FR2689297A1 - Cutting tube lining sleeves which penetrate nuclear reactor vessel cover - to enable repair of welds between sleeves and cover and replace the cut tubes - Google Patents

Abstract

The top cover (1) of a pressurised nuclear reactor vessel is penetrated by upright sleeves (3) welded to the underside of the cover and lined by respective tubes (5) suspended by their upper end flanges (5a) from shoulders at the tops of the sleeves and extending below the bottom ends of the sleeves. To enable examination and repair of a weld (4) between a sleeve (3) and cover (1), the lining tube (5) is cut in two by an inserted tool (10a) and at least its lower part is removed. Subsequently a new tube section is inserted and secured to the lower end of sleeve (3). These actions are performed from below, with cover (1) lifted from the reactor vessel. In one method the tube is cut level with cover (1) and only the lower part is removed, the new tube section extending up inside sleeve (3) almost to the level of the cut. Alternatively the tube (5) is cut below upper flange (5a) and both parts are removed and replaced by a single tube section extending the full length of the sleeve (3). Tube sections are secured to the lower ends of sleeves (3) by welding or by various screw couplings. USE - For repairing welds (4) between the reactor vessel cover (1) and sleeves (3), which accommodate control rod adjusting mechanisms or instrument cables e.g. to thermocouples.

Description

 The invention relates to a method for dismantling and replacing a thermal sleeve of a bushing of a vessel cover of a pressurized water nuclear reactor and a removable thermal sleeve for replacement.

 Pressurized water nuclear reactors generally include a vessel enclosing the reactor core which is immersed in water under reactor cooling pressure.

 The generally cylindrical reactor vessel has a hemispherical cover which can be attached to its upper part.

 The cover is pierced with openings at each of which is fixed by welding, in a vertical position, a tubular crossing piece constituting an adapter ensuring the passage of an extension of a cluster for controlling the reactivity of the core or a passage a measuring means inside the heart, such as a thermocouple column.

 The tubular passage piece has a lower part which projects below the cover of the tank.

 On the upper end parts of each of the adapters, the mechanisms for controlling the movement of the core reactivity control clusters are fixed.

 Inside each of the bushings of the tank cover, is fixed, in a coaxial position with respect to the tubular crossing piece and with a certain radial clearance, a thermal cuff which comprises a development coming to rest on a bearing surface diametrically enlarged located at the upper part of the bore of the tubular crossing piece and which is mounted free in rotation and in translation with a limited stroke inside the crossing.

 The extensions of the reactivity control rods of the reactor and the thermocouple columns pass through the vessel cover inside the thermal sleeves which are themselves arranged coaxially inside the adapters of the control rods or more generally at the l inside the tubular parts of the cover.

 In order to increase the reliability and operational safety of nuclear reactors and to extend the life of these reactors, operators are required to carry out more and more checks and repairs of the various elements constituting the nuclear reactor.

 For example, it may be necessary to check the condition of the parts passing through the cover of the tank to ensure the integrity of these parts after a certain time of operation of the reactor, in particular in the area where these tubular parts are welded to the tank cover.

 These checks, which must make it possible to detect and repair faults on the internal surface of the tubular crossing piece, must be carried out from inside the bore of the tubular piece and consequently require disassembly of the thermal sleeve, to access the interior surface of the crossing bore.

 These checks and repairs must be carried out during a shutdown of the nuclear reactor, the tank cover being removed.

 However, the disassembly of the thermal cuff requires complex operations, insofar as the widening of the thermal cuff resting on a bearing surface disposed at the upper part of the adapter prohibits disassembly of the cuff by pulling towards the bottom, -on its accessible lower part below the cover.

 In addition, the extraction of the cuff from the upper part of the bushing requires the dismantling of the corresponding cluster control mechanism which is fixed to the upper part of the bushing and which comprises a circular sealing weld.

 The intervention therefore requires disassembly and reassembly of a relatively large number of control mechanisms. These interventions are therefore long and delicate.

 In addition, at each intervention, a part of the lips is machined to which the control mechanisms are welded, so that these lips may become insufficient for the sealing welds to be carried out.

 The object of the invention is therefore to propose a method for dismantling and replacing a thermal sleeve of a bushing of the cover of the tank of a nuclear pressurized water reactor, the bushing being constituted by a fixed tubular piece. in vertical position on the tank cover in a through opening, so that a lower end portion of the bushing projects below the tank cover and the cuff being arranged in a coaxial position, at the interior of the crossing and comprising a development bearing on a shoulder in the internal space of the crossing, this process making it possible to carry out in a simple and rapid manner operations for checking and repairing the interior surface of the crossing, by particular, after installing a removable cuff on this crossing.

 For this purpose, a machining tool is introduced inside the thermal cuff from below the cover of the tank, the thermal cuff is machined by removing material, so as to be able to extract at least part of the cuff by a lower end of the bushing, at least part of the cuff of the bushing is extracted, a replacement cuff of the extracted part is introduced into the internal space of the bushing and the replacement cuff is fixed on the part d 'lower end of the crossing.

 The invention also relates to a removable thermal replacement sleeve.

 In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, with reference to the appended figures, several embodiments of the method of the invention and of a removable replacement cuff.

 Figure 1 is a sectional view through a vertical plane of a central passage of the cover of the vessel of a pressurized water nuclear reactor and of its thermal sleeve, during an initial step of the replacement process according to a first mode of realization of the invention.

 Figure 2 is an enlarged view of a lower portion of a peripheral bushing welded to the tank cover.

 Figures 3 and 4 are sectional views through a vertical plane of a passage through a tank cover equipped with a removable replacement cuff according to the first embodiment of the invention.

 Figure 3 relates to a bushing located at the central part of the tank cover.

 Figure 4 relates to a bushing located at the periphery of the cover of the tank.

 Figure 5 is a sectional view through a vertical plane of a crossing, during the implementation of a disassembly process and replacement of a thermal sleeve, according to a second embodiment of the invention.

 Figure 6 is a sectional view through a vertical plane of the bushing shown in Figure 5 equipped with a removable replacement sleeve, according to the second embodiment of the invention.

 FIG. 7 is an enlarged view of part of a bushing fitted with a replacement sleeve as shown in FIGS. 5 and 6.

 Figures 8, 9 and 10 are partial sectional half-views of three alternative embodiments of the means for fixing a replacement cuff in a bushing.

 Figure 8 shows fixing means by welding.

 Figures 9 and 10 show fixing means by screwing and crimping.

 In Figure 1, we see a part of a cover 1 of a tank of a pressurized water nuclear reactor which is crossed by an opening 2 inside which is fixed a tubular part 3 constituting an adapter allowing the passage of an extender ensuring the movement of a control cluster inside the nuclear reactor vessel.

 The adapter 3 of tubular shape comprises a running part 3a whose diameter corresponds to the diameter of the opening 2 and which is first hooped in the cover and then fixed by an annular weld bead 4 on the lower internal face of the cover 1 and a diametrically enlarged upper part 3b situated outside and above the tank cover 1.

 The main part 3a is fixed in the through opening 2 of the tank cover, so that its lower part 3c is projecting relative to the internal lower surface of the cover 1.

 A thermal sleeve 5 of tubular shape is arranged coaxially inside the internal bore of the adapter 3.

 The thermal cuff 5 comprises an upper portion 5a enlarged diametrically constituting a development coming to rest on a bearing surface 6a of frustoconical shape constituting the upper part of the internal bore 6 of the tubular adapter 3.

 The cuff 5 further comprises on its external surface, two rows of three studs 5b projecting radially outwards, arranged at 1200 from each other around the axis of the cuff 5 and coming to bear against the internal surface of the internal bore 6 of the adapter 3, when the sleeve 5 is mounted inside the adapter.

 By this assembly, the cuff remains free in rotation and mobile in translation in a limited manner inside the adapter 3.

 The cuff 5 has an external diameter substantially smaller than the internal diameter of the bore 6, so that there remains an annular space 8 of constant width between the external surface of the cuff 5 maintained in a perfectly coaxial arrangement relative to the bore 6 by the studs 5b and the internal surface of bore 6.

 The diametrically widened upper part 3b of the adapter 3 has on its external surface, a 3rd thread and a 3d lip projecting from the external surface of the part 3b.

 The control mechanism of the adapter 3 allowing the displacement of the extender and the control cluster passing through the cover inside the adapter is fixed on the upper part of the adapter 3 by screwing on the threaded part 3e and supported on the 3d lip.

 The fixing of the control mechanism is completed by a sealing weld bead ensuring the junction between the mechanism and the adapter, along the 3d lip.

 The thermal cuff 5 is integral, at its lower part, below the tank cover and the lower part of the adapter 3, with a centering cone 5e allowing, when fitting the cover 1 on the tank of the reactor, the centering of the ends of the control rods of the clusters constituting extensions of these clusters engaged in the vertical direction inside the reactor core arranged in the tank.

 The bushing 3 shown in FIG. 1 is a bushing located in the vicinity of the central part of the tank cover of substantially hemispherical shape.

 In Figure 2, there is shown, partially and on a larger scale, a bushing located in the vicinity of the periphery of the tank cover 1 crossed by an opening 2 'in which is fixed the tubular crossing piece 3'.

 All of the bushings of a tank cover have the same axis which is in a perfectly vertical position when the cover rests on the tank.

 Therefore, the holes 2 'in which the bushings 3' are fixed, at the periphery of the tank cover open onto the internal underside of the tank cover in the form of an elongated opening along an axis, of substantially elliptical shape. .

 As a result, the weld bead 4 'of the bushing 3' on the tank cover 1 'has a more complex shape and a greater thickness, for the bushings 3' located in the vicinity of the periphery of the tank cover.

 FIG. 2 also shows the projecting lower part 3 ′ c of the tubular crossing piece 3 ′ which is externally threaded, this thread being left on standby and not generally used in the case of reactor vessels currently in service.

 The protruding part 3c of the through tube 3 shown in Figure 1 also has a threaded outer surface.

 After a certain time of operation of the reactor, it is desirable to check the state of the adapters 3 passing through the tank cover and in particular of the part of the adapters at the level of which the fixing on the tank cover is carried out, by the through an annular weld such as 4 or 4 '.

 This control essentially targeting the internal surface of the bore 6 of the adapter in the weld zone can be carried out by an ultrasonic process, using an ultrasound beam emitted by a probe placed opposite the surface. inside the adapter bore.

 As explained above, the dismantling of a thermal sleeve 5, in order to gain access to the internal surface of the bore 6, may require long operations which are difficult to carry out, even after dismantling the tank cover 1.

 According to a first embodiment of the method according to the invention, a tool 10 is introduced inside the internal space of the thermal sleeve 5, by its lower end, below the tank cover, as it is shown in Figure 1.

 During the dismantling and replacement operations of a part of the thermal sleeve which will be described below, the cover of the tank is dismantled and placed so that the bushings 3 have their vertical axis.

 The tool 10 which may be constituted by a mechanical machining tool comprising a cutter îOa is a tool of known type allowing the machining of a tubular part from the inside and in particular the cutting of a cuff thermally arranged coaxially inside a tubular part such as a passage through the tank cover 3.

 The tool 10 makes it possible to cut the wall of the thermal cuff 5, over its entire thickness, along a circular area, so as to be able to extract the lower section 5 ′ of the cuff, by the lower end of the bushing 3, under the tank cover.

 The position of the cut 11 produced by the tool 10 is chosen so that the extraction of the lower section 5 ′ of the thermal sleeve allows access to the surface of the tubular crossing piece 3, in the zone to be checked. which generally corresponds to the area located at the weld 4.

 In the case where one or more defects are detected on the wall of the internal space 6 of the bushing 3, a repair is carried out, for example by electrolytic deposition of a layer of nickel on the surface having defects.

 These operations can be carried out by introducing a repair tool through the lower end of the bushing 3, under the tank cover 1, after extraction of the lower part 5 'from the thermal sleeve.

 The cutting of the thermal sleeve 5 can be carried out by a different means from a mechanical machining tool and for example by using an electro-erosion machining tool.

 After the inspection and repair of the interior surface of the tubular crossing piece 3 have been carried out, a cuff for replacing the extracted section 5 ′ is mounted on the inside of the tubular crossing piece 3.

 Such a replacement sleeve has been shown in FIG. 3, in the case of a bushing 3 located in the vicinity of the central part of the cover similar to the bushing 3 shown in FIG. 1 and, in FIG. 4, in the case a bushing 3 'situated at the periphery of the tank cover, as shown in FIG. 2.

 The cutting of the thermal cuff arranged inside the bushings for which control and repair is carried out, in order to extract the lower section 5 'is carried out at a substantially constant level with respect to a horizontal reference plane.

 As can be seen in FIGS. 3 and 4, due to the convex shape of the tank cover, the lower end 15a (or 15'a) of the part of the thermal cuff 15 (or 15 ') remaining at the The interior of the crossing tube 3 (or 3 ′) at the end of the cutting and extraction of the lower part of the cuff is at a variable distance in the vertical direction from the internal lower surface of the tank cover.

 Generally, as can be seen in FIGS. 3 and 4, the distance between the internal lower surface of the tank cover 1 and the lower edge 15a (or 15'a) of the remaining section of the thermal cuff is greater in the case of peripheral bushings such as 3 'as in the case of bushings 3 located near the center of the cover.

 The replacement cuffs of the lower part extracted from the thermal cuffs themselves have shapes and dimensions adapted to each of the bushings, as can be seen in FIGS. 3 and 4 on which a replacement cuff 12 intended to equip has been shown. a bushing close to the central part of the tank cover (Figure 3) and a bushing 12 'intended to equip a bushing 3' arranged in the vicinity of the peripheral part of the tank cover 1 (Figure 4).

 In addition, the replacement cuffs 12 and 12 'have different dimensions and shapes due to the fact that the lower part of the replacement cuff constituted by the large base of the engagement cone 12e (or 12'e) is located at a constant level with respect to a horizontal reference plane.

 At 1 after the cutting of the thermal cuff, the lower end 15a (or 15'a) of the section remaining in the crossing tube is internally chamfered.

 It should be noted that the cutting off of the thermal sleeve is carried out at a level situated below the centering studs 5d of the thermal sleeve in the bushing tube and above the weld zone of the bushing tube, c ' that is to say in an area located either in the thickness of the tank cover or above the tank cover.

 To install a replacement cuff such as 12 (or 12 ') shown in FIG. 3 or in FIG. 4, and corresponding to a first embodiment of the invention, first, performs a remanufacturing of the 3G projecting lower part (or 3 ′ c) of the tubular crossing piece 3 (or 3 ′) in order to install a piece 13 (or 13 ′) for fixing the cuff of replacement 12 (or 12 ') on the tubular bushing.

 The fixing piece 13 (or 13 ′) has an annular shape and has an internal bore 13a (or 13 ′ a), a part of which is intended to engage on the end part 3c (or 3 ′ c) of the tubular part, after its rework. This part of the bore 13a receiving the end part of the tubular part is limited by a shoulder 13b (or 13'b) intended to come to bear on the end of the tubular passage part 3 (or 3 ') .

 The remanufacturing of the end part of the bushing part aims to adjust this end part to allow its engagement inside the bore of the part 13 and to ensure support and abutment in a position perfectly determined from the part 13, via its shoulder 13b (or 13'b) as well as allowing the production of a corner weld bead having a sufficient section.

 The attachment of the part 13 (or 13 ′) to the end part of the tubular passage piece is ensured by a weld bead 14 (or 14 ′).

 The part 13 (or 13 ') has an external thread 13c (or 13'c) whose axis is coincident with the axis of the bushing, when the part 13 (or 13') is placed at the end of the crossing.

 The replacement sleeve 12 (or 12 ') has an upper part 12a (or 12'a) whose diameter corresponds to the diameter of the upper part of the sleeve and is less than the diameter of the internal bore of the bushing, a part lower 12b (or 12'b) located outside of the bushing whose external diameter is greater than the diameter of the inner bore of the bushing and a middle part 12c (or 12'c) located between the upper parts and lower of the replacement sleeve, by means of which the replacement sleeve is fixed to the end of the bushing. The replacement sleeve 12 (or 12 ′) also has at its upper end an internal chamfer 12d (or 12 ′) and a set of three centering studs 16 (or 16 ′) arranged 1200 from one another around the 'axis of the sleeve 12 coincides with the axis of the crossing.

 In the case of a central crossing 3, the upper part 12a of the cuff is short and the lower part 12b is long. Conversely, in the case of a peripheral crossing, the upper part 12'a of the cuff is long and the lower part 12'b is short.

 The middle part 12c (or 12 ′ c) of the replacement sleeve 12 comprises an external flange 18 (or 18 ′) of annular shape comprising two flat faces, one of which, or the upper face, is intended to come to bear on the underside of the fixing piece 13 (or 13 ′), when the replacement sleeve is placed inside the bushing.

 The upper end of the replacement sleeve 12 (or 12 ') corresponding to the end of the chamfer 12d (or 12'd) is then at a certain distance below the lower end of the chamfered part 15a ( or 15'a) of the remaining part 15 (or 15 ') of the thermal sleeve, an axial clearance 17 (or 17') being maintained between the remaining part of the thermal sleeve and the replacement sleeve.

 The replacement sleeve 12 (or 12 ′) is held in a perfectly coaxial position inside the tubular crossing piece 3 (or 3 ′) by centering the flange 18 (or 18 ′) relative to the piece 13 (or 13 ') and thanks to the studs 16 (or 16').

 An annular space 19 (or 19 ') of substantially constant width is provided between the external surface of the replacement sleeve 12 (or 12') and the surface of the internal bore of the bushing 3 (or 3 ').

 In its central part 12c (or 12 ′ c), the wall of the replacement cuff 12 (or 12 ′) is crossed by a set of channels 20 (or 20 ′) distributed circumferentially around the central part 12 c (or 12 ′) c) and inclined relative to the vertical, either inward (Figure 4) or outward from the cuff (Figure 3); these channels put the space 19 (or 19 ′) into communication with the space located under the tank cover.

 After putting the reactor back into service, after reassembly of the cover on the tank, the channels 20 (or 20 ′) make it possible to supply water to the space between the adapter and the cuff.

 This increases the passage section for water which rises during a fall of the cluster in order to avoid an extension of the fall time of the clusters in the event of an emergency stop.

 The fixing of the replacement sleeve 12 (or 12 ') on the bushing end part 3 or (3') is ensured by a nut 21 (or 21 ') engaging with the threaded external part 13c (or 13 'c) of the fixing piece 13 (or 13') and comprising an internal support face on the underside of the flange 18 (or 18 ') of the replacement sleeve.

 After the flange 18 (or 18 ') has been tightened between the nut 21 (or 21') and the fixing piece 13 (or 13 '), the deformable locking tabs 21a ( or 21'a) integral with the end part of the nut 21 (or 21 ') inside the recesses provided in the fixing piece 13.

The nut 21 is thus blocked in rotation relative to the blocking piece 13 (or 13 ') which is itself fixed to the end part of the bushing.

 Preferably, the nut 21 (or 21 ′) comprises three locking tabs arranged at 1200 which are folded into three recesses arranged at 1200 on the fixing piece 13.

 The method of the invention therefore makes it possible to carry out interventions for control or repair in a simple manner on the internal surface of a crossing piece, after extraction of part of the thermal sleeve.

 The replacement of the thermal sleeve is then carried out in a simple manner using the device which has just been described. The replacement cuff makes it possible to complete the remaining part 15 (or 15 ′) maintained by the terminal development of the thermal cuff and to ensure, in cooperation with this remaining part, all the functions of a thermal cuff according to the prior art.

 The replacement sleeve 12 (or 12 ′) also has the advantage of remaining removable by unscrewing the nut 21 (or 21 ′) which has an external prismatic part allowing the engagement of a tool.

 By exerting sufficient torque on the nut 21 or (21 ′), the tongues 21 a (or 21 ′ a) are deformed or torn off, which makes it possible to unscrew the nut.

 The replacement sleeve 12 (or 12 ′) can then be extracted from the bushing 3 (or 3 ′) from below the cover, so that the internal surface of the bushing is accessible for inspection or repair.

 The successive parts 12a, 12b, 12c (or 12'a, 12'b, 12'c) of the cuff are formed by tubular sections welded end to end. One can design short cuffs which do not have an upper section similar to the part 12a (or 12'a), disposed inside the bushing, in the weld zone of the bushing piece. In this case, the examination and possibly the repair of the interior surface of the bushing can be carried out without dismantling the cuff.

 Referring to Figure 5, we will now describe a second mode of implementation of the disassembly process and replacement of a thermal sleeve of a crossing of the cover of a nuclear reactor vessel.

 In Figure 5, there is shown a tubular crossing piece 23 fixed in the vicinity of the central part of a cover 21 of the vessel of a pressurized water nuclear reactor.

 A thermal cuff 25 is arranged in a coaxial position in the internal bore of the bushing piece 23. The cuff 25 has a bloom 25a at its upper end which comes to bear on a frustoconical bearing of the bore 26.

 The thermal cuff 25 comprises an engagement cone 25e fixed to its lower part.

 As before, a mechanism for controlling the movement of a control cluster, not shown, is fixed to the upper part of the bushing piece 23.

 According to the second embodiment of the method of the invention, the thermal sleeve 25 is extracted in its entirety from the bushing 23, from below the cover 21 by performing machining by removing material from the terminal expansion upper 25a of the thermal cuff.

 Use is made for this of a tool generally designated by the reference 24 and comprising a tubular mandrel 27 in which is slidably mounted in the axial direction and movable in rotation around the axis of the mandrel, a rod 28 coaxial with the mandrel 27. The rod 28 carries a pivoting working grain 28a and a disc-shaped or cylinder-shaped brush 28b in its upper end part.

 The tool 24 also includes a centering and fixing device 29 integral with the lower part of the tubular mandrel 27. The device 29 includes fixing screws on the centering cone 25e of the thermal sleeve 25.

 When the tool 24 is placed inside the cuff 25, the upper part of the sliding rod 28 carrying the working grain 28a and the brush 28b is placed above the opening 25a, so that the machining grain 28a placed in the deployed position comes to be applied, by means of a cutting edge, to the upper part of the opening 25a.

 The rod 28 of the tool is then rotated so as to ensure the machining of the opening 25a inside the bushing 23, without removing the control mechanism (not shown in FIG. 5).

 Initially, the thermal cuff 25 is maintained in the low position in abutment against the frustoconical bearing of the bore 26.

 In a second step, the cuff 25 is raised slightly, so that the opening 25a is located above the frustoconical surface of the bore 26.

The complete elimination of the bloom 25a is then carried out by removal of material by the grain 28a.

 By lifting the cuff 25 above the bearing surface of the bore 26, one avoids damaging this section of the lower section of the bushing, inside the thermal cuff.

 This avoids introducing metallic particles into the control mechanism fixed to the bushing piece 23.

 At the end of the machining by removing material from the opening 25a, the thermal sleeve 25 can be extracted from the bushing 23 by the underside of the cover. The tool 24 can be extracted beforehand from the cuff 25 or simultaneously with the latter.

 It is then possible to carry out any inspection or repair operation on the internal surface of the bushing piece 23.

 After checking and repairing the bushing piece 23, a replacement sleeve 30 is placed inside it as shown in FIGS. 6 and 7 and which is intended to be substituted for the sleeve 25 extracted from the crossing piece 23.

 The replacement cuff 30 comprises a tubular running part 30a whose external diameter is less than the diameter of the internal bore of the bushing piece 23. The external diameter of the running part 30a of the cuff 30 may be identical to the diameter of the cuff 25 which is replaced.

 The replacement cuff comprises an external flange 30b constituting a support part for a fixing nut 31 of the replacement cuff 30.

 The nut 31 is screwed onto the external threaded end portion 23c of the bushing 23, this threaded portion, as explained above, being pre-existing and generally not used.

 The nut 31 is traversed by openings 36 of radial direction distributed circumferentially. The openings 36 make it possible to supply the annular space between the sleeve 30 and the bushing 23 with water.

 A ring 32 fixed by welding points 33 on the external surface of the bushing 23, in its projecting part below the cover 21, comprises deformable tongues 32a which can be folded down on each of the sides of the nut 31 which has a prismatic external surface, for example with a hexagonal section. This produces the rotation lock of the nut 31 relative to the bushing 23, after tightening of this nut. The replacement sleeve 30 which comprises, at its lower part, a centering cone 30e is retained axially by the nut 31 which includes a bearing surface in its internal bore on which the flange 30b rests.

 The replacement cuff 30 also includes centering studs 30c arranged at 1200 from one another around the axis of the cuff 30.

 The upper part 30d of the replacement sleeve 30 does not have any opening and is housed inside the upper part of the bore 26 of the bushing piece 23. The retention of the replacement sleeve 30 n ' is ensured only by the nut 31 cooperating with the flange 30b, which makes it possible to disassemble the replacement cuff by simply unscrewing the nut 31, by exerting sufficient torque to deform the tongues 32a of the crown 32.

 As can be seen in FIG. 7, the nut 31 is produced in two parts and comprises a first external part 31a having an internal thread 31'a over part of its length and an internal part 31b or insert comprising an external thread intended coming to engage in the internal thread 31'a of the external part of the nut.

 The internal bore of the insert 31b produced in annular form includes a tapped part 31'b and radial projecting parts 34 constituting slots ensuring a grip of the insert 31b to carry out its screwing inside the external part 31a.

 The internal thread 31'b is intended to screw the nut 31 onto the thread of the end part 23c of the bushing 23.

 The insert 31b is locked in rotation inside the part 31a of the nut by welding points 35.

 The fitting of the insert 31b delimits a space in which the flange 30b is disposed, which can move due to an axial clearance between the insert and the flange.

 The crown 32 is fixed by welding points 33 on the external surface of the lower part projecting relative to the tank cover, of the bushing piece 23.

 Channels 36 make it possible to supply water to the space between the adapter and the cuff.

 As before, the replacement cuff can be quickly and easily mounted inside the bushing by simply screwing the nut 31 engaged on the replacement cuff 30, below the flange 30b.

 Locking in rotation of the nut 31 is ensured by folding the tabs 32a on the hexagon of the nut 31. In addition, the replacement sleeve 30 can be easily removed by simply unscrewing the nut 31, for example to carry out checks or repairs on the interior surface of the bushing 23 after the nuclear reactor has been put back into service, the cover of which has been fitted with replacement sleeves such as the sleeve 30.

 In Figure 8, there is shown an alternative embodiment of the means for fixing a replacement cuff on the end of a piece of passage of a tank cover.

 The crossing piece 40 is machined at its lower end, to form two frustoconical chamfers 40a and 40b located respectively outside and inside the crossing piece.

 The replacement cuff 41 includes a flange 42 projecting radially from its upper outer edge to form a chamfer 42a.

 When the replacement sleeve 41 is placed in the bushing piece 40, the flange 42 comes to bear on the lower edge of the bushing piece. The chamfers 40a and 42a are opposite to form an annular welding chamfer. The cuff is fixed to the bushing by a weld bead 43 deposited in the chamfer.

 Welding is carried out by an automated orbital TIG process.

 Disassembly of the replacement sleeve requires machining of the weld bead 43, so that this operation is more complex, longer and more costly than in the case of a screwed assembly as described above. On the other hand, the operating behavior of the welded cuff can be guaranteed insofar as the fixing only uses known and proven techniques.

Of course, it is possible to carry out the fixing by a welding process different from the process.
Orbital TIG. It is also possible to fix the cuff by brazing at a temperature close to 600 "C, by heating the parts to be joined by induction.

 In this case, disassembly of the cuff can be achieved by heating the parts by induction to 630 "C and by exerting traction on the cuff.

 FIG. 9 shows a first alternative embodiment of a screwed assembly as described above with reference to FIGS. 3 and 4.

 Machining was carried out on the lower end of the replacement sleeve 50 so as to produce a circumferential groove 50a and an external thread 50b.

 The thread 50b could be constituted by a thread existing on the end of an adapter.

 The replacement cuff 51 includes a collar 52 projecting radially.

 When the replacement sleeve 51 is placed in the bushing 50, the flange 52 comes to bear on the lower edge of the bushing 50.

 The cuff 51 is fixed by a nut 53 screwed onto the threaded part 50b of the bushing piece 50. The nut 53 comprises a support part 53a projecting radially inwards coming into contact with the lower surface of the flange 52.

 At the end of screwing, the flange 52 is clamped between the bearing surface 53a and the lower end of the bushing 50.

 The nut 53 has an upper part 54 of reduced thickness compared to the thickness of the body of the nut constituting a deformable lip which can be folded back inside the groove 50a after screwing the nut 53. crimping of the lip 54 makes it possible to block the nut in rotation.

 In order to disassemble the cuff, a torque is exerted on the nut sufficient to deform and serve the lip 54 or else all or part of the lip 54 is removed by machining before loosening the nut 53.

 In FIG. 10, a second variant of a screwed assembly has been shown as described with reference to FIGS. 3 and 4.

 Was produced by machining the lower part of the crossing piece 60, a groove 60a.

 The replacement cuff 61 includes a collar 62 projecting radially.

 When the replacement sleeve 61 is put in place in the bushing 60, the flange 62 comes to bear on the lower end of the bushing.

 A fixing washer 63 is engaged on the sleeve 61 and on the lower end of the bushing piece 60.

 The washer 63 has an internal bearing surface 63a and an upper part 65 of tubular shape having longitudinal slots delimiting elastic deformable strips 65a having a terminal development 65b comprising a heel intended to come to bear inside the groove 60a .

 The washer 63 is forcibly engaged on the end of the bushing 60, so that the openings 65b ensure the fixing of the washer 63 on the bushing by snap-fastening. The collar 62 resting on the bearing surface 63a is interposed between the washer 63 and the bushing 60.

 A nut 64 having an internal support rim 64a is screwed onto a threaded external part 63b of the washer 63. The rim 64a comes to bear on the strips to maintain them.

 The nut 64 has a lower part 66 of reduced thickness compared to the thickness of the body of the nut constituting a deformable lip which is folded and crimped in an annular recess 63c of the washer 63 after screwing the nut.

 The cuff can be removed by exerting sufficient torque on the nut 64 to deform the lip 66 or by machining the lip 66 before unscrewing the nut.

 The washer 63 can then be removed by simple traction.

 In all cases, the method according to the invention makes it possible to dismantle a thermal sleeve from a bushing of a cover of a nuclear reactor vessel without having to dismantle a fixed cluster control mechanism beforehand. at the top of the crossing.

 It is then possible to carry out any inspection or repair operation on the interior surface of the crossing.

 The method according to the invention also makes it possible to ensure in a simple manner the replacement of the bushing cuff, so that the replacement cuff can be dismantled without difficulty for subsequent operations of checking or repairing the internal surface of the bushing. .

 The invention is not limited to the embodiments which have been described.

 Thus, the machining of the thermal sleeve to extract it from below the cover can be carried out in a manner different from that which has been described.

 It is also possible to use replacement sleeves having a shape different from those which have been described and which are fixed to the lower part of the bushing by screwing or by welding or brazing, using fixing means different from those which have been described above.

 Finally, the invention applies to carrying out control and / or repair operations on the interior surface of any part passing through a vessel cover of a nuclear reactor equipped with thermal sleeves.

Claims (20)

 1.- Method for dismantling and replacing a thermal sleeve (5, 25) of a bushing (3, 3 ', 23) of the cover (1, 21) of the vessel of a pressurized water nuclear reactor , the bushing (3, 3 ', 23) being constituted by a tubular piece fixed on the cover of the tank (1, 21) in a through opening (2) so that a lower end part (3c, 3 'c, 23c) of the bushing is projecting below the cover (1, 21) of the tank and the cuff (5, 25) being arranged in a coaxial position, inside the bushing (3, 3' , 23) and comprising a opening (5a, 25a) coming to bear on a shoulder (6a) in the internal space (6; 26) of the crossing (3, 3 ', 23) characterized by the fact that one introduces a machining tool (10, 24) inside the thermal sleeve (5, 25) from below the cover of the tank (1, 21), which the thermal sleeve (5, 25) is machined by removal of material so that it can be extracted the cuff (5, 25) at least partially through a lower end of the bushing (3, 3 ', 23), which extracts at least part of the cuff (5, 25) from the bushing, which is introduced a replacement sleeve (12, 12 ', 30) of the part extracted from the sleeve (5, 25), in the internal space of the bushing (3, 3', 23) and that the replacement sleeve is fixed (12, 12 ', 30) on the lower part (3c, 3'c, 23c) of the crossing.  2.- Method according to claim 1, characterized in that one introduces a cutting tool inside the thermal cuff (5) and that sections the thermal cuff (5), so as to extract a first section of the sleeve (5 ') via the lower end of the bushing (3) and to leave inside the bushing (3), a second section (15, 15') of the thermal sleeve (5) resting via the opening (5a) on the shoulder (6a) of the internal space of the bushing (3).  3.- Method according to claim 2, characterized in that the replacement cuff (12, 12 ') is arranged coaxially inside the bushing (3, 3') and has an upper end (12d, 12 ' d) inside the bushing (3, 3 ') arranged below a lower end of the remaining part (15, 15') of the thermal sleeve with an axial spacing (17, 17 ').  4.- Method according to claim 1, characterized in that one introduces a machining tool by removal of chips inside the thermal sleeve (25) and that eliminates by removal of chips the development ( 25a) of the thermal cuff, so as to extract the thermal cuff (25) from the bushing (23) in its entirety.  5.- Method according to claim 4, characterized in that fixed in the interior of the crossing (23) over its entire length, between its lower end and the part of its internal space comprising the shoulder support for the development of the thermal cuff (25) which is replaced, a tubular replacement cuff not having a support shoulder.  6.- Method according to any one of claims 1 to 5, characterized in that the replacement cuff (12, 12 ', 30, 51, 61) is fixed on the bushing (3, 3', 23, 50 , 60) using a screw connection.  7.- Method according to any one of claims 1 to 5, characterized in that the replacement sleeve (41) is fixed to the bushing (40) by welding or brazing.  8.- Method according to claim 6, characterized in that fixed on the lower end portion (3c, 3'c) of the bushing (3, 3 ') a fixing piece (13, 13') of annular shape externally threaded and that the replacement cuff (12, 12 ') is fixed by means of a nut (21, 21') on which the replacement cuff (12, 12 ') comes supported by a flange (18, 18 ') projecting radially, the nut (21, 21') being screwed onto the threaded external part (13c, 13'c) of the fixing piece 13.  9.- Method according to claim 8, characterized in that one realizes the rotation lock of the nut (21, 21 ') by folding down deformable tongues (21a, 21'a) integral with the nut (21 ) in recesses of the fixing piece (13, 13 ').  10.- Method according to claim 6, in the case where the projecting lower part of the crossing piece (23) has a thread characterized in that the replacement cuff (30) is fixed by means of a nut (31) on which the replacement sleeve (30) comes to bear via a radially projecting flange (30b), the nut (31) being screwed onto the external thread of the projecting lower part (23c) of the crossing (23).  11.- A method according to claim 10, characterized in that the nut (31) is blocked in rotation by folding down deformable tongues with a flange (32) welded to the end part (23c) of the bushing (23) on external prismatic faces of the nut (31).  12.- Method according to claim 6, charac terized in that one carries out the fixing of the replacement cuff (51) by means of a nut (53) on which the replacement cuff (51) comes support by means of a flange (52) projecting radially, the nut being screwed onto a threaded lower part (50b) of the bushing (50) and locked in rotation by means of a deformable lip (54 ) crimped into a groove (50a) machined in the lower part of the bushing (50).  13.- Method according to claim 6, characterized in that the fixing of the replacement sleeve (61) is carried out by means of a fixing washer (63) on which the replacement sleeve (61) comes supported by a radially projecting flange (62), comprising a thread (63b) on its external surface and a tubular end portion (65) separated by longitudinal slots in a plurality of elastic strips (65a ), a nut (64) screwed onto the thread (63b) of the washer (63) ensuring the retention of end parts (65b) of the lamellae (65a) in a groove (60a) of the bushing (60), after latching of these end parts in the groove (60a) by axial engagement of the washer (63) on the replacement sleeve (61) and on the end of the bushing (60).  14.- Removable sleeve to replace a thermal sleeve (5, 25) of a bushing (3, 23) of the cover (1, 21) of the vessel of a pressurized water nuclear reactor, the bushing (3 , 23) being constituted by a tubular piece fixed on the cover of the tank in a through opening so that a lower end portion (3c, 23c) of the bushing projects below the cover (1, 21 ) of the tank, characterized in that the cuff (12, 12 ', 30, 41, 51, 61) has a first part (12a, 12'a, 30a) internal to the bushing and a second part (12b, 12'b) external to the bushing comprising a radially projecting collar (18, 18 ', 30b, 42, 52, 62) for fixing the cuff to the lower end part of the bushing (3, 3', 23 , 40, 50, 60).  15.- Cuff according to claim 14, characterized in that the part (12b, 12'b) of the cuff external to the bushing (3, 23) has an external diameter greater than the internal diameter of the bushing (3, 23 ).  16.- Cuff according to claim 15, characterized in that it comprises a middle part between its part (12a, 12'a) internal to the bushing (3, 3 ') and its part (12b, 12'b) external to the bushing (3, 3 ') comprising the support flange (18, 18').  17.- Cuff according to claim 16, characterized in that its wall is traversed, in the middle part (12c, 12'c) by channels (20, 20 ').  18.- Replacement sleeve according to any one of claims 14 to 17, characterized in that its part (12a, 12'a) internal to the bushing (3, 3 ') has an axial length less than the length of crossing.  19.- Replacement sleeve according to claim 18, characterized in that the part (12a, 12'a) internal to the bushing has a length substantially zero so as to give access to the surface of the bushing (3, 3 ' ) from the inside of the cuff.  20.- Replacement sleeve according to any one of claims 14 to 17, characterized in that the part (30a) internal to the bushing (23) has a length substantially equal to the length of the bushing (23).