EP0406098A1 - Process for dissolving oxyde deposited on a metallic substrate and its application to decontamination - Google Patents

Abstract

Process for dissolving oxide deposited on a metal substrate. This process comprises the following stages: a) etching with an oxidising solution containing a permanganate anion at a pH of between 2 and 6, preferably between 2 and 4, the etching period being chosen so that the exponential stage is finished, the oxidising solution also containing a sulphate ion at a concentration of between 10<-2> and 10<-5>, preferably between 10<-3> and 10<-4>N; b) a reductive etching by means of an easily degradable organic compound. Application to the decontamination of components of nuclear power stations. <IMAGE>

Description

The present invention relates to a method for dissolving oxide deposited on a metal substrate.

It relates more particularly to a dissolution method of the above type, which is capable of being used for the decontamination of metal parts contaminated during their exposure in a hot zone.

This problem is particularly difficult and clear in the case of the constituent parts of walls or coming into contact with the primary circuit of the steam generators of nuclear reactors.

Most of the radioactivity in the primary circuit, outside the core, is due to corrosion products which are activated in the core and then are precipitated and incorporated into the oxide which forms on the walls of the circuit.

The contamination process being cumulative, it can lead in a few years in the primary circuit to a very high radioactivity.

This contamination considerably hinders maintenance operations during cold shutdowns, while the irradiation of personnel requires protective measures which result in significant economic cost.

The problem is particularly acute at the level of the water box of the steam generator, part of the primary circuit in which the interventions are the longest. It is therefore very important to have a process for rapidly decontaminating components or parts of the primary circuit during cold shutdowns on which personnel must intervene.

The main part of decontamination consists in eliminating the deposits formed on the walls of the circuit during use. This deposit is mainly formed by an oxide layer which incorporates activated or radioactive corrosion products.

This oxide, which may optionally include an external layer which is not very adherent, has an internal layer which is very compact and adherent. To decontaminate, it is necessary to remove this internal oxide, preferably down to the metal.

Among the mechanical, electrochemical or chemical processes envisaged, the chemical processes are the most developed. Indeed, mechanical processes, such as sandblasting, high pressure jets, abrasion, and electrochemical pose problems of implementation and recovery of waste for steam generators.

The principle of the chemical decontamination process consists in dissolving the adherent oxide layer, then in processing the solutions formed from small volume releases and easily storable in protected discharge systems.

The oxides formed in pressurized water reactors are generally very rich in chromium, the oxide and especially the spinels are known to be difficult to dissolve. To dissolve these oxides rich in chromium, it is usually proposed to oxidize chromium from degree 3 to degree 6 in an oxidizing medium and then to dissolve the residual oxide depleted in chromium in a reducing phase.

The problems raised by such a technique are essentially due to the double constraint of non-degradation of the metal parts which will continue their function within the power station and of effluent treatment, after the attack.

From the above, and without being exhaustive, the following constraints follow:

The reagents used must not contain elements which are liable to affect the mechanical properties of the steels to be etched. This is particularly the case for sulfur, the presence of which can cause stress corrosion if this element is in too high a concentration in the reagent.

The reagents used must be either easily separable from the radioactive elements or degradable to give products which are easily separable from the radioactive elements.

The reagents must be such that, after treatment, the effluents to be stored in a protected landfill are as small as possible in volume, or can enter standard containers.

All these constraints must be applied taking into account the size of the devices to be etched, which implies a very high volume of solution involved, and consequently, very dilute solutions to avoid excessive storage of the reagents.

This is why, one of the aims of the present invention is to provide a method which makes it possible to reduce the duration of attack or to improve the efficiency of attack compared to existing techniques.

Another object of the present invention is to provide a process of the above type which avoids the use of sulfur compounds in too large a concentration.

Another object of the present invention is to provide a process in which the reagents are easily reprocessable to give products either pure or storable in easily protected landfills.

• a) attaque au moyen d'une solution oxydante contenant un anion permanganate à un pH compris entre 2 et 6, de préférence entre 2 et 4, la durée de l'attaque étant choisie de manière que la phase exponentielle soit terminée, la solution oxydante contenant également un ion sulfate à une concentration comprise entre 10⁻² et 10⁻⁵, de préférence entre 10⁻³ et 10⁻⁴ N ;
• b) une attaque réductrice au moyen d'un composé organique facilement dégradable.

These aims and others which will appear subsequently are achieved by means of a process for dissolving oxide deposited on a metal substrate comprising the following steps:

• a) attack by means of an oxidizing solution containing a permanganate anion at a pH between 2 and 6, preferably between 2 and 4, the duration of the attack being chosen so that the exponential phase is finished, the oxidizing solution also containing a sulfate ion at a concentration between 10⁻² and 10⁻⁵, preferably between 10⁻³ and 10⁻⁴ N;
• b) a reducing attack by means of an easily degradable organic compound.

The results of the effectiveness of the tests are measured by decontamination factor after the sequence of steps a) and b), this decontamination factor will sometimes be subsequently noted FD.

During the study which led to the present invention, it was shown that after a relatively long incubation time, generally between 5 and 15 hours, the kinetics of destruction of the oxide network became very rapid , assumed an exponential appearance and then returned to a zero value. Thus, the decontamination factor curves as a function of the duration of the oxidizing phase have the appearance of the conventional S curve. The determination of the durations necessary to obtain the so-called exponential phase and the duration necessary to obtain the plateau value can easily be determined in each particular case. In general, a value for the duration of the oxidizing phase is set, such that there is at least 80, advantageously at least 90, preferably at least 99% of the decontamination factor obtained for the value of the plateau.

This value generally ranges from 5 to 20 hours. It is a value of this order which is generally chosen in the case of industrial use (one day).

It should be noted that this phenomenon is difficult to demonstrate, since it can only be measured indirectly after the reducing attack phase. In other words, this effect is only potential until the reducing step has been implemented.

Thus, according to the present invention, it has been demonstrated that it was preferable to increase the duration of the oxidative phase rather than to increase the number of cycles or the other attack phases. However, repeating the cycle several times until the desired FD is obtained remains a solution for difficult cases.

As regards the studies for optimizing the conditions of the oxidative attack, it has also been able to be demonstrated that, surprisingly, the sulfate ion plays a catalytic role during this stage. This is why, although the content of sulfate ions must be limited due to the standards (which depend on the country) intended to avoid stress corrosion by the action of sulfur in the alloy, it is desirable to use a sulfate ion. as a catalyst for the reaction to levels weak but sufficient to be catalytic. Thus, it is preferable to use a sulfate ion concentration of between 10⁻² and 10⁻⁵, preferably from 10⁻³ to 10⁻⁴ M. The pH is preferably adjusted to a value between 2.5 and 3.5. The most acidic values are those which correspond to a better attack of the oxides without significant attack of the substrate.

When the quantities of sulfuric acid are insufficient to obtain the desired pH, the supplement can be supplied by nitric acid.

The permanganate anions are preferably at a concentration of less than 10 g / l and preferably advantageously between 2 and 1/4 of g / l. It should be noted that the total amount of permanganate anions is calculated so as to neutralize, as far as possible, the reducing nature of the attack solution from step b). This neutralization takes into account both strong reducing agents such as ascorbic acid and weak reducing agents such as complexing acids which will be discussed below.

Potassium permanganate is the preferred source of permanganate anions.

The solution of attack b) preferably comprises, in addition to the reducing agent, a complexing agent. The preferred complexing agents are di or polycarboxylic acids containing at least 3 carbon atoms and at most 10 carbon atoms.

The complexing agents preferably have groups which allow their destruction by permanganate. Among such groups, there may be mentioned alcohol groups, in particular when the latter are vicinal to each other or vicinal to other functions such as the ketone or carboxylic function. As another function capable of facilitating degradation by permanganate, mention may be made of ketone and aldehyde functions.

According to the rules of complexation, it is preferable that these acids form a ring with five, six or seven atoms with the cation which they are supposed to complex.

It should be noted that the acids comprising amino functions such as ethylene diamine tetraacetic acid (EDTA) which are to be avoided as far as possible, because of the difficulties which they cause during reprocessing, bring little advantages. in the process according to the invention and their use can be avoided because of this.

For its part, oxalic acid, although it is a good complexing agent, is preferably to be avoided during attack, since it facilitates attack on the substrate itself.

Although other hydroxylated di or polyacids give good results, one of the preferred acids is citric acid.

The reducing agent is preferably chosen so that it is completely degraded by a permanganate or ozone attack. This reducer must also be active vis-à-vis the oxidized elements during the first step, or before. Ascorbic acid and dehydroascorbic acid are reducing agents which give good results. As reducing agents which also give good results, mention may be made of sugars having aldehyde bonds.

The concentration of these elements is of the order of 0.1 to 10 g / l of ascorbic acid. Preferably, around 1 to 2 g / l of ascorbic acid. When using other reducers than ascorbic acid, the same values are used as above by transposing so that there are as many equivalents as when using ascorbic acid.

Following the two steps a) and b), the two etching solutions are mixed in a step e). This step is preferably carried out in such a way that the organic compounds of the reducing attack are destroyed by the oxidation reagents of the oxidizing attack, the permanganate preferably being almost completely transformed into manganous ions (Mn ++).

When this is not the case, and the organic compounds are in excess, it will be possible to use oxidizing reagents such as permanganate itself, but also reagents of the ozone type to destroy the organic compounds.

When the permanganate is in excess it is possible to destroy it by reducing organic compounds, the excess of which is easily eliminated by distillation at low temperature or by thermal degradation at low temperature also. In some cases, hydrogen peroxide can also be used.

The reaction of this mutual destruction of the oxidizing and reducing solutions is preferably carried out at a temperature of 50 ° C to 100 ° C, preferably between 60 and 80 ° C.

Step e) is advantageously followed by a step f) of elimination of the cations present in the solution. This step is advantageously carried out using an anion exchange resin. However, it can also be carried out by a simple cation exchanger, either in the liquid phase or on a solid support.

The active groups of the most effective resins are complexing resins, such as, for example, hydroxydiphosphonic, amino-phosphonic, iminodiacetic, malonic resins, etc. However, if it is desired that these resins can be burnt, it is desirable that these resins contain only groups containing carbon, oxygen as well as hydrogen.

The sulfonic resins give satisfactory results.

These resins have a polystyrene skeleton with crosslinking by means of divinylbenzene.

Advantageously, the solutions resulting from stages a) and b) can be subjected to an exchange of cations before their mixing during stage c) for the attack solution resulting from a) and stage d) for the attack solution from b).

During the study which led to the present invention, it was shown that the reactivity of the two attack solutions resulting respectively from a) and b) was greatly improved by this exchange of cations which seems in particular to potentiate the power oxidizing solutions from a).

It has been surprisingly shown that this exchange of cations and in particular the absorption of potassium, or possibly of the other ions correlated with the permanganate ion, makes it possible to avoid the precipitation of manganese bi-oxide, which avoids having to eliminate a new solid effluent.

The solution from step d) can be partially or intermittently recycled to the reducing attack step b) envisaged.

This recycling can be replaced by a reducing attack in which the resin is kept in suspension.

After destruction of the last remaining organic compounds, it is possible to reject as purified water or to recycle the water used for this depollution operation.

It should be noted that this entire process can be implemented with other techniques capable of improving the decontamination factor, that is to say the rate of dissolution of the activated materials. Thus, the use of ultrasound using a technique known in itself makes it possible to significantly improve the yields.

FIG. 1 represents a block diagram of the whole of a decontamination system in a particular implementation for a primary pump of a pressurized water reactor.

This figure shows a decontamination tank 1, an intermediate storage tank for the oxidizing solution 2, an intermediate storage tank for reducing solution 3, a storage tank for rinsing water and destruction of reagents 4, a filter. 5, the loop of which is advantageously around 3 μm. A cationic resin, that is to say cation exchanger 6, and a system of circuit valves and pumps which makes it possible to circulate the different solutions from a tank to a device, from devices to a tank and from tank with tray.

In operation, this system includes the following different operations:
- heating of deionized water to 80 ° C in tank 1;
- injection of oxidizing reagents into tank 1, mixing with deionized water and reaction with part to be decontaminated to constitute the first oxidizing phase; simultaneously with this operation, the deionized water for the reducing phase is also heated to 80 ° C. in tank 3;
- After reaction, the oxidizing intermediate solution is transferred to the intermediate storage tank while being maintained at 80 ° C;
- the hot water for the reducing phase is transferred from tank 3 to tank 1, while the reducing reagents are also injected into tank 1 and the reactant is left to react with the parts to be decontaminated; once the reaction has been carried out, the reducing solution is transferred to its temporary storage tank 3 or the solution is maintained at 80 ° C. The residual radioactivity is then measured to determine whether a second cycle should take place; in the affirmative, the oxidizing solution already used is transferred after having checked its pH and its KMnO₄ concentration from the intermediate storage tank 2 to the decontamination tank 1 in order to carry out the second oxidation phase;
- once this is completed, the oxidizing solution is transferred from the decontamination tank 1 to the temporary storage tank 2 with maintenance at 80 ° C;
- Then the transfer of the reducing solution already used is carried out after possible verification of the state of the reagents from the intermediate storage tank 3 to the decontamination tank 1 to carry out the second reduction phase once this has been completed, the reducing solution to the temporary storage tank 3 while maintaining at 80 ° C;
- the parts are rinsed with hot water at 80 ° C in the decontamination tank using rinse water sotkcée in tank 4 and heated in the latter;
- then a second rinse with cold water is carried out. These two rinses are after possible contact with a cationic resin sent to the control circuit and discharge of the effluents from the nuclear island. During this time, the oxidizing solution stored in tank 2 is transferred to tank 4 by passing the solution through the filter and the resin, then the same operation is carried out with the reducing solution stored in tank 3. react the two solutions one on the other, then the two solutions are treated by passing over a filter and over resin. Decontaminated effluents are discharged after verifying that the level of residual activity authorizes it. It should be noted that, surprisingly, the mixture of the oxidizing and reducing solutions after elimination of the cations makes it possible to completely destroy the residual oxidizing solution and this without precipitation of MnO₂. The best conditions for carrying out this operation of mixing and mutual destruction are from a temperature of 60 to 80 ° C. in an initial pH at most equal to 3.5. The mixing operation raises the pH.

If ascorbic acid and citric acid or their degradation product remain in the solutions, they can be destroyed by injection of a strong oxidant such as for example sodium persulfate at a temperature of 80 ° C. The effluent is then brought to a pH of 7, with, for example, ammonia, then directed to the reservoirs of the circuit for controlling and discharging the effluents from the nuclear island.

The process is particularly well suited to stainless steels and in particular to those collected in the following table: VS Cr Or Mo Co W stainless steels Z3 CN 18-10 0.03 18 10 - - - Z6 CN 18-10 0.06 18 10 - - - Z3 CND 17-12 0.03 17 12 2.5 - - Z6 CND 17-12 0.06 17 12 2.5 - - Z3 CND 17-12 0.03 20 10 2.5 - - base alloys nickel alloy 600 0.1-0.3 14-17 72 - - - cobalt base alloys like stellite 0.9-1.4 26-32 3 1 60 4

Examples 1 to 19 Operating protocol of the different examples:

The tests were carried out for primary pump parts made of steel 18, 10. The tests were carried out in glass reactors, generally under ultrasound by means of probes whose power has been adapted (25 watts / liter) so as to be transposable to industrial operations. The procedure includes two phases at 80 ° C: an oxidation phase in a KMnO₄ 1 g / l solution acidified to pH 3 or 2.5 with H₂SO₄ and / or HNO₃. A reduction phase in a mixture of organic acids pH3 (ascorbic acid 0.5 to 1 g / l + citric acid 0.5 g / l + oxalic acid 0 to 0.5 g / l).

The main parameters studied were the oxidation and reduction time, the effect of the pH and the nature of the acid in the oxidizing solution, the influence of the number of cycles, the presence of ultrasound and the absence of oxalic acid in the reducing phase.

The contaminated samples come, in 2 separate batches, from a tap of Tihange power plant. The two lots have substantially distinct activities. Heterogeneity has been attributed to a difference in the sampling area.

The results are collated in the following Table I: TABLE I EXPERIMENTAL CONDITIONS AND RESULTS OF DECONTAMINATION TESTS CARRIED OUT AT 80 ° C EXPERIMENTAL CONDITIONS RESULTS No. INITIAL MARK NUMBER OF CYCLES OXIDIZING PHASE REDUCT PHASE. US FD OXIDIZING PHASE MATERIAL ACTIVITY *** pH H2SO4 g / l HNO3 g / l T (h) COMPOSIT. T (h) O NOT Cr (ppb) Fe (ppb) 1 1 1 3 0.05 - 20 AT * 17 * > 190 644 - 1 2 2 20 5 * > 190 439 - 3 3 15 5 * 175 470 - 4 1 ′ 15 3 * > 55 482 3500 2 5 4 10 5 * 9.3 265 - 1 6 4 10 5 * 1.8 150 - 7 2 ′ 10 3 * > 55 465 4830 2 8 5 ′ 5 5 * 19.3 435 4760 9 4 ′ 5 5 * 8.9 85 10320 10 3 ′ 3 5 * 5 171 9450 11 6 ′ 2.5 0.15 5 AT * 5 * > 55 672 4630 12 7 ′ - 0.2 5 5 * 17.5 555 5490 13 11 ′ 0.05 0.13 5 3 * > 55 745 6950 14 13 ′ 0.05 0.13 5 - (**) ---- 3 * > 55 > 390 5830 15 12 ′ - 0.2 10 5 * 25 510 3770 16 8 ′ 2 3 0.05 - 3 AT * 3 * > 55 470 6427 17 9 ′ 2.5 - 0.2 3 2 * > 55 813 6310 18 10 ′ 2.5 - 0.2 3 2 * > 55 780 7410 19 5 1 3 0.05 - 20 B * 5 * > 190 468 - 1 COMPOSITION OF THE REDUCING PHASE. * MEDIA A: ASCORBIC ACIDS 0.5 g + CITRIC 0.5 g / l + OXALIC 0.5 g / l * MEDIUM B: ASCORBIC ACIDS 1g.l + CITRIC 0.5g / l ** DECONTAMINATION WITHOUT CHANGING A BATH, WITH IN SITU DESTRUCTION OF KMn04 BY OXALIC ACID *** MATERIAL ACTIVITY: 1 -> DDD 100 mrem.h 2 -> DDD 30 mrem.h

The comparison of the different tests 1 to 10 shows that, all other things being equal, the duration of the oxidizing phase is very important whereas the duration of the reducing phase plays only a small role, since it is greater than 3 hours. . We see that from 10 hours of attack in the oxidative phase, the decontamination factor begins to become very high and this with a single cycle.

The examination of examples 11 to 15 in comparison with those of l to 10 make it possible to prove that the pH plays a role that the pH of 2.5 gives significantly better results than those of pH 3 and this without there being a significant increase in metal corrosion (see chromium and iron content).

Effect of the nature of the anion associated with the acid

The comparison of tests 11 and 12 makes it possible to show that the sulfate ion is more effective than the nitrate ion. Test 13 shows that it is possible to combine the advantages of the two by using a mixture of nitric acid and sulfuric acid, while avoiding to penalize the process by the excessively high sulfur contents which would be contrary to specifications for chemicals used in pressurized water reactors.

Effect of the duration of the oxidative phase

The comparison of tests 10 and 16 on the one hand, and tests 12, 15, 17 and 18 on the other hand, shows without ambiguity the very favorable effect of 2 cumulative cycles, even if the duration of these is short . However, it should be noted that by increasing the oxidizing phase alone, results in a single cycle are almost as good if not better than with two cycles (see example 7 with example 16 where similar results are obtained for same total duration).

Effect of ultrasound

The comparison of tests 5 and 6 shows that ultrasound has a very favorable effect.

Role of oxalic acid in the reducing phase

The comparison of tests 19 and 2 (Table I) shows that it is possible to remove oxalic acid from the reducing phase without inconvenience. However, this acid is responsible for a large part of the corrosiveness of the reducing solution. Its deletion is therefore interesting.

- Figure 2 is made from the data of Examples 1 to 19. It shows the phenomenon of incubation during the oxidative stage of the process. In fact, for the two samples, the low activity sample and the high activity sample, the decontamination factor as a function of the time of phase a) of the oxidative attack is represented. The ordinate volumes are given on the left for the left curve and on the right for the right curve.

Example 20: Effluent treatment:

The solutions from the previous tests are filtered through two filters of 19 cartridges 475 mm long and 3 µm porosity at a flow rate of 0.5 m³ / hour. This operation is carried out both on oxidizing solutions and on reducing solutions.

After filtration, these solutions are passed over 80 l of a cation exchange resin with sulfonic group 2.2 eq./l sold under the brand Duolite C 20 at the maximum volume flow rate of 7 volumes per volume per hour (i.e. 7 bed volumes per hour or according to the Anglo-Saxon notation 7 V / V / h) or a maximum flow of 0.5 m³ / hour. This passage over resin eliminates all of the soluble activity of the oxidizing and reducing solutions (CO⁵⁸, CO⁶⁰, Mn⁵⁴, etc.) as well as the soluble potassium of the oxidizing solution and the solubilized metals (iron, chromium, nickel) of the reducing solution. A resin allows the elimination of 140 g of iron and about 1 Ci of activity.

The decationated oxidizing and reducing solutions are mixed and react with each other at a temperature between 60 and 80 ° C, which allows the residual oxidizing solution to be completely destroyed without precipitation of the manganese dioxide. The mixture of the two solutions remains at a pH less than or equal to approximately 3.5. The residual ascorbic acid and citric acid are destroyed after dosing by injection of a sufficient quantity of sodium persulfate at a temperature of 80 ° C.

A new passage on resin makes it possible to eliminate all of the manganese thus produced in the stage of destruction of the oxidizing and reducing solutions.

Claims (15)

1. Method for dissolving oxide deposited on a metal substrate, characterized in that it comprises the following steps: a) attack by means of an oxidizing solution containing a permanganate anion at a pH between 2 and 6, preferably between 2 and 4, the duration of the attack being chosen so that the exponential phase is finished, the oxidizing solution also containing a sulfate ion at a concentration between 10⁻² and 10⁻⁵, preferably between 10⁻³ and 10⁻⁴ N; b) a reducing attack by means of an easily degradable organic compound. 2. Method according to claim 1, characterized in that the pH of the attack solution a) is between 2.5 and 3.5. 3. Method according to one of claims 1 or 2, characterized in that at least one of the attacks a) and b) is carried out under ultrasound. 4. Method according to one of claims 1 to 3, characterized in that the manganese content of the attack solution a) is at most equal to 10 g / l, preferably between 2 and 1/4 g / l. 5. Method according to one of claims 1 to 4, characterized in that the permanganate ion is introduced in the form of potassium permanganate. 6. Method according to one of claims 1 to 5, characterized in that the solution of attack b) further comprises a complexing agent. 7. Method according to one of claims 1 to 6, characterized in that said complexing agent is chosen from the group consisting of polycarboxylic acids containing at most carbon atoms and having at least preferably a hydroxyl or amino function. 8. Method according to one of claims 1 to 7, characterized in that said easily degradable compound is chosen from the group consisting of ascorbic acid, dehydroascorbic acid, aldols, di-acids and reducing sugars . 9. Method according to one of claims 1 to 8, characterized in that it further comprises a step e) of mixing the solution from step a) and the solution from step b) . 10. Method according to one of claims 1 to 9, characterized in that it comprises a step f) of treatment of the solution resulting from step e) by elimination of the elements present in the solution in cationic form. 11. Method according to one of claims 1 to 10, characterized in that step f) is carried out by passing the solution from step e) over a bed of cation exchange resin. 12. Method according to one of claims 1 to 11, characterized in that said method comprises an additional step c) of elimination of the cations present in the solution resulting from step a), said step c) taking place before step e). 13. Method according to one of claims 1 to 12, taken separately, characterized in that it comprises an additional step d) of elimination of the cations from the solution resulting from step b), preferably by means of d 'a cation exchange resin. 14. The method of claim 13, characterized in that the solution from step d) is partially or intermittently recycled to step b). 15. Method according to one of claims 1 to 14, characterized in that it comprises a step of removing organic traces in the solution from step f).

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