FR2691282A1 - Eliminating corrosion deposits in sec. part of steam generator of nuclear reactor - by treatment with soln. of mixt. of formic acid and ascorbic acid to dissolve iron oxide(s) - Google Patents

Abstract

Eliminating the deposits of corrosion consisting predominantly of metallic oxides in the sec. part of a steam generator of a nuclear generator cooled by water under pressure comprises dissolving the deposits in a chemical attack soln. Deposits are contacted during at least one phase in which oxides of Fe are dissolved using a soln. contg. 4-12 wt. % of a mixt. of formic acid and ascorbic acid, and the soln. pH being adjusted to 3.5-4.5. Pref. the acid mixt. used to make the treatment soln. comprises 30 wt. % formic acid and 70 wt. % ascorbic acid. Pref. the treatment soln. is maintained at 120-180 deg.C during contact with the corrosion deposits and under a gas pressure to prevent boiling of the soln. Gas pressure may be released suddenly to cause boiling of the treatment soln. during contact with the deposits. Soln. may be heated in the sec. part of the steam generator by the cooling water from the core of the nuclear reactor circulating in the prim. part of the steam generator. An agitating gas such as N2 may be introduced into the sec. part of the steam generator during dissolution of the Fe oxides. Treatment may further comprise the bubbling of ozone into the treatment soln. in the sec. part of the steam generator to cause oxidn. and dilution of Cu contaned in the deposits.

Description

 The invention relates to a method for removing corrosion deposits mainly consisting of metal oxides, in the secondary part of a steam generator of a nuclear reactor cooled by pressurized water.

 The steam generators of nuclear reactors cooled by pressurized water comprise a bundle of tubes fixed at their ends in a very thick tubular plate and arranged in the envelope of the generator delimiting a secondary part.

 The tubular plate of the steam generator separates the secondary part from the primary part of the steam generator comprising a hemispherical water box.

 The envelope of the steam generator of generally cylindrical shape is arranged with its vertical axis and the tube plate is placed horizontally.

 The secondary part of the steam generator is delimited by the upper face of the tube plate.

In addition, the tubes of the bundle which have vertical branches of great length are held transversely by spacer plates regularly spaced along the height of the bundle.

 During the operation of the nuclear reactor, the corrosion products are entrained by the secondary water from the steam generator and are deposited on the upper face of the tube plate, on the tube bundle and on the spacer plates, in particular around the tubes passing through these plates. The deposits are mainly formed by oxides and by corrosive products such as chlorides, sulfates or hydroxide ions. These products can lead to corrosion of the tubes, resulting in pitting, cracks or intergranular attack on the metal of the tubes. Oxides deposited in the interstices between the tubes and the spacer plates can also lead to the formation of a corrosive medium for the steel constituting the spacer plates and the alloys constituting the tubes of the steam generator. The products of this corrosion formed by oxides cause the diameter of the tubes to shrink by local shrinkage at the crossings of the spacer plates.

 It is therefore necessary to periodically remove corrosion deposits in the secondary part of the steam generator and in particular on the upper face of the tube plate, on the spacer plates and on the tube bundle.

 Methods are known for removing deposits, which consists of bringing these deposits into contact with a chemical etching solution which can be introduced into the secondary part of the steam generator and optionally put into circulation. The chemical attack solution is added with a base such as ammonia, so as to limit the aggressiveness of the solution with respect to the metallic materials constituting the steam generator.

 For example, it has been proposed to use a solution of ethylene diamine tetraacetic acid (EDTA) or else a solution containing gluconic acid and citric acid (FR-A-2,562,710).

 The solution circulated in the secondary part of the steam generator is maintained at a moderate temperature, generally between 50 and 100 "C, so that the dissolution rate is as high as possible.

 Whatever the chemical attack process used, its development presupposes several successive phases of attack and dissolution of the different chemical species.

 In particular, a phase of dissolution of the iron oxides and a phase of oxidation and dissolution of the copper compounds and metallic copper contained in the deposits, copper in the metallic state or in the cuprous state, are used. passing to the cupric state to be dissolved and eliminated.

 In a French patent application filed by the FRAMATOME company under the number 92-03534 filed on March 24, 1992, a process has been proposed for de-coppering deposits inside a steam generator, in which bubbling of l ozone in the etching solution inside the secondary part of the steam generator to carry out the oxidation of copper.

 This process significantly increases the rate of dissolution of copper in the etching solution. It is thus possible to reduce the duration of the depoppering phase of the deposits.

 However, in the case of known processes, the dissolution of iron oxides is relatively slow, during the corresponding phase of the process, in particular because the etching solution must be maintained at a moderate temperature between 50 and 100 C required by the chemical nature of the attack solutions used.

 The aim of the invention is therefore to propose a process for eliminating corrosion deposits consisting mainly of metal oxides, in the secondary part of a steam generator of a nuclear reactor cooled by pressurized water, consisting in dissolving the deposits in a chemical attack solution, this process making it possible to increase the speed of dissolution of the deposits, during the phase of dissolution of the iron oxides, so that the duration of this phase is considerably reduced.

 For this purpose, the deposits are brought into contact, during at least one phase of the process in which the dissolution of iron oxides is carried out, with an etching solution containing from 4 to 12% by weight of a mixture of formic acid and ascorbic acid, the pH of the attack solution being adjusted to a value between 3.5 and 4.5.

 Preferably, the formic acid represents substantially 30% by weight and the ascorbic acid 70% by weight in the mixture.

 In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, with reference to the appended figures, an embodiment of the method according to the invention.

 Figure 1 is a sectional view through a vertical plane of a steam generator of a pressurized water nuclear reactor.

 FIG. 1A is a schematic view of the supply circuits of the steam generator.

 FIG. 2 is a view on a larger scale of the lower end of the secondary part of the steam generator showing the means for implementing the method for removing deposits according to the invention.

 In FIG. 1, a steam generator of a pressurized water nuclear reactor is generally designated by the reference 1.

 The steam generator comprises a casing 2 of generally cylindrical shape arranged with its vertical axis and closed at its upper part by a domed bottom. The envelope 2 is fixed, at its lower part, to the upper face of a tubular plate 3, the lower face of which is connected to a water box 5 of hemispherical shape. The tubes 6 of the bundle 4 of the steam generator are bent at their upper part which has two straight branches which are fixed at their ends in holes passing through the tube plate 3, on either side of a partition 7 separating the box in water in two parts.

 The bundle 4 is surrounded by a bundle envelope 10 arranged coaxially inside the envelope 2 of the steam generator.

 The straight branches of the tubes 6 of the bundle are held transversely by horizontal spacer plates 8 fixed inside the bundle casing 10 and pierced by a network of openings allowing the passage of the tubes 6 and a network of openings for the passage of secondary water from the steam generator during heating, against the external surface of the tubes 6.

 In the upper part of the envelope 2 above the bundle 3 are arranged separators and dryers making it possible to dry the vapor formed by the vaporization of the secondary feed water of the steam generator in contact with the tubes 6 of the bundle 4.

 The envelope 2 of the steam generator is traversed in its upper part by a tube 13 for inlet of the secondary feed water into the steam generator connected to a distribution toroid 13a.

The tubing 13 is also connected, outside the steam generator, to a pipe 23 to which the steam generator supply circuits visible in FIG. 1A are connected. The envelope 2 is also crossed in its upper part, at the level of the separators and dryers 12, by a manhole 14 called a secondary manhole, the diameter of which is close to 500 mm and which allows access to the secondary part of the steam generator located above the tube plate 3 and containing the bundle 4.

 The wall of the water box 5 is crossed, in each of the compartments delimited by the partition 7, by an opening at the level of which is fixed a tube such as 15 which makes it possible to connect the primary part of the steam generator to the primary circuit of the nuclear reactor.

 Primary water enters a first compartment of the water box to be distributed in the tubes of the bundle then circulates in these tubes from bottom to top, then from top to bottom to exit in the second compartment of the water box from from which the primary water is removed to return to the reactor vessel.

 The primary water circulating in the tubes heats and vaporizes the secondary supply water introduced by the pipe 23 and the pipe 13; the torus 13a introduces feed water into an annular space located between the bundle envelope 10 and the envelope of the steam generator 2, the feed water then penetrating inside the envelope of the bundle 10 to come into contact with the tubes 6 of bundle 4 through a space formed between the lower end of the bundle casing 10 and the upper face of the tube plate 3.

 In FIG. 1A, we see the pipe 23 connected to the pipe 13 of the steam generator 1 to which is connected the circuit 24 for regulating the feed water flow rate of the power plant called the ARE circuit.

 The ARE circuit includes a circulation pump and adjustment and control means such as valves and valves. The ARE circuit which is connected to the condenser 25 of the power plant, placed at the outlet of the turbine 26, makes it possible to adjust the flow rate of water supplying the steam generator.

The ARE circuit is connected to an auxiliary supply circuit 28 for the steam generators (circuit
ASG) which includes a circulation pump, regulating and control valves and valves and a reserve of emergency water 29.

 The ARE circuit is also connected to a circuit 30 for chemical conditioning and injection of reagents (SIR circuit).

 As will be explained below, the generator supply circuits can be used for implementing the method according to the invention.

 During its circulation in contact with the tube plate 3, then through the spacer plates 8, the supply water which carries in suspension corrosion products such as oxides causes the formation of deposits on the tube plate and on the spacer plates, these deposits being mainly abundant at the level of the openings for crossing the bundle tubes.

These deposits must be removed periodically during periods of shutdown for maintenance of the nuclear reactor, after emptying and cooling of the steam generator.

 The removal of deposits can be carried out using an etching solution which is introduced into the secondary part of the steam generator by the pipe 23 of the ARE circuit and by the pipe 13.

 The etching solution can be introduced into the ARE circuit, via the ASG circuit or the SIR circuit.

In some nuclear power plants, the circuit
ARE and the ASG circuit are separate and are connected to the steam generator via two different pipes. In this case, the attack solution is introduced through the ASG circuit.

 In all cases the attack solution can be evacuated from the secondary part of the steam generator by the tapes and drain pipes which will be described below with reference to FIG. 2.

 In Figure 2, there is shown the lower part of the envelope 2 of the steam generator fixed on the upper face of the tube plate 3 and means provided in the secondary part of the steam generator which can be used concurrently with the hole d man 14 located in the upper part of the envelope to implement the deposit removal process according to the invention.

 These means consist of two inspection holes 18 and 19 similar to manholes but of smaller dimensions passing through the casing 2, by a drain line 20 carried by the tubular plate 3 and a drain pipe 21 leading to the upper face of the tube plate.

 The inspection holes 18 and 19 are located above the upper face of the tube plate 3, substantially at the level of the lower plate of the bundle casing 10 which is connected at its lower part to a distribution plate 10a of the secondary supply water in the bundle 4.

 One of the openings 18 which has a diameter of the order of 50 mm which is usually used to view the lower part of the secondary of the steam generator is called an eye hole. The other opening, the diameter of which is close to 150 to 200 mm, is used for various interventions in the lower part of the bundle of the steam generator and is called fist hole. At each of the openings 18 and 19, a tube is fixed to the casing 2 of the steam generator. Sealing stages 18a and 19a respectively are fixed respectively on the pipes of the inspection holes 18 and 19.

The stages 18a and 19a are each traversed by an opening at the level of which is fixed a pipe making it possible to introduce an attack liquid into the secondary part of the steam generator or to recover this attack liquid. A similar tape fitted with piping is also attached to the tubing of the manhole 14 in order to effect the removal of deposits by chemical means inside the secondary part of the steam generator.

 The purge line 20 comprises a distribution ramp arranged parallel above the upper face of the tube plate 3.

 The drain piping 21 opens onto the tube plate in the vicinity of the peripheral part, at a point constituting a low point on the upper face of the tube plate.

 In the case of an operation aimed at eliminating deposits throughout the secondary part of the steam generator containing the bundle, a chemical attack solution is introduced through the supply water pipe 13 as explained above or also by the manhole 14 so that the upper level of the solution in the envelope 2 of the steam generator is located above the upper end of the bundle. The attack solution can be evacuated by at least one of the inspection openings 18 and 19 or by the drain piping 21.

 In the case of an operation aimed at removing deposits only on the tube plate, the etching solution can be introduced through one of the openings 18 and 19 and then recovered through the other opening; you can also use the drain piping 21.

 At the end of the treatment, the etching solution is completely evacuated from the envelope of the steam generator by the pipes connected to the tapes of the openings 18 and 19 and then by the drain pipes 21.

 The invention can therefore be implemented, either by introducing the etching solution through the steam generator supply circuits, or by using inspection holes fitted with tapes and supply pipes.

In all cases, the attack solution used is an aqueous solution containing from 4 to 12% by weight of a mixture of formic acid or methanoic acid of formula HCO2H and ascorbic acid C6H806, the structural formula of which is given below. below

 Preferably, the mixture contains about 30% by weight of methanoic acid and 70% by weight of ascorbic acid.

In order to limit corrosion of the materials making up the steam generator, an inhibitor can be used such as the product sold under the name
KORENTIN BH from the company BASF (Butyne Diol 1-4) or the product CCI 801 from the company PETROLITE.

 The solution containing the mixture of formic acid and ascorbic acid is used during at least one phase of the elimination of deposits inside the secondary part of the steam generator, during which the oxides are dissolved. iron and especially magnetite formed in the steam generator.

 During this phase, corresponding to the elimination of iron oxides, the secondary part of the steam generator is filled with etching solution, as indicated above.

 The etching solution is brought, inside the secondary part of the steam generator, to a temperature of 120 to 1800C.

 Preferably, to maintain the temperature of the solution in the secondary part of the steam generator, residual heat from the reactor core is used. The elimination of corrosion deposits is in fact carried out during a period of shutdown of the steam generator during which the cooling of the core is ensured by the RRA circuit (cooling circuit of the reactor at shutdown). The heat of the heart is transmitted to the cooling water which is circulated in the primary circuit by the pumps of the RRA circuit. This water passes through the primary part of the steam generator, that is to say in the water boxes and in the U-shaped tubes. By heat exchange through the wall of the U-shaped tubes, the primary water heats the etching solution contained in the secondary part of the steam generator.

 During the phase of dissolution of the iron oxides using the solution according to the invention, the temperature can thus be maintained at a level of between 120 and 1800C.

 In the case of the known processes of the prior art, this temperature had to be maintained between 50 and 100 "C. The use of a mixture of formic acid and ascorbic acid therefore makes it possible to operate at a much higher temperature. and thus to increase the kinetics of the reaction leading to the dissolution of the oxides.

 Consequently, the time required to dissolve the iron oxides in the secondary part of the steam generator is considerably reduced, this period possibly being between two and eight hours.

 The pH of the solution is adjusted to a value between 3.5 and 4.5 using an ammonia solution added to the attack solution.

 During the phase of dissolution of the iron oxides, the circuit being pressurized with nitrogen, the solution does not boil inside the secondary part of the steam generator. However, it is possible to depressurize abruptly in order to bring the solution into boiling, which promotes attack by circulation of the solution, in particular at the interstices.

 Stirring of the solution at high temperature inside the secondary part of the steam generator can also be obtained by introducing bubbles of a gas such as nitrogen, for example by using the purge line 20 located at the lower level of the abutment.

 The dissolution of the copper and the copper oxides contained in the corrosion deposits inside the secondary part of the steam generator can be carried out using a method known from the prior art and for example using an acid solution. citric whose pH is maintained at a value greater than 9.5 by using additions of monoethanol amine as a base, in order to carry out the attack in an alkaline medium. Oxidation of metallic copper or in the form of cuprous oxide, to obtain cupric oxides, is obtained by bubbling air, oxygen or ozone into the solution or by adding water oxygenated to this solution.

 As indicated in patent application 92-03534 from the FRAMATOME company, the injection of ozone bubbles, using for example the purge line of the steam generator makes it possible to accelerate and improve the de-curing in significant proportions .

 In the case where hydrogen peroxide is used as an oxidizing additive to the solution, the solution is stirred by bubbling air introduced through the purge line.

 De-coppering is carried out in all cases at a temperature in the region of 50 "C and its duration can vary between 5 and 20 hours.

 One can also use a solution of ethylene diamine tetraacetic acid (EDTA) in an alkaline medium, a base such as ammonia or ethylene diamine being added to the solution. The oxidation of copper is carried out, as previously, by bubbling an oxidizing gas or by adding hydrogen peroxide.

 The time required to carry out the plucking is then four to six hours at a temperature of 40 "C.

 The main advantage of the method of the invention is to allow the dissolution of iron oxides at a high temperature between 120 and 1800C.

As indicated above, the time necessary to effect the dissolution of the iron oxides and in particular of the magnetite is then between 2 and 8 hours, which can be compared very favorably with the time necessary to carry out the dissolution of the iron oxides in the case of the processes cited according to the prior art (duration of the order of 120 hours).

 The steam generator can then be cleaned by eliminating corrosion deposits, during the time of a normal shutdown of the nuclear reactor unit and without increasing the duration of the shutdown. It is therefore possible to carry out more frequent cleaning of the steam generator, so that the deposits to be eliminated are less abundant.

 The dissolving power of the formic and ascorbic acids used in the context of the invention is substantially twice as high at the reaction temperature, than the dissolving power of the acids or other attack solutions used in the case of the processes of the art prior.

 It is therefore possible to use lower concentrations of acid in the attack solution, which makes it possible to reduce the costs of the products used for cleaning the steam generator.

 In addition, the acids used in the context of the process of the invention degrade more quickly and more easily than the acids used in the case of the processes according to the prior art. The treatment of effluents to remove acids (for example by incineration or ozonation) is therefore easier to implement and less costly.

 In addition, to carry out the treatment, use is made both for filling and emptying of the attack solution as for heating and agitation of this attack solution, only existing circuits of the plant.

 To heat the solution to the optimum reaction temperature, residual heat from the reactor can be used.

 The invention applies to the cleaning of the secondary part of any steam generator of a pressurized water nuclear reactor.

Claims (13)

 1.- Method for eliminating corrosion deposits consisting mainly of metal oxides, in the secondary part of a steam generator (1) of a nuclear reactor cooled by pressurized water, consisting in dissolving the deposits in a chemical attack solution, characterized in that the deposits are brought into contact during at least one phase of the process in which the iron oxides are dissolved, with an attack solution containing from 4 to 12 % by weight of a mixture of formic acid and ascorbic acid, the pH of the solution being adjusted to a value between 3.5 and 4.5.  2.- Method according to claim 1, characterized in that the mixture of formic acid and ascorbic acid in the attack solution contains substantially 30% by weight of formic acid and 70% by weight of ascorbic acid .  3.- Method according to any one of claims 1 and 2, characterized in that the attack solution is maintained at a temperature between 120 and 180 C, during its contact with the corrosion deposits in the part secondary of the steam generator (1).  4.- Method according to claim 3, characterized in that the attack solution is subjected to a pressure exerted by a gas during its contact with the corrosion deposits, in order to avoid boiling of the solution.  5.- Method according to claim 4, characterized in that the gas pressure is suddenly released, so as to bring the etching solution into boiling during its contact with the corrosion deposits.  6.- Method according to any one of claims 3 to 5, characterized in that the attack solution is heated, inside the secondary part of the steam generator, by water cooling the core of the nuclear reactor in circulation in the primary part of the steam generator (1).  7.- Method according to any one of claims 1 to 6, characterized in that a stirring gas such as nitrogen is introduced by bubbling into the secondary part of the steam generator (1), during the phase of dissolution of iron oxides.  8.- Method according to any one of claims 1 to 7, characterized in that it comprises an oxidation and dilution phase of copper contained in the deposits, during which ozone is bubbled through a solution of attack inside the secondary part of the steam generator (1).  9.- Method according to any one of claims 1 to 8, characterized in that the etching solution is introduced into the secondary part of the steam generator (1) by a tube (13) for introducing water supply to the steam generator (1).  10.- Method according to claim 9, charac terized by the fact that the etching solution is introduced into the secondary part of the steam generator by a circuit (24) for regulating the feed water flow rate connected to the generator. steam.  11.- Method according to claim 9, characterized in that the etching solution is introduced into the secondary part of the steam generator by a circuit (28) of auxiliary supply connected to the steam generator.  12.- Method according to any one of claims 1 to 8, characterized in that in the case of elimination of corrosion deposits throughout the secondary part of the steam generator (1) containing the harness (4) of the generator of steam, the chemical attack solution is introduced into the envelope (2) of the steam generator through a manhole (14) disposed at the upper part of the envelope (2).  13.- Method according to any one of claims 1 to 8, characterized in that in the case of elimination of corrosion deposits in contact with the upper face of a tubular plate (3) of the steam generator only , the etching solution is introduced into an envelope (2) of the steam generator through at least one inspection hole (18, 19) located just above an upper face of the tube plate (3).