Method for repairing a steam generator of a nuclear reactor, corresponding steam generator

Abstract

Method for repairing a steam generator of a nuclear reactor, corresponding steam generator. The repair method relates to a steam generator (1) in which at least one tube (24) is engaged in a bore (8) whose internal surface (14) is corroded and has a cavity (32) with a depth greater than a specified depth, said cavity (32) being located below the end portion (26) of the tube (24). The method comprises the following steps: - removal of one end (34) of the end portion (26) of the tube (24), over a length such that said cavity (32) is no longer covered by the end portion (26); - insertion of a plug (36) into the bore (8), the cavity (32) being fluidically isolated from the primary fluid circulation compartment (4) by the plug (36), the cavity (32) communicating fluidly with an internal volume of the tube (24). Figure for the abstract: 4

Description

Title of the invention: Method for repairing a steam generator of a nuclear reactor, corresponding steam generator

[0001] The present invention relates generally to the repair of a steam generator of a nuclear reactor.

[0002] Such a steam generator typically comprises:

[0003] - a tubular plate, having a first large face delimiting a compartment for circulation of a primary fluid, a second large face delimiting a compartment for circulation of a secondary fluid, and a plurality of bores each having a first end opening at the level of the first large face and a second end opening at the level of the second large face, each bore being delimited by an internal surface and having a central axis;

[0004] - at least one primary fluid circulation tube arranged in the second compartment; compartment, the tube having two opposite end portions engaged in two bores.

[0005] The end portions of the tubes are welded to the tube plate at the first large face, in a sealed manner.

[0006] It may happen that the end of the tube is pierced, for example during interventions in the primary fluid circulation compartment. This piercing brings the internal surface of the bore of the tube plate into contact with the primary fluid, which can generate corrosion of the internal surface, and the creation of cavities of varying depths.

[0007] It is possible, to repair the steam generator, to remove the end of the tube, so as to expose the corroded part of the internal surface. A surface layer of the internal surface is then removed, so as to restore a good surface condition and remove the corroded parts. A plug is then inserted into the bore, so as to seal it.

[0008] If one of the cavities has a depth greater than a determined limit, it is not possible to use the repair method described above.

[0009] Indeed, it is not possible to excessively increase the depth of the removed surface layer. This depth is limited by the need to ensure a certain mechanical strength for the veil of material separating the bores from each other. Furthermore, it would become difficult to weld the plug on the first large face, the weld interfering with the welds of the neighboring tubes.

[0010] This could also lead to the use of stoppers that have not yet been qualified. Only Certain plug sizes are currently qualified for a given steam generator, depending on the size of the tube plate bores. The use of unqualified plugs would then make it necessary to justify the safety of the repair on a case-by-case basis. Such a technical solution could lead to limitations in the operating range of the steam generator.

[0011] Another possibility could be to remove the surface layer of the bore to a depth compatible with the already qualified plugs. There would then remain, for the deepest defects, a cavity under the plug.

[0012] Existing plugs are typically fixed by expansion to the internal surface of the bore, such that the external surface of the plug seals the cavity. For the deepest defects, when hot, an overpressure inside the cavity may appear. This phenomenon is referred to as the "boiler effect" and can ultimately lead to the destruction of the plug.

[0013] Thus, there is a need for a method of repairing a steam generator having a bore whose internal surface includes one or more deep cavities, this method not having the above defects.

[0014] To this end, the invention relates to a method of repairing a steam generator of a nuclear reactor, the steam generator comprising:

[0015] - a tubular plate, having a first large face delimiting a compartment for circulation of a primary fluid, a second large face delimiting a compartment for circulation of a secondary fluid, and a plurality of bores each having a first end opening at the level of the first large face and a second end opening at the level of the second large face, each bore being delimited by an internal surface and having a central axis;

[0016] - at least one primary fluid circulation tube arranged in the compartment of circulation of the secondary fluid, the tube having two opposite end portions engaged in two bores;

[0017] the internal surface of one of the bores being corroded and having a cavity of depth greater than a determined depth, said cavity being located under the end part of the tube engaged in said bore;

[0018] the method comprising the following steps:

[0019] - removal of one end of the end portion of the tube, over a length such that said cavity is no longer covered by the end portion; - insertion of a plug into the bore, the cavity being fluidly isolated from the primary fluid circulation compartment by the plug, the cavity fluidly communicating with an internal volume of the tube.

[0020] Thus, according to the method of the invention, the cavity communicates fluidically with the internal volume of the tube. When hot, the steam filling the cavity escapes towards the internal volume of the tube, so that the cavity does not present high pressure. The risk of destruction of the plug is avoided.

[0021] The method may also have one or more of the following characteristics, considered individually or in all technically possible combinations:

[0022] - the plug comprises a first section, engaged in the first end of the bore between the first large face and the cavity, the method comprising a step of fixing the first section in a sealed manner to the tube plate;

[0023] - the plug comprises a second section axially extending the first section and extending in line with the cavity, an external surface of the second section being in contact with the internal surface of the bore, a passage putting the cavity into fluid communication with the internal volume of the tube;

[0024] - the passage comprises at least one groove formed in the internal surface of the bore or in the external surface of the plug;

[0025] - the passage comprises at least one conduit provided inside the plug;

[0026] - the second section of the plug is mechanically fixed to the internal surface of boring by flaring, by screwing or by bayonet fixing;

[0027] - the second section of the plug is rigidly fixed to another plug, engaged in the tube;

[0028] - the method comprises, before the step of inserting the plug, a step of removing a surface layer from the internal surface, from the first large face going up axially beyond the cavity, to a depth less than or equal to the determined depth.

[0029] The invention according to a second aspect relates to a steam generator of a nuclear reactor, the steam generator comprising:

[0030] - a tubular plate, having a first large face delimiting a compartment for circulation of a primary fluid, a second large face delimiting a compartment for circulation of a secondary fluid, and a plurality of bores each having a first end opening at the level of the first large face and a second end opening at the level of the second large face, each bore being delimited by an internal surface and having a central axis;

[0031] - at least one primary fluid circulation tube arranged in the compartment of circulation of the secondary fluid, the tube having two opposite end portions engaged in two bores;

[0032] the inner surface of one of the bores being corroded and having a cavity; - one end of the end part of the tube engaged in said bore having been removed, over a length such that said cavity is no longer covered by the end part; - a plug being inserted into the bore, the cavity being fluidically isolated from the primary fluid circulation compartment by the plug, the cavity fluidically communicating with an internal volume of the tube.

[0033] The steam generator may further have one or more of the following characteristics, considered individually or in all technically possible combinations:

[0034] - the plug comprises a first section, engaged in the first end of the bore between the first large face and the cavity, the first section being fixed in a sealed manner to the tube plate;

[0035] - the plug comprises a second section axially extending the first section and extending in line with the cavity, an external surface of the second section being in contact with the internal surface of the bore, a passage putting the cavity into fluid communication with the internal volume of the tube;

[0036] - the passage comprises at least one groove formed in the internal surface of the bore or in the external surface of the plug;

[0037] - the passage comprises at least one conduit arranged inside the plug;

[0038] - the second section of the plug is mechanically fixed to the internal surface of boring by flaring, by screwing or by bayonet fixing;

[0039] - the second section of the plug is rigidly fixed to another plug, engaged in the tube.

[0040] Other characteristics and advantages of the invention will emerge from the detailed description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which:

[0041] - [Fig.l] [Fig.l] is a partial view of a steam generator, a portion of the upper casing, bowl, and tube plate being omitted to reveal the tube plate bores; - [Fig.2] [Fig.2] is a schematic view, in section, of part of the tube plate of [Fig.l], showing a bore whose internal surface has cavities; - [Fig.3] [Fig.3] is a perspective and sectional view of a plug inserted into a bore, illustrating a first embodiment of the invention; - [Fig.4][Fig.5][Fig.6] Figures 4 to 6 are views similar to that of the [Fig.3], for other embodiments of the invention; and - [Fig.7] [Fig.7] is a schematic view, in section, of an engaged plug in a bore, illustrating a fifth embodiment of the invention.

[0042] The steam generator 1 illustrated in [Fig.l] belongs to a nuclear reactor.

[0043] This steam generator 1 comprises a tubular plate 2, having a first large face 2a delimiting a compartment 4 for circulation of a primary fluid, and a second large face 2b delimiting a compartment 6 for circulation of a secondary fluid.

[0044] The tubular plate 2 also comprises a plurality of bores 8, each having a first end 10 opening at the level of the first large face 2a, and a second end 12 opening at the level of the second large face 2b.

[0045] Each bore 8 is delimited by an internal surface 14 ([Fig.2]), and has a central axis C ([Fig.2]).

[0046] The bores 8 are rectilinear and are distributed over the entire surface of the tube plate 2.

[0047] The steam generator 1 further comprises a bowl 16, fixed to the tube plate 2. The bowl 16 delimits, with the first large face 2a, the primary fluid circulation compartment 4. This compartment 4 is divided by an internal partition 18 into an inlet part and an outlet part. The bowl 16 has a primary fluid inlet 19 opening into the inlet part, and a primary fluid outlet 20 opening into the outlet part.

[0048] [Fig.l] shows that the steam generator 1 comprises an upper casing 22, attached to the tube plate 2. The upper casing 22 delimits, with the second large face 2b, the circulation compartment of the secondary fluid 6.

[0049] The steam generator 1 further comprises at least one primary fluid circulation tube 24, arranged in the secondary fluid circulation compartment 6.

[0050] The tube 24 has two opposite end parts 26, engaged in two bores 8.

[0051] The steam generator 1 comprises a large number of tubes 24.

[0052] These tubes are typically U-shaped tubes.

[0053] The opposite end portions 26 of each tube 24 are engaged in bores 8 which open one into the inlet portion of compartment 4, and the other into the outlet portion of compartment 4.

[0054] As seen in [Fig.l], the end portion 26 penetrates into the bore 8 through the second large face 2b, and extends over the entire height of the bore 8 up to the first large face 2a ([Fig.2]). The end of the portion 26 is fixed to the tube plate 2 by a peripheral weld 28, which creates a seal between the end of the tube and the tube plate at the first large face 2a. Thus, normally, the primary fluid cannot infiltrate between the tube 24 and the internal surface 14 of the bore.

[0055] As illustrated in [Fig.2], in some cases the inner surface 14 of one of the bores is corroded.

[0056] This occurs in particular if the tube 24 is damaged or pierced, causing the primary fluid to come into contact with the internal surface 14 of the bore.

[0057] The internal surface 14 then comprises a plurality of cavities 30, 32.

[0058] Another failure mode causing corrosion of the inner surface 14 and the appearance of cavities 30, 32 is illustrated in [Fig. 2]. The end weld 28 of the tube 24 is damaged at a point 29, thus allowing the primary fluid to infiltrate between the inner surface 14 and the tube 24.

[0059] A particular problem arises in the case where the internal surface 14 carries a cavity 32 of depth greater than a determined depth E.

[0060] The cavities 30, 32 are located under the end part 26 of the tube, engaged in the bore 8. This means that the end part 26 covers the cavities 30, 32.

[0061] Because the cavity 32 has a depth greater than the depth E, the steam generator cannot be repaired using a traditional method.

[0062] As described above, the traditional method consists of removing the surface layer of the internal surface 14 at the corroded area, so as to eliminate the cavities 30 and restore the surface condition of the bore. A plug is then engaged in the bore 8, so as to close this bore and prevent the circulation of primary fluid.

[0063] To restore the surface condition and eliminate the cavity 32, it would be necessary to remove a significant layer of material. The thickness of residual material between the corroded bore 8 and the neighboring bore would be insufficient. Furthermore, it would not be possible in this case to use a plug that has already been qualified to plug the bore 8. The diameter range of qualified plugs is limited, and does not allow the plugging of bores whose surface layer has been removed to a depth greater than the determined depth E.

[0064] The different repair methods which will now be described are particularly suited to the above case, in which the internal surface 14 of one of the bores 8 has a cavity 32 of depth greater than the determined depth E.

[0065] The depth E is a function of the geometry of the tube plate 2, and of the operating conditions of the steam generator 1. It is a function in particular of the thickness of the veil of material separating two neighboring bores 8, and of the size of the plugs already qualified for the repair of the steam generator, in particular for the sealing of a bore.

[0066] For example, the depth E is between approximately 1.2 and 1.3 mm.

[0067] The method illustrated in [Fig.3] comprises the following steps:

[0068] - elimination of one end 34 of the end part 26 of the tube, over a length such that the cavity 32 is no longer covered by the end portion 26;

[0069] - insertion of a plug 36 into the bore 8, the cavity 32 being fluidically isolated from the primary fluid circulation compartment 4 by the plug 36, the cavity 32 fluidly communicating with an internal volume of the tube 24.

[0070] The length of the end 34 of the tube which is removed is a function of the position of the cavity 32 along the bore. The length of tube removed is chosen to expose the cavity 32.

[0071] If the internal surface 14 carries one or more other cavities 30, as illustrated in [Fig.2], the length of tube removed is preferably chosen to also uncover the other cavities 30.

[0072] Advantageously, as illustrated in [Fig. 3], the cavity 32 is remachined before the step of inserting the plug. This remachining step aims to restore the surface condition by eliminating the corroded parts.

[0073] If shallow cavities 30 are present, the internal surface 14 can also be remachined at the level of the cavities 30, to eliminate these corroded areas and restore the surface condition.

[0074] In the embodiment of [Fig. 3], the plug 36 is of short length, said length being taken axially along the central axis C of the bore 8.

[0075] It is engaged in the first end 10 of the bore. It does not extend axially to the cavity 32. It does not extend straight, that is to say opposite the cavity 32.

[0076] In other words, unlike the other embodiments, the plug 36 only comprises a first section engaged in the first end of the bore, and does not comprise a second section, axially extending the first section and extending in line with the cavity 32, that is to say opposite the cavity 32.

[0077] The plug 36 closes the first end 10 of the bore 8. It is welded in a sealed manner to the tube plate 2.

[0078] Typically, as illustrated in [Fig. 3], the plug 36 is substantially level with the first large face 2a and a weld bead 38 secures the plug 36 to the first large face 2a.

[0079] The other end of the tube 24 is closed by a conventional plug. Thus, the internal volume of the tube 24 is entirely isolated from the primary fluid.

[0080] Advantageously, this internal volume is placed in fluid communication with the secondary fluid circulation compartment 6. This is achieved for example by drilling a hole in the tube 24. Thus, the cavity 32 is only in contact with the secondary fluid, which is not or only slightly corrosive with respect to the material constituting the tube plate 2.

[0081] In the embodiment of [Fig. 3], the cavity 32 is not covered by the end portion 26 of the tube nor by the plug 36. The fluid can therefore circulate freely between the cavity 32 and the internal volume of the tube 24.

[0082] It should be noted that, as illustrated in [Fig.3], the tube plate 2 comprises a main part 40, covered by a coating 42. The coating 42 defines the first large face 2a and protects the main part 40 from the primary fluid.

[0083] The main part 40 is made of an alloy steel such as 18MND5 or equivalent.

[0084] The coating 42 is typically made of an alloy of nickel, chromium, iron such as for example Inconel™ 1600 or 1690.

[0085] The plug 36 is housed in the thickness of the coating 42. It is fixed only to this coating 42. The weld 38 therefore has a double function: to create a seal between the plug 36 and the tube plate 2, and to ensure a connection of the plug 36 to the tube plate 2 having sufficient mechanical strength.

[0086] This connection absorbs the forces experienced by the plug and transmits them to the tube plate. These forces result in particular from the pressure difference between the two sides of the plug. The connection prevents the plug from being ejected from the bore.

[0087] Several other embodiments of the invention will now be described.

[0088] A second embodiment is illustrated in [Fig.4].

[0089] Only the points by which this second embodiment differs from the first will be detailed below. Elements that are identical or provide the same function in both embodiments will be designated by the same references.

[0090] In the second embodiment, the plug 36 is longer than in the first embodiment.

[0091] The plug 36 comprises a first section 44, engaged in the first end 10 of the bore, between the first large face 2a and the cavity 32.

[0092] In the first embodiment, the plug 36 comprises only this first section.

[0093] In the second embodiment, the plug 36 comprises, in addition to the first section 44, a second section 46, axially extending the first section 44 and extending in line with the cavity 32, that is to say opposite the cavity 32.

[0094] As in the first embodiment, the method according to the second embodiment comprises a step of fixing the first section 44 in a sealed manner to the tube plate 2.

[0095] Typically, this is achieved by means of a peripheral weld 48, securing the end of the plug 36 to the first large face 2a.

[0096] Unlike the first embodiment, the weld 48 only has the function of providing a seal, and does not have the function of providing a connection between the plug 36 and the tube plate 2 ensuring mechanical strength, in particular the absorption of forces exerted on the plug and transmission to the tube plate.

[0097] The first section 44 has an external surface 50 in contact with the internal surface 14 of the bore.

[0098] The second section 46 also has an external surface 52 in contact with the internal surface 14 of the bore.

[0099] The plug 36 is mechanically fixed to the tube plate 2 by expanding. This expanding creates a sufficiently strong connection to prevent the plug 36 from being ejected from the bore 8 under the effect of a pressure difference between the two sides of the plug.

[0100] More precisely, the second section 46 of the plug is fixed to the internal surface 14 of the bore by expanding.

[0101] Advantageously, the first section 44 is also fixed to the internal surface 14 by expanding.

[0102] Expanding is a mechanical operation consisting of radially deforming the plug, so as to increase its diameter, and to press it against the internal surface 14 of the bore.

[0103] To do this, as visible in [Fig.4], the plug 36 is hollow.

[0104] The expanding is carried out using a tool comprising rotating rollers around the central axis C. These rollers achieve the radial expansion of the plug 36. They are moved axially, along the central axis C, inside the plug 36.

[0105] As visible in [Fig.4], the plug 36 comprises for this purpose an extension 53, axially extending the second section 46 towards the inside of the tube plate. The extension 53 has a reduced diameter compared to the first and second sections 44, 46. It is connected to the second section 46 by an intermediate part 54. It is engaged inside the tube 24.

[0106] The intermediate part 54 is frustoconical in the example shown.

[0107] The extension 53 has an external diameter smaller than the internal diameter of the tube 24, such that a free annular space 55 remains between the extension 53 and the wall of the tube 24.

[0108] The extension 53 has a hollow internal volume, configured to receive an axis (not shown) provided for the movement of the expanding rollers.

[0109] Advantageously, the method comprises, before the step of inserting the plug 36, a step of removing a surface layer from the internal surface 14.

[0110] The surface layer is removed from the first large face 2a, going up axially beyond the cavity 32.

[0111] It is removed to a depth less than or equal to the determined depth E.

[0112] Thus, the cavity 32 remains under the plug 36.

[0113] Advantageously, the step of eliminating the surface layer makes it possible to eliminate the cavities 30 of lesser depth which could be found on the internal surface 14 of the bore, close to the cavity 32.

[0114] Typically, the depth of the removed surface layer, and the length of the removed surface layer, are chosen to allow the use of a plug 36 of an already qualified type.

[0115] This removal step is followed by a visual inspection step of the internal surface 14, making it possible to ensure that the oxidized parts have been completely removed.

[0116] If oxidized parts remain, the reworking is repeated, until visual checks show total elimination of the oxidized parts.

[0117] According to the invention, a passage 56 places the cavity 32 in fluid communication with the internal volume of the tube 24.

[0118] This passage 56 comprises at least one groove 58 formed in the internal surface 14 of the bore 8.

[0119] The passage 56 comprises a single groove 58, or several grooves 58.

[0120] The grooves 58 are typically axial.

[0121] Each groove 58 extends from the cavity 32 to an area of ​​the internal surface 14 located axially beyond the second section 46.

[0122] The or each groove 58 therefore opens into the volume 59 of the bore located around the intermediate part 54. The volume 59 communicates with the internal volume of the tube 24 via the free annular space 55.

[0123] In the example shown, the or each groove 58 is made in the part of the internal surface 14 from which the surface layer has been removed.

[0124] In this second embodiment, the cavity 32 is isolated from the primary fluid circulation compartment 4 by the first section 44 of the plug, fixed in a sealed manner to the first large face 2a.

[0125] The external surface of the second section 46 closes the cavity 32 towards the inside of the bore. On the other hand, the passage 56 puts the internal volume of the cavity 32 in communication with the internal volume of the tube 24. Thus, when hot, the steam which could form inside the cavity 32 can escape towards the inside of the tube 24.

[0126] According to a variant not shown, the passage 56 comprises at least one groove formed in the external surface of the plug 36.

[0127] More precisely, the or each groove 58 is provided in the external surface 52 of the second part 46 of the plug.

[0128] Axially, it extends from a point on the plug located at right angles to the cavity 32, to the end of the second section 46 facing the tube 24.

[0129] Thus, the or each groove 58 places the internal volume of the cavity 32 in communication with the internal volume of the tube 24.

[0130] A third embodiment of the invention will now be described, with reference to [Fig. 5]. Only the points by which this third embodiment differs from the second will be detailed below.

[0131] The third embodiment differs from the second embodiment first by the method of fixing the cap 36.

[0132] As seen in [Fig.5], another plug 60 is engaged in the tube 24, typically in the end portion 26 of the tube.

[0133] This plug 60 is pre-existing. It was installed before the implementation of the present method, to close the tube 24.

[0134] Typically, it was put in place because the tube 24 had a leak, putting the interior of the tube in communication with the secondary fluid circulation compartment 6.

[0135] The plug 36 is fixed to the other plug 60. This fixing is sufficiently strong to take up the forces exerted on the plug 36.

[0136] These forces are transmitted to the tube plate 2 by the other plug 60.

[0137] The attachment of the plug 36 to the other plug 60 comes in addition to or instead of the fixing of the plug 36 to the internal surface of the bore at the level of the second section 46, provided by expanding in the second embodiment.

[0138] The extension 53 comprises a solid barrel 62 constituting the axial end of the extension 53. The barrel 62 carries an external thread 64. It is engaged in an open axial end 66 of the plug 60, which carries an internal thread 68.

[0139] The external thread 64 cooperates with the internal tapping 68 to mechanically lock the plug 36 relative to the tube plate 2.

[0140] In this case, the passage 56 comprises a conduit 70 arranged inside the plug 36.

[0141] The conduit 70 comes in addition to the groove(s) 58 provided at the level of the second section 46 of the plug, either on the internal surface 14 of the bore or on the external surface 52 of the second section 46.

[0142] As described above, the or each groove 58 allows the cavity 32 to be placed in fluid communication with the volume 59, located axially between the second section 46 of the plug and the end portion 26 of the tube. The volume 59 of the bore is located around the intermediate portion 54 and around the base of the extension 53. The free annular space 55, extending between the external surface of the extension 53 and the internal surface of the tube 24, communicates fluidly with the volume 59. On the other hand, the barrel 62 and the plug 60 together close off the entire internal section of the tube 24.

[0143] The conduit 70 is arranged to put the volume 59 into fluid communication with the internal volume 72 of the plug 60.

[0144] More precisely, it places the free annular space 55 in fluid communication with the internal volume 72 of the plug 60.

[0145] The conduit 70 is provided in the extension 53. It has a part opening into the free annular space 55, and another part provided in the barrel 62 and opening into the internal volume 72 of the plug 60.

[0146] In the example shown, the internal volume 72 normally opens out at the end 66, but is closed at the opposite axial end 76. A ring 78 is housed in the internal volume 72. The ring 78 has an axial internal passage 80, carrying a thread.

[0147] In a known manner, the ring 78 is moved axially inside the internal volume 72, so as to expand the plug 60 at the time of its installation.

[0148] According to the invention, an orifice 82 is made at the axial end 76 of the plug 60. The orifice 82 places the internal volume 72 of the plug in fluid communication with the internal volume of the tube 24 located axially beyond the plug 60. The orifice 82 is placed in the extension of the internal passage 80. The internal volume 72 of the plug 60 therefore communicates with the internal volume of the tube 24 through the passage 80 and the orifice 82.

[0149] The orifice 82 is drilled in the plug 60 before the plug 36 is put in place.

[0150] A fourth embodiment will now be described, with reference to [Fig.6].

[0151] Only the points by which the fourth embodiment differs from the second embodiment will be detailed below. Elements that are identical or provide the same function will be designated by the same references.

[0152] In the fourth embodiment, the second section 46 of the plug is fixed to the internal surface 14 of the bore by screwing.

[0153] The external surface 52 of the second section 46 carries a thread 84, the internal surface 14 carrying a tapping 86. The thread 84 and the tapping 86 cooperate to mechanically fix the plug 36 to the tube plate 2.

[0154] As in the second and third embodiments, the passage 56 ensures the fluid communication of the cavity 32 with the internal volume of the tube 24.

[0155] In the fourth embodiment, the passage 56 comprises a conduit 88 formed in the second section 46 of the plug 36.

[0156] In the fourth embodiment, the first section 44 of the plug is hollow. The second section 46 of the plug is solid. However, it comprises a hollow imprint 90, open towards the internal volume of the first section 44. The hollow imprint 90 has a section adapted to receive a tool for screwing the plug 36 into the bore 8.

[0157] In the example shown, this imprint has a hexagonal shape.

[0158] The passage 56 has one or more grooves 58 of the type described above, but of short length.

[0159] The conduit 88 opens at one end into the groove(s) 58, and at an opposite end into a part of the bore 8 communicating with the internal volume of the tube 24, or directly into the internal volume of the tube 24.

[0160] For example, the conduit 88 comprises a diametrical conduit 92, perpendicular to the axis central C, extending over an entire diameter of the second section 46. The diametrical conduit 92 opens at its two opposite ends. At least one of the two ends opens into the groove(s) 58.

[0161] The conduit 88 also comprises an axial conduit 94. This axial conduit extends substantially along the central axis C. At one end, it opens into the diametral conduit 92. At its opposite end, it opens inside the bore 8, or directly inside the tube 24.

[0162] According to an alternative embodiment, the imprint 90 is formed directly at the level of the first section 44.

[0163] In this case, it is possible to remove the grooves 58, and to place the diametrical conduit 92 such that it opens directly into the cavity 32.

[0164] According to another alternative embodiment, the conduit 88 does not comprise a diametrical conduit and an axial conduit. It is arranged so as to pass between the imprint 90 and the cavity 32, and to open directly into the cavity 32.

[0165] A fifth embodiment of the invention will now be described, with reference to [Fig.7].

[0166] Only the points by which the fourth embodiment differs from the second embodiment will be detailed below. Elements that are identical or provide the same function will be designated by the same references.

[0167] In this fifth embodiment, the plug 36 is mechanically fixed to the tube plate by means of a bayonet fixing 96.

[0168] In this case, the end of the second section 46 of the plug closest to the tube 24 carries at least one bayonet 98.

[0169] It carries only one bayonet 98, or two bayonets 98 diametrically opposed, or arranged in any other suitable configuration.

[0170] The bayonet 98 is a lug projecting radially from the external surface 52 of the section 46.

[0171] The bayonet attachment 96 also comprises a groove 100 formed in the internal surface 14 of the bore, axially between the cavity 32 and the tube 24.

[0172] For example, the groove 100 extends over the entire periphery of the bore 8, around the axis C.

[0173] The attachment 96 also comprises, for the or each bayonet 98, a groove 102 allowing said bayonet 98 to be inserted into the groove 100. The groove 102 is for example axially oriented. At one end it opens into the groove 100. At its opposite end it opens at the level of the first large face 2a.

[0174] The plug 36 is fixed to the tube plate 2 as described below.

[0175] Before introduction into the bore 8, it is placed in an orientation around the axis C such that the or each bayonet 98 is in the axial extension of the groove 102 corresponding.

[0176] Then, the plug 36 is engaged in the bore 8, the or each bayonet 98 moving inside the corresponding groove 102.

[0177] Once the bayonet 98 has reached the end of the groove 102, the plug 36 is moved in rotation around the axis C, to cause the or each bayonet 98 to penetrate into the groove 100.

[0178] The sealing weld 48 is then made between the first section 44 of the plug and the tube plate 2.

[0179] The plug 36 does not have an extension 53. For example, it is solid.

[0180] The cavity 32 is placed in fluid communication with the internal volume of the tube 24. by a passage comprising only one or more grooves as in the second embodiment, by a passage comprising one or more grooves plus an internal conduit provided in the plug as described in the fourth embodiment, or by a passage comprising only an internal conduit provided in the plug as described in a variant of the fourth embodiment.

[0181] The steam generator 1, after repair, therefore comprises at least one tube 24 for circulating the primary fluid having an end part 26 engaged in a bore 8 whose internal surface 14 has a cavity 32.

[0182] After repair, the end 34 of said end portion 26 has been removed over a length such that the cavity 32 is no longer covered by the end portion 26.

[0183] A plug 36 is inserted into the bore 8, the cavity 32 being fluidly isolated from the primary fluid circulation compartment 4 by the plug 36. On the other hand, the cavity 32 communicates fluidly with the internal volume of the tube 24.

[0184] The plug 36 comprises a first section 44 engaged in the first end 10 of the bore 8, between the first large face 2a and the cavity 32. The first section 44 is fixed in a sealed manner to the tube plate 2.

[0185] According to the first embodiment, the plug 36 comprises only this first section 44 ([Fig.3]).

[0186] According to the other embodiments (figures 4 to 7), the plug 36 also comprises a second section 46, axially extending the first section 44 and extending in line with the cavity 32, that is to say opposite the cavity 32.

[0187] An external surface 52 of the second section 46 is in contact with the internal surface 14 of the bore 8, a passage 56 putting the cavity 32 into fluid communication with the internal volume of the tube 24.

[0188] The passage 56 comprises at least one groove 58, formed in the internal surface 14 of the bore or in the external surface 52 of the plug.

[0189] Typically, the or each groove 58 is as described in relation to the second embodiment of the invention and illustrated in [Fig.4].

[0190] The or each groove 58, alone, puts the cavity 32 into fluid communication with the internal volume of the tube 24 ([Fig.4]).

[0191] Alternatively, the passage 56 comprises at least one conduit 70, 88 (figures 5 and 6) arranged inside the plug 36.

[0192] This conduit 70, 88 comes in addition to the or each groove 58. Alternatively, the passage 56 does not include a groove 58 and only includes the conduit.

[0193] The conduit is as described in the third embodiment or in the fourth embodiment.

[0194] The second section 46 of the plug is rigidly fixed to the internal surface 14 of the bore 8 by expanding, by screwing or by a bayonet fixing.

[0195] The expansion fixing is of the type described in relation to the second embodiment. The screw fixing is of the type described in relation to the fourth embodiment of the invention. The bayonet fixing is of the type described in relation to the fifth embodiment of the invention.

[0196] Alternatively, the second section 46 of the plug is rigidly fixed to another plug 60, engaged in the tube 24.

[0197] The attachment is of the type described in relation to the third embodiment.

[0198] The method according to the invention has multiple advantages.

[0199] Because the plug comprises a first section, engaged in the first end of the bore between the first large face and the cavity, the method comprising a step of fixing the first section in a sealed manner to the tube plate, the creation of a seal between the cavity and the first compartment can be organized in a simple manner. The fixing of the first section to the tube plate can be carried out conveniently, from the primary fluid circulation compartment, typically by welding. This type of operation is well controlled.

[0200] When the plug comprises a second section, axially extending the first section and extending in line with the cavity, an external surface of the second section being in contact with the internal surface of the bore, it is possible to mechanically fix the plug to the tube plate in a particularly solid manner. This fixing is carried out by different means: expanding, screwing, bayonet fixing. Such fixing methods are convenient and reliable.

[0201] In this case, it is imperative to provide a passage putting the cavity into fluid communication with the internal volume of the tube.

[0202] This passage can be conveniently achieved by a groove provided in the internal surface of the bore or in the external surface of the plug. Such a groove is easy to achieve.

[0203] The passage may further comprise at least one conduit provided inside the cap. Such a passage is also easy to produce.

[0204] When the tube has another plug, which was put in place during a previous episode in the life of the heat exchanger, it is convenient to provide that the second section of the plug is rigidly fixed to this other plug.

[0205] When the method comprises, before the step of inserting the plug, a step of removing the surface layer from the internal surface, from the first large face going up axially beyond the cavity, to a depth less than or equal to the determined depth, it is possible to use a plug that is already qualified. Furthermore, removing the surface layer makes it possible to clean the internal surface of the bore and to eliminate any shallow cavities located near the deep cavity.

Claims

Claims

1. A method for repairing a steam generator (1) of a nuclear reactor, the steam generator (1) comprising: - a tube plate (2), having a first large face (2a) delimiting a compartment (4) for circulation of a primary fluid, a second large face (2b) delimiting a compartment (6) for circulation of a secondary fluid, and a plurality of bores (8) each having a first end (10) opening at the first large face (2a) and a second end (12) opening at the second large face (2b), each bore (8) being delimited by an internal surface (14) and having a central axis (C); - at least one tube (24) for circulation of the primary fluid arranged in the secondary fluid circulation compartment (6), the tube (24) having two opposite end portions (26) engaged in two bores (8);the internal surface (14) of one of the bores (8) being corroded and having a cavity (32) of depth greater than a determined depth, said cavity (32) being located under the end portion (26) of the tube (24) engaged in said bore (8); the method comprising the following steps: - removing one end (34) of the end portion (26) of the tube (24), over a length such that said cavity (32) is no longer covered by the end portion (26); - inserting a plug (36) into the bore (8), the cavity (32) being fluidly isolated from the primary fluid circulation compartment (4) by the plug (36), the cavity (32) fluidly communicating with an internal volume of the tube (24).;

2. Method according to claim 1, in which the plug (36) comprises a first section (44), engaged in the first end (10) of the bore (8) between the first large face (2a) and the cavity (32), the method comprising a step of fixing the first section (44) in a sealed manner to the tube plate (2).

3. Method according to claim 2, in which the plug (36) comprises a second section (46) axially extending the first section (44) and extending in line with the cavity (32), an external surface (52) of the second section (46) being in contact with the internal surface (14) of the bore (8), a passage (56) putting the cavity (32) with the internal volume of the tube (24).

4. A method according to claim 3, wherein the passage (56) comprises at least one groove (58) formed in the inner surface (14) of the bore (8) or in the outer surface (52) of the plug (8).

5. A method according to claim 3 or 4, wherein the passage (56) comprises at least one conduit (70, 88) provided inside the plug (36).

6. A method according to any one of claims 3 to 5, wherein the second section (46) of the plug (36) is mechanically secured to the inner surface (14) of the bore (8) by swaging, screwing or a bayonet fitting (96).

7. Method according to any one of claims 3 to 6, in which the second section (46) of the plug (36) is rigidly fixed to another plug (60), engaged in the tube (24).

8. Method according to any one of the preceding claims, in which the method comprises, before the step of inserting the plug (36), a step of removing a surface layer from the internal surface (14), from the first large face (2a) going up axially beyond the cavity (32), to a depth less than or equal to the determined depth.

9. Steam generator of a nuclear reactor, the steam generator (1) comprising: - a tube plate (2), having a first large face (2a) delimiting a circulation compartment of a primary fluid (4), a second large face (2b) delimiting a circulation compartment of a secondary fluid (6), and a plurality of bores (8) each having a first end (10) opening at the first large face (2a) and a second end (12) opening at the second large face (2b), each bore (8) being delimited by an internal surface (14) and having a central axis (C); - at least one tube (24) for circulation of the primary fluid arranged in the circulation compartment of the secondary fluid (6), the tube (24) having two opposite end portions (26) engaged in two bores (8); the internal surface (14) of one of the bores (8) being corroded and having a cavity (32);- one end (34) of the end part (26) of the tube (24) engaged in said bore (8) having been eliminated, over a length such that; said cavity (32) is no longer covered by the end portion (26); - a plug (36) being inserted into the bore (8), the cavity (32) being fluidly isolated from the primary fluid circulation compartment (4) by the plug (36), the cavity (32) communicating fluidly with an internal volume of the tube (24).

10. Steam generator according to claim 9, in which the plug (36) comprises a first section (44), engaged in the first end (10) of the bore (8) between the first large face (2a) and the cavity (32), the first section (44) being fixed in a sealed manner to the tube plate (2).

11. Steam generator according to claim 10, in which the plug (36) comprises a second section (46) axially extending the first section (44) and extending in line with the cavity (32), an external surface (52) of the second section (46) being in contact with the internal surface (14) of the bore (8), a passage (56) putting the cavity (32) into fluid communication with the internal volume of the tube (24).

12. A steam generator according to claim 11, wherein the passage (56) comprises at least one groove (58) formed in the inner surface (14) of the bore (8) or in the outer surface (52) of the plug (36).

13. A steam generator according to claim 11 or 12, wherein the passage (56) comprises at least one conduit (70, 88) provided inside the plug (36).

14. A steam generator according to any one of claims 11 to 13, wherein the second section (46) of the plug (36) is mechanically secured to the inner surface (14) of the bore (8) by swaging, screwing or a bayonet fitting (96).

15. Steam generator according to any one of claims 11 to 14, in which the second section (46) of the plug (36) is rigidly fixed to another plug (60), engaged in the tube (24).

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