Method for the radioactive decontamination of the metal wall of an item of fluid-circulating equipment

Abstract

The invention relates to a method for the radioactive decontamination of the internal face of the metal wall of an item of fluid-circulating equipment in a nuclear facility, the wall being made of a metal alloy and the internal face of this wall being covered with a surface layer of metal oxides, the method comprising at least one step a) of dissolving the surface layer of metal oxides, followed by a step b) of corroding the internal face of the wall, and being characterised in that: (1) steps a) and b) comprise circulating through the item of equipment a reducing aqueous solution at a temperature of at least 75°C; (2) step a) comprises, simultaneously with circulating the reducing aqueous solution through the item of equipment, biasing the wall to a potential lower than the corrosion potential of the metal alloy in contact with the reducing aqueous solution; and (3) the biasing of the wall is stopped at the end of step a). The invention is intended for use in the nuclear field, in particular for the decommissioning of items of metal equipment as conventional waste or for carrying out thorough decontamination of these items of equipment.

Description

[0001] Description

[0002] METHOD FOR RADIOACTIVE DECONTAMINATION OF THE METAL WALL OF A FLUID CIRCULATION SYSTEM

[0003] technical field

[0004] The invention falls within the field of radionuclide decontamination of metallic equipment.

[0005] More specifically, the invention relates to a method for rapidly but thoroughly removing radioactive contamination fixed on the inner face of the metal wall of equipment used for circulating a fluid (liquid or gas) in a nuclear installation, such as piping composed of a set of tubes and piping accessories (elbows, tees, fittings, taps, pumps, valves, etc.) or a segment of such piping.

[0006] The invention finds application in the nuclear field, particularly in the context of dismantling and remediation operations for end-of-life nuclear facilities, whether for the downgrading of metallic equipment into conventional waste or for carrying out "extensive remediation" of this equipment, that is to say, remediation aimed at giving it a final state compatible with any use. Prior art

[0007] During their use in nuclear facilities, the metal walls of certain equipment used for fluid circulation, such as piping transferring solutions from one unit to another, become contaminated on their inner surface by radionuclides. These radionuclides accumulate primarily in the layer of metal oxides that forms on this surface. It is estimated that approximately 98% of the radionuclides concentrate in the surface layer of metal oxides, and that only about 2% diffuse into the underlying metal. Consequently, erosion of this metal to a thickness of 10 LUYI generally reduces the radionuclide concentration below the mass activity thresholds at which the Nuclear Safety Authority (ASN) can authorize the disposal of metallic equipment as conventional waste. Several methods have been proposed to decontaminate the inner surface of the metal walls of equipment of radionuclides.

[0008] Such processes are, for example, described in references EP-A-1 082 728, US-A-2020 / 0013519, EP-A-1 422 724 and JPH01-173900.

[0009] These processes, which all involve dissolving the surface layer of metal oxides and then corroding the underlying metal, each present a number of drawbacks, including a generally complex implementation, notably:

[0010] - multiple successive steps such as, for example, in US-A-2020 / 00135519, just for the decontamination operations strictly speaking;

[0011] - the use of several different decontamination solutions as in JP-B-7009479 or of a single decontamination solution but whose composition is changed during the process (for example, by means of an ion exchange resin, an inert gas, ozone or by UV irradiation) as in EP-A-1082728, US-A-2020 / 0013519 and JPH01-173900, which requires specific and expensive equipment;

[0012] - the implementation of an oxidation step involving the use of an oxidizing aqueous solution as in US-A-2020 / 0013519, EP-A-1422 724 and JPH-01-173900, with the result of the generation of effluents including oxidizing ions whose management is restrictive;

[0013] - the use of toxic or corrosive reagents such as oxalic acid which is used in EP-A-1 422 724 and / or which generate effluents that are difficult to treat in nuclear facility effluent treatment units such as sulfuric acid which is used in JPH01-173900.

[0014] Due to their implementation methods and the specific equipment they require, these processes also have the disadvantage of being suitable only for small metal parts that can be treated in tanks, which means that, for the decontamination of large metal equipment, this equipment must first be dismantled and cut into sections.

[0015] Description of the invention The invention aims precisely to provide a process which allows for the rapid but very effective decontamination of the inner face of the metal wall of equipment used for the circulation of a fluid in a nuclear installation and which is free from all the disadvantages described above.

[0016] Since the wall is made of a metallic alloy and its inner face is covered with a surface layer of metal oxides, this process comprises at least the following steps: a) dissolving the surface layer of metal oxides, then b) corroding the inner face of the wall to a thickness less than the total thickness of the wall, and is characterized in that:

[0017] - steps a) and b) include circulating in the equipment an aqueous reducing solution with a temperature of at least 75 °C;

[0018] - step a) includes, simultaneously with the circulation of the reducing aqueous solution in the equipment, a polarization of the wall to a potential lower than the corrosion potential of the metallic alloy in contact with the reducing aqueous solution;

[0019] - the polarization of the wall is stopped at the end of step a); and in that

[0020] - step a) is optionally repeated one or more times before proceeding to step b).

[0021] Thus, according to the invention:

[0022] * Step a) of dissolving the surface layer of metal oxides and step b) of corroding the inner face of the wall of the equipment are both carried out with the same solution, in this case a reducing aqueous solution, which is circulated in the equipment, which makes it possible to treat equipment of all sizes;

[0023] * Step a) of dissolution includes, simultaneously with the circulation of the reducing aqueous solution in the equipment, a polarization of the wall of this equipment which allows, during dissolution, the metallic alloy of the wall of the equipment to be placed in its immunity domain and, thus, to prevent the inner face of this wall from being corroded; and

[0024] * It is the cessation of polarization, at the end of step a) of dissolution or the last step a) of dissolution (since step a) may possibly be repeated one or more times), which allows the metallic alloy to spontaneously return to its active corrosion domain and, thus, for the corrosion of the inner face of the wall to occur.

[0025] According to the invention, the metallic alloy can be any metallic alloy suitable for use in a nuclear installation for fluid circulation. Thus, it can in particular be a stainless steel such as an austenitic steel (for example, of the type AISI 304, AISI 304L, AISI 316 or AISI 316L), a martensitic steel, a ferritic steel or an austenitic-ferritic steel, a nickel-chromium superalloy such as those known under the trade name Inconel™, a nickel-iron-chromium superalloy such as those known under the trade name Incoloy™, or even a carbon steel.

[0026] Preferably, the alloy is an austenitic stainless steel, for example of the type AISI 304, AISI 304L, AISI 316 or AISI 316L, or a nickel-chromium superalloy, for example of the type Inconel™ 600.

[0027] Advantageously, the reducing aqueous solution comprises formic acid, ascorbic acid (which is neither toxic nor corrosive unlike oxalic acid), nitric acid and sodium bicarbonate.

[0028] More specifically, the reducing aqueous solution preferably comprises:

[0029] - from 1.8 mol / L to 3 mol / L of formic acid, for example 2 mol / L,

[0030] - from 0.05 mol / L to 0.3 mol / L of ascorbic acid and, even better, from 0.05 mol / L to 0.15 mol / L, for example 0.11 mol / L,

[0031] - from 4.5 mmol / L to 5.5 mmol / L of nitric acid, for example 5 mmol / L, and

[0032] - from 5.5 mmol / L to 22 mmol / L of sodium bicarbonate and, even better, from 9 mmol / L to 12 mmol / L, for example 10.9 mmol / L.

[0033] As previously stated, this reducing aqueous solution has a temperature of at least 75 °C when circulating in the equipment, this temperature preferably being at most 115 °C.

[0034] Preferably, the reducing aqueous solution has a temperature ranging from 75 °C to 95 °C. Typically, the polarization of the equipment wall involves applying a cathode current to this wall using a direct current generator.

[0035] According to the invention, the process advantageously further comprises, prior to step a) or the first step a) if step a) is repeated, a circulation in the equipment of an aqueous solution of a strong acid, preferably nitric acid, or of an aqueous solution of a strong base, preferably sodium hydroxide, to remove any metallic and / or greasy deposits that may be present on the inner face of the wall of the equipment.

[0036] Furthermore, prior to step a) or the first step a) if step a) is repeated, the wall of the equipment is preferably heated to a temperature of at least 75 °C, preferably by circulating water, advantageously distilled, at this temperature through the equipment.

[0037] Preferably, the reducing aqueous solution circulates in a closed circuit during steps a) and b).

[0038] However, it is possible that step b) may include a renewal of the reducing aqueous solution circulating in the equipment, for example every 24 to 72 hours, in which case the flow of reducing aqueous solution to be renewed is taken out of the circuit and replaced by a new flow of reducing aqueous solution.

[0039] Advantageously, step b) further includes a control of the corroded inner face thickness by measuring the quantity of metallic cations that are present in the reducing aqueous solution after it exits the equipment.

[0040] Preferably, the process further comprises, after step b), draining the equipment and circulating through this equipment an aqueous solution of a strong acid, preferably nitric acid, to dissolve the carbon film that may have been deposited on the inner face of the wall during step b).

[0041] According to the invention, the metallic equipment may in particular be a piping for transferring solutions between two units of a nuclear installation or a segment of such piping, a component of a nuclear reactor previously unloaded of its fuel (such as, for example, piping ensuring the transfer of a fluid to a steam generator, a steam generator itself, a nuclear reactor water treatment circuit or a pool water treatment circuit) or a dissolver of a spent nuclear fuel processing plant.

[0042] Other features and advantages of the invention will become apparent from the supplementary description that follows and which refers to the attached figure.

[0043] It goes without saying that this additional description is given as an illustration of the invention and in no way as a limitation of this object.

[0044] Brief description of the figure

[0045] [Fig. 1] schematically represents an example of an installation specially designed for the implementation, in a closed circuit, of the decontamination process according to the invention.

[0046] In this figure, the various elements of the installation are intentionally represented on a non-uniform scale to make the figure more legible. Detailed description of a particular implementation method

[0047] The following relates to an example of an installation specifically designed to decontaminate, in a closed circuit, the inner face of the metal wall of equipment forming an open loop, such as a segment of piping used to transfer solutions between two units of a nuclear installation, and to an example of implementing the decontamination process using this installation.

[0048] The wall of the equipment is made of a metal alloy, for example an austenitic stainless steel of the type AISI 304, AISI 304L, AISI 316 or AISI 316L.

[0049] As can be seen in Figure 1, the installation, labeled 1 in this figure, comprises three units, namely:

[0050] - a unit 10 dedicated to the preparation of the reducing aqueous solution and its circulation towards the equipment to be decontaminated, marked 31 on figure 1 and inscribed in a dotted frame;

[0051] - a unit 30 dedicated to the actual decontamination of the inner face of the wall of the equipment 31; and - a unit 50 dedicated to emptying the equipment 31 as well as storing the final effluents of the process.

[0052] Unit 10 includes a tank 11 which is equipped with:

[0053] * of a system 12 for introducing into this tank the various reagents necessary for the preparation of the reducing aqueous solution;

[0054] * of an agitation system 13, for example of the mechanical agitator type, to obtain a homogeneous mixture of these reactants after their introduction into the tank 11;

[0055] * of a system 14 for supplying the tank 11 with water, preferably distilled, this system comprising, for example, one or more sprinklers suitable for rinsing the wall of this tank, this or these sprinklers being connected to an external water reservoir (not shown in figure 1);

[0056] * of a heating system 15, for example of the immersion heater type, to heat and maintain the reducing aqueous solution at the temperature required for its circulation in the equipment 31;

[0057] * of a system 16 for taking samples of the reducing aqueous solution to measure the redox potential, electrical conductivity, temperature and radiological activity of this solution;

[0058] * of a system 17 to evacuate gaseous emissions (water vapor, dihydrogen) likely to occur in tank 11;

[0059] * of a system 18 allowing the return of all or part of the reducing aqueous solution from unit 30 into tank 11; and

[0060] * of a system 19 allowing a return into the tank 11 of the reducing aqueous solution from the unit 50.

[0061] The wall of tank 11 and all other metallic elements that comprise this tank and that are intended to be in contact with the reducing aqueous solution are made of stainless steel either previously passivated or passivated in situ.

[0062] Unit 10 further includes a system for circulating the reducing aqueous solution to convey it to unit 30 in which equipment 31 is located, for the purpose of looping this solution between tank 11 and equipment 31 during the steps of dissolving the surface layer of metal oxides and corroding the alloy.

[0063] This system comprises a pipe 20 made of a dielectric material such as a fluoropolymer, for example polyvinylidene fluoride (PVDF), or polyethylene, which originates at the bottom of the tank 11 and is equipped with a pump 21, preferably centrifugal to ensure a homogeneous flow without pulsations, a flow meter 22, for example electromagnetic, and two valves, typically two-way, 23 and 24 respectively. Valve 23 is located on the portion of the pipe 20 that runs from the bottom of the tank 11 to the pump 21 and allows the flow of the reducing aqueous solution contained in the tank 11 to be opened or closed to this pipe, while valve 24 is located on the portion of the pipe 20 that runs from the pump 21 to the flow meter 22 and allows the flow rate of the reducing aqueous solution to be regulated in this pipe 20 and, downstream of it, in the equipment. 31.

[0064] The conduit 20 extends to the unit 30 at which it is connected to the end 38 of the equipment 31 by means of a connecting clarinet 32 ​​made of a dielectric material such as a fluoropolymer, for example PVDF, or polyethylene.

[0065] Unit 30 includes a system for polarizing the wall of equipment 31 and monitoring the corrosion potential of this wall.

[0066] This system includes:

[0067] - a direct current generator 33;

[0068] - a reference electrode 34, for example of the Ag / AgCl type;

[0069] - a counter electrode 35, which is made of either platinum or the same alloy as that constituting the wall of the metallic equipment 31;

[0070] - the connecting clarinet 32 ​​having two spigots, respectively 36 and 37, which are also made of a dielectric material such as a fluoropolymer (PVDF for example) or polyethylene like the rest of this clarinet, the spigot 36 being intended to receive the reference electrode 34 while the spigot 37 is intended to receive the counter electrode 35; and - a voltmeter 26 for measuring the potential difference between the reference electrode 34 and the wall of the equipment 31.

[0071] As shown in Figure 1, under operating conditions:

[0072] * the reference electrode 34 is introduced into the tap 36 and has one of its ends which is connected to the common terminal (or "COM" port) of the voltmeter 26 while its other end plunges into the reducing aqueous solution which circulates in the clarinet 32;

[0073] * The counter electrode 35 is introduced into the tapping 37 and has one of its ends connected to the ® terminal of the generator 33 while its other end is in the immediate vicinity (typically less than 1 cm) of the end 38 of the equipment 31 but without being in contact with the end 38 while bathed in the reducing aqueous solution.

[0074] The counter electrode 35 must be electrically isolated from the equipment 31 and this isolation is guaranteed both by the use of a clarinet and taps in a dielectric material and the absence of contact between this counter electrode and the end 38 of the equipment 31.

[0075] The Q terminal of the generator 33 is connected to the external face of the wall of the equipment 31 when this generator is in operation, while it is connected to the ® terminal of the voltmeter 26 when this generator is off.

[0076] Unit 30 also includes a system for removing the reducing aqueous solution from equipment 31 and conveying it either to unit 10 or to unit 50.

[0077] This drainage system includes a pipe 40 which originates at the other end of the equipment 31, namely the end 39, which is fitted with a valve, typically 2-way, and which splits downstream of this valve (in the direction of flow of the reducing aqueous solution) into two branches, respectively 41 and 42, one of which, namely branch 41, joins unit 10 while the other, namely branch 42, joins unit 50.

[0078] As can be seen in Figure 1, branch 41 of pipe 40 itself divides into two branches, 43 and 44 respectively, each equipped with a valve, typically 2-way, 45 and 46 respectively. Branch 43 of pipe 41 allows, if valve 45 is open while valve 46 is closed, a return of the reducing aqueous solution to the tank 11 of unit 10 (for its subsequent recirculation in equipment 31) and is equipped with a circulation heater 25. Branch 44 of pipe 41 is a useful bypass branch, for example, for maintenance operations on the installation.

[0079] Unit 50 comprises two tanks, 51 and 52 respectively, tank 51 serving both as a buffer tank (for example, for temporary storage of the reducing aqueous solution in the event that, the operation of the installation is stopped and then resumed later, it is desired to reuse this solution) and as a drain tank for equipment 31 while tank 52 is dedicated to the storage of the final effluents.

[0080] Tanks 51 and 52, like tank 11 of unit 10, are made of passivated stainless steel.

[0081] Branch 42 of pipe 40 includes a 3-way valve 47, at which point it splits into two branches, 48 ​​and 49 respectively, branch 48 supplying tank 51, for example with reducing aqueous solution, while branch 49 conveys the final effluents to tank 52.

[0082] Tank 51 is equipped with a pipe 53 which originates at the bottom of this tank and which is equipped with a pump 54, for example of the diaphragm type, as well as a 3-way valve 55, at which point it divides into two branches, respectively 56 and 57, branch 56 joining tank 11 and branch 57 joining tank 52. Tank 51 is also equipped with a sprinkler 58 allowing its wall to be rinsed.

[0083] In order to avoid depassivating and, therefore, corroding the stainless steel of the tanks 11, 51 and 52, all the pipes connecting the end 39 of the equipment 31 to these tanks are made of a dielectric material, typically a fluoropolymer, for example a PVDF.

[0084] Installation 1 also includes, upstream of pump 21, a strainer (not shown in Figure 1) to retain large particles resulting from the dissolution of the surface layer and thus protect the pump. This strainer can be supported by a Y-filter (not shown in Figure 1) and can be equipped with a dose rate measurement device (not shown in Figure 1). It also includes various temperature and dose rate measurement points for the reducing aqueous solution as it circulates through the installation (also not shown in Figure 1).

[0085] Optionally, installation 1 may also include a module for the UV decomposition of organic compounds that may be present in the final effluents (not shown in Figure 1), for example of the type consisting of a pipe made of a transparent material around which powerful UV lamps are placed.

[0086] Installation 1 can be in the form of a "skid", i.e. a mobile structure, or chassis, on which units 10, 30 and 50 are fixed, in which case the various pipes of this installation (pipes 20, 40, 41, 42, 43, 44, etc.) are advantageously flexible.

[0087] Implementation of the decontamination process according to the invention:

[0088] Depending on the initial condition of equipment 31, and in particular if the inner surface of the wall of this equipment presents metallic and / or greasy deposits, a preliminary rinse of this wall is carried out using an aqueous solution containing 1 mol / L or more of a strong inorganic acid, preferably nitric acid, or a strong base, preferably sodium hydroxide, at room temperature. To do this, this aqueous solution is circulated several times from tank 11 to equipment 31 and from equipment 31 to tank 11, via pipes 20, 40, 41, and 45. It is then discharged from equipment 31 and sent to tank 52.

[0089] Whether or not this preliminary rinsing has been carried out, water, preferably distilled, is circulated several times from tank 11 to equipment 31 and from equipment 31 to tank 11, also via pipes 20, 40, 41 and 45, this water being heated in tank 11 by the heating system 15 of this tank and being circulated at a temperature and flow rate enabling the inner face of the wall of equipment 31, in its coldest area, to reach a temperature at least equal to the temperature intended to be presented by the reducing aqueous solution for its circulation in equipment 31. Typically, a suitable flow rate is one allowing a water circulation velocity of 0.5 m / s to 1.0 m / s, preferably 0.7 m / s.It should be noted that the coldest area of ​​a metallic piece of equipment will generally be located in its terminal part, but that a simulation using CFD (Computational Fluid Dynamics) software can, in complex cases, make it possible to determine this area.

[0090] In any case, the higher the temperature of the inner surface of the wall of equipment 31, the higher the dissolution rate of the surface layer of metal oxides. It is therefore preferable to maintain a temperature that is not only high, but also as homogeneous as possible within the metal equipment.

[0091] When the internal wall of the equipment 31 reaches the desired temperature, the water is drained from the equipment 31 and sent to the tank 11 in which the reducing aqueous solution is prepared, which advantageously comprises:

[0092] - from 5.5 mmol / L to 22 mmol / L of sodium bicarbonate and, preferably, from 9 mmol / L to 12 mmol / L, for example 10.9 mmol / L,

[0093] - from 0.05 mol / L to 0.3 mol / L of ascorbic acid and, preferably, from 0.05 mol / L to 0.15 mol / L, for example 0.11 mol / L,

[0094] - from 1.8 mol / L to 3 mol / L of formic acid, for example 2 mol / L, and

[0095] - from 4.5 mmol / L to 5.5 mmol / L of nitric acid, for example 5 mmol / L.

[0096] These reagents are introduced into the tank 11 via the system 12, preferably in the order indicated above.

[0097] Such a solution makes it possible to corrode all stainless steels.

[0098] It is also possible to prepare more concentrated reagent solutions and introduce them into tank 11 together with distilled water to bring them to the desired concentration.

[0099] The reducing aqueous solution is homogenized using the stirring system 13 and heated using the heating system 15 to bring it to a temperature of at least 75 °C.

[0100] Dissolution of the surface layer of metal oxides: When the reducing aqueous solution is at the desired temperature, it is conveyed via the pipe 20 to the equipment 31 in which it flows at a speed typically between 0.5 m / s and 1.0 m / s, for example 0.7 m / s.

[0101] Using the generator 33, a direct current is then applied between the counter electrode 35 and the outer face of the wall of the equipment 31 at a current density sufficient to bring the alloy constituting the wall of the element 31 into its immunity range and for the reductive dissolution of the surface oxides to occur. For example, this current density is on the order of 0.1 mA / cm². 2internal face to be decontaminated for AISI 304, AISI 304L, AISI 316 or AISI 316L stainless steel.

[0102] This dissolution stage lasts approximately 30 minutes.

[0103] The electric current is then switched off and the corrosion potential of the alloy is measured using the reference electrode 34.

[0104] If this potential stabilizes in the active corrosion range of the alloy (for example, between -200 mV and -400 mV vs Ag / AgCl for an AISI 304, AISI 304L, AISI 316 or AISI 316L stainless steel), the corrosion stage of the alloy begins.

[0105] If this is not the case, then the dissolution step should be repeated by restarting generator 33, as many times as, at the end of this step, the corrosion potential of the alloy does not stabilize in the active corrosion range of this alloy.

[0106] Typically, a single 30-minute dissolution step will be sufficient for a reducing aqueous solution circulation speed of 0.7 m / s, whereas it will often be necessary to repeat this step once or twice for higher or lower reducing aqueous solution circulation speeds.

[0107] Alloy corrosion:

[0108] At the end of the dissolution step, or the last dissolution step if multiple dissolution steps are performed, the reducing aqueous solution is circulated from tank 11 to equipment 31 and from equipment 31 back to tank 11 as many times as necessary, depending on the thickness of the alloy to be corroded. However, it may be necessary to renew the reducing aqueous solution if it turns dark brown; otherwise, there is a risk that the decomposition products of the organic acids (ascorbic acid and formic acid) will precipitate and adhere to the inner surface of the wall of equipment 31, in which case they can only be removed from this wall using an acidic solution containing an oxidant such as potassium permanganate.

[0109] Generally, this corresponds to a change of reducing aqueous solution every 24 to 72 hours, preferably every 48 hours.

[0110] When a change in the reducing aqueous solution is made, the corrosion potential of the alloy tends to spontaneously shift into its active corrosion range. However, if this is not the case, it is possible to return the alloy on the wall of equipment 31 to its active corrosion range by applying a potential to the wall at which the alloy is within its active corrosion range. Applying such a potential for 5 minutes will generally be sufficient to return the alloy to its active corrosion range.

[0111] Monitoring of the quantity of metallic cations (iron, chromium, nickel and, possibly, molybdenum, manganese and copper) present in the reducing aqueous solution can be carried out, for example by visible spectrometry, in order to assess the thickness of alloy that has been corroded.

[0112] As an example, for an AISI 304 or AISI 304L alloy, corrosion typically has, under the conditions described above, a kinetic of 0.1 pm / h to 1 pm / h, which allows us to estimate the time required to corrode a chosen alloy thickness (depending on the decontamination objective that has been set).

[0113] To stop corrosion and, consequently, the decontamination of the internal surface of equipment 31, it is possible to:

[0114] - to lower the temperature of the reducing aqueous solution, for example by bringing it below 70°C, or even below 65°C; or

[0115] - to empty the equipment 31 of the reducing aqueous solution, which is then sent to the tank 51 of unit 50; or

[0116] - to place the alloy of the wall of the equipment 31 in its passivity domain by reversing the terminals of the generator 33 and applying a current between the counter electrode 35 and the external face of the external wall of the equipment 31.

[0117] It should be noted that, under the conditions described above, the decrease in radiological activity of metallic equipment is generally very rapid initially when radionuclides are highly present in the metal oxide layer and the first micrometer of alloy underlying this layer. Subsequently, the decrease in radiological activity of metallic equipment is slower and fairly linear.

[0118] Rinsing:

[0119] For optimal decontamination of equipment 31 and for dissolution of the carbon film which has been deposited on the inner face of the wall of this equipment, it is advisable, after draining equipment 31, to proceed to rinse it by circulating, at room temperature, an aqueous solution comprising 1 mol / L or more of a strong inorganic acid, preferably nitric acid.

[0120] It is then advisable to rinse all the elements of installation 1 with industrial water.

[0121] Wastewater treatment:

[0122] Depending on the site specifications, the volume of effluent produced (which depends primarily on the size of the treated equipment 31 and the initial radiological activity of its wall) and the radiological activity of the effluent, it may be possible to discharge it after dilution in the site's effluent stream. Toxicity tests (on Vibrio bacteria) have shown that... 3 of effluent in 600 m 3 the amount of discharged effluent does not increase ecotoxicity.

[0123] If effluents require treatment, it is generally necessary to break down the organic compounds present in them. This breakdown can be achieved by injecting hydrogen peroxide and passing the effluent through a UV module (the effluent flows through transparent pipes surrounded by powerful UV lamps). A cooling system is necessary because the oxidation reactions are exothermic and hydrogen peroxide decomposes at around 70°C. It is preferable to maintain the temperature around 60°C for optimal decomposition kinetics. The decomposition reaction will then be catalyzed by the iron released from the corrosion of the alloy (therefore, it is best not to attempt to remove it beforehand).

[0124] The organic acids (and their decomposition products) are then broken down into carbon dioxide and water, resulting in an effluent containing only a low concentration of nitrates and carbonates, the metallic cations resulting from the corrosion of the alloy (mainly Fe). 2+ Fe 3+ , Cr 3+ , Neither 2+ ) and the radionuclides transferred from the equipment to the solution.

[0125] Decomposition must be maximized if a subsequent step involving cation exchange resins is chosen. Ascorbic acid and some of its decomposition products complex with cations and compete with the resin groups (generally sulfonates), particularly with regard to chromium. If decomposition is insufficient, chromium may not be exchanged on the cation exchange resins.

[0126] Optionally, the effluents obtained after the H2O2 / UV decomposition can be treated with cation exchange resins to concentrate the activity, and then with anion exchange resins if nitrate removal is necessary. If the UV treatment was long enough to virtually eliminate Total Organic Carbon (TOC), this treatment will be carried out in a completely conventional manner.

[0127] References cited

[0128] EP-Al 082 728

[0129] US-A-2020 / 0013519

[0130] EP-Al 422 724

[0131] JPH01-173900

Claims

Demands 1. A method for radioactive decontamination of the inner face of the metal wall of equipment used for circulating a fluid in a nuclear installation, the wall being made of a metallic alloy and the inner face of this wall being covered with a surface layer of metal oxides, which method comprises at least the following steps: a) dissolution of the surface layer of metal oxides, then b) corrosion of the inner face of the wall to a thickness less than the total thickness of this wall, and is characterized in that: - steps a) and b) include circulating in the equipment an aqueous reducing solution with a temperature of at least 75 °C; - step a) includes, simultaneously with the circulation of the reducing aqueous solution in the equipment, a polarization of the wall to a potential lower than the corrosion potential of the metallic alloy in contact with the reducing aqueous solution; - the polarization of the wall is stopped at the end of step a), and in that - step a) is optionally repeated one or more times before proceeding to step b).

2. A method according to claim 1, wherein the metal alloy is a stainless steel, a nickel-chromium superalloy, a nickel-iron-chromium superalloy or a carbon steel, preferably an austenitic stainless steel or a nickel-chromium superalloy.

3. A method according to claim 2, wherein the metal alloy is an austenitic stainless steel AISI 304, AISI 304L, AISI 316 or AISI 316L.

4. A method according to any one of claims 1 to 3, wherein the reducing aqueous solution comprises formic acid, ascorbic acid, nitric acid and sodium bicarbonate.

5. A process according to claim 4, wherein the reducing aqueous solution comprises: 1.8 mol / L to 3 mol / L of formic acid, 0.05 mol / L to 0.3 mol / L of ascorbic acid, 4.5 mmol / L to 5.5 mmol / L of nitric acid, and 5.5 mmol / L to 22 mmol / L of sodium bicarbonate.

6. A method according to any one of claims 1 to 5, wherein the reducing aqueous solution has a temperature between 75 °C and 115 °C, preferably between 75 °C and 95 °C.

7. A method according to any one of claims 1 to 6, wherein step a) is repeated one or more times.

8. A method according to any one of claims 1 to 7, wherein the polarization of the wall comprises the application to this wall of a cathode current by means of a direct current generator.

9. A method according to any one of claims 1 to 8, further comprising, prior to step a) or to the first step a) if step a) is repeated, a circulation in the equipment of an aqueous solution of a strong acid, preferably nitric acid, or of an aqueous solution of a strong base, preferably sodium hydroxide.

10. A method according to any one of claims 1 to 9, wherein, prior to step a) or the first step a) if step a) is repeated, the wall of the equipment is heated to a temperature of at least 75 °C, preferably by circulating water, advantageously distilled, at this temperature through this equipment.

11. A method according to any one of claims 1 to 10, wherein the reducing aqueous solution circulates in a closed circuit during steps a) and b).

12. A method according to any one of claims 1 to 10, wherein step b) further comprises a renewal of the reducing aqueous solution circulating in the equipment.

13. A method according to any one of claims 1 to 12, wherein step b) further comprises a control of the thickness of the corroded inner face by measuring the quantity of metallic cations that are present in the reducing aqueous solution after its exit from the equipment.

14. A method according to any one of claims 1 to 13, further comprising, subsequent to step b), draining the equipment and circulating through this equipment an aqueous solution of a strong acid, preferably nitric acid.

15. A method according to any one of claims 1 to 14, wherein the metallic element is a solution transfer piping between two units of a nuclear installation or a segment of such piping, a component of a nuclear reactor previously unloaded of its fuel, or a dissolver of a spent nuclear fuel processing plant.

Images