Nuclear reactor with solid, sealed fuel assemblies individually cooled in nominal operation by heat pipes.

Abstract

A nuclear reactor with sealed, individually cooled solid fuel assemblies operating nominally via heat pipes. The invention essentially consists of a nuclear reactor operating in the fast neutron spectrum, cooled by a network of one or more heat pipes. At least a portion of the evaporator of one or more heat pipes is partially housed with one or more solid fuel pellets confined in a cladding within a sealed assembly partially filled with a liquid metal bath, which can be described as a thermal seal between the fuel rod cladding and the heat pipes. The reactor core consists of a network of these sealed assemblies (fuel rod(s) and heat pipe(s) immersed in a liquid metal bath), arranged within a vessel surrounded by a neutron reflector. Figure for the abstract: Fig. 1

Description

Title of the invention: Nuclear reactor with solid fuel assemblies, sealed and individually cooled in nominal operation by heat pipes. technical field

[0001] The present invention relates to the field of solid fuel nuclear reactors, cooled with a liquid metal, in particular with liquid lead or lead-bismuth, known as FNRs (Fast Neutron Reactors) which are part of the family of so-called fourth generation or GEN-IV reactors.

[0002] More particularly, the invention relates to a type of these nuclear reactors with a simplified architecture because they are cooled by means of heat pipes and aims primarily to improve their passive safety.

[0003] The invention applies to small power reactors or SMRs (acronym for "Small Modular Reactor"), and more specifically to MMRs (acronym for "Micro Modular Reactor") typically with an operating power of less than 50MWth. Previous technique

[0004] Thanks to its high energy concentration and its independence from external influences such as variations in the financial and commercial prices of fossil fuels and weather conditions, nuclear energy has long been a reliable and economically attractive energy source.

[0005] In the current context of climate and energy transition, in particular the decarbonization of energy sources, it seems to have become indispensable due to the extremely low CO2 emissions and the non-intermittent nature of its production method.

[0006] Large-scale nuclear power plants, and therefore high-power plants, supply nuclear energy through a well-developed electrical network, while small-power reactors (SMRs) are dedicated to localized energy production for more limited needs.

[0007] Even smaller reactors (MMR) are intended to meet low energy needs, and therefore represent an ideal solution for decarbonizing geographically isolated areas or areas outside the electricity grid.

[0008] The design of MMR reactors is simpler compared to other known nuclear installations, and also allows for improved safety of a nuclear installation through the use of passive means. A safe design offers large safety margins and significant grace periods. It should be noted here that a A grace period is a period of time following an incident or accident during which, in the absence of any human intervention, a nuclear facility remains in a safe state.

[0009] However, taking into account the economic constraint, penalized by the small power that an MMR reactor can provide, implies significant simplifications in its construction, which must be modular, with modules manufactured in the factory and assembled on site, and in its operation, particularly with regard to the handling of nuclear fuel.

[0010] This economic constraint and the availability requirements necessitate, in particular, longer operating cycles. The objective of protection against the malicious use of nuclear materials from the core (risk of dissemination) must also be taken into account in the design.

[0011] Among MMR nuclear reactor technologies, those with a core containing solid fuel in the form of cylindrical pellets in a cladding (pencil), cooled by a network of heat pipes, exhibit a high level of reliability: [1]. This is due to the passive operation of the heat pipes, which ensure the transport of a significant amount of heat without requiring electrically powered devices (pumps).

[0012] It is recalled here that a heat pipe is a thermal device allowing the transfer of a quantity of energy from a hot source to a cold source, separated by a certain length.

[0013] A heat pipe thus comprises a hermetically sealed enclosure, a capillary network, and a working fluid (heat transfer fluid). Through a phase change of the latter, heat is extracted from one point and transported to another, in a completely passive manner, i.e., without the use of a pump or other mechanical means. During manufacturing, all the air present in the heat pipe is evacuated, and a quantity of pure liquid is introduced to saturate the capillary network. Once the enclosure is sealed and subjected to a heat source, equilibrium is established between the liquid and vapor phases, provided that the established thermo-hydraulic conditions are within the operating limits of the heat pipe: [2].

[0014] The heat pipe is composed of three zones called evaporator, condenser and adiabatic zone between the evaporator and the condenser.

[0015] Under the effect of a hot source applied to a zone at one of the longitudinal ends, which constitutes the evaporator, part of the liquid phase vaporizes and absorbs the latent heat flux, inducing a slight overpressure that causes the vapor to move towards a zone at the other longitudinal end, designated the condenser, where the cold source is applied. At the condenser, the vapor condenses and returns to the liquid phase. The condensed fluid (the condensate) circulates Then, the heat flows through the capillary network and returns to the evaporator under the influence of capillary forces to complete the cycle again, when the heat pipe is not subject to gravity. With a suitable capillary network, a vertical operating position is not required: in other words, the heat pipe can operate in any position and therefore in zero gravity.

[0016] Among the various MMR nuclear reactor technologies with heat pipes, the one featuring a solid steel or graphite matrix ensures good heat transfer efficiency between the fuel pellets and the heat pipes that remove heat during nominal operation. US patent 11515053 B2 illustrates an example of this solution.

[0017] Despite the thermal efficiency of this solution, it has many drawbacks, including:

[0018] - the presence of contact resistance between the fuel pellets and the solid matrix, and also between the heat pipes and the solid matrix, which often requires the presence of a liquid metal used as a thermal seal. The latter can complicate the design of the metal matrix and the sheath, as well as the choice of materials, which must be chemically compatible with the liquid metal forming the thermal seal;

[0019] - the occurrence of thermal expansions in the solid matrix. These expansions These incidents can compromise the reactor's long-term operation, as they can generate significant thermomechanical stresses in the fuel cladding and heat pipes. These thermomechanical stresses can potentially lead to a breach of the primary radiological containment barrier and / or the loss of one or more heat pipes that cool the fuel.

[0020] - a reduced, or even absent, inspection capacity at the level of the conductive medium thermal between the fuel rods and the heat pipes.

[0021] A solution to overcome the aforementioned disadvantages may consist of replacing the solid medium with a liquid, in a stagnant state, which can ensure the thermal transfer of heat from the fuel rods to the heat pipes.

[0022] In fact, the presence of a liquid medium between the fuel rods and heat pipes:

[0023] - ensures the continuous transfer of heat without the need for contact resistances (joints) near the walls of the heat pipe network and the fuel rods;

[0024] - the fact that thermal expansions are absorbed by the liquid, which is free to expand vertically, without generating thermomechanical stresses at the walls of the fuel rods and heat pipes;

[0025] - can guarantee, depending on the type of liquid, a possibility of inspection of the environment.

[0026] According to this solution, patents CN112117016B and US10559389B2 disclose MMR reactors cooled using heat pipes, in which a liquid metal such as sodium is used to maximize heat transfer within the medium between fuel rods and heat pipes. However, they present a major drawback: the possibility of a chemical and exothermic reaction with water and air. This jeopardizes the integrity of the three containment barriers (fuel cladding, primary vessel, reactor building).

[0027] To overcome this drawback, a fluid other than sodium and which is chemically inert with respect to air and water must be considered, as described in patent application CN114121314A.

[0028] More specifically, this patent application discloses the use of lead-bismuth (Pb-Bi) or lead (Pb) instead of sodium, thereby avoiding any risk of chemical reaction between the liquid medium and the surrounding atmosphere. Furthermore, the disclosed solution has the following major advantages:

[0029] - a sufficiently high conductivity, typically in the range of 12 to 18 W / (mK) for Pb-Bi to be able to transfer heat within the liquid medium surrounding the rods and heat pipes;

[0030] - a very high boiling point, typically around 1560 °C for Pb-Bi and 1750 °C for Pb;

[0031] - the possibility of locally controlling the chemistry of the liquid medium and the state of the fuel rods and heat pipes during reactor operation;

[0032] - unlike high-power lead and lead-bismuth reactors, the possibility of drastically reducing the risks associated with erosion and corrosion of materials, thanks to the stagnant state of the liquid.

[0033] However, the solution disclosed in CN114121314A has the following major drawbacks:

[0034] - the absence of any type of Supplementary Safety Device to be triggered in case of an accident, which is passive and distinct from the rotating drums located at the periphery of the heart;

[0035] - in the event of a heat pipe failure, the very probable generation of a hot spot local due to the relative arrangement between heat pipes and fuel rods. Indeed, the arrangement provides for a single heat pipe surrounded by 12 fuel rods distributed in a hexagonal pattern;

[0036] - even though lead is a liquid metal, its ability to transfer heat is when even reduced compared to that of liquid sodium;

[0037] - the replacement of a part of the core and the loading / unloading of the fueling appear to be difficult to achieve, given the presence of heat pipes within the fuel rods.

[0038] It follows from the above that a nuclear MMR reactor cooled using heat pipes can allow for significant design simplifications, such as the absence of primary pumps and devices for removing the heat produced.

[0039] Furthermore, MMR heat pipe reactors in which the matrix or intermediate medium between the heat pipes and the nuclear fuel is a liquid composed of lead (Pb) or lead-bismuth (Pb-Bi) offer significant advantages because they:

[0040] - guarantee a high level of intrinsic safety by avoiding any reaction chemical and exothermic with water and air;

[0041] - promote local control and inspection of nuclear fuel;

[0042] - allow for efficient transfer of the heat produced within the fuel nuclear to the heat pipe network.

[0043] That being said, there is a need to further improve this type of MMR reactor with lead (Pb) or lead-bismuth (Pb-Bi) matrix heat pipes in the liquid state, particularly in order to increase their passive safety so as also to:

[0044] - increase heat transfer within the liquid medium;

[0045] - to promote better distribution of the heat produced and dissipated within the heart ;

[0046] - improve the control of responsiveness at the local level, in normal operation as in an accidental situation;

[0047] - improve fuel management, its placement within the core, its handling;

[0048] - reduce the consequences of freezing of the liquid transfer medium.

[0049] The aim of the invention is to meet at least part of this need. Description of the invention

[0050] To this end, the invention relates, in one of its aspects, to a modular microreactor (MMR) type nuclear reactor, comprising:

[0051] - a central axis (X) tank comprising:

[0052] • a core comprising sealed assemblies, each assembly comprising:

[0053] a casing partially filled with a bath of metal in the liquid state during reactor operation,

[0054] at least one nuclear fuel rod, housed and held in the casing with its axis substantially parallel to the central axis, each rod containing a sealed cladding in which is arranged a stack of nuclear fuel pellets surmounted by a plenum,

[0055] at least one heat pipe, housed and held in the casing with its axis substantially parallel to the central axis and partially passing through the casing in a sealed manner such that at least part of its evaporator is immersed in the bath and at least its condenser extends outside the casing,

[0056] • a fixed neutron reflector, arranged at the periphery of the core to ensure the maintenance of neutron flow in the heart,

[0057] - a closing slab for closing the inside of the primary tank while being traversed by at least part of the condenser of each heat pipe.

[0058] Preferably, the metal in the liquid state is chemically inert to air and water, and, even more preferably, lead (Pb) or lead-bismuth (Pb-Bi) or mercury.

[0059] According to an advantageous embodiment, the housing has a substantially regular hexagonal cross-section, optionally with at least one corner having a bevel.

[0060] Each assembly advantageously comprises a plurality of pencils and a plurality of heat pipes housed and held in the casing so that each heat pipe is adjacent to at least one pencil.

[0061] According to an advantageous configuration, each housing comprises in its center a pencil or a heat pipe surrounded by a ring of at least three pencils or heat pipes respectively.

[0062] According to this configuration and an advantageous embodiment variant, each housing comprises in its center a pencil or a heat pipe surrounded by at least one first ring of hexagonal cross-section homothetic to that of the housing, respectively by at least three pencils or heat pipes, the first ring being surrounded by a second ring of hexagonal cross-section homothetic to those of the housing and the first ring, by an alternation of at least three pencils and heat pipes such that each heat pipe of the center, of the first or of the second ring is adjacent to a pencil.

[0063] Preferably, the sealed sheath of each rod houses at least one neutron absorber below the stack of nuclear fuel pellets.

[0064] According to an advantageous embodiment, the pencil(s) is / are housed and held by at least one grid arranged inside the housing and itself fixed to the heat pipe(s).

[0065] According to this method and an advantageous construction variant, the housing comprises:

[0066] - at least one so-called guide grid, in which the pencil(s) is / are mounted with the possibility of movement parallel to the central axis,

[0067] - at least one grid, called a fusible grid, arranged at a distance from the guide grid and comprising one or more fusible element(s), flexible fusible element(s), the fuel rod(s) being, in normal reactor operation, locked in a direction parallel to the central axis, with axial deflection possible by the fusible element(s) whose flexibility is adapted to compensate for differential expansions axial, the fusible element(s) being adapted to melt beyond a threshold temperature of the bath corresponding to an accidental operation, the melting of the elements causing the pencil(s) to rise by Archimedes' thrust, so as to bring the neutron absorber into the active zone of the core.

[0068] Thus, during normal reactor operation, a fuel rod is fixed and can have axial movement due to the flexibility / spring effect of the fusible means. This axial movement is related to axial thermal expansion. A fuel rod moves parallel to the central axis, beyond its axial movement, only if at least one fusible means melts, which corresponds to an accident situation.

[0069] According to an advantageous embodiment, the housings of the assemblies are arranged in a network within the vessel, being joined together by at least one of their faces, due to their expansion at the normal operating temperature of the reactor.

[0070] Advantageously, the nuclear reactor includes at least one control and / or instrumentation rod, arranged in the space provided between the bevels of adjacent housings.

[0071] Advantageously, it can be provided that at least a portion of the assemblies comprises at least one rod made of inert material, housed and held within the casing with its axis substantially parallel to the central axis. Thus, depending on the requirements for improved heat distribution and / or to avoid hot spots within the casing, a rod, typically made of inert steel, can be used instead of a heating element or heat pipe of the same dimensions.

[0072] Preferably, the reactor vessel and / or the fuel rod cladding and / or the assembly housing is / are made of AISI 316L stainless steel or nickel-based alloy or silicon carbide (SiC).

[0073] Preferably, a heat pipe is cylindrical in shape incorporating one or more solid metal structures, such as steel or copper, and, as a working fluid, a liquid metal at the operating temperature of the reactor, such as potassium or sodium.

[0074] The nuclear fuels of the pellets can be based on depleted uranium dioxide (UO2), slightly enriched, preferably with an enrichment <5%, or reprocessed (URT), and / or plutonium dioxide (PuO2), in particular UPuO2. It can also be UPuZrio or (U,Pu)C.

[0075] The nuclear reactor just described is particularly intended to have a power output between 1 and 50 MWth. The preferred applications of the invention are small-scale Gen IV reactors.

[0076] The invention also relates to a sealed assembly comprising:

[0077] - a central shaft housing, partially filled with a bath of liquid metal, which is preferably chemically inert to air and water,

[0078] - at least one nuclear fuel rod, housed and held in the casing with its axis roughly parallel to the central axis, each fuel rod containing a sealed cladding in which is arranged a stack of nuclear fuel pellets surmounted by a plenum,

[0079] - at least one heat pipe, housed and held in the casing with its axis substantially parallel to the central axis and partially passing through the case in a sealed manner such that at least part of its evaporator is immersed in the bath and at least its condenser extends outside the case.

[0080] The invention also relates to a nuclear installation comprising:

[0081] - a nuclear reactor as described above;

[0082] - at least one heat exchanger comprising a shell forming a shell sealed in which a fluid circulates from an inlet port to an outlet port to remove the heat released by at least a part of the condenser of each heat pipe arranged in a sealed manner in the shell.

[0083] According to an advantageous embodiment, the installation includes at least one thermal storage and / or heat supply and / or heat-to-electricity conversion system, connected to the heat exchanger.

[0084] The invention therefore essentially consists of producing a nuclear reactor operating in the fast neutron spectrum, cooled by a network of one or more heat pipes.

[0085] At least a portion of the evaporator of one or more heat pipes is partially housed with one or more solid fuel pellets contained in a cladding within a sealed assembly partially filled with a liquid metal bath, which can be described as a thermal seal between the fuel rod cladding and the heat pipes. In other words, within a sealed assembly, the volumetric space between the fuel rod(s) and the heat pipe(s) is filled with the metal bath, which forms a heat transfer medium between the fuel rod(s) releasing the heat produced by nuclear fission and the heat pipe(s) ensuring the removal of heat from the reactor vessel. The relative arrangement between the fuel rod(s) and the heat pipe(s) within each sealed assembly ensures a homogeneous temperature distribution within it and optimizes the efficiency of the heat pipes.

[0086] The reactor core consists of a network of these sealed assemblies (fuel rod(s) and heat pipe(s) immersed in a bath of liquid metal), arranged within a vessel surrounded by a neutron reflector.

[0087] At least a portion of the heat pipe condenser passes through the tank's closing slab and is arranged in a shell-and-tube heat exchanger through which a heat removal fluid circulates, preferably to a thermal storage and / or heat supply and / or heat-to-electricity conversion system, connected to the heat exchanger.

[0088] The advantages of the invention compared to existing MMR-type nuclear reactor solutions with heat pipes are numerous in terms of safety, thermal performance and fuel management.

[0089] A reactor according to the invention does not include a heat transfer fluid, pumps, or exchangers integrated inside its tank.

[0090] These advantages derive primarily from the constitution of the core by sealed-casing mixed assemblies in which mixed solid elements (fuel rods / heat pipes) are held and immersed in a bath of metal in a stagnant liquid state which acts as a heat transfer medium.

[0091] The major advantages of a sealed housing assembly according to the invention can be listed as follows: - an assembly can be manufactured and packaged in a factory, transported by land and installed on site more easily than a whole core as per the state of the art; - the transport of assemblies is covered by the regulatory framework relating to the transport of nuclear materials, which is not the case for the hypothetical transport of a fully assembled core which is envisaged in certain MMR nuclear reactor solutions, particularly with heat pipes, according to the state of the art; - handling, loading, unloading or replacement of a part of the core can be carried out more easily than in the case of a whole monolithic core according to the state of the art; - The casing of a sealed assembly behaves like a sealed radiological protection envelope. This envelope confines not only the nuclear fission products, which are initially contained in the fuel rod cladding, but also the liquid metal, notably Pb or Pb-Bi where applicable. This latter product leads, under irradiation, to the isotope 210Polonium, a highly radiotoxic element, the containment of which is an important issue in order to reduce the risks to the health of personnel in a nuclear facility and to the environment; - the local confinement of the pencils in a sealed case allows for a more thorough level of control and inspection; - Detection devices can be placed at the top level in each assembly box, so as to be able to locally monitor the chemistry (oxygen content), temperatures (risk of freezing and / or overheating) temperature) and the integrity of the first containment barrier (pencil sheath); - the small unit size of an assembly housing allows the freezing of the liquid metal bath, in particular Pb (or Pb-Bi), whereas the large dimensions of power reactors according to the state of the art and the components they contain in their main vessel require freezing to be classified as a feared event, jeopardizing their integrity; - The fact that the reactor core is made up of a plurality of identical elements placed side by side, the sealed assemblies, allows, in principle, the power produced by the reactor to be adapted to the specific needs of the user, simply by choosing the number of sealed assemblies to be installed in the vessel. This possibility increases the level of modularity of MMR-type nuclear reactors and makes them more attractive for various applications, since there is no requirement for a specific core and / or boiler design.

[0092] Furthermore, the protection of the reactor vessel and surrounding structures is ensured, on the one hand, by the liquid metal bath inside the assembly housings and, on the other hand, by a fixed neutron reflector, preferably made of steel, around the core. This eliminates the need for rotating drums, as absorbers / reflectors, around the core such as those described in patent application CN114121314 A.

[0093] Reactivity control during normal reactor operation, as well as in accident situations, is ensured by control rods that can be compact and inserted into advantageously designed housings between the assembly boxes. In this way, more precise and localized reactivity control is possible, while simplifying the architecture around the core and increasing the compactness of the vessel.

[0094] The advantageous variant of a fusible grid for retaining fuel rods has the following advantages:

[0095] - ensures contact between the pencils and the heat pipes, so as to be able to reduce the thermal resistance and improve heat transfer during normal reactor operation;

[0096] - increases the local turbulence phenomena of the liquid metal (Pb (or Pb-Bi)) able to circulate by natural convection between the pencils and the heat pipes within each sealed housing;

[0097] - constitutes a passive safety device: when a pencil heats up In an abnormal accidental situation, the fusible link(s) connecting the fuel rod to the heat pipe(s) melt(s). Consequently, the fuel rod is no longer mechanically held by the grid and is free to rise under the effect of the buoyant force in the high-density liquid metal, typically on the order of 10,000 kg / m³ of pressure is contained within the casing. This allows a neutron-absorbing element housed in the lower part of the rod to penetrate the active zone of the heart. In this way, a passive antireactive power supply is automatically provided in the event of an accident.

[0098] The preferred applications of the invention are small-sized reactors of the GenIV series.

[0099] Other advantages and features of the invention will become clearer from the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0100] [Fig-1] [Fig.1] is a schematic cross-sectional view of an installation nuclear comprising a solid fuel rod nuclear reactor cooled by heat pipes according to the invention, a heat exchanger which is connected to the condenser part of the heat pipes to remove the heat produced and a thermal storage and / or conversion system for the heat removed from the exchanger.

[0101] [Fig.2] [Fig.2] is a perspective view of a nuclear reactor as in [Fig.1].

[0102] [Fig.3] [Fig.3] is a detail view of a mixed sealed assembly according to the invention.

[0103] [Fig.4] [Fig.4] is a schematic longitudinal cross-sectional view of a mixed assembly according to an advantageous embodiment of the invention.

[0104] [Fig.5] [Fig.5] is a schematic cross-sectional view at the fuse grid of a mixed assembly according to [Fig.4].

[0105] [Fig.6] [Fig.6] is a schematic cross-sectional view at the top end of the housing of a mixed assembly according to [Fig.4].

[0106] [Fig.7] [Fig.7] is a cross-sectional view of a nuclear reactor according to the invention, at the level of the vessel and the core with a network of sealed mixed assemblies (fuel rods / heat pipes).

[0107] [Fig.8] [Fig.8] is a cross-sectional and perspective view of part of the network of sealed assemblies of [Fig.7].

[0108] [Fig.9] [Fig.9] is a reproduction of a numerical simulation showing the temperature field of the liquid metal bath within a sealed assembly according to the invention. Detailed description

[0109] Throughout this application, the terms “vertical”, “lower”, “upper”, “bottom”, “top”, “below” and “above” are to be understood by reference compared to a primary vessel of a fast neutron nuclear reactor according to the invention, as it is in vertical operating configuration.

[0110] It is specified that on [Fig.2], the crossings of the heat pipes according to the invention are intentionally omitted.

[0111] Figure [Fig. 1] shows a nuclear installation 100 with a heat-pipe-cooled fast neutron nuclear reactor 1 according to the invention.

[0112] Generally, in this installation 100, the nuclear reactor 1 has a vessel 10 housing a core 11 consisting of a network of sealed assemblies 2 whose central axis XI is arranged vertically, parallel to the central axis (X) of the vessel.

[0113] A fixed neutron reflector 12 is arranged at the periphery of the core to ensure the maintenance of the neutron flux in the core 11. Typically, the reflector is made of 316L steel or borated steel.

[0114] Each sealed assembly 2 comprises a housing 20 partially filled with a bath 21 of Pb or Pb-Bi, in liquid state, and houses both nuclear fuel rods 22 and heat pipes 23, as detailed later.

[0115] A heat exchanger 3 is arranged above the tank 10, and its shell 30 houses the condensers 230 of the heat pipes 23, which also pass through the tank 10's closing slab 13 in a sealed manner. Typically, the shell 30 of the exchanger 3 is made of steel (alloy 304 or 316) or another metal, such as Hastelloy® or a nickel-based alloy. The heat transfer fluid F circulating within the shell can be selected from a two-phase liquid / vapor fluid, air, nitrogen (N2), helium, or supercritical CO2.

[0116] A thermal storage and / or heat supply and / or heat-to-electricity conversion system 4, connected to the heat exchanger, is connected to the heat exchanger by a set of conduits, not detailed, which is adapted to the heat transfer fluid F flowing through said exchanger.

[0117] Thus, in the installation 100, the thermal power produced by nuclear fission is evacuated by the heat pipes 23 in normal, incidental and accidental operation to the heat exchanger 3 which, by the heat transfer fluid F which runs through it, transfers it in a loop to the system 4 which can produce electricity or supply heat for industrial needs.

[0118] The primary vessel 10 provides mechanical containment for the core 11 and the neutron reflector, as well as acting as a radiological barrier. It is placed on the base of a concrete structure (not shown), which may be in the form of a concrete vessel well, optionally lined with a layer of thermal insulation material. This layer of thermal insulation material provides thermal insulation for the vessel well. Typically, this layer may be made of polyurethane foam or a silicate-based material.

[0119] The core 11 is supported by a welded mechanical structure called a bed 14, which provides mechanical support for the sealed assemblies 2. Typically, the bed 14 is made of AISI 316L stainless steel. Unlike the bed assemblies in liquid metal fast neutron reactors according to the state of the art, the bed 14 is not traversed by a heat transfer fluid and therefore does not have to ensure flow distribution.

[0120] As shown, the primary tank 10 is a straight cylinder with central axis X. Typically, the primary tank 10 is made of AISI 316L stainless steel.

[0121] The slab 13 is arranged vertically above the core 11 and closes the primary tank 10 and also together with the latter constitutes the third containment barrier for the materials contained in the core 11, the first two containment barriers being made by the sealed assemblies detailed below.

[0122] The closure slab 13 is provided with through-holes for the cylindrical or prismatic control rod components 15, for the heat pipes 23, and, where applicable, for the core control and monitoring elements 11 (not shown). The closure slab 13 is therefore a removable part that meets the sealing requirements of the vessel 10 while allowing through-holes for the heat pipes 23, the control rods 15, and for any control, measurement, and inspection elements that may be present within the vessel.

[0123] Typically, the closing slab 13 is made of AISI 316L stainless steel or concrete. Typically also, the material used for the control bars 15 is B4C.

[0124] The reactor vessel 10, once closed by the slab 13, is put under vacuum to improve thermal insulation and the detection of loss of sealing of a sealed assembly 2.

[0125] A sealed mixed assembly 2 according to the invention is illustrated in detail in figures 3 to 5.

[0126] An assembly 2 is a welded mechanical structure that can be individually removed from or inserted into the bed 14. Each assembly 2 can be manufactured, packaged, factory tested, and transported. It must be resistant to the freezing / thawing of the liquid metal bath 21 it contains. The free volume above the bath 21 accommodates the thermal expansion of the liquid metal inside the housing 20.

[0127] As already explained, each sealed assembly 2 contains, within its housing 20 with a substantially hexagonal cross-section, a network of identical fuel rods 22 and identical heat pipes 23, arranged with each rod 23 adjacent to a heat pipe 22 and in one or more rings of generally hexagonal shape, around a rod or heat pipe at the center of the hexagon of the housing. This ring arrangement of the rods 22 and heat pipes 23 ensures a homogeneous temperature distribution within a housing 20 and optimizes the thermal conduction efficiency of the heat pipes. Depending on the needs for better heat distribution and / or to avoid hot spots within the housing, one or more of these elements (pencil or heat pipe) can be replaced by rods or tie rods, i.e. inert steel elements of the same size as the fuel rods or heat pipes.

[0128] In the example illustrated in [Fig.5], the number of hexagonal rings is equal to 3 with the first ring around a central heat pipe 23 which includes a number of 6 fuel rods 22, the second ring around the first ring which includes 12 rods and alternating heat pipes while the third ring includes 18 rods and alternating heat pipes.

[0129] The space between the rods 22 and heat pipes 23 is filled with a bath 21 of liquid Pb or Pb-Bi, which acts as a thermal seal between the rods that heat up and the heat pipes that ensure the evacuation of heat.

[0130] As illustrated in [Fig.4], the retention of the pencils 22 and heat pipes 23 within a housing 20 is ensured by grids 24, 25 fixed on the heat pipes 23.

[0131] One or more grids 24 arranged in the lower part of the housing 20 form guide grids because they allow axial movement along the central axis XI of the rods 22 to accommodate the differential axial expansion between rods 22 and heat pipes 23, in normal operation of the reactor.

[0132] A grid 25 arranged in the upper part of the housing 20 comprises spring-effect fusible means 250, as shown in [Fig. 5], which axially lock the fuel rods 22 relative to the heat pipes 23 within the housing 20, up to differential axial expansion. These fusible means are designed and dimensioned so that they melt at a given temperature. This temperature corresponds to the temperature of the Pb or Pb-Bi bath 21 at the beginning of a hypothetical accident sequence.

[0133] A hexagonal transverse housing 20 ensures the sealing and mechanical stability of the assembly 2, the sealed passage of the heat pipes 22 from above, and also ensures heat exchange between adjacent housings 20. Typically, a housing 20 is made of AISI 316L stainless steel.

[0134] Each fuel rod 22 is in the form of a cylinder delimited by a steel cladding which is filled with a stack 220 of fuel pellets, having at its upper end a gaseous plenum 221 and at its lower end a neutron absorber 222, generally boron carbide, the height of which is predetermined. The cladding is closed by airtight caps at the top and bottom, not shown.

[0135] The sheath of each pencil 22 is typically made of 316L steel.

[0136] The pencil cladding constitutes the first containment barrier while the sealed housings 20 constitute the second containment barrier, and the tank 10 constitutes the third containment barrier for the radioactive materials contained in the core 11.

[0137] Solid nuclear fuel pellets may be based on depleted, low-enriched uranium dioxide (UO2), preferably with an enrichment <5%, or reprocessed uranium (URT), and / or plutonium dioxide (PuO2), in particular UPuO2. They may also be UPuZriO2 or (U,Pu)C.

[0138] The plenum volume on top of each rod 22 is sized so as to limit the gas pressure inside the rod to an acceptable value, depending on the behavior under irradiation of the fuel and the resistance of the cladding in nominal, incidental and accidental situations.

[0139] The fixation of the pencils 22 in the grid 25 is characterized either by the possibility of axial displacement of the pencils only in the event of an accident, or by axial immobilization by means of fusible means 250 with the possibility of axial deflection, linked to thermal expansion during normal operation, such that the rupture of a portion of these means surrounding a given pencil allows its axial displacement. Since the density of the pencil is less than that of the liquid medium of the bath 21, this displacement movement will be directed upwards.

[0140] The fusible means are typically based on copper or copper alloys, solid at the operating temperature. More generally, the material of the fusible means 250 is chosen so that their melting point corresponds to a temperature limit of the liquid bath 21, linked to the safety of the core 11. If this limit is exceeded, the melting of a means 250 results in the loss of its integrity and thereby releases the mechanical constraint holding the fuel rod axially, which moves upwards under the effect of Archimedes' principle.

[0141] A heat pipe 23 is typically cylindrical in shape and sealed, and incorporates solid metal structures (steel, copper) and a heat transfer fluid, preferably a metal such as potassium or sodium, which is liquid at operating temperature. This metal evaporates upon contact with the wall of the container in the evaporator 231, which is housed in an assembly casing 20. This metal vapor travels through the interior of the heat pipe 23, passing through the sealing plate 12 and reaching the condenser 230 within the heat exchanger 3, where it condenses. The return of the heat transfer fluid from the condenser 230 to the evaporator 231 is achieved by capillary action in a dedicated structure or by gravity.

[0142] The jacket of a heat pipe 23 is a metal tube, the portion of which passing through an assembly housing 20 2 and the portion passing through the shell 30 of the heat exchanger 3 are arranged vertically. A heat pipe jacket may be made of 316 stainless steel or other metal alloys.

[0143] A plate with holes 200 is welded to or forms the upper longitudinal end of the hexagonal housing 20. As illustrated in [Fig.6], each of the holes in this plate allows the passage of at least a portion of the evaporator 231 of a heat pipe 23.

[0144] Each of the heat pipes 23 is hermetically welded to this plate 200. All the hermetically sealed welds of heat pipes to a plate 200 are made during the manufacture of a hermetically sealed assembly 2 so as to be able to transport this assembly 2, which is installed on the site of reactor 1.

[0145] The seal between at least a portion of the condenser 230 of a heat pipe and the shell 30 can be achieved in a similar manner, by welding. These welds to the shell 30 are carried out on the site of reactor 1, at the time of final construction.

[0146] The relative arrangement of sealed assemblies 2 and the network it constitutes to form the core 11 of a reactor 1 is shown in figures 6 and 7.

[0147] The sealing assemblies 2 are arranged in several rings of general hexagonal section around a central assembly 2. In the illustrated example, the number of rings of sealing assemblies 2, all identical, is equal to two with the first ring around the central assembly 2 which includes six sealing assemblies while the second ring around the first ring includes twelve sealing assemblies.

[0148] The dimensions of the housings 20 of all the assemblies 2 are such that the resulting network is compacted at the operating temperature of the reactor 1 by the thermal expansion of the housings 20, which are contiguous (adjacent) and in direct contact by one of their hexagonal faces. In other words, at the operating temperature of the reactor 1, the housings 20 of the sealed assemblies 2 are slightly compressed against each other. This contact force due to mutual compression disappears at a lower temperature, which allows the assemblies 2 to be extracted and inserted individually into the network.

[0149] The beveled edges 200 of the housings 20 allow spaces to be created between them in the network. As illustrated in Figures 6 and 7, each of these spaces can house a neutron absorber bar 15 for reactor control. It can also be a bar dedicated to reactor instrumentation.

[0150] The operation of nuclear reactor 1 and installation 100 is now described in relation to the different normal and accidental operating situations.

[0151] In normal reactor operation, all the heat generated by the fission reactions of the core 11 is removed by heat exchange within each sealed assembly 2 from the fuel rods 22 to the evaporators 231 of the heat pipes 23 via the bath 21 of liquid Pb or Pb-Bi, which acts as a heat transfer medium by conduction. In normal operation, the absorbers neutrons 222 inside the pencils are located below the active core area, without exerting a significant neutron-absorbing effect.

[0152] The heat is removed from the core by the heat pipes 23 to their condenser 230 within the heat exchanger 3.

[0153] The heat transfer fluid F which runs through the inside of the shell 30 of the exchanger 3 removes the heat released by the condensers 230 of the heat pipes 23 to the thermal storage and / or electricity conversion system 4.

[0154] At the beginning of an accident sequence, particularly in the event of a transient with abnormal heating of the liquid Pb or Pb-Bi bath 21 within a group of fuel rods from at least one assembly 2, the heating of this bath 21 leads to the melting of at least some of the holding means 250. As a result, the grid 25 will mechanically release some fuel rods 22. Under the pressure of Archimedes' force, these rods 22 move parallel to the central axis XI upwards. This upward movement brings their neutron absorber 222 into the active zone of the core. This addition of negative neutron reactivity can thus stop or at least slow down the nuclear chain reaction, thereby ending the accident transient.

[0155] The inventors have carried out sizing studies to demonstrate the feasibility of a nuclear reactor 1 cooled by a network of heat pipes with a liquid lead bath, as just described, and propose orders of magnitude on its characteristic elements.

[0156] Thus, the inventors carried out the preliminary calculations using thermal calculation software, such as the COPERNIC software: [3], [4].

[0157] In these calculations, the presence of grids and fusible thermal conductive elements has been neglected.

[0158] Furthermore, the inventors assumed that Pb (or Pb-Bi) was in a liquid state and stagnant in the assemblies. Therefore, only thermal conduction was considered in these calculations, using a simplified and conservative approach, since natural convection circulation in the liquid medium could occur due to axial temperature gradients in the reactor core.

[0159] In these calculations, the inventors based themselves on a power generated in the core of about 5 MWth and on the geometry of the fuel rods, heat pipes and casings shown in Table 1, with a configuration in which a fuel rod is considered at the center of a casing and heat pipe rings of hexagonal cross-section (configuration 1) and a configuration in which a heat pipe is considered at the center of a casing and fuel rod rings of hexagonal cross-section (configuration 2).

[0160] [Tables 1]Assembly Configurations Configuration 1 Configuration 2 Characteristics / Input Data Unit Value Reactor Thermal Power Rating MWth 5 Fuel Type UPuO2 Number of Ring Caps 5 Number of Core Caps 61 Fissile Core Height mm 1500 Number of Ring Caps per Cap mm 4 Cap Thickness 0.3 Cladding Fill Rate (Paplet / Cladder Clearance and Possible Porosity) % 0.983 Surface-to-Surface Distance Between Adjacent Caps mm 3.099 Number of Caps per Cap mm 25 24 Pitch Between Caps mm 20.454 Cladding Outer Diameter mm 15.650 Cladding Inner Diameter mm 15.050 Pellet Outer Diameter mm 14.920 Fissile Core Diameter cm 90.24 Fuel Volume Only dm3 399.96 Fissile Power Average volumetric density W / cm³ 3.215 Average power produced per unit kW 820 Number of heat pipes 732 793 Power per heat pipe kW 6.831 6.305 Surface power kW / m² 92.62 85.49 Total number of core heating elements 762 732 Power generated per pencil kW 3.28 3.42 Surface power of pencils kW / m² 44.46 46.31 External temperature of a pellet °C 604 604 Temperature at the center of a pellet °C 662 664

[0161] These calculations allow us to deduce the power produced by each pencil, the temperature of the external wall of the fuel pencil, as well as the heat flux per pencil and per heat pipe.

[0162] By imposing these last three values ​​as boundary conditions, homogeneously over the height of the liquid bath of the assemblies, a thermal conduction calculation is made with the SolidWorks software of the company Dassault Systèmes on the geometry resulting from the calculations of Table 1.

[0163] The objective of this conduction calculation is to be able to deduce an order of magnitude of temperature difference between the heating rods and the heat pipe network, in order to ensure that the medium of the Pb (or Pb-Bi) bath is at sufficiently high temperatures relative to its solidification point, respectively 327 °C and 125 °C. It is also necessary to verify that the operating point is within an acceptable range, i.e., between freezing and boiling, and has a sufficient margin for transients.

[0164] The results of this calculation showing the temperature field of the liquid bath are illustrated by [Fig.8] for a configuration 1. They confirm the maintenance in the liquid state of the heat transfer medium, i.e. Pb or Pb-Bi, as well as an operating condition compatible with the safety limits mentioned above.

[0165] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0166] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0167] The number of rings and the arrangement of heat pipes / pencils in a sealed assembly may vary depending on the size of the case and other design parameters, including the thermal power per assembly, the constraint on the diameter of the heat pipes, the constraints of maximum axial or radial temperature gradient, at the level of the pens or heat pipes, ...

[0168] In the illustrated examples, a heat pipe is generally in the form of a straight tube arranged vertically, preferably with its adiabatic section occupying a significant portion of the available space between the outlet of each assembly housing and the inlet of the heat exchanger shell, a portion of the heat pipe that is therefore not subjected to heat exchange as such. It goes without saying that within the framework of the invention, other types, shapes and arrangements of heat pipes can be envisaged insofar as at least part of their evaporator is immersed in a liquid metal bath of sealed assemblies and at least part of their condenser is inside the heat exchanger shell.

[0169] Optionally, a fourth barrier is represented by the reactor building not shown. This building may not be classified as a containment structure in view of the safety studies carried out by the inventors. Liste des références citées

[0170] [1]: B. Zohuri, Heat Pipe Design and Technology. “Modem applications for practical thermal management”, Springer, 2016.

[0171] [2]: B. Zohuri, ”Heat pipe design and technology. Modem applications for practical thermal management”, Springer, Second édition, 2021.

[0172] [3] : F. MORIN et al., “COPERNIC, A NEW TOOL BASED ON SIMPLIFIED CALCULATION METHODS FOR INNOVATIVE LWRs CONCEPTUAL DESIGN STUDIES”, ICAPP 2017 Conférence, 2017.

[0173] [4]: P. GAUTHE et al., “Innovative and inherently safe small SFR as a response to the dilemma ’safety vs cost'”, ICAPP 2019 Conférence, 2019.

Claims

Demands

1. A nuclear reactor (1), of the modular microreactor (MMR) type, comprising: - a primary vessel (10) with a central axis (X) comprising: • a core (11) comprising sealed assemblies, each assembly comprising: a casing partially filled with a bath of liquid metal chemically inert to air and water, at least one nuclear fuel rod, housed and retained in the casing with its axis substantially parallel to the central axis, each rod containing a sealed cladding in which is arranged a stack of nuclear fuel pellets surmounted by a plenum, at least one heat pipe, housed and retained in the casing with its axis substantially parallel to the central axis and partially passing through the casing in a sealed manner such that at least a portion of its evaporator is immersed in the bath and at least its condenser extends outside the casing, • a fixed neutron reflector,confined within the vessel and arranged at the periphery of the core to ensure the maintenance of the neutron flux within the core, - a closure slab (13), to close the interior of the primary vessel while being traversed by at least a part of the condenser of each heat pipe.

2. Nuclear reactor (1) according to claim 1, the metal in the liquid state being lead (Pb) or lead-bismuth (Pb-Bi) or mercury.

3. Nuclear reactor (1) according to claim 1 or 2, the casing having a substantially regular hexagonal cross-section, optionally with at least one corner having a bevel.

4. Nuclear reactor (1) according to any one of the preceding claims, each assembly comprising a plurality of fuel rods and a plurality of heat pipes housed and retained in the casing such that each heat pipe is adjacent to at least one fuel rod.

5. Nuclear reactor (1) according to claim 4, each casing comprising in its center a fuel rod or a heat pipe surrounded by a ring respectively of at least three fuel rods or heat pipes.

6. Nuclear reactor (1) according to claim 5 in combination with claim 3, each casing comprising in its center a fuel rod or a heat pipe surrounded by at least a first ring of hexagonal cross-section homothetic to that of the casing, respectively of at least three fuel rods or heat pipes, the first ring being surrounded by a second ring of hexagonal cross-section homothetic to those of the casing and the first ring, of an alternation of at least three fuel rods and heat pipes such that each heat pipe of the center, of the first or of the second ring is adjacent to a fuel rod.

7. Nuclear reactor (1) according to any one of the preceding claims, the sealed cladding of each rod housing at least one neutron absorber below the stack of nuclear fuel pellets.

8. Nuclear reactor (1) according to any one of the preceding claims, the fuel rod(s) being housed and held by at least one grid arranged inside the casing and itself fixed to the heat pipe(s).

9. Nuclear reactor (1) according to claim 8 in combination with claim 7, the casing comprising: - at least one grid, referred to as a guide grid, in which the fuel rod(s) are mounted with the possibility of movement parallel to the central axis, - at least one grid, referred to as a fusible grid, arranged at a distance from the guide grid and comprising one or more fusible and flexible elements, the fuel rod(s) being, in normal reactor operation, locked in the fusible grid in a direction parallel to the central axis with axial movement possible by the fusible element(s) whose flexibility is adapted to compensate for differential axial expansions, the fusible element(s) being adapted to melt beyond a threshold bath temperature corresponding to accidental operation, the melting of the elements causing the fuel rod(s) to rise due to Archimedes' principle,so as to bring the neutron absorber into the active zone of the heart.

10. Nuclear reactor (1) according to any one of claims 3 to 9, the housings of the assemblies being arranged in a network within the vessel and being joined together by at least one of their faces, due to their expansion at the normal operating temperature of the reactor.

11. Nuclear reactor (1) according to claim 10, comprising at least one control and / or instrumentation rod, arranged in the space between bevels of adjacent housings.

12. Nuclear reactor (1) according to any one of the preceding claims, comprising at least one rod of inert material, housed and held in the casing with its axis substantially parallel to the central axis.

13. Nuclear reactor (1) according to any one of the preceding claims, having a power output between 1 and 50 MWth.

14. A sealed assembly comprising: - a central axis housing (XI), partially filled with a liquid metal bath, which is chemically inert to air and water, - at least one nuclear fuel rod, housed and retained in the housing with its axis substantially parallel to the central axis (XI), each rod containing a sealed cladding in which is arranged a stack of nuclear fuel pellets surmounted by a plenum, - at least one heat pipe, housed and retained in the housing with its axis substantially parallel to the central axis and partially passing through the housing in a sealed manner such that at least a portion of its evaporator is immersed in the bath and at least its condenser extends outside the housing.

15. Nuclear installation (100) comprising: - a nuclear reactor (1) according to any one of claims 1 to 13; - at least one heat exchanger (2) comprising a shell forming a sealed shell in which a fluid flows from an inlet port to an outlet port to remove the heat released by at least a portion of the condenser of each heat pipe arranged in a sealed manner in the shell.

16. Nuclear installation according to claim 15, comprising at least one (3) thermal storage and / or heat supply and / or heat-to-electricity conversion system, connected to the heat exchanger.

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