Thermoelectric converter for a nuclear installation

Abstract

Thermoelectric converter for a nuclear installation This description relates to a thermoelectric conversion device (200) comprising: - a primary circuit comprising a first enclosure (220) filled with a primary fluid (23) and adapted to contain fuel elements (204), the first enclosure being adapted to be closed so that the 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; - a first heat exchanger (230; 235) immersed in the primary fluid in the first enclosure;- a secondary circuit (240) configured for the circulation of a secondary fluid (22), coupled to the primary circuit by the first heat exchanger to vaporize the secondary fluid, and comprising a turbo-alternator (241, 243) to generate electrical energy (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. Figure for the abstract: Fig. 2B;

Description

Title of the invention: Thermoelectric converter for a nuclear installation. Technical field

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

[0002] The present 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] The nuclear installation may be a nuclear power plant including at least one nuclear reactor and a storage pool. The nuclear reactor may 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 core of the nuclear reactor, the core being located within a vessel, in a circuit called the primary circuit. The fuel elements are, for example, grouped together in the form of fuel assemblies. A heat transfer fluid circulates in the vessel and in the core, from which it extracts the thermal energy released by the fission of the 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, so as to vaporize 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 reactors, the primary circuit coolant is water, and the secondary circuit coolant is also water. In pressurized water reactors (PWRs), the heat exchanger is incorporated into a steam generator, which is usually part of the primary circuit. The steam generator transfers all or part of the heat from the primary circuit coolant to the water in the secondary circuit, converting it into steam.

[0005] A nuclear installation, 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 the irradiation cycle of these fuel elements is completed, they are removed from the reactor core and vessel, then cooled, i.e., their residual thermal power is reduced, before being removed from the nuclear installation. Fuel elements with residual thermal power can be designated as "spent" or "hot" fuel elements.

[0007] In a nuclear reactor whose fuel elements are compatible with water, typically water-cooled reactors, the fuel elements can be 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 nuclear reactor core, while also ensuring the cooling of these fuel elements before they can be transported and removed from the nuclear facility. The removal of the residual thermal power of the fuel elements relies on their being kept in the water of the storage pool, and on a cooling circuit for this water comprising at least one heat exchanger coupled to a cold source, in order in particular to eliminate the risk of boiling this water.For example, fuel elements are stored at the bottom of the storage pool where a large volume of water actively circulates, cooled by one or more cooling circuits including one or more heat exchangers and one or more circulation pumps. If the water boils, the fuel elements could become submerged, meaning they are no longer completely covered by water (this is called dewatering), leading to varying degrees of degradation, the kinetics of which depend 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] Fig. 1 is a cross-sectional view representing in a simplified manner an example of a nuclear installation 100. This nuclear installation 100 comprises 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 [Fig.1]) of the reactor and fuel buildings rest on the side walls.

[0010] The reactor building 110 includes a vessel 113 which contains a nuclear reactor core containing fuel elements (core and fuel elements not shown in [Fig. 1]), and a water-filled reactor pool 114 for loading and unloading fuel elements into and from the 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 loading compartment 124C allows the fuel elements to be removed once cooled. For example, the cooled fuel elements are removed via a transport package loaded underwater in the loading compartment 124C, or docked beneath the loading compartment 124C.

[0014] The storage compartment 124A includes a storage rack 123 which can hold several fuel elements. The storage rack 123 comprises several locations, or slots, for example between 300 and 1200 locations depending on the type of nuclear reactor. The water depth in the 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 the storage compartment 124A. Indeed, when the pool water heats up in contact with the fuel elements, it participates less effectively in the cooling function of these fuel elements, and may even boil, which can lead to at least partial unflooding of these 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 heat exchanger 142, 142' communicates with a cold source (not shown in [Fig.l]) to which the water in the storage pool 124 releases all or part of its heat and cools continuously. .

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

[0017] Maintaining the water temperature in the storage pool below the threshold, typically around 50°C, is thus contingent upon the proper functioning of at least one of the cooling circuits, in particular 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 resulting in 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 both the main and backup equipment caused by the earthquake and flooding, and it was not possible to measure the temperature of the water in the storage pool and verify that it had reached 90°C, leading to a drop in the water level in the storage pool and the uncovering of the combustible elements.

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

[0019] US patent 9786396B2 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 entering 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 fed from the rack into a turbine where it is expanded to be converted into mechanical energy. The turbine is coupled to an alternator to produce electrical power for supplying electricity outside the pool. A pump (jet pump) coupled 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 cause radioactive contamination problems. Furthermore, the conversion system does not optimize the efficiency, particularly since the combustible 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] An 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 a portion 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 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 inclusive state and the second inclusive 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 comprises 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 inner enclosure; 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 combustible elements immersed in the fluid primary; a closed volume between the chamber and the first enclosure 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 inside 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 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.

[0037] According to one embodiment, the cooling system further comprises a cooling circuit for the storage pool liquid, 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.

[0038] 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.

[0039] One embodiment provides a method for implementing the thermoelectric conversion device as described above, the method comprising: - closing the first enclosure containing the fuel 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 and second states included; - 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 vaporized secondary fluid, so as to generate electrical energy. Brief description of the drawings

[0040] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0041] [Fig.1] is a cross-sectional view representing in a simplified manner an example of a nuclear installation 100;

[0042] [Fig. 2A] is a cross-sectional view representing a thermoelectric conversion device according to one embodiment; and

[0043] [Fig.2B] is a longitudinal cross-sectional view of the thermoelectric conversion device of [Fig.2A]. Description of the implementation methods

[0044] The same elements have been designated by the same reference numerals in the different figures. In particular, the 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.

[0045] For the sake of clarity, only the steps and elements necessary for understanding the described embodiments have been shown and are detailed. In particular, the components of a nuclear reactor are not detailed. Furthermore, not all the components of a storage pool, including one or more cooling circuits, are detailed, as they can be carried out by a person skilled in the art.

[0046] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") together, this means that these two elements can be connected or linked through one or more other elements.

[0047] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0048] Unless otherwise specified, the expressions "approximately", "roughly", and "on the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0049] In the following description, when reference is made to a reactor, it refers, unless otherwise specified, to a nuclear reactor.

[0050] 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.

[0051] The thermoelectric conversion device may be referred to in the following as the "conversion device". The thermoelectric conversion device may also be referred to as the "thermoelectric converter".

[0052] 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.

[0053] 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 displacing the liquid.

[0054] 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.

[0055] 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 elements fuels 204 are 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 arranged in the chamber 230.

[0056] The combustible elements 204 stored in the chamber 230 are preferably isolated from the rest of the pool 20.

[0057] The fuel elements 204 are in the form of irradiated or spent fuel assemblies, referred to as "hot," meaning that each assemblies have residual thermal power. Residual thermal power is understood to mean thermal power exceeding a limit, typically between 5 and 10 kWth, for example, approximately 7 kWth (for fuels of the uranium oxide (UO2) or MOX (mixed oxide) type). Thus, the fuel elements 204 can release heat, or thermal energy.

[0058] 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.

[0059] During operation, the 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).

[0060] The conversion device 200 further includes a cover 206 adapted to cover the outer enclosure 210, the inner enclosure 220, and the chamber 230 to close them, preferably in a hermetic 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 replaced after the combustible elements 204 have been introduced into the chamber 230.

[0061] A person skilled in the art will know how 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 equipped 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.

[0062] 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.

[0063] For example, the heat exchanger elements 235 are integrated into the side walls 231 of the chamber 230.

[0064] 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.

[0065] 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.

[0066] The chamber 230 can form a heat exchanger comprising the heat exchanger elements 235.

[0067] In the example shown, the outer enclosure 210, the inner enclosure 220, and the chamber 230 each have a substantially 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.

[0068] 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.

[0069] The pressure in the 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 the 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.

[0070] The pressure controller 208 can also have the function of raising the pressure inside the chamber 230, for example before the temperature of the primary fluid 23 in the chamber 230 rises, to start pressurizing the chamber 230 before the combustible elements 204 raise the pressure in the chamber 230.

[0071] For example, for water, the upper pressure limit can be set to be greater than the saturated vapor pressure at 200°C, i.e., the upper pressure limit can be greater than approximately 15 bar, so as to maintain the water in a liquid state without it starting to boil. For example, the upper pressure limit is approximately 20 bar for water.

[0072] Pressurizing chamber 230 increases the temperature of the primary fluid 23 in chamber 230, while maintaining this primary fluid in a liquid state. For example, when the primary fluid 23 is water, the temperature of the primary fluid 23 in chamber 230 can exceed 50°C, for example, can reach 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.

[0073] The ability to increase the temperature of the primary fluid 23 makes it possible to increase 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, there would be a very low efficiency (Carnot cycle), typically less than 10%.

[0074] Limiting the pressure in chamber 230 can advantageously allow us to remain below a regulatory threshold, for example related to the regulations for pressure equipment (ESP), for example around 20 bars.

[0075] The pressure in the internal volume 203 can also be limited by the pressure limiter 207.

[0076] The primary fluid 23 in the internal volume 203 can advantageously circulate by natural convection: due to a 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 beneath the rack 205 to circulate again between the fuel elements 204.Thus, this natural convection phenomenon eliminates the need for a forced circulation element, such as a pump, and in particular avoids the need to electrically power such a forced circulation element.

[0077] 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.

[0078] The filling gas 21 of the intercalated volume 201 is preferably at a pressure sufficient to at least expel the liquid that is initially in this intercalated 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 intercalated volume 201. The pressure of the filling gas 21 is, for example, a few bars.

[0079] 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 when shut down, with the fuel elements 204 able to be cooled.

[0080] When the conversion device 200 is stopped, the chamber 230 is unpressurized and is in contact with the water 24 of the storage pool 20. Even if the cover 206 is positioned, the chamber 230 as well as the internal enclosure 210 can be cooled via thermal leaks created at the walls.

[0081] 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.

[0082] The intercalated volume 201 filled with the filling gas 21 allows to form a thermal barrier around the inner enclosure 220, and thus to help to thermally insulate the hot source of the conversion device 200, formed by the primary fluid 23 heated by the fuel elements 204. This makes it possible to increase the efficiency of the thermoelectric conversion.

[0083] 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 from the heated primary fluid 23 to the secondary fluid 22 contained in volume 202. For example, the secondary fluid 22 is hotter in the upper part of volume 202 than in the lower part. For example, the secondary fluid 22 is in a vapor phase at least in the upper part of the volume 202.

[0084] 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.

[0085] Preferably, the secondary fluid 22 has a low vaporization energy (latent heat), to allow its vaporization with a minimum of energy, and its superheating.

[0086] The conversion device 200 further includes a secondary circuit 240, preferably closed, in which the secondary fluid 22 can circulate.

[0087] The secondary circuit 240 is immersed in the liquid 24 of the storage pool 20.

[0088] 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.

[0089] The secondary circuit 240 further includes an alternator 243 (M) coupled to the turbine 241, enabling the transformation of mechanical energy into electrical energy E, which can be extracted from the storage pool 20.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] Similar to the primary fluid 23, the secondary fluid 22 advantageously circulates 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.

[0095] The generated electrical energy E can constitute a backup power source 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 cooling circuit 140 of [Fig. 1]. A power line (not shown) can connect the alternator 243 to the pump(s) of the cooling circuit.

[0096] 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.

[0097] The storage pool 20 can be similar to the storage pool 124 of [Fig. 1], for example, to the storage compartment 124A of [Fig. 1]. The conversion device 200 can be installed in the storage compartment 124A of [Fig. 1].

[0098] 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 exploiting 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.

[0099] 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 and cold sources, and thus the efficiency of the heat engine, can be optimized, for example, by increasing the temperature of the hot source, i.e., the primary fluid 23 in the chamber 230 (and in the inner enclosure 220), under the effect of the temperature of the fuel elements 204, while controlling the pressurization of the chamber 230 (and the inner enclosure 220). The hot source can thus reach, for example, a temperature of approximately 200°C.

[0100] The efficiency of the heat engine can advantageously be 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.

[0101] 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, from the English "Organic Rankine Cycle".

[0102] 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).

[0103] An example of the operation of the conversion device 200 is now described.

[0104] In this example, the fuel elements are organized in the form of fuel assemblies, each fuel assembly consisting of an array of 17x17 pressurized water reactor (PWR) fuel rods.

[0105] Considering a quarter of a core (approximately 50 assemblies) whose residual thermal power as a function of the time elapsed since core unloading is given in Table 1 below (assuming UO2 fuel). The unloaded assemblies are stored in a storage pool.

[0106] [Tables 1] 0.5 day 1 day 3 days 1 week 1 month 3 months 6 months 1 year Core (MWth) 36.4 28.6 20.7 14.5 7.09 3.74 2.06 0.78 Assembly (kWth) 235 185 134 94 46 24 13 5

[0107] It is assumed that the unloaded assemblies have been unloaded for one week, which makes a residual thermal power to be extracted of approximately 3.5 MWth (assuming a quarter of the core is unloaded).

[0108] The primary fluid is considered to be water, the hot source (water heated by the assemblies) is at a temperature of 200°C, the velocity of the primary fluid in the chamber, or the inner enclosure, is between approximately 102 and 101 m / s (first estimates).

[0109] The secondary fluid is considered to be water / steam. The temperature in the cold source, i.e. the water in the storage pool, can vary between 30 and 80°C.

[0110] We consider a temperature difference AT between the hot source and the cold source equal to 100°C (although it may be between 120 and 170°C).

[0111] It is assumed that the heat exchanger between the primary circuit and the secondary circuit generates water vapor at the outlet with a quality greater than or equal to 100% (possibly superheated vapor).

[0112] It is assumed that the heat exchange coefficient h of the heat exchanger between the primary and secondary circuits is between 103 and 104 W / m2 / K, for example equal to about 5000 W / m2 / K, and its exchange surface S is between 5 and 10 m2, for example equal to about 6 m2.

[0113] We can deduce the extracted thermal power Pth, in 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.

[0114] 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.

[0115] 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 become apparent to those skilled in the art. In particular, a nuclear installation may include several thermoelectric conversion devices, for example, dedicated to one or more fuel storage pools.

[0116] Furthermore, although not shown in the figures, there may be a loading / unloading area for fuel elements into / from the thermoelectric conversion device. This loading / unloading area of ​​the thermoelectric conversion device may be located in the storage pool.

[0117] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

1.

2. Demands Thermoelectric conversion device (200) from combustible elements (204) having residual thermal power, the thermoelectric conversion device 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 enclosure so as to be immersed in the primary fluid around the combustible elements; - 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 for introducing the vaporized secondary fluid into the turbo-alternator, so as to generate electrical energy (E); the thermoelectric conversion device being installed in a storage pool (20) at least partially filled with a liquid (24) forming a cold source of the secondary circuit. Thermoelectric conversion device (200) 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. Thermoelectric conversion device (200) according to claim 1 or 2, the storage pool (20) containing other combustible elements, and being 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. Thermoelectric conversion device 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 swimming pool water.

5. Thermoelectric conversion device according to any one of claims 1 to 4, further comprising a second enclosure (210) disposed 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. Thermoelectric conversion device 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. Thermoelectric conversion device 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. A thermoelectric conversion device according to any one of claims 1 to 7, further comprising a lid (206) configured to close the first enclosure (220); the lid being for example equipped with a pressure limiter (207) so as to limit the pressure in the first enclosure.

9. Thermoelectric conversion device according to any one of claims 1 to 8, wherein the first heat exchanger (230; 235) comprises: - a chamber (230) located in the inner 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 enclosure (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. Thermoelectric conversion device according to any one of claims 1 to 9, further comprising 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 a gas accumulator; and / or - adapted to increase the pressure in the first enclosure; and / or - adapted to limit the pressure in the first enclosure to a maximum pressure limit, which is for example equal to about 20 bar.

11. Thermoelectric conversion device 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. Thermoelectric conversion device 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.

13. Fuel element cooling system in a nuclear installation, the cooling system comprising: - a storage pool (20) at least partially filled with a liquid (24), for example water; - a thermoelectric conversion device (200) according to any one of claims 1 to 12, positioned in the storage pool.

14. Cooling system according to claim 13, 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.

15. Cooling system according to claim 13 or 14, further comprising: - at least two thermoelectric conversion devices according to any one of claims 1 to 12, 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.

16. A method for implementing the thermoelectric conversion device (200) according to any one of claims 1 to 12, the method comprising: - closing the first enclosure (220) containing the fuel elements (204) immersed in the primary fluid (23); - passing 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 inclusive state and the second inclusive state; - in the second state, transferring 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; - introducing the vaporized secondary fluid into the turbogenerator, so as to generate electrical energy (E).

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