Tantalum based neutron shield

Abstract

Neutron shielding. The neutron shielding comprises a first section comprising a first neutron absorbing material comprising tantalum, and a second section comprising a second neutron absorbing material, wherein the second neutron absorbing material substantially lacks tantalum. In use, the first section is between a neutron source and the second section.

Description

[0001] Tantalum based neutron shield

[0002] Field of the Invention

[0003] The present invention relates to neutron shielding.

[0004] Background

[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 • Muon catalysed fusion

[0029] • Lattice confinement fusion

[0030] 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.

[0031] 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.

[0032] An effective, compact radiation shield to reduce the absorbed dose of neutrons in sensitive materials (such as in the central column of a spherical tokamak) originating from a high-energy neutron source requires a combination of high Z (atomic number, which implies high atomic mass) and low Z elements in bulk materials. High energy neutrons with kinetic energies greater than a few MeV are effectively moderated by two principal mechanisms: firstly through inelastic nuclear reactions with high Z nuclei, and secondly through elastic scattering with low Z nuclei. After inelastic scattering, the secondary neutron energies are typically below the reaction threshold energies for subsequent inelastic nuclear reactions with high Z elements and can therefore only be effectively moderated by low Z elements. A combination of high-Z and low-Z elements can therefore be used to reduce the high-energy neutron flux incident upon sensitive materials beyond the shield.

[0033] An effective high energy neutron shield requires four main sections, in order from the neutron source to the objects being protected:

[0034] • A material containing elements with high atomic mass, to step down the neutrons energy from the plasma (especially a material with a high cross section for inelastic scatter or neutron multiplication reactions) • A neutron moderator (i.e. a material containing elements with a low atomic mass), to further moderate the neutrons to optimal cross section energies

[0035] • A neutron absorber, to absorb the moderated neutrons

[0036] • A gamma shield, to absorb gamma rays produced by neutron interactions in earlier stages.

[0037] Where a single material or composite material can perform multiple functions, e.g. as a neutron absorber and a gamma shield, then two or more sections may be combined.

[0038] Tungsten is an excellent choice as the high-Z constituent element for the first stage owing to both its high Z (atomic number 74) and its typically high mass and number density in stable compounds as compared to other high-Z elements such as lead. Tungsten borides are especially favourable for shielding applications since they add boron as a constituent element of the neutron shield; boron is an effective neutron absorber at low energies preventing low-energy neutrons from penetrating the shield. In addition, tungsten is an effective absorber of gamma radiation. As such, tungsten borides can act as all of the above sections, with the exception of the neutron moderator (as boron does not have a significant moderating effect compared to, e.g. hydrogen). Tungsten carbides are also favourable for shielding application because the carbon provides neutron energy moderation, albeit not as effective as e.g. hydrogen - as such they may be used for all of the above sections, but will generally be outperformed as moderators by hydrogen-containing materials, or as neutron absorbers by tungsten boride. Additionally, tungsten isotopes have 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. In particular, the 1 MeV neutrons are readily moderated by carbon to energy levels which coincide with absorption resonance peaks of tungsten.

[0039] Hydrogen is the ideal neutron moderator below a few MeV, and is available as a constituent element of many potential materials. However, the most common, water and hydrocarbons, are problematic when integrated into a fusion power plant. Water is difficult to handle during disposal or maintenance of the application - due to the risks of pressured systems, contamination, activation and vaporisation of the water, and the possibility of water from the reactor getting into the environment if mishandled. One issue with neutron shielding is that the neutron irradiation will cause transmutation of the shielding materials, resulting in degradation of their shielding performance - i.e. high absorption isotopes will, on absorbing a neutron, often be transmuted into lower-absorbing isotopes, reducing the ability of the shielding to absorb neutrons particularly in the regions closest to the neutron source.

[0040] Summary

[0041] According to a first aspect, there is provided neutron shielding. The neutron shielding comprises a first section comprising a first neutron absorbing material comprising tantalum, and a second section comprising a second neutron absorbing material, wherein the second neutron absorbing material comprises tungsten and substantially lacks tantalum. In use, the first section is between a neutron source and the second section, and the first section receives neutrons directly from the neutron source.

[0042] According to a second aspect, there is provided an assembly comprising a neutron source; and neutron shielding according to the first aspect, wherein the neutron shielding is arranged such that the first section is between the neutron source and the second section.

[0043] According to a third aspect, there is provided a nuclear fusion reactor comprising a plasma confinement region and neutron shielding. The neutron shielding comprises a first section comprising a first neutron absorbing material comprising tantalum, and a second section comprising a second neutron absorbing material, wherein the second neutron absorbing material comprises tungsten and substantially lacks tantalum. The first section is disposed between the plasma confinement region and the second section, such that in use the first section receives neutrons directly from a plasma confined in the plasma confinement region.

[0044] According to a fourth aspect, there is provided a method. Neutron shielding and a neutron source are provided, the neutron shielding comprising a first section comprising a first neutron absorbing material comprising tantalum and a second section comprising a second neutron absorbing material comprising tungsten and substantially lacking tantalum, the first section of the neutron shielding being disposed closer to the neutron source than the second section. The first section of the shielding is irradiated with neutrons directly 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.

[0045] Further embodiments are presented in claim 2 etseq.

[0046] Brief Description of the Drawings

[0047] Figure 1 A shows a neutron source and a neutron shield.

[0048] Figure 1B shows the neutron source and neutron shield of Figure 1, and the area shielded by the neutron shield.

[0049] Figure 2 shows a further neutron shield.

[0050] Figure 3 shows an apparatus comprising a neutron shield and a heat exchanger.

[0051] Figure 4 shows an assembly comprising a neutron shield.

[0052] Figure 5 shows a tokamak comprising a neutron shield.

[0053] Detailed Description

[0054] As noted in the background, the performance of neutron shielding materials will tend to degrade over time, as neutron exposure transmutes the neutron absorbing isotopes into isotopes with a lower neutron absorption cross section. This effect is particularly pronounced in the outer (i.e. closest to the neutron source) layers of the neutron shielding.

[0055] However, it has been surprisingly found that neutron shielding containing tantalum, a metal with a high atomic mass, will actually increase in neutron absorption under neutron exposure, for example, exposure to a neutron flux having a neutron spectrum of a D-T fusion plasma, which may be, for example, a neutron flux with neutrons at an energy range of 10 to 14 MeV. The isotopes to which tantalum isotopes are transmuted under neutron exposure will on average result in more neutrons being absorbed within the shielding, so that the neutron shielding will absorb a higher proportion of incident neutrons following neutron exposure. As with the degradation of other materials under neutron exposure, this effect is most significant in areas of high neutron flux. In particular, a common transmutation pathway for tantalum isotopes results in them being transmuted into tungsten isotopes - for example, fast neutron bombardment of tantalum results in the capture of a neutron followed by the ejection of two lower energy neutrons, forming Ta-180 which undergoes beta decay to W-180.

[0056] Figure 1A shows an assembly comprising an exemplary neutron source 101 and a neutron shield 102. The neutron shield comprises a first section 110 and a second section 120. In use, the first section is located between the second section and the neutron source, and the first section 110 is exposed directly to the unshielded neutrons from the neutron source 101, with the neutron spectrum of the neutron source. For example, if neutron source 101 is a D-T fusion plasma, the neutrons impinging on first section 110 will have the neutron spectrum of the D-T 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.)

[0057] The first section comprises a first neutron absorbing material that comprises tantalum (e.g. tantalum metal or alloy, a ceramic containing tantalum such as tantalum carbide, tantalum boride, etc., a cermet containing tantalum or a composite material containing tantalum), and the second section comprises a second neutron absorbing material (e.g. tungsten, hafnium, rhenium, and ceramics containing those elements such as their carbides, borides and borocarbides, etc., as well as alloys, cermets, and composite materials including these metals) that substantially lacks tantalum, i.e. has tantalum in only trace or insubstantial amounts. The second neutron absorbing material may comprise less than 5% tantalum, less than 1% tantalum, or less than 1000 parts per million tantalum. The entire second section may substantially lack tantalum (e.g. less than 5% tantalum, less than 1% tantalum, or less than 1000 parts per million tantalum) and any sections of the neutron shielding on a side of the second section opposite the first section and / or any portions of the shielding other than the first section may also substantially lack tantalum.

[0058] For this disclosure, there are two relevant criteria for a “neutron absorbing material”. The first is the performance of the material for interactions with neutrons emitted by the neutron source (i.e. at the energy spectrum emitted by the neutron source), and the second is the performance of the material for interactions with neutrons that have passed through the first section. For the first neutron absorbing material, this performance is achieved by the presence of tantalum in the material in a significant proportion (e.g. at least 10% by number of atoms, at least 20%, at least 50%). The second neutron absorbing material may comprise an element having a total neutron microscopic cross section of at least 5.1 barns for the neutron spectrum of neutrons that have passed through the first section, or a portion thereof (e.g. a portion where the neutron flux is at least 50% of the peak of the spectrum). The “total neutron microscopic cross section” is the cross section for any neutron interactions with atoms of the element, averaged across the isotopes present (e.g. in their natural abundance).

[0059] This arrangement allows for good overall neutron shielding performance, and for neutron shielding performance which degrades more slowly over time (or may increase or remain stable, depending on the first and second neutron absorbing materials and the proportions used of each). The increased neutron absorption of the first section (due to transmutation of tantalum) will counteract the reduced neutron absorption of the second section (due to transmutation of isotopes within the second neutron absorbing material). This results in an overall neutron absorption for the shielding which decays over time more slowly than would be the case for a shield of equivalent thickness entirely made from the second neutron absorbing material, and which may be configured to be substantially constant over a given time period. The first section is located closest to the neutron source so that the tantalum is exposed to higher neutron flux than the second neutron absorbing material, maximising this effect.

[0060] This is of particular use where the second section comprises tungsten, as the transmutation of tantalum into tungsten in the first section will counteract the transmutation of tungsten into other elements in the second section.

[0061] As an example, the first neutron absorbing material may be present in the outer 5-20% of the thickness of the shielding.

[0062] 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.

[0063] 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.

[0064] The 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 1 B. Figure 1 B shows the neutron source 101 and shield 102 (comprising first and second sections 110 and 120) as in figure 1A, and illustrates the shielded region 130 which is protected by both sections. This shielded region 130 may contain components 140 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 102 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 102. 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.

[0065] 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.

[0066] 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.”

[0067] The first and second sections may be directly adjacent, and may be joined to one another at a common boundary surface.

[0068] The first section 110 may be situated so that no other neutron shielding is between it and neutron source 101. The first section 110 may be situated so that no other solid (or liquid) material is between it and neutron source 101 . Alternatively, the first section 110 may be situated in or behind a first wall or behind other solid or liquid material, through which neutrons from neutron source 101 pass before impinging on the first section 110, 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 110). The first section 110 may be directly exposed to neutron radiation at a neutron spectrum emitted by the neutron source 101. For example, if neutron source 101 comprises a fusion plasma, the neutron radiation impinging on the first section 110 may have the neutron spectrum of the fusion plasma.

[0069] The second section may be any suitable neutron shielding arrangement as known in the prior art, comprising a neutron absorbing material that does not comprise tantalum other than perhaps in insubstantial or trace amounts. For example, the second section may comprise tungsten or a ceramic containing tungsten such as tungsten carbide or tungsten boride. As other examples, the second section may comprise a tungsten alloy, a cermet containing tungsten, or a composite material including tungsten. The second neutron absorbing material of the second section may comprise an element having a greater total neutron microscopic cross section than that of tantalum, for the neutron spectrum produced by the neutron source or a portion thereof, for the element and tantalum in their isotopic abundance as present in the respective materials - i.e. the second neutron absorbing material may be initially in some sense a “better” neutron shielding material for the first section than the first neutron absorbing material. However, by the use of a first neutron absorbing material comprising tantalum as the neutron absorbing material of the first section, the performance of the neutron shielding over time is made more consistent - in other words, as the neutron shielding performance (e.g. as measured by the neutron absorption coefficient) of the second section degrades due to transmutation of the second neutron absorbing material, this is counteracted by the neutron shielding performance of the first section increasing due to the transmutation of tantalum in the first neutron absorbing material.

[0070] Where the neutron shielding is used in a nuclear fusion reactor, the neutron spectrum may be the deuterium-tritium fusion neutron spectrum as well studied in the art, or the cross sections may be compared for neutrons of approximately 14 MeV (the peak energy of neutrons produced by deuterium-tritium fusion reactions).

[0071] The use of tantalum as an outer layer of neutron shielding may be combined with a multilayered shielding comprising tungsten carbide or boride and a metal hydride as described in WO / 2021 / 122623. In that application, shielding is described which comprises a plurality of “absorption layers” comprising tungsten boride or tungsten carbide, and a plurality of “moderating layers” comprising a metal hydride. The metal of the metal hydride may be one or more of the metals from groups 4, 5 and 6 from the periodic table and / or yttrium, beryllium, gadolinium, or uranium, and the absorption layers may be an alloy of tungsten metal with the boride and / or carbide.

[0072] In the structures proposed in WO / 2021 / 122623, the absorption layers may make up at least 75% of the total thickness of the shielding (i.e. the combined thickness of the absorption layers and moderating layers), more preferably between 80% and 95%, and the outermost layer may have a thickness of between 30% and 90% of the thickness of the shielding.

[0073] As such, when combining the principles of WO / 2021 / 122623 with the use of tantalum as an outer layer of the shielding, a structure may be used as shown for the neutron shield 202 of Figure 2. The neutron shield comprises a multi-layered structure, with the outermost layer 210 (i.e. the layer facing the neutron source 201) containing tantalum. Behind this layer are one or more further absorption layers 221 comprising tungsten carbide or boride, and one or more moderating layers 222 comprising a metal hydride. Each moderating layer is between two of the further absorption layers, or between a further absorption layer and the outermost layer.

[0074] In the terms of the previous examples, the outermost layer 210 corresponds to the “first section”, as it comprises a first neutron absorbing material comprising tantalum, and the further absorption layers and moderating layers correspond to the “second section” 220, as they comprise a second neutron absorbing material not containing tantalum (i.e. tungsten carbide or boride).

[0075] The outermost layer 210 may extend e.g. 5-20% of the thickness of the shielding. The further absorption layer which is closest to the outermost layer may extend 10-85% of the thickness of the shielding, such that the combined thickness of that layer and the outermost layer is between 30% and 90% (i.e. corresponding to the thickness of the outermost absorption layer in WO / 2021 / 122623).

[0076] When used as a neutron shielding material, tantalum produces a large amount of heat. Depending on the use case, this may be an advantage or a disadvantage. For example, this may require additional cooling where the shielding is used around e.g. a medical neutron source. However, for some use cases such as for a fusion reactor, the generation of heat from neutron flux is particularly useful - the first neutron absorbing material will heat up due to the neutron flux, and this heat can be extracted and used for useful work.

[0077] The extraction of heat may be achieved by the use of a heat exchanger. Figure 3 shows an apparatus comprising a neutron shield 302 and a heat exchanger 334. The neutron shield 302 comprises a first section 310 comprising a first neutron absorbing material that comprises tantalum and a second section 320 comprising a second neutron absorbing material that substantially lacks tantalum, where the first section is between a neutron source 301 and the second section, as described previously. The entire second section 320 may, but need not, be substantially lacking in tantalum. The apparatus 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. The above neutron shielding may be used with any neutron source. Figure 4 shows an example of an assembly comprising a neutron source 401 , neutron shielding 402, and a component 403.

[0078] The neutron source 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.

[0079] The neutron shielding 402 comprises a first section 410 comprising a first neutron absorbing material that comprises tantalum and a second section 420 comprising a material substantially lacking in tantalum as described above, and is arranged to protect the component 403 from the neutron source 401 . The entire second section 420 may, but need not, be substantially lacking in tantalum.

[0080] The component 403 may be, for example, a component susceptible to neutron damage, such as a superconducting magnet or an electronic circuit. Alternatively or additionally, the component 403 may comprise materials susceptible to activation by neutrons (i.e. where neutron irradiation would cause the formation of radioactive isotopes within the material), 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 403.

[0081] Figure 5 shows a schematic example of a tokamak fusion reactor comprising the neutron shielding disclosed above. The tokamak comprises a toroidal plasma chamber 501, a plasma confinement system 502 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 503.

[0082] The neutron shielding comprises a first section 510 of a first material which comprises tantalum, and a second section 520 of a second material which substantially does not comprise tantalum. The entire second section 520 may, but need not, be substantially lacking in tantalum. The first section 510 is between the second section 520 and the plasma chamber. In Figure 5, 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 503 (shown here containing a component 504 to be protected, such as a magnet, etc.) where the use of thinner neutron shielding is most important, with other suitable neutron shielding being used elsewhere.

Claims

CLAIMS:1 . Neutron shielding comprising:a first section comprising a first neutron absorbing material comprising tantalum; a second section comprising a second neutron absorbing material, wherein the second neutron absorbing material comprises tungsten and substantially lacks tantalum;configured such that in use the first section is between a neutron source and the second section, and the first section receives neutrons directly from the neutron source.

2. Neutron shielding according to claim 1 wherein the second section substantially lacks tantalum.

3. Neutron shielding according to claim 2, where all sections of the shielding other than the first section substantially lack tantalum.

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

5. Neutron shielding according to any preceding claim, 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.

6. Neutron shielding according to any preceding claim, wherein, between the first and second neutron absorbing materials, there is no material that is all of:a neutron absorbing material comprising an element having a total neutron microscopic cross section of at least 5.1 barns for a neutron spectrum resulting from a neutron flux with an initial neutron spectrum of a D-T fusion plasma passing through the first section;a solid; andarranged to shield the region shielded by both of the first and second neutron absorbing materials.

7. Neutron shielding according to any preceding claim wherein the first and second sections are joined to one another at a common boundary therebetween.

8. Neutron shielding according to any preceding claim wherein the first and second sections are in conductive thermal contact with one another.

9. Neutron shielding according to any preceding claim, wherein the first section extends across 5 to 20% of the thickness of the neutron shielding.

10. Neutron shielding according to any preceding claim, wherein the first neutron absorbing material is one of:tantalum metal;a ceramic comprising tantalum;a cermet comprising tantalum;an alloy comprising tantalum;a composite comprising tantalum;tantalum carbide; ortantalum boride.

11. Neutron shielding according to any preceding claim, wherein the second neutron absorbing material is one of:tungsten metal;a ceramic comprising tungsten;a cermet comprising tungsten;an alloy comprising tungsten;a composite comprising tungsten;tungsten boride;tungsten carbide.

12. Neutron shielding according to any preceding claim, wherein the second section comprises:a plurality of absorption layers, each comprising the second neutron absorbing material; andat least one moderating layer comprising a metal hydride;wherein:each moderating layer is between at least two absorption layers or between an absorption layer and the first section; andthe outermost absorption layer is between the adjacent moderating layer and the first section; orthe outermost moderating layer is between the adjacent absorption layer and the first section.

13. 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.

14. An apparatus comprising neutron shielding according to claim 13, and a heat exchanger, wherein the coolant channels are configured to transfer heat from the first section to the heat exchanger.

15. An assembly comprising:a neutron source; andneutron shielding according to any of claims 1 to 13 or an apparatus according to claim 14;wherein the neutron shielding is arranged such that the first section is between the neutron source and the second section.

16. An assembly according to claim 15, wherein the neutron shielding is arranged with respect to the neutron source such that in use, any straight line passing from the neutron source to the second neutron absorbing material passes through the first neutron absorbing material.

17. An assembly according to claim 15 or 16, wherein the neutron shielding is arranged such that, in use, the region shielded from the neutron source by both of the first and second neutron absorbing materials is continuous.

18. An assembly according to any one of claims 15 to 17, wherein, between the first neutron absorbing material and the neutron source, there is no material that is a neutron absorbing material comprising an element having a total neutron microscopic cross section of at least 5.1 barns for a neutron spectrum resulting from neutron flux from the neutron source passing through the first section.

19. A nuclear fusion reactor comprising:a plasma confinement region; andneutron shielding comprising:a first section comprising a first neutron absorbing material comprising tantalum; a second section comprising a second neutron absorbing material, wherein the second neutron absorbing material contains tungsten and substantially lacks tantalum;wherein the first section is disposed between the plasma confinement region and the second section, such that in use the first section receives neutrons directly from a plasma confined in the plasma confinement region.

20. The nuclear fusion reactor of claim 19, wherein the neutron shielding is neutron shielding according to any of claims 1 to 13.21 . The nuclear fusion reactor of claim 19 or 20, further comprising a heat exchanger, wherein:the neutron shielding comprises:coolant channels location 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.

22. The nuclear fusion reactor of any one of claims 19 to 21 , 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.

23. The nuclear fusion reactor of any one of claims 19 to 22 wherein:the device further comprises a plasma chamber;the plasma confinement region is interior to the plasma chamber; and the component comprises a plasma confinement system arranged to generate a magnetic field for confining a plasma interior to the chamber in the plasma confinement region.

24. The nuclear fusion reactor of any one of claims 19 to 23 wherein the nuclear fusion reactor is one of:a tokamak;a stellarator;a magnetic confinement nuclear fusion reactor;an inertial confinement nuclear fusion reactor;a magnetic pinch;an electric pinch; oran inertial electrostatic confinement fusion reactor.

25. The nuclear fusion reactor of any of claims 19 to 24 wherein the component to be shielded comprises one of:a magnet; oran electronic device or component.

26. A method comprising:providing neutron shielding and a neutron source, the neutron shielding comprising a first section comprising a first neutron absorbing material comprising tantalum and a second section comprising a second neutron absorbing material comprising tungsten and substantially lacking tantalum, the first section of the neutron shielding being disposed closer to the neutron source than the second section;irradiating the first section of the shielding with neutrons directly from the source; andirradiating 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.

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