact on underlying components) than control protection. from tungsten carbide and water of the same total thickness. Its adjustment for practical tungsten boride and hafnium hydride materials (rather than pure HfHx or the specific tungsten boride composition used in the simulation) gave similar results.

Layers of boride or tungsten carbide can be formed from composite materials containing boride or tungsten carbide, such as cermet with particles of boride or tungsten carbide in a metal matrix. Additionally, as discussed earlier, the tungsten boride/tungsten carbide layer may comprise a mixture of tungsten boride or carbide, which may be a composite material with some other material (eg, cermet having both tungsten boride and tungsten carbide as aggregates).

- 3 043044

Similarly, the metal hydride layer or layers can be formed from metal hydride-containing composite materials, such as a metal-clad metal hydride layer. The metal hydride layers may contain multiple metal hydrides in the alloy. This has the advantage that the thermal decomposition curve of the alloy hydride (i.e., the amount of hydrogen released from the material at temperature) will have a broader temperature profile, allowing any systems designed to absorb or remove thermally decomposed hydrogen. , it is more easy to take hydrogen. As an example, the metal hydride layers may contain any combination of metals known to form solid solutions with each other with a body-centered cubic structure, such as metals found 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.

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

When composite materials are considered, materials with carbide and/or tungsten boride may be referred to as absorber layers and metal hydride layers may be referred to as retardation layers. Although it will be understood that these designations should not be taken as limiting the function of one layer or another - depending on the composition, some delay may occur in the absorbent layer and vice versa. The designations mainly serve to avoid any confusion which might imply that, for example, the tungsten boride layer must be pure tungsten boride.

The protective screen may be continuous (as shown in FIG. 2), ie. with layers of tungsten boride and metal hydride adjacent to each other. Alternatively, as shown in FIG. 3, coolant channels 301, 302 or other elements (eg, temperature, radiation, and/or stress sensors) may be present between the tungsten boride 310 and metal hydride 320 layers, or in one or more of these layers. The coolant passages can be configured to carry liquid or gaseous coolant (water can be used as the coolant and the shield will still provide a useful alternative to existing designs even if water exclusion from the shield is not achieved).

The maximum operating temperature of the protection will generally be determined by the thermal decomposition temperature of the metal hydride (ie, the temperature at which the metal hydride will partially decompose and release hydrogen). The actual operating temperature may be slightly higher than the thermal decomposition temperature, as some release of hydrogen may be allowed by the safety engineering of the reactor. Encapsulation of the metal hydride with stainless steel or the like can be used to prevent the release of hydrogen into the reactor system. For applications with low or transient neutron loads, such as fusion reactors with pulse durations less than 10 s, direct cooling of the shield will not normally be required (although it may be provided as a precautionary measure). In applications with a sustained high neutron load, refrigeration will be required to keep the shielding below the thermal decomposition temperatures of the metal hydride used. In general, for hafnium hydride and yttrium hydride, the temperature must be maintained below about 600°C, for zirconium hydride, the temperature must be maintained below about 300°C, and for lithium hydride, the temperature must be maintained below about 200°C.

Cooling may be provided by coolant passages in the shield (as described briefly above) or by conduction of heat into coolant passages outside the shield layers.

Fig. 4 shows a tokamak plasma chamber for use as a fusion reactor which includes a plasma vessel 41 and a magnetic plasma confinement system 42 to provide plasma confinement 43. The tokamak also includes a neutron shield 44, 45 located between the plasma vessel and the magnetic plasma confinement system, in the neutron shield section 44 of the central column and the outer shield section 45.

While the foregoing has been primarily directed to a multilayer shield, similar principles can be applied to provide a single layer neutron shield containing metal hydrides. For example, a neutron shield may be provided that contains a neutron absorbing and moderating material that is a metal alloy hydride containing at least one neutron absorbing element. In terms of this disclosure, the neutron absorbing element has an average neutron absorption cross section that is greater than 0.1 B (barn) in the neutron energy range between 0.02 and 0.03 eV. The average neutron absorption cross section is the average value of the neutron absorption cross sections in the indicated range of all

- 4,043,044 isotopes of that element present (weighted by their abundance). The isotope distribution of the neutron absorbing element may correspond to their abundance in nature, or it may be some other distribution with the required average neutron absorption cross section. Suitable elements for the alloy are the same as those discussed above for the metal hydride in the multilayer solution. Suitable neutron absorbing elements include hafnium, tungsten, boron, dysprosium, and gadolinium (all of which are suitable at their natural isotopic composition but may be used with other isotopic compositions).

The proportion of the neutron absorbing element may be at least 5 mol%. The ratio of hydrogen to other elements in such a material is between 0.1 and 4, more preferably between 1 and 2. The material may be a high entropy alloy containing at least five elements other than hydrogen (at least one of which is highly neutron absorbing). element), with each of the non-hydrogen elements comprising between 5 and <50 mole % of the alloy (by atomic fraction, excluding hydrogen). The material may be a neutron absorbing element hydride.

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

The material may be graded so that its composition varies across its thickness. For example, there may be a higher proportion of neutron absorbing material closer to the radially outer surfaces of the neutron shield and a higher proportion of hydrogen closer to the radially inner surfaces of the neutron shield.

Claims (14)

1. Neutron shielding, comprising: a plurality of absorbent layers each containing tungsten boride or tungsten carbide; and at least one retardation layer containing a metal hydride; wherein each slowing layer is located between at least two absorbing layers; and configured such that, in use, the outermost absorbing layer is located between the adjacent moderating layer and the neutron source. 2. Neutron protection according to claim 1, wherein the metal hydride contains one or more metals from groups 4, 5 and 6 of the Periodic Table and/or yttrium, beryllium, gadolinium or uranium. 3. Neutron shielding according to claim 2, wherein the metal hydride contains one or more of hafnium, niobium, tantalum, titanium, yttrium, and zirconium. 4. Neutron shielding according to any one of the preceding claims, wherein the metal hydride contains at least five metals, each metal having an atomic fraction of 5 to 50 mol% compared to the total number of metal atoms in the metal hydride. 5. Neutron shielding according to any one of the preceding claims, wherein the metal hydride has a ratio of hydrogen atoms to metal atoms between 0.1 and 4, more preferably between 1 and 2. 6. Neutron shielding according to any one of the preceding claims, wherein each absorbing layer comprises an alloy of tungsten metal and tungsten boride and/or carbide. 7. Neutron shielding according to any preceding claim, wherein the total thickness of the absorbing layers is at least 75% of the total combined thickness of the absorbing layers and moderating layers, more preferably between 80 and 95%. 8. Neutron shielding according to any preceding claim, wherein the outermost absorbing layer has a thickness that is between 30% and 90% of the total combined thickness of the absorbing layers and moderating layers, more preferably between 40% and 80%. 9. Neutron shielding according to claim 8, wherein the additional absorbing layer, which is not the outermost absorbing layer, has a thickness that is at least 10% of the total combined thickness of the absorbing layers and moderating layers. 10. Neutron shielding according to any one of the preceding claims, comprising coolant channels located between the absorbing layers and the moderating layers. 11. Neutron shielding according to any preceding claim, comprising coolant channels embedded in one or more of the absorbing layers and/or moderating layers. 12. A neutron shield assembly comprising a neutron shield according to claim 10 or 11 and a coolant source connected to the coolant channels, the coolant source being configured to maintain the neutron shield at a temperature below the decomposition temperature of the metal hydride. 13. Thermonuclear tokamak reactor, containing: toroidal plasma chamber; - 5 043044 a plasma confinement system configured to create a magnetic field to ensure plasma confinement in the interior of the plasma chamber; a neutron shield according to any one of claims 1 to 11 placed 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. The tokamak fusion reactor according to claim 13, wherein the neutron shield is the neutron shield according to claim 10 or 11 and comprises a coolant source connected to the coolant channels, the coolant source being configured to maintain the neutron shield at a temperature below the decomposition temperature metal hydride.