Valve arrangement and nuclear reactor assembly

Abstract

A valve arrangement 100 comprising: an emergency blowdown valve, controllable to open and close, thereby permitting or prohibiting a flow of coolant therethrough, emergency blowdown valve including:

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 steam generator that would drive 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 (also known as a loss of coolant accident, or LOCA). 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 desirous to keep the emergency blowdown valve open even after the coolant system pressure re-enters the working range, either for a set amount of time or until it is requested that the main valve be closed once again. This may be, for example, for safety reasons, to ensure that the nuclear reactor can be operated safely going forward, or to assess the reasons why the coolant system pressure fell outside the working range in the first place. 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 therethrough, the emergency blowdown valve including: an inlet; an outlet; and 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; the valve arrangement further including a fluid isolator, configured, when the emergency blowdown valve is open, to fluidically isolate the piston chamber on one side of the operating piston head thereby preventing movement of the operating piston head. The fluidic isolation of the piston chamber caused by the fluid isolator causes the emergency blowdown valve to selectively lock in the present state (i.e., open or closed), which can provide a greater degree of control over the valve arrangement and by, extension, the coolant system. This can be referred to as pressure locking, as the fluid within the piston chamber locks the valve in the open position. This kind of pressure locking is applicable to any pilot operated pressure relief valve (POPRV). For example, the POPR may have only one pilot valve, or two pilot vales (a high-pressure pilot valve and a low-pressure pilot valve). Some examples have only a high-pressure pilot valve to trip open which trips open when the pressure upstream of the emergency blowdown valve exceeds a high pressure set point. In contexts such as within nuclear power stations, it can be desirous to keep the POPRV open, once it has tripped open, to ensure full depressurisation of the system. This means it will not close once the pressure drop back below the HP set point. Advantageously, as compared to say, mechanical latching, is that pressure locking does not require as much maintenance and is not subject to as much wear and tear. The fluid isolator has at least two states. The first state may correspond to an open state in which the fluid isolator permits fluid flow therethrough, and the second state may correspond to a closed state in which the fluid isolator prohibits fluid flow therethrough. In some examples, flow through the fluid isolator in the closed state may be prohibited directly by the fluid isolator. In other examples, the fluid isolator may allow flow therethrough, but a secondary component (e.g., a non-return valve) on a first or second side of the fluid isolator may prevent flow in at least a first direction, such that a pressure build up of fluid at the secondary component causes the cessation of further fluid flow through the fluid isolator. However, the fluid isolator is not limited in the above ways, and may have more than two states. The emergency blowdown valve may be a reverse seated valve or a direct seated valve. 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 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. Le., 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 two sides of the operating piston head, which define two parts of the piston chamber, are opposite one another, such that pressurising one of the two sides causes the operating piston head, and therefore the piston as a whole, to move in a first direction, and pressurising the other side causes the piston to move in a second direction opposite the first direction. The two sides of the piston chamber may be referred to as the upper piston chamber and lower piston chamber. The two sides of the operating piston head may be (at least substantially) fluidically isolated from one another by the operating piston head. In some examples they are (at least substantially) fluidically isolated from one another by the operating piston head. In other examples, channels I holes may be present in the operating piston that allows media to pass through from one side to the other. However, the closing or opening force would be dependent on the differential surface area that is smaller on one side than the other (due to the diameter of the stem of the piston). This difference in surface area may cause the operating piston to move due to the media pressure, even if the pressure on both sides of the piston was the same. One or both of the two sides of the operating piston head, defined on opposite sides of the operating piston head respectively, may comprise a respective inlet permitting (e.g., fluid) communication with the external environment. The valve arrangement may include a control system and the control system may be configured to actuate the fluid isolator. The fluid isolator may automatically transition to an open state (e.g., in which fluid communication between the first and second sides of the fluid isolators is permitted). For example, the fluid isolator may transition to the open state after a predetermined time of the pressure upstream of the emergency blowdown valve remaining within a first predetermined pressure range. Alternatively, the fluid isolator may automatically transition to the open state after the pressure enters a second predetermined pressure range representing a subrange of the first predetermined pressure range. However, the present disclosure is not limited in these ways, and the fluid isolator may automatically transition to the open state under a range of other conditions recognisable to the skilled person equipped with the present disclosure. Or alternatively, the fluid isolator may only transition to the open state when operated to do so by a user (manually or otherwise). In some examples, and specifically when the emergency blowdown valve is a direct seated valve, the fluid isolator may be connected to the piston chamber on the side of the operating piston head that lies between the operating piston head and the sealing piston head (i.e., the lower piston chamber). In other examples, the or a further fluid isolator may be connected to the upper piston chamber. The piston chamber on the side of the operating piston head that lies between the operating piston head and the sealing piston head may be referred to as a first, or lower, piston chamber. Meanwhile, the piston chamber on the opposite side of the operating piston head (e.g., the piston chamber which has the operating piston head between itself and the sealing piston head) may be referred to as a second, or upper, piston chamber. The emergency blowdown valve may be a direct seated valve, and the fluid isolator may be connected to the piston chamber on a first side the operating piston head that lies between the operating piston head and the sealing piston head. That is, the fluid isolator may be connected to the lower piston chamber. In some examples, the fluid isolator is a 1 -to-1 valve. 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 one or more pilot valves, and an output port, connecting to the piston chamber with one path therebetween. When the 1-1 valve is in the closed position, there are no ports connecting through the valve (and so in that sense the 1-1 refers to fluid paths through the valve). Alternatively, the emergency blowdown valve may be a reverse seated valve, and the fluid isolator may be connected to the piston chamber on a second side of the operating piston head that has the operating piston head between it and the sealing piston head. That is, the fluid isolator may be connected to the upper piston chamber. In such examples, a piston channel may be connected to the piston chamber on the first side of the operating piston head at a portion of the piston chamber which faces the sealing piston head. This can allow the lower piston chamber to be filled even if the operating piston head abuts the lower face of the piston chamber. Again, in other examples the ora further fluid isolator may be connected to the lower piston chamber. The fluid isolator is movable between an open, de-activated, state in which fluid communication between the inlet and outlet is permitted, and a closed, activated, state in which fluid communication between the inlet and outlet is prohibited. When in the open state, fluid may be permitted to flow in both directions through the fluid isolator, such that in some cases fluid may flow from the ‘output’ port of the fluid isolator to the ‘inlet’ port of the fluid isolator. In this way, for example, pressure in said side of the piston chamber may be released when the fluid isolator is de-activated (e.g., is no longer fluidically isolating the piston chamber). However, the fluid isolator is not limited to the above. For example, the fluid isolator may be a valve having two fluid paths therethrough, such as a 2-to-1 valve in which there are two inlet ports (e.g., respectively connected to the pilot valves and to a vent), and one outlet port (e.g., connected to the piston chamber). The valve may have at least two states. A first state in which the first inlet is connected to the outlet and a second state in which the second inlet is connected to the outlet. In some examples, for example when the valve is implemented as a reverse seated valve, the fluid isolator is connected to a vent port of said side of the piston chamber and is configured to close when the emergency blowdown valve opens such that said side of the piston chamber to which it is connected cannot vent. Advantageously, this allows pressure to be maintained within said side of the piston chamber when the fluid isolator is activated, and a de-pressurised state to be maintained within said side of the piston chamber when the fluid isolator is de-activated. Through this, the movement of the operating piston head, and the piston as a whole can be accurately controlled. In some examples, the valve arrangement further comprises a non-return valve (e.g., a one-way valve) that is fluidly connected to the fluid isolator and arranged to prevent the flow of fluid from the fluid isolator away from the piston chamber. Advantageously, the non-return valve permits fluids to flow into the piston chamber, via the fluid isolator, without being permitted to subsequently flow out, allowing the piston chamber to increase and maintain its pressure. In some examples, such as reverse seated type valves, the non-return valve may be upstream of the fluid isolator, such that the fluid isolator is between the non-return valve and the piston chamber to which the fluid isolator is connected. Generally the non-return valve is suitably positioned between the fluid isolator and the one or more pilot valves. In examples with a non-return valve, the fluid isolator may have only two states, corresponding to the first and second states described above. In some examples, there may be two pilot valves in a respectively parallel arrangement, that are connected upstream of the fluid isolator via a pilot valve flow selector. The pilot valves may operate to control the emergency blowdown valve to open and / or close. For example, there may be a high-pressure pilot valve operable to open the emergency blowdown valve when the pressure upstream of the emergency blowdown valve exceeds an upper threshold pressure and there may be a low-pressure pilot valve operable to open the emergency blowdown valve when the pressure upstream of the emergency blowdown valve goes below a lower threshold pressure. The flow selector may selectively connect one of the two parallel pilot valves to the fluid isolator. E.g., the flow selector may have two states. A first state in which a first pilot valve of the two parallel pilot valves is connected to the pilot valve flow selector, and a second state in which a second pilot valve of the two parallel pilot valves is connected to the pilot valve flow selector (at the exclusion of the other). The pilot valve flow selector may be a three-way shuttle valve. The two parallel pilot valves may be valves which are moveable from a first, closed, state to second, open, state when a predetermined pressure is incident on a port of the valve. Advantageously, this permits fluid to be channelled into the piston chamber to which the fluid isolator is connected. In some examples, the valve arrangement further comprises a feed line that connects an upstream or downstream side of the emergency blowdown valve to said side of the piston chamber. Advantageously, this permits fluid to enter into the piston chamber from the main coolant flow, that has passed through the main valve to a downstream point when the main valve has opened, which can mitigate any losses of fluid from said side of the piston chamber caused by leakage past the operating piston head. For example, this may occur when the operating piston head has not moved fully into an open state in which a port into the piston chamber on the opposite side of the operating piston head to side of the piston chamber is blocked, such that fluid leaking past the operating piston head from said side of the piston chamber may vent out said port. The feed line may be connectable to said side of the piston chamber via the fluid isolator. A non-return valve (for example, the non-return valve described in some of the examples above) may be positioned between the feed line and the fluid isolator. In some examples, the valve arrangement further comprises a feed line selector upstream of the fluid isolator that selectively connects the fluid isolator to one of the feed line and a second channel. In some examples, the second channel connects to a pilot valve assembly. The feed line selector (e.g., a second flow selector) may selectively connect the fluid isolator to the pilot valve flow selector (e.g., a first flow selector). The feed line selector may be a three-way shuttle valve. A non-return valve (for example, the non-return valve described in some of the examples above) may be positioned between the pilot valve assembly and the feed line selector. In some examples, the fluid isolator is or may include a solenoid valve. Advantageously, this allows a user to control the fluid isolator (e.g., actuate the fluid isolator between states) electronically, and as such said user can control the fluid isolator from a distance. Alternatively however, the fluid actuator may be a manual valve. E.g., a valve that requires manual actuation by a user. In some examples, the fluid isolator is actuated to fluidically isolate said side of the piston chamber when the emergency blowdown valve moves to a fully open state. This fully open states corresponds to one in which coolant can flow at a maximum rate. Advantageously, this prevents the fluid isolator from being activated, and thus fluidically isolating the piston chamber to which it is connected, prematurely. Said activation preventing further movement of the piston, as the pressure within the piston chamber may now be fixed. 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. More particularly, the operating piston head is moved in the first / second direction within a piston enclosure, said piston enclosure defining the piston chambers on either side of the operating piston head, until the piston head abuts a wall forming part of the piston enclosure, leaving a first of the two piston chambers at an absolute maximum size and a second of the piston chambers (the second piston chamber being on an opposite side of the operating piston head to the first piston chamber) at an absolute minimum size. The absolute minimum size may refer to the second piston chamber being removed entirely (e.g., reduced to zero volume). Alternatively, or additionally, the fully open state may refer to the operating piston head being moved to a position in which it blocks an inlet channel to a first or second piston chamber, such that fluid is no longer permitted to enter said first or second piston chamber. The inlet may be connected to the piston chamber that is on the opposite side of the operating piston head to the piston chamber which the fluid isolator is connected to. 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, and a controller, configured to control the fluid isolator. 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-D show a direct seated valve arrangement in various configurations. Figs. 2A-C show a variant reverse seated valve arrangement in various configurations. Figs. 3A-C show a further variant reverse seated valve arrangement in various configurations. Fig. 4 shows the valve arrangement of Figs. 3A-3C together with an example pilot valve assembly. Figs. 5A-B show open and closed states of a reverse seated emergency blowdown valve forming part of a valve arrangement of any of the second to forth examples. Fig. 6 shows 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-D show a valve arrangement 100. Within the following Figs., high pressure fluid / coolant is represented by a high density dotted pattern (e.g., see the region 122 and 144 in Fig. 1 A), whilst low pressure fluid / coolant (i.e. fluid / coolant with a lower pressure than the high pressure fluid / coolant) is represented by a low density dotted pattern (e.g., see the region 124 in Fig. 1A). It should be noted that the regions indicated with the same pattern may not be of equal pressure, merely that these areas are at a high pressure than another area (so, for example, the pressure in 144 of Fig. 1A may be lower than 122, but higher than 122). The valve arrangement 100 comprises a direct seated emergency blowdown (EBD) valve 120, which includes an inlet 122 and an outlet 124. The EBD valve 120 also includes a piston 130 which is movable to prohibit / permit a flow of a (media) fluid from the inlet channel 122 to the outlet channel 124 through a channel 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 pressed downward towards the inlet (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 channel 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 that is at least substantially fluidly isolated from the inlet and outlet channels 122 &124, as well as a piston stem 136 which connects the sealing piston head 132 to the operating piston head 134. 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 downward (i.e., closing) force than the force on sealing piston head 132 (i.e., an upward, opening, force) 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 substantially divides the piston chamber 140 into two separate sub-chambers: a first, lower, piston chamber 142 and a second, upper, piston chamber 144. 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 (not shown) that is usable to control the movement of the piston 130, as well as (indirectly) to the inlet channel 122 (connection not shown). Meanwhile, the lower piston channel 143 is connected to a vent channel 112. However, in other examples, the piston channels 143, 145 may be connected to different components. 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 lower piston chamber 142 (e.g., by pumping in fluid through the lower piston channel 143) and / or a decrease in pressure within the upper sub-chamber 144 (e.g., by pumping out fluid via the upper piston channel 145) will cause an upward force to be applied to the operating piston head 134, causing it to be moved toward the top of the piston chamber 140. 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 122. In use, the valve arrangement 100 is (as discussed previously) connected to a pilot valve assembly. 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 working range. For example, when the pressure upstream of the EBD valve 120 is greaterthan a first predetermined threshold and lower than a second predetermined threshold. Variation out of this working range will, through the use of the pilot valve assembly, cause the EBD valve 120 to open. This is achieved by venting the fluid present in the upper piston chamber 144 and / or the introduction of fluid into the lower piston chamber 142. Typically, it is achieved by venting the fluid present in the upper piston chamber 144. The now relatively higher pressure in the lower piston chamber 142 causes the operating piston head 134 to move upwards and so open the EBD valve. The valve arrangement 100 further comprises a fluid isolator 160, which is connected to the lower piston channel 143 such that the fluid isolator 160 is usable to control the flow of fluid into and out of the lower piston chamber 142. However, in other examples, the fluid isolator 160 may instead be connected to the upper piston channel 145. The fluid isolator 160 in this example is a 1 -to-1 solenoid valve, such that the fluid isolator 160 has two channels / ports which feed fluid therethrough. A first of these channels is connected to the vent channel 112, and the second is connected to the lower piston channel 143. Further, the fluid isolator 160 has two states, corresponding to a first, open, state in which fluid is permitted to flow between the vent channel 112 and the lower piston channel 143, and a second, closed, state in which fluid flow between these two channels is prohibited. The former state is represented by the fluid isolator 160 in Figs. 1A, B and D, whilst the latter is represented in Fig. 1C. Through selective switching of the fluid isolator 160 between the open and closed states, the emergency blowdown valve 120 can be locked in an open or closed state. This is now described below in more detail for the valve arrangement 100 with respect to Figs. 1A-D, which show the valve arrangement 100 in different respective states throughout the opening and closing of the emergency blowdown valve 120. In this example, the media pressure in the inlet channel 122 is applied to the upper piston channel 145, and thus to the upper piston chamber 144. As such, the downward pressure applied to the operating piston head 134 is equal to the upward pressure being applied to the sealing piston head 132 by the media at the gap 126 on the inlet channel 122 side. However, the larger size of the operating piston head 134 means that the total downward force experienced by the piston 130 is greater than the total upward force, ensuring that the emergency blowdown valve 120 remains in the closed state shown in Fig. 1A. As is discussed later, the media entering the upper piston channel 145 may be media from the upstream portion of the valve (e.g., from 122) but may also be from a different source. In this instance the pressure of the media in the inlet channel 122 may not be equal to the pressure of the media in the upper piston chamber 144. In Fig. 1B, the emergency blowdown valve 120 is shown as having subsequently opened, which occurs (as discussed above) when the pilot valve assembly connected to the upper piston channel 145 detects that the media pressure within the inlet channel 122 has fallen outside of the predetermined working pressure range, and is thus actuated to open so as to depressurise the upper piston chamber 144 (for example, by connecting the upper piston chamber 144 to a vent, not shown). This causes a net upward force on the piston 130 as the fluid pressure on the sealing piston head 142 is now greater than any fluid pressure on the operating piston head 134, causing the piston 130 to rise within the piston chamber 140 as shown in Fig. 1B. A media load may also be applied to the lower piston channel 143. For example, by connecting said lower channel 143 to the pilot valve assembly in such a way that the media in the inlet channel 122 is connected to the lower piston chamber 142 at the same time as the upper piston chamber 144 is depressurising. This can advantageously speed up the movement of the piston 130 between the closed position shown in Fig. 1A, and the open position shown in Fig. 1B. However, this is not essential, and the lower piston chamber 142 may instead be allowed to maintain its previous pressure (which is intermediate to the inlet channel pressure and the now vented upper piston chamber pressure) as the operating piston head 134 moves upward to the Fig. 1B position. Alternatively, media can be allowed to leak between the piston stem 136 and into the lower piston chamber 142. This would build up a pressure there, such that the open state in Fig 1B is achieved. Once the emergency blowdown valve 120 is open, any media which leaks into the lower piston chamber 142 (e.g., through the sealing, not shown, around the piston stem 136) would vent through the lower channel 143. Subsequently, the media pressure within the inlet channel 122 may fall back within the working pressure range, causing the pilot valve assembly to reconnect the upper piston channel 145 to the inlet channel 122 such that the upper piston chamber 144 will begin to re-pressurise. Absent operation of the fluid isolator 160, this will also mean that the lower piston channel 143 is re-connected to the vent 112 (if not left connected throughout the process), causing the lower piston chamber 142 to de-pressurise. The pressure differential between the upper and lower piston chambers 142, 144 would cause a downward force on the operating piston head, causing the emergency blowdown valve 120 to close again (as shown in Fig. 1A). In some scenarios envisaged by the present inventors, this is undesirable. For example, in the event of a LOCA, it may be desired to lock the EBD open to ensure that the full inventory of coolant is replenished rather than merely enough to bring the fluid pressure back into the lower end of the working range. As such, following the transition of the emergency blowdown valve 120 to the open state, the fluid isolator 160 is actuated from the open state shown in Figs. 1A-B, to the closed state, as shown in Fig. 1C. When the fluid isolator 160 is in the closed state, the lower piston chamber 142 is fluidically isolated from external channels, including the vent channel 112, and so maintains the pressure it had at the time the fluid isolator 160 was initially closed. The lower piston chamber 142 therefore does not become depressurised, even when the media pressure within the inlet channel 122 re-enters the working range, causing the upper piston chamber 144 to repressurise and push down on the operating piston head 134, as the fluid in the lower piston chamber 142 cannot vent and is typically substantially incompressible. Further, leakage from the upper piston chamber 144 into the lower piston chamber 142 via the operating piston head 134 and into the lower piston chamber 142 through the sealing around the piston stem 136 would further increase the pressure within the lower piston chamber 142, bringing it into conformity with the pressure in the upper piston chamber 144. Therefore, the upward and downward forces on the operating piston head 134 will match, and the emergency blowdown valve 120 will remain in the open state so long as the fluid isolator 160 remains in the closed state. In this way, the emergency blowdown valve 120 may be latched / unlatched to the open state using just the fluid isolator 160, without the need for any mechanical latching mechanism. Whilst the above shows the fluid isolator 160 being used to lock the EBD valve 120 in an open position, it is clear that the same mechanism can be used to lock the EBD valve 120 in the closed position. Rather than connecting the fluid isolator 160 to the lower piston chamber 142, it can instead be connected to the upper piston chamber 144 such that the upper piston chamber 144 is isolated from the pilot valve assembly (not shown). Only when the fluid isolator 160 is open can the upper piston chamber 144 be vented, and so the EBD valve 120 opened. Figs. 2A-C show a valve arrangement 200 according to a second example of the present invention. As in Fig. 1, the high / low density dotting patterns represents regions of high / low coolant / fluid pressure respectively. The valve arrangement 200 is similar to the valve arrangement 100 of Figs. 1A-D, and like features are given like reference numerals incremented by a hundred. Similarly to the valve arrangement 100, the valve arrangement 200 comprises an emergency blowdown valve 220 having a valve chamber (including a inlet channel 222, outlet channel 224, and a channel 226 connecting the two), a piston 230 (including a sealing piston head 232, operating piston head 234 and piston stem 236) and a piston chamber 240 (divided into a lower piston chamber 242 and an upper piston chamber 244 by the operating piston head 234 that moves therein). The valve arrangement 200 also again comprises a fluid isolator 260, which is a solenoid valve that controls fluid flow into the piston chamber 240. However, in the valve arrangement 200, the emergency blowdown valve 220 is reverse seated, rather than direct seated as was the emergency blowdown valve 120. In particular, in the valve arrangement 200, the sealing piston head is pulled upward when the EBD valve 220 is closed, rather than being pushed downward, into the channel 226, thereby prohibiting fluid flow between the inlet channel 222 and the outlet channel 224. Additionally, the fluid isolator 260 270 of the second example also differs from the fluid isolator 160 of the first example, both in location and composition. The fluid isolator 260, 270 is connected to the upper piston channel 245. As such, the fluid isolator 260, 270 is usable to fluidically isolate the upper piston chamber 244 from at least the pilot valve assembly, as described in more detail below. The fluid isolator 260 270 includes a 2-to-1 solenoid valve 260, rather than a 1 -to-1 solenoid valve as was the fluid isolator 160, and also includes a non-return valve 270. The 2-to-1 fluid isolator 260 has two states. In a first, shown in Figs. 2A and 2B, fluid is permitted to flow through the non-return valve 270 and solenoid valve 260 to the upper piston chamber 244 via the upper piston channel 245. As discussed previously, when the pressure of the media upstream of the EBD valve 220 is within the operating range, the pilot valve assembly does not connect the upper piston chamber 244 to the inlet channel 222 and so the valve is pressed closed by the fluid pressure acting against the sealing piston head 232. Next, when the pressure deviates from the working range, the pilot valve assembly connects the inlet channel 222 to the upper piston chamber 244 which causes it to pressurise. The operating piston head 234 has a larger surface area than the sealing piston head 232, and so whilst exposed to media of the same pressure, the net force pushes the piston 230 down and so opens the EBD valve 220. The lower piston chamber 242 can vent through lower piston channel 243 (may be, via the pilot valve assembly, selectively connected to a vent). The fluid isolator 260, 270 causes the EBD valve 220 to lock in this open configuration because the non-return valve prohibits fluid from leaving the upper piston chamber 244 via the upper piston channel 245. This arrangement is shown in Fig. 2B, where the solenoid valve 260 connects the non-return valve 270 (which is connected to the pilot valve assembly as discussed) to the upper piston chamber 244. When the EBD valve 220 is to be closed, the solenoid valve 260 moves to a second position in which the upper piston chamber 244 is no longer connected to the non-return valve 270. This is shown in Fig. 2C, where the upper piston chamber 244 is connected to a vent 212. This allows the upper piston chamber 244 to vent, and so the EBD valve 220 will return to the configuration shown in Fig. 2A whereby the fluid pressure upstream of the EBD valve 220 pushes the piston sealing head 232 into its valve seat. A further valve arrangement 300 is shown in Figs. 3A-C. It shares a number of features with the valve arrangement 200 shown in Figs. 2A - 2C, and so like features are indicated by like reference numerals. As in Fig. 1, the high / low density dotting patterns in Figs. 3A-C 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. In Fig. 3A, the emergency blowdown valve 220 is shown in a closed state, in which fluid flow between the inlet channel 222 and outlet channel 224 through the channel 226 is blocked. In particular, there is no media pressure at either the lower or upper sub-chambers 242 244, such that the piston 230 is pushed upward to the closed position purely by the media pressure applied to sealing piston head 232. The valve arrangement 300 differs from that shown in Figs. 2A - 2C in that the fluid isolator further includes (in addition to the non-return valve 270 and solenoid valve 260) a feed line selector (e.g., a shuttle valve) 382. The feed line selector 382 is connected, on one side, to the non-return valve 270, and on another side, to a feed line 380. The feed line 380 connects a downstream portion of the EBD valve 220 (i.e., after the EBD valve, and connected to the outlet channel 224) to the feed line selector 382. When the EBD valve 220 is closed, the feed line 380, and so upper piston chamber 244, is at a lower pressure than the upstream portion of the EBD valve 220 (i.e. the inlet channel 222). As before, in this configuration, the lower piston chamber 242 and lower piston channel 243 are also at a lower pressure. This scenario is changed in Fig. 3B, where the pilot valve assembly has been tripped due to the media pressure within the inlet channel 222 deviating outside of a working range. This causes, via the pilot valve assembly (not shown), high pressure media in the inlet channel 222 to be connected to the upper piston chamber 244, pressurising it. As in the valve arrangement 200, the operating piston head 234 is larger than the sealing piston head 232, such that the equal pressures experienced by the two piston heads 232, 234 results in a net downward force which pushes the sealing piston head 232 away from the channel 226, thus moving the emergency blowdown valve 220 to an open state in which fluid flow is permitted between the inlet channel 222 and the outlet channel 224. Any leaked media from upper piston chamber 244 to lower piston chamber 242 into the lower piston chamber 242 via the sealing around the piston stem 236 would vent through the lower piston channel 243. In some examples, the lower piston channel 243 may be closed by the operating piston head 234 moving fully downward. Therefore, any leaked media from the upper piston chamber 244 would either vent via the lower piston channel 243, or balances off with the media leaked through via the piston stem 236. Further, as before, the non-return valve 270 serves to prevent high pressure media which has passed from the inlet channel 222 to the upper piston chamber 244 from returning back out of the upper piston chamber 244 and toward the pilot valve assembly. Therefore, even if the pilot valve assembly closes and its vent (not shown) opens, for example because the media pressure in the inlet channel 222 returns to the predetermined working pressure range, the high pressure media from the upper piston chamber 244 cannot vent and will instead be trapped within the upper piston chamber 244, meaning that the emergency blowdown valve 220 will remain open. In other words, by keeping the fluid isolators 260, 270, 382 in the first state, the emergency blowdown valve 220 will be latched open as a result of pressure locking. In some examples, the non-return valve can be omitted. For example, the shuttle valve 382 may function as a non-return valve. As before, the emergency blowdown valve 220 may be unlatched by moving the fluid isolator 260 to the second state in which it connects the upper sub-chamber 244 to the vent channel 212, as shown in Fig. 2C. The upper sub-chamber 244 will then de-pressurise, causing a net upward force on the piston 230 (caused by an upward force exerted on the sealing piston head 232 exceeding a now reduced downward force exerted on the operating piston head 234) that will move the emergency blowdown valve 220 back to a closed state. A further addition to the valve arrangement 300 is the feed line 380, which connects the outlet channel 224 to the upper piston chamber 244 via the feed line selector 382 and the solenoid valve 260. The feed line selector 382 selectively connects the solenoid valve 260 to either the non-return valve 270 or the feed line 380. The feed line selector 382 is a three-way shuttle valve, and thus has three ports. A first port which connects to the feed line 380 (and so the outlet channel 324), a second port which connects to the non-return valve 370 and (onwards to) the pilot valve assembly, and a third port which connects to the solenoid valve 260 and so upper piston chamber 344.The feed line selector 382 is movable between two states, a first state in which the feed line 380 is connected to the solenoid valve 260, and a second state in which the pilot valve assembly / non-return valve 270 is connected to the flow solenoid valve 260. In both states, the feed line 380 is blocked from the pilot valve assembly / non-return valve 270, so as to prevent fluid passing from the pilot valve assembly directly into the outlet channel 324 (or vice versa). In the present example, the state of the flow selector is determined by a pressure differential between the first and second ports of the feed line selector 382. In particular, a greater pressure in the first port causes the feed line selector 382 to move to the first state in which the feed line 380 is connected to the solenoid valve 260, and a greater pressure in the second port causes the feed line selector 382 to move to the second state in which the pilot valve assembly / non-return valve 270 is connected to the solenoid valve 260. The EBD valve 220 opens, when the media pressure within the inlet channel 222 deviates from the predetermined working pressure range. The pilot valve assembly senses this, and connects the inlet channel 222 to the non-return valve 270. Media flows through the non-return valve 270, feed line selector 382, and the solenoid valve 260 into the upper piston chamber 244, pressurising it and causing the emergency blowdown valve to move to the open state shown in Fig. 3B. Turing to the feed line selector 382 itself, the high pressure at its second port (the one connecting to the non-return valve 270) due to the higher pressure fluid now passing through the outlet channel 224 will cause feed line flow selector 382 to connect the second port to the third port, which connects the feed line 380 to the solenoid valve 260 and so the upper chamber 244. The feed line 380 thereby supplies the upper piston chamber 244, with media. The inclusion of the feed line selector 382 and feed line 380 thus provides the benefit of allowing the upper piston chamber 244 to remain pressurised even in cases where the operating piston head 234 has not moved to a position in which it blocks the lower piston channel 243 (e.g., corresponding to the piston 330 being in a fully open position). In these cases, the lower piston channel 243 remains accessible, and so any fluid leakage from the upper piston chamber 244 to the lower piston chamber 242 would subsequently be vented. This has the potential to become a particular problem when the media pressure within the inlet channel 222 returns to the predetermined working pressure range, as at this time the pilot valve assembly will cut off the connection (not shown) between the inlet channel 222 and the upper piston chamber 244. As such, the media lost from the upper piston chamber 244 via leakage and venting through the lower piston chamber 242 would not be replaced, meaning that the upper piston chamber 244 will decrease in pressure, and the emergency blowdown valve 220 would return to the closed state. However, as discussed, the presence of the feed line 380 / feed line selector 382 prevents this, as even after the media supply via the pilot valve assembly is cut off, media may still be supplied to the upper piston chamber 244 via the feed line 380. As such, the latching effect, provided by the fluid isolator 260, 270, and 382, by which the emergency blowdown valve 220 is maintained in the open state even after the inlet channel 222 media pressure re-enters the predetermined working pressure range, may be more securely provided with the inclusion of the feed line 380. The latching of the emergency blowdown valve 220 may once again be ended by controlling the solenoid valve 260 to move to the second state shown in Fig. 3C, in which the upper piston chamber 244 is connected to a vent channel 212. This will cause the upper piston chamber 244 to de-pressurise, and the emergency blowdown valve 220 to move to the closed state. Fig. 4 shows the valve arrangement 300 together with an example pilot valve assembly 490. As in Fig. 1, the high / low density dotting patterns in Fig. 4 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 pilot valve assembly 490 in this example includes two pilot valves 492 and 494, as well as a pilot valve flow selector 496. However, it will be understood that the pilot valve assembly might include only one, for example the high-pressure pilot valve. The two pilot valves 492, 494 are a low-pressure pilot valve (LPP) valve 492 and a high-pressure pilot (HPP) valve 494. Each is connected to the inlet channel 222 by a pilot communication line 498. Each pilot valve 492, 494 is operable in at least two states. A first state in which the pilot valve permits flow through the valve (e.g., from the pilot communication line 498 to the pilot valve flow selector 496), and a second state in which the pilot valve blocks flow therethrough. The LPP valve 494 is configured to move to the first state when the media pressure in the inlet channel 222 fall below a first predetermined pressure, and move to the second state when the media pressure in the inlet channel 222 rises above the first predetermined pressure. Meanwhile, the HPP valve 494 is configured to move to the first state when the media pressure in the inlet channel 222 rises above a second predetermined pressure (which is greater than the first predetermined pressure), and move to the second state when the media pressure in the inlet channel 222 falls below the second predetermined pressure. In this way, media from the inlet channel 222 is allowed to flow to the pilot valve flow selector 496 whenever the media pressure within the inlet channel 222 falls outside the predetermined working pressure range defined between the first predetermined media pressure and the second predetermined media pressure. Regarding the pilot valve flow selector 496, this works in a similar manner to the feed line selector 382. In particular, the pilot valve flow selector 496 has three ports. A first port connected to the HPP valve 494, a second port connected to the LPP valve 492, and a third port connected to the non-return valve 270. Further, the pilot valve flow selector 496 is movable between a first state in which the first and third ports are fluidly connected, and a second state in which the second and third ports are fluidly connected, with the state being determine by whether the media pressure is greater at the first port (in which case the pilot valve flow selector 496 moves to the first state) or the second port (in which case the pilot valve flow selector 496 moves to the second state). It is noted that whilst the valve arrangement 300 does include a non-return valve 270, this is not essential to the working of the example, as media fluid in the upper piston chamber 244 will be blocked from passing through the pilot valve assembly 490 by the LPP valve 492 and / or HPP valve 494 when the media pressure within the inlet channel 222 is within the pre-determined working pressure range, since both valves will be closed. Further, the pilot valve assembly 490 shown in Fig. 4 is applicable to the valve arrangements 100 and 200. There, the pilot valve flow selector 496 connects, for example, to the upper piston channel 145 in Figs. 1A - 1D, or to the non-return valve 270 in Figs. 2A - 2C. That is, the pilot valve assembly 490 shown in Fig. 4 is applicable to both direct and reverse seated valves. Figs. 5A-B show closed (Fig. 5A) and open (Fig. 5B) states of a reverse seated emergency blowdown valve 520 which may form part of a valve arrangement of any of the second or third examples. The emergency blowdown valve 520 is similar to the reverse seated emergency blowdown valves 220 or 320 shown in the second and third examples, except that the lower piston channel 543 feeds into the lower piston chamber 542 from a surface of the piston chamber which faces the operating piston 534 (and so in this example, from a bottom surface), rather than from a side face. In this way, pressure may be applied from the lower piston channel 543 to lift the operating piston head 534 even when the operating piston head is abutted against the bottom face of the piston chamber 540, as shown in Fig. 5B. Fig. 6 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 valve arrangements 100, 200, or 300 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 fluid isolators discussed above, for example moving the fluid isolator 160 (e.g. solenoid valve) and / or 260 between their respective positions. Whilst shown in Fig. 6 as connected above the pressurizer 612, a valve arrangement 614 may be connected directly to the coolant circuit 608 and / or below the pressurizer 612. The containment unit 616 provides 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 6161 in the event of a loss of coolant accident (LOCA). Here, as the containment unit 616 provides both functions, the pressure release valve assembly 614 also must 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. *** 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%.

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