Liquid metal or molten salt(s) cooled nuclear reactor incorporating a residual power evacuation (EPuR) system through the primary reactor vessel, comprising a passively or actively triggered pivoting fin module located in the inter-vessel space.

Abstract

A liquid metal or molten salt-cooled nuclear reactor incorporating a residual heat dissipation (RHD) system through the primary reactor vessel, comprising a module of pivoting fins with passive or active activation, located in the inter-vessel space. The invention essentially consists of providing a nuclear reactor incorporating an RHD system that guarantees both: - the removal of residual heat as soon as the reactor is shut down; - the removal of heat through the primary vessel and then behind the secondary vessel; - improved heat removal in a completely passive manner (Seebeck effect) by thermal conduction through fins distributed around the primary vessel in the inter-vessel space, which, when pivoted in their deployed position, essentially create a thermal bridge between the primary and secondary vessels. Figure for the abstract: Fig. 5B

Description

Title of the invention: Nuclear reactor cooled with liquid metal or molten salt(s) integrating a system for evacuating residual power (EPuR) through the primary reactor vessel, comprising a module of pivoting fins with passive or active triggering, located in the inter-vessel space. Technical field

[0001] The present invention relates to the field of fast neutron nuclear reactors cooled with liquid metal, in particular with liquid sodium, known as RNR-Na or SFR (English acronym for “Sodium Fast Reactor”) and which are part of the family of so-called fourth generation reactors.

[0002] More particularly, the invention relates to an improvement in the residual power removal function of these nuclear reactors.

[0003] The invention applies equally well to small or medium power reactors or SMR in English (acronym for “Small Modular Reactor”), typically with an operating power between 50 and 200 MWe, as to high power reactors, typically with an operating power greater than 200 MWe.

[0004] It is recalled here that the residual power of a nuclear reactor ("decay heat" in English) is the heat produced by the core after the nuclear chain reaction has stopped and consists mainly of the disintegration energy of the fission products.

[0005] Although described with reference to a liquid sodium-cooled nuclear reactor, the invention applies to any other liquid metal, such as lead, as a heat transfer fluid for a primary circuit of a nuclear reactor. It also applies to molten salt reactors.

[0006] The invention is described with reference to a liquid metal-cooled nuclear reactor of the integrated primary circuit type, that is to say with all of the primary sodium contained inside the main vessel (primary vessel) in which the primary pumps and the intermediate exchangers (El) are immersed through the closing slab of this vessel.

[0007] It also applies to a reactor with a primary loop circuit, i.e. with the primary pumps and intermediate exchangers placed outside the primary vessel of the reactor, which now only contains the core, which are connected to it by primary pipes. Prior art

[0008] In nuclear reactors, the fundamental safety functions which must be ensured in all circumstances (normal, incidental and accidental operation) are confinement, control of reactivity and evacuation of thermal power from the core.

[0009] For the evacuation of residual power in accident situations, improvement of passivity and diversification of systems are constantly sought to ensure better overall reliability. The objective is to preserve the integrity of the structures and their geometry in all circumstances, namely the first (fuel assembly needle cladding) and second (main vessel) containment barriers, even in the event of a generalized electrical voltage failure over a long period, which corresponds to a Fukushima-type scenario.

[0010] More particularly, the evacuation of the residual power of a liquid metal reactor in a completely passive manner through the main vessel is currently envisaged. If this objective appears not to be fully achievable for a large reactor, because of the excessively high power, it can be considered realistically for small power SMR reactors in order to guarantee an intrinsic improvement in the safety and the residual power evacuation systems, hereinafter referred to as EPUR systems, through the main vessel.

[0011] Generally speaking, the EPUR systems produced or known in the literature for integrated sodium reactors can be classified into several categories:

[0012] - those implementing a sodium / sodium (or sodium / NaK) exchanger arranged at within the cold collector of the reactor, called RRA type (Type A Reactor Cooling) in the publication [1]. We can refer to the acronym DRACS (Anglo-Saxon acronym for “Direct Reactor Auxiliary Cooling System”) in [Fig.2] of this publication [1];

[0013] - those implementing a sodium / sodium (or sodium / NaK) exchanger arranged at within the hot collector, called RRB type (Reactor Cooling Type B) in the publication [1]. We can refer to the acronym DRACS in [Fig.2] of this publication [1];

[0014] - those implementing a sodium / sodium (or sodium / NaK) exchanger arranged at level of the secondary loops, with the acronym IRACS ("Intermediate Reactor Auxiliary Cooling System") in the publication [1]. We can refer to the acronym BPR in [Fig.2] of this publication [1]. To enable the operation of these BPR systems, a fraction of the secondary sodium flow is taken by the secondary loops to make it exchange heat with the intermediate sodium of the EPuR circuit;

[0015] - those implementing cooling by the steam generator (SGS), or by a current of air which cools the external walls or by the water from the cooling system emergency power supply. We can refer respectively to the acronym SGACS (an English acronym for “Steam Generator Outer Shell Decay Heat Removal”) and ASG (an acronym for “Emergency Power Supply”) in [Fig.2] of publication [1];

[0016] - those implementing means of heat evacuation by radiation of the primary (main) reactor vessel in the event of an accident, referred to as RRC type (Type C Reactor Cooling through the Main Vessel) in publication [1]. Reference may be made to the acronym RVACS (Anglo-Saxon acronym for “Reactor Vessel Auxiliary Cooling System”) in [Fig.2] of publication [1];

[0017] - those implementing cooling by natural convection of the air re cooling the external surface of the secondary reactor vessel. Reference may be made to Figure 15 of publication [2].

[0018] All the above-mentioned systems transfer the evacuated heat to a sodium, NaK or thermal oil type refrigerant which exchanges with a cold source in air or water.

[0019] DRACS systems, of the RRA and RRB type, have the major disadvantage of taking up space inside the boiler, which implies a primary tank of larger diameter. This aspect can be restrictive for the transportability of the primary tank by road or river, when it is a design of a small modular power reactor (SMR).

[0020] IRACS and GV cooling systems often require operator action or a power supply. In addition, extracting heat further from the nuclear boiler is not guaranteed in the event of a malfunction, such as a sodium leak, in the secondary circuit in the event of an MdTG (acronym for "Generalized Voltage Loss") type accident.

[0021] Systems for cooling the secondary tank by natural air convection are limited to lower power reactors. In addition, in order to have a sufficient natural air circulation flow rate to cool the external surface of the secondary tank, a significant air draft must be ensured. To achieve this, a significant height of the boiler is necessary, which also imposes construction constraints on the depth of the tank shaft and on the associated civil engineering.

[0022] The main advantage of RVACS type systems is that they are both in the immediate vicinity of the nuclear boiler and do not take up space inside the primary vessel. Thus, these RRC type EPuR systems promote a more compact configuration of the nuclear boiler as well as significant diversification compared to the other aforementioned systems.

[0023] Despite these advantages, RRC type EPuR systems have poor cooling performance because it is carried out by radiation from the secondary tank. In fact, the components of the systems (exchangers, bundles of pipes, or flow air) are mostly arranged outside the secondary tank, opposite its external surface, which limits performance by the presence of thermal resistance of the gas in the inter-tank space (hereinafter inter-tank), i.e. the space between the primary tank and the secondary tank.

[0024] The volume of the inter-tank makes it possible to ensure the function of accessibility to the periodic inspection devices of the state of the external face of the primary (main) tank. For safety reasons, this space is filled with an inerting gas, i.e. with the absence of water vapor, to avoid a possible reaction with the sodium in the event of a leak from the main tank.

[0025] The inerting gas, typically nitrogen, present in the inter-tank has a very low convection coefficient, typically between 100 and 500 W / m2.K, which constitutes a limiting factor:

[0026] - thermal power loss through the main reactor vessel, depending on normal operation. By behaving as an insulator, this gas thus contributes to the overall energy efficiency of the reactor insofar as it allows greater use of thermal power via the energy conversion system,

[0027] - the temperature rise of the secondary tank, in accidental operation, which considerably limits the flow of residual power evacuation.

[0028] Systems other than RVACS systems have been considered for RRCs in order to reduce as much as possible the value of the thermal resistance of the inter-tank and increase the temperature of the secondary tank. Indeed, an increase in the temperature of the secondary tank would in fact lead to an increase in the thermal power PEPuR which can be evacuated in the event of an accident according to equation 1:

[0029] [Equation 1]

[0030] in which TCs denotes the temperature of the secondary tank and TRRC the temperature of the RRC system.

[0031] Patent FR2987487B1 proposed a system that improves heat transfer in the inter-tank by forced circulation of the inerting gas. Despite the advantages it provides, this disclosed system requires operator action, as well as an electricity supply.

[0032] Patent GB2263188A also proposed an RRC system improving heat transfer by filling the inter-tank with a metal, introduced in the solid state and which then melts, improving both the thermal inertia and the heat transfer in the inter-tank. This system according to patent GB2263188A improves the thermal performance of an RRC system, because the thermal resistance of the inter-tank is drastically reduced. However, this type of system has three major drawbacks:

[0033] - the presence of a reservoir where the solid material is stored in operation normal reactor volume. The volume of the tank is almost equivalent to that of the inter-tank, and located near the boiler, at the level of the reactor closure slab. For high-power reactors, the footprint of such a system can be significant, the complex layout and safety studies must take into account tank leak events;

[0034] - the system is not reversible. Therefore, an operational test in non-accidental situation is not possible, because recovery of solid or molten metal is extremely difficult once it is in the inter-tank;

[0035] - in a post-accident dismantling situation, operations in the inter-tank would be even more difficult, because they would be hampered by the presence of a layer or residue of solid metal linked to the primary tank and / or the secondary tank.

[0036] There is therefore a need to improve RRC-type EPuR systems for liquid metal-cooled nuclear reactors, in particular in order to passively increase heat transfer during accidental operation.

[0037] There is further a need for a system that meets the aforementioned need and is reversible to allow:

[0038] - to keep the inter-tank space filled with inerting gas during normal operation and thus improve the energy efficiency of the nuclear facility comprising a liquid metal-cooled nuclear reactor;

[0039] - to guarantee its proper functioning by carrying out periodic checks adapted to test it and ensure its availability in accidental operation of the reactor;

[0040] - to return to normal operation of the reactor in the event of an in trip tempestuous of the system.

[0041] The aim of the invention is to meet at least in part these needs. Statement of the invention

[0042] To this end, the invention relates, in one of its aspects, to a fast neutron nuclear reactor cooled with liquid metal or molten salt, comprising:

[0043] - a tank called a primary tank, filled with a liquid metal or a molten salt as as heat transfer fluid for the primary circuit of the reactor;

[0044] - a tank called a secondary tank, arranged around the primary tank by defining a inter-tank space (E); - a tank well, arranged around the secondary tank; - a closing slab, to enclose the heat transfer fluid inside the primary tank; - a system for evacuating at least part of the power at a time nominal and residual power of the reactor, the system comprising:

[0045] a closed circuit filled with a heat transfer liquid and configured so that the heat transfer liquid circulates therein by natural or forced convection and remains in the liquid state both in nominal operation of the nuclear reactor and in shutdown situations, the closed circuit comprising a coil, arranged between the reactor pit and the secondary vessel, in a helix around the latter;

[0046] a module fixed to the closing slab, comprising: • at least one envelope arranged inside the inter-tank space (E) in contact with the secondary tank, • a plurality of thermally conductive fins, arranged inside the inter-tank space (E) being distributed angularly around the primary tank in columns, each column comprising several fins spaced from each other over at least part of the height of the secondary tank and pivotally mounted along the secondary tank between a retracted position in which they are at a distance from the primary tank and a deployed position in which they are in contact with the primary tank, • one or more Seebeck effect thermo-element(s) arranged inside the envelope and extending along the secondary vessel with their hot side in the lower part of the envelope and their cold side in the upper part of the envelope, the Seebeck effect thermo-element(s) being adapted so that in nominal operation of the nuclear reactor, the current that it(they) generate(s) leaves the fins in their retracted position, while in accidental operation where the residual power must be evacuated, the current that it(they) generate(s) pivots the fins into their deployed position.

[0047] By "in shutdown situations" is understood here and within the scope of the invention a normal shutdown of the reactor and in the event of an accident (accidental operation).

[0048] According to an advantageous embodiment, the module is suspended from the closing slab. Mounting with suspension of the module under the closing slab is advantageous compared to another method, such as welding, because it is easier to carry out. Disassembly is also easier.

[0049] According to an advantageous embodiment, the module comprises a plurality of pivots on each of which a fin is pivotally mounted, each pivot integrating within it an electric motor, preferably with gears, electrically powered by the Seebeck effect thermo-element(s). Such integration is compact and allows reliable operation. Preferably, each pivot is fixed directly, more preferably by welding to the casing.

[0050] Advantageously, the fins are vertical against the secondary tank in their retracted position. This allows the inter-tank space to be cleared as much as possible and thus, with thin fins, typically around a cm, this leaves the maximum possibility of inspecting said space, particularly by a robot, during normal operation of the nuclear reactor.

[0051] The fins may be flat or curved in shape with a curvature defining a contact surface with the primary vessel in their deployed position. Generally, the shape of the fins is adapted to ensure an optimal contact surface between the fin tips and the primary vessel, and therefore thermal conduction between the latter and the secondary vessel during operation for removing residual power from the nuclear reactor.

[0052] According to an advantageous characteristic, the p-type material of the Seebeck effect thermoelement(s) being chosen from lead telluride (PbTe), a mixture (TAGS) of antimony tellurides (Sb2Te3), germanium (GeTe) and silver (Ag2Te), a skutterudite (CeFe4Sbi2). These materials are perfectly suited to generate a current below a desired temperature threshold for the pivoting of the fins in normal operation of the reactor, and above this temperature threshold in an accidental situation of residual power evacuation. Typically, these materials are perfectly suited for a temperature difference between the bottom of the secondary vessel and the closure slab, of approximately 250°C and 450°C, in normal operation and in an accidental situation of residual power evacuation.

[0053] According to an advantageous embodiment, the nuclear reactor comprises mechanical and / or electrical means for returning the fins from their deployed position to their retracted position.

[0054] According to this mode and a first variant, the electrical means may comprise a so-called backup electrical power source for generating an electrical current opposite to that generated by the Seebeck effect thermo-element(s).

[0055] According to this mode and a second variant, the mechanical means may comprise a mechanical device to be operated manually, such as a winch.

[0056] With such mechanical and / or electrical means, the operation of the fin module of the EPuR system is reversible and it can be tested as desired during normal operation of the reactor.

[0057] According to an advantageous configuration, each column of fins extends substantially over the height of the cylindrical part of the secondary tank. Thus, the possible thermal conduction surface between the primary and secondary tanks is optimized.

[0058] According to a complementary embodiment, the reactor comprises a system for filling with liquid metal the part of the inter-tank space (E) separating the non-cylindrical parts of the primary and secondary tanks, the system being able to be activated in an accident situation of the nuclear reactor or on the decision of an operator following the shutdown of the reactor. This avoids having to install fins between the hemispherical parts of the primary and secondary tanks, which can be complex for the arrangement of said fins.

[0059] Preferably, the fins and the casing are made of steel or aluminum. More preferably, the fins may be made of stainless steel such as AISL 316L steel, which has good mechanical strength and corrosion resistance.

[0060] For the fins, aluminum alloys, for example, 1060 aluminum can be used in preference to steel because aluminum has a higher conductivity, up to 10 times higher depending on the alloy, compared to that of steel. In addition, aluminum is about 3 times lighter than steel, which advantageously facilitates the location of the fins in the inter-tank space. In addition, aluminum and its alloys are corrosion resistant and can be easily welded with standard methods.

[0061] The melting temperature, of approximately 650°C for 1060 aluminium, makes it suitable for the production of fins in the event of accidental operation, with a maximum temperature of the reactor vessel below 600-650°C in the event of an accident.

[0062] In order to reduce as much as possible the thermal resistance between the primary and secondary tanks during residual power evacuation operation (fins in the deployed position), the annular distribution coefficient of the fins in the inter-tank space, defined as being the percentage of the occupation rate of the cross-section of this space by the fins in their deployed position, is greater than 60%, preferably greater than 80%.

[0063] Thus, the EPuR system with pivoting fin module according to the invention performs the residual power evacuation function (EPUR) and ensures the confinement of radioactivity by preserving the entirety of the first radiological barrier (fuel cladding), as well as the second (main vessel).

[0064] Thanks to the pivoting fin module, the thermal resistance between the primary and secondary tanks is reduced in the event of an accidental discharge of residual power by the contact, which may be direct, of the fins with the primary tank in their deployed position, which therefore allows the heat to be transmitted directly by conduction to the secondary tank, which in turn transmits the heat to the coil of the EPuR system around the latter.

[0065] Furthermore, actuation of the fins may be actively triggered by an operator following a normal shutdown of the reactor.

[0066] The invention therefore essentially consists of producing a nuclear reactor integrating an EPuR system which guarantees both:

[0067] - evacuation of residual power as soon as the reactor is shut down;

[0068] - heat evacuation through the primary tank then behind the tank secondary;

[0069] - improved heat dissipation in a completely passive manner (effect Seebeck) by thermal conduction through fins distributed around the primary tank in the inter-tank space, which, when pivoted in their deployed position, create a sort of thermal bridge between the primary and secondary tanks.

[0070] The EPUR system according to the invention is therefore distinguished from systems according to the state of the art by the way of evacuating the power passively, from the outside of the primary tank, by exploiting its radiation at high temperatures towards the inter-tank space and its propagation both by radiation and thermal conduction by means of the fins in their deployed position in the secondary tank.

[0071] The EPUR system operates continuously, both during normal operation of the reactor at nominal power, and during accidental operation with the fins acting as an active heat sink.

[0072] The invention applies to all nuclear reactors cooled with liquid sodium or other liquid metals or salts, whatever their configuration, characterizing the mode of the primary circuit, of small or medium power or SMR in English (acronym for "Small Modular Reactor"), typically with an operating power between 50 and 200 MWe, or of high power, namely:

[0073] - the integrated RNRs for which the primary pumps and the exchangers are in entirely contained inside the main tank containing the core and are immersed in the cooling fluid of said main tank through the closing slab of this tank.

[0074] - partially integrated ("hybrid") RNRs for which only the pumps primaries are contained inside the main tank containing the core;

[0075] - the so-called “loop” RNRs for which the primary pumps and the exchangers Intermediate heat sources are placed in dedicated tanks outside the main reactor vessel which now only contains the core and internal structure, the main vessel and the component vessel being connected by primary pipes.

[0076] The heat transfer liquid of the primary circuit is preferably a liquid metal chosen from a binary lead-bismuth alloy (Pb-Bi), a binary sodium-potassium alloy (NaK), sodium or other ternary alloys of liquid metals.

[0077] The preferred applications of the invention are small-sized reactors of the GenIV sector, in particular reactors cooled with sodium and lead and salt.

[0078] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0079] [Fig.l] [Fig.l] is a schematic partial sectional view of a liquid sodium cooled nuclear reactor (SFR) with an RRC type EPuR system intended to implement the invention.

[0080] [Fig.2] [Fig.2] is a partial longitudinal sectional view showing the primary vessel and part of the fuel assemblies of an SFR nuclear reactor as well as part of the pipe network of an EPuR system according to the invention.

[0081] [Fig.3] [Fig.3] is a schematic view of an integrated RNR in partial longitudinal section showing the primary and secondary vessels, the reactor core and part of a layer of pipes of an EPuR system around the secondary vessel of an integrated type SFR nuclear reactor in accordance with the invention.

[0082] [Fig.4A], [Fig.5A], [Fig.6A] Figures 4A, 5A and 6A are views respectively from above, from the side and from the front of a module including a column of fins of an EPuR system according to the invention, the fins being in their retracted position vertically against the secondary tank which corresponds to normal operation of an SFR nuclear reactor.

[0083] [Fig.4B], [Fig.5B], [Fig.6B] Figures 4B, 5B and 6B are views respectively of above, from the side and from the front of a module including a column of fins of an EPuR system according to the invention, the fins being in their deployed position in contact against the primary vessel which corresponds to accidental operation of an SFR nuclear reactor.

[0084] [Fig.7A], [Fig.7B] Figures 7A and 7B illustrate the figure of merit of different p-type thermoelectric materials, known as z, which are suitable within the scope of the invention, and the range of temperatures concerned respectively in normal operation and in accidental operation of the SFR or RNR nuclear reactor.

[0085] [Fig.8] [Fig.8] illustrates in the form of a curve the evolution of the ratio of the thermal resistance between primary and secondary tanks as a function of the annular distribution coefficient of the fins according to the invention, with a thickness equal to 1 cm.

[0086] [Fig.9] [Fig.9] is an enlargement of [Fig.8].

[0087] [Fig. 10] [Fig. 10] illustrates in the form of a curve the evolution of the ratio of the thermal resistance between primary and secondary tanks as a function of the increase in thickness of the fins according to the invention.

[0088] [Fig. 11] [Fig. 11] illustrates in the form of a curve the evolution of the ratio of the thermal resistance between primary and secondary tanks as a function of the annular distribution coefficient of the fins according to the invention, with a thickness equal to 10 cm.

[0089] [Fig. 12] [Fig. 12] is a side view of a finned module of a system of EPuR according to a variant of the invention, the fins with curved contact surface being in their deployed position in contact against the primary vessel which corresponds to accidental operation of an SFR nuclear reactor. Detailed description

[0090] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a primary tank filled with liquid sodium of an integrated RNR, as it is in the vertical operating configuration.

[0091] Figures 1 to 3 show a liquid sodium-cooled nuclear reactor 1 (SFR), with an integrated type architecture, with a system 2 for removing residual power (EPuR) from the reactor, both in accordance with the invention.

[0092] Such a reactor 1 comprises a primary vessel 10 or reactor vessel filled with liquid sodium, called primary liquid, and inside which is present the core 11 where a plurality of fuel assemblies 110 are installed which generate thermal energy by the fissions of the fuel, and assemblies 11 of lateral neutron protections (PNL).

[0093] The primary tank 10 supports the weight of the sodium in the primary circuit as well as the internal components.

[0094] The support of the core 11 is provided by two separate structures making it possible to separate the functions of support and supply of cooling fluid to the core:

[0095] - a first pressure-welded mechanical structure called a bed base 12 in in which the feet of the fuel assemblies 110 are positioned and which is supplied with cold sodium (400°C) by primary pumps 100;

[0096] - a second welded mechanical structure called decking 13 on which the bed base comes into support; the decking generally rests on a part of the internal wall in the lower part of the primary tank 10.

[0097] Typically, the bed base 12 and the decking 13 are made of AISI 316L stainless steel.

[0098] The sheaths of the assemblies 110 constitute the first containment barrier while the tank 10 constitutes the second containment barrier.

[0099] As shown, the primary tank 10 is cylindrical in shape with a central axis X extended by a hemispherical bottom. Typically, the primary tank 10 is made of AISI 316L stainless steel with a very low boron content in order to protect against the risk of cracking at high temperature. Its external surface is made highly emissive by a pre-oxidation treatment, carried out to facilitate the radiation of heat to the outside during the residual power evacuation phase.

[0100] A plug 18, called a core cover plug, is arranged directly above the core 10.

[0101] In such a reactor 1, the extraction of the heat produced during the nuclear reactions within the core 11 is carried out by circulating the primary sodium using pumping means 100, arranged in the reactor vessel 10, towards intermediate exchangers 15 arranged inside the primary vessel 10 in the example illustrated.

[0102] Thus, under normal operating conditions of the reactor, heat extraction is carried out by the secondary sodium arriving cold through its supply pipe 152 to an intermediate exchanger 15 before emerging hot through its outlet pipe 151.

[0103] The extracted heat is then used to produce water vapor in steam generators not shown, the steam produced being fed into one or more turbines and alternators also not shown. The turbine(s) transform(s) the mechanical energy of the steam into electrical energy.

[0104] The reactor vessel 10 is separated into two distinct zones by a separation device consisting of at least one tank 16 arranged inside the reactor vessel 10. This separation device is also known as a redan and is made of AISI 316L stainless steel. In general, as illustrated in [Fig. 3], the separation device consists of a single inner tank 16 whose shape is cylindrical at least in its upper part.

[0105] The step 16 is generally welded to the bed 12 as shown in [Fig.3].

[0106] As illustrated in [Fig.3], the primary sodium zone delimited internally by the inner vessel 16 collects the sodium leaving the core 11: it constitutes the zone in which the sodium is the hottest and is therefore commonly called the hot zone 160 or hot collector. The primary sodium zone 161 delimited between the inner vessel 16 and the reactor vessel 10 collects the primary sodium and supplies the pumping means: it constitutes the zone in which the sodium is the coldest and is therefore commonly called the cold zone or cold collector 161.

[0107] As illustrated in [Fig.3], the reactor vessel 10 is anchored or placed in the upper part and closed by a closing slab 17 supporting the various components, such as the pumping means not shown, certain components of the evacuation system 2, as specified below, and the core cover plug 18. The closing slab 17 is therefore an upper cover which encloses the liquid sodium inside the primary vessel 10. Typically, the slab 17 can be made of non-alloy steel (A42). This tight closure allows the vessel roof to be inerted.

[0108] The sealing of the primary tank 10 is guaranteed by a metal seal between the closing slab 17 and the core cover plug 18.

[0109] The core cover plug 18 is a rotating plug which carries all the handling systems as well as all the instrumentation necessary for monitoring the core including the control rods, the number of which depends on the type of core and of its power, as well as thermocouples and other monitoring devices. Typically, the cover cap 18 is made of AISI 316L stainless steel.

[0110] The space between the closing slab 17 and the free sodium levels, commonly called the cell ceiling, is filled with a gas that is neutral with respect to sodium, typically Argon.

[0111] A support and containment system 3 is arranged around the primary tank 10 and below its closing slab 17.

[0112] More precisely, as shown in figures 2 and 3, this system 3 comprises a vessel well 30, inside which are inserted from the outside towards the inside a layer of thermal insulating material 31, a secondary vessel 32 and the primary vessel 10 of the reactor.

[0113] The tank well 30 is a block with a generally parallelepiped exterior shape which supports the weight of the slab 17 and therefore of the components which it itself supports. The functions of the tank well 30 are to provide biological protection and protection against external aggression, and also to ensure cooling of the external environment to maintain low temperatures. Typically, the tank well 30 is a concrete block.

[0114] The layer of thermal insulating material 31 guarantees the thermal insulation of the tank well 30. Typically, the layer 31 is made of polyurethane foam or silicate-based.

[0115] The secondary tank 32 guarantees the retention of the primary sodium in the event of a leak through the primary tank 10 and the protection of the tank well 30. The secondary tank 32 rests against the tank well 30 and its upper part is welded to the closing slab 17. Typically, the secondary tank 32 can be made of AISI 316L stainless steel.

[0116] The space E between the secondary tank 32 and the primary tank 10, called the inter-tank space, is filled with a thermally conductive gas, such as nitrogen. It must be sufficient to allow the location of the inspection systems used. Typically, the thickness of the inter-tank space E is approximately 20 cm.

[0117] The system 2 according to the invention for evacuating residual power (EPuR) through the primary tank 10 will now be described, more particularly with reference to FIGS. 2 and 3.

[0118] The EPuR system according to the invention 2 will make it possible to completely passively evacuate the residual power outside the primary tank 10 by capturing the high-temperature radiation in the inter-tank space E.

[0119] The system 2 firstly comprises a closed circuit 4 filled with a liquid metal, which comprises: - a coil 40, arranged in a helix in the inter-tank space E around the primary tank 10, - a first cold collector 41, welded directly to one of the ends of the coil 40, the cold collector being arranged outside above the closing slab 17, - a first hot collector 42, welded directly to the other end of the coil 40, the hot collector being arranged outside above the closing slab 17, and preferably directly above the first cold collector 41.

[0120] The collectors 41 and 42 are connected to the cold source of the system 50 by the pipes 451 and 452.

[0121] The closing slab 17 supports, on its upper part, the weights of the parts which support the cold 41 and hot 42 collectors.

[0122] The closing slab 17 has openings of different types to allow the insertion of the coil 40, which enters and exits through the top of the slab 17.

[0123] The coil 40 has a diameter which is a function of the diameter of the primary tank 10 and a height sufficient to have the surface area necessary for the evacuation of the desired heat.

[0124] In other words, the total number of turns, the spacing and the diameter of these turns which make up the coil 40 depend on the diameter of the primary vessel 10 and the power of the core 11 of the nuclear reactor. For example, the pitch of the turns of the coil 40 can be equal to 10 cm, which is a good compromise for the manufacture and absorption of heat by radiation.

[0125] For example also, the external diameter of the coil 40 is fixed at a standard dimension of 5 cm, in order to minimize the pressure losses, reduce the size of the pipes in the inter-tank E and maximize the surface exposed to the primary tank 10. The thickness of the coil 40 depends on the mechanical stresses exerted by the internal liquid metal and by its weight.

[0126] The material of the coil 40 must have good emissivity characteristics. Typically, the material of the coil is chosen from AISI 316L stainless steel, ferritic steels, nickel, inconel, Hastelloy. This material depends on the internal fluid used for the closed circuit 4.

[0127] This internal heat transfer fluid C is a liquid metal, chemically stable, of low viscosity, good conductor and heat transfer agent, chemically compatible with all the piping of circuit 4 and capable of operating in natural or forced convection in a temperature range between 150-600 °C. Typically, the liquid metal of circuit 4 can be chosen from an NaK, Pb-Bi alloy, sodium or one of the ternary alloys of liquid metals, ...

[0128] As shown in [Fig. 1], the cold 41 and hot 42 collectors have a general toroidal shape centered on the central axis (X) of the primary tank 10. These collectors 41, 42 are supported on support pieces directly welded to the slab of closing 17.

[0129] As illustrated in [Fig.l], the EPuR system according to the invention 2 also comprises a cold source 5 configured to absorb the heat evacuated by the radiation from the primary vessel 10 through the entire coil 40. The sizing of the cold source 5 depends both on the power of the core 11 of the reactor which in fact determines the residual power to be evacuated, and on the duration of the transient to be considered, which therefore requires a substantially proportional thermal inertia.

[0130] The cold source 5 comprises at least one reservoir 50, arranged at a distance from the primary tank 10 and at a higher level relative to the closing slab 17. Preferably, the reservoir 50 is confined in a containment building 52.

[0131] In order to place the cold source 5 at an optimal distance from the primary tank 10, the hydraulic circuit 2 comprises a connecting loop 45 comprising a set of piping and, where appropriate, valves between the cold 41 and hot 42 collectors and the exchangers of the cold source 5.

[0132] More precisely, as illustrated in [Fig.l], the connecting loop 45 comprises a hydraulic branch 451 which connects the first cold collector 41 to the cold end of an exchanger of the cold source 5 and a hydraulic branch 452 which connects the first hot collector 42 to the hot end of the exchanger of the cold source 5.

[0133] Thus, the first cold collector 41 distributes the flow of internal liquid metal from the cold branch 451 to each cold branch 401 of the coil 40 and the first hot collector 42 collects the internal liquid metal which comes from each hot branch 401 of the coil 40 to bring it to the hot branch 452.

[0134] The cold 451 and hot 452 branches are preferably sized so that they are of the shortest possible length in order to reduce the pressure losses and to increase the natural convection flow rate in the closed hydraulic circuit 4, in the case where the flow is intended to be passive by the design of the EPuR system.

[0135] Thus, the closed hydraulic circuit 4 which has just been described is configured so that the heat-transfer liquid metal remains in the liquid state both during nominal operation of the nuclear reactor and during operation when the nuclear reactor is stopped, releasing the residual power.

[0136] According to the invention, the RRC type EPuR 2 system which has just been described comprises a module 6 of thermally conductive fins 60, with passive triggering, arranged in the cylindrical part of the inter-tank space E, between the primary tank 10 and the secondary tank 32.

[0137] Advantageously, the module 6 is suspended from the closing slab 17 by an envelope 61, typically made of steel, which matches the shape of the secondary tank 32, as shown in Figures 5A, 5B.

[0138] As visible in Figures 4A and 4B, the fins 60 of the module 6, typically made of steel or aluminum, are grouped in columns 62 arranged inside the inter-tank space (E) and preferably distributed regularly around the primary tank 10. By way of example, each fin 60 is a right parallelepiped with dimensions (thickness, width, length) equal to 1*5*25 cm.

[0139] In each column 62, the fins 60 are spaced from each other over the height H of the cylindrical part of the secondary tank 32 and are pivotally mounted around pivots 64 along the secondary tank between a retracted position in which they are vertical against the secondary tank 32 (FIGS. 4A, 5A, 6A) and a deployed position in which they are each in contact with the primary tank 10 along a contact surface SC (FIGS. 4B, 5B, 6B).

[0140] The pivots 64 are welded to the steel casing which is in contact with the CS.

[0141] As detailed in [Fig.4B], thermo-elements 63 with Seebeck effect are arranged inside the casing 61 arranged inside the casing and extend along the secondary tank 32 with their hot side in the lower part of the casing 61 and their cold side in the upper part of the casing 61. For thermo-elements, reference may be made to [3]. Publication [4] mentions PbTe as a thermoelectric material which can be welded, and therefore have potentially elongated dimensions.

[0142] In operation of the nuclear reactor, these Seebeck effect thermo-elements 63 permanently generate an electric current thanks to the temperature difference between the closing slab 17 at which their cold side is arranged and the bottom of the secondary vessel 32 at which their hot side is arranged. This current I generated within the thermo-elements 32 is also propagated within the electrically conductive pivots 64.

[0143] Each of the pivots 64 houses an electric motor, preferably geared, powered directly by the electric current. Thus, when the current generated by the thermoelements 63 exceeds a threshold value, the pivots 64 are pivoted from their vertical retracted position to their deployed position of contact with the primary tank 10.

[0144] In normal operation of the reactor, the temperature difference AT between the bottom of the secondary vessel 32 and the closure slab 17, symbolized by the heat flux Q nom in [Fig.5A], implies a current generated by the thermo-elements 63 which is lower than the threshold value beyond which the pivoting of the fins 60 is triggered. Under these conditions, the fins 60 remain in their vertical retracted position (Figures 4A, 5A, 6A). Given the possible small thickness of the fins 60, it is always possible to carry out an inspection, in particular by robot, of the inter- E tanks in this normal operation of the reactor.

[0145] In a nuclear reactor shutdown situation, the pivoting of the fins 60 is entirely passive and depends solely on the increase in temperature. Thus, in this operating case the temperature difference AT between the bottom of the secondary vessel 32 and the closure slab 17, symbolized by the heat flux Qacc in [Fig.5B], implies a current generated by the thermo-elements 63 which is greater than the threshold value beyond which the pivoting of the fins 60 is triggered.

[0146] The pivoting of the fins 60, by the rotation of their pivot 64, is aided by the force of gravity, which contributes to causing the fins 60 to fall until a physical contact SC is generated with the primary tank 10 (Figures 4B, 5B, 6B). As a result, the heat transfer by thermal conduction is accelerated between the primary tank 10 and the secondary tank 32 and the heat flux is greater than with an EPuR system which would not include fins. The generated contact points SC promote the rise in temperature of the secondary tank 32 and the increase in the power transferred by radiation to the EPuR system 2.

[0147] The shape of the fins 60 is preferably adapted so as to generate a contact surface and therefore a thermal conduction surface which is maximum between the primary 10 and secondary 32 tanks. At the same time, care is taken to ensure that this adapted shape does not generate mechanical forces on the external surface of the primary tank 10 in order to guarantee its integrity in all circumstances. Of course, the radial thermal expansions of the primary 10 and secondary 32 tanks are advantageously taken into account in the shape of the fins 60.

[0148] In order for the operation of the fin module 60 to be reversible, mechanical and / or electrical means are provided for returning the fins 60 from their deployed position to their vertical retracted position. The reversible nature of the module 6 makes it possible to test it during normal operation of the nuclear reactor. As electrical means, a backup electrical source, such as a battery, may be provided which would generate a current opposite to that generated by the Seebeck effect of the thermo-elements 63. As mechanical means, manually controlled members may be provided, such as a manually actuated winch-type mechanism.

[0149] To improve thermal conduction in accidental operation, it is possible to provide, at the time of pivoting of the fins 60, a filling of liquid metal in the lower part of the inter-tank space E which is not cylindrical.

[0150] For the application of liquid sodium SFR reactor, thermo-elements of the lead tellurium type can perfectly suit the required operation and the temperature level below 650°C, as is evident from the curves of the p-type thermoelectric materials for an AT of the order of 250°C and 450°C respectively.

[0151] The inventors carried out preliminary calculations using known thermal calculation software, such as the COPERNIC software: [5], [6].

[0152] in order to look at the influence of the following different parameters on the ratio of the thermal resistance of the inter-tank E:

[0153] - the annular distribution coefficient of the fins, which can be defined as being the percentage of surface area or occupancy rate of the section of the inter-tank occupied by the fins in their deployed position;

[0154] - the thickness of a fin.

[0155] Results of these calculations are illustrated in the form of curves in Figures 8 to 11. It is specified that in Figures 8 and 9, the thickness of fins 60 is approximately 1 cm while in [Fig. 11], it is 10 cm.

[0156] From these curves, it can be seen that the thickness of the fin 60 is not a parameter which contributes to a significant reduction in the thermal resistance of the inter-tank space E, provided that there is a low occupancy rate of the fins 60 in this space.

[0157] On the other hand, a denser annular distribution of the fins 60 can promote the reduction of this thermal resistance of the inter-tank space E by up to a factor of 10 or more compared to a configuration without fins.

[0158] As enlarged in [Fig.9], it can be seen for example that there is a 50% reduction in the overall thermal resistance in the inter-tank space E, if the annular distribution factor of the fins is between 30% and 40%. Beyond 60% of this factor, the reduction is even greater.

[0159] This means that if the heat flux is imposed on the wall of the primary vessel 10, the temperature difference between the latter and the secondary vessel 32 becomes 10 times lower, with the favorable consequence of a strong increase in the power evacuated by the EPuR 2 system in a reactor shutdown situation.

[0160] This power can be further increased by increasing the contact surface (SC) between the primary tank 10 and the secondary tank 32. One possibility is shown in [Fig. 12] with fins 60 having a curved shape 600 which ensures a larger contact surface SC when they pivot.

[0161] It is specified here that the results from the above studies are preliminary and should be confirmed by 2D or 3D thermal conduction calculations.

[0162] The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the examples illustrated within non-illustrated variants.

[0163] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0164] If in all the examples illustrated, the EPuR 2 system with its module 6 of fins 60 with passive triggering which has just been described in relation to an integrated type nuclear reactor can quite easily be implemented in a loop nuclear reactor with intermediate exchangers arranged outside the primary vessel.

[0165] If in all the examples illustrated, the pivoting of the fins 60 is exclusively passive by the threshold current which circulates in the Seebeck effect thermo-elements 63, it is possible to envisage a configuration with active triggering, in particular in cases of shutdown operation (accidental or programmed) of the nuclear reactor, by injection of a current not coming from the thermo-elements 63.

[0166] Other more optimal and efficient pairs of materials may be chosen depending on the design and concept of the reactor (the operating temperature levels in normal and accidental modes may vary depending on the configurations).

[0167] Instead of a reservoir 50, other means can be considered as a cold source.

[0168] For example, the cold source can be a pool of water, if the internal fluid in the circuit is thermal oil, confined or not in a building. It can also be a chimney with a NaK / air exchanger, if the internal fluid in the circuit is NaK. List of cited references

[0169] [1]: https: / / www.nrc.gov / docs / ML1914 / ML19149A378.pdf

[0170] [2]: BS TRIPLETT et al., “PR1SM: A competitive small modular sodium-cooled reactor,” Nuclear Technology, vol. 178, p. 186-200, 2012.

[0171] [3]: Thermoelectrics - Northwest Materials Science and Engineering:

[0172] http: / / thermoelectrics.matsci.northwestern.edu / thermoelectrics / index.html.

[0173] [4]: ​​X. REALES FERRERES “Fabrication and Characterization of High Tem pérature PbTe-based Thermoelectric Modules for Waste Heat Recovery Applications, Thermoelectric Modules for Waste Heat Recovery Applications”. University of Wollongong Thesis Collection 2017+.

[0174] https: / / ro.uow.edu.au / cgi / viewcontent.cgi?article=1325&context=thesesl

[0175] [5] : F. MORIN et al., "COPERNIC, A NEW TOOL BASED ON SIMPLIFIED CAL CULATION METHODS FOR INNOVATIVE LWRs CONCEPTUAL DESIGN STUDIES”, ICAPP 2017 Conférence, 2017.

[0176] [6]: P. GAUTHE et al., ''Innovâtive and inherently safe small SFR as a response to the dilemma ’safety vs cost'”, ICAPP 2019 Conférence, 2019.

Claims

1. Claims Fast neutron nuclear reactor (1) cooled with liquid metal or molten salt, comprising: - a tank (10) called the primary tank, filled with a liquid metal or a molten salt as a heat transfer fluid for the primary circuit of the reactor; - a tank (32) called a secondary tank, arranged around the primary tank, defining an inter-tank space (E); - a tank well (30), arranged around the secondary tank (32); - a closing slab (17), to enclose the heat transfer fluid inside the primary tank; - a system (2) for evacuating at least part of both the nominal power and the residual power of the reactor, the system comprising: a closed circuit (4) filled with a heat transfer fluid and configured so that the heat transfer fluid circulates therein by natural or forced convection and remains in the liquid state both during nominal operation of the nuclear reactor and in reactor shutdown situations, the closed circuit comprising a coil (40), arranged between the reactor well and the secondary tank, in a helix around the latter; a module (6) fixed to the closing slab, comprising: • at least one envelope (61) arranged inside the inter-tank space (E) while being in contact with the secondary tank (32), • a plurality of thermally conductive fins (60), arranged inside the inter-tank space (E) being distributed angularly around the primary tank (10) in columns (62), each column comprising several fins spaced from each other over at least part of the height of the secondary tank and pivotally mounted along the secondary tank between a retracted position in which they are at a distance from the primary tank and a deployed position in which they are in contact with the primary tank, • one or more Seebeck effect thermo-element(s) (63) arranged inside the envelope and extending along the secondary tank with their hot side in the lower part of the envelope and their cold side in the upper part of the envelope, the Seebeck effect thermo-element(s) (64) being adapted so that in nominal operation of the nuclear reactor, the current that it (they) generate (s) leaves the fins in their retracted position, while in accidental operation where the residual power must be evacuated the current that it (they) generate (s) pivots the fins into their deployed position.

2. Nuclear reactor (1) according to claim 1, the module being suspended from the closing slab (17).

3. Nuclear reactor (1) according to claim 1 or 2, the module comprising a plurality of pivots (64) on each of which a fin (60) is pivotally mounted, each pivot integrating within it an electric motor, preferably with gears, electrically powered by the Seebeck effect thermo-element(s) (63).

4. Nuclear reactor (1) according to claim 3, each pivot being fixed directly, preferably by welding to the casing.

5. Nuclear reactor (1) according to one of the preceding claims, the fins being vertical against the secondary vessel (32) in their retracted position.

6. Nuclear reactor (1) according to one of the preceding claims, the fins being of a flat or curved shape according to a curvature defining a contact surface with the primary vessel in their deployed position.

7. Nuclear reactor (1) according to one of the preceding claims, the p-type material of the Seebeck effect thermoelement(s) (63) being chosen from lead telluride (PbTe), a mixture (TAGS) of antimony tellurides (Sb2Te3), germanium (GeTe) and silver (Ag2 Te), a skutterudite (CeFe4Sbi2).

8. Nuclear reactor (1) according to one of the preceding claims, comprising mechanical and / or electrical means for returning the fins from their deployed position to their retracted position.

9. Nuclear reactor (1) according to claim 9, the electrical means comprising a so-called backup electrical power source for generating an electrical current opposite to that generated by the thermo-element(s) (63) with Seebeck effect.

10. Nuclear reactor (1) according to claim 9, the mechanical means comprising a manually actuated mechanical device, such as a winch.

11. Nuclear reactor (1) according to one of the preceding claims, each column of fins extending substantially over the height of the cylindrical part of the secondary vessel.

12. Nuclear reactor (1) according to one of the preceding claims, comprising a system for filling with liquid metal the part of the inter-tank space (E) separating the non-cylindrical parts of the primary and secondary tanks, the system being able to be actuated in an accident situation of the nuclear reactor or on the decision of an operator following the shutdown of the reactor.

13. Nuclear reactor (1) according to one of the preceding claims, the fins and the casing being made of steel or aluminum.

14. Nuclear reactor (1) according to one of the preceding claims, the annular distribution coefficient of the fins (60) in the inter-tank space, defined as being the percentage of the occupancy rate of the cross-section of this space by the fins in their deployed position, being greater than 60%, preferably greater than 80%.

15. Nuclear reactor (1) according to one of the preceding claims, of loop or integrated type.

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