Nuclear reactor flow calming assembly and associated reactor and method

Abstract

The present invention relates to a flow calming assembly (10) for a nuclear reactor including a vessel (12) and an enclosure (14) located within the vessel (12), with fluid circulating from the vessel (12) to the enclosure (14) at a bottom portion of the vessel (12). The flow calming assembly (10) includes an outer calming device (28) having an opening for fluid passage therethrough, the outer calming device (28) including an upper portion (34) and a lower portion (36), the upper portion (34) including a flared or conical annular portion extending between an upper end (38) and a lower end (40), the lower portion (36) having a shape exhibiting symmetry along a major axis (D), the lower portion (36) extending from the lower end (40) of the upper portion (34). The present invention further relates to an associated nuclear reactor and installation method.

Description

[Technical Field]

[0001] The present invention relates to a flow calming assembly for a nuclear reactor that includes a vessel and an enclosure located within the vessel, with fluid circulating from the vessel to the enclosure at a bottom portion of the vessel.

[0002] The present invention further relates to an associated nuclear reactor and an associated method for installing an external calming device within the reactor. [Background technology]

[0003] When the fluid reaches the bottom of the vessel and turns around to enter the enclosure containing the reactor core, the fluid flow rate may fluctuate, which can lead to observed nuclear fluctuations and, as a result, further limit the reactor power output.

[0004] EP 3729464 describes a nuclear reactor including a vessel, an enclosure positioned within the vessel having a lower end that is open to allow fluid to circulate from an injection circuit to the vessel bottom and then enter and circulate through the enclosure, and a flow diffuser element configured to create a uniform flow entering the enclosure, the diffuser element defining a mixing zone free of any obstructions between the diffuser element and the vessel bottom.

[0005] The diffusion element includes a substantially circular or polygonal flat surface portion with a plurality of orifices, the flat portion positioned a first distance from the support and a second distance from a lower limit of the container bottom, the second distance being greater than the first distance.

[0006] However, this results in a confluence region at the bottom of the vessel of flows coming from different injection circuits, which can cause unstable and strong vortices. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent No. 3729464 Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a flow calming assembly for effectively calming flows between a container and an enclosure. [Means for solving the problem]

[0009] To this end, the present invention relates to a flow calming assembly of the type described above, comprising an outer calming device, the outer calming device having an opening for the passage of a fluid, the outer calming device comprising an upper part and a lower part, the upper part comprising a flared or conical annular part extending between the upper and lower ends, the lower part having a shape that is symmetrical along a major axis and extending from the lower end of the upper part.

[0010] The presence and shape of the external calming device effectively calms the flow and favors proper redirection of the flow inside the lower plenum. Proper redirection results in stable and controlled vortices in the confluence region, which improves mixing and limits spatial and temporal inhomogeneities in the mass flow rate at the enclosure inlet. Indeed, the better the mixing, the better the operation of the core inside the enclosure.

[0011] According to particular embodiments of the invention, the flow calming assembly has one or more of the following characteristics, considered alone or according to any technically possible combination: - including a flow distributor for distributing the flow of fluid from the container to the enclosure, the outer calming device radially surrounding the flow distributor; - the outer calming device is configured so as not to come into contact with any of the pressure elements of the container; - the upper end of the upper part has a larger dimension than the lower end; - The superior part contains a flared or conical annulus with a circular base, and the inferior part is cylindrical; - the lower portion is provided with a plurality of rows of through holes, the rows being staggered with respect to adjacent rows; - the upper part is provided with a number of holes; - the lower part radially surrounds the flow distributor; - the outer calming device is adapted to extend from the bottom portion of the enclosure to a distance of 0.5 centimeters to 10 centimeters from the bottom surface of the container; - the outer calming device is provided with a plurality of fasteners attached to an upper end of said outer calming device for fastening said outer calming device inside the container; and / or The outer calming device is provided with a plurality of indexing elements, which are adapted to cooperate with an insertion and / or extraction tool for inserting or extracting said outer calming device into or from a container.

[0012] The present invention further relates to a nuclear reactor comprising a vessel and an enclosure located within the vessel, with a fluid circulating from the vessel to the enclosure at a bottom portion of the vessel, the reactor being provided with a flow calming assembly, wherein the flow calming assembly includes an outer calming device, the outer calming device having an opening for the passage of the fluid, the outer calming device including an upper portion and a lower portion, the upper portion including a flared or conical annular portion extending between the upper and lower ends, the lower portion having a shape exhibiting symmetry along a major axis, and the lower portion extending from the lower end of the upper portion.

[0013] The present invention further provides a method for installing an external calming system in a nuclear reactor including a vessel and an enclosure located within the vessel, the reactor including a reactor core positioned within the enclosure, with fluid circulating from the vessel to the enclosure at a bottom portion of the vessel, the method comprising: - removing the internal structure and the nuclear fuel assemblies from the vessel; - providing an outer calming device, the outer calming device having an opening for passage of a fluid, the outer calming device including an upper portion and a lower portion, the upper portion including a flared or conical annular portion extending between the upper and lower ends, the lower portion having a shape exhibiting symmetry along a major axis, the lower portion extending from the lower end of the upper portion; - lowering an outer calming device within the vessel; - fastening the outer calming device inside the container; - reinstalling the internal structure and the nuclear fuel assemblies into the vessel; The present invention relates to a method comprising:

[0014] According to particular embodiments of the invention, the method comprises one or more of the following characteristics, considered alone or according to any technically possible combination: - the outer quenching device is fastened to a protrusion projecting radially from the inner surface of the vessel and / or to the lower plate of the reactor core; and / or - The outer calming device is fastened so as to extend from the bottom part of the enclosure to a distance of 0.5 cm to 10 cm relative to the bottom surface of the container.

[0015] Other characteristics and advantages of the invention will become apparent upon reading the following description, given purely by way of example and made with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a bottom portion of a nuclear reactor vessel with an example of a flow calming assembly of the present invention; [Figure 2] 2 is a three-dimensional representation of the outer calming device of the flow calming assembly of FIG. 1; [Figure 3]3 shows a three-dimensional representation of the external pacifying device of FIG. 2 together with an insertion and / or extraction tool. DETAILED DESCRIPTION OF THE INVENTION

[0017] An example of a flow calming assembly is shown in FIG.

[0018] The flow calming assembly 10 is adapted for a nuclear reactor including a vessel 12 and an enclosure 14 located within the vessel 12 .

[0019] The container 12 is bounded by an outer wall 16 .

[0020] The outer wall 16 defines an inner surface 17 of the container 12. The inner surface 17 here includes a protrusion 18 that protrudes radially from the inner surface 17 towards the interior of the container, and in particular towards the enclosure 14.

[0021] The outer wall 16 defines a bottom surface 19 of the container 12 .

[0022] The bottom surface 19 is part of the inner surface 17 .

[0023] The enclosure 14 is bounded by an inner wall 20 .

[0024] The inner wall 20 exhibits symmetry along the major axis D.

[0025] The inner wall 20 corresponds to a given profile displaced along the axis.

[0026] In the exemplary embodiment, the inner wall 20 is cylindrical here.

[0027] The enclosure 14 is open at a lower end thereof.

[0028] The outer wall 16 includes a side portion 22 that radially surrounds the inner wall 20 of the enclosure 14 .

[0029] The side portion 22 is substantially cylindrical.

[0030] The reactor core is located inside the enclosure 14 .

[0031] The reactor core includes, for example, nuclear fuel assemblies containing nuclear fuel rods.

[0032] The core is positioned above a lower plate 24 which is provided with holes.

[0033] Fluid circulates from the vessel 12 to the enclosure 14 at the bottom portion of the vessel 12 .

[0034] More specifically, the fluid circulates downwardly through the space between the container 12 and the enclosure 14, more precisely the space between the outer wall 16 and the inner wall 20, and then from the container 12 to the enclosure 14 at the lower end of the enclosure 14.

[0035] Thus, the fluid enters the enclosure 14 and circulates within the enclosure 14 .

[0036] The space between the container 12 and the enclosure 14, particularly between the side portion 22 of the outer wall 16 and the inner wall 20, corresponds to an injection circuit having a substantially circular cross section.

[0037] The flow calming assembly 10 here includes an outer calming device 28 .

[0038] In the depicted embodiment, the flow calming assembly 10 further includes a flow distributor 26 .

[0039] The flow distributor 26 distributes, for example, in a known manner, the flow of fluid from the vessel 12 to the enclosure 14. In particular, the flow distributor 26 is configured to uniformly distribute the flow of fluid entering the enclosure 14.

[0040] The flow distributor 26 includes, for example, a sidewall 30 and an interface 32 .

[0041] The side wall 30 does not have any openings for the passage of fluid.

[0042] The sidewall 30 exhibits symmetry along the major axis D.

[0043] The sidewall 30 corresponds to a given profile displaced along an axis.

[0044] In the exemplary embodiment, the sidewall 30 is substantially cylindrical.

[0045] Alternatively, the given contour is a polygon.

[0046] The side wall 30 and the inner wall 20 are coaxial.

[0047] An interface 32 closes the lower end of the sidewall 30 .

[0048] The interface 32 is perpendicular to the axis around which the sidewall 30 extends.

[0049] The interface 32 may, for example, be disc-shaped. Alternatively, the interface 32 may be polygon-shaped.

[0050] The interface 32 is, for example, a grating.

[0051] The grid has openings of a size between, for example, the estimated size of the fuel assemblies and half the size of the pitch between the fuel assemblies.

[0052] An outer calming device 28 radially surrounds the flow distributor 26 .

[0053] The outer calming device 28 is adapted to calm the fluid flow upstream of the enclosure 14 and, if applicable, before the flow distributor 26 .

[0054] The outer calming device 28 has openings for the passage of fluid, one embodiment of which is described below.

[0055] The outer calming device 28 extends from the bottom portion of the enclosure 14, in particular from the lower end of the enclosure 14, to a distance of less than 10 centimetres, more precisely less than 5 centimetres, relative to the bottom surface of the container.

[0056] More specifically, the distance between the lower end of the outer calming device 28 and the bottom surface 19 of the container 12 is between 0.5 centimeters and 10 centimeters.

[0057] With the exception of the protrusions 18, there is no contact between the outer calming device 28 and the container 12.

[0058] The outer calming device 28 is not in contact with any pressurized walls of the container and the protrusions are not under pressure.

[0059] The outer calming device 28 does not come into contact with any of the pressure elements of the container 12 .

[0060] In the exemplary embodiment, the outer calming device 28 includes an upper portion 34 and a lower portion 36 .

[0061] The upper portion 34 includes, and more precisely is, a flared or conical annulus.

[0062] The annular portion, more precisely the upper portion 34, has for example the shape of a portion of a cone tapering from the base to an apex, said apex being located here below the upper portion. Alternatively, the annular portion, more precisely the upper portion 34, has for example a shape tapering from the base.

[0063] In the exemplary embodiment, the upper portion 34 has an overall shape that is substantially rotationally symmetric about one axis, specifically about the major axis D.

[0064] The base is here circular, and therefore the upper portion 34 comprises a conical annulus with a circular base, or more precisely an annulus.

[0065] Alternatively, the base is not circular.

[0066] The base has, for example, a polygonal shape.

[0067] The upper portion 34 extends between an upper end 38 and a lower end 40 .

[0068] The superior end 38 has a larger size, specifically a larger diameter, than the inferior end 40 .

[0069] The upper portion 34 is provided with a plurality of holes 42 .

[0070] The holes 42 are arranged in columns, with the holes in one column being, for example, staggered with respect to the holes in adjacent columns.

[0071] Each hole 42 is, for example, round.

[0072] The total surface of the hole is equal to 10% to 60% of the surface of the upper part 34, here the surface of the cone.

[0073] Each hole has a diameter of, for example, 30 to 100 mm.

[0074] Said diameter is governed by the hydraulic optimization of the reactor under investigation.

[0075] Fluid passing through holes 42 is replenished directly to the core periphery. Specifically, fluid passing through holes 42 does not pass through flow distributor 26.

[0076] The shape of the lower portion 36 exhibits symmetry along the major axis D.

[0077] The shape of the lower portion 36 corresponds to a given contour displaced along said major axis.

[0078] The given contour is, for example, the contour of the lower end 40 of the upper portion 34 .

[0079] In the exemplary embodiment, the lower portion 36 is cylindrical.

[0080] Alternatively, the contour has the shape of a polygon.

[0081] The lower portion 36 here extends about a main axis D.

[0082] The main axis D is a cylindrical axis.

[0083] The lower portion 36 extends from a lower end 40 of the upper portion 34 .

[0084] The lower end 40 of the upper portion 34 coincides with the upper end of the lower portion 36 .

[0085] The lower portion 36 is integral with or secured together with a lower end 40 of the upper portion 34. In the exemplary embodiment, the lower portion 36 is integral with the upper portion 34 and the outer calming device 28 is made integral therewith.

[0086] The lower portion 36 has the same diameter as the lower end 40 of the upper portion 34 .

[0087] The lower part has a height of, for example, 40% to 60% of the height of the outer calming device 28 along the main axis D.

[0088] The lower portion 36 is provided with a plurality of rows of through holes 44, the rows being, for example, staggered with respect to adjacent rows.

[0089] For example, there are at least three rows 44 of through-holes, such as three or four rows.

[0090] Each row corresponds to the periphery of the lower portion 36 .

[0091] The through-hole 44 here has a rectangular shape with borders at the corners.

[0092] The through-holes 44 are disposed within a strip 46 of the lower portion 36 .

[0093] The strip portion 46 extends from the lower end of the lower portion 36 .

[0094] The strip portion 46 corresponds to at least half of the lower portion 36 .

[0095] The through holes 44 cover at least 75% of the surface of the strip 46 .

[0096] Each through hole 44 has a length of 150 cm 2 ~350cm 2 It has a surface area of

[0097] Each through-hole 44 has a length of 15 cm to 40 cm and a width of 5 cm to 15 cm, for example.

[0098] The lower portion 36 radially surrounds the flow distributor 26 , and more particularly the interface 32 of the flow distributor 26 .

[0099] Fluid passing through the through-holes 44 further passes through the interface 32 of the flow distributor 26 .

[0100] In the exemplary embodiment, the outer calming device 28 is provided with a plurality of fasteners 48 attached to an upper end of the outer calming device 28, here upper end 38, for connecting the outer calming device 28 to the container 12.

[0101] The fasteners 48 are attached, and more specifically bolted here, to ears 50 projecting from the upper end of the outer silencer 28 .

[0102] The ears 50 are provided with holes, specifically threaded holes, here for bolting, to fasten the fasteners 48 to the corresponding ears 50 .

[0103] The ears project radially.

[0104] There are a pair of ears 50 for each fastener 48 .

[0105] In the exemplary embodiment, the fastener 48 is, for example, a U-shaped plate.

[0106] The fastener 48 includes two arms 52 and a central portion 54 connecting the two arms 52 .

[0107] Each arm 52 is attached to an ear 50, specifically here by a bolt.

[0108] Each arm 52 extends upwardly from ear 50 .

[0109] Each arm 52 here extends substantially parallel to the main axis D.

[0110] The central portions 54 are therefore higher than the corresponding arms 52. Each central portion 54 here extends orthogonally to the main axis D along a direction local tangential to the outer calming device.

[0111] Each central portion 54 has a surface adapted to face and rest on the upper surface of the corresponding projection 18 .

[0112] In an alternative embodiment, the fastener 48 further includes a lower connecting portion that connects the two arms.

[0113] The lower connecting portion is adapted to partially face the distal surface of the corresponding projection 18, in particular to contact this surface.

[0114] In the exemplary embodiment, there are as many fasteners 48 as there are projections 18 .

[0115] Alternatively, there are more lugs 18 than fasteners 48. Specifically, the number of fasteners is between half the number of lugs and the number of lugs. It is contemplated that fastenings will occur to between half and all of the lugs.

[0116] Each fastener 48 rests on a respective projection 18 .

[0117] Each fastener 48, and more particularly each central portion 54, is further attached to a respective projection, and more particularly here bolted.

[0118] An outer silencer 28 is fastened to the projection 18 .

[0119] The outer calming device 28 further comprises, for example, a notch 55 for the passage of the protrusion 18 .

[0120] In particular, between the ears of each ear pair 50, the outer silencer 28 includes one notch 55.

[0121] Thus, each projection 18 passes through the fastener 48 below the central portion 54 and through the notch 55 .

[0122] Each protrusion 18 is therefore disposed between a corresponding fastener 48 positioned above and on most of its side and a corresponding notch 55 positioned below and partially on its side.

[0123] If applicable, the lower connecting portion of the fastener extends through the corresponding notch to surround the corresponding protrusion.

[0124] Alternatively, the outer calming device 28 is fastened to the lower plate of the reactor core.

[0125] In the exemplary embodiment, the outer calming device 28 further comprises a plurality of indexing elements 56 .

[0126] The indicator element 56 is, for example, here a hole in the superior end of the outer calming device.

[0127] The indicator elements 56 are more precisely pairs of holes provided in extensions extending from some of the ear pairs 50, for example.

[0128] The indexing element 56 is adapted to cooperate with an insertion and / or extraction tool for inserting or extracting the outer calming device 28 into or from the container 12, as will be described below.

[0129] In operation, fluid flowing from the injection circuit towards the enclosure 14 first passes through the outer calming device 28 , specifically through the lower portion 36 of the outer calming device 28 , and then through the through-holes 44 .

[0130] The distance between the lower end of the outer calming device 28 and the bottom surface of the vessel allows the majority of the fluid to travel through the outer calming device 28 first.

[0131] The outer calming device 28 calms the fluid flow upstream of the enclosure and, if applicable, the flow distributor 26 .

[0132] The majority of the fluid passes through the perforations 44 in the outer calming device 28 and then through the flow distributor 26, specifically the interface 32, thus distributing the fluid flow from the vessel 12 to the enclosure 14. The fluid finally passes through the flow distributor 26 and into the enclosure 14.

[0133] The fluid is therefore first allowed to settle and then distributed, thus reducing turbulence.

[0134] In the exemplary embodiment, a portion of the fluid passes through holes 42 and is replenished directly to the periphery of the core. The fluid is calmed directly by outer calming device 28.

[0135] The shape of the outer calming device 28 is particularly advantageous for calming the fluid flow upstream of the enclosure, as this shape favors proper redirection of the flow within the lower plenum.

[0136] DETAILED DESCRIPTION OF THE INVENTION A method for installing an external calming system in a nuclear reactor, and particularly in an existing nuclear reactor, is now described.

[0137] The nuclear reactor, as previously described, includes a vessel and an enclosure located within the vessel.

[0138] In the illustrated embodiment, the reactor is further equipped with a flow distribution device, for example as described above, for distributing the flow of fluid from the vessel to the enclosure.

[0139] However, the reactor is not provided with an external calming system, as previously explained.

[0140] The method includes the following steps: - removing the internal structure and fuel assemblies from the vessel; - providing an external calming device; - lowering an outer calming device within the container; - fastening the outer calming device inside the container; and - Reinstalling the internal structure and fuel assemblies into the vessel.

[0141] The step of removing the internal structure and nuclear fuel assemblies from the vessel provides access to the bottom of the vessel.

[0142] All internal structures of the container are removed from said container.

[0143] The internal structure includes an inner wall 20 .

[0144] The internal structure includes a lower internal structure and a higher internal structure.

[0145] The internal structure, and more particularly the subinternal structure, includes a flow distributor 26 .

[0146] The lower internals support the weight of the core, hold the fuel assemblies, control the alignment of the clusters and instrumentation, direct the flow of heat transfer fluids, shield the vessel from radiation emitted by the core, and necessarily maintain a high degree of rigidity to allow for the required alignment accuracy.

[0147] The upper internal structure positions the control cluster in close alignment with the fuel assemblies.

[0148] An external calming device as previously described is provided and lowered inside the vessel.

[0149] The outer silencer device is handled by an insertion and / or extraction tool 100, as depicted, for example, in FIG.

[0150] The tool 100 includes a plurality of, at least three, beams 102 attached together at a center 104 .

[0151] The beams 102 are connected together so that the center of each beam is at center 104 .

[0152] The beams 102 extend in the same plane.

[0153] Beam 102 has a similar length.

[0154] The ends of the beam 102 are such that a circumscribing circle passes through all the ends of the beam.

[0155] Tool 100 further includes a reinforcing structure 106 that connects beams 102 at center 104 .

[0156] At some ends of the beam 102, complementary elements 108 of the indexing elements 56 are provided.

[0157] The complementary element 108 is adapted to cooperate with the indexing element 56 to securely fasten the tool 100 and the outer silencer 28 together.

[0158] The tool 100 and the outer silencer 28 are fixed together, for example by screwing or bolting.

[0159] Here, the tool 100 is fixedly secured to the outer calming device 28 .

[0160] Thus, the tool 100 lowers the outer calming device 28 inside the vessel 12 .

[0161] The outer calming device 28 is lowered so that it radially surrounds the flow distributor and extends from the bottom part of the enclosure to a distance of less than 10 centimeters, more precisely less than 5 centimeters, to the bottom surface of the container.

[0162] The distance is between 0.5 centimeters and 10 centimeters.

[0163] Specifically, the outer silencer 28 is lowered until the fastener 48 contacts the projection 18 .

[0164] The tool 100 is then separated and removed from the outer silencer 28 .

[0165] The outer calming device 28 is then fastened inside the container 14, for example against the projection 18.

[0166] In particular, each fastener 48 is fastened to a corresponding projection 18 .

[0167] In one embodiment, at least one bolt is threaded through each fastener 48, here through the central portion 54, and, if applicable, through the lower connecting portion, directly into the corresponding projection 18.

[0168] Alternatively, the outer calming device 28 is fastened to the lower plate of the reactor core.

[0169] All elements removed from the container during the removal step are then reinstalled inside the container.

[0170] This method allows for the addition of an external calming device to an existing nuclear reactor, so that the existing reactor has a flow calming assembly according to the present invention.

[0171] Moreover, said tool 100 is further adapted to extract the outer calming device 28 from the vessel 12, in particular for any maintenance operations.

[0172] In such a case, the outer calming device 28 may, for example, be decoupled from the fastener 48, so that the fastener 48 remains on the protrusion 18, and the outer calming device 28 may be withdrawn from the container 12 without the fastener.

[0173] Later, the outer silencer 28 is reinstalled inside the container and the fasteners are reattached to the outer silencer 28, thus to the ears 50.

[0174] Alternatively, the fastener 48 is decoupled from the protrusion 18, and the outer calming device 28, along with the fastener, is then withdrawn from the container 12, the outer calming device 28 is reinstalled inside the container, and the fastener is repositioned on the protrusion.

[0175] The external calming device is not directly attached to the pressure vessel wall and may be removed specifically to inspect the entire vessel bottom.

[0176] The flow calming assembly of the present invention can be subjected to maintenance procedures by withdrawing the outer calming device from the vessel.

[0177] The flow calming device of the present invention is highly advantageous because it thus first calms and then distributes the fluid flow, thereby reducing turbulence in the fluid flow at the inlet to the reactor core. [Explanation of symbols]

[0178] 10. Flow Calming Assembly 12 containers 14 Enclosure 26 Fluid distribution device 28 External calming device 34 Upper part 36 Lower part 38 Upper end 40 Lower end

Claims

1. A flow calming assembly (10) for a nuclear reactor including a vessel (12) and an enclosure (14) located within the vessel (12), with fluid circulating from the vessel (12) to the enclosure (14) at a bottom portion of the vessel (12), 1. A flow calming assembly (10) comprising an outer calming device (28), the outer calming device (28) having an opening for the passage of a fluid, the outer calming device (28) comprising an upper portion (34) and a lower portion (36), the upper portion (34) comprising a flared or conical annulus extending between an upper end (38) and a lower end (40), the lower portion (36) having a shape exhibiting symmetry along a major axis (D), the lower portion (36) extending from the lower end (40) of the upper portion (34), the upper portion (34) comprising a flared or conical annulus with a circular base, and the lower portion (36) being cylindrical.

2. 2. The flow calming assembly (10) of claim 1, including a flow distributor (26) for distributing fluid flow from the vessel (12) to the enclosure (14), the outer calming device (28) radially surrounding the flow distributor (26).

3. 3. A flow calming assembly according to claim 1 or 2, wherein the outer calming device (28) is configured so as not to come into contact with any pressure element of the vessel (12).

4. A flow calming assembly according to any one of claims 1 to 3, wherein the upper end (38) of the upper portion (34) has a larger dimension than the lower end (40).

5. 5. A flow calming assembly according to any one of claims 1 to 4, wherein the lower portion (36) is provided with a plurality of rows (44) of through holes, the rows being staggered with respect to adjacent rows.

6. A flow calming assembly according to any one of claims 1 to 5, wherein the upper portion (34) is provided with a plurality of holes (42).

7. A flow calming assembly according to any one of claims 1 to 6, wherein the lower portion (36) radially surrounds the flow distributor (26).

8. 8. A flow calming assembly according to any one of claims 1 to 7, wherein the outer calming device (28) is adapted to extend from a bottom portion of the enclosure (14) to a bottom surface (19) of the vessel (12) a distance of between 0.5 centimeters and 10 centimeters.

9. 9. A flow calming assembly according to any one of claims 1 to 8, wherein the outer calming device (28) is provided with a plurality of fasteners (48) attached to an upper end of the outer calming device (28) for fastening the outer calming device (28) inside the vessel (12).

10. 10. A flow calming assembly according to any one of claims 1 to 9, wherein the outer calming device (28) is provided with a plurality of indexing elements (56), the indexing elements (56) being adapted to cooperate with an insertion and / or extraction tool (100) for inserting or extracting the outer calming device (28) into or from a container.

11. 1. A nuclear reactor comprising a vessel (12) and an enclosure (14) located within the vessel (12), with fluid circulating from the vessel (12) to the enclosure (14) at a bottom portion of the vessel (12), the reactor comprising a flow calming assembly (10), 1. A nuclear reactor, comprising: a flow calming assembly (10) including an outer calming device (28), the outer calming device (28) having an opening for passage of a fluid, the outer calming device (28) including an upper portion (34) and a lower portion (36), the upper portion (34) including a flared or conical annulus extending between an upper end (38) and a lower end (40), the lower portion (36) having a shape exhibiting symmetry along a major axis (D), the lower portion (36) extending from the lower end (40) of the upper portion (34), the upper portion (34) including the flared or conical annulus with a circular base, and the lower portion (36) being cylindrical.

12. A method for installing an external calming system (28) in a nuclear reactor including a vessel (12) and an enclosure (14) located within the vessel (12), the reactor including a reactor core positioned within the enclosure (14), and fluid circulating from the vessel (12) to the enclosure (14) at a bottom portion of the vessel (12), the method comprising: - removing the internal structure and the nuclear fuel assemblies from the vessel (12); providing an outer calming device (28), the outer calming device (28) having an opening for the passage of a fluid, the outer calming device (28) comprising an upper portion (34) and a lower portion (36), the upper portion (34) comprising a flared or conical annulus extending between an upper end (38) and a lower end (40), the lower portion (36) having a shape exhibiting symmetry along a major axis (D), the lower portion (36) extending from the lower end (40) of the upper portion (34), the upper portion (34) comprising a flared or conical annulus with a circular base, and the lower portion (36) being cylindrical; - lowering the outer calming device (28) in the container (12); - fastening the outer calming device (28) inside the container (12); - reinstalling the internals and the nuclear fuel assemblies in the vessel (12); A method comprising:

13. 13. The method of claim 12, wherein the outer calming device (28) is fastened to a protrusion (18) projecting radially from the inner surface (17) of the vessel (12) and / or to a lower plate (24) of the reactor core.

14. 14. The method according to claim 12 or 13, wherein the outer calming device (28) is fastened in such a way that it extends from the bottom portion of the enclosure (14) to the bottom surface (19) of the container (12) at a distance of between 0.5 centimeters and 10 centimeters.

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