Valve arrangement and nuclear reactor assembly

Abstract

A valve arrangement 100 for a nuclear reactor, the valve arrangement 100 comprising: a direct or reverse seated emergency blowdown valve 20, controllable, by movement of a piston 30 of the emergency b

Description

Field of the Invention The present invention relates to a valve arrangement for a nuclear reactor and a nuclear reactor assembly. 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 that takes heat energy from the nuclear core and transports it to one or more turbines, where the heat energy is converted into electrical energy. More particularly, interaction of the coolant (e.g., a coolant fluid) with the nuclear core will cause the coolant to absorb heat energy generated by the nuclear reactions occurring in the nuclear core. As a result, the coolant will heat up and expand, in which state it can be used to drive one or more turbines to generate electrical energy. This will cause the coolant to cool down again, at which point it can be recirculated to the nuclear core to repeat the process. In some cases, the coolant might become too hot, causing a pressure level within the coolant system to rise above a preset maximum pressure threshold. If not dealt with, this can cause ruptures / explosions in the coolant system. On the other hand, sometimes the coolant system pressure might drop below a recommended threshold level (for example, because of a leak somewhere in the system), in which case there will be an insufficient quantity of coolant circulating within the coolant system and past the nuclear core. Such an event can be referred to a loss of coolant accident (LOCA). The present invention has been devised in light of the above considerations. Summary of the Invention Accordingly, in a first aspect, embodiments of the invention provide a valve arrangement for a nuclear reactor, the valve arrangement comprising: a direct seated emergency blowdown valve, controllable, by movement of a piston of the emergency blowdown valve in a piston chamber, to open and close thereby permitting or prohibiting a flow of coolant therethrough; and a pilot valve assembly, configured to control the emergency blowdown valve to open and close, the pilot valve assembly comprising: a low-pressure pilot valve, configured to operate when a fluid pressure on an upstream side of the emergency blowdown valve drops below a first predetermined pressure; a high-pressure pilot valve, configured to operate when the fluid pressure on the upstream side of the emergency blowdown valve exceeds a second predetermined pressure; and a controller valve, configured to connect the piston chamber of the emergency blowdown valve to a vent when either the low-pressure pilot valve or high-pressure pilot valve is operated thereby opening the emergency blowdown valve. The provision of the controller valve within the pilot valve assembly permits the emergency blowdown valve to be accurately, simply and automatically controlled to close whenever the pressure of the coolant flow upstream of the emergency blowdown valve falls outside of a predetermined range (e.g., the range defined between the first predetermined pressure and the second predetermined pressure), ensuring increased system safety and reliability. The controller valve also allows the same type of pilot valve to be used for both the low-pressure pilot valve and high-pressure pilot valve, albeit with a different set point for the high-pressure pilot valve (by varying the force by which the valve is held closed). The use of the term ‘operate’ in relation to the pilot valves is taken to mean the pilot valves causing the emergency blowdown valve to move to an open position. For example, the low-pressure pilot (LPP) valve is configured to operate when a fluid pressure on an upstream side of the emergency blowdown valve drops below a first predetermined pressure refers to the LPP valve so as to cause the emergency blowdown valve to open. The same is true for the high-pressure pilot (HPP) valve, being configured to operate when the fluid pressure on the upstream side of the emergency blowdown valve exceeds a second predetermined pressure. In the following, the use of the term ‘activation’ or ‘actuation’ in relation to the pilot or emergency blowdown valves refers to the movement of the respective valve between a first and a second state. For example, the actuation of the emergency blowdown valve may refer to the piston of the emergency blowdown valve moving from an open state, in which the flow of coolant is permitted, to a closed state, in which the flow of coolant is prohibited (or vice versa). The piston of the emergency blowdown valve includes a sealing piston head which is movable to permit / prohibit the flow of coolant therethrough, an operating piston head which moves in the piston chamber, and a stem connecting the operating piston head to the sealing piston head. The operating piston head divides the piston chamber into a first, upper, chamber on a first side of the operating piston head, and a second, lower, chamber on an opposite side of the piston head to the first chamber. The upper chamber is located such that the operating piston head is between the upper chamber and the sealing piston. The lower chamber is located between the operating piston head and the operating piston head. The pilot valve assembly may be connected to at least the upper chamber. A ‘direct’ seated emergency blowdown valve is understood to mean a valve in which the piston is in a ‘closed’ state (e.g., a state in which flow through the valve is prohibited) where the sealing head of the piston is pushed against a sealing surface when the piston is in an extended state. In the present case, a higher pressure in the upper piston chamber than the lower piston chamber causes the operating piston head to move towards the lower piston chamber (reducing its volume). Thus, the sealing piston head of the piston of the emergency blowdown valve moves away from the piston chamber and toward its main seat in which the piston prohibits the flow of coolant. On the other hand, a decrease of pressure in the upper piston chamber (for example, when the piston chamber is connected to the vent) or an increase in the pressure of the lower piston chamber (relative to the upper piston chamber) causes the piston to be actuated toward an open position, in which the sealing piston head of the piston is retracted toward the piston chamber and the flow of coolant is permitted. The lower piston chamber may include one or more leakage channels, which allow it to be filled with liquid from upstream of the emergency blowdown valve. The second predetermined pressure is higher than the first predetermined pressure. When the emergency blowdown valve is open, the coolant flows through a coolant chamber of the emergency blowdown valve. The coolant chamber comprises a first, upstream, part connected to an inlet channel of the valve, and a second, downstream, part connected to an outlet channel of the valve. Pressure variations in the piston chamber cause movement of the piston in the coolant chamber. Higher pressures in the piston chamber cause the piston to block a pathway through the coolant chamber (e.g., the emergency blowdown valve is moved to a closed state), thereby inhibiting the flow of coolant from the inlet to the outlet. Similarly, lower pressures in the piston chamber (or pressures higher than the second predetermined pressure) cause the piston to unblock the pathway through the coolant chamber (e.g., the emergency blowdown valve is moved to an open state), thereby permitting the flow of coolant from the inlet to the outlet. In some examples, the upstream side of the emergency blowdown valve is connected to the piston chamber when the fluid pressure on the upstream side of the emergency blowdown valve is above the first predetermined pressure. Connecting the piston chamber to the upstream side creates an equalisation of pressure between the upstream flow of coolant and the piston chamber, and more generally increases the pressure in the piston chamber, in which case the emergency blowdown valve piston is actuated to the closed position in which the flow of coolant is prohibited. At the same time, the piston chamber may be blocked off from the vent when the upstream coolant pressure is above the first predetermined pressure, so as not to again become de-pressurised. This provides a simple and accurate way of closing the emergency blowdown valve at the correct pressure. On the other hand, when the pressure of the upstream coolant is below the first predetermined pressure, the upstream side isolated (e.g., by the low-pressure LPP in a first state) from the piston chamber. In this way, the piston chamber can be de-pressurised and maintained at a lower pressure (e.g., by connecting the piston chamber to a vent), so as to actuate the emergency blowdown valve piston to the open state in which the flow of coolant is permitted, without causing any de-pressurisation of the coolant. In some examples, the flow of coolant on the upstream side of the emergency blowdown valve is connected to the vent when the fluid pressure on the upstream side of the emergency blowdown valve is above the second predetermined pressure. In such examples however, the main venting is still through the emergency blowdown valve. For example, an opening between the inlet and outlet channels of the emergency blowdown valve may be larger than a passageway through one or more of the LPP valve, the controller valve and the HPP valve, such that a greater volume of coolant is movable through from the inlet channel of the emergency blowdown valve to the outlet channel of the emergency blowdown valve compared to from the inlet channel of the emergency blowdown valve to the vent. This advantageously ensures that a pressure differential is maintained between the coolant chamber and the piston chamber (e.g., wherein the piston chamber has a lower pressure than the coolant chamber), allowing the emergency blowdown valve to remain in an open state. The flow of coolant may be connected to the vent via a pathway that extends through each of the LPP valve, the HPP valve and the controller valve. For example, the coolant may first pass through the LPP valve, proceed to pass through the controller valve, and then pass through the HPP valve before arriving at the vent. This allows multiple fail-safes to be placed in between the flow of coolant and the vent to prevent coolant being vented at the wrong times / pressures. However, the disclosure is not limited in this way, and the path between the coolant and the vent may pass through each of the pilot valves in the pilot valve assembly in a different order, or may only pass through a subsection of the pilot valves. For example, the coolant may pass through only the HPP valve and the coolant valve on its way to the vent. In this way, the pathway for venting the coolant may be advantageously simplified. Moreover, there may be multiple pathways connecting the coolant to the vent existing at the same time, which may allow for quicker coolant venting. Below the second predetermined pressure, the coolant is inhibited / blocked from accessing the vent. The coolant may be blocked from the vent by different parts of the pilot assembly at different pressures. For example, below the first predetermined pressure, the coolant may be blocked from the vent by the LPP valve and / or the HPP valve, whilst between the first and second predetermined pressures, the coolant may be blocked by one or more of the LPP valve and the controller valve. This allows the coolant to pass into different compartments of the pilot assembly at different pressure points (e.g., below and above the first predetermined pressure) whilst maintaining a barrier between the coolant and the vent. However, the present disclosure is not limited in this way, and the coolant may instead be consistently blocked at the same point or points in the pilot assembly at all pressures below the second predetermined pressure. In some examples, the piston chamber, and specifically the upper piston chamber, is connected to the vent either: via the low-pressure pilot valve when the pressure of the coolant on the upstream side of the emergency blowdown valve is below the first predetermined pressure; or via at least the high-pressure pilot valve when the pressure of the coolant on the upstream side of the emergency blowdown valve exceeds the second predetermined pressure. More particularly, the connection of the piston chamber to the vent via either the LPP valve or the HPP valve may be permitted when particular pressure conditions of the coolant upstream of the emergency blowdown valve have caused the respective valves to be actuated to a state in which a previously inaccessible path between the piston chamber and vent has been unblocked. When the pressure of the upstream side of the emergency blowdown valve exceeds the second predetermined pressure, the connecting path between the piston chamber, and specifically the upper piston chamber, and the vent may also be through the controller. Between the first and second pressures, all connections between the piston chamber and the vent are blocked by one or more of the pilot valves in the pilot valve assembly, thereby preventing depressurisation of the piston chamber and opening of the emergency blowdown valve. In some examples, a rest state of the emergency blowdown valve is where the flow of coolant past the emergency blowdown valve piston is prohibited. This advantageously prevents undue changes to the coolant in the coolant system, so as to increase coolant stability. The rest state is the state in which the emergency blowdown valve will reside in the absence of any external forces (e.g., when the pressure in the upper and lower piston chambers is the same). The piston may be held in a rest position (e.g., a position of the piston which corresponds to the emergency blowdown valve being in the rest state) by its own weight. Alternatively, the piston may be biased toward the rest position by a biasing means. The biasing means may be a spring-loaded bias. For example, the biasing means may be a compressed spring. However, the present disclosure is not limited in this way, and the skilled person, equipped with the present disclosure, will recognise other suitable biasing means. One or more of the pilot valves in the pilot valve assembly (e.g., the LPP valve, the HPP valve and the controller valve) may comprise at least three channels. These channels may include an inlet channel, a valve channel and a vent channel. In some examples, the LPP valve and the HPP valve may be the same valve type (e.g., a valve having the same ports, configuration and operating conditions). Albeit the HPP valve will typically have a higher setpoint (i.e., a higher pressure at which it is opened relative to the LPP valve). In such cases, different pilot valves may utilise different ports of the common valve type. In some examples, each of the pilot valves comprise at least an inlet, a valve channel and a vent channel. An inlet channel is understood to be a port via which the valve can be controlled / actuated between states (e.g., between a first, rest, state, and a second, actuated, state). For example, via the application of pressure to the inlet channel. In some examples, the LPP and HPP valves may be actuated (e.g., to the second state) when a pressure at the inlet exceeds either the first or second predetermined pressure valves. The inlet channel may be connected (e.g., directly or indirectly) to the upstream side of the emergency blowdown valve. In this way, one or more of the LPP, HPP and controller valves directly sense, and respond to (via actuation), pressure changes in the upstream coolant. In one example, the upstream side of the emergency blowdown valve is directly connected to the inlets of the LPP and HPP valves. A direct connection being one where there is no intermediate valve between the piston chamber and the inlet channel. In other words, the valve channel is always fluidly connected to the piston chamber. The coolant flow on the upstream side of the emergency blowdown valve may further be indirectly connected to the controller. Indirectly connected meaning where there is one or more elements, such as, e.g., another valve, between the valve in question and the piston coolant, such that the valve channels connection to the coolant may at times be blocked. In the above example, the controller inlet channel may be indirectly connected to the coolant via the HPP valve. In one example, the valve channels of the LPP and controller valves are directly connected to the piston chamber. Meanwhile, a piston chamber channel of the HPP valve is indirectly connected to the piston chamber through the controller valve. A vent channel is understood to be a port which is between the valve in question and the vent (e.g., an outlet connected to ambient pressure, which allows the free flow of coolant therethrough). In other words, the vent channel connects the valve in question to the vent. Once again, the vent channel may directly or indirectly connect the valve in question to the vent. For example, a first pilot valve vent channel may be connected to a valve channel of a second pilot valve, which may be connectable to a vent channel of the second pilot valve. This vent channel of the second pilot valve may itself be directly connected to the vent. In one example, the vent channels of the LPP and HPP valves are directly connected to the vent. Meanwhile, the vent channel of the controller valve is indirectly connected to the vent through the HPP valve. In some examples, one or more (or each) of the pilot valves in the pilot valve assembly (e.g., the LPP valve, the HPP valve and the controller valve) comprise at least an inlet channel, a vent channel and a valve channel as described. The valve channel is connectable to at least one of the inlet channel and the vent channel, wherein said connection is dependent on whether the valve in question is in a first state or a second state. In some cases, the valve channel is connectable only to the inlet channel. For example, in the first state the valve channel may be blocked from the inlet channel, and in the second state the valve channel may be connected to the inlet channel. In other cases, the valve channel may be connectable to only the vent channel. For example, in a first state of the valve in question the valve channel may be connected to the vent channel, and in a second state of the valve in question the valve channel may be blocked from the vent channel. In still further cases, the valve channel may be connectable to both the inlet channel and the vent channel. For example, in the first state the valve channel may be connected to the vent channel and blocked from the inlet channel, and in the second state the valve channel may be connected to the inlet channel and blocked from the vent channel. Each or a subset of the valves encompassed within the present disclosure (for example, different pilot valves of the pilot valve assembly) may each comprise the same version of a valve channel. Alternatively, the different respective valves (for example, each of the pilot valves in the pilot valve assembly) may comprise a different respective type of valve channel. The LPP and controller valves may each have just an inlet, a valve channel and a vent channel (i.e., no further channels). The LPP valve may be movable between a first (e.g., rest) state and a second (e.g., actuated) state. In the first state, the LPP inlet is blocked from the valve channel of the LPP valve and the LPP vent channel, and the valve channel of the LPP valve is connected to the LPP vent channel. The LPP valve may adopt the first state when the fluid pressure upstream of the main valve is below the first predetermined pressure. In the second state, the LPP vent channel is blocked from the LPP inlet and valve channel of the LPP valve, and the LPP inlet is connected to the valve channel of the LPP valve. The LPP valve may adopt the second state when the fluid pressure upstream of the main valve is above the first predetermined pressure. The controller valve may also be movable between a first (e.g., rest) state and a second (e.g., actuated) state. In the first state, the valve channel of the controller valve may be blocked from the controller vent channel, whilst in the second state the valve channel of the controller valve may be connected to the controller vent channel. The controller valve may adopt the first state when the fluid pressure upstream of the main valve is above the first predetermined pressure, and may adopt the second state when the fluid pressure upstream of the main valve is above the second predetermined pressure. The controller inlet, that is, the port connected or connectable to the upstream side of the main valve, may be blocked from the valve channel and / or vent channel in both the first state and the second state. But the present disclosure is not limited in this way, and the controller inlet may instead be connected to the valve channel and / or vent channel in the second state. In some examples, the HPP valve comprises an inlet, a piston chamber channel, a valve channel and a vent channel. In these cases, the HPP inlet is connectable to the HPP valve channel, and the HPP piston chamber channel is connectable to the HPP vent channel. The HPP inlet and HPP valve channel may be completely blocked from the HPP piston chamber channel and the HPP vent channel. The present disclosure is not limited in this way however. The HPP valve may be movable between a first (e.g., rest) state and a second (e.g., actuated) state. In the first state the HPP inlet may be blocked from the HPP valve channel, whilst in the second state the HPP inlet may be connected to the HPP valve channel. The HPP piston chamber channel is connected the HPP vent channel at least in the second state of the HPP valve, and may be connected to the HPP vent channel in both the first and second states. One or more of the pilot valves may be direct seated valves, which can simplify the pilot valve assembly. However, the present disclosure is not limited in this way, and the pilot valves may instead be reverse seated valves. One or more of the pilot valves may be piston valves. In some examples, each of the pilot valves are piston valves. In some examples, one or more of the pilot valves are biased to a rest position. The biasing may be achieved by the weight of the valve (for example, where the pilot valve is a piston valve, this may be the weight of the piston), or may be via a biasing means. For example, the one or more pilot valves may be biased to the rest position by a spring-loaded biasing means, such as a compressed spring. The present disclosure is not limited in this way, however. In some examples, a rest position of the pilot valves is one in which the coolant is blocked from the vent. In some examples, the controller valve is connected to the HPP valve. In other words, the actuation of the controller valve is linked to the actuation of the HPP valve. This may mean that, for example, the controller valve is actuated at the second predetermined pressure threshold. The controller valve may be separate entity to the HPP valve. For example, a vent channel of the controller valve (e.g., directly) connects to the piston chamber channel of the HPP valve, and the valve channel of the HPP valve connects to the inlet channel of the controller. In this way, the controller is positioned to be between the piston chamber and the piston chamber channel of the HPP valve, whilst the HPP valve is positioned between the coolant on the upstream side of the emergency blowdown valve and the inlet of the controller valve. In some examples, a vent channel of the controller valve is connected to a vent channel of the HPP valve, and the vent channel of the HPP valve is connected directly to the vent However, the present disclosure is not limited in this way. For example, the controller valve may instead be integrated into the HPP valve. For example, the controller valve may be positioned so as to prohibit / permit a flow from the piston chamber channel of the HPP valve to the vent channel of the HPP valve, and be directly controlled by movement of the HPP valve between the open and closed state. In other words, the HPP valve may control flow from the HPP inlet to the HPP valve channel, whilst the control valve controls flow from the HPP piston chamber channel to the HPP vent channel. This may advantageously simplify the design of the pilot valve arrangement. Moreover, the controller valve may instead be connected (e.g., as a separate entity to or an integrated part of) the LPP valve. In some examples, a valve channel of the controller valve is connected to a valve channel of the low-pressure pilot valve. In some examples, wherein a vent channel of the low-pressure pilot valve connects directly to the vent In a second aspect of the present invention, there is provided a valve arrangement for a nuclear reactor, the valve arrangement comprising: a reverse seated emergency blowdown valve, controllable, by movement of a piston of the emergency blowdown valve in a piston chamber, to open and close thereby permitting or prohibiting a flow of coolant therethrough; and a pilot valve assembly, configured to control the emergency blowdown valve to open and close, the pilot valve assembly comprising: a low-pressure pilot valve, configured to operate when a fluid pressure on an upstream side of the emergency blowdown valve drops below a first predetermined pressure; a high-pressure pilot valve, configured to operate when the fluid pressure on the upstream side of the emergency blowdown valve exceeds a second predetermined pressure; and a controller valve, configured to connect the piston chamber of the emergency blowdown valve to pressurise the piston chamber when either the low-pressure pilot valve or high-pressure pilot valve is operated thereby opening the emergency blowdown valve. The piston includes a sealing piston head which is movable to permit / prohibit the flow of coolant therethrough, an operating piston head which moves in the piston chamber, and a stem connecting the operating piston head to the sealing piston head. The operating piston head divides the piston chamber into a first, upper, chamber on a first side of the operating piston head, and a second, lower, chamber on an opposite side of the piston head to the first chamber. The upper chamber is located such that the operating piston head is between the upper chamber and the sealing piston. The lower chamber is located between the operating piston head and the operating piston head. The pilot valve assembly may be connected to at least the lower chamber. A ‘reverse’ seated emergency blowdown valve is understood to mean a valve in which the piston is in a ‘closed’ state (e.g., a state in which flow through the valve is prohibited) when the sealing head of the piston is pulled against a sealing surface when the piston is in a retracted state. In the present case, a higher pressure in the lower piston chamber than in the higher chamber retracts the sealing head of the piston of the emergency blowdown valve toward from the piston chamber and away from a main seat in which the piston prohibits the flow of coolant. On the other hand, a decrease of pressure in the lower piston chamber (for example, when the piston chamber is connected to the vent) or an increase in the pressure of the upper piston chamber (relative to the lower piston chamber) causes the piston to be actuated toward a closed position, in which a portion of the piston is extended away from the piston chamber and the flow of coolant is prohibited. The valve arrangement according to the second aspect of the present invention may be combined with one or more of the features described in relation to the valve arrangement according to the first aspect of the present invention. In particular, the features described in relation to the pilot valve assembly of the valve arrangement according to the first aspect of the present invention are equally applicable to the pilot valve assembly of the valve arrangement according to the second aspect of the present invention. In a third aspect, there is provided a nuclear reactor assembly comprising the valve arrangement of the first and / or second aspect connected to a primary cooling loop. 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: Fig. 1 is a schematic of a direct seated emergency valve. Fig. 2 is a schematic of a low-pressure pilot valve. Fig. 3 is a schematic of a high-pressure pilot valve connected to a controller valve. Fig. 4 is a schematic of a valve arrangement in a first state. Fig. 5 is a schematic of a valve arrangement in a second state. Fig. 6 is a schematic of a reverse seated emergency valve. Fig. 7 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. Fig. 1 shows a direct seated emergency blowdown valve (e.g., ‘main valve’) 20 which forms part of a valve arrangement of the type discussed below. The main valve 20 comprises a coolant chamber 22, formed from a first, upstream, part 24 connected to a main valve inlet channel 25, and a second, downstream, part 26 connected to a main valve outlet channel 27. The upstream part of the coolant chamber 24 is further connected to a control channel 28, which is usable to control the main valve 20 as will be described in more detail below. The main valve 20 further comprises a main valve piston 30 and a main valve piston chamber 32. The main valve piston chamber 32 has two regions, an upper main valve piston chamber and a lower main valve piston chamber. The upper main valve piston chamber is fluidically connected to a piston chamber channel 33 discussed below, and is distalmost from a main valve sealing piston head 31a as compared to the lower piston chamber. The lower main valve piston chamber includes one or more leakage pathways, so that liquid upstream of the main valve leaks into the lower main valve piston chamber and pressurises it. The main valve piston 30 extends into the coolant chamber 24, and is usable to control the flow of coolant between the main valve inlet channel 25 and the main valve outlet 26. More particularly, the main valve piston 30 comprises a main valve sealing piston head 31a which is movable in the coolant chamber 22, an upper piston 31 b which is movable in the main valve piston chamber 32 (also referred to as a control chamber) which is above the upper surface of an upper piston 31 b. Lower piston chamber 32b is part of the control chamber 32, and is below the lower surface of the upper piston 31 b. In some examples, there are channels through body 37 from the valve main chamber 24 to the lower piston chamber 32b. Whilst media pressure is applied to the top of the main piston 312a, the same pressure is also applied to the bottom and top parts of the upper piston chamber due to the channels which are within body 36 (also referred to as a separator) and the upper piston 31b, where channel 33 is connected to channel 28, and containing the same pressure as the main valve inlet channel 25. Once the channel 33 opens(by means explained below), channel 28 will be disconnected from channel 33 and will be closed. Then, upper piston chamber 32 begins depressurising. This then creates an unbalanced pressure load applied to the top and bottom of the upper piston 31b, while the main media pressure from the main valve 36 is applied to the bottom surface of the upper piston 31b. The lower surface area of the upper piston 31 b is larger than the upper surface of the main piston 31a, and therefore equal media pressure applied on these two surfaces causes an unbalanced load, with the resultant force being upwards on the main piston 30 as there is no media pressure at the upper surface of the upper piston 31b at upper piston chamber 32. This causes the main valve (i.e., the main piston 31a) to open. To close the main valve, it is enough to connect channel 33 to the main media pressure at channel 28. This would balance the pressure and load the upper and lower surface off the upper piston 31 b, btu the main media pressure within the main valve chamber 36 would apply and try to close the main valve by applying a downward load at the upper surface of the main piston 31a. A spring load, discussed below, can also be used to facilitate closing the main valve. In this example, the main valve 20 is biased to the closed state by a main valve compression spring 38, which acts to press the main valve sealing piston head 31a against the main seat 34 as shown in Fig. 1. In this way, the main valve 20 remains in the closed state whilst the main valve piston chamber 32 is pressurised. Advantageously, this prevents communication between the main valve inlet channel 25 and the main valve outlet channel 27 when the pressure at the main valve inlet channel 25 is within a working range (this will be elaborated on in more detail below). Alternatively, the main valve compressed spring 38 may not be present at all in some embodiments, and the main valve piston 30 may instead rest on the main seat 34 in the absence of external forces by virtue of its own weight. Movement of the main valve piston 30 between the first state and the second state is controlled via changes in pressure within the main valve piston chamber 32 and specifically the upper main valve piston chamber. These pressure changes can be caused by introducing to or removing fluids from the upper main valve piston chamber via a piston chamber channel 33, which connects to the upper main valve piston chamber. An increase in the pressure in the main valve upper piston chamber causes the main valve piston 30 to move toward the closed position, in which the main valve piston 30 is pressed against the main seat 34. In contrast, a decrease in pressure in the main valve upper piston chamber 32 causes the main valve piston 30 to move toward the open position, and abut the second seat 36. In this way, the main valve 20 may be easily and accurately controlled via the pressure chamber 32. Figs. 2 and 3 show a series of pilot valves 40 and 60 which are usable to form a pilot valve assembly which controls the main valve in Fig. 1 to open or close channel 33 in Fig. 1. Fig. 2 shows a pilot valve referred as a low-pressure pilot (LPP) valve 40 which remains in a closed state (one in which a LPP valve inlet 42 is not connected to a valve channel 44 of the LPP valve) until a predetermined pressure is applied. The LPP valve 40 comprises an LPP valve inlet channel 42, a valve channel of the LPP valve 44 and a LPP valve vent channel 46. The valve channel 44 of the LPP valve being connectable to both the LPP valve inlet channel 42 or the LPP valve vent channel 46 under different conditions as discussed below. The LPP valve 40 further comprises an LPP valve piston 50, which can be actuated between a first and second position via pressure changes in the LPP valve inlet channel 42, said first and second positions corresponding to ‘closed’ and ‘open’ states of the LPP valve 40 respectively. More particularly, the LPP valve piston 50 will be actuated from the first position (e.g., the closed state of the LPP valve 40) to the second position (e.g., the open state of the LPP valve 40) when a pressure in the LPP valve inlet channel 42 rises above a first predetermined pressure. In the first position, the LPP valve piston 50 blocks the connection between the LPP valve inlet channel 42 and the valve channel 44 of the LPP valve, whilst leaving a passage between the valve channel 44 of the LPP valve and LPP valve vent channel 46 unblocked (e.g., the valve channel 44 of the LPP valve and the LPP valve vent channel 46 are fluidly connected when the LPP valve piston 50 is in the first position). In the second position of the LPP valve piston 50, a pathway between the LPP valve inlet channel 42 and the valve channel 44 of the LPP valve is opened (e.g., the LPP valve inlet channel 42 and the valve channel 44 of the LPP valve are fluidly connected), whilst the previous connection between the valve channel 44 of the LPP valve and the LPP valve vent channel 46 is now blocked off. In this example, the LPP valve piston 50 is biased to the first position by a LPP valve compression spring 52. Fig. 3 shows a HPP valve 60 and a controller valve 80 connected together. More particularly, a plurality of the channels of the respective valves directly connect to one another without any intermediate object therebetween, as is described in more detail below. Similarly to the LPP valve 40, the HPP valve 60 comprises a HPP valve inlet channel 62, a valve channel 64 of the HPP valve and a HPP valve vent channel 66, as well as a HPP valve piston 70 which is movable from a first position (e.g., a closed state of the HPP valve 60) to a second position (e.g., an open state of the HPP valve 60) when the pressure in the HPP valve inlet channel 62 reaches and / or exceeds a second predetermined pressure. Said second predetermined pressure of the HPP valve 60 being greater than the first predetermined pressure of the LPP valve 40. Once again, the valve channel 64 of the HPP valve is connected to the HPP valve vent channel 66 and blocked from the HPP valve inlet channel 62 when the HPP valve piston 70 is in the first position, and the valve channel 64 of the HPP valve is connected to the HPP valve inlet channel 62 and blocked from the HPP valve vent channel 66 when the HPP valve piston 70 is in the second position. Further, the HPP valve piston 70 is biased to the first position by a HPP valve compression spring 72. In addition to the above, the HPP valve 60 further comprises a HPP valve piston chamber channel 68, which indirectly connects the HPP valve 60 to the main valve piston chamber 32 via channel 33 as described below in relation to Fig. 4. The HPP valve piston chamber channel 68 is connected to the HPP valve vent channel 66. More particularly, in the present embodiment the HPP valve piston chamber channel 68 connects to the HPP valve vent channel 66 regardless of whether the HPP valve piston 70 is in the first or second position. The controller valve 80 also comprises a controller valve inlet channel 82, a valve channel 84 of the controller valve and a controller valve vent channel 86, as well as a controller valve piston 90 which is movable from a first position (e.g., a closed state of the controller valve 80) to a second position (e.g., an open state of the controller valve 80) when the controller valve inlet channel 82 reaches and / or exceeds a predetermined pressure. Said predetermined pressure may be any pressure equal to or below the second predetermined pressure of the HPP valve 60. As shown in Fig. 3, the HPP valve 60 and controller valve 80 are connected together, with two respective channels of each respective valve joined together directly. In particular, the valve channel 64 of the HPP valve 60 is fluidly connected to the controller valve inlet channel 82, and the controller valve vent channel 86 is fluidly connected to the HPP valve piston chamber channel 68. The use of this arrangement is described in relation to Figs. 4 and 5 below. Once the media pressure exceeds the set pressure, it moves piston 70 upwards, and connects channel 62 to channel 64, and closes the connection of 64 to 68 or 66, where 68 is still connected to 66 (channel 66 is always connected to 66). The presence of pressurised media in channel 64 from 62, transferred to piston 90 via channel 82, and pushes the piston 90 upwards, opening channel 84 to 86. Whilst channel 84 is connected to channel 33 of the main valve (in Fig. 4), the pressurised media from Upper Piston Chamber of the main valve would be depressurised through channel 33 into 84, that passes through 86 into 68 and then into vent channel 66. During this, LPP 40 (see Fig. 4) is open and its channel 44 is connected to channel 33 of the main valve and of the controller valve on HPP, and feeds high pressure media through this channel into the upper piston chamber 32 of the main valve via channel 33, but this pressurised media will be vented through open channel 84, to 86, 68 and 66. The size of channel 84 to 66 is designed to be larger than the feeding channel 44 to 33, therefore more media will be vented through 66 than reaching to Upper Piston Chamber 32 via channel 33. This would cause depressurisation of the upper piston chamber of the main valve and opens it as explained before. Figs. 4 and 5 show a valve arrangement 100. The valve arrangement 100 includes the main valve 20 shown in Fig. 1, as well as the pilot valve assembly 40, 60 and 80 shown in Figs. 2-3. The pilot valves 40 and 60 are connected to the main valve 20, as well as to each other, in such a way that the main valve piston 30 of the main valve 20 is moved to the second position in which the main valve piston 30 abuts the second seat (i.e., the main valve 20 is actuated to the second, open, state) only when the pressure in the coolant chamber upstream part 24 / emergency blowdown valve inlet channel 25 is below the first predetermined pressure (a so called low-pressure or LPP trip), or above the second predetermined pressure (a so called high-pressure or HPP trip). More particularly, the piston chamber 32 (and more particularly the upper piston chamber) and piston chamber channel 33 of the main valve 20 is directly connected to both the valve channel 44 of the LPP valve 40 and the valve channel 84 of the controller valve 80 (as shown by the connecting lines 102), and the coolant chamber upstream part 24 / coolant channel 28 is connected to the inlet channels 42 and 62 of both the LPP valve 40 and HPP valve 60 (as shown by the connecting lines 104). These connections 102 and 104 also mean that the valve channels 44 and 84 of the LPP and controller valves 40 and 80 are connected to one another, as are the inlets 42 62 of the LPP and HPP valves 40 60. In addition to this, the vent channels 46 66 of the LPP and HPP valves 40 60 are both connected to each other and a vent 110, as shown by the connecting lines 106. The HPP and controller valves 60 80 are also connected together, as described previously in relation to Fig. 3. The working of this arrangement to achieve the desired affect described above can be explained in three stages, in which the pressure in the upstream part of the coolant chamber 24 is respectively: below the first predetermined pressure (so called LPP trip), between the first predetermined pressure and the second predetermined pressure (within the so called working pressure range), and above the second predetermined pressure (so called HPP trip). The state of the main valve 20 when the pressure in the coolant chamber upstream part 24 is above the first predetermined pressure but below the second predetermined pressure will be described with reference to Fig. 5, which shows the state of the valve arrangement 100 in this scenario. Once the media pressure reaches above the first predetermined pressure, the LPP valve piston 50 is pushed upwards by the media pressure under it, pressing onto a secondary seat. In other words, the LPP valve 40 will transition to the open state, in which a pathway between the valve channel 44 of the LPP valve and LPP valve vent channel 46 is blocked, and a pathway between the LPP inlet channel 42 and the valve channel 44 of the LPP valve is opened. This allows coolant from the LPP valve inlet channel 42 to pass through the gap in between the LPP valve piston 50 and its lower seat, and travel through the valve channel 44 of the LPP valve to the main valve piston chamber 32. The same media also travels towards the valve channel 84 of the controller valve, but while its controller valve piston 90 is sitting down on its main seat, the high-pressure flow cannot pass through the controller valve 80. Therefore the upper main valve piston chamber becomes pressurised, transitioning the main valve 20 to the closed state in which the piston 30 abuts its main seat 34 (as the pressure above and below the main valve operating piston head 31 b is equal, and so the weight of the valve or the biasing means closes the main valve 20). The controller valve piston 90 sits tight on its main seat (e.g., its position when the controller valve 80 is in the closed state) by its own weight, or by a spring load that in some examples may be applied by a compression spring (not shown) at the top of the controller valve piston 90. The high-pressure coolant from the valve channel 84 of the controller valve enters the controller valve 80 from the side of the controller valve piston 90, as shown in Fig. 5, and any leakage to top and bottom of the controller valve piston 90 helps the controller valve piston 90 sit tighter on its main seat, preventing any depressurisation of the main valve upper piston chamber 32. Next, the case where the pressure in the upstream part of the coolant chamber 24 is below the first predetermined pressure is discussed, this is shown in Fig. 4. First, it is noted that when the pressure in the coolant chamber upstream part 24 is below the first predetermined pressure, both the LPP and HPP valves will be in the closed states. As such, the paths between the inlet channels 42 62 and valve channels 44 64 of both the LPP and HPP valves will be blocked by the respective LPP / HPP valve pistons 50 70, and so coolant (travelling down the connecting lines 102 and 104) will be trapped in the inlet channels 42 62 of each valve. In contrast, the closed state of the LPP / HPP valves 40 and 60 means that the valve channel 44 of the LPP valve is connected to the LPP valve vent channel 46, such that the main valve piston chamber 32 is connected to the vent 110 via the connecting line 102, the valve channel 44 of the LPP valve, the LPP valve vent channel 46, and the connecting line 106 pathway. Therefore, the upper piston chamber of the main valve 20 will depressurise via the LPP valve 40, and the main valve 20 will open to connect the upstream part 24 and downstream part 26 of the coolant chamber 22 (as the lower main valve piston chamber is pressurised, and so can overcome the force of spring 38). Therefore, at pressures above the first predetermined pressure but below the second predetermined pressure, the main valve 20 remains closed, and stays closed, as the HPP valve piston 70 also sits on its lower seat by the spring load from the HPP valve compression spring 72, which opens a gap in between the HPP valve piston 70 and a HPP valve secondary seat at its upper part. This allows any leaked coolant from a bottom seat of the HPP valve 60 to vent through the HPP valve vent channel 66, preventing any pressure increase in the valve channel 64 of the HPP valve, as shown in Fig. 3. Finally, the state of the main valve 20 when the pressure in the coolant chamber upstream part 24 is above the second predetermined pressure will be described, again with reference to Fig. 5. At pressures at and exceeding the second predetermined pressure, the LPP valve piston 50 has already been moved upwards, created a gap between the LPP valve piston 50 and its main seat (as the pressure upstream of the main valve will have exceeded the first predetermined pressure). Now, the HPP valve piston 70 also moves to its second position as the pressure load on the bottom surface of the HPP valve piston 70 overcomes the downward spring load of the HPP valve compression spring 72, creating a gap between the HPP valve piston 70 and its main seat. This allows the coolant to flow from the HPP valve inlet channel 62, through the gap, and into the valve channel 64 of the HPP valve. The HPP valve vent channel 66 is closed off from the valve channel 64 of the HPP valve due to the movement of the HPP valve piston 70 to the second position sitting on the secondary seat. Once pressurised coolant reaches the controller valve inlet channel 82 via the valve channel 64 of the HPP valve, it will apply pressure at the lower surface of the controller valve piston 90, pushing the HPP valve piston 90 to its second position by overcoming its weight, pressure load gathered on top of the controller valve piston 90 by the leaked coolant through the valve channel 84 of the controller valve, and also any compressed spring if used (not shown) on top of the controller valve piston 90. This opens a gap in between the controller valve piston 90 and its main seat, allowing the high-pressure coolant from the valve channel 33 into 84 of the controller valve to pass through the gap into the controller valve vent channel 86. The coolant then flows to the vent 110 via the valve channel 64 of the HPP valve and the HPP valve vent channel 66, allowing the main valve upper piston chamber 32 to depressurise and the main valve piston 30 to move to its second position (i.e., allowing the main valve 20 to transition to its open state). Note that any leakage from the bottom of piston 90 through 82 will be vented to 66, and cannot go the other way into 84. Summarising the above, the main valve 20 therefore: opens when the pressure upstream of the main valve drops below the first predetermined pressure, or rises above the second predetermined pressure, and remains closed when the pressure upstream of the main valve is in between the first predetermined pressure and the second predetermined pressure. The example shown in Figs. 4 and 5 is for a direct seated main valve, but the teaching is equally applicable to reverse seated main valves. In such examples, the pilot valve assembly is configured so that the upper piston chamber of the main valve is pressurised when the pressure upstream of the valve is outside of the working range (or, alternatively, the lower piston chamber of the main valve is depressurised). Fig. 6 shows an example of a reverse seated valve 620, which comprises many of the same features as the direct seated main valve 20 shown in Fig. 1 (like features are given like reference numerals incremented by 600). In particular, the reverse seated main valve 600 comprises a coolant chamber 622, formed from a first, upstream, part 624 connected to a main valve inlet channel 625, and a second, downstream, part 626 connected to a main valve outlet channel 627. The upstream part of the coolant chamber 624 is further connected to a control channel 628, which is usable to control the main valve 620 via a pilot valve assembly 638. Said pilot valve assembly 638 may be formed in the same arrangement as the pilot valve assembly of Figs. 4 and 5, though is not limited in this way. The reverse seated main valve 620 further comprises a reverse seated main valve piston 630, including a reverse seated main valve sealing piston head 631a, a reverse seated main valve operating piston head 631b and a reverse seated main valve piston shaft 631c, and a reverse seated main valve piston chamber 632. The main valve piston chamber 632 has two regions, an upper main valve piston chamber which is distalmost from the reverse seated main valve sealing piston head 631a and a lower main valve piston chamber which is proximal most to the reverse seated main valve sealing piston head 631a. Finally, the reverse seated main valve 620 comprises a reverse seated main valve piston chamber channel 633, connecting the reverse seated main valve piston chamber 630 to the pilot valve assembly 638. However, whilst the piston chamber channel 33 of the direct seated main valve 20 connected to the upper piston chamber of the direct seated main valve 20, the reverse seated main valve piston chamber channel 633 instead connects to the lower piston chamber of the reverse seated main valve 620, such that it is the lower, rather than upper, piston chamber 632 of the reverse seated main valve 620 that is connected to a vent (i.e., de-pressurised) when the pressure upstream of the reverse seated main valve 620 is outside the working range. Further, the reverse seated main valve 620 is configured such that the reverse seated main valve 620 is in closed state when the reverse seated main valve piston 630 is contracted toward the reverse seated main valve piston chamber 630. In this contracted state, the reverse seated main valve sealing piston head 631a is pulled against a main seat between the upstream 624 and downstream 626 parts of the reverse seated main valve coolant chamber so as to forbid flow between the upstream 624 and downstream 626 parts of the coolant chamber 622. This is the opposite of the direct seated main valve 20, which is configured to be in the closed state when the main valve sealing piston head 31a is in an extended state in which the main valve sealing piston head 31a is distalmost from the main valve piston chamber 32 and pushed against its main seat. Whilst not shown, there may be a separate feed line for the upper piston chamber 632 which is also connected to the pilot valve assembly 638, and which is provided with fluid when the operating pressure deviates from the working range. Further, the reverse seated operating piston head 631b may have a first surface, located in or defining the upper piston chamber, which is larger than a second surface, located in or defining the lower piston chamber. As such, when the lower piston chamber is depressurised by connection to the vent, the pressure in the upper piston chamber does not need to be significantly larger than the residual pressure in the lower piston chamber. As such, de-pressurisation of the reverse seated main valve lower piston chamber (e.g., when the pressure upstream of the reverse seated main valve 620 is outside the working range) causes the reverse seated main valve sealing piston head 631a (via corresponding movement of the reverse seated main valve operating piston head 631b) to be pushed away from both the reverse seated main valve piston chamber 630 and its main seat, thus moving the reverse seated main valve 620 to an open state in which flow between the upstream 624 and downstream 626 parts of the reverse seated main valve coolant chamber 622 is permitted. As discussed, this can be aided in some examples by pressurisation or enhanced pressurisation of the upper piston chamber. Fig. 7 shows a nuclear reactor assembly 702 which includes a reactor pressure vessel 704, connected to a steam generator 706 via a coolant circuit 708. In use, fissile material in the reactor pressure vessel 704 is used to heat coolant (e.g., water.) within the coolant circuit 708 which is then used to heat water within the steam generator 706 to generate steam. The steam is used to generate electricity via one or more turbines (not shown). The coolant circuit 708 includes a pressurizer 712, as well as a reactor coolant pump 710. The pressurizer 712 maintains the fluid pressure of the coolant, for example to ensure the coolant remains in the liquid phase. The pressurizer 712 is fluidically connected to a containment unit 716 via a pressure release valve arrangement 714 which includes two valve arrangements 100 connected in parallel. Control and instrumentation system 718 is connected to the pressure release valve arrangement 714 and includes (for example) electrical command lines and sensor lines. Whilst in the example shown, the valve arrangement 714 is shown as connected above the pressurizer 712, it can (in addition, or instead) be connected below the pressurizer 712 and / or directly to the coolant circuit 708. The containment unit 716 provides two functions: (i) to allow for the overflow of coolant in the event of overpressure from the coolant circuit 708 into the containment unit 716; and (ii) to allow for the provision of emergency coolant from storage in the containment unit 716 in the event of a loss of coolant accident (LOCA). Here, as the containment unit 716 provides both functions, the pressure release valve assembly 714 also must provide two functions: (i) to open and allow coolant to flow from the coolant circuit 708 into the containment unit 716 in the event of over pressure; and (ii) to open and allow reserve coolant to flow from the containment unit 716 to the coolant circuit 708 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 5 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 10 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 15 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:a direct seated emergency blowdown valve, controllable, by movement of a piston of the emergency blowdown valve in a piston chamber, to open and close thereby permitting or prohibiting a flow of coolant therethrough; anda pilot valve assembly, configured to control the emergency blowdown valve to open and close, the pilot valve assembly comprising:a low-pressure pilot valve, configured to operate when a fluid pressure on an upstream side of the emergency blowdown valve drops below a first predetermined pressure;a high-pressure pilot valve, configured to operate when the fluid pressure on the upstream side of the emergency blowdown valve exceeds a second predetermined pressure; anda controller valve, configured to connect the piston chamber of the emergency blowdown valve to a vent when either the low-pressure pilot valve or high-pressure pilot valve is operated thereby opening the emergency blowdown valve.

2. The valve arrangement according to claim 1, wherein the upstream side of the emergency blowdown valve is connected to the piston chamber when the fluid pressure on the upstream side of the emergency blowdown valve is above the first predetermined pressure.

3. The valve arrangement according to any previous claim, wherein the piston chamber is connected to the vent either:via the low-pressure pilot valve when the pressure of the coolant on the upstream side of the emergency blowdown valve is below the first predetermined pressure; orvia at least the high-pressure pilot valve when the pressure of the coolant on the upstream side of the emergency blowdown valve exceeds the second predetermined pressure.

4. The valve arrangement according to any previous claim, wherein a rest state of the emergency blowdown valve is where the flow of coolant past the emergency blowdown valve piston is prohibited.

5. The valve arrangement according to any previous claim, wherein each of the pilot valves and controller valve comprises at least an inlet channel, a valve channel, and a vent channel.

6. The valve arrangement according to any previous claim, wherein each of the pilot valves and controller valve are piston valves.

7. The valve arrangement according to any previous claim, wherein one or more of the pilot valves are biased to a rest position.

8. The valve arrangement of any previous claim, wherein a rest position of the pilot valves and controller valve is one in which the coolant is blocked from the vent.

9. The valve arrangement according to any previous claim, wherein a vent channel of the controller valve is connected to a vent channel of the high-pressure pilot valve, and wherein the vent channel of the high-pressure pilot valve is connected directly to the vent.

10. The valve arrangement of claim 9, wherein a valve channel of the controller valve is connected to a valve channel of the low-pressure pilot valve.

11. The valve arrangement of either claim 9 or 10, wherein a vent channel of the low-pressure pilot valve connects directly to the vent.

12. A valve arrangement for a nuclear reactor, the valve arrangement comprising:a reverse seated emergency blowdown valve, controllable, by movement of a piston of the emergency blowdown valve in a piston chamber, to open and close thereby permitting or prohibiting a flow of coolant therethrough; anda pilot valve assembly, configured to control the emergency blowdown valve to open and close, the pilot valve assembly comprising:a low-pressure pilot valve, configured to operate when a fluid pressure on an upstream side of the emergency blowdown valve drops below a first predetermined pressure;a high-pressure pilot valve, configured to operate when the fluid pressure on the upstream side of the emergency blowdown valve exceeds a second predetermined pressure; anda controller valve, configured to connect the piston chamber of the emergency blowdown valve to pressurise the piston chamber when either the low-pressure pilot valve or high-pressure pilot valve is operated thereby opening the emergency blowdown valve.

13. A nuclear reactor assembly comprising the valve arrangement of any preceding claim connected to a primary cooling loop.A

Images