Absorber-reflector unit

Abstract

An absorber-reflector unit 100 for a nuclear reactor control drum (200, Fig. 9), comprises: a neutron-reflecting material 20, e.g. nuclear-grade graphite, beryllium oxide or aluminium oxide; a neutron

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 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 unit for a nuclear reactor as set out in claim 8, and a nuclear reactor as set out in claim 11. 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 neutron-reflecting material; a neutron-absorbing material; a shaft at least partially housed within the neutron-reflecting material and coupled to the neutron-reflecting material such that when the shaft is rotated the neutron-reflecting material is rotated; at least one positioning member, the at least one positioning member being at least partially housed within the neutron-absorbing material, and oriented parallel to the shaft. The neutron-absorbing material may comprise a plurality of sections of neutron-absorbing material. The neutron-reflecting material may comprise a plurality of sections of neutron-reflecting material. The neutron-reflecting material may comprise nuclear-grade graphite, beryllium oxide, or aluminium oxide. The neutron-absorbing material may comprise boron carbide or borated stainless steel. The at least one shaft of the absorber-reflector unit may comprise stainless steel, nuclear-grade graphite, or aluminium oxide. The at least one positioning member of the absorber-reflector unit may comprise stainless steel, boron carbide, or borated stainless steel. In 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 may 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 of the control drum unit may be a spring. In 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 absorber-reflector unit; FIG. 2 is a schematic plan view of an example absorber-reflector unit; FIG. 3 is a schematic isometric view of an example absorber-reflector unit; FIG. 4 is a schematic plan view of an example absorber-reflector unit; FIG. 5 is a schematic isometric view of an example absorber-reflector unit; FIG 6 is a schematic isometric view of an example absorber-reflector unit; FIG 7 is a schematic isometric view of an example absorber-reflector unit; FIG 8 is a schematic exploded view of a control drum unit; and FIG 9 is a schematic of an example 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 shows a schematic plan view, of an example absorber-reflector unit 100 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 positioning member 60. 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, such that rotation of the base plate 10 causes rotation of the shaft 40. The shaft 40 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 materials for the shaft include stainless steel, nuclear-grade graphite, and aluminium oxide. 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 positioned relative to the base plate 10 by a positioning member 60. The positioning member 60 is oriented parallel to the shaft 40. The positioning member 60 is connected to the base plate 10, and passes through the neutron-absorbing material 30. In the example of FIG.1 and FIG. 2, the positioning member 60 and the hole through the neutron-absorbing material 30 through which the positioning member 60 passes, are shaped to interlock with one another, such that once the neutron-absorbing material 30 is fitted into place around the positioning member 60 and within the channel on the surface of the neutron-reflecting material 20, the neutron-absorbing material 30 will remain in that position relative to the positioning member 60 and the neutron-reflecting material 20, even during rotation of the base plate 10. It will however be appreciated that providing the neutron-absorbing material 30 is generally shaped to match the shape of the channel in the outer surface of the neutron-reflecting material 20, the positioning member 60 and the hole through the neutron-absorbing material 30 through which the positioning member 60 passes do not have to be shaped to interlock with one another in order for the neutron-absorbing material 30 to remain in position relative to the neutron-reflecting material 20 during rotation of the base plate 10. For example, a cylindrical positioning member, in combination will the complimentary shaping of the channel in the outer surface of the neutron-reflecting material and the neutron-absorbing material 30, will serve to keep the neutron-absorbing material 30 in position relative to the neutron-reflecting material 20. The skilled person will further appreciate that it is not essential for both the shaft 40 and the positioning member 60 to interlock with the neutron-reflecting material 20 and neutron-absorbing material 30 respectively in the case where there is just a single shaft 40 and a single positioning member 60, providing that the neutron-reflecting material 20 and neutron-absorbing material 30 have complimentary shapes, and at least one of the shaft 40 or the positioning member 60 interlock with the neutron-reflecting material 20 and neutron-absorbing material 30 respectively. In other words, if the shaft 40 interlocks with the neutron-reflecting material 20, and the neutron-absorbing material interlocks with the neutron-reflecting material (for example, by fitting into a channel in the surface of the neutron-reflecting material), then the positioning member 60 does not need to interlock with the neutron-absorbing material, but can instead have other forms, such as a form with a circular cross-section. Equally, if the positioning member 60 interlocks with the neutron-absorbing material 30, and the neutron-reflecting material 20 interlocks with the neutron-absorbing material 30 (for example, by encompassing the neutron-absorbing material), then the shaft 40 does not need to interlock with the neutron-reflecting material in order to keep the neutron-reflecting material in place with respect to the neutron-absorbing material 30. In addition, although not shown in the figures, the shaft 40, positioning member 60, 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. The skilled person will appreciate it is possible, when required, to achieve the necessary degree of interlock between parts such as the shaft 40 and neutron-reflecting material 20, or the neutron-reflecting material 20 and the neutron-absorbing material 30, whilst still allowing for enough room between the parts to accommodate the small amount of thermal expansion, or differences in rates of thermal expansion, expected from components of such a device. FIG. 3 shows a schematic isometric view, and FIG. 4 a schematic plan view, of a further example of an absorber-reflector unit 100 for a nuclear reactor control drum. 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 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 the individual shafts do not need 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, 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 example of FIG. 3 and FIG. 4 shows three shafts 40, but the skilled person will appreciate that this is purely illustrative and that two, four, or more shafts 40 can be used. Furthermore, the example absorber-reflector unit 100 of FIG. 3 and FIG. 4 also has an additional positioning member 60. By having two or more positioning members 60 connected to the base plate 10 and passing through the neutron-absorbing material 30, the individual positioning members 60 do not have to interlock with the neutron-absorbing material 30, increasing the range of potential shapes that the positioning members 60 can take. For example, each positioning member 60 can have a circular cross-section, thus removing points of stress-concentration from the surfaces of the two or more positioning members 60. The example of FIG. 3 and FIG. 4 shows two positioning members 60, but the skilled person will appreciate that this is purely illustrative and that three, four, or more positioning members 60 can be used. FIG. 5 shows a schematic isometric view of a further example absorber-reflector unit 100. In this example, the neutron-reflecting material 20 is divided up into sections. This may be necessary if it is difficult to acquire single pieces of neutron-reflecting material 20 of a size required for the absorber-reflector unit 100. Advantageously, because of the orientation of the shafts 40, the neutron-reflecting material 20 can be stacked over the shafts 40, and the shafts 40 can be used to align the sections of neutron-reflecting material 20 with one another and with the neutron-absorbing material 30. Thus, the examples shown herein allow for the use of smaller pieces of neutron-reflecting material, without compromising the accuracy of their positioning within the absorber-reflector unit 100. FIG. 6 shows a schematic isometric view of a further example absorber-reflector unit 100. In this example, the neutron-absorbing material 30 is divided up into sections. As with the sections of neutron-reflecting material 20 in the example of FIG. 5, it may be useful to be able to stack portions of a neutron-absorbing material 30 in such a way if, for example, single pieces of neutron-absorbing material 30 of a size required for the absorber-reflector unit 100 are not available, are difficult to machine into shape, or difficult to transport. FIG. 7 shows a schematic isometric view of a further example absorber-reflector unit 100. In this example, both the neutron-reflecting material 20 and the neutron-absorbing material 30 take the form of sections of neutron-reflecting material 20 and section of neutron-absorbing material 30 which have been stacked on top of one another. As with the examples of FIG. 5 and FIG. 6, it may be useful to be able to stack portions of neutron-reflecting material 20, neutron-absorbing material 30, or both the neutron-reflecting material 20 and 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. 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. 7, 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. 8 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 relative 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 due to its location with respect to 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 thermal 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 can be 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. 9 shows a schematic of an example 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 at least one 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. 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 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 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 neutron-reflecting material;a neutron-absorbing material;a shaft at least partially housed within the neutron-reflecting material and coupled to the neutron-reflecting material such that when the shaft is rotated the neutron-reflecting material is rotated;at least one positioning member, the at least one positioning member being at least partially housed within the neutron-absorbing material, and oriented parallel to the shaft.

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

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

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

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

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

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

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

9. The control drum unit of claim 8, 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 100.5 10. The control drum unit of claim 9, wherein the retaining element is a spring.

11. A nuclear reactor comprising the control drum unit of any of claim 8, claim 9, or claim 10.

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