Device for filtering surface impurities for a coolant of a nuclear reactor vessel

Abstract

The invention relates to a device (12) for filtering surface impurities for a coolant of a nuclear reactor vessel, the device extending between a first end (13) and a second end (14) and comprising: a) a handling interface (15) located at the first end, b) an enclosure (16) defining an internal space (17) and provided with at least one intake opening (18) for the coolant and with a discharge opening (19) for the coolant, c) a filter (20) accommodated in the internal space of the enclosure between the intake opening and the discharge opening, and d) a sheath (21) for the passage of an actuator shaft from the first end to the second end in the internal space through the filter, wherein the enclosure is configured to move freely in translation in the longitudinal direction with respect to the sheath in order to allow the intake opening to be kept at a surface of the coolant by flotation in the coolant.

Description

[0001] Surface impurity filtration device for a heat transfer fluid in a nuclear reactor vessel

[0002] CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0003] This application claims priority from French patent application no. 2413580 filed on December 6, 2024, the content of which is incorporated herein by reference.

[0004] DOMAIN

[0005] This presentation relates to the field of nuclear reactors, in particular fast neutron reactors (FNRs), and especially those of the integrated or loop type. Such reactors can be cooled by a coolant, typically a liquid metal, and most particularly lead or a lead-bismuth mixture. These are referred to as lead-cooled fast neutron reactors (LFRs), also known by their English acronym, "Lead Fast Reactor."

[0006] More specifically, the presentation concerns the removal of impurities located on the surface of the coolant of a nuclear reactor.

[0007] The presentation applies in particular to so-called small modular reactors (SMRs), also known by the English name "Small Modular Reactor" abbreviated as SMR. These are small-sized, low-power nuclear fission reactors — on the order of 10 to 1000 MWth.

[0008] STATE OF THE ART

[0009] The operating principle of fast neutron reactors has been known for several decades. A fast neutron reactor is a nuclear reactor that uses fast neutrons (with a kinetic energy greater than 0.907 MeV), as opposed to thermal neutrons (with a kinetic energy less than 0.025 MeV). Unlike conventional second- and third-generation nuclear reactors, the core of a fast neutron reactor is not moderated (there is no slowing down or thermalization of the neutrons). Furthermore, although other technologies have been studied, the majority of fast neutron reactors use liquid metal as a coolant, particularly lead or a lead-bismuth mixture, which notably has a high boiling point.Lead-cooled nuclear reactors typically consist of a reactor vessel containing the core, which is generally made up of a large number of fuel assemblies. Heat is extracted by circulating liquid metal (primary circuit) through the core using pumping systems. This heat is transferred to a secondary circuit via a heat exchanger, typically a steam generator (SG) to produce steam. This steam is then used to drive a turbine, which converts it into mechanical energy, which is in turn converted into electrical energy.

[0010] The reactor vessel is sealed by a slab to prevent the molten metal from coming into contact with the outside air. All components (heat exchangers, pumps, pipes, etc.) pass vertically through this slab so they can be removed by lifting them vertically using a lifting device. The circuit layout differs depending on whether the reactor is "integrated" or "looped." It should be noted that this description primarily concerns "integrated" type fast neutron reactors, but it also applies to "looped" type fast neutron reactors.

[0011] During the operation of a nuclear reactor, the nuclear fuel contained in the fuel assemblies is depleted, so the fuel must be replaced periodically.

[0012] During its circulation within the reactor vessel and as it passes through the core, the molten metal can gradually become contaminated with impurities. These impurities are of various kinds and originate primarily from corrosion products. It is essential to remove them as they appear to prevent incidents, particularly to avoid degrading the thermal performance of steam generators and residual heat removal (RHR) systems, and to ensure proper cooling of all parts of the core. Complex devices exist for removing impurities, capable of extracting a portion of the heat transfer fluid (i.e., molten metal), cooling it, and passing it through a filter made of metallic fibers, such as stainless steel fibers (see, in particular, documents FR2518301 Al, FR2246942 Al, FR2624032 Al, and FR2573563 Al).Impurities in the heat transfer fluid preferentially precipitate onto the metal fibers of the filter if the heat transfer fluid temperature is sufficiently low. This results in the cold trapping of impurities.

[0013] Such devices can be immersed in the reactor vessel and are then called "integrated", or alternatively they can be mounted outside the vessel on a bypass coolant circuit and are then called "derived".

[0014] Other filter-based devices (FR3018459 Al, WO 2023 / 018350 Al) positioned, for example, at the core level have also been developed.

[0015] Finally, patent CN111739671 proposes using a magnetic separator to clean lead-based coolant of impurities.

[0016] All these devices are complex in their structure; this increases the costs of implementing and maintaining these devices.

[0017] When the heat transfer fluid is lead or a lead-bismuth mixture, impurities tend to rise to the surface due to the density difference between the fluid and the impurities. The devices described above are relatively effective at removing impurities circulating within the heat transfer fluid. However, they are not suitable for impurities floating on the surface.

[0018] In accidental operating conditions of a reactor using a liquid metal such as lead as a coolant, air may enter the vessel, resulting in the production of corrosion products that will constitute impurities.

[0019] The chemical composition of these impurities typically includes iron, nickel, aluminum, chromium, or long carbon chains (lubricants degraded at high temperatures). Generally, these impurities are less dense than the heat transfer fluid, particularly when the latter is lead or a lead-bismuth mixture. They therefore migrate to the surface of the heat transfer fluid and are located at a maximum depth of a few centimeters below the surface, or even a maximum depth of 5 millimeters or less.

[0020] Patent CN113284640 proposes a surface impurity removal device divided into two parts: an impurity trapping assembly and a heat transfer fluid extraction system. The extraction system includes a bell-shaped inlet immersed in the heat transfer fluid. The extraction system draws the heat transfer fluid through this inlet and transfers it to the trapping assembly, which is configured to filter the impurities. The filtered heat transfer fluid then returns to the reactor. This device has a very large footprint in the area at the top of the reactor vessel. Therefore, such a device is particularly unsuitable for small modular reactors (SMRs). Furthermore, installing this type of device is complex and time-consuming.

[0021] Patent RU181860 U1 describes a housing with openings in the surface of the heat transfer fluid and a screw conveyor whose upper part is located above the surface. The screw is configured to draw the heat transfer fluid towards the lower part of the housing, where impurities are trapped in a filter medium. Openings in the lower part of the housing allow the filtered heat transfer fluid to exit.

[0022] Finally, the EFIT (lead-cooled European Facility for Industrial Transmutation) project proposes a gravitational filtration system (Eighth International Topical Meeting on Nuclear Applications and Utilization of Accelerators. Pocatello, Idhao, USA' 2007.

[0023] These devices allow for continuous cleaning of the heat transfer fluid surface. The main drawback of these filtration systems is that they cease to function if the heat transfer fluid level changes.

[0024] Publication CN114496323 A describes a filtration device comprising a casing placed in the reactor's primary heat transfer fluid. This casing includes an axially movable intake section and a filter section integrated into the casing. The heat transfer fluid enters the filter section through the intake section, which has an inlet opening. This intake section floats in the heat transfer fluid, so the inlet opening is always at the fluid's free surface. When the fluid level fluctuates, the inlet opening moves axially with the intake section, thus remaining at the free surface. The filter section, on the other hand, is fixed and does not move with the level variations. The filter is therefore not located near the free surface, where impurities tend to concentrate.Therefore, especially when a liquid metal is used as a heat transfer fluid and the density differences between the fluid and the impurities are significant, the impurities are likely to settle far from the filter, making their capture and retention extremely difficult, if not impossible.

[0025] The purpose of this presentation is to propose a device for removing surface impurities from liquid coolant metal which continues to function in the event of a variation in the level of the heat transfer fluid.

[0026] More generally, the presentation aims to propose a compact device to simplify and improve surface impurity filtration operations, while ensuring a high level of safety, in particular by preventing these impurities from being carried into the reactor core during a fuel change operation.

[0027] EXPOSED

[0028] One aim of this presentation is to propose a device for removing surface impurities from the liquid coolant metal in a reactor vessel, the device continuing to function in the event of a variation in the level of the coolant.

[0029] The objective is achieved by means of an impurity filtration device for a heat transfer fluid in a nuclear reactor vessel, the device extending in a longitudinal direction between a first end and a second end and comprising: a) a handling interface configured to allow manipulation of the filtration device by a handling system, preferably a fuel assembly handling system, the handling interface being located at the first end, b) an enclosure defining an internal space, and provided with at least one heat transfer fluid inlet opening and one heat transfer fluid outlet opening which define a flow direction in the longitudinal direction from the upstream inlet opening to the downstream outlet opening, c) a filter housed in the internal space of the enclosure between the inlet opening and the outlet opening.and d) a sleeve for the passage of an actuator shaft from the first end to the second end in the internal space through the filter, wherein the enclosure is configured to move freely in translation in the longitudinal direction relative to the sleeve in order to maintain the inlet opening to a surface of the heat transfer fluid by flotation in the heat transfer fluid.

[0030] Such a device is advantageously and optionally complemented by the following various features, taken alone or in combination:

[0031] - the device is configured to be placed in place of a fuel assembly in the nuclear reactor vessel for access to a hot collector of the vessel or alternatively in place of a waste heat removal system for access to a hot collector of the vessel;

[0032] - the enclosure includes spaces for receiving ballast elements configured to modify the buoyancy of the enclosure in the heat transfer fluid;

[0033] - the filter blocks the flow direction in the internal space;

[0034] - the filter comprises filter elements following one another in the direction of flow, a first filter element upstream of a second filter element having a mesh size greater than a mesh size of the second filter element;

[0035] - the first filter element leaves a first opening free in the direction of flow, said first opening being opposite the second filter element in the direction of flow, and the second filter element leaves a second opening free in the direction of flow, opposite the first filter element; and

[0036] - the sheath is vertically centered in relation to the enclosure.

[0037] The presentation also covers an impurity filtration system for a heat transfer fluid in a nuclear reactor vessel, comprising:

[0038] - a filtration device as described above, and

[0039] - an actuator configured to cause the flow of heat transfer fluid in the flow direction from the inlet opening to the outlet opening, the actuator comprising a motor disposed outside the internal space beyond the first end and a shaft extending from the motor into the internal space downstream of the filter through the sleeve.

[0040] As an option for this system, the shaft is terminated by a propeller located between the heat transfer fluid inlet opening and the outlet opening.

[0041] The presentation also relates to a nuclear reactor vessel comprising a heat transfer fluid and a heat transfer fluid impurity filtration system as previously described, and a nuclear reactor comprising such a vessel.

[0042] Finally, the presentation also covers a process for filtering impurities from a heat transfer fluid in a nuclear reactor vessel, the process comprising the following steps:

[0043] - to attach to the nuclear reactor vessel a filtration system according to one of the two preceding claims, so as to insert the filtration device into the vessel, the inlet opening of the containment being located, by flotation, at a surface of the vessel's heat transfer fluid; and

[0044] - operate the actuator so as to create a flow of heat transfer fluid through the filter between the inlet opening and the outlet opening.

[0045] Advantageously and optionally, the impurity filtration is carried out to a depth from the surface of 5 centimeters or less, preferably 1 centimeter or less, and even more preferably 5 millimeters or less. DESCRIPTION OF FIGURES

[0046] Other features and advantages will become apparent from the following description, which is purely illustrative and not limiting, and should be read in conjunction with the accompanying drawings in which: Figure 1 schematically illustrates a cross-section of a lead-cooled fast neutron nuclear reactor; Figures 2, 3 and 4 schematically illustrate a device for filtering impurities in the coolant; Figures 5, 6 and 7 schematically illustrate a filter; Figure 8 schematically illustrates a system for filtering impurities in the coolant; and Figure 9 schematically illustrates a process for filtering impurities in the coolant.

[0047] DETAILED DESCRIPTION OF THE INVENTION

[0048] Nuclear reactor vessel

[0049] Figure 1 describes an example of a nuclear reactor vessel. In relation to Figure 1, a nuclear reactor main vessel 1 comprises a vessel body 2 configured to receive a coolant 4, which may be a molten metal, such as molten lead or a lead-bismuth mixture. The coolant extends vertically within the vessel from the bottom to a free level 27. The vessel 1 includes a sealing slab 48 that hermetically seals the vessel body 2.

[0050] The closing slab 48 includes a fixed part and a rotating plug 5 incorporating a through passage 6 for handling fuel.

[0051] Core 3 comprises a plurality of fuel assemblies arranged vertically and parallel to each other.

[0052] When the reactor is in operation, the core 3 is immersed inside the coolant 4. The main vessel 1 includes a pumping system 7 which is configured to set the coolant 4 in motion.

[0053] Tank 1 includes:

[0054] - a hot collector 8 which is located above the core 3 and below the pumping system 7,

[0055] - a heat exchanger 9,

[0056] - a cold manifold 10, which is located above the core 3 and below the pumping system 7, and

[0057] - a wall 11 which separates the hot collector 8 and the cold collector 10.

[0058] The cold collector 10 is fluidly connected to the hot collector via the exchanger 9. The cold collector is also fluidly connected to the bottom of the core 3.

[0059] Tank 1 may also include a waste heat removal system located above the hot collector 8 or above the cold collector 10. The waste heat removal system passes through the closing slab 48. The closing slab 48 has a secondary passage which allows the waste heat removal system to be inserted through the closing slab.

[0060] Impurity filtration device

[0061] Referring to Figures 2 and 3, a filtration device 12 extends primarily along a longitudinal axis L between a first end 13 and a second end 14. When the device is used in the reactor, the longitudinal axis L corresponds to the vertical direction, with the first end 13 located above the second end 14. The filtration device 12 extends primarily along the longitudinal axis L, meaning that its longitudinal dimension along the axis L is greater than its dimension along transverse directions orthogonal to the axis L. Advantageously, the horizontal dimension of the filtration device is less than or equal to the horizontal dimension of a fuel assembly. Thus, a fuel assembly can be replaced by a filtration device in the reactor vessel. Advantageously, the device is configured to be placed in place of a fuel assembly in the nuclear reactor vessel.The device therefore has a footprint in each dimension that is less than or equal to the footprint of an assembly. Furthermore, the upper end of the device 12 is geometrically and functionally sufficiently close to the upper end of a fuel assembly to allow a fuel assembly handling system to manipulate a filtration device 12.

[0062] The device can also be configured to be placed in place of a waste heat removal system located above the hot collector 8 or above the cold collector 10.

[0063] The device can be configured to be placed in place of a fuel assembly or in place of a waste heat removal system for access to the hot collector of the tank.

[0064] The filtration device 12 is centered on the longitudinal axis L, which is therefore also called the central axis L.

[0065] The vertical dimension of a filtration device can be chosen to be greater than the vertical distance separating the free level 27 of the heat transfer fluid in the tank 1 and the closing slab 48. In this way, when the device is hung on the closing slab, the bottom of the device is immersed in the heat transfer fluid 4.

[0066] The device 12 includes a handling interface 15 for a fuel assembly handling system, the handling interface being located at the first end 13. The interface 15 of the device 12 can correspond to a handling interface for a fuel assembly. The shape of the interface 15 allows fuel assembly lifting equipment to move the device 12, for example, between a storage location and a usage location. The dimension of the handling interface 15 in a horizontal direction is greater than or equal to 180 mm and less than or equal to 210 mm.

[0067] The interface 15 advantageously features a central orifice that passes vertically through the interface. This central orifice leaves the central axis free of any material. The orifice is configured, for example, to allow passage for a shaft of an actuator, such as a mechanical shaft or a gas conduit, with this actuator shaft centered on the central axis.

[0068] Located between interface 15 and second end 14, the device 12 includes a sleeve 21 for the passage of an actuator shaft from first end 13 to second end 14. The sleeve 21 is rigidly fixed to interface 15, for example, via a relay piece 24 located longitudinally between interface 15 and sleeve 21. Both sleeve 21 and relay piece 24, when present, leave the central axis free of any material. Sleeve 21 extends longitudinally from second end 14 to an end 22 of the sleeve. Advantageously, the sleeve has a radial cross-section with respect to the annular central axis. The sleeve extends radially from an inner cylinder to an outer cylinder, both cylinders having a circular radial cross-section.

[0069] The device 12 includes an enclosure 16 defining an internal space 17. The enclosure 16 radially surrounds the sleeve 21 with respect to the central axis. In other words, the sleeve 21 is located between the central axis and the enclosure 16.

[0070] Advantageously, the sheath is vertically centered with respect to the enclosure. This means that the sheath 21 is centered on the central axis L and the enclosure 16 is also centered on the central axis L.

[0071] The enclosure 16 extends longitudinally from one end 29 to the second end 14. The second end 14 of the device 12 is defined by the end of the enclosure 16.

[0072] The end 29 of the enclosure 16 is located longitudinally between the second end 14 and the handling interface 15.

[0073] The end 22 of the sleeve 21 is located longitudinally between the second end 14 and the end 29 of the enclosure 16

[0074] The enclosure 16 advantageously defines a drain 26 at the second end 14. The drain 26 is an opening in the enclosure 16 which connects the internal space 17 and the outside of the enclosure 16. The drain 26 is oriented longitudinally and centered on the central axis L. The enclosure 16 includes an external radial wall 28, which is advantageously cylindrical, centered on the central axis and of circular radial section. The enclosure 16 comprises an internal radial wall 34 which is located between the external radial wall 28 and the sleeve 21. The material of the enclosure 16 is located between the external radial wall 28 and the internal radial wall 34. The internal space 17 is located between the internal radial wall 34 and the sleeve 2. The internal space 17 communicates fluidly with the exterior of the enclosure 16. The enclosure 16 has at least one inlet opening 18 and one outlet opening 19.

[0075] The openings are configured to allow passage of the heat transfer fluid which moves radially, that is to say, approaching the central axis L or moving away from the central axis L.

[0076] The enclosure 16 advantageously comprises, for defining the inlet opening 18, two vertical walls and two horizontal walls joining the outer radial wall 28 and the inner radial wall 34. A vertical wall is a wall parallel to the central axis L. A horizontal wall is a wall orthogonal to the central axis L. Referring to Figure 5, the enclosure 16 comprises the vertical wall 31 and the horizontal wall 32, which define the inlet opening 18. The horizontal wall 32 is the lowest vertically aligned horizontal wall and corresponds to the lower horizontal wall. The lower horizontal wall defines the level at which impurities are admitted.

[0077] When the enclosure 16 has several inlet openings 18, they are located longitudinally in the same position. The inlet openings are then distributed radially around the central axis; advantageously, they are distributed regularly around the central axis L. Advantageously, the enclosure 16 comprises a plurality of lower horizontal walls which are all vertically located in the same position, a position which defines the level of impurity admission.

[0078] When the enclosure 16 has several discharge openings 19, they are located longitudinally in the same position. The discharge openings 19 are then distributed radially around the central axis. The inlet opening 18 is located longitudinally between the first end 13 and the discharge opening 19.

[0079] The evacuation opening 19 is located longitudinally between the intake opening 18 and the second end 14.

[0080] The inlet opening 18 and the outlet opening 19 define between them a flow direction in the longitudinal direction L from the inlet opening 18 upstream to the outlet opening 19 downstream.

[0081] The device 12 includes a filter 20 housed in the internal space 17 of the enclosure 16 between the inlet opening 18 and the outlet opening 19. The filter is located radially between the sleeve 21 and the external wall 28 of the enclosure 16. The filter 20 is located longitudinally between the inlet opening 18 and the outlet opening 19.

[0082] The sheath 21 passes longitudinally through the filter 20, the sheath extends longitudinally in the internal space 17 through the filter 20.

[0083] Filter 20 is rigidly fixed to enclosure 16.

[0084] The enclosure 16 is configured to move freely in longitudinal translation relative to the sleeve 21. The enclosure 16 and therefore the filter 20 are mounted freely in translation along the central axis L relative to the sleeve 21.

[0085] Advantageously, the filtration device includes a key system configured to prevent rotation about the vertical axis of the housing 16 relative to the sleeve 21. For example, the sleeve 21 has a groove parallel to the vertical axis on its outer surface, and the housing includes a projection parallel to the vertical axis with a shape complementary to the shape of the groove. The housing can be mounted on the sleeve so that the projection is inserted into the groove. The projection thus positioned prevents rotation of the housing 16 relative to the sleeve 21.

[0086] We can thus define within the device 12 a fixed part and a mobile part. The fixed part is the one intended to be rigidly fixed to the closing slab 48. The fixed part includes in particular the first end 13, the handling interface 15, the sleeve and possibly the relay part 24.

[0087] The moving part is the part designed to slide relative to the fixed part along the central axis L. The moving part includes in particular the second end 14, the enclosure 16 and the filter 20.

[0088] The presence of a fixed and a moving part in the device 12 allows the moving part to float on the surface of the heat transfer fluid. Thus, even if the free level 27 of the heat transfer fluid 4 in the tank varies, the moving part remains on the surface of the heat transfer fluid 4. The filtration device therefore maintains the same filtration performance if the free level 27 of the heat transfer fluid 4 in the tank varies.

[0089] The sheath includes two rings that protrude radially outwards. The two rings are located at two different heights.

[0090] The moving part includes a stop located vertically between the two rings of the sheath. A horizontal projection of the stop has a non-zero overlap with a horizontal projection of each of the two rings. A horizontal projection is understood here as a projection along the vertical direction onto a horizontal plane.

[0091] The stop and the two rings are configured to limit the vertical translational travel of the moving part.

[0092] Ballast

[0093] The enclosure 16 includes spaces 30 for receiving ballast elements. The ballast elements are configured to modify the buoyancy of the enclosure in the heat transfer fluid. For example, and as illustrated in Figure 3, the enclosure 16 includes at its upper end 29 one or more rods 30 that extend vertically upwards from the body of the enclosure 16. The rods 30 can, in particular, be evenly distributed around the central axis L. Advantageously, the spaces 30 are aligned vertically with the body of the enclosure 16 so that the projection of the enclosure 16 in a horizontal plane is not extended when the spaces 30 are added. This means that the horizontal projection of the spaces 30, and in particular of the rods 30, is entirely contained within the horizontal projection of the enclosure 16.

[0094] Alternatively, and with reference to Figures 4 and 6, the ballast elements 49 are discs, or cylindrical pieces with a circular cross-section, configured to be received at the upper end 19 of the housing 16. The upper end 19 of the housing 16 and the discs 49 are shaped so that a disc 49 placed on the housing 16 remains stably positioned. For example, the upper end 19 and the disc 49 have complementary reliefs 52. A second disc 49 can be placed on top of a first disc 49 already received at the upper end 19 of the housing 16. The ballast elements 49 are thus simply placed on the housing 16 without being rigidly fixed to either the housing 16 or the sleeve 21. The ballast elements can therefore be lifted along the sleeve 21.

[0095] Still in this alternative mode, a first ballast element 49 can define the inlet opening 18. In this case, it is the enclosure 16 and the first ballast element 19 placed on the enclosure 16 that together define the internal space 17. The first ballast element 49 is then a disc which has a vertical height sufficient to define the inlet opening 18. The first ballast element 49 is hollow so that the inlet opening communicates fluidly with the interior of the enclosure 16.

[0096] The device can also be configured so that the filter 20 is positioned vertically between the first ballast element 49 and the enclosure 16. The filter can, for example, be inserted into a groove 50 in the enclosure 16. The groove 50 corresponds to a variation in the internal radius of the internal radial wall 34. The groove 50 thus prevents the filter 20 from moving vertically from top to bottom. Furthermore, the first ballast element 49 can include a lip 51 that covers the upper end of the filter 20, thereby preventing the filter 20 from moving vertically from bottom to top.

[0097] This design facilitates filter replacement. Since the first ballast element 49 is not mechanically connected to the housing 16, it can be lifted to allow the removal of the filter 20 for replacement with a clean one. This ease of replacement is also achieved when multiple ballast elements 49 are present. It is thus possible to add or remove ballast elements from the housing 16. Such addition or removal allows for modification of the buoyancy of the housing 16 within the heat transfer fluid 4.

[0098] In particular, it is possible to adjust the buoyancy of the chamber 16 to set a distance between the free level 27 of the heat transfer fluid 4 in the tank and the impurity intake level defined by the lower horizontal wall(s) of the intake opening. Referring to Figure 5, the free level 27 is represented by a dashed curved line, and the distance 33 corresponds to the vertical distance between the free level and the lower horizontal wall 32 of the intake opening 18. This distance 33 corresponds to the depth at which impurities are admitted by the filtering device 12.

[0099] Depending on the chosen depth of impurity intake, the device

[0100] 12 allows for the treatment of a more or less large surface area of ​​the heat transfer fluid. The distance 33 or inlet depth is equal to a few centimeters, preferably less than or equal to one centimeter, and even more preferably less than or equal to 5 millimeters.

[0101] By choosing such values, only the part of the heat transfer fluid containing most of the impurities is treated, while the rest of the uncontaminated heat transfer fluid is not treated by the device.

[0102] It should also be noted that the intake depth also determines the static pressure on the filter. The greater the depth, the greater the static pressure the heat transfer fluid applies to the filter. Therefore, choosing the intake depth allows you to adjust this static pressure.

[0103] Filtered

[0104] The filter 20 is arranged in the internal space 17 radially between the internal wall 34 of the enclosure 16 and the sleeve 21 and vertically between the inlet opening 18 and the outlet opening 19.

[0105] The filter 20 is configured to remove impurities from the heat transfer fluid flowing through the enclosure 16 in the direction of flow from the inlet opening 18 to the outlet opening 19. Advantageously, the filter 20 obstructs the flow direction within the internal space. In other words, the filter material 20 is distributed so as to impede the heat transfer fluid as it flows in the direction of flow. In particular, the filter 20 can, for example, occupy an entire vertical slice of the internal space; that is, between a low vertical position and a high vertical position, the filter material 20 occupies the entire ring defined between the sleeve and the enclosure 16.

[0106] Referring to Figure 7, the filter 20 can comprise a plurality of filter elements 35, 36, 38, and 40. These elements are arranged horizontally and therefore parallel to each other. These filter elements are arranged successively in the flow direction. The flow direction, referenced as E in Figure 7, is a downward vertical direction opposite to the upward orientation of the central axis L. Thus, there is a first filter element 35 placed above a second filter element 36, itself placed above a second filter element 38, and so on. With respect to the downward flow direction E, a filter element placed upstream of another filter element has a mesh size larger than one mesh size of the other filter element. Each filter element is a more or less perforated material with gaps of varying sizes.The larger the gaps, the larger the mesh size of the filter element. The smaller the impurity size is compared to the size of the gaps in the filter element, the more easily impurities pass through.

[0107] Each filter element corresponds to a horizontal filter layer and filter 20 comprises several horizontal filter layers, with decreasing mesh size in the direction of flow.

[0108] Using different mesh sizes that decrease in size following the direction of flow allows for the filtration of increasingly smaller impurities in progressively deeper layers of the filter. This optimizes the operation of the filter, and in particular maximizes the volume of impurities retained by the filter. The larger this volume, the less frequently the filtration process needs to be repeated, thus reducing the installation and removal of the filtration system. The filter elements can be made of stainless steel wool.

[0109] Advantageously the first filter element, that is to say the one placed furthest upstream in relation to the direction of flow, is made of steel wool.

[0110] The downstream filter elements can then be made of sintered steel with a mesh size that decreases progressively in the direction of flow.

[0111] The filter elements can be rings which fully occupy the entire radial space between the sleeve 21 and the enclosure 16 over a certain height.

[0112] Alternatively, some filter elements are rings which occupy the radial space between the sleeve 21 and the enclosure 16 over a certain height but they define a vertical opening with the enclosure 16. These filter elements are in continuous contact with the enclosure 16 around the central axis L except in a certain angular sector where the edge of the filter element is moved away from the enclosure 16 defining in this angular sector a vertical passage or vertical opening.

[0113] With reference to Figure 7, filter element 36 defines a vertical passage 37, filter element 38 defines a vertical passage 39, and filter element 40 defines a vertical passage 4L

[0114] In other words, filter 20 comprises a first filter element that leaves a first opening in the flow direction and a second filter element that leaves a second opening in the flow direction. The two filter elements are directly opposite each other, meaning that one of the two filters is located immediately upstream or downstream of the other.

[0115] The filter is arranged so that the first opening is opposite the second filter element in the direction of flow, and the second opening in the direction of flow is opposite the first filter element.

[0116] In relation to figure 7, the opening 37 of the filter element 36 is vertically above the filter element 38, the opening 39 of the filter element 38 is vertically above the filter element 40. In particular, the filter elements can be arranged so that the opening of the first filter element is located in a radial plane opposite the opening of the second filter element.

[0117] By arranging the vertical openings in alternating radial positions, the total pressure drop in filter 20 is limited, as the filter 20 as a whole maintains good retention capacity. This configuration also has the advantage of creating circulation within the filtration device when it is in operation.

[0118] Advantageously, the horizontal projection of a filtration device is included within the horizontal projection of a fuel assembly, so that a filtration device can easily be placed in place of a fuel assembly in the nuclear reactor vessel. In this case, the filtration device will be located at the hot collector of the vessel.

[0119] Advantageously, the horizontal projection of a filtration device is included within the horizontal projection of a heat transfer fluid extraction system, so that a filtration device can easily be placed in place of such a system within the nuclear reactor vessel. In this case, the filtration device will be located at the cold manifold of the vessel.

[0120] Impurity filtration system

[0121] A filtration device 12 such as has been presented so far can be used in a filtration system 42 of impurities.

[0122] With reference to Figure 8, the system 42 includes an actuator 47 configured to cause the flow of heat transfer fluid in the flow direction E from the inlet opening 18 to the outlet opening 19. The actuator 47 comprises a motor 43 located outside the internal space 17 beyond the first end 13 and a shaft 23 extending from the motor 43 to the internal space 17 downstream of the filter 20 through the sleeve 21. The motor 43 may be located outside the tank. Optionally, the actuator 47 further comprises a propeller 25 whose shaft is located at the lower end of the shaft 23, the propeller 25 being situated between the inlet opening 18 and the outlet opening 19.

[0123] In relation to figure 8, the system 42 is fixed to the main reactor vessel by a housing 46, that is to say a metal structure connecting the system 42 to the closing slab 48 of the reactor.

[0124] The actuator 47 includes a portion located above the closing plate 48. The motor 43 is located above the plate 5. It is configured to rotate the shaft 23, which extends downwards from the motor 43 into the tank, around a vertical direction. The shaft 23 passes through the plate 5. The actuator 47 is preferably arranged at an opening in the plate 5, and in particular at the opening of the rotating cap 5. The actuator 47 is configured to move the heat transfer fluid from the inlet opening 18 to the outlet opening 19.

[0125] This movement can be ensured by a motor which drives in rotation the shaft 23 which in turn drives a propeller placed in the heat transfer fluid located in the internal space 17 between the filter 20 and the discharge opening 19. The heat transfer fluid is locally moved from the filter towards the opening 19 which sets the heat transfer fluid in motion within the entire enclosure 16, from the inlet opening 18 to the discharge opening 19.

[0126] The combination of the shaft motor and propeller acts as a vertical pump, which is interesting because it allows for easy regulation of the propeller's rotation speed.

[0127] The pump is held in place by mechanical support elements, which can be at one or more points.

[0128] Alternatively, actuator 47 can be implemented using an argon bubble pump. Argon gas is used in the reactor vessel to occupy the space above the coolant. Its inert nature prevents oxidation of the coolant. Actuator 47 comprises a vertically oriented conduit that is fluidically connected at its upper end to the pump and whose lower end opens into the internal space 17 below the filter 20. The enclosure 16 is configured so that all argon bubbles from the conduit exit the internal space through the discharge opening 19. Generating an argon gas flow from the pump to the internal space 17 produces bubbles in the internal space that exit the internal space 17 below the filter.These bubbles carry the heat transfer fluid from the internal space 17 to the outside, which sets the heat transfer fluid in motion in the internal space 17 from the inlet opening 18 to the outlet opening 19 through the filter 20.

[0129] When the actuator 47 is arranged at the opening of the rotary stopper 5, then the filtration of impurities can be implemented in the hot collector 8. The constraints on the hot collector are stricter compared to the constraints on the cold collector, in particular because the hot collector corresponds to a more densely populated area of ​​the tank than the area of ​​the cold collector.

[0130] To implement impurity filtration in the cold collector 10, a penetration into the dedicated tank can be installed. The dimensional constraints are not as stringent as in the case of the hot collector 8. Therefore, the penetration can be shared with other equipment (for example, a pump or a residual heat evacuation system).

[0131] Impurity filtration process

[0132] A filtration system 42 as just presented allows the implementation of a process P for filtering impurities from a heat transfer fluid in a nuclear reactor vessel.

[0133] Process P is implemented following a preliminary assessment of surface impurities in the heat transfer fluid. This assessment constitutes a preliminary step in the process. It can be carried out by sampling and analyzing the heat transfer fluid or by visual inspection using a camera.

[0134] In relation to figure 9, we present a method of implementing process P for filtering impurities from a heat transfer fluid in a nuclear reactor vessel.

[0135] If impurity filtration is decided upon, the reactor is shut down during the first step El of the process. This shutdown may correspond to scheduled preventive maintenance or a fuel assembly change.

[0136] During a second step E2 of the process, one of the reactor slab inlets is opened. For example, the inlet of the residual power evacuation system above the cold collector or the inlet of the rotary plug above the hot collector.

[0137] In a third step E3 of the process, the filtration device is moved from its designated storage area in the reactor building and inserted into the reactor vessel via one of the inlets opened during the second step E2. A fuel assembly reloading machine can be used to handle the filtration device. This step can advantageously be carried out once the coolant level in the vessel has been uniformed. The moving part of the filtration device is chosen to be less dense than the coolant. This moving part floats on the surface of the coolant.

[0138] In a fourth step, E4, of the process, the filtration device is fixed and adjusted to the reactor vessel. Specifically, the inlet depth is adjusted; this is the vertical distance between the free level of the coolant in the vessel and the lower edge defining the inlet opening. This depth is preferably less than or equal to 5 centimeters, more preferably less than or equal to 1 centimeter, and ideally less than or equal to 5 millimeters. This adjustment can be made by adding or removing ballast elements from the vessel. The filtration device is connected to an actuator to form a filtration system.

[0139] In a fifth step E5 of the process, the actuator is activated to move the heat transfer fluid through the internal space and the filter from the inlet opening to the outlet opening. When the filtration system includes a mechanical shaft, the motor is started to drive the impeller located in the heat transfer fluid. When the filtration system includes an argon bubble pump, an argon gas flow is generated from the pump into the internal space to produce argon bubbles within the internal space. As these bubbles exit the internal space through the outlet opening 19, they carry heat transfer fluid with them. In all cases, this movement causes a decrease in pressure, which lowers the free level of the heat transfer fluid in the chamber, drawing the surrounding heat transfer fluid towards the inlet opening.The heat transfer fluid passing through the filter is purified, with the impurities it carries being retained in the filter mesh. Simultaneously, the filter captures impurities from the heat transfer fluid in the cavity of the upper section. After passing through the filter, the purified heat transfer fluid exits the enclosure and the filtration system through the discharge opening. The duration of this fifth stage depends on the amount of impurities present on the surface in the operating zone of the filtration system.

[0140] During a sixth step, E6, of the process, the filtration system is removed from the reactor vessel through the same inlet as in the third step. It should be noted that, thanks to the drain, the filtration device is emptied of all heat transfer fluid as it is raised above the heat transfer fluid.

[0141] During a seventh step E7 of the process, the filter is removed from the filtration device and then washed or disposed of, the washed filter or a new filter is placed in the filtration device.

[0142] If necessary, steps E3, E4, E5 and E6 are repeated as long as there remains too much impurity on the surface of the heat transfer fluid.

[0143] During an eighth step E8 of the process, the filter of the filtration device is placed in its dedicated storage area.

Claims

DEMANDS 1. A surface impurity filtration device (12) for a heat transfer fluid (4) of a nuclear reactor vessel (1), the device extending in a longitudinal direction (L) between a first end (13) and a second end (14) and comprising: a) a handling interface (15) configured to allow manipulation of the filtration device by a handling system, preferably a fuel assembly handling system, the handling interface being located at the first end, b) an enclosure (16) defining an internal space (17), and provided with at least one inlet opening (18) for the entry, during use, of a heat transfer fluid and an outlet opening (19) for the exit, during use, of the heat transfer fluid which define a flow direction in the longitudinal direction from the upstream inlet opening to the downstream outlet opening,(c) a filter (20) housed in the internal space of the enclosure between the inlet opening and the outlet opening and rigidly fixed to the enclosure (16), and (d) a sleeve (21) for the passage of a shaft (23) of an actuator (47) from the first end to the second end in the internal space through the filter, wherein the enclosure (16) is configured to move freely in translation in the longitudinal direction, with the filter (20), relative to the sleeve (21) so as to allow, when the enclosure (16) is immersed, during use, in the heat transfer fluid, the inlet opening (18) to remain at a surface of the heat transfer fluid by flotation in the heat transfer fluid.

2. Device according to claim 1, wherein the device is configured to be disposed in place of a fuel assembly in the nuclear reactor vessel for access to a hot collector (8) of the vessel or in place of a waste heat removal system for access to the hot collector of the vessel.

3. Device according to any one of the preceding claims, wherein the enclosure comprises spaces (30) for receiving ballast elements, the ballast elements being configured to modify a buoyancy of the enclosure in the heat transfer fluid.

4. Device according to any one of the preceding claims, wherein the filter blocks the flow direction in the internal space.

5. Device according to any one of the preceding claims, wherein the filter comprises filter elements (36, 38) following one another in the direction of flow, a first filter element (36) upstream of a second filter element (38) having a mesh size greater than a mesh size of the second filter element.

6. Device according to the preceding claim, wherein the first filter element (36) leaves a first opening (37) free in the flow direction, said first opening being opposite the second filter element (38) in the flow direction, and the second filter element leaves a second opening (39) free in the flow direction, opposite the first filter element.

7. Device according to any one of the preceding claims, wherein the sheath is vertically centered with respect to the enclosure.

8. Impurity filtration system (42) for a heat transfer fluid in a nuclear reactor vessel, comprising: - a device according to any one of the preceding claims, and - an actuator (47) configured to cause the flow of heat transfer fluid in the flow direction from the inlet opening to the outlet opening, the actuator comprising a motor (43) disposed outside of the internal space beyond the first end and a shaft (23) extending from the motor to the internal space downstream of the filter through the sleeve.

9. System according to the preceding claim, in which the shaft is terminated by a helix (25) located between the heat transfer fluid inlet opening and the outlet opening.

10. Nuclear reactor vessel (1) comprising a heat transfer fluid (4) and a system (42) for filtering impurities from the heat transfer fluid according to any one of claims 8 and 9.

11. Nuclear reactor comprising a reactor vessel according to the preceding claim.

12. A method for filtering impurities from a heat transfer fluid (4) in a nuclear reactor vessel (1), the method comprising the following steps: - (E4) attaching to the nuclear reactor vessel a filtration system (42) according to any one of claims 8 and 9, so as to insert the filtration device (12) into the vessel (1), the inlet opening (18) of the enclosure (16) being located, by flotation, at a surface of the vessel's heat transfer fluid; and - (E5) operate the actuator (47) so as to create a flow of heat transfer fluid through the filter (20) between the inlet opening and the outlet opening (19).

13. Method according to claim 12, wherein the filtering is carried out to a depth from the surface of the heat transfer fluid, the depth being less than or equal to 5 centimeters, preferably less than or equal to 1 centimeter, and even more preferably less than or equal to 5 millimeters.

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