Nuclear power plant housing two molten salt reactor(s), with means of transferring fuel liquid between the two reactor vessels; Start-up and operation method of the power plant.

Abstract

A nuclear power plant with a nuclear building housing at least two molten salt nuclear reactors of the fast neutron type and means for transferring molten salt fuel liquid between the two reactor vessels; related method for starting and operating the installation. The invention relates to a nuclear power plant comprising: - a reactor building (10); - at least two reactor vessel shafts, located inside the reactor building, - at least two reactor vessels (2) of molten salt nuclear reactors (1) of the fast neutron type, axisymmetric about a central axis, internally delimiting a primary circuit of fuel in liquid form in which at least one salt is molten, the interior of the vessel being devoid of a moderator material; each of the vessels being housed in one of the two reactor vessel shafts; - means of transferring the molten salt fuel from the inside of one of the two tanks to the inside of the other of the two tanks.Figure for the abridged version: fig. 4.

Description

Title of the invention: Nuclear power plant housing two molten salt reactor(s), with means for transferring fuel liquid between the two reactor vessels; Starting and operating procedure for the power plant. technical field

[0001] The present invention relates to the field of molten salt reactors (MSR). More particularly, it relates to the field of small or medium power MSRs, or Advanced Modular Reactors (AMR).

[0002] The main objective of the invention is therefore to propose a nuclear installation with such reactors, more particularly those with fast neutrons, which guarantees continuity of service in operation.

[0003] By "molten salt reactor(s)", we mean here and within the framework of the invention, the usual technological meaning, namely a nuclear reactor in which the nuclear fuel is in liquid form, dissolved in a molten salt, at a temperature typically between 500 and 900 °C, which acts as a heat transfer fluid. Previous technique

[0004] Molten salt reactors are based on the use of a molten salt, for example of lithium fluoride (LiF) and beryllium fluoride (BeF2) or of sodium chloride (NaCl) and magnesium (MgCl2), serving both as a heat transfer fluid and as a moderator as the primary fluid within the reactor vessel, which is metallic or ceramic, such as SiC.

[0005] The tank contains the molten salt at high temperature, typically between 500 and 900 °C, generally at ambient pressure.

[0006] The fissile fuel can be uranium-235, plutonium, or uranium-233, the latter being obtained from the conversion of thorium. A molten salt reactor can perform its own breeder reactor using a fertile blanket containing the fertile isotope to be irradiated.

[0007] The nuclear reaction is triggered by the concentration of fissile material of the fuel within the reactor vessel or by passing through a graphite moderator block.

[0008] A molten salt reactor can therefore be moderated by graphite, producing thermal neutrons, or without a moderator producing fast neutrons.

[0009] The presence or absence of moderators thus defines the two main families of molten salt reactors, respectively thermal neutron and fast neutron reactors.

[0010] From the 2000s onwards, molten salt reactors were evaluated and then selected within the framework of the Generation IV International Forum. They are now the subject of international research with a view to deployment as fourth generation reactors, in particular as small modular reactors (SMRs) which are advanced nuclear reactors (AMR for "Advanced Nuclear Reactors"), whose power capacity can reach up to 300 MWe per unit.

[0011] Although promising in terms of safety potential, molten salt reactors may require expensive and complex systems and components.

[0012] Indeed, in a molten salt reactor, the primary fuel circuit, containing dissolved uranium or plutonium, constitutes the first safety barrier and must therefore meet very demanding design criteria in terms of leak-tightness. This primary circuit must include a core zone, in which the nuclear fission reactions take place in a chain reaction, and a heat exchange zone fluidically connected to the core, in which the heat generated in the core is transferred to a secondary circuit.

[0013] In conventional designs, the core is connected to a plurality of fluid circulation loops, each comprising an exchanger and a pump adapted to ensure circulation to and from the associated exchanger.

[0014] For example, among the programs selected for Generation IV, the homogeneous indirect-cooled reactor resulting from research at the LPSC laboratory in Grenoble, designated by the Anglo-Saxon acronym MSFR (for "Molten Salt Fast Reactor"), whose fuel is a liquid fluorinated salt with breeder production provided by Thorium, comprises twelve or sixteen fluidic circulation loops. Each of the loop components adds complexity to the overall fluidic circuit: [1],

[0015] For the design of a molten salt reactor(s), in particular of the SMR type, the inventors of the present invention initially sought to develop a design reducing to a minimum the number of pipes and components, in particular to retain the major advantage inherent in SMRs, namely the increased modularity capacity by manufacturing the components in a factory for transport to the construction site, and also to increase operational safety.

[0016] They have thus designed a molten salt nuclear reactor of the fast neutron type, described and claimed in patent application filed on December 19, 2022, under number FR2213882, entitled "Molten salt nuclear reactor of the fast neutron type, whose primary circuit is by natural convection circulation." The proposed reactor may have a reactor vessel incorporating a primary circuit. The fuel is of reduced size, typically with a diameter of less than 2.5m and an overall height of less than 5m, making the reactor compliant with the requirements for modular AMR reactors. Thus, a primary circuit with a reactor vessel, the inner cylindrical shell, and its primary / secondary heat exchanger, as described in this patent application, can be manufactured in a factory, transported to the site, and then used throughout the reactor's lifetime.

[0017] In theory, fast neutron molten salt reactors have the advantage of great versatility both in terms of fuel that can be used (uranium, plutonium, thorium, minor actinides) and in operating mode (burner or regenerator).

[0018] It is recalled here that a "burner" mode corresponds to a reactor operation where there is an intensive consumption of fissile isotopes with limited regeneration of fissile material.

[0019] A "regenerating" mode of a nuclear reactor is an operation in which it produces all or part of the fissile fuel it consumes from fertile material. Thus, neutrons, generated by fission in the reactor core, are absorbed by fertile material which in turn produces new fissile materials.

[0020] In burner mode, a fast neutron molten salt reactor can use as fissile isotopes: uranium 235, uranium 233, fissile isotopes of plutonium and fissile isotopes of minor actinides.

[0021] In regenerator mode, a fast neutron molten salt reactor can use the same fissile isotopes listed above and as fertile isotopes: uranium 238, thorium 232, fertile isotopes of plutonium and fissile isotopes of minor actinides.

[0022] With the design that the inventors have proposed in the aforementioned patent application FR2213882, the exploitation of the great versatility of fast neutron molten salt reactors is made possible.

[0023] In particular, a reactor according to this design can be designed to be of sufficient size to operate in iso-generator mode, a mode in which for each fission produced, a fissile nucleus is produced by fertile capture.

[0024] One of the main requirements for major industrial players likely to use molten salt reactors, particularly of the SMR type, is continuity of service. Indeed, these economic players, such as data centers, steel plants, and chemical plants, need a continuous power supply to avoid problems with their production equipment. For example, an arc furnace that stops for any reason shuts down permanently.

[0025] In countries where the electricity grid is unstable, manufacturers use contingency strategies that are illogical and inefficient but pragmatic, such as the The implementation of backup solutions, such as large quantities of electrochemical batteries coupled with uninterruptible power supplies (UPS), is a significant advantage. It is therefore reasonable to assume that a robust and integrated solution would allow these manufacturers to do without these backup systems.

[0026] However, a single molten salt reactor, particularly an SMR type operating on-site, does not guarantee continuity of service in the event of an emergency shutdown. In such a case, the energy supply to the industrial site connected to this single reactor would be interrupted.

[0027] In fact, a nuclear power plant must have at least two reactors to guarantee a permanent energy production.

[0028] In addition, maintenance operations may take place on a reactor, which, with the risk of emergency shutdown, ultimately requires the implementation of a third reactor on a nuclear power plant site.

[0029] The applicant is therefore considering nuclear power plants with one or more sets of three reactors.

[0030] With the isogenerator operating mode envisaged as described above, a reactor operates within the limits of the presence of fertile nuclei. The maximum operating time then corresponds to the depletion of the fertile material, which is equivalent to approximately 25 years of operation at full power for a reactor as envisaged by the inventors. However, the molten salts used in the reactor are corrosive. Thus, it is unlikely that at least some of the applicant's reactor vessels would be able to withstand corrosion over such a long period of exposure.

[0031] In parallel with this, the safety requirements are extremely stringent in the case of equipment lasting several decades.

[0032] Consequently, the inventors chose to replace the reactor vessel after a shorter period of operation. This ensures its resistance to corrosion while being technologically and economically advantageous.

[0033] With this choice, the issue of continuity of service is even more critical.

[0034] There is therefore a need to improve molten salt reactors, of the type fast neutron reactors, particularly when considered as AMR reactors, are used to overcome the disadvantages mentioned above, especially to ensure continuity of service for a nuclear power plant that incorporates them.

[0035] The aim of the invention is therefore to meet at least part of this need. Description of the invention

[0036] To this end, the invention relates, in one of its aspects, to a nuclear power plant comprising - A reactor building; - at least two reactor vessel wells, located inside the reactor building, - at least two molten salt nuclear reactor vessels, of type fast neutron reactors, axisymmetric around a central axis, internally delimiting a primary circuit of a fuel in liquid form in which at least one salt is melted, the interior of the vessel being devoid of a moderator material; each of the vessels being housed in one of the two vessel wells; - means of transferring the molten salt fuel from the inside of one of the two tanks to the inside of the other of the two tanks.

[0037] The term "reactor building" is understood in its usual sense, namely a building that contains the reactor itself and at least part of the circuits and systems ensuring the operation and safety of the reactor. These systems may include one or more residual heat evacuation systems, usually referred to as EPUR systems, and / or fission gas management system(s).

[0038] By "worn reactor vessel" is meant a reactor vessel that is considered to need replacing due to its level of irradiation and / or corrosion. Typically, a reactor vessel according to the invention that has been irradiated for a period of 5 to 10 years can be considered worn.

[0039] According to an advantageous embodiment, the nuclear power plant comprises: - at least three reactor vessel shafts, located inside the reactor building,

[0040] - at least three molten salt nuclear reactor vessels of the neutron type fast,

[0041] - means for transferring the molten salt(s) fuel from inside one of the three tanks inside another of the three tanks.

[0042] According to an advantageous embodiment, the nuclear power plant comprises:

[0043] -at least one additional shaft, separate from the reactor vessel shafts and located in the reactor building, adapted to house a container, called a transport castle, adapted to contain a nuclear reactor vessel; - at least one initial handling corridor, connecting an entrance to the reactor building to the additional shaft; - at least one second handling corridor, connecting the additional shaft to each tank shaft; - a handling chain either to bring, via the first and / or second handling corridors, a new reactor vessel into a reactor vessel well or a transport tower in the additional well, or a spent reactor vessel devoid of its fuel into an open transport tower housed in the additional well, or to evacuate, by the first handling corridor, a closed transport castle housing a used reactor vessel devoid of its fuel, leading to the entrance of the reactor building.

[0044] According to this method, the entrance to the reactor building preferably includes an airlock located in a so-called truck door, through which a transport cask containing a spent reactor vessel can be handled on a road vehicle, such as a truck.

[0045] Advantageously, the handling system includes at least one overhead crane, referred to as a polar crane, integrated internally at the top of the reactor building. For the purposes of this invention, "polar crane" means in its usual sense, namely a handling crane located under the dome of the reactor building and resting on brackets fixed to the structure of the reactor building.

[0046] According to a first advantageous configuration, the reactor building is generally cylindrical in shape, the additional shaft being arranged in the center of the reactor building, three reactor vessel shafts being arranged at 120° to each other around the additional shaft and each connected to the latter by one of the second handling corridors.

[0047] According to this first configuration, the handling chain is advantageously adapted to vertically evacuate a transport tower housing a reactor vessel in the second and then in the first handling corridor.

[0048] According to a second advantageous configuration, the reactor building is generally of a right parallelepiped shape, with three reactor vessel shafts arranged parallel to each other and each connected to an additional shaft by one of the second handling corridors, the three additional shafts being connected by a first handling corridor common to the entrance of the reactor building.

[0049] According to this second configuration, the handling system is adapted to vertically evacuate a transport cask housing a reactor vessel into the second aisle, tilt it, and then evacuate it horizontally into the first handling aisle. Thus, in this configuration, the handling system, particularly its overhead crane, vertically brings the spent reactor vessel into the location, the additional shaft, provided for a transport cask. Once the cask containing this vessel is hermetically sealed, the assembly is removed from this additional shaft and tilted horizontally into the handling aisle. The assembly is then evacuated to an entrance of the reactor building, where an airlock provides access for a truck for road transport.

[0050] Several alternatives for the transfer of fuel between two tanks can be considered.

[0051] For transfer in liquid form, the transfer means may include either one or more pipes connecting two reactor vessels to transfer the fuel in liquid form or one or more drums into which the fuel in liquid form is evacuated from one reactor vessel to be poured into another reactor vessel.

[0052] For a transfer in solid form, the transfer means comprising at least one ingot mold in which the fuel in liquid form evacuated from a reactor vessel is solidified and then remelted to be poured into another reactor vessel.

[0053] According to a preferred application, each nuclear reactor of the power plant is according to the teaching of the aforementioned patent application FR2213882.

[0054] Thus, each nuclear reactor advantageously comprises: - at least one heat exchanger between the reactor's primary circuit and a secondary circuit, arranged inside the reactor vessel; - a shell in the form of at least one hollow cylinder, with the central axis coinciding with that of the reactor vessel, the shell being arranged in the reactor vessel to separate the interior of the latter into a central zone and a peripheral zone in which the heat exchanger is arranged so that in reactor operation, the molten salt(s) fuel liquid circulates by natural convection in a loop from the bottom of the central zone defining the reactor core (C) in which the fission reactions occur, from which it rises by heating to the top of the central zone where it is deflected to the top of the peripheral zone to pass through the exchanger (ZE) and then descends to the bottom of the peripheral zone where it is deflected to the reactor core.

[0055] In the context of the invention, "free of moderator material" means any material that allows a nuclear reactor to be classified as a thermal neutron reactor. In the usual sense, the kinetic energy of a fast neutron is greater than leV, while that of a thermal neutron is less than leV, typically on the order of 0.025 eV. Reference may be made to publication [2], and in particular to [Fig. 4], which indicates, for several types of reactors, the thermal fraction and the fast fraction of the neutron flux.

[0056] Thus, a molten salt reactor according to the invention is described as a fast neutron reactor.

[0057] Typically, a molten salt reactor according to the invention can exhibit a thermal neutron fraction of 0 to 0.05 and a fast fraction of 0.6 to 0.65.

[0058] A nuclear reactor may have one or both of the following dimensional characteristics for a typical power output of 150 MWth: - the diameter of the reactor vessel is between 1.5 and 2m; - the height of the primary circuit inside the reactor vessel is between 2.5 and 4m.

[0059] The largest existing and qualified transport towers have usable internal dimensions of 2.5 m in diameter and 7 m in length. A reactor vessel must therefore fit within this space. The dimensions of the reactor vessel cannot therefore exceed 2.5 m in diameter and 7 m in height, which is met by the aforementioned dimensions. The weight of the reactor vessel is low compared to that of a transport tower, which is ideal because the overall weight is limited by radiation protection considerations that necessitate the presence of shielding. However, this shielding, generally made of lead, is very heavy. Furthermore, the weight of this radiation protection increases if the diameter of the vessel increases. Given the dimensions and weight envisaged for a reactor vessel with its internal components, the transport of the entire transport tower and reactor vessel assembly by truck is feasible.

[0060] Preferably, the molten salt fuel(s) of the primary circuit is selected from a mixture of NaCl-UCl3, preferably in proportions of 25 to 30 mol% for UCl3, and PuCl3, preferably in proportions of 5 to 36 mol%, as salts, with depleted uranium U235, preferably less than 0.3%, atomic, or a mixture of NaCl-UCl3, preferably at 34 mol%, as a salt with enriched uranium U235 (HALEU), preferably in proportions of 5 to 20%. The molten salt(s) may also contain ThC14.

[0061] During reactor operation, the temperature of the molten salt(s) fuel liquid in the primary circuit can be between 500 and 750°C.

[0062] The power of a nuclear reactor is advantageously between 10 and 500 MWth, which corresponds to a power range sought for AMR type reactors.

[0063] The invention also relates to a method for starting up and operating a nuclear power plant as described above, comprising the following steps:

[0064] i / handling of a reactor vessel of a first reactor in a first of the three vessel wells;

[0065] ii / divergence and operation in iso-generator mode of the first reactor;

[0066] iii / handling of a reactor vessel from a second reactor in a second of the three vessel wells;

[0067] iv / at the end of a first predetermined duration, divergence and operation in iso-generator mode of the second reactor;

[0068] v / before or at the end of a second predetermined period during which the reactor vessel of the first reactor is worn out, shutdown of the first reactor and handling of a reactor vessel of a third reactor in a third of the three vessel wells;

[0069] vi / transfer of fuel from the reactor vessel of the first reactor into that of the third reactor;

[0070] vii / divergence and operation in iso-generator mode of the third reactor;

[0071] viii / at the end of a third predetermined period in which the reactivity within the spent reactor vessel of the first reactor has decreased, and said vessel has been inspected and cleaned, handling of said vessel inside a transport castle housed in the additional shaft, sealing of the transport castle and then removal of the transport castle containing the spent, inspected and cleaned reactor vessel from the reactor building;

[0072] ix / handling of a new reactor vessel in the first of the three vessel wells;

[0073] x / before or at the end of the second predetermined period in which the reactor vessel of the second reactor is worn out, shutdown of the second reactor;

[0074] xi / transfer of fuel from the reactor vessel of the second reactor into the new one of the first reactor;

[0075] xii / divergence and operation in iso-generator mode of the first reactor;

[0076] steps v / to xii / can be repeated so as to prolong operation permanent nominal capacity of at least two nuclear reactors.

[0077] The first predetermined period can be around 2 years, the second 5 years, the third around 1 year.

[0078] Thus, the invention essentially consists of constructing a nuclear power plant with a reactor building comprising at least two reactor vessel shafts, each dedicated to housing a reactor vessel of a fast neutron molten salt nuclear reactor intended to operate in iso-generator mode, and means for transferring fuel from one vessel to another.

[0079] By combining this with the replacement of reactor vessels considered to be "worn out", within reasonable timeframes, the continuity of service in operation of the power plant can be guaranteed.

[0080] In the three-nuclear-reactor embodiment, the continuity of service of the nuclear power plant is guaranteed, even if maintenance operations may take place on a reactor, and / or there is a risk of an emergency shutdown of a reactor.

[0081] Indeed, with an iso-generator mode operation, a molten salt nuclear reactor (MSR) operates within the limit of the presence of fertile nuclei.

[0082] In other words, the maximum theoretical operating time of such a reactor then corresponds to the exhaustion of the fertile material, which is equivalent to 25 years of operation at full power.

[0083] However, RSFs are known for the corrosive nature of their salts. Therefore, the Applicant opted for a system where the reactor vessel is replaceable before its maximum lifespan. This ensures its resistance to corrosion while also being technologically and economically advantageous.

[0084] Therefore, to guarantee the energy supply of an industrial site located downstream of a nuclear power plant or the stability of the network, the inventors conceived of a reactor building with three tank shafts, each intended to house a tank of a nuclear reactor, with a predetermined nominal life, preferably of 5 years in a first approach, with two charges of salt(s) to be melted to be transported to the site at a time interval, also predetermined, preferably equal to 3 years.

[0085] The operation according to this mode is in substance as follows.

[0086] A first reactor in a first reactor vessel well diverges and operates in mode iso-generator to start producing energy.

[0087] After a predetermined period, typically one year later, the second reactor in a second vessel well.

[0088] After a predetermined period, typically five years from the divergence of the first reactor, its vessel is considered worn and must be replaced.

[0089] A new tank is therefore installed in the third tank well and the fuel salt of the first reactor is transferred from the used tank to said new tank.

[0090] The third reactor immediately diverges and operates in iso-generator mode to take over for energy production.

[0091] The spent reactor vessel can be left in the first reactor well to allow its radioactivity to decay, and to be inspected and cleaned. It is then removed and placed in a transport tower housed in a secondary well, and replaced with a new vessel. This new vessel will receive the salt from the second reactor in the second reactor well when the vessel in that well is considered spent.

[0092] This cyclic operating mode minimizes nuclear transport while ensuring continuous operation of the power plant. The reactor vessels are transported empty between the factory where they are manufactured and at least some of the components are assembled, and the site where the nuclear power plant is installed. They are then installed and filled with the fuel salt(s) once placed in their reactor vessel well.

[0093] After a period of operation, typically 5 years, the salt(s) is transferred into another, new tank, and the original tank, considered to be worn out, is kept housed in its tank well to achieve the decay of radioactivity.

[0094] After a period, typically of one year, the radioactivity of the tank has decreased to the point where it is now transportable, and therefore removed from the reactor building and then from the site of the power plant.

[0095] The combustible salt is brought to the site in a quantity and packaging compatible with the requirements of the local safety authority. It is part of a transport nuclear power plants must comply with nuclear safety regulations. These regulations mandate the presence of specifically qualified personnel to escort convoys. One of the anticipated limitations for the development of SMRs is the availability of these personnel, as in conventional reactors, a third of the core is replaced every 12 to 18 months.

[0096] Thanks to the invention, transport is limited to the original supply in two stages separated by a predetermined period, typically 3 years for the return with the replacement of the fissile and fertile material 25 years later, or faster if necessary.

[0097] The invention therefore allows a reduction in the number of fuel transports compared to the SMR reactor solutions currently envisaged, which is, in addition to being a logistical advantage, a factor of social acceptance.

[0098] In conclusion, a molten salt fast neutron nuclear reactor plant according to the invention offers numerous advantages, including: - the guarantee of continuity of operational service, even in the event of significant maintenance operations and / or unscheduled shutdowns; - quick and easy installation using handling equipment that can be standard for reactors within the reactor building; - quick and easy loading of a reactor vessel considered to be used and once the radioactivity is at a low level into a transport castle directly within the reactor building.

[0099] Other advantages and features of the invention will become clearer from 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

[0100] [Fig-1] [Fig.1] is a view from a simulation coupling the mechanics of computational fluid dynamics (CFD) and 3D neutronics, showing the circulation of the primary fluid with the temperature field within a molten salt nuclear reactor, of the fast neutron type installed within a reactor building of a power plant according to the invention.

[0101] [Fig.2] [Fig.2] is a schematic partial perspective view of a building reactor of a nuclear power plant, according to a first cylindrical building configuration according to the invention.

[0102] [Fig.3] [Fig.3] is a partial perspective view of a reactor building of a nuclear power plant, according to a second first building configuration, in a straight parallelepiped shape according to the invention.

[0103] [Fig.4] [Fig.4] is a longitudinal cross-sectional view of a reactor building of a power plant according to the invention, made at the level of one of the reactor vessel shafts intended to house a nuclear reactor and an additional shaft housing a transport castle intended to house and transport a spent reactor vessel. Detailed description

[0104] Throughout this application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a fast neutron molten salt nuclear reactor, as provided for in a vertical operating configuration in a nuclear power plant according to the invention.

[0105] It should be noted that the various temperatures, power outputs, volumes, flow rates, etc., indicated are for guidance purposes only. For example, other temperatures may be considered depending on the configuration, particularly the power of the molten salt reactor(s), the volume of molten salt fuel liquid, and the power requirements for the intended application.

[0106] With reference to [Fig.1], a molten salt(s) fast neutron nuclear reactor 1 is described, according to a primary circuit configuration as described and claimed in patent application FR2213882. This [Fig.1] is a numerical simulation view obtained by coupling Computational Fluid Dynamics (CFD) and 3D neutronics, as explained below.

[0107] The reactor 1 with central axis X comprises a tank 2 with a metal jacket preferably made of stainless steel or nickel-based alloy, with a thickness of approximately 10 to 20 mm, and formed of a hemispherical tank bottom and a vertical cylinder.

[0108] This reactor vessel 2 internally delimits a primary circuit of fuel in liquid form in which at least one salt is molten. The interior of vessel 2 is devoid of moderator material. In other words, the molten salt(s) fuel liquid fills and circulates inside the vessel without being moderated.

[0109] A single annular heat exchanger 3 between the reactor primary circuit and a secondary circuit is arranged inside the reactor vessel 2.

[0110] A first shell 4 in the form of at least one hollow cylinder, with its central axis coinciding with that of the reactor vessel, is arranged in the reactor vessel 2 to separate the interior of the latter into a central zone and a peripheral zone in which the heat exchanger 3 is arranged.

[0111] The thickness of the bottom of the ferrule 4, in the core area C, can be reduced compared to that of the top of the ferrule 4. As an example, for a total height H equal to 2.5m, the reduced height H1 of the bottom of the ferrule 4 is equal to 1m.

[0112] A second ferrule 5 is arranged concentrically inside the first ferrule 4. The interior of the second ferrule 5 defines a space in which control and / or safety bars for nuclear reactions can extend.

[0113] The ferrules 4, 5 can be made of stainless steel or nickel-based alloy.

[0114] The ferrules 4, 5 are advantageously fixed by suspension to the cap-lid closing reactor vessel 2.

[0115] At the bottom of the reactor vessel 2, below the first shell 4, a first deflector 6, in the form of a portion of a torus.

[0116] At the top of the reactor vessel 2, above the first shell 4, a second deflector 7, also in the form of a portion of a torus.

[0117] As symbolized by the arrows in [Fig. 1], with the shells 4, 5 and the deflectors 6, 7 as arranged, in reactor operation, the molten salt(s) fuel liquid circulates solely by natural convection in a loop from the bottom of the central zone defining the reactor core C in which the fission reactions occur, from which it rises by heating to the top of the central zone between the shells 4 and 5 where it is deflected by the deflector 7 towards the top of the peripheral zone to pass through the exchanger 3 and then descends towards the bottom of the peripheral zone where it is deflected by the deflector 7 towards the reactor core C.

[0118] The ferrule 5 allows the fuel liquid to be guided as it rises between the two areas where it is diverted, i.e. in the central area of ​​the reactor from the deflection area by the deflector 6 through the core C to the deflection area by the deflector 7.

[0119] The deflectors 6, 7, by their shapes and arrangement, each allow the flow of the diverted molten salt(s) combustible liquid to be distributed.

[0120] As shown in [Fig.1], the thickness of the part of the first shell, arranged above the exchanger 4, can be greater than that of its part arranged below the exchanger, i.e. at the level of the core C.

[0121] The dimensional, temperature and power characteristics, and of the molten salt fuel liquid obtained are as follows: - dimensions: tank diameter 2 between 1.5 and 2m, primary circuit height between 2.5 and 4m; - power between 10 and 300 MWth; - primary circuit operating temperature between 550 and 750°C; - Molten salt fuel liquid of the primary circuit to be chosen from a mixture of NaCl-UCl3 of 25 to 30% mol-PuCl3 of 9 to 11% mol with depleted uranium U235 at 0.7%, or a mixture of NaCl-UCl3 at 34% mol with natural uranium U235 enriched to 20%.

[0122] Advantageously, elements such as MgCl2, minor actinide chlorides or other elements from the periodic table of elements may be added in varying proportions.

[0123] A nuclear reactor 1 as just described, operating alone, does not allow for the continuity of service of a nuclear power plant. Indeed, if an emergency shutdown of this single reactor were to occur, the energy supply to an industrial site located downstream of the power plant would be interrupted.

[0124] To overcome this drawback, a nuclear power plant according to the invention has at least two reactors to guarantee a permanent energy production.

[0125] Nevertheless, maintenance operations may take place on a reactor.

[0126] Therefore, the inventors considered constructing a nuclear power plant comprising one or more sets of three nuclear reactors 1 housed in a reactor building.

[0127] To achieve the function of continuity of energy supply of the nuclear power plant, the nuclear power plant according to the invention combines the following aspects: - an operation of each reactor 1 in isogenerator mode; - a transfer of combustible salt from one tank to another; - replacement of a tank considered to be worn out within reasonable timeframes.

[0128] Thus, as illustrated in figures 2 to 4, the power plant 10 comprises a reactor building 11 inside which three reactor vessel shafts 12 are located. As an example, the diameter 0 of a reactor building of generally cylindrical shape can be on the order of 22m.

[0129] Each of these three reactor vessel shafts 12 can house a reactor vessel 2 as illustrated in [Fig. 1]. A reactor vessel shaft 12, which is a concrete structure in the reactor building 11, can include a neutron reflector 101.

[0130] And to ensure the continuity of service of operation of the power plant, means are provided for transferring the molten salt fuel from inside one of the three tanks to inside another of the three tanks, as detailed below.

[0131] This transfer of the fuel salt can be carried out either in its liquid or solid form. For liquid transfer, reactor building 11 can incorporate one or more pipelines connecting two reactor vessels to transfer the fuel in liquid form. It is also possible to provide one or more transfer drums into which the fuel in liquid form is discharged from one reactor vessel to be poured into another reactor vessel. For solid transfer, at least one ingot mold can be provided in which the fuel in liquid form discharged from one reactor vessel is solidified and then remelted before being poured into another reactor vessel.

[0132] According to an advantageous embodiment, the reactor building 11 incorporates at least one additional shaft 13, separate from the reactor vessel shafts, adapted to house a transport castle 100, adapted to contain a nuclear reactor vessel 2.

[0133] The largest existing and qualified transport towers have usable internal dimensions of 2.5 m in diameter and 7 m in length. The reactor vessel of a reactor 2 must therefore fit within this space. The weight of the vessel is small compared to that of the tower; the overall weight is indeed limited by radiation protection considerations, which necessitate the presence of shielding.

[0134] A first handling corridor 14 is located in the reactor building 11 and connects an entrance 15 of the reactor building to the additional shaft 13.

[0135] A second handling corridor 16 connects the additional shaft 13 to each tank shaft 12.

[0136] A handling chain is provided within reactor building 11 either to bring, via the first 15 and / or second 16 handling corridors, a new reactor vessel into a vessel well or a transport castle in the additional well or a spent reactor vessel devoid of its fuel into an open transport castle housed in the additional well, or to evacuate, via the first handling corridor, a closed transport castle housing a spent reactor vessel devoid of its fuel to the entrance 15 of the reactor building.

[0137] As illustrated in [Fig.4], the handling chain preferably includes a polar overhead crane 102 integrated inside at the top of the reactor building 11.

[0138] The entrance 15 of the reactor building 11 advantageously comprising an airlock 17 located in a truck door 18, through which a transport cask containing a spent reactor vessel can be handled on a road vehicle, such as a truck.

[0139] Several reactor building 11 configurations can be envisaged.

[0140] One approach involves constructing the reactor building 11 in a cylindrical shape, as illustrated in [Fig. 2]. A significant advantage of this configuration is that the cylindrical shape of the reactor building minimizes handling lengths and allows for only straight handling paths. Another advantage is that the additional shaft 13, which can accommodate a transport tower, is centrally located, i.e., at the center of the diameter of the reactor building 11. The removal of an assembly consisting of a spent reactor vessel 2 is carried out vertically via the handling corridor 14. Thus, during the handling of such an assembly, it does not pass over any nuclear components, which is a safety factor for the power plant.

[0141] A second configuration consists of constructing the reactor building 11 in a general rectangular shape, as illustrated in [Fig. 3]. The three reactor vessel shafts 12 are then arranged parallel to each other and each connected to the additional shaft 13 by one of the second handling corridors 16, the three additional shafts 13 being connected by a first handling corridor 14 common to the entrance 15 of the reactor building.

[0142] In this configuration, the polar crane 102 can vertically lift a transport cask 100 housing a reactor vessel 2 into the second aisles 16, tilt it, and then evacuate it horizontally into the first handling aisle 14. Thus, in this configuration, the polar crane 102 vertically lifts the spent reactor vessel 2 into the additional shaft 13, designed for a transport cask 100. Figure 4 illustrates the transfer of a spent vessel 2 from its vessel shaft 12 to the additional shaft 13 containing a transport cask 100. Once the cask containing this vessel 2 is hermetically sealed, the assembly is lifted out of this additional shaft and tilted horizontally into the handling aisle. The assembly is then evacuated to an entrance 15 of the reactor building, where the airlock 17 provides access for a truck for road transport.A new, non-irradiated tank 2 can quickly be handled, made the reverse journey, and connected to the cooling, heating, and fuel salt circuits.

[0143] The divergence of the nuclear reactors 1 and the continuous operation of the nuclear power plant 10 are carried out according to the following steps.

[0144] Step i / : handling of a reactor vessel 2 of a first reactor 1 in a first of the three vessel wells 12.

[0145] Step ii / : The first reactor 1 diverges and operates in iso-generator mode.

[0146] Step iii / : handling of a reactor vessel 2 of a second reactor 1 in a second of the three vessel wells 12.

[0147] Step iv / at the end of a first predetermined duration, the second reactor diverges and operates in iso-generator mode.

[0148] Step v / : before or at the end of a second predetermined period during which the reactor vessel of the first reactor is worn out, the first reactor is shut down and a reactor vessel of a third reactor is handled in a third of the three vessel wells.

[0149] Step vi / : the fuel is transferred from the reactor vessel of the first reactor to that of the third reactor.

[0150] Step vii / : the third reactor diverges and operates in iso-generator mode.

[0151] Step viii: At the end of a third predetermined period during which the reactivity within the spent reactor vessel of the first reactor has decreased, and said vessel has been inspected and cleaned, said vessel is handled inside a transport tower 100 housed in the additional shaft, and then the seal is hermetically sealed. transport castle then evacuation of the transport castle containing the used reactor vessel, inspected and cleaned out of the reactor building.

[0152] Step ix / : handling of a new reactor vessel 2 in the first of the three vessel wells 12.

[0153] Step x / : before or at the end of the second predetermined time during which the reactor vessel of the second reactor is worn out, the second reactor is stopped.

[0154] Step xi / : the fuel is transferred from the reactor vessel of the second reactor into the new one of the first reactor.

[0155] Step xii / : The first reactor diverges and operates in iso-generator mode.

[0156] All steps v / to xii / can be repeated as many times as desired so as to prolong the permanent nominal operation of at least two nuclear reactors.

[0157] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0158] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0159] A reactor building 11 can house various other components. Other shafts can be provided for this purpose in the building structure. For example, as illustrated in [Fig.2], for each nuclear reactor, a shaft 110 can be provided for the installation of an active or passive device for evacuating residual heat, a shaft 111 for the installation of a heat exchanger, and a shaft 112 for the installation of a fission gas management system. List of cited references

[0160] [1]: E. Merle-Lucotte, M. Allibert, M. Brovchenko, D. Heuer, V. Ghetta, A. Laureau, P.Rubiolo, Chapitre "Introduction to the Physics of Thorium Molten Sait Fast Reactor (MSFR) Concepts”, Thorium Energy for the World, Springer International Publishing, Switzerland (2016).

[0161] [2]: Jiri Krepel et al. "Selj-Sustaining Breeding in Advanced Reactors: Characterization ofSelected Reactors”, Encyclopedia of Nuclear Energy 2021, Pages 801-819. https: / / www.sciencedirect.com / science / article / pii / B9780128197257001239? via%3Dihub

Claims

Demands

1. Nuclear power plant comprising: - a reactor building (10); - at least two reactor vessel shafts, located inside the reactor building, - at least two reactor vessels (2) of molten salt nuclear reactor(s), of the fast neutron type, axisymmetric about a central axis, internally delimiting a primary circuit of fuel in liquid form in which at least one salt is molten, the interior of the vessel being devoid of a moderator material; each of the vessels being housed in one of the two reactor vessel shafts; - means for transferring the molten salt fuel from the interior of one of the two vessels to the interior of the other of the two vessels.

2. Nuclear power plant according to claim 1, comprising: - at least three reactor vessel shafts, located inside the reactor building, - at least three molten salt nuclear reactor vessels, of the fast neutron type, - means for transferring the molten salt fuel from inside one of the three vessels to inside another of the three vessels.

3. A nuclear power plant according to claim 1 or 2, comprising: - at least one additional shaft, separate from the reactor vessel shafts and located in the reactor building, adapted to house a container, referred to as a transport tower, adapted to contain a nuclear reactor vessel; - at least one first handling corridor, connecting an entrance of the reactor building to the additional shaft; - at least one second handling corridor, connecting the additional shaft to each reactor vessel shaft; - a handling chain either for bringing, via the first and / or second handling corridors, a new reactor vessel into a reactor vessel shaft or a transport castle in the additional shaft or a spent reactor vessel devoid of its fuel inside an open transport castle housed in the additional shaft, or to evacuate, via the first handling corridor, a closed transport castle housing a spent reactor vessel devoid of its fuel to the entrance of the reactor building.

4. Nuclear power plant according to claim 3, the reactor building entrance comprising an airlock located in a so-called truck door, through which a transport cask containing a spent reactor vessel can be handled on a road vehicle, such as a truck.

5. Nuclear power plant according to claim 3 or 4, the handling chain comprising at least one overhead crane, referred to as a polar crane, integrated inside at the top of the reactor building.

6. Nuclear power plant according to any one of claims 3 to 5, the reactor building being generally cylindrical in shape, the additional shaft being arranged in the center of the reactor building, three reactor vessel shafts being arranged at 120° to each other around the additional shaft and each connected to the latter by one of the second handling corridors.

7. Nuclear power plant according to claim 6, the handling chain being adapted to vertically evacuate a transport castle housing a reactor vessel into the second and then into the first handling corridor.

8. Nuclear power plant according to any one of claims 3 to 5, the reactor building being generally of a right parallelepiped shape, three reactor vessel shafts being arranged parallel to each other and each connected to an additional shaft by one of the second handling corridors, the three additional shafts being connected by a first handling corridor common to the entrance of the reactor building.

9. Nuclear power plant according to claim 8, the handling chain being adapted to vertically evacuate a transport castle housing a reactor vessel into the second corridors, to tilt it and then evacuate it horizontally into the first handling corridor.

10. Nuclear power plant according to any one of the preceding claims, the transfer means comprising one or more conduits connecting two reactor vessels for transferring fuel in liquid form.

11. Nuclear power plant according to any one of claims 1 to 9, the transfer means comprising one or more drums into which the fuel in liquid form is evacuated from one reactor vessel to be poured into another reactor vessel.

12. Nuclear power plant according to any one of claims 1 to 9, the transfer means comprising at least one ingot mold in which the fuel in liquid form evacuated from a reactor vessel is solidified and then remelted for being poured into another reactor vessel.

13. Nuclear power plant according to any one of the preceding claims, each nuclear reactor (1) comprising: - at least one heat exchanger (3) between the primary circuit of the reactor and a secondary circuit, arranged inside the reactor vessel;- a shell (4) in the form of at least one hollow cylinder, with its central axis coinciding with that of the reactor vessel, the shell being arranged in the reactor vessel to separate the interior of the latter into a central zone and a peripheral zone in which the heat exchanger is arranged so that in reactor operation, the molten salt(s) fuel liquid circulates by natural convection in a loop from the bottom of the central zone defining the reactor core (C) within which the fission reactions occur, from which it rises by heating to the top of the central zone where it is deflected to the top of the peripheral zone to pass through the exchanger (ZE) then descends to the bottom of the peripheral zone where it is deflected to the reactor core.;

14. Method of starting up and operating a nuclear power plant according to any one of claims 2 to 13, comprising the following steps: i / handling a reactor vessel of a first reactor in a first of three vessel shafts; ii / divergence and operation in iso-generator mode of the first reactor; iü / handling of a reactor vessel from a second reactor in a second of the three vessel wells; iv / at the end of a first predetermined period, divergence and operation in iso-generator mode of the second reactor; v / before or at the end of a second predetermined period during which the reactor vessel of the first reactor is worn out, shutdown of the first reactor and handling of a reactor vessel of a third reactor in a third of the three vessel wells; vi / transfer of fuel from the reactor vessel of the first reactor into that of the third reactor; vii / divergence and operation in iso-generator mode of the third reactor; viii / at the end of a third predetermined period in which the reactivity within the spent reactor vessel of the first reactor has decreased, and said vessel has been inspected and cleaned, handling of said vessel inside a transport castle housed in the additional shaft, sealing of the transport castle and then removal of the transport castle containing the spent, inspected and cleaned reactor vessel from the reactor building; ix / handling of a new reactor vessel in the first of the three vessel wells; x / before or at the end of the second predetermined period during which the reactor vessel of the second reactor is worn out, shutdown of the second reactor; xi / transfer of fuel from the reactor vessel of the second reactor into the new one of the first reactor; xii / divergence and operation in iso-generator mode of the first reactor; steps v / to xii / can be repeated so as to extend the permanent nominal operation of at least two nuclear reactors.

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