Absorber-reflector unit

Abstract

An absorber-reflector unit 100 for a nuclear reactor control drum (200, Fig. 15), comprises: a neutron-reflecting material 20, e.g. nuclear-grade graphite, beryllium oxide or aluminium oxide; a neutro

Description

FIELD OF THE DISCLOSURE The present disclosure relates to a reflector-absorber unit for use in a control drum unit, and more specifically a reflector-absorber unit for a control drum unit suitable for use in a nuclear reactor, and more specifically in a micro nuclear reactor. BACKGROUND Nuclear energy is viewed as an important contributor in the race to reduce global carbon emissions. Nuclear fission reactors are one type of device which can harness nuclear energy to provide “green” power. The amount of energy produced a by a fission-based nuclear reactor depends on the rate of fission events occurring within the reactor core. One common way to control the rate of fission events is to use “control rods” - rods of a neutron-absorbing material that can be inserted into the nuclear reactor core to absorb “stray” neutrons that would otherwise proliferate the chain reaction of nuclear fission events. An alternative device suggested for use to control of the amount of energy produced by a fission reactor is known as a control drum. The idea of a control drum is that it contains at least a neutron-absorbing material and a neutron-reflecting material, with a plurality of control drums being positioned around the periphery of the core of the nuclear reactor. By changing the orientation of the control drums, the nature of the material facing towards the nuclear core - either neutron-reflecting or neutron-absorbing - is varied. When the neutron-reflecting material faces towards the core, stray neutrons are reflected back towards the core to induce further fission events, and so the reactivity level is greater. When the neutron-absorbing material faces towards the core, stray neutrons are absorbed by the drum, reducing the number of fission events in the core, and therefore reducing the reactivity level. Control rods are by far the most common means of controlling the reactivity level of nuclear reactors, with comparatively few control drum devices actually existing today. Designing control drums is a challenging process, as the control drums inherently require the integration of different material types having different properties, which have to maintain repeatable, predictable, long-term functionality, whilst residing in an extremely challenging environment. It is therefore essential that control drums incorporate extremely robust component structures which will allow the safe control of the nuclear reactor during its lifetime. Whilst many theoretical designs of control drum exist, they often only consider the neutronics of the reactor, and frequently overlook the engineering challenges associated with the physical construction of such devices. The work described herein seeks to address this issue. SUMMARY The present disclosure provides an absorber-reflector unit for a nuclear reactor control drum as set out in claim 1, a control drum as set out in claim 13, and a nuclear reactor as set out in claim 16. Optional features are included in the dependent claims. According to a first aspect there is provided an absorber-reflector unit for a nuclear reactor control drum, the absorber-reflector unit comprising: a base plate; a neutron-reflecting material; a neutron-absorbing material; at least one shaft connected to the base plate and passing through the neutron-reflecting material, the at least one shaft being coupled to the neutron-reflecting material such that when the at least one shaft is rotated the neutron-reflecting material is rotated; at least one attachment member, the at least one attachment member being at least partially housed within both the neutron-reflecting material and the neutron-absorbing material so as to couple the neutron-reflecting material to the neutron-absorbing material. The absorber-reflector unit can comprise one or more positioning elements, each of which can be connected to the base plate and pass through the neutron-absorbing material. The at least one attachment member of the absorber-reflector unit can be oriented orthogonally to the shaft. The at least one attachment member of the absorber-reflector unit can be fixedly secured to the neutron-reflecting material, whilst allowing relative movement between the neutron-absorbing material and the at least one attachment member. The at least one attachment member of the absorber-reflector unit can be oriented parallel to the at least one shaft. The neutron-absorbing material of the absorber-reflector unit can comprise a plurality of sections of neutron-absorbing material. The neutron-reflecting material of the absorber-reflector unit can comprise a plurality of sections of neutron-reflecting material. The neutron-reflecting material of the absorber-reflector unit can comprise nuclear-grade graphite, beryllium oxide, or aluminium oxide. The neutron-absorbing material of the absorber-reflector unit can comprise boron carbide or borated stainless steel. The at least one shaft of the absorber-reflector unit can comprise stainless steel, nuclear-grade graphite, or aluminium oxide. The at least one attachment member of the absorber-reflector unit can comprise nuclear-grade graphite, beryllium oxide, or aluminium oxide. The at least one positioning element of the absorber-reflector unit comprises stainless steel, borated stainless steel, or boron carbide. According to a second aspect there is provided a control drum unit for a nuclear reactor, the control drum unit comprising the absorber-reflector unit of the first aspect. The control drum unit can further comprise a retaining element, the retaining element being configured to resist movement of the absorber-reflector unit within the control drum unit, whilst allowing expansion of the absorber-reflector unit. The retaining element can be a spring. According to a third aspect, there is provided a nuclear reactor comprising the control drum unit of the second aspect. The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and / or combined with any other feature described herein. BRIEF DISCRIPTION OF THE DRAWINGS Embodiments will now be described by way of example only, with reference to the Figures, in which: FIG. 1 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 2 is a schematic plan view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 3 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 4 is a schematic plan view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 5 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 6 is a schematic plan view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 7 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 8 is a schematic plan view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 9 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 10 is a schematic plan view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 11 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 12 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 13 is a schematic isometric view of an example of an absorber-reflector unit for a nuclear reactor control drum unit; FIG. 14 shows a schematic exploded view of an example control drum unit; and FIG. 15 shows a schematic example of a nuclear reactor. DETAILED DESCRIPTION Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. FIG. 1 shows a schematic isometric view, and FIG. 2 a schematic plan view, of an example of an absorber-reflector unit for a nuclear reactor control drum unit. The absorber-reflector unit comprises a base plate 10, a neutron-reflecting material 20, a neutron-absorbing material 30, a shaft 40, and a number of attachment members 50. The base plate 10 is connectable to a rotation means (not shown), such as a gear or actuator, which can be used to rotate the base plate. The shaft 40 is connected to the base plate 10, and passes through the neutron-reflecting material 20. In the example of FIG. 1 and FIG. 2, the shaft 40 and the hole through the neutron-reflecting material 20 through which the shaft 40 passes are shaped to interlock with one another, such that rotation of the shaft causes rotation of the neutron-reflecting material 20. Suitable materials for the base plate include stainless steel, low-alloy steel, and nickel-based alloys. Suitable neutron-reflecting materials include ceramics such as nuclear-grade graphite, beryllium oxide, and aluminium oxide. These materials are all approved for use in nuclear reactors. In the example absorber-reflector unit 100 of FIG.1 and FIG. 2, the neutron-reflecting material 20 has a channel on its outer surface, into which is positioned a neutron-absorbing material 30. The neutron-absorbing material is generally shaped to match the shape of the channel in the outer surface of the neutron-reflecting material. Suitable neutron-absorbing materials include boron carbide and borated stainless steel. In the example of FIG. 1 and FIG. 2, the neutron-absorbing material is coupled to the neutron-reflecting material by a number of attachment members 50. The attachment members 50 pass through apertures in the neutron-absorbing material 30 and into apertures in the neutron-reflecting material 20. The attachment members 50 are then secured to the neutron-reflecting material 20, such that the attachment members 50 are fixed in place with relation to the neutron-reflecting material 20. As a consequence of this, the neutron-absorbing material 30 becomes fixed in position in relation to the neutron-reflecting material 20. Suitable attachment members for the example absorber-reflector units 100 of FIG.1, FIG.2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 11, FIG. 12, and FIG. 13 include screws, bolts, and rivets, i.e. means that can be fixed to the neutron-reflecting material 20 in such a way as to hold the neutron-absorbing material 30 in position in relation to the neutron-reflecting material 20. Suitable materials for the attachment members include nuclear-grade graphite, beryllium oxide, and aluminium oxide, i.e. the same materials as can be used for the neutron-reflecting material 20. Indeed, by making the attachment members 50 from the same material as the neutron-reflecting material 20, the amount of stress induced by heat expansion where the attachment members are secured to the neutron-reflecting material can be minimised. Depending on the materials used, it is envisaged that the apertures in the neutron-reflecting material 20 through which the attachment members 50 pass may be untapped, and may have a small clearance around the section of the attachment member passing through the neutron-reflecting material 20 so as to accommodate any relative movement between the material of the attachment member(s) 50 and the neutron-absorbing material 30 at least in an axial direction of the attachment member(s) 50. Such relative movement will only be on a small scale, for example relative movement owing to any difference in the rates of thermal expansion of the material of the attachment member(s) 50 and the neutron-absorbing material 30. As such, the neutron-absorbing material 30 is still held in position relative to the neutron-reflecting material 20, but the possibility of thermal expansion is accounted for, to reduce the chance of stress-induced fractures in the materials during operation. In addition, although not shown in the figures, the shaft 40, neutron-reflecting material 20 and neutron-absorbing material 30 are dimensioned such as to allow for a degree of thermal expansion when the absorber-reflector unit is exposed to the high temperatures surrounding a nuclear reactor core. For example, there may be a small gap between the neutron-absorbing material 30 and the neutron-reflecting material 20 so as to allow for a degree of expansion of the neutron-absorbing material 30 during use of the absorber-reflector unit. FIG. 3 shows a schematic isometric view, and FIG. 4 a schematic plan view, of a further example of an absorber-reflector unit for a nuclear reactor control drum unit. In the example absorber-reflector unit of FIG.3 and FIG.4, additional shafts 40 have been coupled to the base plate 10. The additional shafts 40 also pass through the neutron-reflecting material 20. The addition of further shafts 40 are a way to spread the mechanical stress induced in the one or more shafts overcoming the inertia of the neutron-reflecting material when the base plate is rotated. Having more than one shaft 40 connected to the base plate 10 and passing through the neutron-reflecting material 20 also means that each individual shaft does not have to be shaped such that rotation of a single shaft causes rotation of the neutron-reflecting material 20. That is to say, each individual shaft does not have to interlock with the neutron-reflecting material 20. As machining interlocking shapes into these materials can be challenging, a design which allows for the use of shapes or cross-sections which are easier to machine can be advantageous. For example, by having two or more shafts connected to the base plate 10 and passing through the neutron-reflecting material 20, the rotation of the base plate 10 will still cause rotation of the neutron-reflecting material 20 via the shafts 40, but each shaft can have, for example, a circular cross-section, which is simpler to produce, and removes any points of stress-concentration from the surfaces of the two or more shafts 40. The shaft or shafts 40 may comprise stainless steel, nuclear-grade graphite, and aluminium oxide, as they have suitable structural and neutron-interacting characteristics. In the example absorber-reflector unit of FIG.3 and FIG.4, only four attachment members 50 are shown, compared with the eight shown in the example absorber-reflector unit of FIG.1 and FIG.2. This is to demonstrate that the actual number and location of the attachment members can be varied, providing there is at least one attachment member to fix the section of neutron-absorbing material 30 into position in relation to the section of neutron-reflecting material 20. FIG. 5 shows a schematic isometric view, and FIG. 6 a schematic plan view, of a further example of an absorber-reflector unit for a nuclear reactor control drum unit. In the example absorber-reflector unit of FIG.5 and FIG.6, in addition to the features of the example shown in FIG. 3 and FIG. 4, positioning elements 60 have been added. The positioning elements are attached to the base plate 10, and pass through the neutron-absorbing material, such that at least part of each positioning element 60 is enclosed by the neutron-absorbing material. Adding one or more positioning elements 60 to the absorber-reflector unit can reduce the stress induced in the one or more attachment members 50 overcoming the inertia of the neutron-absorbing material 30 when the base plate is rotated. Suitable materials for the one or more positioning elements 60 include boron carbide and borated stainless steel, i.e. the same as the neutron-absorbing material 30. By making the one or more positioning elements 60 out of the same neutron-absorbing material 30, the potential for mechanical stresses caused by having neighbouring materials undergoing different rates of thermal expansion is minimised. FIG. 7 shows a schematic isometric view, and FIG. 8 a schematic plan view, of a further example of an absorber-reflector unit for a nuclear reactor control drum. In the example absorber-reflector unit of FIG.7 and FIG.8, as with the example shown in FIG. 1 and FIG.2, there is a single shaft 40 passing through the neutron-reflecting material 20. The shaft 40 and the hole through the neutron-reflecting material 20 through which the shaft 40 passes are shaped to interlock with one another, such that rotation of the shaft causes rotation of the neutron-absorbing material 30. The skilled person will appreciate a wide variety of interlocking shapes and holes can be used depending on, amongst other things, machining capabilities and materials used for the shaft and neutron-reflecting material 20. In the example absorber-reflector unit of FIG.7 and FIG.8, the attachment member 50 takes the form of a columnar interlock, which is to say the attachment member 50 has a cross-sectional profile which allows it to be slid into grooves on the surface of both the neutron-reflecting material 20 and the neutron-absorbing material 30, and having been positioned within those grooves, it holds the neutron-reflecting material 20 and the neutron-absorbing material 30 in a fixed positional relationship with respect to one another. The schematic example shown in FIG.7 and FIG. 8 has a rounded hourglass-like cross-sectional profile, which is illustrative of a such shapes having a narrow waist and comparatively wider extremities, which serve to hold the neutron-reflecting material 20 and the neutron-absorbing material 30 in a fixed positional relationship with respect to one another. Suitable materials for the attachment member include the same materials that can be used for neutron-reflecting material or the neutron-absorbing material, including stainless steel, boron carbide, and beryllium oxide. Using the same material for the attachment member 50 as that used of either the neutron-reflecting material 20 or neutron-absorbing material 30 reduces the issues associated with using materials having differing coefficients of thermal expansion within the absorber-reflector unit. However, in order to account for any remaining differences in thermal expansion, it is envisaged that a small gap (not shown) could be present between the attachment member 50 and one or more of the neutron-reflecting material 20 and neutron-absorbing material 30. The neutron-reflecting material 20 and the neutron-absorbing material 30 in the example absorber-reflector unit of FIG.7 and FIG.8 are shown having different cross-sectional profiles compared with earlier examples. It is to be noted that the exact profiles of the neutron-reflecting material 20 and the neutron-absorbing material 30 can vary, and will depend on the materials used, the configuration of the control drum and nuclear reactor in which the absorber-reflector unit is used, and the associated neutronics. It will be appreciated that the means of holding and positioning the components of the absorber-reflector unit described herein enable a wide variety of cross-sectional profiles to be used in the absorber-reflector unit. FIG. 9 shows a schematic isometric view, and FIG. 10a schematic plan view, of a further example of an absorber-reflector unit for a nuclear reactor control drum. In the example absorber-reflector unit 100 of FIG.9 and FIG. 10, an additional shaft 40 has been coupled to the base plate 10. The additional shaft 40 also passes through the neutron-reflecting material 20. The addition of a further shaft 40 is a way to spread the mechanical stress induced in the one or more shafts overcoming the inertia of the neutron-reflecting material when the base plate is rotated. Having more than one shaft 40 connected to the base plate 10 and passing through the neutron-reflecting material 20 also means that each individual shaft does not have to be shaped such that rotation of a single shaft causes rotation of the neutron-absorbing material 30. For example, by having two or more shafts connected to the base plate 10 and passing through the neutron-reflecting material 20, each shaft can have a circular cross-section, thus removing points of stress-concentration from the surfaces of the two or more shafts 40. In the example absorber-reflector unit 100 of FIG.9 and FIG. 10, a positioning element 60 has been attached to the base plate 10, and passes through the neutron-absorbing material, such that at least part of each positioning element 60 is enclosed by the neutron-absorbing material. Adding one or more positioning elements 60 to the absorber-reflector unit can reduce the stress induced in the one or more attachment members 50 overcoming the inertia of the neutron-absorbing material 30 when the base plate is rotated. FIG. 11 shows a schematic isometric view of a further example of an absorber-reflector unit for a nuclear reactor control drum. This example is similar to the example shown in FIG. 5, with the additional feature that the neutron-absorbing material 30 has been divided into a number of sections, which in this example have been stacked on top of one another. It may be useful to be able to use sections of a neutron-absorbing material 30 in such a way if, for example, single pieces of neutron-absorbing material of a size required for the absorber-reflector unit 100 are not available, are difficult to machine into shape, or difficult to transport. In cases where the neutron-absorbing material is divided into sections, the addition of one or more positioning elements 60 can be beneficial as they can help with alignment of the sections of the neutron-absorbing material 30. However, as will be appreciated, the addition of one or more positioning elements is not essential, as the sections of neutron-absorbing material 30 also can be positioned using the one or more attachment members 50. FIG. 12 shows a schematic isometric view of an example absorber-reflector unit 100 where the neutron-reflecting material 20 is divided into sections. As with the section of neutron-absorbing material 30 in the example of FIG. 11, it may be useful to be able to stack portions of a neutron-reflecting material 20 in such a way if, for example, single pieces of neutron-reflecting material of a size required for the absorber-reflector unit 100 are not available, are difficult to machine into shape, or difficult to transport. Attachment members 50 can be used to position the neutron-absorbing material 30 with respect to the neutron-reflecting material 20. FIG. 13 shows a schematic isometric view of a further example of an absorber-reflector unit for a nuclear reactor control drum. In the example absorber-reflector unit 100 of FIG. 13, both the neutron-reflecting material 20 and the neutron-absorbing material 30 take the form of sections of neutron-reflecting material 20 and neutron-absorbing material 30 which have been stacked on top of one another. As with the examples of FIG. 11 and FIG. 12, it may be useful to be able to stack portions of neutron-reflecting material 20 and / or neutron-absorbing material 30 in such a way if, single pieces of a size required for the absorber-reflector unit 100 are not available, are difficult to machine into shape, or difficult to transport, for example. Attachment members 50 can be used to position the neutron-absorbing material 30 with respect to the neutron-reflecting material 20. It will be appreciated that, whilst the examples shown herein have shown the neutron-reflecting material 20 and neutron-absorbing material 30 divided into sections along planes perpendicular to the axis of the shaft, it is also possible to divide both or either of the neutron-reflecting material 20 and the neutron-absorbing material 30 into sections along other axes too, for example along a plane running parallel to the axis of the shaft, providing (in the case of a section of neutron-reflecting material 20) there is at least one shaft passing through the section neutron-reflecting material 20, and (in the case of a section of neutron-absorbing material 30) there is at least one attachment member 50 passing through the section of neutron-absorbing material 30 to secure the section neutron-absorbing material 30 to a section of neutron-reflecting material 20. In the example of FIG. 13, the base plate has a smaller footprint than the footprint of the neutron-reflecting material 20 and neutron-absorbing material combined. It is not necessary for the base plate to have the same footprint as the footprint of the neutron-reflecting material 20 and neutron-absorbing material 30 combined, as a combination of shafts 40 and attachment members 50 can be used to keep the positional relationship between the base plate, neutron-reflecting material 20, and neutron-absorbing material 30 fixed, such that rotation of the base plate causes rotation of the neutron-reflecting material 20 and neutron-absorbing material 30. It is possible the base plate 10 and one or more of the one or more shafts 40 can be formed from a single piece of material, However, the base plate 10 having the same or similar footprint to the combined neutron-reflecting material 20 and neutron-absorbing material can be beneficial in terms of spreading the load if the absorber-reflector unit is oriented vertically in a gravitational field. The size and shape of the base plate 10 can vary depending on the environment(s) it will be expected to operate in. The sections of neutron-absorbing material 30 and neutron-reflecting material 20 do not have to have the same length along the axis of the shaft: as illustrated in the example of FIG. 13, the lengths of the sections of the neutron-absorbing material 30 and the neutron-reflecting material 20 can be different from each other; the lengths of the sections of the neutron-absorbing material 30 do not all have to be the same as each other; and lengths of the sections of the neutron-reflecting material 20 do not all have to be the same as each other either. The absorber-reflector unit 100 described herein is configured for use within a control drum unit. FIG. 14 shows a schematic exploded view of a control drum unit 200 to illustrate an example of how the absorber-reflector unit 100 may be incorporated into a control drum unit 200. The control drum unit 200 comprises a bottom cap 210 to which the absorber-reflector unit 100 is attached. The absorber-reflector unit 100 can be fixed to the bottom cap 210 such that rotation of the bottom cap leads to rotation of the entire control drum unit 200, including the absorber-reflector unit contained therein. A retaining plate 220 is then positioned atop the absorber-reflector unit 100, which provides a flat surface against which a retaining element 230 can be placed. The retaining element provides a resistive force to keep the components of the absorber-reflector unit 100 in place within the control drum unit 200, whilst allowing the absorber-reflector unit 100 to expand as a result of being heated by the energy absorbed from the nuclear reactor core. In other words, the retaining element resists movement of the absorber-reflector unit 100 within the control drum unit 200, whilst allowing a degree of expansion of the absorber-reflector unit 100. The resistive element can take the form of a spring, for example a coil spring or leaf spring. An outer sleeve 240 is positioned to surround the absorber-reflector unit, and is brought into abutment with the bottom cap 210 by the attachment of a top cap 250. The top cap 250 can also serve to compress the retaining element 230 against the retaining plate 220. The top cap 250, bottom cap 210, and outer sleeve 240 can then be welded together in order to hermetically seal the control drum unit 200. The absorber-reflector unit can be contained within a hermetically sealed vessel so as to contain any toxic gas created and released by the neutron-absorbing material 30 during operation of the nuclear reactor. To that end, the sealed vessel created by the combination of the bottom cap 210, outer sleeve 240, and top cap 250 can be dimensioned to include enough space to accommodate both the thermal expansion of the materials encased within, and any outgas products produced during the operational lifetime of the control drum unit 200. The inclusion of the retaining element 230 means that the absorber-reflector unit 100 is held in the same relative position within the absorber-reflector unit 100 whilst still being able to undergo thermal expansion. Suitable materials for the bottom cap 210, retaining plate 220, retaining element 230, outer sleeve 240, and top cap 250 include stainless steel, low-alloy steels, and nickel-based alloys. FIG. 15 shows a schematic example of a nuclear reactor 300 comprising a nuclear reactor core 350 and a number of control drum units 200. One or more of the control drum units 200 may contain an absorber-reflector unit 100 according to the description herein. The control drum units 200 are positioned around the periphery of the nuclear reactor core 350. The rotational position of the control drum units 200 is controlled by a control drum unit control unit 260. The reactivity level in the nuclear reactor core 350 can be controlled by rotating the control drum units 200 containing absorber-reflector units 100 to vary whether it is the neutron-reflecting material 20 or neutron-absorbing material 30 that is facing towards the nuclear reactor core 350, as described earlier. As such, the absorber-reflector unit 100 described herein is capable of being used as part of a control drum control system for a nuclear reactor. The absorber-reflector unit described herein benefits from design features which can be constructed using only materials which are already approved for use in nuclear reactors. In particular, the absorber-reflector unit described herein does not require the use of any adhesives or bonding layers, which generally involve materials which have not been approved for use in nuclear reactors, and therefore require extensive testing a validation before they could be considered for use in a nuclear reactor. By comparison, the absorber-reflector unit described herein uses mechanical joins and welds, and as such represents a realistic and practical solution for nuclear reactor activity level control. 5 The absorber-reflector unit described herein is suitable for use in one or more control drums within a nuclear reactor. The absorber-reflector unit described herein may be particularly suitable for use in a microreactor, where space and mass limitations are particularly challenging. The absorber-reflector unit described herein may be particularly suitable for use in a microreactor configured for extra-terrestrial use, for example on a 10 lunar base, non-terrestrial planetary base, or space station, as control drums are able to withstand the challenging conditions, such as vibrations and G-forces, encountered during launch from Earth in a space vessel. It will be understood that the invention is not limited to the embodiments above described and various modifications and improvements can be made without departing 15 from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. An absorber-reflector unit for a nuclear reactor control drum, the absorber-reflector unit comprising:a base plate;a neutron-reflecting material;a neutron-absorbing material;at least one shaft connected to the base plate and passing through the neutron-reflecting material, the at least one shaft being coupled to the neutron-reflecting material such that when the at least one shaft is rotated the neutron-reflecting material is rotated;at least one attachment member, the at least one attachment member being at least partially housed within both the neutron-reflecting material and the neutron-absorbing material so as to couple the neutron-reflecting material to the neutron-absorbing material.

2. The absorber-reflector unit of claim 1, further comprising one or more positioning elements, each of the one or more positioning elements being connected to the base plate and passing through the neutron-absorbing material.

3. The absorber-reflector unit of claim 1 or claim 2, wherein the at least one attachment member is oriented orthogonally to the shaft.

4. The absorber-reflector unit of any preceding claim, wherein the at least one attachment member is fixedly secured to the neutron-reflecting material, whilst allowing relative movement between the neutron-absorbing material and the at least one attachment member.

5. The absorber-reflector unit of claim 1 or claim 2, wherein the at least one attachment member is oriented parallel to the at least one shaft.

6. The absorber-reflector unit of any preceding claim, wherein the neutron-absorbing material comprises a plurality of sections of neutron-absorbing material.

7. The absorber-reflector unit of any preceding claim, wherein the neutron-reflecting material comprises a plurality of sections of neutron-reflecting material.

8. The absorber-reflector unit of any preceding claim, wherein the neutron-reflecting material comprises nuclear-grade graphite, beryllium oxide, or aluminium oxide.

9. The absorber-reflector unit of any preceding claim, wherein the neutron-absorbing material comprises boron carbide or borated stainless steel.

10. The absorber-reflector unit of any preceding claim, wherein the at least one shaft comprises stainless steel, nuclear-grade graphite, or aluminium oxide.

11. The absorber-reflector unit of any preceding claim, wherein the at least one attachment member comprises nuclear-grade graphite, beryllium oxide, or aluminium oxide.

12. The absorber-reflector unit of any preceding claim, wherein the at least one positioning element comprises stainless steel, borated stainless steel, or boron carbide.

13. A control drum unit for a nuclear reactor, the control drum unit comprising the absorber-reflector unit of any preceding claim.

14. The control drum unit of claim 13, further comprising a retaining element, the retaining element being configured to resist movement of the absorber-reflector unit 100 within the control drum unit 200, whilst allowing expansion of the absorber-reflector unit 100.

15. The control drum unit of claim 14, wherein the retaining element is a spring.

16. A nuclear reactor comprising the control drum unit of any of claim 13, claim 14,or claim 15.

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