Liquid metal cooled nuclear reactor including a fully passive decay heat removal (DHR) system with a modular cold source

Abstract

The present invention relates to a liquid metal cooled nuclear reactor comprising a fully passive decay heat removal (DHR) system with a modular cold source. The invention mainly consists in manufacturing a nuclear reactor comprising a system which ensures at the same time: - decay heat removal in a fully passive way from the moment of the beginning of the accident; - heat removal through the primary vessel; - reduction of the risk of chemical interaction between sodium (or NaK) and the material used as final cold source; - presence of a final cold source which provides the same function as the sodium / air or NaK / air exchanger used in the state of the art but with different properties (storage with PCM).

Description

Technical Field

[0001] This invention relates to the field of liquid metal cooled fast neutron nuclear reactors, particularly sodium fast reactors known as SFRs or Na-cooled FNRs and forming part of a family of reactors known as the fourth generation.

[0002] More specifically, the present invention relates to improving the decay heat removal capabilities of these nuclear reactors.

[0003] This invention is particularly applicable to small modular reactors (SMRs) that typically have an operating power of 50 MWe to 200 MWe.

[0004] It is important to reiterate here that the decay heat of a nuclear reactor is the heat generated by the reactor core after the nuclear chain reaction stops, and it consists of the decay energy of the fission products.

[0005] Although described with reference to a liquid sodium-cooled nuclear reactor, the invention is applicable to any other liquid metal (e.g., lead) as a heat transfer fluid in the primary loop of a nuclear reactor. Background Technology

[0006] In a nuclear reactor, the essential safety functions that must always be ensured are containment, control of reactivity, and removal of heat and decay heat.

[0007] To mitigate decay heat, efforts have been made to enhance the system's passivity and versatility to ensure better overall reliability. The aim is to maintain structural integrity, particularly the integrity of the first containment barrier (fuel assembly cladding) and the second containment barrier (main vessel), even in the event of a prolonged power outage at the nuclear power plant (corresponding to the Fukushima scenario).

[0008] More specifically, it is currently envisioned that decay heat be removed from liquid metal-cooled reactors in a completely passive manner via the main vessel. While this may seem impossible for large reactors (due to their high power), it could be practically considered for low-power SMRs to ensure a substantial improvement in the safety of removing decay heat via the main vessel and decay heat removal system (DHR system).

[0009] The DHR systems currently used in sodium-cooled reactors are not entirely passive, as they actually utilize instrumentation and control systems and / or human intervention. Furthermore, these systems typically use sodium flow loops with a cold air source, which are susceptible to failure. Additionally, current systems lack diverse solutions for the radiators (also known as final cold sources) that provide final cooling to the reactor in the event of an accident. These systems can be highly sensitive to internal and external attacks and malicious acts.

[0010] Existing or known DHR systems in the literature can generally be divided into three categories:

[0011] A / The DHR system is located in the loop upstream of the energy conversion system;

[0012] B / The DHR system is located at least partially within the main vessel of the reactor;

[0013] C / The DHR system is located outside the primary or secondary vessel of the reactor.

[0014] The A / system releases heat to the liquid metal / air exchanger. [1] The main drawbacks of these systems are that they require at least two exchangers, operate primarily through forced convection with limited natural convection, and require a final cooling source of the liquid metal / air exchanger type, thus posing a risk of chemical interactions in the event of liquid metal leakage or external attack on the final cooling source.

[0015] The B / system also releases the removed heat to a final cold source of the liquid metal / air exchanger type.

[0016] Some of these B / systems include arranging cold or heat collectors within the main container. [1] In addition to the main drawbacks of the A / systems mentioned above, they also pose a risk of contact with radioactive liquid metals in the container, and require reactor shutdown when handling components of these B / systems.

[0017] Patent application JP2013076675A also discloses a B / system, which is presented as a passive cooling system, a portion of which passes through a plate. The proposed solution has several drawbacks, namely the sealing provided by the plate, the potential for heat transfer to the dome, the need to shut down the reactor when handling components of the system, and the additional weight that needs to be supported by the plate.

[0018] The C / system includes exchangers, tube bundles, or airflows arranged outside the primary or secondary container.

[0019] Known C / systems outside of secondary containers have the following main disadvantages:

[0020] - It must be operated actively, that is, through forced convection;

[0021] - Limited efficiency because the internal fluid (hot oil) used is not a good heat transfer medium;

[0022] - Chemical instability of heat transfer fluids at temperatures above 300°C to 350°C;

[0023] - The cooling performance is poor because it is achieved through radiation from the secondary container.

[0024] The aforementioned patent application JP2013076675A discloses a C / system located outside a secondary container: it includes a heat collector and downward and upward flow channels surrounding the primary container, formed respectively between the heat collector and the silo, and between the heat collector and the protective container. External air is introduced into the downward flow channel to flow downwards, then upwards to the bottom of the silo, and finally discharged to the outside. This system design suffers from the aforementioned disadvantages, particularly low efficiency because air is not a good heat transfer medium, and low cooling performance because it is handled by the secondary container. Furthermore, the final cold source, exposed to the outside, is at risk of external attack.

[0025] Patent application KR20150108999 A discloses a C / system outside the secondary vessel. Here, the final cold source is also exposed externally. Furthermore, the disclosed solution has several drawbacks. First, system components must be welded to the secondary vessel. Additionally, the operation of this system requires a phase change in the heat transfer fluid, resulting in significant density variations, which in turn lead to large variations in mechanical stress within the piping system, making it inefficient in the stages leading to vessel puncture and core meltdown.

[0026] In application FR3104311A1, the applicant proposed a liquid metal-cooled nuclear reactor comprising a DHR system utilizing a cold source of phase change material (PCM). This system overcomes the shortcomings of the aforementioned A / , B / , and C / systems, requiring little or no modification to the nuclear reactor (including its structure). However, the solutions for the cold source proposed in the aforementioned patent application are not entirely satisfactory for the following reasons:

[0027] The heat distribution in the PCM is not optimal;

[0028] - The thermal expansion of the structure was not adequately considered;

[0029] - Difficult to implement;

[0030] - Handling, replacing, and inspecting PCMs can be difficult;

[0031] - The cold source is not compact enough.

[0032] Therefore, there is a need to improve the DHR system of liquid metal-cooled nuclear reactors, especially the solution proposed in patent application FR3104311A1. Summary of the Invention

[0033] Therefore, one aspect of the present invention relates to a liquid metal-cooled fast neutron nuclear reactor, comprising:

[0034] - A container referred to as the main container, which is filled with liquid metal as the heat transfer fluid for the primary loop of the reactor;

[0035] - Container pits that define the space between containers arranged around the main container;

[0036] - A sealing plate used to confine liquid metal within the main container;

[0037] - A venting system for simultaneously venting at least some of the nominal heat and decay heat of the reactor, the system comprising:

[0038] A closed loop filled with a heat transfer fluid, the closed loop comprising at least:

[0039] - An array of multiple U-shaped tubes arranged in the space between containers, distributed around the main container, with each U-shaped tube extending along the main container, wherein the bottom of the U-shape faces the bottom of the main container.

[0040] - A first collector, referred to as the first cold collector, is connected to a branch of the U-shaped branch of each tube in the array, referred to as the cold branch. The cold collector is arranged on the outside above the enclosed plate.

[0041] - A second collector, referred to as the first heat collector, is connected to another branch, called the heat branch, in the U-shaped branch of each tube in the array. The heat collector is arranged on the outside above the enclosed plate.

[0042] - Multiple single-tube heat exchangers, one end of which is connected to the first cold collector and the other end to the first heat collector.

[0043] The closed loop is configured such that the heat transfer fluid flows through it by natural convection and remains liquid during the nominal operation of the nuclear reactor and during the shutdown operation to release decay heat from the nuclear reactor.

[0044] The cold source includes at least one modular storage unit disposed at a distance from the main container and above a sealing plate, the storage unit including a third collector, referred to as the second cold collector, connected to the first cold collector; a fourth collector, referred to as the second heat collector, connected to the first heat collector disposed above the second cold collector and connected to the second cold collector via a plurality of single-tube exchangers; a plurality of modules disposed in at least one row of housings, each module containing a solid-liquid phase change material (PCM) confined within a housing and arranged to be cooled by one of the plurality of single-tube exchangers; and a support structure supporting at least the second heat collector and the plurality of housings, wherein the PCM present in each module is designed to be solid during nominal operation of the nuclear reactor and to transform into a liquid state in the event of an accident involving the release of decay heat from the nuclear reactor during exchange with liquid metal circulating in the single-tube exchangers.

[0045] According to an advantageous variant, the tubes of a single-tube exchanger have a straight shape.

[0046] According to an advantageous embodiment, the support structure also supports a second cold collector.

[0047] Advantageously, each module has an overall cube shape, with individual corners or cutouts designed to allow single-tube exchangers to pass through.

[0048] Typically, the housing of each module is made of stainless steel 439 (at least 17% Cr and 1% Ti). This ferritic steel is corrosion-resistant, a good thermal conductor, and has a lower coefficient of thermal expansion than type 304 / 316 stainless steel. It is also suitable for achieving good magnetic hold so that it can be gripped by operating machines.

[0049] According to a preferred embodiment variation, the PCM filling each module is in the form of one or more blocks in the solid state, preferably made of Zamak, and preferably arranged to be secured in place within each module by one or more springs. Zamak has a melting temperature of approximately 385°C and, due to its significant thermal inertia and latent heat, can absorb most of the heat while still maintaining the temperature of the heat transfer fluid below the critical operating temperature of the main vessel. Preferably, a gap is provided between the module walls and the Zamak blocks to compensate for operational expansion in accident scenarios.

[0050] In an advantageous embodiment, a sliding mechanism, preferably in the form of a rolling bearing, is arranged below each module and included in the support structure. This mechanism allows the module to move automatically using compensation for its expansion after it has been heated during an accident.

[0051] Advantageously, the arrangement of the containers limits the space for forming processing corridors.

[0052] According to an advantageous embodiment, a double wall is included within a single-tube heat exchanger. Positioned in free contact with the basic module cooling it, the double wall is filled with an inert gas, typically helium. The double wall is advantageously made of type 316 austenitic stainless steel. Its function is to prevent direct contact between the heat transfer fluid and air, or, in the event of leakage of the heat transfer medium through the heat exchanger tubes, to prevent leakage of the heat transfer medium and direct contact with the liquid PCM. The space between the walls is filled with helium to improve thermal conductivity and allow for leak detection.

[0053] Therefore, the system according to the invention (as in the system of the aforementioned application FR3104311A1) performs decay heat removal (DHR) function and ensures radioactivity containment by maintaining the integrity of the first radiation barrier (fuel cladding) and the second radiation barrier (main container).

[0054] Therefore, the present invention is mainly about manufacturing a nuclear reactor that includes a system that simultaneously ensures the following:

[0055] - The decay heat is discharged in a completely passive manner from the moment the accident begins;

[0056] -Heat is dissipated through the main container;

[0057] - Reduce the risk of chemical interactions between sodium (or NaK) and the material used as the final cold source;

[0058] - There exists a final cold source (with a PCM storage unit) that provides the same function as the sodium / air or NaK / air exchangers used in the prior art but has different properties.

[0059] Therefore, the difference between the DHR system according to the present invention and prior art systems lies in the fact that, by utilizing the fact that heat radiates to the space between containers at high temperatures, heat is passively discharged through the outside of the main container. Compared with prior art systems, no auxiliary system is required to assist in the circulation of internal fluids.

[0060] Unlike the aforementioned application FR3104311A1, the cold source is modular and comprises multiple modules, each filled with PCM and combined together to form an assembly. Each module is cooled by a single-tube heat exchanger, and additional heat and cold collectors ensure the distribution of heat transfer fluids in the multiple heat exchangers.

[0061] Therefore, the DHR system according to the present invention, compared with the system of application FR3104311A1, can improve the heat distribution within the PCM and increase the outward exchange surface area, which is useful for dispersing some of the radiated heat through the walls of the enclosure.

[0062] In other words, compared to the system applying for FR3104311A1, the following is ensured:

[0063] - By utilizing the connection of fixed and equal number of modules (and enclosures) with heat exchanger tubes, heat can be distributed more efficiently in the PCM;

[0064] - Thermal expansion of the structure and PCM is taken into account in accident scenarios and during phase transitions;

[0065] -PCM (Zamak), advantageously in block form, performs better than cadmium in terms of volumetric latent heat and thermal inertia.

[0066] - This is easier to implement by using modules stored in a tiered manner on the supporting structure. The storage is done using an operating machine, which can pass through aisles created by the spaces between the enclosures. It is also easier to inspect each module, and therefore the entire system.

[0067] Therefore, compared to other known and used DHR systems, this system possesses strong versatility and disruption capabilities, which endows it with improved passive safety characteristics and no intervention delay due to the permanent circulation of internal fluids. The idea is that in the event of a nuclear power plant blackout (SBO), the DHR can be maintained without command control, operator intervention, or external cooling sources. It is therefore referred to as an intrinsically safe or "unattended safe" reactor.

[0068] The DHR system operates permanently during both normal operation and accident operation of the reactor at nominal heat.

[0069] During nominal operation, the DHR system according to the invention dissipates heat originating from the temperature difference between the main container and the tube array.

[0070] From the onset of the accident, decay heat is discharged in a completely passive manner through the permanent natural circulation of the internal heat transfer fluid, which also occurs during normal operation. This permanent natural circulation is made possible by the significant difference in fluid density between the hot and cold branches of the U-tube, as well as the height of the hot and cold branches.

[0071] Exhausting heat through the main container is advantageous because it inherently ensures this function in the event of a severe accident or earthquake (which would cause significant deformation of the container's internal structure). Under such extreme conditions, systems within the container, such as existing systems, would not be able to properly perform this safety function.

[0072] The presence of a cold source with a double-walled single-tube heat exchanger ensures a reduced risk of chemical interaction between sodium (or NaK) and the material (PCM) that provides the cooling function.

[0073] The diversity of cooling sources and the passive operation of the DHR system according to the invention enhance the concept of installation security relative to external attacks and failures of another system.

[0074] Furthermore, PCM allows for a more compact size than liquid metal / air-type final cold sources.

[0075] If necessary, it is conceivable to add a heat pump to increase the circulation rate of the heat transfer fluid within the closed loop.

[0076] This invention applies to all liquid sodium-cooled nuclear reactors, regardless of their structure, characterized by a primary loop mode, small modular reactor, or SMR type, typically with an operating power of 50 MWe to 200 MWe, particularly:

[0077] - An integrated FNR, whose main pumps and exchangers are completely contained within a main container surrounding the reactor core and are immersed in the cooling fluid of the main container through the closure plate of the container;

[0078] - Partially integrated (“hybrid”) FNR, in which only the main pump is contained within the main container surrounding the reactor core;

[0079] - The FNR, known as the "loop-type FNR", has its main pumps and intermediate heat exchangers placed in a dedicated container outside the reactor's main container, which now only contains the core and internal structure. The main container and component containers are connected by a main piping system.

[0080] The heat transfer liquid in the circuit is preferably a liquid metal selected from binary lead / bismuth (Pb-Bi) alloys, binary sodium-potassium (NaK) alloys, sodium, or other ternary alloys of liquid metals.

[0081] The PCM used to fill one or more reservoirs is preferably selected from lead, cadmium, Zamak, and a mixture of salts consisting of 53% KNO3, 40% NaNO2, and 7% NaNO.

[0082] The heat collectors and cold collectors and piping of the loop, as well as the components of the loop (if appropriate), are preferably made of materials selected from stainless steel AISI 316L, ferritic steel, nickel-based alloys, chromium-nickel-iron alloys, and Hastelloy.

[0083] The preferred application of this invention is small reactors of the GenIV family, particularly sodium and lead-cooled reactors.

[0084] Beyond safety, this invention can also be used in normal operation to provide greater flexibility in load tracking.

[0085] Other advantages and features of the invention will become clearer and more apparent after reading the detailed description of exemplary embodiments of the invention, which are given by way of non-limiting illustration with reference to the following drawings. Attached Figure Description

[0086] [ Figure 1 ] Figure 1 This is a perspective schematic diagram of a liquid sodium-cooled nuclear reactor (SFR) with a DHR system according to the present invention.

[0087] [ Figure 2 ] Figure 2 Repeated in some section views Figure 1 Part of it.

[0088] [ Figure 3 ] Figure 3 This is a partial longitudinal cross-sectional view showing the main vessel and some fuel assemblies of the nuclear reactor SFR, as well as a portion of the tube array of the DHR system according to the invention.

[0089] [ Figure 4 ] Figure 4 repeat Figure 3 However, there is no insulating material layer.

[0090] [ Figure 5 ] Figure 5 This is a schematic side view of a modular storage unit that serves as a cold source for a DHR system according to the present invention.

[0091] [ Figure 6 ] Figure 6 yes Figure 5 A schematic cross-sectional view of the storage unit passing through a single module (box).

[0092] [ Figure 6A ] Figure 6A yes Figure 6 Detailed view.

[0093] [ Figure 7 ] Figure 7 It is a schematic diagram of a longitudinal section of a row of boxes passing through the support structure of a modular storage unit according to a variant of the present invention.

[0094] [ Figure 8 ] Figure 8 It is a cross-sectional view through a module whose shape allows free contact with a single-tube exchanger, showing its relative arrangement to a block made of Zamak as a PCM.

[0095] [ Figure 9 ] Figure 9 It is a schematic cross-sectional view of the module, the PCM block inside the module, and the spring that restricts the lateral movement of the PCM block. Detailed Implementation

[0096] Throughout this application, the terms “vertical,” “lower,” “upper,” “low,” “high,” “bottom,” and “top” shall be understood with reference to the main container filled with liquid sodium, such as the main container in a vertical operating configuration.

[0097] Figure 1 and Figure 2 A liquid sodium-cooled nuclear reactor 1 (SFR) is shown, having a loop-type architecture and a discharge system 2 for simultaneously discharging at least some of the nominal heat and decay heat of the reactor according to the invention.

[0098] Such a reactor 1 has a main container 10 or reactor vessel, which is filled with liquid sodium called the main liquid, and inside it is a core 11 containing multiple fuel assemblies 110 that generate heat through the fission of fuel, and a transverse neutron shield (LNS) assembly 111.

[0099] Container 10 supports the weight of sodium in the primary circuit and the weight of internal components.

[0100] Core 11 is supported by two separate structures, which allows the support and cooling fluid supply functions to be separated from the core:

[0101] - A first mechanically pressure-welded structure, referred to as diagrid 12, in which the base of fuel assembly 110 is located, and diagrid 12 is supplied with cold sodium (400°C) by the main pump;

[0102] - A second mechanically welded structure, referred to as positioning plate 13, on which the core grid plate is supported; the positioning plate is typically supported on a portion of the inner wall of the bottom portion of the main container 10.

[0103] Typically, the core grid plate 12 and the positioning plate 13 are made of stainless steel AISI 316L.

[0104] The cladding of component 110 constitutes a first containment barrier, while container 10 constitutes a second containment barrier.

[0105] As shown in the figure, the main container 10 has a cylindrical shape along its central axis X, which is connected to a hemispherical bottom. Typically, the main container 10 is made of AISI 316L stainless steel with a very low boron content to prevent the risk of cracking at high temperatures. Its outer surface is given high emissivity through a pre-oxidation treatment, which is performed during the dissipation of decay heat to promote heat radiation to the outside.

[0106] A plug 18, called the core cap plug, is vertically arranged above the core 11.

[0107] In such a reactor 1, the heat generated during the nuclear reaction within the core 11 is extracted by using a pumping device 150 arranged in the reactor vessel 10, so that primary sodium is circulated toward an intermediate exchanger 15 arranged outside the vessel 10 in the illustrated example.

[0108] Therefore, heat is extracted through the second loop sodium, cooled at the intermediate exchanger 15 through its supply pipe 152, and then heated through its outlet pipe 151.

[0109] The extracted heat is then used to generate steam in a steam generator (not shown), and the generated steam is supplied to one or more turbines and an alternator (also not shown). The turbines convert the mechanical energy of the steam into electrical energy.

[0110] The reactor vessel 10 is divided into two distinct areas by a separation device consisting of at least one container 16 arranged inside the reactor vessel 10. This separation device, also known as a baffle, is made of stainless steel AISI 316L. Typically, as... Figure 2 As shown, the separation device consists of a single internal container 16, which has a cylindrical shape at least at its top portion.

[0111] like Figure 3 and Figure 4 As shown, the baffle 16 is typically welded to the core grid plate 12.

[0112] like Figure 1 As shown, sodium exiting the reactor core 11 is collected in the primary loop sodium zone defined within the inner container 16; the primary loop sodium zone constitutes the hottest area for sodium and is therefore commonly referred to as hot zone 160 or hot collector. The primary loop sodium zone 161, defined between the inner container 16 and the reactor vessel 10, collects primary loop sodium and supplies it to the pumping system; it constitutes the coldest area for sodium and is therefore commonly referred to as cold zone or cold collector 161.

[0113] like Figure 2 As shown, the reactor vessel 10 is anchored and sealed by a sealing plate 17, which supports various components, such as pumping devices (not shown), some components of the evacuation system 2 described below, and the core plug 18. Therefore, the sealing plate 17 is the top cover that confines liquid sodium within the main vessel 10. Typically, the plate 17 is made of non-alloy steel (A42).

[0114] The main container 10 is sealed by a metal seal between the sealing plate 17 and the core cap plug 18.

[0115] The core plug 18 is a rotating plug that is included in all the instruments required for all handling systems and core monitoring, including control rods (the number of which depends on the type and power of the core), as well as thermocouples and other monitoring devices. Typically, the plug 18 is made of AISI 316L stainless steel.

[0116] The space between the sealing plate 17 and the sodium-free layer (often called the covering gas chamber) is filled with a gas that is inert to sodium, usually argon.

[0117] The support and containment system 3 is arranged around the main container 10 and below the enclosure plate 17 of the main container 10.

[0118] More specifically, such as Figure 3 and Figure 4 As shown, the system 3 includes a container pit 30 in which an insulation layer 31, a lining-type sheath 32, and the main container 10 of the reactor are inserted from the outside to the inside.

[0119] The container pit 30 is a block with an overall parallelepiped external shape, supporting the weight of the support plate 17 and thus the weight of the components it supports. The container pit 30 provides biological protection and protection against external attacks, as well as cooling the external environment to maintain a low temperature. The container pit 30 is typically a concrete block.

[0120] The insulation layer 31 ensures the insulation of the container pit 30. Layer 31 is typically made of polyurethane foam or silicate.

[0121] The liner sleeve 32 ensures that primary sodium is retained and protects the container pit 30 in the event of a leak in the main container 10. The liner 32 is supported on the container pit 30, and its top portion is welded to the closure plate 17. Typically, the liner 32 is made of AISI 316L stainless steel.

[0122] The space E between the lining sheath 32 and the main container (referred to as the inter-container space) is filled with a thermally conductive gas, such as nitrogen, to cool the surface of the main container 10. Space E must be sufficient to allow for the positioning of the inspection system used. The thickness of the inter-container space E is typically approximately 30 cm.

[0123] The decay heat removal (DHR) system 2 according to the invention for discharging decay heat through the main container 10 will now be described, with more specific reference to... Figure 3 , Figure 6 and Figure 7 To describe.

[0124] By capturing high-temperature radiation in the space E between the containers, the DHR system 2 according to the invention can exhaust decay heat outside the main container 10 in a completely passive manner.

[0125] System 2 first includes a closed loop 4 filled with liquid metal, which includes:

[0126] - An array 40 of multiple U-shaped tubes 400 arranged in the space E between containers, the tubes being distributed around the main container 10 and each tube extending along the main container 10, wherein the bottom of the U-shape faces the bottom of the main container 10.

[0127] - A first cold collector 41, which is directly welded to one branch 401 (referred to as a cold branch) of the U-shaped branch of each tube in the array, is arranged on the outside above the enclosure plate 17.

[0128] - A first heat collector 42, which is directly welded to another branch 402 (referred to as a hot branch) in the U-shaped branch of each tube in the array, is arranged on the outside above the enclosure plate 17, and preferably vertically above the first cold collector 41.

[0129] - Multiple single-tube exchangers 43, one end 431 of which is connected to the first heat collector 42 and the other end 432 of which is connected to the first cold collector 41.

[0130] The upper portion of the enclosed plate 17 supports the weight of the components that support the cold collector 41 and the heat collector 42.

[0131] The sealing plate 17 has various types of openings to allow each tube 400 of the array 40 to be inserted. Thus, each tube 400 enters and exits through the top of the plate 17.

[0132] In the case of the loop reactor shown, if some pipes 400 exit / enter through the side of the main vessel 10, they bypass the branches of the primary loop.

[0133] like Figure 4 As shown, the cold branch 401 of the U-shaped tube 400 is fully inserted into the insulation layer 31 to reduce the temperature of the insulation layer 31 and prevent fluid backflow, ultimately allowing the liquid metal to flow naturally within each tube 400.

[0134] The diameter of the tube array 40 is based on the diameter of the main container 10, and the height is sufficient to provide the surface area required for the desired heat dissipation.

[0135] In other words, the total number and size of the U-shaped tubes 400 constituting the array 40 depend on the diameter of the main vessel 10 and the power of the reactor core 11. For example, the spacing between the tubes in the array can be equal to 10 cm, which is a good compromise for manufacturing and absorbing heat through radiation.

[0136] Similarly, for example, the outer diameter of each tube 400 is fixed at a standard size of 5 cm to minimize head loss, reduce the tube volume in the space E between the containers, and maximize the surface exposed to the main container 10. The thickness of each tube depends on the mechanical stress exerted by the liquid metal inside and its weight.

[0137] The material of each tube 400 must have good emissivity characteristics on the side of the heat-absorbing branch 402. Typically, the tube material is selected from stainless steel AISI 316L, ferritic steel, nickel, chromium-nickel-iron alloys, and Hastelloy. This material depends on the internal fluid used in the closed loop 4.

[0138] The internal heat transfer fluid C is a chemically stable liquid metal with low viscosity, making it a good thermal conductor and heat transfer medium. It is chemically compatible with all piping systems in loop 4 and can operate within a temperature range of 150°C to 600°C via natural convection. Typically, the liquid metal in loop 4 can be selected from NaK alloys, Pb-Bi alloys, ternary alloys of sodium or liquid metals, etc.

[0139] like Figure 3 As shown, the cold collector 41 and the hot collector 42 have an annular integral shape centered on the central axis (X) of the main container 10. These collectors 41, 42 are supported on a support member 44 that is directly welded to the closure plate 17.

[0140] Each single-tube heat exchanger 43 has the function of dissipating the heat absorbed by the fluid within system 2 by cooling the fluid as it leaves and by allowing the dissipation of decay heat, performing better than the cold source proposed in patent application FR1913942. As shown, each single-tube heat exchanger 43 is preferably a straight tube. Typically, each single-tube heat exchanger 43 is made of stainless steel AISI 316.

[0141] like Figure 2 and Figure 5 As shown, the DHR system 2 according to the invention also includes a cold source 5, which is configured to absorb heat dissipated from the main vessel 10 by radiation through the entire array 40 of tubes 400. The size of the cold source 5 depends both on the power of the reactor core 11 (which in effect determines the decay heat to be dissipated) and on the envisioned transport duration to be supported, and therefore requires substantially proportional thermal inertia.

[0142] The cold source 5 includes at least one storage container 50, which is arranged at a distance from the main container 10 and at a level above the enclosure 17.

[0143] According to the present invention, the storage device 50 is a modularly designed storage device, in which multiple modules 5000 are combined together in the form of a housing 500 arranged in at least one row, and each module 5000 is formed in the form of a block by a shell 50000 containing a solid-liquid phase change material (PCM) 50001.

[0144] exist Figure 5 and Figure 6 In the example shown, the housings 500 are arranged in multiple rows, and their weight is supported by a support structure 503. Also as shown, each module 5000 has an overall cubic shape, for example, with sides of approximately 60 cm. The walls 50000 of each module 5000 are preferably made of ferritic stainless steel, typically steel 439. Processing is performed using a magnetized arm machine, which can easily grip the modules through their outer surface.

[0145] More specifically, the storage container 50 is arranged at a certain distance from the main container 10 and above the closing plate 17.

[0146] The modular storage unit 50 includes a second cold collector 501 connected to a first cold collector 41, and a second heat collector 502 connected to a first heat collector 42 and disposed above the second cold collector 501, and connected to the second cold collector 501 via a plurality of single-tube exchangers 43. More specifically, the end 432 of the single-tube exchanger 43 is connected to the second cold collector 501, and the second cold collector 501 itself is connected to the first cold collector 41. Furthermore, the end 431 of the single-tube exchanger 43 is connected to the second heat collector 502, and the second heat collector 502 itself is connected to the first heat collector 42.

[0147] Using a cold collector 501 and a heat collector 502 at the end of the tubes of the heat exchanger 43 allows for optimized distribution of the flow rate of the liquid metal heat transfer medium, thereby promoting its cooling.

[0148] Each group of four modules 5000 forming a housing 500 is vertically passed through one of the straight single-tube switches 43.

[0149] The modular storage unit 50 finally includes a support structure 503, which supports the second cold collector 501, the second heat collector 502, and multiple housings 500.

[0150] The support structure 503 can advantageously be a mechanically welded assembly of the metal profile 504. The support structure 503 is typically made of steel 304.

[0151] like Figure 5As shown, the housings 500 are preferably arranged opposite each other to form processing corridors 505. These corridors 505 can be sized, for example, to allow processing units equipped with magnets to pass through, which can lock each module 5000 and move each module 5000 as it is attracted to the outer ferromagnetic layer of the housing 50000. Such movement can be performed, for example, with the PCM fused within the module 5000, or through simple control after system testing.

[0152] For example Figure 5 As shown, the collectors 501 and 502 of the modular storage are positioned to completely empty the processing corridor 505 and thus not to complicate the processing operation.

[0153] The shape of each module 5000 allows it to pass vertically through the single-tube heat exchanger 43, in which the heat transfer medium C flows through the housing 500 by natural convection.

[0154] like Figure 7 As shown, the single-tube exchanger 43 can be isolated from direct contact with the module 5000 by inserting a double-walled 506 filled with inert gas, thereby improving the thermal conductivity of the double-walled 506 and, if appropriate, allowing for leak detection.

[0155] The double-walled 506 has the function of preventing direct contact between air and heat transfer fluid C in the event of leakage from pipe 43, and preventing possible chemical reactions between heat transfer fluid and PCM block 50001 in the event of an accident.

[0156] Typically, the double walls are made of 316 or 439 stainless steel, with helium as the inert gas, and the space between the 506 double walls has a thickness of 0.5 mm.

[0157] Modular storage 50 disperses the small amount of heat discharged during the accident phase and all the heat discharged by system 2 when the reactor is operating at nominal power through natural convection and radiation from the walls of all enclosures 500.

[0158] The dimensions of each module 5000 and enclosure 500 component depend on the PCM it contains and the heat to be dissipated during normal operation and in the event of an accident. The normal operating point (under nominal conditions) affects the size and total number of modules, while the accident scenario affects the required volume of the PCM.

[0159] PCM is used as a heat buffer. During the exchange of liquid metal with a single-tube exchanger, PCM is selected to be in a solid state during the nominal operation of the nuclear reactor and to be converted to a liquid state during the shutdown operation of the nuclear reactor to release decay heat.

[0160] In other words, during the nominal operation phase of the reactor, the PCM is in a solid state and it transfers the heat released by each single-tube heat exchanger 43 through conduction, and then releases the heat to the walls of each module 5000 through convection and radiation.

[0161] During the accident and nominal phases, the liquid PCM must store the heat released by the single-tube exchanger 43, thus cooling the liquid metal of the cooling circuit 2.

[0162] Preferably, such as Figure 8 As shown, in its solid state, the PCM consists of blocks 50001 arranged and fixed in place within each module 5000. More specifically, as... Figure 9 As shown, blocks 50001 are stacked one on top of the other in a stacking manner within each module 5000. Specifically, due to the slightly concave shape of the PCM blocks 50001 when in a solid state, they can interlock.

[0163] Advantageously, a gap is provided between the inner wall of module 5000 and block 50001 to compensate for operational expansion under accident scenarios. The atmosphere inside each module 5000 is preferably composed of an inert gas (e.g., helium) to ensure good internal thermal conductivity and to effectively transfer heat in the initial phase of a transient accident involving a nuclear power plant blackout (SBO).

[0164] Optional spring 50002 can be arranged at the stack end between the end block 50001 and the wall of module 5000 to limit lateral movement of all blocks. Figure 9 ).

[0165] In order to operate correctly in a static state, the PCM must have high thermal conductivity at nominal reactor power.

[0166] To ensure proper operation under accident conditions, PCMs possess the following characteristics: increased thermal inertia (increased specific heat capacity and density), melting point between 250°C and 400°C, operating temperature between 150°C (solid) and 600°C (liquid), and increased latent heat.

[0167] Of course, the PCM must also be chemically compatible with the fluid within the closed loop 2 so that there will be no problems in the event of interaction following a leak in the single-tube exchanger 43.

[0168] Typically, when the heat transfer fluid in closed loop 2 is a NaK alloy, the PCM is made of Zamak, or when the heat transfer fluid is a Pb-Bi alloy, the PCM is made of lead.

[0169] During the accident phase, the exchanger tube 43 will expand radially after being heated.

[0170] To compensate for this expansion, a sliding mechanism 507 can be arranged below each module 5000, the sliding mechanism 507 forming part of the support structure 503. For example... Figure 7 As schematically shown, the sliding mechanism 507 may consist of rolling bearings, which allow the module 5000 to move in contact with the tube 43 in each layer of the support structure 504.

[0171] The modular storage unit 50 is preferably contained within a containment building 52. Therefore, the final cold source 5 of the system 2 according to the invention is protected from possible external attacks.

[0172] The inner walls of the containment building 52 preferably have high emissivity to facilitate the dissipation of heat radiated from the outer walls of the modular storage unit 50 housed therein.

[0173] In order to place the cold source 5 at an optimal distance from the main container 10, the hydraulic circuit 2 includes a connecting loop 45, which includes components of the piping system and, if appropriate, valves between the cold collector 41 and the heat collector 42 and each individual tube exchanger 43.

[0174] More specifically, such as Figure 1 and Figure 2 As shown, the connection loop 45 includes a hydraulic branch 451 that connects the first cold collector 41 to the cold end 431 of each single-tube exchanger 43 via the second cold collector 501 and a hydraulic branch 452 that connects the first heat collector 42 to the hot end 432 of each single-tube exchanger 43 via the second heat collector 502.

[0175] Therefore, the first cold collector 41 distributes the liquid metal flow within the cold branch 451 toward the U-shaped bottom of each cold branch 401 of each pipe 400, and the first hot collector 42 collects the internal liquid metal from the U-shaped bottom of each hot branch 402 of each pipe 400 to supply it to the hot branch 452.

[0176] Advantageously, the cold branch 451 and the hot branch 452 are sized to have the shortest possible length in order to reduce head loss from them and increase the natural convection velocity in the closed hydraulic circuit 4.

[0177] Therefore, according to the present invention, the closed hydraulic circuit 4 just described is configured such that the heat transfer liquid metal flows therein by natural convection and remains liquid during the nominal operation of the nuclear reactor and during the shutdown operation of the nuclear reactor that releases decay heat.

[0178] During nominal power operation of the reactor, each single-tube exchanger is in free contact with each module 5000 that constitutes a housing 500. The heat released by the single-tube exchanger 43 is transferred primarily by convection and conduction to the solid PCM, typically in the form of a Zamak block 50001.

[0179] During the accident phase, PCM 50001 becomes liquid and stores the heat released by each exchanger 43, thereby cooling the liquid metal within the closed hydraulic circuit 4.

[0180] The configuration just presented can be applied to any type of liquid metal cooled reactor and is designed to diversify the DHR functionality relative to existing solutions for nuclear power plant outages lasting several days (typically around 7 days) through a compact and fully passive system 2.

[0181] The duration of the application varies depending on the reactor power and the size of the modular storage unit 50.

[0182] If one seeks to further extend (or conversely shorten) the operating duration of the thermal buffer, a larger (or smaller) modular storage unit 50 is required to increase (or decrease) the required thermal inertia and total storage capacity, which depends primarily on the total available latent heat.

[0183] On the other hand, the reactor power directly affects the total energy to be stored. Therefore, without changing the storage capacity and thus the overall volume of the PCM, increasing the reactor power will shorten the service life of the modular storage unit 50.

[0184] For an example of a draft of an advanced modular reactor named AMR-SFR ATRIUM, the inventors have calculated the dimensions of Module 5000 as the basic module of the reservoir and the dimensions of DHR System 2, wherein the power is 400 MWth, the container diameter is 8m, and the height is 12m.

[0185] It should be noted that, for the calculations under consideration, the geometry of module 5000 is cubic and made of 439 steel.

[0186] The results of these calculations are given in Tables 1 and 2 below.

[0187] [Table 1]

[0188]

[0189] [Table 2]

[0190]

[0191] This application represents an exemplary case. Due to the increased compactness of module 5000 or modifications to its layout, optimization studies can be conducted to reduce its footprint.

[0192] The invention is not limited to the examples just described, and features of the examples shown can be combined with each other in particular in variations not shown.

[0193] Other variations and implementations may be conceived without departing from the scope of the invention.

[0194] Although in all the examples shown, DHR system 2 is described as having a single modular storage unit 50 and associated piping system, which has the advantage of producing a smaller footprint, it goes without saying that multiple modular storage units can also be provided for replication (safety) purposes.

[0195] The DHR system described earlier regarding the loop-type nuclear reactor can be fully implemented in an integrated nuclear reactor.

[0196] In the case of an integrated reactor design, the array 40 of pipes is uniformly arranged around the entire main vessel 10.

[0197] In some loop reactors, the tubes 400 on the primary loop side can converge in miniature collectors at the branch points to avoid potential hot spots in the involved U-shaped tubes 400.

[0198] List of cited references:

[0199] [1]: HOURCADE E et al., "ASTRID Nuclear Island design: update in French-Japanese joint team development of Decay Heat Removal system", 2018, ICAPP.

Claims

1. A liquid metal cooled fast neutron nuclear reactor (1) comprising: - a vessel (10), called primary vessel, filled with a liquid metal as heat transfer fluid of a primary circuit of the reactor; - a vessel pit (30) defining a space between vessels arranged around the primary vessel; - a closure plate (17) for confining the liquid metal within the primary vessel; - an evacuation system (2) for simultaneously evacuating at least some of the nominal heat and decay heat of the reactor, the system comprising at least: a closed circuit (4) filled with a heat transfer liquid, the closed circuit comprising: - an array (40) of a plurality of U-shaped tubes (400) arranged in the space between vessels, distributed around the primary vessel and each extending along the primary vessel with the bottom of the U facing the bottom of the primary vessel, - a first collector (41), called first cold collector, connected to one branch (401), called cold branch, of the U-shaped branch of each tube of the array, the cold collector being arranged outside above the closure plate, - a second collector (42), called first hot collector, connected to the other branch (402), called hot branch, of the U-shaped branch of each tube of the array, the hot collector being arranged outside above the closure plate, - a plurality of single tube exchangers (43) having one end (431) connected to the first cold collector and the other end (432) connected to the first hot collector, the closed circuit being configured so that the heat transfer liquid flows therein by natural convection and remains in liquid state both during nominal operation of the nuclear reactor and during shutdown operation of the nuclear reactor releasing decay heat; a cold source (5) comprising at least one modular reservoir (50) arranged at a distance from the primary vessel and above the closure plate, the reservoir comprising a third collector (501), called second cold collector, connected to the first cold collector, a fourth collector (502), called second hot collector, connected to the first hot collector arranged above the second cold collector and to the second cold collector through the plurality of single tube exchangers, a plurality of modules (5000) arranged in at least one row of boxes (500), each module containing a solid-liquid type phase change material PCM (50001) confined in an enclosure (50000) and arranged to be cooled by one of the plurality of single tube exchangers, and a support structure (503) supporting at least the second hot collector and the plurality of boxes, the PCM present in each module being designed to pass from solid state during nominal operation of the nuclear reactor to liquid state in case of accident releasing decay heat during heat exchange with the liquid metal circulating in the single tube exchanger.

2. The nuclear reactor (1) according to claim 1, wherein, The nuclear reactor is of toroidal type.

3. The nuclear reactor (1) according to claim 1 or 2, wherein The tubes of the single tube exchangers have a straight shape.

4. The nuclear reactor (1) of claim 1, wherein, The support structure (503) also supports the second cold collector (501).

5. The nuclear reactor (1) of claim 1, wherein, Each module (5000) has the overall shape of a cube, with a single corner or cut-out of each module having a shape designed to allow the passage of a single tube exchanger.

6. The nuclear reactor (1) of claim 1, wherein, At least a portion of the outer wall of each module is coated with a layer of ferromagnetic material.

7. The nuclear reactor (1) of claim 1, wherein, The PCM filling each module is in the form of one or more blocks in the solid state.

8. The nuclear reactor (1) according to claim 7, wherein The one or more blocks are arranged within each module while being held in place by one or more springs (50002).

9. The nuclear reactor (1) of claim 1, wherein, A sliding mechanism (507) is arranged below each module and is included in the support structure.

10. The nuclear reactor (1) of claim 1, wherein, The arrangement of the boxes defines a space that forms a processing gallery (505).

11. The nuclear reactor (1) of claim 1, wherein, A double wall (506) is arranged between the single tube exchanger (43) and the module that it cools, said double wall being filled with an inert gas.

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