EP1192623A1

EP1192623A1 - Method and device for radioactive decontamination of a steel wall

Info

Publication number: EP1192623A1
Application number: EP00922729A
Authority: EP
Inventors: Jean-Michel Fulconis; Jacques Delagrange; Francis Dalard
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1999-04-26
Filing date: 2000-04-25
Publication date: 2002-04-03
Anticipated expiration: 2020-04-25
Also published as: US6702902B1; EP1192623B1; FR2792763B1; DE60040737D1; WO2000065606A1; FR2792763A1; JP2002543401A

Abstract

The invention relates to a method for radioactive decontamination of a steel wall and to a device which is used to carry out said method. The inventive method consists in bringing the wall which is to be decontaminated into contact with a stripping solution comprising a first iron-oxidation reactant, whereby a gas comprising a second oxidation reactant is directly and continuously added to the stripping solution.

Description

METHOD AND DEVICE FOR RADIOACTIVE DECONTAMINATION OF A STEEL WALL

DESCRIPTION

Technical field of the invention

The present invention relates to a process for radioactive decontamination of a steel wall, in particular to a process for radioactive decontamination of a steel wall by chemical pickling. It concerns for example the radioactive decontamination of an internal circuit, a metal wall, a pipe or an apparatus of a reprocessing plant for irradiated nuclear fuels called hereinafter "wall". It particularly concerns the decontamination of an austenitic steel surface which constitutes most of the walls of these factories.

The present invention also relates to a device for implementing this method. At the origin of the radioactive contamination fixed on the walls of factories for reprocessing irradiated fuels, there are essentially surface adsorption phenomena. This contamination includes a labile radioactive contamination, consisting of Pu ²⁴¹ , Am, U, ²⁴² ' ²⁴⁴ Cm, ¹³⁷ Cs, ⁹⁰ Sr and a particulate radioactive contamination, consisting in particular of ruthenium and insolubles of the following types: cesium phosphomolybdate, phosphate zirconium, zirconium molybdate, plutonium phosphate, mixed zirconium and plutonium molybdate, oxides of Mo, Sb, Al, Fe, colloidal plutonium oxides, etc ... It is therefore not necessary to cause ----. A strong erosion to decontaminate these walls. Thus, it is generally accepted that, depending on the walls, erosion on the order of 2 to 10 μm is sufficient. In general, and in particular in the aforementioned example of decontamination of the walls of a plant for reprocessing irradiated nuclear fuels, radioactive decontamination can be broken down into two major steps: - a first step which aims to eliminating labile contamination and the main deposits adhering to the walls, and - a second step which aims to eliminate on the one hand the particulate contamination fixed on the walls and on the other hand the residual deposits. The first stage is a rinsing stage which implements various rinsing sequences which are not corrosive to the wall, and the second stage is an erosive stage which implements corrosive reagents for the wall which are mainly oxidative mixtures of nitric acid / acid. hydrofluoric, nitric acid / cerium nitrate IV, or mixtures including chromic acid, nitric acid and cerium nitrate.

The nitric acid / hydrofluoric acid mixture is a mixture which has the advantage of attacking refractory contamination deposits such as molybdates or various phosphates, for example of Zr ⁴⁺ , Mo0 ₂ ²⁺ , Pu ⁴⁺ , and antimony oxides .

During the corrosion stage, the best decontamination factors are generally reached when erosion is slow, that is to say with a kinetics of the order of 1 μm / h or less, and regular. This is quite difficult to obtain when the circuits to be decontaminated are complex and have zones more sensitive to corrosion such as for example welding zones, zones of mechanical stresses, etc.

Likewise, the oxidant used for corrosion must not be too aggressive. This is why the oxidizing mixture of nitric acid / hydrofluoric acid cannot always be used because it is difficult to control in particular on large developed surfaces.

In addition, certain walls of the internal circuits of plants for reprocessing irradiated nuclear fuels are made of austenitic steel. It is therefore necessary for these walls to use an oxidant which does not cause an intergranular attack on the steel. The redox couple Ce ^τv / Ce ^ττι is one of the rare oxidizing agents which allows predictable erosion on these austenitic steel surfaces without causing too much intergranular attack.

However, the processes of the prior art using the above-mentioned redox couple have in particular the drawbacks of requiring, for effective decontamination, large quantities of cerium, of the order of 0.5 mole / m ² / μm ^"1 , and thereby entailing a high cost of treating effluents since high concentrations of cerium in the vitreous confinement matrices are not admissible, the concentration of cerium required in these processes is therefore a limiting factor.

In the processes of the prior art, the concentration of Ce ^IV decreases throughout the reaction which implies an evolutionary corrosion kinetics: this is too strong at the beginning, too weak at the end. This drawback eliminated by the present invention because the Ce ^IV concentration is almost constant and therefore also the kinetics.

In addition, the methods of the prior art, and in particular the method described in the aforementioned document, are intended to treat oxides and not metal walls with 0-valence such as walls made of austenitic steel. The nature of the alloy to be eroded, or corroded, which in the case of a reprocessing plant is exclusively of austenitic steel and not of the INCONEL (trademark) or INCOLOY (trademark) type means that this chromic acid in the decontaminating solutions of the processes described for the latter is not desirable.

Thus, patent application EP-A-0 174 317 describes a method for decontaminating chromium oxides on the surface of INCONEL steam generators in power plants. In this patent, Ce ^IV used as an oxidizing agent and ozone is used as an agent for regenerating Cerium IV from Cerium III formed during oxidation. This process describes the use as an corrosion fluid of an aqueous solution of nitric acid, chromic acid and cerium nitrate in which ozone has been dissolved. It also recommends using a gas-liquid contactor to dissolve the ozone.

In the practice of decontamination of a steam generator according to the process described in

EP-0 174 317, a regeneration box for Cerium IV is coupled to the steam generator, this box allows regeneration of Cerium IV by injecting ozone until saturation. this is not the case of a reprocessing plant due to the high radiochemical activity α, β and γ of the elements to be decontaminated, the inaccessibility of certain cells and the complexity and variety of the elements to be decontaminated.

In addition, the process described in application EP-0 174 317 reduces without eliminating the variability of the corrosion rate due to its direct dependence on the value of the concentration of Ce ^IV which evolves during the reaction since the regeneration Cerium IV is done in batch.

Disclosure of the invention The present invention overcomes the aforementioned drawbacks by providing a method for decontaminating a steel wall comprising bringing the wall to be decontaminated into contact with a pickling solution comprising nitric acid and a first iron oxidizing agent, at a suitable temperature, so that the surface of said wall is eroded by oxidation of its metallic constituents such as Fe °, Cr °, Ni ⁰ , Mn ° which it contains, said setting in contact being carried out with a direct introduction, continuously, into the pickling solution, of a gas comprising a second oxidizing agent so as to oxidize, at least in part, continuously, the first oxidizing agent reduced by the oxidation of the metallic constituents of steel. During the process of the invention, the first agent for oxidizing the metallic constituents of steel is reduced when it oxidizes the metallic constituents of the steel wall and it is regenerated to continuous by the second oxidizing agent, - _^. by oxidation. The first oxidizing agent is therefore chosen so as to be able to oxidize metallic constituents of the steel wall, and the second oxidizing agent is chosen so as to be able to oxidize the first oxidizing agent reduced by the oxidation of metallic constituents of steel to regenerate it.

For example, the first oxidizing agent can be chosen from CelV, Ag ²⁺ , etc. For example, the gas comprising the second oxidizing agent can be chosen from a gas comprising ozone.

Advantageously, according to an embodiment of the process of the present invention, the first oxidizing agent can be cerium IV, for example in the form of cerium nitrate IV, and the second oxidizing agent can be a gas comprising ozone.

According to this embodiment, the present invention makes it possible, in particular, to eliminate the aforementioned drawbacks relating in particular to the processes of the prior art using cerium.

In fact, the present invention provides a means for regenerating IV cerium continuously throughout the reaction and at constant speed. Thus throughout the reaction, the concentration of IV cerium is constantly equal to the optimal value in terms of decontamination. The present invention describes a process by which a second oxidizing agent, for example ozone, is dissolved in the pickling solution also called, hereinafter, decontamination solution directly in the element to be decontaminated. In addition, the process described in the present invention makes it possible to dissolve the second oxidizing agent, for example ozone in., the pickling solution, or decontamination solution, without making any specific modification or arrangement to an installation to be decontaminated. According to the invention, the pickling solution can be an aqueous solution comprising nitric acid at a concentration of approximately 0.5 to 5 mol. I ^"1 and cerium nitrate at a concentration of about 0.001 to 0.1 mol.l ^{" 1} . According to the invention, the gas comprising ozone can also comprise at least one gas chosen from oxygen and nitrogen.

According to the invention, the gas comprising ozone can comprise approximately 1 to 20% of ozone. According to the invention, the suitable temperature can be approximately 10 to 80 ° C., for example ambient temperature, for example 25 ° C.

According to the invention, the steel wall may be a wall of an internal circuit of a plant for reprocessing irradiated nuclear fuels, for example a wall of austenitic steel.

The ozone introduced into the pickling solution can, for example, be dissolved therein by using two variants of a member which is very common in reprocessing plants and usually used for setting in motion liquid solutions. In the context of the application of the process of the present invention, the introduction of ozone into the erosion solution can be carried out by means of two gas-liquid contact members which can be, on the one hand, air- transfer lift, with natural submergence or vacuum, and / or on the other hand, the mixing air-lift. However, failing these two systems, the invention can be carried out by injecting gas using any plunging pipe for introducing liquid or gaseous reagent. Due to their use as described in the present invention, these devices play a dual role during decontamination:

- by the mixing which they provide, they allow the continuous renewal of the solid liquid interface at which the steps of oxidation of the metal support and of dissolution of the oxidized species are carried out,

- Through the close gas / liquid contact made, they ensure the transfer of the second oxidizing agent, for example ozone, into the pickling solution at the threshold of which the regeneration step of cerium IV takes place.

According to the method of the invention, due to the introduction, or injection, of the gas comprising the second oxidizing agent, for example ozone, in the two air-lift systems towards the decontamination solution, or pickling solution, gas for example ozone plays a double role:

- it is used as an energy source in hydrodynamic mixing, - it provides the solution with the second oxidizing agent, for example ozone, which is necessary for the regeneration of IV cerium.

The present invention also provides a device for radioactive decontamination of a steel wall of an internal circuit of a reprocessing plant for irradiated fuel / nuclear according to the method of the present invention, the internal circuit being provided with air. - brewing lift and / or a transfer air lift, device in which the air lift is used both as a means of introducing the gas comprising the second oxidizing agent into the pickling solution comprising the first oxidizing agent, and as a means of homogenization of the pickling solution when this solution comes into contact with the steel wall to be decontaminated.

According to the invention, the device can also comprise a unit for producing the second oxidizing agent, for example when the first oxidizing agent is cerium IV, the second oxidizing agent can be ozone and l The above production set can therefore be an ozone production set. This assembly can be a conventional assembly, known to those skilled in the art for the production of ozone.

Due to the use as described in the present invention of two types of air lift, the method makes it possible to choose precisely and simply the portion of an installation that one wishes to decontaminate.

Other advantages will become apparent on reading the following description and the following illustrative and nonlimiting examples, with reference to the diagrams in the appendix.

Brief description of the figures

- Figure 1 is an example of an assembly of ozone production and monitoring of the ozone concentration in the process according to one invention; - Figure 2 is a diagram illustrating the process of the present invention applied to a tank of reprocessing plant for irradiated nuclear fuels.

The advantages of the process of the present invention are multiple:

- Due to the remote injection of ozone into the element being decontaminated, the process is completely autonomous and therefore does not require intervention on the decontaminating solution in action on a "process" element. The conduct of the process is limited to a possible analytical monitoring of the concentration of corrosion elements by sampling the pickling solution using already existing sampling systems.

- From a practical point of view, given the low ozone flow rates required, the ozone production system is compact, light and can be easily placed on a mobile support. The ozone production device is supplied either with compressed air available in the installation, it may then be necessary to provide on the assembly a device for deoiling and drying the compressed air, or by means of an air bottle. reconstituted or even pure oxygen. It is advisable to add to the device a means of monitoring the ozone concentration at the outlet of the ozonator so as to regulate the smooth running of the process of regenerating Ce ^ιv . The diagram of FIG. 1 in the appendix presents., An assembly of ozone production and monitoring of the ozone concentration in the gaseous mixture injected towards the pickling solution contained in the element to be decontaminated. In this figure, the reference 1 indicates an ozone production device, the reference 3 a supply of oxygen or of reconstituted air, the reference 5 of the control valves the reference 7 of the control manometers, the reference 9 an ozonator of the type conventional, the reference 11 of the means for adjusting the flow rate of injected gas, the reference 13 an analyzer and the reference 15 a gas supply pipe comprising ozone to the pickling solution contained in the element to be decontaminated. The same ozone production device can therefore be successively used for all of the elements to be decontaminated.

- Due to the principle of direct injection using already existing pipes, the direction of flow of fluids respects the confinement barriers already present in the process.

The ozone production device is under overpressure with respect to the element to be decontaminated, so there is no risk of breaking the containment barriers. The excess ozone directly joins the existing vent circuits. Given the low mass flow of ozone and the dilution factor by the ventilation network, this process does not require any device for neutralizing excess ozone. Chemical aspect of the process of the invention

In general, we can write the chemical reactions involved as follows: 3Ce ⁺⁴ + Fe ° => 3Ce ⁺³ + Fe ³⁺

6Ce ⁺⁴ + Cr ° <=> 6Ce ⁺³ + Cr ⁶⁺

2This ⁺⁴ + Ni ⁰ = 2This ⁺³ + Ni ²⁺

7Ce ⁺⁴ + Mn ° <=> 7Ce ⁺³ + Mn ⁷⁺

2H ⁺ + 2Ce ⁺³ + 0 ₃ <=> 2Ce ⁺⁴ + H ₂ 0 + 0 ₂ Each element of the reactive mixture system has a well defined role:

- nitric acid is used to make the passivation layer of metal walls permeable to ions and therefore make stainless steel susceptible to corrosion;

- CelV is the engine of corrosion, it is it which will exchange electrons with the constituent elements of the alloy to be corroded;

- ozone is the regenerating agent for CelV because the redox potential of the couple 0 ₃ , H ⁺ / H ₂ 0, 0 ₂ is greater than that of the couple Ce ^τv / Ce ^lτι :

2.075 V / ENH versus 1.72 V / ENH.

A key parameter in controlling the corrosion rate is the concentration of CelV. Thus, depending on the desired thickness of corrosion, which depends on the depth of contamination generally between

2 and 10 μm, as well as the surface / volume ratio of the circuit considered, the initial CelV concentration will be chosen to jointly optimize the corrosion rate and the duration of the operation. The present invention therefore makes it possible to control the concentration of Ce ^IV by in situ oxidation of Ce ¹¹¹ formed during corrosion by ozone dissolved in the pickling solution.

From a total concentration of cerium nitrate given, the balance of the Ce ^IV / Ce ¹¹¹ ratio is reached as soon as the two consumption and production speeds of Ce ^IV are stationary, the time taken to reach this stationary state the shorter the quantity of cerium is. Under the conditions of implementation of the invention, taking into account the small quantities of cerium used, the stationary state is reached quickly; moreover, the equilibrium reached is strongly displaced towards the valence IV of the cerium which is thus largely in the majority (more than 90%) compared to the Cerium III. The use of the cerium introduced is therefore optimal since almost all of the cerium introduced is permanently in its "useful" form. This condition, to be fulfilled, requires a slight excess production of ozone. The regeneration reaction of Ce ^IV with ozone from Ce ^{111 is} written:

0 ₃ + 2H ⁺ + 2e ^" <t = 0 ₂ + H ₂ 0

2Ce ⁺³ <= 2Ce ⁴⁺ + 2 e ^" or: 0 ₃ + 2H ⁺ + 2Ce ³⁺ → 0 ₂ + H ₂ 0 + 2Ce ⁺

The speed of regeneration of Ce ^IV from Ce ¹¹¹ depends on many parameters. We can express this equation by a classic Arrhenius law of the type: d [Ce ^IV ] / dt = A _app exp (-E _app / RT) [0 _3dlssous ] ^a [Ce ^III ] ^b with:

• A _pp and E _app respectively the pre-exponential factor and the activation energy of Arrhenius of the reaction: these quantities - _^ have been called apparent since in our case, they depend on many parameters such as acidity, • R the constant of ideal gases,

• T the thermodynamic temperature,

• [O ₃ _below ] the ozone concentration in the solution,

On the order of the reaction with respect to the concentration of dissolved ozone,

• [Ce ¹¹¹ ] the cerium concentration of valence III,

• b the order of the reaction relative to the concentration of cerium at valence III. If the temperature and the acidity of the medium are fixed, the rate of regeneration of the CelV is dependent on the concentration of ozone dissolved in the solution. This concentration is only constant when the solution is saturated with ozone. The saturation speed of the ozone decontaminating solution depends on many parameters such as the acidity of the medium, the temperature or the mode of contactor used. Note that under the operating conditions of the process of the invention and in the absence of an ozone consumption reaction, the ozone concentration is stable after a few minutes: ten minutes in the worst case. The speed of passage of ozone in the liquid phase therefore does not constitute a limiting factor in the speed of regeneration of CelV.

The ozone concentration at equilibrium with a nitric solution depends on many parameters, one the temperature can for example be cited: the solubility decreases with temperature; the nitric acid concentration: the solubility decreases slightly with the nitric acid content. We give in Tables (I) and (II) below some values measured under the conditions of use of the process of the invention.

Table I [O _3] have _ssous depending on the temperature

0.5 <HNO ₃ <4 mol.l ^"1 , [0 ₃ ] _ga2 = 35 g. M ^{" 3}

Table II [O _3] dissou _s according to _[3 0] _ga z

0.5 <NHO ₃ <4 mol.l ^"1 , Θ = 20 ° C

The process described in the present invention makes it possible to control each of the following parameters: temperature for example between 10 and 50 ° C. for example, ambient temperature, for example around 25 ° C.

- acidity of, for example, between 0.5 and 4 mol.l ^"1 , for example HN0 ₃ 4 mol.l ^{" 1} , - initial concentration of cerium of, for example, between 1.10 ^"3 and 10 ^{" 1} mol.l ^"1 , for example 10 ^"2 mol.l ^{" 1} ,

- ozone gas flow rate adapted to the volume of the element to be decontaminated and to the type of contactor,

- concentration of ozone in the gas, for example between 1 to 20% by weight.

Thus, the concentration of dissolved ozone can vary between 1 and 20 mg.l ^"1. Due to the use of just sufficient quantity of cerium with adequate kinetics, the residual corrosion at the end of the operation is reduced: stopping ozone injection means almost immediate stopping of regeneration of CelV and quickly stopping corrosion due to the small amount of CelV present in the medium, on the other hand, the amount of IV cerium present at the stop of the ozone injection is low and known thus, the final monitoring of corrosion is very easy. The corrosion rate can thus be controlled making analytical monitoring easier to implement by 'the set of regular samples and analyzes of corrosion solution samples. Figure 2 in the appendix is a diagram illustrating the process of the present invention applied to a tank of a reprocessing plant for irradiated nuclear fuels. In this figure, the reference 17 indicates the wall to be decontaminated, the references 19 and 21 and 101 indicate the air or ozone oxygen supply pipes, the reference AC corresponds to a compressed air intake of the air. -lift 102, the reference AL corresponds to a natural AL circuit, it relates to the air-lift with natural submergence (cf. reference 102), the reference 23 indicates the pickling solution according to the invention, the reference 25 a privileged area ozone solubilization, the references 29 and 102 indicate other privileged zones of ozone solubilization, the references 27 and 103 indicate the overflow pipes of the separator pots, the references 31 and 104 indicate a separator pot, the reference 33 a vacuum production circuit, the reference 105 a vent circuit, the references 35 and 106 the return pipes of the regenerated solutions to the tank to be decontaminated, the reference C indicates a circuit under empty, the reference ALI an immersed brewing air-lift, the reference R a return to the initial tank and the reference E shows the outlet to the vent. In this figure, the dotted line referenced 40 highlights, the decontamination limited to the walls of the tank and its equipment, and the dotted line referenced 50 the decontamination of all the walls of the circuit comprising the tank and the additional equipment such as the piping of the transfer elements and the separator pots. The compressed air intake AC can also be used for injecting a gas or a gaseous mixture. It has the same functions as those relating to elements referenced 19 and 21.

The present invention makes it possible to use three types of air lift:

- a vacuum air lift,

- a submerged air lift, and

- an air-lift with natural submergence.

Examples

As examples, we give in table (III) following the evolution of the thickness of corroded steel as a function of the CelV concentration after 12 hours during the corrosion of a stainless steel test piece 304 L with a Surface / Volume ratio of 10: HN0 ₃ 4 mol.l ^"1 , ozone flow (do ₃ ) = 6 g. H ^{" 1} , volume of solution 3 liters.

Table III

Thickness of 304 L corroded in 12 hours depending on [CelV]

HN0 ₃ 4 mol.l ^-1 , d ₀₃ = 6 g. h ^"1 , v = 3 1

For comparison, we give in the tab] _. Water (IV) according to the quantity of cerium necessary to decontaminate 1 m ² of surface (ratio S / V = 10) according to the thickness of corrosion desired according to which one uses the conventional cerium alone process or the process set out in the present invention.

Table IV Comparison of the quantities of cerium required to decontaminate 1 m ² of surface

(S / V ratio = 10) depending on the desired corrosion thickness

In the case of the conventional process, the quantity of cerium required increases with the thickness of steel to be corroded. The present invention makes it possible, for example, to obtain a corrosion rate of 0.5 μm.h ^"1 , or 10 μm in 24 hours, with a solution of 1.10 ^{" 2} mol.l ^"1 of cerium in nitric acid 4 mol.l ^"1 under bubbling oxygen ozone.

Claims

1. A method of radioactive decontamination of a steel wall comprising bringing the wall to be decontaminated into contact with a pickling solution comprising nitric acid and a first agent for oxidizing the metallic constituents of the steel, at a adequate temperature, so that the surface of said wall is eroded by oxidation of the metal constituents of the steel it contains, said contacting being carried out with a direct, continuous introduction into the pickling solution, d a gas comprising a second oxidizing agent so as to oxidize, at least in part, continuously, said first oxidizing agent reduced by the oxidation of the metallic constituents of the steel.

2. The method of claim 1, wherein the first oxidizing agent is cerium nitrate IV and the second oxidizing agent is 1 ozone.

3. The method of claim 2, wherein the pickling solution is an aqueous solution comprising nitric acid at a concentration of about 0.5 to 5 mol.l ^"1 and cerium nitrate at a concentration of about 0.001 to 0.1 mol.l ^"1 .

4. The method of claim 2, wherein the gas comprising ozone further comprises at least one gas selected from oxygen and nitrogen.

5. Method according to claim 2 or 4, -in which the gas comprising ozone comprises approximately 1 to 20% of ozone.

6. The method of claim 2, wherein the temperature is about 10 to 80 ° C.

7. Method according to any one of the preceding claims, in which the steel wall is a wall of an internal circuit of a plant for reprocessing irradiated nuclear fuels.

8. Device for radioactive decontamination of a steel wall of an internal circuit of a plant for reprocessing irradiated nuclear fuels according to the method of claim 1, the circuit being provided with a stirring air-lift and / or a transfer air-lift, a device in which the air-lift is used both as a means of introducing the gas comprising the second oxidizing agent into the pickling solution comprising the first oxidizing agent, and as a means of homogenizing the pickling solution when this solution is brought into contact with the steel wall to be decontaminated.

9. Device according to claim 8, further comprising an assembly for producing the second oxidizing agent.

10. Device according to claim 9, in

/ which the first oxidizing agent being cerium IV, the production set of the second oxidizing agent is an ozone production set.