Nuclear power plant with a nuclear building housing at least one molten salt nuclear reactor(s), of the fast neutron type and a fission gas management system accumulated in the reactor pile ceiling.

Abstract

A nuclear power plant with a nuclear building housing at least one molten salt fast neutron reactor and a fission gas management system for the accumulated gases in the reactor pile headspace. The invention relates to a nuclear power plant comprising: a reactor building (10); at least one reactor vessel shaft (9), located inside the reactor building; at least one reactor vessel (2) (1) filled with a molten salt fuel bath and defining within it a gas space above the free level of the bath, called the pile headspace; another vessel (8), arranged around the reactor vessel and inside the reactor vessel shaft, defining an inter-vessel space (E) with the reactor vessel; at least one fission gas management system (gaseous PF) for the gases accumulated in the reactor vessel pile headspace during normal reactor operation. Figure for the abstract: Fig. 2

Description

Title of the invention: Nuclear power plant with a nuclear building housing at least one molten salt nuclear reactor(s), of the fast neutron type, and a fission gas management system for gases accumulated in the reactor pile head. 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 invention thus has as its main objective to propose a nuclear installation with at least one such reactor, more particularly a fast neutron reactor, which guarantees management of the fission gases which accumulate in the reactor pile head in such a way as to have normal operation of the reactor at limited low pressure.

[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.

[0004] By "pile roof", we mean here and within the framework of the invention, the usual technological meaning, namely the upper part of the internal volume of the main vessel of a molten salt reactor, located above the liquid volume of molten salt and usually filled with an inert gas (argon or helium). Previous technique

[0005] 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.

[0006] The tank contains the molten salt at high temperature, typically between 500 and 900 °C, generally at ambient pressure.

[0007] 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 operation using a fertile blanket containing the fertile isotope to be irradiated.

[0008] 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.

[0009] A molten salt reactor can therefore be moderated by graphite, producing thermal neutrons, or without a moderator producing fast neutrons.

[0010] The presence or absence of moderators thus defines the two main families of molten salt reactors, respectively thermal neutron and fast neutron reactors.

[0011] 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.

[0012] Although promising in terms of safety potential, molten salt reactors may require expensive and complex systems and components.

[0013] 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.

[0014] 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.

[0015] 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],

[0016] 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.

[0017] 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 can have a reactor vessel incorporating a reduced-size primary fuel circuit, typically with a diameter of less than 2 m and an overall height of less than 4 m, which makes the reactor compliant with the requirements of modular AMR reactors. Thus, a primary circuit with a reactor vessel, the inner cylindrical shell, and its primary / secondary heat exchanger according to this patent application can be manufactured in a factory, transported to the site, and then used for the lifetime of the reactor.

[0018] 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).

[0019] 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.

[0020] 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 materials.

[0021] 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.

[0022] 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.

[0023] 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.

[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] Regardless of the operating mode, during normal operation, heavy atoms (uranium, plutonium, and minor actinides) undergo fission, producing energy and fission products (FPs). These FPs consist of remains of the fissioned half-atoms. They are statistically distributed over a large part of the periodic table with species remaining in solution in the fuel salt (liquid PF), others depositing on the structural parts of the vessel (solid PF), and the rest escaping and joining the reactor pile head by accumulating there (gaseous PF).

[0026] The gaseous PF will therefore gradually increase the pressure in the head of the fuel cell and therefore in the reactor vessel.

[0027] However, to avoid the restrictive regulations of nuclear pressure equipment (ESPN) and the associated costs, it is imperative to keep the latter at a value below 1.5 bar absolute.

[0028] It should be noted that pressurized water reactors (PWRs) in the existing fleet are not subject to this constraint because the first containment barrier, namely the cladding of a solid fuel, is not subject to ESPN regulations. Furthermore, these PWRs operate at high pressure, and the fuel cladding is naturally subjected to high pressures. This applies to all solid fuel reactor designs. In other words, solid fuel reactors accept a gradual pressure increase in the fuel cladding that forms the first containment barrier. For example, a PWR fuel rod begins a period of normal operation with an internal pressure of 30 bar and ends it at an internal pressure of 60 bar.

[0029] Thus, in this type of reactor, the question of managing fission gases (gaseous PF) in the pile head does not arise.

[0030] There is therefore a need to improve molten salt type reactors, fast neutron type reactors, particularly when considered as AMR reactors, and more particularly those intended to operate in natural convection as envisaged in application FR2213882, in order to manage the fission gases (gaseous PF) in the pile head so that the normal operation of the reactors is at limited pressure, typically less than 1.5 bar absolute.

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

[0032] To this end, the invention relates, in one of its aspects, to a nuclear power plant comprising: - a reactor building; - at least one reactor vessel well, located inside the reactor building, - at least one nuclear reactor vessel filled with a liquid bath fuel with molten salt(s) and delimiting within it a space of gas above the free level of the bath, called the pile head; - another tank, arranged around the reactor tank and inside the tank well, defining an inter-tank space (E) with the reactor tank; - at least one fission gas (FP gas) management system for gases accumulated in the reactor vessel stack ceiling during normal reactor operation, including: • at least one cold trap, arranged in the inter-tank space and connected to the reactor vessel head by a first fluidic line, the cold trap being adapted to condense chlorides and, where applicable, aerosols, • at least one first reservoir, called a buffer tank, arranged in the inter-tank space and connected to the cold trap by a second fluidic line, the buffer tank being adapted to store the purified gaseous PFs in the cold trap, during an initial period of radioactive decay, • at least one second tank, called a storage tank, arranged in the space between the reactor vessel shaft and the reactor building and connected to the buffer tank by a third fluid line, the storage tank being adapted to store the purified gaseous PFs in the cold trap, during a second period of radioactive decay, following the first period and at the end of which the gaseous PFs are likely to be evacuated outside the reactor building.

[0033] 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.

[0034] Preferably, the nuclear reactor is of the fast neutron type, the reactor vessel is 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.

[0035] According to a preferred application, each nuclear reactor of the power plant is according to the teaching of the aforementioned patent application FR2213882.

[0036] Thus, each nuclear reactor advantageously comprises a shell in the form of at least one hollow cylinder, its central axis coinciding with that of the reactor vessel. The shell is arranged within the reactor vessel to separate its interior into a central zone and a peripheral zone in which the heat exchanger is located, so that during reactor operation, the molten salt(s) fuel 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, and from which it rises by heating to the top of the peripheral zone. central where it is diverted up the peripheral zone to cross the interchange (ZE) then down the peripheral zone where it is diverted towards the reactor core.

[0037] Advantageously still, at least one heat exchanger between the primary circuit of the reactor and a secondary circuit is arranged inside the reactor vessel.

[0038] 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.

[0039] Thus, a molten salt reactor according to the invention can be described as a fast neutron reactor.

[0040] 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.

[0041] According to an advantageous embodiment, the nuclear power plant includes at least one Very High Efficiency (VHE) filter and / or at least one Iodine Trap (IAT), arranged in the space between the reactor vessel shaft and the reactor building and connected to the buffer tank and the storage tank. The filter(s) filter the headspace and prevent the entrainment of aerosols during the deflation of the headspace into the buffer tank(s).

[0042] According to an advantageous embodiment, the nuclear power plant includes a unit for testing and controlling the gases present in the storage tank before release to the outside, through a chimney.

[0043] According to another advantageous embodiment, the nuclear power plant includes at least one Very High Efficiency (VHE) filter and / or at least one Iodine Trap (IPT), connected to the storage tank and to the gas testing and control unit.

[0044] Advantageously, the nuclear power plant includes at least three buffer tanks per nuclear reactor, preferably between three and ten buffer tanks, with a unit volume between 0.06 m3 and 0.2 m3.

[0045] Advantageously, the nuclear power plant includes at least three storage tanks per nuclear reactor, preferably between three and eight storage tanks, with a unit volume between 0.5 m3 and 3 m3.

[0046] Preferably, the nuclear power plant includes a cooling device for each buffer tank and / or a cooling device for each storage tank.

[0047] Advantageously, the nuclear power plant includes at least one shut-off valve on each of the first, second and third fluidic lines.

[0048] 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.

[0049] Preferably, the molten salt fuel liquid 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.

[0050] During reactor operation, the temperature of the molten salt(s) fuel liquid in the primary circuit can be between 500 and 750°C.

[0051] The power of a nuclear reactor is advantageously between 10 and 500 MWth, which corresponds to a power range sought for AMR type reactors.

[0052] The invention also relates to a method for operating a nuclear power plant as described above, comprising the following steps, during normal operation of the nuclear reactor:

[0053] i / closing the shut-off valves on the first, second and third fluidic lines until a predetermined threshold pressure value is reached within the reactor pile head;

[0054] ii / opening of the shut-off valves on the first and second fluidic lines and keeping the shut-off valve closed on the third fluidic line so that the neutral gas and fission gases within the pile head pass through the cold trap(s) where the chlorides and aerosols are condensed, then are evacuated into the buffer tank(s);

[0055] iii / closing of the shut-off valves on the first and second fluidic lines;

[0056] iv / once the first radioactive decay period is completed, the shut-off valves on the first and second fluidic lines are kept closed and the shut-off valve on the third fluidic line is opened so that the gases present in the buffer tank(s) are evacuated into the storage tank(s);

[0057] v / closure of the shut-off valve on the third fluidic line;

[0058] vi / once the second radioactive decay period is completed, opening of a shut-off valve on a fourth line connecting the storage tank(s) to the outside of the reactor building, so as to vent the gases outside the reactor building, preferably through a discharge chimney to the outside air.

[0059] Thus, the invention essentially consists of constructing a nuclear power plant with a molten salt reactor(s) with a reactor building which houses a pressurized radioactive fission gas management system which is fluidly connected to the pile head and allows the transfer of these to buffer and storage tanks in which the gases present undergo subsequent radioactive decays.

[0060] Insofar as it is also necessary to reduce the quantity of equipment subject to the ESPN nuclear regulations, the tanks dedicated to radioactive decay have the function of storing gaseous products at ambient temperature and pressure.

[0061] Therefore, the gas management system includes at least one cold trap, typically cooled to 60°C, to condense chlorides and aerosols. Furthermore, to minimize the volume of the storage tank(s) and allow their integration into the space between the safety vessel and the reactor building, one or more buffer tanks are provided downstream of the cold trap(s).

[0062] At the end of the last period of decay within the storage tank(s), typically after several weeks, the activity of these gases has greatly reduced and they can be released outside the reactor building, into the environment, in accordance with the regulations in force.

[0063] Due to their radioactivity, fission gases must be contained before being released into the environment. To achieve this, the fluid circuit and the buffer and storage tanks must be separated from the environment by several containment barriers.

[0064] The fission gas buffer tank(s) are thus housed in the space between the reactor vessel and a safety tank around the reactor vessel.

[0065] The larger capacity storage tank(s), which allow for radioactive decay over a longer period than the buffer tanks, are housed outside the well of this vessel for space reasons. Advantageously, they can be housed in a casing forming a bunker.

[0066] Thus, the invention makes it possible to avoid the pressure operation of the molten salt reactor(s) by decreasing the gas pressure within the head of the fuel cell at regular time steps.

[0067] The operation of the power plant according to the invention must allow gaseous fission products to accumulate in the headspace of the fuel cell until a predetermined threshold value, preferably equal to 1.35 bar absolute, is reached.

[0068] .In the end, a nuclear reactor plant with molten salt(s), in particular with fast neutrons according to the invention, has many advantages, among which we can mention: - a lower cost because the fission gas management system avoids complex and costly specific pressurized equipment to meet nuclear regulations; - a small footprint because it is completely integrated within the reactor building; - high safety because any untimely opening of a valve on one of the circuits, or any failure of a circuit or of a cold tank / trap, does not lead to a dispersion of radioactive products in the reactor building or outside of it.

[0069] 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

[0070] [Fig.1] [Fig.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.

[0071] [Fig.2] [Fig.2] is a schematic longitudinal cross-sectional view of a building reactor of a nuclear power plant, housing a fission gas management system according to the invention.

[0072] [Fig.2A] [Fig.2] is a schematic longitudinal cross-sectional view illustrating a bayonet tube heat exchanger, with its inlet and outlet manifolds, as it may be arranged in a reactor of the power plant according to [Fig.2].

[0073] [Fig.3] [Fig.3] is a schematic view showing the different elements of a fission gas management system as well as their associated cooling devices and their arrangement in relation to the reactor building and the vessels it houses.

[0074] [Fig.4] [Fig.4] is a schematic view of an alternative embodiment of the floor of the cold traps of the fission gas management system according to the invention.

[0075] [Fig. 5] [Fig. 5] is a schematic view of an alternative embodiment of the floor of the buffer tanks of the fission gas management system according to the invention.

[0076] [Fig.6] [Fig.6] is a schematic view of an alternative embodiment of the floor of the buffer tanks of the fission gas management system according to the invention. Detailed description

[0077] 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.

[0078] Similarly, the terms "inlet", "outlet", "upstream", "downstream", are to be understood by reference to the fission gas circuit according to the invention from their evacuation from the reactor pile ceiling to their extraction into the environment outside the reactor building.

[0079] It should be noted that the various temperatures, power ratings, 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.

[0080] In figures 4 to 6, the acronyms MT and MP designate at least one sensor respectively for the temperature and pressure of the gases passing through the component concerned.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] A single annular heat exchanger 3 between the primary circuit of the reactor and a secondary circuit is arranged inside the reactor vessel 2.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] The ferrules 4, 5 can be made of stainless steel or nickel-based alloy.

[0089] The ferrules 4, 5 are advantageously fixed by suspension to the cap-lid closing reactor vessel 2.

[0090] 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.

[0091] At the top of the reactor vessel 2, above the first ferrule 4, a second deflector 7, also in the form of a portion of a torus.

[0092] 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.

[0093] 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.

[0094] The deflectors 6, 7, by their shapes and arrangement, each allow the flow of the diverted molten salt(s) combustible liquid to be distributed.

[0095] 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.

[0096] The dimensional, temperature and power characteristics 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 450 and 750°C; - molten salt fuel liquid for the primary circuit to be selected from a mixture of 25 to 30 mol% NaCl-UCl3-9 to 11 mol% PuCl3 with depleted uranium U235 at 0.7%, or a mixture of NaCl-UCl3 at 34 mol% with natural uranium U235 enriched to 20%.

[0097] Advantageously, elements such as MgCl2, minor actinide chlorides or other elements from the periodic table of elements may be added in varying proportions.

[0098] As illustrated in [Fig. 2], the reactor vessel 2 comprises a headspace, usually referred to as the fuel cell headspace 20, filled with an inert gas, such as argon or helium, above the molten salt(s) fuel liquid. Depending on the power of reactor 1, the free volume of the fuel cell headspace can typically be between 1 and 4 m³.

[0099] As illustrated in [Fig. 2A], the heat exchanger(s) 3 may comprise a bundle of bayonet tubes defining the exchange portion with the secondary circuit. The secondary fluid circulating in the heat exchanger(s) 3 may be based on a mixture of molten salts NaCl-MgCl2 or NaCl-MgCl2-KCl or NaCl-MgCl2-KCl-ZnCl2.

[0100] Each bayonet tube comprises a hollow tube 30 opening into the inside of a blind tube 31.

[0101] Each tube 30, 31 is immersed substantially vertically in the molten salt(s) combustible liquid with a partial immersion height Hi.

[0102] Each open hollow tube 30 is connected to an inlet manifold 32 while each blind tube is connected to an outlet manifold 33 of the secondary fluid.

[0103] The inlet manifolds 32 and outlet 33 of the secondary fluid are advantageously arranged in the stack crown 20.

[0104] A nuclear reactor 1 as just described operates with molten salt(s) whose temperature varies between 450°C and 750°C.

[0105] Thus, mechanically, the blanket gas above the bath of molten salt(s) combustible liquid is also at this temperature. This high temperature therefore induces the presence in the blanket gas of all fission products having a lower boiling point.

[0106] Table 1 below gives a non-exhaustive list of the boiling points of the main fission gases that can be found in the headspace of pile 20 for a molten salt fuel liquid bath of the primary circuit to be chosen from a mixture of NaCl-UC13 of 25 to 30% mol-PuC13 of 9 to 11% mol with depleted uranium U235 at 0.7%, or a mixture of NaCl-UC13 at 34% mol with natural uranium U235 enriched to 20%.

[0107] [Tables] Chemical element Boiling point Chloride Ge 83 GeCl4 As 130 AsC13 Se 127 Se2Cl2 Br 5 BrCl Zr 331 ZrCl4 Sn 623 SnCl2 Sb 223 SbCl3 Te 380 TeCl4 I 97 ICI Tb 180 TbCl3

[0108] To manage all the fission gases present in the pile head, the inventors considered installing in the reactor building 10 of the nuclear reactor a system 100 adapted to regularly deflate the pile head 20 so that the pressure within it remains below 1.5 bars, preferably around 1.35 bars.

[0109] The deflation takes place during periods of radioactive decay in tanks 102, 103.

[0110] Fission gases (gaseous PF) being radioactive and their transfer requires maintaining their containment.

[0111] Also to guarantee this containment, the fluidic connections and the tanks must be separated from the external environment by several containment barriers.

[0112] Thus, the inventors chose an architecture with another tank 8, called a safety tank, arranged around the reactor tank and inside the tank well 9, defining an inter-tank space (E) with the reactor tank.

[0113] Consequently, the reactor according to the invention implements four fission gas containment barriers, namely the reactor vessel 2 as the first barrier, the safety vessel 8 as the second barrier, the vessel well 9 as the third barrier and finally the reactor building 10 as the fourth reactor building.

[0114] And this architecture with four containment barriers allows for a judicious placement of the 102, 103 radioactive decay tanks within the reactor building 20, as detailed below.

[0115] Since it is also necessary to reduce the amount of equipment subject to ESPN regulations, the decay tanks 102, 103 should only be used for gaseous products at ambient temperature and pressure. It is therefore advisable to pass the deflation products from the headliner through cold traps to condense the chlorides and, if necessary, the aerosols.

[0116] More specifically, as illustrated in Figures 2 and 3, the fission gas management system 100 accumulates in the reactor vessel stack ceiling during normal reactor operation, comprising at least one cold trap 101, arranged in the inter-vessel space and connected to the reactor vessel stack ceiling 20 by a first fluid line 104 on which a two-way valve VI is mounted. Typically, the cold trap cools the gases discharged from the stack ceiling 20 to a temperature of 60°C. Each cold trap 101 is equipped with a cooling device 105 that cools the temperature of the gases entering it and maintains the desired cooling temperature.

[0117] To minimize the volume of one or more storage tanks 103 and allow their integration into the inter-tank annular space, at least one buffer tank 102 is placed downstream of the cold trap 101.

[0118] More specifically, the buffer tank 102 is arranged in the inter-tank space and connected to the cold trap 101 by a second fluid line 106 on which a two-way valve V2 is mounted. This buffer tank 102 is adapted to store the purified gaseous PFs in the cold trap 101 during an initial radioactive decay period. Each buffer tank 102 is equipped with a cooling device 107 that cools the temperature of the gases entering it and maintains the desired cooling temperature.

[0119] At least one other tank, referred to as storage tank 108, is arranged in the space between the reactor vessel shaft 8 and the reactor building 10 and is connected to the buffer tank 101 by a third fluid line 109 on which at least one two-way valve V3 is mounted. This storage tank 108 is adapted to store the purified gaseous PFs in the cold trap 101 during a second radioactive decay period, following the first period, at the end of which the gaseous PFs are likely to be vented outside the reactor building 10. This second period is longer than the first period within the buffer tank(s) 102. Each storage tank 108 is equipped with a cooling device 110 that cools the temperature of the gases entering it and maintains the desired cooling temperature.

[0120] Preferably, each storage tank 108 is arranged and placed in a sealed case forming a casemate outside the tank well 8, for reasons of space.

[0121] Preferably, between a buffer tank 102 and a storage tank 103, at least one Very High Efficiency (VHE) filter and / or an Iodine Trap (IAT) 111 can be installed. Thus, these gases are filtered by this / these filter(s) prior to discharge. and / or these iodine 111 traps. On the fluidic line 109, downstream of these filter(s) and / or these iodine 111 traps, a V4 two-way valve can be provided.

[0122] The power plant advantageously includes a unit 112 for testing and controlling the gases present in the storage tank 103 before discharge to the outside, via a chimney 113. Valves V5, V6 may be provided on the fluid line on either side of the unit 112.

[0123] An advantageous embodiment is to have three storage tanks 103. Indeed, such an embodiment allows one to be filled while another undergoes radioactive decay, and a third serves as a safety reserve. Thus, three storage tanks 103 ensure the safe operation of the gaseous PF management system.

[0124] Figures 4 to 6 illustrate such an advantageous mode with several possible embodiments of the stages respectively of cold trap 101, buffer tank 102, storage tank 103.

[0125] In [Fig. 4], a single stage of three independent, parallel-flow cold traps 101.1, 101.2, and 101.3 is provided at the outlet of the reactor vessel head 20. The condensate can be discharged from each cold trap by its own fluidic circulator 114 and returned via a fluidic line, optionally including a two-way valve V8, to the reactor vessel head 20 by passing through the reactor vessel head 2. As an indicative example, a cold trap may have a unit volume of 60 L and be made of Inconel®. The fluidic circulator 114 may be a sealed, magnetically driven pump. The pump's operating flow rate may be on the order of lm³ / h.

[0126] In [Fig. 5], a single stage of four independent buffer tanks 102.1, 102.2, 102.3, 102.4, connected in parallel with each other, is provided at the outlet of the cold trap stage. More precisely, each buffer tank is connected to a single cold trap 101.1, 101.2, 101.3, 101.4. It is advantageous to install a safety reservoir 115 in bypass of the cold traps in case of excessive overpressure in the reactor vessel head 20. This safety reservoir 115, under permanent vacuum, is directly connected to the reactor vessel head 20 through the reactor vessel lid 200. Preferably, one or more isolation valves V9 with default opening setpoint and / or one or more rupture discs D are arranged in parallel, if necessary, on the fluid line connecting the reactor vessel head 20 and this safety reservoir 115. As an indicative example, a cold trap here could have a unit volume of 5 liters and be made of Inconel®.A buffer tank can have a unit volume of 140 l, while the single safety tank 155 can have a volume of 300 l.

[0127] In [Fig.6], a single stage of three independent storage tanks 103.1, 103.2, 103.3, operating in parallel with each other, is provided at the outlet of the stage of buffer tanks. At least one circulation pump 116.1, 116.2, preferably a vacuum pump, which can be duplicated and in this case operate in parallel, supplies the storage tanks 103.1, 103.2, 103.3 with gaseous PF that has already undergone the first radioactive decay in the buffer tanks. The gases exiting each of the storage tanks 103.1, 103.2, 103.3 are analyzed in the test and control unit 112. This unit 112 may advantageously incorporate an iodine cartridge, a paper filter, and a radioactive measurement detector. Once the gases have been analyzed and have a radioactivity level that meets the applicable discharge standards, the gases are released into the external environment through a stack 113 via the gaseous effluents (GE). As an indicative example, a storage tank may have a unit volume of 2 m³.

[0128] As an indicative example, corresponding to the detailed mode in Figures 4 to 6, for a molten salt reactor with a power of 100MW, with a pile head 20 of a volume on the order of 1 to 4m3, an optimal configuration can be considered as follows:

[0129] The optimal configuration considered here uses the following data:

[0130] - a cold trap 101 upstream of a buffer tank, cooling to 60°C;

[0131] - four buffer tanks 102 with a unit volume of 0.2 m3;

[0132] - three storage tanks 103 with a unit volume of 2 m3;

[0133] - radioactive decay time of 1 month in buffer tanks and 6 months in storage tanks.

[0134] We now briefly describe the operation of the fission gas management system of a power plant with a reactor 1 as just described.

[0135] The operating reactor 1 produces fission gases, which accumulate in the pile head 20 until they reach a predetermined threshold value, typically equal to 1.35 bar absolute. The temperature in the pile head is high, typically between 620 and 750°C.

[0136] At this moment all valves VI to V9 are closed.

[0137] During the deflation of the headliner of pile 20, the valve(s) VI, V2, V3 separating the The headspace of the reactor and the cold trap(s) 101, the buffer tank(s) 102, and the filters are opened. This causes pressure equalization within these previously evacuated volumes. The headspace gases, both neutral and fission gases, pass through the cold traps where chlorides and, if present, aerosols are condensed. The gases are then sufficiently purified to be introduced into one or more buffer tanks 102. The valves are then closed, and reactor 1 begins to pressurize the headspace 20 again, as the fission gases are once more accumulated in this volume.

[0138] The radioactivity of the gases in the buffer tank(s) 102 decreases during an initial period, typically of about thirty days. The radioactive decay produces heat, and it is necessary to cool the buffer tank(s) 102 using their dedicated devices 107.

[0139] A buffer tank 102 allows for complete decoupling of the deflation process, thus avoiding the use of active systems requiring precautions and maintenance, such as blowers. These could become clogged and corroded by salt aerosols. Vacuum pumps can nevertheless be used to evacuate the fluid circuits before opening the valves.

[0140] After the first period of decay in the buffer tank(s) 102, the gases are transferred to the storage tanks 103, outside the tank well 8. The same evacuation procedure for the circuits can be implemented.

[0141] The gases then pass through the Very High Efficiency (HHE) filters and / or the Iodine Traps (IAT) 111 before reaching the storage tank(s) 103.

[0142] After a second period of radioactive decay, typically of several months, the gases are directed to the Gaseous Effluents circuit to be discharged at stack 113. Preferably, prior to discharge, they are passed through HEPA and PAL filters. Measurements are then carried out in unit 112 to validate the conformity of the discharges with the regulations in force.

[0143] A system 100, which has just been described, makes it possible to reduce the activity of fission gases, after a decay period of 7 months, to that of krypton (85Kr) for 92% of the total and to that of tritium (3H) for 8% of the total. Such releases, with a 30 m high stack, induce an effective dose of 1 pSv outside the site, which is acceptable from a regulatory point of view.

[0144] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0145] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0146] Several variants can be considered for the fission gas management system, with scaling according to the reactor size and filter composition depending on the salts used as fuel liquid in the reactor. Thus, it is possible to increase the size and number of tanks to extract the fission gases produced by a 1000 MW molten salt reactor or to reduce the volume for a 50 MW reactor.

[0147] If, in the illustrated examples, the nuclear reactor operates with a fuel salt based on plutonium chloride, uranium and minor actinides, The invention can be implemented for a combustible liquid made from fluoride, iodide and bromide salts.

[0148] A large number of configurations of cold traps, buffer tanks and storage tanks other than those illustrated in Figures 4 to 6 can be envisaged, such as:

[0149] - a larger number of buffer tanks. The multiplication of tanks can While it has an interest in risk dispersion, it introduces complexity within the constrained space of the second barrier. Nevertheless, it is possible to plan for up to ten reservoirs;

[0150] - a volume of buffer tanks related to their number. All configurations between one with three 0.2 m3 tanks and one with ten 0.06 m3 tanks are considered viable by the inventors;

[0151] - Cold traps operating at a colder temperature are possible. Warmer cold traps are not desirable because they would not condense all species.

[0152] - the decay time of fission gases extracted from storage tanks This is directly linked to regulations on environmental discharges. Strict legislation will result in long retention periods and therefore large tanks. All tank sizes are compatible with the reactors but not with the regulations. Three 2 m³ tanks are compliant with French regulations. It would also be possible to use more small tanks. Thus, configurations ranging from three 0.5 m³ tanks to eight 3 m³ tanks are feasible.

[0153] Furthermore, for safety reasons, all the valves of the fluidic circuit of system 100 according to the invention are advantageously doubled. List of cited references

[0154] [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).

[0155] [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

1.

2. Demands Nuclear power plant comprising: - a reactor building (10); - at least one reactor vessel well (9), located inside the reactor building, - at least one nuclear reactor vessel (2) (1) filled with a bath of molten salt(s) fuel liquid and delimiting within it a space of gas above the free level of the bath, called the pile head; - another tank (8), arranged around the reactor tank and inside the tank well, defining an inter-tank space (E) with the reactor tank; - at least one (100) fission gas management system (gaseous PF) accumulating in the reactor vessel stack ceiling during normal reactor operation, comprising: • at least one cold trap (101), arranged in the inter-tank space and connected to the reactor vessel stack head by a first fluidic line, the cold trap being adapted to condense chlorides and, where applicable, aerosols, • at least one first tank (102), called a buffer tank, arranged in the inter-tank space and connected to the cold trap by a second fluidic line, the buffer tank being adapted to store the purified gaseous PFs in the cold trap, during a first period of radioactive decay, • at least one second tank (103), called a storage tank, arranged in the space between the reactor vessel shaft and the reactor building and connected to the buffer tank by a third fluidic line, the storage tank being adapted to store the purified gaseous PFs in the cold trap, during a second period of radioactive decay, following the first period and at the end of which the gaseous PFs are likely to be evacuated outside the reactor building. Nuclear power plant according to claim 1, the nuclear reactor being of the fast neutron type, the reactor vessel being axisymmetric around a central axis (X), internally delimiting a primary circuit of a fuel in liquid form in which at least one salt is melted, the interior of the tank being devoid of a moderating material.

3. Nuclear power plant according to claim 1 or 2, the nuclear reactor (1) comprising 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 thereof 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) and then descends to the bottom of the peripheral zone where it is deflected to the reactor core.

4. Nuclear power plant according to any one of the preceding claims, comprising at least one Very High Efficiency (VHE) filter (111) and / or at least one Iodine Trap (IPT), arranged in the space between the reactor vessel shaft and the reactor building and connected to the buffer tank and the storage tank.

5. Nuclear power plant according to one of the preceding claims, comprising a unit for testing and controlling the gases present in the storage tank before release to the outside, through a chimney.

6. Nuclear power plant according to claim 5, comprising at least one Very High Efficiency (VHE) filter and / or at least one Iodine Trap (IPT), connected(s) to the storage tank and to the gas testing and control unit.

7. Nuclear power plant according to any one of the preceding claims, comprising at least three buffer tanks per nuclear reactor, preferably between three and ten buffer tanks, with a unit volume between 0.06 m3 and 0.2 m3.

8. A nuclear power plant according to any one of the preceding claims, comprising at least three storage tanks per reactor nuclear, preferably between three and eight storage tanks, with a unit volume between 0.5 m3 and 3 m3.

9. Nuclear power plant according to any one of the preceding claims, comprising a cooling device for each buffer tank.

10. Nuclear power plant according to any one of the preceding claims, comprising a cooling device for each storage tank.

11. Nuclear power plant according to any one of the preceding claims, comprising at least one shut-off valve (VI, V2, V3, V4) on each of the first, second and third fluidic lines.

12. A method of operating a nuclear power plant according to claim 11, comprising the following steps, during normal operation of the nuclear reactor: i / closing the shut-off valves on the first, second and third fluidic lines until a predetermined threshold pressure value is reached within the reactor pile head; ii / opening the shut-off valves on the first and second fluidic lines and keeping the shut-off valve on the third fluidic line closed so that the neutral gas and fission gases within the pile head pass through the cold trap(s) where chlorides and aerosols are condensed, and are then discharged into the buffer tank(s); iü / closing the shut-off valves on the first and second fluidic lines;iv / once the first radioactive decay period is completed, keep the shut-off valves on the first and second fluid lines closed and open the shut-off valve on the third fluid line so that the gases present in the buffer tank(s) are evacuated into the storage tank(s); v / close the shut-off valve on the third fluid line; vi / once the second radioactive decay period is completed, open a shut-off valve on a fourth line connecting the storage tank(s) to the outside of the reactor building, so as to evacuate the gases outside the reactor building, preferably through a vent stack to the outside air.