Valve arrangement and nuclear reactor assembly

Abstract

A valve arrangement 100 suitable for a nuclear reactor 602 (Figure 9) and a nuclear reactor assembly; the valve arrangement comprising: an emergency blowdown valve 120, controllable to open and close,

Description

Field of the Invention The present invention relates to a valve arrangement for a nuclear reactor, and a nuclear reactor assembly including the same. Background Nuclear reactors are a desirable addition to a power grid, as they present ideal base load stations. This is because they are considered a low carbon source of electricity, and are not dependent upon variable weather conditions (which are limiting factors on other low carbon sources). These features allow them to be used as the backbone of a complete electricity network. One of the most common types of nuclear reactors used around the world is the pressurized water reactor (PWR) in which a primary circuit of pressurized water is used as coolant, moderator, and heat transfer fluid to the steam generator. The relative simplicity of the system provides them with the advantage that this system can be scaled. Consequently, they are suitable for both large scale power plants as well as small modular reactors. However, as with all nuclear power stations, they require a robust safety system to prevent accidents. As discussed, nuclear reactors require a coolant system to circulate a media (e.g., coolant) round the nuclear core, both to prevent nuclear meltdowns and to transfer energy from the nuclear core to a turbine system for conversion to usable electrical energy. In some scenarios, the pressure within the coolant system can become either too great or two low. The former case can lead to ruptures / explosions within the reactor, and the latter case can lead to nuclear meltdowns as a result of insufficient coolant circulation within the coolant system. To mitigate these risks, coolant systems may make use of valve arrangements in which a main (emergency blowdown) valve is configured to automatically open the main flow of coolant up to an external channel whenever the coolant system drops below a first predetermined pressure, or exceeds a second, higher, predetermined pressure. In this way, the pressure can be corrected either by the removal or addition of coolant to the coolant system, bringing it back into a predetermined working range between the first and second predetermined pressures. At which time the emergency blowdown valve will automatically shut again, disconnecting the coolant system from the external channel. However, in some cases it may be desirable to have a greater degree of control over the emergency blowdown valve so that the pressure within the coolant system can be better maintained within the predetermined working range. For example, it may be desirous that there be a greater degree of control over the opening of the emergency blowdown valve, such that it is not merely movable between the open and closed states in a binary fashion. This may be particularly difficult to implement whilst also meeting the requirement that electrical commands sent to the emergency blowdown valve be binary in signal strength, which is preferrable as it reduces the risk of system failure and increases system reactivity in cases where speed is critical to preventing disaster. The present invention has been devised in light of the above considerations. Summary of the Invention Accordingly, in a first aspect, embodiments of the present invention provide a valve arrangement for a nuclear reactor, the valve arrangement comprising: an emergency blowdown valve, controllable to open and close, thereby permitting or prohibiting a flow of coolant from an inlet to an outlet of the emergency blowdown valve, and a staged opening mechanism, configured to cause the emergency blowdown valve to transition from a fully closed to a fully open state via one or more predefined, increasingly open, intermediate states, the emergency blowdown valve pausing at each intermediate state before continuing the transition to the next state. The provision of intermediate states between the fully closed and fully open states allows the emergency blowdown (EBD) valve to better control the flow of fluid therethrough. In particular, it allows the EBD valve to be moved to varying degrees of openness, which will permit varying levels of fluid through. This can be advantageous in scenarios where careful control of the fluid flow is required. The provision of the intermediate states also allows the valve arrangement to be more reactive in some scenarios. For instance, if a command is sent to the EBD valve to close, the EBD valve will be able to implement this command more quickly when initially in an intermediate state (i.e., already only partially open) compared to when the EBD valve is fully open. The fully open state corresponds to one in which fluid / coolant can flow through the EBD valve at a maximum rate for a given pressure (that is, the cross-section of the outlet is at its maximum extent). The fully closed state corresponds to one in which fluid / coolant is prohibited from flowing through the EBD valve. In some examples, there are two or more intermediate states, or only one intermediate state between the fully closed and fully open states. In some examples, the emergency blowdown valve is a direct seated valve and includes: a piston arrangement, configured to selectively seal the inlet from the outlet, the piston arrangement including: a sealing piston head which selectively seals the inlet from the outlet; an operating piston head, operatively connected to the sealing piston head; and a piston chamber, within which the operating piston head is movable; and wherein the staged opening mechanism comprises one or more vent channels connected to the piston chamber, each of the one or more vent channels being openable by a respective electronically controllable valve to vent the piston chamber on one side of the piston head, thereby transitioning the emergency blowdown valve to the next state. The term ‘direct seated’ is understood to mean that the emergency blowdown valve is in a closed state, e.g., a state in which flow from an upstream side to downstream side of the valve is prohibited, when a sealing piston head of the valve is extended furthest from the piston chamber. Le., the valve is in the closed state when the piston is being pushed into a main seat, in which position the piston prohibits fluid flow from an upstream to downstream side of the emergency blowdown valve. On the other hand, the valve is in an open state, e.g., in which flow from an upstream side to downstream side of the valve is permitted, when the sealing piston head is retracted toward the piston chamber. The vent channels being ‘openable’ to transition the direct seated EBD valve to the next state is understood to mean that the one or more vent channels associated with a particular intermediate state maintain the direct seated EBD valve in said intermediate state when said one or more vent channels are closed, and allow the direct seated EBD valve to transition to a different state when said one or more vent channels are opened. More particularly, the vent channels in this example prevent fluid from flowing out of the piston chamber, such that a compression / extension of the portion of the piston chamber to which the one or more vent channels are connected, caused, for example, by the operating piston head moving within the piston chamber, causes a change in pressure within said portion of the piston chamber which counters the compression / extension. For example, if the operating piston head is moved so that the portion of the piston chamber (for example, on a first side of the operating piston head) to which the vent channel is attached is increased in size, whilst the fluid contained therein is kept constant (because the one or more vent channels are closed), the pressure within said portion of the piston chamber decreases, and a restoring force will act on the operating piston head to move it back to its original position. Likewise, when the operating piston head acts to compress the portion of the piston chamber which the vent channels are trapping fluid within, the pressure within said portion of the piston chamber will increase, which will act to push the operating piston head back again. However, the present disclosure is not limited in this way. For example, the one or more vent channels connected to the piston chamber of the direct seated EBD valve may instead be closable (that is, by a respective controllable, electrically, mechanically, or in any other way, valve) to pressurise the piston chamber on one side of the piston head, thereby transitioning the EBD valve to the next state. In this case, the vent channels being ‘closable’ to transition the EBD valve to the next state is understood to mean that the one or more vent channels associated with a particular intermediate state maintain the EBD valve in said intermediate state when said one or more vent channels are open, and allow the EBD valve to transition to a different state when said one or more vent channels are closed. This may work on the basis that the vent channels connected to a portion of the piston chamber (for example, on a second side of the operating piston head), whilst open, vent out fluid from said portion of the piston chamber, such that the pressure is unable to increase even if additional fluid is being input into said portion of the piston chamber via another channel. Thus, said portion of the piston chamber will be unable to exert a force on the operating piston head to expand the size of said portion of the piston chamber whilst the one or more vent channels are open, and so the EBD valve will be maintained in the intermediate state associated with said one or more vent channels. It is then not until the vent channels are closed that the pressure within said portion of the piston chamber will be able to increase to the point that the operating piston head is actuated onward to another state. Alternatively, the emergency blowdown valve is a reverse seated valve and includes: a piston arrangement, configured to selectively seal the inlet from the outlet, the piston arrangement including: a sealing piston head which selectively seals the inlet from the outlet; an operating piston head, operatively connected to the sealing piston head; and a piston chamber, within which the operating piston head is movable; and wherein the staged opening mechanism comprises one or more vent channels connected to the piston chamber, each of the one or more vent channels being closable by a respective electronically controllable valve to pressurise the piston chamber on one side of the piston head, thereby transitioning the emergency blowdown valve to the next state. The term ‘reverse seated’ is understood to mean that the valve is in a closed state, e.g., a state in which flow from an upstream side to downstream side of the valve is prohibited, when a sealing piston head of the valve is retracted toward the piston chamber. I.e., the valve is in the closed state when the piston is being pulled into a main seat, in which position the piston prevents fluid flow from an upstream to downstream side of the emergency blowdown valve. On the other hand, the valve is in an open state, e.g., in which flow from an upstream side to downstream side of the valve is permitted, when the sealing piston head is extended furthest from the piston chamber. The vent channels being ‘closable’ to transition the reverse seated EBD valve to the next state is understood to have the same meaning as described above. In particular, that the one or more vent channels associated with a particular intermediate state maintain the reverse seated EBD valve in said intermediate state when said one or more vent channels are open, and allow the reverse seated EBD valve to transition to a different state when said one or more vent channels are closed. However, the present disclosure is not limited in this way, and, similarly to the direct seated EBD valve, the one or more vent channels extending from the piston chamber of the reverse seated EBD valve may instead be openable by a respective electronically controllable valve to vent the piston chamber on one side of the piston head, thereby transitioning the EBD valve to the next state. Again, the vent channels being ‘openable’ to transition the reverse seated EBD valve to the next state is understood to have the same meaning as described above. In particular, that the one or more vent channels associated with a particular intermediate state maintain the direct seated EBD valve in said intermediate state when said one or more vent channels are closed, and allow the direct seated EBD valve to transition to a different state when said one or more vent channels are opened. The use of one or more vent channels provides a simple and accurate approach to implementing a (continuous or discrete) series of intermediate states in the piston arrangement. There may by a series (plurality) of vent channels, each with a corresponding electronically controllable valve. For example, the valve arrangement may comprise two, three, four, five, or six or more vent channels. The inclusion of a series of vent channels allows for a plurality of well-defined intermediate states to be provided with the EBD valve. Each of the series of vent channels may be associated with a different intermediate state. However, the present disclosure is not limited in this way, and multiple vent channels of the series of vent channels may be associated with a single intermediate state. The operating piston head may be mechanically coupled to the sealing piston head by a piston stem positioned between the operating piston head and sealing piston head. The sealing piston head may refer to a component of the piston that is pressed or pulled onto a peripheral region of a gap between the inlet and the outlet of the EBD valve, or may be slid across a gap between an inlet and outlet. In other words, the sealing piston head may move in a direction that is parallel to a line connecting the inlet to the outlet of the EBD valve, or may move in a direction that is perpendicular to a line connecting the inlet to the outlet of the EBD valve. The valve arrangement may comprise a pilot valve assembly fluidly connected to the piston chamber. The pilot assembly may control the flow of fluid into the piston chamber. The pilot assembly may control a fluid flow from the inlet of the EBD valve, or another region upstream of the EBD valve, to the piston chamber. The pilot assembly may be configured to cause the flow of fluid into the piston chamber when a pressure at the inlet of the EBD valve deviates from a predetermined working range defined by a first predetermined pressure and a second predetermined pressure (the second being higher than the first). In this way the EBD valve can be controlled to move to an open state when the pressure upstream of the EBD valve deviates from the predetermined working range. The operating piston head may divide the piston chamber into two portions (for example, the portions of the piston chamber described above). More particularly, the operating piston head may divide the piston chamber into a first, upper, piston chamber on a first side of the operating piston head, and a second, lower, piston chamber on an opposite side of the operating piston head to the upper piston chamber. The upper piston chamber may be located such that the operating piston head is between the upper piston chamber and the sealing piston head. The lower piston chamber may be located between the operating piston head and the sealing piston head. The operating piston head may (substantially) fluidically isolate the upper piston chamber from the lower piston chamber. The upper piston chamber may include an upper piston channel and the lower piston chamber may include a lower piston channel. The upper piston channel and lower piston channel may respectively connect the upper piston chamber and lower piston chamber to one of the pilot assembly and a vent. In this way, the piston chamber may be suitably pressurised / de-pressurised in order to actuate the piston and move the EBD valve between the open and closed states. When the EBD valve is a direct seated valve, the pilot assembly may be attached to the lower piston channel. In this way, fluid may be introduced into the lower piston chamber to pressurise it, causing the operating piston head (and sealing piston head) to be actuated upward thus opening the EBD valve. When the EBD valve is a reverse seated valve, the pilot assembly may be attached to the upper piston channel. The diameter of one or more or all of the respective vent channels may be equal to or greater than the diameter of the upper piston channel and / or the lower piston channel. The operating piston head may be movable along a longitudinal direction between a first, upper, end of the piston chamber and a second, lower, end of the piston chamber. The operating piston head may be movable between first and second end positions that are respectively proximal to the first and second ends of the piston chamber, or may be directly placed against the first / second ends of the piston chamber. The first end may face into and / or partially define the upper piston chamber, and the second end may face into and / or partially define the lower piston chamber. When the operating head is at the first or second end position, the EBD valve will be in one of the fully open and fully closed states. In other words, the fully open state may be one in which the operating piston head is pushed / pulled to the furthest extent in a first or second direction, such that the sealing piston head cannot be any further retracted toward / extended from the piston chamber. When the EBD valve is a direct seated valve, the operating piston head being at the first end position corresponds to the EBD valve being in a fully open state and the operating piston head being at the second end position corresponds to the EBD valve being in a fully closed state, and vice versa for a reverse seated valve. As the operating piston head is moved between the first and second end positions, the relative sizes (enclosed volumes) of the upper and lower piston chambers change. For example, when the operating piston head is moved from the first end position to the second end position, the upper piston chamber will increase in size and the lower piston chamber will decrease in size, and vice versa for when the operating piston head is moved from the second end position to the first end position. In this way, when the operating piston head is at the first end position, the upper chamber is at its minimum size, and the lower chamber is at its maximum size, and vice versa when the operating piston head is at the second end position. The absolute minimum size may refer to the first or second piston chambers being absent entirely (e.g., reduced to a zero, or effectively zero, volume). In some examples, when the operating piston head is at the first end position, the upper piston channel is blocked (e.g., by the operating piston head). Similarly, when the operating piston head is at the second end position, the lower piston channel may be blocked. In some examples, the upper piston channel may be located in a surface of the first end of the piston chamber. Similarly, in some examples the lower piston channel may be located in a surface of the second end of the piston chamber. This advantageously allows the upper and lower piston channels to apply pressure to the operating piston head even in cases where one of the respective upper and lower piston chambers has been removed entirely (as discussed above). That is, fluid passed down the upper / lower piston channel will directly abut against the operating piston head whilst moving in the direction of movement of the operating piston head, pushing it downward / upward. However, the present disclosure is not limited in this way. For example, one or more of the upper and lower piston channels may instead be located in a side (e.g., wall) of the piston chamber that runs between the first and second ends of the piston chamber. The one or more vent channels are arranged between the first and second ends of the piston chamber (e.g., in a side wall of the piston chamber). In some examples, the one or more vent channels may be arranged along a direction of movement of the operating piston head between the first and second ends of the piston chamber (e.g., a longitudinal direction). An intermediate state of the EBD valve corresponds to a point at which the operating piston head passes the vent channel associated with said intermediate state. In some examples, an intermediate state may be classified as the point at which the operating piston head fully passes the one or more vent channels associated with the intermediate state as the EBD valve moves from the fully closed state to the fully open state (that is, once the operating piston head surface facing the end of the piston chamber associated with the EBD being in the closed state has entirely passed the one or more vent channels associated with the intermediate state). However, the present disclosure is not limited in this way. For example, the intermediate state may instead correspond to a point at which the operating piston head surface facing the end of the piston chamber end associated with the EBD being in the closed state has passed half of the one or more vent channels associated with the intermediate state. Alternatively, an intermediate state may correspond to any position in which the operating piston head surface facing the end of the piston chamber end associated with the EBD being in the closed state is longitudinally aligned with at least a portion of the one or more vent channels associated with said intermediate state. This may be the case where varying levels of fluid introduction into the upper or lower piston chambers (via the upper or lower piston channel) causes a varying degree of pressure on the operating piston head, which the currently exposed portion(s) of the (one or more) vent channel(s) are either unable to cope with (i.e., are unable to vent as much fluid as is being introduced) or in excess of what is required (i.e., more is vented than is being introduced). As such, the operating piston head may be constantly moving upward and downward to balance the inflow and outflow of fluid, whilst still remaining in the intermediate state associated with the respective one or more vent channels. The intermediate state may be defined in one of the ways described above when the EBD valve is a reverse seated valve and the staged opening mechanism comprises one or more vent channels that are closable by a respective electronically controllable valve to pressurise the piston chamber and transition the EBD valve to the next intermediate state, or when the EBD valve is a direct seated valve and the staged opening mechanism comprises one or more vent channels that are openable by a respective electronically controllable valve to vent the piston chamber and transition the EBD valve to the next intermediate state In some (other) examples, an intermediate state may be classified as the range (or a particular point with said range) within which the operating piston head is in the process of passing the one or more vent channels associated with the intermediate state as the EBD valve moves from the fully open state to the fully closed state (i.e., once the operating piston head surface facing the end of the piston chamber associated with the EBD being in the open state is in the process of passing the one or more vent channels associated with the intermediate state). The intermediate state may defined in one of these ways when the EBD valve is a reverse seated valve and the staged opening mechanism comprises one or more vent channels that are openable by a respective electronically controllable valve to vent the piston chamber and transition the EBD valve to the next intermediate state, or when the EBD valve is a reverse seated valve and the staged opening mechanism comprises one or more vent channels that are closable by a respective electronically controllable valve to pressurise the piston chamber and transition the EBD valve to the next intermediate state. In some examples, the diameter of one or more of the respective vent channels is adjustable. Advantageously, adjustment of the one or more vent channels allows the operating piston head to be stopped at different longitudinal points relative to the vent channel (i.e., allows adjustment of the intermediate states). The one or more vent channels may be adjusted using a nozzle or adjustable aperture fitted into the or each vent channel. In some examples, one or more of the respective electronically controllable valves are biased to either: a closed position in which fluid flow through the one or more respective vent channels is blocked; or an open position in which fluid flow through the one or more respective vent channels is permitted. In some examples, all of the respective electronically controllable valves are either biased to the open or closed position. Control of the biased state can allow the EBD to automatically pass straight to the fully open state unless otherwise controlled, which can improve safety. The electronically controllable valves may be biased by a biasing means. For example, the biasing means may be a spring such that the valves are spring loaded. Alternatively, the electronically controllable valves may be biased to the open / closed position by their own weight. In some examples, the one or more electronically controllable valves are solenoid valves. In some examples, the one or more electronically controllable valves are 1 -to-1 valves. A 1 -to-1 valve is understood to be a valve having no more than one fluid path through the valve. E.g., an input port, connecting to the piston chamber, and an output port, connecting to the vent with one path therebetween. When the 1 -to-1 valve is in the closed position, there are no ports connecting through the valve (and so in that sense the 1 -to-1 refers to fluid paths through the valve). The one or more vent channels being ‘connected to’ the piston chamber is understood to mean that the vent channels are either directly or indirectly (e.g., through a valve and / or one or more other components) fluidically connected to the piston chamber. In some examples, the one or more vent channels are directly connected to the piston chamber. That is, the one or more vent channels extend from the piston chamber. In such examples, one or more vent channels may be blocked from the piston chamber on at least a first side of the operating piston head when the emergency blowdown valve is in the closed state. In some cases, the vent channels may be blocked from the piston chamber on a side of the operating piston head whose pressurisation (i.e., the increase in the pressure within the piston chamber on said side of the operating piston head) would cause the EBD valve to move from the fully closed state toward the fully open state. Blocked is understood to mean (substantially) fluidically disconnected / isolated. The blocking may be caused by the operating piston head. The operating piston head may block one or more of the vent channels from the piston chamber entirely, and / or the operating piston head may block fluid flow from the piston chamber on a second side of the operating piston head, which may be connected to one or more vent channels, to the piston chamber on the first side of the operating piston head. The piston chamber on the first side of the operating piston head may be one of the upper and lower piston chambers described above. When the emergency blowdown valve is a direct seated valve, the vent channels may be blocked from the lower piston chamber when the emergency blowdown valve is in the closed state. When the emergency blowdown valve is a reverse seated valve, the vent channels may be blocked from the upper piston chamber when the emergency blowdown valve is in the closed state. However, the present disclosure is not limited to the above cases, and the vent channels may instead be blocked from the piston chamber on a side of the operating piston head whose de-pressurisation (i.e., the decrease in the pressure within the piston chamber on said side of the operating piston head) would cause the EBD valve to move from the fully closed state toward a fully open state. When the emergency blowdown valve is a direct seated valve, the vent channels may be blocked from the upper piston chamber when the emergency blowdown valve is in the closed state. When the emergency blowdown valve is a reverse seated valve, the vent channels may be blocked from the lower piston chamber when the emergency blowdown valve is in the closed state. In some examples, when the one or more vent channels are blocked from the piston chamber on at least a first side of the operating piston head (when the emergency blowdown valve is in the closed state), one or more of the vent channels may be connected to the piston chamber on a second side of the operating piston head opposite the piston chamber on the first side of the operating piston head. However, the present disclosure is not limited in this way, and the vent channels may be blocked from both the piston chambers on both sides of the operating piston head (e.g., upper and lower piston chambers). When the one or more vent channels are blocked from the piston chamber on at least a first side of the operating piston head when the emergency blowdown valve is in the closed state, all of the one or more vent channels are exposed on the first side of the operating piston head when the EBD valve is in the fully open state. In other words, in the fully open state, all of the vent channels are connected to the piston chamber on said side of the operating piston head that they were blocked from when the EBD valve was in the fully closed state. Moreover, in some examples in which the one or more vent channels extend from the piston chamber, and when the staged opening mechanism comprises a series of vent channels, at least two of the vent channels of the series of vent channels are arranged along a direction of movement of the operating piston head. That is, at least two vent channels of the series of vent channels may be arranged at different longitudinal points along the piston chamber (e.g., between the first end and the second end). In this way, the operating piston head, when moving along the longitudinal direction, will pass each of the at least two vent channels at a different time (i.e., the vent channels will be revealed in sequence). Each of the at least two vent channels arranged at different longitudinal points may be associated with a different intermediate state of the EBD valve. In some examples, each of the vent channels of the series of vent channels may be arranged at different longitudinal points between the first and second ends of the piston chamber. Alternatively, multiple points along the longitudinal direction include two or more vent channels. The series of vent channels may be arranged at regular intervals along the longitudinal direction. In this way, the intermediate states associated with the vent channels at each longitudinal point will be evenly spaced, increasing the control over fluid flow through the EBD valve across the whole range of possible fluid flows (e.g., from no fluid flow when the EBD is fully closed, to maximum fluid flow when the EBD valve is fully open). However, the present disclosure is not limited in this way, and the series of vent channels may be arranged at irregular intervals along the longitudinal direction. For example, each of the vent channels may be grouped within a half of the piston chamber closest to the first end of the piston chamber, with no vent channels present in the second half of the piston chamber closest to the second end of the piston chamber. The series of vent channels may be arranged along a single side of the piston chamber running between the first end and the second end, or at least two of the vent channels may be arranged on different sides of the piston chamber running between the first end and second end. The series of vent channels may be arranged along a single line (e.g., a line running parallel to the longitudinal direction), or may be spread on either side of said line (e.g., in a zig zag pattern along the line running parallel to the longitudinal direction). In some examples, the one or more vent channels may be connected to the piston chamber via the upper piston channel (discussed above) and / or the lower piston channel (discussed above). In some examples, the one or more vent channels are indirectly connected to the piston chamber via a pilot valve assembly (that is, the pilot valve assembly described above). That is, the pilot valve assembly controls flow between the piston chamber and the one or more vent channels. In some examples, when the one or more vent channels are indirectly connected to the piston chamber, the staged opening mechanism comprises a series of vent channels arranged in parallel with respect to one another. That is, each vent channel may each be connected to a single unifying channel (for example, a channel connecting the staged opening mechanism to the pilot assembly) at one end, and may each be connected to a vent at the other end. In some examples, when the valve arrangement comprises a series of vent channels, at least two of the vent channels have respectively different channel diameters. In some examples, the one or more respective electronically controllable valves are controllable to one or more intermediate points between a fully open state of the respective electronically controllable valve and a fully closed state of the respective electronically controllable valve. ‘Controllable to one or more intermediate states’ is understood to mean that the electronically controllable valve can be controlled to transition to said one or more intermediate points between the fully open and closed states of the electronically controllable valve, and then controlled to remain at said one or more states of the electronically controllable valve for an indefinite period of time (said indefinite period of time may be controlled through the use of a control signal, which may be provided by a user or an automated system). Advantageously, this reduces the number of vent channels that are required to transition the EBD valve between any number of EBD valve intermediate states, simplifying the design of the valve arrangement. For example, only one vent channel and electronically controllable valve is enough to transition the EBD valve through several EBD valve intermediate stages. The respective electronically controllable valves may be transitioned between the one or more intermediate electronically controllable intermediate points between the fully open and closed states of the electronically controllable valve, and the fully open and closed states of the electronically controllable valve) using a varying-strength electronic signal, wherein the varying strengths of the varying-strength electronic signal may correspond to different electronically controllable valve state / point. The respective electronically controllable valves may be controllable to a series of discrete electronically controllable valve intermediate points (for example, a series of discrete electronically controllable valve intermediate states which are evenly or irregularly spaced), or the respective electronically controllable valves may be controllable through a continuous series of electronically controllable valve intermediate points. Alternatively, the one or more respective electronically controllable valves are binarily movable between the fully open state and the fully closed state. In some examples, the commands for transitioning the EBD valve into each respective intermediate state are binary. More particularly, upon arriving at a particular intermediate state, the EBD valve is either controlled to remain in said intermediate state, or permitted to move straight past said state. In some examples, the emergency blowdown valve is biased to the closed state. The EBD valve may be biased to the closed position by a biasing means. For example, the biasing means may be a spring such that the valves are spring loaded. Alternatively, the EBD valve may be biased to the closed position by their own weight. In some examples, the valve arrangement may further comprise a controller for controlling the staged operating mechanism. In a second aspect, embodiments of the invention provide a nuclear reactor assembly comprising a primary coolant loop connected to the valve arrangement of the first aspect. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figs. 1A-B is a schematic of an emergency blowdown valve. Figs. 2A-B is a schematic of a first valve arrangement in a fully closed state. Figs. 3A-B is a schematic of a first valve arrangement in a first intermediate state. Figs. 4A-B is a schematic of a first valve arrangement in a second intermediate state. Figs. 5A-B is a schematic of a first valve arrangement in a fully open state. Fig. 6 is a schematic of a second valve arrangement. Figs. 7A-B is a schematic of a third valve arrangement. Figs. 8A-D is a schematic of a fourth valve arrangement. Fig. 9 is a schematic of a nuclear reactor assembly. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. Figs. 1A-B show a reverse seated emergency blowdown (EBD) valve 120. In the following Figs., high pressure fluid / coolant is represented by a high density dotted pattern (e.g., see the region indicated by 122 in Fig. 1A), whilst low pressure fluid / coolant is represented by a low density dotted pattern (e.g., see the region indicated by 124 in Fig. 1A). The EBD valve 120 includes an inlet channel 122 and an outlet channel 124. The EBD valve 120 also includes a piston 130 which is movable to prohibit / permit a flow of a (media) fluid, typically coolant, from the inlet channel 122 to the outlet channel 124 through a gap 126. The piston 130 does so by including a sealing piston head 132 for selectively sealing the gap 126. More particularly, the sealing piston head 132 is pulled upward (relative to the page) onto the gap 126, as shown in Fig. 1A, such that a periphery of the sealing piston head 132 abuts against the walls around the edge of the gap 126 (i.e., a valve seat), sealing it so as to block fluid flow therethrough. The piston 130 further includes an operating piston head 134, enclosed within a piston chamber 140, as well as a piston stem 136 which connects the sealing piston head 132 to the operating piston head 134. Piston stem seals are provided to minimise leakage into the piston chamber 140, and further seals are provided on the operating piston head 132 to minimise leakage from a lower piston chamber 142 into an upper piston chamber 144 (discussed below). In this example, the operating piston head 134 is larger (e.g., has a larger surface area) than the sealing piston head 132, such that the operating piston head 134 will experience a greater total force than the sealing piston head 132 when each has the same pressure fluid acting on the relevant surfaces. The operating piston head 134 is movable within the piston chamber 140 along a longitudinal line 102, which causes a corresponding movement in the sealing piston head 132 (via the piston stem 136) between a first, fully-closed, position (shown in Fig. 1A) and a second, fully-open, position (shown in Fig. 1B). The operating piston head 134 also divides the piston chamber 140 into two separate sub-chambers: the first, upper, piston chamber 144 and the second, lower, piston chamber 142. Each of the upper and lower piston chambers 142, 144 includes a respective piston channel 143, 145 (a lower piston channel 143 for the lower piston chamber 142, and an upper piston channel 145 for the upper piston chamber 144). In this present example, the upper piston channel 145 is connected to a pilot valve assembly 190 that is usable to control the movement of the piston 130 (for example, by controlling the fluid flow into the upper piston chamber 144), as well as (indirectly) to the inlet channel 122 (connection not shown). Meanwhile, the lower piston channel 143 is connected to a vent (not shown). However, in other examples, the piston channels 143, 145 may be connected to different components usable to control the movement of the piston 130 via the operating piston head 134. The movement of the operating piston head 134 is controllable through control of the pressure within one or both of the upper and lower piston chambers 142, 144 (which change in volume as the operating piston head 134 moves in the piston chamber 140, one growing larger whilst the other gets smaller). For example, in Fig. 1A, an increase in pressure within the upper piston chamber 144 (e.g., by pumping in fluid through the upper piston channel 145) and / or a decrease in pressure within the lower piston chamber 142 (e.g., by pumping out fluid via the lower piston channel 143) will cause a downward force to be applied to the operating piston head 134, causing it to be moved toward the bottom of the piston chamber 140 (relative to the page). Control of the pressures within one or more of the upper and lower piston chambers 142, 144 is also usable to maintain the piston 130 in a current state (e.g., either open or closed), as will be described in more detail below. Typically, the pressure exerted to operate the piston (i.e., the fluid pressure provided into the relevant side of the piston chamber 140) is the same as the fluid pressure upstream of the EBD valve i.e., the fluid pressure at the inlet channel 122. In use, the EBD valve 120 is (as discussed previously) connected to a pilot valve assembly 190. The EBD valve 120 is typically arranged to remain closed when the pressure of fluid or media upstream of the EBD valve 120 is within a predetermined working range. For example, when the pressure upstream of the EBD valve 120 is greater than a first predetermined threshold, or lower setpoint, and lower than a second predetermined threshold, or upper setpoint. Deviation out of this predetermined working range will, through the use of the pilot valve assembly 190, cause the EBD valve 120 to open. This is achieved by venting the fluid present in the lower piston chamber 142 and / or the introduction of fluid into the upper piston chamber 144 (or vice versa, where the valve is a direct seated valve). In the example shown in Figs. 2-5, it is the introduction of fluid into the upper piston chamber 144 that causes the EBD valve 120 to open. In either case though, the now relatively higher pressure in the upper piston chamber 144 causes the operating piston 134 to move downwards and so open the EBD valve (see Fig. 1B). Figs. 2A-5B show a first valve arrangement 100 including the EBD valve 120 of Fig. 1 (only a part of which is shown in each of Figs. 2A-5B), as well as a staged opening mechanism 160 which is integrated with (e.g., the piston chamber 140 of) the EBD valve 120. As in Fig. 1A, the high / low density dotting patterns in Figs. 2A-5B represent regions of high / low coolant / fluid pressure respectively. Further, the dotted lines represent pathways through which low pressure fluid / coolant is travelling, whilst dashed lines represent pathways through which high density fluid is travelling. The staged opening mechanism 160 in this example comprises two vent channels 162a, 162b, each having a respective electronically controllable valve 164a, 164b which is located along, and controls flow through, the respective vent channel 162a, 162b. More particularly, the electronically controllable valves are movable between two states: a first state in which fluid may flow through the respective controllable valve 164a, 164b, and thus along the respective vent channel 162a, 162b (shown, for example in Figs. 2A-2B) , and a second state in which fluid is prohibited from flowing through the respective electronically controllable valve 164a, 164b, and thus is prohibited from flowing along the respective vent channel 162a, 162b (shown, for example, in Figs. 5A-5B). It is important to note that whilst the present example shows the staged opening mechanism 160 comprising two vent channels 162a, 162b / two electronically controllable valves 164a, 164b, the present disclosure is not limited in this way. In other examples, the staged opening mechanism 160 may comprise only one vent channel / electronically controllable valve. In still further examples, the staged opening mechanism 160 may comprise three, four, five, or any larger number of vent channels / electronically controllable valves. As previously mentioned, the staged opening mechanism 160 is integrated into the EBD valve 120. More particularly, the vent channels 162a, 162b are connected to the piston chamber 140 of the EBD valve 120 in such a way that, depending on the position of the operating piston head 134, one or both of the vent channels 162a, 162b may fluidly communicate with the upper piston chamber 144 (discussed in more detail below). The first vent channel 162a is connected at a different point along the longitudinal direction 102 to the second vent channel 162b (i.e., the first vent channel 162a is connected to the piston chamber 140 at a higher point, relative to the page, than the second vent channel 162b). As such, when the operating piston head 134, starting from the top (relative to the page) of the piston chamber 140, moves downward (relative to the page), the first vent channel 162a becomes fluidly connected to the upper piston chamber 144 first, and the second vent channel 162b becomes fluidly connected to the upper piston chamber 144 second. In other words, the first / second vent channels 162a, 162b become connected to the upper piston chamber 144 at first / second points when the operating piston head 134 is moved from the top of the piston chamber 140 to the bottom, with the first point preceding to the second point. The operation of the first valve arrangement 100 will now be described in detail, with reference to Figs. 2A-5B. In Figs. 2A-B, the EBD valve 120 is in a fully closed state (e.g., the pressure within the inlet channel, see Fig. 1: 122, is within the predetermined working range), in which fluid / coolant flow between the inlet channel and outlet channel of the EBD valve 120 is prohibited. This is equivalent to the state of the EBD valve 120 in Fig. 1A. In this state, the operating piston head 134 is towards the top (relative of the page) of the piston chamber 140, such that the upper piston chamber 144, which is de-pressurised, is at its minimum size and the lower piston chamber 142 is at its maximum size. In this example, as the upper piston channel 145 is in a sidewall of the piston chamber 140, the operating piston 134 does not directly abut the upper surface of the piston chamber 140. However, clearly, if the upper piston channel 145 were instead in the upper surface of the piston chamber 140 the operating piston 134 could abut that surface. Further, both the first 162a and second 162b vent channels are blocked (fluidically isolated) from the upper piston chamber 144. More particularly, the first vent channel 162a is blocked from the piston chamber 140 entirely by the operating piston head 134, whilst the second vent channel 162b is fluidically connected to the lower piston chamber 142 but again blocked from the upper piston chamber by the operating piston head 134. In this example, the electronically controllable valves 164a, 164b of the respective vent channels 162a, 162b both remain in the open state when the EBD valve 120 is in the closed state. This may be because they are biased to the open position (e.g., by a spring load, not shown), or because they are actively being controlled to remain in the open state. This is useful for explanatory purposes of the invention, as will become clear below, but does not represent a limitation of the present disclosure. For example, the electronically controllable valves of the staged opening mechanism 160 may instead remain in the closed stated (e.g., by biasing or active control) when the EBD valve 120 is in the closed state. In Figs. 3A-B, the upper piston chamber 144 has begun to pressurise. This might be, for example, because the pressure within the inlet channel (see Fig. 1: 142) has moved to a value outside the predetermined working range, tripping the pilot valve assembly (see Figs. 1A-B: 190) and causing an inflow of fluid into the upper piston chamber 144. The increased pressure within the upper piston chamber 144 exerts a downward (relative to the page) force on the operating piston head 134 which exceeds an upward force exerted on the sealing piston head (see Fig. 1: 132), causing the piston 130 to begin moving downward. As shown in Fig. 3A, the operating piston head 134 has been actuated past the first vent channel 162a, such that the first vent channel 162a is now in fluid communication with the upper piston chamber 144. As a result, pressurised fluid in the upper piston chamber 144 has begun to flow through the first vent channel 162a and first electronically controllable valve 164a (which is in an open state as discussed above) to a vent (not shown), decreasing the pressure within the upper piston chamber 144. This outflow of fluid enters an equilibrium with the inflow of fluid from the upper piston channel 145, bringing the piston 130 to a stop at a point proximal to the first vent channel 162a. This stable position of the operating piston head 134 corresponds to the first intermediate state of the EBD valve 120, in which the EBD valve 120 is neither fully closed nor fully open. In Figs 4A-4B, the first electronically controllable valve 164a has been moved to the closed state, such that the flow of fluid through the first vent channel 162a is now blocked. As a result, fluid that has flowed into the upper piston chamber 144 via the upper piston channel 145 would again become trapped within the upper piston chamber 144, causing an increase in pressure within the upper piston chamber 144 that has actuated the operating piston head 134 further downwards (relative to the page). At the point shown in Fig. 4A, the operating piston head 134 has moved past the second vent channel 162b. As the second electronically controllable valve 164b is still in the open state, fluid from the upper piston chamber 144 is now able to flow through the second vent channel 162b and out a vent (not shown), in the same manner as was previously described in relation to the first vent channel 162a. A second equilibrium has then been established in the upper piston chamber 144 between the inflow of fluid from the upper piston channel 145 and the outflow of fluid through the second vent channel 162b (mirroring the first equilibrium established when the first electronically controllable valve 164a was in the open state in Figs. 3A-B), causing the operating piston head 134 to stop and the EBD valve to settle in a second intermediate state shown in Fig. 4A. In Figs. 5A-5B, the second electronically controllable valve 164b has been closed, such that fluid flow through the second vent channel 162b is now blocked. As such, fluid from the upper piston channel 145 once again begins to build up within the upper piston chamber 144, increasing the pressure and actuating the operating piston head 134 downwards (relative to the page) until it reaches a bottom of the piston chamber 140 as shown in fig. 5A. At this point, when the upper piston chamber 144 is at a maximum size and the lower piston chamber 142 is at a minimum size, the EBD valve 120 is in the fully open state shown in Fig. 1B, completing the transition from the fully closed state shown in Fig. 1A. Note that in the above, the staged opening mechanism 160 was configured to control the pressure within the upper piston chamber 144. However, in other examples (particularly examples in which the EBD valve is a direct seated valve), the staged opening mechanism 160 may instead be configured to control pressure within the lower piston chamber 142. Fig. 6 shows a second valve arrangement 200. The second valve arrangement 200 is similar to the first valve arrangement 100 in several respects, and like features are given like reference numerals incremented by one hundred. The second valve arrangement 200 includes a gate valve as the emergency blowdown valve (EBD) 220, which includes, an inlet channel 222, and outlet channel 224, a piston 230, and a piston chamber 240. As with first valve arrangement 100, the piston 230 comprises a sealing piston head 232 which is movable to selectively seal a gap 226 between the inlet channel 222 and the outlet channel 224, an operating piston head 234 which is movable within the piston chamber 240, and a piston stem 236 coupling the sealing piston head 232 to the operating piston head 234. The operating piston head 234 divides the piston chamber 240 into an upper piston chamber 244 and a lower piston chamber 242, each of which includes a respective piston channel 243, 245, and is movable within the piston chamber 240 along a longitudinal line 202. When the operating piston head 234 is at the top (relative to the page) of the piston chamber 240, the sealing piston head 232 is retracted toward the piston chamber 240, opening the gap 226 between the inlet channel 222 and outlet channel 224 to permit fluid flow through the EBD valve 220. When the operating piston head 234 is at a bottom (relative to the page) of the piston chamber 240, the sealing piston head is extended from the piston chamber 240 so as to be pushed across the gap 226 into a holding chamber 228, thereby blocking the gap 226 and prohibiting flow through the EBD valve 220. This is the opposite scenario to the reverse seated EBD valve 120 of the first valve arrangement 100, in which the fluid flow was permitted between the inlet channel 122 and the outlet channel 124 when the operating piston head 134 was at the bottom (relative to the page) of the piston chamber 140, and prohibited when the operating piston head 134 was at the top (relative to the page) of the piston chamber 140. The second valve arrangement 200 further includes a staged opening mechanism 260 and a pilot valve assembly 290. The staged opening mechanism 260 comprises first and second vent channels 262a, 262b arranged in a side of the piston chamber 240 between the top and bottom ends of the piston chamber (relative to the page, and along the longitudinal line 202). The staged opening mechanism 260 further includes first and second electronically controllable valves 264a, 264b which control fluid flow through the first and second vent channels 262a, 262b respectively. In particular, the first and second electronically controllable valves 264a, 264b are movable between a closed state, in which fluid flow through the respective vent channels 262a, 262b is prohibited, and an open state, in which fluid flow through the respective vent channels 262a, 262b is permitted. As with the vent channels 162a, 162b of the first valve arrangement 100, the first vent channel 264a is arranged above (relative to the page) the second vent cannel 262b. The pilot valve assembly 290 includes a first pilot valve 292 and a second pilot valve 294. The first pilot valve 292 is connected to the lower piston channel 243 on a first side and the inlet channel 222 and second pilot valve 294 on a second side, and is movable between a first, closed, state in which the first and second first pilot valve side connections are disconnected, and a second, open, state in which the first and second first pilot valve side connections are connected. Meanwhile, the second pilot valve 294 is connected to the upper piston channel 245 on a first side, the first pilot valve 292 and inlet channel 222 on a second side, and a vent 295 on a third side. The second pilot valve 294 is movable between a first state in which the first side connection of the second pilot valve is connected to the vent 295, and a second state in which the first side connection of the second pilot valve side is connected to the second side connection of the second pilot valve. The valve arrangement 200 also includes an additional lower piston chamber vent channel 296, which extends from a bottom end of the piston chamber 240 (relative to the page). Fluid flow through said lower piston chamber vent channel 296 is controlled by a vent valve 298, which is movable between an open and closed state. The operation of the valve arrangement 200 is now described, starting from a fully open state of the EBD valve 220. In particular, the second pilot valve 294 is in the first state, such that the upper piston chamber 244 is connected to the vent 295 and is thus in a de-pressurised state. Meanwhile, the first pilot valve 292 is open (that is, the lower piston chamber 242 is connected to the inlet channel 222) and the vent valve 298 is closed (that is, the lower piston chamber 242 is not venting via the lower piston chamber vent channel 296), such that the lower piston chamber 242 is in a pressurised state. As such, the conditions within the piston chamber 240 force the operating piston head 234 to the top of the piston chamber 240 (relative to the page), such that the sealing piston head 232 is retracted from the gap and flow between the inlet channel 222 and the outlet channel 224 is permitted. To begin closing the EBD valve 220 (that is, to begin moving the operating piston head 234 to the bottom of the piston chamber 240, relative to the page), the second pilot valve 294 is moved to the second state in which the upper piston chamber 244 is disconnected from the vent 295 and connected to the inlet channel 222. Meanwhile, the first pilot valve 292 is moved to the closed state in which the lower piston channel 242 is disconnected from the inlet channel 222, and the vent valve 298 is moved to the open state in which fluid is permitted to flow through the lower piston chamber vent channel 296. As such, the lower piston chamber 242 becomes de-pressurised. Under these conditions, the operating piston head 234 begins moving toward a bottom of the piston chamber 240, until it passes the first vent channel 262a, having the first electronically controllable valve 264a in an open state. Similarly to the case described in relation to the first valve arrangement 100, fluid will be able to vent from the upper piston chamber 244 via the first vent channel 262a, such that the operating piston head 234 will be prevented from travelling further down, and settle into a first intermediate state. Once the first electronically controllable valve 264a is closed, pressure will again begin building up in the upper piston chamber 244, forcing the operating piston head 234 down to the second vent channel 262b. Similarly to the scenario at the first vent channel 262a, if the second electronically controllable valve 264b is in the open state, the fluid will vent from the upper piston chamber 244 via the second vent channel 262b and the operating piston head 234 will be stopped in a second intermediate state. It is not then until the second electronically controllable valve 264b is moved to the closed state that the operating piston head 234 will be able to continue downward to the bottom of the piston chamber 240, corresponding to the EBD valve 220 being in the closed state. It is further noted that once the operating piston head 234 has passed one or both of the respective vent channel 262a 262b, the respective electronically controllable valve 264a, 264b may be again moved to the open state. In this way, one or both of the first and second vent channels 262a, 262b may aid in the venting of the upper piston chamber 244, and will also be prepared for controlling the EBD valve 220 through the intermediate states as it is moved back toward the fully open state, as is described below. To begin moving the EBD valve 220 back toward the fully open state, the first pilot valve 292 is opened (connecting the lower piston chamber 242 to the inlet channel 222), the vent valve 298 is closed (preventing further venting from the lower piston chamber 242 via the lower piston chamber vent valve 296), and the second pilot valve 294 is moved back to the first state (connecting the upper piston chamber 244 to the vent 295). Under these conditions, the lower piston chamber 242 will begin pressurising and the upper piston chamber 244 will begin de-pressurising, moving the operating piston head 234 upward (relative to the page). As the operating piston head 234 moves upward, it will pass each of the first and second vent channels 262a, 262b in turn, this time beginning with the second vent channel 262b. Once again, as long as the respective first / second electronically controllable valve 264a, 264b is in the open state, the operating piston head will pause at the respective first / second vent channel 262a, 262b due to the venting of fluid out of the first / second vent channel 262a, 262b, causing the EBD valve 220 to settle in a first / second intermediate state. It will not then be until the respective first / second electronically controllable valve 264a, 264b is moved to the closed state that the operating piston head 234 will be able to continue upward, eventually arriving at the top of the piston chamber 240 (in which the EBD valve 220 is in the fully open state) when both the first and second respective electronically controllable valves 264a, 264b are closed. Figs. 7A-7B show a third valve arrangement 300. The third valve arrangement 300 is similar to the second valve arrangement 200 in several respects, and like features are given like reference numerals incremented by one hundred. In particular, the third valve arrangement 300 again comprises a direct seated EBD valve 320, which includes an inlet channel 322, an outlet channel 324, a piston 330, and a piston chamber 340. Said piston 330 including a sealing piston head 332 that is configured to close a gap 326 between the inlet channel 322 and the outlet channel 324 when it is maximally extended from the piston chamber 340, an operating piston head 334 which divides the piston chamber 340 into a lower piston chamber 342 and an upper piston chamber 344, and a piston stem 336 coupling the sealing piston head 332 and the operating piston head 334. In addition to these features, the EBD valve 320 also includes lower piston chamber leakage channels 352 connecting the lower piston chamber 342 to the inlet channel 322 through a bottom end of the piston chamber 340, and upper piston chamber leakage channels 354 connecting the upper piston chamber 344 to the lower piston chamber 342 through the operating piston head 334. These upper piston chamber leakage channels 354 are smaller in diameter than a upper piston channel 345 (which connects the upper piston chamber 344 to the inlet channel 322 / vent, not shown, described below) and the lower piston chamber leakage channels 352, for reasons that are explained below. Further similarly to the second valve arrangement 200, the third valve arrangement 300 further includes a staged opening mechanism 360 and a pilot valve assembly 390, which are configured to control the movement of the piston 330 between the top and bottom of the piston chamber 340 (and thus the EBD valve 320 between a fully open state, a closed state, and one or more intermediate states). However, in the third valve arrangement 300, the pilot valve assembly 390 is configured differently to the pilot valve assembly 290 of the second valve arrangement 200. In particular, the pilot valve assembly 390 includes a low pressure pilot (LPP) valve 392 and a high pressure pilot (HPP) valve 394 connected in parallel between the inlet channel 322 and the upper piston chamber 344. Said respective LPP / HPP valves 392, 394 being respectively movable between a first state in which the respective LPP / HPP valve 392, 394 permits flow between the inlet channel 322 and the upper piston chamber 344, and a second state in which the respective LPP / HPP valve 392, 394 prohibits flow between the inlet channel 322 and the upper piston chamber 344. The LPP valve 392 occupies the first state when the fluid pressure within the inlet channel 322 is below or equal to a first predetermined pressure, and occupies the second state when the fluid pressure within the inlet channel 322 is above the first predetermined pressure. The HPP valve 394 occupies the first state when the fluid pressure within the inlet channel 322 is equal to or above a second predetermined pressure (which is greater than the first predetermined pressure), and occupies the second state when the fluid pressure within the inlet channel 322 is below the second predetermined pressure. Said first and second predetermined pressures defining a working pressure range of the EBD valve 320, within which the EBD valve 320 remains closed. The staged opening mechanism 360 of the third valve arrangement 300 is also different to the staged opening mechanism 260 of the second valve arrangement 200, both in composition and location. Starting with composition, the staged opening mechanism 260 comprises only a single vent channel 362, and a single electronically controllable valve 364 which controls fluid flow through said vent channel 362. Further, the electronically controllable valve 364 is electronically controllable (for example, by varying the strength of the electronic signal sent to the electronically controllable valve 364) to move to one or more intermediate points between the closed and fully open states, such that fluid flow through the vent channel 362 may be varied. Said intermediate points between the closed and fully open states may be a number of discreet values, or may be continuous spectrum. Turning to the location of the staged opening mechanism 360, this is now only indirectly connected to the piston chamber 340 via the pilot valve assembly 390 (as opposed to directly connected to the piston chamber 340 as in the first and second valve arrangements 100, 200). More particularly, in the present example, the vent channel 362 is connected to both the LPP and HPP valves 392, 394 in such a way that the vent channel 362 is connected to the upper piston chamber 344 when one of the LPP and HPP valves 392, 394 is in the second state described above (that is when, the LPP / HPP valve 392, 394 in question prohibits flow between the upper piston chamber 344 and the inlet channel 322). The operation of the third valve arrangement 300 will now be described with reference to Figs. 7A-B, starting from a closed state of the EBD valve 320 in which the operating piston head 334 is at the bottom of the piston chamber 340 (relative to the page). The EBD valve 320 will occupy the closed state, or at least be capable of moving to the closed state, when the pressure within the inlet channel 322 is above the first predetermined pressure and below the second predetermined pressure (that is, when both the LPP and HPP valves 392, 394 are in the first state defined above). In other words, the EBD valve 120 will be in or capable of moving to the closed state when the upper piston chamber 344 is connected to the inlet channel 322 via both the LPP and HPP valves 392, 394, as this is when the upper piston chamber 344 may be pressurised to exert a downward (relative to the page) force on the operating piston head 334 to push the sealing piston head 332 away from the piston chamber 340. However, once the pressure in the inlet channel 322 falls outside of the predetermined working range (for example, above or below the first and second predetermined pressures), one of the LPP valve 392 and HPP valve 394 will be moved to the second state in which the upper piston chamber 344 is connected to the vent channel 362. An example of this is shown in Figs. 7A-B, where the pressure within the inlet channel 322 is either equal to or below the first predetermined pressure, causing the LPP valve 392 to move to the second state in which the upper piston chamber 344 is connected to the vent channel 346 therethrough (see the dotted lines in Figs. 7A-B). Control of the state of the EBD valve 320 will then be controllable using the electronically controllable valve 364, as this controls venting of the upper chamber fluid through the vent channel 362. In particular, when the electronically controllable valve 364 is closed, there will be no venting through the vent channel 362. Furthermore, the upper piston chamber 344 is still connected to the lower piston chamber 342 through the upper piston leakage channels 354, and so will be maintained at at least an equal pressure to the lower piston chamber 342, such that the operating piston head 334 will continue to block the gap 326 between the inlet channel 322 and the outlet channel 324. This is shown in Fig. 7A. However, as the electronically controllable valve 364 is controlled to transition to an open state, fluid will begin to vent from the upper piston chamber 344 via the vent channel 362, with said vented outflow being greater than the inflow through the upper piston leakage channels 354 due to the greater diameter of the upper piston channel 345 as described above. Therefore, the pressure within the upper piston chamber 344 will be relatively smaller than the pressure in the lower piston chamber 342, which is being fed with fluid from the inlet channel 322 via the lower piston leakage channels 352 at a greater rate than the fluid which is being lost to the upper piston chamber 344 via the upper piston chamber leakage channels 354 due to the smaller diameter of the upper piston chamber leakage channels 354. This will result in a net upward force being applied to the operating piston head 334, moving it, and the sealing piston head 342, upwards toward the top of the piston chamber 340, opening the gap 326. By subsequently controlling the exact size of the opening of the electronically controllable valve 364, it will be understood that the exact pressure in-balance between the upper and lower piston chambers 342 344 will be controllable, allowing the operating piston head 334, and thus the EBD valve 320, to be moved to one or more intermediate states between the closed and fully open states, before arriving at the fully open state shown in Fig. 7B. Figs. 8A-D show a fourth arrangement 400. The fourth valve arrangement 400 is similar to the third valve arrangement 300 in several respects, and like features are given like reference numerals incremented by one hundred. In particular, the fourth valve arrangement 400 comprises an EBD valve 420 and a pilot valve assembly 490 that are identical in structure and functionality to the EBD valve 320 and pilot valve assembly 390 of the third valve arrangement 300. A description of these components is thus not repeated here. The fourth valve arrangement 400 further comprises a staged opening mechanism 460 that is positioned relative to the EBD valve 420 and the pilot valve assembly 490 in the same way as the staged opening mechanism 360 is positioned relative to the EBD valve 320 and the pilot valve assembly 390 of the third valve arrangement 300. However, instead of one vent channel 362, the staged opening mechanism 460 of the fourth valve arrangement 400 includes three vent channels 462a, 462b, 462c arranged in parallel between the pilot valve assembly 490 and a vent (not shown). More particularly, each vent channel 462a, 462b, 462c connects at a first end to the pilot valve assembly 490, and at a second end to the vent. Further, each vent channel 462a, 462b, 462c comprises an electronically controllable valve 464a, 464b, 464c, each of which is binarily controllable between a closed and fully open state (as opposed to be adjustable to intermediate points between the closed and fully open states, as was the case with the electronically controllable valve 364 of the third valve arrangement 300). The total possible fluid throughput of the three electronically controllable valves 464a, 464b, 464c in combination may be equal to the total possible fluid throughput of the electronically controllable valve 364, though the present disclosure is not limited in this way. The operation of the fourth valve arrangement 400, which is similar to the operation of the third valve arrangement 300, is now described in relation to Figs. 8A-D. In Fig. 8A, the EBD valve 420 is in the closed state, in which the sealing piston head 432 is blocking a gap 426 between the inlet channel 422 and the outlet channel 424. This is because, although the pressure within the inlet channel 422 has fallen below the first predetermined pressure (described in relation to the third valve arrangement 300 above), thus tripping the LPP valve 492 to connect the upper piston chamber 444 to the staged opening mechanism 460 (as shown by the dotted lines in Fig. 8A), each of the electronically controllable valves 464a, 464b, 464c are in the closed state. As such, the upper piston chamber 444 remains disconnected from the vent, and so is maintained at an equal pressure to the lower piston chamber 442 by the upper piston leakage channels 454. The situation changes in Fig. 8B, where a first electronically controllable valve 464a of the three electronically controllable valves 464a, 464b, 464c has been controlled to transition to the open state. Now, a certain proportion of fluid within the upper piston chamber 444 is vented via the vent channel 462a in a given amount of time, which is greater than the total amount of fluid inflow into the upper piston chamber 444 (i.e., via the upper piston chamber leakage channels 454) in the same time period. This will cause an at least temporary pressure imbalance between the upper and lower piston chambers 442, 444, which will cause the operating piston head 434 to move upward before settling in a first intermediate state shown in Fig. 8B. Subsequently, a second electronically controllable valve 464b of the three electronically controllable valves 464a, 464b, 464c is opened, such that fluid is able to vent out of the upper piston chamber 444 via both the first and second electronically controllable valves 464a, 464b. This will create a greater pressure differential between the lower and upper piston chambers 442, 444, moving the operating piston head 434 further upwards so that the EBD valve 420 enters a second intermediate state. This is shown in Fig. 8C. Finally, the third electronically controllable valve 464c of the three electronically controllable valves 464a, 464b, 464c is opened, such that fluid is able to be vented via each of the three electronically controllable valves 464a, 464b, 464c. This causes the operating piston head 434 to move to the top (relative to the page) of the piston chamber 440, in which the EBD valve 420 is in the fully open state, as shown in Fig. 8D, completing the transition of the ERB valve 420. Of course, the skilled person will recognise that the any number of vent channels / electronically controllable valves may be includable in the valve arrangement shown in Figs. 8A-D, so as to introduce additional intermediate states to the EBD valve 420 between the closed and fully open states. Fig. 9 shows a nuclear reactor assembly 602 which includes a reactor pressure vessel 604, connected to a steam generator 606 via a coolant circuit 608. In use, fissile material in the reactor pressure vessel 604 is used to heat coolant (e.g., water) within the coolant circuit 608 which is then used to heat water within the steam generator 606 to generate steam. The steam is used to generate electricity via one or more turbines (not shown). The coolant circuit 608 includes a pressurizer 612, as well as a reactor coolant pump 610. The pressurizer 612 maintains the fluid pressure of the coolant, for example to ensure that the coolant remains in the liquid phase. The pressurizer 612 is fluidically connected to a containment unit 616 via a pressure release valve arrangement 614 which includes two of the valve arrangements as discussed above connected in parallel. Control and instrumentation system 618 is connected to the pressure release valve arrangement 614 and includes (for example) electrical command lines and sensor lines. It is this control and instrumentation system 618 which may issue commands to the electronically controllable valves 1 of any of the valve arrangements 100, 200, 300, 400 discussed above, for example moving the electronically controllable valves 164a and / or 164b of the first valve arrangement 100 between their respective positions. Whilst the pressure release valve arrangement 614 in this example is shown as connected above the pressurizer 612, it may instead be connected below the pressurizer, or to the coolant circuit 608 directly. The containment unit 616 can provide two functions: (i) to allow for the overflow of coolant in the event of overpressure from the coolant circuit 608 into the containment unit 616; and (ii) to allow for the provision of emergency coolant from storage in the containment unit 616 in the event of a loss of coolant accident (LOCA). In some examples, it may provide only function (i). Here, as the containment unit 616 may provide both functions, the pressure release valve assembly 614 also may also provide two functions: (i) to open and allow coolant to flow from the coolant circuit 608 into the containment unit 616 in the event of over pressure; and (ii) to open and allow reserve coolant to flow form the containment unit 616 to the coolant circuit 608 in the event of a LOCA. In some examples it may only provide function (i). The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A valve arrangement for a nuclear reactor, the valve arrangement comprising:an emergency blowdown valve, controllable to open and close, thereby permitting or prohibiting a flow of coolant from an inlet to an outlet of the emergency blowdown valve, anda staged opening mechanism, configured to cause the emergency blowdown valve to transition from a fully closed to a fully open state via one or more predefined, increasingly open, intermediate states, the emergency blowdown valve pausing at each intermediate state before continuing the transition to the next state.

2. The valve arrangement of claim 1, wherein the emergency blowdown valve is a direct seated valve and includes:a piston arrangement, configured to selectively seal the inlet from the outlet, the piston arrangement including:a sealing piston head which selectively seals the inlet from the outlet;an operating piston head, operatively connected to the sealing piston head; anda piston chamber, within which the operating piston head is movable; andwherein the staged opening mechanism comprises one or more vent channels connected to the piston chamber, each of the one or more vent channels being openable by a respective electronically controllable valve to vent the piston chamber on one side of the piston head, thereby transitioning the emergency blowdown valve to the next state.

3. The valve arrangement of claim 1, wherein the emergency blowdown valve is a reverse seated valve and includes:a piston arrangement, configured to selectively seal the inlet from the outlet, the piston arrangement including:a sealing piston head which selectively seals the inlet from the outlet;an operating piston head, operatively connected to the sealing piston head; anda piston chamber, within which the operating piston head is movable; andwherein the staged opening mechanism comprises one or more vent channels connected to the piston chamber, each of the one or more vent channels being closable by a respective electronically controllable valve to pressurise the piston chamber on one side of the piston head, thereby transitioning the emergency blowdown valve to the next state.

4. The valve arrangement of claim 2 or claim 3, wherein the diameter of one or more of the respective vent channels is adjustable.

5. The valve arrangement of any one of claims 2 to 4, wherein one or more of the respective electronically controllable valves are biased to either:a closed position in which fluid flow through the one or more respective vent channels is blocked; oran open position in which fluid flow through the one or more respective vent channels is permitted.

6. The valve arrangement of any one of claims 2 to 5, wherein the one or more electronically controllable valves are solenoid valves.

7. The valve arrangement of any one of claims 2 to 6, wherein the one or more electronically controllable valves are 1-to-1 valves.

8. The valve arrangement of any one of claims 2 to 7, wherein the vent channels extend from the piston chamber.

9. The valve arrangement of claim 8, wherein the one or more vent channels are blocked from the piston chamber on at least a first side of the operating piston head when the emergency blowdown valve is in the closed state.

10. The valve arrangement of claim 8 or 9, wherein when the staged opening mechanism comprises a series of vent channels, at least two of the vent channels of the series of vent channels are arranged along a direction of movement of the operating piston head.

11. The valve arrangement of any one of claims 2 to 7, wherein the one or more vent channels are indirectly connected to the piston chamber via a pilot valve assembly.

12. The valve arrangement of claim 11, wherein the staged opening mechanism comprises a series of vent channels arranged in parallel.

13. The valve arrangement of any one of claims 2 to 12, wherein when the valve arrangement comprises a series of vent channels, at least two of the vent channels have respectively different channel diameters.

14. The valve arrangement of any previous claim, wherein the one or more respective electronically controllable valves are controllable to one or more intermediate points of the electronically controllable valve between the fully open state of the respective electronically controllable valve and the fully closed state of the respective electronically controllable valve.

15. The valve arrangement of any one of claims 1 to 13, wherein the one or more respective electronically controllable valves are binarily movable between the fully open state and the fully closed state.

16. The valve arrangement of any previous claim, wherein the emergency blowdown valve is biased to the fully closed state.

17. The valve arrangement of any previous claim, further comprising a controller for controlling the staged 5 opening mechanism.

18. A nuclear reactor assembly, comprising a primary coolant loop connected to the valve arrangement of any preceding claim.

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