PRESSURE-CONTAINING SILO FOR A PRESSURISED WATER REACTOR NUCLEAR POWER PLANT

Abstract

A pressure-containing silo for one or more components on a primary coolant circuit of a nuclear reactor having nuclear fuel assemblies cooled by coolant circulating the primary coolant circuit, the silo defining a release space which, in a loss-of-coolant accident releasing the pressurised coolant water from the one or more components therein, receives and contains the released water and steam, at increasing pressure, formed therefrom; wherein the silo is formed from plural, substantially identical, stacked and joined modular units, each having: a concrete body, a metal liner which lines a surface of the concrete body, and which, when the units are stacked and joined, is sealed edge-to-edge with metal liners of neighbouring units forming an inward-facing, pressure-containing skin surrounding the release space, and plural conduits which, when the units are stacked, align with the conduits of neighbouring units to receive elongate tensioning members for post-stressing the concrete of the bodies.

Claims

1. A pressure-containing silo for one or more components on a primary coolant circuit of a nuclear power plant having a nuclear reactor containing fuel assemblies which are cooled by pressurised coolant circulating around the primary coolant circuit, the silo defining a release space which, in the event of a loss-of-coolant accident releasing the pressurised coolant from the one or more components contained therein, receives and contains the released coolant; wherein the silo is formed from plural, substantially identical, stacked and joined modular units, each modular unit having: a concrete body, a metal liner which lines a surface of the concrete body, and which, when the units are stacked and joined, is sealed edge-to-edge with the metal liners of neighbouring units to form an inward-facing, pressure-containing skin surrounding the release space, and plural conduits which, when the units are stacked, align with the conduits of neighbouring units to receive elongate tensioning members for post-stressing the concrete of the bodies.

2. The pressure-containing silo according to claim 1, wherein each modular unit further has alignment fixtures which engage with corresponding alignment fixtures of neighbouring units to ensure that the units, when stacked, are correctly located relative to each other.

3. The pressure-containing silo according to claim 1, wherein each modular unit further has alignment markings which align to corresponding alignment markings of neighbouring units to ensure that the units, when stacked, are correctly located relative to each other.

4. The pressure-containing silo according to claim 1, wherein the elongate tensioning members extend in three orthogonal directions in the aligned conduits.

5. The pressure-containing silo according to claim 1, wherein the release space is a cylindrical space.

6. The pressure-containing silo according to claim 5, wherein two of the orthogonal directions are perpendicular to the cylinder axis and the third orthogonal direction is parallel to the cylinder axis.

7. The pressure-containing silo according to claim 5, wherein the cylindrical space extends vertically, and is capped at its upper end by a domed head.

8. The pressure-containing silo according to claim 7, wherein the domed head is secured to the silo by bolting at the ends of the elongate tensioning members which extend parallel to the cylinder axis.

9. The pressure-containing silo according to claim 5 wherein each of the modular units extends circumferentially around the cylindrical release space by at least 60°.

10. The pressure-containing silo according to claim 1, wherein the metal liners are sealed edge-to-edge by welding, brazing, gaskets and/or mechanical fasteners.

11. The pressure-containing silo according to claim 1, wherein grouting is inserted between faces of neighbouring modular units when the units are stacked.

12. The pressure-containing silo according to claim 11, wherein the modular units have integral retention formations to shutter the inserted grouting.

13. The pressure-containing silo according to claim 1 wherein the nuclear power plant is a PWR nuclear power plant and the pressure containing silo contains the one or more components on the primary coolant circuit of the PWR nuclear power plant.

14. An array of plural of the pressure-containing silos according to claim 1, each silo being for containing respective components on the primary coolant circuit of the PWR nuclear power plant, wherein components in neighbouring silos are connected by pipework of the primary coolant circuit to transfer the pressurised coolant water therebetween, the neighbouring silos having aligned apertures formed in selected of the modular units through which apertures the connecting pipework extends.

15. The array of claim 14, wherein the neighbouring silos are in close contact such that the entire length of the connecting pipework between the release spaces of neighbouring silos is surrounded by the concrete bodies of the selected modular units of those neighbouring silos.

16. The array of claim 14, wherein a first one of the silos is for containing a reactor pressure vessel of the PWR nuclear power plant, and a second one of the silos is for containing a steam generator of the PWR nuclear power plant, in use the steam generator receiving pressurised coolant water from the nuclear reactor, extracting heat therefrom to generate steam for use in power generation, and returning the pressurised coolant water to the nuclear reactor; wherein the reactor pressure vessel is confined by and positioned within the first silo such that, in the event of the loss-of-coolant accident of the reactor pressure vessel, nuclear fuel elements within the nuclear reactor remain fully covered by the coolant water when the steam pressure within the release space of the first silo reaches an equilibrium level limiting further steam formation; wherein the steam generator is confined by the second silo such that, in the event of the loss-of-coolant accident of the steam generator, the nuclear fuel elements within the nuclear reactor remain fully covered by the coolant water when the steam pressure within the release space of the second silo reaches an equilibrium level limiting further steam formation; and wherein the release spaces of the first and second silos are isolated from each other such that the increasing pressure from the contained steam in either release space is not communicated to the other release space.

17. A PWR nuclear power plant having a reactor pressure vessel containing fuel assemblies which are cooled by pressurised coolant water circulating around a primary coolant circuit, components of the power plant on the primary coolant circuit being contained in respective silos of the array of claim 14.

18. A method for the manufacture of a pressure-containing silo for one or more components of a primary coolant circuit of a nuclear power plant, the nuclear power plant having a nuclear reactor containing fuel assemblies which are cooled by pressurised coolant circulating around the primary coolant circuit, wherein the silo defining a release space which, in the event of a loss-of-coolant accident releasing the pressurised coolant from the one or more components is contained therein, the method comprising: providing a plurality of stacked and joined modular units, each modular unit having: a concrete body, comprising plural conduits to align with conduits of neighbouring units, and a metal liner which lines a surface of the concrete body; stacking the modular units with the metal liners of neighbouring units to form an inward-facing, pressure-containing skin surrounding the release space, and with the conduits of neighbouring units aligned; joining the units; and inserting tensioning members into the concrete bodies to apply a post stressing load to the concrete of the bodies of the modular units.

19. The method according to claim 18, wherein joining the units comprises joining the metal liners edge-to-edge by welding, brazing, gaskets and/or mechanical fasteners.

20. The method according to claim 18 wherein joining the units comprises inserting grouting between faces of neighbouring modular units when the units are stacked.

21. An array of plural pressure-containing silos for one or more components on a primary coolant circuit of a nuclear power plant having a nuclear reactor containing fuel assemblies which are cooled by pressurised coolant circulating around the primary coolant circuit, each silo in the array of plural pressure-containing silos being formed from plural, substantially identical, stacked and joined modular units, and being for containing at least one respective component on the primary coolant circuit of the nuclear power plant, wherein components in neighbouring silos are connected by pipework of the primary coolant circuit to transfer the pressurised coolant water therebetween, each silo defining a release space which, in the event of a loss-of-coolant accident releasing the pressurised coolant from the component contained therein, receives and contains the released coolant, the neighbouring silos having aligned apertures formed in selected of the modular units through which apertures the connecting pipework extends.

22. The array of claim 21, wherein the nuclear power plant comprises a pressurised water reactor, PWR, and wherein a first one of the silos is for containing a reactor pressure vessel of the PWR nuclear power plant, and a second one of the silos is for containing a steam generator of the PWR nuclear power plant, in use the steam generator receiving pressurised coolant water from the nuclear reactor, extracting heat therefrom to generate steam for use in power generation, and returning the pressurised coolant water to the nuclear reactor; wherein the reactor pressure vessel is confined by and positioned within the first silo such that, in the event of the loss-of-coolant accident of the reactor pressure vessel, nuclear fuel elements within the nuclear reactor remain fully covered by the coolant water when the steam pressure within the release space of the first silo reaches an equilibrium level limiting further steam formation; wherein the steam generator is confined by the second silo such that, in the event of the loss-of-coolant accident of the steam generator, the nuclear fuel elements within the nuclear reactor remain fully covered by the coolant water when the steam pressure within the release space of the second silo reaches an equilibrium level limiting further steam formation; and wherein the release spaces of the first and second silos are isolated from each other such that the increasing pressure from the contained steam in either release space is not communicated to the other release space.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0055] FIG. 1 shows a schematic perspective view of a PWR power plant;

[0056] FIG. 2 shows a perspective view of an array of pressure-containing silos of the present invention;

[0057] FIG. 3A-3E show a plan view of a number of arrangements of the pressure-containing silos of the present invention;

[0058] FIG. 4 shows a perspective view of a modular unit of the present invention;

[0059] FIG. 5 shows a perspective view of a domed head which caps a cylindrical release space of a pressure-containing silo of the present invention;

[0060] FIG. 6 shows steps in an example construction sequence for assembling the modular units to form a silo; and

[0061] FIG. 7 shows schematically a cross-section through an end piece for closing a non-opening end of a silo.

DETAILED DESCRIPTION

[0062] FIG. 1 is a schematic diagram of a PWR nuclear power plant 10 having an output which may be in the range from 300 to 1000 MWe. An RPV 12 containing fuel assemblies is centrally located in the plant. Clustered around the RPV are three steam generators 14 connected to the RPV by pipework 16 of the pressurised water, primary coolant circuit. Coolant pumps 18 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.

[0063] A pressurizer 13 maintains the water pressure in the primary coolant circuit at about 155 bar.

[0064] In the steam generators 14, heat exchangers transfer heat from the pressurised water to feed water circulating in pipework 19 of a secondary coolant circuit, thereby producing steam which is used to drive turbines which in turn drive an electricity-generator. The steam is condensed before returning to the steam generators.

[0065] FIG. 2 shows an array 20 of vertically extending, pressure-containing silos 21 for components of a similar power plant. Each silo is capped with a respective domed head 22 and delimits an interior, cylindrical release space, having a typical diameter of about 5 to 7 m. In the example power plant of FIG. 2, an RPV 12, two steam generators 14 and a pressuriser 13 are located within the release spaces of respective silos and connected to one another by pipework 16 of the primary coolant circuit to transfer the pressurised coolant water therebetween. Neighbouring pressure-containing silos have aligned apertures in their walls through which the connecting pipework 16 extends. Coolant pumps 18 are also included on the primary circuit to drive the pressurised water around the primary coolant circuit.

[0066] In the event of a loss-of-coolant accident from one of the components 12, 13, 14, 18 or the pipework 16 of the primary circuit, pressurised coolant water is released into the release space of one of the silos. The released water forms steam which increases in pressure until it reaches an equilibrium level which inhibits further water release and steam formation. The water level in the RPV 12 drops during this release episode, but the pressure-containing silos 21 are configured such that, whichever silo receives the released water, the fuel assemblies within the RPV 12 remain fully covered by the coolant water over the entire episode. In particular, the cylindrical release spaces are relatively low in volume as they are approximately matched to the sizes of the components which they contain, and are also isolated from one another other such that the increasing pressure from the contained steam in a given release space is not communicated to the other release spaces. Relatively small components, such as the pressuriser 13 may not fully fill the release space of the silo 21 in which they are located. In this case, the remainder of the release space may be used to house additional equipment such as chemistry and volume control equipment, heat exchanger equipment, air conditioning equipment, and cooling water tanks.

[0067] The RPV 12 shown in FIG. 2 is for a small modular reactor (SMR) with an output of between 300-1000 MWe requires a silo with a diameter of between 5-7 m. In the event of a loss-of-coolant accident the equilibrium pressure for the pressure-containing silos 21 is approximately 30-50 bar.

[0068] An advantage of the silo arrangement over conventional PWR containment structures is that in the event of a loss-of-coolant accident, equilibrium pressure is reached much more rapidly than in conventional PWR containment structures and thus less coolant water is lost from the primary circuit, such that rapid refilling of water into the RPV to maintain coverage of the nuclear fuel assemblies maybe unnecessary.

[0069] As shown in FIG. 2, it is preferable that neighbouring pressure-containing silos 21 of the array 20 are in close contact such that the entire length of the connecting pipework 16 between the release spaces of the neighbouring silos is surrounded by the walls of the neighbouring pressure-containing silos. This close contact is facilitated by forming the silos with a prismatic outer surface. For example, the silos shown in FIG. 2 have rectangular prismatic outer surfaces, but another option for close-packing the silos is to form them with hexagonal prismatic outer surfaces.

[0070] As also shown in FIG. 2, the pipework 16 may have hot and cold legs which are arranged at spaced vertical heights with the silo walls having sufficient concrete ligaments between the apertures for the respective legs such that any hoop stress in the walls is kept below a safe predetermined level.

[0071] Other options are for the hot and cold legs of the pipework 16 to be arranged at the same vertical height with horizontally-spaced respective apertures, or to be arranged as coaxial, nested pipes. Having coaxial pipes for the respective legs advantageously reduces the number of apertures needed between neighbouring silos 21. When the silos are made of reinforced concrete (as discussed below), reducing the number of apertures advantageously reduces the need to reposition concrete reinforcement members.

[0072] Optionally, part of a boundary of the pressure-containing silo 21 containing the RPV 12 may comprise a heat exchanger configured to convey heat to an exterior of the silo. In the event of the loss-of-coolant accident in the RPV, the released steam condenses on the heat exchanger and runs back to the lowest part of the silo beneath the RPV. Thus, as water escapes the primary circuit, it fills the silo. A valve on the RPV can be set so that when the silo water pressure and the primary circuit water pressure are in a similar range, it opens, to equalise pressure and allow the water in the silo to flow back into the RPV. The silo bottom under the reactor vessel can be sized so that if the total water inventory of the primary circuit is emptied into its volume the reactor core will remain covered by water.

[0073] Another option, however, is for plural of the silos 21 to have respective such heat exchangers. In this case, the silos can be arranged to allow cross-flooding between the silos of the condensed steam in order that the water from the primary circuit can still run to the lowest part of the silo containing the RPV 12. Having more than one heat exchanger installed in different silos provides redundancy in case of damage or blockage to a heat exchanger.

[0074] FIGS. 3A-3E show different possible close-packed arrangements of the pressure-containing silos 21 and the main components (i.e. RPV, steam generator SG, and pressurizer PZR) which they house. FIG. 3A shows the linear arrangement of FIG. 2. FIG. 3B shows a 2 by 2 square layout with two diagonally disposed steam generators. FIG. 3C shows a cross-shaped layout with three steam generators and a pressurizer surrounding a centrally positioned RPV. FIG. 3D shows a 2 by 3 rectangular arrangement with four steam generators located at the corners of the rectangle. FIG. 3E shows an arrangement which has only one steam generator, and the RPV and pressurizer (which may be integral with the RPV) are sized such that they are contained in the same silo.

[0075] Close-packed arrangements other than those shown in FIGS. 3A-3E are possible, for example if the silos 21 have non-rectangular (e.g. hexagonal prismatic) outer surfaces.

[0076] In other arrangements or more of the silos 21 may extend horizontally instead of vertically, and the components within those silos may be suitably adapted for a horizontal orientation.

[0077] The pressure-containing silos 21 shown in FIG. 2 are identical in size, however it is possible to have arrangements wherein the silos are differently sized.

[0078] Each silo 21 is formed from plural, substantially identical, stacked and joined modular units. FIG. 4 shows a perspective view of one such modular unit 40 comprising: (i) a concrete body 41, (ii) a metal (e.g. stainless steel) liner 42 which lines a surface of the concrete body which, when the units are stacked and joined, is sealed edge-to-edge with the metal liners of neighbouring units to form an inward-facing, pressure-containing skin surrounding the release space, and (iii) plural conduits 43, 44, 45 which, when the units are stacked, align with the conduits of neighbouring units to receive elongate tensioning members for post-stressing the concrete of the bodies. Extended reinforcement (i.e. re-bar) 46, 47 projects from the surface of the concrete body of the modular unit. The extended reinforcements of neighbouring modular units are offset when the units are stacked so that they do not mechanically interfere with each other. When the units are joined together by grouting, the extended reinforcement thus fixes the thickness of the grouted joint and can also help to strengthen the joint. The modular unit shown in FIG. 4 extends circumferentially around the cylindrical release space of its silo by 180° and forms a rectangular prismatic outer surface of the silo. A variant unit also compatible with a rectangular prismatic outer surface has a 90° circumferential extent. If the silo has a hexagonal prismatic outer surface of the silo, the circumferential extent of the respective units can be 60°, 120° or 180°.

[0079] The modular units 40 can be manufactured by a moulding process. For example, the units can be cast by pouring concrete into a mould, followed by setting and release. Conveniently, the metal liners can be cast-in as part of such a moulding process. The moulding process facilitates the manufacture of the modular units on standardized production lines either on site or off site in a controlled factory setting.

[0080] Optional features may be included in the modular units 40. Examples are: alignment fixtures (e.g. cones and slots) which engage with corresponding alignment fixtures of neighbouring units to ensure that the units, when stacked, are correctly located relative to each other; alignment markings which align to corresponding alignment markings of neighbouring units to ensure that the units, when stacked, are correctly located relative to each other; sensors which function during the construction of the silos 21 from the units to monitor the handling of the units; sensors which monitor the silos in normal service and in accident scenarios; integral retention formations to shutter the grouting inserted between faces of neighbouring modular units when the units are stacked; integrated heat exchangers for cooling the release space defined by the silo which contains the RPV 12; and apertures in selected modular units through which pipework 16 extends.

[0081] FIG. 5 shows a perspective view of one of the domed heads 22 of FIG. 2. The head has a dome portion 51 in the form of a depressed hemisphere which caps the cylindrical release space of the silo, and a surrounding bolting flange 52. The head, which may be formed of stainless steel, stainless steel-coated carbon steel, or even just suitably painted carbon steel, may have diameter of about 6 m and a thickness of about 60 mm. Conveniently, the head can be secured to the silo by bolting the ends of elongate tensioning members 53 which extend parallel to the cylinder axis through conduits 43 to the bolting flange 52. In other arrangements the domed head may be held in place by a bolting joint or by hinged pressure doors. The head is configured to resist anticipated pressure and temperature loads, without degrading during service.

[0082] Rather than securing the heads 22 to the silos 21 by bolting their flanges 52 to the ends of elongate tensioning members 53, other forms of closure can be adopted, such as forming the heads as hinged pressure doors.

[0083] In some arrangements, a component which is contained within a silo 21 can be fixed to the domed head 22 to allow convenient insertion and removal of the component from the release space of the pressure-containing silo. If a silo contains plural components, then the components can be fixed together to enable their combined insertion or removal in a single operation. For example, the reactor coolant pumps may be located beneath a steam generator, and therefore one potential method of maintenance of the coolant pumps is to unbolt the steam generator and lift the generator and reactor coolant pumps out of the reactor as one unit.

[0084] The bottom end of each silo 21 can be sealed using a concrete plug. The elongate tensioning members may wrap under or through the concrete plug to hold it in place.

[0085] FIG. 6 shows steps in an example construction sequence for assembling the modular units 40 to form a silo 21.

[0086] The modular units 40 are firstly transported from a manufacturing location to a reactor site. The modular units can be sized so that they are transportable by road, rail, water or air. This allows the units to be manufactured at a location or locations away from the reactor site and then transported to a reactor site for assembly. Manufacture of modular units at more than one location allows standardized modular units to be manufactured in parallel which correspondingly reduces the overall construction times. Alternatively, the modular units may be manufactured at the reactor site.

[0087] The pre-cast units may then joined together by grouting them together using concrete. In this example the exposed re-bar (i.e. extended reinforcement 47) is configured so that when two pieces are placed in close proximity these re-bar overlap, whereby when the concrete grouting is set the two units are strongly bound together by the re-bar.

[0088] The modular pre-cast units may be placed directly into their final positions and then fixed in place. Pre-cast units may lifted by crane into their final positions. However, it can be more efficient to assemble two or four units together in a dedicated jig, where they can be precisely aligned, their liners 42 joined, the units grouted together and the completed assembly inspected, before the assembly is craned into position and bonded (i.e. by further welding and grouting) to other previously installed assemblies to build up the silo. Conveniently, the liners 42 are welded together edge-to-edge before the grouting. In this example, only two weld types (horizontal and vertical) need to be managed on site, which facilitates automated welding.

[0089] Once a silo is constructed, post tensioning cables (i.e. elongate tensioning members) are inserted within the conduits 43, 44, 45, before being tensioned. These post tensioned cables apply a large compressive load on the concrete of the units so that even when the silo is fully stressed in an accident scenario, the concrete remains under a compressive load.

[0090] According to possible variants to the approach described above: [0091] The steel liner 42 may incorporate features such as grooves and overlaps which allow it to be sealed to neighbouring liners without welding the liners together. These methods may involve methods such as elastomeric seals and infusing the joint with thermoplastic resins or brazing materials. [0092] The steel liners 42 may be joined together by means of a bolting flange or other similar mechanical joint. [0093] The units 40 may be joined without the use of grouting. For example, mechanical interlocks, such as grooves and crenulations, may be used to hold the units together, these joints being kept under compressive load in-service by the post tensioning cables and may include gaskets. [0094] An end piece 60 for closing the non-opening end of the silo may be provided, as shown schematically in FIG. 7. The end piece can have one on more turn-around passages 62 through which the post tensioning cables can be inserted. This can then avoid a need for a cable stressing gallery at that end of the silo.

[0095] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.