FR2938964A1 - Method for electrons discharging drilling of cap of tube of steam generator of pressurized nuclear power plant, involves carrying out to and fro movement of electrode for drilling base of cap, and verifying drilling of base - Google Patents

Abstract

The method involves powering electrons discharging drilling electrode (22) by electric current. The electrons discharging drilling electrode is displaced in a tube (5) for placing a free end at a proximity of an interior base (12) of a cap (10). The electrons discharging drilling electrode is lead through the interior base. A to and fro movement of the electrode is carried out for drilling the base of the cap by a disintegration arc created between the free end and the base. The drilling of the base is verified. An independent claim is also included for a device for electrons discharging drilling of a cap of a tube of a steam generator of a pressurized nuclear power plant, comprising fixation units that are formed by an expandable sleeve made of a stainless alloy.

Description

A method and apparatus for electron discharge percaae of a plug of a steam generator tube of a pressurized water nuclear power plant. The present invention relates to a method and an electron discharge piercing device of a plug of a steam generator tube of a pressurized water nuclear power plant. The pressurized water nuclear reactors comprise a primary circuit in which circulates the pressurized cooling water of the nuclear reactor, which generally consists of several loops on each of which is arranged a steam generator ensuring, by heat exchange between the pressurized cooling water of the nuclear reactor and the feed water, heating and vaporizing the feed water, so as to produce steam which is sent to the turbine of the nuclear power plant. The steam generators of pressurized water nuclear reactors comprise a pressure envelope of generally cylindrical shape arranged with its vertical axis in which is placed a bundle of heat exchange tubes and a water box of substantially hemispherical shape constituting the part bottom of the steam generator, below the envelope enclosing the bundle. A thick plate, disposed horizontally and traversed by vertically arranged holes arranged in a network, ensures the separation between the upper part of the envelope of the steam generator and the water box. The tubes of the steam generator bundle are engaged and sealed at their end portion in the holes of the tube plate. The water box is delimited, on the one hand, by the hemispherical wall constituting the lower part of the envelope of the steam generator and, on the other hand, by the horizontal lower surface of the tubular plate following which the passage and fixation of the tubes of the beam. Each of the tubes of the bundle is bent at its upper part and has two straight branches whose ends are engaged in holes through the tube plate on either side of a partition separating the water box into two compartments each connected by a tubing, to a pipe of the primary circuit, so that the cooling water of the reactor enters the water box inside one of the compartments, to be distributed in each of the tubes of the bundle. The pressurized cooling water circulating inside the tubes of the bundle is recovered in the second compartment of the water box to be taken up by a pipe of the primary circuit connected to the second compartment of the water box. In order to access the water box, the hemispherical envelope of this water box is traversed by a visit opening or manhole in each of the two parts of the hemispherical wall delimiting a compartment of the box. to water.

The circulating cooling water inside the tubes of the bundle ensures, in the upper part of the steam generator defined by the vertical cylindrical envelope of this steam generator, the heating and the vaporization of feed water. which is introduced inside the envelope of the steam generator, in its upper part.

The water vapor formed in the steam generators is recovered in the upper part of the steam generator casing. After a certain time of operation of the steam generator, the tubes of the bundle which are in contact with fluids with high temperature and high pressure containing chemical species and having a certain aggressiveness, are likely to be deteriorated and to present, by for example, cracks crossing their walls. These cracks may result in leakage of pressurized water with some radioactivity in the portion of the steam generator containing the feed water.

In addition, the cracks are likely to evolve and lead to breaks in the tubes of the beam. In order to avoid the risk of tube rupture which would require an immediate shutdown of the nuclear reactor and an unscheduled repair of the tubes which would entail significant capital costs of the nuclear power plant, during a period of programmed shutdown of the reactor, it is realized. nuclear, clogging the tubes of steam generators with leaks.

Clogging of the defective tubes of the steam generator is achieved by placing in this tube at the tube plate, a plug adapted to secure the pipe deemed cracked or crackable. But, during some leak tests, it has been found that plugs can pose doubts about their effectiveness and the maintenance of the seal. In order to avoid the risks of a leak of the radioactive water of the primary circuit in the water of the secondary circuit, the operators of the nuclear power stations require, before restarting the wafer, the installation of a second plug said shaving, c ' that is, at the bottom of the thickness of the tubular plate. Before placing this second stopper, it is imperative to pierce the first plug placed above so as to avoid a so-called boiler effect, that is to say a difference in pressure of the volume contained between the two plugs during a commissioning of the wafer and thus an increase in the temperature of the circuits, primary and secondary respectively, in the steam generator. Indeed, such a pressure difference can cause damage to the tube. To avoid the occurrence of this phenomenon of overpressure, the solution is to pierce the dubious plug placed, or in a position called altitude, at a distance of about 80 mm from the underside of the tube plate, either in a position called super altitude, at a distance of about 200 mm. Until now, the drilling of plugs is a mechanical drilling using a drill or a drill carried by a robotic arm. This technique has the disadvantage of producing chips that are recovered and require the implementation of large means as qualified materials and personnel. This type of robotic interventions can be designed if a large number of caps must be drilled on the same steam generator. In the case of several plugs distributed over several steam generators of the same nuclear power plant, the dismantling, transport and reassembly of all the intervention means must be carried out several times which causes a prolonged immobilization of the unit. of the nuclear power station.

The object of the invention is to propose an electron discharge piercing device which avoids these drawbacks and which is light and easily movable from one steam generator to another, which reduces the immobilization time of the wafer of the nuclear plant. In the case where a drill bit is broken inside the stopper during mechanical drilling, the device according to the invention also makes it possible to break up the broken drill bit and finish drilling the stopper. The subject of the invention is therefore a process for piercing electrons from a plug of a steam generator tube of a pressurized water nuclear power plant, said steam generator comprising an external integral pressure envelope. a tubular plate whose bottom face delimits a feed water box of the primary fluid steam generator and whose upper face delimits with the outer envelope a space of the secondary circuit containing a bundle of U-shaped curved tubes whose branches straight lines are vertical and parallel to each other and fixed in holes passing through the tube plate, said method comprising the following steps: - an electron discharge piercing electrode is mounted on a support and guide assembly, - the position is adjusted of the electrode on said assembly with respect to the position of the bottom of the plug in the tube, is introduced into the water box and fixed to-or s of the tube plate the support and guide assembly for placing the electrode in the tube comprising the plug to be pierced, - the electrode is supplied with electric current, - the electrode is moved in the tube to place its end free near the inner bottom of the plug and the electrode is advanced through said inner bottom and, simultaneously, it moves the electrode back and forth to perform the drilling of the bottom of the plug by a disintegration arc created between the free end of said electrode and the bottom of said plug, and - it checks the drilling of the bottom of said plug. According to other features of the invention: - the position of the electrode is adjusted by means of an auxiliary template, - the drilling is verified at the bottom of said plug by means of a control rod advance electrode and a camera placed inside the water box, and - the electrode is simultaneously supplied with electric current and dielectric fluid to drill the plug bottom by electroerosion.

The invention also relates to an electron discharge piercing device of a plug of a steam generator tube of a pressurized water nuclear power plant, said steam generator comprising an external pressure casing integral with a tubular plate whose bottom face delimits a feed water box of the primary fluid steam generator and whose upper face delimits with the outer envelope a space of the secondary circuit containing a bundle of U-shaped curved tubes whose branches straight lines are vertical and parallel to each other and fixed in holes through the tube plate, said device comprising: - an electron discharge piercing electrode, - a support and guide assembly of the electrode in the tube comprising said plug to drill, - a centering element of the electrode in said tube, - fasteners of said assembly below the tubular plate re, - a connecting part of the electrode with means for supplying electric current, - a unit for moving the electrode in the tube and applying a movement back and forth to the electrode for placing its free end near the inner bottom of the plug and advancing the electrode through said inner bottom and simultaneously to perform the drilling of the bottom by a disintegration arc created between the free end of said electrode and the bottom of said cap . According to other characteristics of the invention: the device comprises an auxiliary template for adjusting the position of the electrode on said assembly with respect to the position of the bottom of said plug in the tube, the device comprises verification means piercing the bottom of the plug formed by an electrode advance indicator rod and by a camera placed inside the water box, the support and guide assembly of the electrode comprises a bearing base the fasteners below the tube plate, a cylinder extending below the base and in which is slidably mounted a support of the connecting piece, and the centering element extending above the base formed by a tubular centralizer adapted to be placed in the tube carrying the plug to be pierced, the support and the tubular centering device are made of an insulating material, the electrode is hollow and the connecting piece is connected to power supply entation in electric current and dielectric fluid, - the connecting piece is connected to the electrode by a hollow cylindrical electrode holder and whose outer diameter is substantially equal to the inner diameter of the tubular centering device, - the support, the connecting piece the electrode holder and the electrode are movable by said unit between a rest position in which the free end of the electrode is spaced from the bottom of the plug and a working position in which said free end is positioned in the vicinity of said bottom, - the unit is separated from the cylinder and connected to the support by a transmission cable, - the unit is mounted directly on the cylinder, - the centering element comprises at least one discharge passage of the dielectric fluid containing the slag drilling the bottom of the plug, - the fasteners are formed by expandable sleeves each suitable for being placed in a tube of the beam of t ubes, and - the expandable sleeves are made of stainless alloy, one of said sleeves constituting a grounding. The invention will be better understood on reading the description which will follow, given by way of example and with reference to the appended drawings, in which: FIG. 1 is a schematic sectional view through a vertical plane of the lower part of a steam generator comprising a water box in which is positioned a device for piercing electrons from a plug of a tube of this generator steam, - FIG. 2 is an axial sectional view of the plug mounted within a tube of the steam generator; FIG. 3 is a diagrammatic sectional view through a vertical plane of the steam generator tube plate and showing the different positions of a plug in tubes of this steam generator; FIG. 4 is a diagrammatic view partly in section through a vertical plane of an electron discharge piercing device of the plug, according to the invention, said device being in the rest position, FIG. 5 is a diagrammatic sectional view in a vertical plane of the plug piercing device in a working position; FIG. 6 is a schematic perspective view of the support and guide assembly of an electrode of the piercing device; FIG. 7 is a sectional view through a vertical plane of the electrode support, FIG. 8 is a diagrammatic sectional view through a vertical plane of the electrode connecting piece with means for supplying electrical current and dielectric fluid, FIG. Fig. 9 is a schematic perspective view of the centering element of the electrode in the steam generator tube; 10 is a diagrammatic view in axial section of the electrode holder of the electrode, FIG. 11 is a diagrammatic view in axial section of the electrode, FIG. 12 is a schematic perspective view of a control rod for advancing the electrode, and FIG. 13 is a schematic view in axial section of an auxiliary template for adjusting the position of the electrode on the support and guide assembly. In FIG. 1, there is shown the lower part of a steam generator generally designated by the reference numeral 1, which comprises an outer shell 1 of generally cylindrical in which is disposed a heat exchange beam of the steam generator constituted by 5. Each of the tubes 5 of the exchange bundle of the steam generator has a U-shape and comprises a curved upper part, not shown, and two straight branches whose lower ends are engaged and fixed in through holes 6 of vertical direction of a tubular plate 4 separating the upper part of the steam generator enclosing the bundle of tubes 5 of a lower portion delimited by a substantially hemispherical wall 3 constituting the water box 2 of the steam generator.

The water box 2 of the steam generator is delimited at its upper part by the horizontal lower face 4a of the tube plate 4. The water box 2 of the steam generator is separated into two compartments by a vertical partition 7 arranged in a plane diametrical tubular plate 4 constituting a plane of symmetry on either side of which are fixed the ends of the straight branches of each tube 5 of the steam generator beam. The partition 7 separates the water box 2 from the steam generator into a first and a second compartment which are respectively connected to an inlet pipe and to a cooling water supply pipe of the pressurized water nuclear reactor which circulates inside the tubes 5 between the inlet compartment and the outlet compartment separated by this partition 7. The section plane of FIG. 1 is a plane parallel to the partition 7, so that only one of the two compartments of the water box 2 is visible in this figure. The wall 3 of the water box is traversed by a visit opening 8 or manhole to access the interior of the compartment of the water box, for example to perform control or maintenance operations or even to clog one or more tubes 5 at risk of leakage or leakage. For this, a plug 10 of the type shown in FIG. 2 is introduced into the defective tube 5 by means of a suitable system of known type. As shown in FIG. 2, the plug 10 fixed inside a tube 5 comprises a tubular body 11 closed by a bottom 12 at one of its ends. The tubular body 11 has, on its outer surface, protruding parts 13 generally of annular shape, obtained by machining the body 11 of the plug 10. A core 14 having a tapped internal bore 14a is engaged in the bore of the tubular body 11 the plug so as to achieve the diametral expansion of the body 11 inside the tube 5 and ensure the maintenance of the plug 10 in said tube 5, the projecting portions 13 of the body of the plug 10 being applied with a strong pressure against the inner surface of the corresponding tube 5. The diametrical expansion of the body 11 of the plug 10, is achieved by moving the core 14 in the axial direction inside the plug 10, using for example a threaded pull rod, not shown. The plug 10 is placed in the corresponding tube 5 at the level of the tubular plate 4, as shown in FIG. 3. The plug 10 can be placed in a position called at altitude, that is to say at a distance 11 of the order of 80 mm from the lower face 4a of the tube plate 4 or in a position called super altitude, that is to say at a distance 12 of the order of 200 mm from the lower face of the face 4a of said tubular plate 4. In the case where the plug 10 placed in a tube 5 to secure, present, when a leak test, a doubt about its own tightness, a second plug 10 is placed after the first plug 10 to seal the defective tube 5, as shown in FIG. 3. Before placing the second plug 10, the first plug 10 must be pierced so as to avoid an effect called boiler effect, that is to say an overpressure of the volume contained between the two plugs 10 during a delivery in service of the portion of the nuclear power plant and thus avoid an increase in the temperature of the circuits, respectively primary and secondary in the steam generator. To pierce the bottom 12 of the plug 10, an electron discharge piercing device generally designated 20 and shown in Figs. 4 and 5 is used. The electron discharge piercing device according to the invention is a device for drilling by electroerosion or spark erosion. In the following, the device according to the invention will be described in the case of an EDM drilling device.

Referring now to Figs. 4 to 13, the electroerosion drilling device designated by the general reference 20 will be decried.

This device 20 comprises an assembly 21 for supporting and guiding an electrode 22 which, in this case, is a hollow electrode, as shown in FIG. 11. The guide support assembly 21 comprises a base 23 intended to be applied to the lower face 4a of the tubular plate 4, as shown in FIGS. 4 and 5. This base 23 carries fasteners 24 below the tubular plate 4 which are preferably two in number. Each fastener 24 is of known type and commonly used to fix an element below the tube plate 4.

Each fastener 24 is formed by an expandable sleeve 25 actuated by an expander core, not shown, controlled by an actuating rod integral with a jack 26 carried by the base 23. The actuation of the cylinder 26 allows to realize the diametrical expansion of the sleeve 25 and the fixing of the base 23 below the tubular plate 4. The expandable sleeve 25 of each fastener 24 is made of either a synthetic material or a stainless alloy. The base 23 of the support and guide assembly 21 also carries a cylinder 29 (FIG 6) which extends below said base 23 and in which is slidably mounted a support 30 of a connecting piece 31 with an electrode holder 32 integral with the electrode 22. The support 30 shown in more detail in FIG. 7, has the shape of a bowl having in its lower part an extension 30a and whose upper part delimits a housing 30b for the positioning of the connecting piece 31. The support 30 comprises projecting on its outer wall a finger 33 for to be placed in a groove 34 formed in the cylinder 29 over its entire height (FIG 6) so as to guide the sliding of the support 30 during its displacement in the cylinder 29. The support 30 is made of an insulating material. The base 23 also carries a centering element 35 shown in FIG. 9. This centering element 35 comprises a plate 36 intended to be fixed below the base 23, as shown in Figs. 4 and 5, and a tubular centering device 37 traversing the base 23 and extending above said base 23 after its mounting. The tubular centralizer 32 has an outer diameter substantially equal to the inner diameter of the tube 5. The centering element 35 is also made of an insulating material. The connecting piece 31 shown on a larger scale in FIG. 8, comprises a circular plate 38 intended to be placed in the bottom of the housing 30b of the support 30. The plate 38 has a diameter substantially equal to the inner diameter of the housing 30b. The connecting piece 31 also comprises a body 39 extending above the plate 38. This connecting piece 31 makes it possible to connect the electrode 22 via the electrode holder 32, to means for supplying the electric current and dielectric fluid supply means, not shown. For this, the body 39 of the connecting piece 31 comprises a tapped hole 40 for fixing a terminal, not shown, connected to the power supply means and a channel 41 intended to be connected to the power supply means. dielectric fluid. The electrode holder 32 shown in FIG. 10 comprises a lower nozzle 45 for fixing in the channel 41 of the connecting piece 31 and a rod 46 comprising an axial channel 47 for the passage of the dielectric fluid between the connecting piece 31 and the electrode 22. This rod 46 is provided with at its end opposite to the tip 45, a housing 48 for fixing the electrode 22. The electrode 22 comprises at its lower part a fixing end 50 in the housing 48 of the electrode holder 32 and an axial channel 51 for the dielectric fluid. The end of the electrode 22 is provided with at least one outlet orifice 52 of this dielectric fluid. The assembly constituted by the connecting piece 31, the electrode holder 32 and the electrode 22 is made of a conductive material of the electric current.

The piercing device also comprises, as shown in FIG. 4, a unit 60 for moving the electrode 22 in the tube 5 and applying a movement back and forth to this electrode 22 to perform the drilling of the bottom 12 of the plug 10. This unit 60 is connected to the electrode 22 by a transmission cable 61, a first end 61a is connected to said unit 60 and a second end 61b is connected to the support 30. This transmission cable 61 is of known type. The unit 60 has a dual function, the first consisting through the transmission cable 61 to move the support 30, the connecting piece 31, the electrode holder 32 and the electrode 22 between a rest position, shown in FIG. 4, in which the free end of the electrode 22 is spaced from the bottom 12 of the plug 10 and a working position shown in FIG. 5, wherein said free end of the electrode 22 is positioned near this bottom 12. The second function of the unit 60 is to print through the transmission cable 61 a movement back and forth to the electrode 22 for drilling the bottom 22, as will be seen later. The piercing device comprises means for checking the drilling of the bottom 12 of the plug 10 formed by a control rod 65 (FIG 12) for advance and end of travel of the electrode 22 and by a camera 9 (FIG. ) placed inside the water box 2. As shown in Figs. 4 to 5, the control rod 65 is fixed on the support 30 and extends below this support 30. The length of the control rod 65 is determined so that the position of its free end 65a is at the bottom 29a of the cylinder 29 when the electrode 22 has pierced the bottom 12 of the plug 10, as shown in FIG. 5. The piercing device also includes an auxiliary template 70 (Fig. 13) for adjusting the position of the electrode 22 on the support and adjustment assembly 21, as will be seen later.

The piercing of the plug 10 is carried out as follows. Since the position of the plug 10 in the tube 5 is known, the operators first mount the electrode 22 on the support and guide assembly 21 and adjust the position of this electrode on said assembly. For this, the operators use the auxiliary template 70 (FIG 13) whose height corresponds to the idle stroke that the electrode must perform to go from the bottom of the tubular plate 4 to the lower edge of the plug 10. The operators also determined the length of the control rod 65 and fixed this control rod 65 on the support 30. After these various adjustment operations, the operators introduce into the water box 2, the support assembly 21 and guide provided with the electrode holder 32 carrying the electrode 22. The connecting piece 31 is connected to the power supply means and the dielectric fluid and the support 30 is connected to the unit 60 by the transmission cable 61.

Fixing the assembly 21 below the tubular plate 4 is performed by first positioning the tubular centralizer 35 in the corresponding tube 5 and introducing the expandable sleeves 25 of the fasteners 24 into the tubes 5 of the beam tube, as shown in FIG. 4.

The jacks 26 of the fasteners 24 are actuated so that the sleeves 25 expand and secure the base 23 of the assembly 21 with the tube plate 4. At this stage, the various components of the piercing device are in the position shown in FIG. 4.

Then, the electrode 22 is moved via the electrode holder 32 by sliding the support 30 and the connecting piece 31 in the cylinder 29 so as to bring the free end of said electrode 22 close to the inner bottom. 12 of the cap 10. The support guide 30 is provided by the finger 33 which slides in the groove 34 of the cylinder 29.

The electrode 22 is supplied with electric current and dielectric fluid so that this dielectric fluid passes through the channels 47 and 51, respectively of the electrode holder 32 and the electrode 22, and then spreads inside the plug 10 after having through the orifice 52 formed at the free end of the electrode 22. Simultaneously, the unit 60 moves and moves back and forth by means of the transmission cable 61, the support 30, the connecting piece 31, the electrode holder 32 and the electrode 22 for drilling the bottom 12 of the plug 10 by a disintegration arc thus created between the free end of said electrode 22 and the bottom 12 of said plug 10. The electrode 22 moves towards the bottom 12 of the plug 10 and performs its drilling and the various components of the drilling device are in the position shown in FIG. 5. As soon as the operators note by examining the images transmitted by the camera 9, the control rod 65 is in the position shown in this FIG. 5, they cut the supply of electric current and dielectric fluid and they bring the support 30, the connecting piece 31, the electrode holder 32 and the electrode 22 to the initial position shown in FIG. 4 and they stop the unit 60. The bottom of the plug 12 is thus pierced.

After removal of the device from the water box 2 of the steam generator, the operators can proceed to a control drilling of the bottom 12 of the plug 10 by introducing into the corresponding tube 5 a control rod, not shown.

Thanks to the direct supply of the electrode 22 in dielectric fluid through channels in the electrode holder and the electrode, the device according to the invention is simple to implement because it avoids the use of an apparatus complex to maintain the assembly in the dielectric fluid. In the case of expanding sleeves made of stainless alloy, one of these sleeves can be used as a grounding plug. According to one variant, the unit 60 can be mounted directly on the cylinder 29 of the support and guide assembly 21, which avoids the use of a transmission cable 61. According to yet another variant, the tubular centralizer 35 forming the centering element may comprise at least one discharge passage of the dielectric fluid containing the drilling dross of the bottom 12 of the plug 10. The drilling device according to the invention may also be a spark-piercing device and in this case case no dielectric fluid supply means is used.

In the case of a drill broken in the bottom of the plug, the drilling device according to the invention can save a critical situation by providing drilling. Finally, the device according to the invention can also be adapted to perform the reconforming of a steam generator tube or the shaving of a weld of a plug or tube below the tube plate of this steam generator.

Claims (17)

CLAIMS1.- A process for the electron discharge piercing of a plug (10) of a steam generator tube (5) (1) of a pressurized water nuclear power plant, said steam generator (1) comprising an outer casing (1 a) pressure integral with a tubular plate (4) whose lower face (4a) defines a water box (2) for supplying the steam generator (1) in primary fluid and whose face upper limit defines with the external envelope (1 a) a space of the secondary circuit containing a bundle of tubes (5) curved in a U whose straight branches are vertical and parallel to each other and fixed in holes (6) passing through the tube plate ( 4), said method comprising the steps of: - mounting on a support and guide assembly (21), an electron discharge piercing electrode (22), - adjusting the position of the electrode (22) on said assembly (21) with respect to the position of the bottom (12) of the plug (1) 0) in the tube (5), - is introduced into the water box (2) and is fixed below the tubular plate (4), the assembly (21) support and guide to place the electrode (22) in the tube (5) having the plug (10) to be pierced, - the electrode (22) is supplied with electric current, - the electrode (22) is moved in the tube (5) to place its end free near the inner bottom (12) of the plug (10) and the electrode (22) is advanced through said inner bottom (12) and, simultaneously, the electrode (22) is moved back and forth to piercing the bottom (12) of the plug (10) by a disintegration arc created between the free end of said electrode (22) and the bottom (12) of said plug (10), and - checking the bottom hole (12) of said plug (10). 2. Method according to claim 1, wherein the position of the electrode (22) is adjusted by means of an auxiliary jig (70). 3. Method according to claim 1, wherein it verifies the drilling of the bottom (12) of the plug (10) by means of a control rod (65) for advance and end of travel of the electrode (22) and a camera (9) placed inside the water box (2). 4. A process according to any one of claims 1 to 3, wherein the electrode (22) is simultaneously supplied with electric current and dielectric fluid to effect piercing of the bottom (12) of the plug (10) by electroerosion. 5. ù Device for drilling by electron discharge of a plug (10) of a steam generator tube (5) (1) of a pressurized water nuclear power plant, said steam generator (1) comprising an outer casing (1 a) pressure integral with a tubular plate (4) whose lower face (4a) defines a water box (2) for supplying the steam generator (1) in primary fluid and whose face upper limit defines with the external envelope (1 a) a space of the secondary circuit containing a bundle of tubes (5) centered in U whose straight branches are vertical and parallel to each other and fixed in holes (6) passing through the tube plate ( 4), said device comprising: - an electron discharge piercing electrode (22); - an assembly (21) for supporting and guiding the electrode (22) in the tube (5) comprising said plug (10); ) to drill, - a centering element (35) of the electrode (22) in said tube (5), - gold fasteners (24) of said assembly (21) below the tubular plate (4), - a piece (31) for connecting the electrode (22) with means for supplying electrical power, - a unit (60, 61) for moving the electrode (22) in the tube (5) and for reciprocating the electrode (22) to place its free end near the inner bottom of the plug (10) and advance the electrode (22) through said inner bottom (12) and, simultaneously, to pierce the bottom (12) by a disintegration arc created between the free end of said electrode ( 22) and the bottom (12) of said plug (10). 6. Device according to claim 5, said device comprising an auxiliary template (70) for adjusting the position of the electrode (22) on said assembly (21) relative to the position of the bottom (12) of the plug (10). in the tube (5). 7. Device according to claim 5, said device comprising means for verifying the drilling of the bottom (12) of the plug (10) formed by a control rod (65) for advance and end of travel of the electrode (22) and by a camera (9) placed inside the water box (2). 8. Device according to any one of claims 5 to 7, wherein the assembly (21) for supporting and guiding the electrode (22) comprises a base (23) carrying: - the fasteners (24) below the tubular plate (4), - a cylinder (29) extending below the base (23) in which is slidably mounted a support (30) of the connecting piece (31), and - The centering element (35) extending above the base (23) formed by a tubular centralizer (37) adapted to be placed in the tube (5) having the plug (10) to be pierced. 9. Device according to claim 8, wherein the support (30) and the tubular centralizer (35) are made of an insulating material. 10. Device according to any one of claims 7 to 9, wherein the electrode (22) is hollow and the connecting piece 31 is connected to means for supplying electrical current and dielectric fluid. 11. Device according to any one of claims 5 to 10, wherein the connecting piece 31 is connected to the electrode 22 by a cylindrical electrode holder 32, hollow and whose outer diameter is substantially equal to the inner diameter of the tubular centering device. (35). 12. Device according to any one of claims 8 to 11, wherein the support (30), the connecting piece (31), the electrode holder (32) and the electrode (22) are movable by said unit ( 60, 61) between a rest position in which the free end of the electrode (22) is spaced from the bottom (12) of the plug (10) and a working position in which said free end is positioned near said bottom (12). 13. Device according to any one of claims 5 to 12 wherein the unit (60) is separated from the cylinder (29) and connected to the support (30) by a transmission cable (61). 14. Device according to any one of claims 5 to 12, wherein the unit (60) is mounted directly on the cylinder (29). 15. Device according to any one of claims 5 to 14, wherein the centering element (35) comprises at least one discharge passage of the dielectric fluid containing the drilling dross of the bottom (12) of the plug (10). . 16. Device according to any one of claims 5 to 15, wherein the fasteners (24) are formed by expandable sleeves (25) each adapted to be placed in a tube (5) of the tube bundle. 17. Device according to claim 16, wherein the expandable sleeves (25) are made of stainless alloy, one of said sleeves (25) constituting a grounding.