EP0167513B1 - Process for the restoration of steam generator tubes - Google Patents

Description

The present invention relates to a method for repairing tubes of a steam generator by adding a sealed internal metallic coating, covering a defective area of the internal wall of the tube.

It finds its main application in a pressurized water nuclear power plant of the PWR type, during repairs by means of a sleeve of the tubes of a steam generator suffering from localized corrosion.

The steam generator is a heat exchanger, in which the primary liquid which cools the reactor transfers its heat under a pressure of more than 155 bars and a temperature of approximately 325 ° C, to a secondary circuit, which by evaporation, delivers the steam required to drive the turbine.

Although the tubes of a steam generator are made of special stainless steels, in order to withstand severe service stresses and stresses and in particular corrosion, they can be damaged and present microscopic perforating cracks, sources of circuit leaks primary to secondary circuit.

Corrosion phenomena introduce by themselves the danger that the primary liquid, which contains polluting radioactive compounds, mixes with the secondary liquid with the unfortunate consequence of the impossibility of maintaining the radioactivity below a determined level. .

The currently most widely used repair method for repairing such steam generator tubes is to mask the damaged area of each tube with an internal metal sleeve.

• manchon perméable à l'eau : à une des extrémités, on réalise la fixation par emboutissage d'un manchon contre le tube avec déformation permanente. Ce procédé laisse subsister un petit interstice qui ne supprime pas toute fuite mais limite la perte par infiltration, et
• manchon étanche : au voisinage des deux extrémités au moins, le manchon est fixé de manière étanche pour soudage ou brassage.

After insertion and precise positioning, the metal sleeve is fixed to the tube, generally in the vicinity of its two ends. Depending on the method of attachment, the repairs thus carried out are of one or other of the following types:

• sleeve permeable to water: at one end, fixing is carried out by stamping a sleeve against the tube with permanent deformation. This process leaves a small gap which does not eliminate any leakage but limits the loss by infiltration, and
• waterproof sleeve: at least in the vicinity of the two ends, the sleeve is tightly fixed for welding or patching.

The main disadvantage of these known repair methods consists in the fact that they can introduce into the tube, in the vicinity of the fixing zones, internal tensions by expansion and / or heating.

From the article by Korbaciok and Aphahaboc published in the journal Tr. Kishinev S-KH Inst. 1974 part 123, pages 99 to 102 is known a method for repairing drawers of hydraulic distributors by coating the inner wall of the drawers with a layer of chromium by electrolysis.

The purpose of depositing electrolytic chromium is to replace the metal removed by wear, with a layer of fresh metal, in this case chromium which ensures excellent adhesion, so as to be able to rework the part and restore it to its original shape.

This process, widely used in the mechanical engineering industry, is however not suitable for parts used at high temperatures.

Chromium has a thermal expansion coefficient very different from that of austenitic stainless steel tubes and in particular of inconel 600.

A tube of a steam generator, which would have been repaired by cold chromium plating and which would then be brought to a temperature of 300 ° would on the one hand see its layer of chromium crack. We would even see the appearance in the body of the tube, that is to say the base metal, of new cracks caused by excessive tensile stress resulting from differential expansions.

In addition, the replacement of chromium plating by another electrolytic process, such as nickel plating did not seem to be an acceptable technical solution either because to ensure optimal adhesion of nickel, it is necessary to add to the electrolytic bath approximately 10% of metal nickel chloride.

Chlorine is not disturbing for the electro-deposition process but constitutes a potentially dangerous element for austenitic stainless steels present in the primary circuit. The presence of a minimum quantity of chlorine stops corrosion under voltage at severe operating conditions of pressure and temperature, under which the primary liquid is found.

We also know from the article by Stratulak and Kettar published in the review Tr. Kishinev. S-KH, inst. 1974, part 123, pages 102-107, a method of restoring the cylinder liners of tractor engines by covering with a layer of wear-resistant chrome.

To improve the adhesion of electrolytic chromium to the support, the authors recommend anodic pickling in the chromium plating bath.

This document confirms that it was known to repair a tubular steel element by coating the interior wall with a layer of chromium by electrolysis, in order to replace the metal removed by wear in order to be able to rework the part and give it back to its original shape.

However, in the case of steam generator tubes, the quantity of metal disappeared is very small.

Corrosion cracks under stress are so small that we cannot consider them recap industrially.

The in situ production of an electrolytic sleeve has never been suggested because it goes against the unfavorable prejudices stated above.

By French patent application No. 2421359, a process is known for nickel-plating protection of the surfaces of a recovery boiler in contact with a heating fluid. The method also applies to the internal surfaces of the tubes of the tube bundle.

The coating layer is advantageously obtained by chemical nickel plating.

• bonne résistance à de nombreuses sortes de corrosion ;
• dureté appréciable ;
• résistance à l'abrasion ;
• résistance aux hautes températures ;
• revêtement de qualité constante quel que soient les reliefs de la pièce à traiter ;
• adhérence excellente dans le temps grâce à la pénétration superficielle de l'acier ou du carbone.

The French patent application cites the following properties:

• good resistance to many kinds of corrosion;
• appreciable hardness;
• abrasion resistance;
• resistance to high temperatures;
• constant quality coating whatever the reliefs of the part to be treated;
• excellent adhesion over time thanks to the surface penetration of steel or carbon.

This aforementioned nickel-plating process when applied to the internal surface of a steam generator tube is however not capable of providing sufficient protection since it does not completely exclude corrosion phenomena in the tubes. This process absolutely does not prevent the formation of microcracks by corrosion under tension under the extremely severe conditions of use of pressure and temperature to which said generators are subjected. The thermal and mechanical stresses are also much more severe than in a conventional recovery boiler.

It can therefore be said that conventional nickel-plating methods are not satisfactory for preventively protecting a steam generator against the formation of microcracks by corrosion under tension.

The invention aims to remedy the above drawbacks. It proposes a method which makes it possible to obtain a sealed connection without deformation or local heating of the tube to be repaired. It relates to a process for repairing tubes of austenitic stainless alloys based on nickel of a steam generator by the addition of a sealed internal metallic coating, covering a defective area of the internal wall of the tube, characterized in that the coating is carried out by depositing nickel from a chlorine-free nickel sulfamate solution.

According to a feature of the invention, the closed cavity is arranged in the defective tube, by closing the defective zone of the tube over a length of 20 to 80 cm in the manner of an electrolytic cell, using upper parts ( 6) and lower (7) of a consumable nickel anode or nickel alloy support, which is immersed in the aforementioned nickel sulfamate solution and is gripped to the tube to be treated by two plastic buffers.

At the top of the electrolytic cell, a capillary release tube is provided, through which the gases formed during the process can escape.

These characteristics and particularities of the invention, as well as others, will appear during the following detailed description of a particular form of the invention, with reference to the following drawings which schematically represent the latter.

• la figure 1, est une coupe axiale longitudinale d'une cellule électrolytique destinée à la réparation de tubes suivant l'invention ;
• la figure 2 est une vue schématique du dispositif montré à la figure 1, monté dans le faisceau de tubes du générateur de vapeur.

In these drawings:

• Figure 1 is a longitudinal axial section of an electrolytic cell for repairing tubes according to the invention;
• Figure 2 is a schematic view of the device shown in Figure 1, mounted in the bundle of tubes of the steam generator.

In these two figures, the same reference notations designate identical or analogous elements.

As illustrated in FIG. 1, a steam generator annotated as a whole by the reference number 1, of a pressurized water nuclear power station, of a stainless steel heat exchanger, the bundle of tubes of which is traversed internally under a pressure of 155 bar by a primary circuit and surrounded externally by a secondary circuit. The tubes 2 have a diameter of approximately 20 mm and a length of approximately 12 m, folded in the shape of an inverted U. These tubes are made of austenitic stainless alloys based on nickel, sold under the brand name INCONEL 600 ^R or Inconel SB 163 comprising 75% nickel and 13 to 18% chromium and iron.

The ASME nomenclature is entitled “Nickel-chromium-iron alloy UNS 600 according to SB 163 •.

After some time, some 2 'tubes can be damaged by stress corrosion. This is caused by the severe operating conditions of pressure and temperature, under which the primary liquid is found, which consists of pure water added with boric acid and lithium hydroxide.

Corrosion phenomena mainly take place at the level of the tube plate 3.

Repair work can of course only be carried out during a shutdown of the nuclear power plant, from the moment when one has access to the tube plate 3 at the inlet of the steam generator 1.

As illustrated in FIG. 1, an electrolytic cell represented as a whole by the reference notation 4 ′, is formed on the spot along the interior wall of the damaged tube 2 ′.

• une pièce supérieure 6, réalisée de préférence en matière synthétique et pourvue de doigts ou de lamelles élastiques destinés à centrer l'appareil ;
• une bague 9 en mousse synthétique qui assure une connexion étanche entre la pièce supérieure 6 et le tube 2' ;
• un conduit capillaire d'évacuation 10 pour les gaz formés aux cours du procédé ;
• un conduit de liaison 11 entre la partie externe et interne de l'anode 12, afin d'assurer une circulation de l'électrolyte ;
• une électrode creuse tubulaire 12 réalisée en nickel, dont la longueur est adaptée à la longueur de la pièce frontale 5 à réparer. Cette pièce est fixée par un filetage 13 ;
• une conduite d'amenée 14 pour assurer l'alimentation en solution électrolytique fraîche ;
• une conduite d'évacuation 15 de la solution électrolytique fraîche ;
• une pièce inférieure 16 pourvue de doigts élastiques 8 et d'une bague étanche 9 en matière synthétique.

The method according to the invention provides a front piece for closing the tube 2 'with two plugs. The equipment necessary for this purpose includes:

• an upper part 6, preferably made of synthetic material and provided with fingers or elastic strips intended to center the device;
• a ring 9 of synthetic foam which provides a sealed connection between the upper part 6 and the tube 2 ';
• a capillary discharge duct 10 for the gases formed during the process;
• a connecting conduit 11 between the external and internal part of the anode 12, in order to ensure circulation of the electrolyte;
• a tubular hollow electrode 12 made of nickel, the length of which is adapted to the length of the front piece 5 to be repaired. This part is fixed by a thread 13;
• a supply line 14 for supplying fresh electrolytic solution;
• an evacuation pipe 15 for the fresh electrolytic solution;
• a lower part 16 provided with elastic fingers 8 and a sealed ring 9 made of synthetic material.

The lower part is part of a gripping device 16 intended to fix the lower part of two elastic pads 17 to a tubular plate 3.

The gripping device comprises two elastic pads 17 which are introduced and brought into neighboring tubes.

The defective front section 5 of the tube 2 'on which a sleeve must be provided must be pretreated, before subjecting it to a surface treatment in order to repair it. The inner surface of the tube 2 ′ must in particular be sufficiently clean to make possible perfect fixing of the deposited metal, which generally requires prior cleaning of the surface.

The sleeve along the defective zone 26 of the inner wall of the tube 2 'is formed, using a surface treatment carried out on the spot, in the tube 2', for example by deposition from a phase liquid, which is in contact with the surface to be covered.

If gases are released during treatment, which is generally the case, the method does not apply to horizontal tubes, because the contact between the metal and the liquid phase is no longer ensured in the vicinity of the upper layer.

As numerous compositions of electrolytic baths have been described in the literature, in order to meet the various requirements of the mechanical characteristics, the thickness and the deposition rate, the invention consists in describing the precise operating conditions, in order to create a coating resistant to corrosion using an electrolytic bath free of halides, in particular chlorides.

The presence of chlorine ions, generally in the form of 250 to 300 g of hydrated nickel chloride, is today still considered an essential condition to avoid the passivation of a nickel anode and thus ensure a sufficient growth rate. of the deposited nickel layer.

For the rest of the primary circuit, the use of chlorine ions is absolutely excluded due to the possible formation of stress corrosion attacks.

In the following paragraphs, the compositions and the main operating characteristics of two modes of implementation of electrodeposition baths are presented.

The internal surface of the steam generator is cleaned over a distance corresponding to the first sixty centimeters of the tube bundle. The deposits and oxides which have formed after commissioning are removed by cleaning with a cylindrical wire brush.

Finally we carry out a careful cleaning, using a bristle brush.

et procure une solution concentrée à 48° Baumé contenant 550 à 650 g/I de sulfamate de nickel. La pureté de la solution sulfamate de nickel atteint plus de 99%.The electrolytic solution is prepared by mixing sulfamic (amino-sulfonic) acid in crystallized form and nickel carbonate in the presence of hydrolysis inhibitors. The creation is carried out according to the following equation:

and provides a concentrated solution at 48 ° Baumé containing 550 to 650 g / I of nickel sulfamate. The purity of the nickel sulfamate solution reaches more than 99%.

The composition of the electrolytic bath greatly influences the quality of the coating in terms of attachment to a support, constant thickness, corrosion resistance, hardness and weldability.

By way of example, the addition of a few additives, such as an inhibitor, prevents the decomposition of nickel sulfamate. Decomposition is also avoided by adding small amounts of boric acid (about 25 g / l), by which the pH is adjusted between 3 and 5.

The addition of colloids improves the brightness of the nickel layer.

To avoid hyrogenic stings, small amounts of wetting agents are added. For this purpose, alkyl sulphates are used such as for example lauryl sulphate (0.15-0.30 g / 1).

As shown in Figure 2, the freshly prepared electrolytic solution is poured into the container 19 and flows under gravity, through the valve 20, the flow meter 21, the adjustment valve 22, the thermostat 23, the pH meter 24 and enters the electrolysis cell 1 via the supply line 14.

The fresh electrolytic solution propagates in the annular space 25 formed by the wall to be repaired from the tube 2 'of the steam generator. This wall is limited by the upper and lower parts 7. This space 25 is entirely filled with liquid.

The liquid overflows through an upper connecting pipe and flows through the annular space 27 from the hollow cavity of the electrode 12 towards the discharge pipe 15.

The used solution is collected by the pipe and the three-way valve 28 in glass requests 29 from which the solution is rejected or regenerated so that it can be used again in one of the containers 19 or 32. The flow rate is adjusted to the target value of the desired flow velocity in electrolysis, approximately 1 to 15 cm / s.

The temperature is kept constant at 55-65 ° C and at a pH value of 3 to 5 Soerensen.

As anode, a small hollow tube based on nickel, preferably nickel S, is used. This anode is produced for example from an extruded titanium tube, on which a coating of nickel to be consumed 30 is electrolytically deposited with a sulfur content of 0.02%. Nickel S provides the advantage of not passivating. The upper end of the tube 12 is provided with a thread to allow separation from the upper part 6.

As soon as the operating conditions are reached, thanks to the stabilized external source, there is a potential difference between the two electrodes, equal to 2 to 5 V.

The operating conditions must be carefully maintained for the temperature at the inlet of the cell, the potential difference, the current density, the flow rate and the composition.

The gases which are possibly formed in the electrolysis cell, escape at the top by the capillary tube, while the liquid is retained by its higher viscosity.

• - vidange : ouvrir l'obturateur 20, ainsi que la vanne de détente 31 et la vanne d'évacuation 35 ;
• - rinçage : fermer la vanne d'évacuation 35 et la vanne de détente 31, dévier la vanne 28 vers le bac de réception 34 du liquide de rinçage et ouvrir la vanne 33 ; et
• - vidange : fermer la vanne 33, ouvrir la vanne d'évacuation 35 et la vanne de détente 31.

As soon as the desired thickness of the sleeve is reached, the electrical supply current is interrupted, the installation is emptied and rinsed with distilled water as follows:

• - draining: open the shutter 20, as well as the expansion valve 31 and the drain valve 35;
• - rinsing: close the drain valve 35 and the expansion valve 31, deflect the valve 28 towards the receiving tank 34 of the rinsing liquid and open the valve 33; and
• - drain: close the valve 33, open the drain valve 35 and the expansion valve 31.

The device is removed from the repaired tube. This removal from the device is made possible by the fact that the upper part comprises an elastic polyurethane foam, which is applied below the sleeve.

• 1) diminution draconnienne des temps d'exposition à l'irradiation dans le générateur de vapeur du personnel chargé de la réparation, comparativement à la mise en place de manchons mécaniques connus actuellement ;
• 2) simplicité des opérations requises à l'intérieur de l'espace fermé du générateur de vapeur ;
• 3) possibilité de réparer plusieurs tubes simultanément ;
• 4) épaisseur uniforme de 0,10 à 0,15 millimètre de nickel pur ;
• 5) contrairement aux procédés connus, on ne provoque dans la paroi du tube une tension interne dans le matériau supérieur à 100 MPa ;
• 6) grande résistance du manchon ainsi réalisé, à l'encontre de corrosion du type alcalin.

In addition to the fact that the sleeve does not cause any internal tension in the repaired tube, the method provides the following advantages:

• 1) drastic reduction in exposure times to irradiation in the steam generator of repair personnel, compared to the installation of known mechanical sleeves currently;
• 2) simplicity of the operations required inside the closed space of the steam generator;
• 3) possibility of repairing several tubes simultaneously;
• 4) uniform thickness of 0.10 to 0.15 millimeters of pure nickel;
• 5) unlike known methods, no internal tension in the material is caused in the material greater than 100 MPa;
• 6) high resistance of the sleeve thus produced, against corrosion of the alkaline type.

The resistance of the sleeve has been proven by numerous corrosion tests under tension in an alkaline medium and in pure water which is charged with hydrogen, at a temperature of 350 ° C.

Profilometric tests, eddy current measurements, flicker tests measured using a photoelectric cell do not reveal any fault with the power supply.

The current intensity is a quantity dependent on the immersed surface of the cathode.

The current intensity is adjusted using the potential difference between the electrodes. The conductivity of the bath and the distance between the hollow nickel anode and the wall of the tube are decisive in this case.

If several tubes are repaired simultaneously, the current intensity must be set separately in the various cells.

In special cases, the metal deposits take place according to a completely current-free process. The nickel coating can be deposited, for example, by catalytic reduction from a solution of nickel salts using sodium hypophosphite free of chlorine.

It is obvious that the invention is in no way limited to the embodiment described above, which is rather described by way of example. Many modifications can be made without departing from the scope of the following claims.

Claims (4)

1. Process for repairing tubes (2'), made of stainless austenitic alloys based on nickel, of a steam generator, by providing a leakproof internal metallic coating, so as to cover a defective zone (26) of the internal wall of the tube (2'), characterized in that the metallic coating is produced by electrolytic deposition of nickel from a. solution of nickel sulphamate which is free from chlorine.

2. Process according to claim 1, characterized in that a closed cavity is formed in the defective tube (2') by closing off the detective zone (26) of the tube (2') over a length of 20 to 80 cm, in the manner of an electrolytic cell, by means of upper (6) and lower (7) pieces and a ring (9) which holds in place a consummable anode (12) of nickel or a nickel alloy (30), immersed in the abovementioned nickel sulphamate solution and fastened to the tube to be treated (2') by two plugs of synthetic material (17) inserted into adjoining tubes (2).

3. Process according to one of the preceding claims, characterized in that at the upper part of the electrolytic cell there is provided a capillary orifice (10) through which the gases evolved during the process can escape.

4. Process according to any one of the claims, characterized in that the electrolytic solution of nickel sulphamate is regenerated to prevent the exhaustion of the said solution.

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