Process for the repair of at least one connecting area between a partition plate and a tube plate of a water chamber of a heat exchanger

The invention relates to a process for repairing at least one connecting area between a partition plate (8) and a tube plate (5) of a water chamber (7) of a heat exchanger (1), in which process the area of weld requiring repair is identified, at least one rail (11) is inserted into the water chamber, the said rail (11) is attached below the tube plate (5), a robot arm (20) is inserted, the robot arm is hoisted within the water chamber, the robot arm is remotely controlled, the connecting area requiring repair is machined and a check is made to ensure that cracks have been eliminated from the repaired area.

TECHNICAL FIELD

This invention relates to a process for the repair of at least one connecting area between a partition plate and a tube plate of a water chamber of a heat exchanger and, in particular, a steam generator in a pressurised water nuclear reactor.

BACKGROUND TO THE INVENTION

Pressurised water nuclear reactors include a primary circuit in which pressurised water cooling the nuclear reactor circulates, this generally comprising several loops in each of which there is a steam generator which heats and vaporises the feed water through the exchange of heat between the pressurised nuclear reactor cooling water and the feed water to generate steam which is delivered to the nuclear power station's turbine. Steam generators for pressurised water nuclear reactors comprise an envelope of generally cylindrical shape with its axis vertical, within which is located a heat exchanger tube bundle and a substantially hemispherical water chamber comprising the lower part of the steam generator, below the envelope enclosing the bundle.

A thick tube plate located horizontally and perforated by systematically arranged holes in a vertical direction provides separation between the upper part of the steam generator and the water chamber. The tubes in the steam generator bundle are engaged and fixed into the holes in the tube plate by means of their extremities in a leaktight manner. The water chamber is bounded by both the hemispherical wall constituting the lower part of the steam generator envelope and the horizontal undersurface of the tube plate into which the holes for the passage and attachment of the tubes of the bundle open.

Each of the tubes in the bundle is arched at the top and has two straight branches whose extremities engage in the holes passing through the tube plate on either side of a partition plate separating the water chamber into two compartments each connected via a pipe to a primary circuit duct in such a way that the reactor cooling water enters the water chamber in one of the compartments in order to be distributed to each of the tubes in the bundle.

The pressurised cooling water circulating within the tubes of the bundle is collected in the second compartment of the water chamber so that it can be collected by a duct in the primary circuit connected to the second compartment of this water chamber.

In order that access may be gained to the water chamber the hemispherical envelope of this water chamber is traversed by an inspection opening or manhole in each of the two parts of the hemispherical envelope bounding one compartment of the water chamber.

On its underside the tube plate comprises a band forming an additional thickness, also known as a partition stub, with which the upper edge of the partition plate is aligned.

The partition plate is attached to the tube plate by a weld at each of its longitudinal edges.

While the nuclear reactor is in operation, the tube plate and the partition plate are subjected to mechanical and thermal stresses. These stresses also have an effect in the connecting areas between the tube plate and the partition plate, that is to say in the weld itself and above all in the areas adjacent to that weld.

After operation for some time these stresses can give rise to the appearance of cracks or incipient cracks in the connecting areas which need to be detected to prevent any leakage of pressurised cooling water into the inner space of the steam generator envelope to which feed water is fed so that they will not prejudice the integrity of the pressurised envelope should cracks propagate.

In the situation where cracks or incipient cracks are detected in these areas, a repair has to be made in order to avoid the risk of leaks or compromising the integrity of the pressurised envelope when the steam generator is brought back into operation.

These repairs are generally performed during scheduled shutdowns of the nuclear power station for recharging the reactor vessel with fuel assemblies.

These repairs may be carried out by operators working within the water chamber of the steam generator.

This technique has the disadvantage that operators have to work in a highly radioactive area containing deposits of activated materials which are carried into the water chamber by the nuclear reactor cooling water circulating in contact with the core's fuel assemblies. This being the case the doses received by operators may be large, even when work is carried out very quickly with each of the operators only being present in the water chamber of the steam generator for a very short time.

Many operators therefore have to carry out the work, so that the doses which they receive are limited.

In order to avoid the disadvantages associated with the need for operators to enter into the water chamber it is known that devices remotely controlled from outside the water chamber can be used to carry out inspection operations and to carry out various tasks, for example eliminating cracks or incipient cracks, or for any other work.

These remote controlled devices can reduce the time during which operators are present within the steam generator water chamber, or even do away with the need for operators to work within the water chamber.

For this purpose an anthropomorphic robot arm which is caused to enter the water chamber through the inspection opening and which includes an attachment portion attached to the wall of the water chamber in the vicinity of that opening is known.

Devices for working within a water chamber of a steam generator which incorporate a plate attaching the device to the undersurface of the tube plate and a turret which can be mounted on the attachment plate so that it can rotate about an axis perpendicular to the plate, that is to say a vertical axis perpendicular to the tube plate while the work is being carried out, are also known.

The device also comprises one or two telescopic arms which rotate as one piece with the turret in such a way that they can be caused to move in rotation about the axis of the turret in order to sweep through one or two annular fields.

The devices used hitherto are in general fairly bulky and extremely complex in their structure and require long and difficult operations, in particular for inserting and positioning them within the steam generator's water chamber.

Furthermore, these devices are generally provided for specific operations and are not intended to withstand large forces, particularly in the course of machining operations.

Also, fitting the device within the water chamber in an initial stage of the operation is a complex task which may require at least one operator to be present within the water chamber.

SUMMARY OF THE INVENTION

The invention therefore has the object of providing a process for the repair of at least one connecting area between a partition plate and a tube plate in a heat exchanger water chamber which avoids the disadvantages mentioned above.

The invention therefore relates to a process for the repair of at least one connecting area between a partition plate and a tube plate of a heat exchanger water chamber, the said water chamber comprising a wall of substantially hemispherical shape provided with at least one inspection opening and bounded at the top by the tube plate through which a plurality of vertical holes pass, and transversely by the partition plate, wherein,the connecting area requiring repair is identified,at least one rail supporting at least one carriage which can move along the said rail and which is equipped with pendular lifting means is inserted into the water chamber through the inspection opening,the said rail is secured below the tube plate,a robot arm is passed in through the opening,the robot arm is hoisted within the water chamber by the lifting means and the said robot arm is secured below the tube plate,the robot arm is remotely controlled,a reference associated with the position of the arm in the water chamber is obtained and the connecting area requiring repair is machined to a predetermined depth using a tool in order to eliminate cracks or incipient cracks in the said area, andelimination of these cracks or incipient cracks is checked.

According to other features of the invention:after elimination of cracks or incipient cracks has been monitored, the machined area is if appropriate refilled by a welding torch carried by the robot arm,after the check that cracks or incipient cracks have been eliminated, or after the machined area has been filled by welding, hammering of the repaired area is carried out by means of a tool carried by the robot arm in order to place that area under compression.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1shows the lower part of a heat exchanger, such as for example a steam generator in a nuclear power station, referred to in general by reference1. This steam generator comprises an external envelope2of general cylindrical shape in which there is a heat exchange bundle comprising a set of tubes3.

Each of tubes3in the steam generator exchanger bundle is conventionally U-shaped and incorporates an arched top part and two straight branches whose lower extremities are engaged in and secured to vertical holes in a tube plate5separating the top part of the steam generator enclosing tube bundle3from a lower part bounded by a substantially hemispherical wall6comprising water chamber7of the steam generator. This water chamber7is bounded at the top by the undersurface of tube plate5.

As illustrated more particularly inFIG. 2, water chamber7is separated into two compartments A and B by a vertical partition forming a partition plate8. This partition plate is located in a diametral plane of tube plate5constituting a plane of symmetry, on either side of which the extremities of the straight branches of each of tubes3in the bundle of steam generator1are secured.

Partition plate8therefore separates water chamber7into a first and a second compartment, respectively A and B, respectively connected to an inlet pipe and an outlet pipe, not shown, for water cooling the pressurised water nuclear reactor which circulates within tubes3between the inlet compartment and the outlet compartment separated by partition plate8.

As illustrated inFIG. 2, tube plate5comprises, substantially in the middle thereof, a band5aforming an excess thickness, also known as a partition stub and generally welded to the tube plate through a full penetration weld, and with which the upper edge of partition plate8is aligned.

Partition plate8is attached to tube plate5at each of its longitudinal edges by a weld5bextending over the full length of partition plate8.

Each weld5band the areas adjacent to this weld form attachment areas in which cracks or incipient cracks may occur after the steam generator has been in operation for a length of time.

An inspection opening or manhole9providing access to the interior of this water chamber passes through wall6of water chamber7.

Inspection opening9(FIGS. 1 and 4) can in particular be used to insert a repair device, indicated by the general reference10and used to eliminate cracks or incipient cracks in each connecting area between tube plate5and partition plate8, into water chamber7of the steam generator.

Repair device10comprises several independent elements through which working members can be caused to enter within water chamber7, as well as moved within the said working chamber7.

As illustrated inFIGS. 1 to 4, the repair device comprises at least one rail11fitted with attachment members18below tube plate5.

As illustrated inFIG. 3, rail11comprises a flange12which is designed to be applied to the underside of tube plate5and a rolling track13supported by flange12on which at least one carriage15moves. Rolling track13extends over the entire length of flange12of rail11.

Preferably, rail11is formed of two portions of rail to assist insertion into water chamber7through inspection opening9and each rail11comprises members for attachment below tube plate5.

These attachment members preferably comprise three flange clamps18distributed over the entire length of flange12of rail11, as illustrated inFIG. 3. Flange clamps18are of the conventional type and each comprise two radially expandable pins and each have a nominal outside diameter which is very much smaller than the inside diameter of vertical holes4. The pins of clamps18can be caused to expand from outside water chamber7through an appropriate known drive device.

As illustrated inFIG. 3, carriage15is equipped by pendular lifting means19comprising a chain or cable or any other appropriate member of a known type.

Repair device10also comprises a robot arm, indicated as a whole by general reference20. This robot arm20of a known type comprises several arm members21articulated to the others in order to allow the free extremity22to move over 360° in space. Free extremity22of robot arm20is provided with means for the attachment of an appropriate repair tool for the work which has to be done.

Robot arm20comprises a base30for attachment below tube plate5and, as illustrated inFIG. 5, this attachment base30comprises a bearing plate31of generally rectangular shape. Bearing plate31of base30providing attachment beneath tube plate5comprises a central housing32within which drive means33acting together with lifting means19are located in such a way as to abut base30against the said tube plate. Drive means33comprises a wheel, not shown, placed in a box34and this wheel is supported by a shaft35driven in rotation by a motor/gearbox36. This wheel is fitted with means gripping lifting means19, and in the situation where this lifting means19comprises a chain, the wheel is a toothed wheel. In order to permit passage of chain19, box34incorporates an opening34afor the entry of chain19and an opening34bfor the exit of this chain19.

Furthermore, bearing plate31comprises a recess37for positioning rail11on both its opposite sides. Bearing plate31also comprises at least two hydraulic flange clamps40, of a known type, each of which are designed to enter a vertical hole4in tube plate5. Preferably, and as shown inFIG. 5, bearing plate31has four hydraulic flange clamps40each located at one corner of this bearing plate31. These flange clamps40each comprise a pin which is radially expandable in such a way as to immobilise the corresponding clamp within a vertical hole4in order to hold base30and the robot arm below tube plate5within water chamber7.

So that robot arm20can be more easily inserted into water chamber7the device comprises an assembly50(FIGS. 1 and 4) for supporting and introducing this robot arm20. This assembly50is movable and is provided with members51for attachment to the outer periphery of inspection opening9and comprises a plate52supporting robot arm20with its base30opposite inspection opening9. This carriage52can be moved on assembly50.

The repair of one or more connecting areas between partition plate8and tube plate5of water chamber7is carried out as follows.

First of all the connecting area which has to be repaired, in which cracks or incipient cracks are present, is identified by appropriate known means.

Then an operator located in this water chamber7receives through inspection opening9a first rail11on which a carriage15is placed. The operator locks flange clamps18. He then performs the same operation with second rail11so that rails11form a continuous rolling track. Rail11carries a carriage15from which a chain19hangs within water chamber7.

The operator then clamps assembly50onto the outer edge of inspection opening9through fixing attachments51as shown inFIG. 4. Carriage52on assembly50supports robot arm20, the attachment base30of this robot arm being placed opposite inspection opening9.

The operator inserts a free end of a chain19into opening34aof drawing means33located in base30of robot arm20and engages this chain19with the toothed wheel of this drawing means33. The end of the chain leaves through outlet opening34b. The toothed wheel of drawing means33is driven by motor/gearbox36.

Carriage52moves on assembly50through a winch, not shown, to insert robot arm20within water chamber7as far as its first joint.

Robot arm20is hoisted by chain19and toothed wheel of drawing means33until the supports of plate31on attachment base30are in contact with the underside of tube plate5. Flange clamps40are each positioned in a vertical hole4in this tube plate5. As soon as attachment base30abuts against the underside of tube plate5, robot arm20is locked onto said tube plate5using hydraulic flange clamps40.

As robot arm20is equipped with a machining tool, such as for example a mill, a grinder or a jet of abrasive water, an operator remotely controls robot arm20from the outside of water chamber7. The profile and flatness of the connecting area is detected by means of robot arm20which is remotely controlled by the operator from the outside of water chamber7. This detection, performed for example by means of a sensor, makes it possible to create a reference before any machining takes place. Robot arm20adopts a reference associated with its position in water chamber7and machines the connecting area requiring repair to a particular depth to eliminate the cracks or incipient cracks in that area using a tool which it carries.

The operator then checks that there are no indications of cracks or incipient cracks within the connecting area so repaired and robot arm20equipped with an appropriate system of the known type if necessary fills by welding using a welding torch carried on the robot arm if the results of the inspection require it, or hammering of the machined surface to place the repaired area under compression.

These operations can therefore be performed without the need for operators being present within the steam generator's water chamber.

In fact insertion, positioning and attachment of the robot arm within the water chamber can be carried out wholly from outside the water chamber.