Low pressure valve

Abstract

A pilot valve 100 for a nuclear reactor, comprising a valve body 110 including an inlet 111, a valve outlet 112, and a vent outlet 113; a first piston arrangement 120 configured to adopt a closed conf

Description

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. Modern safety systems for nuclear reactors aim to be both active and passive. Active systems operate under the control of the operator and / or running systems, such as pumps and generators, which in normal operation are associated with emergency control. Passive safety systems do not require any external operator input or active systems running in order to operate. This latter system is beneficial as it allows for automatic self-control of the system that is not requisite on external power or user input. In emergency situations this is desirable as, in certain cases, power to the reactor may be disrupted or it may not be possible for operators to control the system manually in which case passive control systems allow the system to remain safe. In the case of a pressurized water reactor, a key fault which requires protective action is a Loss of Coolant Accident (LOCA) event, in which the reactor coolant inventory is lost, limiting the heat removal capacity of the reactor. If not rectified, a LOCA fault could lead to the failure of a nuclear reactor, due to the heat produced by the radioactive decay within the fuel rods causing damage to the reactor plant, including fuel clad melt and the release of fission products. Consequently, to prevent this from happening, nuclear reactors are equipped with emergency cooling systems that can replace the coolant inventory if there is a fault. In a PWR, the system to protect against this is known as the Emergency Core Cooling System (ECCS). These systems typically involve the opening of pipelines to discharge the present reactor coolant inventory. The discharge process reduces the reactor circuit pressure, allowing fresh coolant to be injected into the system at low pressure under the force of gravity. The initial discharge pipelines are normally isolated from the reactor using isolation valves, which can be opened upon the detection of a LOCA. Typically this involves instrumentation to monitor the parameters of the plant, a control system to generate initiation signals on reaching set points and valve actuators to change the valve positions. Systems to achieve this isolation of the coolant from the emergency supply of cooling fluid in the event of a LOCA are known in the art. Accumulator Isolation Passive Valves (AlPVs) are used to isolate a pressurized accumulator at 125 bar and the core at 165bar and typically includes two such valves in series. During normal operation, when there is a reduction of pressure in the reactor circuit, the valve opens proportionally to the difference in the pressure between the accumulator located upstream and the reactor circuit and the core downstream. For the AIPV, since the valve position is proportional to the pressure difference, once the pressure equalizes (either due to a recovery of reactor circuit pressure or from the discharge of the accumulator pressure) the valve shuts, isolating the line once again. The valve therefore does not remain latched open to allow for complete system depressurization. Alternatively, an Automatic Safety Valve for Accumulator Depressurization (ASVAD) can be used. These are used to vent gas from the gas space of an accumulator by opening a valve when the force applied from the pressure in the system drops below a level, which is set by the force applied by the spring acting on the valve plunger. The ASVAD is not an isolation valve in the usual sense but is specifically designed for the venting of gas. As such, it is not suitable for the isolation of high pressure, high temperature water. As a further example, the AP1000 reactor design by Westinghouse features a valve for discharging the heater coolant - termed the Squib valve. The Squib valve is equipped with an explosive charge that is used to open the valve. However, spurious operation of the squib valve can result in a major radiological hazard. Consequently, the safety justification of the plant design is dependent upon a highly reliable Control and Instrumentation (C&l) system to prevent spurious operation, and as such adds significant cost to the plant design. Accordingly, there is a desire to develop a simplified valve. SUMMARY OF THE INVENTION Accordingly, in a first aspect, embodiments of the invention provide a pilot valve for a nuclear reactor, a pilot valve for a nuclear reactor, the pilot valve comprising: a valve body including an inlet, a valve outlet and a vent outlet; a first piston arrangement configured to: adopt a closed configuration where the inlet is fluidically isolated from the valve outlet when a fluid pressure on an upstream side of the inlet is above a predetermined operating pressure; and adopt an open configuration where the inlet is fluidically connected to the valve outlet when the fluid pressure on the upstream side of the inlet is below the predetermined operating pressure; and a second piston arrangement configured to: adopt an open configuration where the vent outlet is fluidically connected to a downstream side of the inlet when the fluid pressure on the upstream side of the inlet is above the predetermined operating pressure; and adopt a closed configuration where the vent outlet is fluidically isolated from the downstream side of the inlet when the fluid pressure on the upstream side of the inlet is below the predetermined operating pressure. In this way, any fluid leaking through the first piston arrangement in the closed configuration is able to flow out of the vent outlet via the second piston arrangement when in the open configuration. Thus, such a pilot valve may prevent a build-up of pressure at the valve outlet of the pilot valve which may otherwise actuate a main valve to which the valve outlet of the pilot valve may be connected. The terms upstream and downstream should be understood in the context of fluid flow through the pilot valve from the inlet to the valve outlet and / or the vent outlet. Thus, by definition, the valve outlet and the vent outlet are, in normal use, each downstream of the inlet and / or the inlet is upstream of both the valve outlet and the vent outlet. The valve body may include or may be provided by a main housing. The inlet, valve outlet and vent outlet may be located through the valve body, that is, allowing fluid passage therethrough. The valve body may also include (e.g. at least partially or entirely define) a fluid chamber. The fluid chamber may be fluidically connected to the inlet, valve outlet and / or vent outlet. Thus, the fluid chamber may be downstream of the inlet, upstream of the valve outlet and / or upstream of the vent outlet. The first piston arrangement is arranged to control fluid flow through the inlet. The first piston arrangement may include a first piston configured to engage with a first piston seat. In the closed configuration, the first piston engages with the first piston seat thereby fluidically isolating the inlet from the fluid chamber. The first piston and / or the first piston seat may be cylindrical or annular, or may have at least partially complementary opposing surfaces. The second piston arrangement is arranged to control fluid flow through the vent outlet. The second piston arrangement may include a second piston configured to engage with a second piston seat. In the closed configuration, the second piston engages with the second piston seat thereby fluidically isolating the fluid chamber from the vent outlet. The second piston and / or the second piston seat may be cylindrical or annular, or may have at least partially complementary opposing surfaces. The valve outlet may be fluidically between the first piston arrangement and the second piston arrangement. The first piston arrangement may be mechanically coupled to the second piston arrangement such that transition of one piston arrangement between configurations causes a transition of the other piston arrangement between configurations. Thus, the first piston may be coupled to the second piston (e.g. via a stem or shaft). The second piston may be connected to the shaft via a manifold. The first piston may be mounted on an (axial) end of the shaft. The second piston and / or the second piston seat may encircle the shaft. The shaft may pass through one or more of the first piston, the first piston seat, the second piston and the second piston seat. The first piston arrangement may be configured to move from the open configuration to the closed configuration when the fluid pressure on the upstream side of the inlet exceeds a predetermined upper pressure (e.g., 165 Bar). The second piston arrangement may be configured to move from the closed configuration to the open configuration when the fluid pressure on the upstream side of the inlet exceeds the predetermined upper pressure. The predetermined upper pressure is greater than the predetermined operating pressure. In this way, the opening and closing pressures of each piston arrangement may be different such that each piston arrangement may exhibit hysteresis which may thereby reduce chattering. When, the first piston arrangement is in the closed configuration and the second piston arrangement is in the open configuration, the pilot valve may be referred to as being in a high-pressure configuration in that the fluid pressure upstream of the inlet is higher than the predetermined operating pressure. The first piston arrangement may be configured to move from the closed configuration to the open configuration when the fluid pressure on the upstream side of the inlet is below a predetermined lower pressure (e.g., 125 Bar). The second piston arrangement may be configured to move from the open configuration to the closed configuration when the fluid pressure on the upstream side of the inlet is below the predetermined lower pressure. The predetermined lower pressure is less than the predetermined operating pressure. In this way, the opening and closing pressures of each piston arrangement may be different such that each piston arrangement may exhibit hysteresis which may thereby reduce chattering. When the first piston arrangement is in the open configuration and the second piston arrangement is in the closed configuration, the pilot valve may be referred to as being in a low-pressure configuration in that the fluid pressure upstream of the inlet is lower than the predetermined operating pressure. The first piston arrangement may be biased into the open configuration. The second piston arrangement may be biased into the closed configuration. Thus, the pilot valve may be biased into the low-pressure configuration. Biasing may be achieved via one or more springs. When the first piston is coupled to the second piston via a shaft, the shaft may be biased via a spring (e.g. a coil spring). The spring may be located between the shaft and the valve body. The pilot valve may include a spring seat. The spring may be located between the shaft and the spring seat. Thus, the spring seat may retain the spring. The spring seat may be connected to the main housing of the valve body via an adjustor. The adjustor may allow the displacement between the spring seat and the valve body (e.g. via rotation of the adjustor). Thus, the spring force applied by the spring to the shaft may be adjusted (and therefore the dynamic behaviour of the piston assemblies of the pilot valve). The motion of a rigid body (e.g. the shaft body including the shaft and any components attached to the shaft e.g. the first piston and the second piston) is governed by the resultant force acting on the rigid body. The main forces acting on the shaft body may include spring force(s) and pressure force(s). The pressure force(s) may be calculated by multiplying the local pressure by the local surface area over which the local pressure acts. The shaft may be constrained to move axially such that only axial forces acting on the shaft contribute to movement of the shaft body. The spring may apply a varying downward spring force on the shaft body and fluid pressure may apply a varying upward pressure force on the shaft body such that the resultant force on the shaft body may vary between upward and downward depending on the fluid pressure. In general, the pressure force will be dominated by the pressure force from the first piston but there may also be minor contributions from the second piston. This explanation is merely provided to aid understanding and the disclosure should not be bound by such theory. Thus, the first piston may include upper and lower axial faces. The first piston lower axial face may be upstream of the first piston lower axial face. The first piston upper axial face may be adjacent the shaft (i.e. when the first piston is mounted on the shaft). Thus, the first piston lower axial face may have a greater surface area than the first piston upper axial face (when the first piston is cylindrical, the surface area of the first piston lower axial face may be equal to the surface area of the first piston upper axial face plus the cross-sectional area of the shaft). Hence, when the first piston is surrounded by fluid at a substantially constant pressure (e.g. when the first piston arrangement is in an open configuration), the first piston may generate an (upward) force on the shaft. Such a force on the shaft will increase as the fluid pressure increases (e.g. the fluid pressure upstream of the inlet). The second piston may include upper and lower axial faces. The second piston lower axial face may have approximately the same surface area as the second piston upper axial face. Thus, when the second piston is surrounded by fluid at a constant pressure (e.g. when the first piston arrangement is in an open configuration), the second piston may generate negligible force on the shaft. Thus, the predetermined lower pressure and / or the predetermined higher pressure may be controlled by tuning the spring force (e.g. via the adjustor) and / or the piston geometries (e.g. the first piston upper / lower axial surface ratios). The pilot valve may include one or more (chatter-reducing) flow disruptors to induce turbulence (e.g. local vortices) in the fluid flowing through the pilot valve thereby creating a more uniform pressure distribution to reduce the likelihood of component chattering. Chattering may be understood to mean repeated contact between components which often results in damage to the components, for example caused by fluctuating operating pressures around the piston arrangements. The one or more flow disruptors may be located on the first piston arrangement. Thus, the one or more flow disruptors may be located on the first piston and / or the first piston seat (e.g. between the first piston and the first piston seat). The one or more flow disruptors may be one or more projections upstanding from a surface of the first piston and / or the first piston seat. The one or more projections may be an annular portion or annular projection, for example when the first piston and / or first piston seat is cylindrical. In this case, the annular projections may be concentric with a central axis of the first piston and / or the first piston seat. Each of the one or more flow disruptors may include a plurality of annular portions (e.g. two or three). There may be circumferential gaps between adjacent potions which may be the same in size, similarly each of the plurality of annular portions may have a same circumferential extent. In these ways, pressure distribution uniformity over the first piston arrangement may be improved thereby reducing the likelihood of chattering. Each of the one or more flow disruptors may be tapered (e.g. away from a surface from which they upstand). Thus, each of the one or more flow disruptors may have a triangular cross-section in a circumferential direction. Alternatively, the one or more flow disruptors may have a smooth and continuous cross-section in a circumferential direction. The one or more flow disruptors may have rounded edges. In this way, uniformity of pressure distribution may be improved. A radially outer surface of each the one or more flow disruptors may be convex or concave. A radially inner surface of each of the one or more flow disruptors may be concave or convex. Such curvature may direct fluid flow to create a more uniform pressure distribution on the first piston arrangement thereby reducing chattering. One or more flow disruptors on the first piston seat may be located radially inward from one or more flow disruptors on the first piston. A minimum distance between the / each of the first piston seat flow disruptor(s) and the first piston seat may be much less than (e.g. less than two thirds, less than half, less than one third) a minimum distance between the first piston flow disruptor and the / each of the first piston seat flow disruptor(s). The minimum distance between the / each of the first piston seat flow disruptor(s) and the first piston seat may be equal to or greater than the radial distance between the shaft and the inner curved surface of the first piston seat. In this way, fluid pressure through the flow disruptors may be maintained to better maintain downward force on the first piston to mitigate chattering. The minimum distance may depend on the media conditions, for example if the media is a gas / vapour then more turbulence may be required, meaning more lift / depth of the grooves are required. In contrast, if the media were liquid, then less depth / height may be required, and when saturated media then it may differ if the grooves are filled with liquid, the gas can escape without any circular turbulence which can cause chattering. In a particular example, the first piston seat includes a plurality of annular projections around a shared circumference of the first piston seat with circumferential gaps therebetween. In the same example, the first piston includes a single circumferentially complete projection. The single projection on the first piston may be radially outward from the plurality of annular projections on the first piston seat. Where they have convex or concave surfaces, the single projection of the first piston may have a concave surface which faces (and so is directed radially inwards towards) a concave surface on each of the plurality of annular projections on the first piston seat. A pilot valve may be understood to be a valve that controls a separate main valve also known as a piloted valve. Thus, in a second aspect, embodiments of the invention provide a nuclear reactor comprising: a primary coolant loop; a main valve through which coolant can flow from the primary coolant loop; and the pilot valve of the first aspect, wherein the pilot valve is configured to control operation of the main valve and the primary coolant loop pressure is the fluid pressure upstream of the inlet. The valve outlet of the pilot valve may be connected to the main valve such that the main valve opens when the pressure at the valve outlet of the pilot valve exceeds a predetermined trigger pressure. Thus, the pilot valve may operate as follows. In ordinary operation, the main valve is closed and the primary coolant loop of the nuclear reactor may be pressurised such that the pilot valve is in the high-pressure configuration (i.e. first piston arrangement closed, second piston arrangement open). Any fluid leaking through the first piston arrangement will pass through the vent outlet via the second piston arrangement thereby preventing a build-up of pressure at the valve outlet which would otherwise cause the main valve to open. If a loss of coolant accident occurs, the loss of coolant will cause a pressure drop in the main coolant loop. This drop in pressure will cause the pilot valve to transition from the high-pressure configuration to the low-pressure configuration (i.e. first piston arrangement open, second piston arrangement closed). This will result in the pressure at the valve outlet of the pilot valve exceeding the predetermined trigger pressure and the main valve opening. With the main valve open, coolant can be continually pumped into the main coolant loop thereby cooling the reactor and reducing the likelihood of a meltdown incident. 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. 1A shows a pilot valve 100 according to the first aspect in a first configuration; Fig. 1Bshows the pilot valve 100 of Fig. 1Ain a second configuration; Fig. 2A shows a pilot valve 170 according to the first aspect in a first configuration; Fig. 2B shows the pilot valve 170 of Fig. 2A in a second configuration; Fig. 3 shows a pilot valve 200 according to the first aspect; Fig. 4 shows the first piston 211 of the pilot valve 200 of Fig. 3; Fig. 5 shows the first piston seat 212 of the pilot valve 200 of Fig. 3; and Fig. 6 shows a 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 and 1B show a pilot valve 100 for a nuclear reactor according to the first aspect. The pilot valve 100 includes a valve body 110, a first piston arrangement 120 and a second piston arrangement 130. The valve body 110, providing a main housing, includes an inlet 111, valve outlet 112, vent outlet 113, adjustor 116 (to which a spring seat 115 is connected) and fluid chamber 117. The inlet 111, valve outlet 112 and vent outlet 113 are each located through valve body 110 such that fluid may pass therethrough. The first piston arrangement 120 is arranged to control fluid flow through the inlet 111 and the second piston arrangement 130 is arranged to control fluid flow through the vent outlet 113. The valve outlet 112 and the vent outlet 113 are each located downstream of inlet 111. The first piston arrangement 120 includes a first piston 121 and a first piston seat 122. The first piston 121 is engageable with the first piston seat 122 to fluidically isolate the inlet 111 from the fluid chamber 117. The second piston arrangement 130 includes a second piston 131 and a second piston seat 132. The second piston 131 is engageable with the second piston seat 132 to fluidically isolate the vent outlet 113 from the fluid chamber 117. In this example, the first piston 121 is mechanically coupled to the second piston 131 via a stem 140 (although it will be appreciated that this does not need to be the case, and each piston arrangement may operate independently). The stem 140 has a cross-sectional area A2 as labelled in Fig. 1 The stem 140 is connected to the second piston 131 via a manifold 134 which is sealed against valve body 110 (but is moveable relative thereto, whilst retaining the seal). The first piston 121 is cylindrical and arranged upstream of the first piston seat 122 which is annular. The stem 140 passes through the first piston seat 122, second piston seat 132 and second piston 131. The second piston 131 is annular to define an axial fluid flow path between the second piston 131 and the stem 140. The second piston seat 132 is annular and the stem 140 is arranged with respect to the second piston seat 132 such that fluid cannot pass between the stem 140 and the second piston seat 132. Each of the first piston seat 122 and the second piston seat 132 are fixed to the valve body 110. The first piston 121 includes a first piston upper axial face 123 and a first piston lower axial face 124. The first piston lower axial face has a surface area A1 as labelled in Fig. 1B. The first piston upper axial face has a surface area equal to A1 minus A2. Thus, the first piston upper axial face 123 has a smaller surface area than the first piston lower axial face 124. Adjustment of the ratio A1 :A2 allows the dynamic behaviour of the stem 140, and thus first piston 121 and second piston 132, to be controlled. Without wishing to be bound by theory, in the open configuration, the resultant (upward) pressure force acting on the first piston 121 is approximately equal to the fluid pressure surrounding the first piston 121 multiplied by A2 plus the spring load 150. Fig. 1A shows the pilot valve 100 in a high-pressure configuration where the first piston arrangement 120 is in a closed configuration such that the inlet 111 is (substantially) fluidically isolated from the fluid chamber 117 (and thus the valve outlet 112) and the second piston arrangement 130 is in an open configuration such that the inlet 111 is fluidically connected to the vent outlet 113. This is an open vent channel, where a pressure is required through 112 to operate another valve, but this pressure cannot be built up due to the open channel to vent 113 via 130. In this high-pressure configuration, any fluid leaking through the first piston arrangement 120 is vented through the vent outlet 113 via the second piston arrangement 130 to prevent a build up of fluid pressure at the valve outlet 112 thereby preventing actuation of a main valve (not shown) connected to the valve outlet 112. Fig. 1B shows the pilot valve 100 of Fig. 1 in a low-pressure configuration where the first piston arrangement 120 is in an open configuration such that the inlet 111 is fluidically connected to the fluid chamber 117 (and thus the valve outlet 112) and the second piston arrangement 130 is in a closed configuration such that the vent outlet 113 is fluidically isolated from the fluid chamber 117. In this low-pressure configuration, fluid passing through the first piston arrangement 120 is fluidly isolated from the vent outlet 113 by the second piston arrangement 130 to enable a build up of fluid pressure at the valve outlet 112 thereby enabling actuation of the main valve (not shown) connected to the valve outlet 112. A spring 150 is located between the spring seat 115 and the stem 140. The spring seat 115 is connected to the valve body 110 via the adjustor 116. The adjustor 116 allows the spring force exerted on the stem 140 by the spring 150 to be adjusted by varying the displacement of the spring seat 115 from the valve body 110. Thus, when a fluid pressure on an upstream side of the main inlet 111 is below a predetermined operating pressure, the spring 150 expands and urges the pilot valve 100 to adopt the low-pressure configuration. Accordingly, when a fluid pressure on an upstream side of the inlet 111 exceeds a predetermined operating pressure, the spring 150 is forced to contract and the fluid pressure urges the pilot valve 100 to adopt the high-pressure configuration. It will be appreciated that movement of the stem 140, first piston 121 and second piston 131 is dictated by the balance of forces acting on this rigid body of components. Wetted surfaces of the first piston 121 and the second piston 131 will experience a force equal to the local pressure multiplied by the local area over which the local pressure acts. The pressure force from the second piston 131 on the stem 140 is negligible due to the second piston’s upper and lower axial faces (not labelled) having equal or substantially equal surface areas. The resulting pressure force combines with the spring force from spring 150 to give a resultant force acting on the rigid body thereby dictating its motion. Thus, motion of stem 140 and associated components can be controlled by varying the wetted areas of the first piston 121 and second piston 131 and by adjusting the spring force of the spring 50. It is well known to adjust spring force by changing the amount of compression or extension of a spring, in this case, spring 150 via adjustor 116. Fig. 2A and 2B show a further pilot valve 170. Where it shares features with the pilot valve 100 shown in Figs. 1A and 1B, like features are indicated by like reference numerals. It differs from the previous pilot valve in that manifold 134 no longer moves with stem 140, but is rather fixed and integrally formed with the valve body 110. Instead, a passage is provided around the second piston 131 and between it and the valve body 110 in the first configuration shown in Fig. 2A. Passages 172A and 172B are provided through the second piston seat 132, as shown, to allow fluid to flow through and to the vent outlet 113. When the pilot valve moves to the second configuration, as shown in Fig. 2B, the second piston moves down and seals the passages 172A and 172B thereby preventing fluid flow from the inlet 111 through to the vent outlet 113. Fig. 3 shows a pilot valve 200 according to the first aspect. The pilot valve 200 is largely similar to the pilot valve 100 of Figs. 1 and 2 and like features have like reference numerals. Thus, the description of the pilot valve 100 applies equally to the pilot valve 200 insofar as no differences are explicitly identified. The pilot valve 200 differs from the pilot valve 100 in that it includes flow disruptors on a first piston 221 and a first piston seat 222 of a first piston arrangement 220 in a form of ripples or waves. Fig. 3 is a side view of the pilot valve 200 with the first piston arrangement 220 in an open configuration. The first piston 221 is attached to a stem 240. The first piston seat 222 is annular and the stem 240 passes through the first piston seat 222. The first piston 221 includes a first piston flow disruptor 223 which is an annular projection upstanding from an upper surface of the first piston 221. The first piston seat 222 includes a set of first piston seat flow disruptors 224A, 224B, 224C which are each partial (in a circumferential sense) annular projections upstanding from a lower surface of the first piston seat 222. Fig. 4 shows a plan view of the first piston 221 from above including the first piston flow disruptor 223. Fig. 5 shows a plan view of the first piston seat 222 from below including the first piston seat flow disruptors 224A, 224B, 224C. The first piston seat 222 includes circumferential gaps 228A, 228B, 228C between adjacent pairs of the first piston seat flow disruptors 224A, 224B, 224C. The first piston flow disruptor 223 is located radially outward of the first piston seat flow disruptors 224A, 224B, 224C. The dashed circles in Fig. 4 indicate where the first piston seat flow disruptors 224A, 224B, 224C would align with the lower surface of the first piston 221 when the first piston arrangement 220 is in the closed configuration. Similarly, the dashed circles in Fig. 5 indicate where the first piston flow disruptor 223 would align with the upper surface of the first piston seat 222 when the first piston arrangement 220 is in the closed configuration. The first piston flow disruptor 223 is defined by a convex radially outer surface 225 and a concave radially inner surface 227 such that first piston flow disruptor 223 tapers in an outward radial direction. Each of the first piston seat flow disruptors 224A, 224B, 224C includes a respective concave outer surface 226A, 226B, 226C and a respective convex inner surface 228A, 228B, 228C such that each of the first piston seat flow disruptors 224A, 224B, 224C tapers in an inward radial direction. A minimum distance D1 between each of the first piston seat flow disruptors 224A, 224B, 224C and the first piston seat 221 is much less than the minimum distance D2 between the first piston flow disruptor 223 and the respective first piston seat flow disruptors 224A, 224B, 224C. The minimum distance D2 between each of the first piston seat flow disruptors 224A, 224B, 22C and the first piston seat 221 is equal to the radial distance D3 between the stem 240 and the inner curved surface 229 of the first piston seat 221. This configuration prevents chattering of the first piston 221 against the first piston seat 222. In use, when the first piston arrangement 220 is in an open configuration, fluid enters the first piston arrangement 220 radially between the first piston 221 and the first piston seat 222. Fluid is urged upwards by the convex outer surface 225 of the first piston flow disruptor 223. After passing the first piston flow disruptor 223, the fluid is urged downwards by the concave outer surfaces 226A, 226B, 226C of the first piston seat flow disruptors 224A, 224B, 224C. This downward flow imparts a downward force on the first piston 221 which serves to stabilise the first piston 221 and the stem 240 and so help prevent chattering. After passing the first piston seat flow disruptors 224A, 224B, 224C, the fluid moves upward between the stem 240 and the first piston seat 222. Such a configuration of flow disruptors induces turbulent flow in the fluid, resulting in a more stable pressure distribution through the first piston arrangement 220, thereby reducing the likelihood of chattering. When the first piston arrangement 220 is in a closed configuration, the upper edge of the first piston flow disruptor 223 engages with the lower surface of the first piston seat 222 such that fluid is prevented from entering the first piston arrangement 220. Fig. 6 shows a reactor assembly 302 which includes a reactor pressure vessel 304, connected to a steam generator 306 via a primary coolant loop 308. In use, fissile material in the reactor pressure vessel 304 is used to heat coolant (e.g., water) within the primary coolant loop 308 which is then used to heat water within the steam generator 306 to generate steam. The steam is used to generate electricity via one or more turbines (not shown). The primary coolant loop 308 includes a reactor coolant pump 310. A pressurizer 312 is connected to primary coolant loop 308. The pressurizer 312 maintains the fluid pressure of the coolant, for example to ensure the coolant remains in the liquid phase. The pressurizer 312 is fluidically connected to a containment unit 316 via a main valve 314. The main valve 314 is controlled by the pilot valve 100 of Figs. 1 and 2. The containment unit 316 allows for the provision of emergency coolant in the event of a loss of coolant accident (LOCA). When such an accident occurs, the fluid pressure in the primary coolant loop 308 will drop thereby causing the pilot valve 100 to open the main valve 314. This allows coolant from the containment unit 316 to drain into the primary coolant loop 308 thereby cooling the fission material in the reactor pressure vessel 3O4.Whilst shown in Fig. 6 as being connected above the pressurizer 312, the main valve may be connected directly to the primary coolant loop 308 or at the bottom of the pressurizer 312. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about”, it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example ±10%.

Claims

1. A pilot valve (100) for a nuclear reactor, the pilot valve comprising:a valve body (110) including an inlet (111), a valve outlet (112) and a vent outlet (113);a first piston arrangement (120) configured to:adopt a closed configuration where the inlet (111) is fluidically isolated from the valve outlet (112) when a fluid pressure on an upstream side of the inlet (111) is above a predetermined operating pressure; andadopt an open configuration where the inlet (111) is fluidically connected to the valve outlet (112) when the fluid pressure on the upstream side of the inlet (111) is below the predetermined operating pressure; anda second piston arrangement (130) configured to:adopt an open configuration where the vent outlet (113) is fluidically connected to a downstream side ofthe inlet (111) when the fluid pressure on the upstream side of the inlet (111) is above the predetermined operating pressure; andadopt a closed configuration where the vent outlet (113) is fluidically isolated from the downstream side ofthe inlet (111) when the fluid pressure on the upstream side ofthe inlet (111) is below the predetermined operating pressure.

2. The pilot valve (100) of claim 1, wherein the first piston arrangement(120) includes a first piston (121) and a first piston seat (122) and the second piston arrangement (130) includes a second piston (131) and a second piston seat (132).

3. The pilot valve (100) of claim 2, wherein the first piston (121) is mechanically coupled to the second piston (131) via a shaft (140) such that:when the first piston arrangement (120) is in the open configuration the second piston arrangement (130) is in the closed configuration; andwhen the first piston arrangement (120) is in the closed configuration and the second piston arrangement (130) is in the open configuration.

4. The pilot valve (100) of claim 3, wherein the shaft (140) is biased via a spring (150) such that the first piston arrangement (120) is urged to adopt the open configuration.

5. The pilot valve (100) of claim 4, wherein the valve body (110) includes an adjustor (116) such that the spring force delivered to the shaft (150) is adjustable.

6. The pilot valve (100) of any one of claims 3 to 5, wherein the first piston (121) includes an upper axial face (123) and a lower axial face (124), the first piston (121) is mounted to the shaft (140) at the upper axial face (123) and the surface area ofthe upper axial face (123) is less than the surface area of the lower axial face (124).

7. The pilot valve (100) of any preceding claim, wherein the first piston arrangement is configured to move from the open configuration to the closed configuration when the fluid pressure on the upstream side of the inlet exceeds a predetermined upper pressure, wherein the predetermined upper pressure is greater than the predetermined operating pressure.

8. The pilot valve (100) of any preceding claim, wherein the first piston arrangement is configured to move from the closed configuration to the open configuration when the fluid pressure on the upstream side of the inlet drops below a predetermined lower pressure, wherein the predetermined lower pressure is less than the predetermined operating pressure.

9. The pilot valve (100) of any preceding claim, wherein the first piston arrangement includes one or more chatter-reducing flow disruptors.

10. The pilot valve (100) of claim 9 when dependent on claim 2, wherein a first piston flow disruptor is located on the first piston.

11. The pilot valve (100) of claim 10, wherein the first piston flow disruptor is an annular projection upstanding from a first surface of the first piston.

12. The pilot valve (100) of any of claims 9 to 11 when dependent on claim 2, wherein a plurality of first piston seat flow disruptors are located on the first piston seat.

13. The pilot valve (100) of claim 12, wherein each of the first piston seat flow disruptors is an annular portion with circumferential gaps therebetween.

14. The pilot valve (100) of any one of claims 9 to 13, wherein each of the one or more flow disruptors includes a convex or concave outer surface and / or a convex or concave inner surface.

15. A nuclear reactor (302) comprising:a primary coolant loop (308);a main valve (314) through which coolant can flow to the primary coolant loop (308); andthe pilot valve (100) of any preceding claim, wherein the pilot valve (100) is configured to control operation of the main valve (314) and the primary coolant loop pressure is the fluid pressure upstream of the inlet (111).14

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