Vessel filled with liquid metal having a cover plug with decoupled sealing and mechanical strength, method for opening and reclosing the vessel of a fast neutron nuclear reactor cooled by liquid metal

Abstract

The invention relates to a device for removably closing a vessel (10) filled with liquid metal, - a plug (12) removably fastened to the vessel while closing it by means of a device with flange(s) / counter-flange(s) assembled together by a detachable connection; - sealing means between the vessel and the plug in its fastened position, comprising at least means for freezing the same liquid metal and keeping the metal in the frozen state over at least part of the height of the annular space between the plug and the vessel.

Description

[0001] Description

[0002] Title: Liquid metal filled tank, with decoupled sealing and mechanical resistance cap-lid, Related process of opening and resealing the tank of a liquid metal cooled fast neutron nuclear reactor.

[0003] technical field

[0004] The present invention generally relates to the field of so-called fast neutron nuclear reactors.

[0005] It relates in particular to a fast neutron nuclear reactor cooled with liquid metal, in particular liquid sodium, known as SFR (Sodium Fast Reactor), lead or a lead alloy, or generally known as LMBFR (Liquid Metal Breeder Fast Reactor), which is part of the family of so-called fourth-generation reactors (GEN IV).

[0006] The invention relates even more particularly to the aforementioned loop-type reactor, that is to say, one in which the heat exchangers and the means for pumping the primary fluid, such as sodium, are located outside the reactor vessel.

[0007] The invention is preferably applicable to small or medium power reactors or SMRs (acronym for "Small Modular Reactor"), typically with an operating power between 50 and 300 MWe.

[0008] For the purposes of this invention, "SMR reactor" means the usual technological meaning, namely a nuclear fission reactor, smaller in size and power than conventional SLR reactors, some of whose blocks are manufactured in a factory and transported to a nuclear site for installation.

[0009] For the purposes of this invention, "reactor block" means the vessel, also known as the reactor vessel or primary vessel, and all the components and parts of the fluidic circuit, including the reactor core which generates heat through nuclear fission reactions and is housed inside the reactor vessel.

[0010] More specifically, the invention relates to a new design of the cap-lid of such a reactor vessel. Although described with reference to a liquid sodium cooled reactor, the invention can be implemented for other metals, such as lead, a lead-bismuth alloy.

[0011] Previous technique

[0012] The currently known sodium-cooled fast neutron reactors (SFRs) can be classified into two types:

[0013] - The most common type is called integrated because the primary liquid sodium circuit, including pumping equipment and heat exchangers, is entirely contained within a single vessel called the reactor vessel. An integrated reactor also includes secondary liquid sodium circuits (piping, pumps and steam generators) arranged outside the reactor vessel and a tertiary water / steam circuit which is associated with a turbine for electricity production;

[0014] - the other is called looped because the primary sodium circuit is arranged in several tanks with intermediate heat exchangers and means of pumping out of the reactor tank. A looped reactor also includes secondary liquid sodium circuits (piping, pumps and steam generators) and a tertiary water / steam circuit which is associated with a turbine for the production of electricity.

[0015] These two types of reactors have architectures that require the choice of a removable cover to allow the unloading of fuel assemblies from the top of the reactor vessel.

[0016] A primary vessel closure cover for a sodium-cooled reactor, which is leak-proof and removable, cannot be designed as a primary vessel cover for a pressurized water reactor (PWR).

[0017] Indeed, the high sodium temperatures (typically between 550 and 630°C) and the high-amplitude temperature transients (typically 150°C in one minute) make a bolted fastening and sealing design, such as that used in a PWR, incompatible due to creep and the risk of bolt loosening. Furthermore, the absence of a gas head beneath the lid exacerbates the thermomechanical loads on the lid.

[0018] A closing cover, usually called a Core Cover Plug (CCP), must perform the following functions: - a mechanical function: to support and position the control mechanisms of the rods and the monitoring instrumentation of the reactor core;

[0019] - a sealing function: to contribute to the sealing of the upper closure of the reactor block;

[0020] - a hydraulic function: to deflect the sodium jets at the exit of the core and to calm the surface of the sodium in the hot collector of the reactor block;

[0021] -a protective function that is both biological and thermal.

[0022] Several types of Core Cover Plug (CCP) have already been proposed for liquid sodium-cooled reactors, which can be summarized as follows:

[0023] - a straight-shaped BCC as in the SUPERPHENIX reactor project;

[0024] - a conical-shaped BCC as in the EFR reactor project;

[0025] - a conical BCC allows a reduction in the diameter of the primary tank;

[0026] - a split BCC as in the JSFR reactor project, the slot of which is necessary for the passage of a pantograph arm for handling fuel assemblies;

[0027] - a two-part BCC which is integrated on one side into a Small Rotating Plug (PBT) and on the other side into a Large Rotating Plug (GBT), in order to significantly reduce the size of the rotating plugs and potentially the diameter of the primary tank.

[0028] Not all of these solutions are really suitable for an SMR reactor, for which one seeks in particular optimal flexibility when changing fuel assemblies within it.

[0029] There is therefore a need to further improve the core plugs of fast neutron nuclear reactors, particularly those cooled with liquid metal, especially of the loop type, so that they are suitable for SMRs.

[0030] More generally, there is a need to improve the sealing plugs of tanks filled with liquid metal.

[0031] The aim of the invention is to meet at least part of this need.

[0032] Description of the invention

[0033] To this end, the invention relates, in one of its aspects, to a removable closure device for a tank filled with liquid metal, comprising: - a closure plug, fixed in a removable manner to the reactor tank by closing it, by means of a device with flange(s) / counter-flange(s) assembled together by a removable connection;

[0034] - means of sealing between the tank and the stopper in its fixed position, including at least means for freezing the same liquid metal and maintaining the metal in a frozen state over at least part of the height of the annular space between the stopper and the tank.

[0035] According to an advantageous configuration, the tank includes in its upper part, a ferrule forming a chimney in which the plug is mounted, the sealing means including at least one cooling circuit to freeze the liquid metal, arranged over at least part of the peripheral height of the chimney.

[0036] The cooling system is advantageously liquid-based, preferably oil-based.

[0037] According to an advantageous embodiment, the sealing means further comprise a portion of an inert gas circuit, preferably argon, arranged over at least part of the height of the annular space between the sealing plug and the tank, above the frozen metal. This embodiment thus improves the seal obtained by freezing the metal.

[0038] To further enhance the seal, the sealing means also include a double gasket arranged between the flanges and counter-flange when they are assembled together.

[0039] According to another advantageous embodiment, the stopper includes heating means for thawing the metal, so that the heart-shaped stopper-lid can be opened. In this embodiment, the thawing means preferably comprise an electric heating system arranged on the outer ring of the stopper.

[0040] In an advantageous variant, the device includes heat-insulating material arranged inside and against the outer ring of the cap. This ensures good thermal insulation, particularly to prevent the metal from freezing.

[0041] The liquid metal could be liquid sodium.

[0042] According to a preferred application, the device is integrated into a liquid-metal cooled fast neutron nuclear reactor. Such a reactor comprises a reactor block and at least one device as described above, the vessel constituting the reactor vessel, and the plug being the reactor core cover plug. The invention also relates, in another aspect, to a method for opening and resealing the reactor vessel of a nuclear reactor as described above, comprising the following steps:

[0043] i / shutdown of reactor operation;

[0044] ii / thawing of the metal by activating heating means;

[0045] iii / disassembly of the flange(s) of the counter-flange(s);

[0046] iv / docking of a transfer chamber above the cap-cover on the flange(s), v / docking of a first gate valve above the chamber, then putting the chamber under inert gas;

[0047] vi / docking, above the first gate valve, of a withdrawal hood (8) equipped in its lower part with a second gate valve (80), then putting the hood under inert gas;

[0048] vii / opening of the first and second gate valves, then lifting of the cap-lid in the hood;

[0049] viii / closing of the first and second gate valves;

[0050] ix / moving the hood to a storage location.

[0051] steps ii / to ix / being reversed to achieve the resealing of the reactor vessel.

[0052] By "reverse" we mean that the steps are carried out in the opposite direction.

[0053] Thus, the invention essentially consists of proposing, for a tank containing a liquid metal, a stopper whose function of retention and mechanical resistance is ensured by a flanged and counter-flange device, which is decoupled from the sealing achieved first by a freezing of the metal in the annular space between tank and stopper, sealing advantageously improved by a circuit of inert gas above the frozen metal.

[0054] The invention finds particular application for a nuclear reactor vessel cooled with liquid metal, which is the same metal which in its frozen state will serve as a means of sealing between the reactor vessel and the core cap-lid.

[0055] Ultimately, compared to those of liquid metal-cooled nuclear reactors according to the state of the art, a core cap-lid for a nuclear reactor according to the invention offers numerous advantages, including:

[0056] - a core-shaped cap-lid with decoupled mechanical resistance and sealing that can be installed quickly and reliably;

[0057] - the absence of risk of contamination of the liquid metal bath contained in the tank, due to the fact that the liquid metal ensuring the seal is the same metal;

[0058] - a perfect fit for SMR type reactors.

[0059] Other advantages and features of the invention will become clearer upon reading the detailed description of examples of implementations of the invention given by way of illustration and not limitation with reference to the following figures.

[0060] Brief description of the drawings

[0061] [Fig 1] Figure 1 is a schematic view of a liquid sodium cooled nuclear reactor, Na-NR with a cap-lid according to the invention.

[0062] [Fig 2] Figure 2 is a schematic view of a core cap-lid according to the invention in its configuration fixed on the primary vessel of a Na-NR reactor.

[0063] [Fig 3] Figure 3 is a schematic view of a nuclear power plant with a nuclear reactor as shown in Figure 1 and the core cap-lid handling system for the different steps implemented for opening and reopening the reactor vessel.

[0064] [Fig 4] Figure 4 is a schematic view of a core cap-lid with different embodiment variants according to the invention, in its configuration fixed on the primary vessel of a Na-NR reactor.

[0065] Detailed description

[0066] Throughout this application, the terms "horizontal", "vertical", "lower", "upper", "below" and "above" are to be understood by reference to a reactor vessel, or primary vessel, arranged vertically and to an arrangement in relation to the cold or hot zone.

[0067] Similarly, throughout this application, the terms "upstream" and "downstream" are to be understood with reference to the direction of sodium flow.

[0068] Figure 1 shows a reactor block of a loop-type Na-RF nuclear reactor, with a globally designated by the reference 1. The reactor block 1 includes first of all a reactor vessel 10 with central axis X, inside of which is arranged a core 11 in which the heat is released following the nuclear reactions.

[0069] The vessel 10 is closed by a plug 12, commonly called the "core lid plug," which includes the instrumentation necessary for the control and proper functioning of the nuclear reactions, and detailed below.

[0070] The core 11 consists of fuel assemblies 13. Advantageously, these assemblies are devoid of a box or hexagonal tube around the needles which constitute them, and made as described and claimed in the patent application filed by the applicant on November 19, 2024 under number FR2412649 and entitled "Fuel assembly for nuclear reactor, of type RNR-Na, devoid of a box, comprising a bundle of fuel needles held laterally by grids with cells and longitudinally by stiffening rods, passing through the grids".

[0071] The core 11 may advantageously include in its center an assembly or rod 14 of an autonomous system that ensures the automatic shutdown of the reactor. The control motor for this system is housed in the plug 12, the control linkage extending from the plug to the assembly or rod 14.

[0072] The core 11 is supported by a base 15 into which are embedded the feet of the assemblies 13 constituting the core and which has fluidic passages, this base 15 being supported by a non-watertight platform 16 resting on the bottom of the tank 10.

[0073] A so-called hot collector 17 is separated from the so-called cold collector 18 below by a suitable separating device 19, called a step, in the form of a metal ferrule. As illustrated, this ferrule 19 has a frustoconical shape in its upper part, which is extended by a cylindrical shape in its lower part, down to the level of the spring 15.

[0074] Empty spaces of fuel assemblies 13 can be provided between the core 11 and the step 19.

[0075] The reactor advantageously includes a reactivity control system 2 implemented as described and claimed in the patent application filed by the applicant on December 6, 2024 under number FR2413620 and entitled "Reactivity control system of a rotating non-fueled nuclear reactor assembly, at the periphery of the core". In substance, this system 2 is arranged partly in the annular space between the core 11 and the reactor vessel 10, and it comprises a plurality of cylindrical locations distributed, preferably regularly, in the annular space.

[0076] Each cylindrical location houses a part of a subset 20 of system 2 comprising at least one non-combustible assembly or at least one non-combustible monobloc bar, generally cylindrical in shape, comprising at least one neutron-absorbing material.

[0077] The assemblies or control bars 200 of each sub-assembly 20 of the reactivity control system 2 are arranged in the cold manifold 18 to be traversed by the heat transfer fluid not passing through the core 11 so as to be cooled by it, as detailed below.

[0078] As illustrated in Figure 1, in order to occupy the maximum space in the cold collector 18, the identical subassemblies 20 can be arranged so that they are joined to each other.

[0079] The cold collector 18 can also house assemblies of pencils or monobloc bars of neutron reflector material 180, preferably assembled in the form of baskets, in the spaces not occupied by the sub-assemblies 20 of the reactivity control system 2.

[0080] The heat dissipation circuit followed by the sodium S in normal operation of the core 11 is schematically represented by the dashed arrows in figure 1: - the cold sodium arriving in the cold collector 18 descends between the tank 10 and the step 19 but without passing through the assemblies or bars 200 which are housed in a cylindrical tube, 210, sealed on its periphery except for its lower end, forming a jacket which therefore isolates the assemblies or bars 200 from the cold sodium circulating in this space between cold collector 18 and tank 2,

[0081] - the cold sodium reverses at the level of the bed 15 so that a major flow reaches the core 11 after passing through the decking 16 and the bed 15 and the remaining flow reaches the assemblies or control bars 200,

[0082] - the main flow of sodium becomes hot as it passes through the core 11 and rises to be evacuated from the vessel 10 from the hot collector 17,- the remaining flow of cold sodium cools by direct contact all the jackets 210 and the assemblies or bars 200 within which it circulates, thanks to the presence of an inlet plug at its lower end and one at its upper end allowing the passage of sodium.

[0083] The primary / secondary exchange loops 2 are all arranged inside a sealed enclosure 4 of axisymmetric shape around the X axis of the reactor vessel 11, which is illustrated in Figure 3. This sealed enclosure 4 constitutes a third containment barrier, with the fuel assemblies and the reactor vessel constituting the first and second containment barriers respectively.

[0084] Reactor block 1 and containment structure 4 are arranged inside a reactor building 5 consisting of an underground concrete infrastructure, which performs various functions, including contributing to the containment safety function. Thus, reactor building 5 constitutes a fourth containment barrier.

[0085] Typically, the building envelope of reactor 5 can consist of several layers. As illustrated in Figure 3, the envelope can be a non-prestressed concrete wall, serving as the interface with the outside.

[0086] As also illustrated in Figure 3, reactor building 5 can be fully buried in soil S.

[0087] According to the invention, as shown in figure 2, the heart-shaped cap-lid 12 firstly comprises an external cylindrical ferrule 120 extended downwards by a domed shape.

[0088] This ferrule 120 is housed in a cylindrical ferrule 100 forming a chimney in the upper part of the reactor vessel 10.

[0089] The shell 120 is removably fixed to the reactor vessel 10 by closing the latter, by means of a device 121 with flange 122 and counter-flange 123 assembled together by a removable connection, in particular by means of bolts 124. The flange 122 is preferably one piece with the shell 100 of the vessel 10.

[0090] To ensure the seal between the reactor vessel 10 and the core cover plug 12 in its fixed position, at least one cooling circuit 125, preferably oil-cooled, is arranged over at least part of the peripheral height of the chimney 100. This cooling circuit 125 is configured to freeze the liquid sodium and to keep it frozen over at least part of the height of the annular space between the core cover plug 12 and the reactor vessel 10, more precisely in the annular space between the ferrules 100, 120.

[0091] As an additional means of sealing the reactor during operation, a section of an inert gas circuit, preferably argon, can be installed along at least part of the height of the annular space between the core cover 12 and the reactor vessel 10, above the frozen metal. A supply pipe 101 can thus pass through the shell 100 to supply inert gas, in particular argon, into said annular space.

[0092] To further improve the seal, a double seal 126 can be fitted between the flange 122 and counter-flange 123 when they are assembled. This can consist of two O-rings arranged in concentric grooves and compressed when the counter-flange 123 is tightened onto the flange 122. The seal between the reactor vessel 10 and the cap 12 can therefore be achieved by two O-rings, particularly those made of Inconel® or silver-plated stainless steel.

[0093] Another supply pipe 102 can pass through the flange and open into the inter-seal space to supply inert gas, such as argon. A flow meter can be fitted to this pipe: in the event of a leak in the double seal 126, the flow rate can vary. The flow meter can thus be connected to an alert system in case of a leak exceeding a certain threshold.

[0094] To enable the opening of the tank 10, the heart-shaped cap-lid 12 also incorporates an electric heating system 127 arranged on the outer ferrule 120 of the cap-lid. This system 127 is configured to defrost the sodium, thus allowing the heart-shaped cap-lid 12 to be opened.

[0095] The cap-lid 12 can finally integrate a heat-insulating material 128, arranged inside and against its external ferrule 120.

[0096] A pipe 129 may be provided passing through the shell 100 to supply or remove liquid sodium in order to maintain the desired sodium level in the annular space between the shell 120 and the shell 100. A process for opening and resealing the reactor vessel of a nuclear reactor, initially closed by a core cap 12 as just described, is now described. Figure 3 illustrates all the steps of the process.

[0097] n shuts down the reactor.

[0098]

[0099] the removal of the sealing plug 50 from the slab 51 of the reactor building 5.

[0100] the cover 40 of enclosure 4 is removed.

[0101] before or during steps 111, 1111, the metal is thawed by activating the electric heating system 127. The cooling circuit has been stopped beforehand and preferably during step 111.

[0102] We then proceed to disassemble the flange 122 from the counter-flange 123 and remove the latter.

[0103] we carry out the docking of a transfer chamber 6 above the cap-cover 12 on the flange 122.

[0104] The first gate valve 7 is positioned above the airlock. Then the airlock 6 is filled with inert gas, preferably argon.

[0105] We then carry out the docking, above the first gate valve 7, of a withdrawal hood 8 equipped in its lower part with a second gate valve 80. Then we carry out the inert gas, preferably argon, of the hood 8. It is specified here that the hood 8 is equipped in its upper part with handling means, such as a lifting winch.

[0106] The gate valves 7, 80 are opened and then the cap-lid 12 is lifted into the hood 8, thanks to the actuation of the handling means within the hood 8.

[0107] we perform the closing of the gate valves 7, 80.

[0108]

[0109] The hood 8 is then moved to a storage location.

[0110] All the handling steps for the various components, including the closing plug 50, the cover 40, the airlock 6, the gate valve 7, the hood 8, and the plug-cover 12, can be carried out by one or more handling devices. This could be a single overhead crane 90 within a dedicated handling / maintenance building 90, located above the slab of the reactor building 5.

[0111] Once the cap-lid 12 has been removed from the reactor vessel 10, it is possible to unload spent fuel assemblies and replace them with new fuel assemblies within the same reactor vessel 10.

[0112] Once the new fuel assemblies are in place within reactor block 1, the vessel 10 can be closed. To do this, steps ii / to ix / are reversed, i.e. the cap-lid 12 which has been stored in the hood 8 is put back in place, and the assembly with flange 122 and counter-flange and the sealing by liquid sodium gel is carried out again.

[0113] A new reactor operating cycle can then take place.

[0114] Figure 4 illustrates different embodiments of the core cap 12. A support plate 129 is arranged within the cap 12, below the heat-insulating material 128. This plate 129 can support various mechanisms 130 such as mechanisms for controlling and shutting down the core, instrumentation mechanisms, etc. During reactor operation, this support plate 129 is maintained at an acceptable temperature relative to the flanges of the mechanisms 130. More specifically, one or more plates or blocks 131 made of heat-insulating material can be arranged below the support plate 129 to limit its temperature.

[0115] Watertight sleeves 132 pass through the support plate 129, where applicable the thermal insulation plates / blocks 131 and the shell 120 to house various handling means including pilot and stop bars, to allow their translational guidance by the mechanisms 130. The sleeves 132 can also provide mechanical support for a possible grid positioned above the reactor core to prevent the latter from being lifted.

[0116] The sealed volume V below the support plate 129 and, where applicable, the insulating plates / blocks 131, is advantageously filled with a gas, preferably an inert gas, which will act as a mechanical energy absorber in the event of a severe accident originating from the core. Indeed, such an accident could lead to the core melting and the creation of a gas bubble generating a mechanical shock at the plug 12. Other variations and embodiments may be considered without departing from the scope of the invention.

[0117] The cap-lid 12 which is described with its flange / counter-flange fixing device and its metal gel sealing system can be considered as a closing / opening device in a confined environment of a tank filled at least partially with a liquid metal (sodium, lead).

[0118] The tank can, for example, be that of a so-called test loop in which various components / equipment are tested under liquid metal, particularly under sodium, such as sensors, valves, pumps, tanks, flow meters,...

Claims

Demands 1. Removable closure device for a tank (10) filled with liquid metal, - a plug (12), removably fixed to the tank when closing the tank, by means of a device with flange(s) / counter-flange(s) assembled together by a removable connection; - sealing means between the tank and the plug in its fixed position, comprising at least means for freezing the same liquid metal and maintaining the metal in a frozen state over at least part of the height of the annular space between the plug and the tank.

2. Device according to claim 1, the tank comprising in its upper part, a ferrule (100) forming a chimney in which the plug is mounted, the sealing means comprising at least one cooling circuit for freezing the liquid metal, arranged on at least a part of the peripheral height of the chimney.

3. Device according to claim 2, the cooling circuit being liquid, preferably oil-based.

4. Device according to any one of the preceding claims, the sealing means further comprising a portion of an inert gas circuit, preferably argon, arranged over at least a portion of the height of the annular space between the plug and the reactor vessel, above the frozen metal.

5. Device according to any one of the preceding claims, the sealing means further comprising a double seal arranged between flanges and counter-flange when assembled together.

6. Device according to any one of the preceding claims, the stopper comprising heating means for thawing the metal, so as to be able to open the stopper.

7. Device according to claim 6, the defrosting means comprising an electric heating system arranged on the outer ring of the cap.

8. Device according to any one of the preceding claims, comprising a heat-insulating material arranged inside and against the outer ring of the cap.

9. Device according to any one of the preceding claims, the liquid metal being liquid sodium.

10. Liquid metal cooled fast neutron nuclear reactor, comprising a reactor block (1), and at least one device according to any one of the preceding claims, the vessel constituting the reactor vessel, the plug being the reactor core plug-lid.

11. A method for opening and resealing the reactor vessel of a nuclear reactor according to claim 10, comprising the following steps: i / shutdown of reactor operation; ii / thawing of the metal by activating heating means; iii / disassembly of the flange(s) of the counter-flange(s); iv / docking of a transfer chamber (6) above the cap-cover on the flange(s), v / docking of a first gate valve (7) above the chamber, then putting the chamber under inert gas; vi / docking, above the first gate valve, of a withdrawal hood (8) equipped in its lower part with a second gate valve (80), then putting the hood under inert gas; vii / opening of the first and second gate valves, then lifting of the cap-lid in the hood; viii / closing of the first and second gate valves; ix / moving the hood to a storage location; steps ii / to ix / being reversed to achieve the resealing of the reactor vessel.

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