Method of servicing nuclear reactor

Abstract

The invention relates to a maintenance method of a control rod drive assembly. The method comprises the steps of: removing an initial weld (67) of the Omega seal (65) and removing the sheath (25); performing at least one maintenance operation on the drive assembly (17); cutting an edge (69) of a first lip (61) of the omega seal (65); obtaining a replacement sheath (25R) comprising a new second lip (71); a new sealing weld (73) is formed between the first lip (61) and the new second lip (71), the new weld (73) being angularly offset by an angle (alpha) of between 15 DEG and 80 DEG with respect to the initial weld.

Description

Technical Field

[0001] The present invention relates to maintenance of a control mechanism of a control rod assembly of a nuclear reactor. Background Art

[0002] A nuclear reactor generally comprises a pressure vessel in which are arranged nuclear fuel assemblies which together form the core of the nuclear reactor. The reactivity of the core is controlled inter alia by driving control rod assemblies into the core, each rod assembly comprising a plurality of rods made of neutron absorbing material.

[0003] Each control rod assembly is attached to the end of a drive rod which is moved by a rod assembly drive mechanism (MCG in French: Mécanisme de Commande de Grappe).

[0004] The rod assembly drive mechanisms (MCGs) are each used to extract, hold or insert control rod assemblies in the reactor core. The rod assembly drive mechanisms (MCGs) must also allow the rod assembly drive rods to fall freely when the reactor's automatic cut-off circuit breaker is opened.

[0005] The nuclear reactor thus comprises a plurality of control assemblies, each dedicated to one control rod assembly. Each drive assembly comprises an adapter rigidly attached to the cover of the pressure vessel, a housing for receiving a lifting mechanism for lifting the drive rods of the control rod assembly, and a sheath for receiving the drive rods. The housing, lifting mechanism and sheath form an MCG.

[0006] The adapter extends through the container cover and is rigidly attached to the container cover. The adapter defines a channel through which the drive rod moves.

[0007] The lifting mechanism is often called the lifting mechanism assembly (EML in French: Ensemble Mécanisme de Levée). The mechanism is an actuator whose function is to control, insert or extract the drive rods of the control rod assembly in the reactor core.

[0008] The housing and jacket together form a pressure enclosure in fluid communication with the interior of the pressure vessel through the adapter.

[0009] The housing has a housing lower end that is usually screwed to the upper end of the adapter. The seal between the lower end of the housing and the upper end of the adapter is achieved by a welded seal, which is usually called a "Canopy" seal.

[0010] The jacket forms the upper end of the pressure housing. The jacket defines an inner volume in which the drive rods are housed at a high position of the control rod assembly. The inner volume is closed at the top and communicates with the inner volume of the casing at the bottom.

[0011] In order to form a sealed connection between the sheath and the housing, a first lip may be provided at the upper end of the housing and a second lip may be provided at the lower end of the sheath. The first lip and the second lip seal against each other and form a seal known as an "Omega" seal.

[0012] Since the EML consists of moving parts that clamp and move the drive rod, it is subject to wear phenomena which make it necessary to replace it after millions of cycles.

[0013] In most cases, replacement is of a rod assembly drive mechanism (MCG) that has reached the end of its service life. Sometimes, an MCG can be replaced due to a fault.

[0014] One solution for replacing the rod assembly drive mechanism in the field (RMCG in French: Remplacement d'un Mécanisme de Commande de Grappe) is to replace the entire MCG by intervening at the "Canopy" seal. This operation involves replacing the lifting mechanism assembly EML and the pressure housing (jacket + casing).

[0015] RMCG's "Canopy" solution can include the following operations:

[0016] - cutting of the "Canopy" seal (located between the housing and the adapter of the cover),

[0017] - Remove the entire MCG,

[0018] - machining the adapter to restore the weldable profile,

[0019] - reinstall new MCG (possible adjustments exist to pair with the adapter),

[0020] - Reconstruction of welded "Canopy" seals,

[0021] - Associated re-identification.

[0022] Another solution could be to intervene at the “Ω-type” seal by cutting along the axis of the original weld.

[0023] The “Ω-type” scheme of RMCG may include the following operations:

[0024] - cutting of the "Ω-type" seal (between the jacket and the casing of the MCG) along the axis of the weld in order to maintain the initial design of the weld seal (vertical cut),

[0025] -Remove the sheath and EML,

[0026] - machining the lip of the housing in order to restore a weldable profile identical to the initial profile,

[0027] - Reinstallation with new EML and new sheath or re-processing of the original sheath on site,

[0028] - Welding of the same "Ω-type" seal as the original design,

[0029] -Re-identification of associations.

[0030] Such a solution is not entirely satisfactory.

[0031] RMCG’s “Canopy” solution requires:

[0032] - Replacement of the complete MCG including the pressure casing (jacket + shell),

[0033] -Remove / reinstall the cover insulation which has an impact on the operational plan.

[0034] The "Ω-type" RMCG utilizing vertical cuts to the weld seal is also unsatisfactory in the following respects.

[0035] The EML replacement test was performed by intervention at the "Ω-type" seal.

[0036] After cutting along the axis of the weld of the "Ω" seal (vertical cut), dye penetrant inspection was performed to ensure that the surface finish was free of defects. In an assembly of more than 50 cut MCGs, 38% of the dye penetrant inspections showed unacceptable surface defects in the thin lip.

[0037] The random and unpredictable presence of defects makes it impossible to prepare adequate controls for the site. Defects require systematic repair and justification. In most cases, this may not even prove to be repairable, or be repairable at the expense of developing complex methods.

[0038] As an emergency, intervention at the "Canopy" seal will be required.

[0039] In this context, the present invention aims to propose a method for maintaining a nuclear reactor which enables reliable and economical maintenance of the rod assembly drive assembly. Summary of the invention

[0040] To this end, the invention relates to a method for maintaining a nuclear reactor comprising at least one drive assembly for a control rod assembly, the drive assembly or each drive assembly comprising an adapter rigidly attached to a cover of a pressure vessel of the nuclear reactor, a housing receiving a lifting mechanism for lifting drive rods of the control rod assembly and a jacket for receiving the drive rods, the housing having a housing lower end connected to the adapter in a leakproof manner, the housing having a first lip at the housing upper end, the jacket having a second lip at the jacket lower end, the first lip and the second lip being welded to each other by an initial weld and forming together an omega-shaped seal, the omega-shaped seal having an initial shape obtained by rotating an initial arc about an axis of rotation, the initial arc having an initial geometric centre, an initial first arc belonging to the first lip and an initial second arc belonging to the second lip, the initial second arc being connected to the initial first arc by an initial welding point, the initial welding point defining together with the initial geometric centre a line extending along a reference direction, the maintenance comprising the following steps:

[0041] - removing the initial weld and removing the sheath;

[0042] - performing at least one maintenance operation on the drive assembly;

[0043] - cutting the edge of said first lip;

[0044] - Obtain a replacement sheath including a new second lip;

[0045] -A new sealing weld is formed between the first lip and the new second lip, the first lip and the new second lip together defining a new Ω-type seal, the new Ω-type seal having a shape obtained by rotating a new arc around the rotation axis, the new arc having a new geometric center, a new first arc belonging to the first lip and a new second arc belonging to the new second lip, the new second arc being connected to the new first arc via a new welding point, the new welding point together with the geometric center defining a line extending along a new direction, the new direction being angularly offset relative to the reference direction by an angle comprised between 15° and 80°.

[0046] Thus, the operation is carried out at the "Ω-type" seal. The housing therefore remains in place and does not have to be replaced. Only the EML and, if appropriate, the sheath are replaced.

[0047] No need to remove the cover's insulation. It produces a dose gain.

[0048] This saves planning time compared to RMCG's "Canopy" solution.

[0049] Furthermore, the inherent defect risk in the “Ω-type” RMCG described above is more limited.

[0050] The sealed welded connection between the housing and the sheath is re-formed between the new second lip and the surface of the first lip obtained by removing the edge of the first lip. This surface is free of defects that have been removed by cutting the edge of the original first lip.

[0051] Since the edge of the original first lip has been removed, this particular surface is not an area that is affected by the heat of the formation of the original weld seal.

[0052] The maintenance method may also have one or more of the following features, considered individually or in all technically possible combinations:

[0053] - the replacement sheath is an initial sheath on which the new second lip has been formed;

[0054] - the replacement sheath is a new sheath;

[0055] - the method comprises a step of inspecting the surface condition of said first lip between the step of removing said initial weld and the step of cutting in order to detect any defects, the width of the edge of said first lip cut at said cutting step being selected in order to remove defects, if any;

[0056] - the rod moves along a movement axis, the reference direction being parallel to the movement axis;

[0057] - in the step of removing the initial weld, cutting the initial weld along the reference direction or along a direction forming an angle of less than 15° with the reference direction;

[0058] - the new second arc is longer than the initial second arc;

[0059] - the new omega seal has the same shape as the original omega seal;

[0060] - said new sealing weld is angularly offset from said initial weld towards said first lip. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Other characteristics and advantages of the invention will become apparent from the following description of the invention given by way of example and in no way limiting with reference to the accompanying drawings, in which:

[0062] Figure 1 It is a simplified diagram of a nuclear reactor;

[0063] Figure 2 yes Figure 1 A cross-sectional view of a rod assembly drive mechanism of a nuclear reactor is shown;

[0064] Figure 3 is an enlarged view of the "Canopy" seal of the welded connection between the housing and the adapter, Figure 2 The rod assembly drive mechanism is shown mounted on the adapter;

[0065] Figure 4 Before implementing the maintenance method of the present invention, Figure 2 An enlarged view of an "Ω-type" seal of a welded connection between the housing and the sheath of a rod assembly control mechanism is shown; and

[0066] Figure 5 After implementing the method of the present invention Figure 4 A view similar to the one shown. DETAILED DESCRIPTION

[0067] Figure 1 The nuclear reactor 1 shown comprises a pressure vessel 3 in which nuclear fuel assemblies 5 are arranged to form a core 7 of the nuclear reactor.

[0068] The pressure vessel 3 comprises a lower part 9 and a cover 11 which is removably attached to the lower part 9 .

[0069] The nuclear reactor 1 further comprises a control rod assembly 13 which can be inserted into or withdrawn from the core 7 in order to control the reactivity of the core.

[0070] Each rod assembly 13 includes a plurality of rods made of neutron absorbing material.

[0071] Each control rod assembly 13 is attached to the end of a drive rod 15 and as such is moved by a rod assembly drive mechanism (MCG).

[0072] The rod assembly drive mechanisms (MCGs) are each used to extract, hold or insert control rod assemblies in the reactor core. The rod assembly drive mechanisms (MCGs) must also allow the rod assembly drive rods to fall freely when the reactor's automatic cut-off circuit breaker is opened.

[0073] The nuclear reactor 1 thus comprises a plurality of control assemblies 17 , each dedicated to one control rod assembly 13 .

[0074] Each drive assembly 17 includes an adapter 19 rigidly attached to the cover 11 of the pressure vessel, a housing 21 receiving a lifting mechanism 23 for lifting the drive rods 15 of the control rod assembly 13, and a sheath 25 ( Figure 1 and Figure 2 ).exist Figure 1 In FIG. 1 , the sheath is not shown so that the upper end of the drive rod 15 is visible.

[0075] The adapter 19 engages in a through hole of the container's cover 11 and is rigidly attached to the container's cover 11. The adapter 19 extends through the container's cover 11 and defines a passage through which the drive rod 15 moves.

[0076] The lifting mechanism 23 is generally referred to as a lifting mechanism assembly (EML). This mechanism is an actuator that functions to control, insert or extract the drive rod 15.

[0077] The housing 21 and the jacket 25 together form a pressure enclosure which is in fluid communication with the interior of the pressure vessel via the adapter 19 .

[0078] The lifting mechanism 23 , the housing 21 and the sheath 25 form a subassembly generally denoted by the term “rod assembly drive mechanism” (MCG: Mécanisme de Commande de Grappe) .

[0079] The housing 21 is a tubular portion, the central axis of which corresponds substantially to the axis of movement X of the drive rod 15. This housing 21 delimits internally an inner volume in which the drive rod 15 engages. A lifting mechanism 23 is housed between the rod and the wall of the housing.

[0080] In the present description, the terms “lower” and “upper”, “bottom” and “top” are to be understood along the axis X of movement of the drive rod 15 .

[0081] This axis is usually vertical.

[0082] The housing 21 has a housing lower end 27 which is connected to the adapter 19 in a leak-proof manner.

[0083] More precisely, the housing lower end 27 is usually screwed onto the upper end 29 of the adapter 19 .

[0084] The seal between the lower end 27 of the housing and the upper end 29 of the adapter is provided by a welded seal 31, which is commonly referred to as a "Canopy" seal ( Figure 3 ).

[0085] The jacket 25 forms the upper part of the pressure vessel.

[0086] The jacket 25 defines an inner volume body 33 in which the drive rod 15 is accommodated at a high position of the control rod assembly 13. The inner volume body 33 is closed upward and communicates with the inner volume body of the housing 21 downward.

[0087] The housing 21 has an upper housing end 35 having a tubular shape around the axis X of movement.

[0088] from Figure 2 and Figure 4As can be seen in FIG. 1 , the housing upper end 35 has a substantially annular housing upper surface 37 which is perpendicular to the displacement axis X and rotated away from the adapter 19 .

[0089] The housing upper end portion 35 also has a housing radial inner surface 39 and a housing radial outer surface 41;

[0090] The sheath 25 has a sheath lower end 43 which engages in the housing upper end 35 and is fastened thereto.

[0091] To this end, the lower end 43 of the sheath has an external thread 45 cooperating with a thread 47 supported by the radial inner surface 39 of the housing.

[0092] The sheath 25 also has a main portion 49 that is located outside the housing 21 .

[0093] An annular rib 51 is provided on the lower end 43 of the sheath at the junction with the main portion 49. The annular rib 51 is located above the external thread 45.

[0094] The sheath lower end 43 has a substantially annular sheath upper surface 53 which is perpendicular to the displacement axis X and which rotates relative to the adapter 19 .

[0095] The jacket upper surface 53 is defined by the annular rib 51 .

[0096] The jacket upper surface 53 extends substantially in the same plane as the housing upper surface 37 .

[0097] The jacket lower end 43 also has a radially outer surface 55 .

[0098] The radially outer surface 55 is defined by an annular rib 51 .

[0099] The radially outer surface 55 is pressed against the radially inner surface 39 of the housing.

[0100] The annular rib 51 also defines a jacket lower surface 57 which is substantially annular and perpendicular to the movement axis X. The jacket lower surface 57 bears on a shoulder 59 formed in the radially inner surface 39 of the upper end 35 of the housing.

[0101] like Figure 4 As shown, before the method of the present invention is implemented, the housing 21 has a first lip 61 at the housing upper end 35 .

[0102] The sheath 25 has a second lip 63 at the sheath lower end 43 .

[0103] The first lip 61 and the second lip 63 are welded to each other in a sealed manner by means of an initial weld 67 .

[0104] The first lip 61 and the second lip 63 together form an omega-shaped seal 65 .

[0105] The housing upper surface 37 supports the first lip 61 .

[0106] The jacket upper surface 53 supports the second lip 63 .

[0107] The omega-type seal 65 has an initial shape obtained by rotating an initial arc line 65 a around a rotation axis.

[0108] This axis of rotation corresponds substantially to the axis of movement X of the drive rod.

[0109] Figure 4 The initial arc 65a is shown in FIG. The initial arc 65a substantially corresponds to a cross section of the Ω-type seal 65 in a radial plane containing the axis of rotation X.

[0110] The initial arc 65 a has a certain thickness, which corresponds to the thickness of the Ω-shaped seal 65 .

[0111] The initial arc 65a has an initial geometric center C.

[0112] When the initial arc line 65a is a circular arc, the initial geometric center C corresponds to the center of the circular arc.

[0113] When the initial arc 65a is not a circular arc, the initial geometric center C is, for example, the isocenter of all points of the initial arc. In a variant, the geometric center C is the isocenter of all points of the inner surface having the initial arc 65a, or the isocenter of all points of the outer surface having the initial arc 65a.

[0114] The initial arc line 65 a has an initial first arc 61 a belonging to the first lip 61 and an initial second arc 63 a belonging to the second lip 63 , and the initial second arc 63 a is connected to the initial first arc 61 a through an initial welding point P.

[0115] The initial welding point P belongs to the initial weld seam 67 .

[0116] The initial weld bead 67 has a shape obtained by rotating the initial welding point P around the rotation axis X.

[0117] The initial welding point P together with the initial geometric center C defines a line L extending along a reference direction.

[0118] exist Figure 4 It can be seen that the initial welding point P has a certain surface area.

[0119] To define the line L, for example the isobarycenters of all points forming part of the surface are considered.

[0120] Typically, the reference direction is parallel to the axis of movement X of the rod.

[0121] The first lip 61 and the second lip 63 each extend around the axis X and have a closed profile. They are curved towards each other.

[0122] The initial first arc 61a and the initial second arc 63a each generally have a shape of a quarter circle. The initial arc 65a is a semicircle with point C as the center.

[0123] The initial shape of the omega seal 65 thus corresponds to a semi-annular surface coaxial with the axis X.

[0124] The first lip 61 and the second lip 63 delimit, together with the housing upper surface 37 and the jacket upper surface 53 , a cavity extending around the axis X.

[0125] The seal 65 is referred to as omega-shaped due to its shape.

[0126] The purpose of the maintenance method is to access the drive assembly 17 in order to perform maintenance operations thereon.

[0127] The maintenance method includes the following steps:

[0128] - remove the initial weld 67 and remove the sheath 25;

[0129] - performing at least one maintenance operation on the drive assembly 17;

[0130] - cutting edge 69 of first lip 61;

[0131] - obtaining a replacement sheath 25R comprising a new second lip 71 ;

[0132] A new sealing weld 73 is formed between the first lip 61 and the new second lip 71 , which together define a new omega-shaped seal 73 .

[0133] During the step of removing the initial weld 67 , said initial weld 67 is cut along the reference direction L or along a direction forming an angle with the reference direction L of less than 15°.

[0134] Removal is usually performed by remote operation, by any suitable means (grinding wheel, saw, machining machine, etc.).

[0135] After removal of the sheath 25, the EML 23 can be accessed.

[0136] At least one maintenance operation typically includes one or more of the following operations:

[0137] - Replace or repair the sheath 25,

[0138] - Replace or repair EML 23,

[0139] - Repair the upper end 35 of the housing.

[0140] The method preferably comprises a step of inspecting the surface condition of the first lip 61 to detect possible defects between the step of removing the initial weld 67 and the step of cutting.

[0141] Such defects are typically the presence of cracks or cavities which may result in the rejection of the new sealing weld 73 .

[0142] Inspection shall be carried out by dye penetrant inspection, radiography or any other suitable method.

[0143] The edge 69 is the edge of the first lip that was initially welded to the second lip by the weld 67. This edge 69 is cut over the entire periphery of the first lip 61. Therefore, the portion of the initial weld that remains attached to the first lip 61 can be removed at the end of the removal step. Therefore, a new surface with a good surface condition can be formed to weld the first lip 61 to the new second lip 71.

[0144] The edge 69 of the first lip is cut to a width selected so as to eliminate any defects detected during the inspection step.

[0145] The cut width herein corresponds to the dimension taken about the axis of rotation in a radial plane.

[0146] In a variation, the inspection step is not performed and the edge of the first lip is cut to a predetermined width.

[0147] Cutting is usually performed remotely, by any suitable means (laser, saw, machining center, etc.).

[0148] The replacement sheath 25R is, for example, an initial sheath 25 on which a new second lip 71 has been formed.

[0149] The new second lip 71 is formed by any suitable means: welding an extension to the initial second lip 63 , removing the initial second lip 63 and fastening the new second lip 71 instead of the initial second lip, etc.

[0150] In a variant, the replacement sheath 25R is a new sheath that has never been used in a nuclear reactor. This replacement sheath 25R is manufactured directly with a new second lip 71 having the required dimensions.

[0151] According to another variant, the replacement sheath 25R is a reused sheath, but coming from another drive assembly 17, from the same nuclear reactor or from another nuclear reactor. The sheath is then modified so as to include a new second lip of the desired dimensions.

[0152] New welds are usually made by remote operation or manually. The welding process is the same as the initial weld. Typically, the welding method is TIG / GTAW welding.

[0153] The new Ω-type seal 75 has a shape obtained by rotating the new arc line 75 b around the rotation axis X.

[0154] New arc 75b in Figure 5 This new arc line 75b substantially corresponds to a cross section of the new Ω-type seal 75 in a radial plane containing the axis of rotation X.

[0155] The new arc 75 b has a certain thickness, which corresponds to the thickness of the new Ω-shaped seal 75 .

[0156] The new arc 75b has a new geometric center C'.

[0157] The new geometric center C' is defined as the initial geometric center C.

[0158] The new arc line 75b has a new first arc 61b belonging to the first lip 61 and a new second arc 71b belonging to the new second lip 71, and the new second arc 71b is connected to the new first arc 61b through a new welding point P'.

[0159] The new spot weld P′ belongs to the new weld seam 73 .

[0160] The new weld seam 73 has a shape obtained by rotating the new weld point P′ around the axis of rotation X.

[0161] The new welding point P' is defined as the initial welding point P.

[0162] The new welding point P′ defines, together with the new geometrical centre C′, a line extending along a new direction L′ which is angularly offset relative to the reference direction L by an angle α comprised between 15° and 80°.

[0163] This offset results from the fact that the edge 69 of the first lip 61 has been cut.

[0164] The angle α is preferably between 25° and 70°, otherwise preferably between 30° and 60°.

[0165] The new second arc 71b is longer than the initial second arc 63a.

[0166] The new first arc 61b is shorter than the initial first arc 61a.

[0167] Thus, the new weld 73 is angularly offset relative to the initial weld 67 towards the first lip 61 , ie, angularly offset from the first lip side.

[0168] On the other hand, the new omega seal 75 preferably has substantially the same shape as the original omega seal 65 .

[0169] In particular, the new geometric center C' is substantially at the same position as the initial geometric center C.

Claims

1. A method of maintaining a nuclear reactor (1), the nuclear reactor comprising at least one drive assembly (17) for a control rod assembly (13), the drive assembly (17) or each drive assembly (17) comprising an adapter (19) rigidly attached to a cover (11) of a pressure vessel (3) of the nuclear reactor (1), a housing (21) receiving a lifting mechanism (23) for lifting a drive rod (15) of the control rod assembly (13), and a jacket (25) for receiving the drive rod (15), the housing (21) having a housing lower end (27) connected to the adapter (19) in a leakproof manner, the housing (21) having a first lip (61) at the housing upper end (35), the jacket (25) having a jacket lower end (43) a second lip (63), the first lip (61) and the second lip (63) being welded to each other by an initial weld (67) and forming together an omega seal (65), the omega seal (65) having an initial shape obtained by rotating an initial arc (65a) around an axis of rotation (X), the initial arc (65a) having an initial geometric center (C), an initial first arc (61a) belonging to the first lip (61) and an initial second arc (63a) belonging to the second lip (63), the initial second arc (63a) being connected to the initial first arc (61a) by an initial welding point (P), the initial welding point (P) together with the initial geometric center (C) defining a line extending along a reference direction (L), the maintenance comprising the following steps: - removing the initial weld (67) and removing the sheath (25); - performing at least one maintenance operation on the drive assembly (17); - cutting the edge (69) of said first lip (61); - obtaining a replacement sheath (25R) comprising a new second lip (71); - forming a new sealing weld (73) between the first lip (61) and the new second lip (71), the first lip (61) and the new second lip (71) together defining a new Ω-shaped seal (75), the new Ω-shaped seal (75) having a shape obtained by rotating a new arc (75b) around the rotation axis (X), the new arc (75b) having a new geometric center (C'), a new first arc (61b) belonging to the first lip (61) and a new second arc (71b) belonging to the new second lip (71), the new second arc (71b) being connected to the new first arc (61b) via a new welding point (P'), the new welding point (P') together with the geometric center (C') defining a line extending along a new direction (L'), the new direction (L') being angularly offset relative to the reference direction (L) by an angle (α) comprised between 15° and 80°.

2. Maintenance method according to claim 1, wherein the replacement sheath (25R) is the initial sheath (25) on which the new second lip (71) has been formed.

3. The maintenance method according to claim 1, wherein the replacement sheath (25R) is a new sheath.

4. The maintenance method according to any one of claims 1 to 3, wherein the method comprises: A step of monitoring the surface condition of the first lip (61) between the step of removing the initial weld (67) and the step of cutting in order to detect defects, the width of the edge (69) of the first lip (61) cut at the cutting step being selected in order to eliminate possible defects.

5. Maintenance method according to any one of claims 1 to 4, wherein the rod (25) moves along a movement axis (X), the reference direction (L) being parallel to the movement axis (X).

6. The maintenance method according to any one of claims 1 to 5, wherein in the step of removing the initial weld (67), the initial weld (67) is cut along the reference direction (L) or along a direction forming an angle less than 15° with the reference direction (L).

7. The maintenance method according to any one of claims 1 to 6, wherein the new second arc (71b) is longer than the initial second arc (63a).

8. The maintenance method according to any one of claims 1 to 7, wherein the new omega seal (75) has the same shape as the original omega seal (65).

9. Maintenance method according to any one of claims 1 to 8, wherein the new seal (73) is angularly offset from the initial seal (67) towards the first lip (61).

Images