Neutron shielding

Abstract

Neutron shielding. The neutron shielding (600) comprises first (601) and second (602) sections, each comprising boron. The neutron shielding is configured such that in use the first section is between a neutron source (620) and the second section. At least 75% of boron in the first section is boron-11, and at least 70% of boron in the second section is boron-10. Also provided are assemblies incorporating the neutron shielding, a method comprising providing and irradiating the neutron shielding, and a method of designing neutron shielding.

Description

[0001] Neutron Shielding

[0002] Field of the Invention

[0003] The present invention relates to neutron shielding.

[0004]

[0005] In high neutron flux environments, neutron shielding is required to protect the environment and delicate components from neutron emissions. High neutron flux environments include, for example, nuclear fission reactors and nuclear fusion reactors.

[0006] Nuclear fusion reactors and methods include, for example:

[0007] • Magnetic confinement, such as

[0008] o Tokamaks

[0009] o Stellarators

[0010] o Magnetic mirror

[0011] o Field-reversed configuration

[0012] o Spheromaks

[0013] o Reversed field pinch

[0014] • Inertial confinement, such as

[0015] o Indirect drive

[0016] o Direct drive

[0017] o Fast ignition

[0018] o Magneto-inertial fusion

[0019] o Z-machine

[0020] • Pinch devices, such as

[0021] o Z-pinch

[0022] o Theta-pinch

[0023] o Screw pinch

[0024] • Inertial electrostatic confinement devices, such as

[0025] o Fusors

[0026] o Polywells

[0027] Magnetised target fusion

[0028] Beam fusion

[0029] Muon catalysed fusion

[0030] 38214647-3• Lattice confinement fusion

[0031] While the eventual goal is to provide fusion as an energy source, this disclosure applies equally to experimental devices used for scientific purposes, such as those used in current research.

[0032] Some applications for neutron shielding present particular problems, e.g. spherical tokamaks, where shielding is required in the central column of the tokamak, but it is important to minimise the thickness of that central column, and thereby minimise the aspect ratio of the tokamak (or reduce the overall size - and cost - required to obtain a given aspect ratio). Minimising the aspect ratio of a spherical tokamak is important, as this significantly improves the stability and energy confinement of the plasma. Therefore, the shielding must be as thin as possible while still providing adequate protection for the delicate components inside. Relatively thin shielding may also be useful in other neutron shielding applications.

[0033] Boron-containing compounds are known to be effective neutron absorbers. Natural boron comprises two isotopes - typically, 19.9% boron-10 and 80.1% boron-11 as an overall abundance. Boron- 10 has significantly greater neutron capture cross section at almost all energies than boron-11, as shown in Figure 1. As such, the use of “enriched borides”, i.e. boron-containing compounds having a proportion of boron-10 greater than the natural abundance (e.g. greater than 25%, up to 100% boron-10), have been proposed as materials for neutron shielding. Where a percentage of a boron isotope is given in this disclosure, that percentage refers to the proportion of that isotope as a percentage of boron present, unless otherwise stated.

[0034] Figure 2 shows simulated magnet lifetime for a monolithic tungsten boride shield used to protect a high temperature superconducting (HTS) magnet in a spherical tokamak. Greater magnet lifetime corresponds to better shielding performance. As can be seen, the magnet lifetime increases significantly with increased proportion of boron-10 in the tungsten boride, particularly above 70% boron-10 as a percentage of total boron.

[0035] For this simulation, a shield composed of e-phase tungsten boride, hereafter e-phase W2BX, was used, where x represents a value which is currently the subject of investigation in the literature, “e-phase W2BX” refers to the phase of tungsten boride

[0036] 38214647-3commonly referred to in the literature as W2B5. However, the exact stoichiometry of E-phase W2BX is currently the subject of experimental investigation; for example, it may be more correctly presented as “W2B4+X”, or “W2B4-X” (for values of x from 0 to 1), and the stoichiometric formula is likely to be in the range W2B3.6 to W2B5. Regardless of the uncertainty as to the exact composition, s-phase W2BX can be synthesised as by methods known in the art, e.g. by the methods disclosed in the following papers:

[0037] • Bing Dai et al “Synthesis of W2B5 powders by the reaction between WO3 and amorphous B in NaCI / KCI flux” Ceramics International, Volume 46, Issue 10, Part A, 2020, Pages 14469-14473, ISSN 0272-8842, doi: 10.1016 / j.ceramint.2020.02.244.

[0038] • C.L. Yeh, H.J. Wang, “Preparation of tungsten borides by combustion synthesis involving borothermic reduction of WO3”, Ceramics International, Volume 37, Issue 7, 2011, Pages 2597-2601, ISSN 0272-8842, doi: 10.1016 / j. ceram i nt.2011.04.006.

[0039] • Coskun, Selim & Ovegoglu, Lutfi. (2012). Room-Temperature Mechanochemical Synthesis of W2B5 Powders. Metallurgical and Materials Transactions A. 44A.

[0040] 2012, doi: 10.1007 / s11661-012-1551-4.

[0041] The resulting s-phase W2BX may then be incorporated into a composite material e.g. as a cemented tungsten boride.

[0042] Other parameters of the simulation are as follows:

[0043] • Major radius = 380 cm

[0044] • Minor radius = 172.728 cm

[0045] • Aspect Ratio = 2.2

[0046] • Elongation = 3.0

[0047] • Fusion power = 1GW-Fusion Full-Power-Year

[0048] These simulation parameters are given for illustration only - the improved neutron shielding effect for greater boron-10 enrichment would be expected for any fusion device utilising a boron containing shield.

[0049] Boron may be provided as a metal boride, a metal borocarbide or as boron carbide. Any metal boride or borocarbide will provide effective shielding due to the presence of boron, though this may be improved by the use of a metal which is itself effective as a neutron absorber, or has favourable neutron interactions which allow the boron to more easily absorb neutrons. Tungsten is an excellent choice as the metal in a metal boride or

[0050] 38214647-3borocarbide, as its compounds tend to have high density, and tungsten isotopes have significant cross sections for both elastic and inelastic scattering, and a significant cross section for neutron multiplication (n, 2n) reaction which results in the absorption of a 14 MeV neutron and the emission of two neutrons with a distribution of energies peaking at 1 MeV. The neutrons resulting from such multiplication events are lower energy and more likely to be moderated and absorbed by other processes, e.g. absorbed by boron. Boron carbide and metal borocarbides are effective shielding materials due to carbon being an effective moderator of neutrons, reducing high energy neutrons to energies which are more easily absorbed by boron.

[0051] These materials may be provided as ceramics, cermets, or composite materials.

[0052] Summary

[0053] According to a first aspect, there is provided neutron shielding. The neutron shielding comprises first and second sections, each comprising boron. The neutron shielding is configured such that in use the first section is between a neutron source and the second section. At least 75% of boron in the first section is boron-11 , and at least 70% of boron in the second section is boron-10.

[0054] According to a second aspect, there is provided an assembly. The assembly comprises a neutron source and neutron shielding according to the first aspect. The neutron shielding is arranged such that the first section is between the neutron source and the second section.

[0055] According to a third aspect, there is provided a nuclear fusion reactor. The nuclear fusion reactor comprises a plasma confinement region and neutron shielding. The neutron shielding comprises first and second sections, each comprising boron. At least 75% of boron in the first section is boron-11, and at least 70% of boron in the second section is boron-10. The first section is disposed between the plasma confinement region and the second section.

[0056] According to a fourth aspect, there is provided a method. Neutron shielding and a neutron source are provided, the neutron shielding comprising first and second sections, each comprising boron. The first section is between the neutron source and the second

[0057] 38214647-3section, at least 75% of boron in the first section is boron-11, and at least 70% of boron in the second section is boron-10. The first section of the shielding is irradiated with neutrons from the source. The second section of the shielding is irradiated with neutrons passing from the source through the first section, wherein substantially all neutrons entering the second section of the shielding do so only after passing through the first section.

[0058] According to a fifth aspect, there is provided a method of designing neutron shielding. A shielding performance measure is simulated for each of a plurality of neutron shields. Each neutron shield comprises first and section sections, each comprising boron. Each neutron shield is configured such that in use, the first section is between a neutron source and the second section. At least 75% of boron in each first section is boron-11, and at least 70% of boron in each second section is boron-10. The first section of each neutron shield has a respective first thickness, the respective first thickness being a respective percentage of the thickness of each neutron shield. A neutron shield of the plurality of neutron shields is selected based on the first thickness and the shielding performance measure.

[0059] Further embodiments are presented in claim 2 et seq.

[0060] Brief Description of the Drawings

[0061] Figure 1 is a graph showing the neutron capture cross section of boron-10 and boron-11;

[0062] Figure 2 is a graph showing simulated magnet lifetime for a monolithic tungsten boride shield at various boron enrichment levels;

[0063] Figure 3A is a graph showing the (n, alpha) cross section of boron-10 and boron-11; Figure 3B is a graph showing hydrogen production against depth for simulated neutron shields;

[0064] Figures 4A, 4B, and 4C show loss of boron through transmutation for an example enriched boride (4A), a natural boride (4B) and an example depleted boride (4C); Figures 5A and 5B show the elastic scattering cross sections of boron-10 and boron-11 ;

[0065] Figure 6 schematically illustrates a neutron shield;

[0066] Figures 7A, 7B, and 7C are graphs showing the magnet lifetime achieved in simulations by shields constructed according to figure 6;

[0067] 38214647-3Figure 8 shows the neutron flux incident on three cross sections of a neutron shield constructed according to figure 6;

[0068] Figure 9 schematically shows a neutron shield and the region shielded by it;

[0069] Figure 10 schematically shows a further example neutron shield;

[0070] Figure 11 shows a schematic example of a tokamak fusion reactor neutron shielding; and

[0071] Figure 12 shows an assembly comprising a further neutron shield.

[0072] Detailed Description

[0073] As discussed in the background, boron- 10 is a more effective neutron absorber than boron-11. While this means that boron-10 is preferred for neutron shielding properties, this also results in material instabilities due to gas production and transmutation of boron-10 resulting from that neutron absorption. One of the reactions responsible for this is the (n, alpha) reaction, for which the cross section is shown in Figure 3A for boron-10 and boron-11. This reaction results in the boron atom absorbing a neutron and emitting an alpha particle, resulting in the production of helium. As can be seen in the figure, the cross section is several orders of magnitude lower for boron-11 than for boron-10. This (and other neutron interactions) means that for greater enrichment (i.e. greater proportion of boron-10), more gas will be produced and more transmutation will occur, requiring systems for dealing with the gas and shortening the useful lifetime of the shielding materials.

[0074] Where a percentage of a boron isotope is given in this disclosure, that percentage refers to the proportion of that isotope as a percentage of boron present, unless otherwise stated (e.g. “a tungsten boride with 100% boron-11” may have any suitable ratio of tungsten to boron, but all the boron present will be boron-11). Borides comprising greater proportions of boron-10 than natural boron will be referred to as “enriched” borides, and borides comprising greater proportions of boron-11 than natural boron will be referred to as “depleted” borides. All other isotopes of boron have a half life of less than 1 second (772 ms for boron-9, 20 ms or less for all other isotopes), so it can be assumed that the proportions of boron-10 and boron-11 add up to 100% within reasonable margin of error.

[0075] Figure 3B is a graph showing hydrogen production against depth into the shield for three different isotopic compositions of e-phase W2BX. The compositions are an example

[0076] 38214647-3enriched boride 311 (94% boron-10, 6% boron-11), a boride comprising natural boron 312 (19.9% boron-10, 80.1% boron-11), and an example depleted boride 313 (94% boron-11, 6% boron-10). As can be seen, noting the logarithmic scale on the Y axis, the hydrogen production increases significantly with the enrichment of boron.

[0077] Figures 4A, 4B, and 4C show the loss of boron through transmutation for the example enriched boride (4A), the natural boride (4B) and the example depleted boride (4C), with each graph showing the boron loss for each of boron-10 and boron-11 as a percentage of total boron. In each graph, the upper line represents the loss of boron-10, and the lower line shows the loss of boron-11.

[0078] As can be seen in the figures, the gas production and boron losses are significantly lower for boron-11 than for boron-10 (and consequently lower for the depleted boride than for the enriched boride), and most of the gas production and boron loss occurs at low depths into the shield (i.e. in the parts of the shield closest to the neutron source).

[0079] Figures 5A and 5B show the elastic scattering cross sections of boron-10 and boron-11. Figure 5A shows neutron energies from 10 eV to 20 MeV and Figure 5B shows neutron energies from 10 keV to 20 MeV. As can be seen in the figures, for most of the range of neutron energies from 1 MeV to 20 MeV, boron-11 has a much greater elastic scattering cross section than boron-10, and the elastic scattering cross sections are similar between 10 and 20 MeV. Even where the elastic scattering cross section for boron-10 is greater in some places in the 1 MeV to 10 MeV range, it is still very close to the elastic scattering cross section for boron-11. Elastic scattering of neutrons is the primary method of neutron moderation for boron-10 and boron-11, so this shows that boron-11 is a good neutron moderator over the 1 to 20 MeV range compared to boron-10, which is important for moderating high energy neutrons down to less than 1 MeV, to energy ranges where they are more easily absorbed.

[0080] Figure 6 schematically illustrates a neutron shield. The neutron shield 600 is installed to protect a component 610 from neutrons emitted by a neutron source 620. The neutron shield comprises a first section 601 and a second section 602, where the first section is located between the second section and the neutron source. Both the first section and the second section comprise a boron-containing compound, e.g. a metal boride, metal borocarbide, boron carbide, or metal borohydride. In the first section, the boron-

[0081] 38214647-3containing compound has a percentage of boron-11 greater than 75%. This may be natural boron e.g. with a percentage of boron-11 close to 80% (e.g. 75%-85%), or depleted boron, i.e. having a percentage of boron-11 which is greater than that of natural boron, e.g. greater than 85%. In the second section, the boron-containing compound is enriched, i.e. having a percentage of boron-10 which is greater than that of natural boron, e.g. greater than 70%. High-purity (e.g. greater than 99%) boron-10 and boron-11 are readily available from suppliers, and both isotopes are stable, so the proportions of boron-10 in the first section and / or the proportion of boron-11 in the second section may be higher, e.g. at least 85%, at least 90%, or at least 95%.

[0082] This mitigates the gas production and transmutation of boron within the shielding, as boron-11 is less prone to gas production and transmutation than boron-10, and the majority of gas production and transmutation happen within the parts of the shielding closest to the neutron source. It has been surprisingly found that a significant portion of the thickness of the shielding can be formed from a depleted boron-containing compound without a significant reduction in overall shielding performance compared to a shield formed fully from an otherwise chemically identical enriched boron-containing compound, and in fact with an increased shielding performance in some cases.

[0083] The above neutron shielding may be used with any neutron source. The neutron source 620 may be, for example, a fusion reaction taking place in a plasma confinement region of a fusion reactor. In a magnetic confinement fusion reactor, the reaction may take place at a location within the plasma chamber. In an inertial confinement fusion reactor, the reaction may take place at the location of a target (e.g. a fuel pellet). In a magnetic or electric pinch, the reaction may take place at the location of maximum compression of the plasma. The plasma may be a D-T (deuterium and tritium) plasma. The spectrum of neutrons as emitted from the D-T plasma, before such neutrons have interacted with neutron shielding or with other solid or liquid materials that may absorb or scatter them, may be termed the “initial” neutron spectrum of the D-T plasma, which may be, for example, a neutron flux with neutrons at an energy range of 10 to 14 MeV.

[0084] The component 610 may be, for example, a component susceptible to neutron damage, such as a superconducting magnet or an electronic circuit. Alternatively or additionally, the component 610 may comprise materials susceptible to activation by neutrons (i.e. where neutron irradiation would cause the formation of radioactive isotopes within the

[0085] 38214647-3material), such as cobalt. The neutron shielding may also be used to prevent neutrons from reaching the environment outside of the device comprising the neutron source, or to protect locations where people may be present during routine operation and / or maintenance operation, instead of or in addition to protecting specific components 610.

[0086] Surface 815 that faces the neutron source, interface 825 between the first and second sections, and surface 835 furthest from the neutron source are shown in Figure 6 for later reference.

[0087] The distribution of boron isotopes within the first section may be uniform and the distribution of boron isotopes within the second section may be uniform, i.e. the proportions of boron-10 and boron-11 as a percentage of total boron may be the same in all parts of the first section, and the proportions of boron-10 and boron-11 as a percentage of total boron may be the same in all parts of the second section (though different to the first section, as the first section comprises depleted boron and the second section comprises enriched boron).

[0088] Figures 7A, 7B, and 7C are graphs showing the magnet lifetime achieved in simulations by shields constructed according to figure 6, with the x-axis showing the percentage thickness of the second section (i.e. the thickness of the second section, as a fraction of the total thickness of the first and second section). The shield used in the simulations of figure 7A and 7C are formed from e-phase W2BX, and the shield used in the simulation of Figure 7B is formed from WB4. The shields in figures 7A and 7B each have a first section containing depleted boron - where 94% of the boron in the first section is boron-11, with the remaining 6% being boron-10. The shield in figure 7C has a first section containing natural boron - where 80% of the boron in the first section is boron-11, with the remaining 20% being boron-10. In each of figures 7A, 7B, and 7C, the second section is formed from enriched boron, with 94% of the boron in the second section being boron-10, and the remaining 6% being boron-11. Each shield has a total thickness of 56 cm. As expected, at close to 0% thickness of the second section (corresponding to a shield where almost the entire thickness comprises depleted boron-containing compounds), the shielding performance is relatively poor, and at close to 100% thickness of the second section (corresponding to a shield where almost the entire thickness comprises enriched boron-containing compounds), the shielding performance is relatively good. However, as can be seen from the graph, shielding performance

[0089] 38214647-3comparable to a fully-enriched shield (i.e. a shield comprising enriched boron-containing compounds throughout) is obtained from 100% thickness of the second section (i.e. a fully-enriched shield) all the way to 30-40% thickness of the second section for both materials when using depleted boron for the first section, and 20% thickness of the second section for e-phase W2BXwhen using natural boron for the first section. In fact, for each of the shields, there is a peak at 40-60% thickness of the second section (43% for figure 7A, 57% for figure 7B, and 46% for figure 7C) showing that the shield has better shielding performance than a shield of equivalent thickness and composition with enriched boron throughout.

[0090] While the increase in magnet lifetime shown in Figures 7A, 7B and 7C is small, introduction of the depleted boron-containing compound into the first section results in decreased gas production and transmutation of boron, as compared to a shield comprising enriched boron-containing compounds throughout - i.e. an overall improvement is present if more depleted boron-containing compounds are used without a significant reduction in shielding performance, as the useful lifetime of the neutron shield is enhanced. Any gas production within the shield can cause swelling, requiring gaps to be left when designing the shield to allow for this swelling to occur. This results in the neutron shield requiring more space than would be required just for its shape prior to neutron irradiation, or limits the amount of neutron shielding that can be present in a predetermined space (i.e. because some of that space will be required for expansion). The size of the expansion gap is a trade-off between the amount of space which is not occupied by shielding materials (smaller gaps means more shielding material and hence better shielding performance, or more compact shielding for the same amount of initial performance) and the useful lifetime of the shield (with a smaller gap, the shield will swell to fill that gap sooner, and will require replacement at that time). If gas production is reduced, then the rate of swelling of the shield will also be reduced, and so the size of expansion gap required to allow for a required lifetime of the shield before it needs to be replaced will also be reduced. For example, when designing an apparatus incorporating the neutron shielding, this allows for either more compact shielding for similar shielding performance and usable lifetime, better shielding performance for the same space requirement and usable lifetime, or better useable lifetime for the same space requirement and shielding performance (or some other more favourable trade-off between the three factors than would be achieved with higher gas production). The gas production of the shield used for the simulation in Figure 7C would be comparatively

[0091] 38214647-3higher than that of an equivalent shield from the simulations used in Figure 7B, due to the higher proportion of boron-10 in natural boron compared to depleted boron, but still significantly lower than a shield with the same overall proportions of boron-10 and boron-11 evenly distributed throughout.

[0092] Additionally, enriched boron-containing compounds are relatively expensive, but depleted boron-containing compounds are a by-product of that enrichment with few specific uses, so expected to be relatively cheap. As such, by the use of a greater proportion of depleted boron, the overall cost of the shielding is reduced and use is made of the depleted boron by-products of boron enrichment. Natural boron is also cheaper than enriched boron, and is readily available. Even if there were no improvement in magnet lifetime for a shield with the first section as compared to a shield with enriched boron throughout, these advantages would justify the use of a first section comprising depleted or natural boron, particularly where the magnet lifetime is close to that achieved by a fully enriched shield. Note that in the simulations described herein, “magnet lifetime” is measured assuming that the shield itself does not require replacement - i.e. the “magnet lifetime” is a measure of the performance of the neutron shield. The operating lifetime (e.g. time between maintenance cycles) of the device comprising the shield may be shorter than this “magnet lifetime” due to the performance of the neutron shield becoming suboptimal before the neutron damage to the magnet causes the magnet performance to become suboptimal. For example, for a neutron shield formed from E-phase W2BX with a thickness of less than 20 cm, it is expected that there would be little if any benefit to “magnet lifetime” (i.e. the initial neutron shielding performance) for a shield comprising first and second sections as described above as compared to an equivalent shield using enriched boron throughout. However, such a shield would still have a longer operating lifetime due to reduced gas production, and likely be cheaper to manufacture due to the lower cost of natural or depleted boron.

[0093] A neutron shield may be designed with this in mind, for example by simulating a shielding performance measure for each of a plurality of neutron shields of varying designs, each such neutron shield comprising a first section and a second section as described above, each having a different thickness of the first section (hereafter “first thickness”). A neutron shield design may then be selected based on the shielding performance measure and first thickness of each shield.

[0094] 38214647-3For example, selecting a neutron shield design “based on the shielding performance measure and first thickness of each shield” may include selecting a neutron shield design where the first thickness is non-zero, and the shielding performance measure is the best out of those neutron shield designs. Alternatively, selecting the neutron design “based on the shielding performance measure and first thickness of each shield” may include comparing the shielding performance of each neutron shield design to a threshold, and selecting the neutron shield having the greatest first thickness (and therefore the least amount of enriched boron) among those neutron shields for which the performance is better than the threshold. Where a threshold is used, it may be, for example, the shielding performance measure simulated for a “fully enriched” neutron shield design where the second section is the full thickness of the shield (i.e. lacking a first section, and having enriched boron-containing compounds throughout). Alternatively, the threshold may be based on the shielding performance measure for the “fully enriched” neutron shield design, e.g. a fixed proportion of that measure such as 90% of that measure.

[0095] The shielding performance measure may be a magnet lifetime for a simulated tokamak, or more generally may be a lifetime for a sensitive component protected from a neutron source by the shield. Alternatively, the shielding performance measure may be a proportion of neutrons (by number, total energy, or similar) transmitted through the shield from a neutron source.

[0096] Without wishing to be bound by theory, the peak in magnet lifetime seen in Figures 7A and 7B appears to be due to boron-11 providing some additional moderating effect to neutrons in the first section, resulting in a lower energy spectrum and therefore neutrons which are more easily absorbed by the boron-10 in the second section, as well as an overall reduction in the neutron flux reaching the second section as compared to the neutron flux incident on the first section.

[0097] This is shown in Figure 8, which shows the neutron flux incident on three cross sections of a shield comprising a first section, comprising depleted boron (in this case, e-phase W2BX), having a thickness of 24 cm, and a second section, comprising enriched boron, having a thickness of 32 cm. The first cross section 810 is at the surface 815 of the first section facing the neutron source. The second cross section 820 is at the interface 825 between the first and second section. The third cross section 830 is at the surface 835

[0098] 38214647-3of the second section furthest from the neutron source. (Reference numerals 815, 825, 835 are shown in Figure 6.)

[0099] As can be seen in Figure 8, each neutron spectrum has a primary peak 811, 821, 831 at 14 MeV and a secondary peak 812, 822, 832 somewhere between 0.1 and 1 MeV. Comparing the first and second cross sections 810, 820:

[0100] • the overall neutron flux at the second cross section is reduced at all energy levels compared to the primary cross section

[0101] • for the first cross section, the primary peak 811 is at a higher neutron flux than the secondary peak 812, whereas for the second cross section the primary peak 821 is at a lower neutron flux than the secondary peak 822

[0102] • the secondary peak 822 of the second cross section is at a lower neutron energy than the secondary peak 812 of the primary cross section.

[0103] A further advantage of the use of a first section containing an enriched boron-containing compound and a second section containing a depleted boron-containing compound is that the two sections can be made such that the boron-containing compounds differ only in their levels of enrichment of boron. Furthermore, the two sections can be made with substantially the same composition (e.g. the same binder materials in a cemented boride). This allows for the materials to be easily joined together - as they are chemically identical, they can be bonded more easily and without the need for e.g. interlayers to account for differing thermal expansion.

[0104] The first and second sections may comprise other components - e.g. coolant channels, moderator layers, etc. Such other components may also be provided between the first and second sections. The first and second sections may be integrally formed, may be placed adjacent to each other, may be placed with a gap between them, or any other suitable arrangement such that, when in use, the first section is between the neutron source and the second section.

[0105] The first and second sections may be in conductive thermal contact with one another, so that the temperature of the sections (which can, for example, be layers in a layered structure) can be controlled by thermal conduction between the sections and by cooling channels in one or more of the sections. This conductive thermal contact may be via support structures separating the sections.

[0106] 38214647-3The first and second sections may be constructed such that the first and second neutron absorbing materials both shield a continuous region of interest situated behind the shielding, as shown schematically in Figure 9. Figure 9 shows the neutron source 901 and shield 902 (comprising first and second sections 910 and 920) as in figure 5, and illustrates the shielded region 930 which is protected by both sections. This shielded region 930 may contain components 940 to be protected from neutron radiation, such as magnets (e.g. high temperature superconducting magnets), electronics, etc. Some portions of these components may extend beyond the shielded region - for example, in a tokamak, if the shielding 902 of this description is used only in the central column and different neutron shielding is used elsewhere, a central column portion of a toroidal field coil may be within the shielded region, and other portions of the toroidal field coil (e.g. the return limbs) may be outside of the shielded region for the shielding 902. Any straight line between the neutron source and the shielded region will pass through both the first neutron absorbing material and the second absorbing material (as shown by example dotted lines in the figures), and the shielded region may be continuous, i.e. a single unbroken volume, or a “connected space” in the topological sense.

[0107] The first and second materials themselves may be continuous (e.g. each provided as respective monolithic structures, or as contiguous elements forming a continuous structure), but this need not be the case to ensure that the shielded region is continuous; for example, non-contiguous blocks formed from the first material may still shield a continuous region if they overlap when viewed from the neutron source.

[0108] The first and second sections may be neighbouring, i.e. where other components are placed between the first and second sections (e.g. cooling systems), these other components may not be especially relevant to shielding performance - for example, they will not be made of neutron absorbing materials, will not be solid, and / or will not be arranged to shield the whole volume shielded by the first and second shielding materials. For example, coolant channels will typically be made from and carry materials with low neutron absorption, the coolant will typically be a liquid or gas, and the channels will cover only a portion of the area of the shielding so will not shield regions not directly behind a channel, so first and second sections separated by coolant channels and the vacuum / gas / support structures between them would be considered “neighbouring.”

[0109] 38214647-3The first and second sections may be directly adjacent, and may be joined to one another at a common boundary surface.

[0110] The first section 910 may be situated so that no other neutron shielding is between it and neutron source 901. The first section 910 may be situated so that no other solid (or liquid) material is between it and neutron source 901. Alternatively, the first section 910 may be situated in or behind a first wall or behind other solid or liquid material, through which neutrons from neutron source 901 pass before impinging on the first section 910, where that material is not neutron shielding (e.g. where the material has no significant effect on the neutron spectrum which reaches the first section 910). The first section 910 may be directly exposed to neutron radiation at a neutron spectrum emitted by the neutron source 901. For example, if neutron source 901 comprises a fusion plasma, the neutron radiation impinging on the first section 910 may have the neutron spectrum of the fusion plasma.

[0111] Figure 10 shows a further example neutron shield 1000 comprising a first section 1001, a second section 1002, and a third section 1003, which provides shielding for a component 1010 from neutrons emitted by a neutron source 1020. Each of the first, second, and third sections comprises boron. The first section comprises depleted boron as previously discussed. The second section comprises enriched boron as previously discussed. The third section comprises boron having a proportion of boron-10 greater than the proportion of boron-10 in the first section and less than the proportion of boron-10 in the second section, and a proportion of boron-11 greater than the proportion of boron-11 in the second section and less than the proportion of boron-11 in the first section, i.e. it is at a level of enrichment or depletion intermediate between the first and second sections. The third section may comprise boron at or close to its natural isotopic abundance, e.g. having between 15% and 25% boron-10 with the remaining boron being boron-11. The first section is located between the second and third sections and the neutron source, and the third section is located between the first and second sections.

[0112] The boron-containing compounds may be metal borides, metal borocarbides, metal borohydrides, or boron carbide. The metal of the metal borides, metal borocarbides, or metal borohydrides may be tungsten or tantalum.

[0113] 38214647-3Enriched and depleted boron-containing compounds may be produced by obtaining enriched (i.e. high B-10) and depleted (i.e. high B-11) boron, which is readily available from chemical suppliers or may be produced by means known in the art such as those described in A V Khoroshilov and P I Ivanov “Boron isotope separation by extraction method: features of the phase composition and flow reflux", J. Phys.: Conf Ser. 2147 012018 (2022), which may then be used as a reagent in the synthesis of the boron-containing compound by methods known in the art, for example various tungsten borides may be synthesised by the methods of synthesis of e-phase W2BX disclosed previously, or those presented in Itoh, H., Matsudaira, T., Naka, S. et al. Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron. J Mater Sci 22, 2811-2815 (1987). The boron-containing compound may be used as the only material in the first and second sections, or may be used as part of a composite material in the first and second sections, e.g. a cermet containing a metal binder and particles of the boron-containing compound.

[0114] In general, for any known method of creating a boron-containing compound and incorporating such a compound into a neutron shield, a section of a neutron shield comprising enriched or depleted boron may be produced by substituting enriched or depleted boron for the boron used as a reactant when producing the boron-containing compound.

[0115] Figure 11 shows a schematic example of a tokamak fusion reactor comprising the neutron shielding disclosed above. The tokamak comprises a toroidal plasma chamber 1101, a plasma confinement system 1102 arranged to generate a magnetic field for confining a plasma in an interior of the plasma chamber, and neutron shielding as described above. In this example, the tokamak is shown as a spherical tokamak with a central column 1103.

[0116] The neutron shielding comprises a first section 1110, and a second section 1120 each comprising boron. At least 85% of boron in the first section is boron-11 , and at least 70% of boron in the second section is boron-10. In Figure 11, the neutron shielding is shown surrounding the whole plasma chamber, but neutron shielding with the structure described herein may be used only for a portion of the tokamak, e.g. only for the central column 1103 (shown here containing a component 1104 to be protected, such as a

[0117] 38214647-3magnet, etc.) where the use of thinner neutron shielding is most important, with other suitable neutron shielding being used elsewhere.

[0118] Figure 12 shows an assembly comprising a further neutron shield 1200. The neutron shield 1200 comprises first 1201 and second 1202 sections as described above, where the first section 1201 is between a neutron source 1220 and the second section 1202. The assembly further comprises a heat exchanger 1234, and the neutron shield further comprises coolant channels (e.g. pipes or the like) positioned within the first section (coolant channel 331), within the second section (coolant channel 333), and / or or between the first and second sections (coolant channel 332). The coolant and heat exchanger may be of any suitable type as known in the art.

[0119] The above disclosure is further described by the following numbered clauses

[0120] 1. Neutron shielding comprising:

[0121] first and second sections, each comprising boron;

[0122] configured such that in use the first section is between a neutron source and the second section;

[0123] wherein at least 75% of boron in the first section is boron-11 , and at least 70% of boron in the second section is boron-10.

[0124] 2. Neutron shielding according to clause 1 , wherein 75% to 85% of boron in the first section is boron-11.

[0125] 3. Neutron shielding according to clause 1 , wherein at least 85% of boron in the first section is boron-11.

[0126] 4. Neutron shielding according to any of clauses 1 to 3, wherein the boron is provided as one or more of:

[0127] a metal boride;

[0128] a metal borocarbide;

[0129] a metal borohydride;

[0130] boron carbide.

[0131] 38214647-35. Neutron shielding according to clause 4, wherein the boron is provided as one or more of:

[0132] a tungsten or tantalum boride;

[0133] a tungsten or tantalum borocarbide; or

[0134] a tungsten or tantalum borohydride..

[0135] 6. Neutron shielding according to clause 5, wherein the boron is provided as a bitungsten boride of an epsilon phase.

[0136] 7. Neutron shielding according to any preceding clause, and comprising a third section between the first section and the second section, the third section comprising boron, wherein:

[0137] the third section has a proportion of boron- 11 as a percentage of total boron less than the proportion of boron-11 as a percentage of total boron of the first section, and the third section has a proportion of boron- 10 as a percentage of total boron less than the proportion of boron- 10 as a percentage of total boron of the second section.

[0138] 8. Neutron shielding according to clause 7, wherein the proportion of boron-10 as a percentage of total boron in the third section is between 15% and 25%, with the remaining boron being boron-11.

[0139] 9. Neutron shielding according to clause 7 or 8, wherein:

[0140] the first section is bonded to the third section and / or the second section is bonded to the third section.

[0141] 10. Neutron shielding according to any of clauses 1 to 6, wherein the first and second sections are bonded together.

[0142] 11. Neutron shielding according to any preceding clause, wherein the first section comprises one or more boron-containing chemical compounds, the second section comprises the boron-containing chemical compounds of the first section, and the third section, if present, comprises the boron-containing chemical compounds of the first section,

[0143] whereby the boron-containing compounds in each section are chemically though not isotopically the same as the boron-containing compounds in each other section.

[0144] 38214647-312. Neutron shielding according to any preceding clause, further configured such that in use the first section receives neutrons directly from the neutron source.

[0145] 13. Neutron shielding according to any preceding clause, further configured such that in use, neutrons from the neutron source impinge on the first section without first passing through other neutron shielding.

[0146] 14. Neutron shielding according to any preceding clause, and further configured such that in use, neutrons having an initial neutron spectrum of a D-T fusion plasma impinge on the first section.

[0147] 15. Neutron shielding according to any preceding clause, and further configured such that in use, neutrons having a spectrum comprising an energy range of 10 to 14 MeV impinge on the first section.

[0148] 16. Neutron shielding according to any preceding clause, wherein the first section extends across 20% to 50% of the combined thickness of the first and second sections.

[0149] 17. Neutron shielding according to any preceding clause, and further comprising coolant channels located between the first and second section and / or coolant channels integrated within the first and / or second section.

[0150] 18. An apparatus comprising neutron shielding according to clause 17, and a heat exchanger, wherein the coolant channels are configured to transfer heat from the first section to the heat exchanger.

[0151] 19. An assembly comprising:

[0152] a neutron source; and

[0153] neutron shielding according to any one of clauses 1 to 17, or an apparatus according to clause 18;

[0154] wherein the neutron shielding is arranged such that the first section is between the neutron source and the second section.

[0155] 20. A nuclear fusion reactor comprising:

[0156] 38214647-3a plasma confinement region; and

[0157] neutron shielding comprising:

[0158] first and second sections, each comprising boron;

[0159] wherein at least 75% of boron in the first section is boron- 11 , and at least 70% of boron in the second section is boron- 10; wherein the first section is disposed between the plasma confinement region and the second section.

[0160] 21. The nuclear fusion reactor of clause 20, wherein the neutron shielding is neutron shielding according to any of clauses 1 to 17.

[0161] 22. The nuclear fusion reactor of clause 20 or 21, further comprising a heat exchanger, wherein:

[0162] the neutron shielding comprises:

[0163] coolant channels located between the first and second section and / or coolant channels integrated within the first and / or second section; and the coolant channels are configured to transfer heat from the neutron shielding to the heat exchanger

[0164] 23. The nuclear fusion reactor of any one of clauses 20 to 22, further comprising a component to be shielded from neutron radiation, wherein the neutron shielding is arranged between the plasma confinement region and the component to be shielded.

[0165] 24. The nuclear fusion reactor of clause 23 wherein:

[0166] the device further comprises a plasma chamber;

[0167] the plasma confinement region is interior to the plasma chamber; and the component to be shielded comprises a plasma confinement system arranged to generate a magnetic field for confining a plasma interior to the chamber in the plasma confinement region.

[0168] 25. The nuclear fusion reactor of any one of clauses 20 to 24 wherein the nuclear fusion reactor is one of:

[0169] a tokamak;

[0170] a stellarator;

[0171] a magnetic confinement nuclear fusion reactor;

[0172] an inertial confinement nuclear fusion reactor;

[0173] 38214647-3a magnetic pinch;

[0174] an electric pinch;

[0175] an inertial electrostatic confinement fusion reactor;

[0176] a magnetised target fusion reactor;

[0177] a beam target fusion reactor;

[0178] a muon catalysed fusion reactor; or

[0179] a lattice confinement fusion reactor.

[0180] 26. The nuclear fusion reactor of any of clauses 20 to 25 wherein the component to be shielded comprises one of:

[0181] a magnet; or

[0182] an electronic device or component.

[0183] 27. A method comprising:

[0184] providing neutron shielding and a neutron source, the neutron shielding comprising first and second sections, each comprising boron;

[0185] wherein the first section is between the neutron source and the second section, at least 75% of boron in the first section is boron-11, and at least 70% of boron in the second section is boron-10;

[0186] irradiating the first section of the shielding with neutrons from the source; and irradiating the second section of the shielding with neutrons passing from the source through the first section, wherein substantially all neutrons entering the second section of the shielding do so only after passing through the first section.

[0187] 28. A method of designing neutron shielding, the method comprising:

[0188] simulating a shielding performance measure for each of a plurality of neutron shields, each neutron shield comprising:

[0189] first and section sections, each comprising boron;

[0190] configured such that in use, the first section is between a neutron source and the second section;

[0191] wherein at least 75% of boron in the first section is boron- 11 , and at least 70% of boron in the second section is boron-10;

[0192] 38214647-3wherein the first section of each neutron shield has a respective first thickness, the respective first thickness being a respective percentage of the thickness of each neutron shield; and

[0193] selecting a neutron shield of the plurality of neutron shields based on the first thickness and the shielding performance measure.

[0194] 29. The method of clause 28, wherein selecting the neutron shield of the plurality of neutron shields comprises:

[0195] identifying a first set of neutron shields of the plurality of neutron shields, wherein each neutron shield of the first set has a shielding performance measure better than a threshold; and

[0196] selecting the neutron shield of the first set having a greater first thickness than the first thickness of each other neutron shield in the set.

[0197] 30. The method of clause 29, wherein the threshold is the shielding performance measure for an enriched neutron shield, the enriched neutron shield being a neutron shield of the plurality of neutron shields, the enriched neutron shield having a first thickness of zero.

[0198] 31. The method of any one of clauses 28 to 30, and comprising manufacturing a neutron shield according to the selected neutron shield.

[0199] 38214647-3

Claims

1. 23CLAIMS:

1. Neutron shielding comprising:first and second sections, each comprising boron;configured such that in use the first section is between a neutron source and the second section;wherein at least 75% of boron in the first section is boron-11 , and at least 70% of boron in the second section is boron-10.

2. Neutron shielding according to claim 1, wherein 75% to 85% of boron in the first section is boron-11.

3. Neutron shielding according to claim 1, wherein at least 85% of boron in the first section is boron-11.

4. Neutron shielding according to any of claims 1 to 3, wherein the boron is provided as one or more of:a metal boride;a metal borocarbide;a metal borohydride;boron carbide;a tungsten or tantalum boride;a tungsten or tantalum borocarbide;a tungsten or tantalum borohydride; ora bi-tungsten boride of an epsilon phase.

5. Neutron shielding according to any preceding claim, and comprising a third section between the first section and the second section, the third section comprising boron, wherein:the third section has a proportion of boron-11 as a percentage of total boron less than the proportion of boron-11 as a percentage of total boron of the first section, and the third section has a proportion of boron- 10 as a percentage of total boron less than the proportion of boron- 10 as a percentage of total boron of the second section.38214647-36. Neutron shielding according to claim 5, wherein the proportion of boron-10 as a percentage of total boron in the third section is between 15% and 25%, with the remaining boron being boron-11.

7. Neutron shielding according to claim 5 or 6, wherein:the first section is bonded to the third section and / or the second section is bonded to the third section.

8. Neutron shielding according to any of claims 1 to 4, wherein the first and second sections are bonded together.

9. Neutron shielding according to any preceding claim, wherein the first section comprises one or more boron-containing chemical compounds, the second section comprises the boron-containing chemical compounds of the first section, and the third section, if present, comprises the boron-containing chemical compounds of the first section,whereby the boron-containing compounds in each section are chemically though not isotopically the same as the boron-containing compounds in each other section.

10. Neutron shielding according to any preceding claim, further configured such that in use the first section receives neutrons directly from the neutron source and / or neutrons from the neutron source impinge on the first section without first passing through other neutron shielding.

11. Neutron shielding according to any preceding claim, wherein the first section extends across 20% to 50% of the combined thickness of the first and second sections.

12. Neutron shielding according to any preceding claim, and further comprising coolant channels located between the first and second section and / or coolant channels integrated within the first and / or second section.

13. An assembly comprising:a neutron source; andneutron shielding according to any one of claims 1 to 12;38214647-3wherein the neutron shielding is arranged such that the first section is between the neutron source and the second section.

14. A nuclear fusion reactor comprising:a plasma confinement region; andneutron shielding according to any of claims 1 to 12.

15. A method comprising:providing neutron shielding and a neutron source, the neutron shielding comprising first and second sections, each comprising boron;wherein the first section is between the neutron source and the second section, at least 75% of boron in the first section is boron-11, and at least 70% of boron in the second section is boron-10;irradiating the first section of the shielding with neutrons from the source; and irradiating the second section of the shielding with neutrons passing from the source through the first section, wherein substantially all neutrons entering the second section of the shielding do so only after passing through the first section.38214647-3

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