Thermoelectric converter for a nuclear plant

Abstract

The present description relates to a thermoelectric conversion device (200) comprising: - a primary circuit comprising a first enclosure (220) which is filled with a primary fluid (23) and is suitable for containing fuel elements (204), the first enclosure being designed to be closed such that the conversion device changes from a first state, in which the temperature of the primary fluid is at a first temperature and the pressure in the first enclosure is at a first pressure, to a second state, in which the temperature of the primary fluid is at a second temperature higher than the first temperature and the pressure in the first enclosure is at a second pressure higher than the first pressure, the primary fluid remaining in the liquid state; - a first heat exchanger (230; 235) immersed in the primary fluid in the first enclosure; - a secondary circuit (240) which is configured for the flow of a secondary fluid (22), is coupled to the primary circuit by the first heat exchanger in order to vaporize the secondary fluid, and comprises a turbo-alternator (241, 243) for generating electrical power (E) from the vaporized secondary fluid; the thermoelectric conversion device being installed in a storage pool (20) filled with a liquid (24) forming a cold source for the secondary circuit.

Description

DESCRIPTION Thermoelectric converter for a nuclear installation This application claims priority from French patent application FR2413712 filed on December 9, 2024 and entitled "Thermoelectric Converter for a Nuclear Installation", which is considered to be an integral part of this description within the limits provided by law. technical field

[0001] This description relates in general to the conversion of thermal energy into electrical energy intended for use, for example, in a nuclear installation.

[0002] This description relates in particular to the conversion into electrical energy of the thermal energy of fuel elements being cooled after use in a nuclear reactor, for example in a storage pool, or cooling pool, of a nuclear installation.

[0003] A nuclear facility can be a nuclear power plant including at least one nuclear reactor and a storage pool. The nuclear reactor can be, for example, a power reactor and / or a research reactor. Previous technique

[0004] A nuclear reactor uses fuel elements containing energetic fissile materials, or even fertile materials which, under the action of neutrons, can partially transform into fissile materials. The fuel elements are generally installed in the reactor core, which is itself located within a reactor vessel, in a circuit called the primary circuit. The fuel elements are, for example, grouped into of fuel assemblies. A heat transfer fluid circulates in the reactor vessel and the core, from which it extracts the thermal energy released by the fission of fissile materials. The heat transfer fluid carries the extracted heat and transfers it via a heat exchanger to a secondary fluid circulating in a secondary circuit, thereby vaporizing the secondary fluid. The secondary circuit typically includes a turbine and a generator (turbo-generator) to generate electricity from the vaporized secondary fluid. In water-cooled reactors, the primary circuit heat transfer fluid is water, and the secondary circuit fluid is also water. In pressurized water reactors (PWRs), the heat exchanger is integrated into a steam generator, which is generally part of the primary circuit.The steam generator transfers all or part of the heat from the heat transfer fluid in the primary circuit to the water in the secondary circuit to transform it into steam.

[0005] A nuclear facility, for example a nuclear power plant, may include one or more nuclear reactors.

[0006] When fuel elements are no longer in use, for example when their irradiation cycle is complete, they are removed from the reactor core and vessel, then cooled—that is, their residual thermal power is reduced—before being removed from the nuclear facility. Fuel elements with residual thermal power can be referred to as "spent" or "hot" fuel elements.

[0007] In a nuclear reactor whose fuel elements are compatible with water, typically water reactors, the fuel elements can be The fuel elements are cooled in a storage pool, or cooling pool, which is filled with water. A storage pool ensures the storage of fuel elements removed from the core of the nuclear reactor, while also cooling these fuel elements before they can be transported and removed from the nuclear facility. Removing the residual heat from the fuel elements relies on keeping them submerged in the water of the storage pool and on a water cooling circuit that includes at least one heat exchanger coupled to a cold source, in order to eliminate the risk of the water boiling. For example, the fuel elements are stored at the bottom of the storage pool where a large volume of water actively circulates, being cooled by one or more cooling circuits that include one or more heat exchangers and one or more circulation pumps.In the event of water boiling, combustible materials could become exposed, meaning they are no longer completely submerged (a process known as dewatering), leading to varying degrees of degradation, the rate of which depends on their residual heat output at the time of partial or total dewatering. Each cooling circuit for the storage pool water is typically sized to maintain the average temperature of the storage pool water, under normal operating conditions, at approximately 50°C.

[0008] Figure 1 is a cross-sectional view representing in a simplified manner an example of a nuclear installation 100. This nuclear installation 100 includes a reactor building 110 and an annex building 120, or fuel building, adjoining the reactor building 110.

[0009] The reactor building 110 includes side walls 111 and a bottom wall, or base, 112. The fuel building 120 includes side walls 121 and a bottom wall, or base, 122. In general, roofs (not shown in Figure 1) of the reactor and fuel buildings rest on the side walls.

[0010] Reactor building 110 includes a vessel 113 which contains a nuclear reactor core containing fuel elements (core and fuel elements not shown in Figure 1), and a reactor pool 114 filled with water for loading and unloading fuel elements into and from vessel 113.

[0011] The fuel building 120 includes a water-filled storage pool 124. In the example shown, the storage pool 124 has three compartments that can be separated from each other by a door or a cofferdam 125: a storage compartment 124A, a transfer compartment 124B and a loading compartment 124C.

[0012] The transfer compartment 124B communicates with the reactor pool 114 by means of a transfer device 131 which passes in a sealed manner through the side walls 111, 121 of the reactor and fuel buildings 110, 120 opposite, so as to connect the reactor pool 114 with the transfer compartment 124B of the storage pool 124, in particular to transport the fuel elements between the reactor pool 114 and the storage pool 124.

[0013] The 124C loading compartment allows for the removal of fuel elements once they have cooled. For example, the cooled fuel elements are removed via a transport package loaded underwater into the 124C loading compartment, or docked beneath the 124C loading compartment.

[0014] Storage compartment 124A includes a storage rack 123 that can hold several fuel elements. Storage rack 123 comprises several locations, or bays, for example, between 300 and 1200 locations depending on the type of nuclear reactor. The water depth in storage compartment 124A is, for example, between 10 and 20 meters, between 12 and 20 meters, or between 12 and 14 meters.

[0015] The nuclear installation 100 includes a cooling circuit 140 connected to an outlet of the storage pool 124, for example, to an outlet of storage compartment 124A. Indeed, when the pool water heats up in contact with the fuel elements, it contributes less effectively to cooling these fuel elements and can even boil, which can lead to at least partial unflooding of the fuel elements. It must therefore be continuously cooled. The cooling circuit 140 includes a pump 141 for circulating the water and a heat exchanger 142 for cooling the water in the storage pool 124. Another cooling circuit 140', including another pump 141' and another heat exchanger 142', can be connected in parallel with the cooling circuit 140.Each exchanger 142, 142' communicates with a cold source (not shown in Figure 1) to which the water from the storage pool 124 gives up all or part of its heat and is continuously cooled.

[0016] Each cooling circuit 140, 140' is connected to an inlet of the storage pool 124 by an injection circuit 143 designed to reinject the cooled water into each of the compartments 124A, 124B, 124C of the storage pool 124. The injection circuit 143 comprises several pipes, each dipping into one of the compartments 124A, 124B, 124C of storage pool 124, in particular a pipe 144 plunging into the lower part of storage compartment 124A, under storage rack 123.

[0017] Maintaining the water temperature in the storage pool below the threshold, typically around 50°C, is therefore contingent upon the proper functioning of at least one of the cooling circuits, particularly the cooling circuit pump, the heat exchanger, and even the availability of the cold source. For example, if no pump is operating, the water can boil, and combustible materials can be exposed, leading to their degradation to varying degrees, depending on their residual heat output at the time of exposure, and potentially causing damage to the storage pool and related installations.For example, during the Fukushima accident in 2011, the cooling system became inaccessible due to failures of the main equipment, but also of the backup equipment, due to the earthquake and flooding, and it was not possible to measure the temperature of the water in the storage pool and to verify that it had reached 90°C, leading to a drop in the water level in the storage pool, and a dewatering of the combustible elements.

[0018] Therefore, a system for cooling fuel elements in a storage pool is sought that is not dependent on external sources, typically electricity and / or cold sources, while allowing safe and continuous cooling of the fuel elements.

[0019] US patent US9786396B2 describes a heat conversion system in which combustible elements are stored in a rack submerged in a water-filled pool. The pool water is pressurized before The water enters the rack, and the pressurized water is heated by the combustible elements as it passes through the rack, causing it to vaporize. The steam is then introduced at the rack's outlet into a turbine where it is expanded and converted into mechanical energy. The turbine is coupled to an alternator to produce electrical power, providing electricity to the area outside the pool. A pump (jet pump) connected to the turbine pressurizes the water to supply the rack with pressurized water.

[0020] The conversion system described above, however, has drawbacks. For example, the vaporized water passing through the turbine comes directly from the fuel elements, which can pose radioactive contamination problems. Furthermore, the conversion system does not optimize efficiency, particularly since the fuel elements are immersed directly in the pool. Summary of the invention

[0021] There is a need for a thermoelectric conversion device from the thermal energy of spent fuel elements, whose efficiency can be improved, and which is not subject to radioactive contamination problems.

[0022] One embodiment overcomes all or part of the drawbacks of known thermoelectric devices for converting the thermal energy of combustible elements.

[0023] One embodiment aims at emergency electricity production via thermoelectric conversion from the thermal energy of some of the fuel elements present in a nuclear installation, for example in a storage pool. It would be advantageous if the thermoelectric conversion device could limit the thermal load borne by the cold source of the nuclear installation, for example the storage pool.

[0024] One embodiment provides for a thermoelectric conversion device using combustible elements having residual thermal power, the thermoelectric conversion device comprising: - a primary circuit comprising a first enclosure at least partially filled with a primary fluid; the first enclosure being adapted to be pressurized and to contain the combustible elements immersed in the primary fluid, so as to transfer all or part of the heat from the combustible elements to said primary fluid;the first enclosure being adapted to be closed, so that, when the first enclosure is closed, the thermoelectric conversion device passes from a first state, in which the temperature of the primary fluid is at a first temperature and the pressure in the first enclosure is at a first pressure, to a second state, in which the temperature of the primary fluid is at a second temperature higher than the first temperature and the pressure in the first enclosure is at a second pressure higher than the first pressure, the primary fluid remaining in the liquid state between the first included state and the second included state; - a first evaporator heat exchanger positioned in the first chamber so as to be immersed in the primary fluid around the combustible elements; - a secondary circuit configured for the circulation of a secondary fluid; the secondary circuit being coupled to the primary circuit by the first heat exchanger to transfer all or part of the heat from the primary fluid to the secondary fluid, so as to vaporize all or part of the secondary fluid; the secondary circuit comprising a turbo-alternator having a fluid inlet to introduce the vaporized secondary fluid into the turbo-alternator, so as to generate electrical energy; the thermoelectric conversion device being installed in a storage pool at least partially filled with a liquid forming a cold source of the secondary circuit.

[0025] According to one embodiment, the transition from the first state to the second state is obtained at least partially by the heat of the combustible elements.

[0026] According to one embodiment, the storage pool contains other combustible elements, and is coupled to a cooling circuit for the storage pool liquid, the cooling circuit comprising at least one critical element, such as a pump, electrically connected to the turbo-alternator to be powered at least partially by the generated electrical energy.

[0027] According to one embodiment, the primary fluid and the liquid are water, the primary fluid coming for example from the pool water.

[0028] According to one embodiment, the thermoelectric conversion device further comprises a second enclosure disposed around the first enclosure, defining an intercalated volume between the second enclosure and the first enclosure; the intercalated volume being intended to be filled with a filling gas, for example nitrogen or argon, forming a thermal barrier around the first enclosure.

[0029] According to one embodiment, the filling gas has a defined pressure to expel from the intercalated volume a portion of liquid from the storage pool.

[0030] According to one embodiment, the secondary circuit includes a third enclosure connected to a fluid outlet of the turbogenerator and a second heat exchanger configured to couple the secondary circuit with the cold source so as to transfer all or part of the heat from the secondary fluid to the cold source; the second heat exchanger being, for example, integrated into at least one wall of the third enclosure.

[0031] According to one embodiment, the thermoelectric conversion device further includes a lid configured to close the first chamber; the lid being, for example, equipped with a pressure limiter so as to limit the pressure in the first chamber.

[0032] According to one embodiment, the first heat exchanger comprises: - a chamber located within the first containment structure; the chamber defining an internal volume at least partially filled with the primary fluid; the chamber being adapted to be pressurized and to contain the fuel elements immersed in the primary fluid; a closed volume between the chamber and the first containment structure being at least partially filled with the secondary fluid and being connected to the secondary circuit; and - at least one heat exchanger element positioned in at least one side wall of the chamber between the closed volume and the internal volume.

[0033] According to one embodiment, the thermoelectric conversion device further comprises a pressure controller connected to the inside of the first chamber to control the pressure in said first chamber; the pressure controller being, for example: - a pressurizer or gas accumulator; and / or - adapted to increase the pressure in the first chamber; and / or - adapted to limit the pressure in the first chamber to a maximum pressure limit, which is for example equal to approximately 20 bars.

[0034] According to one embodiment, the second pressure is greater than the saturated vapor pressure of the primary fluid at the second temperature.

[0035] According to one embodiment, the secondary fluid is water or an organic fluid, preferably having a low vaporization energy.

[0036] One embodiment provides for a fuel element cooling system in a nuclear installation, the cooling system comprising: a storage pool adapted to be at least partially filled with a liquid, for example water; - a thermoelectric conversion device as described above, adapted to be positioned in the storage pool.

[0037] One embodiment provides for a fuel element cooling system in a nuclear installation, the cooling system comprising: - a storage pool at least partially filled with a liquid, for example water; - a thermoelectric conversion device as described previously, positioned in the storage pool.

[0038] According to one embodiment, the cooling system further comprises a cooling circuit for the storage pool liquid, the cooling circuit comprising a critical element, like a pump, electrically connected to the thermoelectric conversion device to be powered by the generated electrical energy; the cooling circuit comprising a fluid inlet and outlet connected to the storage pool.

[0039] According to one embodiment, the cooling system further comprises: - at least two thermoelectric conversion devices as described above, positioned in the storage pool; and / or - at least one fuel element loading zone in the first enclosure of each thermoelectric conversion device.

[0040] One embodiment provides a method for implementing the thermoelectric conversion device as described above, the method comprising: - the closure of the first enclosure containing the combustible elements immersed in the primary fluid; - the transition from the first state of the thermoelectric conversion device to the second state of the thermoelectric conversion device; the primary fluid remaining in the liquid state between the first included state and the second included state; - in the second state, the transfer of all or part of the heat from the primary fluid to the secondary fluid, so as to vaporize all or part of the secondary fluid; the introduction into the turbo-alternator of the vaporized secondary fluid, so as to generate electrical energy. Brief description of the drawings

[0041] These features and advantages, as well as others, will be detailed in the following description of modes of specific implementations carried out, by way of non-exhaustive list, in relation to the attached figures, including:

[0042] Figure 1 is a cross-sectional view representing in a simplified way an example of a nuclear installation 100;

[0043] Figure 2A is a cross-sectional view representing a thermoelectric conversion device according to one embodiment; and

[0044] Figure 2B is a longitudinal cross-sectional view of the thermoelectric conversion device of Figure 2A. Description of the implementation methods

[0045] The same elements have been designated by the same reference numerals in the different figures. In particular, structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0046] For the sake of clarity, only the steps and elements necessary for understanding the described implementation methods have been shown and are detailed. In particular, the components of a nuclear reactor are not detailed. Furthermore, not all components of a storage pool, including one or more cooling circuits, are detailed, as they can be implemented by a person skilled in the art.

[0047] Unless otherwise specified, when referring to two connected elements, this means directly connected without any intermediate elements other than conductors, and when referring to two coupled elements, this means that these two elements can be connected or linked through one or more other elements.

[0048] In the description that follows, when referring to absolute positional qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative positional qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientational qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0049] Unless otherwise specified, the expressions "approximately", "roughly", "about", and "on the order of" mean within 10% or 10°, preferably within 5% or 5°.

[0050] In the description that follows, when a reactor is referred to, unless otherwise specified, it refers to a nuclear reactor.

[0051] Figure 2A is a cross-sectional view of a thermoelectric conversion device 200 according to one embodiment. Figure 2B is a longitudinal cross-sectional view of the thermoelectric conversion device 200 of Figure 2A. The cross-sectional view of Figure 2A is formed along the section plane AA shown in Figure 2B. The cross-sectional view of Figure 2B is formed along the section plane BB shown in Figure 2A.

[0052] The thermoelectric conversion device may be referred to hereafter as a "conversion device". The thermoelectric conversion device may also be referred to as a "thermoelectric converter".

[0053] The conversion device 200 can be positioned in a storage pool 20 filled with a liquid 24, which is water in this example, forming a cold source of the conversion device 200, as explained later.

[0054] The conversion device 200 comprises an outer enclosure 210 arranged around an inner enclosure 220. The outer enclosure 210 has side walls 211 and a bottom wall, or base, 212. The inner enclosure 220 has side walls 221 and a bottom wall, or base, 222, which may correspond to a portion of the bottom wall 212 of the outer enclosure 210. The intercalated volume 201 between the outer enclosure 210 and the inner enclosure 220 is intended to be filled with a filling gas 21, forming a thermal barrier, as explained later. The filling gas 21 is preferably a substantially inert gas, for example, nitrogen or argon. A gas inlet (not shown) is connected to the intercalated volume 201.The intercalated volume 201 between the outer enclosure 210 and the inner enclosure 220 can initially be filled with a liquid, for example the liquid 24 from the storage pool 20, the filling gas then driving out the liquid.

[0055] The conversion device 200 further includes a chamber 230, delimited by side walls 231 and a bottom wall 232, which is positioned on the bottom wall 222 of the inner enclosure 220, or which corresponds to a portion of the bottom wall 222 of the inner enclosure 220.

[0056] Chamber 230 defines an internal volume 203 which, during operation, is filled with a primary fluid 23. The primary fluid is water in the example of Figures 2A and 2B, originating from water 24 in the storage pool 20. The water is, for example, demineralized water. Chamber 230 is configured to accommodate fuel elements 204 immersed in the primary fluid 23 of the internal volume 203. For example, the fuel elements 204 are arranged in a rack 205 which is sized to be placed in room 230.

[0057] The combustible elements 204 stored in chamber 230 are preferably isolated from the rest of the pool 20.

[0058] Fuel elements 204 are in the form of irradiated or spent fuel assemblies, referred to as "hot," meaning that each assembly has residual thermal power. Residual thermal power is defined as thermal power exceeding a limit, typically between 5 and 10 kWth, for example, approximately 7 kWth (for fuels such as uranium oxide (UO2) or mixed oxides (MOX). Thus, fuel elements 204 can release heat, or thermal energy.

[0059] The inner enclosure 220 further includes in its upper part a ring 223 which connects the side walls 221 of the inner enclosure 220 to the side walls 231 of the chamber 230. The ring 223 makes it possible to form a closed volume 202 between the chamber 230 and the inner enclosure 220, more precisely between the side walls 221 of the inner enclosure 220 and the side walls 231 of the chamber 230.

[0060] During operation, volume 202 is filled with a second fluid 22, or secondary fluid 22. The secondary fluid 22 is, for example, water, in particular demineralized water, or an organic fluid preferably with low energy of vaporization, or low latent heat. The organic fluid could, for example, be a fluid known as Opteon™ or YF(R-1234yf).

[0061] The conversion device 200 further includes a cover 206 which is adapted to cover the outer enclosure 210, the inner enclosure 220 and the chamber 230 to close them, preferably in a sealed manner. The cover 206 is removable. For example, the cover 206 is removed to introduce the combustible elements 204 into the chamber 230, and then it is repositioned after the combustible elements 204 have been introduced into the chamber 230.

[0062] A person skilled in the art will be able to provide a means of sealing between the cover 206 and the external enclosure 210 and internal enclosure 220 and the chamber 230. The cover 206 may be fitted with a pressure limiter 207, such as a valve (S), intended to limit the pressure in the chamber 230, and thus also in the internal enclosure 220.

[0063] Heat exchanger elements 235 are integrated into the chamber 230, preferably at an interface between the internal volume 203 and the volume 202. The heat exchanger elements 235 are immersed in the primary fluid 23.

[0064] For example, the heat exchanger elements 235 are integrated into the side walls 231 of the chamber 230.

[0065] For example, the heat exchanger elements 235 have a length L4 less than the length L3 of the chamber 230. For example, the heat exchanger elements 235 do not extend to the lower wall 232 of the chamber 230. For example, the heat exchanger elements 235 do not extend to the upper end of the chamber 230. For example, the heat exchanger elements 235 do not extend to the lid 206.

[0066] For example, the heat exchanger elements 235 have a length L4 greater than or equal to, for example greater than, the length L5 of the rack 205. For example, the heat exchanger elements 235 are substantially centered in their length with respect to the length of the rack 205 and / or the fuel elements 204.

[0067] Chamber 230 can form a heat exchanger comprising heat exchanger elements 235.

[0068] In the example shown, the outer enclosure 210, the inner enclosure 220, and the chamber 230 each have a roughly parallelepiped shape with a square cross-section. Furthermore, the outer enclosure 210, the inner enclosure 220, and the chamber 230 are concentric. This example is not limiting; the outer enclosure, the inner enclosure, and the chamber could have other geometries, for example, parallelepipeds with a non-square cross-section, a right circular or non-circular cylinder, or any other geometry conceivable by a person skilled in the art, and, for example, they might not all necessarily have the same geometry. Moreover, the outer enclosure, the inner enclosure, and the chamber might not be concentric.

[0069] Once the chamber 230 is closed with the hot combustible elements 204 inside, the pressure inside the chamber 230 can rise at least due to the temperature of the combustible elements 204. The temperature of the primary fluid 23 in the internal volume 203 of the chamber 230 can also rise by heat transfer from the combustible elements 204 to the primary fluid 23.

[0070] The pressure in chamber 230 and in the internal enclosure 220 are advantageously controlled, for example by means of a pressure controller 208 (G), or a pressurizer or a gas accumulator, connected by a line 209 to the internal volume 203 of chamber 230. The pressure in the internal volume 203 containing the primary fluid 23 can advantageously be controlled to limit this pressure below an upper pressure limit.

[0071] The pressure controller 208 can also be used to increase the pressure inside chamber 230, for example before the fluid temperature primary 23 in chamber 230 rises, to begin pressurizing chamber 230 before the fuel elements 204 raise the pressure in chamber 230.

[0072] For example, for water, the upper pressure limit can be set to be higher than the saturated vapor pressure at 200°C; that is, the upper pressure limit can be greater than approximately 15 bar, so as to keep the water in a liquid state without it starting to boil. For example, the upper pressure limit is approximately 20 bar for water.

[0073] Pressurizing chamber 230 increases the temperature of the primary fluid 23 within chamber 230, while maintaining this primary fluid in a liquid state. For example, when the primary fluid 23 is water, its temperature in chamber 230 can exceed 50°C, reaching, for instance, 200°C, without reaching the boiling point of water, provided that the pressure in chamber 230 remains above the saturated vapor pressure at 200°C.

[0074] The ability to increase the temperature of the primary fluid 23 allows for an increase in the efficiency of the thermoelectric conversion. Indeed, otherwise, for example with a water temperature of approximately 50°C, considering that the cold source is at 20°C, the efficiency (Carnot cycle) would be very low, typically less than 10%.

[0075] Limiting the pressure in chamber 230 can advantageously allow us to stay below a regulatory threshold, for example related to the regulations for pressure equipment (ESP), for example around 20 bars.

[0076] The pressure in the internal volume 203 can also be limited by the pressure limiter 207.

[0077] The primary fluid 23 in the internal volume 203 can advantageously circulate by natural convection: due to the thermal gradient between the temperature of the fuel elements 204, which is higher than the temperature of the primary fluid 23 beneath these fuel elements, the primary fluid 23 circulates from the underside of the rack 205 containing the fuel elements 204, through the rack 205, between the fuel elements 204, rising and being heated. The primary fluid 23, thus heated, then descends between the rack 205 and the lateral walls 231 of the chamber 230, transferring all or part of its heat to the secondary fluid 22 contained in the volume 202 via the heat exchanger elements 235. The cooled primary fluid 23 then returns under the rack 205 to circulate again between the fuel elements 204.Thus, this natural convection phenomenon makes it possible to do without a forced circulation element, such as a pump, and in particular makes it possible not to have to electrically power such a forced circulation element.

[0078] The chamber 230 with the combustible elements 204 inside and the primary fluid 23 around the combustible elements 204 is part of a primary circuit of the conversion device 200.

[0079] The filling gas 21 of the intermediate volume 201 is preferably at a pressure sufficient to at least displace the liquid that is initially in this intermediate volume, which in this example is the water 24 from the storage pool 20. The pressure of the filling gas 21 is, for example, defined as a function of the height of the water 24 initially contained in the intermediate volume 201. The pressure of the filling gas 21 is, for example, a few bars.

[0080] When the conversion device 200 is shut down, the intercalated volume 201 is filled with water 24 from the storage pool 20, and the fuel elements 204 can be cooled. This allows for safe operation during shutdown, with the fuel elements 204 able to be cooled.

[0081] When the conversion device 200 is stopped, chamber 230 is unpressurized and is in contact with the water 24 of the storage pool 20. Even if the cover 206 is positioned, chamber 230 as well as the internal enclosure 210 can be cooled via thermal leaks created at the walls.

[0082] In operation of the conversion device 200, to produce electrical energy, the intercalated volume 201 is put under a defined pressure in filling gas 21.

[0083] The intercalated volume 201 filled with the filling gas 21 helps to form a thermal barrier around the inner enclosure 220, and thus helps to thermally insulate the hot source of the conversion device 200, formed by the primary fluid 23 heated by the fuel elements 204. This helps to increase the efficiency of the thermoelectric conversion.

[0084] During operation of the conversion device 200, after filling the intercalated volume 201 with the filling gas 21 at the defined pressure, and closing the external enclosure 210, internal enclosure 220, and chamber 230 with the lid 206, the pressure inside chamber 230 increases due to the heat from the fuel elements 204 and / or the pressure controller 208. The fuel elements 204 transfer all or part of their heat to the primary fluid 23 contained in the internal volume 203, and the heat exchanger elements 235 transfer the heat of the primary fluid 23 heated to the secondary fluid 22 contained in the volume 202. For example, the secondary fluid 22 is hotter in the upper part of the volume 202 than in the lower part of the volume 202. For example, the secondary fluid 22 is in vapor phase at least in the upper part of the volume 202.

[0085] Since the temperature of the primary fluid 23 in the volume 203 can reach 200°C for water, at a pressure greater than the saturated vapor pressure of water at 200°C to remain in the liquid state, it is possible to obtain a very good heat exchange between the primary fluid 23 and the secondary fluid 22, allowing the secondary fluid 22 to be vaporized.

[0086] Preferably, the secondary fluid 22 has a low vaporization energy (latent heat), to allow its vaporization with a minimum of energy, and its superheating.

[0087] The conversion device 200 further includes a secondary circuit 240, preferably closed, in which the secondary fluid 22 can circulate.

[0088] The secondary circuit 240 is immersed in the liquid 24 of the storage pool 20.

[0089] The secondary circuit 240 includes a turbine 241 (T) having a fluid inlet connected to the volume 202, for example to the upper part of the volume 202. For example, a recovery circuit 242 for the vaporized secondary fluid 22 connects the turbine 241 to the upper part of the volume 202. The vaporized secondary fluid 22 can thus drive the turbine 241. All or part of the thermal energy of the fuel elements 204 is thus indirectly transformed into mechanical energy.

[0090] The secondary circuit 240 further includes an alternator 243 (M) coupled to the turbine 241, enabling transforming mechanical energy into electrical energy E, which can be extracted from storage pool 20.

[0091] The secondary circuit 240 includes another enclosure 244 connected to the turbine 241, for example containing the turbine 241, or even the alternator 243. For example, the turbine 241 and the alternator 243 are installed in the upper part of the enclosure 244.

[0092] Another heat exchanger element 245 is integrated into at least one of the walls of the enclosure 244, the enclosure 244 with the heat exchanger element 245 being able to form a heat exchanger.

[0093] The heat exchanger element 245 is configured to extract all or part of the heat from the secondary fluid 23, in condensed / liquid form at the outlet of the turbine 241, to the liquid 24 in the storage pool 20 which forms the cold source. Thus, the secondary fluid 23 can be cooled.

[0094] The secondary fluid 23, thus cooled, circulates in the enclosure 244 to the lower part of the enclosure 244, from where it is reinjected into the volume 202, for example into the lower part of the volume 202, by a reinjection circuit 246.

[0095] Similar to the primary fluid 23, the secondary fluid 22 circulates advantageously in the secondary circuit 240 by natural convection, by thermal gradient, which makes it possible to do without a forced circulation element, such as a pump.

[0096] The generated electrical energy E can constitute a source of backup electricity and be used, for example, to at least partially power one or more pumps of a water cooling circuit 24 of the storage pool 20, which may be similar to the circuit of cooling 140 of figure 1. An electrical line (not shown) can connect the alternator 243 to the pump(s) of the cooling circuit.

[0097] The enclosures 210, 220 and 244 of the conversion device 200 can be fixed to the bottom of the storage pool 20, or at a distance from the bottom of the storage pool 20.

[0098] The storage pool 20 can be similar to the storage pool 124 in Figure 1, for example to the storage compartment 124A in Figure 1. The conversion device 200 can be installed in the storage compartment 124A in Figure 1.

[0099] Thus, even in the event of a loss of external power sources, one (or more) pump(s) can be operated to cool (at least partially) the water 24 in the storage pool 20, by utilizing the residual thermal power of the fuel elements 204 to cool other fuel elements stored in the storage pool 20, in particular by preventing the temperature of the water 24 from rising above a critical limit, for example, between 50 and 80°C. More broadly, the thermoelectric conversion device 200 can electrically supply any critical element to ensure the cooling of fuel elements stored in the storage pool 20.

[0100] The conversion device 200 is based on the principle of a Rankine cycle heat engine, in which the hot source is the primary fluid 23 circulating around the fuel elements 204, and the cold source is the liquid 24 in the storage pool 20. The cold source is, for example, at a maximum temperature of 50°C, or even 80°C in degraded conditions. The temperature difference between the hot source and the cold source, and thus the efficiency of The heat engine's performance can be optimized, for example, by increasing the temperature of the heat source, i.e., the primary fluid 23 in chamber 230 (and in the inner enclosure 220), under the effect of the temperature of the fuel elements 204, while controlling the pressurization of chamber 230 (and the inner enclosure 220). The heat source can thus reach, for example, a temperature of approximately 200°C.

[0101] The efficiency of the thermal machine can be advantageously optimized by the thermal insulation of the inner enclosure 220, made possible by the outer enclosure 210 and the filling gas 21 forming a thermal barrier between the outer enclosure 210 and the inner enclosure 220.

[0102] When the secondary fluid is water, it is possible to speak of a steam Rankine cycle heat engine. When the secondary fluid is an organic fluid, it is possible to speak of an organic Rankine cycle heat engine, or ORC.

[0103] The control of the thermal machine can be achieved by controlling the pressure of the filling gas 21 forming a thermal barrier, and the pressure in the chamber 230 (and the internal enclosure 220).

[0104] An example of the operation of the 200 conversion device is now described.

[0105] In this example, the fuel elements are organized into fuel assemblies, each fuel assembly consisting of a network of 17x17 pressurized water reactor (PWR) fuel rods.

[0106] We consider a quarter of a core (approximately 50 assemblies) whose residual thermal power in function of the time elapsed since unloading of the core, is given in Table 1 below (assumption fuel UO2) • The unloaded assemblies are stored in a storage pool.

[0107] [Table 1]

[0108] It is assumed that the unloaded assemblies have been unloaded for a week, which makes a residual thermal power to be extracted of approximately 3.5 MWth (assuming a quarter of the core is unloaded).

[0109] The primary fluid is assumed to be water, the hot source (water heated by the assemblies) is at a temperature of 200°C, and the velocity of the primary fluid in the chamber, or inner enclosure, is approximately 10 -2 at 10 -1 m / s (preliminary estimates).

[0110] The secondary fluid is considered to be water / vapor. The temperature in the cold source, i.e. the water in the storage pool, can vary between 30 and 80°C.

[0111] We consider a temperature difference AT between the hot source and the cold source to be equal to 100°C (although it may be between 120 and 170°C).

[0112] The heat exchanger between the primary and secondary circuits is considered to generate at the outlet of Water vapor with a purity greater than or equal to 100% (possibly superheated vapor).

[0113] The heat transfer coefficient h of the heat exchanger between the primary and secondary circuits is considered to be between 10 3 and 10 4 W / m 2 / K, for example equal to approximately 5000 W / m 2 / K, and its exchange surface S is between 5 and 10 m 2 for example equal to approximately 6 m 2 .

[0114] We can deduce the extracted thermal power Pth, as a first approximation, Pth = h-SAT, i.e. about 3 MWth, i.e. an efficiency of about 85%, much higher than the efficiency of the Carnot cycle without pressurization and at a hot source temperature of 50°C.

[0115] It is understood from the entire preceding description that the conversion device according to the embodiments makes it possible to increase the thermoelectric conversion efficiency, and this without the risk of contamination of the secondary circuit, the combustible elements being isolated from the secondary circuit, and in particular from the turbine and the alternator.

[0116] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will be apparent to them. In particular, a nuclear installation may include several thermoelectric conversion devices, for example, dedicated to one or more spent fuel pools.

[0117] Furthermore, although not shown in the figures, there may be a loading / unloading zone for fuel elements into / from the thermoelectric conversion device. This loading / unloading zone of the conversion device thermoelectric can be positioned in the storage pool.

[0118] Finally, the practical implementation of the described methods and variants is within the reach of a person in the trade, based on the functional indications given above.

Claims

29 DEMANDS 1. Fuel element cooling system in a nuclear installation, the cooling system comprising: - a storage pool (20) adapted to be at least partially filled with a liquid (24); and - a thermoelectric conversion device (200) from combustible elements (204) having residual thermal power, the thermoelectric conversion device being adapted to be positioned in the storage pool and comprising: - a primary circuit comprising a first enclosure (220) at least partially filled with a primary fluid (23); the first enclosure being adapted to be pressurized and to contain the combustible elements (204) immersed in the primary fluid, so as to transfer all or part of the heat from the combustible elements to said primary fluid;the first enclosure being adapted to be closed, so that, when the first enclosure is closed, the thermoelectric conversion device passes from a first state, in which the temperature of the primary fluid is at a first temperature and the pressure in the first enclosure is at a first pressure, to a second state, in which the temperature of the primary fluid is at a second temperature higher than the first temperature and the pressure in the first enclosure is at a second pressure higher than the first pressure, the primary fluid (23) remaining in the liquid state between the first included state and the second included state; - a first heat exchanger (230; 235) evaporator positioned in the first chamber so as to be immersed in the primary fluid around the elements 30 fuels; - a secondary circuit (240) configured for the circulation of a secondary fluid (22); the secondary circuit being coupled to the primary circuit by the first heat exchanger (230; 235) to transfer all or part of the heat from the primary fluid to the secondary fluid, so as to vaporize all or part of the secondary fluid; the secondary circuit comprising a turbo-alternator (241, 243) having a fluid inlet to introduce the vaporized secondary fluid into the turbo-alternator, so as to generate electrical energy (E); the liquid (24) forming a cold source of the secondary circuit.

2. Cooling system according to claim 1, wherein the transition from the first state to the second state is obtained at least partially by the heat of the combustible elements (204).

3. Cooling system according to claim 1 or 2, wherein the storage pool (20) contains other combustible elements, and is coupled to a cooling circuit for the liquid (24) of the storage pool, the cooling circuit comprising at least one critical element, such as a pump, electrically connected to the turbo-alternator to be powered at least partially by the electrical energy (E) generated.

4. Cooling system according to any one of claims 1 to 3, wherein the primary fluid (23) and the liquid (24) are water, the primary fluid being for example from pool water.

5. A cooling system according to any one of claims 1 to 4, wherein the thermoelectric conversion device further comprises a second enclosure (210) arranged around the first enclosure (220), defining an intercalated volume (201) between the second enclosure (210) and the first enclosure (220); the intercalated volume being intended to be filled with a filling gas (21), for example nitrogen or argon, forming a thermal barrier around the first enclosure.

6. Cooling system according to claim 5, wherein the filling gas (21) has a defined pressure to expel from the intercalated volume (201) a portion of liquid (24) from the storage pool (20).

7. Cooling system according to any one of claims 1 to 6, wherein the secondary circuit (240) comprises a third enclosure (244) connected to a fluid outlet of the turbogenerator (241, 243) and a second heat exchanger (245) configured to couple the secondary circuit (240) with the cold source (24) so ​​as to transfer all or part of the heat from the secondary fluid to the cold source; the second heat exchanger (245) being, for example, integrated into at least one wall of the third enclosure.

8. Cooling system according to any one of claims 1 to 7, wherein the thermoelectric conversion device further comprises a cover (206) configured to close the first enclosure (220); the cover being, for example, provided with a pressure limiter (207) so as to limit the pressure in the first enclosure.

9. Cooling system according to any one of claims 1 to 8, wherein the first heat exchanger (230; 235) comprises: a chamber (230) located in the first enclosure (220); the chamber defining an internal volume (203) at least partially filled with the primary fluid (23); the chamber being adapted to be pressurized and to contain the fuel elements (204) immersed in the primary fluid; a closed volume (202) between the chamber (230) and the first containment (220) being at least partially filled with the secondary fluid (22) and being connected to the secondary circuit (240); and - at least one heat exchanger element (235) positioned in at least one side wall (231) of the chamber (230) between the closed volume (202) and the internal volume (203).

10. Cooling system according to any one of claims 1 to 9, wherein the thermoelectric conversion device (200) further comprises a pressure controller (208) connected inside the first enclosure (220) to control the pressure in said first enclosure; the pressure controller being, for example: - a pressurizer or gas accumulator; and / or adapted to increase the pressure in the first chamber; and / or - adapted to limit the pressure in the first chamber to a maximum pressure limit, which is for example equal to approximately 20 bars.

11. Cooling system according to any one of claims 1 to 10, wherein the second pressure is greater than the saturated vapor pressure of the primary fluid (23) at the second temperature.

12. Cooling system according to any one of claims 1 to 11, wherein the secondary fluid (22) is water or an organic fluid, preferably having a low vaporization energy. 33 13. Cooling system according to any one of claims 1 to 12, further comprising a cooling circuit for the liquid (24) of the storage pool (20), the cooling circuit comprising a critical element, such as a pump, electrically connected to the thermoelectric conversion device to be powered by the generated electrical energy; the cooling circuit comprising a fluid inlet and outlet connected to the storage pool.

14. Cooling system according to any one of claims 1 to 13, further comprising: - at least two thermoelectric conversion devices (200), positioned in the storage pool (20); and / or - at least one fuel element loading zone (204) in the first enclosure (220) of each thermoelectric conversion device.

15. A method for implementing the thermoelectric conversion device (200) of the cooling system according to any one of claims 1 to 14, the method comprising: - the closure of the first enclosure (220) containing the combustible elements (204) immersed in the primary fluid (23); - the transition from the first state of the thermoelectric conversion device to the second state of the thermoelectric conversion device; the primary fluid (23) remaining in the liquid state between the first included state and the second included state; - in the second state, the transfer of all or part of the heat from the primary fluid (23) to the secondary fluid (22), so as to vaporize all or part of the secondary fluid; 34 the introduction into the turboalternator of the vaporized secondary fluid, so as to generate electrical energy (E).

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