Process for dissolving oxides containing silver and cobalt contamination on a metallic substrate

Abstract

The present invention relates to a process for dissolving oxides containing silver and cobalt contamination on a metallic substrate.

Description

Title of the invention: Method for dissolving oxides containing silver and cobalt contamination on a metal substrate TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method for dissolving oxides deposited on a metal substrate, this method being intended more particularly for the radioactive decontamination of metal parts, contaminated during their exposure in a hot zone.

[0002] It specifically aims at the radioactive decontamination of stainless metal surfaces, made of iron, nickel or cobalt-based alloys, covered with complex oxides composed essentially of iron, nickel and chromium and essentially containing radioactive contamination in radioisotopes 60Co, 58Co and 110mAg. STATE OF THE ART

[0003] It may be necessary to decontaminate parts, particularly parts constituting the walls or pipes of the primary circuit and auxiliary circuits of nuclear reactors, both to reduce the dosimetry of personnel involved in maintenance and to remove these parts from the controlled area.

[0004] Most of the radioactivity in the primary circuit and the auxiliary circuits results from the activation, in the reactor core, of corrosion products dissolved during operation. They are deposited by precipitation on the walls of the circuits and are gradually incorporated into the oxide which grows on the surface of the materials on the walls of the circuits.

[0005] The oxides formed on the walls of the circuits are generally composed of two more or less compact layers. The inner layer comes from the direct corrosion of the metal surface. The outer layer is formed by diffusion of metallic species through the inner layer and by precipitation or deposition of corrosion products transported by the fluid. It can therefore have a particulate appearance.

[0006] The circuits of pressurized water reactors are covered with thin oxides (between 1 and 7 μm). The inner layer has a significant enrichment in chromium (30 to 55% by weight) as well as a notable enrichment in cobalt (0.2 to 0.9% by weight). These values ​​can represent ten times the core content of the material. The oxide (NixCOyCrzFe3_x_y_zO4) is described as a substituted spinel or a mixture Cr2 O3 + NiO + Fe3O4. The radioactive materials are therefore mainly the isotopes 60 Co, 58 Co, 51 Cr, 54 Mn, 59 Fe, 122 Sb, 124 Sb, 110 Ag.

[0007] The radioisotope 60Co comes from the activation of 59Co present in cobalt-based alloys called Stellites or as an impurity in iron-based alloys or of nickel. The radioisotope 58Co comes mainly from the activation of 58Ni in nickel-based alloys constituting the tube bundles of the steam generators of pressurized water reactors. The radioisotope 110mAg comes particularly from the degradation of the nuclear reactor control rods made of silver-indium-cadmium alloy which participate in the control of the nuclear reaction. It can also come from the degradation of silver Helicoflex® seals.

[0008] The deposition process is cumulative and within a few years, the parts of the primary circuit and the auxiliary circuits exhibit very high radioactivity.

[0009] Contamination considerably hinders maintenance operations during cold shutdowns. In addition, personnel irradiation requires protective measures which result in significant additional economic costs. The problem is particularly acute for interventions in auxiliary circuits such as the chemical and volumetric control circuit, the cooling circuit of the reactor during shutdown or possibly the sampling circuit. It is therefore very important to have a process for quickly decontaminating these circuits or parts of these circuits on which personnel must intervene, during cold shutdowns. Decontamination consists of removing the complex oxide layers formed on the walls of the circuits during their use.To decontaminate, it is necessary to remove not only the external oxide layer, which is generally porous and poorly adherent, but also the internal layer, which is compact and strongly adherent to the metal walls.

[0010] Among the mechanical, electrochemical or chemical processes considered, chemical processes are the most developed. Indeed, mechanical processes such as sandblasting, high-pressure jets, abrasion and electrochemical processes pose problems of implementation and recovery of waste in the circuits.

[0011] Chemical decontamination consists of using chemical solutions to dissolve the layer of oxides adhering to the metal walls, then treating the collected solutions in order to concentrate them and obtain only low-volume waste. This waste can then be easily stored in landfill systems suitable for this purpose, i.e. protected and where radioactivity is controlled.

[0012] The oxides enclosing the contamination are therefore made up of magnetite, hematite (iron oxides) and complex spinels of the nickel ferrite, iron and nickel chromite types. They are therefore generally very rich in iron, nickel and chromium. However, spinels and especially chromium oxides are difficult to dissolve.

[0013] A method for dissolving oxides is already known from document EP 0 406 098, comprising the steps of carrying out:

[0014] a) an oxidative attack with a solution of potassium permanganate and acids sulfuric and nitric at a pH between 2 and 6, and

[0015] b) a reductive attack using a reducing agent consisting in particular of ascorbic acid and a complexing agent chosen from polycarboxylic acids, of the oxalic acid or citric acid type.

[0016] The process optionally includes a subsequent step of treating the effluents obtained at the end of the oxidizing and reducing attacks, by passing these effluents over a cation exchange resin.

[0017] This process is not sufficiently effective to completely eliminate the oxide layers deposited on the walls of the treated metal pipes.

[0018] Indeed, from the beginning of the oxidizing phase, the pH rises to values ​​of 2.8 to 3.2, which makes this oxidizing phase less effective. As a result, chromium oxides and spinels loaded with chromium are less solubilized in depth.

[0019] However, the reducing phase will dissolve these oxides less the more chromium they contain.

[0020] As a result, a greater quantity of chromium oxide remains in this case on the walls, after the reducing phase, thus making the decontamination imperfect.

[0021] Furthermore, the efficiency of the process is greatly reduced by the excessively high pH value of the reducing phase which reduces the solubility of the spinels loaded with chromium.

[0022] Finally, the use of a complexing agent such as oxalic acid constitutes a significant source of pollution since this acid remains partly trapped in the form of cationic complexes on the cation exchange resins, after treatment of the effluents or in the sludge in the case of treatment on an evaporator.

[0023] Also known from document FR 2 590 716 is a method for decontaminating the walls of nuclear reactors, in particular the walls of the primary circuit of nuclear reactors with a pressurized water circuit. This method consists of increasing the efficiency of renewal of the solution by the emission of ultrasound throughout the duration of the oxidizing and reducing phases. Moreover, the effectiveness of ultrasound appears especially in the oxidizing phase where the penetration of the solution in depth plays a more important role than in the reducing phase.

[0024] This method also has a number of disadvantages. Only surfaces directly in the vicinity of the ultrasonic probes are properly decontaminated. In addition, the positioning of the ultrasonic probes is difficult to implement. It may even be impossible with certain surface profiles to be decontaminated (small diameter pipes, presence of elbows, etc.)

[0025] Furthermore, a process for dissolving oxides is known (FR 2 699 936) comprising at least one cycle consisting of:

[0026] a) Carry out an oxidative attack using a solution comprising a permanganate anion and nitric acid, at a pH between 1.8 and 2.3 and at a temperature between 80 and 85°C, this pH being maintained throughout the duration of the oxidative attack;

[0027] b) Carry out a reductive attack using a solution comprising a reducing agent and nitric acid, at a pH lower than 2 and at a temperature between 80 and 85°C.

[0028] One to three treatment cycles can be carried out successively until the targeted effectiveness is achieved.

[0029] However, during decontamination with this type of process, stellites, cobalt-based alloys frequently present in nuclear reactors, are particularly sensitive to corrosion due to their chemical composition and their particular metallurgical structure. Indeed, significant generalized corrosion is noted after a single treatment cycle with this process, even if a pH of 2.5 is chosen to reduce the acid aggressiveness of the environment. Stellites with only one deposit layer appear to be more attacked than those with two deposit layers due to a greater dilution of iron.

[0030] The stellites therefore have, at the end of this process, a surface state incompatible with a return to the operating state.

[0031] The term stellite is the trade name for the cobalt-based alloy from CABOR Inc.

[0032] Nuclear valves have numerous sealing and guiding surfaces made of cobalt-based hard alloys of the stellite type. This material is used for its excellent wear resistance, high hardness and good corrosion resistance under operating conditions in the units.

[0033] Stellite is a cobalt-based alloy deposited on the sealing surfaces of valves by various welding processes. The filler metal is initially in the form of powder or rod. During the deposition of the first pass by thermal processes, a mixture is made between a thin layer of support metal, stainless steel, and the filler metal, stellite. A modification of the chemical composition of the deposited material is then observed. Depending on the energy input of the deposition method (torch, Tungsten Inert Gas (TIG), electrode) and the depth of the support metal melted, the phenomenon of mixing of the two metals is more or less marked. In addition, the different nature of the materials brought into contact leads to a dilution of metallic elements (mainly iron) at the boundary zone.

[0034] Several passes may be necessary to obtain a deposited stellite on the surface that is identical in composition to the powder or rod-formed stellite. A gradient of concentrations of the different elements is therefore observed within the deposit. In the event of a manufacturing defect or mechanical wear of the material, the stellite has a very different chemical composition on the surface from that of the filler mixture.

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[0043] In order to guarantee certain mechanical properties and their resistance to corrosion during operation, a particular chemical composition is required for grade 6 and 12 stellite deposits. These values ​​are presented in the following table 1. [Tables 1] Alloy Ni Co Cr C Si Fe Mo Stellite Gr 6 <3 Balance (Bal.) 26-32 0.9-1.2 <2.0 <3.0 <1.0 Stellite Gr 12 >3 Bal. 26-32 1.2-1.7 <2.0 <3.0 <1.0 Table 1: Chemical composition of grades 6 and 12 stellites Four zones of different chemical composition must be considered within the stellite ranges: - the base metal made of stainless steel, - the Heat Affected Zone (HAZ) of the base metal. This thin thickness of stainless steel undergoes a high temperature rise during the deposition of stellite (up to 680°C). It can then be susceptible to intercrystalline corrosion, - the stellite dilution zone. In this zone, the stellite has an excessive iron content which promotes corrosion and low carbon and chromium contents, - the stellite in its nominal composition. The last two zones mentioned can be observed within the same deposition pass or cover several passes. The deposition methods used during the manufacture of stellite bearing surfaces lead to the melting of the filler metal. The stellite thus has a dendritic cooling microstructure from this liquid phase. The solidification dendrites are a cobalt-rich solution of Face-Centered Cubic (FCC) structure with a fine precipitation of M7C3-type carbides (where M is mainly chromium but also iron and tungsten). The dendritic structure is coarser in the torch deposit than in the plasma deposit. A eutectic solid solution surrounds these dendrites. It is also cobalt-based, of CFC structure and enriched in tungsten. A coarse precipitation of M7C3 and M23C6 type carbides is observed. This precipitation is favored in the initially carbon-rich stellites. The precipitation of carbides leads to a significant consumption of chromium in the interdendritic zone and more precisely in the peripheral zone of the carbides. While the nominal chromium concentration of the stellite is greater than 26%, the peripheral zones of the carbides may have chromium contents of less than 12%. The material is vulnerable to corrosion in these areas, losing its stainless character.

[0044] An excessive iron content from the support metal can also be encountered in the dilution zone. This iron is mainly concentrated in the eutectic. It also promotes the precipitation of carbides during solidification and generates dechromization of the eutectic by dilution of the chromium. This zone is then subject to corrosion reactions.

[0045] The deposition processes influence the microstructure and properties of the stellites. The two main defects encountered are overcarburization and excessive dilution, which increase the risk of corrosion of the material.

[0046] Other defects appear on the stellite deposits of valves during operation. These defects are mainly generalized corrosion and cracking.

[0047] Generalized corrosion begins with a disappearance of the eutectic matrix without affecting the large carbides. The chromium-depleted zones, on the periphery of the eutectic carbides, are mainly attacked. The phenomenon continues with the loosening of the large carbides and the final presence of only dendrites. These end up disintegrating and lead to the total disappearance of the stellite.

[0048] Mechanical stresses can also cause cracks crossing both the eutectic and the dendrites. Fracture of the large carbides in the eutectic phase is observed.

[0049] Document FR 2 699 936 also describes a process for dissolving oxides comprising at least one cycle consisting of:

[0050] a) carrying out an oxidative attack using a solution comprising a permanganate anion and sodium hydroxide, at a pH greater than or equal to 12 and at a temperature between 80 and 85°C, this pH being maintained throughout the duration of the oxidative attack;

[0051] b) carrying out a reductive attack using a solution comprising a reducing agent and nitric acid, at a pH of less than 2 and at a temperature of between 80 and 85°C.

[0052] However, this process is only indicated as usable in the case where the metal substrate on which it is applied comprises at least one part made of Inconel®. In addition, the decontamination factors obtained are lower than those of the process using an acid oxidizing phase. Furthermore, the literature and feedback from international decontaminations show that processes operating in an alkaline medium have lower results than those using an acid oxidizing phase.

[0053] Finally, stellites are more sensitive to corrosion than Inconel®. The man of the The trade would therefore be led to think that the oxidative attack and / or the acidity of the reducing attack of the process could cause corrosion of this type of material, making it incompatible with an application of the process in an industrial environment.

[0054] Indeed, Inconel®, thanks to its chemical composition (chromium content > 12% and its nickel base), has good corrosion resistance. This is not the case for stellites due to the possible presence in its interdendritic zones of low chromium contents (< 12%) which may be subject to degradation. This is all the more true since, due to the reduction in the efficiency of such a process compared to the same process with an acid oxidizing phase, two or three cycles may be necessary to obtain suitable efficiency. This leads to the production of a significant quantity of contaminated effluents, the subsequent treatment of which is extremely longer.

[0055] Furthermore, for the oxide dissolution process to have suitable efficiency, the duration of the oxidizing attack must be at least 5 hours and the duration of the reducing attack must be at least 5 hours, for each cycle.

[0056] The process described in document FR 2 699 936 makes it possible to dissolve oxides containing a double contamination of cobalt 60Co / 58Co and silver 110mAg but it cannot be applied to metal substrates comprising at least one part of stellite because the stellite degrades significantly in the decontamination solutions used in this process.

[0057] Document FR 2 850 673 also describes a process for dissolving oxides comprising at least one cycle consisting of:

[0058] a) carrying out an oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH between 9 and 13, advantageously 12, and at a temperature above 60°C, this pH being maintained throughout the duration of the oxidative attack.

[0059] b) carrying out a reductive attack using a solution comprising a reducing agent and nitric acid, at a pH lower than 2 and at a temperature higher than 60°C.

[0060] However, this method is only indicated as usable in the case where the metal substrate on which it is applied comprises at least one part made of stellite and does not comprise Inconel®. This method makes it possible to decontaminate such a metal substrate industrially without the corrosion of the stellite being too significant. However, for the oxide dissolution process to be suitably effective, the duration of the oxidizing attack must be 8 hours and the duration of the reducing attack must be 5 hours for each cycle. In addition, for the decontamination to be suitably effective, two or three cycles of this process must be carried out, resulting in the production of a significant quantity of contaminated effluent.

[0061] Furthermore, the process for dissolving oxides described in document FR 2 850 673 allows the extraction of radioactive materials 60Co, 58Co, 51Cr, 54Mn, 59Fe, 122Sb, 124Sb excluding 110mAg which is a metastable nuclide, which cannot be dissolved in a basic oxidizing medium, or in an acid reducing medium.

[0062] Depending on the level of contamination in 60Co / 58Co or in 110m Ag on all the circuits or circuit loops made of stainless steel and stellites to be decontaminated, the method as described in document FR 2 850 673 is usually applied in the case of a majority contamination in 60Co / 58Co and the method as described in document FR 2 699 936 is usually applied in the case of a majority contamination in 110mAg.

[0063] In this context, it is necessary to improve the effectiveness of chemical surface decontamination processes for components or parts of the primary circuit or auxiliary circuits of the nuclear reactor while preserving the safety of the decontamination solutions with respect to the surfaces / components to be decontaminated.

[0064] There is therefore an interest in being able to propose a unique process capable of solubilizing oxides containing a double contamination 60Co / 58Co and 110mAg on metallic substrates, preferably comprising both stainless steel and stellites, while:

[0065] - preserving the integrity of the materials of the metal substrate so that these materials can be maintained in operation (minimum corrosion criteria to be respected),

[0066] - producing as little effluent as possible to treat and waste to manage, and while

[0067] - acting over relatively short periods of time, because these decontamination operations are carried out when the nuclear reactor is shut down. These decontamination operations must last as little time as possible in order to allow the reactor to be restarted as quickly as possible.

[0068] An objective of the present invention is to remedy at least in part the drawbacks of the prior art mentioned above. SUBJECT OF THE INVENTION

[0069] One of the aims of the present invention is to provide a method capable of removing double contamination of silver and cobalt present on a metallic substrate, preferably comprising both stainless steel and stellites, without degrading said metallic substrate.

[0070] One of the aims of the present invention is to improve the effectiveness of at least one of the attacks compared to existing techniques, to provide a method which makes it possible to reduce the attack times, or to reduce the quantity of effluents during circuit decontamination.

[0071] Another object of the present invention is to provide a process whose reactants are easily reprocessable to give products that are either pure or easily storable in protected landfills.

[0072] These aims and others which will appear subsequently are achieved by means of a process of dissolving oxide deposited on a metallic stellite substrate.

[0073] The present invention therefore relates to a method for dissolving oxides deposited on a metal substrate, preferably a metal substrate comprising both stainless steel and stellites, characterized in that said method comprises the following steps consisting of: a. Carry out, for a period of less than 2 hours, an acid oxidative attack using a solution comprising a permanganate anion and nitric acid, at a temperature between 78 and 82°C, at a pH between 2 and 3, preferably at a pH of 2.5, this pH being maintained throughout the duration of the oxidative attack. b. Optionally, rinse with demineralized water acidified with nitric acid, at a pH between 2 and 3, preferably 2.5, at a temperature greater than or equal to 50°C, c. Carry out, for a period of between 3 and 5 hours, an acid reduction attack using a solution comprising a reducing agent and nitric acid, at a pH less than or equal to 2, and at a temperature of between 78 and 82°C, d. Optionally, carry out at least one rinse with demineralized water, at a temperature greater than or equal to 50°C, e. Carry out, for a period of between 5 hours and 8 hours, preferably 5 hours, a basic oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously 12, and at a temperature of between 78 and 82°C, this pH being maintained throughout the duration of the oxidative attack. f. Carry out, for a period of between 3 hours and 5 hours, preferably 3 hours, an acid reductive attack using a solution comprising a reducing agent and nitric acid, at a pH of less than 2 and at a temperature of between 78 and 82°C. g. Optionally, rinse with demineralized water, at a temperature greater than or equal to 50°C, h. Carry out, for a period of between 5 hours and 8 hours, preferably 5 hours, a basic oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously 12, and at a temperature of between 78 and 82°C, this pH being maintained throughout the duration of the oxidative attack. i. Carry out, for a period of between 3 hours and 5 hours, preferably 3 hours, an acid reduction attack using a solution comprising a reducing agent and nitric acid, at a pH below 2 and at a temperature between 78 and 82°C. j. Optionally, carry out at least one rinse with demineralized water, at a temperature greater than or equal to 50°C.

[0074] Advantageously, the steps are carried out from e) to j) then from a) to d) or are carried out in order, from a) to j).

[0075] Advantageously, when steps a) to j) are carried out in order, i.e. when the steps are carried out in the following order a), b), c), d), e), f), g), h), i) and j), step j) is carried out.

[0076] Advantageously, when steps a) to j are carried out, steps b) and j) are carried out.

[0077] Advantageously, when steps e) to j) and then a) to d) are carried out, step d) is carried out. Advantageously, when steps e) to j) and then a) to d) are carried out, steps b) and d) are carried out.

[0078] Advantageously, steps b) and j) are carried out when steps a) to j) are carried out in order.

[0079] Advantageously, the duration of the acid oxidative attack in step a) is 1 hour.

[0080] Advantageously, the duration of the basic oxidative attack in step e) and in step h) is 5:00 a.m.

[0081] Advantageously, the duration of the acid reduction attack in step c) is 5 hours.

[0082] Advantageously, the duration of the acid reductive attack in step f) and / or in step i) is 5:00 a.m.

[0083] Advantageously,

[0084] - the acid oxidative attack of step a) is carried out at a temperature of 80°C, and / or

[0085] - the acid reductive attack of step c) is carried out at a temperature of 80°C, and / Or

[0086] - the basic oxidative attack of step e) is carried out at a temperature of 80°C, and / Or

[0087] - the acid reductive attack of step f) is carried out at a temperature of 80°C, and / Or

[0088] - the basic oxidative attack of step h) is carried out at a temperature of 80°C, and / or

[0089] - the acid reductive attack of step i) is carried out at a temperature of 80°C. Advantageously, the solution used in step a) comprising a permanganate anion and nitric acid, has a pH of between 2 and 3, and preferably has a pH of 2.5.

[0090] Advantageously, the solution comprising a permanganate anion and sodium hydroxide used in step e) and / or h), has a pH of 12.

[0091] Advantageously, in step a), the solution comprising a permanganate anion and nitric acid has a permanganate anion concentration of between 0.8 g / l and 1.2 g / l, preferably has a permanganate anion concentration of 1 g / l. Advantageously, the permanganate anion of the solution used in step a) comprising a permanganate anion and nitric acid is potassium permanganate.

[0092] Advantageously, in step e) and / or in step h), the solution comprising a permanganate anion and sodium hydroxide has a permanganate anion concentration of between 0.8 g / l and 1.4 g / l, preferably has a permanganate anion concentration of 1 g / l. Advantageously, the permanganate anion used in step e) and / or in step h) is potassium permanganate.

[0093] Advantageously, in step c) and / or in step f) and / or in step i), the solution comprising a reducing agent and nitric acid has a reducing agent concentration of between 0.8 g / l and 1.4 g / l, preferably has a reducing agent concentration of 1 g / l. Advantageously, the reducing agent used in step c) and / or in step f) and / or in step i) is ascorbic acid.

[0094] Advantageously, the solution used in step c) and / or in step f) and / or in step i), comprising a reducing agent and nitric acid, has a pH of 1.8.

[0095] Advantageously, the demineralized water used during rinsing in step b) and / or d) and / or g) has a temperature of 80°C.

[0096] Advantageously, the demineralized water used during at least one rinse in step j) has a temperature of 80°C.

[0097] Advantageously, Permanganate Fanion is introduced in step a) and / or in step e) and / or in step h), in the form of potassium permanganate.

[0098] Advantageously, the reducer in step c) and / or in step f) and / or in step i) is ascorbic acid.

[0099] Advantageously, when steps a) to j) are carried out in order, step j) is carried out until the rinsing water has a conductivity threshold less than or equal to 25 pS / cm, preferably less than or equal to 10 pS / cm.

[0100] Advantageously, when steps e) to j) and then a) to d) are carried out, step d) is carried out until the rinsing water has a conductivity threshold of less than or equal to 25 pS / cm, preferably less than or equal to 10 pS / cm. DETAILED DESCRIPTION OF THE INVENTION Definitions

[0101] In the present invention, the term "dose rate" is intended to denote the quantity which measures the effective impact of radioactive particles and rays on the human body. The dose rate may be individual, for an exposed worker. The dose rate may be collective, in which case it represents the sum of the individual doses received by a specific group of exposed workers. According to the laws and regulations of the countries, the dose rate, whether individual or collective, is a limit quantity not to be exceeded.

[0102] The term "dose rate reduction factor", known by the acronym FRDD, ​​means the ratio of the dose rate value before decontamination to the dose rate value after decontamination. This dose reduction factor makes it possible to evaluate the effectiveness of a decontamination process, in particular an oxide dissolution process. A decontamination process having a FRDD of approximately 10, preferably 10, has suitable effectiveness. A decontamination process having a FRDD of 10 means that 90% of the contamination has been removed.

[0103] According to the invention, the term “attack duration” means the time during which the attack is carried out, whether the attack is an acid oxidizing attack, a reducing attack or a basic oxidizing attack. The duration of the attack corresponds to the time between the circulation in the circuit to be decontaminated of the chemical reagents, at the set temperature and the emptying of the circuit to be decontaminated.

[0104] The method of dissolving oxide deposited on a metal substrate according to the invention comprises the following steps consisting of: a. Carry out, for a period of less than 2 hours, an acid oxidizing attack using a solution comprising a permanganate anion and nitric acid, at a temperature between 78 and 82°C, at a pH between 2 and 3, preferably at a pH of 2.5, this pH being maintained throughout the duration of the oxidizing attack, b. Optionally, rinse with demineralized water acidified with nitric acid, at a pH between 2 and 3, preferably 2.5, at a temperature greater than or equal to 50°C, c. Carry out, for a period of between 3 hours and 5 hours, an acid reduction attack using a solution comprising nitric acid and a reducing agent, such as ascorbic acid, at a pH less than or equal to 2, and at a temperature of between 78 and 82°C, d. Optionally, carry out at least one rinse with demineralized water, at a temperature greater than or equal to 50°C, e. Carry out, for a period of between 5 hours and 8 hours, preferably 5 hours, a basic oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously 12, and at a temperature of between 78 and 82°C, this pH being maintained throughout the duration of the oxidative attack, f. Carry out, for a period of between 3 hours and 5 hours, preferably 3 hours, an acid reduction attack using a solution comprising a reducing agent and nitric acid, at a pH lower than 2 and at a temperature between 78 and 82°C, g. Optionally, rinse with demineralized water, at a temperature greater than or equal to 50°C, h. Carry out, for a period of between 5 hours and 8 hours, preferably 5 hours, a basic oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously 12, and at a temperature of between 78 and 82°C, this pH being maintained throughout the duration of the oxidative attack, i. Carry out, for a period of between 3 hours and 5 hours, preferably 3 hours, an acid reductive attack using a solution comprising a reducing agent and nitric acid, at a pH of less than 2 and at a temperature of between 78 and 82°C, j. Optionally, carry out at least one rinse with demineralized water, at a temperature greater than or equal to 50°C.

[0105] Each of the steps of the process will be detailed below.

[0106] Steps a), b), c) and d) of the method according to the invention aim to specifically remove the 110mAg contamination present in the form of metallic silver on the internal surface of the walls and pipes of the circuits to be decontaminated, it being understood that steps b) and d) are optional.

[0107] Steps e), f) and g) of the method according to the invention aim to weaken the oxide layer on the surface of the materials and to essentially remove the 58 / 60 Co contamination from the internal surface of the walls and pipes of the circuits to be decontaminated, it being understood that step g) is optional.

[0108] Steps h), i) and j) of the method according to the invention make it possible to continue, on the one hand, the embrittlement of the oxide layer on the surface of the materials and, on the other hand, the dissolution of the 58Co and / or 60Co contamination in the decontamination solutions, it being understood that step j) is optional.

[0109] The method according to the invention comprises a step, i.e. step a), where the acid oxidative attack is carried out using a solution comprising a permanganate anion and nitric acid, at a temperature of between 78 and 82°C, at a pH of between 2 and 3, preferably at a pH of 2.5, this pH being maintained throughout the duration of the oxidative attack. Step a) according to the invention makes it possible to easily and quickly solubilize the metallic silver, i.e. the 110mAg contamination, present on the internal surface of the walls and pipes of the circuits to be decontaminated. The permanganate anion, acting as an oxidant, is advantageously potassium permanganate. The concentration of oxidant, i.e. potassium permanganate, is advantageously between 0.8 g / l and 1.2 g / l, and is ideally 1 g / l.

[0110] Step a) according to the invention is carried out for a period of less than 2 hours, preferably for a period of 1 hour, in order to preserve the stellite materials. Indeed, metallic silver, like oxidized silver, dissolves very quickly in an acidic oxidizing medium. Carrying out the oxidizing attack for a period of 1 hour advantageously makes it possible to preserve the stellite materials as well as the flexible metallic joints, such as silver joints, in particular of the Helicoflex® type, still used to ensure sealing thanks to the deformation of their structure in pipeline networks, in the nuclear field. The fact of using a solution comprising a permanganate anion and nitric acid at a pH between 2 and 3, preferably at a pH of 2.5 makes it possible to both easily and quickly solubilize the metallic silver, i.e.contamination in 110mAg, present on the internal surface of the walls and pipes of the circuits to be decontaminated and to preserve the stellite materials as well as the flexible metal joints present in these pipes of the circuits to be decontaminated.

[0111] Step b) according to the invention is optional and is advantageously carried out after step a). It comprises rinsing with demineralized water acidified with nitric acid, at a pH of between 2 and 3, preferably 2.5, and this, at a temperature greater than or equal to 50°C. Step b) according to the invention is advantageously carried out with demineralized water at a temperature of 80°C; this makes it possible to optimize the rinsing of dissolved metallic species, particularly silver contamination. The demineralized water is advantageously acidified to a pH of between 2 and 3, preferably 2.5, by any appropriate means, preferably by adding nitric acid to the demineralized water. This rinsing with demineralized water, at a pH between 2 and 3, preferably 2.5, prevents the precipitation of residual Ag+ ions, and thus facilitates the removal of solubilized metallic silver. Step b), although optional, is recommended.

[0112] Step c) according to the invention is advantageously carried out after step b) when the latter is carried out. It comprises carrying out, for a period of between 3 hours and 5 hours, an acid reductive attack using a solution comprising nitric acid and a reducing agent, such as ascorbic acid, at a pH less than or equal to 2, preferably 1.8, and at a temperature of between 78 and 82°C, preferably at a temperature of 80°C. The duration of the acid reductive phase of step c) according to the invention is preferably 5 hours. The reducing agent is advantageously ascorbic acid. The concentration of reducing agent in step c), preferably the concentration of ascorbic acid, is advantageously between 0.8 and 1.4 g / l and is ideally 1 g / l.

[0113] Advantageously, the reducing agent is preferably chosen so that it is completely degraded by a permanganate attack. This reducer must also be active towards the elements oxidized during step a) step. Ascorbic acid is a reducing agent giving good results.

[0114] Step c) makes it possible to solubilize the manganese oxide deposits which formed during the acid oxidizing attack of step a). During step c), certain oxides, including in particular iron and nickel-based oxides such as magnetite, hematite and nickel ferrite, present on the surface of the materials and containing the 58 Co / 60 Co contamination, begin to solubilize.

[0115] Step d) according to the invention is optional and is advantageously carried out after step c). It comprises at least one rinsing with demineralized water, at a temperature greater than or equal to 50°C, preferably at a temperature of 80°C. Step d) according to the invention is advantageously carried out with demineralized water at a temperature of 80°C; this makes it possible to optimize the rinsing of dissolved metal species, contamination and the remaining reagents from the decontamination solution. The means used to carry out the rinsing of step d) is advantageously chosen as a function of the decontamination loop created by all of the circuits or portions of circuits to be decontaminated, in particular the presence on all of these circuits or portions of circuits to be decontaminated of pipes that are difficult to drain.

[0116] The basic oxidative attack of step e) and / or step h) is carried out using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously at a pH of 12, and at a temperature of between 78 and 82°C, preferably at a temperature of 80°C. The basic oxidative attack of step e) and / or h) is maintained at a pH of between 9 and 13, advantageously at a pH of 12, throughout the duration of the oxidative attack. The permanganate anion, acting as an oxidant, is advantageously potassium permanganate. The concentration of oxidant, i.e. potassium permanganate, during step e) and / or step h) is advantageously between 0.8 g / 1 and 1.4 g / 1 and is ideally 1 g / 1.

[0117] The duration of the basic oxidative attacks according to the invention, i.e. steps e) and / or h), is between 5 hours and 8 hours, preferably approximately 5 hours. The duration of the basic oxidative attacks according to the invention cannot be greater than 8 hours in order to avoid any corrosion of the metal substrate during the attack. This represents a clear advantage, in particular a time saving, compared to the method for dissolving oxides deposited on a stellite metal substrate as described in application FR 2 850 673 where the oxidative attack lasted 8 hours.

[0118] The basic oxidative attack of step e) and / or h) makes it possible in particular to dissolve the chromium oxides present on the surface of the walls and pipes of the circuits to be decontaminated. The dissolution of the chromium oxides can be monitored throughout the duration of the basic oxidative attack of step e) and / or h) by carrying out periodic analyses of chromium in solution. During the basic oxidative attack of step e) and / or h), the chromium content in the solution will increase until it reaches a plateau; this plateau indicates that there is no more chromium capable of being dissolved by the basic oxidative attack, i.e. that there are no more chromium oxides present on the surface of the walls and pipes of the circuits being decontaminated or that the solution is saturated with chromium with the achievement of an equilibrium between the chromium put into solution and the chromium still present in the oxides on the surface of the walls. Advantageously, the duration of the basic oxidative attack of step e) and / or h) can be set as a function of the dissolution of the chromium oxides; the basic oxidative attack is preferably stopped when the chromium content in the solution reaches said plateau.

[0119] The acid reductive attack of step f) and / or i) is carried out, for a period of between 3 hours and 5 hours, preferably 3 hours, using a solution comprising a reducing agent and nitric acid, at a pH of less than 2 and at a temperature of between 78 and 82°C. The reducing agent may be ascorbic acid. The concentration of reducing agent, preferably ascorbic acid, is advantageously between 0.8 and 1.4 g / l and is ideally 1 g / l.

[0120] The acidic reductive attack of step f) and / or i) makes it possible in particular to dissolve the nickel and / or iron oxides present on the surface of the walls and pipes of the circuits to be decontaminated. During the acidic reductive attack, in particular during step f) and / or i), the nickel and / or iron content in the solution will increase until it reaches a plateau; this plateau indicates that there is no more nickel and / or iron capable of being dissolved by the reductive attack, i.e. that there are no more nickel and / or iron oxides present on the surface of the walls and pipes of the circuits being decontaminated or that the solution is saturated with iron and / or nickel with the achievement of an equilibrium between the iron and / or nickel put into solution and the iron and / or nickel still present in the oxides on the surface of the walls.Advantageously, the duration of the acid reduction attack of step f) and / or i) can be set as a function of the dissolution of the iron and / or nickel oxides; the acid reduction attack is preferably stopped when the iron and / or nickel content in the solution reaches said plateau. The duration of the acid reduction phase of step f) and / or i) is advantageously between 3 hours and 5 hours.

[0121] This represents a clear advantage, in particular a saving in time, compared to the process for dissolving oxides deposited on a stellite metal substrate as described in FR 2 850 673 where, to dissolve the iron and / or nickel oxides present on the surface of the walls and pipes to be decontaminated, the reductive attack was carried out over a period of at least 5 hours.

[0122] The duration of the acid reducing phase of step f) and / or i) according to the invention is preferably 5:00 a.m.

[0123] Step g) according to the invention is optional, it is advantageously carried out after step f). It comprises rinsing by any appropriate means, preferably with demineralized water, and this, at a temperature greater than or equal to 50°C, preferably at a temperature of 80°C. Step g) according to the invention is advantageously carried out with demineralized water at a temperature of 80°C; this makes it possible to optimize the rinsing of dissolved metal species, contamination and the rest of the reagents from the decontamination solution.

[0124] Step j) according to the invention is optional; it is advantageously carried out after step i). It comprises at least one rinsing with demineralized water, at a temperature greater than or equal to 50°C, preferably at a temperature of 80°C. The at least one rinsing carried out in step j) makes it possible to ensure that the solubilized contaminations, initially present on the walls of the circuits, as well as the chemical reagents used to solubilize the contaminations have been effectively eliminated.

[0125] Advantageously, a measurement of the conductivity of the rinsing water obtained at the end of step j) when steps a) to j) are carried out in order, or a measurement of the conductivity of the rinsing water obtained at the end of step d) when steps e) to j) and then a) to d) are carried out makes it possible to ensure the level of cleanliness of the walls and pipes of the circuits and to ensure that the reagents used for decontamination have been eliminated. The presence of reagents used in this process could be harmful and pose a problem when restarting the reactor or when operating the latter. The measurement of the conductivity of the rinsing water is the criterion which makes it possible to guarantee that the chemical reagents have been eliminated from the circuit.

[0126] A conductivity threshold is advantageously predefined, and is preferably less than or equal to 25 pS / cm, ideally less than or equal to 10 pS / cm. When the measured conductivity is higher than this conductivity threshold, decontamination of the walls and pipes of the circuits is not sufficient; another rinse with demineralized water must be carried out, preferably at room temperature, or at a temperature greater than or equal to 50°C, preferably at a temperature of 80°C. Step j) according to the invention can be repeated as many times as necessary, until the measurement of the conductivity of the rinsing water obtained at the end of the last rinse of step j) or d) is lower than the fixed conductivity threshold.

[0127] When the measured conductivity is lower than this conductivity threshold, the walls and pipes of the circuits are considered decontaminated and therefore clean.

[0128] Advantageously, when the metal substrates such as the walls and / or pipes of the circuits to be decontaminated have on the surface 110mAg contamination and at least 58Co and / or 60Co contamination, the method according to a first variant of the invention comprises in order steps a), b), c), d), e), f), g), h), i) and j), it being understood that step j) is carried out and that rinsing steps g), b) and d) are optional.

[0129] Advantageously, when the metal substrates such as the walls and / or the pipes of the circuits to be decontaminated have 58 Co and / or 60 Co contamination on the surface, preferably have mainly 58 Co and / or 60 Co contamination on the surface, i.e. preferably have a surface contamination of between 80% and 100% 58 Co and / or 60 Co, the method according to a second variant of the invention comprises in the following order steps e), f), g), h), i), j), a), b), c) and d), preferably comprises in the following order steps e), f), h), i), j), a), b) and c), d) it being understood that the rinsing steps g), j) and b) are optional.

[0130] Step b) according to the first and / or second variant of the invention makes it possible to easily remove the solubilized silver ions by rinsing so that they do not reprecipitate during the following acid reduction phase.

[0131] Step d) according to the first and / or second variant of the invention makes it possible to easily remove by rinsing the contaminants solubilized during steps a) and / or c).

[0132] Step g) according to the first and / or second variant of the invention makes it possible to easily remove by rinsing the contaminants solubilized during steps e) and / or f) as explained above.

[0133] The method according to the first or second variant of the invention has numerous optional rinsing steps. The fact of avoiding several rinsing steps makes it possible to reduce the quantity of effluents produced which are subsequently treated by expensive processes, without calling into question the effectiveness of the decontamination. Furthermore, the method according to the first or second variant of the invention makes it possible to effectively decontaminate the walls and pipes of circuits over a reduced period of time. In this context, the maintenance times for decontamination of nuclear power plants are reduced, leading to an optimization of electrical production.

[0134] The decontamination method according to the invention advantageously has a FRDD of approximately 10, preferably 10; it thus has suitable efficiency.

Claims

Claims

1. A method of dissolving oxides deposited on a metal substrate, preferably a metal substrate comprising both stainless steel and stellites, characterized in that said method comprises the following steps: a. Carry out, for a period of less than 2 hours, an acid oxidative attack using a solution comprising a permanganate anion and nitric acid, at a temperature between 78 and 82°C, at a pH between 2 and 3, preferably at a pH of 2.5, this pH being maintained throughout the duration of the oxidative attack. b. Optionally, carry out a rinse with demineralized water acidified with nitric acid, at a pH between 2 and 3, preferably 2.5, at a temperature greater than or equal to 50°C, c. Carry out, for a period of between 3 hours and 5 hours, an acid reductive attack using a solution comprising a reducing agent and nitric acid, at a pH less than or equal to 2, and at a temperature between 78 and 82°C, d. Optionally, carry out at least one rinse with demineralized water, at a temperature greater than or equal to 50°C, e. Carry out, for a period of between 5 hours and 8 hours, preferably 5 hours, a basic oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously 12, and at a temperature of between 78 and 82°C, this pH being maintained throughout the duration of the oxidative attack. f. Carry out, for a period of between 3 hours and 5 hours, preferably 3 hours, an acid reductive attack using a solution comprising a reducing agent and nitric acid, at a pH of less than 2 and at a temperature of between 78 and 82°C. g. Optionally, rinse with demineralized water, at a temperature greater than or equal to 50°C, h. Carry out, for a period of between 5 hours and 8 hours, preferably 5 hours, a basic oxidative attack using a solution comprising a permanganate anion and sodium hydroxide at a pH of between 9 and 13, advantageously 12, and at a temperature of between 78 and 82°C, this pH being maintained throughout the duration of the oxidative attack. i. Carry out, for a period of between 3 hours and 5 hours, preferably 3 hours, an acid reductive attack using a solution comprising a reducing agent and nitric acid, at a pH of less than 2 and at a temperature of between 78 and 82°C. j. Optionally, carry out at least one rinse with demineralized water, at a temperature greater than or equal to 50°C.

2. Method according to claim 1, characterized in that the steps are carried out from e) to j) then from a) to d) or are carried out in the order from a) to j).

3. Method according to claim 2, characterized in that: - when steps a) to j) are carried out in order, step j) is carried out, or - when steps e) to j) and then a) to d are carried out, step d) is carried out.

4. Method according to any one of the preceding claims, characterized in that steps b) and j) are carried out when steps a) to j) are carried out in order.

5. Method according to any one of the preceding claims, characterized in that the duration of the acid oxidative attack in step a) is 1 hour.

6. Method according to any one of the preceding claims, characterized in that the duration of the basic oxidative attack in step e) and in step h) is 5 hours.

7. Process according to any one of the preceding claims, characterized in that the duration of the acid reductive attack in step c) is 5 hours.

8. Process according to any one of the preceding claims, characterized in that the duration of the acid reductive attack in step f) and / or in step i) is 5 hours.

9. Process according to any one of the preceding claims, characterized in that: - the acid oxidizing attack of step a) is carried out at a temperature of 80°C, and / or - the acid reducing attack of step c) is carried out at a temperature of 80°C, and / or - the basic oxidizing attack of step e) is carried out at a temperature of 80°C, and / or - the acid reducing attack of step f) is carried out at a temperature of 80°C, and / or - the basic oxidative attack of step h) is carried out at a temperature of 80°C, and / or - the acidic reductive attack of step i) is carried out at a temperature of 80°C.

10. Method according to any one of the preceding claims, characterized in that the solution used in step a) comprising a permanganate anion and nitric acid, has a pH of between 2 and 3, and preferably has a pH of 2.

5.

11. A method according to any one of the preceding claims, characterized in that the solution comprising a permanganate anion and sodium hydroxide used in step e) and / or h), has a pH of 12.

12. A method according to any one of the preceding claims, characterized in that, in step a), the solution comprising a permanganate anion and nitric acid has a permanganate anion concentration of between 0.8 g / l and 1.2 g / l, preferably has a permanganate anion concentration of 1 g / l.

13. Method according to any one of the preceding claims, characterized in that, in step e) and / or in step h), the solution comprising a permanganate anion and sodium hydroxide has a permanganate anion concentration of between 0.8 g / l and 1.4 g / l, preferably has a permanganate anion concentration of 1 g / l.

14. Method according to any one of the preceding claims, characterized in that, in step c) and / or in step f) and / or in step i), the solution comprising a reducing agent and nitric acid, has a reducing agent concentration of between 0.8 g / l and 1.4 g / l, preferably has a reducing agent concentration of 1 g / l.

15. Method according to any one of the preceding claims, characterized in that the solution used in step c) and / or in step f) and / or in step i), comprising a reducing agent and nitric acid, has a pH of 1.

8.

16. Method according to any one of the preceding claims, characterized in that the demineralized water used during rinsing in step b) and / or d) and / or g) has a temperature of 80°C.

17. Method according to any one of the preceding claims, characterized in that the demineralized water used during the at least one rinse in step j) has a temperature of 80°C.

18. Method according to any one of the preceding claims, characterized in that permanganate pennant is introduced in step a) and / or step e) and / or step h), in the form of potassium permanganate.

19. A method according to any one of the preceding claims, characterized in that the reducing agent in step c) and / or in step f) and / or in step i) is ascorbic acid.

20. Method according to any one of the preceding claims, characterized in that: - when steps a) to j) are carried out in order, step j) is carried out until the rinsing water has a conductivity threshold less than or equal to 25 pS / cm, preferably less than or equal to 10 pS / cm, or - when steps e) to j) and then a) to d are carried out, step d) is carried out until the rinsing water has a conductivity threshold less than or equal to 25 pS / cm, preferably less than or equal to 10 pS / cm.