Flow distribution device

Abstract

A flow distribution device is provided for a nuclear reactor pressure vessel which encloses a reactor core comprising a core barrel containing fuel assemblies. The core barrel is spaced from the pressure vessel to define a downcomer flow annulus within the pressure vessel but outside the core barrel and further define a plenum beneath a lower core support plate of the core barrel. In use, reactor coolant flows in sequence downwardly through the downcomer flow annulus, through the plenum, upwardly through apertures in the lower core support plate and into the core barrel. The flow distribution device firstly comprises a bowl-shaped flow distribution element having a bowl wall which extends upwardly from a base of the bowl to a rim of the bowl. The rim is attachable, in use, beneath the lower core support plate such that the bowl occupies a portion of the plenum with the bowl wall is spaced from a bottom of the pressure vessel. The bowl wall contains plural flow openings through which the reactor coolant flows into the bowl en route through the plenum to the lower core support plate. The flow distribution device secondly comprises a vortex suppression element located within the bowl. The vortex suppression element is configured to suppress vortices in the flow of reactor coolant entering the bowl through the flow openings before the flow passes upwardly through the apertures in the lower core support plate. In particular, the vortex suppression element comprises a circumferential row of fins which each extend radially across the bowl to suppress circumferential rotational vortex flow of the reactor coolant

Description

[0001] FLOW DISTRIBUTION DEVICE

[0002] This application claims priority from GB 2402620.5 filed 23 February 2024, the contents and elements of which are herein incorporated by reference for all purposes.

[0003] Field of the Invention

[0004] The present invention relates to a flow distribution device for a nuclear reactor pressure vessel.

[0005] Background

[0006] Nuclear power plants convert heat energy from the nuclear fission of fissile material contained in fuel assemblies in a nuclear reactor into electrical energy. Nuclear reactors have moderators to condition released neutrons to increase the rate of fission of the fissile material in the fuel assemblies. They also have control mechanisms to control the neutron flux and thereby control the rate of fission as required.

[0007] Moderators operate by slowing neutrons to increase the collision cross section of neutrons with the selected fuel. Water moderated reactors use light or heavy water as a moderator. A light water reactor (LWR) uses normal water as both coolant and neutron moderator. Known types of light water reactors include boiling water reactors (BWRs) and pressurised water reactors (PWRs). Heavy water reactors (HWRs) use water with an increased deuterium content, as deuterium has a lower absorption cross section, permitting the use of fuel with a lower fissile content.

[0008] Figure 1 is a schematic diagram of a PWR 20. An RPV 22 enclosing a reactor core containing fuel assemblies is centrally located in the reactor. Clustered around the RPV are three steam generators 24 connected to the RPV by pipework 26 of the pressurised water primary coolant circuit. A pressuriser 28 maintains the water pressure in the primary coolant circuit. Coolant pumps suspended beneath the steam generators circulate pressurised water around the primary coolant circuit, taking heated water from the RPV to the steam generators, and cooled water from the steam generators to the RPV. In the steam generators, heat is transferred from the pressurised water to feed water circulating in pipework of a secondary coolant circuit 26, thereby producing steam which is used to drive turbines which in turn drive an electricity-generator. The steam is then condensed before returning to the steam generators. The pressuriser maintains a pressure of around 155 bar in the primary circuit.

[0009] BWR power plants differ primarily from PWRs in that steam is generated directly in a primary circuit and used to drive a turbine. BWRs therefore do not require steam generators, but otherwise share many features with PWRs. To allow the water to boil at a useful operating temperature, the pressure in a BWR is lower than a PWR, being typically 70 to 75 Bar.

[0010] Conventionally, high capital costs are associated with the construction and operation of large LWR plants, which typically have power outputs significantly in excess of 1 GWe. Due to these high capital costs, and a desire to service small electricity grids, the industry is moving towards the development of smaller units. Small modular reactors (SMRs) are reactors having power outputs of less than about 700MWe. SMRs are seen as much more manageable investments than larger reactors. This is because SMRs benefit from economies of series production, relatively short construction times, and remote factory-based prefabrication before transportation of reactor elements to site.

[0011] In all water moderated reactors, water must flow through the primary circuit with constrained geometries at high pressures and temperatures. This causes turbulent flow patterns. Control of the flow behaviour of the water is important to ensure the nuclear reactor is operating correctly and efficiently, and to mitigate any damage an uncontrolled flow pattern would cause to the nuclear reactor and its components.

[0012] In more detail, Figure 2 schematically shows the flow path of the coolant flow through a lower portion of an RPV 2. Within the RPV, the flow inlet to the reactor core 5 is typically located at a lower core support plate 9 of a core barrel 6, the lower core support plate being spaced from a bottom wall 4 of the RPV by a lower plenum volume 8. Coolant flow enters the lower plenum volume downwardly from an annular downcomer outlet . It then changes direction in the lower plenum volume to flow upwardly through the lower core support plate 9, where it accepts heat from and moderates the fuel assemblies 10 of the reactor core. The flow behaviour at the reactor core inlet presents specific challenges. The flow here can be unstable due to the complex flow geometry and change in flow direction at this part of the primary circuit. This unstable flow can cause vibration and degradation of the nuclear reactor and its components, especially the fuel assemblies. Ideally the flow should be stable and even on entry into the reactor core 5, such that all the fuel assemblies 10 experience a similar flow environment. Thus control of the flow behaviour can be important for achieving metrics needed for safe and efficient operation of the nuclear reactor.

[0013] To address such issues, it is known to introduce flow calming or conditioning devices in the lower plenum volume of an RPV. However, improved solutions to the problem of flow instability at entry into the reactor core are still needed.

[0014] The present invention has been conceived in view of the above considerations.

[0015] Summary of the Invention

[0016] Accordingly, a first aspect of the present invention provides a flow distribution device for a nuclear reactor pressure vessel which encloses a reactor core comprising a core barrel containing fuel assemblies, the core barrel being spaced from the pressure vessel to define a downcomer flow annulus within the pressure vessel but outside the core barrel and further define a plenum beneath a lower core support plate of the core barrel. In use, reactor coolant flows in sequence downwardly through the downcomer flow annulus, through the plenum, upwardly through apertures in the lower core support plate and into the core barrel. The flow distribution device comprises: a bowl-shaped flow distribution element having a bowl wall which extends upwardly from a base of the bowl to a rim of the bowl, the rim being attachable, in use, beneath the lower core support plate such that the bowl occupies a portion of the plenum with the bowl wall spaced from a bottom of the pressure vessel, wherein the bowl wall contains plural flow openings through which the reactor coolant flows into the bowl en route through the plenum to the lower core support plate; and a vortex suppression element located within the bowl, the vortex suppression element being configured to suppress vortices in the flow of reactor coolant entering the bowl through the flow openings before the flow passes upwardly through the apertures in the lower core support plate.

[0017] Advantageously, the distribution element can even the flow of coolant through the reactor core, helping to ensure that all of the fuel assemblies experience a similar flow of coolant, while the vortex suppression element can suppress vortices in the coolant flow which could otherwise cause vibration and degradation of the fuel assemblies. The combination of these functionalities in a single device can simplify manufacture and installation in the reactor pressure vessel, and can produce only a relatively small pressure drop in the coolant flow.

[0018] Conveniently, the vortex suppression element may be welded or mechanically fastened (e.g. bolted or riveted) to the bowl wall.

[0019] The total flow cross-sectional area of the flow openings may be at least 15% of the total area of the bowl wall (the total area including the area of the openings formed in the bowl wall). A total flow cross-sectional area at or greater than this limit can help to ensure adequate control of the coolant flow, whilst also ensuring that the pressure drop from the coolant flow’s interaction with the device is not excessive. In particular, if the total flow cross-sectional area of the flow openings is less than 15% the increased pressure drop caused by the device may cause inefficiencies in the primary circuit due to the difficulty in pumping sufficient coolant flow through the primary circuit to overcome the pressure drop. The maximum total flow cross-sectional area of the flow openings is generally limited by the structural integrity requirements of the device.

[0020] The vortex suppression element may comprise a circumferential row of fins which each extend radially across the bowl to suppress circumferential rotational vortex flow of the reactor coolant. For example, the circumferential row of fins may be extended such that the fins adopt a star-shaped configuration. Such a configuration can efficiently suppress circumferential swirling flows, which can be a significant cause of vibrational deterioration of the fuel assemblies.

[0021] The fins may increase in vertical height with decreasing radial distance, e.g. to match the increasing depth of the bowl towards its centre. The vertical height of the fins should be such that they adequately suppress large scale vortices.

[0022] There may be a gap between the top of the vortex suppression element and the lower core support plate. The minimum gap between the top of the vortex suppression element and the lower core support plate may be approximately equal to the pitch between adjacent fuel assemblies. If this gap is insufficient, there may be inadequate distance for flow equilibration across the device before arrival at the lower core support plate. The vortex suppression element may comprise one or more support rings coaxially located within the bowl to support the fins. These support rings can improve the strength and rigidity of the device. The flow distribution device may have further openings in the support rings to allow coolant flow to pass through. In contrast, the fins may be formed as continuous solid walls without flow openings formed therein to better inhibit circumferential rotational vortex flow.

[0023] The flow distribution device may have a first portion of the flow openings which are inside a predetermined radial distance from a central axis of the bowl, and a second portion of the flow openings which are outside the predetermined radial distance from the central axis. For example, the first portion of the flow openings may be located in a central area of the bowl, and the second portion of the flow openings may be located in an annular area of the bowl that surrounds the central area.

[0024] In this case, the area ratio of all of the flow openings of the first portion relative to the area of the bowl wall inside the predetermined radial distance may be less than the area ratio of all of the flow openings of the second portion relative to the area of the bowl wall outside the predetermined radial distance (the respective bowl wall area including the area of the openings formed in that part of the bowl wall). Such an arrangement can encourage coolant flow to fuel assemblies which are more distant from the central axis of the reactor core, counteracting a tendency for the coolant flow to otherwise be biased towards the central axis.

[0025] Each flow opening of the first portion of the flow openings may have a smaller flow cross-sectional area than that of each flow opening of the second portion of the flow openings. This can further encourage coolant flow to fuel assemblies which are more distant from the central axis of the reactor core.

[0026] Conveniently, each flow opening of the first portion of the flow openings may be circular.

[0027] Each flow opening of the second portion of the flow openings may be slot-shaped. For example, each flow opening may be stadium shaped. Preferably, the length directions of the slot shapes may extend in circumferential directions relative to the central axis of the bowl.

[0028] The bowl wall of the flow distribution device may be smoothly curved over substantially its entire extent. A smoothly curved bowl wall avoids surface discontinuities and associated flow disruptions, thereby promoting efficient and effective conditioning of the flow. The bowl wall may follow a portion of a substantially spherical surface. For example, the bowl wall may substantially form a hemisphere.

[0029] The rim of the flow distribution device can be weldable to the underside of the lower core support plate, e.g. in such a manner that the rim is effectively integral with the lower core support plate. In particular, the rim may be weldable to the underside of the lower core support plate in such a manner that all the flow from the downcomer flow annulus must pass through the flow distribution device to arrive at the lower core support plate. An annular portion of the bowl wall may form the rim. The annular portion may be frustoconical in shape and / or may be a solid wall portion without any flow openings formed therein. These configurations are consistent with forming a strong weld to the underside of the lower core support plate.

[0030] A second aspect of the present invention provides a nuclear reactor pressure vessel which encloses a reactor core comprising a core barrel containing fuel assemblies, the core barrel being spaced from the pressure vessel to define a downcomer flow annulus within the pressure vessel but outside the core barrel and further define a plenum beneath a lower core support plate of the core barrel. In use, reactor coolant flows in sequence downwardly through the downcomer flow annulus, through the plenum, upwardly through apertures in the lower core support plate and into the core barrel. The vessel further comprises the flow distribution device of the first aspect, the rim of the bowl-shaped flow distribution element of the flow distribution device being attached beneath the lower core support plate such that the bowl occupies a portion of the plenum with the bowl wall of the bowl-shaped flow distribution element spaced from a bottom of the pressure vessel, whereby, in use, the reactor coolant flows into the bowl through the flow openings of the bowl wall en route through the plenum to the lower core support plate, and the vortex suppression element suppresses vortices in the flow of reactor coolant entering the bowl through the flow openings before the flow passes upwardly through the apertures in the lower core support plate.

[0031] The downcomer flow annulus may have a substantially constant radial width around its circumference at its junction with the plenum, and the rim of the bowl may be radially spaced from the pressure vessel by the same radial width. This configuration is consistent with a smoothly uninterrupted flow path for the coolant flow at the transition from the downcomer flow annulus to the plenum. Starting at the rim and ending at the base of the bowl, the spacing of the bowl wall from the bottom of the pressure vessel may remain at a distance equal to that width. Alternatively, from the rim to the base, the spacing of the bowl wall from the bottom of the pressure vessel may reduce (e.g. smoothly taper) to a distance which is less than that width. Relative to the spacing at the rim, the reduction in the spacing may be at least 20% and / or at most 50%, e.g. about 35%.

[0032] The entire weight of the flow distribution device (ignoring buoyancy effects in operation) may be carried by the core barrel. For example, the flow distribution device may be mounted solely on the lower core support plate of the core barrel, without any attachments to other parts of the reactor pressure vessel. By mounting the flow distribution device this way, the device can reduce the number of welds to the reactor pressure vessel, reducing the likelihood of any one weld experiencing weld decay, and thereby extending the vessel’s service life. Avoiding attachments which would otherwise traverse the gap between the bowl wall and the bottom of the pressure vessel also helps to avoid disruptions to the flow streams in this gap.

[0033] The rim of the bowl may be welded to the underside of the lower core support plate. For example, the rim may be weldable to the underside of the lower core support plate in such a manner that all the flow from the downcomer flow annulus must pass through the flow distribution device to arrive at the lower core support plate. Moreover, the rim may be weldable to the underside of the lower core support plate in such a manner that the rim is effectively integral with the lower core support plate.

[0034] A third aspect of the present invention provides a method of operating the nuclear reactor pressure vessel of the second aspect, the method comprising: flowing reactor coolant in sequence: downwardly through the downcomer flow annulus, through the plenum, upwardly through the apertures in the lower core support plate, and into the core barrel; wherein the reactor coolant flows into the bowl through the flow openings en route through the plenum, and wherein vortices in the flow of reactor coolant entering the bowl through the flow openings are suppressed by the vortex suppression element before the flow passes upwardly through the apertures in the lower core support plate.

[0035] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0036] Summary of the Figures

[0037] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0038] Figure 1 shows a schematic diagram of a pressurised water reactor;

[0039] Figure 2 shows schematically a coolant flow through a reactor pressure vessel;

[0040] Figure 3 shows a perspective view of a flow distribution device;

[0041] Figure 4 shows a side view of the flow distribution device;

[0042] Figure 5 shows a top-down view of the flow distribution device;

[0043] Figure 6 shows bottom-up view of the flow distribution device; and

[0044] Figure 7 shows a schematic longitudinal cross-section through the lower internals of a reactor pressure vessel comprising the flow distribution device.

[0045] Detailed Description of the Invention

[0046] 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. All documents mentioned in this text are incorporated herein by reference. The present disclosure relates to a flow distribution device for an RPV in which the device helps to evenly distribute a flow of coolant to plural fuel assemblies in a nuclear reactor core, by incorporating a bowlshaped flow distribution element. Vortices in the coolant flow are suppressed by a vortex suppression element located within the bowl.

[0047] Figures 3 to 6 show respectively perspective, side, top-down and bottom-up views of the flow distribution device 100. The flow distribution device 100 comprises a bowl-shaped flow distribution element 110, and a vortex suppression element 120 which is located within the flow distribution element 110. The bowlshaped flow distribution element 110 has a bowl wall 111 perforated by plural flow openings 112, 113, which allow the coolant to flow through the device, the upper edge of the bowl terminating at an unperforated annular rim 150.

[0048] Figure 7 shows a schematic longitudinal cross-section through a bottom portion of an RPV 200 comprising the flow distribution device 100. Some features of the RPV 200 are hidden for clarity. A flow path of coolant flow through the RPV is indicated by bold, dashed, arrowed lines. The coolant, after leaving steam generators of a PWR, enters the RPV 200 through upper inlet nozzles (not shown) and flows downwardly through a downcomer annulus 207, which surrounds a reactor core 201 formed by a core barrel 209 containing plural fuel assemblies 202. The core barrel 209 is suspended within the RPV 200 through mountings to an upper portion of the RPV (not shown). The downcomer anulus 207 is thus bounded on the inside by a wall of the core barrel 207 and on the outside by the outer wall 203 of the RPV.

[0049] The coolant flow exits the downcomer annulus 207 at its lower end to enter a plenum 206, which is a cavity between a hemispherical bottom wall 208 of the RPV 200 and a lower core support plate 204 forming the bottom of the core barrel 207. The flow distribution device 100 is located within the plenum 206 and is attached by the rim 150 to the underside of the lower core support plate 204. The coolant flow, after entering the plenum 206 at its junction with the downcomer annulus 207, flows along a gap between the bowl wall 111 and the bottom wall 208, turns upwardly to flow through the openings 112, 113 of the flow distribution element 110, and then flows through channels in the lower core support plate 204 to continue upwards through the reactor core 201 , cooling the fuel assemblies 202 and moderating the neutrons of the fission reaction. The heated coolant flow then leaves the reactor core 201 , and subsequently exits the RPV 200 through outlet nozzles not shown in Figure 7 to return to the steam generators.

[0050] Returning to Figure 3, the bowl-shaped flow distribution element 110 is configured such that, in use, unnecessary disturbance of the coolant flow is reduced as it flows through the flow distribution device 100. For example, the bowl wall of the flow distribution element 110 is smoothly curved, as discontinuities may disrupt the coolant flow. For example, the bowl wall may be substantially hemispherical. Accordingly, when we discuss the total area of the bowl wall or the area of a part of the bowl wall, we mean the area including the area of any openings formed in the bowl wall or part thereof. In the example of Figure 3, the flow openings are slot-shaped (more specifically stadium shaped) 112 or circular 113. However, the flow openings can have other shapes. The size, shape and placement of these openings influence the flow paths the coolant can take through the flow distribution element 110 and thus the relative distribution of the flow as it leaves the flow distribution element. The openings also help to straighten the flow and align it with the central axis 101 of the bowl. For example, as shown best in Figure 4, the circular flow openings 113 are all inside a predetermined radial distance 102 from the central axis 101 of the bowl and thus form a first portion 130 of the openings, while the slot-shaped openings 112 are all outside that radial distance and form a second portion 140 of the openings. The circular flow openings 113 have a smaller cross-sectional area than the slot-shaped openings 112. In addition, the area ratio of all of the flow openings 113 of the first portion 130 relative to the area of the bowl wall inside the radial distance 102 is less than the area ratio of all of the flow openings 112 of the second portion 140 relative to the area of the bowl wall outside the radial distance. With such an arrangement, the openings 112, 113 encourage more of the flow to be directed to radially outer parts of the reactor core 201 than would otherwise be the case. That is, it counteracts a tendency for the flow to predominantly service the central parts of the core. At a local level, the size and shape of each hole can also affect the turbulence and straightness of the flow passing through that hole.

[0051] The total flow cross-sectional area of the flow openings 112, 113 may be at least 15% of the total area of the bowl wall 111 , with the maximum total flow cross-sectional area being limited by a need for structural integrity of the device. As shown in Figures 3 and 4, each flow opening 112, 113 has an axis for flow through the opening that is parallel to the central axis 101 of the bowl, and thus also parallel with the fuel assemblies 202 of the reactor core 201. This arrangement helps to guide the flow upwards towards the fuel assemblies. As the bowl wall 111 has a finite thickness, the effect of orientating the axes of the flow openings in this way is to create at each opening a bore through the thickness of the curved bowl wall that, with the exception of the bore formed by the one opening located on the central axis, extends non- perpendicularly to the local plane of the bowl wall.

[0052] The rim 150 of the flow distribution element 110 is welded to the underside of the lower core support plate 204 to integrate the rim and the plate. As mentioned above, the rim 150 does not contain flow openings, which is consistent with forming a strong weld.

[0053] The rim 150 may be frustoconical in shape, as shown best in Figure 4. A frustoconical rim may simplify both the process of welding the flow distribution device to the underside of the lower core support plate, as well as the geometry of the weld itself, as it limits the geometry of the weld to a single line of curvature (i.e. around the circumferential direction).

[0054] Located within the bowl-shaped flow distribution element 110 is the vortex suppression element 120. As well as helping to remove unstable flow patterns, e.g. vortices that form due to flow swirl from the downcomer annulus, this element also provides mechanical stability and rigidity to the flow distribution element 110. The vortex suppression element 120 comprises a circumferential row of fins 121 , forming a star-shape at the centre of the flow distribution element. Each fin is an unperforated plate orientated such that any normal to the plate has no vertical component. The flow into the plenum 206 can be highly turbulent with large scale rotational vortices centred on the central axis 101 as well as high-speed eddies in all directions. The fins of the vortex suppression element constrain the coolant flow to suppress the large scale rotational vortices and some of the high-speed eddies that would be detrimental to the functioning of the nuclear reactor.

[0055] There can be any number of fins 121 in the vortex suppression element 120, but in general five, six, seven (as shown in Figures 3, 5 and 6), eight or nine fins combine effective vortex suppression with a low pressure drop. The fins can be formed as flat plates, evenly spaced from each other and extending in the radial direction. However, as shown in Figures 3 and 5, the inner edges of the fins may be radially offset from the central axis to avoid forming a blockage to vertical flow at this position were all the fins to meet at the axis. The vertical height of the fins can decrease in height with increasing radial distance from the central axis. As also shown in Figures 3 and 5, the fins can be formed as continuous solid walls without flow openings formed therein (i.e. unlike the bowl wall and the support rings discussed below). This improves the ability of the fins to suppress rotational vortex flow.

[0056] The fins 121 are supported by at least one support ring. Preferably, as shown in Figures 3 and 5, the fins are supported by two support rings, i.e. an outer support ring 122 and an inner support ring 123. The support rings may have plural flow hole openings 124 to allow coolant to flow through them in the radial direction relatively unimpeded and thus not counteracting the flow redistribution performed by the flow distribution element 110.

[0057] For flexibility of manufacture, the fins 121 can be mounted to the flow distribution element 110 for example through welding or bolted connections.

[0058] The height of the fins relative depth of the bowl may influence the coolant flow. Specifically, as the height of the fins is increased relative to the depth of the bowl, the fins are better able to suppress vortices in the coolant flow.

[0059] However, the top of the vortex suppression element 120 does not project beyond the bottom edge of the rim 150. This produces a gap between the top of the element and the lower core support plate 204, as shown in Figure . The minimum gap between the top of the vortex suppression element and the lower core support plate may be approximately equal to the pitch spacing of adjacent fuel assemblies. This allows the coolant flow to horizontally distribute to a meaningful extent before flowing through the perforations in the lower core support plate, thereby further helping to ensure that all fuel assemblies 202 in the reactor core 201 are adequately and evenly supplied by the coolant flow from the lower core support plate.

[0060] The flow into the plenum should have unimpeded access to all of the flow openings 112, 113 so that the flow distribution device 100 is able to condition substantially all the flow flowing into the plenum from the downcomer annulus. Thus the spacing between the bowl wall 111 and the RPV wall 208 may be substantially constant and equal to the radial width of the downcomer flow annulus 207 at its junction with the plenum 206, providing a smooth continuation of the geometry of the flow path from the downcomer annulus 207 into the plenum 206. Alternatively, as shown in Figure 7, the spacing may smoothly taper to a minimum spacing at the bottom of the bowl. Relative to the spacing at the rim, the reduction in the spacing may be at least 20% and / or at most 50%, e.g. about 35%. This reduction can help to better direct the coolant flow through the device.

[0061] The flow distribution device 100 may be mounted solely to the reactor core 201 , e.g. to the lower core support plate 204 of the core barrel 202 of the reactor core, as shown in Figure 7. Mounting in this manner, i.e. without any attachments to other parts of the RPV 200, ensures that the entire weight of the flow distribution device (ignoring buoyancy effects in use) is carried by the core barrel, avoiding the need for additional connections or contact with the RPV walls 203, 208.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0066] 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.

[0067] 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

Claims:

1. A flow distribution device (100) for a nuclear reactor pressure vessel (200) which encloses a reactor core (201 ) comprising a core barrel (209) containing fuel assemblies (202), the core barrel (209) being spaced from the pressure vessel (200) to define a downcomer flow annulus (207) within the pressure vessel (200) but outside the core barrel (209) and further define a plenum (206) beneath a lower core support plate (204) of the core barrel (209), in use, reactor coolant flowing in sequence downwardly through the downcomer flow annulus (207), through the plenum (206), upwardly through apertures in the lower core support plate (204) and into the core barrel (209); wherein the flow distribution device (100) comprises: a bowl-shaped flow distribution element (110) having a bowl wall (111) which extends upwardly from a base of the bowl to a rim (150) of the bowl, the rim (150) being attachable, in use, beneath the lower core support plate (204) such that the bowl occupies a portion of the plenum (206) with the bowl wall (111) spaced from a bottom of the pressure vessel (208), wherein the bowl wall (111) contains plural flow openings (112, 113) through which the reactor coolant flows into the bowl en route through the plenum (206) to the lower core support plate (204); and a vortex suppression element (120) located within the bowl, the vortex suppression element (120) being configured to suppress vortices in the flow of reactor coolant entering the bowl through the flow openings (112, 113) before the flow passes upwardly through the apertures in the lower core support plate (204); wherein the vortex suppression element (120) comprises a circumferential row of fins (121) which each extend radially across the bowl to suppress circumferential rotational vortex flow of the reactor coolant.

2. The flow distribution device (100) according to claim 1 , wherein the total flow cross-sectional area of the flow openings (112, 113) is at least 15% of the total area of the bowl wall (111).

3. The flow distribution device (100) according to claim 1 or 2, wherein the fins are formed as continuous solid walls without flow openings formed therein.

4. The flow distribution device (100) according to any one of the previous claims, wherein the vortex suppression element (120) comprises one or more support rings (122, 123) coaxially located within the bowl to support the fins (121).

5. The flow distribution device (100) according to any preceding claim having a gap between the top of the vortex suppression element and the lower core support plate, and preferably wherein the gap is equal to or greater than the pitch between adjacent fuel assemblies.

6. The flow distribution device (100) according to any preceding claim having: a first portion of the flow openings (130) which are inside a predetermined radial distance (102) from a central axis (101) of the bowl, and a second portion of the flow openings (140) which are outside the predetermined radial distance(120) from the central axis (101), wherein the area ratio of all of the flow openings (113) of the first portion (130) relative to the area of the bowl wall (111) inside the predetermined radial distance (102) is less than the area ratio of all of the flow openings (112) of the second portion (140) relative to the area of the bowl wall (111) outside the predetermined radial distance (102). . The flow distribution device (100) according to claim 6, wherein each flow opening (113) of the first portion of the flow openings (130) has a smaller flow cross-sectional area than that of each flow opening (112) of the second portion of the flow openings (140).

8. The flow distribution device (100) according to claim 6 or 7, wherein each flow opening (113) of the first portion of the flow openings (130) is circular.

9. The flow distribution device (100) according to any of claims 6 to 8, wherein each flow opening (112) of the second portion of the flow openings (140) is slot-shaped.

10. The flow distribution device (100) according to any preceding claim, wherein the bowl wall (111) is smoothly curved over substantially its entire extent, and preferably may substantially form a hemisphere.

11. The flow distribution device (100) according to any preceding claim, wherein the rim (150) is weldable to the underside of the lower core support plate (204).

12. The flow distribution device (100) according to any preceding claim, wherein an annular portion of the bowl wall (111) forms the rim (150), the annular portion being frustoconical in shape and / or the annular portion is a solid wall portion without any flow openings formed therein.

13. A nuclear reactor pressure vessel (200) which: encloses a reactor core (201 ) comprising a core barrel (209) containing fuel assemblies (202), the core barrel (209) being spaced from the pressure vessel (200) to define a downcomer flow annulus (207) within the pressure vessel (200) but outside the core barrel (209) and further define a plenum (206) beneath a lower core support plate (204) of the core barrel (209), in use, reactor coolant flowing in sequence downwardly through the downcomer flow annulus (207), through the plenum (206), upwardly through apertures in the lower core support plate (204) and into the core barrel (209); and further comprises the flow distribution device (100) according to any preceding claim, the rim (150) of the bowl-shaped flow distribution element (110) of the flow distribution device (100) being attached beneath the lower core support plate (204) such that the bowl wall (111 ) of the bowl-shaped flow distribution element (110) spaced from a bottom of the pressure vessel (208); whereby, in use, the reactor coolant flows into the bowl through the flow openings (112, 113) of the bowl wall (111 ) en route through the plenum (206) to the lower core support plate (204), and the vortex suppression element (120) suppresses vortices in the flow of reactor coolant entering the bowlthrough the flow openings (112, 113) before the flow passes upwardly through the apertures in the lower core support plate (204).

14. The nuclear reactor pressure vessel (200) according to claim 13, wherein: the downcomer flow annulus (207) has a substantially constant radial width around its circumference at its junction with the plenum (206), and the rim (150) of the bowl is radially spaced from the pressure vessel (200) by the same radial width.

15. The nuclear reactor pressure vessel (200) according to claim 13 or 14 wherein the entire weight of the flow distribution device (100) is carried by the core barrel (209).

16. The nuclear reactor pressure vessel (200) according to any of claims 13 to 15, wherein the rim (150) is welded to the underside of the lower core support plate (204).

17. A method of operating the nuclear reactor pressure vessel (200) according to any of claims 13 to 16, the method comprising: flowing reactor coolant in sequence: downwardly through the downcomer flow annulus (207), through the plenum (206), upwardly through the apertures in the lower core support plate (204), and into the core barrel (209); wherein the reactor coolant flows into the bowl through the flow openings (112, 113) en route through the plenum (206), and wherein vortices in the flow of reactor coolant entering the bowl through the flow openings (112, 113) are suppressed by the vortex suppression element (120) before the flow passes upwardly through the apertures in the lower core support plate (204).