Control rod housing column

Abstract

A control rod housing column (100, fig. 4) for internals of a pressure vessel of a pressurised water reactor. The column is configured to contain a rod cluster control assembly which is slidably movab

Description

Field of the Invention The present invention relates to a control rod housing column for internals of a pressure vessel of a Pressurised Water Reactor (PWR). Background Nuclear reactors are a useful addition to a power grid. Specifically, they provide base load stations, but are also considered a low-carbon source of electricity and are not dependent upon variable weather conditions, which is a limiting factor for other low-carbon sources. One of the most common types of nuclear reactor is the PWR. PWRs are well-understood and are scalable, and thus are suitable for both large-scale power plants as well as for small modular reactors. Figure 1 is a schematic diagram of a PWR 20 that uses boron-free pressurised water as its primary coolant. A reactor pressure vessel (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 23 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 electricitygenerator. The steam is then condensed before returning to the steam generators. The pressuriser maintains a pressure of around 15.5 MPa in the primary circuit. Figure 2 shows schematically a cutaway longitudinal section through the RPV 22, including its lower and upper internals. The lower internals include a core barrel 30 which houses the fuel assemblies (not shown). The upper internals include control rod housing columns (CRHCs) 46 for the fuel assemblies. In operation, the pressurised water primary coolant from the steam generators 24 is received through inlet nozzles 32 of the RPV 22, and flows through the downcomer annulus 34 formed between the inside of the RPV and the outside of the core barrel 30, before turning up through a flow distribution device 36 located in the lower head of the RPV. This device diverts, straightens and distributes the primary coolant flow prior to its entry into the core barrel through apertures in a lower core support plate 38 which interfaces mechanically with the fuel assemblies, providing them with radial and axial restraint. A radial neutron reflector 40 surrounds the fuel assemblies in the core barrel 30, helping to even the burnup of fuel at the core periphery by reflecting fast neutrons and also helping to reduce radiation damage to the RPV. The primary coolant is heated by the fuel assemblies as it flows upwards through them to exit the core barrel 30 through respective apertures in an upper core plate 42. The primary coolant then enters an outlet plenum containing the RPV’s CRHCs 46 before exiting though outlet nozzles 48 of the RPV and circulating back to the steam generators 24. The CRHCs 46 align and support respective rod cluster control assemblies (RCCAs), each RCCA having a cluster of control rods which are slidably movable through the corresponding fuel assembly below via a drive shaft at the top of the RCCA to regulate the rate of fission in the fuel assembly. As well as maintaining the vertical free-path of the control rods by guiding them as they are inserted into and withdrawn from the reactor core, a further important function of the CRHCs 46 is protecting withdrawn control rods from primary coolant flow-induced loads. Thus the CRHCs need to have a high stiffness so that deformation under extreme loading conditions is kept within acceptable limits. In PWR’s that use boronated pressurised water as the primary coolant, a portion of the fuel assemblies do not have corresponding CRHCs, and the primary coolant exiting these fuel assemblies enters the outlet plenum directly after passing through the corresponding apertures in the upper core plate. For other fuel assemblies with corresponding CRHCs located above them, primary coolant exiting these assemblies must first pass through their CRHCs before it can join the rest of the primary coolant in the outlet plenum. However, relative to a PWR that uses boronated pressurised water as its primary coolant, a PWR that uses boron-free pressurised water, such as illustrated in Figures 1 and 2, generally requires a larger proportion of its fuel assemblies to have corresponding RCCAs and CRHCs in order to exercise adequate control over the rate of fission in the reactor core. Summary of the Invention An insight of the present inventor was that, particularly in a PWR that has a high density of CRHCs, it is desirable that any pressure drop of the primary coolant in its passage through the CRHCs is minimised so that overall performance of the PWR 20 can be maintained. Moreover, when there is a greater reliance on control rods to control the rate of fission, the need for the CRHCs to tolerate extreme loading increases. Accordingly, in a first aspect, the present invention provides a control rod housing column for internals of a pressure vessel of a pressurised water reactor; wherein the column is configured to contain a rod cluster control assembly such that the control assembly is slidably movable along the axial direction of the column, whereby, in use, with the axial direction of the column extending vertically in the pressure vessel, the rate of fission in a corresponding fuel assembly of a reactor core located in the pressure vessel below the column can be regulated by control rods of the control assembly, a lower end portion of the column terminating in a bottom face through which the control rods insert into the corresponding fuel assembly and also through which an upward flow of pressurised water, heated by the fuel assembly, is received into the column; and wherein the lower end portion of the column provides one or more flow windows spaced above the bottom face through which the flow of heated pressurised water received into the column through the bottom face subsequently exits the column, the flow windows being configured such that the direction of the exiting flow is substantially parallel to the axial direction of the column. Advantageously, by enforcing a direction of the exiting flow that is substantially parallel to the axial direction of the column, flow disturbances and sudden turbulent mixing can be reduced, helping to reduce primary coolant pressure drops and better ensuring desirable flow behaviour in the outlet plenum. In particular, by avoiding radially directed exiting flows, flows from CRHCs can be prevented from immediately impinging on adjacent CRHCs, or similar radial flows from these CRHCs, with consequent turbulent mixing and pressure loss. Moreover, the configuration of the one or more flow windows to allow axial exit flow facilitates the adoption of geometries at the base of the CRHC having enhanced lateral stiffness. Consistent with achieving these advantages, preferably the column is configured such that substantially all of the flow of heated pressurised water received into the column through the bottom face subsequently exits the column through the one or more flow windows. For example, between its ends, and apart from the flow windows, the column may present, in use, a substantially continuous external surface without openings. Conveniently, the column may form an external shoulder region spaced from the bottom face at a top of the lower end portion, the column being narrower above the shoulder region, and the one or more flow windows being provided in the shoulder region. To provide at least some of the enhanced lateral stiffness, the second moment of area of the lower end portion about the axis of the column may be greater than the second moment of area of the column immediately above the lower end portion about the axis of the column. For example, it may be greater by a factor of at least 1.3, and preferably by at least 1.5. At the lower end portion, the column may have an external surface that forms a square shape on a crosssection perpendicular to the axis of the column. Such a shape can provide the lower end portion with a high second moment of area. The corresponding cross-sectional shape of the column immediately above the lower end portion can then be, for example, circular, octagonal, square or square with rounded corner, but fitting within the perimeter of the square shape of the lower end portion. In this way, the one or more flow windows can be formed by the transition of the column between these cross-sectional shapes, e.g. at the above-mentioned shoulder region. The column typically contains channels configured to slidably receive the control rods and a drive shaft of the rod cluster control assembly, the channels thereby providing lateral support to the control rods and drive shaft. For example, the channels may be formed by plural guide members which are axially spaced along the column. Such an arrangement can conveniently be produced by forming the column in sections, which are subsequently joined together to produce the complete column. The column may carry one or more temperature measuring devices, such as thermocouples, inside the flow windows arranged to measure the temperature of the pressurised water exiting the corresponding fuel assembly. As the direction of the exiting flow is substantially parallel to the axial direction of the column, which is typically also the direction of flow of water in the outlet plenum from fuel assemblies without corresponding control rod housing column, mixing of the exiting flow with the surrounding water is generally quite gradual and the measured temperature can thus be a good proxy for the amount of heating provided by the corresponding fuel assembly. This in turn allows the measured temperature to be used to detect failure of correct movement of the control rods of the control assembly in the corresponding fuel assembly, as discussed in more detail below in relation to the fifth aspect. In a second aspect, the present invention provides upper internals of a pressure vessel of a pressurised water reactor, the upper internals including: an upper core plate which, in use, locates above a reactor core of lower internals of the pressure vessel, the upper core plate containing plural apertures which accept an upward flow of pressurised water heated by respective fuel assemblies of the reactor core; plural control rod housing columns of any one of the previous claims, the bottom face of each column being fixed at a respective aperture of the upper core plate to receive the upward flow of pressurised water accepted by that aperture; and plural rod cluster control assemblies respectively contained in the control rod housing columns. In a third aspect, the present invention provides a pressure vessel of a pressurised water reactor, the pressure vessel containing: lower internals including a lower core support plate and a reactor core formed from plural fuel assemblies located on the lower core support plate; and the upper internals of the second aspect located above the lower internals such that the apertures of the upper core plate are respectively aligned with the fuel assemblies. In a fourth aspect, the present invention provides the use of a pressurised water reactor having the pressure vessel of the third aspect, wherein the pressurised water in the pressure vessel is boron-free pressurised water. In other words, the fourth aspect provides a method of operating a pressurised water reactor having the pressure vessel of the third aspect, the method comprising using boron-free pressurised water as a neutron moderator and coolant in the pressure vessel. Advantageously, boron-free pressurised water allows the core duty of the reactor to be increased. Furthermore, without boric acid as a soluble shim, less waste is produced by the reactor. In a fifth aspect, the present invention provides a method of detecting failure of correct movement (e.g. insertion or withdrawal) of control rods into a fuel assembly of the pressure vessel of the third aspect, at least some of the control rod housing columns of the pressure vessel carrying one or more temperature measuring devices inside their flow windows arranged to measure the temperature of the pressurised water exiting the corresponding fuel assemblies, the method including: receiving measurements of the temperature of the pressurised water exiting a given control rod housing column at its one or more flow windows using the one or more temperature measuring devices of that column; comparing the measured temperature with a corresponding predicted temperature for the pressurised water exiting the given column at the one or more flow windows, the predicted temperature being calculated on the basis of the axial position of the rod cluster control assembly in the column; and detecting a failure of correct movement of the control rods of the control assembly into the corresponding fuel assembly when the measured temperature of the pressurised water diverges from the corresponding predicted temperature by more than a predetermined amount. As previously noted, PWRs which use boron-free pressurised water are more reliant on control rods for control of the rate of fission. Thus the importance of the ability to be able to identify and monitor for individual control rod positioning failures (known as a single control rod withdrawal event, or SCRWE, errors) is increased. The amount of heating provided by a fuel assembly is directly dependent on the axial position of its control rods. Thus advantageously, the presence of a predictable relationship between the temperature of the exiting water and the amount of heating allows SCRWE errors to be detected in a manner that is independent of any actual measurement of rod axial position. In a sixth aspect, the present invention provides a control system (e.g. an analogue control system or a computer control system) for detecting failure of correct movement of control rods into a fuel assembly, the system being arranged (e.g. programmed) to perform the method of the fifth aspect. In a seventh aspect, the present invention provides computer program comprising code which, when the code is executed on a computer, causes the computer to perform the method of the fifth aspect. In an eight aspect, the present invention provides computer readable medium storing the computer program of the seventh aspect. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows a schematic diagram of a pressurised water reactor; Figure 2 shows schematically a cutaway longitudinal section through a pressure vessel of the reactor; Figure 3 is a longitudinal cross section through the upper internals of a pressurised water reactor; Figure 4 is a perspective view of a control rod housing column of the reactor of Figure 3; Figure 5 shows a cutaway longitudinal section through the column of Figure 4; and Figure 6A shows a perspective view of the lower portion of the column of Figures 4 and 5; Figure 6B shows a cutaway longitudinal section through the lower portion, Figure 6C shows a longitudinal cross section through the lower portion; and Figure 6D shows a transverse cross section through the lower portion on plane A-A of Figure 6C. 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. All documents mentioned in this text are incorporated herein by reference. Figure 3 is a longitudinal cross section through the upper internals of a boron-free PWR 60. Illustrated are: the PWR’s pressure vessel 62, an inlet nozzle 64, the downcomer annulus 66, an outlet nozzle 68, the upper core plate 70, and an upper plenum bulkhead72. An array of CRHCs 100 is mounted in an outlet plenum formed between the topside of the upper core plate and the underside of the upper plenum bulkhead which provides a means of securing the upper internals components, including the CRHCS, in their correct axial and radial positions. Fuel assemblies (not shown) of the reactor core are located below the upper core plate with their top outlet nozzles connecting to respective apertures formed in the upper core plate. Figure 4 is a perspective view of one of the CRHCs 100. Each CRHC has a bottom face 102 that joins to one of the apertures in the upper core plate 70, so that the column has a one-to-one relationship with a corresponding fuel assembly below. The CRHC 100 is installed in the pressure vessel 62 with the axial direction of the column extending vertically in the pressure vessel. The CRHC contains an RCCA (not shown) that is slidably movable along the axial direction of the column. A known form of RCCA, illustrated for example in EP 0316631 A (hereby incorporated by reference), comprises a spider assembly having a central hub for connection with a drive mechanism, a plurality of vanes radially extending outwardly from the hub, and connection fingers secured to the vanes. The upper ends of control rods are attached to the fingers so that in use the rods suspend downwards from the spider assembly for insertion, via the bottom face of the CRHC, into corresponding guides in the fuel assembly below. By operating the drive mechanism, the RCCA can be slidably moved along the axial direction of the CRHC, with the result that the control rods can be inserted or withdrawn as needed from the fuel assembly to control the rate of fission in the fuel assembly and thus its heat production. This produced heat is transferred to the pressurised water coolant which flows upwards through the reactor core, leaving each fuel assembly through its top outlet nozzle. A portion of the fuel assemblies do not have corresponding CRHCs 100, and the flow of coolant from these assemblies passes through the upper core plate 70 and directly enters the outlet plenum of pressure vessel. However, for a further portion of the fuel assemblies that do have corresponding CRHCs, the flow of coolant passes through the upper core plate, enters the columns through their bottom faces 102, and only subsequently exits the columns into the outlet plenum, as discussed in more detail below. The CRHC 100 has a lower end portion 106 that provides the bottom face 102, a main central portion 108 that extends from the lower end portion to a flange 110 that joins to an underside of the upper plenum bulkhead72, and a top portion 112 that extends upwards from the topside of the upper plenum bulkhead. The top face of the top portion contains a central aperture 114 providing drive mechanism access to the central hub of the RCCA contained in the CRHC. Figure 5 shows a cutaway longitudinal section through the CRHC 100. Spaced along the column axis of the central portion are guide members 104 each containing channels 128 (best seen in Figure 6D discussed below) which slidably receive the control rods and drive shaft of the RCCA. Similar channels are also formed in the bottom face 102 of the lower end portion and the flange 110. Conveniently, the central portion can be formed from plural machined sections which are then fabricated together, each section providing a respective guide member. Figure 6A shows a perspective view of the lower end portion 106 of the CRHC 100, including its junction with the central portion 108; Figure 6B shows a cutaway longitudinal section through the lower end portion, Figure 6C shows a longitudinal cross section through the lower end portion; and Figure 6D shows a transverse cross section through the lower end portion on plane A-A of Figure 6C. The central portion 108 has an external surface that produces a rounded corner square shape on a crosssection perpendicular to the axis of the column. In contrast, the lower end portion 106 has an external surface that produces a square shape on a cross-section perpendicular to the axis of the column. This shape is formed by four flat walls 116 which are sized so that the bottom end of the central portion inserts coaxially a short distance into the lower end portion with a perimeter gap between the central portion and the flat walls of the lower end portion. Elongate connecting ribs 118, circumferentially spaced around the central portion and oriented with their length directions parallel to the axial direction of the CRHC 100, are each welded along one edge to one of the walls and along an opposite edge to the external surface of the central portion. In this way the central portion is securely fixed to the lower end portion while, viewed along the axial direction, there is little obstruction of the perimeter gap. The insertion of the central portion into the lower end portion effectively provides the column with an external shoulder region 126 at the top of the lower end portion, the column being narrower above the shoulder region. The bottom face 102 of the lower end portion 106 is formed by a square end block 120, with the walls 116 extending upwards from respective sides of this square. The block has a large central aperture 122 that allows the upward flow of coolant from the corresponding fuel assembly to pass relatively unimpeded into the CRHC 100, the perimeter of the aperture being shaped to provide guidance to the outermost control rods of the RCCA. The perimeter gap between the lower end portion 106 and the central portion 108 is divided by the connecting ribs 118 into a number of flow windows 124 through which the flow of coolant received through the central aperture 122 subsequently exits the column, as indicated by the grey arrowed line on Figure 6C. In particular, the flow windows 124 at the corners of the square cross-sectional shape have the largest areas and receive the majority of the flow. The windows are configured such that the direction of the exiting flow is substantially parallel to the axial direction of the column, and being at the aforementioned external shoulder region 126, the flow can continue upwards unimpeded therefrom into the outlet plenum and parallel to the external surface of the central portion. Substantially all the flow exits the column in this way as there are no openings in the walls 116 to produce radially-directed flows. The internal and external surfaces of the CRHC 100 exposed to the upward coolant flow provide little obstruction to the flow or encouragement for turbulence, and thus significant pressure drops produced by the coolant’s passage through the column can be avoided. The smooth external surface of the central portion 108 with its avoidance of abrupt corners is particularly significant in this regard. In addition, however, the axially directed flow of the coolant on exit from the column avoids a situation where the exiting coolant directly impinges on corresponding flows from adjacent columns or on the columns themselves. This helps to avoid further pressure losses associated with turbulent mixing. A further benefit associated with the flow out of the CRHCs 100 is that the amount of flow through a given column can be regulated by appropriate sizing control of the geometries of the flow windows 124. In association with control of the sizing of the apertures in the upper core plate 70, this provides a means to regulate the upwards coolant flow across the entire upper core plate, e.g. with an aim of providing a uniform flow rate across the area of the plate. A primary function of the CRHCs 100 is to maintain the vertical free-path of the control rods under all conceivable conditions so that the control rods can control the rate of fission in the fuel assemblies. For example, in the unlikely event of catastrophic failure of the pipework connecting the pressure vessel 62 to the steam generators there could be sudden loss of coolant from the vessel in a so-called LB-LOCA (large break loss of cooling accident) that imposes high sudden sideways loads on the CRHCs. Because the inlet nozzles 64 and outlet nozzles 68 are positioned approximately midway up the columns, these sideways loads can produce high turning moments on the base of columns, i.e. at the lower end portion 106. However, due to its expanded cross-sectional area relative to the central portion, the second moment of area of the lower end portion about the axis of the column is greater than the corresponding second moment of area of the central portion. Moreover, this second moment of area of the lower end portion is maintained over the entire axial length of the lower end portion by the avoidance of openings in the walls 116 for producing radially-directed flows. Thus the column has a geometrically-enhanced lateral stiffness at its base which helps it to resist deformation under a LB-LOCA and maintain the vertical free-path. For example, the second moment of area of the lower end portion about the axis of the column may be greater than the corresponding second moment of area of the central portion by a factor of at least 1.3 and preferably by at least 1.5. In some examples the second moment of area may be increased by a factor of 1.6. A further advantage that follows from the configuration of the flow windows 124 is that, as the mixing of the exiting flow with the surrounding coolant in the outlet plenum is gradual, the temperature of the coolant inside the windows is a good proxy for the temperature of the exiting flow from the corresponding fuel assembly and thus the amount of heating provided by the fuel assembly. Thus, temperature measuring instrumentation such as thermocouples may be beneficially positioned inside the windows of at least some of the CRHCs 100. Conveniently, power and / or communication cabling for the instrumentation can be routed down the external surfaces of these CRHCs 100. The temperatures measured by this instrumentation can then be compared with corresponding predicted temperatures for the coolant calculated on the basis of the axial positions of the RCCAs in their CRHCs 100. For example, a SCRWE failure can be detected independently of any actual measurement of control rod axial position by monitoring for divergence of one or more measured temperatures from the corresponding predicted temperatures by more than a predetermined amount. *** The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

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