EP0158566B1 - Process for eliminating deposits in a steam generator of a nuclear pressurized water reactor - Google Patents

Abstract

Process for the elimination of corrosion products formed on the tube plate and in the gaps between the tubes and the spacer plates of a steam generator of a pressurized water nuclear reactor, in order to prevent the appearance of a corrosion phenomena, which can lead to necking or denting of tubes by oxide growth. The process consists of reacting with said oxides at between 50 DEG and 100 DEG C., an aqueous solution containing 6 to 8% gluconic acid, 3 to 5% citric acid, approximately 0.5% of a corrosion inhibitor and ammonia until a pH between approximately 3 and 9.5 is obtained.

Description

The present invention relates to a method for removing deposits formed in a steam generator of a pressurized water nuclear reactor. It relates more particularly to a process for removing deposits formed on the tube plate and in the interstices of tube-plate spacers of a pressurized water nuclear reactor steam generator.

It is known that in a steam generator of this type, the primary fluid coming from the reactor circulates in a bundle of tubes fixed by swaging and welding in a tube plate surmounting the water box of the steam generator. These tubes are held in place by pierced spacer plates. During the operation of the reactor, it is observed that the steam generators are the source of damage due to the deposits of oxides and corrosive products accumulating on the tube plate and in the interstices of the tube-plate spacers. Deposits on the tube plate allow the accumulation and concentration of harmful products such as chlorides, sulfates and hydroxide ions. These products lead to corrosion of the tubes by forming either pits, or cracks, or an intergranular attack. Oxides in the interstices between the tube-spacer plates can also lead to the creation of a very corrosive environment for the steel of the plates. The oxides produced by this corrosion cause the diameter of the tubes to shrink, which can lead to their cracking.

This phenomenon, generally known as "DENTING" and which will be further examined in detail below, entails the obligation to plug the affected tubes.

It is practically not possible to mechanically remove these deposits because there is an impossibility of access due to the geometry of the steam generators. On the other hand, it is possible to act chemically and several methods are currently used for this purpose.

Among the documents likely to illustrate the state of the art, mention may be made of patent FR-A-2 262 099 (PFIZER INC.) Which relates to the preparation of metal surfaces such as those of petroleum storage tanks or bilges of oil tankers, for the application of a protective coating. This preparation consists in treating surfaces in order to rid them of iron oxides and other impurities and they are made passive to resist rust. For this purpose, 0.1% of an organic acid such as citric and gluconic is used to dissolve in particular iron oxide, and the pH is adjusted between 5.5 and 7.5.

We can also cite a study by J. Gonzalez-Velasco from 1978 published in Anales de Quimica-Madrid and relating to the electrophoresis applied to gluconic acid complexes with Fe (II) and Cu (II) ions.

Among the known processes making it possible to act chemically and to dissolve the corrosive products of the secondary circuit, one can cite that which involves the use of a solution of ethylenediaminetetracetic acid (EDTA).

• a) Dissolution des produits de corrosion déposés sur la plaque à tubes La solution contient:
  
  • Le pH est ajusté à 7,0 avec de l'ammoniaque. Le temps de contact est de 7 h à une température de 93° C.
• b) Dissolution du cuivre contenu dans les produits de corrosion déposés La solution contient:
  
  • Le pH est ajusté à 7,0 avec de l'ammoniaque puis à 10,0 avec de l'éthylènediamine. Le temps de contact est de 4 à 6 h à une température de 38° C.
• c) Dissolution des produits de corrosion présents dans les interstices tubes-plaques entretoises La solution contient:
  
  • Le pH est ajusté à 6,0 avec de l'ammoniaque. Le temps de contact est de 120 h environ à une température de 121°C.

The steps of this process are indicated below and can be repeated and possibly combined with several rinses.

• a) Dissolution of the corrosion products deposited on the tube plate The solution contains:
  
  • The pH is adjusted to 7.0 with ammonia. The contact time is 7 h at a temperature of 93 ° C.
• b) Dissolution of the copper contained in the deposited corrosion products The solution contains:
  
  • The pH is adjusted to 7.0 with ammonia and then to 10.0 with ethylenediamine. The contact time is 4 to 6 h at a temperature of 38 ° C.
• c) Dissolution of the corrosion products present in the interstices of the tube-spacer plates The solution contains:
  
  • The pH is adjusted to 6.0 with ammonia. The contact time is approximately 120 h at a temperature of 121 ° C.

Such a method has the drawback of requiring numerous washes resulting in very large quantities of effluents. In addition, after a certain operating time, there is the appearance of pitting corrosion, in particular of manganese steel, nickel, molybdenum which constitutes the ferrules.

The object of the present invention is to eliminate these drawbacks by providing a washing process by means of a solution making it possible to dissolve the corrosion products present in the secondary circuit of a steam generator without causing the damage recalled above. .

The subject of the invention is a process for removing the oxides formed on the tube plate and in the interstices of the tube-plate spacers of a steam generator of a pressurized water nuclear reactor in order to reduce the risks of corrosion of the tubes (pitting). , corrosion under tension and intergranular) and to avoid the appearance of a phenomenon of corrosion which could lead to shrinking of the tubes by growth of oxides, process consisting in making act on these oxides, between 50 and 100 ° C, an aqueous solution containing 6 to 8% gluconic acid, 3 to 5% citric acid, approximately 0.5% of a corrosion inhibitor and of ammonia until a pH of between 3 and 9.5.

• - la figure 1 est une représentation schématique d'un des générateurs de vapeur qui sont habituellement montés sur le circuit primaire d'un réacteur nucléaire à eau pressurisée;
• - les figures 2A et 2B représentent en coupe verticale le passage d'un tube dans une plaque entretoise respectivement avant fonctionnement et au bout d'un certain temps de fonctionnement pour illustrer le phénomène de "DENTING";
• - la figure 3 représente un appareil d'étude de la corrosion simulant le fonctionnement d'un générateur de vapeur;
• - la figure 4 représente l'appareil d'étude équipé d'un circuit provisoire permettant la mise en oeuvre du procédé de nettoyage chimique.

Other advantages and characteristics of the invention will also emerge from the description which follows, given with reference to the appended drawings in which:

• - Figure 1 is a schematic representation of one of the steam generators which are usually mounted on the primary circuit of a pressurized water nuclear reactor;
• - Figures 2A and 2B show in vertical section the passage of a tube in a spacer plate respectively before operation and after a certain period of operation to illustrate the phenomenon of "DENTING";
• - Figure 3 shows a corrosion study device simulating the operation of a steam generator;
• - Figure 4 shows the study device equipped with a provisional circuit allowing the implementation of the chemical cleaning process.

The steam generator shown in FIG. 1 comprises a vertical cylindrical enclosure in which is disposed a bundle of tubes 3 in a U shape, and a tube plate 5 on which the tubes 3 are fixed and which delimits with a vertical partition 6, at the lower part of the steam generator, on the one hand, a distribution chamber in the tubes 3 of the primary fluid introduced by the pipe 7, and on the other hand, a chamber for collecting the primary fluid leaving the tubes 3, this fluid being then evacuated outside the steam generator by the pipe 9. Above the plate 5, the tube bundle 3 is held by spacer plates 11 provided with different types of openings, the first type of openings being intended when mounting the tubes 3 and the second type of openings being intended for the passage of the secondary fluid between the tubes 3. This secondary fluid which is introduced into the steam generator by the pipe 13 is transformed into steam pa r the heat coming from the primary fluid circulating in the tubes 3, the vapor being evacuated through the pipe 15 after having passed through the water-steam separators 17.

In Figure 2A, there is shown a tube 3 at a spacer plate 11 before operation of the steam generator. We see in this figure that the tube 3 which is usually made of Inconel 600 passes with a play of the order of a few tenths of a millimeter through the spacer plate 11 usually made of carbon steel, which defines inside the steam generator small annular spaces in which accelerated corrosion of carbon steel is observed during operation of the steam generator. In Figure 28, the same tube 3 is shown at a spacer plate 11, after several months of operation of the steam generator. In this case, it can be seen that the annular space has been filled with corrosion products 12 which develop during the operation of the plant. The expansion of these corrosion products 12 ends up causing stresses on the tubes 3 and causing deformation, which gives rise to local shrinking of the tubes 3, this shrinking phenomenon of the tube 3 is usually designated by the term "DENTING".

Furthermore, the growth of these corrosion products in the interstices between the plate 11 and the tubes 3 generates tensions which also lead to deformations of the spacer plates 11, which causes stresses and distortions on some of the tubes 3 of the bundle. .

The process according to the invention which makes it possible to remedy the phenomena of "DENTING" is implemented by calling upon the action of an acidic complexing medium constituted by gluconic acid used at a concentration of at least 0 , 1 M whose complexing power, especially in alkaline medium, is very high vis-à-vis the ferric ions which it complex from a minimum pH of 3.0.

As gluconic acid does not dissolve iron oxides, an acid is added to it to achieve this result, namely citric acid.

The presence of this acid requires the addition of ammonia to obtain a pH greater than 3.0 compatible with the formation of gluconate-iron complexes. In addition, to prevent corrosion of non-stainless steels, an inhibitor is added.

As an example of a solution having given excellent results, the solution of the following composition may be given without limitation from the point of view of the proportions of the constituents:

Apart from ferric ions, gluconic acid also complex cupric ions, hence the possibility of dissolving, with the same solution, without intermediate draining, the copper corrosion products. This property therefore makes it possible to avoid the formation of too large quantities of effluents. To do this, the pH must be adjusted to 9.2 by addition of ammonia and the potential of the solution to around 200 mV / DHW (saturated calomel electrode) by addition of hydrogen peroxide or by bubbling compressed air to oxidize CUO and CU ^I to CU ". The corrosion inhibitor consists of a mixture of amines having a sulfur content about 5% by weight. Under these alkaline pH conditions, the corrosion inhibitor must remain soluble and for this reason the product sold under the commercial designation P ₆ made by the company SOMAFER is chosen.

It is specified that during the phase of dissolution of the iron oxides, the temperature of the treatment must be between 80 and 95 ° C. while during the dissolution of the copper oxides the temperature must be close to 50 ° C.

To verify the effectiveness of the process, tests were carried out in devices such as those represented in FIGS. 3 and 4 which can simulate the conditions of use of a steam generator of the type of those currently used on reactors pressurized water.

In FIG. 3, a steam generator is seen comprising an insulated enclosure 21 having an internal diameter of approximately 400 mm and a capacity of approximately 100 liters. At the base of this enclosure 21, is arranged a tubular plate 23 of approximately 200 mm thick, which is made of manganese-nickel-mobybdenum steel. In the plate 23 are fifteen tubes 25 welded and welded representing the bundle of tubes in which the primary fluid of the steam generator circulates. These tubes 25 are bent in a U with a radius of 55.6 mm, which corresponds to the smallest radius used in the steam generators equipping FESSENHEIM type ordinary water nuclear power plants, and they are held by spacer plates 27 placed on the along the bundle of tubes above the tube plate 23, the height of the tubes above this plate 23 being approximately 1 meter. Electric heating rods 29 are arranged inside the U-shaped tubes 25, at only one of their ends, so as to dissipate a thermal flux through the wall of the tubes to simulate the heating of the steam generator by a primary fluid . These heating rods 29 have a height of approximately 150 mm and they are located immediately above the tube plate 23. In the tubes 25, a helium pressure is maintained.

At its base, the enclosure 21 is provided with a pipe 31 for reintroducing into this enclosure the condensed vapor discharged at the top of the enclosure by the outlet pipe 33.

The enclosure also includes a drain pipe 35 and a pipe 37 into which an auxiliary heating rod can be introduced at will or for circulating the solution according to the invention back to a tank.

In an installation of this type, artificially causes corrosion similar to that which occurs in a steam generator for a pressurized water nuclear reactor by operating the installation in the following manner: it is introduced into the enclosure 21 via the pipe 31 demineralized and degassed water by sparging with nitrogen, the installation comprising a pressure injection pump (not shown in the drawing) to establish a pressure of 47 bars in the enclosure and maintain the water level substantially constant above the bundle of tubes 25. In the tubes 25, the quantity of heat produced by the heating rods 29 is adjusted, so that the temperature of the water in the enclosure is maintained at 260 ° C. and that the heat flux through the tubes 25 is 20 to 40 W / cm ² . The steam produced is evacuated via line 33 and recycled after condensation in the introduction line 31.

The steam generator was operated for 1030 hours under the temperature and pressure conditions of a pressurized water reactor steam generator, but in the presence of a secondary medium polluted by sea water of conductivity included between 120 and 240 _k LS.cm- ¹ and sludge taken from the tube plate of an industrial steam generator at the time of a stop for recharging, this phase being intended to produce a comparable state of soiling of the device to that of an operating steam generator.

A number of corrosion specimens in various materials such as steels A42, A533, Z10C13, Inconel 600 in the state or having undergone several types of heat treatments were placed in the enclosure of the steam generator or in the sample box 36 which can be seen in FIG. 4. In this same figure, it can be observed that the liquid from the reservoir 39 into which a solution according to the invention is introduced is sucked up by a pump P and sent through line 35 into the steam generator. This liquid then enters either the line 33 and passes through the sample box 36 or the line 37 to return in both cases to the tank 39.

An example of the use of this device is given here.

• - 0,82 litre d'inhibiteur de corrosion 10,4 kg d'acide citrique
• - 39 litres d'une solution commerciale d'acide gluconique à 50 %
• - 5,2 litres d'ammoniaque à 20 % (d = 0,920)
• - 0,2 litre d'inhibiteur de corrosion.

In the tank 39 containing about 215 liters of water, the following products were added in order:

• - 0.82 liter of corrosion inhibitor 10.4 kg of citric acid
• - 39 liters of a commercial 50% gluconic acid solution
• - 5.2 liters of 20% ammonia (d = 0.920)
• - 0.2 liter of corrosion inhibitor.

After homogenization, the pH of the solution was established at 3.2.

The solution was heated to a temperature of 80 ± 2 ° C and maintained at this temperature through the use of heating rods; then the washing solution was circulated in the device by actuating the pump P. During the entire treatment, which lasted 170 hours, the flow rate ratio in the steam generator model and in the sample box 36 was maintained constant so that the linear velocities of the fluid are identical throughout the circuit.

The following quantities were measured continuously: pH, potential / DHW, iron content and temperature of the solution as well as the aggressiveness of the solution with respect to manganese-nickel-molybdenum steel which has been measured electrochemically in situ by means of the use of a probe known as name CORRATER.

• - 1 kg d'acide citrique pour le cas où le Ph deviendrait supérieur à 4,0 afin d'éviter une diminution d'efficacité de la solution de nettoyage.
• - 0,4 litre d'inhibiteur de corrosion pour le cas où l'indication de la sonde CORRATER évoluerait brutalement vers des valeurs supérieures traduisant une augmentation de l'agressivité de la solution.

We had planned to add if necessary:

• - 1 kg of citric acid in case the Ph becomes higher than 4.0 in order to avoid a reduction in the effectiveness of the cleaning solution.
• - 0.4 liter of corrosion inhibitor for the case where the indication of the CORRATER probe suddenly changes to higher values reflecting an increase in the aggressiveness of the solution.

In fact, no addition of reagent was necessary during treatment.

• - la dissolution quasi complète des 1800 grammes de produit de corrosion déposé sur la plaque à tubes,
• - l'aspect satisfaisant des parois des tubes et de la maquette,
• - le débouchage à 90 % des interstices tubes-plaques entretoises,
• - l'absence de corrosion par piqûres des tubes et des échantillons d'Inconel 600 quel que soit l'état de traitement thermique de cet alliage,
• - une légère corrosion par piqûres de l'acier au manganèse-nickel-molybdène,
• - une faible corrosion généralisée des aciers non inoxydables, inférieure à 0,2 µm · h^-1.

After 170 hours of treatment, we opened the model of the steam generator and we could see:

• - the almost complete dissolution of the 1800 grams of corrosion product deposited on the tube plate,
• - the satisfactory appearance of the walls of the tubes and of the model,
• - uncorking at 90% of the interstices of the tube-spacer plates,
• - the absence of pitting corrosion of the tubes and samples of Inconel 600 whatever the state of heat treatment of this alloy,
• - slight pitting corrosion of the manganese-nickel-molybdenum steel,
• - generalized low corrosion of non-stainless steels, less than 0.2 µm · h ^-1 .

Claims (6)

1. Process for the elimination of corrosion products formed on the tube plate and in the gaps between tubes and spacer plates of a steam generator of a pressurized water nuclear reactor, in order to avoid the appearance of a corrosion phenomenon which can lead to necking or denting of tubes through oxide growth, comprising causing to act on these oxides at between 50 and 100°C, an aqueous solution containing 6 to 8 % gluconate acid, 3 to 5 % citric acid, approximately 0.5 % corrosion inhibitor and ammonia until a pH between approximately 3 and 9.5 is obtained.

2. Process according to claim 1, characterized in that the corrosion inhibitor, which must be soluble in the washing solution is constituted by a mixture of several amines.

3. Process according to either of the claims 1 and 2, characterized in that use is made of a washing solution containing 7.5 % by weight of gluconic acid, 4 % by weight of citric acid, 0.4 % by weight of corrosion inhibitor and ammonia in a quantity sufficient to have a pH of 3.1.

4. Process according to any one of the preceding claims, characterized in that in order to eliminate the cupric ions, the treatment temperature is kept at 50° C and the potential of the solution is regulated to approximately 200 mV /SEC.

5. Process according to claim 4, characterized in that the potential is regulated by adding hydrogen peroxide or by bubbling air.

6. Process according to any one of the claims 1 to 3, characterized in that the temperature of the washing water is kept at between 80 and 95" C to dissolve the oxides, other than those of copper and in particular iron oxide.

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