Control drum drive mechanism

Abstract

A control drum drive mechanism for a nuclear reactor, comprising: a pressure boundary structure 7; first and second stators 1 and 4 located outside the pressure boundary structure; a rotor located ins

Description

TITLE Control Drum Drive Mechanism CROSS REFERENCE TO RELATED APPLICATIONS This represents the first application directed towards the subject-matter. FIELD This disclosure relates to control drum drive mechanisms, and a reactivity control system for a nuclear reactor. BACKGROUND Nuclear reactors require a means by which to control the rate at which nuclear reactions occur within the nuclear reactor core, which in turn will control the temperature of the nuclear reactor core. Whilst control rods are most commonly used for this purpose, for some applications control drums are also being considered, owing to their comparative volume requirements and movement characteristics. However, as design experience for control drums is relatively limited, improvements to control drum drive mechanisms, which control the rotational position of control drums, are desirable. SUMMARY The present disclosure provides a control drum drive mechanism (CDDM) as set out in claim 1, a reactivity control system as set out in claim 13, and a method for operating a control drum drive mechanism as set out in claim 15. Optional features are included in the dependent claims. According to a first aspect there is provided a control drum drive mechanism for a nuclear reactor, comprising a pressure boundary structure having an internal volume; a first stator and second stator located external to the pressure boundary structure; a rotor located within the internal volume, the rotor having a first rotor section and a second rotor section, the rotor being rotatably securable to a control drum drive shaft; a latch mechanism connected to the second rotor section, the latch mechanism being biased to an open configuration; and a SCRAM spring mechanism connectable to the control drum drive shaft; wherein the first stator is configured to control the rotational position of the rotor around the axis of the control drum drive shaft via rotation of the first rotor section, and the second stator is configured to close the latch mechanism, wherein the rotor can only rotate the control drum drive shaft when the latch mechanism is closed, and when the latch mechanism is open, the control drum drive shaft will rotate to a predetermined rotational position under the rotational force exerted by the SCRAM spring mechanism. The latch mechanism of the control drum drive mechanism may comprise a plurality of lever arms, with each lever arm comprising a latch connection pad configured to engage with a sub-arc of the control drum drive shaft circumferential surface. The latch mechanism of the control drum drive mechanism may comprise four lever arms, the lever arms being located 90 degrees apart from one another, with the latch connection pad of each lever arm configured to engage with a sub-arc of the control drum drive shaft circumferential surface. At least a subregion of the circumferential surface of the control drum drive shaft of the control drum drive mechanism may be indented, and at least a subregion of the surface of the latch connection pad may have protrusions complementary to the indentations on the subregion of the control drum drive shaft, so as to increase the frictional force applied between the control drum drive shaft and the latch connection pad. The indentations on the surface of the subregion of the control drum drive shaft of the control drum drive mechanism may be longer than the length of the latch connection pad. The SCRAM spring mechanism of the control drum drive mechanism may comprise a pair of torsional coil springs, each torsional coil spring having one end connected to the pressure boundary structure, and the other end connected to the control drum drive shaft, the pair of torsional coil springs being coiled in opposing directions, such that when the control drum drive shaft is rotated, the tension in one of the torsional coil springs is increased, and the tension in the other torsional coil spring is decreased. Alternatively, the SCRAM spring mechanism of the control drum drive mechanism may comprise a pair of tension wires, each end of each tension wire of the pair of tension wires being connected to an extension spring, and each extension spring being connected to the pressure boundary structure, such that the control drum drive shaft is contacted by, and passes between, the pair of tenson wires, wherein each tension wire is also attached to the control drum drive shaft at a point proximal to the middle of the tension wire, such that when the control drum drive shaft is rotated, the tension wires are pulled in opposite directions. Alternatively, the SCRAM spring mechanism of the control drum drive mechanism may comprise a pair of tension wires, with each tension wire being connected to the control drum drive shaft at one end, and an extension spring at the other end, with each extension spring being connected to the pressure boundary structure. Where the SCRAM spring mechanism comprises a pair of tension wires connected to the control drum drive shaft at one end and an extension spring at the other end, the points where the extension springs are connected to the pressure boundary structure may be separated by an obtuse angle. The SCRAM spring mechanism of the control drum drive mechanism may be attached to the pressure boundary structure by means of two or more pre-load tensioners, each pre-load tensioner being capable of increasing or decreasing the tension force in the attached SCRAM spring mechanism. The control drum drive mechanism may further comprise a drive shaft position indicator system comprising an indicator sensor and a shaft indicator, the shaft indicator being connected to the control drum drive shaft, and the indicator sensor being positioned outside of the pressure boundary structure. The position indicator system may comprise a Hall effect sensor. According to a second aspect, there is provided a reactivity control system for a nuclear reactor, the reactivity control system comprising one or more electronic control units connected to one or more control drum drive mechanisms according to any preceding claim. The reactivity control system may further comprise one or more sensor systems, the one or more sensor systems including one or more of a neutron detector, a thermal probe, a pressure sensor, and a load requirement gauge, wherein the reactivity control system may control the one or more control drum drive mechanisms based at least in part on the input of the one or more sensor systems and the position indicator system. According to a third aspect, there is provided a method for operating the control drum drive mechanism of any preceding claim, the method comprising the steps of: rotating the control drum drive shaft from the safety shutdown position, and recording the direction of rotation using the drive shaft position indicator system; rotating the control drum drive shaft back to the safety shutdown position; and rotating the control drum drive shaft from the safety shutdown position in the opposite direction of rotation to the direction that the control drum drive shaft was rotated in the previous time it rotated away from the safety shutdown position. Where the control drum drive mechanism comprises two or more pre-load tensioners, the method may further comprise opening the latch mechanism and allowing the control drum drive shaft to rotate just under the influence of the SCRAM spring mechanism springs until the control drum drive shaft comes to rest, such that the tension in the springs of the SCRAM spring mechanism is balanced, and measuring the rotational position of the control drum drive shaft using the drive shaft position indicator system. The method may further comprise adjusting one or more of the pre-load tensioners until the position of the control drum drive shaft corresponds to the safety shutdown position if, when the latch mechanism is opened and the control drum drive shaft is allowed to rotate under just the influence of the SCRAM spring mechanism springs until the control drum drive shaft comes to rest, the drive shaft position indicator system indicates that the position of the control drum drive shaft does not correspond to the safety shutdown position. 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 DESCRIPTION OF THE DRAWINGS Embodiments will now be described by way of example only with reference to the accompanying drawings, in which: FIG. 1 shows a schematic sectional view of an example control drum drive mechanism (CDDM); FIG. 2 shows a schematic sectional view of the example CDDM of FIG. 1 in a configuration where the latch mechanism is closed; FIG. 3 shows a schematic sectional view of a nuclear reactor including CDDMs; FIG. 4 shows an isometric view of a section of the CDDM having an alternative design of SCRAM spring mechanism; FIG. 5 shows a sectional schematic view of a further alternative design of SCRAM spring mechanism; FIG. 6 shows a sectional schematic view of a further alternative design of SCRAM spring mechanism; FIG. 7 shows a sectional schematic view of a further alternative design of SCRAM spring mechanism; FIG. 8 shows a cross-sectional schematic view along the line A-A’ in FIG. 2; FIG. 9 shows a schematic sectional plan view of an example reactivity control system for a nuclear reactor; FIG. 10 shows a schematic sectional plan view of another example reactivity control system (ROS) for a nuclear reactor; FIG. 11 shows a schematic sectional plan view of another example reactivity control system (RCS) for a nuclear reactor; and FIG. 12 shows a flow chart detailing a method of operating the CDDM. DETAILED DESCRIPTION FIG. 1 shows a schematic sectional view of an example control drum drive mechanism (CDDM) 100. The CDDM is built around a pressure boundary structure 7. Located outside of, or external to, the pressure boundary structure 7 are an indicator sensor 8 of a drive shaft position indicator system, a first stator 1 comprising first stator cooling coils 2 and first stator windings 3, and a second stator 4 comprising second stator cooling coils 5 and a second stator electromagnet 6. Within the internal volume of the pressure boundary structure 7 there is a rotor, comprising a first rotor section 11 and a second rotor section 16. Each control drum 150 (see FIG. 3) within a nuclear reactor 200 is rotated by means of a control drum drive shaft 18 to which the control drum 150 is attached. The rotor is rotatably secured to a control drum drive shaft 18, which is to say that the rotor is fixed in position relative to the control drum drive shaft, but able to rotate freely about the control drum drive shaft. In the example CDDM of FIG. 1 and FIG. 2, this is achieved using a combination of bearings 10, including thrust bearings 9, to maintain the positional relationship and axial alignment between the rotor and the control drum drive shaft 18, whilst still allowing the rotor to rotate freely around the control drum drive shaft 18. Connected to the second rotor section 16 is a latch mechanism, which in the example CDDM of FIG. 1 and FIG. 2 comprises four lever arms 20, with each lever arm comprising a ferromagnetic latch pad 19, a latch pin 21, a latch spring 22, and a latch connection pad 23. The skilled person will appreciate more or fewer than four lever arms could be used as part of the latch mechanism without affecting the latch mechanism’s function. The example CDDM of FIG. 1 is shown with the latch mechanism 20 open, which is to say that none of the latch connection pads 23 are engaged with the control drum drive shaft 18. The lever arms 20 of the latch mechanism are biased towards an open configuration by the latch springs 22. In the example CDDM of FIG. 1, the latch springs take the form of a simple extension spring, with one end of each latch spring connected to the lever arm 20 between the latch pin 21 and ferromagnetic latch pad 19, and the other end of each latch spring connected to the second rotor section 16, but the skilled person will appreciate that other suitable resistive elements could be used to bias the lever arm towards an open configuration. The movement of the latch mechanism 20 between open and closed configurations is controlled by the second stator 4, and more specifically the activation or deactivation of the second stator electromagnet 6. When electric current is passed through the second stator electromagnet 6, a magnetic field is created, which pulls the ferromagnetic latch pads 19 towards the second stator electromagnet 6. As the ferromagnetic latch pads 19 are pulled towards the second stator electromagnet 6, the lever arms 20 pivot around the latch pins 21, and as a result the latch connection pads 23 (which are at the opposite end of the lever arm to the ferromagnetic latch pad 19) will move into contact with a sub-arc of the control drum drive shaft 18 circumferential surface, creating a frictional join between the latch connection pads 23 and the control drum drive shaft 18, thus closing the latch mechanism. As passing a current through the second stator electromagnet 6 will cause the second stator electromagnet 6 to heat up, a second stator cooling mechanism 5 is provided in close proximity to the second stator electromagnet 6, so as to absorb and remove heat energy from the second stator electromagnet 6. The second stator cooling mechanism 5 may comprise a fluid circuit cooling mechanism, whereby fluid is circulated in close proximity to the second stator electromagnet 6 to absorb and remove heat energy from the second stator electromagnet 6. Suitable fluids for this purpose include gaseous fluids such as hydrogen or nitrogen, liquid fluids such as deionised water, liquid metal, or oils. FIG. 2 shows a schematic sectional view of the example CDDM of FIG. 1 in a configuration where the latch mechanism is closed. The force of the second stator electromagnet 6 pulling on the ferromagnetic latch pads 19 is balanced by the force of the latch connection pads 23 pushing against the control drum drive shaft 18, effectively creating a friction join between the latch connection pads 23 and the control drum drive shaft. Once the latch mechanism is closed and the friction join between the latch connection pads 23 and the control drum drive shaft 18 is established, the control drum drive shaft (and therefore the attached control drum) can be rotated using the first stator 1. The rotation of the control drum drive shaft is achieved using electromagnetic principles familiar to the skilled person. The first stator 1 and second stator 4 can be controlled independently when operating the CDDM. A current will be passed through the first stator windings to create a magnetic field, which extends through the pressure boundary structure to envelop the first rotor section 11. The first rotor section 11, or at least a sub-section of the first rotor section, is embedded with permanent magnets or ferromagnetic material, or can be made using a magnetic material or a material treated to increase its ferromagnetic properties, in order for the first rotor section to be magnetic, and therefore be influenceable by an externally-applied magnetic field. Suitable materials include Inconel®, chromium-iron magnetic stainless steel alloys, and duplex materials, which can be treated to exhibit magnetic properties. Being magnetic, the first rotor section 11 will try to align itself to the magnetic field created by the first stator 1. By controlling the flow of electric current through the first stator windings 3 of the first stator 1, different alignments of magnetic field can be created, allowing the first rotor section to be rotated to any desired angle. This provides a means for the CDDM to control the rotational position of the rotor around the axis of the control drum drive shaft. Given that the first 11 and second 16 rotor sections are fixed to one another (i.e. the rotor is made up of the first 11 and second 16 rotor sections), rotation of the first rotor section 11 will mean rotation of the whole rotor, including second rotor section 16. Rotation of the second rotor section 16 will mean rotation of the attached latch mechanism (including the lever arms 20 in the example CDDM of FIG. 1 and FIG. 2), which, when the latch mechanism is closed such that a friction join is created between the latch connection pad(s) 23 of the latch mechanism and the control drum drive shaft 18, will in turn lead to rotation of the control drum drive shaft, and therefore the control drum itself. To ensure the latch mechanism stays closed during rotation of the control drum drive shaft, the control of the first stator 1 and second stator 4 will need to be synchronised such that when the electromagnetic field created by first stator 1 is used to rotate the drive shaft 18, the electromagnetic field created by the second stator 4 is also rotated to keep the latch mechanism closed during rotation. In this way, the CDDM can be used to control the rotational position of the control drum drive shaft, and therefore the rotational position of the control drum. As passing a current through the first stator windings 3 will cause the first stator windings 3 to heat up, a first stator cooling mechanism 2 is provided in close proximity to the first stator windings 3, so as to absorb and remove heat energy from the first stator windings 3. The first stator cooling mechanism 2 may comprise a fluid circuit cooling mechanism, whereby fluid is circulated in close proximity to the first stator windings 3 to absorb and remove heat energy from the first stator windings 3. Suitable fluids for this purpose include gaseous fluids such as hydrogen or nitrogen, liquid fluids such as deionised water, liquid metal, or oils. It is an advantage of the design of the present invention that such fluid cooling mechanisms for the CDDM components are deployed outside of, or external to, the pressure boundary structure 7, and do not require cooling fluid inside, or internal to, of the pressure boundary structure. This simplifies the design of the CDDM, and makes the first and second cooling mechanisms 2, 5 easier to service or replace, as such activities do not require the pressure boundary structure 7 to be removed from the high pressure volume 160 (see FIG. 3) of the nuclear reactor 200. The rotational position of the control drum drive shaft (and therefore the control drum attached to it) can be measured by the drive shaft position indicator system. This comprises a shaft indicator 17, which is attached to the control drum drive shaft 18 and will rotate as the control drum drive shaft rotates, and an indicator sensor 8 located outside of the pressure boundary structure 7. The rotational position of the shaft indicator 17 can be detected by the indicator sensor 8. An example drive shaft position indicator system could include a Hall effect sensor system, with the shaft indicator 17 comprising a multipole ring magnet, and the indicator sensor 8 comprising a Hall effect latch to detect the position and rotation of the multipole ring magnet. It is important to know the rotational position of the control drum drive shaft so that the CDDM can correctly position the control drum to have the desired effect on the reactivity levels within the nuclear reactor core. Also visible in FIG. 1 and FIG. 2 is a sectional view of the SCRAM spring mechanism (SSM). An important safety feature of all nuclear reactors is the ability to rapidly reduce the rate of nuclear reactions occurring within the nuclear reactor core, in order to reduce the temperature of the nuclear reactor core. This rapid shutdown of the nuclear reactor core is known as a SCRAM. It should always be possible to SCRAM the nuclear reactor during normal operation. Furthermore, should the control systems of the nuclear reactor lose power, the nuclear reactor should undergo a SCRAM by default, to ensure that the loss of power to the control systems does not lead to the nuclear reactor core overheating. The SSM shown in FIG. 1 and FIG. 2 is a way to achieve this important safety feature. In the example CDDM of FIG. 1 and FIG. 2, the SSM comprises a pair of torsional coil springs 25. Each torsional coil spring has one end connected to the pressure boundary structure 7, and the other end connected to the control drum drive shaft 18. The pair of torsional coil springs are coiled in opposing directions around the control drum drive shaft, so that when the latch mechanism is closed and the control drum drive shaft is rotated by energising the first stator 1 to rotate the rotor, the tension in one of the torsional coil springs 25 is increased, and the tension in the other torsional coil spring is decreased. The SSM of FIG. 1 and FIG. 2 is set up so that when the tension in the torsional coil springs is balanced, and the sum of the rotational force applied to the control drum drive shaft by the torsional coil springs is zero, the rotational position of the control drum drive shaft is at a predetermined rotational position. The predetermined rotational position can be where the control drum drive shaft, and therefore control drum it is attached to, which comprises both a neutron-absorbing material and a neutron-reflecting material, is positioned so that the neutron-absorbing material is closest to and facing the nuclear reactor core. This orientation of the control drum, and therefore the control drum drive shaft, will be referred to as the safety shutdown position. As understood by the skilled person, having the neutronabsorbing material facing the nuclear reactor core will lead to a reduction in the rate at which nuclear reactions occur within the nuclear reactor core, ultimately leading to the safe shutdown of the nuclear reactor. Therefore, when the latch of the CDDM is open, and the rotational position of the control drum drive shaft is dictated solely by the SSM, the control drum will rotate to a position so as to reduce the rate at which nuclear reactions occur within the nuclear reactor core. This will be the case if power is cut to the CDDM, as the latch mechanism is held shut by the second stator electromagnet, meaning that without the CDDM receiving power, the latch mechanism will open, releasing the control drum drive shaft from the grip of the latch connection pad(s) 23, and leaving the SSM as the sole mechanism controlling the rotational position of the control drum drive shaft 18. As a result of the pair of torsional coil springs 25 being coiled in opposing directions around the control drum drive shaft, no matter which way the control drum drive shaft is rotated, the tension in one of the torsional coil springs will increase, and as such will start storing some elastic potential energy, whilst the tension in the other torsional coil spring is decreased. If the CDDM undergoes a SCRAM, the latch mechanism will open, and the torsional coil spring in which the tension was increased can release this elastic potential energy by rotating the control drum drive shaft such that the tension in that torsional coil spring is decreased, until a point is reached where the tension in the pair of torsional coil springs is balanced, and that the sum of the rotational force applied to the control drum drive shaft by the torsional coil springs is at its minimum, i.e. zero. This allows the CDDM to operate in both clockwise and anticlockwise directions. This ability allows for the stresses on the springs of the SCRAM spring mechanism to be balanced, as the CDDM can alternate between clockwise and anticlockwise rotation during its operational lifetime. For example, an electronic control unit (see FIG. 9, FIG. 10, or FIG. 11) used to control the CDDM can record the direction (clockwise or anticlockwise) the CDDM has been rotated, so that each time the CDDM is returned to its neutral or safety shutdown position, the next time the CDDM is operated, the CDDM can be rotated in the opposite direction. By repeating this process each time the CDDM returns to its neutral or safety shutdown position, wear on the SSM can be balanced throughout the operational lifetime of the nuclear reactor. FIG. 3 shows a schematic sectional view of a nuclear reactor 200 including CDDMs 100. The nuclear reactor 200 includes a nuclear reactor core 170, around which are positioned control drums 150. The nuclear reactor core 170 and the control drums 150 are housed within a high-pressure volume 160. The control drums 150 are connected via control drum drive shafts 18 to the CDDMs. FIG. 4 shows an isometric view of a section of the CDDM having an alternative design of SCRAM spring mechanism 25. In the example of FIG. 4, the SSM comprises a pair of tension wires 26, each end of each tension wire of the pair of tension wires being connected to an extension spring 27, and each extension spring being connected to the internal surface of the pressure boundary structure 7. The extension springs are connected to the pressure boundary structure in pairs, which is to say that the extension spring 27 at a first end of a first tension wire 26 of the pair of tension wires is connected to the pressure boundary structure 7 at approximately the same location as the extension spring 27 at a first end of a second tension wire 26 of the pair of tension wires, and the extension spring at a second end of the first tension wire of the pair of tension wires is connected to the pressure boundary structure at approximately the same location as the extension spring at a second end of the second tension wire of the pair of tension wires. The points on the interior wall of the pressure boundary structure to which the pairs of extension springs are connected are 180 degrees apart, i.e. diametrically opposed, or a straight angle. The control drum drive shaft 18 is contacted by, and passes between, the pair of tension wires, such that the centre of the region where the first tension wire of the pair of tension wires contacts the surface of the control drum drive shaft is approximately 180 degrees around the surface of the control drum drive shaft from the centre of the region where the second tension wire of the pair of tension wires contacts the surface of the control drum drive shaft. Each tension wire is also attached to the surface of the control drum drive shaft at a point proximal to the middle, or half-way point, along the length of the tension wire, such that the two points on the surface of the control drum drive shaft where the tension wires are attached are 180 degrees apart. This means that when the control drum drive shaft is rotated, the tension wires are moved with the control drum drive shaft, and more specifically, are pulled in opposite directions. Consequently, when the control drum drive shaft 18 is rotated, the tension in the extension spring at one end of each tension wire is increased, and the tension in the extension spring at the other end of each tension wire is decreased, with the increase or decrease in tension depending on whether the point where the extension spring was attached to the surface of the control drum drive shaft was rotated towards or away from the extension spring in question. The SSM will be set up so that when the tension in the extension springs 27 at the two ends of a single tension wire 26 is balanced, which in the case of the SSM of FIG. 4 is also when the sum of the rotational force applied to the control drum drive shaft by the extension springs is equal to zero, the rotational position of the control drum drive shaft 18 will be such that the attached control drum 150 will be positioned with the neutron-absorbing material closest to and facing the nuclear reactor core, i.e. in the safety shutdown position. This ensures that, if the CDDM loses power, the latch mechanism will open and the SSM will rotate the control drum back to its safety shutdown position. The skilled person will appreciate that whilst the example SSM in FIG. 4 is shown with just one pair of tension wires 26, the SSM could use two pairs of tension wires, the pairs of tension wires being spaced apart along the longitudinal axis of the control drum drive shaft 18, or three or more pairs of tension wires spaced apart along the longitudinal axis of the control drum drive shaft, with the strength of the extension springs 27 attached to the ends of the tension wires being chosen to provide an appropriate level of resistance and tension. FIG. 5 shows a sectional schematic view of a further alternative design of SCRAM spring mechanism. In the example of FIG. 5, the SSM comprises a pair of tension wires 26 (shown as dashed lines in FIG. 5), with each tension wire being connected to the control drum drive shaft 18 at one end, and connected to an extension spring 27 at the other end, with each of the extension springs being connected to the pressure boundary structure 7. When the control drum drive shaft 18 is rotated, whether it be clockwise or anti-clockwise, the tension wires 26 attached to the control drum drive shaft 18 will be pulled in opposite directions, with both extension springs being stretched, and consequently experiencing an increase in tension and elastic potential energy. If the latch mechanism is opened, such that the force rotating the control drum drive shaft is removed, elastic potential energy in the extension springs can be released, and the extension springs will contract so as to reduce the elastic potential energy stored within them, until each extension spring has released as much elastic potential energy as possible, and the force exerted by the pair of tension springs is balanced. This is the situation shown in FIG. 5, where each point on the surface of the control drum drive shaft where a tension wire is connected is facing towards a point on the inner surface of the pressure boundary structure 7 where an extension spring connected to that tension wire is connected, and when each tension wire 26 and extension spring 27 combination is at its minimum extension length. When this happens, the control drum drive shaft 18 will be positioned such that the attached control drum 150 will be positioned with the neutron-absorbing material closest to and facing the nuclear reactor core, i.e. in the safety shutdown position. This ensures that, if the CDDM loses power, the SSM will rotate the control drum back to its safety shutdown position. FIG. 6 shows a sectional schematic view of a further alternative design of SCRAM spring mechanism. In the example of FIG. 6, the SSM comprises a pair of tension wires 26 (shown as dashed lines in FIG. 6), with each tension wire being connected to the control drum drive shaft 18 at one end, and connected to an extension spring 27 at the other end, with each of the extension springs being connected to the internal surface of the pressure boundary structure 7. The design of the SSM shown in FIG. 6 is similar to that shown in FIG. 5, with the exception that the points where the extension springs 27 are attached to the inner surface of the pressure boundary structure 7 are not located 180 degrees apart, but rather are at an obtuse angle, i.e. an angle between 90 and 180 degrees, for example in a range from a minimum of 100, 110, or 120 degrees, to a maximum of 140, 150, or 160 degrees. The SSM of FIG. 6 works in exactly the same way as the SSM of FIG. 5, in that rotation of the control drum drive shaft will increase the elastic potential energy in the pair of extension springs 27. Then, if the latch mechanism is opened, and the force rotating the control drum drive shaft 18 is removed, elastic potential energy in the extension springs can be released, and the extension springs will contract so as to reduce the elastic potential energy stored within them, until each extension spring has released as much elastic potential energy as possible, and the force exerted by the pair of extension springs is balanced. However, by locating the points where the extension springs 27 are attached to the inner surface of the pressure boundary structure 7 at an obtuse angle to one another, when the latch mechanism is opened and the SSM controls the rotation of the control drum drive shaft, the amount of resonance or “wobbling” about the safety shutdown position will be reduced. This is because the symmetry of the SSM arrangement shown in FIG. 5 is no longer present, and so the rotational forces applied to the control drum drive shaft by the tension wires are not being applied in a symmetrical fashion. This lack of symmetry reduces the potential for resonance in the system, meaning the control drum drive shaft, and hence the control drum, will settle at the safety shutdown position faster in the event of a SCRAM using the SSM of FIG. 6 than a CDDM where the components of the SSM are positioned symmetrically about the control drum drive shaft. FIG. 7 shows a sectional schematic view of a further alternative design of SCRAM spring mechanism. In the example of FIG. 7, the SSM comprises a pair of tension wires 26 (shown as dashed lines in FIG. 7), with each tension wire being connected to the control drum drive shaft 18 at one end, and connected to an extension spring 27 at the other end. Each of the extension springs is then connected to a pre-load tensioner 28. The pre-load tensioners 28 are used to increase or decrease the amount of tension in the SSM. As such, devices that can produce controlled, small changes in linear position, such as small linear actuators, worm gear arrangements, a leadscrew with a nut actuated by a DC motor, or a latch and lift linear driver having a default closed position, could be used to perform the function of a pre-load tensioner. Each of the pre-load tensioners 28 is attached to the inner surface of the pressure boundary structure 7. As with the SSM of Fig. 6, the angle between the two points in the inner surface of the pressure boundary structure where the pre-load tensioners are attached is an obtuse angle, i.e. between 90 and 180 degrees, for example in a range from a minimum of 100, 110, or 120 degrees, to a maximum of 140, 150, or 160 degrees, to reduce the potential for resonance in the SSM as much as possible. The purpose of the pre-load tensioners 28 is to counteract any plastic deformation the extension springs 27 may undergo during their operational lifetime. This is achieved by periodically measuring the rotational position of the control drum drive shaft when there are no rotational forces acting on the control drum drive shaft other than the SSM, e.g. when the latch mechanism is opened and the SSM is allowed to rotate the control drum drive shaft until the tension in the extension springs of the SSM is balanced. If no plastic deformation has occurred, or plastic deformation has occurred but in a way that the deformation is balanced across the extension springs of the SSM, then the drive shaft position indicator system should read that the control drum drive shaft is in the safety shutdown position, for example at zero degrees, which is an acceptable outcome. However, if the drive shaft position indicator system reads that the control drum drive shaft is not in the safety shutdown position, e.g. it is not at zero degrees, it is likely plastic deformation of one or more extension springs has occurred in a way that the tension in the extension springs of the SSM is no longer correctly balanced, and therefore needs to be adjusted. This can be done via the pre-load tensioners, which can act as force-measurement devices in addition to acting as pre-load tensioners. The preload tensioners can be adjusted to increase (or decrease) the amount of tension in the SSM, i.e. (in the example CDDM of FIG. 7) the amount of tension in the extension spring(s), so that when the latch mechanism is open and the control rum drive shaft is rotated solely by the forces exerted by the SSM until the tension in the springs of the SSM is balanced, the control drum drive shaft (and therefore the control drum itself) will be in the safety shutdown position. By alternating between clockwise and anticlockwise rotation of the CDDM during operations, and periodically adjusting (if necessary) the tension in the SSM springs using the pre-load tensioners, the accurate positioning of the control drums during normal operation and during a SCRAM can be maintained during the lifetime of the CDDM. It will be understood that pre-load tensioners 28 can be used with any of the SCRAM spring mechanisms described herein. In the case of the SSM indicated in FIG. 1 and FIG. 2, comprising pairs of torsional coil springs, the torsional coil springs can be attached to the interior surface of the pressure boundary structure via pre-load tensioners, so that the preload tensioners can periodically measure and adjust the tension in the torsional coil springs of the SSM. It will be appreciated that, as with the example SSM of FIG. 1 and FIG. 2, a CDDM comprising the SSM of any of FIG. 4, FIG. 5, FIG. 6, or FIG. 7 can operate in both clockwise and anticlockwise directions, which allows for the stresses on the springs of the SCRAM spring mechanism to be balanced, as the CDDM can alternate between clockwise and anticlockwise rotation during its operational lifetime. For example, an electronic control unit (see FIG. 9, FIG. 10, or FIG. 11) used to control the CDDM can record the direction (clockwise or anticlockwise) the CDDM has been rotated, so that each time the CDDM is returned to its neutral or safety shutdown position, the next time the CDDM is operated, the CDDM can be rotated in the opposite direction. By repeating this process each time the CDDM returns to its neutral or safety shutdown position, wear on the SSM can be balanced throughout the operational lifetime of the nuclear reactor. Furthermore, alternating the direction of rotation of the control drum from its safety shutdown position means the effect on the burnup rate of radioactive material within the nuclear reactor core caused by the reflector material facing the nuclear reactor core is spread over a larger volume of the nuclear reactor core, leading to increased operational lifetime and power output compared with a reactor using control drums that always rotate in the same direction from the safety shutdown position. FIG. 8 shows a cross-sectional schematic view along the line A-A’ in FIG. 2 of an optional design of interface between the latch connection pads 23 and the circumferential surface of the control drum drive shaft 18 when the latch mechanism is in the closed configuration. To increase the frictional force between the latch connection pads 23 and the control drum drive shaft 18, indentations have been made in a subregion of the surface of the control drum drive shaft 18, and complimentary protrusions are present on a subregion of the surface of the latch connection pads 23. The indentations in the surface of the control drum drive shaft may take the form of grooves extending lengthwise along the surface of the control drum drive shaft 18, and the protrusions on the surface of the latch connection pads 23 may take the form of ridges or teeth. The size and profile of the indentations and protrusions can be optimised to provide the optimum combination of clean interfacing regardless of the rotational position of the control drum drive shaft, and increased contact surface area leading to increased fictional force between the two surfaces (the ridges and grooves shown in FIG. 8 have been exaggerated for illustrative purposes). As the interior volume of the pressure boundary structure will be at an elevated temperature, owing to the heat travelling up the control drum drive shaft from the control drum, and the proximity to the core, items within the interior volume of the pressure boundary structure may undergo expansion. To accommodate this, the axial length of the indentations on the surface of the control drum drive shaft can be increased such that they are longer than the length of the latch connection pads, so as to provide a complimentary surface profile along the entire length of the latch connection pad surface, even if the control drum driveshaft and / or latch connection pads undergo expansion as a result of the being located in such a hot environment. FIG. 9 shows a schematic sectional plan view of an example reactivity control system 50 for a nuclear reactor 200 comprising a number of control drums 150 arranged around the periphery of the nuclear reactor core 170. The reactivity control system (ROS) 50 comprises an electronic control unit (ECU) 60. The electronic control unit (ECU) 60 is connected to each of the CDDMs, which in turn are connected to the control drums. The RCS can be configured to receive data from, for example, one or more of the following sources within the nuclear reactor core 170: neutron detectors; thermal probes; pressure sensors. The RCS may also receive data relating to the energy demands on the nuclear reactor (for example, from a power delivery control unit), the control drum position, and the CDDM expected position. The RCS may receive this data by incorporating one or more sensor systems, including one or more of a neutron detector, a thermal probe, a pressure sensor, and a load requirement gauge (to gauge the amount of energy, or load, being requested of the nuclear reactor), and by being connected to the position indicator system. The RCS 50 can determine from the data received the optimum rotational position for each control drum so as to best influence the reaction rate within the nuclear reactor core (and therefore the heat energy generated by the nuclear reactor core and the nuclear reactor itself), and instruct the ECU to control each of the CDDMs 100 to position the control drums 150 accordingly. Also, by monitoring these data, the RCS can detect if the nuclear reactor is moving to an operational state outside of acceptable limits, and in extreme cases, can initiate a SCRAM by instructing the ECU return the control drum to the safety shutdown position, or to stop the supply of power to the second stator in each CDDM. Without power, the second stator will no longer create a magnetic field, and the latch spring 22 will be able to open the latch mechanism, leaving the SSM as the only source of force being applied to the control drum drive shaft. As explained earlier, this will lead to the control drums being rotated to their safety shutdown position by the SSM, which in turn will lead to the cooling of the nuclear reactor core. FIG. 10 shows a schematic sectional plan view of another example reactivity control system (RCS) 50 for a nuclear reactor 200. In the example of FIG. 10, the RCS comprises multiple ECUs, with each ECU operating a pair of adjacent CDDMs. The RCS and ECUs operate in the same way as the RCS and ECU of the example system of FIG. 9, in that the RCS uses data received from one or more system sensors to determine how to instruct the ECU to control the CDDM. Having multiple ECUs provides a degree of system redundancy, as the RCS would still be able to function, albeit to a diminished degree, in the event of the failure of an ECU. It is likely that the failure of an ECU, or a breakdown in the communication channel between the RCS and an ECU, would lead to the CDDM under the control of the ECU going into a failsafe mode, and the CDDMs connected to the ECU releasing their latch mechanisms so that their control drums can return to the safety shutdown position. Should a pair of control drums return to their safety shutdown position, the RCS may be able to instruct the remaining ECUs to control the remaining control drums so as to continue the safe and controlled operation of the nuclear reactor 200. Whilst in the example of FIG. 10 each ECU operates a pair of adjacent CDDMs, the skilled person will understand that the CDDMs controlled by a single ECU do not have to be adjacent, and could be separated by CDDMs controlled by different ECUs instead. FIG. 11 shows a schematic sectional plan view of another example reactivity control system (RCS) 50 for a nuclear reactor 200. In the example of FIG. 11, the RCS comprises multiple ECUs, with each ECU operating a single CDDM. This arrangement provides yet further redundancy, as the failure of an ECU, or the communication channel between the RCS and an ECU, would only lead to the single CDDM under the control of the ECU going into a failsafe mode. It is to be understood that whilst the examples of FIG.9, FIG. 10, and FIG. 11 show a nuclear reactor 200 having eight control drums, the illustrated principles may just as readily be applied to nuclear reactors having more or fewer control drums. Equally, whilst the example of FIG. 10 shows each ECU being connected to two CDDMs 100 on adjacent control drums 150, each ECU may be connected to more than two CDDMs, and the CDDMs the ECU is connected to do not have to be adjacent to one another, but can instead be interspersed between CDDMs connected to different ECUs. The skilled person will understand that the configuration of the RCS may be further varied, in that each ECU can control different numbers of CDDMs, and each ECU of the RCS does not need to control the same number of CDDMs. For example, in a nuclear reactor with eight control drums, two ECUs may control two CDDMs each, and one ECU could control the remaining four CDDMs, or four of the CDDMs could be controlled by individual ECUs, with the remaining four CDDMs being controlled by a single ECU, and arranged to alternate around the nuclear reactor core 170 with the CDDMs which are each controlled by their own ECU. Further optional arrangements will be apparent to the skilled person. FIG. 12 shows a flow chart detailing a method 300 of operating the CDDM so as to ensure accurate positioning of the control drum 150, including the repeatability of achieving an optimal safety shutdown position in the event of a SCRAM. In a first step 310 during operation of the nuclear reactor 200 the control drum drive shaft is rotated from the safety shutdown position, and the direction of rotation (either clockwise or anticlockwise) is recorded by the drive shaft position indicator system 30. Ina second step 320, the control drum drive shaft is rotated back to the safety shutdown position. This may be done as part of a SCRAM, or as part of routine operations. In a third step 330 the control drum drive shaft is rotated (either clockwise or anticlockwise) from the safety shutdown position in the opposite direction of rotation to the direction of rotation it was rotated in the previous time it rotated away from the safety shutdown position. By recording the direction of rotation of the control drum drive shaft when it leaves the safety shutdown position, the direction of rotation of the control drum drive shaft can be alternated between incidences where the CDDM is returned to its neutral or safety shutdown position. By alternating the direction of rotation of the control drum drive shaft between incidences where the CDDM is returned to its neutral or safety shutdown position, the stresses on the springs of the SCRAM spring mechanism during its operational lifetime can be balanced. For example, the electronic control unit 60 used to control the CDDM can record the direction (clockwise or anticlockwise) the control drum drive shaft has been rotated, so that each time the control drum drive shaft is returned to its neutral or safety shutdown position, the next time the CDDM is operated, the control drum drive shaft can be rotated in the opposite direction. By repeating this process each time the control drum drive shaft returns to its neutral or safety shutdown position, wear on the SSM can be balanced throughout the operational lifetime of the nuclear reactor. In a first optional step 340, the latch mechanism is opened and the control drum drive shaft is allowed to rotate just under the influence of the SSM springs until the control drum drive shaft comes to rest, i.e. once the tension in the springs of the SSM is balanced. The drive shaft position indicator system is then used to measure the rotational position of the control drum drive shaft, and therefore the control drum fixed to it. Periodically measuring the rotational position of the control drum drive shaft when solely determined by the forces of the SSM can help determine if the SSM springs have undergone any elastic deformation, and if any corrective measures need to be taken. In a second optional step 350, if the drive shaft position indicator system indicates that the position of the control drum drive shaft (and therefore the control drum) does not correspond to the safety shutdown position when the latch mechanism is opened and the control drum drive shaft is allowed to rotate under just the influence of the SSM springs until the control drum drive shaft comes to rest, one or more of the pre-load tensioners can be adjusted until the position of the control drum drive shaft does correspond to the safety shutdown position. Using the pre-load tensioners to adjust the tension in the SSM springs in this way ensures that the forces exerted by the SSM springs are balanced when the control drum is in the safety shutdown position. The first 310, second 320, and third 330 steps are then repeated during the 5 operational lifetime of the CDDM, with the first 340 and second 350 optional steps being added as required or desired. Utilising this method will help maintain accurate positioning of the control drums during normal operation and during a SCRAM for the lifetime of the CDDM. Various examples have been described, each of which comprise one or more 1 o combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein.

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