Nuclear reactor having solid fuel assemblies that are sealed and cooled individually in nominal operation by heat pipes

Abstract

The invention relates to a nuclear reactor (1) having solid fuel assemblies (2) that are sealed and cooled individually in nominal operation by heat pipes (23). The invention substantially consists of a nuclear reactor (1) that operates in the fast neutron spectrum and is cooled by an array of one or more heat pipes (23). At least part of the evaporator (231) of one or more heat pipe(s) (23) is held and partially housed with one or more rods (22) of solid fuel pellets which are enclosed in a sheath in a sealed assembly which is partially filled with a liquid metal bath (21), which can be defined as a thermal seal between the rod sheaths and the heat pipes (23). The core (11) of the reactor (1) consists of an array of these sealed assemblies (2) (fuel rod(s) and heat pipe(s) immersed in a liquid metal bath) arranged within a tank (10) surrounded by a neutron reflector (12).

Description

[0001] Description

[0002] Title: Nuclear reactor with solid fuel assemblies, sealed and individually cooled in nominal operation by heat pipes.

[0003] Technical field

[0004] 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 RNR (Fast Neutron Reactors) which are part of the family of so-called fourth generation or GEN-IV reactors.

[0005] More particularly, the invention relates to a type of these nuclear reactors with simplified architecture because they are cooled by means of heat pipes and aims mainly to improve their passive safety.

[0006] The invention applies to small power reactors or SMRs in English (acronym for “Small Modular Reactor”), and more specifically to MMRs (acronym for “Micro Modular Reactor”) typically with an operating power of less than 50MWth.

[0007] Prior art

[0008] Thanks to its high energy concentration and its independence from external influences such as fluctuations in financial and commercial prices of fossil fuels and weather conditions, nuclear energy has long been a reliable and economically attractive source of energy.

[0009] In the current context of climate and energy transition, particularly the decarbonization of energy sources, it seems to have become indispensable due to its extremely low CO2 emissions and the non-intermittent nature of its production method.

[0010] Large nuclear power plants, and therefore high power plants, provide nuclear energy through a well-developed electricity network, while small power reactors (SMR) are dedicated to the production of localized energy for more limited needs.

[0011] Even smaller reactors (MMRs) are designed to meet low energy needs and are therefore an ideal solution for decarbonizing geographically isolated areas or areas outside the electricity grid. The design of MMR reactors is simpler compared to other known nuclear facilities and also improves the safety of a nuclear facility through the use of passive means. A safe design offers large safety margins and significant grace periods. A grace period is a period of time after an incident or accident during which, in the absence of any human intervention, a nuclear facility remains in a safe state.

[0012] However, taking into account the economic constraint, penalized by the low 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 its operation, particularly with regard to the handling of nuclear fuel.

[0013] This economic constraint and availability requirements impose, in particular, extended operating cycles. The objective of protection against malicious use of nuclear materials in the core (risk of dissemination) must also be taken into account in the design.

[0014] Among the MMR nuclear reactor technologies, those with a solid fuel core, in the form of cylindrical pellets in a cladding (pencil), cooled by a network of heat pipes, have 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 the need for devices requiring electrical power (pumps).

[0015] We recall here that a heat pipe is a thermal device allowing a quantity of energy to be transferred from a hot source to a cold source, a certain distance apart.

[0016] A heat pipe thus comprises a hermetically sealed enclosure, a capillary network and a working fluid (heat transfer fluid). Thanks to a phase change of the latter, the heat is taken from one point and transported to another, completely passively, i.e. without using a pump or other mechanical means. During manufacturing, all the air present in the heat pipe tube is evacuated and a quantity of pure liquid is introduced to saturate the capillary network. Once the enclosure is closed, and subjected to a heat source, an equilibrium is established between the liquid phase and the vapor phase, if the thermo-hydraulic conditions established are within the operating limits of the heat pipe: [2].

[0017] The heat pipe is composed of three zones called evaporator, condenser and adiabatic zone between the evaporator and the condenser.

[0018] Under the effect of a heat source applied in an area at one of the longitudinal ends, which constitutes the evaporator, part of the liquid phase vaporizes and absorbs the heat flux in latent form by inducing a slight overpressure which causes the movement of the vapor towards an area at the other longitudinal end, designated condenser where the cold source is applied. At the condenser, the vapor condenses and returns to the liquid phase. The condensed fluid (the condensates) then circulates in the capillary network and returns to the evaporator under the effect of capillary forces to repeat a cycle, when the heat pipe is not subject to gravity. With a suitable capillary network, it is possible to avoid a vertical operating position: in other words, the heat pipe can operate in all positions and therefore outside of gravity.

[0019] Among the various MMR nuclear reactor technologies with heat pipes, the one with a solid matrix of steel or graphite guarantees good efficiency of heat transfer between the fuel pellets and the heat pipes that evacuate the heat during nominal operation. US patent 11515053 B2 illustrates an example of this solution.

[0020] Despite the thermal efficiency of this solution, it has many drawbacks including:

[0021] - 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 cladding, as well as the choice of materials, which must be chemically compatible with the liquid metal forming the thermal seal;

[0022] - the occurrence of thermal expansions in the solid matrix. These expansions can compromise the long-term operation of the reactor, as they can generate significant thermomechanical stresses in the fuel cladding and heat pipes. Thermomechanical stresses can potentially cause a rupture of the first radiological containment barrier and / or a loss of one or more heat pipes that cool the fuel. - reduced or even absent inspection capacity in the thermally conductive medium between the fuel rods and the heat pipes.

[0023] A solution to overcome the above-mentioned drawbacks 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.

[0024] In fact, the presence of a liquid medium between fuel rods and heat pipes:

[0025] - ensures continuous heat transfer without the need for contact resistors (gaskets) near the walls of the heat pipe network and rods;

[0026] - the fact that thermal expansions are absorbed by the liquid, which is free to expand vertically, without generating thermomechanical forces at the level of the walls of the rods and heat pipes;

[0027] - can guarantee, depending on the type of liquid, a possibility of inspection of the environment.

[0028] 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 improve as much as possible the heat transfer within the medium between rods and heat pipes. However, they have the major drawback of a possibility of chemical and exothermic reaction with water and air. This endangers the integrity of the three containment barriers (fuel cladding, primary vessel, reactor building).

[0029] To overcome this drawback, a fluid other than sodium and which is chemically inert towards air and water must be considered, as described in patent application CN114121314A.

[0030] More specifically, this patent application discloses the use of lead-bismuth (Pb-Bi) or lead (Pb) instead of sodium, which avoids any risk of chemical reaction between the liquid medium and the surrounding atmosphere. In addition, the disclosed solution has the following major advantages:

[0031] - a sufficiently high conductivity, typically of the order 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; - a very high boiling point, typically around 1560 °C for Pb-Bi and 1750 °C for Pb;

[0032] - the possibility of locally controlling the chemistry of the liquid medium and the state of the rods and heat pipes during operation of the reactor;

[0033] - unlike high-power lead and lead-bismuth reactors, the possibility of drastically reducing the risks linked to erosion and corrosion of materials, thanks to the stagnant state of the liquid.

[0034] However, the solution disclosed in CN114121314A has the following major drawbacks:

[0035] - the absence of any type of Additional Safety Device to be triggered in the event of an accident, which is passive and distinct from the rotating drums located on the periphery of the core;

[0036] - in the event of a heat pipe failure, the very likely generation of a local hot spot due to the relative arrangement between heat pipes and fuel rods. Indeed, the arrangement provides for a single heat pipe surrounded by a number of 12 rods distributed in a hexagonal arrangement;

[0037] - even though lead is a liquid metal, its ability to transfer heat is still reduced compared to that of liquid sodium;

[0038] - replacing part of the core and loading / unloading the fuel appear to be difficult to carry out, given the presence of heat pipes within the rods.

[0039] It is clear from the above that an MMR nuclear 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.

[0040] In addition, MMR heat pipe reactors whose matrix or intermediate medium between the heat pipes and the nuclear fuel is a liquid made of lead (Pb) or lead-bismuth (Pb-Bi), have significant advantages because they:

[0041] - guarantee a high level of intrinsic safety by avoiding any chemical and exothermic reaction with water and air;

[0042] - promote local control and inspection of nuclear fuel; - enable efficient transfer of heat produced within the nuclear fuel 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, in particular in order to increase their passive safety so as also to:

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

[0045] - promote better distribution of the heat produced and evacuated within the core;

[0046] - improve control of reactivity at the local level, in normal operation as well as in accident situations;

[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.

[0050] Statement of the invention

[0051] To this end, the invention relates, in one of its aspects, to a nuclear reactor of the modular microreactor (MMR) type, comprising:

[0052] - a tank with a central axis (X) comprising:

[0053] • a core comprising sealed assemblies, each assembly comprising: a casing partially filled with a bath of metal in the liquid state during operation of the reactor, 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 sheath in which is arranged a stack of nuclear fuel pellets surmounted by a plenum, at least one heat pipe, housed and held in the casing with its axis substantially parallel to the central axis and passing partly 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,

[0054] • a fixed neutron reflector, arranged at the periphery of the core to ensure the maintenance of the neutron flux in the core, - a closing slab to close the interior of the primary vessel while being crossed by at least part of the condenser of each heat pipe.

[0055] Preferably, the metal in the liquid state is chemically inert to air and water, and more preferably, lead (Pb) or lead-bismuth (Pb-Bi) or mercury.

[0056] According to an advantageous embodiment variant, the housing has a substantially regular hexagonal cross-section, where appropriate with at least one corner having a bevel.

[0057] Each assembly advantageously comprises a plurality of rods and a plurality of heat pipes housed and held in the housing such that each heat pipe is adjacent to at least one rod.

[0058] According to an advantageous configuration, each housing comprises in its center a rod or a heat pipe surrounded by a ring of at least three rods or heat pipes respectively. According to this configuration and an advantageous embodiment variant, each housing comprises in its center a rod or a heat pipe surrounded by at least a first ring of hexagonal section homothetic to that of the housing, respectively at least three rods or heat pipes, the first ring being surrounded by a second ring of hexagonal section homothetic to those of the housing and the first ring, an alternation of at least three rods and heat pipes such that each heat pipe of the center, of the first or of the second ring is adjacent to a rod.

[0059] Preferably, the sealed sheath of each rod houses at least one neutron absorber below the stack of nuclear fuel pellets.

[0060] 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).

[0061] According to this mode and an advantageous construction variant, the housing comprises:

[0062] - 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,

[0063] - at least one grid, called a fuse grid, arranged at a distance from the guide grid and comprising one or more fuse elements, the rod(s) being, in normal operation of the reactor, blocked in a direction parallel to the central axis, with axial movement possible by the fuse element(s) whose flexibility is adapted to compensate for axial differential expansions, the fuse element(s) being adapted to melt beyond a threshold temperature of the bath corresponding to accidental operation, the melting of the elements causing the rod(s) to rise by Archimedes' thrust, so as to bring the neutron absorber into the active zone of the core.

[0064] Thus, in normal operation of the reactor, a rod is fixed and can have axial movement due to the flexibility / spring effect of the fuse means. The axial movement is linked to axial thermal expansion. A rod moves parallel to the central axis, beyond the axial movement, only in the case where at least one fuse means melts, which corresponds to an accident situation.

[0065] According to an advantageous embodiment, the housings of the assemblies are arranged in a network within the tank, being joined together by at least one of their faces, due to their expansion at the normal operating temperature of the reactor.

[0066] Advantageously, the nuclear reactor comprises at least one control and / or instrumentation rod, arranged in the space provided between the bevels of adjacent housings.

[0067] Advantageously, it can be provided that at least part of the assemblies comprises at least one rod made of inert material, housed and held in the housing with its axis substantially parallel to the central axis. Thus, depending on the needs for better heat distribution and / or to avoid hot spots within the housing, a rod, typically made of inert steel, can be put in place instead of a pencil or a heat pipe, of the same dimensions.

[0068] Preferably, the reactor vessel and / or the rod cladding and / or the assembly housing is / are made of AISI 316L stainless steel or nickel-based alloy or silicon carbide (SiC).

[0069] More preferably, a heat pipe is cylindrical in shape incorporating one or more solid metal structures, such as steel or copper, and, as working fluid, a liquid metal at the operating temperature of the reactor, such as potassium or sodium.

[0070] The nuclear fuels of the pellets can be based on depleted uranium dioxide (UO2), low enrichment, preferably with an enrichment <5%, or reprocessing (URT), and / or plutonium dioxide (PuCh), in particular UPuCh- It can also be UPuZno or (U,Pu)C. The nuclear reactor which has just been described is particularly intended to have a power of between 1 and 50 MWth. The preferred applications of the invention are small reactors of the Gen IV sector.

[0071] The invention also relates to a sealed assembly comprising:

[0072] - a central axis housing, partially filled with a bath of metal in the liquid state, which is preferably chemically inert to air and water,

[0073] - 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 sheath in which is arranged a stack of nuclear fuel pellets surmounted by a plenum,

[0074] - at least one heat pipe, housed and held in the housing with its axis substantially parallel to the central axis and partly passing through the housing 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 housing.

[0075] The invention also relates to a nuclear installation comprising:

[0076] - a nuclear reactor as described above;

[0077] - at least one heat exchanger comprising a casing forming a sealed shell in which a fluid circulates from an inlet orifice to an outlet orifice to evacuate the heat released by at least part of the condenser of each heat pipe arranged in a sealed manner in the shell.

[0078] According to an advantageous embodiment, the installation comprises at least one thermal storage system and / or heat supply system and / or heat conversion system into electricity, connected to the heat exchanger.

[0079] 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.

[0080] At least part of the evaporator of one or more heat pipes is held and partially housed with one or more solid fuel pellet rods confined in a cladding in a sealed assembly partially filled with a bath of liquid metal, which can be described as a thermal seal between the rod claddings and the heat pipes. In other words, within a sealed assembly, the volume space between the rod(s) and the heat pipe(s) is filled with the metal bath which forms a heat transfer medium between the rod(s) releasing the heat produced by nuclear fission and the heat pipe(s) which ensure the evacuation of the heat from the vessel. The relative arrangement between the rod(s) and the heat pipe(s) within each sealed assembly guarantees a homogeneous distribution of the temperature within it and optimizes the efficiency of the heat pipes.

[0081] The core of the reactor is made up of a network of these sealed assemblies (fuel rod(s) and heat pipe(s) immersed in a bath of liquid metal), arranged within a tank surrounded by a neutron reflector.

[0082] At least a portion of the condenser of the heat pipes passes through the closing slab of the tank and is arranged in a shell-and-tube type heat exchanger in which a heat removal fluid circulates to preferably bring it into a thermal storage system and / or heat supply and / or heat conversion into electricity, connected to the heat exchanger.

[0083] The advantages of the invention compared to existing solutions for MMR-type nuclear reactors with heat pipes are numerous in terms of safety, thermal performance and fuel management.

[0084] A reactor according to the invention does not include a heat transfer fluid, nor pumps, nor exchangers integrated inside its tank.

[0085] These advantages arise firstly from the constitution of the core by mixed assemblies with a sealed casing in which mixed solid elements (fuel rods / heat pipes) are maintained and immersed in a bath of metal in a liquid and stagnant state which plays the role of a heat transfer medium.

[0086] The major advantages of a sealed housing assembly according to the invention can be listed as follows:

[0087] - an assembly can be manufactured and packaged in the factory, transported by land and installed on site more easily than an entire core as according to the state of the art;

[0088] - 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 completely assembled core which is envisaged in certain MMR nuclear reactor solutions, in particular with heat pipes, according to the state of the art;

[0089] - handling, loading, unloading or replacement of a part of the core can be carried out more easily than in the case of an entire monolithic type core according to the state of the art;

[0090] - the casing of a sealed assembly behaves like a sealed radiological protection envelope. This envelope confines not only the nuclear fission products, which are confined first in the rod cladding, but also the liquid metal, in particular Pb or Pb-Bi where appropriate. The latter product leads, under irradiation, to the isotope 210Polonium, an element with high radiotoxicity, the containment of which is an important issue in order to reduce risks to the health of personnel in a nuclear installation and to the environment;

[0091] - the local confinement of the pencils in a sealed case allows a more in-depth level of control and inspection;

[0092] - detection devices can be placed at the upper level in each assembly box, so as to be able to locally control the chemistry (oxygen content), temperatures (risk of freezing and / or overtemperature) and the integrity of the first containment barrier (rod sheath);

[0093] - the small unit dimension of an assembly box allows the freezing of the liquid metal bath, in particular Pb (or Pb-Bi), to be accepted, whereas the large dimensions of power reactors according to the state of the art and of the components they contain in their main tank require freezing to be classified as a feared event, endangering their integrity;

[0094] - the fact of having a reactor core consisting of a plurality of identical juxtaposed elements, the sealed assemblies, makes it possible in principle to adapt the power produced by the reactor to the specific needs of the user, by simply 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 several applications, since there is no obligation for a specific core and / or boiler design. Furthermore, the protection of the reactor vessel and the surrounding structures is ensured on the one hand by the liquid metal bath inside the assembly boxes and on the periphery of the core by a fixed neutron reflector, preferably made of steel. This eliminates the need for rotating drums, as absorbers / reflectors, on the periphery of the core such as those described in patent application CN114121314 A.

[0095] Reactivity control during normal reactor operation, as well as in accident situations, is ensured by control rods that can be compact and fit into advantageously arranged housings between the assembly boxes. In this way, more precise and local control of reactivity is possible, while simplifying the architecture around the core and increasing the compactness of the vessel.

[0096] The advantageous variant of a fuse grid for holding fuel rods has the following advantages:

[0097] - ensures contact between the rods and the heat pipes, so as to reduce thermal resistance and improve heat transfer during normal operation of the reactor;

[0098] - increases the local turbulence phenomena of the liquid metal (Pb (or Pb-Bi)) which can circulate in natural convection between the rods and the heat pipes within each sealed box;

[0099] - constitutes a passive safety device: when a rod heats up abnormally in an accidental situation, the fusible means(s) which connect(s) the rod to the heat pipe(s) melt(s). As a result, the rod is no longer mechanically held by the grid and is free to rise under the effect of Archimedes' thrust in the high-density liquid metal, typically of the order of 10,000 kg / m 3 contained in the casing. This allows a neutron absorber element housed in the lower part of the rod to be inserted into the active zone of the core. In this way, an anti-reactivity supply is automatically provided passively in the event of an accident.

[0100] The preferred applications of the invention are small reactors of the GenIV sector.

[0101] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0102] [Fig 1] Figure 1 is a schematic cross-sectional view of a nuclear installation 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 evacuate the heat produced and a system for thermal storage and / or conversion of the heat evacuated from the exchanger.

[0103] [Fig 2] Figure 2 is a perspective view of a nuclear reactor as in Figure 1.

[0104] [Fig 3] Figure 3 is a detailed view of a mixed sealed assembly according to the invention.

[0105] [Fig 4] Figure 4 is a schematic longitudinal sectional view of a mixed assembly according to an advantageous embodiment of the invention.

[0106] [Fig 5] Figure 5 is a schematic cross-sectional view at the fuse grid of a mixed assembly according to Figure 4.

[0107] [Fig 6] Figure 6 is a schematic cross-sectional view at the upper end of the housing of a mixed assembly according to Figure 4.

[0108] [Fig 7] Figure 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 mixed sealed assemblies (fuel rods / heat pipes).

[0109] [Fig 8] Figure 8 is a cross-sectional and perspective view of a portion of the network of sealed assemblies of Figure 7.

[0110] [Fig 9] Figure 9 is the reproduction of a numerical simulation showing the temperature field of the liquid metal bath within a sealed assembly according to the invention.

[0111] Detailed description

[0112] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a primary vessel of a fast neutron nuclear reactor according to the invention, as it is in the vertical operating configuration.

[0113] It is specified that in Figure 2, the crossings of the heat pipes according to the invention are voluntarily omitted. Figure 1 shows a nuclear installation 100 with a fast neutron nuclear reactor 1 cooled by heat pipes, according to the invention.

[0114] Generally speaking, 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.

[0115] 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 boron steel.

[0116] Each sealed assembly 2 comprises a housing 20 partially filled with a bath 21 of Pb or Pb-Bi, in the liquid state, and houses both nuclear fuel rods 22 and heat pipes 23, as detailed below.

[0117] A heat exchanger 3 is arranged above the tank 10 and its shell 30 tightly houses the condensers 230 of the heat pipes 23 which also pass tightly through the closing slab 13 of the tank 10. 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 which circulates within the shell can be chosen from a two-phase liquid / vapor fluid, air, nitrogen (N2), helium, supercritical CO2.

[0118] A system 4 for thermal storage and / or heat supply and / or conversion of heat into electricity, 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.

[0119] 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 flows through it, transfers it in a loop to the system 4 which can produce electricity or provide heat for industrial needs.

[0120] The primary vessel 10 provides the mechanical confinement function for the core 11 and the neutron reflector as well 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, possibly coated with a layer of thermal insulation material. The layer of thermal insulation material ensures the thermal insulation of the vessel well. Typically, this layer may be made of polyurethane foam or silicate-based foam.

[0121] The core 11 is supported by a welded mechanical structure called a base 14 which provides mechanical support for the sealed assemblies 2. Typically, the base 14 is made of AISI 316L stainless steel. Unlike the bases in fast neutron reactors and liquid metal heat transfer fluid according to the state of the art, the base 14 is not crossed by a heat transfer fluid and therefore does not have to ensure flow distribution.

[0122] 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.

[0123] The slab 13 is arranged directly above the core 11 and closes the primary tank 10 and also constitutes with the latter the third containment barrier for the materials contained in the core 11, the first two containment barriers being produced by the watertight assemblies detailed below.

[0124] The closure slab 13 is provided with passages for the components of the control rods 15 of cylindrical or prismatic shape, for the heat pipes 23 and, where appropriate, for the control and monitoring elements of the core 11, not shown. The closure slab 13 is therefore a removable part making it possible to meet the sealing requirements of the tank 10 while allowing passages for the heat pipes 23, the control rods 15 and for the control, measurement and inspection elements possibly present within the tank.

[0125] Typically, the closing slab 13 is made of AISI 316L stainless steel or concrete. Also typically, the material used for the control bars 15 is B4C.

[0126] The reactor vessel 10, once closed by the slab 13, is placed under vacuum to improve the thermal insulation and the detection of loss of sealing of a sealed assembly 2.

[0127] A sealed mixed assembly 2 according to the invention is illustrated in detail in Figures 3 to 5.

[0128] An assembly 2 is a mechanically welded structure that can be removed or inserted individually into the bed 14. Each assembly 2 can be manufactured, packaged, tested in the factory, and transported. It must be resistant to freezing / thawing of the liquid metal bath 21 that it contains. The free volume above the bath 21 makes it possible to accommodate the thermal expansions of the liquid metal, inside the casing 20. As already explained, each sealed assembly 2 contains in its casing 20 of substantially hexagonal cross-section, a network of fuel rods 22 all identical and heat pipes 23 all identical, arranged with each rod 23 adjacent to a heat pipe 22 and according to one or more rings of generally hexagonal shape, around a rod or heat pipe in the center of the hexagon of the casing.This arrangement in crown(s) of the rods 22 and heat pipes 23 guarantees a homogeneous distribution of the temperature within a casing 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 casing, one or more of these elements (rod or heat pipe) can be replaced by rods or tie rods, that is to say inert steel elements of the same size as the fuel rods or the heat pipes.

[0129] In the example illustrated in Figure 5, the number of hexagonal rings is equal to 3 with the first ring around a central heat pipe 23 which comprises a number of 6 fuel rods 22, the second ring around the first ring which comprises 12 rods and alternating heat pipes while the third ring comprises 18 rods and alternating heat pipes.

[0130] 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 which heat and the heat pipes which ensure the evacuation of the heat.

[0131] As illustrated in Figure 4, the holding of the rods 22 and heat pipes 23 within a housing 20 is ensured by grids 24, 25 fixed on the heat pipes 23.

[0132] One or more grids 24 arranged in the lower part of the housing 20, form guide grids because they allow axial displacement along the central axis XI of the rods 22 to accommodate the differential axial expansion between rods 22 and heat pipes 23, during normal operation of the reactor.

[0133] A grid 25 arranged in the upper part of the housing 20 comprises spring-effect fusible means 250, as shown in FIG. 5, which block the rods 22 axially relative to the heat pipes 23 within the housing 20, apart from differential axial expansions. 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 start of a hypothetical accidental sequence. A hexagonal cross-sectional 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 the exchange of heat 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, with 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 sealed plugs at the top and bottom, not shown.

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

[0136] The rod claddings constitute the first containment barrier while the sealed boxes 20 constitute the second containment barrier, and the vessel 10 constitutes the third containment barrier for the radioactive materials contained in the core 11.

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

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

[0139] The fixing of the rods 22 in the grid 25 is characterized either by the possibility of axial displacement of the rods only in the event of an accident, or by axial immobilization by means of the fusible means 250 with the possibility of axial movement, linked to thermal expansions in normal operation, so that the rupture of a part of these surrounding a given rod allows its axial displacement. Given that the density of the rod is lower 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 leads to the loss of its integrity and thereby releases the mechanical stress of axial maintenance of the rod, which moves upwards under the effect of Archimedes' thrust.

[0141] A heat pipe 23 is typically of generally sealed cylindrical shape, 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 on contact with the wall of the container in the evaporator 231 which is housed in an assembly housing 20. This metal vapor travels through the interior of the heat pipe 23 by crossing the closing slab 12 reaching the condenser 230 within the exchanger 3 by condensing. The return of the heat transfer fluid from the condenser 230 to the evaporator 231 is done by capillary force in a dedicated structure or by gravity.

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

[0143] A hole plate 200 is welded to or constitutes 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 welded in a sealed manner to this plate 200. All the sealed welds of heat pipes to a plate 200 are made during the manufacture of a sealed assembly 2 so as to be able to transport this assembly 2, which is installed on the site of the reactor 1.

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

[0146] The relative arrangement of sealed assemblies 2 and the network that it constitutes to form the core 11 of a reactor 1 is shown in figures 6 and 7. The sealed assemblies 2 are arranged in several rings of generally hexagonal section around a central assembly 2. In the example illustrated, the number of rings of sealed assemblies 2, all identical, is equal to two with the first ring around the central assembly 2 which comprises a number of six sealed assemblies while the second ring around the first ring comprises twelve sealed assemblies.

[0147] The sizing of the housings 20 of all the assemblies 2 is done so that the network formed is compacted at the operating temperature of the reactor 1 by the thermal expansion of the housings 20 which are adjoining (adjacent) and are 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 between them. This contact force by mutual compression disappears at a lower temperature, which makes it possible to extract and insert individually assemblies 2 into the network.

[0148] The beveled edges 200 of the housings 20 allow spaces to be provided between them in the network. As illustrated in Figures 6 and 7, each of these spaces can house a neutron absorber rod 15 for reactor control. It can also be a rod dedicated to reactor instrumentation.

[0149] The operation of nuclear reactor 1 and installation 100 is now described in relation to the various normal and accidental operating situations.

[0150] In normal operation of the reactor, 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 Pb or Pb-Bi in the liquid state, which acts as a heat transfer medium by conduction. In normal operation, the neutron absorbers 222 inside the rods are located under the active core zone, without exerting a significant neutron absorbing effect.

[0151] The heat is removed from the core by the heat pipes 23 to their condenser 230 within the heat exchanger 3.

[0152] The heat transfer fluid F which flows through the interior of the shell 30 of the exchanger 3 evacuates the heat released by the capacitors 230 of the heat pipes 23 to the thermal storage and / or electricity conversion system 4. At the start of an accidental sequence, in particular in the event of a transient with abnormal heating of the bath 21 of liquid Pb or Pb-Bi within a group of rods of at least one assembly 2, heating of this bath 21 causes the melting of at least part of the holding means 250. Consequently, the grid 25 will mechanically release certain fuel rods 22. Under the thrust of the Archimedes force, these rods 22 move upwards parallel to the central axis XI. 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, thus putting an end to the accidental transient.

[0153] 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 type bath, as just described, and propose orders of magnitude on its characteristic elements.

[0154] Thus, the inventors carried out the preliminary calculations using thermal calculation software, such as COPERNIC software: [3], [4],

[0155] In these calculations, the presence of grids and fusible thermal conductive elements was neglected.

[0156] Furthermore, the inventors assumed that Pb (or Pb-Bi) was in a liquid and stagnant state in the assemblies. Therefore, only thermal conduction was considered in these calculations, keeping a simplified and penalizing approach, because circulation by natural convection in the liquid medium could be established due to axial temperature gradients in the reactor core.

[0157] In these calculations, the inventors based themselves on a power generated in the core of approximately 5 MWth and on the geometry of the rods, heat pipes and casings presented in Table 1, with a configuration in which a fuel rod is considered at the center of a casing and crowns of heat pipes of hexagonal section (configuration 1) and a configuration in which a heat pipe is considered at the center of a casing and crowns of fuel rods of hexagonal section (configuration 2). [Table 1]

[0158] These calculations make it possible to deduce the power produced by each rod, the temperature of the external wall of the fuel rod, as well as the heat flow per rod and per heat pipe.

[0159] By imposing these last three values ​​as boundary conditions, in a homogeneous manner over the height of the liquid bath of the assemblies, a thermal conduction calculation is made with the SolidWorks software from the company Dassault Systèmes on the geometry resulting from the calculations in table 1.

[0160] The objective of this conduction calculation is to be able to deduce an order of magnitude of temperature difference between the rods and the heat pipe network, in order to be able to ensure that the Pb (or Pb-Bi) bath environment is at sufficiently high temperatures compared to its solidification point, respectively equal to 327 °C and 125 °C. It is also necessary to check that the operating point is within an admissible range, i.e. between freezing and boiling, and has sufficient margin for transients.

[0161] The results of this calculation showing the temperature field of the liquid bath are illustrated in Figure 8 for configuration 1. They confirm the maintenance of the heat transfer medium in the liquid state, i.e. Pb or Pb-Bi, as well as an operating condition compatible with the safety limits mentioned previously.

[0162] The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants. Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0163] The number of rings and the arrangement of heat pipes / pencils in a sealed assembly can vary depending on the package size and other design parameters, including the thermal power per assembly, the constraint on the heat pipe diameter, the maximum axial or radial temperature gradient constraints, at the pencils or heat pipes, ...

[0164] In the examples illustrated, a heat pipe is in the general form of a straight tube arranged vertically, with preferably its adiabatic part occupying a significant part of the space available between the outlet of each assembly box and the inlet of the heat exchanger shell, part of the heat pipe which is therefore not subject to heat exchange as such. It goes without saying that within the scope 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 bath of liquid metal of sealed assemblies and at least part of their condenser is inside the heat exchanger shell.

[0165] A fourth barrier is possibly represented by the reactor building, which is not shown. This building may not be classified as a containment building, given the safety studies carried out by the inventors.

[0166] Liste des références citées

[0167] [1]: B. Zohuri, Heat Pipe Design and Technology. “Modem applications for practical thermal management”, Springer, 2016.

[0168] [2]: B. Zohuri, "Heat pipe design and technology. Modem applications for practical thermal management", Springer, Second edition, 2021.

[0169] [3]: F. MORIN et al., “COPERNIC, A NEW TOOL BASED ON SIMPLIFIED CALCULATION METHODS FOR INNOVATIVE LWRs CONCEPTUAL DESIGN STUDIES”, ICAPP 2017 Conference, 2017.

[0170] [4]: P. GAUTHE et al., “Innovative and inherently safe small SFR as a response to the dilemma 'safety vs cost’", ICAPP 2019 Conference, 2019.

Claims

Claims 1. Nuclear reactor (1), of the modular microreactor (MMR) type, comprising: - a tank (10) with a central axis (X) comprising: • a core (11) comprising sealed assemblies, each assembly comprising: a housing partially filled with a bath of metal in the liquid state during operation of the reactor, at least one nuclear fuel rod, housed and held in the housing with its axis substantially parallel to the central axis, each rod containing a sealed sheath in which is arranged a stack of nuclear fuel pellets surmounted by a plenum, at least one heat pipe, housed and held in the housing with its axis substantially parallel to the central axis and passing partly through the housing 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 housing, • a fixed neutron reflector, arranged on the periphery of the core to ensure the maintenance of the neutron flux in the core, - a closing slab (15), to close the interior of the primary tank while being crossed by at least 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 housing being of substantially regular hexagonal cross-section, where appropriate with at least one corner having a bevel.

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

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

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

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

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

9. Nuclear reactor (1) according to claim 8 in combination with claim 7, the housing comprising: - at least one so-called guide grid, in which the pencil(s) are mounted with the possibility of movement parallel to the central axis, - at least one grid, called a fuse grid, arranged at a distance from the guide grid and comprising one or more flexible fusible elements, the rod(s) being, in normal operation of the reactor, blocked in the fuse 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 axial differential expansions, the fusible element(s) being adapted to melt beyond a threshold temperature of the bath corresponding to accidental operation, the melting of the elements causing the rod(s) to rise by Archimedes' thrust, so as to bring the neutron absorber into the active zone of the core.

10. Nuclear reactor (1) according to one of claims 3 to 9, the housings of the assemblies being arranged in a network within the vessel while 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 bar, arranged in the space provided between bevels of adjacent housings.

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

13. Nuclear reactor (1) according to one of the preceding claims, the power of which is between 1 and 50 MWth.

14. Waterproof assembly comprising: - a central axis housing (XI), partially filled with a bath of metal in the liquid state, which is preferably chemically inert to air and water, - at least one nuclear fuel rod, housed and held in the casing with its axis substantially parallel to the central axis (XI), each rod containing a sealed sheath in which is arranged a stack of nuclear fuel pellets surmounted by a plenum, - at least one heat pipe, housed and held in the housing with its axis substantially parallel to the central axis and partly passing through the housing 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 housing.

15. Nuclear installation (100) comprising: - a nuclear reactor (1) according to one of claims 1 to 13; - at least one heat exchanger (2) comprising a casing forming a sealed shell in which a fluid circulates from an inlet orifice to an outlet orifice to evacuate the heat released by at least part 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 system (3) for thermal storage and / or supply of heat and / or conversion of heat into electricity, connected to the heat exchanger.

Images