The invention relates to a method of securing a tube of substantially cylindrical shape in a bore passing through a spherical wall, and to a device for depositing welding material in a facing. In particular, the invention relates to a method of securing an adapter in the vessel head of a pressurized water nuclear reactor.
Nuclear reactors cooled by pressurized water generally comprise a vessel of cylindrical shape placed, in service, with its axis vertical and having a bottom end constituted by a round bottom secured to a bottom end of the cylindrical wall of the vessel, and a top end constituting a support flange for a head which is generally of hemispherical shape and which can be secured to the vessel in a manner that is proof against the pressurized water contained in the vessel while the nuclear reactor is in operation. After the primary circuit of the nuclear reactor has been cooled and depressurized, the head can be dismounted in order to give access to the inside of the vessel which contains the core of the nuclear reactor.
In general, the reactivity of the nuclear reactor core is adjusted by means of control rods of absorbent material which are moved vertically inside the nuclear reactor core. The nuclear reactor control rods are secured to the bottom ends of drive shafts that pass through the vessel head inside tubular adapters that are of generally cylindrical shape and that have mechanisms secured thereto for moving the control rods in the vertical direction.
While the nuclear reactor is in operation, temperature measurements are taken inside its core by means of columns of thermocouples which likewise pass through the vessel head inside adapters.
The vessel head thus has a plurality of cylindrical tubular adapters passing through it, each being secured to the inside of a bore having a vertical axis (when the head is in its in-service position on the vessel), with the various adapters being distributed in a plurality of rows and in a plurality of zones of the head that are annular about the vertical axis that is common to the vessel and the head and on which the center of the spherical wall of the vessel head is situated. Depending on their positions through the vessel head, the bores through which the adapters pass (which are all parallel to the axis of the vessel head), themselves have axes at various acute angles relative to the radii of the hemispherical head passing through respective points on the axes of the bores. In particular, one of the annular rows of bores passing through the vessel head is disposed in such a manner that the axes of the bores are at an angle of about 38° relative to the corresponding radii of the hemispherical vessel head. In general, the bores have axes that do not pass through the center of the hemispherical head (with the exception of one bore that is placed on the vertical axis of the vessel), and the intersections between the bore of cylindrical shape with the outside and inside walls of the vessel head present shapes that are complex.
Each adapter passing through the vessel head presents both a top portion that projects above the vessel head, said portions having secured thereto, in particular, the mechanisms for moving the control rods, and a bottom portion that projects beneath the vessel head, which bottom portion is shorter relative to the inside surface of the head than is the corresponding top portion, and serves in particular to receive a cone for re-engaging a drive shaft.
The adapter tubes are generally made of a nickel-based alloy such as 690 alloy, and the vessel head is made of a low-alloy ferritic steel and coated on its inside surface in a layer of stainless steel. The adapter tubes must be secured in their bores passing through the head in such a manner as to be completely proof against the pressurized water that fills the vessel while the nuclear reactor is in operation (at a temperature of about 320° C. and at a pressure of about 155 bars) and they must be capable of withstanding the pressure inside the vessel.
The adapter tubes are engaged tightly in the bores passing through the vessel head and they are secured by being welded to the inside portion of the vessel head which is constituted by low-alloy steel coated in stainless steel. In each of the zones of the inside portion of the head in which a passage is made for an adapter tube, an annular facing is machined to surround the bore through which the adapter tube passes, and a welding material that is metallurgically compatible with the material of the adapter tube is deposited in the facing by welding (generally by melting a wire). Thereafter, the head is drilled to form the bore for passing the adapter, the adapter is secured tightly in the bore, and finally the adapter is welded by depositing a welding material in a portion of the facing around the adapter, in order to secure the adapter to the previously-deposited layer of welding material.
The operation of depositing a layer of welding material in the annular facing prior to drilling the bore is generally referred to by the term “buttering”.
Until now, the operations of initially depositing a first welding material in the annular facing prior to drilling the bore, and of welding the adapter tube by depositing a second welding material in the remaining portion of the facing after the bore has been drilled and the adapter tube mounted therein, have been performed manually, in particular because of the complex shape of the connection surfaces between the adapters and the inside portion of the vessel head.
Such operations are lengthy and expensive and require numerous inspections, since the welding must be free from defects. The number of adapter tubes secured to a vessel head is generally large (e.g. 65 or 77 adapter tubes, depending on the type of nuclear reactor), which makes this operation extremely lengthy and expensive.
It is thus very desirable to have automatic welding methods available for performing the prior buttering and/or the welding of the adapters to the concave spherical wall of a vessel head.
More generally, it can be desirable to have automatic welding methods available for welding tubes of generally cylindrical shape to spherical walls, in particular when the tubes are secured in such a manner that their axes do not pass through the center of the spherical wall.
The object of the invention is thus to provide a method of securing a tube of substantially cylindrical shape in a bore passing through a spherical wall, the method comprising: machining an annular facing in an inside surface of the spherical wall at the periphery of a zone of the spherical wall through which the bore for securing the tube passes; depositing a first welding material in the facing; machining the welding material and making the bore by drilling through a portion of the spherical wall surrounded by the facing; engaging the tube tightly in the bore; and depositing a second welding material in the facing at the periphery of the tube in order to weld the tube to the first welding material deposited in the facing, it being possible to perform this method with reduced execution time and with very good quality.
To this end, at least one of the first and second welding materials is deposited in the facing automatically by performing the following operations:
In particular implementations taken singly or in combination:
The invention also provides a device for depositing a welding material in an annular facing machined in a spherical wall about a zone where a bore passes through the spherical wall in order to secure a cylindrical tube to the spherical wall, the device comprising a shaft having means for securing it to the wall on the axis of the bore for securing the tube, a welding head mounted on the shaft to rotate about the axis of the bore and including rotary drive motor means, a second motor-driven carriage carried by the welding head and movable in a direction parallel to the axis of the bore for securing the tube, a welding torch pivotally mounted about an axis orthogonal to the axis of the bore for securing the tube, and a motor-driven device for tilting the welding torch by pivoting it about the pivot axis on the second carriage, and control means for controlling the displacements of the second carriage and of the tilt device in order to keep the position of the welding torch constant in angle of tilt and in distance relative to the facing of the spherical wall.
In particular embodiments, taken in isolation or in combination:
In order to make the invention well understood, there follows a description given by way of example with reference to the accompanying figures of an automatic method in accordance with the invention for securing an adapter in the vessel head of a nuclear reactor.
The vessel head has an annular flange la of very great thickness for fitting onto a flange constituting the top portion of the vessel. The flange la is pierced by openings 2 for passing studs for securing the vessel head on the vessel flange.
The central portion 1b of the vessel head is in the form of a spherical cap and is pierced by openings for securing a plurality of adapters 3 that are disposed so that their axes are parallel to the axis of symmetry of the vessel head. In
As can be seen in
After the wall 4 of the head has been forged and machined, a layer of stainless steel (24% chromium and 12% nickel, or 20% chromium and 10% nickel) is deposited on the concave inside surface of the vessel head using a submerged arc welding method employing a machine fed with stainless steel strip.
After making the stainless steel layer over the entire inside surface of the wall 4 of the vessel head, an annular facing 7 is made in each of the zones for securing an adapter 3 to the inside surface of the head, with the section of the facing being asymmetrical, as shown in
The bore 5 for passing an adapter as shown in
After the asymmetrical annular facing 7 has been made by machining around the radial direction 9 with the profile that is shown in
Thereafter, by drilling along the axial direction 10, the bore 5 is made through the wall 4.
As can be seen in
Thereafter, the adapter 3 is welded to the vessel head by depositing welding material 12 in the remaining portion of the facing 7 that remains after the buttering layer 8 has been deposited and machined, so as to provide metallurgical bonding between the adapter 3 of nickel alloy and the buttering layer 8 which is also of nickel alloy. The second welding material 12 is generally constituted by the same nickel alloy as the buttering layer 8.
In the invention, at least one of the operations of buttering by depositing a layer 8 of a first welding material, and of welding the adapter 3 by depositing a second welding material, can be performed in a manner that is entirely automatic.
As with
After the inside of the wall 4 of the vessel head has been coated in a layer of stainless steel, an annular facing 7 is machined around the zone through which the adapter is to pass.
The annular facing 7 is machined in such a manner as to conserve a reserve of metal 11 in the central portion of the facing 7. Compared with prior methods in which the buttering is performed manually, in order to perform this operation automatically and as shown in
Thereafter, the central portion of the reserve of metal 11 is drilled in the direction 10 of the theoretical geometrical axis for the bore in which the adapter is to be secured after the buttering layer 8 has been made and the wall 4 of the head has been drilled. For example, for an adapter as shown in
The welding wire 23 is preferably a coated wire with its metal portion being made of a nickel alloy, e.g. alloy 152.
In order to perform welding automatically, the welding head 16 is set into rotation as represented by arrow 16′ by its own drive means 28. In order to make a weld bead in a portion of the facing 7, the position of the position torch 20 is initially adjusted along the Y direction perpendicular to the axis of rotation of the head 16 by means of the carriage 17. Thereafter, the carriage 17 remains stationary, and while the head 16 is rotating, an elliptical weld bead is deposited in the bottom of the facing by moving the welding torch 20 in controlled manner in the axial direction Z parallel to the axis 10 by means of the axial displacement carriage 18.
In order to ensure that the electrode 20′ remains in an accurately constant position relative to the surface on which the welding material is being deposited, the inclination of the torch 20 about an axis orthogonal to the theoretical axis of the bore and perpendicular to the plane containing the axes Y and Z is adjusted by means of the torch tilt device 19. This ensures that the distance between the end of the electrode 20′ and the deposition surface, and the angle of tilt of the electrode 20′ of the torch relative to the direction of the theoretical axis of the bore are adjusted to values that remain accurately constant. The weld wire 23 which is fed in regulated manner into the arc produced by the melting electrode 20′ thus serves to deposit a weld bead of constant characteristics.
Throughout the rotation of the welding head 16, the displacement of the carriage in the axial direction Z and the angle of inclination of the torch 20, and also the welding current and the linear feed rate of the weld wire 23 are controlled in programmed manner by a control unit 30 of the automatic welding device 14 so as to produce a weld bead having characteristics that are accurately constant. In particular, the welding current and the feed rate of the weld wire are adapted during rotation of the welding head in order to obtain buttering that is regular.
As described above, after the layer of buttering 8 has been deposited in the annular facing 7, the layer of buttering that has been deposited is machined by being milled so as to obtain a layer 8 having a finished state, after which the bore 5 is drilled in the previously-defined axial direction 10 through the reserve of metal 8 in the central portion of the wall 4 left in position after making the facing 7. The adapter tube 3 is then engaged and secured tightly, as shown in
As can be seen in
The operation of the automatic welding device 14 for making weld beads in the chamfer 24 is analogous to the operation of the welding device for making a buttering layer as described above. The chamfer 24 is filled with the welding material 12 constituted by juxtaposed welding beads having accurately constant characteristics. Between two welding passes, for the purpose of depositing two successive beads all along the circumference of the chamfer 24, the welding torch is displaced in the Y direction perpendicular to the axis 10 by a determined amount corresponding to the width of a welding bead by means of the motor-driven carriage 17. As before, the axial displacement carriage 18 and the device 19 for adjusting the angle of inclination of the torch are programmed and adjusted throughout the time welding is taking place. The same applies to the welding current and to the wire feed rate of the automatic welding machine. The control unit 30 enables the displacements of the welding torch to be controlled by the motor-driven carriages 17 and 18 and the tilting device 19 and enables welding conditions to be adjusted in programmed manner (welding current and feed rate of the welding material).
This enables buttering and welding for securing an adapter to be performed in automatic manner with quality that is accurately constant and with increased execution speed.
The method of the invention for securing an adapter can be implemented by performing automatically in the manner described above both the preliminary buttering of the facing and the welding of the adapter, or else by performing only one of those two buttering and welding operations in automatic manner, as described.
The buttering could optionally be performed manually or automatically by using a method other than that described above. In particular, the buttering could be performed by an automatic method as described in a patent application filed on the same day as the present application, in which the welding material is deposited by causing the welding torch to turn about an axis extending radially relative to the spherical surface of the vessel head.
In order to implement the method of the invention, it is also possible to perform buttering in automatic manner as described above, by causing the welding head to turn about an axis that is inclined relative to the radial direction of the spherical surface of the vessel head and to perform automatic welding in a manner that is different, or even to perform welding manually.
Under all circumstances, performing at least one of the buttering and welding operations in automatic manner makes it possible to reduce significantly the time required for executing the operations of securing the adapter and obtaining a bond of very good metallurgical quality.
The invention is not limited to securing adapters for passing through the vessel head, but can also be used for securing any cylindrical tubular element that passes through a spherical wall.
The invention may be applied outside the field of building or repairing nuclear reactors.
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04 01187 | Feb 2004 | FR | national |
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