FR2648946A1 - METHOD OF DISSOLVING OXIDE DEPOSITED ON 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.

Description

The present invention relates to a dissolution process

  oxide deposited on a metal substrate.

  It relates more particularly to a dissolution process of the above type, which may be used for the decontamination of contaminated metal parts during their exposure in a hot zone. This problem is particularly arduous and sharp in the case of parts constituting walls or coming into contact with the primary circuit

  nuclear reactor steam generators.

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

walls of the circuit.

  Since the contamination process is cumulative, it can lead in a few years in the primary circuit to a

very high radioactivity.

  This contamination considerably hampers maintenance operations during cold shutdowns, while the irradiation of personnel requires protective measures which result in

significant additional economic cost.

  The problem is particularly acute in 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 decontaminating rapidly during cold shutdown components or parts of the primary circuit on which the personnel

must intervene.

  Most of the decontamination is to remove deposits formed on the walls of the circuit during use. This deposit is fo.rmé mainly a layer of oxide that incorporates corrosion products

activated or radioactive.

  This oxide, which may optionally comprise a slightly adhesive outer layer, has a very compact and adherent inner layer. To decontaminate, it is necessary to remove this internal oxide,

preferably 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 problems pose problems of implementation and recuperation

  waste ration for steam generators.

  The principle of the chemical decontamination process consists of a dissolution of the adherent oxide layer, then treatment of the solutions formed of small volume discharges and easily storable in

  protected discharge systems.

  The oxides formed in pressurized water reactors are generally very rich in chromium whose oxide and especially spinels are known to be difficult to dissolve. To solubilize these chromium-rich oxides, it is ordinarily proposed to oxidize chromium of degree 3 to 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 essential

  due to the double constraint of no deterioration of metal parts that will continue to function within the plant and

  effluent treatment, after the attack.

  From the foregoing, and without being exhaustive, the following constraints ensue: The reagents used must not contain elements that

  are likely to alter the mechanical properties of the steels to be stripped.

  This is particularly the case for sulfur, the presence of which can cause stress corrosion if this element is too concentrated.

in the reagent.

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

  separable from radioactive elements.

  The reagents should be such that, after treatment, the effluents to be stored in a protected landfill are as small as possible in volume,

  or can enter standardized containers.

  All these constraints must be applied taking into account the size of the devices to be stripped, which implies a very high volume of solution involved, and consequently, very difficult solutions.

  diluted to avoid excessive storage of reagents.

  Therefore, one of the aims of the present invention is to provide a method that reduces the attack time or

  improve the effectiveness of the attack compared to existing techniques.

  Another object of the present invention is to provide a method of the above type which avoids the use of sulfur compounds in too large concentration. Another object of the present invention is to provide a process whose reagents are easily processable to give products either

  pure, or storable in landfills protected easily.

  These and other objects which will become apparent are achieved by means of an oxide dissolution process deposited on a metal substrate comprising the following steps: a) etching with 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 reached; b) a reducing attack using an organic compound

easily degradable.

  The results of the efficiency of the tests are measured by decontamination factor after the sequence of steps a) and b), this factor of

  Decontamination will sometimes be noted as FD.

26ó8946

  During the study which led to the present invention, it was shown that after a relatively long incubation period, generally between 5 and 15 hours, the kinetics of destruction of the oxide network became very fast. , took an exponential pace then ironed by a null value. Thus, the decontamination factor curves as a function of the duration of the oxidizing phase have the shape of the conventional S curve. Determining the times required to obtain the so-called exponential phase and the time required to obtain the plateau value can easily be determined in each particular case. In general, an oxidizing phase time value of at least 80, advantageously at least 90, preferably at least 99%, is preferred.

  Decontamination factor obtained for the value of the bearing.

  This value generally ranges from 5 to 20 hours. It is a value of this order which is generally chosen in cases of industrial use (one day) It should be noted that this phenomenon is difficult to highlight, since it is measurable only indirectly after the reductive attack phase. In other words, this effect is only potential as long as the stage

  reductive has not been implemented.

  Thus, according to the present invention, it has been demonstrated that it is preferable to increase the duration of the oxidizing phase rather than increase the number of cycles or the other phases of attack. However, repeating the cycle several times until the desired FD is still

solution for difficult cases.

  With regard to the optimization studies of the conditions of the oxidative attack, it could moreover be demonstrated that in a way

  surprisingly, the sulfate ion played a catalytic role during this step.

  Therefore, although the sulphate ion content should be limited because of the (country-dependent) standards for avoiding stress-induced corrosion of sulfur in the alloy, it is desirable to use a sulphate ion. as a catalyst for the reaction at low levels but sufficient to be catalytic. Thus, it is preferable to use a sulphate 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 that correspond to a better attack of oxides without significant attack

of the substrate.

  When the quantities of sulfuric acid are insufficient for

  to obtain the desired pH, the complement can be provided by nitric acid.

  The permanganate anions are preferably at a concentration of

  less than 10 g / l and preferably preferably between 2 and 1/4 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 character of the etching solution of step b). This neutralization also takes into account strong reducing agents such as ascorbic acid and reducing agents.

  such as the complexing acids which will be discussed below.

  Potassium permanganate is the preferred source of permanganate anions. The solution of the 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

not more than 10 carbon atoms.

  The complexing agents preferably have groups which allow their destruction by permanganate. Among such groups, mention may be made of alcohol groups, especially when the latter are vicinal to one another or vicinal to other functions such as the ketone or carboxylic function. As another function likely to facilitate degradation by permanganate, mention may be made of the ketone and aldehyde functions. According to the rules of complexation, it is preferable that these acids form a five, six or seven-membered ring with the cation they

are supposed to complex.

  It should be noted that acids with amino functions such as ethylene diamine tetraacetic acid (EDTA) which are to be avoided as far as possible, because of difficulties they cause during reprocessing, bring little benefit in the process according to the invention and their use can be avoided thereby. For its part, oxalic acid, although it is a good complexing agent, is preferably to be avoided during the attack because it facilitates the attack of the

substrate itself.

  Although other hydroxy di or polyacids give good

  As a result, one of the preferred acids is citric acid.

  The reducing agent is preferably selected 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 dehydroasc acid

  are good reducing agents. Reducers which also give good results include sugars with

aldehyde bonds.

  The concentration of these elements is of the order of 0.1 to 10 g / l of ascorbic acid. Preferably at around 1 to 2 g / day of ascorbic acid. When other reducing agents than ascorbic acid are used, the same values as above are used by transposition 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 being

  preferably, 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 ozone-type reagents for

destroy the organic compounds.

  When the permanganate is in excess it is possible to destroy it with reducing organic compounds whose excess is easily removed by distillation at low temperature or thermal degradation at low temperature as well. 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.degree.

   C, preferably between 60 and 80 C.

  Step e) is advantageously followed by a step f) of eliminating

  nation of the cations present in the solution. This step is

  carried out using an anion exchange resin. It can, however, also be conducted by a simple cation exchanger, either in

  liquid phase, either on solid support.

  The active groups of the most effective resins are the

  complexing resins, such as hydroxydiphos resins, for example

  phonic, amino-phosphonic, iminodiacetic, malonic etc. However, if it is desired that these resins can be burned, it is desirable that these resins contain only groups containing

  carbon, oxygen and hydrogen.

  The sulfonic resins give satisfactory results.

  These resins have a polystyrene backbone with crosslinking at

medium of divinylbenzene.

  Advantageously, the solutions from steps a) and b) can be cation exchanged before mixing in step c) for the etching solution from a) and step d) for the

attack solution from b).

  During the study that led to the present invention, it was shown that the reactivity of the two attack solutions from a) and b) respectively was greatly improved by this cation exchange, which seems to potentiate, in particular, the power of oxidizing solutions from a). It has been surprisingly shown that this exchange of the cations and in particular the absorption of potassium, or possibly

  other ions correlated with the permanganate ion, prevented the precipitation

  bi-manganese oxide, which eliminates the need to eliminate a new

solid effluent.

  The solution from step d) may be partially or intermittently recycled to the envisaged reductive attack step b). This recycling can be replaced by a reducing attack in which the resin is kept in suspension.

  After destruction of the last residual organic compounds, it.

  It is possible to reject as purified water or to recycle the water that has served

  to this depollution operation.

  It should be noted that this whole process can be implemented with other techniques to improve the decontamination factor, ie the dissolution rate of the activated materials. Thus, the use of ultrasound according to a known technique in itself makes it possible to improve

significantly the returns.

  Figure I shows a block diagram of the whole of a decontamination system in a particular implementation for a

  primary pump of pressurized water reactor.

  In this figure, there is shown a decontamination tank 1, an intermediate storage tank of the oxidizing solution 2, an intermediate storage tank of reducing solution 3, a storage tank of the rinsing and reagent destruction water 4, a filter 5 whose passer is preferably about 3 pm. A cationic resin, ie a cation exchange resin 6, and a system of circuit and pump valves which makes it possible to circulate the different solutions of a tank to a tank.

  device, one-bin and bin-tray devices.

  In operation, this system comprises the following different operations: - heating of the deionized water at 80 C in the tank; injection of the oxidizing reagents into the tank I, mixing with the deionized water and reaction with a 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 the tank 3. After reaction, the oxidizing intermediate solution is transferred to the intermediate storage tank while being maintained at C; the hot water for the reducing phase is transferred from the tank 3 to the tank 1, while the reducing reagents are also injected into the tank 1 and the reagent is allowed to react with the parts to be decontaminated; once the reaction is made, 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 if a second cycle must take place; in the affirmative, the oxidizing solution already used is transferred after checking its pH and KMnO4 concentration of the intermediate storage tank 2 to the decontamination tank I in order to carry out the second oxidation stage - once this is complete the oxidizing solution is transferred from the decontamination tank I to the temporary storage tank 2 with holding at 80 C; the transfer of the reducing solution already used is then carried out after possible verification of the state of the reagents of the intermediate storage tank 3 towards the decontamination tank I in order to carry out the second reduction phase; once this is complete, the reducing solution is transferred to the temporary storage tank 3 while maintaining C; - Rinsing with hot water 80 C parts in the decontamination tank from rinsing water sotkcée in the tray 4 and heated in it - then a second rinse with cold water. These two rinses are optionally contacted with a cationic resin sent to the control circuit and effluent discharge of the nuclear island. During this time, the oxidizing solution stored in the tank 2 is transferred to the 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 the tank 3. react the two solutions one on the other, then both solutions are treated by passage on filter and resin. Decontaminated effluents are released 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 removal of the cations makes it possible to totally destroy the residual oxidizing solution and without the precipitation of MnO 2. The best conditions for performing this operation of mixing and mutual destruction are at a temperature of 800C at an initial pH of at most 3.5. The mixing operation makes

raise the pH.

  If the ascorbic acid and the citric acid or their degradation product remain in the solution, they can be destroyed by injection of a strong oxidant such as for example sodium persulfate at the temperature of 800C. The effluent is then brought to a pH of 7, for example with ammonia and then directed to the control circuit tanks.

  and effluent discharge from the nuclear island.

  The process is particularly well suited to stainless steels and especially to those in the following table C Cr Ni 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 - ..

  basic cobalt alloys such as stellite 0.9-1.4 26-32 3 1 60 4 Examples I to 19 Operating procedure of the various examples The tests were carried out for steel primary pump parts 18, 10. were made in glass reactors, usually under ultrasound using probes whose power was adapted

  (25 watts / liter) so as to be transposable to industrial operations.

  The procedure has two phases at 80 C: an oxidation phase in

  a solution KMnO4 I g / l acidified to pH 3 or 2.5 with H2SO4 and / or HNO3.

  A reduction phase in a mixture of organic acids pH3 (acid

  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 acid nature of the oxidizing solution, the influence of the number of cycles, the presence of ultrasound and

  the absence of oxalic acid in the reducing phase.

  Contaminated samples come, in 2 separate batches, from a Tihange Centrale tap. Both lots have substantially different activities. Heterogeneity has been attributed to a difference

sampling area.

  The results are collated in Table I according to 0 ',,. N 0 kCn o c'

  TABLE I.EXPERIMENTAL CONDITIONS AND RESULTS OF DECONTAMINATION TESTS

CARRIED AT 80'C

EXPERIMENTAL CONDITIONS5 RESULTS

  NUMBER NUMBER OXIDIZING PHASE REDUCING PHASE U F.D OXIDIZING PHASE ACTIVITY OF

  INITIAL CYCLES pH H2504 g9 / l HNO39g / I T (h) COMPOSIT. T (h) 0 N Cr (ppb) Fe (pDo) M1ATERIAU "*

I I 20 17> 190 644

2 2 20 5 *> 190 439

3 3 IS 5 "175 470 -

4 1 '15 3 *> 55 482 3500 2

4 3 0.05 - 10 5 * 9.3 265 -

6 4 10 5 1.8 150 -

7 2 'I 10 A 3 "> 55 465 4830

8 5 '5 5 19.3 435 4760

9 4 '5 5 "8.9 85 10320

3 '3. 5 "5 171 9450

It is 6 '0.15 5 5> 55 672 4630

12 7 '- 0.2 5 5 K 17.5 555 5490

  13 I the 2.5 0.05 0.13 S A 3 "> 55 745 6950 2

  14 13 '0.05 0.13 5 - (*) -... 3 "> 55> 390 5830

12 '- 0.2 10 5 _ 25 510 3770

16 8 '3 0.05 - 3 3 "> 55 470 6427

  17 9 '2 2.5 - 0.2 3 AU 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 -

COMPOSITION OF THE REDUCTIVE PHASE

  * MEDIUM A: ASCORBULATED ACIDS 0.5 g / 1 + CITRILE 0.5 g / 1 * OXALIOUE 0.5 9/1 * MILIEUB: ACIDESASCORBIOUE lg / I * CITRIOUS 0.5 g / l

  ** DECONTAMINATION WITHOUT CHANGING BATH.AYEC DESTRUCTION

  IN SITU OF KMNO4 BY OXALIUCED ACID ** ACTIYITY OF THE MATERIAL: I -> DDD 100 mrem / h 2 -> DDD 30 mrem / h The comparison of the different tests I to 10 shows that everything else is equal, the duration of the oxidizing phase is very important while the duration of the reducing phase plays little role, since it is greater than 3 hours. It can be seen that starting from 10 hours of attack in the oxidizing phase, the decontamination factor starts at

  become very high with only one cycle.

  Examination of Examples 11 to 15 in comparison with those of I to allow to prove that the pH plays a role that the pH of 2.5 gives the results sensiblenient better than those of the pH 3 and that without there being a significant increase in metal corrosion (see chromium content

and iron).

  Effect of the nature of the anion associated with the acid The comparison of the tests 11 and 12 shows that the sulfate ion is more effective than the nitrate ion. Test 13 shows that it is possible to combine the advantages of both by using a mixture of nitric acid nitrate and sulfuric acid, while avoiding to penalize the process by too high sulfur contents which would be contrary to specifications for the chemicals used in reactor at

pressurized water.

  Effect of the duration of the oxidizing phase The comparison of the tests 10 and 16 on the one hand, and the tests 12, 17 and 18, on the other hand, shows unambiguously the very favorable effect of 2 cumulated cycles, even if the duration of these is short. However, it should be noted that by increasing the single oxidizing phase, results obtained in one cycle are almost as good or better than two cycles (see Example 7 with Example 16 where results are obtained).

  similar for the 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 the tests 19 and 2 (Table 1) shows that the oxalic acid of the reducing phase can be removed without inconvenience. But this acid is responsible for much of the corrosivity of the reducing solution. Its deletion is therefore interesting. - Figure 2 is made from the data of Examples I to 19. It shows the incubation phenomenon during the oxidizing step 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 the a) phase of the oxidative etching, are 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 resulting from the preceding tests are filtered on two filters of 19 cartridges with a length of 475 mm and a porosity of 3 μm at a flow rate of 0.5 m 3 / hour. This operation is carried out both on the solutions

  oxidizing only on reducing solutions.

  After filtration, these solutions are passed over 80 l of a cation exchange resin with 2.2 eq./l sulfonic group sold under the brand name Duolite C 20 at the maximum volume flow rate of 7 volumes per volume and hour (ie 7 volumes of the bed per hour or in the English notation 7 V / V / h) or a maximum flow rate of 0.5 m '/ hour. This passage over resin makes it possible to eliminate the totality of the soluble activity of the solutions

58 60 54

  oxidizing and reducing agents (CO58, CO60, Mn54, etc.) as well as the soluble potassium of the oxidizing slution and the solubilized metals (iron, chromium, nickel) of the reducing solution. A resin allows elimination of 140

  g of iron and about I Ci of activity.

  Decanted oxidizing and reducing solutions are mixed and react with one another at a temperature of between 80 ° C. and 100 ° C., which makes it possible to completely destroy the residual oxidizing solution without precipitation of the manganese dioxide. The mixture of the two solutions remains at a pH of less than or equal to about 3.5. The remaining ascorbic acid and citric acid are, after dosing, destroyed by injection of a sufficient amount of sodium persulfate into a

temperature of 80 ° C.

  A new passage over resin eliminates all the manganese thus produced in the destruction step of the oxidizing solutions

and reductive.

Claims (15)

CLAIMS I. Process for dissolving oxide deposited on a metal substrate, characterized in that it comprises the following steps: a) etching with an oxidizing solution containing a permanganate anion at a pH of between 2 and 6, preferably between 2 and 4, the duration of the attack being chosen so that the exponential phase is complete; 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 etching solution a) is between 2.5 and 3.5.   3. Method according to one of claims I and 2, characterized by   the concentration of sulphate group is between 10-2 and 10-5, preferably between 10-3 and 10-4N.   4. Method according to one of claims I to 3, characterized by the   at least one of the attacks a) and b) is performed under ultrasound. -   5. Method according to one of claims I to 4, characterized by the   the manganese content of the attack solution a) is at most   equal to 10 g / l, preferably between 2 and 1/4 g / l.   6. Method according to one of claims I to 5, characterized by the   that the permanganate ion is introduced as potassium permanganate.   7. Method according to one of claims I to 6, characterized by the   the solution of attack b) further comprises a complexing agent.   25. Process according to one of claims I to 7, characterized by   said complexing agent is chosen from the group consisting of polycarboxylic acids containing at most carbon atoms and   at least preferably having a hydroxyl or amine function.   9. Method according to one of claims 1 to 8, characterized by the   said easily degradable compound is selected from the group consisting of ascorbic acid, dehydroascorbic acid, aldols,   di-acids and reducing sugars.   10. Method according to one of claims 1 to 9, characterized by   the fact that it further comprises a step e) of mixing the resulting solution   of step a) and the solution resulting from step b).   Method according to one of Claims 1 to 10, characterized by   the fact that it comprises a step f) of treating the solution resulting from step e) by eliminating the elements present in the solution in cationic form.   12. Method according to one of claims I to 11, characterized by   the fact that step f) is carried out by passing the solution resulting from the step   e) on a bed of cation exchange resin.   13. Method according to one of claims I to 12, characterized by   the fact that said method comprises an additional step c) of eliminating   tion of the cations present in the solution resulting from step a), said step c) taking place before step e).   14. Method according to one of claims I to 13 taken   separately, characterized by the fact that it comprises an additional step d) of removing the cations from the solution resulting from step b), preferably   by means of a cation exchange resin.   15. Process according to claim 14, characterized in that   the solution resulting from step d) is partially or intermittently tent recycled to step b).   Method according to one of Claims 1 to 15, characterized by   the fact that it involves a step of elimination of organic traces in the solution resulting from step f).