Nuclear power plant with nuclear building housing at least two molten salt fast neutron nuclear reactors, with means for transferring molten salt liquid fuel between the two reactor vessels; and related method for starting and operating the installation

Abstract

The invention relates to a nuclear power plant comprising: - a reactor building (10); - at least two reactor pits implanted inside the reactor building, - at least two reactor vessels (2) of molten salt fast neutron nuclear reactors (1), the reactor vessels exhibiting axial symmetry about a central axis, internally delimiting a primary circuit for a fuel in liquid form in which at least one salt is melted, the interior of the vessel being devoid of moderator material; each of the reactor vessels being housed in one of the two reactor pits; and - means for transferring the molten salt fuel from inside one of the two reactor vessels into the other of the two reactor vessels.

Description

[0001] Description

[0002] Title: Nuclear power plant with a nuclear building housing at least two molten salt nuclear reactors, of the fast neutron type and with means of transferring molten salt fuel liquid between the two reactor vessels; Related method of starting and operating the installation.

[0003] technical field

[0004] 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).

[0005] The main objective of the invention is therefore to provide a nuclear installation with such reactors, more particularly fast neutron reactors, which guarantees continuity of service in operation.

[0006] 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.

[0007] Previous technique

[0008] Molten salt reactors rely on the use of a molten salt, for example lithium fluoride (LiF) and beryllium fluoride (BeF2) or sodium chloride (NaCl) and magnesium (MgCh), 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.

[0009] The tank contains the molten salt at high temperature, typically between 500 and 900 °C, generally at ambient pressure.

[0010] The fissile fuel can be uranium-235, plutonium, or uranium-233, the latter being produced from the conversion of thorium. A molten salt reactor can perform its own breeder reactor function using a fertile blanket containing the fertile isotope to be irradiated. The nuclear reaction is triggered by the concentration of fissile material in the fuel within the reactor vessel or by its passage through a graphite moderator block.

[0011] A molten salt reactor can therefore be moderated by graphite, producing thermal neutrons, or without a moderator producing fast neutrons.

[0012] The presence or absence of moderators thus defines the two main families of molten salt reactors, respectively thermal neutron and fast neutron reactors.

[0013] 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.

[0014] Although promising in terms of safety potential, molten salt reactors may require expensive and complex systems and components.

[0015] 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.

[0016] In conventional designs, the core is connected to a plurality of fluid circulation loops, each comprising a heat exchanger and a pump adapted to ensure circulation to and from the associated heat exchanger.

[0017] For example, among the programs selected for Generation IV, the homogeneous indirect-cooled reactor developed by the LPSC laboratory in Grenoble, known by the acronym MSFR (for "Molten Salt Fast Reactor"), whose fuel is a liquid fluorinated salt with thorium-based breeder fuel, comprises twelve or sixteen fluidic circulation loops. Each component of the loops adds complexity to the overall fluidic circuit: [1]. For the design of a molten salt reactor, particularly of the SMR type, the inventors of the present invention initially sought to develop a design that minimizes the number of pipes and components, notably to preserve the major advantage inherent in SMRs, namely the increased modularity achieved by manufacturing components in a factory for transport to the construction site, and also to enhance operational safety.

[0018] 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, with a primary circuit by natural convection circulation." The proposed reactor can feature a reactor vessel incorporating a reduced-size primary fuel circuit, typically with a diameter of less than 2.5 m and an overall height of less than 5 m, making the reactor compliant with the requirements for modular AMR reactors. Therefore, 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.

[0019] 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).

[0020] 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.

[0021] A "regenerating" mode of a nuclear reactor is an operating mode 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 material.

[0022] In burner mode, a fast neutron molten salt reactor can use the following as fissile isotopes: uranium-235, uranium-233, the fissile isotopes of plutonium, and the fissile isotopes of minor actinides. In breeder mode, a fast neutron molten salt reactor can use the same fissile isotopes listed above and the following as fertile isotopes: uranium-238, thorium-232, the fertile isotopes of plutonium, and the fissile isotopes of minor actinides.

[0023] With the design that the inventors proposed in the aforementioned patent application FR2213882, the exploitation of the great versatility of fast neutron molten salt reactors is made possible.

[0024] 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.

[0025] One of the main requirements for major industrial players likely to use molten salt reactors (MSRs), 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 prevent problems with their production equipment. For example, an arc furnace that shuts down for any reason is permanently out of service.

[0026] In countries with unstable power grids, manufacturers employ contingency strategies that are illogical and inefficient but pragmatic, such as implementing backup solutions like large quantities of electrochemical batteries coupled with uninterruptible power supplies (UPS). It is therefore reasonable to assume that a robust, integrated solution would allow these manufacturers to do away with these backup systems.

[0027] However, a single molten salt reactor, particularly an SMR type operating on-site, cannot 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.

[0028] In fact, a nuclear power plant must have at least two reactors to guarantee a continuous energy production.

[0029] 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.

[0030] The applicant therefore envisions nuclear power plants with one or more sets of three reactors. With the isogenerator operating mode envisaged above, a reactor operates within the limits of the availability 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 envisioned by the inventors. However, the molten salts used in reactors 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, safety requirements are extremely stringent in the case of equipment lasting several decades.

[0032] Therefore, the inventors opted to replace the reactor vessel after a shorter operating period. This ensures its resistance to corrosion while also being technologically and economically advantageous.

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

[0034] There is therefore a need to improve molten salt reactors, fast neutron reactors, especially when considered as AMR reactors, in order to overcome the disadvantages mentioned above, particularly to guarantee the continuity of service of a nuclear power plant that integrates them.

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

[0036] Description of the invention

[0037] To this end, the invention relates, in one of its aspects, to a nuclear power plant comprising

[0038] - A reactor building;

[0039] - at least two reactor vessel wells, located inside the reactor building,

[0040] - at least two molten salt nuclear reactor vessels of the fast neutron type, axisymmetric around 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 vessel shafts; - means of transferring the molten salt fuel from the interior of one of the two vessels to the interior of the other of the two vessels, so as to extend the permanent nominal operation of at least two nuclear reactors.

[0041] The term "reactor building" is used in its usual sense, namely a building that contains the reactor itself and at least some of the circuits and systems ensuring the reactor's operation and safety. These systems may include one or more residual heat evacuation systems, usually called EPUR systems, and / or fission gas management system(s).

[0042] A "worn reactor vessel" is defined as a reactor vessel that is considered to require replacement 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.

[0043] According to an advantageous embodiment, the nuclear power plant comprises:

[0044] - at least three reactor vessel wells, located inside the reactor building,

[0045] - at least three molten salt nuclear reactor vessels of the fast neutron type,

[0046] - means of transferring the molten salt fuel from the inside of one of the three tanks to the inside of another of the three tanks.

[0047] According to an advantageous embodiment, the nuclear power plant comprises:

[0048] -at least one additional well, separate from the reactor vessel wells and located in the reactor building, adapted to house a container, called a transport castle, adapted to contain a nuclear reactor vessel;

[0049] - at least one initial handling corridor, connecting an entrance to the reactor building to the additional shaft;

[0050] - at least one second handling corridor, connecting the additional shaft to each tank shaft;

[0051] - a handling chain either to bring, via the first and / or second handling corridors, a new reactor vessel to the inside of a vessel well or a transport castle in the additional well or a used reactor vessel devoid of its fuel to the inside of an open transport castle housed in the additional well, or to evacuate, via the first handling corridor, a closed transport castle housing a used reactor vessel devoid of its fuel to the entrance of the reactor building.

[0052] 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.

[0053] 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.

[0054] According to a first advantageous configuration, the reactor building is generally cylindrical in shape, with the additional shaft arranged in the center of the reactor building, three reactor vessel shafts arranged at 120° to each other around the additional shaft and each connected to the latter by one of the second handling corridors.

[0055] 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.

[0056] According to a second advantageous configuration, the reactor building is generally rectangular in 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.

[0057] According to this second configuration, the handling system is adapted to vertically move a transport cask containing a reactor vessel into the second handling corridor, tilt it, and then move it horizontally into the first handling corridor. Thus, in this configuration, the handling system, including its overhead crane, vertically lifts the spent reactor vessel into the designated location, the additional shaft, for a transport cask. Once the cask containing the vessel is hermetically sealed, the entire assembly is removed from this additional shaft and tilted horizontally into the handling corridor. The assembly is then moved to an entrance of the reactor building, where an airlock provides access for a truck for road transport.

[0058] Several alternatives for transferring fuel between two tanks can be considered.

[0059] For a transfer in liquid form, the means of transfer 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.

[0060] For a transfer in solid form, the transfer means include at least one ingot mold in which the fuel in liquid form removed from a reactor vessel is solidified and then remelted to be poured into another reactor vessel.

[0061] According to a preferred application, each nuclear reactor in the power plant is, according to the teachings of the aforementioned patent application FR2213882.

[0062] Thus, each nuclear reactor advantageously includes:

[0063] - at least one heat exchanger between the reactor's primary circuit and a secondary circuit, arranged inside the reactor vessel;

[0064] - 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.

[0065] For the purposes of this 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 Figure 4, which shows, for several types of reactors, the thermal fraction and the fast fraction of the neutron flux.

[0066] Thus, a molten salt reactor conforming to the invention is described as a fast neutron reactor.

[0067] 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.

[0068] A nuclear reactor may have one or both of the following dimensional characteristics for a typical power output of 150MWth:

[0069] - the diameter of the reactor vessel is between 1.5 and 2m;

[0070] - the height of the primary circuit inside the reactor vessel is between 2.5 and 4m.

[0071] 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. Consequently, the reactor vessel's dimensions cannot exceed 2.5 m in diameter and 7 m in height, which is met by the aforementioned dimensions. The reactor vessel's weight 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 use of shielding. This shielding, generally made of lead, is very heavy. Furthermore, the weight of this radiation protection increases with the diameter of the vessel. Given the dimensions and weight considered for a reactor vessel with its internal components, the entire transport tower and reactor vessel assembly can be transported by truck.

[0072] Preferably, the molten salt fuel(s) of the primary circuit is selected from a mixture of NaCl-UCh, preferably in proportions of 25 to 30 mol% for UCh, and PuCl, 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-UCh, preferably at 34 mol%, as salt with enriched uranium U235 (HALEU), preferably in proportions of 5 to 20%. The molten salt(s) may also contain ThC14.

[0073] During reactor operation, the temperature of the molten salt(s) fuel liquid in the primary circuit can be between 500 and 750°C.

[0074] The power output of a nuclear reactor is advantageously between 10 and 500 MWth, which corresponds to a power range sought for AMR-type reactors. The invention further relates to a method for starting up and operating a nuclear power plant as described above, comprising the following steps: i / handling a reactor vessel of a first reactor in the first of three reactor vessel shafts; ii / criticality and operation in isogenerator mode of the first reactor; iii / handling a reactor vessel of a second reactor in the second of three reactor vessel shafts; iv / at the end of a first predetermined period, criticality and operation in isogenerator 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 a reactor vessel of a third reactor in a third of three reactor vessel shafts;vi / transfer of fuel from the reactor vessel of the first reactor to that of the third reactor; vii / criticality and operation in isogenerator mode of the third reactor; 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, handling of said vessel inside a transport cask housed in the additional shaft, hermetically sealing of the transport cask and then removal of the transport cask 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 shafts; x / before or at the end of the second predetermined period during which the reactor vessel of the second reactor is spent, shutdown of the second reactor; xi / transfer of fuel from the reactor vessel of the second reactor to the new one of the first reactor;xii / divergence and operation in iso-generator mode of the first reactor; steps v / to xii / may be repeated so as to extend the permanent nominal operation of at least two nuclear reactors. The first predetermined period may be on the order of 2 years, the second 5 years, the third on the order of 1 year.

[0075] 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 of transferring fuel from one vessel to another.

[0076] By combining this with the replacement of reactor vessels considered "worn out" within reasonable timeframes, the continuity of service in operation of the power plant can be guaranteed.

[0077] In the three-reactor nuclear power plant embodiment, continuity of service 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.

[0078] Indeed, with an iso-generator mode operation, a molten salt nuclear reactor (MSR) operates within the limit of the presence of fertile nuclei.

[0079] 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.

[0080] However, reactive fuel cells (RFCs) 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.

[0081] 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.

[0082] The operation according to this mode is essentially as follows.

[0083] A first reactor in the first reactor vessel well diverges and operates in isogenerator mode to begin producing energy. After a predetermined period, typically one year later, the second reactor in a second reactor vessel well begins operation.

[0084] After a predetermined period, typically five years from the criticality of the first reactor, its vessel is considered worn and must be replaced.

[0085] A new tank is therefore installed in the third tank well and the fuel salt from the first reactor is transferred from the old tank to the new tank.

[0086] The third reactor immediately diverges and operates in iso-generator mode to take over for energy production.

[0087] 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, itself 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.

[0088] This cyclical operating method 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 their 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.

[0089] After a period of operation, typically 5 years, the salt(s) is transferred into another, new tank, and the original tank, considered worn out, is kept housed in its tank well to achieve the decay of radioactivity.

[0090] 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 power plant site.

[0091] The fuel salt is delivered to the site in the volume and packaging required by the local safety authority. It is part of a nuclear transport operation, which must comply with nuclear safety regulations. These regulations mandate the presence of specifically qualified personnel to escort the 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.

[0092] Thanks to the invention, transport is limited to the original supply in two stages spaced a predetermined period apart, typically 3 years for the return with the replacement of fissile and fertile material 25 years later, or faster if necessary.

[0093] The invention therefore allows a reduction in the number of fuel transports compared to the SMR reactor solutions currently envisaged, which, in addition to being a logistical advantage, is a factor in social acceptance.

[0094] In conclusion, a nuclear power plant with molten salt fast neutron reactors according to the invention offers numerous advantages, including:

[0095] - the guarantee of continuity of operational service, even in the event of significant maintenance operations and / or unscheduled shutdowns;

[0096] - quick and easy installation using handling equipment that can be standard for reactors within the reactor building;

[0097] - 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.

[0098] Other advantages and features of the invention will become clearer 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.

[0099] Brief description of the drawings

[0100] [Fig 1] Figure 1 is a view from a simulation coupling 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] Figure 2 is a schematic partial perspective view of a reactor building in a nuclear power plant, according to a first cylindrical building configuration of the invention. [Fig. 3] Figure 3 is a partial perspective view of a reactor building in a nuclear power plant, according to a second first building configuration, a rectangular parallelepiped in shape of the invention.

[0102] [Fig 4] Figure 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.

[0103] 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 output of the molten salt reactor(s), the volume of molten salt fuel(s), and the power requirements of the intended application.

[0106] With reference to Figure 1, a molten salt fast neutron nuclear reactor 1 is described, according to a primary circuit configuration as described and claimed in patent application FR2213882. This Figure 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 includes 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 liquid fuel in which at least one salt is molten. The interior of vessel 2 is devoid of moderator material. In other words, the liquid fuel containing molten salt(s) fills and circulates inside the vessel without being moderated. A single annular heat exchanger 3 between the reactor's primary circuit and a secondary circuit is arranged inside reactor vessel 2.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] Ferrules 4, 5 can be made of stainless steel or nickel-based alloy.

[0113] The ferrules 4, 5 are advantageously fixed by suspension to the cap-lid closing the reactor vessel 2.

[0114] At the bottom of reactor vessel 2, below the first shell 4, a first deflector 6, in the form of a portion of a torus.

[0115] At the top of reactor vessel 2, above the first shell 4, a second deflector 7, also in the form of a portion of a torus.

[0116] As symbolized by the arrows in Figure 1, with the shells 4, 5 and the deflectors 6, 7 as arranged, in reactor operation, the molten salt(s) fuel liquid circulates only 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 core of reactor C.

[0117] The shell 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 diversion area by the deflector 6 through the core C to the diversion area by the deflector 7. The deflectors 6, 7 by their shapes and their arrangement each allow the flow of the diverted molten salt fuel liquid to be distributed.

[0118] As shown in Figure 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.

[0119] The dimensional, temperature and power characteristics of the molten salt fuel liquid obtained are as follows:

[0120] - dimensions: tank diameter 2 between 1.5 and 2m, primary circuit height between 2.5 and 4m;

[0121] - power between 10 and 300 MWth;

[0122] - primary circuit operating temperature between 550 and 750 °C;

[0123] - Molten salt fuel liquid of the primary circuit to be chosen from a mixture of NaCl-UCL of 25 to 30% mol-PuCL of 9 to 11% mol with depleted uranium U235 at 0.7%, or a mixture of NaCl-UCL at 34% mol with natural uranium U235 enriched to 20%.

[0124] Advantageously, elements such as MgCh, minor actinide chlorides or other elements from the periodic table of elements can be added in varying proportions.

[0125] 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 plant would be interrupted.

[0126] To overcome this drawback, a nuclear power plant according to the invention has at least two reactors to guarantee a continuous energy production.

[0127] However, maintenance operations can take place on a reactor.

[0128] Therefore, the inventors considered building a nuclear power plant comprising one or more sets of three nuclear reactors 1 housed in a reactor building.

[0129] To achieve the function of ensuring continuity of energy supply to the nuclear power plant, the nuclear power plant according to the invention combines the following aspects:

[0130] - operation of each reactor 1 in isogenerator mode; - transfer of combustible salt from one tank to another;

[0131] - replacement of a tank considered to be worn out within reasonable timeframes.

[0132] Thus, as illustrated in figures 2 to 4, the power plant 10 includes a reactor building 11 inside which are located three reactor vessel shafts 12. As an example, the diameter 0 of a reactor building of general cylindrical shape can be on the order of 22m.

[0133] Each of these three vessel shafts 12 can house a reactor vessel 2 as illustrated in Figure 1. A vessel shaft 12 which is a concrete structure in the reactor building 11 can include a neutron reflector 101.

[0134] And to guarantee the continuity of service of operation of the power plant, means are planned for transferring the molten salt fuel from inside one of the three tanks to inside another of the three tanks, as detailed below.

[0135] This transfer of fuel salt can be carried out in either 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. Alternatively, one or more transfer drums can be used to transfer the fuel in liquid form from one reactor vessel to another. For solid transfer, at least one ingot mold can be used to solidify the fuel in liquid form from one reactor vessel, then remelt it before transferring it to another reactor vessel.

[0136] 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.

[0137] 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 Type 2 reactor must therefore fit within this space. The weight of the vessel is small compared to that of the tower; the overall weight is limited by radiation protection considerations, which necessitate the use of shielding.

[0138] 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 3.

[0139] A second handling corridor 16 connects the additional shaft 13 to each reactor vessel shaft 12. A handling chain is provided within the reactor building 11 either to bring, via the first 15 and / or second 16 handling corridors, a new reactor vessel into a reactor vessel shaft or a transport castle into the additional shaft or a spent reactor vessel devoid of its fuel into 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 15 of the reactor building.

[0140] As illustrated in Figure 4, the handling chain preferably includes a polar overhead crane 102 integrated inside at the top of the reactor building 11.

[0141] The entrance 15 of the reactor building 11 advantageously includes 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.

[0142] Several reactor building configurations can be considered.

[0143] One approach involves constructing reactor building 11 in a cylindrical shape, as illustrated in Figure 2. A significant advantage of this configuration is that the cylindrical shape of the reactor building minimizes handling distances 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 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.

[0144] A second configuration consists of constructing the reactor building 11 in a general right parallelepiped shape, as illustrated in Figure 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.

[0145] In this configuration, the polar crane 102 can vertically lift a transport cask 100 housing a reactor vessel 2 into the second handling corridor 16, tilt it, and then evacuate it horizontally into the first handling corridor 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 reactor 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 corridor. 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 tank 2, therefore not irradiated, can quickly be handled and made the reverse journey and be connected to the cooling, heating and fuel salt circuits.

[0146] The divergence of nuclear reactors 1 and the continuous operation of nuclear power plant 10 are carried out according to the following steps.

[0147] Step i / : handling of a reactor vessel 2 from a first reactor 1 in one of the first of the three vessel wells 12.

[0148] Step ii / : The first reactor 1 diverges and operates in iso-generator mode.

[0149] Step iii / : handling of a reactor vessel 2 of a second reactor 1 in a second of the three vessel wells 12.

[0150] Step iv / at the end of a first predetermined duration, the second reactor diverges and operates in iso-generator mode.

[0151] 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.

[0152] Step vi / : the fuel is transferred from the reactor vessel of the first reactor to that of the third reactor.

[0153] Step vii / : the third reactor diverges and operates in iso-generator mode.

[0154] 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, the vessel is handled into a transport cask 100 housed in the additional well. The transport cask is then hermetically sealed, and the transport cask containing the spent, inspected, and cleaned reactor vessel is removed from the reactor building. Step ix: A new reactor vessel 2 is handled into the first of the three vessel wells 12.

[0155] Step x / : before or at the end of the second predetermined period during which the reactor vessel of the second reactor is worn out, the second reactor is stopped.

[0156] Step xi / : the fuel is transferred from the reactor vessel of the second reactor into the new one of the first reactor.

[0157] Step xii / : The first reactor diverges and operates in iso-generator mode.

[0158] All steps v / to xii / can be repeated as many times as desired so as to extend the permanent nominal operation of at least two nuclear reactors.

[0159] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0160] Other variants and embodiments may be considered without departing from the scope of the invention.

[0161] A reactor building 11 can house various other components. Additional shafts can be provided in the building structure for this purpose. For example, as illustrated in Figure 2, each nuclear reactor can have a shaft 110 dedicated to the installation of an active or passive residual heat evacuation device, a shaft 111 dedicated to the installation of a heat exchanger, and a shaft 112 dedicated to the installation of a fission gas management system.

[0162] List of cited references

[0163] [1]: E. Merle-Lucotte, M. Allibert, M. Brovchenko, D. Heuer, V. Ghetta, A. Laureau, P. Rubiolo, Chapter “Introduction to the Physics of Thorium Molten Salt Fast Reactor (MSFR) Concepts'”, Thorium Energy for the World, Springer International Publishing, Switzerland (2016).

[0164] [2]: Jiri Krepel et al. “ Self-Sustaining Breeding in Advanced Reactors: Characterization of Selected Reactors”, Encyclopedia of Nuclear Energy 2021, Pages 801-819. https: / / www.sciencedirect.com / science / article / pii / B97801281972570012397via%3Dihub

Claims

Demands 1. Nuclear power plant comprising: - a reactor building (10); - at least two reactor vessel wells, located inside the reactor building, - at least two reactor vessels (2) of molten salt (1) nuclear reactors, of the fast neutron type, axisymmetric around a central axis, internally delimiting a primary circuit of a 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 vessel shafts; - means of transferring molten salt fuel from inside one of the two tanks to inside the other of the two tanks, so as to extend the permanent nominal operation of at least two nuclear reactors.

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

3. Nuclear power plant according to claim 1 or 2, comprising: - at least one additional well, separate from the reactor vessel wells 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 aisles, 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 well additional, either to evacuate, via the first handling corridor, a closed transport castle housing a used reactor vessel devoid of its fuel to the entrance of the reactor building.

4. Nuclear power plant according to claim 3, the entrance to the reactor building 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, of the truck type.

5. Nuclear power plant according to claim 3 or 4, the handling chain comprising at least one overhead crane, called 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 of general 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 to be 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 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 operation of the reactor, 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.

14. A method for 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 reactor vessel shafts; ii / criticality and operation in isogenerator mode of the first reactor; iii / handling a reactor vessel of a second reactor in a second of three reactor vessel shafts; iv / at the end of a first predetermined period, criticality and operation in isogenerator mode of the second reactor; v / before or at the end of a second predetermined period in which the reactor vessel of the first reactor is worn out, shutdown of the first reactor and handling a reactor vessel of a third reactor in a third of three reactor vessel shafts; vi / transfer of fuel from the reactor vessel of the first reactor to that of the third reactor; vii / criticality and operation in isogenerator mode of the third reactor; 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, handling of said vessel inside a transport cask housed in the additional shaft, hermetically sealing of the transport cask and then removal of the transport cask 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 shafts; x / before or at the end of the second predetermined period during which the reactor vessel of the second reactor is spent, shutdown of the second reactor; xi / transfer of fuel from the reactor vessel of the second reactor to 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 prolong the permanent nominal operation of at least two nuclear reactors.;

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