Tungsten boride neutron shielding improvement material

Abstract

The use of ditungsten pentaboride W2B5 in neutron shielding is disclosed.

Description

[Technical Field]

[0001] This invention relates to neutron shielding materials for nuclear fusion reactors. In particular, this invention relates to neutron shielding materials containing tungsten boride. [Background technology]

[0002] The challenges of nuclear fusion power generation are extremely complex. Fusion neutrons are needed when a deuterium-tritium (DT) plasma or deuterium-deuterium (DD) plasma is heated and has enough energy for the atomic nuclei to overcome Coulomb electrostatic repulsion and fuse, and when high-energy neutrons and fusion products are produced (for example, in the case of DT) 4 It is produced when it begins to emit He). Currently, the most promising way to achieve this is to use a tokamak device. In conventional tokamak approaches to nuclear fusion (such as those embodied by ITER), the plasma needs to have a long confinement time, high temperature, and high density in order to optimize this process.

[0003] A tokamak is a powerful toroidal magnetic field B T And, high plasma current I p This typically involves a large plasma volume and a combination of considerable auxiliary heating, providing a high-temperature, stable plasma in which fusion can occur. The auxiliary heating (e.g., by injecting a neutral beam of several tens of megawatts of high energy H, D, or T) is necessary to raise the temperature to a sufficiently high value required for fusion to occur and / or to maintain the plasma current.

[0004] To make the reactor as compact as possible (which allows for higher efficiency, especially in the case of a "spherical tokamak" plasma configuration), the thickness of the radiation shielding must be reduced as much as possible while maintaining sufficient protection for other components. Minimizing the distance between the plasma and the magnetic field coils allows for higher magnetic fields in the plasma at lower currents in the coils.

[0005] Figure 1 shows a cross-section of the central column, illustrating the problems that the shielding material must overcome. The central column includes the central core of a high-temperature superconducting (HTS) coil 11 and the outer layer 12 of the shielding body. Depending on the material used for the shielding body, if the shielding body is exposed to air while operating at high temperatures, a layer of oxidized shielding material 13 may be present on the outer surface. There are three main causes of damage due to the plasma 14. Firstly, high-energy neutrons 15 produced by the nuclear fusion reaction can essentially knock atoms out of the shielding structure, creating a damage cascade 16 that propagates through the material and degrades material properties (such as mechanical, thermal, or superconducting properties). Secondly, the heat flux 17 from the nuclear fusion reaction is large and can damage the shielding body due to uneven heating and thermal stress induced by the HTS core. This is because at high temperatures, the amount of current that can flow while maintaining superconductivity decreases, and the resistance of the coil can suddenly increase, potentially causing the magnet to quench. Finally, high-energy particles in the plasma can strip away the outer surface of the shielding body 18. This can not only damage the shielding material itself, but also contaminate the plasma if the shielding material is directly exposed to it. It is desirable to have a shielding material that can resist these effects and prevent neutrons from reaching the superconducting coil.

[0006] Current shielding designs also often utilize water channels for both cooling the shield and slowing neutrons (to enhance the shield's effectiveness). However, this presents problems because handling water during disposal or maintenance of the application is difficult due to the risks of pressurizing the system, contamination, activation and evaporation of the water, and the possibility of reactor water entering the environment if mishandled. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Therefore, an effective neutron shield that does not require water for deceleration is needed. [Means for solving the problem]

[0008] According to a first aspect of the present invention, the use of ditungsten pentaboride W2B5 in a neutron shield is provided.

[0009] According to a second embodiment, a neutron shield containing ditungsten pentaboride W2B5 is provided.

[0010] According to a third embodiment, a tokamak fusion reactor is provided, comprising a plasma chamber, a toroidal magnetic field coil, a plurality of poloidal magnetic field coils, and a neutron shield located inside the plasma chamber and between the toroidal magnetic field coil or the poloidal magnetic field coil, wherein the neutron shield is a shield according to a second embodiment. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram of the shielding layer of the central column of the tokamak. [Figure 2] This graph shows the neutron flux of tungsten boride and tungsten carbide shielding materials. [Figure 3] This graph shows the energy storage from neutrons and gamma rays in tungsten boride and tungsten carbide shielding materials. [Figure 4] This graph shows the atomic densities of tungsten and boron or carbon within a shielding material, as well as their sums. [Figure 5] This graph shows the percentage of 10B isotopes remaining in tungsten boride material after 30 years of operation, as a function of different levels of boron content within the neutron shield. [Figure 6] This graph shows the peak neutron flux in the HTS core (same as in Figure 1) for different isotopic concentrations of 10B isotope in the shielding material. [Modes for carrying out the invention]

[0012] The concept of conventional neutron shields was based on tungsten carbide and / or tungsten boride rich in tungsten. Tungsten, due to its high Z number and usually high-density tungsten compounds, is an effective photon absorber. Tungsten is also effective as an inelastic scatterer that reduces the energy of incident neutrons of about 14 MeV. Tungsten carbide provides an additional advantage in that carbon is a moderately effective neutron moderator (briefly speaking, it slows down neutrons to make them easier to be absorbed by tungsten). Tungsten boride provides an additional advantage in that boron is an effective absorber of low-energy neutrons that can generally pass through tungsten-based shields.

[0013] During the study of possible compositions of tungsten carbide and tungsten boride, surprisingly, W2B5 (ditungsten pentaboride), a specific stoichiometric composition of tungsten boride, was found to be a significantly more effective shielding material against both gamma rays and neutrons than other tungsten borides or tungsten carbides in the intensity and energy range expected in a tokamak fusion reactor.

[0014] Figure 2 shows the simulation results of various tungsten boride materials (along with tungsten and tungsten carbide for comparison) regarding neutron absorption with a water moderator layer present (201) or absent (202). Since the measurement is the neutron flux to the high-temperature superconducting HTS central column of a tokamak fusion reactor, the lower the value, the better. The scale is logarithmic. The tungsten borides examined are W2B, WB, W2B5, and WB4. Also, tungsten carbide (WC, shown as a horizontal line from the Y-axis) and more complex composite materials, B 0.329 C 0.074 Cr 0.024 Fe 0.274 W 0.299 are being examined.

[0015] As can be seen from the graph, W2B5 is significantly superior in neutron absorption performance compared to other compositions. In fact, W2B5 is an excellent absorber such that its performance improves when more water moderators are replaced with W2B5. This is because the moderation effect of water does not sufficiently push back the substances that the remaining W2B5 constitutes in order to create space for the moderators. That is, usually, the presence of moderators enables more neutrons to be absorbed due to a larger cross-sectional area for absorbing slower neutrons, but this effect is completely offset by the increase in the absorption ability of W2B5.

[0016] Figure 3 shows the actual energy deposition into HTS materials in the same simulation as in Figure 2 for both direct energy deposition by neutrons and secondary energy deposition by gamma rays. The graphs shown are for gamma-ray energy deposition with water moderators (301) and without water moderators (302), and for neutron energy deposition with water moderators (303) and without water moderators (304). As before, the lower the value, the better, and the same compounds are plotted. As in this graph as well, it can be seen that W2B5 is the best in all cases. Since the neutrons reaching HTS have high energy, the direct energy deposition by neutrons becomes high even when there is no water moderator, despite the low neutron flux. However, the secondary deposition by gamma rays is low for W2B5 when there is no moderator, and considering the logarithmic scale of the graph, naturally the total energy deposition is also low in this case.

[0017] This is due to the unusually high density (about 13 g / cm 3It is theorized that this occurs due to the particularly close-packed crystal structure of W2B5, which has such high atomic number densities (i.e., number of atoms per unit volume) both tungsten and boron that they are higher than expected when compared to other stoichiometric compositions. This is shown in Figure 3, which shows the atomic densities (atoms per cubic centimeter) of tungsten 401 and boron or carbon 402, and their sum 403 for various stoichiometric compositions of pure tungsten, tungsten carbide, and tungsten boride. As can be seen, W2B5 has the highest atomic density of boron among all the stoichiometric compositions considered, far exceeding the trend line for tungsten atomic density. The total atomic density including both tungsten and boron is also the highest. This is important because both boron and tungsten play important roles in shielding.

[0018] There is some debate in the scientific community regarding the precise structure of W2B5. Structures with space group P63 / mmc (metal matrix composites) are known to contain a tungsten boride phase consisting of alternating layers of boron layers made up of graphite-like planar layers and condensed cyclohexane-like chairs containing tungsten atoms positioned between the boron layers. For this structure to be W2B5, the center of each cyclohexane-like ring must contain an additional boron atom, and the main point of discussion is whether this configuration is stable. If the additional boron atom is completely absent, the structure is correctly identified as W2B4; if there is only partial occupancy (i.e., boron atoms are present in some units of the structure but not in others), the structure is W2B 4+x It is correctly identified as W2B5. However, since W2B5 is the most common description of this structure in the literature, it is the term used herein. W2B4 or W2B 4+xIf the structure is correct, the proportion of boron in the phase will be slightly lower than described herein, but the overall conclusion that this is the best phase for use in neutron shielding remains unchanged, and the description of W2B5 herein can be replaced with the description of the correct formula. Other phases may be present in smaller proportions within the boride, but the desired phase (W2B4, W2B 4+x , or W2B5) make up the majority.

[0019] In general, W2B5 can be incorporated into existing designs that use other tungsten boride formulations. For example, W2B5 can be incorporated as solid sintered W2B5 or as a tungsten boride component in cemented tungsten boride containing W2B5 particles in a metal binder. Although the above results indicate that a moderator is not required, W2B5-based shielding can still be provided with a moderator such as water or other hydrogen-containing materials, or any other suitable neutron moderator known in the art. For example, providing a moderator may be beneficial if W2B5 is included as part of a composite material such as cermet, ceramic, or cemented tungsten boride, and the combination of the composite and moderator provides better neutron absorption in the target range than the composite alone. Moderators may also be beneficial if the expected neutron energy differs from that of the 14.1 MeV fusion neutrons used in the above simulation, and / or if water (or other moderators) is used both as a moderator and to cool a neutron shield or other nearby components.

[0020] W2B5 can be provided as a single component on a composite shield, for example, including additional materials to provide further absorption for gamma rays, neutrons of different energies, or any other type of radiation. The W2B5 shield may include structural and cooling components that can be made from any suitable material.

[0021] It should be understood that the advantages of W2B5 are not specific to particular shielding applications (e.g., shape or structure), but rather lie primarily in its performance as a shielding material.

[0022] The increase in neutron absorption for a neutron shield of a given thickness can be used to improve absorption for a shield of a set thickness compared to other tungsten boride-based solutions, or to provide a comparable neutron shield at a reduced thickness compared to other tungsten boride-based solutions. The latter is particularly useful in applications such as the central column of a spherical tokamak fusion reactor, where minimizing the thickness of the shield is a critical design objective (as part of minimizing the overall diameter of the central column).

[0023] A potential problem with existing shielding materials that benefit from neutron absorption by boron is that the absorption... 10 B isotope 7 Li and 4 It is converted into He alpha particles, 10 The proportion of boron-10 isotopes gradually decreases over time. This is shown in Figure 5, which plots the proportion of boron-10 remaining in several tungsten borides after 30 years of operation at 200 MW against the material at several locations within the shielding, from the plasma-facing surface 501 to the HTS core-facing surface 505, with intermediate depths 502, 503, and 504 as schematically shown in Figure 500. Partial losses are highest at the outer plasma-facing surface, and neutron flux is also highest at the outer plasma-facing surface, and is reduced by the shielding. W2B5 shows the best performance among all materials considered in this regard, with minimal partial decrease in isotopic content across the entire shielding.

[0024] Natural boron is, 11 Compared to the 80% isotope content of B, it absorbs neutrons. 10 It has an isotopic content of 19% to 20% of boron (other isotopes of boron have half-lives of at most tens to hundreds of milliseconds). In many applications, 18% to 20% is used. 10The use of natural boron containing B or other boron is sufficient, but the performance of the boride shielding is, 10 Increasing the B content, that is, a higher proportion than that present in natural boron 10 B, for example, at least 25% 10 This can be enhanced by providing B. This effect on the peak neutron flux in the HTS core of each tungsten boride material is 10 B / B 全体 Figure 6 shows the percentages for 0% (601), 20% (602), 40% (603), 60% (604), 80% (605), and 100% (606). 10 As the proportion of B increases, the shielding performance of each tungsten boride improves by more than twofold, but W2B5 remains the best tungsten boride at all enrichment levels. Similar results are obtained with neutron and gamma-ray energy accumulation in the core (not shown).

[0025] W2B5 can be formed as a pure solid material by manufacturing techniques such as sintering, melting, and casting. Sintering of W2B5 can be carried out by discharge plasma sintering, high-temperature compression of W2B5 powder, non-pressurized (atmospheric pressure) sintering, or other suitable methods.

[0026] Alternatively, a relatively inexpensive manufacturing route is composite cemented tungsten boride.

[0027] Pure W2B5 has excellent neutron shielding properties, but is brittle overall. To mitigate this, W2B5 can be provided within metal-reinforced composite materials to offer suitable physical properties for structural (e.g., load-bearing) applications of W2B5 composite materials.

[0028] Since some of the advantages of W2B5 derive from its structure, the alloying metal elements used to improve its structural performance must be selected so as not to react strongly with borides. If a large portion of its structure reacts with other elements in the composite to form other borides, the structure will be impaired or lost. In particular, suitable metals for providing W2B5 in metal-reinforced composites include transition metals (e.g., tungsten), preferably transition metals of Group 11 of the periodic table (copper, silver, gold), zinc, or lead, more preferably copper. Alloys with such metals as the main component, such as gilded alloys, bronze and brass such as phosphor bronze or aluminum bronze, red brass, beryllium copper and cupronickel, or gold and / or silver alloys such as electrum or goloid, are also suitable. Aluminum reacts to form borides, but specific composition and cooling rates are required to form large amounts of WAlB. Therefore, aluminum can be used as an alloying metal element by controlling the composition and cooling rate to limit the formation of WAlB.

[0029] As an example of a metal-reinforced composite material, W2B5 can be provided as a component in aggregates of cemented tungsten boride, including a metal matrix and aggregates, as described for WB in International Publication No. 2016 / 009176.

[0030] Metal-reinforced composite materials can contain, for example, a high proportion of W2B5 of at least 70% by weight, at least 80% by weight, or at least 90% by weight. Since W2B5 is 12.8% by weight of boron, a composite material containing N% by weight of W2B5 will contain at least 0.128N% by weight of boron, which will result in a considerable proportion of boron in the material. Thus, metal-reinforced composite materials can contain at least 9% by weight of boron, at least 10% by weight of boron, or at least 11.5% by weight of boron.

[0031] The neutron attenuation performance of metal-reinforced composite materials generally improves with increasing boron content.

[0032] Metal-reinforced composite materials can be formed in various ways, for example by liquid-phase sintering (LPS) as shown in Figure 7. To form a composite material by LPS, W2B5 powder is mixed with the powder of a selected metal 701, and if necessary, additives such as stearic acid (about 1% by weight of W2B5) are added to reduce the frequency of cold welding during pretreatment. The powders can be ground together in an inert atmosphere to reduce their average particle size. The mixed powder is pressed to form a "compacted body" 702, and then heated above the melting point of the selected metal so that it becomes liquid. Capillary forces due to wetting of the solid W2B5 by the liquid metal draw the liquid into the interparticle voids, rearranging the particles 703. Once porosity is eliminated and the rearrangement stage begins to slow down, the diffusion mechanism becomes dominant as W2B5 diffuses through the liquid and re-precipitation on other particles 704. This causes larger particles to grow at the expense of smaller particles, and the contacting curved particle surfaces tend to flatten. Due to these shape changes, the W2B5 particles solidify more densely. In the final stage 705, the composite material reaches its maximum density as the W2B5 structure is strengthened along with the formation of a solid microstructure in a manner similar to solid-phase sintering. The composite material can then be cooled so that the liquid metal solidifies into a continuous matrix around the W2B5 structure.

[0033] Sintering can be performed under pressure, for example by hot pressing, or by "pressureless" sintering techniques, in which the material to be sintered is placed in a mold and the mold is vibrated while being heated to a temperature sufficient for sintering to occur. The advantage of pressureless sintering is that it allows for finer control over the metal content of the final material, as pressure sintering can extrude liquid metal from the material.

[0034] Depending on the neutron and gamma ray fluxes incident on the shield and the duration of the pulses (when the fusion device is not operating in steady mode), it may be desirable to integrate a cooling system with the shield to maintain the shield within its thermal operating limits. Such a cooling system can take the form of a channel within the shield through which a coolant, such as gaseous helium, is pumped. Water cooling can also be used to extract heat from the system via a suitable metal interface to minimize corrosion, which is optional. Alternatively, heat can be extracted from the shield's thermal conduction by maintaining a heat sink in one or more areas of the shield.

[0035] W2B5 offers further advantages over the use of WC or pure W shielding due to its significantly superior oxidation resistance. This is a critical safety consideration for worst-case accident scenarios combining loss of coolant (LOCA) and loss of vacuum (LOVA).

[0036] W2B5 shielding is particularly advantageous in situations where space for neutron shielding is highly constrained. One such example is neutron shielding in tokamak fusion reactors, especially spherical tokamaks. In such reactors, the shielding protects the poloidal or toroidal field coils from neutrons emitted by the fusion plasma in the plasma chamber. Since the coils can be made of relatively delicate high-temperature superconducting materials, effective shielding is required. However, making this shielding as thin as possible allows for a more preferable spherical geometric shape, bringing the field coils closer to where the magnetic field is needed, thereby improving the efficiency of the reactor.

Claims

1. A neutron shielding material comprising a tungsten boride compound in a phase having alternating boron layers including graphite-like planar layers and condensed cyclohexane-like chair layers and tungsten atoms located between said alternating boron layers, wherein the boron in said boron layers has a ratio of boron-10 content to total boron content that is greater than the ratio of boron-10 content to total boron content in natural boron.

2. A neutron shielding material as described in claim 1, wherein the ratio of the boron-10 content to the total boron content is 25% or more.

3. The neutron shielding material as described in claim 1, wherein the tungsten boride compound has a structure with space group P6 3 / mmc.

4. The neutron shielding material of claim 1, wherein the tungsten boride compound has a chemical formula of W 2 B 5-x , where x is a real value greater than or equal to 0 and less than 4.

5. The neutron shielding material of claim 1, wherein the tungsten boride compound is a solid sintered tungsten boride compound.

6. A composite material comprising the neutron shielding material described in claim 1 and a metal.

7. The composite material of claim 6, wherein the tungsten boride compound comprises a weight percentage of the composite material selected from the group consisting of at least 70%, at least 80%, and at least 90%.

8. The composite material of claim 6, wherein the metal is selected from the group consisting of transition metals, copper, silver, gold, zinc, lead, and aluminum.

9. The composite material described in claim 6, comprising a matrix and aggregates, the matrix comprising a metal, and the aggregates comprising the neutron shielding material described in claim 1 in the form of a cermet, ceramic or cement.

10. A neutron shielding body comprising the neutron shielding material described in claim 1.

11. A nuclear fusion reactor comprising a plasma chamber and a neutron shielding body as described in claim 10, wherein the neutron shielding body is positioned outside the plasma chamber and configured to block neutrons emitted from the plasma chamber during operation of the nuclear fusion reactor.

12. A fusion reactor as described in claim 11, wherein the neutron shield does not have a coolant flow path.

13. The fusion reactor of claim 11, wherein the fusion reactor is a magnetic confinement reactor and further includes a plurality of magnetic field coils arranged outside the plasma chamber, and the neutron shielding is arranged between the plasma chamber and at least one of the magnetic field coils.

14. The fusion reactor of claim 13, wherein the fusion reactor is a spherical tokamak.

15. A composite neutron shielding material comprising a metal and a tungsten boride compound in a phase having alternating boron layers including graphite-like planar layers and condensed cyclohexane-like chair layers and tungsten atoms located between said alternating boron layers, said tungsten boride compound accounting for at least 70% by weight of said composite neutron shielding material.

16. The composite neutron shielding material of claim 15, wherein the tungsten boride compound has a chemical formula of W 2 B 5-x , where x is a real value greater than or equal to 0 and less than 4.

17. The composite neutron shielding material of claim 15, wherein the boron in the boron layer has a ratio of boron-10 content to total boron content that is greater than the ratio of boron-10 content to total boron content in natural boron.

18. A composite neutron shielding material as described in claim 17, wherein the ratio of the boron-10 content to the total boron content is 25% or more.

19. The composite neutron shielding material of claim 15, wherein the metal is selected from the group consisting of transition metals, copper, silver, gold, zinc, lead, aluminum, and alloys containing a transition metal, copper, silver, gold, zinc, lead, or aluminum.

20. A neutron shielding material comprising ditungsten pentaboride W 2 B 5 , having a ratio of boron-10 content to total boron content greater than the ratio of boron-10 content to total boron content in natural boron.

21. A neutron shielding material as described in claim 20, wherein the ratio of the boron-10 content to the total boron content is 25% or more.

22. A composite neutron shielding material comprising ditungsten pentaboride W 2 B 5 and a metal, said ditungsten pentaboride accounting for at least 70% by weight of said composite neutron shielding material.

23. The composite neutron shielding material of claim 22, wherein the ditungsten pentaboride has a ratio of boron-10 content to total boron content that is greater than the ratio of boron-10 content to total boron content in natural boron.

24. The composite neutron shielding material of claim 22, wherein the metal is selected from the group consisting of transition metals, copper, silver, gold, zinc, lead, aluminum, and alloys containing a transition metal, copper, silver, gold, zinc, lead, or aluminum.