RU2850060C1 - Improved materials for neutron shielding based on tungsten biride - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention relates to neutron shielding material and can be used in thermonuclear reactor development technologies. The material contains a tungsten boride compound in a phase having alternating layers of boron, including graphite-like flat layers and condensed cyclohexane-like chains, with tungsten atoms located between the alternating layers of boron. Moreover, the boron in the boron layers has a higher proportion of boron-10 relative to the total boron content than the proportion of boron-10 relative to the total boron content in naturally occurring boron.

EFFECT: simultaneous increase in the resistance of the neutron shielding material to damage by particle cascades, thermal stresses introduced by uneven heating and the HTS core, as well as ablation of the outer surface of the shielding.

22 cl, 7 dwg

Description

Field of technology to which the invention relates

The present invention relates to neutron shielding materials for thermonuclear reactors. Specifically, this invention relates to neutron shielding containing tungsten boride.

Background of the invention

The problem of producing fusion energy is extremely complex. Neutrons are produced in nuclear fusion when deuterium-tritium (DT) or deuterium-deuterium (DD) plasma is heated to the point where the nuclei have sufficient energy to overcome Coulomb electrostatic repulsion and fuse together, releasing energetic neutrons and fusion products (e.g., ⁴ He for DT). Currently, the most promising way to achieve this is through the use of a tokamak device; in the traditional approach to tokamak fusion (as embodied in ITER), the plasma must have a long confinement time, high temperature, and high density to optimize the process.

A tokamak is characterized by a combination of a strong toroidal magnetic field B _T , a high plasma current I _p , and, typically, a large plasma volume and significant co-heating to ensure a stable plasma hot enough to support nuclear fusion. Co-heating (e.g., by injecting tens of megawatts of high-energy neutral particle beams (H, D, or T)) is necessary to increase the temperature to values high enough to initiate nuclear fusion and/or maintain the plasma current.

To ensure the reactor's compactness (which ensures higher efficiency, especially for the "spherical tokamak" plasma configuration), the radiation 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.

Figure 1 shows a cross-section of the central column and illustrates the challenges that the shielding material must overcome. The central column contains a central core of high-temperature superconductor (HTS) coils 11 and an outer shielding layer 12. Depending on the shielding material used, a layer of oxidized shielding material 13 may be present on the outer surface if the shield is exposed to air during high-temperature operation. There are three main causes of damage that arise from plasma 14. First, high-energy neutrons 15 generated by the nuclear fusion reaction can essentially knock atoms out of the shielding structure, creating damage cascades 16 that propagate through the material and degrade material properties (such as mechanical, thermal, or superconducting properties). Secondly, the heat flux 17 from the nuclear fusion reaction is significant and can damage the shielding due to thermal stresses introduced by uneven heating and the HTS core. Higher temperatures reduce the current that can be carried while maintaining superconductivity and can lead to a sharp increase in coil resistance, causing the magnet to lose superconductivity. Finally, energetic plasma particles will cause ablation 18 of the shielding's outer surface. This not only causes damage to the shielding itself but can also contaminate the plasma if the shielding is directly exposed to it. It is desirable to have a shielding material that can resist these effects and also prevent neutrons from reaching the superconducting coils.

Modern containment structures also often use water channels for both cooling the shield and moderating neutrons (which increases the shield's effectiveness). However, this creates problems, as water is difficult to manage during facility decommissioning or maintenance, due to the risks of pressurized systems, contamination, activation, and evaporation of the water, as well as the potential for reactor water to leak into the environment if improperly handled.

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

The essence of the invention

According to a first aspect of the invention, the use of tungsten pentaboride, W ₂ B ₅ , in neutron shielding is provided.

According to the second aspect, neural protection is provided, containing ditungsten pentaboride, W ₂ B ₅ .

According to a third aspect, a tokamak fusion reactor is provided, comprising a plasma chamber, a toroidal field coil, a plurality of poloidal field coils, and a neutron shield located between the interior of the plasma chamber and the toroidal or poloidal field coils, wherein the neutron shield is the shield according to the second aspect.

Brief description of the drawings

Figure 1 is a schematic illustration of the shielding layer in the central column of the tokamak;

Figure 2 is a graph showing the neutron flux for tungsten boride and tungsten carbide shielding materials;

Figure 3 is a graph showing the energy release from neutrons and gamma rays for tungsten boride and tungsten carbide shielding materials;

Figure 4 is a graph showing the atomic densities in shielding materials for tungsten, for boron or carbon and their sum;

Figure 5 is a graph showing the fraction of ^10B isotope remaining after 30 years of service for tungsten boride materials as a function of boron content at various levels in neutron shielding;

Figure 6 is a graph showing the peak neutron flux in a HTS core (as in Figure 1) for different isotopic concentrations of ^10B in the shielding materials.

Detailed description

Previous neutron shielding concepts were based on tungsten-rich tungsten carbides and/or tungsten borides. Tungsten is an effective photon absorber due to its high atomic number Z and the typically high density of tungsten compounds. Tungsten is also effective as an inelastic scatterer, reducing the energies of incident neurons with energies of ~14 MeV. Tungsten carbide offers the additional advantage of being a fairly effective neutron moderator (in short, it slows neutrons, making them easier for tungsten to absorb). Tungsten boride offers the additional advantage of being an effective absorber of low-energy neutrons, which may be able to penetrate shielding made entirely of tungsten.

During the study of possible tungsten carbide and boride compositions, it was unexpectedly discovered that a particular stoichiometric composition of tungsten boride, _W2B5 ( _tungsten pentaboride), is a significantly more effective shielding material than other tungsten borides or carbides for both gamma rays and neutrons, at the intensity and energy ranges expected in a tokamak-type nuclear fusion reactor.

Figure 2 shows the results of modeling the neutron absorption of various tungsten boride-based materials (with tungsten and tungsten carbide as references), either with or without a 202 water moderator layer. The measured result is the neutron flux onto the high-temperature superconducting (HTSC) central column of a tokamak fusion reactor, with lower values being better. The scale is logarithmic. The tungsten borides considered are W ₂ B, WB, W ₂ B ₅ , and WB ₄ . Additionally, tungsten carbide (WC, indicated by horizontal lines from the y-axis) and a more complex composite material, B _0.329 C _0.074 Cr _0.024 Fe _0.274 W _0.299 , are considered.

As can be seen from the diagram, _W2B5 significantly outperforms other _compounds in neutron absorption. In fact, it is a fairly good absorber, the efficiency of which increases when _the water moderator is replaced by more _W2B5 , since the moderating effect of the water does not provide sufficient reinforcement to _the remaining _W2B5 , given the material removed to make room for the moderator—i.e., normally the presence of a moderator would allow more neutrons to be absorbed due to the larger absorption cross-section for slower neutrons, but this effect is completely offset by the increased absorption capacity _{of W2B5} .

Figure 3 shows the actual energy deposition on the HTSC material in the same simulation as Figure 2, for both direct neutron deposition and secondary gamma-ray deposition. The graphs shown are for gamma-ray deposition with and without a 301 water moderator, and for neutron deposition with and without a 304 water moderator. As before, lower values are better, and the same compounds are plotted. In this diagram, it can be seen that _W2B5 again performs best in all cases. Direct neutron deposition is higher without a water _moderator , despite the neutron flux being lower, since the neutrons that reach the HTSC have higher energies. However _, the secondary gamma-ray release is lower for _W2B5 without a moderator, and given the logarithmic scale of the graph, it will be clear that the total energy release in this case will also be lower.

This is theoretically proposed to be due to the particularly close-packed crystal structure of W ₂ B ₅ , which has an anomalously high density among tungsten borides (~13 g/cm ³ ) and, consequently, a higher atomic number density (i.e., the number of atoms per unit volume) of both tungsten and boron than would otherwise be expected, compared with other stoichiometries. This is illustrated in Figure 4, which shows the atomic density (in atoms per cubic centimeter) of tungsten 401 and boron or carbon 402, and their sums 403, for various stoichiometries of pure tungsten, tungsten carbide, and tungsten boride. As can be seen, W ₂ B ₅ has the highest atomic density of boron of all the stoichiometries considered and lies well above the trend line for the atomic density of tungsten. The total atomic density, including tungsten and boron, is also the highest. This is important because both boron and tungsten play important roles in protection.

It should be noted that there is some debate in the scientific community regarding the exact structure of W ₂ B ₅ . It is known that a tungsten boride phase exists containing alternating boron layers consisting of graphite-like planar sheets and fused cyclohexane-like chains with tungsten atoms interspersed between the boron layers, in a structure with the space group P6 ₃ /mmc. For this structure to be W ₂ B ₅ , the center of each cyclohexane-like ring must contain an extra boron atom, and the debate centers on whether this arrangement is stable. In the case where the extra boron atom is completely absent, the structure would be correctly identified as W ₂ B ₄ , while in the case where there is only partial occupancy (i.e., a boron atom is present in some cells of the structure, but not all), the structure would be correctly identified as W ₂ B _4+x . However _{, W2B5} is the most commonly accepted description of this structure in the literature, and that is why this term is used here. If _{the W2B4 or W2B4 +x} structure is correct, the proportion of boron in such a phase will be slightly lower than that described here, but the general conclusions that this is the best phase for neutron shielding applications remain the same, and references _{to W2B5} in this document can be replaced by references to the correct formula. Other phases may be present in this boride in smaller proportions, but the desired phase (i.e. _{, W2B4} , _{W2B4 +x,} or _{W2B5 )} will predominate.

In general, _W2B5 can be incorporated into any existing designs using other tungsten boride compositions. For example, it can be incorporated as solid _{sintered W2B5} or as _a tungsten boride component in a cemented tungsten boride containing _W2B5 particles in a metallic binder. Although the above results indicate that a moderator is not necessary, _{W2B5 -} based shielding can still be provided with a moderator, such as water or another hydrogen-containing material, or any other suitable neutron moderator known in the art. For example, the presence _of a _moderator can be beneficial when _W2B5 is part of a composite material, such as a cermet, ceramic, or cemented tungsten boride, so that the combination of the composite material and the moderator provides better neutron absorption in the target range than the composite material alone. A moderator may also be useful where the expected neutron energy differs from the 14.1 MeV fusion neutrons used for the simulations discussed above, and/or where water (or other moderating material) is used both as a moderator and to cool the neutron shield or other nearby components.

_W2B5 can be used as a component of composite shielding, for example, incorporating other materials to provide additional absorption of gamma rays, neutrons of various energies, or any other types _{of radiation. Shielding with W2B5 can} include _structural components and cooling components, which can be made of any suitable material.

It should be understood that the advantages of W ₂ B ₅ lie primarily in its performance as a protective material, rather than in its specificity to any particular protective application (e.g. geometry or structure).

The increased neutron absorption at a given neutron shielding thickness can be used to provide improved absorption for a given shielding thickness compared to other tungsten boride-based solutions, or it can be used to provide a similar degree of neutron shielding with a reduced thickness compared to other tungsten boride-based solutions. The latter is particularly useful in applications such as the central column of a fusion reactor—a spherical tokamak—where minimizing shielding thickness (as part of minimizing the overall diameter of the central column) is an important design goal.

A potential problem with existing shields that benefit from the absorption of neutrons by boron is that the absorbing isotope ^10B is converted to ^7Li and the alpha particle ^4He , so that the fraction of the isotope ^10B gradually decreases with time. This is illustrated in Figure 5, which shows the fraction of boron-10 remaining for several tungsten borides after 30 years of operation at 200 MW, depending on the material, for several positions within the shield from the plasma-facing surface 501 to the HTS core-facing surface 505, with intermediate _depths 502, 503, 504, as shown in diagram 500. The relative loss is highest at the outer, plasma-facing surface, where the neutron flux is highest, and decreases across the shield. _W2B5 exhibits the best performance of all the materials considered in this regard, with the smallest decrease in the fraction of the isotope content throughout the shield.

Natural boron has an isotopic content of the neutron-absorbing ¹⁰ B of 19-20%, compared to 80% for ¹¹ B (other boron isotopes have half-lives of tens to hundreds of milliseconds, at most). While the use of natural boron or other boron with 18-20% ¹⁰ B will be sufficient for many applications, the performance of boride shields can be improved by enriching them in ¹⁰ B, i.e., providing a higher proportion of ¹⁰ B than is present in naturally occurring boron, for example at least 25% ¹⁰ B. The effect of this on the peak neutron flux in the HTS core for each of the tungsten boride materials is shown in Figure 6, for ¹⁰ B/B proportions _{as low as} 0% 601, 20% 602, 40% 603, 60% 604, 80% 605, and 100% 606. Higher percentages of ¹⁰ B improve the shielding performance by more than a factor of 2 for each of the tungsten borides, but W ₂ B ₅ remains the best tungsten boride at all enrichment levels. Similar results are obtained for the neutron and gamma ray energy deposition in the core (not shown).

_W2B5 can be formed as _a pure solid material through production methods such as sintering or melting and casting. Sintering _{of W2B5 can} be accomplished by spark plasma sintering, hot pressing _{of W2B5} powders, pressureless sintering, or other suitable methods.

An alternative, relatively inexpensive manufacturing technology would be composite cemented tungsten boride.

Pure _W2B5 has excellent neutron shielding properties, but is typically brittle. To mitigate this, _W2B5 can be incorporated into _a metal _- reinforced composite to provide the appropriate physical properties for structural (e.g., load-bearing) applications of the _{W2B5 composite} .

The additional alloying metal element for improving the structural characteristics should be chosen so that it does not react strongly with the borides, since part of the advantages _{of W2B5} derives from its structure, and this structure will be disrupted or lost if a large proportion of it reacts with other elements in the composite to form other _borides . In particular, suitable metals for providing _W2B5 in a metal-reinforced composite include transition metals (e.g., tungsten), preferably the transition metals from Group 11 of the Periodic Table (copper, silver, and gold), zinc or lead, and more preferably copper. Alloys consisting primarily of such metals, such as bronzes and brasses such as tombac, phosphor or aluminum bronze, red brass, beryllium copper alloy, and cupronickel, or alloys of gold and/or silver such as electrum or holoid, are also suitable. Although aluminum does react to form borides, the formation of significant amounts of WAlB requires specific compositions and cooling rates. In fact, by adjusting compositions and cooling rates to limit WAlB formation, aluminum can be used as an additional alloying element.

As an example of a metal-reinforced composite, W ₂ B ₅ may be provided as a component in a cemented tungsten boride filler comprising a metal matrix and a filler, as described for WB in WO 2016/009176 A1.

_The metal-reinforced composite may contain a high proportion of _W2B5 , such as at least 70% by weight, at least 80% by weight, or at least 90% by weight. This will result in a significant proportion of boron in the _material , since _W2B5 has 12.8% boron by weight, so a composite containing N% _W2B5 by _weight contains at least 0.128N% boron by weight. In essence, the metal-reinforced composite may contain at least 9% boron by weight, at least 10% boron by weight, or at least 11.5% boron by weight.

The neutron flux attenuation performance of metal-reinforced composite generally improves with increasing boron content.

Metal-reinforced composites can be formed by a variety of methods, such as liquid phase sintering (LPS), as illustrated in Figure 7. To form the composite via LPS _{, W2B5} powder is mixed with the powder of the selected metal 701 and, optionally, additives such as stearic acid (approximately 1% by weight of _{W2B5 )} to reduce the incidence of cold welding during pre-treatment. The powders can be milled together under an inert atmosphere to reduce their average particle size. The mixed powders are pressed 702 to form a "green compact," which is then heated to a temperature above the melting point of the selected metal so that it becomes liquid. Capillary forces due to the wetting of solid _{W2B5 by} the liquid metal will draw liquid into the voids between the particles and force the particles to rearrange 703. As porosity is eliminated and the rearrangement phase begins to slow, diffusion mechanisms become dominant as _W2B5 diffuses through the _liquid and redeposits on other particles 704. This causes larger grains to grow at the expense of smaller grains and tends to straighten the curved surfaces of particles that are in contact. These shape changes force the _W2B5 particles to pack more tightly. At the final stage 705, the composite reaches its highest density as _{the W2B5} structure hardens to form a hard _{microstructure} , similar to solid-state sintering. The composite is then allowed to cool so that the liquid metal solidifies into a continuous matrix around the W ₂ B ₅ structure.

Sintering can be performed under pressure, such as in a hot press, or "pressureless" sintering methods can be used, where the material to be sintered is placed in a mold that is vibrated while heated to a temperature sufficient for sintering to occur. The advantage of pressureless sintering is finer control of the metal content in the final material, as sintering under pressure can cause liquid metal to be squeezed out of the material.

Depending on the neutron and gamma ray flux incident on the shield, as well as the duration of any pulses (if the fusion facility is not operating in steady state), it may be desirable to integrate a cooling system with the shield to maintain the shield within its operating temperature limits. Such a cooling system may take the form of channels in the shield through which a coolant, such as gaseous helium, is pumped. Water cooling may also be used to remove heat from the system, optionally through a suitable metal interface to minimize corrosion. Alternatively, by maintaining a heat sink in one or more areas of the shield, heat can be removed from the shield by conduction.

_W2B5 offers the additional advantage of significantly improved oxidation resistance over WC or pure W protection. This is an important safety consideration for the worst-case scenario of _a combined loss of coolant (LOCA) and loss of vacuum (LOVA) accident.

_{W2B5 shielding} is particularly useful in situations where space for neutron shielding is very limited. One such example is neutron shielding in a tokamak fusion reactor, specifically a spherical tokamak. In such a reactor, the shielding protects the poloidal or toroidal field coils from neutrons emitted by the fusion plasma in the plasma chamber. The coils can be made of relatively fragile high-temperature superconducting material, so effective shielding is necessary. However, reactor efficiency can be improved by making the shielding as thin as possible, as this allows for a more favorable spherical geometry and allows the excitation coils to be closer to where the magnetic field is needed.

Claims (22)

1. A neutron shielding material comprising a tungsten boride compound in a phase having alternating layers of boron comprising graphite-like flat layers and condensed cyclohexane-like chains, with tungsten atoms located between the alternating layers of boron, wherein the boron in the boron layers has a greater proportion of boron-10 content relative to the total boron content than the proportion of boron-10 content relative to the total boron content in naturally occurring boron. 2. The neutron shielding material according to paragraph 1, wherein the proportion of boron-10 content in relation to the total boron content is greater than or equal to 25%. 3. The neutron shielding material according to paragraph 1, wherein the tungsten boride compound has a structure with the space group P6 ₃ /mmc. 4. The neutron shielding material according to claim 1, wherein the tungsten boride compound is a solid sintered tungsten boride compound. 5. A composite neutron shielding material containing a neutron shielding material according to item 1 and a metal. 6. The neutron shielding composite material of claim 5, wherein the tungsten boride compound constitutes a percentage by weight of the composite material selected from the group consisting of at least 70%, at least 80%, or at least 90%. 7. A composite neutron shielding material according to claim 5, wherein the metal is selected from the group consisting of a transition metal, copper, silver, gold, zinc, lead, and aluminum. 8. A composite neutron shielding material according to paragraph 5, wherein the composite material contains a matrix and a filler, wherein the matrix contains the said metal, and the filler contains the neutron shielding material according to paragraph 1 in metal-ceramic, ceramic or cemented form. 9. Neutron shielding containing neutron shielding material according to paragraph 1. 10. A thermonuclear reactor comprising a plasma chamber and a neutron shield according to claim 9, located between the plasma chamber and the toroidal or poloidal field coils. 11. A thermonuclear reactor according to paragraph 10, wherein the neutron shield does not have channels for the coolant. 12. The thermonuclear reactor of claim 10, wherein the reactor is a magnetic confinement reactor and further comprises a plurality of magnetic field excitation coils located on the outer side of the plasma chamber, wherein the neutron shield is located between the plasma chamber and at least one of the magnetic field excitation coils. 13. A thermonuclear reactor according to claim 12, wherein the reactor is a spherical tokamak. 14. A neutron shielding composite material comprising a metal and a tungsten boride compound in a phase having alternating layers of boron comprising graphite-like flat layers and condensed cyclohexane-like chains, with tungsten atoms located between the alternating layers of boron, wherein the tungsten boride compound constitutes at least 70% by weight of the composite material. 15. The composite material of claim 14, wherein the boron in the boron layers has a greater proportion of boron-10 content relative to the total boron content than the proportion of boron-10 content relative to the total boron content in naturally occurring boron. 16. A composite material according to claim 15, wherein the proportion of boron-10 content in relation to the total boron content is greater than or equal to 25%. 17. The composite material of claim 14, wherein the metal is selected from the group consisting of a transition metal, copper, silver, gold, zinc, lead, aluminum, and an alloy comprising a transition metal, copper, silver, gold, zinc, lead, or aluminum. 18. A neutron shielding material comprising ditungsten pentaboride, _W2B5 , having _a boron-10 content relative to the total boron content greater than the boron-10 content relative to the total boron content of naturally occurring boron. 19. The neutron shielding material according to paragraph 18, wherein the proportion of boron-10 content in relation to the total boron content is greater than or equal to 25%. 20. A composite neutron shielding material comprising tungsten pentaboride, W ₂ B ₅ , and a metal, wherein the tungsten pentaboride constitutes at least 70% by weight of the composite material. 21. The composite material of claim 20, wherein the tungsten pentaboride has a greater proportion of boron-10 content relative to the total boron content than the proportion of boron-10 content relative to the total boron content of naturally occurring boron. 22. The composite material of claim 20, wherein the metal is selected from the group consisting of a transition metal, copper, silver, gold, zinc, lead, aluminum, and an alloy comprising a transition metal, copper, silver, gold, zinc, lead, or aluminum.